Dynamic study of W atoms and clusters on W (1 1 1) surfaces

Using a field ion microscope, the diffusion behaviors and atomic processes of W atoms and clusters on W (1 1 1) surfaces were observed directly. The activation energy of W clusters diffusion on W (1 1 1) as a function of cluster size has an oscillatory and increasing behavior. But, the activation energy of a single W atom is especially high. The compact geometric structures are more stable and have higher activation energies of surface diffusion than structures with extra atoms at the periphery. Besides the terrace diffusion, other atomic processes such as the ascending, descending, detachment motion on W (1 1 1) surfaces were also observed. Unlike the general systems, their occurrence temperatures are quite near. These experimental results were used to discuss the formation mechanism of single atom W tips.     

Electronic changes of dysprosium and tungsten during growth of Dy on W(100)

Dy growth on W(100) has been studied with W 4f_7/2 core-level shift (CLS) and Dy/W(100) valence band analysis. CLS measurements of the W 4f_7/2 peaks during the growth resulted in three Dy-induced features at 31.24±0.02, 30.92±0.02 and 30.74±0.02 eV binding energies. The shift in the W 4f_7/2 peak is consistent with the reconstruction seen in the Dy thin film. Valence band study of Dy/W(100) showed an interfacial hybridization between Dy and W(100) which may be responsible for the high thermal stability of Dy superstructures on W(100). The same direction of shift in Dy 4f levels and the W 4f_7/2 level during growth suggest that these shifts are mainly strain dependent. Increasing the Dy overlayer thickness to more than 2 ML resulted in Dy 5d–6s hybridization which has been attributed to the formation of islands of Dy.     

Atomic structure and electronic properties of Ta(1 1 2) and W(1 1 2) surfaces

We present results of theoretical and experimental studies of the structural and electronic properties of Ta(1 1 2) and W(1 1 2) surfaces. Atomic geometries of these surfaces and their electronic structures have been obtained from the first-principles pseudo-potential calculations based on the density functional theory and the use of the plane wave basis set. In the experimental part of our studies we have performed measurements of the photofield emission spectroscopy. The calculated density-of-states distributions are compared and discussed with energy distributions of electrons extracted from photofield emission characteristics. The photofield emission spectra measured for the surface regions of considered systems present very dense structures of features, while the corresponding dependences obtained for the bulk have much more smooth shape. Experimental results correspond very well with calculated density-of-states distributions.     

A theoretical study of hydrogen adsorption and diffusion on a W(1 1 0) surface

We have used density functional theory to investigate hydrogen adsorption and diffusion on a W(1 1 0) surface. Hydrogen adsorption structures were examined from low coverage to one monolayer, and a threefold hollow site was found to be the most stable site at all coverages. In contrast to previous assertions, the work function decrease is not due to electron transfer from the hydrogen atoms to the W surface, but due to electron depletion at the vacuum region above the hydrogen atoms. Hydrogen atoms can diffuse via short-bridge sites and long-bridge sites at a coverage of θ = 1.0. Although the calculated activation energy for hydrogen diffusion via a short-bridge site is as small as 0.05 eV, field emission microscope experiments have shown that the activation energy for hydrogen diffusion is about 0.20 eV, which agrees fairly well with our calculated value of the activation energy via a long-bridge site. This discrepancy can be related to hydrogen delocalization on the W(1 1 0) surface, which has been suggested by electron energy loss spectroscopy experiments.     

Structural and electronic properties of the Ti/W(1 1 1) adsorption system

We present theoretical and experimental study of the structural and electronic properties of the Ti/W(1 1 1) adsorption system. Atomic arrangements of the considered surfaces and their electronic structures have been obtained from first-principles pseudopotential calculations based on the density functional theory in a plane-wave-basis implementation. The corresponding experimental data have been provided by photofield emission spectroscopy. Investigations of the clean and Ti-covered W(1 1 1) surfaces indicate substantial structural relaxations deep into the substrate, and a noticeable modification of the surface electronic properties of the system induced already by a thin film of titanium. Configuration with adatoms positioned in substrate-lattice-continuation (i.e., deep-hollow) sites is found to be energetically most favorable. A good agreement between the measured photofield emission spectra and the computed local-density-of-states distributions confirms our theoretical predictions for a clean W(1 1 1) substrate as well as Ti coverages of 0.25 and 1 ML.     

On the preparation and electronic properties of clean W(1 1 0) surfaces

We have studied the influence of oxygen pressure during the cyclic annealing used for the cleaning of W(1 1 0) surfaces. For this purpose the surface morphology and electronic properties are measured by means of scanning tunneling microscopy (STM) and spectroscopy (STS), respectively. It is found that the surfaces with impurity atom densities as low as 2 × 10⁻³ can be obtained by gradually reducing the oxygen pressure between subsequent annealing cycles down to about 2 × 10⁻⁸ mbar in the final cycle. Only on the clean surface a bias-dependent spatial modulation of the local density of states (LDOS) is observed at step edges and around impurity sites by STS. In addition, we find a pronounced peak in the occupied states. In combination with density functional theory calculations these features can be traced back to a dispersive p_z-d_xz-type surface resonance band and the lower band edge of a surface state, respectively.     

Surface mass diffusion and step stiffness on an anisotropic surface; Mo(0 1 1)

Results of step fluctuation experiments for Mo(0 1 1), using low-energy electron microscopy, are re-examined using recently developed procedures that offer accurate coefficients of surface mass diffusion. By these means, surface diffusion D_s is documented at T/T_m ∼ 0.5, while the crossover to relaxation driven by bulk vacancy diffusion is inferred for T/T_m ∼ 0.6. Here, T_m is the melting temperature T_m = 2896 K. We obtain D_s = 4 × 10⁻⁴ exp(−1.13 eV/k_BT) cm²/s for the temperature interval 1080-1680 K. Possible indications of diffusion along step edges appear for T/T_m ∼ 0.4. The same measurements of step fluctuation amplitudes determine also the step stiffness, which by symmetry is anisotropic on Mo(0 1 1). It is shown that three independent procedures yield mutually consistent step stiffness anisotropies. These are (1) step fluctuation amplitudes; (2) step relaxation rate anisotropies; and (3) the observed anisotropies of islands in equilibrium on the Mo(0 1 1) surface. The magnitude of the step stiffness obtained from step edge relaxation is consistent with earlier measurements that determine diffusion from grain boundary grooving.     

Surface modification by vacuum annealing for field emission from heavily phosphorus-doped homoepitaxial (1 1 1) diamond

The relationship between field emission properties and C 1s core level shifts of heavily phosphorus-doped homoepitaxial (1 1 1) diamond is investigated as a function of annealing temperature in order to optimize surface carbon bonding configurations for device applications. A low field emission threshold voltage is observed from surfaces annealed at 800 °C for hydrogen-plasma treated surface, while a low field emission threshold voltage of wet-chemical oxidized surface is observed after annealing at 900 °C. The C 1s core level by X-ray photoelectron spectroscopy (XPS) showed a shoulder peak at 1 eV below the main peak over 800 and 900 °C annealing temperature for hydrogen-plasma treated and wet-chemical oxidized surfaces, respectively. When the shoulder peak intensity is less than 10% of the main peak intensity, lower threshold voltages are observed. This is due to the carbon-reconstruction which gives rise to a small positive electron affinity. By increasing annealing temperature, the shoulder peak ratios also increase, which indicates that a surface graphitization takes place. This leads to higher threshold voltages.     

Surface diffusion during decay of nano-island on Si(1 0 0) at high temperature

Surface diffusion during decay of a two-dimensional nano-island formed on Si(1 0 0) surface at 750-800 K is studied using STM and a kinetic Monte Carlo simulation. From a surface diffusion point of view, decay proceeds so that the total diffusion rate of atoms on a surface decreases. Atoms at step edges move more frequently than terrace atoms, which results in decay from step edges of the island. In addition, a terrace atom takes part in surface diffusion in the same way as an atom from steps of the island once it hops up on a terrace leaving a vacancy. The mass transport is not a specific atom process but terrace atoms and vacancies on the terrace are involved. Repeated upward and downward hops of atoms and their difference are combined with surface diffusion, which leads to the mass transport. Some tracks of atom using simulation show random walk with preferential diffusion along step edges, re-entering to the island, exchange of diffusing atom and filling in a vacancy on the terrace. The motion of the center of the island to the upper side of the terrace observed by STM is also well reproduced in the simulation.     

Growth and oxidation of Cr films on the W(1 0 0) surface

The growth and oxidation of Cr films on the W(1 0 0) surface have been studied with low energy electron microscopy (LEEM) and diffraction (LEED). Cr grows in a Stranski-Krastanov (SK) mode above about 550 K and in a kinetically limited layer-by-layer mode at lower temperature. Stress relief in the highly strained pseudomorphic (ps) Cr film appears to be achieved by the formation of (4 × 4) periodic inclusions during the growth of the third layer between 575 and 630 K and by growth morphological instabilities of the third layer at higher temperature. Kinetic or stress-induced roughening is observed at lower temperature. In the SK regime, three-dimensional (3D) Cr islands nucleate after the growth of three Cr layers. 3D island nucleation triggers dewetting of one layer from the surrounding Cr film. Thus, two ps Cr layers are thermodynamically stable. However, one and two layer ps Cr films are unstable during oxidation. 3D clusters, that produce complex diffraction features and are believed to be Cr₂O₃, are formed during oxidation of one Cr layer at elevated temperature, T ? 790 K. The single layer Cr film remains intact during oxidation at T ? 630 K. 3D bulk Cr clusters are formed predominantly during oxidation of two ps Cr layers.     

Surface diffusion of k-mers in one-dimensional systems

The surface diffusion of interacting k-mers is studied both through analytical and Monte Carlo simulation methods in one-dimensional systems. Adsorption isotherms, jump diffusion coefficients and collective diffusion coefficients are obtained for attractive and repulsive k-mers, showing a variety of behaviors as a function of the size of particles, k. The following main results are found: (a) diffusion coefficients increase with k for non-interacting particles; (b) for fixed k, diffusion coefficients increase as the interaction energy increases from negative (attractive) to positive (repulsive) values; (c) for attractive interactions diffusion coefficients increase with k in the whole range of coverage; (d) for repulsive interactions diffusion coefficients decrease with k up to moderately high coverage and increase with k at high coverage. Results are rationalized in terms of the behavior of the vacancy probability distribution.     

Diffusion Dynamics of Cux Cluster on Cu(111) Surface

利用分子动力学与修正分析型嵌入原子模型研究了从200K至800 K范围内,Cux (1·x·8)小团簇在(111)面的自扩散动力学行为,计算了Cu小团簇的扩散系数与激活能. 计算结果表明,密排的Cu7团簇的激活能最高,其扩散前因子比单个吸附原子的高3个数量级,这与其他类似金属的实验结果一致. 另外,还定量讨论了薄膜生长与团簇扩散的关系.     

Systematical study of dimer diffusion on metal fcc(0 0 1) surfaces

We study systematically the dimer diffusion on a series of metal fcc(0 0 1) surfaces. The atomic interactions are modeled by the realistic model potentials including embedded-atom method potential, surface-embedded-atom method potential, and Rosato-Guillopé-Legrand potential. Based on the results of the static calculations and the molecular dynamics simulations, three different kinds of fcc(0 0 1) surfaces can be distinguished named hard, middle, and soft. On the different kind of surfaces, not only the dominant diffusion mechanism but also the physical model for exchange mechanism is different. In addition, besides the conventional hopping and exchange mechanisms, some other interesting diffusion mechanisms for dimers are observed in our molecular dynamics simulations such as exchange rotation mechanism, cooperative hopping mechanism, and cooperative exchange mechanism.     

Hydrogen adsorption and diffusion on Pd(1 1 1)

The adsorption, diffusion and ordering of hydrogen on Pd(1 1 1) was studied by scanning tunneling microscopy in the temperature range of 37-90 K. At low coverage isolated hydrogen atoms were observed. They formed √3×√3-1H islands as the coverage increased. Above 1/3 monolayer (ML) coverage areas of a new phase with √3×√3-2H structure were formed, with both structures coexisting between 1/3 and 2/3 ML. Finally a 1 × 1 structure was formed after high exposures of hydrogen above 50 K, with a coverage close to 1 ML. Atomically resolved images reveal that H binds to fcc hollow sites.     

Vibrationally assisted diffusion of H2O and D2O on Pd(1 1 1)

The adsorption and diffusion of isolated H₂O and D₂O molecules on Pd(1 1 1) were studied using scanning tunneling microscopy at low temperature (∼40 K). At low tunneling voltage the diffusion is thermally activated. The diffusion rate is enhanced by several orders of magnitude when the tunneling electron has enough energy to excite the vibrational “scissor” mode of the molecule. An isotope effect was observed in the threshold voltage for electron-assisted diffusion of H₂O and D₂O.     

Core-level shifts at the Pt/W(1 1 0) monolayer bimetallic interface

We have measured W and Pt 4f_7/2 core-level photoemission spectra from interfaces formed by ultrathin Pt layers on W(1 1 0), completing our core-level measurements of W(1 1 0)-based bimetallic interfaces involving the group-10 metals Ni, Pd, and Pt. With increasing Pt coverage the sequence of W spectra can be described using three interfacial core-level peaks with binding-energy (BE) shifts (compared to the bulk) of −0.220 ± 0.015, −0.060 ± 0.015, and +0.110 ± 0.010 eV. We assign these features to 1D, 2D pseudomorphic (ps), and 2D closed-packed (cp) Pt phases, respectively. For ∼1 ps ML the Pt 4f_7/2 BE is 71.40 ± 0.02 eV, a shift of +0.46 ± 0.09 eV with respect to the BE of bulk Pt metal. The W 4f_7/2 core-level shifts induced by all three adsorbates are semiquantitatively described by the Born-Haber-cycle based partial-shift model of Nilsson et al. [39]. As with Ni/W(1 1 0), the difference in W 4f_7/2 binding energies between ps and cp Pt phases has a large structural contribution. The Pt 4f lineshape is consistent with a small density of states at the Fermi level, reflective of the Pt monolayer having noble-metal-like electronic structure.     

Synthesis and field-emission properties of the tungsten oxide nanowire arrays

High-density, uniformly distributed and quasi-aligned tungsten oxide nanowire arrays have been synthesized by a conventional thermal evaporation approach on indium tin oxide (ITO) coated glass substrates without any catalysts. The temperature of the substrate was . The tungsten oxide nanowires are single crystalline with growth direction of [0 1 0]. For commercial applications, field emission properties of tungsten oxide nanowires were studied under a poor vacuum at room temperature. The electron field-emission turn-on field (E_to), defined as the macroscopic field required to produce a current density of , is about . The performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for commercial application in field-emission displays.     

Adsorption and diffusion of a Si adatom on the H/Si(1 1 1) surface: comparison with the H/Si(0 0 1) surface

Using a first-principles method, we investigate the adsorption and diffusion of a Si adatom on the H-terminated Si(1 1 1) substrate, which would be useful in understanding the initial stages of Si homoepitaxy using a H surfactant. The adatom substitutes H atom(s) to form a monohydride structure or a dihydride structure. In forming the monohydride structure, the energy barrier for H substitution is absent. The adatom migrates on the surface with alternating its chemical state between monohydride and dihydride. These behaviors of the adatom are quite similar to those on the H/Si(0 0 1)2 × 1 surface, despite the significant difference in the substrate structure between both orientations. The resulting diffusion barrier is 1.30 eV, which is also comparable to that on the H/Si(0 0 1)2 × 1 surface.     

Study of two types of Ir or Rh covered single atom pyramidal W tips

The growth of a single-atom sharp pyramid tips on a tungsten substrate by depositing noble metals of the Pt family was investigated by field ion microscopy earlier. The Pd or Pt covered single-atom W pyramidal tips have been successfully prepared and established by our group. They are thermally and chemically stable and can easily be regenerated. In this study, we report the establishing and structural analysis of Rh and Ir covered single-atom W pyramidal tips. Two types of stable structures with bcc {2 1 1} facets are found for both metals. One is stacked by 1, 3, 10 atoms, and the other is stacked by 1, 6, 15 atoms for the top three layers and so on in series from the top to the deeper layers. The single atom tip is destroyed layer by layer after field evaporation to observe the structures of the different layers. However, the tip can be regenerated after it is annealed again and the two types of structures appear systematically depending on the annealing temperature. The regeneration process is investigated and the growth parameters of the two different types of Rh or Ir covered W tip are determined. The differences in the activation barrier and binding energy of these two types are calculated to be 0.08 eV and 0.064 eV for Rh covered single atom tips, and 0.03 eV and 0.14 eV for Ir covered single atom tips, respectively. Possible mechanisms and the relevance for application are discussed.     

Valence band and surface core-level shift studies of the oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

Using synchrotron radiation, W 4f_7/2 and valence-band (VB) photoelectron spectroscopy was used to study the oxidation states of the p(3 × 1) reconstructed surface of W(1 0 0) produced by oxygen adsorption at 1500 K. The W 4f_7/2 study showed two features at 0.40 and 1.50 eV higher binding energies relative to the bulk feature which represent shifts to lower binding energies compared to the W 4f_7/2 features of the as-grown oxygen films on W(1 0 0). Co-existence of these features is a characteristic of the formation of the p(3 × 1) structure. The VB studies of this system and as-grown oxygen films at room temperature (RT) showed an oxygen-induced feature for the p(3 × 1) structure for which the peak maximum had shifted by 0.9 eV to higher binding energies compared to the as-grown films. The direction of shifts in the W 4f_7/2 and VB spectra for the p(3 × 1) structure confirmed the reduction of the charge transfer from W to oxygen compared to the as-grown films. A study of the VB changes with photon energy shows that the main component of the oxygen-induced feature in the p(3 × 1) VB at 6.6 ± 0.2 eV is related to a hybridized state between W 5d and O 2p electrons.