The electronic structure of alkali-metal layers on semiconductor surfaces

Alkali-metal layers on semiconductor surfaces are model systems for metal-semiconductor contacts, Schottky barriers, and metallization processes. The strong decrease of the work function as a function of alkali-metal coverage is also technically made use of. Recently, however, interest in these systems is growing owing to ongoing controversial discussions about questions like: Is the adsorbate system at monolayer coverage metallic or semiconducting, and does the metallization take place in the alkali overlayer or in the top layer of the semiconductor? Is the bonding ionic or covalent? What ist the absolute coverage at saturation? What are the adsorption sites? Do all alkali metals behave similar on the same semiconductor surface? We try to answer some of the questions for Li, Na, K and Cs on Si(111)(2×1), K and Cs on Si(111)(7×7) and on GaAs(110), and Na and K on Si(100)(2×1) employing the techniques of direct and inverse photoemission.     

Negative-ion formation from surface scattering at finite temperatures

Temperature effects on negative-ion formation in positive-ion-surface scattering are studied within the framework of the time-dependent Anderson-Newns model. It is shown that the negative-ion formation is significantly enhanced at finite temperature T, provided k_BT is not less than the Anderson correlation energy U, where k_B is the Boltzmann constant. In the transient region (femtosecond timescale), temperature effects are, however, masked by large energy fluctuations.     

Negative-ion formation from surface scattering and the Anderson correlation energy U

A theoretical investigation of negative-ion formation from positive-ion-surface scattering is presented from a unified point of view. Based on the time-dependent Anderson-Newns model, the correlation energy U is seen to play an important role in the two-electron transfer process. Calculations of the probability of negative-ion formation are in good agreement with experiments on the conversion of H⁺(D⁺) to H^?(D^?) by scattering from a cesiated W(100) surface.     

Pattern formation on carbon nanotube surfaces

Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine forms varying surface superlattices at increasing temperatures. The ordering transition is controlled by the surface migration barrier for fluorine atoms to pass through next neighbor sites on the nanotube, explaining the transition from semi-ionic low coverage to covalent high coverage fluorination observed experimentally for gas phase fluorination between 200 and 250 degrees C. The effect of solvents on fluorine binding and surface diffusion is explored.     

Hillock formation on ion-irradiated graphite surfaces

Abstract

Scanning probe microscopy experiments show that ion irradiation of (0001) graphite results in the formation of isolated defects comprising of a few tens of atoms. We use molecular dynamics simulations and density-functional theory calculations to study the formation probabilities of these defects. We identify different defect structures which correspond to experimentally observed hillocks on graphite surfaces. We find that the predominant source of defects are vacancies and interlayer interstitials, and identify a three-atom carbon ring defect on the graphite surface.     

负离子元素杂质破坏模型

探讨了HfO₂薄膜中负离子元素杂质破坏模型，并得出薄膜中的杂质主要来源于 镀膜材料. 用电子束蒸发方法沉积两种不同Cl元素含量的HfO₂薄膜，测定薄膜 弱吸收和损伤阈值来验证负离子元素破坏模型. 结果表明，随着Cl元素含量的增加薄膜的弱 吸收增加损伤阈值减小. 这主要是因为负离子元素在蒸发过程中形成挥发性的气源中心而产 生缺陷，缺陷在激光辐照过程中又形成吸收中心. 因此负离子元素的存在将加速薄膜的破坏 . 关键词： 负离子元素杂质 缺陷 吸收     

Oxide formation and reduction on rhodium surfaces

The growth of thin oxide layers on Rh and their reduction by CO has been investigated by imaging atom-probe mass spectroscopy and field-ion microscopy. Surface oxides were produced by heating Rh field-emitter tips in oxygen at pressures between 0.01 and 1.0 Torr and temperatures between 400 and 650 K. The oxidized samples were transferred under ultrahigh vacuum to an imaging atom-probe/field-ion microscope for compositional and structural analysis. Oxygen uptake was found to follow a logarithmic law with an initial activation energy of 3.1 kcal/mol. Imaging atom-probe analysis indicated that the oxide formed in 1 Torr O₂ was stoichiometric Rh₂O₃ for temperatures of 500 K and above. The onset pressure for oxidation at 500 K was found to be ≈0.01 Torr, with only a weak pressure dependence in the range from 0.01 to 1 Torr. Field-ion microscope images of the oxide showed ring structures suggestive of epitaxial growth above the (111) plane, and images of the substrate after removal of the oxide indicated that the oxide was thicker above the stepped regions of the surface than above the low-index planes. The oxide was quickly reduced in 1 Torr CO at temperatures above 420 K, and partially-reduced oxides were found to be substoichiometric throughout the oxide region. CO reduction exhibited a much stronger temperature dependence than surface oxide formation indicating a different rate-controlling step for the two processes. The time dependence for CO reduction at 418 K suggested that the COO surface reaction was rate-determining in the reduction process.     

Pattern formation on anisotropic and heterogeneous catalytic surfaces

We review experimental and theoretical work addressing pattern formation on anisotropic and heterogeneous catalytic surfaces. These systems are typically modeled by reaction-diffusion equations reflecting the kinetics and transport of the involved chemical species. Here, we demonstrate the influence of anisotropy and heterogeneity in a simplified model, the FitzHugh-Nagumo equations. Anisotropy causes stratification of labyrinthine patterns and spiral defect chaos in bistable media. For heterogeneous media, we study the situation where the heterogeneity appears on a length scale shorter than the typical pattern length scale. Homogenization, i.e., computation of effective medium properties, is applied to an example and illustrated with simulations in one (fronts) and two dimensions (spirals). We conclude with a discussion of open questions and promising directions that comprise the coupling of the microscopic structure of the surface to the macroscopic concentration patterns and the fabrication of nanostructures with heterogeneous surfaces as templates. (c) 2002 American Institute of Physics.     

On the formation of periodic structures on solid surfaces

A model is proposed to explain the formation of periodic structures produced on solid surfaces by laser radiation. The model gives rise to a system of two linear integrodifferential equations with difference kernels for temperature correction due to the specific absorption of electromagnetic energy at a certain solid surface profile and at a surface profile formed due to heat expansion resulting from temperature correction. The solution of this system reveals, that, first, periodic structures are formed as a result of the propagation of periodic profiles generated from a certain original non-periodic profile over the body surface. Second, the amplitudes of these waves grow with time only for a laser density exceeding certain critical value, i.e. the formation of periodic structures is a threshold effect relative to the laser density.     

Vacancy formation and CO adsorption on gold-doped ceria surfaces

We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO₂ is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation.