Scanning tunneling microscopy study of growth of Pt nanoclusters on thin film Al2O3/NiAl(1 0 0)

The growth of Pt nanoclusters on thin film Al₂O₃ grown on NiAl(1 0 0) was studied by using scanning tunneling microscopy (STM). The samples were prepared by vapor depositing various amounts of Pt onto the Al₂O₃/NiAl(1 0 0) at different substrate temperatures in ultra high vacuum (UHV). The STM images show that sizeable Pt nanoclusters grow solely on crystalline Al₂O₃ surface. These Pt clusters appear to be randomly distributed and only a few form evident alignment patterns, contrasting with Co clusters that are highly aligned on the crystalline Al₂O₃. The size distributions of these Pt clusters are rather broader than those of the Co clusters on the same surface and the sizes are evidently smaller. With increasing coverage or deposition temperature, the number of larger clusters is enhanced, while the size of the majority number of the clusters remains around the same (0.4 nm as height and 2.25 nm as diameter), which differs drastically from the Pt clusters on γ-Al₂O₃/NiAl(1 1 0) observed earlier. These Pt cluster growth features are mostly attributed to smaller diffusion length and ease to form stable nucleus, arising from strong Pt-Pt and Pt-oxide interactions and the peculiar protrusion structures on the ordered Al₂O₃/NiAl(1 0 0). The thermal stability of Pt nanoclusters was also examined. The cluster density decreased monotonically with annealing temperature up to 1000 K at the expense of smaller clusters but coalescence is not observed.     

Growth of Al2O3 thin films on NiAl(1 0 0) by gas-phase oxidation and electro-oxidation

The growth and structures of aluminum oxides on NiAl(1 0 0) have been investigated by RHEED (reflection high energy electron diffraction), complemented by LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and STM (scanning tunneling microscopy). Crystalline θ-Al₂O₃ phase grows through gas-phase oxidation on the NiAl(1 0 0) substrate with its a and b-axes parallel to [0 −1 0] and [0 0 1] direction of the substrate, respectively, forming a (2 × 1) unit cell. Whilst, three-dimensional nano-sized NiAl(1 0 0) protrusions and Al₂O₃, NiAl(0 1 1) clusters were found to co-exit at the surface, evidenced by extraordinary transmission spots superposed to the substrate reflection rods in the RHEED patterns. Particularly, the NiAl(0 1 1) clusters develop with their (0 1 1) plane parallel to the NiAl(1 0 0) surface, and [1 0 0] axis parallel to the [0 −1 1] direction of the substrate surface. STM observation combined with information from AES and TPD (temperature programmed desorption) suggest the formation of these 3D structures is closely associated with partial decomposition of the crystalline oxides during annealing. On the other hand, smoother (2 × 1) oxide islands with thickness close to a complete monolayer of θ-Al₂O₃ can be formed on NiAl(1 0 0) by electro-oxidation, in contrast with the large crystalline films formed by gas-oxidation.     

Structures of Co and Pt nanoclusters on a thin film of Al2O3/NiAl(1 0 0) from reflection high-energy electron diffraction and scanning-tunnelling microscopy

With reflection high-energy electron diffraction (RHEED) and scanning-tunnelling microscopy (STM), we made measurements on Co and Pt nanoclusters grown by vapour deposition on a thin film of Al₂O₃/NiAl(1 0 0). The results show that the annealed Co nanoclusters (with mean diameters 2.5, 3.4, 5.8 nm and heights 0.7, 1.5, 1.5 nm, respectively) and Pt nanoclusters (with mean diameter 2.25 nm and height 0.4 nm) are highly crystalline and that their structures are significantly affected by the oxide substrate. Structural analysis based on the RHEED patterns indicates that both Co and Pt clusters have a fcc phase and grow with their (0 0 1) facets parallel to the θ-Al₂O₃(1 0 0) surfaces, and with their [1 1 0] and [−1 1 0] axes along the [0 1 0] and [0 0 1] directions of the oxide surface, respectively, so (Co(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0] and Pt(0 0 1)[1 1 0]∥Al₂O₃(1 0 0)[0 1 0]). This growth is optimal as the Co and Pt fcc (0 0 1) facets match well with the oxygen mesh. To minimize the lattice mismatch, the lattice parameter of the Co clusters expands 4-5% relative to fcc Co bulk, whereas the lattice parameter of the Pt clusters remains near the bulk value, as the Pt fcc (0 0 1) plane has a close lattice match with the oxide surface.     

Influence of [S2O3]/[Sm] on Sm2S3 thin films deposited by liquid phase deposition method on self-assembled monolayers

Sm₂S₃ thin films were prepared on Si (1 0 0) substrates using SmCl₃ and Na₂S₂O₃ as precursors by liquid phase deposition method on self-assembled monolayers. The influence of the molar concentration ratio of [S₂O₃²⁻]/[Sm³⁺] on the phase compositions, surface morphologies and optical properties of the as-deposited films were investigated. The as-deposited Sm₂S₃ thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), ultraviolet-visible (UV-vis) and photoluminescence spectrum (PL). Results show that it is important to control the [S₂O₃²⁻]/[Sm³⁺] during the deposition process and monophase Sm₂S₃ thin films with orientation growth along (0 1 1) direction can be achieved when [S₂O₃²⁻]/[Sm³⁺] = 2.0, pH 3.0, with citric acid as a template agent. The as-deposited thin films exhibit a dense and crystalline surface morphology. Good transmittance in the visible spectrum and excellent absorbency of ultraviolet light of the thin films are observed, and the band gap of the thin films first decrease and then increase with the increase of the [S₂O₃²⁻]/[Sm³⁺]. The as-deposited thin films also exhibit red photoluminescence properties under visible light excitation. With the increase of the [S₂O₃²⁻]/[Sm³⁺] in the deposition solution, the PL properties of Sm₂S₃ thin films are obviously improved.     

First-principles study of elastic and thermo-physical properties of kesterite-type Cu2ZnSnS4

The lattice constants and elastic constants of the kesterite-type Cu₂ZnSnS₄ have been calculated using density-functional theory (DFT). The calculated lattice constants are in good agreement with the experimental data. The calculated elastic constants indicate that the bonding strength along the [1 0 0] and [0 1 0] directions is as strong as the one along the [0 0 1] direction. The high B/G ratio shows that the kesterite-type Cu₂ZnSnS₄ compound has ductile behavior. Finally, using the Debye model, the volume, bulk modulus and heat capacity as a function of temperature for the kesterite-type CZTS have been estimated at different pressures. The Debye temperature and Gruneisen parameter are 157 K and 2.28 at 300 K temperature, respectively. The present results can give some information for the design of the kesterite-type CZTS compounds, and these can also be used to stimulate future experimental and theoretical work.     

Facile synthesis of Nb2O5 nanorod array films and their electrochemical properties

Nb₂O₅ nanorod array films were synthesized by a facile hydrothermal process using niobium metal foil and NH₄F as precursors. The Nb₂O₅ nanorods stand on the niobium metal foil substrate and are less than 100 nm in diameter and about 1 μm in length. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) characterizations indicate that these nanorods have orthorhombic structure and grew longitudinally along 〈0 0 1〉 direction. The nanorod growth mechanism was discussed. Thermal annealing at a temperature below 500 °C did not change the microstructure of nanorods but improve the crystallinity. The Nb₂O₅ nanorod array films have been tested as cathode material for lithium battery, which showed a good specific capacity up to 380 mAh g⁻¹ even after 50 charge/discharge cycles.     

Modulation structure in the STM images of the non-metallic misfit-layer compound (PbS)1.12VS2

This paper presents the scanning tunneling microscopy (STM) results of the misfit-layer compound (PbS)_1.12VS₂, which is constructed of alternately stacking of PbS (Q) and VS₂ (H) layers. Temperature dependent resistivity measurements show a semiconducting behavior with small activation energies. Unlike the metallic 1Q/1H type of compounds we have succeeded to take both the STM images of a Q layer and a H layer, because electron tunneling from the underlying H layer is suppressed when intermediate positive bias voltage (V_b) is applied to a tip. At V_b = 0.15 V the image shows pseudo-tetragonal arrays of bright spots, although it is obscure with decreasing bias voltage and disappears at less than 10 mV. A modulation structure is found on the H layer of a stepped surface on which surface atoms are undulated in a period being twice the V-V interatomic distance in the [1 0]_H or the [1 1]_H direction.     

Structural and optical properties of Er, Yb co-doped Y2O3 thin films

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.     

Influence of epitaxial growth on phase competition in Pr0.5Sr0.5MnO3 films

A series of Pr_0.5Sr_0.5MnO₃ (PSMO) films with various thickness were epitaxially grown on substrates of (0 0 1)-oriented (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT), LaAlO₃ (LAO) and SrTiO₃ (STO), and (0 1 1)-oriented STO using pulse laser deposition. Influence of epitaxial growth on phase competition was investigated. A ferromagnetic metal to antiferromagnetic insulator (FMM-AFI) transition upon cooling is present in both largely compressed situations deposited on LAO (0 0 1) and tensile cases deposited on STO (0 0 1) but absent in little strained films grown on LSAT (0 0 1), indicating that the antiferromagnetic insulating state is favored by strains. On the other hand, the 400 nm films deposited on (0 1 1)-oriented STO as well as LAO substrates show FMM-AFI transition. These results reveal that both the orientation of epitaxial growth and substrate-induced strain affect the FMM-AFI transition.     

Molecular dynamics study on surface structure and surface energy of rutile TiO2 (1 1 0)

The formula for surface energy was modified in accordance with the slab model of molecular dynamics (MDs) simulations, and MD simulations were performed to investigate the relaxed structure and surface energy of perfect and pit rutile TiO₂(1 1 0). Simulation results indicate that the slab with a surface more than four layers away from the fixed layer expresses well the surface characteristics of rutile TiO₂ (1 1 0) surface; and the surface energy of perfect rutile TiO₂ (1 1 0) surface converges to 1.801±0.001 J m⁻². The study on perfect and pit slab models proves the effectiveness of the modified formula for surface energy. Moreover, the surface energy of pit surface is higher than that of perfect surface and exhibits an upper-concave parabolic increase and a step-like increase with increasing the number of units deleted along [0 0 1] and [1 1 0], respectively. Therefore, in order to obtain a higher surface energy, the direction along which atoms are cut out should be chosen in accordance with the pit sizes: [] direction for a small pit size and [0 0 1] direction for a big pit size; or alternatively the odd units of atoms along [1 1 0] direction are removed.     

Preparation and photoluminescence characteristics of Eu-doped MgAl1.8Y0.2O4 nanocrystals

A simple combustion route was employed for the preparation of Eu³⁺-doped MgAl_1.8Y_0.2−xO₄ nanocrystals using metal nitrates as precursors and urea as a fuel in a preheated furnace at 500 °C. The powders thus obtained were then fired at 1000 °C for 3 h to get better luminescent properties. The incorporation of Eu³⁺ activator in these nanocrystals was checked by luminescence characteristics. These nanocrystals displayed bright red color on excitation under 254 nm UV source. The main emission peak was assigned to the transition [⁵D₀→⁷F₂] at 615 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were carried out to understand surface morphological features and the particle size. Crystal structures of the nanocrystals were investigated by the X-ray diffraction (XRD) technique. The crystallite size of the as-prepared nanocrystals was around 29 nm, which was evaluated from the broad XRD peaks. The crystallite size increased to ∼45 nm on further heat treatment at 1000 °C.     

Thermally activated magnetization reversal process of self-assembled Fe55Co45 nanowire arrays

We have investigated the temperature dependence of the magnetic properties and the magnetic relaxation of the Fe₅₅Co₄₅ nanowire arrays electrodeposited into self-assembled porous alumina templates with the diameter about 10 nm. X-ray diffraction (XRD) pattern indicates that the nanowire arrays are BCC structure with [1 1 0] orientation along the nanowire axes. Owing to the strong shape anisotropy, the nanowire arrays exhibit uniaxial magnetic anisotropy with the easy magnetization direction along the nanowire axes. The coercivity at 5 K can be explained by the sphere chains of the symmetric fanning mechanism. The temperature dependence of coercivity can be interpreted by thermally activated reversal mechanism as being the localized nucleation reversal mechanism with the activation volume much smaller than the wire volume. Strong field and temperature-dependent magnetic viscosity effects were also observed.     

Synthesis and CO2 capture property of high aspect-ratio Li2ZrO3 nanotubes arrays

High aspect-ratio Li₂ZrO₃ nanotubes were prepared by hydrothermal method using ZrO₂ nanotubes layers as templates. Characterizations of SEM, XRD, TEM and CO₂ adsorption were performed. The results showed that tetragonal Li₂ZrO₃ nanotubes arrays containing a little monoclinic ZrO₂ can be obtained using this simple method. The mean diameter of the nanotubes is approximately 150 nm and the corresponding specific surface area is 57.9 m² g⁻¹. Moreover, the obtained Li₂ZrO₃ nanotubes were thermally analyzed under a CO₂ flow to evaluate their CO₂ capture property. It was found that the as-prepared Li₂ZrO₃ nanotubes arrays would be an effective acceptor for CO₂ at high temperature.     

Influence of substrate temperature on structures and dielectric properties of pyrochlore Bi1.5Zn1.0Nb1.5O7 thin films prepared by pulsed laser deposition

The influence of substrate temperature on structural and dielectric properties of cubic pyrochlore Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films prepared by pulsed laser deposition process has been investigated. BZN thin films were deposited on Pt/Ti/SiO₂/Si(1 0 0) substrate and in situ annealed at 700 °C. The results indicate that the substrate temperature has a significant effect on the structural and dielectric properties of BZN thin films. The films exhibit a cubic pyrochlore structure in the substrate temperature range from 550 °C to 700 °C and at the annealing temperature of 700 °C. With further increase of substrate temperature to 750 °C, the phases of Bi₂O₃, BiNbO₄ and Bi₅Nb₃O₁₅ can be detected in the XRD pattern due to the Zn loss. The dielectric constant and loss tangent of the films deposited at 650 °C are 192 and 6 × 10⁻⁴ at 10 kHz, respectively. The tunability is 10% at a dc bias field of 0.9 MV/cm.     

First-principles calculations on Al/AlB2 interfaces

The AlB₂ (1 1 1) surfaces and Al (1 1 1)/AlB₂ (0 0 0 1) interface were studied by first-principles calculations to clarify the heterogeneous nucleation potential of α-Al grains on AlB₂ particles in purity aluminium and hypoeutectic Al-Si alloys. It is demonstrated that the AlB₂ (0 0 0 1) surface models with more than nine atomic layers exhibit bulk-like interior, wherein the interlayer relaxations localized within the top three layers are well converged. The outmost layer of AlB₂ free surface having a preference of metal atom termination is evidenced by surface energy calculations. With Al atoms continuing the natural stacking sequence of bulk AlB₂, Al-Al metallic bonds are formed across interface during the combination of Al atoms with Al-terminated AlB₂ surface. The calculated interfacial energy of the Al/AlB₂ interface is much larger than that between the α-Al and aluminium melts, elucidating the poor nucleation potency of α-Al grains on AlB₂ particles from thermodynamic considerations.     

The crystallographic texture and magnetic anisotropy of melt-spun CeCo5.4 ribbon

CeCo_5.4 ribbons have been prepared by melt spinning at wheel speeds v=5, 15, 25 and 35 m/s. The ribbons are essentially single 1:5 phase, and have significant crystallographic texture and magnetic anisotropy. The ribbon's longitudinal direction is easy direction, and normal to the ribbon plane direction is hard direction. For v=5 m/s [1 1 1]-axes of the grains near non-contact surface of the ribbon are normal to the ribbon plane. With increase of v, [1 1 0] and [2 0 0]-axes of the grains near non-contact surface rotate toward the normal direction and the c-axes parallel to the ribbon plane. The anisotropy increases up to v=25 m/s and then decreases. The grains near contact-wheel surface are randomly oriented for all v. The coercivity increases with increase of v due to decrease of the grain size. The values of coercivity are smaller in the easy direction and are larger in the hard direction, meaning that the coercivity mechanism is mainly characterized by domain wall pinning.     

Study of polycrystalline bulk CuIn1-xGaxTe2

Polycrystalline CuIn_1−xGa_xTe₂ bulk films were synthesized by reacting, in stoichiometric proportions, high purity Cu, In, Ga and Te in a vacuum sealed quartz ampoule. The phase structure and composition of the bulk films were analysed by X-ray diffraction and energy-dispersive X-ray analysis, respectively. The bulk samples, of p-type conductivity, are found to be near-stoichiometric, polycrystalline, with tetragonal chalcopyrite structure, predominantly oriented along a direction perpendicular to the (1 1 2) plane. Photoluminescence spectra were recorded at 7 K and 700 mW to characterize the defects and the structural quality. The main peak as a function of composition has been studied.     

Rotational spectra of gauche perfluoro-n-butane, C4F10; perfluoro-iso-butane, (CF3)3CF; and tris(trifluoromethyl)methane, (CF3)3CH

The microwave spectra of the gauche conformer of perfluoro-n-butane, n-C₄F₁₀, of perfluoro-iso-butane, (CF₃)₃CF, and of tris(trifluoromethyl)methane, (CF₃)₃CH, have been observed and assigned. The rotational and centrifugal distortion constants for gauche n-C₄F₁₀ are: A = 1058.11750(7) MHz, B = 617.6832(1) MHz, C = 552.18794(1) MHz, Δ_J = 0.0257(5) kHz, δ_J = 0.0052(3) kHz. A C-C-C-C dihedral angle, ω, of ∼55° has been determined. These values agree well with those obtained from a coupled cluster (CCSD/cc-PVTZ) calculation. The rotational and centrifugal distortion constants for iso-C₄F₁₀ and iso-C₄HF₉ are: B_o = 816.4519(4) MHz, D_J = 0.023(2) kHz, and B_o = 903.6985(25) MHz, D_J = 0.043(4) kHz, respectively. The dipole moment of iso-C₄F₁₀ and iso-C₄HF₉ have been measured and found to be 0.0338(8) and 1.69(9) D, respectively.     

Structural and optical properties of SrCu2O2 films deposited on sapphire substrates by pulsed laser deposition

We deposited SrCu₂O₂ (SCO) films on sapphire (Al₂O₃) (0 0 0 1) substrates by pulsed laser deposition. The crystallographic orientation of the SCO thin film showed clear dependence on the growth temperature. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis showed that the film deposited at 400 °C was mainly oriented in the SCO [2 0 0] direction, whereas when the growth temperature was increased to 600 °C, the SCO film showed a dominant orientation of SCO [1 1 2]. The SCO film deposited at 500 °C was obvious polycrystalline, showing multi peaks from (2 0 0), (1 1 2), and (2 1 1) diffraction in the XRD spectrum. The SCO film deposited at 600 °C showed a band gap energy of 3.3 eV and transparency up to 80% around 500 nm. The photoluminescence (PL) spectra of the SCO films grown at 500 °C and 600 °C mainly showed blue-green emission, which was attributed to the intra-band transition of the isolated Cu⁺ and Cu⁺–Cu⁺ pairs according to the temperature dependent-PL analysis.     

Evolution of SiGe nanoclusters and micro defects in the Si1−xGex layer fabricated by two-step ion implantation and subsequent thermal annealing

The Si_1−xGe_x thin layer is fabricated by two-step Ge ion implantation into (0 0 1) silicon. The embedded SiGe nanoclusters are produced in the Si_1−xGe_x layer upon further annealing. The number and size of the nanoclusters changed due to the Ge diffusion during annealing. Micro defects around the nanoclusters are illustrated. It is revealed that the change of Si-Si phonon mode is causing by the nanoclusters and micro defects.