Applied Surface Science (v.252, #13)
Subject Index (VIII-XXV).
Author Index (I-VII).
Guest Editors’ Preface by Juergen Reif; Ioanna Zergioti; Valentin Craciun; Thomas Lippert; Jacques Perrière (4359).
Spectral and temporal characteristics of metallic nanoparticles produced by femtosecond laser pulses by D. Scuderi; R. Benzerga; O. Albert; B. Reynier; J. Etchepare (4360-4363).
We study the time of flight optical emission from titanium and tungsten nanosized particles, generated through femtosecond laser-matter interaction in vacuum, in the wavelength spectral range from 300 to 900 nm. Typical spectra consist of broadband structureless signals similar to black body emission from a macroscopic object. Nanoparticles temperature, deduced from their emission spectra, decreases drastically as a function of their time of arrival at a given distance from the target. This behaviour is seen to be independent of individual particle velocities.
Keywords: Metallic nanoparticles; Femtosecond laser pulses; Optical emission;
Influence of ambient gas ionization on the deposition of clusters formed in an ablation plume by A. Bailini; P.M. Ossi (4364-4367).
The effect of inert gas ionization on the dynamics of a laser ablation plume expanding through a background inert gas is studied. Charge transfer reactions between ablated ions and neutral background gas atoms yield to the formation of a charged layer on the plume expansion front. The energy lost by ablated ions when the plume is slowed down is calculated. The observed microstructure differences between carbon films prepared by pulsed laser deposition in helium, where the ionization mechanism is absent and respectively in argon, where it is present, are well correlated to model predictions.
Keywords: Pulsed laser deposition; Plume dynamics; Ionization; Cluster-assembled films; Carbon;
Measurements of nanoparticle size distribution produced by laser ablation of tungsten and boron-carbide in N2 ambient by Miklós Bereznai; Péter Heszler; Zsolt Tóth; Ola Wilhelmsson; Mats Boman (4368-4372).
Nanoparticles (NPs) were produced by ablating tungsten and boron-carbide (B4C) target materials in atmospheric pressure nitrogen ambient using ArF excimer laser pulses. The size distributions of the NPs formed during the ablation were monitored—within a 7–133 nm size window—by a condensation particle counter connected to a differential mobility analyzer. The laser repetition rate was varied between 1–50 Hz, and the fluence was systematically changed in the range of 0.5–15 J/cm2, for both materials, allowing a comparative study in an extended laser parameter regime. The multishot ablation threshold (Φ th) of B4C was determined to be ∼1.9 J/cm2 for the laser used (ArF excimer, λ = 193 nm). Similarly to earlier studies, it was shown that the size distributions consist of mainly small nanoparticles (<∼20 nm) attributed to a non-thermal ablation mechanism below Φ th. An additional broad peak appears (between 20 and 40 nm) above Φ th as a consequence of the thermally induced macroscopic ablation. Chemical composition of deposited polydisperse nanoparticles was studied by X-ray photoelectron spectroscopy showing nitrogen incorporation into the boron-carbide.
Keywords: Nanoparticle; Laser ablation; Tungsten; Boron-carbide; Size distribution;
Laser induced synthesis of nanoparticles in liquids by P.V. Kazakevich; A.V. Simakin; V.V. Voronov; G.A. Shafeev (4373-4380).
The review of results on nanoparticles formation is presented under laser ablation of Ag, Au, and Cu-containing solid targets in liquid environments (H2O, C2H5OH, C2H4Cl2, etc.). X-ray diffractometry (XRD), UV–vis optical transmission spectrometry, and high resolution transmission electron microscopy (HRTEM) characterize the nanoparticles. The morphology of nanoparticles is studied as the function of both laser fluence and nature of the liquid. The possibility to control the shape of nanoparticles by ablation of an Au target by an interference pattern of two laser beams is demonstrated. Formation of alloyed Au–Ag and Ag–Cu nanoparticles is reported under laser exposure of a mixture of individual nanoparticles. The effect of internal segregation of brass nanoparticles is discussed due to their small lateral dimensions. The factors are discussed that determine the distribution function of particles size under laser ablation. The influence of laser parameters as well as the nature on the liquid on the properties of nanoparticles is elucidated.
Keywords: Laser ablation; Nanoparticles; Plasmon resonance; Colloidal solutions;
Microstructure investigation and heat treatment of nanocrystalline laser-deposited carbon fibers by C. Fauteux; R. Longtin; J. Pegna; M. Boman (4381-4386).
Carbon fibers were deposited by laser-assisted chemical vapor (LCVD) deposition from ethylene at sub-atmospheric pressure. Prior Raman spectroscopy analysis allowed the determination of the crystal size (average value of 3 nm in the edge region and from 8 to 113 nm in the center region). In this analysis, the microstructure of as-grown and heat treated fibers was examined using micro-Raman spectroscopy and transmission electron microscopy. The analysis revealed the degree of three-dimensional ordering of graphite planes and that the fibers were porous.
Keywords: Carbon fibers; Laser-assisted CVD; Raman spectroscopy; Transmission electron microscopy;
Laser ablation of toluene liquid for surface micro-structuring of silica glass by H. Niino; Y. Kawaguchi; T. Sato; A. Narazaki; T. Gumpenberger; R. Kurosaki (4387-4391).
Microstructures with well-defined micropatterns were fabricated on the surfaces of silica glass using a laser-induced backside wet etching (LIBWE) method by diode-pumped solid state (DPSS) UV laser at the repetition rate of 10 kHz. For a demonstration of flexible rapid prototyping as mask-less exposure system, the focused laser beam was directed to the sample by galvanometer-based point scanning system. Additionally, a diagnostics study of plume propagation in the ablated products of toluene solid film was carried out with an intensified CCD (ICCD) camera.
Keywords: Laser-induced backside wet etching (LIBWE); Diode-pumped solid state UV laser; Silica glass; UV absorption;
In situ reflectivity investigations of solid/liquid interface during laser backside etching by R. Böhme; T. Otto; K. Zimmer (4392-4396).
In situ reflectivity measurements of the solid/liquid interface with a pump-probe setup were performed during laser-induced backside wet etching (LIBWE) of fused silica with KrF excimer laser using toluene as absorbing liquid. The intensity, the temporal shape, and the duration of the reflected light measured in dependence on the laser fluence are discussed referring to the surface modification and the bubble formation.The vaporisation of the superheated liquid at the solid interface causes a considerable increase of the reflectivity and gives information about the bubble lifetime. The alterations of the reflectivity after bubbles collapse can be explained with the changed optical properties due to surface modifications of the solid surface. Comparative studies of the reflectivity at different times and the etch rate behaviour in dependence on the laser fluence show that the in situ measured surface modification begins just at the etch threshold fluence and correlates further with etch rate behaviour and the etched surface appearance. The already observed surface modification at LIBWE due to a carbon deposition and structural changes of the near surface region are approved by the changes of the interface reflectivity and emphasizes the importance of the modified surface region in the laser-induced backside wet etching process.
Keywords: Laser; Etching; Fused silica; Solid/liquid interface; Time-resolved reflectivity;
Surface functionalisation of polypyrrole films using UV light induced radical activation by P. Lisboa; D. Gilliland; G. Ceccone; A. Valsesia; F. Rossi (4397-4401).
Electrochemically deposited polypyrrole (PPy) films were functionalised with amine or carboxylic function. The functionalisation was done by grafting allylamine or acrylic acid (AAc) using UV light radical activation. The active groups of the surface were quantified by X-ray photoelectron spectroscopy (XPS) after chemical derivatisation with trifluoroethanol (TFE) or 4-trifluoromethylbenzaldehyde (TFBA), respectively. Grafting with AAc completely covered the PPy film introducing high levels of carboxylic function. In the case of allylamine grafting, a saturation point at low amine carbon level was achieved. Further characterisation of the surfaces was done by time of flight secondary ion mass spectroscopy (TOF-SIMS), atomic force microscope (AFM) and scanning electron microscope (SEM).
Keywords: Conductive polymer; Polypyrrole; UV grafting; Chemical derivatisation; Surface micro-patterning;
Advanced excimer-based crystallization systems for production solutions by F. Simon; J. Brune; L. Herbst (4402-4405).
Line beam excimer laser annealing (ELA) is a well-known technique for thin Si-film crystallization and established in LTPS mass production. With introduction of sequential lateral solidification (SLS) some aspects such as crystalline quality, throughput and flexibility regarding the substrate size could be improved, but for OLED manufacturing still further process development is necessary. This paper discusses line beam ELA and SLS-techniques that might enable process engineers to make polycrystalline-silicon (poly-Si) films with a high degree of uniformity and quality as required for system on glass (SOG) and active matrix organic light emitting displays (AMOLED). Equipment requirements are discussed and compared to previous standards. SEM-images of process examples are shown in order to demonstrate the viability.
Keywords: Excimer laser annealing; ELA; SLS; Poly-Si; LTPS; SOG;
Diode laser soldering using a lead-free filler material for electronic packaging structures by C. Chaminade; E. Fogarassy; D. Boisselier (4406-4410).
As of today, several lead-free soldering pastes have been qualified for currently used soldering process. Regarding the new potential of laser-assisted soldering processes, the behaviour of the SnAgCu soldering paste requires, however, new investigations. In the first part of this study, the specific temperature profile of a laser soldering process is investigated using a high power diode laser (HPDL). These experimental results are compared to a thermal simulation developed for this specific application. The second part of this work deals with the diffusion of the tin-based filler material through the nickel barrier using the information extracted from the temperature simulations.
Keywords: Laser; Soldering; Lead-free; Thermal modelling; Electronic packaging;
Molecular dynamics simulation study of deep hole drilling in iron by ultrashort laser pulses by N.N. Nedialkov; P.A. Atanasov (4411-4415).
Classical molecular dynamics simulation technique is applied for investigation of the iron ablation by ultrashort laser pulses at conditions of deep hole for the first time. Laser pulse duration of 0.1 ps at wavelength of 800 nm is considered. The evolution of the ablated material in deep hole geometry differs completely from the free expansion regime as two major mechanisms are important for the final hole shape. The first one is the deposition of the ablated material on the walls, which narrows the hole at a certain height above its bottom. The second mechanism is related to ablation of the material from the walls (secondary ablation) caused by its interaction with the primary ablated particles. Properties of the secondary ablated particles in terms of the velocity and the angular distribution are obtained. The material removal efficiency is estimated for vacuum or in Ar environment conditions. In the latter case, the existence of well-defined vapor cloud having low center of the mass velocity is found. The processes observed affect significantly the material expulsion and can explain the decrease of the drilling rate with the hole depth increase, an effect observed experimentally.
Keywords: Molecular dynamics simulation; Hole; Iron;
Dynamics of laser-induced desorption from dielectric surfaces on a sub-picosecond timescale by Florenţa Costache; Sebastian Eckert; Juergen Reif (4416-4419).
The impact of ultra-short laser pulses induces intensity-dependent non-equilibrium processes in the surface region of dielectric targets, resulting in desorption of surface constituents. We report time-resolved studies on particle ejection from CaF2 and BaF2 targets.Pump–probe time-of-flight mass spectrometry (ToF MS) was used to measure the particle yields as a function of the delay time between pairs of sub-damage threshold laser pulses, thus obtaining the temporal dynamics of the laser-excited charged particle emission.In a correlative manner, the positive ion, electron and negative ion desorption yields dependence on the pump–probe delay time reveal a coherence peak around zero delay (similar to a two-pulse autocorrelation), accounting for the increase of the ion yield with laser intensity. Additionally, the measurements reveal a delayed peaks at ∼900 fs (BaF2) and ∼300 fs (CaF2). For comparison, we present time-resolved studies on electron emission from aluminum targets with pump and probe pulses below the damage threshold. The measurements show, again, the autocorrelation peak in the coherence region and an additional increase in the electron yield when the pulses are several picoseconds apart.The autocorrelation could also stand for the dephasing time of the electronic coherence, while the delayed peaks may reveal for the time needed for the collisional energy to be transferred to the lattice. The pump pulse induces a new unstable phase, which is further destabilized by the probe pulse.A corresponding qualitative picture for temporal dynamics of femtosecond (fs) laser-induced particle emission is proposed.
Keywords: Femtosecond laser desorption; Time-of-flight mass spectrometry; Temporal dynamics; Dielectrics;
Microstructural analyses of the nanoparticles obtained after laser irradiation of Ti and W in ethanol by M.S.F. Lima; F.P. Ladário; R. Riva (4420-4424).
The laser ablation technique has been employed to prepare titanium (Ti) and tungsten (W) colloids from the elemental solids in ethanol using a copper-hydrogen bromide (CuHBr) vapour laser. The obtained nanoparticles were spherical with most diameters ranging from 0.05 to 0.5 μm. Some particles, notably when using Ti targets, are bigger than 0.9 μm, indicating resolidification of liquid droplets. The mechanism of material removal was characterized by photo-ablation where some particles were generated by rapid solidification from the melt. The W particles had tendency to coalescence, since small clusters merge to larger spheres. On the other hand, the Ti particles were coalescence-free, perhaps due to the thin high-resistant oxide layer at their surface.
Keywords: Nanomaterials; Nanoparticles; Laser; Colloids;
Mechanism of the formation of metal nanoclusters during pulsed laser deposition by M.A. Pushkin; V.V. Lebid’ko; V.D. Borman; V.N. Tronin; V.I. Troyan; I. Smurov (4425-4428).
The geometrical structure of Au, Ni, Co and Cr nanoclusters self-assembled on NaCl and HOPG surfaces under pulsed laser deposition (PLD) has been experimentally investigated. The PLD technique is characterized by an extremely high instantaneous deposition rate. Unlike for the thermal evaporation (TE) process, formation of fractal nanoclusters under PLD conditions has been observed with scanning tunneling microscopy (STM). The driving mechanism for this phenomenon occurring at high deposition rate is thought to be the evolution of the initial interacting-adatom states in a system far from thermodynamic equilibrium. The obtained results can be explained by proposing a new mechanism of condensed phase formation under the conditions of strong deviation from thermodynamic equilibrium.
Keywords: Nanoclusters; Pulsed laser deposition; Scanning tunneling microscopy;
Au cluster growth on ZnO thin films by pulsed laser deposition by E. György; J. Santiso; A. Figueras; A. Giannoudakos; M. Kompitsas; I.N. Mihailescu; C. Ducu (4429-4432).
Nanostructures formed by Au nanoparticles on ZnO thin film surface are of interest for applications which include medical implants, gas-sensors, and catalytic systems. A frequency tripled Nd:YAG laser (λ = 355 nm, τ FWHM ∼ 10 ns) was used for the successive irradiation of the Zn and Au targets. The ZnO films were synthesized in 20 Pa oxygen pressure while the subsequent Au coverage was grown in vacuum. The obtained structures surface morphology, crystalline quality, and chemical composition depth profile were investigated by acoustic (dynamic) mode atomic force microscopy, X-ray diffraction, and wavelength dispersive X-ray spectroscopy. The surface is characterized by a granular morphology, with average grain diameters of a few tens of nanometers. The surface roughness decreases with the increase of the number of laser pulses applied for the irradiation of the Au target. The Au coverage reveals a predominant (1 1 1) texture, whereas the underlying ZnO films are c-axis oriented. A linear dependence was established between the thickness of the Au coverage and the number of laser pulses applied for the irradiation of the Au target.
Keywords: Au/ZnO nanostructures; Pulsed laser irradiation;
Synthesis of nanoclusters by nanosecond laser ablation: Direct simulation Monte Carlo modelling by Tatiana E. Itina; Marc Sentis; Wladimir Marine (4433-4438).
Collisional processes leading to the formation of nanoparticles in a laser-ablated plume are numerically simulated with the aid of an atomistic-level model based on direct simulation Monte Carlo (DSMC) method. The formation of nanoparticles in nanosecond laser ablation of a mono-atomic target is investigated in the presence of an inert background gas. The DSMC procedure is modified in order to account for numerous plume species and to describe several reactions (i.e., recombination/dissociation, sticking, evaporation) taking place in the plume and affecting the size and spatial distribution of the produced nanoclusters. Calculation results allow us to visualize the nanoparticles and to correlate their space distributions with plume dynamics. In addition, cluster size distributions are investigated at different pressures. The effects of the background gas on cluster formation within the plume are furthermore shown.
Keywords: Nanoparticles; Nanosecond laser ablation; Simulation; Clusters;
Synthesis of nanosized particles during laser ablation of gold in water by N.V. Tarasenko; A.V. Butsen; E.A. Nevar; N.A. Savastenko (4439-4444).
In this paper we report the formation of gold nanoparticles during laser ablation of gold target in water in the absence of any additives. The experiments were carried out by using the radiation of the pulsed Nd:YAG laser, operating at the second (532 nm, 10 ns, 10 Hz), or the fourth harmonic (266 nm) wavelengths. The properties of the nanoparticles were found to be susceptible to the additional 532 and 266 nm laser irradiation. It has been established that both the mean size of the nanoparticles and their stability could be varied by proper selection of the parameters of laser ablation and postirradiation such as laser fluence and wavelength combinations.
Keywords: Nanoparticles; Gold colloids; Laser ablation; Laser-induced modification;
Pulsed laser deposition of nanostructured Ag films by Tony Donnelly; Brendan Doggett; James G. Lunney (4445-4448).
Ultra-thin (0.5–5 nm) films of Ag have been prepared by pulsed laser deposition in vacuum using a 26 ns KrF excimer laser at 1 J cm−2. The deposition was controlled using a Langmuir ion probe and a quartz crystal thickness monitor. Transmission electron microscopy showed that the films are not continuous, but are structured on nanometer size scales. Optical absorption spectra showed the expected surface plasmon resonance feature, which shifted to longer wavelength and increased in strength as the equivalent film thickness was increased. It is shown that Maxwell Garnett effective medium theory can be used to calculate the main features of optical absorption spectra.
Keywords: Pulsed laser deposition; Nanostructured Ag films; Maxwell Garnett theory;
Laser synthesis of nanostructures based on transition metal oxides by S.A. Mulenko; V.P. Mygashko (4449-4452).
Nanostructures based on iron oxides in the form of thin films were synthesized while laser chemical vapor deposition (LCVD) of elements from iron carbonyl vapors (Fe(CO)5) under the action of Ar+ laser radiation (λ L = 488 nm) on the Si substrate surface with power density about 102 W/cm2 and vapor pressure 666 Pa. Analysis of surface morphology and relief of the deposited films was carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM). This analysis demonstrated their cluster structure with average size no more than 100 nm. It was found out that the thicker the deposited film, the larger sizes of clusters with more oxides of higher oxidized phases were formed. The film thickness (d) was 10 and 28 nm. The deposited films exhibited semiconductor properties in the range 170–340 K which were stipulated by oxide content with different oxidized phases. The width of the band gap E g depends on oxide content in the deposited film and was varied in the range 0.30–0.64 eV at an electrical field of 1.6 × 103 V/m. The band gap E g was varied in the range 0.46–0.58 eV at an electrical field of 45 V/m. The band gap which is stipulated by impurities in iron oxides E i was varied in the range 0.009–0.026 eV at an electrical field of 1.6 × 103 V/m and was varied in the range 0–0.16 eV at an electrical field 45 V/m. These narrow band gap semiconductor thin films displayed of the quantum dimensional effect.
Keywords: Laser deposition; Thin films; Quantum dimensional effect;
3D periodic structures grown on silicon by radiation of a pulsed Nd:YAG laser and their field emission properties by A.V. Karabutov; G.A. Shafeev; N. Badi; A.M. Nair; A. Bensaoula (4453-4456).
Periodic three-dimensional structures were successfully grown on single crystal Si wafers either bare or Au-covered under their exposure to a pulsed radiation of a Nd:YAG laser in vacuum. The structures protrude above the initial wafer surface for 10 μm while their spatial period is about 70 μm. The coupling of the laser radiation to Si surface is related to the thermal non-linear absorption of the near band gap radiation. The structures exhibit an efficient field emission with an average emission current of 5 mA/cm2 and is sensitive to the post-treatment of samples. The drawbacks of the emission current densities are discussed.
Keywords: Laser ablation; Periodic structures; Field emission;
Formation of periodic structures by laser ablation of metals in liquids by P.V. Kazakevich; A.V. Simakin; G.A. Shafeev (4457-4461).
Experimental results are presented on ablation of metals (W, Cu, brass and bronze) in a liquid environment (e.g., ethanol or water) by irradiation with either a pulsed copper vapor laser (0.51 μm) or a pulsed Nd:YAG laser (1.06 μm). The target material is ejected into surrounding liquid in the form of nanoparticles. In a certain range of laser parameters (fluence and number of laser shots) the surface of the solid target is composed of micro-cones having a regular structure. The distance between neighboring micro-cones in the structure depends on the laser spot size. The structures allow the observation of up-conversion of the laser frequency due to generation of the second harmonics in the eye retina.
Keywords: Laser ablation; Periodic structures;
Ultraviolet laser microstructuring of silicon and the effect of laser pulse duration on the surface morphology by E. Skantzakis; V. Zorba; D.G. Papazoglou; I. Zergioti; C. Fotakis (4462-4466).
The study of the laser pulse duration effect on the silicon micro-spikes morphology is presented. The microcones were produced by ultraviolet (248 nm) laser irradiation of doped Si wafers in SF6 environment. The laser pulse duration was adjusted at 450 fs, 5 ps and 15 ns. We have analyzed the statistical nature of the spikes’ morphological characteristics, such as periodicity and apex angle by exploiting image processing techniques, on SEM images of the irradiated samples. The correlation of the quantitative morphological characteristics with the laser parameters (pulse duration, laser fluence and number of pulses) provides new insight on the physical mechanisms, which are involved on the formation of Si microcones.
Keywords: Laser materials processing; Silicon microcones;
Surface control of optical properties in silicon nanocrystals produced by laser pyrolysis by E. Trave; V. Bello; G. Mattei; M. Mattiazzi; E. Borsella; M. Carpanese; F. Fabbri; M. Falconieri; R. D’Amato; N. Herlin-Boime (4467-4471).
Macroscopic quantities (g/h) of Si nanoparticles were prepared by laser pyrolysis of silane and showed photoluminescence (PL) emission in the range 700–1050 nm after oxidation in air at a temperature T ≥ 700 °C. Two different strategies were followed to reduce as-produced particle agglomeration which hinders most of the applications, namely etching with either acid or alkaline solutions. Well isolated single particles were detected after acid etching in HF. Disaggregation was also achieved by the combined effect of the high power sonication and alkaline etching by tetra-methyl ammonium hydroxide (TMAH), which leaves OH terminated surfaces. However, in both cases re-aggregation was observed within a few hours after oxide removal. Stable dispersions of Si nanoparticles in different solvents were obtained by treatments of H-terminated surfaces with the surfactant TOPO (C24H51P＝O, trioctylphospine oxide) and by treatment of OH-terminated surfaces with Na3PO4.
Keywords: Silicon nanocrystals; Photoluminescence; Tetra-methyl ammonium hydroxide;
Surface temperature evolution in pulsed laser action of millisecond range by M. Doubenskaia; I. Smurov (4472-4476).
An originally developed multi-wavelength pyrometer (12 wavelengths in the range 1.001–1.573 μm, 50 μs acquisition time for each photodiode, 800 μm spatial resolution, 900–3500 °C brightness temperature range) is used to measure brightness temperature under the pulsed action of Nd:YAG laser (HAAS–HL62P) on stainless steel (INOX 304L) substrates. Specially developed “notch” filters (10−6 transparency at 1.06 μm wavelength) are applied to avoid the influence of laser radiation on temperature measurements. The true temperature is restored on the basis of method of multi-colour pyrometry. The accuracy of brightness temperature measurements is examined by comparing the temperature evolution for pulses with different durations but with the same value of energy density flux.The influence of the following parameters is studied keeping the remaining ones constant: pulse duration (6–20 ms, rectangular pulse shape), energy per pulse (10–33 J, rectangular pulse shape), pulse shape (three types of triangulars and one rectangular). Finally the evolution of surface temperature for pulses with more complex shapes but with the same pulse duration and energy per pulse is compared.
Keywords: Surface temperature measurements; Multi-wavelength pyrometry; Laser pulse shape; Melting; Solidification;
Cathodoluminescence and epitaxy after laser annealing of Cs+-irradiated α-quartz by P.K. Sahoo; S. Gąsiorek; S. Dhar; K.P. Lieb; P. Schaaf (4477-4480).
In the course of a systematic investigation of dynamic, chemical, and laser-induced solid phase epitaxy of α-quartz after ion implantation, we have studied epitaxy and cathodoluminescence emission after 250 keV Cs-ion implantation and subsequent pulsed excimer laser treatment in air. Rutherford backscattering channelling analysis showed partial epitaxy for all the laser-irradiated samples; however, no full epitaxy was achieved. The optical properties of these samples were analyzed using cathodoluminescence spectroscopy, giving evidence of five emission bands at 2.42, 2.79, 3.25, 3.65, and 4.30 eV photon energy. Their intensity relation to the laser power and retained Cs-ion fraction are discussed and the present results will be compared with those obtained after chemical and dynamic epitaxy of quartz after alkali-ion, Ge, and Ba implantation.
Keywords: Epitaxy; Ion implantation; Quartz; Excimer laser annealing;
The p–n junction formation in Hg1−x Cd x Te by laser annealing method by L. Dumanski; M. Bester; I.S. Virt; M. Kuzma (4481-4485).
The formation of p–n junctions in Hg1−x Cd x Te is still an open research task. In this paper, laser treatment of n-type Hg1−x Cd x Te samples resulting in the formation of a p–n junction is studied. The YAG:Nd3+ laser with pulse duration of 250 μs or 40 ns was used. The energy density of laser beam was below the threshold of sample surface melting. The interpretation of the results is based on a model of defects formation related to interstitial mercury diffusion following laser irradiation. The Hall measurements clearly point out to a simple p–n junction. The resistance of samples shows the long time relaxation described by the 1/2 power law, which is attributable to the defect diffusion processes, but not to the changes in the electron–hole systems.
Keywords: p–n Junctions; Laser annealing; HgCdTe;
Effect of laser-remelting of surface cracks on microstructure and residual stresses in 12Ni maraging steel by J. Grum; J.M. Slabe (4486-4492).
The paper presents the results of a study on possible application of laser-remelting to repair of narrow and comparatively deep cracks at the surface of highly thermo-mechanically loaded parts made of 12% Ni hot-working maraging tool steel. Laser-remelting of maraging steel is, due to very good weldability and flexibility of the process, very prospective for repair of fatigued surfaces of parts made of this steel at which the presence of surface microcracks may be observed. In addition to the efficiency of crack remelting, the influence of laser-remelting on the heat-affected zone in terms of its microstructure and residual stresses was also studied. The microstructure in the laser-remelted track is cellular/dendritic. In the heat-affected zone surrounding the laser-remelted track, the microstructure varies considerably. A microstructure analysis revealed, in the heat-affected zone, five microstructural zones and sub-zones. Residual stresses measured after laser-remelting are with reference to gradual through-depth changing of the stresses favourable.
Keywords: Laser-remelting; Maraging steel; Microstructure; Residual stresses; Surface cracks;
Structural modification of laser annealed a-Si1−x C x :H films by U. Coscia; G. Ambrosone; C. Minarini; V. Parisi; S. Schutzmann; A. Tebano (4493-4496).
Hydrogenated amorphous silicon carbon films, with relatively low hydrogen content and carbon fraction x, C/(C + Si), ranging from 0.20 to 0.57 have been deposited by RF-plasma enhanced chemical vapor deposition (PECVD) for excimer laser annealing experiments. After the laser treatments all the films show structural modifications. It has been obtained that with increasing x the crystallinity degree of the Si phase decreases, while that of the SiC phase increases and becomes predominant for x = 0.39. In the overstoichiometric samples only the c-SiC phase has been observed. In all the treated samples 3C-SiC crystallites have been detected.
Keywords: Silicon–carbon alloys; Pulsed laser treatment; Crystallization;
Structural and electrical characterization of strontium bismuth tantalate (SBT) thin films by S. O’Brien; G.M. Crean; L. Cakare; M. Kosec (4497-4501).
Ferroelectric strontium bismuth tantalate (SBT) thin films were deposited by thermal metalorganic chemical vapour deposition (MOCVD) onto a complex layered Pt/IrO2/Ir/Ti(Al)N substrate. A study of ultra-violet (UV)-assisted rapid thermal processing (RTP) annealing strategies of the SBT thin films was performed. The influence of UV irradiation temperature and annealing atmosphere on the crystallinity of the deposited films was evaluated using both microstructural and electrical analysis techniques. A UV-RTP strategy in an oxygen atmosphere above 400 °C, followed by a furnace treatment at 700 °C, provided an optimum remnant polarization figure of merit.
Keywords: Ferroelectric; SBT thin films; Polarization;
Mechanisms and application of the Excimer laser doping from spin-on glass sources for USJ fabrication by S. Coutanson; E. Fogarassy; J. Venturini (4502-4505).
In this work was investigated numerically and experimentally a simple laser doping method employing borosilicate (BSG) glass films as dopant sources which are deposited onto Si by the spin-coating technique. Both short (20 ns) and long (200 ns) pulse duration Excimer laser beams were used to deposit a large amount of energy in short time onto the near-surface region. Under suitable conditions, the irradiation leads to surface melting and dopant incorporation by liquid phase diffusion from the surface. Boron distribution profiles in the two-pulse duration regimes were studied as well as their electrical properties, and the junction formation of less than 25 nm in depth was demonstrated.
Keywords: Excimer laser; Spin-on glass; USJ fabrication;
On the theory of explosive boiling of a transparent liquid on a laser-heated target by S.N. Andreev; A.A. Samokhin; I. Smurov (4506-4510).
The possible manifestations of thermodynamical instability (explosive vaporisation) are discussed for different regimes of laser heating of the metal/transparent liquid system. The present calculations show that the explosive vaporisation in the metastable region may occur if the nucleation rate is high enough. This condition is achievable if the surface tension of the superheated liquid tends to zero near the spinodal. It is also shown that the dependence of the phase explosion time on laser intensity markedly changes its behaviour when the water temperature reaches the spinodal.
Keywords: Laser heating; Explosive vaporisation; Spinodal;
The influence of pulse duration on the stress levels in ablation of ceramics: A finite element study by A. Vila Verde; Marta M.D. Ramos (4511-4515).
We present a finite element model to investigate the dynamic thermal and mechanical response of ceramic materials to pulsed infrared radiation. The model was applied to the specific problem of determining the influence of the pulse duration on the stress levels reached in human dental enamel irradiated by a CO2 laser at 10.6 μm with pulse durations between 0.1 and 100 μs and sub-ablative fluence. Our results indicate that short pulses with durations much larger than the characteristic acoustic relaxation time of the material can still cause high stress transients at the irradiated site, and indicate that pulse durations of the order of 10 μs may be more adequate both for enamel surface modification and for ablation than pulse durations up to 1 μs. The model presented here can easily be modified to investigate the dynamic response of ceramic materials to mid-infrared radiation and help determine optimal pulse durations for specific procedures.
Keywords: Mesoscopic modelling; Laser ablation; Enamel; Finite elements; CO2 laser; Pulse duration; Stress transients;
Laser treatment of white China surface by K. Osvay; I. Képíró; O. Berkesi (4516-4522).
The surface of gloss fired porcelain with and without raw glaze coating was radiated by a CO2 laser working at 10.6 μm, a choice resulted from spectroscopic studies of suspensions made of China. The shine of the untreated sample was defined as the distribution of micro-droplets on the surface. The surface alterations due to laser heating were classified by the diameter of the completely melted surface, the ring of the surface at the threshold of melting, and the size of microscopic cracks. The diameter of the laser treated area was in the range of 3 mm, while the incident laser power and the duration of laser heating were varied between 1 and 10 W and 1–8 min, respectively. The different stages of surface modifications were attributed primarily to the irradiating laser power and proved to be rather insensitive to the duration of the treatment. We have found a range of parameters under which the white China surface coated with raw glaze and followed by laser induced melting exhibited very similar characteristics to the untreated porcelain. This technique seems prosperous for laser assisted reparation of small surface defects of unique China samples after the firing process.
Waveguide structures written in SF57 glass with fs-laser pulses above the critical self-focusing threshold by V. Díez-Blanco; J. Siegel; J. Solís (4523-4526).
Waveguide writing with femtosecond (fs) laser pulses in glasses is compromised by uncontrollable non-linear propagation phenomena for powers exceeding a certain threshold, P cr, which is inversely proportional to the material linear and non-linear refractive indices, n 0 and n 2, respectively. We have studied the behaviour of a commercial glass, Schott SF57, with high values of n 0 and n 2 when it is processed above P cr with 800 and 1260 nm, 100 fs laser pulses to produce waveguide structures. Two types of structures, longitudinal (l-) and transversal (t-) ones depending on the sample translation, were obtained. In both of them, material damage surrounded by guiding regions was generated. We attribute the formation of the guiding regions of increased refractive index to the high local pressure reached in the focal volume where catastrophic damage occurs. This pressure can be high enough to compress the material nearby thus generating regions with a permanent refractive index increase up to 5 × 10−4. The efficiency of the guiding region is conditioned by the pulse energy. Mode profile analyses reveal double-sided exponential guided modes in the l-waveguides. As an alternative route to obtain suitable mode shapes and, simultaneously, to increase the coupling efficiency, the use of multiple structures has been investigated. Various l-structures have been written close to each other generating a region of increased refractive index large enough to effectively support a Lorentzian mode at 633 nm.
Keywords: Femtosecond micromachining; SF57 glass; Waveguide structures;
High-resolution electron microscopy study of SiGeC thin films grown on Si(1 0 0) by laser-assisted techniques by Ch.B. Lioutas; N. Frangis; S. Soumelidis; S. Chiussi; E. López; B. León (4527-4530).
PLIE was used for rapid crystallisation of a-SiGeC films deposited by LCVD on Si(1 0 0) substrates. HRTEM study of thin films grown with several laser energies shows that the combination of the two laser techniques gives an almost completely crystallised alloy, even for the lowest laser fluence. Island formation is observed below a certain threshold of fluence (about 450 mJ/cm2). In the case of the lowest energy (100 mJ/cm2) the material was partially crystallised (with the crystalline material being the predominant state), to a nanocrystalline alloy with a considerable amount of epitaxialy grown grains and with grain sizes of several tens of nanometers. Above the threshold of 450 mJ/cm2 a rather smooth thin film is grown. The crystallisation is almost complete and the alloy is grown in an almost perfect epitaxial way.
Keywords: SiGeC; HRTEM; Excimer laser; Laser CVD; PLIE; Thin film processing;
Toward long lifetime photoinduced spatial distribution of quadratic optical properties in doped photopolymers by Jean-Philippe Bombenger; Loïc Mager; Alain Fort; Christiane Carre (4531-4535).
A new approach for the spatial photopatterning of the quadratic non-linear optical properties of novel doped photopolymers is presented. The material is composed of a mixture of triacrylic monomers and copolymerizable push–pull chromophores exhibiting quadratic hyperpolarizability. The methods used to perform the photopatterning of the NLO properties and to improve their lifetime is presented along with a model for the thermally induced chromophore orientation relaxation.
Keywords: Quadratic nonlinear optical properties; Photopatterning; Triacrylic monomers;
Fabrication of the hydrogen resistive ferroelectric film of the (Pb0.72La0.28)Ti0.93O3/Pb(Zr0.52Ti0.48)O3/(Pb0.72La0.28)Ti0.93O3 heterostructure by a pulsed laser deposition method by Eun Sun Lee; Dong Hua Li; Hyun Woo Chung; Sang Yeol Lee (4536-4540).
(Pb0.72La0.28)Ti0.93O3 (PLT)/Pb(Zr0.52Ti0.48)O3 (PZT)/PLT heterostructure was fabricated by using a pulsed laser deposition method. After depositing this structure, the hydrogen annealing process was performed in the forming gas (95% N2 + 5% H2) at a substrate temperature of 400 °C for 30 min to study the effects of hydrogen passivation.The heterostructure was not degraded by the hydrogen annealing in contrast with the case of PZT film without buffer layers. This heterostructure showed almost no degradation in terms of the remanent polarization even after the H2 annealing, while the PZT film exhibited 64% reduction, which is from 20.1 to 7.3 μC/cm2 after the annealing. The leakage current was decreased by an order in the case of the heterostructure, while the leakage current of the PZT film increased by an order.These can be explained that the PLT bottom buffer layer works as a seeing layer to help the PZT growth and the top PLT buffer layer acts as a barrier for penetrating hydrogen atoms.
Keywords: PZT heterostructure; Hydrogen resistance;
Comparison of the effect of PLT and PZT buffer layers on PZT thin films for ferroelectric materials applications by Dong Hua Li; Eun Sun Lee; Hyun Woo Chung; Sang Yeol Lee (4541-4544).
In order to study the effect of different buffer layers on the Pb(Zr0.52Ti0.48)O3 (PZT) thin films, 10-nm thick (Pb0.72La0.28)Ti0.93O3 (PLT) and Pb(Zr0.52Ti0.48)O3 buffer layers have been deposited on the Pt(1 1 1)/Ti/SiO2/Si substrates by pulsed laser deposition, respectively. The top buffer layers were also deposited on PZT thin films with the same thickness of the seed layers in order to enhance the fatigue characteristics of PZT thin films. We compared the results of dielectric constant, hysteresis loops and fatigue resistance characteristics. It was found that the dielectric properties of PZT thin films with PLT buffer layers were improved by comparing with PZT thin films with PZT buffer layers. The polarization characteristics of PZT thin films with PLT buffer layers were observed to be superior to those of PZT thin films using PZT buffer layers. The remanent polarization of PZT thin films showed 36.3 μC/cm2 and 2.6 μC/cm2 each in the case of use PLT and PZT buffer layers. For the switching polarization endurance analysis, PZT thin films with PLT buffer layers showed more excellent result than that of PZT thin films with PZT buffer layers.
Keywords: PZT thin films; Buffer layers; Ferroelectric characteristics;
Effect of buffer layers on the property of Pb(Zr,Ti)O3–Pb(Mn,W,Sb,Nb)O3 thin films grown by pulsed laser deposition by Hyun Woo Chung; Eun Sun Lee; Dong Hua Li; Byung Du Ahn; Sang Yeol Lee (4545-4548).
New ferroelectric Pb(Zr,Ti)O3–Pb(Mn,W,Sb,Nb)O3 (PZT–PMWSN) thin film has been deposited on a Pt/Ti/SiO2/Si substrate by pulsed laser deposition. Buffer layer was adopted between film and substrate to improve the ferroelectric properties of PZT–PMWSN films. Effect of a Pb(Zr0.52Ti0.48)O3 (PZT) and (Pb0.72La0.28)Ti0.93O3 (PLT) buffer layers on the stabilization of perovskite phase and the suppression of pyrochlore phase has been examined. Role of buffer layers was investigated depending on different types of buffer layer and thickness. The PZT–PMWSN thin films with buffer layer have higher remnant polarization and switching polarization values by suppressing pyrochlore phase formation. The remnant polarization, saturation polarization, coercive field and relative dielectric constant of 10-nm-thick PLT buffered PZT–PMWSN thin film with no pyrochlore phase were observed to be about 18.523 μC/cm2, 47.538 μC/cm2, 63.901 kV/cm and 854, respectively.
Thickness effect in Pb(Zr0.2Ti0.8)O3 ferroelectric thin films grown by pulsed laser deposition by M. Lisca; L. Pintilie; M. Alexe; C.M. Teodorescu (4549-4552).
Epitaxial Pb(Zr,Ti)O3 (PZT) thin films with thicknesses in the range of 50–200 nm and with 0.2 Zr/(Zr + Ti) ratio, were grown by pulsed laser deposition (PLD).The substrates used for PLD deposition are single crystalline 0.5% Nb-doped (1 0 0) SrTiO3 (STON). SrRuO3 (SRO) thin films were deposited as bottom and top electrodes in order to have minimum structural misfit, to insure on one side high quality growth, and on the other side to minimize the influence of the extended structural defects. Structural and electrical characterization was performed. The epitaxial PZT films are c-axis oriented and have an average roughness of 0.4 nm. The ferroelectric behavior was proved in all investigated films by the presence of the hysteresis loops and by the butterfly shape of the capacitance–voltage (C–V) characteristics. The ferroelectricity was present even in the samples with relative high leakage currents, down to a thickness of 50 nm. These results are essential when small thickness is needed for miniaturization of ferroelectric devices using PZT.
Keywords: Ferroelectric; Pulsed laser deposition; Thickness reduction;
Pulsed laser deposition of perovskite relaxor ferroelectric thin films by N. Scarisoreanu; M. Dinescu; F. Craciun; P. Verardi; A. Moldovan; A. Purice; C. Galassi (4553-4557).
Structural, dielectric and ferroelectric properties of thin films of La-doped lead zirconate titanate (PLZT) and sodium bismuth titanate–barium titanate (NBT–BT) perovskite relaxor ferroelectric have been investigated. PLZT films were deposited on Pt/Si substrates in oxygen atmosphere by pulsed laser deposition (PLD) and radio frequency (RF) discharge-assisted PLD, using sintered targets with different La content and Zr/Ti ratio, near or at the boundary relaxor ferroelectric. The films are polycrystalline with perovskite cubic or slightly rhombohedral structure. A slim ferroelectric hysteresis loop, typical for relaxors, has been measured for all film sets. Dielectric characterization shows a large value of capacitance tunability and low dielectric loss. However, common problems related to lead diffusion into the metallic electrode layer do not allow one to obtain high capacitance values, due to the formation of an interface layer with low dielectric constant. Lead-free NBT–BT thin films have been deposited on single crystal (1 0 0)-MgO substrates starting from targets with composition at the morphotropic phase boundary between rhombohedral and tetragonal phase. Films deposited by PLD are polycrystalline perovskite with a slight (1 0 0) orientation. Capacitance measurements were performed using interdigital metallic electrodes deposited on the film's top surface and showed high relative dielectric constant, on the order of 1300.
Keywords: PLZT films; NBT–BT films; Perovskite; Relaxor ferroelectric films;
Correlation between structural and mechanical properties of PbTiO3 thin films grown by pulsed-laser deposition by A. Roemer; E. Millon; W. Seiler; D. Ruch; A. Riche (4558-4563).
Tetragonal lead titanate (PbTiO3, PT) thin films are grown on (1 0 0) MgO substrate by pulsed-laser deposition (PLD) for expected applications in integrated optics. The realisation of outstanding and reliable devices into integrated circuits requires sufficient mechanical resistance despite that the obtained PT films display interesting waveguiding properties associated with low optical losses. Two mechanical properties characteristic of elasticity and hardness of PT films are studied. The elastic modulus (E or Young's modulus) and the hardness (H) are measured by the nanoindentation technique. These mechanical properties are correlated to the crystalline quality of PT/MgO thin films. The films show epitaxial relationship with the MgO substrate and the orientation of crystallites perpendicularly to the surface substrate may be the consequence of a growth process along c-axis, a-axis or both. Differences on curves plotting hardness and elastic modulus as a function of indentation depth are observed as the curves are less dispersed for the films mainly c-axis oriented.
Keywords: Pulsed-laser deposition; Lead titanate; Waveguide; Thin films; Hardness; Elastic modulus;
Crystallinity and morphology dependent luminescence of Li-doped Y2−x Gd x O3:Eu3+ thin film phosphors by Jong Seong Bae; Jung Hyun Jeong; Kyoo Sung Shim; Byung Kee Moon; Soung-soo Yi; Jung Hwan Kim; Young Soo Kim; Jeong Sik Lee (4564-4568).
The influence of lithium doping on the crystallization, the surface morphology, and the luminescent properties of pulsed laser deposited Y2−x Gd x O3:Eu3+ thin film phosphors was investigated. The crystallinity, the surface morphology, and the photoluminescence (PL) of films depended highly on the Li-doping and the Gd content. The relationship between the crystalline and morphological structures and the luminescent properties was studied, and Li+ doping was found to effectively enhance not only the crystallinity but also the luminescent brightness of Y2−x Gd x O3:Eu3+ thin films. In particular, the incorporation of Li and Gd into the Y2O3 lattice could induce remarkable increase in the PL. The highest emission intensity was observed Li-doped Y1.35Gd0.6O3:Eu3+ thin films whose brightness was increased by a factor of 4.6 in comparison with that of Li-doped Y2O3:Eu3+ thin films.
Keywords: Y2−x Gd x O3:Eu3+; Thin film phosphors; Surface morphology; Photoluminescence (PL);
Structural and optical properties of Er, Yb co-doped Y2O3 thin films by A.Og. Dikovska; P.A. Atanasov; I.G. Dimitrov; C. Vasilev; T. Kocourek; M. Jelinek (4569-4572).
Thin Er3+, Yb3+ co-doped Y2O3 films were grown on (1 0 0) YAG substrates by pulsed laser deposition. Ceramic targets having different active ion concentration were used for ablation. The influence of the rare-earth content and oxygen pressure applied during the deposition on the structural, morphological and optical properties of the films were investigated. The films deposited at the lower pressure, 1 Pa, and at 1/10 Er to Yb doping ratio are highly textured along the (1 1 1) direction of the Y2O3 cubic phase. In addition to the crystalline structure, these films possess smoother surface compared to those prepared at the higher pressure, 10 Pa. All other films are polycrystalline, consisting of cubic and monoclinic phases of Y2O3. The rougher surface of the films produced at the higher-pressure leads to higher scattering losses and different behavior of the reflectivity spectra. Optical anisotropy in the films of less than 0.004 was measured regardless of the monoclinic structure obtained. Waveguide losses of about 1 dB/cm at 633 nm were obtained for the films produced at the lower oxygen pressure.
Keywords: Y2O3; PLD; Optical properties;
In situ grown epitaxial YBa2Cu3O7 − x thin films by pulsed laser deposition under reduced oxygen pressure during cool-down time by Maria Branescu; A. Vailionis; I. Ward; J. Huh; G. Socol (4573-4577).
We report novel pulsed laser deposition conditions that were used to obtain superconducting epitaxial YBCO thin films, grown in situ using an oxygen pressure lower than the usual one during the cool-down time. We studied the influence of the PLD conditions as substrate temperature, oxygen pressure, laser fluence, and number of laser pulses on the crystallographic and morphological features, and on the superconducting properties of the films. Good superconducting properties were obtained without a high temperature post-deposition annealing process. A maximum critical temperature of 88.6 K was obtained.
Keywords: Pulsed laser deposition; YBCO; Thin films; Oxygen pressure;
Doped thin metal oxide films for catalytic gas sensors by E. György; E. Axente; I.N. Mihailescu; C. Ducu; H. Du (4578-4581).
TiO2 and Pt doped TiO2 thin films were grown by pulsed laser deposition on 〈0 0 1〉 SiO2 substrates. The doped films were compared with undoped ones deposited in similar experimental conditions. An UV KrF* (λ = 248 nm, τ FWHM ≅ 20 ns, ν = 2 Hz) excimer laser was used for the irradiation of the TiO2 or Pt doped TiO2 targets. The substrate temperatures were fixed during the growth of the thin films at values within the 300–500 °C range. The films’ surface morphology was investigated by atomic force microscopy and their crystalline quality by X-ray diffractometry. The corresponding transmission spectra were recorded with the aid of a double beam spectrophotometer in the spectral range of 400–1100 nm. No contaminants or Pt segregation were detected in the synthesized anatase phase TiO2 thin films composition. Titania crystallites growth inhibition was observed with the increase of the dopant concentration. The average optical transmittance in the visible-infrared spectral range of the films is higher than 85%, which makes them suitable for sensor applications.
Keywords: Doped oxide thin films; Pulsed laser deposition; Gas sensors;
Growth of oxide thin films for optical gas sensor applications by D. Caiteanu; E. György; S. Grigorescu; I.N. Mihailescu; G. Prodan; V. Ciupina (4582-4586).
Tungsten trioxide and titanium dioxide thin films were synthesised by pulsed laser deposition. We used for irradiations of oxide targets an UV KrF* (λ = 248 nm, τ FWHM ≅ 20 ns, ν = 2 Hz) excimer laser source, at 2 J/cm2 incident fluence value. The experiments were performed in low oxygen pressure. The (0 0 1) SiO2 substrates were heated during the thin film deposition process at temperature values within the 300–500 °C range. The structure and crystalline status of the obtained oxide thin films were investigated by high resolution transmission electron microscopy. Our analyses show that the films are composed by nanoparticles with average diameters from a few to a few tens of nm. Moreover, the films deposited at substrate temperatures higher than 300 °C are crystalline. The tungsten trioxide films consist of a mixture of triclinic and monoclinic phases, while the titanium dioxide films structure corresponds to the tetragonal anatase phase. The oxide films average transmittance in the visible-infrared spectral range is higher than 80%, which makes them suitable for sensor applications.
Keywords: Thin films; Pulsed laser deposition; Optical gas sensors;
Pulsed laser deposited iron fluoride thin films for lithium-ion batteries by Yoshinari Makimura; Aline Rougier; Jean-Marie Tarascon (4587-4592).
Iron fluoride thin films were successfully grown by Pulsed Laser Deposition (PLD), and their physico-chemical properties and electrochemical behaviours were examined by adjusting the deposition conditions, such as the target nature (FeF2 or FeF3), the substrate temperature (T s ≤ 600 °C), the gas pressure (under vacuum or in oxygen atmosphere) and the repetition rates (2 and 10 Hz). Irrespective of the FeF2 or FeF3 target nature, iron fluoride thin films, deposited at 600 °C under vacuum, showed X-ray diffraction (XRD) patterns corresponding to the FeF2 phase. On the other hand, iron fluoride thin films deposited at room temperature (RT) from FeF2 target were amorphous, whereas the thin films deposited from FeF3 target consisted of a two-phase mixture of FeF3 and FeF2 showing sharp and broad diffraction peaks by XRD, respectively. Their electrochemical behaviour in rechargeable lithium cells was investigated in the 0.05–3.60 V voltage window. Despite a large irreversible capacity on the first discharge, good cycling life was observed up to 30 cycles. Finally, their electrochemical properties were compared to the ones of iron oxide thin films.
Keywords: FeF2; FeF3; Pulsed Laser Deposition; Lithium-ion battery; Thin film;
Cobalt and tantalum additions for enhanced electrochromic performances of nickel-based oxide thin films grown by pulsed laser deposition by Yoshinari Makimura; Aline Rougier; Jean-Marie Tarascon (4593-4598).
Aiming at improving the durability of anodic electrochromic nickel oxide thin films, Ni–M–O (M = Co, Ta) thin films were grown by pulsed laser deposition (PLD), using optimized conditions, namely room temperature and 10−1 mbar oxygen pressure. For low Co and Ta contents (<5%), both additions lead to a loss of the [1 1 1] preferred orientation of the NiO rock-salt structure followed by a film amorphization with increasing Ta amount. Among the two series of metal additions (M ≤ 20%), the Ni–Co–O (5% Co) and Ni–Ta–O (10% Ta) thin films show the highest electrochemical performances especially in respect of improved durability. If the enhanced properties are associated with a limited dissolution of the oxidized phase for the Ni–Ta–O system, the opposite trend is observed for the Ni–Co–O system as compared to pure NiO.
Keywords: Nickel oxide; Ni–Co–O thin films; Ni–Ta–O thin films; Pulsed laser deposition; Electrochromism;
Structural characterization and magnetoresistance of manganates thin films and Fe-doped manganates thin films by S. Canulescu; Th. Lippert; H. Grimmer; A. Wokaun; R. Robert; D. Logvinovich; A. Weidenkaff; M. Doebeli (4599-4603).
Perovskites thin films with the composition La0.6Ca0.4MnO3 doped with 20% Fe, were prepared by pulsed reactive crossed beam laser ablation, where a synchronized reaction gas pulse interacts with the ablation plume. The films were grown on various substrates and the highest colossal magnetoresistance ratio (CMR) was detected by Hall measurements for films grown on LaAlO3 (1 0 0), which was selected as substrate for further investigations.Several growth parameters, such as substrate temperature and target to substrate distance were varied to analyze their influence on the film properties.The structure of the deposited thin films was characterized by X-ray diffraction and atomic force microscope, while Rutherford backscattering (RBS) was used to determine the film stoichiometry. The electrical properties were determined by Hall effect measurements in a magnetic field of 0.51 T.These measurements reveal that the amplitude of the CMR ratio depends strongly on the substrate and that the oxygen content influences the temperature where the transition from semiconductor to metal is observed.
Keywords: Magnetoresistance; Manganates thin films; Fe-doped manganates thin films; Pulsed laser deposition;
Deposition of zinc oxide thin films by reactive pulsed laser ablation by P. Bilkova; J. Zemek; B. Mitu; V. Marotta; S. Orlando (4604-4609).
Thin films of zinc oxide have been deposited by reactive pulsed laser ablation of Zn and ZnO targets in presence of a radio frequency (RF) generated oxygen plasma. The gaseous species have been deposited at several substrate temperatures, using the on-axis configuration, on Si (1 0 0). Thin films have been characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and infrared spectroscopy. A comparison among conventional PLD and reactive RF plasma-assisted PLD has been performed.
Keywords: Reactive pulsed laser deposition; Zinc oxide; Thin films;
PLD of Fe3O4 thin films: Influence of background gas on surface morphology and magnetic properties by M.L. Paramês; J. Mariano; Z. Viskadourakis; N. Popovici; M.S. Rogalski; J. Giapintzakis; O. Conde (4610-4614).
Ablation of Fe3O4 targets has been performed using a pulsed UV laser (KrF, λ = 248 nm, 30 ns pulse duration) onto Si(100) substrates, in reactive atmospheres of O2 and/or Ar, with different oxygen partial pressures. The as-deposited films were characterised by atomic force microscopy (AFM), X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS) and extraction magnetometry, in order to optimise the deposition conditions in the low temperature range. The results show that a background mixture of oxygen and argon improves the Fe:O ratio in the films as long as the oxygen partial pressure is maintained in the 10−2 Pa range. Thin films of almost stoichiometric single phase polycrystalline magnetite, Fe2.99O4, have been obtained at 483 K and working pressure of 7.8 × 10−2 Pa, with a high-field magnetization of ∼490 emu/cm3 and Verwey transition temperature of 112 K, close to the values reported in the literature for bulk magnetite.
Keywords: Fe3O4 thin film; Pulsed laser deposition; Magnetic characterisation;
Epitaxial ZrC thin films grown by pulsed laser deposition by V. Craciun; J. Woo; D. Craciun; R.K. Singh (4615-4618).
ZrC thin films were grown on (0 0 1)Si, (1 1 1)Si and (0 0 0 1)sapphire substrates by the pulsed laser deposition (PLD) technique. X-ray diffraction, X-ray reflectivity and Auger electron spectroscopy investigations were used to characterize the structure and composition of the deposited films. It has been found that films grown at temperatures higher than 700 °C under very low water vapor pressures were highly textured. Films deposited on (0 0 1)Si grew with the (0 0 1) axis perpendicular to the substrate, while those deposited on (1 1 1)Si and (0 0 0 1)sapphire grew with the (1 1 1) axis perpendicular to the substrate. Pole figures investigations showed that films were epitaxial, with in-plane axis aligned to those of the substrate.
Keywords: ZrC; Laser ablation; Epitaxial films;
Nickel–titanium alloy: Cytotoxicity evaluation on microorganism culture by V.C. Dinca; S. Soare; A. Barbalat; C.Z. Dinu; A. Moldovan; I. Stoica; T. Vassu; A. Purice; N. Scarisoareanu; R. Birjega; V. Craciun; V. Ferrari DeStefano; M. Dinescu (4619-4624).
High purity nickel (Ni) and titanium (Ti) targets have been used to form well-defined thin films of nitinol on Ti substrate by pulsed laser deposition (PLD) technique. Their chemical composition, crystalline structure and surface properties have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM).We have shown that by varying the deposition parameters such as laser fluence and number of laser pulses, we are able to control the film thickness as well as film's uniformity and roughness.Cytocompatibility tests have been performed through in vitro assays using microorganisms culture cells such as yeasts (Saccharomyces cerevisiae) and bacteria (Escherichia coli), in order to determine the thin film's toxic potential at the in vitro cellular level. Microorganism's adhesion on the nitinol surface was observed and the biofilm formation has been analyzed and quantified.Our results have shown no reactivity detected in cell culture exposed to NiTi films in comparison with the negative controls and a low adherence of the microorganisms on the nitinol surface that is an important factor for biofilm prevention. We can, therefore, conclude that NiTi is a good candidate material to be used for implants and medical devices.
Keywords: NiTi thin films; Surface analyses; Biofilms; Microorganism; Cytotoxicity;
Superlattice CoCrPt/Ru/CoFe structure fabricated by pulsed laser deposition by X.F. Hu; Q. Liang; H.Q. Li; X.X. He; Xiaoru Wang; W. Zhang (4625-4627).
The synthetic antiferromagnets (SAF) have been used in spin-valve sensor in data storage industry . We report a new hard/Ru/soft sandwich structure (SHBL) fabricated by pulsed lased deposition to replace current single layer structure for information recording application. SHBL consists of two magnetic layers separated by thin nonmagnetic layers, typically with Ru layers of 0.7–1.2 nm, through which antiferromagnetic coupling is induced. Varying the relative thickness of the magnetic layers, the spacer layers, and the type of magnetic materials can alter magnetic properties of CoCrPt/Ru/CoFe superlattice. The coercivity H c and grain size of magnetic layer is also dependent on the laser fluence. High laser fluence results in both small grain size and high H c. The observed phenomena are related to high quenching and deposition rates during PLD at high fluence, resulting in more pronounced phase segregation.
Keywords: Superlattice; CoCrPt/Ru/CoFe; Pulsed laser;
Enhancement of coercivity with reduced grain size in CoCrPt film grown by pulsed laser deposition by Q. Liang; X.F. Hu; H.Q. Li; X.X. He; Xiaoru Wang; W. Zhang (4628-4631).
We report a pulsed laser deposition (PLD) growth of VMn/CoCrPt bilayer with a magnetic coercivity (H c) of 2.2 kOe and a grain size of 12 nm. The effects of VMn underlayer on magnetic properties of CoCrPt layer were studied. The coercivity, H c, and squareness, S, of VMn/CoCrPt bilayer, is dependent on the thickness of VMn. The grain size of the CoCrPt film can also be modified by laser parameters. High laser fluence used for CoCrPt deposition produces a smaller grain size. Enhanced H c and reduced grain size in VMn/CoCrPt is explained by more pronounced surface phase segregation during deposition at high laser fluence.
Keywords: CoCrPt; Coercivity; Pulsed laser deposition;
Pulsed laser ablation of indium tin oxide in the nano and femtosecond regime: Characterization of transient species by A. De Bonis; A. Galasso; V. Marotta; S. Orlando; A. Santagata; R. Teghil; S. Veronesi; P. Villani; A. Giardini (4632-4636).
Tin doped indium oxide (ITO) is a n-type highly degenerate, wide band-gap semiconductor that is extensively used for many engineering applications. Pulsed laser ablation of indium tin oxide in the nano and femtosecond regime has been performed in our laboratory. Plume diagnostics has been carried out by means of a fast Intensified Coupled Charge Device (ICCD) camera. Optical emission spectroscopy has been applied to characterize the transient species produced in the nano and femtosecond regime. The time evolution of emission lines, in the femto and nanosecond regime, have been compared and discussed. In the mass spectrometry, of the ionized species, the presence of mixed metal oxide clusters has been detected. This fact is an indication that chemical reactions can occur during the plasma expansion or on the ITO surface.
Keywords: Pulsed laser ablation; Indium tin oxide; Transient species;
Multilayered metal oxide thin film gas sensors obtained by conventional and RF plasma-assisted laser ablation by B. Mitu; V. Marotta; S. Orlando (4637-4641).
Multilayered thin films of In2O3 and SnO2 have been deposited by conventional and RF plasma-assisted reactive pulsed laser ablation, with the aim to evaluate their behaviour as toxic gas sensors. The depositions have been carried out by a frequency doubled Nd–YAG laser (λ = 532 nm, τ = 7 ns) on Si(1 0 0) substrates, in O2 atmosphere. The thin films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrical resistance measurements. A comparison of the electrical response of the simple (indium oxide, tin oxide) and multilayered oxides to toxic gas (nitric oxide, NO) has been performed. The influence on the structural and electrical properties of the deposition parameters, such as substrate temperature and RF power is reported.
Keywords: Thin film; Semiconducting oxides; Laser deposition; Toxic gas sensors;
Pulsed reactive crossed beam laser ablation of La0.6Ca0.4CoO3 using 18O by M.J. Montenegro; K. Conder; M. Döbeli; T. Lippert; P.R. Willmott; A. Wokaun (4642-4646).
The composition of thin perovskite films, especially the oxygen content, is a crucial parameter which influences many physical properties, such as conductivity and catalytic activity. Films produced by pulsed laser deposition are normally annealed in an oxygen atmosphere after deposition to achieve a desired oxygen content. In pulsed reactive crossed beam laser ablation, no annealing step is necessary, but a fundamental question regarding this deposition technique is still open: where does the oxygen in the films come from?There are three possibilities, i.e. from the target, from the gas background, or from the gas pulse. To answer this question two experiments were performed: 18O2 was used during the deposition process as background gas with 16O anions in the target and 16O2 gas pulse, and a 18O2 gas pulse with 16O from the target and background. These experiments revealed that the quantification of the oxygen origin is only possible, when no oxygen exchange occurs at the deposition temperature. The films are characterized after deposition by elastic recoil detection analysis (ERDA) to determine the 16O/18O ratio. Experiments with different oxidizing species in the gas pulse (N2O and O2) confirm that the oxidizing potential (N2O > O2) as well as the number of molecules are important.
Keywords: Pulsed reactive crossed beam laser ablation; Perovskite; 18O;
Matrix assisted pulsed laser evaporation processing of triacetate-pullulan polysaccharide thin films for drug delivery systems by R. Cristescu; G. Dorcioman; C. Ristoscu; E. Axente; S. Grigorescu; A. Moldovan; I.N. Mihailescu; T. Kocourek; M. Jelinek; M. Albulescu; T. Buruiana; D. Mihaiescu; I. Stamatin; D.B. Chrisey (4647-4651).
We report the first successful deposition of triacetate-pullulan polysaccharide thin films by matrix assisted pulsed laser evaporation. We used a KrF* excimer laser source (λ = 248 nm, τ ≈ 20 ns) operated at a repetition rate of 10 Hz. We demonstrated by FTIR that our thin films are composed of triacetate-pullulan maintaining its chemical structure and functionality. The dependence on incident laser fluence of the induced surface morphology is analysed.
Keywords: Polysaccharide; Pullulan; Thin films; Laser deposition; Matrix assisted pulsed laser evaporation; Drug delivery;
Laser deposition of cryoglobulin blood proteins thin films by matrix assisted pulsed laser evaporation by R. Cristescu; T. Kocourek; A. Moldovan; L. Stamatin; D. Mihaiescu; M. Jelinek; I. Stamatin; I.N. Mihailescu; D.B. Chrisey (4652-4655).
We report the first successful deposition of type II cryoglobulin blood protein thin films by matrix assisted pulsed laser evaporation (MAPLE) using a KrF* excimer laser source (λ = 248 nm, τ FWHM ≈ 20 ns) operated at a repetition rate of 10 Hz. We demonstrate by AFM and FTIR that MAPLE-deposited thin films consist of starting type II cryoglobulin only, maintaining its chemical structure and biological functionality, being properly collected and processed. The dependence on incident laser fluence of the induced surface morphology is presented. The presence of type II cryoglobulin was revealed as aggregates of globular material in the MAPLE-deposited thin films and confirmed by standard cryoglobulin tests.
Keywords: Blood protein; Cryoglobulin; Laser deposition; Matrix assisted pulsed laser evaporation; Medical physics;
On the orientation independence of inverse pulsed laser deposition by L. Égerházi; Zs. Geretovszky; T. Szörényi (4656-4660).
Carbon nitride films have been deposited in the inverse pulsed laser deposition (IPLD) geometry by ablating a graphite target in nitrogen atmosphere while the spatial orientation of the target (and substrate) normal was varied. Two different orientations were tested, in one of which the axis of the plasma plume was made to point downwards, imposing the maximum gravitational barrier on the ablated species and make them move against the gravitational field while growing the film in order to verify the extent of a possible orientational effect. The thickness distribution of films obtained in different orientations was sampled along their axes of symmetry by stylus profilometry. The results indirectly proved that the kinetic energy of the species responsible for building the IPLD films surpassed the effect of gravitational field, even in the outer regions of the films, where the ablated species were believed to be thermalised. Evidences are also provided that utmost care should be taken to keep experimental conditions, like process pressure, spot size, etc., constant in order to get reproducible results.
Keywords: Inverse pulsed laser deposition; Orientation of target; Stylus profilometry; Carbon nitride;
Atomic force microscopic characterization of films grown by inverse pulsed laser deposition by L. Égerházi; Zs. Geretovszky; T. Csákó; T. Szörényi (4661-4666).
Carbon nitride films have been deposited by KrF excimer laser ablation of a rotating graphite target in 5 Pa nitrogen ambient in an inverse pulsed laser deposition configuration, where the backward motion of the ablated species is utilised for film growth on substrates lying in the target plane. Topometric AFM scans of the films, exhibiting elliptical thickness distribution, have been recorded along the axes of symmetry of the deposition area. High resolution AFM scans revealed the existence of disk-like, or somewhat elongated rice-like features of 5–10 nm average thickness and ∼100 nm largest dimension, densely packed over the whole, approximately 14 × 10 cm2 deposition area. The RMS roughness of the film decreased from 9 nm near to the laser spot down to 2 nm in the outer regions. Even the highest RMS value obtained for IPLD films was less than half of the typical, 25 nm roughness measured on simultaneously deposited PLD films.
Keywords: Carbon nitride films; Pulsed laser deposition; Atomic force microscopic characterization; Film nanostructure; Particulates;
Femtosecond pulsed laser deposition of diamond-like carbon films: The effect of double laser pulses by Nikoletta Jegenyes; Zsolt Toth; Bela Hopp; Jozsef Klebniczki; Zsolt Bor; Costas Fotakis (4667-4671).
The bonding structure of carbon films prepared by pulsed laser deposition is determined by the plasma properties especially the change of the kinetic energy. Using double laser pulses the ablation process and the characteristics of the generated plasma can be controlled by the setting of the delay between the pulses. In our experiments, amorphous carbon films have been deposited in vacuum onto Si substrates by double pulses from a Ti:sapphire laser (180 fs, λ = 800 nm, at 1 kHz) and a KrF laser system (500 fs, λ = 248 nm, at 5 Hz). The intensities have been varied in the range of 3.4 × 1012 to 2 × 1013 W/cm2. The morphology and the main properties of the thin layers were investigated as a function of the time delay between the two ablating pulses (0–116.8 ps) and as a function of the irradiated area on the target surface. Atomic force microscopy, spectroscopic ellipsometry and Raman-spectroscopy were used to characterize the films. It was demonstrated that the change of the delay and the spot size results in the modification of the thickness distribution of the layers, and the carbon sp2/sp3 bonding ratio.
Keywords: Pulsed laser deposition; Diamond-like carbon; Double laser pulses;
Growth and characterization of β-SiC films obtained by fs laser ablation by C. Ghica; C. Ristoscu; G. Socol; D. Brodoceanu; L.C. Nistor; I.N. Mihailescu; A. Klini; C. Fotakis (4672-4677).
We achieved the growth of cubic silicon carbide (SiC) films on (1 0 0)Si substrates by pulsed laser deposition (PLD) at moderate temperatures such as 750 °C, from a SiC target in vacuum. The as-deposited films are morphologically and structurally characterized by scanning electron microscopy (SEM), conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The morphology of deposited films is dominated by columns nucleated from a thin nanostructured beta silicon carbide (β-SiC) interface layer. The combined effects of columnar growth, tilted facets of the emerging columns and the presence of particulates on the film surface, lead to a rather rough surface of the films.
Keywords: Laser ablation; SiC; Transmission electron microscopy;
Production of nanoparticles of different materials by means of ultrashort laser pulses by G. Ausanio; S. Amoruso; A.C. Barone; R. Bruzzese; V. Iannotti; L. Lanotte; M. Vitiello (4678-4684).
Ultrashort pulsed laser ablation in vacuum of different targets was performed in order to investigate the possibility of producing nanoparticles with controlled size and shape. A systematic morphology characterization of deposited products was performed for nickel and silicon as a function of laser pulse intensity and wavelength, at a fixed pulse repetition rate. The nanoparticles were investigated by atomic force microscopy, and clear trends for their size and shape anisotropy were evidenced. The best conditions to obtain nanosized particles of oblate ellipsoidal shape, with the minor axis below 10 nm, were determined in the case of nickel targets. Our results show that ultrashort pulse laser deposition can be considered as an interesting technique for the tailoring of nanogranular films with the desired particles dimension and shape, according to the peculiar properties required in specific applications. Moreover, the preliminary features are very promising from the point of view of the production of magnetoresistive films with specific anisotropy.
Keywords: Nanoparticles; Ultrashort laser ablation;
Fs/ns-dual-pulse orthogonal geometry plasma plume reheating for copper-based-alloys analysis by A. Santagata; A. De Bonis; P. Villani; R. Teghil; G.P. Parisi (4685-4690).
Plasma plume emission spectroscopy signal enhancements between 12- and 280-fold were obtained in air at atmospheric pressure by reheating the fs-laser ablation plume (energy 0.75 and 3.0 mJ) with a 45 mJ ns-pulse in orthogonal geometry. The emission enhancements induced by the double pulse configuration (DP) at various inter-pulse delay times and distances of the second laser beam from the target surface were investigated for copper-based-alloy standards. Temporal surveys of the plasma plume temperatures induced by both fs-single pulse (fs-SP) and DP placed at a fixed distance of 0.5 mm from the target surface were carried out. Several copper-based-alloy standards were employed for drawing Zn calibration curves by using either fs-SP or DP configurations and considering Cu as internal standard. The experimental data show that, for high Zn contents, the fs-SP set-up is affected by a self-absorption phenomenon so that a deviation from the assumed calibration single linear response is observed and two linear regressions are considered. Conversely, it has been observed that the DP configuration is not affected by any self-absorption effect and provides an improvement of the Zn limit of detection (LOD) but worse calibration linear regressions than the fs-SP. Thus, the DP scheme can increase the analytical sensitivity of fs-SP and, furthermore, its process can be supposed to be independent from the matrix composition even for largely different Zn contents of the Cu-based-alloy standards used.
Keywords: Laser induced optical emission spectrometry; Fs laser pulsed ablation; Emission enhancement; Copper-based-alloys analysis; Dual-pulse LIBS; Orthogonal reheating laser pulse;
X-ray analysis of mechanical and thermal effects induced by femtosecond laser treatment of aluminum single crystals by S. Valette; R. Le Harzic; E. Audouard; N. Huot; R. Fillit; R. Fortunier (4691-4695).
Surface marking of aluminum single crystal is performed with femtosecond laser pulses. X-ray analysis allows to measure thermal and mechanical effects induced by the femtosecond laser pulses. These effects are estimated by comparing the pole figures (crystallinity) and the broadening of the diffraction peaks (mechanical contribution) before and after the laser irradiation. The results show that the femtosecond laser treatment ensures a re-crystallization of the structure and the presence of mechanical residual stresses. The analysis of the pole figures provides the sign of a re-crystallization on smaller volumes compared to initial ones. After the laser irradiation, the crystallization is perfectly oriented like the (1 1 0) orientation of the massive sample. Moreover, following the laser treatment, we show that the crystallographic structure is purer than the initial one. We also prove that the laser effect is persistent on a typical scale of 10 μm beyond the surface.
Keywords: Femtosecond laser; Metals; Thermal effects; Mechanical effects; X-ray analysis; Microstructure characterization; Re-crystallization;
Influence of femtosecond laser marking on the corrosion resistance of stainless steels by S. Valette; P. Steyer; L. Richard; B. Forest; C. Donnet; E. Audouard (4696-4701).
Marking is of prime importance in the field of biomaterials to allow the identification of surgical tools as well as prostheses. Nowadays, marking is often achieved by means of laser beam, which may modify the characteristics of the treated surfaces. The use of laser devices delivering nanosecond pulses is known to induce dramatic corrosion degradations during sterilization or decontamination processes of the biomaterials. The aim of the present study is to investigate the ability of femtosecond (pulse duration in the 10−15 s range) laser treatments to avoid preferential corrosion processes of the marked areas, in order to extend the durability and the reliability of biomaterials. Experiments have been performed on martensitic Z30C13 and austenitic 316L stainless steels. Electrochemical measurements (cyclic polarization curves) were carried out to determine the passive state of samples before and after engraving, their corrosion rate and their susceptibility to localized corrosion. Further protracted immersion tests were also carried out to evaluate the natural long-term degradation of engraved parts. The electrochemical behavior is then explained on the basis of surface characterizations. Femtosecond laser marking is shown to provide an electrochemical ennoblement. Moreover, the chemical composition is not affected so that the passive character of both stainless steels is maintained, even improved if we consider the susceptibility to localized corrosion.
Keywords: Femtosecond laser; Marking; Corrosion resistance; Stainless steels;
Self-organized pattern formation upon femtosecond laser ablation by circularly polarized light by Olga Varlamova; Florenta Costache; Jürgen Reif; Michael Bestehorn (4702-4706).
Surface ripples generation upon femtosecond laser ablation is attributed to self-organized structure formation from instability. We report that linear arrangements are observed not only for linearly polarized light but also for ablation with circularly polarized light. Long ordered chains of spherical nanoparticles, reminding of bead-strings are almost parallel but exhibit typical non-linear dynamics features such as bifurcations. In a first attempt to understand the self-assembly, we rely on models recently developed for the description of similar structures upon ion beam erosion and for the simulation of instabilities in thin liquid films. Our picture describes an unstable surface layer, non-uniformly eroded through Coulomb repulsion between individual positive charges.
Keywords: Femtosecond laser ablation; Ripples; Circular polarization; Self-organization;
Property improvement of pulsed laser deposited boron carbide films by pulse shortening by T. Csákó; J. Budai; T. Szörényi (4707-4711).
Growth characteristics and surface morphology of boron carbide films fabricated by ablating a B4C target in high vacuum with a traditional KrF excimer laser and a high brightness hybrid dye/excimer laser system emitting at the same wavelength while delivering 700 fs pulses are compared. The ultrashort pulse processing is highly effective. Energy densities between 0.25 and 2 J cm−2 result in apparent growth rates ranging from 0.017 to 0.085 nm/pulse. Ablation with nanosecond pulses of one order of magnitude higher energy densities yields smaller growth rates, the figures increase from 0.002 to 0.016 nm/pulse within the 2–14.3 J cm−2 fluence window. 2D thickness maps derived from variable angle spectroscopic ellipsometry reveal that, when ablating with sub-ps pulses, the spot size rather than the energy density determines both the deposition rate and the angular distribution of film material. Pulse shortening leads to significant improvement in surface morphology, as well. While droplets with number densities ranging from 1 × 104 to 7 × 104 mm−2 deteriorate the surface of the films deposited by the KrF excimer laser, sub-ps pulses produce practically droplet-free films. The absence of droplets has also a beneficial effect on the stoichiometry and homogeneity of the films fabricated by ultrashort pulses.
Keywords: PLD; Thin films; Thickness distribution; Ultrashort pulse processing; Surface morphology; Nanostructure;
Propagation of LaMnO3 laser ablation plume in oxygen gas by S. Amoruso; A. Sambri; M. Vitiello; X. Wang (4712-4716).
The propagation of LaMnO3 laser ablation plume in oxygen background has been investigated using fast photography of overall visible plume emission and time-resolved optical emission spectroscopy. The plume expansion was studied with ambient oxygen pressures ranging from vacuum level to 100 Pa. Free-expansion, splitting, sharpening and stopping of the plume were observed at different pressures and time delays after the laser pulse. Time-resolved optical emission spectroscopy showed that oxides are mainly formed through reaction of the atomic species ablated from the target with oxygen in the gas-phase. These reactions mainly affect the content of lanthanum oxide in the plume, while emission of manganese oxide is barely observed in all the range of pressure investigated.
Keywords: Laser ablation; Spectroscopy; Fast imaging; Manganites;
Recognition of biological systems by mass-resolved laser spectroscopy by S. Orlando; A. Giardini; A. Paladini; S. Piccirillo; F. Rondino; M. Satta; D. Catone; M. Speranza (4717-4721).
Molecular clusters are non-covalent aggregates widely recognized as a new state of matter, whose properties are neither those of the individual constituents nor those of their condensed phases. Tailor made molecular clusters have proved to be ideal systems for modelling molecular recognition phenomena and their applications in many scientific fields interlocking the physical and life sciences are now well assessed. In the last few years, it has become possible, through the use of advanced laser techniques, to study the interactions between individual components of a cluster, produced by laser desorption. The studies were carried out developing laser spectroscopic methodologies, capable of characterizing molecular clusters and probing the chemical bond breaking and forming on an extremely short time scale.This paper deals with chiral recognition in gas phase clusters of biological interest through the application of the mass-resolved Resonant Two Photon Ionization (1 and 2 color R2PI) laser technique.The measurement of the spectroscopic shifts and of the fragmentation thresholds of diasteromeric clusters allows the determination of the nature of the interactions which control the formation of biological material and affect their stability and reactivity.
Keywords: Biological systems; Mass-resolved laser spectroscopy; Chirality;
Excimer laser ablation of thin titanium oxide films on glass by O. Van Overschelde; S. Dinu; G. Guisbiers; F. Monteverde; C. Nouvellon; M. Wautelet (4722-4727).
Thin titanium dioxide films are deposited on glass substrates by magnetron sputter deposition. They are irradiated in air, by means of a KrF excimer laser. The ablation rate is measured as a function of the laser fluence per pulse, F, and of the number of pulses, N. Above a fluence threshold, the films are partially ablated. The ablated thickness does not vary linearly with N. This is the signature of a negative feedback between the film thickness and the ablation rate. The origin of this negative feedback is shown to be due to either thermal or electronic effects, or both. At high F, the film detachs from the substrate.
Keywords: Titanium dioxide films; Glass substrates; Magnetron sputter deposition; Laser processing; Thin films; Laser ablation;
On determining the spot size for laser fluence measurements by B. Farkas; Zs. Geretovszky (4728-4732).
Energy fluence, defined as pulse energy over irradiated area, is a key parameter of pulsed laser processing. Nevertheless, most of the authors using this term routinely do not realize the problems related to the accurate measurement of the spot size. In the present paper we are aiming to approach this problem by ablating crystalline Si wafers with pulses of a commercial KrF excimer laser (λ = 248 nm, τ = 15 ns) both in vacuum and at ambient atmosphere. For any pulse energy, the size of the ablated area monotonously increases with increasing number of pulses. The difference in the ablated area could be as high as a factor of three when 2000 consecutive pulses impinge on the surface. The existence and extent of the gradual lowering of multi-pulse ablation threshold queries the applicability of routinely used procedure of dividing the pulse energy with the size of the ablated area exposed into either carbon-paper or a piece of Si with one or a few pulses when determining the fluence. A more quantitative way is proposed allowing comparison of results originating from different laboratories.
Keywords: PLA; PLD;
Particles movement and surface quality in PLD/PR systems by A. Marcu; C. Grigoriu; K. Yatsui (4733-4737).
A three-dimensional model based on Monte-Carlo and Finite Elements techniques has been used for simulating plume behavior, ‘micron-sized particles’ movement and interaction with obstacles in a Pulsed Laser Deposition with Plasma Reflection (PLD/PR) system. Have been investigated the influences of mass, surface size and emission time on trajectory and film surface quality as well. Droplet and ‘big-size particles’ deposition statistics are presented and a comparison between theoretical and experimental results upon thin film surface quality as well. One can observe that particles mass and surface size have a strong influence on the particles trajectory by affecting the collisions parameters during the entire propagation process. The emission time should influence the particles trajectory by affecting the probability of interaction with other particles. By making a 10,000 particles statistic for a normal distribution of these investigated parameters, we obtain reasonable good results in modeling ‘big-size particles’ tendency to be deposited at lower reflection angles. These results sustain assumption of ‘big particles’ deflection by plume fine particles during the propagation process.
Keywords: PLD/PR systems; Surface quality; Monte-Carlo model; Ablation plume;
Study on metal microparticle content of the material transferred with Absorbing Film Assisted Laser Induced Forward Transfer when using silver absorbing layer by T. Smausz; B. Hopp; G. Kecskeméti; Z. Bor (4738-4742).
Absorbing Film Assisted Laser Induced Forward Transfer (AFA-LIFT) is a modified LIFT method where a high absorption coefficient thin film coating of a transparent substrate is used to transform the laser energy into kinetic in order to transfer the “target” material spread on it. This method can be used for the transfer of biomaterials and living cells, which could be damaged by direct irradiation of the laser beam. In previous experiments, ∼50–100 nm thick metal films have been used as absorbing layer. The transferred material can also contain metal microparticles originating from the absorbing thin film and acting as non-desired impurities in some cases. The aim of our work was to study how the properties (number, size and covered area) of metal particles transferred during the AFA-LIFT process depend on film thickness and the applied fluence. Silver thin films with different thickness (50–400 nm) were used as absorbing layers and real experimental conditions were modeled by a 100 μm thick water layer. The particles transferred without the use of water layer were also studied. The threshold laser fluence for the complete removal of the absorber from the irradiated area was found to strongly increase with increasing film thickness. The deposited micrometer and submicrometer particles were observed with optical microscope and atomic force microscope. Their size ranged from 100 nm to 20 μm and depended on the laser fluence. The increase in fluence resulted in an increasing number of particles of smaller average size.
Keywords: AFA-LIFT; Excimer laser; Silver;
Excimer laser forward transfer of mammalian cells using a novel triazene absorbing layer by A. Doraiswamy; R.J. Narayan; T. Lippert; L. Urech; A. Wokaun; M. Nagel; B. Hopp; M. Dinescu; R. Modi; R.C.Y. Auyeung; D.B. Chrisey (4743-4747).
We present a novel laser-based approach for developing tissue engineered constructs and other cell-based assembly's. We have deposited mesoscopic patterns of viable B35 neuroblasts using a soft direct approach of the matrix assisted pulsed laser evaporation direct write (MAPLE DW) process. As a development of the conventional direct write process, an intermediate layer of absorbing triazene polymer is used to provide gentler and efficient transfers. Transferred cells were examined for viability and proliferation and compared with that of as-seeded cells to determine the efficacy of the process. Results suggest that successful transfers can be achieved at lower fluences than usual by the incorporation of the intermediate absorbing layer thus avoiding any damage to cells and other delicate materials. MAPLE DW offers rapid computer-controlled deposition of mesoscopic voxels at high spatial resolutions, with extreme versatility in depositing combinations of natural/synthetic, living/non-living, organic/inorganic and hard/soft materials. Our approach offers a gentle and efficient transfer of viable cells which when combined with a variety of matrix materials allows development of constructs and bioactive systems in bioengineering.
Keywords: Laser forward transfer; MAPLE DW; Triazene polymer;
Two-dimensional differential adherence of neuroblasts in laser micromachined CAD/CAM agarose channels by A. Doraiswamy; T. Patz; R.J. Narayan; M. Dinescu; R. Modi; R.C.Y. Auyeung; D.B. Chrisey (4748-4753).
Laser micromachining of hydrophobic gels into CAD/CAM patterns was used to develop differentially adherent surfaces and induce the attachment of B35 rat neuroblasts that would later form engineered nerve bundles. Narrow channels, 60–400 μm wide, were micromachined in a 2% agarose gel using an ArF laser, and subsequently filled with an extracellular matrix gel. Upon the addition of 1 ml of a 2 × 104 cells/ml neuroblast suspension, the cells selectively adhered to the ECM-lined channels in a non-confluent manner and we monitored their growth at various time points. The adherent neuroblasts were fluorescently imaged with a propidium iodide live/dead assay, which revealed that the cells were alive within the channels. After 72 h growth, the neuroblasts grew, proliferated, and differentiated into nerve bundles. The fully grown 1 cm long nerve bundle organoids maintained an aspect ratio on the order of 100. The results presented in this paper provide the foundation for laser micromachining technique to develop bioactive substrates for development of three-dimensional tissues. Laser micromachining offers rapid prototyping of substrates, excellent resolution, control of pattern depth and dimensions, and ease of fabrication.
Keywords: Cellular channels; Micromachining; Nerve bundles;
Removal of doped poly(methylmetacrylate) from tungsten and titanium substrates by femto- and nanosecond laser cleaning by L. Urech; T. Lippert; A. Wokaun; S. Martin; H. Mädebach; J. Krüger (4754-4758).
The influence of different laser pulse lengths on the removal of a polymer layer from metal substrates was investigated. As model systems, doped poly(methylmetacrylate) (PMMA) on titanium and tungsten substrates were selected.The ablation threshold and irradiation spot morphology of titanium and tungsten were compared for femtosecond (fs) and nanosecond (ns) laser irradiation and different pulse numbers. Nanosecond laser treatment resulted in a non-homogeneous surface morphology for both titanium and tungsten substrates. Femtosecond irradiation of tungsten revealed a homogeneous ablation spot with little changes in the surface morphology. For titanium, the formation of columnar structures within the irradiation spot was observed.Two different dopant concentrations were used for PMMA to achieve an equal linear absorption coefficient for the femto- and nanosecond laser wavelengths of 790 and 1064 nm. The best results were achieved for the removal of doped PMMA by femtosecond laser irradiation, where only a minimal modification of the metal surface was detected. In the case of nanosecond laser exposure, a pronounced change of the structure was observed, suggesting that damage-free cleaning of the selected metal may only be possible using femtosecond laser pulses. Different experimental parameters, such as laser fluence, pulse repetition rate and sample speed were also investigated to optimize the cleaning quality of doped PMMA from tungsten substrates with femtosecond laser pulses.
Keywords: Poly(methylmetacrylate); Laser treatment; Tungsten; Titanium; Femtosecond;
Laser ablation lithography on thermoelectric semiconductor by O.Yu. Semchuk; V.N. Semioshko; L.G. Grechko; M. Willander; M. Karlsteen (4759-4762).
In this paper, experimental results of the investigation of the periodic structure on thermoelectric semiconductor Cu2Se are presented. Periodic structures were formed on surfaces of semiconductors due to multi-beam interaction of Q-switched Nd:YAG laser, which was operated in the lowest order of Gaussian mode and pulse duration 7 ns. Surface temperature evolution and transient reflectivity are studied during laser treatment. Creation of Cu islands in the maximal intensity of interference pattern was found.
Keywords: Laser ablation; Fluence; Laser-induced structures; Surface; Thermoelectric semiconductors; Cooper islands;
Laser etching of transparent materials at a backside surface adsorbed layer by R. Böhme; D. Hirsch; K. Zimmer (4763-4767).
The laser etching using a surface adsorbed layer (LESAL) is a new method for precise etching of transparent materials with pulsed UV-laser beams. The influence of the processing parameters to the etch rate and the surface roughness for etching of fused silica, quartz, sapphire, and magnesium fluoride (MgF2) is investigated. Low etch rates of 1 nm/pulse and low roughness of about 1 nm rms were found for fused silica and quartz. This is an indication that different structural modifications of the material do not affect the etching significantly as long as the physical properties are not changed. MgF2 and sapphire feature a principal different etch behavior with a higher etch rate and a higher roughness. Both incubation effects as well as the temperature dependence of the etch rate can be interpreted by the formation of a modified near surface region due to the laser irradiation. At repetition rates up to 100 Hz, no changes of the etch rate have been observed at moderate laser fluences.
Keywords: Excimer laser; Laser etching; Adsorbed layer; Fused silica; Quartz; Sapphire; Magnesium fluoride; MgF2;
Comparing study of subpicosecond and nanosecond wet etching of fused silica by Cs. Vass; D. Sebők; B. Hopp (4768-4772).
The effectiveness of the laser induced backside wet etching (LIBWE) of fused silica produced by subpicosecond (600 fs) and nanosecond (30 ns) KrF excimer laser pulses (248 nm) was studied. Fused silica plates were the transparent targets, and naphthalene–methyl-methacrylate (c = 0.85, 1.71 M) and pyrene–acetone (c = 0.4 M) solutions were used as liquid absorbents. We did not observe etching using 600 fs laser pulses, in contrast with the experiments at 30 ns, where etched holes were found. The threshold fluences of the LIBWE at nanosecond pulses were found to be in the range of 360–450 mJ cm−2 depending on the liquid absorbers and their concentrations. On the basis of the earlier results the LIBWE procedure can be explain by the thermal heating of the quartz target and the high-pressure bubble formation in the liquid. According to the theories, these bubbles hit and damage the fused silica surface. The pressure on the irradiated quartz can be derived from the snapshots of the originating and expanding bubbles recorded by fast photographic setup. We found that the bubble pressure at 460 mJ cm−2 fluence value was independent of the pulse duration (600 fs and 30 ns) using pyrene–acetone solution, while using naphthalene–methyl-methacrylate solutions this pressure was 4, 5 times higher at 30 ns pulses than it was at 600 fs pulses. According to the earlier studies, this result refers to that the pressure should be sufficiently high to remove a thin layer from the quartz surface using pyrene–acetone solution. These facts show that the thermal and chemical phenomena in addition to the mechanical effects also play important role in the LIBWE procedure.
Keywords: Wet etching; Fused silica; Transparent materials; Subpicosecond krF laser system;
Surface plasmon resonance spectroscopy on rotated sub-micrometer polymer gratings generated by UV-laser based two-beam interference by M. Csete; G. Szekeres; Cs. Vass; N. Maghelli; K. Osvay; Zs. Bor; M. Pietralla; O. Marti (4773-4780).
Two-beam interference method was applied to generate gratings having periods of 416 nm and 833 nm by the forth harmonic of a Nd:Yag laser on thin poly-carbonate films spin-coated onto silver layer-covered substrates. The dependence of the modulation depth on the fluence and number of laser pulses was investigated by atomic force microscopy. A secondary pattern appeared on very thin polymer layers thanks to the “p” polarized laser beam illumination induced self-organized processes. The conditions of the emergence of grating-coupling caused additional plasmon resonance peak were determined for the sub-micrometer periodic polymer gratings. Surface plasmon resonance measurements were performed in attenuated total reflection arrangement to determine the effect of the angle between the plasmon propagation direction and the polymer groves on the grating-coupling. The effect of the modulation depth on the grating-coupling caused additional resonance minimum was also analyzed. We found coupling phenomena according to our calculations, the differences between the measured and theoretically predicted resonance curves were explained by the scattering on the complex surface structure.
Keywords: Surface plasmons; Sub-micrometer polymer gratings; Grating-coupling;
Local chemical transformations in poly(dimethylsiloxane) by irradiation with 248 and 266 nm by Vera-Maria Graubner; Oskar Nuyken; Thomas Lippert; Alexander Wokaun; Sylvain Lazare; Laurent Servant (4781-4785).
Poly(dimethylsiloxane) (PDMS) has been irradiated with a frequency quadrupled Nd:YAG laser and a KrF*-excimer laser at a repetition rate of 1 Hz. The analysis of ablation depth versus pulse number data reveals a pronounced incubation behavior. The thresholds of ablation (266 nm: 210 mJ cm−2, 248 nm: 940 mJ cm−2) and the corresponding effective absorption coefficients α eff (266 nm: 48900 cm−1, 248 nm: 32700 cm−1, α lin = 2 cm−1) were determined. The significant differences in the ablation thresholds for both irradiation wavelengths are probably due to the different pulse lengths of both lasers. Since the shorter pulse length yields a lower ablation threshold, the observed incubation can be due to a thermally induced and/or a multi-photon absorption processes of the material or impurities in the polymer.Incubation of polymers is normally related to changes of the chemical structure of the polymer. In the case of PDMS, incubation is associated with local chemical transformations up to several hundred micrometers below the polymer surface. It is possible to study these local chemical transformations by confocal Raman microscopy, because PDMS is transparent in the visible. The domains of transformation consist of carbon and silicon, as indicated by the appearance of the carbon D- and G-bands between 1310 and 1610 cm−1, a band appearing between 502 and 520 cm−1 can be assigned to mono- and/or polycrystalline silicon.The ablation products, which are detected in the surroundings of the ablation crater consist of carbon and amorphous SiO x (x ≈ 1.5) as detected by infrared spectroscopy.
Keywords: Poly(dimethylsiloxane) (PDMS); Irradiation and laser ablation; Chemical transformation;
Influence of laser pulse shape on dry laser cleaning by D. Grojo; M. BoyoMo-Onana; A. Cros; Ph. Delaporte (4786-4791).
The influence of laser pulse shape and pulse duration on dry laser cleaning is investigated by means of combined experimental and theoretical approaches. Possibilities to enhance damage-free particle removal efficiency by adjusting the temporal parameters of laser irradiations are discussed. The numerical description predicts that the removal of sub-micron particles by excimer lasers occurs in the ‘quasi-static’ inertia force regime. Experiments are based on an electro-optic gating system capable of switching off either leading or trailing edges of an excimer laser pulse. It is demonstrated that the inertia force corresponding to the pulse trailing front may play a role in laser cleaning but not as the dominant mechanism. Both the surface temperature and the expansion dynamics of materials are calculated in order to examine their role in dry laser cleaning. This study helps to determine the dominant mechanism responsible for particle removal by nanosecond laser pulses.
Keywords: Laser pulse shaping; Dry laser cleaning; Particle removal;
Comparison of mechanisms and effects of Nd:YAG and CO2 laser cleaning of titanium alloys by M.W. Turner; P.L. Crouse; L. Li (4792-4797).
The paper compares laser cleaning trials performed using a Q-switched Nd:YAG laser, λ = 1.064 μm and a continuous wave (CW) CO2 laser, λ = 10.6 μm, applied to aerospace-grade, contaminated titanium alloys. The mechanisms for cleaning using each laser system are modelled to determine the mode and extent of contaminant removal. The model results are then compared with the surface chemistry and micro-structural results from the cleaning trials performed. The results show the dominant cleaning process for Nd:YAG cleaning to be by evaporation of the contaminant via conduction through surface heating, while for CO2 laser cleaning the small fraction of the beam coupling directly with the contaminant is sufficient for direct heating and selective evaporation. The results for experimental cleaning, electron beam (EB) welding and diffusion bonding align well with the model, particularly when secondary reactions are taken into account.
Keywords: Laser; Titanium; Cleaning; Joining;
Investigation into CO2 laser cleaning of titanium alloys for gas-turbine component manufacture by M.W. Turner; P.L. Crouse; L Li; A.J.E. Smith (4798-4802).
This paper reports results of the investigation into the feasibility of using a CO2 laser technology to perform critical cleaning of gas-turbine aero-engine components for manufacture. It reports the results of recent trials and relates these to a thermal model of the cleaning mechanisms, and describes resultant component integrity. The paper defines the experimental conditions for the laser cleaning of various aerospace-grade contaminated titanium alloys, using a continuous wave CO2 laser. Laser cleaning of Ti64 proved successful for electron beam welding, but not for the more sensitive Ti6246. For diffusion bonding the trials produced a defective standard of joint. Effects of oxide formation is modelled and examined experimentally.
Keywords: Laser; Titanium; Cleaning; Joining;
Combining wire and coaxial powder feeding in laser direct metal deposition for rapid prototyping by Waheed Ul Haq Syed; Andrew J. Pinkerton; Lin Li (4803-4808).
Powder and wire deposition have been used separately in many laser-cladding, rapid prototyping and other additive manufacturing applications. In this paper, a new approach is investigated by simultaneously feeding powder from a coaxial nozzle and wire from an off-axis nozzle into the deposition melt pool. Multilayer parts are built from 316L steel using a 1.5 kW diode laser and different configurations of the powder and wire nozzles are compared in terms of surface roughness, deposition rate, porosity and microstructure. The parts are analysed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and optical microscopy techniques. Results show that deposition efficiency increased and surface roughness decreased with the combined process; some porosity was present in samples produced by this method, but it was 20–30% less than in samples produced by powder alone. Wire injection angles into the melt pool in both horizontal and vertical planes were found to be significant for attaining high deposition efficiency and good surface quality. Reasons for the final sample characteristics and differences between the combined process and the separate powder and wire feeding techniques are discussed.
Keywords: Coaxial powder feeding; Wire feeding; Diode laser; Direct metal deposition;
Solid freeform fabrication of alumina using laser-assisted ESAVD by Yiquan Wu; Kwang-Leong Choy (4809-4813).
Electrostatic spray assisted vapour deposition (ESAVD) has been used successfully to produce films and coatings. A combination of laser-assisted (LA) deposition and ESAVD has been developed for the direct fabrication of alumina ceramics. This novel laser-assisted ESAVD technique involves spraying a liquid precursor onto a substrate whilst a laser beam is simultaneously used to heat the precursor and cause the decomposition and chemical reactions of the precursor in order to produce a solid deposit. A desired three-dimensional (3-D) object can be formed through layer-by-layer deposition using a computer-controlled platform. Dense or porous alumina ceramics can be deposited on the substrate by optimizing the processing parameters. This paper reports the fabrication of alumina parts by the laser-assisted ESAVD method. The effect of processing parameters including laser scanning speed, laser beam size and laser power on the microstructure will be presented. Scanning electron microscope (SEM) and X-ray diffraction (XRD) have been used to characterise the phase and structure of the deposited alumina parts.
Keywords: Solid freeform fabrication; ESAVD; Ceramics; Laser deposition;
Selective ablation of thin films with short and ultrashort laser pulses by J. Hermann; M. Benfarah; G. Coustillier; S. Bruneau; E. Axente; J.-F. Guillemoles; M. Sentis; P. Alloncle; T. Itina (4814-4818).
Micromachining of CuInSe2 (CIS)-based photovoltaic devices with short and ultrashort laser pulses has been investigated. Therefore, ablation thresholds and ablation rates of ZnO, Mo and CuInSe2 thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti:sapphire laser. The experimental results were compared to the theoretical evaluation of the samples heat regime. In addition, the cells photo-electrical properties were measured before and after laser machining. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were employed to characterise the laser-induced ablation channels. Using nanosecond laser pulses, two phenomena were found to limit the laser-machining process. Residues of Mo that were projected onto the walls of the ablation channel and the metallization of the CuInSe2 semiconductor close to the channel lead to a shunt. The latter causes the decrease of the photovoltaic efficiency. As a consequence of these limiting effects, only subpicosecond laser pulses allowed the selective or complete ablation of the thin layers without a relevant change of the photo-electrical properties.
Keywords: Laser ablation; Micromachining; Femtosecond laser; Solar cells;
New aspects on pulsed laser deposition of aligned carbon nanotubes by S. Tamir; Y. Drezner (4819-4823).
We have grown carbon nanotubes (CNT) by pulsed laser deposition (PLD) at 1000 °C in Ar atmosphere. A Nd/YAG laser was used for irradiation of a graphite target containing Ni and Co rods. High-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM) images showed that “closed” carbon nanotubes were grown between clusters of metallic particles, so that the individual nanotubes were arranged in parallel to each other forming a shape of “Rope-Bridge”. The nanotubes structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and their type was found to be of MWNT, containing about five SWNT. Total diameter was 5–20 nm and their length was about 1 μm. High homogeneous distribution carbon nanotubes were grown and different structures were observed such as well-aligned carbon nanotubes, bamboo-like and Y-junction carbon nanotubes.
Keywords: Pulsed laser deposition; Carbon nanotubes; Laser ablation;
Surface morphology of polyethylene glycol films produced by matrix-assisted pulsed laser evaporation (MAPLE): Dependence on substrate temperature by Katarzyna Rodrigo; Pawel Czuba; Bo Toftmann; Jørgen Schou; Roman Pedrys (4824-4828).
The dependence of the surface morphology on the substrate temperature during film deposition was investigated for polyethylene glycol (PEG) films by matrix-assisted pulsed laser evaporation (MAPLE). The surface structure was studied with a combined technique of optical imaging and AFM measurements. There was a clear difference between the films produced below and above the melting point of PEG. For temperatures above the melting point, the polymer material was distributed non-uniformly over the substrate with growths areas, where cluster-like structures merge into large islands of micrometer size. At these temperatures, the islands in the investigated growth areas cover most of the bottom layer which has a typical height of 50–150 nm.
Keywords: Morphology; MAPLE; PEG;
Fabrication and characterization of epitaxial Sr0.6Ba0.4Nb2O6/La0.7Sr0.3CoO3 heterostructures by Tsun-Yu Yum; Chee-Leung Mak; Kin-Hung Wong (4829-4833).
Epitaxial Sr0.6Ba0.4Nb2O6(SBN60)/La0.7Sr0.3CoO3 heterostructures were fabricated on LAO(0 0 1) substrates using pulsed laser deposition (PLD). Their structural properties were investigated by X-ray diffraction. The θ–2θ scans showed single crystalline Sr1−x Ba x Nb2O6 (SBN) and La x Sr1−x CoO3 (LSCO) layers with a 〈0 0 1〉 orientations perpendicular to the substrate plane. Phi scans on the (2 2 1) plane of SBN layer indicated that the films have two in-plane orientations with respect to the substrate. The SBN unit cells were rotated in the plane of the film by ±18.4° as well as ±45° with respect to the LAO substrate. This rotation was explained by considering the lattice matching between films and substrate, and minimization of electrostatic energy. Spectroellipsometry (SE) was used to characterize the depth profile, the microstructural inhomogeneities, including voids and surface roughness, refractive indices and extinction coefficients of the films.
Keywords: Epitaxial; Heterostructures; Pulsed laser deposition;
Electrical, structural, and optical properties of ITO thin films prepared at room temperature by pulsed laser deposition by Jong Hoon Kim; Kyung Ah Jeon; Gun Hee Kim; Sang Yeol Lee (4834-4837).
Indium tin oxide (ITO) thin films were prepared by pulsed laser deposition (PLD) on glass substrate at room temperature. Structural, optical, and electrical properties of these films were analyzed in order to investigate its dependence on oxygen pressure, and rapid thermal annealing (RTA) temperature. High quality ITO films with a low resistivity of 3.3 × 10−4 Ω cm and a transparency above 90% were able to be formed at an oxygen pressure of 2.0 Pa and an RTA temperature of 400 °C. A four-point probe method, X-ray diffraction (XRD), atomic force microscopy (AFM), and UV–NIR grating spectrometer are used to investigate the properties of ITO films.
Keywords: Transparent conducting oxide (TCO); Indium tin oxide (ITO); Pulsed laser deposition (PLD); Rapid thermal annealing (RTA);
Laser impulse coupling at 130 fs by C. Phipps; J. Luke; D. Funk; D. Moore; J. Glownia; T. Lippert (4838-4844).
We measured the momentum coupling coefficient C m and laser-generated ion drift velocity and temperature in the femtosecond (fs) region, over a laser intensity range from ablation threshold to about one hundred times threshold. Targets were several pure metals and three organic compounds. The organic compounds were exothermic polymers specifically developed for the micro-laser plasma thruster, and two of these used “tuned absorbers” rather than carbon particles for laser absorption. The metals ranged from Li to W in atomic weight. We measured time of flight (TOF) profiles for ions. Specific impulse reached record values for this type of measurement and ablation efficiency was near 100%. These measurements extend the laser pulsewidth three orders of magnitude downward in pulsewidth relative to previous reports. Over this range, we found C m to be essentially constant. Ion velocity ranged from 60 to 180 km/s.
Keywords: Femtosecond; Laser momentum coupling; Lithium; Tungsten; Gold; Optimum coupling fluence; Ion time of flight;
Investigation of the two-photon polymerisation of a Zr-based inorganic–organic hybrid material system by B. Bhuian; R.J. Winfield; S. O’Brien; G.M. Crean (4845-4849).
Two-photon polymerisation of photo-sensitive materials allows the fabrication of three dimensional micro- and nano-structures for photonic, electronic and micro-system applications. However the usable process window and the applicability of this fabrication technique is significantly determined by the properties of the photo-sensitive material employed. In this study investigation of a custom inorganic–organic hybrid system, cross-linked by a two-photon induced process, is described. The material was produced by sol–gel synthesis using a silicon alkoxide species that also possessed methacrylate functionality. Stabilized zirconium alkoxide precursors were added to the precursor solution in order to reduce drying times and impart enhanced mechanical stability to deposited films. This enabled dry films to be used in the polymerisation process. A structural, optical and mechanical analysis of the optimised sol–gel material is presented. A Ti:sapphire laser with 80 MHz repetition rate, 100 fs pulse duration and 795 nm is used. The influence of both material system and laser processing parameters including: laser power, photo-initiator concentration and zirconium loading, on achievable micro-structure and size is presented.
Keywords: Two-photon polymerisation (2PP); Material system; Sol–gel;
Diagnostics of picosecond laser pulse absorption in preformed plasma by Nadja Vogel (4850-4856).
We present results where highly supersonic plasma jets and accelerated plasma fragments are generated by interaction of an intense picosecond laser pulse with a metallic target (Al, Cu, W, and Ta) in gas atmosphere. The formation of jets and well-localized massive plasma fragments occurs when a strong forward shock from a main laser pulse and a reverse shock from a pre-pulse meet to. Interferometric and shadow graphic measurements with high temporal (100 ps) and spatial (1 μm) resolution yield information about the formation and evolution of plasma jets and plasma fragments. The excitation of the electric and self-generated magnetic field by ponderomotive force during propagation of the laser pulse in a gas atmosphere was investigated as well. It had been shown previously that under certain conditions a hollow current channel can be generated in laser-produced plasma. The azimuthal magnetic field in such a micro-channel was determined by Faraday rotation of a probing laser beam to be 7.6 MGauss (MG). Ion acceleration in a pinched annular current channel up to 8 MeV analogous to micro-“plasma focus” conditions, may be realized at lengths of 100 μm. Self-generated magnetic fields of 4–7 MG have also been measured in thin skin layers in front of shock waves, where well-collimated plasma blocks were separated and accelerated away from the plasma body. The velocity of dense plasma blocks reaches values of order of 3 × 108 cm/s and they are stable during acceleration and propagation in gas.
Keywords: Laser-plasma interactions; Shock waves in plasma; Self-generated mega-gauss magnetic fields; Interferometry; Faraday rotation measurements;
Femtosecond pulse shaping for phase and morphology control in PLD: Synthesis of cubic SiC by C. Ristoscu; G. Socol; C. Ghica; I.N. Mihailescu; D. Gray; A. Klini; A. Manousaki; D. Anglos; C. Fotakis (4857-4862).
Pulse shaping introduces the method that makes possible the production of tunable arbitrary shaped pulses. We extend this method to control the prevalent growth of cubic SiC films on Si (1 0 0) substrates by pulsed laser deposition at temperatures around 973 K from a SiC target in vacuum. We used a laser system generating 200 fs pulses duration at 800 nm with 600 μJ at 1 kHz. The obtained structures are investigated by electron microscopy, X-ray diffraction and profilometry. We observed grains embedded in an amorphous texture, characteristic in our opinion to the depositions obtained with very short pulses. We present a comparison of deposited films with and without pulse shaping. Pulse shaping promotes increased crystallization and results in the deposition of thin structures of cubic SiC with a strongly reduced density of particulates, under similar deposition conditions.
Keywords: Pulse shaping; PLD; Nanostructured coatings; Cubic SiC;
Characterization of laser ablation of solid targets with near-infrared laser pulses of 100 fs and 1 ps duration by S. Amoruso; G. Ausanio; R. Bruzzese; L. Gragnaniello; L. Lanotte; M. Vitiello; X. Wang (4863-4870).
The process of laser ablation of silicon targets with ≈1 ps/1055 nm and ≈100 fs/780 nm laser pulses has been investigated by exploiting optical emission spectroscopy and fast photography of the ablated species in the gas phase, and atomic force microscopy of less than one layer deposits of the ablated material. We have observed, for both durations, the presence of a fast atomic component followed by a much slower plume of nanoparticles of the target material. For both laser pulses, experimental ablation thresholds have been compared with the prediction of a simple theoretical model, resulting in pretty good agreement. The properties of size distribution of the silicon nanoparticles have been analyzed as a function of the laser pulse intensity, and also compared with the results for metallic targets. Our experimental results show that the generation of nanoparticles, with pretty narrow size distributions, is a characteristic feature of the interaction of ultrashort laser pulses with solids (namely, metals and semiconductors), for both ≈1 ps and ≈100 fs laser pulse durations.
Keywords: Ultrashort laser ablation; Nanoparticles; Spectroscopy; Fast imaging;
Functionalized polysiloxane thin films deposited by matrix-assisted pulsed laser evaporation for advanced chemical sensor applications by E.J. Houser; D.B. Chrisey; M. Bercu; N.D. Scarisoreanu; A. Purice; D. Colceag; C. Constantinescu; A. Moldovan; M. Dinescu (4871-4876).
High-quality thin films of fragile chemoselective polymers with precise and accurate thickness, density and chemical integrity are required for advanced chemical sensor applications. While these attributes are difficult to achieve by conventional methods, we have successfully demonstrated the matrix-assisted pulsed laser evaporation (MAPLE) deposition of thin films of especially synthesized fluoro-alcohol substituted carbo-polysiloxane polymer coatings. The quadrupled output of a Nd:YAG laser (265 nm) served as the laser source and depositions were done in a background pressure of N2. Using various solvents appropriate to solvate this polymer (e.g. tetrahydrofuran, acetone and chloroform) and varying the laser fluence, we optimized the deposition of high-quality thin films on 1 cm2 double-polished silicon substrates. The best solvent used as matrix was proved to be acetone. Under these conditions, the important functional groups were reproduced and observed by Fourier Transform Infrared Spectroscopy (FTIR) as compared to the drop cast films and the surface roughness was analysed using Atomic Force Microscopy (AFM) and found to be much smoother than conventional wet deposition techniques.
Keywords: MAPLE; Polymer; Pulsed laser;
Influence of substrate temperature and atmosphere on nano-graphene formation and texturing of pulsed Nd:YAG laser-deposited carbon films by C. Scilletta; M. Servidori; S. Orlando; E. Cappelli; L. Barba; P. Ascarelli (4877-4881).
Pulsed laser ablation of a pyrolytic graphite target (Nd:YAG laser, second harmonic: λ = 532 nm, hν = 2.33 eV, τ = 7 ns, ν = 10 Hz, Φ ≈ 7 J/cm2), operating at different experimental conditions (i.e. fluence, substrate temperature, working pressure, inert sustaining gases) has been used to prepare films of nano-structured carbon. Detailed characterisation of synchrotron X-ray measurements, performed at grazing incidence, established the formation of nano-sized graphene structures at high deposition temperatures (∼900°C). A longitudinal growth of parallel graphene layers, with c ˆ axis parallel to the substrate, was also attained when the plume of activated carbon species was allowed to expand in the vacuum (base level ∼5 × 10−4 Pa). The presence of He sustaining gas, on the other side, gave rise to nano-graphitic particles, but with their c ˆ axis randomly oriented in the film. The mass density of the deposited films, as measured by laboratory X-ray reflectivity (XRR), is also strongly dependent on the experimental settings: films grown in the inert gas have density lower than the vacuum-deposited films. The highest density was obtained at room temperature, where an amorphous carbon structure is formed.
Keywords: Nd:YAG-pulsed laser deposition; Carbon nano-structures; Grazing incidence XRD; Graphene cluster orientation;
Thickness determination of large-area films of yttria-stabilized zirconia produced by pulsed laser deposition by N. Pryds; B. Toftmann; J.B. Bilde-Sørensen; J. Schou; S. Linderoth (4882-4885).
Films of yttria-stabilized zirconia (YSZ) on a polished silicon substrate of diameter up to 125 mm have been produced in a large-area pulsed laser deposition (PLD) setup under typical PLD conditions. The film thickness over the full film area has been determined by energy-dispersive X-ray spectrometry in a scanning electron microscope (SEM) with use of a method similar to one described by Bishop and Poole. The attenuation of the electron-induced X-rays from the Si wafer by the film was monitored at a number of points along a diameter and the thickness was determined by Monte Carlo simulations of the attenuation for various values of film thickness with the program CASINO. These results have been compared with direct measurements in the SEM of the film thickness on a cross-section on one of the wafers. The results of these measurements demonstrate the ability of this technique to accurately determine the thickness of a large film, i.e. up to diameters of 125 mm, in a relatively short time, without destroying the substrate, without the need of a standard sample and without the need of a flat substrate. We have also demonstrated that by controlling the deposition parameters large-area YSZ films with uniform thickness can be produced.
Keywords: Yttria-stabilized zirconia (YSZ); Pulsed laser deposition (PLD); Large-area deposition; Solid oxide fuel cells (SOFC);
New PLAD apparatus and fabrication of epitaxial films and junctions of functional materials: SiC, GaN, ZnO, diamond and GMR layers by Hachizo Muto; Takeshi Kusumori; Toshiyuki Nakamura; Takashi Asano; Takahiro Hori (4886-4896).
We have developed a new pulsed laser ablation–deposition (PLAD) apparatus and techniques for fabricating films of high-temperature or functional materials, including two short-wavelength lasers: (a) a YAG 5th harmonic (213 nm) and (b) Raman-shifted lasers containing vacuum ultraviolet light; also involved are (c) a high-temperature heater with a maximum temperature of 1350 °C, (d) dual-target simultaneous ablation mechanics, and (e) hybrid PLAD using a pico-second YAG laser combined with (c) and/or (d). Using the high-T heater, hetero-epitaxial films of 3C–, 2H– and 4H–SiC have been prepared on sapphire-c. In situ p-doping for GaN epitaxial films is achieved by simultaneous ablation of GaN and Mg targets by (d) during film growth. Junctions such as pGaN (Mg-doped)-film/n-SiC(0 0 0 1) substrate and pGaN/n-Si(1 1 1) show good diode characteristics. Epitaxial films with a diamond lattice can be grown on the sapphire-c plane by hybrid PLAD (e) with a high-T heater using a 6H-SiC target. High quality epitaxial films of ZnO are grown by PLAD by introducing a low-temperature self-buffer layer; magnetization of ferromagnetic materials is enforced by overlaying on a ferromagnetic lattice plane of an anti-ferromagnetic material, showing the value of the layer-overlaying method in improving quality. The short-wavelength lasers are useful in reducing surface particles on functional films, including superconductors.
Keywords: PLAD apparatus; Epitaxial film; Functional material;
Microfabrication in free-standing gallium nitride using UV laser micromachining by E. Gu; H. Howard; A. Conneely; G.M. O’Connor; E.K. Illy; M.R.H. Knowles; P.R. Edwards; R.W. Martin; I.M. Watson; M.D. Dawson (4897-4901).
Gallium nitride (GaN) and related alloys are important semiconductor materials for fabricating novel photonic devices such as ultraviolet (UV) light-emitting diodes (LEDs) and vertical cavity surface-emitting lasers (VCSELs). Recent technical advances have made free-standing GaN substrates available and affordable. However, these materials are strongly resistant to wet chemical etching and also, low etch rates restrict the use of dry etching. Thus, to develop alternative high-resolution processing for these materials is increasingly important. In this paper, we report the fabrication of microstructures in free-standing GaN using pulsed UV lasers. An effective method was first developed to remove the re-deposited materials due to the laser machining. In order to achieve controllable machining and high resolution in GaN, machining parameters were carefully optimised. Under the optimised conditions, precision features such as holes (through holes, blind or tapered holes) on a tens of micrometer length scale have been machined. To fabricate micro-trenches in GaN with vertical sidewalls and a flat bottom, different process strategies of laser machining were tested and optimised. Using this technique, we have successfully fabricated high-quality micro-trenches in free-standing GaN with various widths and depths. The approach combining UV laser micromachining and other processes is also discussed. Our results demonstrate that the pulsed UV laser is a powerful tool for fabricating precision microstructures and devices in gallium nitride.
Keywords: Gallium nitride; Pulsed UV lasers; Micromachining;
Influence of experimental parameter on the Li-content of LiMn2O4 electrodes produced by pulsed laser deposition by Th. Dumont; T. Lippert; M. Döbeli; H. Grimmer; J. Ufheil; P. Novák; A. Würsig; U. Vogt; A. Wokaun (4902-4906).
Thin film deposition by pulsed reactive crossed beam laser ablation has often advantages such as no need for annealing after deposition over conventional pulsed laser deposition. However, when depositing material from a target containing a low atomic mass component such as lithium, the resulting films are frequently sub-stoichiometric in the light elements. This is most probably due to scattering of the plasma plume in the crossed gas pulse, which creates a high local pressure. The light species are scattered more than the heavier elements, which leads to Li-deficiencies in the resulting film. Depositing lithium-spinels without the use of a crossed gas pulse is still strongly dependent on the applied background pressure. A background pressure below 10 Pa results also in lithium-deficient films. However, this is not due to scattering effects but very probably to the evaporation of lithium oxide from the growing film. These boundary conditions allow only a window of conditions for the growth of stoichiometric LiMn2O4 films that is useful as test electrodes for Li-batteries research. A strong influence of the experimental parameters was observed. The films deposited at different background pressures and different substrate/target distances showed very different behaviors in the electrochemical measurements. Extraction and insertion of lithium was only detected for films deposited at a distance between 3 and 4 cm and at a background pressure of 20 Pa, whereas films deposited at other conditions did not show this electrochemical activity.
Keywords: Pulsed laser deposition (PLD); Lithium; Cyclic voltammetry (CV); Lithium-deficiency;
Magnetic damping in Fe-based films deposited by laser ablation in magnetic field by A.P. Caricato; M. Fernandez; A. Luches; S. Luby; E. Majkova; L. Chitu; P. Svec; Z. Frait; D. Fraitova; R. Malych; P. Mengucci (4907-4913).
Magnetic films were deposited on oxidized Si by KrF laser ablation of amorphous ribbons (composition Fe60Ni20B20, Fe64−x Nb x Co21B15 with x = 0–5 and Fe73.5−x Ni x Cu1Nb3Si13.5B9 with x = 0–40). The deposited films are amorphous. Deviation from stoichiometry is low. The magnetization motion damping parameter λ and magnetization of the films were obtained from ferromagnetic resonance (FMR) spectra at 18, 25, 49 and 70 GHz. The lowest λ value obtained is 5.1 × 107 rad/s for a Fe33.5Ni40Nb3Cu1Si13.5B9 film. Here, the magnetization was only 7.99 kG because of low content of Fe. But high magnetization around 16 kG, combined with acceptable damping parameter (λ = 9.1–9.8 × 107 rad/s), was measured in Fe60Ni20B20 films. For the samples deposited in the external magnetic field of 30 mT parallel to the substrate surface an in plane uniaxial anisotropy is observed, which is 5 × 10−3 erg/cm3 in the film with the lowest damping parameter. The Fe-rich film Fe73.5Nb3Cu1Si13.5B9 shows a much higher anisotropy of 17.9 × 10−3 erg/cm3. Such combinations of properties are appropriate for the application of this kind of films in fast magnetic devices.
Keywords: Magnetization; Laser ablation; Fe-based films;
Laser micromachining of sputtered DLC films by Y.Q. Fu; J.K. Luo; A.J. Flewitt; S.E. Ong; S. Zhang; W.I. Milne (4914-4918).
DLC films with different thicknesses (from 100 nm to 1.9 μm) were deposited using sputtering of graphite target in pure argon atmosphere without substrate heating. Film microstructures (sp2/sp3 ratio) and mechanical properties (modulus, hardness, stress) were characterized as a function of film thickness. A thin layer of aluminum about 60 nm was deposited on the DLC film surface. Laser micromachining of Al/DLC layer was performed to form microcantilever structures, which were released using a reactive ion etching system with SF6 plasma. Due to the intrinsic stress in DLC films and bimorph Al/DLC structure, the microcantilevers bent up with different curvatures. For DLC film of 100 nm thick, the cantilever even formed microtubes. The relationship between the bimorph beam bending and DLC film properties (such as stress, modulus, etc.) were discussed in details.
Keywords: DLC; Microcantilever; Laser micromachining;
Optical components based on two-photon absorption process in functionalized polymers by S. Klein; A. Barsella; G. Taupier; V. Stortz; A. Fort; K.D. Dorkenoo (4919-4922).
We report on the fabrication of basic elements needed in optical circuits in a photopolymerizable resin, using a two-photon absorption (TPA) process to perform a selective polymerization. By taking advantage of the high spatial selectivity of the TPA approach, we can control the value of the local index of refraction in the material and realize permanent optical pathways in the bulk of photopolymerizable matrices. The computer-controlled design of such pathways allows creating optical circuits. As an example of application, optical fibers separated by millimetric distances and placed in arbitrary positions have been connected with moderate losses. Moreover, active components, such as electro-optical Mach–Zehnder interferometers, can be fabricated using photopolymers functionalized with non-linear optical chromophores, in order to be integrated in micro-optical circuits.
Keywords: Two-photon absorption; Photopolymerization; Micro-optical components;