The valence of Sm in a monolayer on Al(100)

Submonolayer and monolayer films of Sm deposited on an Al(100) crystal surface have been studied by LEED and X-ray photoemission spectroscopy. Layers deposited at — 100°C or RT and without subsequent annealing were disordered and composed of Sm in both divalent and trivalent forms. Annealed films showed a pseudo hexagonal structure and were predominantly trivalent. We conclude that a perfect monolayer of Sm on Al(100) is composed wholly of trivalent atoms.     

Valence variations in the monolayer regime of Sm on the Nb(110) surface

Sm overlayers in the monolayer regime, deposited on a Nb(110) single-crystal surface at room temperature, have been studied by means of high-resolution photoelectron spectroscopy of the Nb 3d_5/2 core level and the Sm valence band. In the submonolayer regime, the Sm valence varies from mainly divalent for very low coverages to essentially trivalent close to a complete monolayer. Above 1 ML, a new divalent component appears in the Sm 4f spectra, corresonding to divalent Sm in the second layer. The mixed valence in this overlayer system is concluded to be heterogeneous (all Sm atoms have integer but site-dependent valence). Sm forms ordered overlayers on Nb(110) and the Sm growth is consistent with a layer-by-layer growth mode (Frank–van der Merwe growth).     

6H-SiC(0001)表面graphene逐层生长的分子动力学研究

利用经典分子动力学方法和模拟退火技术分析研究了6H-ＳiC（0001）表面graphene的逐层生长过程及其形貌结构特点．研究表明,经过高温蒸发表面硅原子后,6H-ＳiC（0001）表面的碳原子能够通过自组织过程生成稳定的局部单原子层graphene结构．这种过程类似于6H-ＳiC（0001）表面graphene的形成,其生长和结构形貌演化主要取决于退火温度和表面碳原子的覆盖程度． 研究发现,当退火温度高于1400K时,6H-ＳiC（0001）表面碳原子能形成局部的单原子层graphene结构．这一转变温 关键词： graphene 碳化硅 分子动力学     

Au钝化Si(100)表面的电子特性

用TB-LMTO方法研究单层的Au原子在理想的Si(100)表面的化学吸附.计算了Au原子在不同位置的吸附能,吸附体系与清洁Si(100)表面的层投影态密度, 以及电子转移情况.结果表明, Au原子在吸附面上方的A位(顶位)吸附最稳定, Au钝化Si(100)表面可以取得明显的钝化效果, 这一结论与实验事实相符合.     

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E _e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p _3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E _e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E _e ≤ 66 eV. Deposition of Sm atoms at room temperature (～300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.     

Comparative study of adsorption characteristics of Cs on the GaN （0001） and GaN （0001） surfaces

The adsorption characteristics of Cs on GaN （0001） and GaN （0001） surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave uttrasoft pseudopotential method based on first-principles calculations. The results show that the most stable position of the Cs adatom on the GaN （0001） surface is at the N-bridge site for 1/4 monolayer coverage. As the coverage of Cs atoms at the N-bridge site is increased, the adsorption energy reduces. As the Cs atoms achieve saturation, the adsorption is no longer stable when the coverage is 3/4 monolayer. The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer, and then rises with Cs atomic coverage. The most stable position of Cs adatoms on the GaN （000i） surface is at H3 site for 1/4 monolayer coverage. As the Cs atomic coverage at H3 site is increased, the adsorption energy reduces, and the adsorption is still stable when the Cs adatom coverage is 1 monolayer. The work function reduces persistently, and does not rise with the increase of Cs coverage.     

Adsorption of Ag and Au atoms on wurtzite ZnO (0001) surface

We perform first-principles calculations to investigate various surface structures in the absorption of Ag and Au atoms on wurtzite ZnO (0001) surface. The results show that both Ag and Au atoms prefer to be absorbed on the H₃ sites (the center of Zn–O ring) of the surface, and the most favorable monolayer (ML) coverage is 1. The calculated electron structure shows that the Ag- and Au-adsorbed ZnO (0001) surfaces exhibit metallic characteristics even the ML coverage of the adatoms is very low. Finally, the work functions of Ag- and Au-adsorbed ZnO (0001) surfaces are calculated and discussed for the first time in the present work.     

Comparative study of adsorption characteristics of Cs on the GaN(0001) and GaN(000) surfaces

The adsorption characteristics of Cs on GaN(0001) and GaN(000) surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave ultrasoft pseudopotential method based on first-principles calculations.The results show that the most stable position of the Cs adatom on the GaN(0001) surface is at the N-bridge site for 1/4 monolayer coverage.As the coverage of Cs atoms at the N-bridge site is increased,the adsorption energy reduces.As the Cs atoms achieve saturation,the adsorption is no longer stable when the coverage is 3/4 monolayer.The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer,and then rises with Cs atomic coverage.The most stable position of Cs adatoms on the GaN(000) surface is at H3 site for 1/4 monolayer coverage.As the Cs atomic coverage at H3 site is increased,the adsorption energy reduces,and the adsorption is still stable when the Cs adatom coverage is 1 monolayer.The work function reduces persistently,and does not rise with the increase of Cs coverage.     

α-Al2O3(0001)基片表面结构与能量研究

对α-Al₂O₃(0001)晶体表层三种不同终止原子结构的计算模型， 在三维周期边界条件下 的κ空间中，采用超软赝势平面波函数描述多电子体系.应用基于密度泛函理论的局域密度 近似，计算了不同表层结构的体系能量，表明最表层终止原子为单层Al的表面结构最稳定. 对由10个原子组成的菱形原胞进行了结构优化，得到晶胞参数值(a₀＝0.48178n m)与实验 报道值误差小于1.3%.进一步计算了超晶胞(2×2)表面弛豫，弛豫后原第2层O原子层成为最 表层； 对不同表层O，Al原子最外层电子进行了布居分析，表面电子有更大的概率被定域在 O原子的周围，表面明显地表现出O原子的电子表面态. 关键词： 2O₃(0001)')" href="#">α-Al₂O₃(0001) 超软赝势 表面结构 表面态     

First-principles study of Sr adsorption on InN (0001)

Structures of Sr adsorbed on InN (0001) surfaces are theoretically investigated by first-principles calculations. The adsorption energies of Sr on InN (0001) decrease with decreasing Sr coverage. An InN (0001)-(2 ×2) surface structure covered by a 1/4 monolayer of Sr at the T₄ sites may be the most energetically favourable. Sr atoms may substitute indium atoms, or accumulate at the voids inside InN films. The interstitial Sr defects may act as a potential source of compensation for the p-type behaviour of Sr-doped InN at the surface.     

Chemisorption of Au on Si（001） surface

The chemisorption of one monolayer of Au atoms on an ideal Si(001) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of the adsorption system of a Au atom on different sites are calculated. It is found that the most stable position is A site (top site) for the adsorbed Au atoms above the Si(001) surface. It is possible for the adsorbed Au atoms to sit below the Si(001) surface at the B_1 site(bridge site), resulting in a Au－Si mixed layer. This is in agreement with the experiment results. The layer projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated.     

Quantitative aspects of Auger electron spectroscopy; On the importance of attenuation of the primary electron beam

Measurements of Auger electron currents at different energies of the primary electrons and at different angles of incidence suggest that attenuation of the primary electron beam in the crystal has a significant influence on the measured Auger currents. This makes it possible to distinguish between a monolayer and a multilayer system, i.e. between impurity atoms in the top atomic layer and impurity atoms dispersed through the surface region. Comparison of AES and ellipsometric measurements of monolayer and multilayer systems on silicon and germanium surfaces shows that AES results can be interpreted to give quantitative information about the number of atoms of the type investigated.     

H,Cl和F原子钝化Cu_2ZnSnS_4(112)表面态的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法系统研究了Cu_2ZnSnS_4体相的晶格结构、能带、态密度及表面重构与H,Cl和F原子在Cu_2ZnSnS_4(112)表面上的吸附和钝化机理.计算结果表明:表面重构出现在以金属原子Cu-Zn-Sn终止的Cu_2ZnSnS_4(112)表面上,并且表面重构使表面发生自钝化;当单个H,Cl或F原子吸附在S原子终止的Cu_2ZnSnS_4(112)表面上时,相比于桥位(bridge)、六方密排(hcp)位和面心立方(fcc)位点,三种原子均在特定的顶位(top)吸附位点表现出最佳稳定性.当覆盖度为0.5 ML时,无论H,Cl还是F原子占据Cu_2ZnSnS_4(112)表面的2个顶位均具有最低的吸附能.以S原子终止的Cu_2ZnSnS_4(112)表面在费米能级附近的电子态主要由价带顶部Cu-3d轨道和S-3p轨道电子贡献,此即表面态.当H,Cl或F原子在表面的覆盖度达0.5 ML时,费米能级附近的表面态降低,其中H原子钝化表面态的效果最佳,Cl原子的效果次之,F原子的效果最差.表面态降低的主要原因在于吸附原子从S原子获得电子致使表面Cu原子和S原子在费米能级处的态密度峰几乎完全消失.     

The surface reconstructions of the (100) crystal faces of iridium,platinum and gold: II. Structural determination by LEED intensity analysis

The investigation, in a companion paper, of the reconstructions of the Ir(100), Pt(100), and Au(100) crystal surfaces is completed here with an extensive analysis of low energy electron diffraction (LEED) intensities, using dynamical (multiple scattering) calculations. It is found that a hexagonal rearrangement of the top monolayer is a likely explanation of the surface reconstruction. At least for Ir and Pt (no calculations were made for Au), this hexagonal layer would have a registry involving bridge sites on the next square unit cell metal layer and it is contracted and buckled. Bond length contractions parallel and perpendicular to the surface occur; the Pt top layer is rotated by a small angle (0.7°) with respect to the substrate. A second model that cannot be ruled out by the LEED analysis, but disagrees with ion-scattering data, involves shifted close-packed rows of top-layer atoms and requires domain structures in the case of Pt and Au. Charge-density-wave and missing-row models are ruled out by our structure analysis. A correlation is found between the occurrence of surface reconstructions on metals and a small ratio of their Debye temperature to their melting point. This correlation singles out mainly the 5d metals as having a propensity to surface reconstruction. The effects of adsorbates on the reconstructions are also discussed.     

ZnO 极性表面及其N原子吸附机理的第一性原理研究

基于密度泛函理论的第一性原理赝势法计算了ZnO极性表面的几何结构和电子结构特性,对比分析了ZnO(0001)和ZnO(0001)表面结构弛豫、能带结构、电子态密度及N吸附ZnO极性表面的形成能情况.计算结果表明: 相对于ZnO(0001)表面,ZnO(0001)表面受结构弛豫影响更加明显,而ZnO(0001)表面完整性更好.相对于体相ZnO结构,ZnO(0001)表面的能带带隙变窄,同时价带顶附近能级非局域性增强使晶体表面的导电性能变得更好;而ZnO(0001)表面的能带带隙变宽,由于O^{-关键词： 密度泛函理论 第一性原理 ZnO极性表面 N吸附}     

ATLEED研究6H-SiC(0001)-(3×3)R30°重构表面

低能电子衍射(LEED)对6H-SiC(0001)-(3×3)R30°表面的研究结果表明,该表面有1/3单层的Si原子吸附在T4空位上与第一个SiC复合层中的三个Si原子键接,它们之间的垂直距离为0.171nm.通过对该表面10个非等价垂直入射衍射束的自动张量低能电子衍射(ATLEED)计算,得到“最佳结构”由于表面SiC复合层堆积顺序不同而产生的三种表面终止状态(surface termination)的混合比例为S1∶S2∶S3=15∶15∶70,理论计算与实验I-V曲线比较得到可靠性因子RVHT=0.165,RP=0.142,表明表面生长符合能量最小化的台阶生长机制.     

石墨烯在Al_2O_3(0001)表面生长的模拟研究

基于密度泛函理论的广义梯度近似法,对用化学气相沉积法在蓝宝石(α-Al_2O_3)(0001)表面上生长石墨烯进行理论研究.研究结果表明:CH_4在α-Al_2O_3(0001)表面上的分解是吸热过程,由CH_4完全分解出C需要较高能量及反应能垒,这些因素不利于C在衬底表面的存在.在α-Al_2O_3(0001)表面,石墨烯形核的活跃因子并不是通常认为的C原子,而是CH基团.通过CH基团在α-Al_2O_3(0001)表面上的迁移聚集首先形成能量较低的(CH)_x结构.模拟研究(CH)_x对揭示后续石墨烯的形核生长机理具有重要意义.     

Formation of graphene on Ru（0001） surface

We report on the formation of a graphene monolayer on a Ru(0001) surface by annealing the Ru(0001) crystal. The samples are characterized by scanning tunnelling microscopy (STM) and Auger electron spectroscopy (AES). STM images show that the Moir\'{e} pattern is caused by the graphene layer mismatched with the underlying Ru(0001) surface and has an $N\times N$ superlattice. It is further found that the graphene monolayer on a Ru(0001) surface is very stable at high temperatures. Our results provide a simple and convenient method to produce a graphene monolayer on the Ru(0001) surface, which is used as a template for fabricating functional nanostructures needed in future nano devices and catalysis.     

Use of the continued fraction method for the study of adsorption: Hydrogen on graphite

The local densities of states and energies of adsorption of one, two and a monolayer of hydrogen atoms on the (0001) surface of graphite are calculated. A Hückel type Hamiltonian with pi orbitals for the carbon atoms and 1s orbitals for the hydrogens is used. The densities of states are calculated by the continued fraction method, with an enlarged energy band obtained by surrounding graphite with appropriate pseudoatoms, which proved to be a suitable procedure to obtain the bound states.     

Passivation effects of phosphorus on 4H-SiC(0001) Si dangling bonds: A first-principles study

The effect of phosphorus passivation on 4H-SiC(0001) silicon(Si) dangling bonds is investigated using ab initio atomistic thermodynamic calculations. Phosphorus passivation commences with chemisorption of phosphorus atoms at high-symmetry coordinated sites. To determine the most stable structure during the passivation process of phosphorus, a surface phase diagram of phosphorus adsorption on SiC(0001) surface is constructed over a coverage range of 1/9–1 monolayer(ML). The calculated results indicate that the 1/3 ML configuration is most energetically favorable in a reasonable environment. At this coverage, the total electron density of states demonstrates that phosphorus may effectively reduce the interface state density near the conduction band by removing 4H-SiC(0001) Si dangling bonds. It provides an atomic level insight into how phosphorus is able to reduce the near interface traps.