Nanocrystallite formation in Pd capped PrH3−δ films

Restricting the palladium cap layer thickness to ≤9 nm on top of 170 nm Pr films during in situ hydrogen loading has been shown to result in nanocrystallite size PrH_3−δ films even though the deposited Pr films are of large crystallite size. The effect is attributed to hydrogen-induced stresses in the PrH_3−δ films, which trigger structural rearrangement. These nanocrystalline films show a blue shift of the transmittance edge with respect to PrH_3−δ films of large crystallite size. The approximate size of the nanocrystallites calculated from the blue shift using an effective mass approximation (EMA) theory is supported by XRD, TEM and AFM measurements.     

Comparative studies on structural and optical properties of ZnO films grown on c-plane sapphire and GaAs (001) by MOCVD

ZnO thin films were grown on c-plane sapphire and GaAs (001) substrates by metalorganic chemical vapor deposition. Atomic force microscopy and double-crystal X-ray diffractometry were utilized to investigate the structural properties of the ZnO films. The optical properties of ZnO films were also investigated in terms of time integrated and resolved photoluminescence (TIPL and TRPL). Large hexagonal crystallites and better crystalline quality were observed from the ZnO film on sapphire. Also, both the TIPL and TRPL showed a significant difference as the substrate changed. In particular, a detected sharp contrast in the result of TRPL measurement is due to the different defect structure and the lattice strain and stress of ZnO films on different substrates.     

Deactivation of TiO2 photocatalytic films loaded on aluminium: XPS and AFM analyses

TiO₂ films were loaded on aluminium substrates by dip-coating method. Based on cyclic photocatalytic degradation experiments using benzamide as model molecule, XPS and AFM tests, the deactivating behaviour of the samples was studied. Experiment results show that the samples with less coating times (one to four times) deactivated very quickly, while the samples coated more than five times did not lose activity. Al element was proved to segregate from substrate and diffuse into TiO₂ films during calcination and annealing treatment, existing as mixture of Al₂O₃ and Al(OH)₃ at the boundaries among TiO₂ particles. During photocatalytic reactions in aqueous phase, the transformation of Al from Al₂O₃ to Al(OH)₃ and the leaching of the latter brought out serious alternation of surface morphology to the samples coated one to three times, on whose surface Ti³⁺ and Ti²⁺ centers were also detected after six cycles of photocatalytic reactions, while fresh films and the tested films which did not deactivate possess unique +4 valence Ti. The alteration of surface morphology, together with the change of valence of surface Ti element, resulted in the deactivation encountered in this research.     

Electrostatic assembly of Cu2O nanoparticles on DNA templates

In this paper, a method for highly ordered assembly of cuprous oxide (Cu₂O) nanoparticles (NPs) by DNA templates was reported. Cetyltrimethylammonium bromide (CTAB)-capped Cu₂O NPs were adsorbed onto well-aligned λ-DNA chains to form necklace-like one-dimensional (1D) nanostructures. UV-vis, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the nanostructure. The Cu₂O nanostructures fabricated with the method are both highly ordered and quite straight.     

Photoalignment of liquid crystals by cinnamate polyelectrolyte layer-by-layer ultrathin film

In this paper, a new layer-by-layer (LBL) ultrathin film with photosensitive cinnamoyl chromophores was prepared. It could induce uniform alignment of liquid crystals (LC) after irradiated by linearly polarized ultraviolet (LPUV) light. First, a photosensitive polycation containing cinnamoyl side groups was synthesized. Then a layer-by-layer ultrathin film was prepared in an aqueous solution of the polycations and poly (sodium 4-styrenesulphonate). The spectra of UV-vis absorption and ellipsometric measurement showed that the LBL film was uniform and suitable for the photoalignment of LC. When the film was irradiated by LPUV light, the cinnamoyl units in the film underwent [2 + 2] cycloaddition along the electric vector direction of the LPUV light. The polarized UV-vis spectra also proved that the film was anisotropic, and then the film could induce the homogeneous alignment of nematic LC. With the number of bilayers increasing, the alignment effect of multilayer film became better, and the contrast ratio became higher.     

Atomic scale characterization of oxygen adsorbates on Al(111) by scanning tunneling microscopy

We present atomic scale STM pictures of clean and oxygen containing Al(111) surfaces. Little influence of the surface oxygen on the topography of the surfaces is found. Three different oxygen species can be distinguished. One of them is associated with adsorbed oxygen and found to grow in small islands upon adsorption at 300 K. Characteristic hexagonal nuclei, created upon annealing of a dilute oxygen adlayer, represent the second one. By comparison with existing spectroscopic data these are assigned to nuclei of a surface oxide.     

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.     

Composition dependent structural modification in relaxed Si1−xGex films by 100 MeV Au beam

We report the modification of molecular beam epitaxy grown strain-relaxed single crystalline Si_1−xGe_x layers for x=0.5 and 0.7 as a result of irradiation with 100 MeV Au ions at 80 K. The samples were structurally characterized by Rutherford backscattering spectrometry/channeling, transmission electron microscopy (TEM) and high-resolution X-ray diffraction before and after irradiation with fluences of 5×10¹⁰, 1×10¹¹ and 1×10¹² ions/cm², respectively. No track formation was detected in both the samples from TEM studies and finally, the crystalline to amorphous phase transformation at 1×10¹² ions/cm² was examined to be higher for Si_0.3Ge_0.7 layers compared to Si_0.5Ge_0.5 layers.     

Scanning probe microscopy (STM/AFM) and applications in biology

Received: 27 March 1998     

Frequency shifts of cantilever in AFM

The frequency shift and frequency shift image of cantilever in AFM have been studied by numerical integration of the equation of motion of cantilever for silicon tip with rutile TiO₂(0 0 1) surface in UHV conditions and by the Hamaker summation method for the tip-surface interaction forces. The effects of the excitation frequency at the cantilever base and the equilibrium position of the tip on the frequency shift have been calculated and the results showed the same phenomena as those measured, e.g., the frequency shift increased dramatically or rapidly before the contact point and was then almost level off after the contact point. The effects of scanning speed and the initial closest distance of tip to the contact point have been calculated at different excitation frequencies at the cantilever base and the results showed that proper frequency shift image could be obtained either by noncontact mode at the excitation frequency slightly less than the resonance frequency of free cantilever, or by tapping mode at the excitation frequency a few times smaller than the resonance frequency of free cantilever.     

Diamond-like carbon layer formation on graphite by excimer laser irradiation

2 fluence results in the formation of a thin hydrogen-free diamond-like carbon (DLC) film on the surface. The density of the laser-formed DLC layer is approximately 2.8 g/cm³, as calculated from area-selective Raman spectroscopy and atomic force microscopic (AFM) measurements. The proportion of the sp³ hybrid states of carbon is estimated to be approximately 50%. Annealing in air at 650 °C for 30 min recovers the graphite structure. Received: 26 March 1998/Accepted: 27 March 1998     

X-ray and AFM studies of polydisperse TiO2 (anatase) particles

Titanium dioxide (TiO₂) materials of a high chemical purity, as-prepared by the thermal hydrolysis, as well as subsequently modified by adsorption of different metal cations (Fe³⁺, Co²⁺, Cu²⁺), have been investigated by the X-ray diffraction, X-ray fluorescence and AFM microscopy methods. All TiO₂ powders have a fine-dispersated anatase structure and consist of grown together nanocrystallites of ∼8-17 nm. TiO₂ particles, usually ranging from 100 to 600 nm, show the ability to form large agglomerates, up to 2 μm in size. Contrary to the pure anatase, metal-modified TiO₂ particles possess a positive charge on their surface and can be lifted away by the AFM tip from the substrate surface during the scanning. This effect is mostly pronounced for the Fe-modified TiO₂ sample, where particles up to 250 nm are removed. The possible interaction mechanisms between different TiO₂ particles and the silicon tip are discussed. The electrostatic force has been found to play an essential role in the sample-tip interaction processes, and its value depends on the type of metal cation used.     

Preparation, characterization and application of a nanostructured composite: Octakis(cyanopropyldimethylsiloxy)octasilsesquioxane

Octakis(cyanopropyldimethylsiloxy)octasilsesquioxane was prepared and characterized by ¹³C, ²⁹Si NMR (MAS), SEM, FT-IR, XRD and thermogravimetric techniques. The four groups α, β, γ, κ (to the terminal silicon atom), associated with an acrylonitrile, were clearly seen in the ¹³C NMR (α-CH₂ at 17.9; β-CH₂ at 31.3; γ-CH₂ at 50.4; κ-CN at 59.0 ppm). The ²⁹Si NMR spectrum of the final product, exhibits only Q type silicon signal, ascribed to Q⁴ (−118.0 ppm). The presence of acrylonitrile substituted for octameric cube confers a relative change phase and thermal stability to the material. With regard to the applications for this new material, it was intended, in this case, to react with Na₂[Fe(CN)₅NH₃] by chemical substitution. This composite was incorporated into a carbon paste electrode and the electrochemical studies were performed by cyclic voltammetry. The cyclic voltammogram of the modified graphite paste electrode, showed one redox couples with formal potential () = 0.24 V versus SCE.     

Enhancing the phase stability and ionic conductivity of scandia stabilized zirconia by rare earth co-doping

Effect of co-doping Yb, Gd and Ce in scandia stabilized zirconia (SSZ) on the phase stability, high temperature aging behavior and ionic conductivity was studied. Both binary (10 mol% SSZ) and the ternary (co-doped) compositions were found to be in single cubic phase in the as-processed condition. However, the binary composition exhibited the rhombohedral ‘β’ phase after sintering whereas the ternary compositions remained in the single cubic phase. The sintered pellets were aged at 900 °C for 500 h in air to study the phase stability at high temperature. Transmission electron microscopy revealed that the aged samples of Yb and Gd co-doped compositions contain small amount of the tetragonal phase which resulted in considerable degradation in conductivity (more than 20%). The Ce co-doped sample, on the other hand, was in single cubic phase after aging and this ensured that conductivity reduction was minimal in this composition. The co-doped samples however, showed higher conductivity before and after aging compared to the binary composition. The rhombohedral ‘β’ phase was absent in all the co-doped ternary compositions even after high temperature aging.     

Investigation of the structural distortion in FeSex crystals by X-ray absorption near edge structures (XANES) spectroscopy

Crystals of the new superconductor FeSe were investigated by X-ray absorption near edge structures spectroscopy (XANES) in the temperature range 300-77 K. Crystals with Se content varying from x=1-0.8 were used. The Fe K-edge spectra show a structure distortion and increase in hole content as x decreases. The Fe K-edge spectra also show changes when the crystals are cooled to 100 K which may be related to the orthorhombic distortion reported in these crystals. The data is examined in conjunction with XRD measurements on the crystals to seek a correlation.     

True atomic resolution under ambient conditions obtained by atomic force microscopy in the contact mode

2 ) has been investigated by contact-mode atomic force microscopy (AFM) in air. Both the terraces and the monolayer step itself were reproducibly imaged at atomic resolution in the repulsive-force regime at forces between tip apex and sample of the order of 10^-9 N. Several kinks were also imaged at atomic resolution. Details of the atomic registry of subsequent Se-Nb-Se sandwich layers as well as the arrangement of the individual atoms at the kink sites were resolved. The results are in perfect quantitative agreement with the lattice structure known from X-ray analysis and indicate that true atom-by-atom lateral resolution of microscopic defects is feasible by AFM in the contact mode and under ambient conditions. Published online: 10 February 1999     

High-throughput study of the influence of H2O and CO2 on the performance of nitrogen storage and reduction (NSR) catalysts

This paper describes the application of parallel high-throughput experimentation based on FTIR spectral imaging to a tolerance study for NO_x storage and reduction (NSR) catalysts with respect to CO₂ and H₂O in the feed. It was found that both gases decrease the storage capacity of platinum/barium based NSR catalysts, with H₂O having a stronger effect than CO₂.     

Time relaxation of dielectric constant in the commensurate phase of TlGaSe2

The results of measurements of the time dependences of the dielectric constant of TlGaSe₂ in the commensurate ferroelectric phase are presented. From the result of the observation of the decay of ε at different stabilized temperatures below the commensurate phase transition temperature after cooling from the incommensurate phase, the presence of two different characteristic relaxation time constants with the same temperature behaviour has been revealed. This peculiarity is considered as a result of a coexistence of two polar sublattices in the temperature range below 110 K. According to these results, the previously reported dielectric anomaly at about 103 K is considered as a final lock-in phase transition accompanied by the forming of the antiferroelectric state in TlGaSe₂.     

Synthesis and electrochemical properties of Li4Ti5O12/C composite by the PVB rheological phase method

Spinel Li₄Ti₅O₁₂/C powders were synthesized successfully by a simple rheological phase method using polyvinylbutyral (PVB) as both template and carbon source. The structure and morphology characteristics of the composite were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy and transmission electron microscopy. The XRD results showed that the composite had a good crystallinity. Its average particle size was about 2.1 μm with a narrow size distribution as a result of homogeneous mixing of the precursors. The in situ carbon coating produced by decomposition of PVB played an important role in improving electrical conductivity, thereby enhancing the rate capacity of Li₄Ti₅O₁₂ as anode material in Li-ion batteries. The Li₄Ti₅O₁₂/C composite, synthesized at 800 °C for 15 h under argon, containing 0.98 wt% of carbon, exhibited better electrochemical properties in comparison with the pristine Li₄Ti₅O₁₂, which could be attributed to the enhanced electrical conductive network of the carbon coating on the particle surface.