Applied Surface Science (v.164, #1-4)

Preface by Margrit Hanbücken; Jacques Perrocheau (xi).

Introduction: Micro and Nanotechnologies by Huguette Launois (1).

The literature on ‘compliant’ substrates is critically reviewed. Such substrates are aimed at avoiding the propagation of threading dislocations (TDs) into epitaxial layer of large misfit. The principle is to introduce a thin intermediate layer able to glide freely under the strain developed by the epitaxy. The two most successful techniques applied up to now, are the intermediate oxide layer (silicon oxide mostly) and the large angle twist-bonded layer. The origins of the ‘compliance’ effect are very poorly known. It is shown that the simplistic explanations generally given are totally insufficient. The very few experiments reporting detailed analysis of the various interfaces and the mechanisms are presented. A new mechanism, involving the grain boundary (GB) steps, is proposed, however, more experimental data is needed to check its validity. Experiments are suggested to improve our knowledge of the various mechanisms involved.
Keywords: Epitaxial growth; Strain relaxation; Compliant substrate;

GaAs/GaAs twist-bonding for compliant substrates: interface structure and epitaxial growth by G. Patriarche; C. Mériadec; G. LeRoux; C. Deparis; I. Sagnes; J.-C. Harmand; F. Glas (15-21).
We investigated by transmission electron microscopy (TEM) the structure of the interface fabricated by twist-bonding two GaAs wafers in order to obtain a compliant substrate. The interface contains a dense network of pure screw dislocations even for twist angles as large as 16°. Then, one side of the assembly was thinned down to a few nanometers with mismatched In x Ga1−x As layers grown on it. The structural quality of these layers has been studied by X-ray diffraction (XRD) and by TEM. The structural quality generally appears weakly improved for the growths realised on our compliant substrates. One of the problems encountered is that the thinnest compliant layers appear to be unstable during the heating stage required by the epitaxy process.
Keywords: Epitaxy; Dislocations; Twin boundaries; Compliant substrate; Transmission electron microscopy; X-ray diffraction;

Anion effect in Co/Au(111) electrodeposition: structure and magnetic behavior by L. Cagnon; A. Gundel; T. Devolder; A. Morrone; C. Chappert; J.E. Schmidt; P. Allongue (22-28).
In-situ scanning tunneling microscopy (STM) and in-situ alternating gradient field magnetometry (AGFM) are used in combination to characterize the structure and magnetic behavior of Co/Au(111) layers electrodeposited from CoSO4 solutions, with anions X=Cl and SCN added in amount traces. In the Cl solution, the 2-D growth process is briefly recalled. In the thiocyanate (SCN) solution, preliminary observations show that nanometer clusters accumulate on top of a Co underpotential deposition (UPD) monolayer. Both structures solution/Co/Au(111) exhibit perpendicular magnetic anisotropy (PMA) for a Co thickness tt*, with t*∼2 ML in the Cl solution and ∼4–5 ML in the SCN one. Results are briefly discussed in light of classical models for PMA in ultrathin films.
Keywords: Anion effect; Co/Au(111) electrodeposition; Structure and magnetic behavior;

Low pressure chemical vapor deposition growth of silicon quantum dots on insulator for nanoelectronics devices by T. Baron; F. Martin; P. Mur; C. Wyon; M. Dupuy; C. Busseret; A. Souifi; G. Guillot (29-34).
We present a comparative study of nucleation and growth of Si quantum dots on SiO2, SiO x N y and Si3N4 substrates using silane low pressure chemical vapor deposition (LPCVD) at low temperature (570–610°C). The samples are investigated by atomic force micoscopy (AFM), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and spectroscopic ellipsometry (SE). We show that the chemical nature of the surface, precisely, the presence of SiO bonds, decreases the Si quantum dot density. By optimising the deposition parameters, a Si dot density of 1012 cm−2 can be obtained below 600°C on a pure Si3N4 surface. The influence of hydrogen, provided by silane decomposition, on the Si nucleation mechanism will be discussed.
Keywords: Low pressure chemical vapor deposition; Silicon quantum dots; Nanoelectronics devices;

Structural characterization of Si1−x Ge x /Si strained superlattices and relaxed virtual substrates grown by chemical vapor deposition by S. Bozzo; J.-L. Lazzari; B. Holländer; C. Coudreau; A. Ronda; S. Mantl; F.A. D'Avitaya; J. Derrien (35-41).
We present SiGe/Si (001) heteroepitaxy performed in a commercial Chemical Vapor Deposition (CVD) cold-wall vertical reactor using SiH4 and GeH4 as precursor gases. This versatile machine operates in a wide range of temperature and pressure, allowing both the Ultra High Vacuum (UHV) and the Low Pressure (LP)-CVD. The growth of Si0.71Ge0.29/Si strained superlattices (SLs) at 600°C and Si0.75Ge0.25/Si relaxed pseudo-substrates at 700°C is demonstrated using UHV and LP, respectively, and their structural properties are discussed. Finally, results of a first attempt of growing an undoped resonant tunneling like heterostructure onto a Si0.75Ge0.25 virtual substrate (VS) are reported.
Keywords: Chemical Vapor Deposition; SiGe; Superlattices; Pseudo-substrates;

Controlled nucleation of Co clusters on Au(111): towards spin engineering by I. Chado; S. Padovani; F. Scheurer; J.P. Bucher (42-47).
The morphology and distribution of Co clusters on Au(111) can be controlled: from very small (a few atoms) randomly distributed clusters to bigger (several 100 atoms) self-organized clusters on a regular network. The substrate temperature imposed during growth turns out to be the leading parameter, deciding for spontaneous organization of adsorbed metal atoms. From a detailed variable temperature scanning tunneling microscopy (STM) study, it is suggested that this way of organizing matter at the nanoscale may be used to achieve new material properties. In particular, from in situ magnetic measurements on the example of Co on Au(111), it is shown that the direction of the magnetization can be adjusted from in-plane to out-of-plane, depending on the growth condition.

We compare the 3D Monte-Carlo simulations of atom aggregation on a surface with TEM micrographs of real cobalt deposits obtained by sputter deposition on amorphous alumina. The discrepancy between deposited masses in otherwise similar calculated and real deposits suggests the existence of a layer of trapped cobalt atoms in the latter.
Keywords: Cobalt; Alumina; Monte-Carlo simulations;

Nanometric gold clusters have been produced in a laser vaporisation source and deposited on HOPG and gold (111) surfaces. We have found that coalescence and diffusion of the incident clusters depend on the nature of the substrate. The influence of these processes on thin film morphologies is evidenced experimentally. Among those two substrates, the HOPG appears as a suitable candidate for gold cluster ordering in order to make devices at nanoscale level.
Keywords: Deposition; Gold clusters; HOPG and gold substrates;

We report on the influence of the length of thiol alkyl chains used to coat silver sulfide particles on the self-organization of nanoparticles in monolayers. The variation of the average distance between particles with the length of the thio derivatives alkyl chain is derived from direct measurements of TEM patterns.
Keywords: Silver sulfide; Nanoparticles; Self-assemblies; Thiol alkyl chains length;

This contribution concerns surface stress effects on the equilibrium shape (ES) of nano-crystals. It is based on an extended Wulf–Kaishew theorem including lattice mismatch and surface stress properties as well.
Keywords: Epitaxial stress; Wulf–Kaishew theorem; Nano-crystals;

Plasma etching: principles, mechanisms, application to micro- and nano-technologies by Christophe Cardinaud; Marie-Claude Peignon; Pierre-Yves Tessier (72-83).
Nowadays, plasma-etching processes are asked to produce patterns from the nanometer to the micrometer range with the same efficiency. The very severe requirements in terms of etch rate, selectivity, profile control and surface damage plasma-etching processes lead to, have been at the origin of the development of mechanistic studies by means of plasma diagnostics and surface analysis, as well as the development of new etching devices. We review here the basic concepts of plasma etching, and using examples, we describe more in details important features. We recall, in particular, the important role of the surface layer, the ion bombardment and the substrate temperature.
Keywords: Plasma etching; Micro-technology; Nano-technology;

A short review of recent experimental and theoretical studies of atomic and molecular manipulations of individual adsorbates by scanning tunneling microscopy (STM) is given. The different types of manipulations are detailed, with an analysis of the interactions responsible for the displacement of the adsorbate.
Keywords: Atomic and molecular manipulations; Adsorbates; scanning tunneling microscopy (STM);

Self-rearrangements of vicinal silicon surfaces by Margrit Hanbücken; Bernd Röttger; Henning Neddermeyer (91-96).
Vicinal silicon surfaces auto-organise in a very systematic way, creating stepped surfaces and facets. The driving forces for these morphological changes are the intrinsic material properties governed by the need to minimise the surface free energy. Depending on their initial crystallographic orientation (hkl) and the individual misorientation, very different final morphologies have been observed. Systematic changes in the surface morphology and the associated atomic surface structures can be studied in a very straightforward way on concave-shaped silicon surfaces. These samples contain a wide range of polar misorientation angles and all azimuthal directions. In the present paper, we summarise previous results obtained with scanning tunnelling microscopy on concave silicon samples with an initial (111) and (100) orientation and additional measurements on Si(211) and (322). Some remarks on the more open Si(110) surface will also be given.

Microfabrication: LIGA-X and applications by R.K. Kupka; F. Bouamrane; C. Cremers; S. Megtert (97-110).
X-ray LIGA (Lithography, Electrogrowth, Moulding) is one of today's key technologies in microfabrication and upcoming modern (meso)-(nano) fabrication, already used and anticipated for micromechanics (micromotors, microsensors, spinnerets, etc.), micro-optics, micro-hydrodynamics (fluidic devices), microbiology, in medicine, in biology, and in chemistry for microchemical reactors. It compares to micro-electromechanical systems (MEMS) technology, offering a larger, non-silicon choice of materials and better inherent precision. X-ray LIGA relies on synchrotron radiation to obtain necessary X-ray fluxes and uses X-ray proximity printing. Inherent advantages are its extreme precision, depth of field and very low intrinsic surface roughness. However, the quality of fabricated structures often depends on secondary effects during exposure and effects like resist adhesion. UV-LIGA, relying on thick UV resists is an alternative for projects requiring less precision. Modulating the spectral properties of synchrotron radiation, different regimes of X-ray lithography lead to (a) the mass-fabrication of classical nanostructures, (b) the fabrication of high aspect ratio nanostructures (HARNST), (c) the fabrication of high aspect ratio microstructures (HARMST), and (d) the fabrication of high aspect ratio centimeter structures (HARCST). Reviewing very recent activities around X-ray LIGA, we show the versatility of the method, obviously finding its region of application there, where it is best and other competing microtechnologies are less advantageous. An example of surface-based X-ray and particle lenses (orthogonal reflection optics (ORO)) made by X-ray LIGA is given.
Keywords: LIGA; MEMS; ORO; HARMST; X-ray; Microfabrication; Microtechnology; Diffraction; High aspect ratio structures; Surface lenses; Microchannel plates;

Electron beam lithography: resolution limits and applications by C. Vieu; F. Carcenac; A. Pépin; Y. Chen; M. Mejias; A. Lebib; L. Manin-Ferlazzo; L. Couraud; H. Launois (111-117).
We report on the resolution limits of Electron Beam Lithography (EBL) in the conventional polymethylmethacrylate (PMMA) organic resist. We show that resolution can be pushed below 10 nm for isolated features and how dense arrays of periodic structures can be fabricated at a pitch of 30 nm, leading to a density close to 700 Gbit/in2. We show that intrinsic resolution of the writing in the resist is as small as 3 to 5 nm at high incident electron energy, and that practical resolution is limited by the development of the resist after exposure and by pattern transfer. We present the results of our optimized process for reproducible fabrication of sub-10 nm lines by lift-off and 30-nm pitch pillar arrays by lift-off and reactive ion etching (RIE). We also present some applications of these nanostructures for the fabrication of very high density molds for nano-imprint lithography (NIL) and for the fabrication of Multiple Tunnel Junction devices that can be used for single electron device applications or for the connection of small molecules.
Keywords: Nanolithography; Electron beam lithography; Nanofabrication; Single electron devices; Molecular electronics; Data storage devices;

Nanometer scale apertureless near field microscopy by S. Grésillon; S. Ducourtieux; A. Lahrech; L. Aigouy; J.C. Rivoal; A.C. Boccara (118-123).
It is necessary to use the information contained in the near field to get sub-wavelength details in optical imaging which are not revealed through the far-field image. We have designed and built various setups able to perform near-field measurements in the UV, visible and IR, both in transmission, reflection and dark field with a resolution of 10 nm, independent of the wavelength but related to the tip size. Images revealing local dielectric contrasts, small particle effects, as well as local field enhancements in random structures, are shown.
Keywords: Near field; Field enhancement; Local properties;

Detection of the optical magnetic field by circular symmetry plasmons by Eloı̈se Devaux; Alain Dereux; Eric Bourillot; Jean-Claude Weeber; Yvon Lacroute; Jean-Pierre Goudonnet; Christian Girard (124-130).
We report on the influence of coating a sharpened optical fiber tip with Au when observing nanofabricated dielectric structures with a Photon Scanning Tunneling Microscope (PSTM) in constant-height mode. For well-defined incident wavelengths and coating thicknesses, we found that such tips detect the distribution of the magnetic field associated with the optical wave in the near-field zone. A simple tip model indicates that this phenomenon is related to the excitation of circular symmetry plasmons in Au coated tips.
Keywords: Optical magnetic field; Circular symmetry plasmon; Au;

Time-resolved investigation of the vibrational dynamics of metal nanoparticles by C. Voisin; N. Del Fatti; D. Christofilos; F. Vallée (131-139).
Time resolved excitation and detection of the coherent vibrational motion of metal nanoparticles are discussed in the light of femtosecond pump-probe experiments performed in silver nanoparticles with radius ranging from 2.1 to 15.3 nm. Analysis of the phase of the observed sample absorption oscillations shows that coherent excitation is dominated by an indirect displacive mechanism due to subpicosecond heating of the lattice by fast electron-lattice energy transfer for large nanoparticles (R>10 nm). For smaller particles, the results suggest an additional contribution from direct coupling with the non-equilibrium electron gas. Both mechanisms, being related to an isotropic particle expansion, the fundamental radial mode is preferentially excited because of its better spatial matching with the excitation process. Optical control of the acoustic nanoparticles vibration is also demonstrated.
Keywords: Metal nanoparticles; Acoustic modes; Femtosecond spectroscopy;

New advances in theoretical background as well as recent experimental results associated to nanometer scale monometallic clusters are presented. Regarding the calculation of their electronic structure, the significant physical parameters, which control the relaxation of the intermetallic distances are introduced. From an experimental point of view, the limitations and advantages of a study based on an Extended X-ray Absorption Fine Structure (Exafs)–Anomalous Wide Angle X-ray Scattering (Awaxs) approach are discussed. Finally, through a review of recent papers, we try to make a link between metallic surfaces and nanometer scale monometallic clusters in the case of the interaction with a small molecule namely nitric oxide (NO). This parallel could be used to classify and thus predict the behaviour of the nanometer scale monometallic particles during adsorption of this molecule.
Keywords: Nanometer metallic cluster; Metallic surface; Nitric oxide (NO) adsorption;

Roughness and chemistry of silicon and polysilicon surfaces etched in high-density plasma: XPS, AFM and ellipsometry analysis by Laëtitia Rolland; Christophe Vallée; Marie-Claude Peignon; Christophe Cardinaud (147-155).
Surface chemistry and morphology of polysilicon thin films etched in a high-density fluorocarbon plasma under various conditions are studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and spectroscopic ellipsometry. XPS reveals the presence of a fluorocarbon layer, which composition and thickness depend on the plasma conditions. Ellipsometry measurements show the need to consider the superficial roughness. Surface roughness and morphology obtained by AFM are used to define geometric models suitable to represent the top layer when processing the ellipsometry data. Results are discussed and compared to that given by the Bruggeman effective medium approximation (BEMA). The BEMA model always agrees with the geometric model, which is the closest to the observed surface morphology. However, if a good agreement is obtained between surface roughness and top layer thickness for unetched or weakly damaged samples, a discrepancy is observed for the etched samples. Formation of a non-transparent fluorocarbon layer on these sample is put forward to explain this behaviour.
Keywords: XPS; AFM; Ellipsometry analysis;

Reaction between CO and a pre-adsorbed oxygen layer on supported palladium clusters by Laurent Piccolo; Conrad Becker; Claude R. Henry (156-162).
The transient kinetics of reaction between carbon monoxide and an oxygen monolayer pre-adsorbed on palladium clusters supported on MgO(100), has been studied for various cluster sizes (4–15 nm), in the temperature range 120–400°C, using molecular beam and mass spectrometry under ultrahigh vacuum. When the CO beam is opened, the CO2 production rate first increases instantaneously, and then increases slowly to its maximum, before decreasing to zero due to the lack of oxygen. The period of slow increase of the reaction rate, namely the induction period, appears at about 200°C and becomes longer when temperature increases. Although it is not observed on Pd(111), this peculiar reaction kinetics does not depend on cluster size, and is attributed to a precursor state of CO chemisorption. The oxygen coverage at saturation is found equal to 0.4. At this high oxygen coverage, gaseous CO physisorbs above the oxygen adlayer. At high temperature, the precursor is more likely to desorb, which reduces its chemisorption probability, and thus the CO2 production rate. The temperature-dependant kinetics of CO adsorption and reaction with oxygen has been simulated thanks to a simple kinetic model accounting for the precursor mechanism.
Keywords: Oxidation of carbon monoxide; Heterogeneous catalysis; Clusters; Palladium; Magnesium oxide; Simulation of reaction kinetics;

Characterization and reactivity of Pd–Pt bimetallic supported catalysts obtained by laser vaporization of bulk alloy by J.L. Rousset; F.J. Cadete Santos Aires; F. Bornette; M. Cattenot; M. Pellarin; L. Stievano; A.J. Renouprez (163-168).
Bimetallic Pd–Pt clusters produced by laser vaporization of bulk alloy have been deposited on high surface alumina. Energy dispersive X-ray (EDX) analysis and transmission electron microscopy (TEM) show that they have a perfectly well-defined stoichiometry and a narrow range of size. Therefore, they constitute ideal systems to investigate alloying effects towards reactivity. Pd–Pt alloys are already known for their applications in the hydrogenation of unsaturated hydrocarbons, especially aromatics, because this system is highly resistant to sulfur and nitrogen poisoning. In this context, the catalytic properties of this system have been investigated in the hydrogenation of tetralin in the presence of hydrogen sulfide. Preliminary results show that this model catalyst is more sulfur-resistant than each of the pure supported metals prepared by chemical methods.
Keywords: Laser vaporisation; Well-defined bimetallic catalysts; TEM; Reactivity;

STM studies: spatial resolution limits to fit observations in nanotechnology by Pierre David Szkutnik; Agnès Piednoir; Antoine Ronda; Florence Marchi; Didier Tonneau; Hervé Dallaporta; Margrit Hanbücken (169-174).
Atomically resolved Scanning Tunneling Microscopy (STM) images are nowadays currently obtained on flat surfaces without any special STM tip preparation. On the contrary, imaging of three-dimensional nanometer-scale objects exhibiting very much inclined facets, requires the use of STM tips with an appropriate geometry. The tip shape has to be controlled over the whole length that corresponds to the height of the object to be imaged. The cone angle of the tip has to be smaller than the tilt angle of the imaged facets. These criteria are also required when STM tips are used for patterning of nanometer-scale dots. In this paper, we will present the spatial resolution limits determined for these tips on different calibration gratings as well as first results obtained on nanometer scale patterning.
Keywords: STM; Spatial resolution limits; Nanotechnology;

Non linear optics and magneto-optics in ultrathin metallic films by Gilles Tessier; Pierre Beauvillain (175-185).
Ultrathin magnetic cobalt-based Au/Co/Au trilayers have been studied by optical second harmonic generation (SHG) and by linear magneto-optical Kerr effect (MOKE). We show that SHG has a selective interface and surface sensitivity, whereas MOKE is bulk sensitive. SHG was used to probe the surface plasmon (SP) resonance in Au/Co/Au films. The resonant coupling of SPs with SHG results in an enhancement and sign reversal of the non linear magneto-optical effects. On simple gold surfaces, we also demonstrate the existence of a strong relation between SHG and roughness measured by atomic force microscopy (AFM).
Keywords: Second harmonic generation; Magneto-optics; Ultrathin metallic films; Surface plasmons; Interface roughness;

We report fabrication, structural and magnetic properties of cobalt nanocrystals. They are synthesized in colloidal assemblies. The coated cobalt particles are air stable. Due to the narrow size distribution of these 8 nm particles, they self-assembled after deposition on a flat substrate. The magnetic response depends on the organization. When they are dispersed in a solution, the saturation is not reached at 2T even at 3 K, they are still a superparamagnetic part in the signal. When particles are organized in 2D superlattices, saturation is reached at lower applied field, compared to isolated particles. This is attributed to an increase of dipolar interaction. However, the hysteresis loop markedly differs with the sample orientation in the applied field. This confirms the dipolar magnetic interactions between cobalt nanoparticles.
Keywords: Magnetism; Nanoparticles; Self-assembly; Cobalt;

The hysteresis curve at very low temperature of Co nanoparticles organized in a two-dimensional network on a graphite substrate is determined both experimentally and from a numerical calculation. The Co particles are modeled as spherical particles of uniaxial symmetry and the easy axes are randomly distributed. We focus on the effect of the dipolar interactions between particles on the hysteresis loop: the magnetization curve is thus compared to the case of non-interacting particles, which corresponds to particles dispersed in solution at vanishing concentration. The magnetization curve is calculated for two orientations of the applied field: normal and parallel to the substrate surface. The ratio between the remanence magnetizations obtained with the field, normal and parallel to the surface, appears to be a convenient parameter to estimate the importance of the dipolar interactions. A satisfactory agreement with experiment is obtained for this ratio.
Keywords: Magnetism; Nanoparticles; Numerical calculations;

Modelling of SAW filter based on ZnO/diamond/Si layered structure including velocity dispersion by M.B. Assouar; O. Elmazria; R. Jiménez Riobóo; F. Sarry; P. Alnot (200-204).
A simulator based on the coupling of mode (COM) theory, previously developed for modelling the bulk substrate surface acoustic wave (SAW) devices, was modified to be adapted for layered structures. The frequency response of ZnO/diamond/Si SAW filter was calculated and the results were compared with experimental ones extracted from the literature. A good agreement is obtained for the frequencies within and close to the pass-band of the filter. Outside of this pass-band, the experimental frequency response exhibits an asymmetry, which is not reproduced by the simulation. This asymmetry is attributed to the dispersion, as a function of frequency, of SAW velocity (V P) and electromechanical coupling coefficient (K 2), which cannot be neglected in the case of layered structures. In the original program developed for bulk structures, K 2 and V were assumed to be constant. To take into account the effect of dispersion, the program was modified by the introduction of a dispersive model. The confrontation between the results obtained by simulation, including the dispersive model, and by experimental measurements shows a good agreement.
Keywords: Surface acoustic wave; Diamond; Velocity dispersion; COM theory;

Wavelength-scale periodically structured dielectrics in two or three dimensions, the so-called photonic crystals (PCs), may acquire outstanding electromagnetic properties, due to the appearance of a photonic gap and of the peculiar photon dispersion relations around such gaps. One may take advantage of these properties to elaborate novel devices based on microresonators, integrated mirrors, etc. In this paper, we start with a brief introduction to two-dimensional (2D) crystals and to defects in these crystals. We next discuss the physical and technological issues raised by some recent realisations. The incorporation of PCs into various devices is then examined, restricting ourselves to applications to light-emitters and integrated optics, a case for which radiation losses of PCs, are discussed.
Keywords: Photonic crystals; Microcavities; Semiconductors; Light-emitting devices;

Synthesis and use of a novel SnO2 nanomaterial for gas sensing by Céline Nayral; Eric Viala; Vincent Collière; Pierre Fau; François Senocq; André Maisonnat; Bruno Chaudret (219-226).
Decomposition of the organometallic precursor [Sn(NMe2)2]2 in a controlled water/anisol mixture leads to the formation of monodisperse nanocomposite particles of Sn/SnO x . Full oxidation of the particles into SnO2 occurs at 600°C without size or morphology change. These particles can be deposited onto silicon nitride covered microelectronic platforms and used as sensitive layers of gas sensors. Doping of the sensors with palladium can be achieved either by co-decomposition of organometallic precursors (doping in volume) or by deposition of palladium on preformed SnO2 nanoparticles (doping in surface). The doped sensors display an unusually high sensitivity for CO sensing.
Keywords: SnO2; Gas sensing; Palladium;

Two types of complex nanotubes produced by the arc-discharge method are investigated in this study: multi-walled carbon nanotubes filled with metallic nanowires and composite BN–C nanotubes. Their multi-element character yields specific spatial chemical arrangements — deduced from transmission electron microscopy (TEM) studies — which give precious information about the growth mechanism of nanotubes. Concerning filled nanotubes, if the metal–graphite cathode is carbon free, no filling is obtained, whereas if it contains sulfur — either as an impurity of graphite or when added under controlled quantities — complete or very long fillings are achieved, even for metals with very high melting temperatures. The chemical analyses revealed various types of fillings: pure sulfides, grains of sulfides alternating with grains of pure metals, or in some specific cases, pure metals. As for the B–C–N tubes, a total phase separation is observed between BN and C, and these two phases form concentric shells typically of the C/BN/C type. In this paper, we show that an approach combining the vapor–liquid–solid (VLS) scheme and the characteristics of the solidification given by phase diagrams account very well for the observed structures. The contrast between the absence of filling, when a sulfur-free carbon–metal rod is used for the cathode, and the successful fillings, when sulfur is added, is explained by metal–sulfur phase diagrams: adding sulfur to a liquid metal decreases the solidification temperature. The different types of fillings are also explained by the nature of the sulfur–metal phase diagram. An eutectic solidification, such as in Ni–S, yields two phases — the pure metal and the first sulfide — within a given tube, whereas the existence of a miscibility gap in the liquid, such as in Cr–S, leads to two separate liquids and, therefore, to two different fillings. In the same way, the eutectic-like pseudo-binary C–BN phase diagram explains not only the complete phase separation between BN and C, but also the observed organisation between layers: we propose that the latter is due to a sequential solidification of the two phases. As a perspective, this phase diagram approach is also discussed in the context of the formation of ropes of single-walled carbon nanotubes from the solidification of a metal–carbon liquid particle.
Keywords: Filled and mixed nanotubes; TEM studies; Phase-diagram approach;

GaN and GaInN quantum dots: an efficient way to get luminescence in the visible spectrum range by B. Damilano; N. Grandjean; J. Massies; F. Semond (241-245).
It is shown that both GaN and Ga0.8In0.2N quantum dots (QDs) can be grown by molecular beam epitaxy on silicon or sapphire substrates making use of the strain-induced two-dimensional (2D)–three dimensional (3D) growth mode occurring for mismatched materials (Stranski–Krastanov, SK, mode of growth). GaN and Ga0.8In0.2N QDs were embedded in an AlN and a GaN matrix, respectively. Despite the dislocation density (which can exceed 1010 cm−2 on silicon substrate), strong visible room temperature photoluminescence (PL) is observed owing to the QD related carrier localization and to the high QD density. Although GaN and AlN have band-gaps yielding to ultra-violet emission, the PL related to the GaN QDs is in the visible part of the electromagnetic spectrum. This is due to the presence of a large built-in electric field, which induces a strong quantum-confined Stark effect, and thereby an important red shift of the PL. It is demonstrated that the emission wavelength can be tuned in almost the whole visible spectrum range by simply varying the GaN or the GaInN QD size. The luminescence efficiency is found to be significantly larger in QD structures than in standard quantum well (QW) structures.
Keywords: Quantum dots; GaN; GaInN; Si substrate;

Use of microtechnology for DNA chips implementation by E. Souteyrand; J.P. Cloarec; J.R. Martin; M. Cabrera; M. Bras; J.P. Chauvet; V. Dugas; F. Bessueille (246-251).
Keywords: DNA chips; Silicon; Hybridisation; Microtechnology;

The understanding of the reactivity of supported nanometer-sized particles, which are used in heterogeneous catalysis, can be approached in two ways: by studying the reactivity of size selected and soft-landed small metal clusters containing 2 to 50 atoms (‘molecular’ approach) or by studying the reactivity of extended single crystal surfaces (‘surface science’ approach). We discuss the advantages and the limitations of these two approaches. We show in particular that often these two approaches cannot explain quantitatively the reaction kinetics for supported clusters a few nanometers in diameter. The peculiarity of these nanometer-sized supported clusters is due to their intrinsic heterogeneities: the presence of different types of facets, the presence of edges and the presence of the support. Taking account the recent experimental results on model supported catalysts, we show how the role of these heterogeneities can be investigated.
Keywords: Clusters; Heterogeneous catalysis; Reaction kinetics; Model catalysts;

Shape control of copper nanocrystals by A Filankembo; M.P Pileni (260-267).
In this short paper, it is demonstrated that addition of various salts differing by their counter ions markedly changes the shape of copper metal nanocrystals. These follow the Hofmeister series.
Keywords: Shape control; Copper nanocrystal; Salt;

We report the results of STM-induced photon emission experiments performed on several types of nanostructures deposited or fabricated onto atomically flat (111) gold surfaces. We demonstrate the simultaneous observation of contrasts both from chemically inhomogeneous and homogeneous nanostructures. In the latter case the mechanisms of the contrast involves only geometrical factors and we observe a resonance in excitation spectroscopy that we attribute to a localized plasmon oscillation mode. Our results illustrate the opportunities offered by STM-induced photon emission experiments as a photophysical probe technique with high-spatial resolution.
Keywords: Photon emission; Scanning tunneling microscopy; Photophysical properties; Metal nanostructures;

Author index (275-277).

Subject index (279-285).