On the stability of Caesium-covered ZnO surfaces

The characterisation of the Cs/ZnO system using LEED and AES, results in the observation of new superstructures on the (000$\text{1}$)—O polar surface, a (2 × 2) structure and a combined (2 × 2) and (√3 × √3) configuration. A possible explanation is proposed for the existence of these superstructures in terms of electrostatic energy calculations.     

Kondo temperature of magnetic impurities at surfaces

Based on the experimental observation that only the close vicinity of a magnetic impurity at metal surfaces determines its Kondo behavior, we introduce a simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au. Excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated. The Kondo temperature is shown to depend on the occupation of the d level determined by the hybridization between the adatom and the substrate with a minimum around single occupancy.     

Magnetic states at the oxygen surfaces of ZnO and Co-doped ZnO

First principles calculations of the O surfaces of Co-ZnO show that substitutional Co ions develop large magnetic moments which long-range order depends on their mutual distance. The local spin polarization induced at the O atoms is 3 times larger at the surface than in the bulk, and the surface stability is considerably reinforced by Co. Moreover, a robust ferromagnetic state is predicted at the O (0001) surface even in the absence of magnetic atoms, correlated with the number of p holes in the valence band of the oxide, and with a highly anisotropic distribution of the magnetic charge even in the absence of spin-orbit coupling.     

Aluminum and nitrogen impurities in Wurtzite ZnO: first-principles studies

First-principles calculations have been performed to investigate the doping behaviors of Al and N dopant impurities in ZnO. According to the results, in the Al mono-doping case, the impurity states are quite delocalized, the corresponding effective masses are small, and the formation energy is as low as −9.71 eV. In the N mono-doping case, the impurity states are localized, the effective masses are large, and the formation energy is high (4.55 eV in the most favorable extreme O-rich conditions). In the Al-N codoping case, the corresponding effective masses are marked decreased compared to the N mono-doping situations, and the formation energy of the N-Al-N system is as low as −2.54 eV in the O-rich condition. The above results can explain the electrical behaviors of the doped or codoped ZnO systems observed in experiments.     

Kondo state of Co impurities at noble metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the properties of magnetic Co adatoms on noble metal surfaces at 6 K. Due to spin-flip scattering of the substrate electrons at the impurity the many-body Kondo state forms. This state is characterized by an energy, the Kondo temperature T_K. We measure T_K of adatom systems and a resonant scattering phase shift locally and are thus able to discuss the coupling of the Co adatom to the metal electronic system. From the resonant scattering phase shift of the surface-state electrons scattering off a Co adatom on Ag(111), we find that the coupling to the surface state is rather weak. On the other hand, increasing the number of nearest neighbor substrate atoms increases the coupling of a Co adatom to the host metal and increases T_K. This shows the dominant character of the coupling of the Co atom to the bulk states of the substrate crystal. PACS 72.10.Fk; 68.37,Ef; 72.15.Qm     

Influence of impurities on phase stability of martensites in titanium

The influence of H, C, N and O impurities on the phase stability of titanium was studied by first principles total energy calculations. The occupation energies of the impurities were estimated to identify their site preferences. All impurities considered prefer to occupy the octahedral site except for H, which tends to occupy the tetrahedral site in the β phase. Electronic structures were analyzed to clarify the intrinsic influence mechanisms of impurity on the stability of martensitic phases. It was found that the density of states around the Fermi energy, which was affected dramatically by impurity, and the bonding interactions between impurity and titanium were connected to the phase stability of Ti. Elastic constants of impurity-containing systems were estimated from curves of energy against strain to evaluate the mechanical stability of these systems. It was shown that the α″ phase can not be stabilized by impurities considered here, while the α′ phase (regardless of the impurities) and H- and C-containing β phase are thermodynamically stable and also satisfy the mechanical stability criteria.     

The role of structural inhomogeneities in the temperature behavior of the thermopower in metallized nanotubes with impurities

High values of the thermopower and its derivative with respect to temperature in metallized carbon nanotubes at low temperatures are shown to result from the following factors: structural inhomogeneities and modification of the electronic structure both due to local structural regions of disrupted long-range order and to electron-electron interaction. The former factor plays an important role in carbon-nanotube bundles, whereas the latter is of substantial significance in single-walled nanotubes. Statistical inhomogeneities and all broken bonds are essential for electron scattering that gives rise to electron transport in metallized carbon nanotubes. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 31–41, March, 2009.     

The stability of partially rigid two-dimensional surfaces in uniform incompressible flow

A theoretical analysis is made of the stability of a partially rigid two-dimensional surface embedded in the uniform flow of an incompressible inviscid fluid. Membranes, simply supported panels and clamped panels, attached at their leading and trailing edges to rigid flat extensions aligned with the undisturbed flow direction, are considered and numerical results are obtained, by using the Galerkin method, showing how the stability varies with the change in length of the upstream and downstream rigid elements. Similar results are obtained for a cantilever panel attached to a leading edge rigid surface modelling the aerofoil or splitter plate used in experiments. The effects of structural damping are included where appropriate and comparisons made with other relevant theoretical and experimental results.     

The role of residual technological impurities in the formation of radiative recombination centres in irradiated germanium

A comparison study of the photoluminescent (PL) spectra in pure and ultra-pure irradiated and annealed germanium has been performed. Some radiative recombination centres including impurities have been found. The spectra of samples bombarded by neutrons were found to include, along with the lines from intrinsic defects (Z, Y, X, W, V), the centres due to hydrogen and oxygen impurities (the lines T₁, T₂, U). A considerable number of the lines (A₁-A₃, B₁, B₂, C, D₁, D₂) in the PL spectra of crystals exposed to gamma-quanta and electrons were identified with the centres including carbon atoms. Such a great set of lines, presumably, results from the transformation of the above mentioned complexes in the course of annealing. The observed differences in the PL spectra within 0.72 to 0.74 eV range probably indicate a contribution of several types of intrinsic defects and other impurities, along with carbon, to complexing.     

无序杂质区域对沉积在胶体基底表面的金原子凝聚体分形结构的影响

由于受基底表面无规分布杂质的影响,沉积在熔融玻璃基底表面的金原子凝聚形成了具有特殊结构的分形凝聚体.根据这一实验结果,建立了各向异性的团簇团簇凝聚模型,对此类胶体基底表面的金原子分枝状凝聚体的生长过程进行了计算机模拟,研究了无规分布的杂质区域对凝聚体各种参数的影响,其结果与实验相符合 关键词： 薄膜生长 MonteCarlo模拟 分形     

Superhydrophobic surfaces based on dandelion-like ZnO microspheres

This study presents a simple method to fabricate superhydrophobic surface based on ZnO nanoneedles. ZnO nanoneedles had been constructed on zinc layers by immersing in an aqueous NH₄OH solution at 80 °C. The ZnO films were characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The ZnO films exhibited excellent superhydrophilicity (contact angle for water was 0°), while they changed wettability to superhydrophobicity with a water contact angle greater than 150° after further chemical modification with n-dodecanoic acid. The procedure reported here only needs readily available reagents and laboratory equipments, which can be applied to various substrates of any size and shape.     

Characteristic interactions of 111Cd probes with nonradioactive In impurities doped in ZnO

Local structures produced in 0.5 at.% In-doped ZnO were investigated by means of the time-differential perturbed angular correlation method using the ¹¹¹Cd probe nuclei generated in the disintegration of different parents, ¹¹¹In and ^111mCd. From distinct perturbation patterns obtained with the probes descended from the different parents, it was ascertained that the doped In atoms locally form a unique structure dispersed in ZnO matrix without forming macroscopic agglomerates of their own.     

Substitutional impurities near surfaces. A perturbation approach to the segregation energies

We present a perturbation analysis for the segregation energy of a substitutional impurity near a surface. We show that there exists a linear contribution in ΔZ, the difference of valence between impurity and host, due to the presence of the surface, even if the local neutrality is ensured. The series effect for the fcc (001) and bcc (001) surfaces is dicussed, within this frame work.     

Electron energy-loss spectroscopy from polar ZnO surfaces

Reflection electron energy-loss spectra (ELS) in the 2–30-eV range were obtained from ZnO ($000\overline{1}$)O and (0001)Zn faces. Measurements were performed on atomically clean surfaces and on surfaces subjected to different treatments, including exposures to atomic hydrogen and molecular oxygen. The spectra were compared with ELS, UPS, XPS and UV reflection results obtained by other investigators. An identification of the observed electronic transitions is attempted.     

Electrostatic-field-driven alignment of organic oligomers on ZnO surfaces

We study the physisorption of organic oligomers on the strongly ionic ZnO(1010) surface by using first-principles density-functional theory and nonempirical embedding methods. It turns out that the in-plane variation of the molecule-substrate interaction energy and the bonding dipole in the vertical direction are linked up by a linear relationship originating from the electrostatic coupling of the molecule with the periodic dipolar electric field generated by the Zn-O surface dimers. Long oligomers with a highly axial π-electron system such as sexiphenyl become well oriented with alignment energies of several 100 meV along rows of a positive electric field, in full agreement with recent experiments. These findings define a new route towards the realization of highly ordered self-assembled arrays of oligomers or polymers on ZnO(1010) and similar surfaces.     

On the role of group I elements in ZnO

Diffusion of Li, Na and K into single-crystal substrates of ZnO was performed. We compare the results with ZnO epitaxial films doped with the respective elements during growth. The diffused and in-situ doped layers were studied using mass spectroscopy and low temperature photoluminescence spectroscopy. Li and Na are known to produce deep acceptor centers which give rise to shallow donor to deep acceptor recombinations in the visible spectral region. We will demonstrate that shallow acceptors are also introduced, having binding energies around 300 meV. A donor–acceptor pair recombination (zero phonon line at 3.05 eV) with LO phonon replica is observed. We further investigated bulk ZnO crystals which contained the deep Li acceptor by thermal treatments under H₂ atmospheres. With increasing annealing temperature shallow donors are introduced as monitored by EPR while the EPR signal of the neutral Li acceptors decreases. Quite unexpectedly, the shallow Li acceptor centre which is not present in the as-grown state is also created. PACS 71.55.Gs; 78.55.Et; 76.30.Da     

The influence of atomic impurities on the orientational transitions in localized monolayer films of diatomic molecules adsorbed on crystalline surfaces

The orientational ordering in localized films consisting of a mixture of diatomic and mono-atomic particles and formed on the (100) face of a cubic crystal is considered. It is demonstrated that the presence of atomic impurities influence the formation of orientationally ordered states. In particular, it is shown that even a simple mean-field theory leads to a qualitatively correct picture of changes in orientational properties in the film when the molar ratio of spherically symmetric particles increases.