Hydrogen adsorption on Gd(0001)

The electronic structure of hydrogen adsorbate-induced states on Gd(0001) was investigated by means of photoelectron spectroscopy with linearly polarized radiation. The E vector of the incoming photon beam is rotatable. Clean and well-ordered rare-earth (0001) surfaces exhibit a highly localized surface state near the Fermi edge. After the adsorption of hydrogen, the surface state disappears and an additional sharp feature at about 4 eV binding energy is observed. For this latter state, the ratio of the radial matrix elements as well as the relative phase shifts were determined to be R=R_p/R_f=2.4±0.3 and δ_f−δ_p=310±10°, respectively. The removal of the Gd surface state by hydrogen adsorption was investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). The removal of the surface state exhibits domain-like behavior, with surface steps acting as domain boundaries. The tunneling spectra reveal that hydrogen adsorption causes a dramatic reduction in the differential conductivity near the Fermi level.     

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.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

Structural characterization of a (krypton-methane) film adsorbed on (0001) graphite

Krypton-methane films adsorbed on (0001) graphite have been studied by neutron diffraction, between 45 and 98 K. A two-dimensional solid solution has been observed. Its melting temperature has been determined for two compositions of the layer. The melting appears to be a first-order phase transition.     

The interaction of D2O with Zr(0001) at 80 K

The adsorption of D₂O on Zr(0001) at 80 K and its subsequent reactions at higher temperatures have been studied by thermal desorption spectroscopy (TDS), work-function measurements (Δф), nuclear reaction analysis (NRA), LEED, infrared reflection spectroscopy (FTIR-RAS), Auger electron spectroscopy (AES), and static secondary ion mass spectroscopy (SSIMS). D₂O adsorption on Zr(0001) at 80 K is accompanied by a Δф of −1.33 eV. The adsorbed D₂O can be characterized into three layers by TDS: a chemisorbed layer (up to 0.23 ML), a second adsorbed layer, and an ice layer. The chemisorbed D₂O dissociates into OD_ad and D_ad at 80 K (possibly also into O_ad) and no desorption products could be detected, implying that the reaction products dissolved into the zirconium at temperatures appropriate for each component. The ice layer and most of the second adsorbed layer desorb as molecular water during heating. The water adsorbed at 80 K did not form any long-range ordered structure, but a (2 × 2) LEED pattern that was formed by heating the sample to temperatures above 430 K is believed due to be an ordered oxygen superstructure.     

A molecular beam study of the adsorption dynamics of CO on Ru(0001), Cu(111) and a pseudomorphic Cu monolayer on Ru(0001)

We have investigated the sticking coefficient of CO on Ru(0001), a pseudomorphic Cu monolayer on Ru(0001), and a fully relaxed Cu(111) multilayer as function of kinetic energy, surface coverage, and surface temperature. At a low kinetic energy of 0.09 eV, the initial sticking coefficients, S₀, on these surfaces are determined to be 0.92, 0.96 and 0.87, respectively. In all cases, a decrease of S₀ with increasing beam energy was observed, yielding values of 0.58, 0.14 and 0.07, respectively, at a kinetic energy of 2.0 eV. For all three surfaces the coverage dependent sticking coefficients, S(Θ), display very characteristic behavior at low kinetic energies: S(Θ) remains more or less constant up to coverages close to saturation, indicative of precursor adsorption kinetics. However, characteristic minima at intermediate coverages are observed, which are correlated to the formation of well ordered adsorbate phases. For high kinetic energies we observe a transition towards a linear decrease of S(Θ) for Ru(0001). In contrast, for the pseudomorphic Cu monolayer and for Cu(111) we find an increase in the sticking coefficients at low coverages, followed by a decrease close to saturation. This behavior is attributed to adsorbate assisted sticking, that is, to a higher sticking coefficient on adsorbate covered regions than on the bare surface. The comparison between the pseudomorphic monolayer and Cu(111) reveals that the CO bond strength to the former is larger by 40%. The initial sticking coefficients for both surfaces are very similar at low kinetic energies; at high kinetic energies, S₀ for the pseudomorphic Cu monolayer is, however, larger by a factor of two.     

A new 9 K superconducting organic salt composed of the bis (ethylenedithio) tetrathiafulvalene (ET) electron-donor molecule and the tetrakis (trifluoromethyl) cuprate (III) anion, [Cu(CF3)4]

We report the discovery of a second, higher-T_c superconducting organic charge-transfer salt derived from the electron-donor molecule BEDT-TTF (or ET), the novel organometallic anion [Cu(CF₃)₄]⁻, and the neutral solvent molecule 1, 1, 2-trichloroethane (TCE). We have very recently reported that this charge-transfer system yields a new superconducting phase salt, _L- (ET)₂Cu(CF₃)₄·TCE, with inductive onset T_crmc=4.0 K at ambient pressure. This phase salt ( denotes a particular packing arrangement of the ET organic donor molecules) is electrocrystallized in the habit of hexagonal plates. Crystals possessing a needle-like habit electrocrystallize simultaneously with these plates, and we find these needles to be a distinctly different superconducting phase with diamagnetic onset T_c=9.2±0.1 K at ambient pressure. On the basis of our experiments, we denote this new superconducting phase as _H−(ET)₂Cu(CF₃)₄·(TCE)_x,X<1.     

ELECTRONIC STRUCTURE STUDIES OF THE C60 FILM CONDENSED ON 2H-MoS2(0001) SURFACE

The electronic structure and vibrational spectrum of the C₆₀ film condensed on a 2H- MoS₂(0001) surface have been investigated by X-ray photoelectron spectroscopy (XPS), ul-traviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES) and infrared high-resolution electron-energy-loss spectroscopy (HREELS). AES analysis showed that at low energy side of the main transition, C₆₀ contains a total of three peaks just like that of graphite. However, the energy position of the KLL main Auger transition of C₆₀ looks like that of diamond, indicating that the hybridization of the carbon atoms in C₆₀ is not strictly in sp²- bonded state but that the curvature of the molecular surface introduces some sp²pz- bonded character into the molecular orbitals. XPS showed that the C 1s binding energy in C₆₀ was 285.0eV, and its main line was very symmetric and offered no indication of more than a single carbon species. In UPS measurement the valence band spectrum of C₆₀ within 10eV below the Fermi level (E_F) shows a very distinct five-band structure that character-izes the electronic structure of the C₆₀ molecule. HREEL results showed that the spectrum obtained from the C₆₀ film has very rich vibrational structure. At least, four distinct main loss peaks can be identified below 200 meV. The most intense loss was recorded at 66 meV, and relatively less intense losses were recorded at 95, 164 and 197meV at a primary energy of electron beam E_P = 2.0eV. The other energy-loss peaks at 46, 136, 157 and 186meV in HREEL spectrum are rather weak. These results have been compared to infrared spectrum data of the crystalline solid C₆₀ taken from recent literatures.     

Adsorption and angle-resolved electron-stimulated desorption of CCl4 on Ru(0001)

The adsorption and electron-stimulated desorption of CCl₄ on Ru(0001) have been studied at 100 K using a variety of surface analytical techniques, including thermal desorption spectroscopy, Auger electron spectroscopy, low-energy ion scattering, and mass and angle-resolved electron-stimulated desorption (ESDIAD). CC1₄ is found to dissociate partially for fractional monolayer coverages, and to adsorb molecularly above a coverage of 13%. Upon electron bombardment, Cl⁺ ions are found to desorb with their angular distribution directed along the surface normal; this implies that CC1₄ orients with one C-C1 bond normal to the surface. The total yield of Cl⁺ increases linearly with increasing molecular CC1₄ coverage up to one monolayer, which implies that Cl⁺ stems mainly from molecular CCl₄ and that interadsorbate quenching is not significantly affecting the desorption yield. Beyond a coverage of 1 ML, the Cl⁺ yield continues to increase, and starts leveling off after 2 ML. We estimate the yield from a thick layer of CCl⁴ to be of the order of 3 × 10⁻⁸ ions/electron. The angular distribution of the desorbing Cl⁺ ions widens with increasing CCl₄ exposure. Besides Cl⁺, higher mass fragments of CCl₄, such as CCl⁺, CCl₂⁺ and CCl₃⁺ are also found to desorb from multilayers upon electron bombardment.     

酞菁铜与MoS_2(0001)范德瓦耳斯异质结研究

利用光电子能谱、原子力显微镜以及低能电子衍射等表面研究手段系统研究了真空沉积生长的酞菁铜薄膜与衬底MoS2(0001)之间的范德瓦耳斯异质结界面电子结构和几何结构.角分辨光电子能谱清楚地再现了MoS2(0001)衬底在Γ点附近的能带结构.低能电子衍射结果表明,CuPc薄膜在MoS2(0001)表面沿着衬底表面[11ˉ20],[1ˉ210]和[ˉ2110]三个晶向有序生长,反映了衬底对CuPc的影响.原子力显微镜结果表明,CuPc在MoS2衬底上遵循层状-岛状生长模式:在低生长厚度下(单层薄膜厚度约为0.3 nm),CuPc分子平面平行于MoS2表面上形成均匀连续的薄膜;在较高的沉积厚度下,CuPc沿衬底晶向形成棒状晶粒,表现出明显的各向异性.光电子能谱显示界面偶极层为0.07 eV,而且能谱在膜厚1.2 nm饱和,揭示了酞菁铜与MoS2(0001)范德瓦耳斯异质结的能级结构.     

Total Cross Sections for Electron Scattering from CF4, CF3H,CF2H2, and CFH3 Molecules in Energy Range from 100 to 3000 eV

The additivity rule has been employed to calculate the total cross sections for electron scattering by CF4,CF3 H, CF2 H2, and CFH3 molecules over an incident energy range from 100 to 3000 e V. Compared with other calculations and experimental data for CF4, excellent agreement has been obtained. Above 1000 eV, there are no experimental data for CF3H, CF2H2, and CFH3, so the present results can provide comparison and prediction for experimental research.     

Kr2+辐照导致的Ca8LnNa(PO4)6F2(Ln=La,Nd和Sm)磷灰石结构陶瓷材料的非晶转变

采用传统固相烧结法成功制备出磷灰石结构材料Ca₈LnNa（PO₄）₆F₂（Ln=La,Nd和Sm）,并通过常规X射线衍射（X-ray diffraction,XRD）进行表征确定三种样品都为单一相。在室温条件下用800 keVKr2+对三种样品进行辐照实验,辐照后的样品利用掠入射X射线衍射（grazing incidence X-ray diffraction,GIXRD）进行表征获取其表面辐照损伤层的结构信息。发现在实验辐照剂量范围内（1.0×1014~7.0×1014cm-2）三种样品都发生了从晶体结构到非晶结构的转变,不过三种样品的抗辐照非晶性能却有差异,抗辐照非晶能力大小关系是Ca₈LaNa（PO₄）₆F₂ > Ca₈NdNa（PO₄）₆F₂ > Ca₈SmNa（PO₄）₆F₂。这是由于在Ca₈LnNa（PO₄）₆F₂中,当镧系核素半径越小时,将有更大概率占据Ca（2）位置与F形成比Ca-F更弱的离子键。因此,镧系核素半径越小,辐照下将有更多的点缺陷保留下来,更容易发生非晶相变。     

Photoinduced processes on alkali covered surfaces: NO desorption from K/Cr2O3(0001)

We have studied the ultraviolet laser induced desorption of NO/Cr₂O₃(0001), K/Cr₂O₃(0001) and the coadsorbate system NO/K/Cr₂O₃(0001) using resonance enhanced multiphoton ionization spectroscopy for state selective detection of the desorbing species after excitation with nanosecond laser pulses. The goal of our experiments was to study the influence of surface electronic modifications via alkali adsorption on the photodynamics of a simple molecule. The photochemistry of the isolated and the coadsorbate systems is strongly dependent on the coverage of the diverse components. In this paper we shall mainly focus on data for the low coverage regime of potassium. From the two adsorbate species of NO, a chemisorbed and a physisorbed species, we present data of the chemisorbed species. The velocity distributions show a strong dependence on the excitation energy which we interpret on the basis of electron energy loss spectra as being due to surface charge transfer states. This is corroborated with our coadsorption experiments with low coverages of potassium which alter the velocity distributions.     

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.     

Cs/GaN(0001)吸附体系电子结构和光学性质研究

采用基于第一性原理的密度泛函理论平面波超软赝势方法计算了 1/4ML Cs原子吸附 (2 × 2) GaN(0001) 表面的吸附能、能带结构、电子态密度、电荷布居数、功函数和光学性质. 计算发现, 1/4ML Cs 原子在 GaN(0001) 表面最稳定吸附位为 N 桥位, 吸附后表面仍呈现为金属导电特性, Cs原子吸附GaN(0001)表面后主要与表面 Ga 原子发生作用, Cs6s 态电子向最表面 Ga 原子转移, 引起表面功函数下降. 研究光学性质发现, Cs 原子吸附 GaN(0001) 表面后, 介电函数虚部、吸收谱、反射谱向低能方向移动.     

Methane adsorption on graphite（0001） films： A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite(0001) films. The surface energy and the interlayer relaxation of the clean graphite(0001) as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.     

Fluorination of the silicon dioxide surface during reactive ion and plasma etching in halocarbon plasmas

Surface sensitive spectroscopies were employed to investigate the surface modifications which occur on SiO₂ during actual reactive ion and plasma etching in CF₄ and CF₄/O₂ plasmas. Photoemission and electron energy loss spectroscopies were used to characterize the composition and bonding in the modified layer. Core level photoemission measurements indicated a reaction layer 10–15 Å thick with a mean composition of SiOF₂. Photoemission and EELS studies of the valence bands identified several features due to Si---F bonding. Comparisons with the SiO₂ valence bands and implications for bonding are discussed. Electron energy loss spectra were also used to determine if ion induced “defect” species formed during reactive etching persist in the reaction layer. A comparison of reactive ion etching samples with those exposed to plasma etching conditions (negligible ion bombardment) indicated that the ion bombarded surfaces exhibit a reduced level of fluorination during steady state etching. Low energy ion scattering was used to determine the composition of the outermost atomic layer. These measurements indicated a fluorine terminated surface with virtually no remaining surface oxygen.     

Ru(0001) 表面BaO吸附层的原子结构和氮分子的吸附性质

采用密度泛函理论研究了Ru(0001) /BaO表面的原子层结构和氮分子的吸附性质. 研究结果表明, 在低覆盖度下氧化钡倾向于以相同的构型形成Ru(0001) 表面原子层. 在此构型中, 氧原子位于表面p(1× 1) 结构的hcp谷位, 而钡原子则位于同一p(1× 1) 结构的顶位附近. 钌氧键键长等于0.209 nm, 比EXAFS的实验值大0.018 nm. 在Ru(0001) /BaO表面氮分子倾向吸附于钡原子附近. 相应位置的氮分子吸附能位于0.70到0.87 eV之间, 大于氧原子附近的氮分子吸附能. 钡原子附近的钌原子对氮分子具有更强的活化性能. 相应位置的氮分子拉伸振动频率等于1946 cm^{- 1}, 比氧原子附近的最大分子振动频率小约130 cm^-1. Ru(0001) /BaO表面氮分子键强度介于清洁Ru(0001) 和Ru(0001) /Ba表面之间. Ru(0001)/BaO表面不同位置的氮分子吸附性质差异是由钡和氧原子化学性质不同造成的. 表面钡原子的作用能够减少吸附氮分子的σ^*轨道电子密度, 增加π^*轨道电子密度, 从而增强氮分子和钌原子间的轨道杂化作用, 弱化氮分子键.     

Argon ion stimulated conversion between CFx (x = 0–3) chemical states and fluorine depletion in fluorocarbon films studied by X-ray photoelectron spectroscopy

Surface chemical composition of fluorocarbon films deposited onto indium-tin-oxide (ITO) substrates was modified by 2.0 keV Ar⁺ irradiation with doses of up to 2.6×10¹³ ions/cm². The effect of ion irradiation on the chemical composition and bonding configuration of the upper-surface and sub-surface regions were monitored by high-resolution angle-resolved X-ray photoelectron spectroscopy (AR-XPS). It was found that the as-deposited films compose of a distribution of CF, CF₂, CF₃ and C–C chemical states. C 1s line-shape analysis of XPS spectra measured at grazing and normal emission angles shows that as a result of the irradiation, the population of the CF₃ and CF₂ chemical states were preferentially depleted from the film’s surface resulting in an increased population of the CF and C–C chemical states in the upper-surface region of the film. It is suggested that low energy ion irradiation can be used to chemically modify the surface of fluorocarbon films.     

LEED and Auger electron observations of the SiC(0001) surface

LEED and AES experiments of the SiC{0001} crystal surfaces show that on heat-treatment these surfaces are easily “covered” with a layer of graphite by evaporation of silicon. The graphite layer, which has a distinct crystallographic relation to the SiC crystal, is monocrystalline on the Si-face and mostly polycrystalline on the C-face. A speculation about the mechanism of the initial graphitization of the basal faces of SiC is given.