Applied Surface Science (v.177, #3)

Fabrication of Nd3+, Cr4+ co-doped Gd3Ga5O12 thin film waveguide by two-target pulsed laser deposition by Sachiyo Fukaya; Tetsu Hasegawa; Yoshihisa Ishida; Tomoyuki Shimoda; Minoru Obara (147-151).
The growth of Nd3+,Cr4+ co-doped gadolinium gallium garnet (Gd3Ga5O12:GGG) (1 1 1) thin films on YAG (1 1 1) substrate has been demonstrated with two-target pulsed laser deposition and post-annealing. The concentrations of Nd and Cr in the co-doped thin film are well controlled by changing respective KrF laser ablation fluence for Nd:GGG and Cr,Ca:GGG sintered targets. The structure of Nd,Cr:GGG thin films on the YAG substrate shows a waveguide structure with high numerical aperture. It is confirmed that the Nd3+ and Cr4+ ions in co-doped films act as the laser active ion and saturable absorber, respectively, at 1.06 μm to be able to be used for monolithic self Q-switched waveguide laser to generate high peak power and short pulse output.
Keywords: KrF excimer laser; Pulsed laser deposition; Optical waveguide; Co-doping; Self Q-switched laser;

Solution growth and characterization of amorphous selenium thin films by Biljana Pejova; Ivan Grozdanov (152-157).
A novel solution growth methodology for fabrication of amorphous, red selenium thin films and their conversion to the hexagonal, gray modification is presented. The method is based on deposition process from aqueous solution of selenosulfate by citric or ascorbic acid as oxidizing agents. The X-ray diffraction method is used for identification of the deposited materials. Ultra-thin films of both amorphous and hexagonal selenium are highly transparent in the Vis–NIR spectral region.
Keywords: Selenium thin film; Solution growth method; Optical band gap energy;

Angle dependent X-ray photoemission study on UV-ozone treatments of indium tin oxide by Weijie Song; S.K So; Daoyuan Wang; Yong Qiu; Lili Cao (158-164).
The surface chemistry of ITO thin-films before and after UV-ozone treatment was characterized using angle dependent X-ray photoelectron spectroscopy (ADXPS). After solvent cleaning, the ITO surface was covered with a thin nonconducting carbon contamination layer of ∼7 Å. This contamination layer was removed efficiently by UV-ozone treatment, and the chemical states of the residual carbon at ITO surface after UV-ozone treatment were quite different from contaminated carbon. UV-ozone treatment modified ITO surface by introducing O2− ions into ITO surface. The modified depth was about 50 Å. The modification decreased the carrier concentration at ITO surface, and thus decreased the conductivity of ITO surface.
Keywords: Angle dependent X-ray photoelectron spectroscopy; Indium tin oxide; UV-ozone treatment;

A deposition system combining pulsed laser deposition (PLD, cooled Ga target) and a source of atomic nitrogen was developed to grow epitaxial gallium nitride on sapphire. The layers obtained with this system were characterized using high-resolution X-ray diffraction, atomic force microscopy (AFM), Hall effect, and secondary ion mass spectroscopy (SIMS). It is found that the crystal quality greatly depends on the atomic nitrogen flux incident on the substrate during growth. After optimization of the atomic nitrogen-to-gallium flux ratio, samples showing very narrow GaN(0 0 0 2) rocking curves (full width at half maximum, FWHM=80 arcsec) have been synthesized at a low substrate temperature (T s=750°C). Surface analysis of these thin films, using AFM, also show a very low roughness (R rms=14 Å).
Keywords: GaN thin films; Hall effect; Epitaxial;

The chemical states of Cu and Zn in an ultra-fine high performance Cu/ZnO/Al2O3 catalyst at various preparation stages for the methanol synthesis by CO2/H2 were investigated by X-ray photoelectron spectroscopy. It was found that copper was presented as metallic state and zinc still as ZnO during reaction or reduction. Moreover, it shows that the metallic Cu can be partially oxidized to Cu δ+ species by pure CO2. The active center of the Cu/ZnO/Al2O3 catalyst for methanol synthesis was discussed.
Keywords: X-ray photoelectron spectroscopy; Cu/ZnO/Al2O3; Methanol synthesis; Synergism; Active site;

Adsorption properties of SO2 on ultrafine precious metal particles studied using density functional calculation by Nobumoto Ohashi; Kentaro Yoshizawa; Akira Endou; Seiichi Takami; Momoji Kubo; Akira Miyamoto (180-188).
The microscopic adsorption properties of molecules including SO2 on ultrafine precious metal particles such as Pd and Pt were investigated using density functional quantum chemical calculations. The precious metal particles which are used as the activation sites in a three-way catalyst (TWC) were modeled by Pd3 and Pt3 clusters. The adsorption energies (E ads) of the molecules on the metal clusters were calculated. Different possible adsorption sites of SO2 on the Pd3 and Pt3 clusters were considered. It was found that the SO2 adsorption states on the Pd3 cluster are energetically more stable than those on the Pt3 cluster when SO2 molecule was initially located perpendicular to the cluster plane. There were only small differences in the values of E ads for the SO2 adsorption on each adsorption site of the Pd3 clusters. However, in the case of the Pt3 cluster, the values of E ads for SO2 adsorption depended on the adsorption sites. Moreover, when SO2 molecule and the cluster were on the same plane, the SO2 adsorption state on the Pd3 cluster was energetically less stable than that on the Pt3 cluster. These results indicate that the adsorption stabilities of SO2 on the Pd3 cluster strongly depend on the adsorption geometries. The difference in the adsorption stabilities of SO2 on the Pd3 cluster can be explained by the difference in the orbital interaction near the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels.
Keywords: Density functional calculations; Metal cluster; Pt; Pd; Molecular adsorption; Adsorption energy; SO2;

Morphological control of the supramolecular pseudoisocyanine J-aggregates by the functions of a mica/solution interface by Shoko S Ono; Sadaaki Yamamoto; Hiroshi Yao; Osamu Matsuoka; Noboru Kitamura (189-196).
The effect of a mica/solution interface on the formation of supramolecular pseudoisocyanine chloride (PIC-Cl) J-aggregates was investigated by AFM observing the morphology and alignment of the J-aggregates relative to a mica surface upon applying a PIC-Cl solution onto a mica surface. When a PIC-Cl solution was placed directly on freshly cleaved mica (Method 1), the resulting J-aggregates formed leaf-like islands that are anisotropically oriented with respect to the lattice of a mica substrate. These anisotropic alignment might results from the epitaxial interaction between the positively charged N atoms of the molecules and the regularly aligned negative holes left by the dissociating K+ ions during the nucleation process. Based on their epitaxial interactions, the supramolecular structure of the J-aggregate has been proposed that the long axis of PIC-Cl molecules is parallel to the long axis of the island. In contrast, when a PIC-Cl solution was poured onto an aliquot of pure water already present on the mica surface (Method 2), the J-aggregates formed streaks of needle-like islands that were oriented in only one direction which corresponded well to the stream of the poured PIC-Cl solution. Based on these results, possible aggregation mechanisms were proposed where the stream of the solution is participating. It is revealed for the first time that both the morphology and alignment of the J-aggregate islands relative to a mica surface can be controlled by how the PIC-Cl solution is applied onto the mica surface.
Keywords: Atomic force microscopy; Surface structure; Adsorption; Solid-fluid interfaces; Epitaxy; Growth; Self-assembly;

Local overheating near the interface of a growing droplet may lead to a decrease in the rate of droplet growth if the heat conductivity is sufficiently small. Moreover, this rate may even change sign, so that droplets of radius larger than a critical radius will shrink. The existence of a thermal bath with heat loss through thermal dissipation narrows the range of existence of this phenomenon.
Keywords: Droplet; “Time lag”; Landau–Ginzburg equation;

Properties of HgCdTe films obtained by laser deposition on a sapphire by I.S Virt; M Bester; Ł Dumański; M Kuźma; I.O Rudyj; M.S Frugynskyi; I.V Kurilo (201-206).
Films of HgCdTe have been obtained by pulsed laser deposition (PLD) using: Nd:YAG pulse laser (40 ns, 1 J/pulse, λ=1.06 μm) and XeCl excimer laser (25 ns, 150 mJ/pulse, λ=0.308 μm). Layers were deposited on monocrystalline and amorphous surfaces of Al2O3. Samples were obtained when the substrate temperature was 300 and 500 K. Monocrystalline Hg1−x Cd x Te (x=0.2) were used as a target. A thickness of layers obtained was in the range of 0.05–0.5 μm, depending on a type of laser used and on a number of shots. Surface morphology was investigated by electron scanning microscopy. The chemical composition of layers was determined by X-ray micro-analyses. The samples obtained were of good homogeneity and they reflected well the composition of the target. The structural properties of the samples were compared with results of the electrophysical measurements.
Keywords: Thin solid films; HgCdTe; Pulsed laser deposition;

The dispersive component of the surface free energies (γ s d) of commercial active carbons (AC) from various origins were determined by inverse gas chromatography at infinite dilution (IGC-ID). This method discriminates clearly the AC produced from wood (and activated/carbonised with phosphoric acid) and those from coconut-shell (carbonised and steam activated at 850°C). The values for the last AC (from coconut) are twice higher than the values for AC of wood origin. The structure and shape of the pores have to be considered to explain these values. It seems that for AC, IGC-ID globally characterises the most energetic micropores. This can be observed, in this work, by two ways: (i) washing of commercial AC (chemically activated) allows to liberate a part of the micropores blocked by soluble phosphate and consequently increases the γ s d value; (ii) modifying coconuts AC by chemical treatment (formamide) results in a strong decrease of both microporosity and γ s d value. On the contrary, thermal activation of the modified AC increases at the same time the microporosity and the surface free energy. Finally, we demonstrate that the IGC method is also an useful tool to monitor in situ the evolutions of the surface properties of carbonaceous materials.
Keywords: Surface free energy; Inverse gas chromatography; Active carbon;