Surface molecules and chemisorption: I. Adatom density of states

A useful picture of chemisorption on metal surfaces is one in which a localized molecule is formed between the adatom and its nearest neighbor substrate atoms. The interaction responsible for the molecule formation is treated as the coupling between the adsorbate state and a group orbital formed from a linear combination of atomic orbitals on the substrate atoms. Within the surface molecule picture, level width and level shift functions, given by Newns modification of the Anderson theory, have been calculated and the resulting adatom density of states function has been obtained. This has been done for model systems in which the substrate is either a free electron metal or a tightbinding p-band metal and the adsorbate is s or p like. The results show how it is possible to simultaneously have narrow virtual levels due to chemisorption (～ 1 eV) which previously implied weak interactions and also high binding energies (? 3 eV) as are observed experimentally.     

钾钡共掺菲分子结构和电子特性的第一性原理研究

采用基于密度泛函理论的平面波赝势法,对钾钡共掺杂情形下菲分子晶体的结构、能带、电子态密度、形成能、电荷转移等电子特性进行系统的研究.通过对比计入范德瓦尔斯力作用前后晶体结构的差异,说明计算中包含范德瓦尔斯力修正的重要性.从形成能的角度证明了共掺杂的可行性和稳定性.在钾钙、钾锶、钾钡等共掺杂元素组合中,K_1Ba_1-菲中平均每个金属原子的形成能为-0.25 eV,远大于K_1Sr_1-菲和K_1Ca_1-菲中的-0.13和-0.04 eV,钾钡共掺杂是最合理的方案.只有单双价金属共掺杂,才能使分子呈现负三价.此时,菲分子最低未占据轨道(LUMO)和LUMO+1轨道组成的能带正好位于费米能级处,K_1Ba_1-菲呈现金属性.费米能级处的态密度为17.3 eV~(-1),电子态主要来自于碳原子的2p轨道,钡原子的5d轨道也有少许贡献.从理论模拟的角度研究了K_1Ba_1-菲的晶体结构和电子特性,在已有实验和理论研究尚未涉及共掺杂的背景下,提出了不同价态金属共掺杂方案,为制备芳烃有机超导体样品和调制体系电子结构提供了新的研究思路.     

Initial growth at the F16CoPc/Ag(111) interface

An extended, combined (STM/STS)–(UPS/XPS) study was carried out towards a comprehensive understanding of the mechanism responsible for the F₁₆CoPc/Ag(111) interface formation. The evolution of the morphology and the electronic properties at the organic/metal interface is investigated for the early-stage growth of the ultrathin molecular film. Template-guided molecular structures are formed via a strong molecule–substrate interaction which leads to the formation of a new adsorption-induced interface state close to the Fermi energy (E_F). With increasing the thickness the molecular coupling to the metal surface states becomes less important while the more dominant molecule–molecule interaction governs the second layer formation. The quenching of the interface state upon increasing the molecular thickness, together with the changes observed in the Co 2p and F 1s core levels, is explained based on a charge transfer at the interface and a corresponding charge redistribution within the molecular ligand. A detailed “picture” of the energy level alignment close to E_F is achieved by correlating the high resolution UPS and highly localized STS data.     

Spectroscopic states of PTCDA negative ions and their relation to the maxima of unoccupied state density in the conduction band

Electron attachment spectroscopy is employed to demonstrate that the scattering of slow (0–15 eV) electrons from perylenetetracarboxilic dianhydride (PTCDA) molecules in the gas phase leads to the resonant formation of molecular and fragment negative ions detected in the mass-spectrometric experiment. Depending on the electron energy, currents of anions have clearly manifested peaks at 0.14, 1.9, 3.0, 4.8, and 5.7 eV. In addition, resonant states are also detected at thermal energy (0 eV) of scattered electrons, as well as at 0.4 and 1.0 eV, as shoulders on experimental curves. The spectroscopic states of PTCDA anions at energies exceeding 0 eV are interpreted in terms of the formation of shape resonances on the basis of calculated values of energies of π*-type unoccupied molecular orbitals. It is found that the positions of unoccupied orbitals of an isolated PTCDA molecule correspond to the peaks in the density of states of the conduction band of PTCDA films provided that the energies of the orbitals are shifted by 1.4 eV. The latter value can be interpreted as the binding energy of a molecule in the film due to the polarization interaction with the surroundings.     

Effect of plasma treatment on interface property of BCN/GaN structure

Interface properties of BCN/GaN metal-insulator-semiconductor (MIS) structures are investigated by X-ray photoelectron spectroscopy (XPS) and capacitance versus voltage (C-V) characteristics measurements. The BCN/GaN samples are fabricated by in situ process consisting of plasma treatment and deposition of BCN film in the plasma-assisted chemical vapor deposition (PACVD) apparatus. XPS measurement shows that the oxide formation at the BCN/GaN interface is suppressed by nitrogen (N₂) and hydrogen (H₂) plasma treatment. The interface state density is estimated from C-V characteristics measured at 1 MHz using Terman method. The minimum interface state density appears from 0.2 to 0.7 eV below the conduction band edge of GaN. The minimum value of the interface state density is estimated to be 3.0 × 10¹⁰ eV⁻¹ cm⁻² for the BCN/GaN structure with mixed N₂ and H₂ plasma treatment for 25 min. Even after annealing at 430 °C for 10 min, the interface state density as low as 6.0 × 10¹⁰ eV⁻¹ cm⁻² is maintained.     

Adsorption of carbon monoxide on the molybdenum (100) surface

The interaction of CO with Mo(100) has been studied by means of thermal desorption spectrometry, work function measurements and electron stimulated desorption, in conjunction with LEED and AES. Results have been obtained for adsorption at room temperature and at temperatures down to 200 K. The study confirms previous results, showing that the β-states formed at room temperature are atomic. The thermal desorption data for the β-states are analyzed to give directly the desorption activation energy as a function of coverage. This energy is found to vary smoothly from an initial value of 3.7 to a final value of 2.9 eV molecule, indicating an average repulsive interaction between a pair of adjacent adatoms of 0.2 eV. The data at low temperature indicate that a molecular state, virgin-CO, is produced in competition with β-CO and probably one other state, from a common precursor. The step leading to virgin-CO has both a low activation energy and a low pre-exponential factor, suggesting that a reorientation of the molecule is required.     

All electron ab initio investigations of the electronic states of the FeC molecule

The low-lying electronic states of the molecule FeC have been investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) and multi reference configuration interaction (MRCI) calculations. The relativistic corrections for the one-electron Darwin contact term and the relativistic mass-velocity correction have been determined in perturbation calculations. The electronic structure of the FeC molecule is interpreted as antiferromagnetic couplings of the localized angular momenta of the ions and resulting in a triple bond in the valence bond sense. The electronic ground state is confirmed as being . The spectroscopic constants for the ground state and eleven excited states have been derived from the results of the MRCI calculations. The spectroscopic constants for the ground state have been determined as and ,and for the low-lying state as and . The values for the ground state agree well with the available experimental data. The FeC molecule is polar with charge transfer from Fe to C. The dipole moment has been determined as in the ground state and as 1.51 D in the state. From the results of the MRCI calculations the dissociation energy, , is determined as 2.79 eV, and D₀ as 2.74 eV. Received: 2 October 1998 / Received in final form: 30 March 1999     

低覆盖度CO分子在 Ni（110）面的吸附研究

利用密度泛函理论研究了低覆盖度下CO分子在Ni（110）表面的吸附结构和电子态。研究结果表明：在低覆盖度情况下, CO分子优先垂直吸附在短桥位,其次是顶位和长桥位。垂直短桥位吸附、顶位吸附相应的振动频率分别是1850.52 cm-1、1998.08cm-1。态密度的研究结果表明：CO分子和Ni原子在-10 eV -8 eV,-8 eV—-6 eV及1 eV -5 eV能量范围内发生了杂化作用。-10ev -8ev能量范围内的杂化主要来源于C、O原子的s轨道、pz轨道与Ni原子s、p、d轨道的杂化作用。-8ev—-6ev能量范围内的杂化作用主要来源于C、O原子的py、 px轨道与Ni原子d、s轨道的杂化作用。轨道间的杂化作用是吸附作用的主要来源。 我们计算的吸附位置与相应的振动频率与相关实验结果基本一致。     

Physisorption of positronium on quartz surfaces

Whether positronium (Ps) can be physisorbed on a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a Ps2 molecule on a material host. Experiments in the past have produced evidence for physisorbed Ps on a quartz surface, but firm theoretical support for such a conclusion was lacking. We present a first-principles density-functional calculation of the key parameters determining the interaction potential between Ps and an alpha-quartz surface. We show that there is indeed a bound state with an energy of 0.14 eV, a value which agrees very well with the experimental estimate of approximately 0.15 eV. Further, a brief energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of physisorbed atoms shows that the formation and desorption of a Ps2 molecule in that picture is consistent with the above results.     

Interaction and local magnetic moments of metal phthalocyanine and tetraphenylporphyrin molecules on noble metal surfaces

In order to understand the Kondo effect observed in molecular systems, first-principles calculations have been widely used to predict the ground state properties of molecules on metal substrates. In this work, the interaction and the local magnetic moments of magnetic molecules （3d-metal phthalocyanine and tetraphenylporphyrin molecules） on noble metal surfaces are investigated based on the density functional theory. The calculation results show that the dz2 orbital of the transition metal atom of the molecule plays a dominant role in the molecule-surface interaction and the adsorption energy exhibits a simple declining trend as the adsorption distance increases. In addition, the Au（111） surface generally has a weak interaction with the adsorbed molecule compared with the Cu（ll 1） surface and thus serves as a better candidate substrate for studying the Kondo effect. The relation between the local magnetic moment and the Coulomb interaction U is examined by carrying out the GGA＋U calculation according to Dudarev＇s scheme. We find that the Coulomb interaction is essential for estimating the local magnetic moment in molecule-surface systems, and we suggest that the reference values of parameter U are 2 eV for Fe and 2-3 eV for Co.     

Method of observation of low density interface states by means of X-ray photoelectron spectroscopy under bias and passivation by cyanide ions

X-ray photoelectron spectroscopy (XPS) measurements under bias can observe low density interface states for metal-oxide-semiconductor (MOS) diodes with low densities. This method can give energy distribution of interface states for ultrathin insulating layers for which electrical measurements cannot be performed due to a high density leakage current. During the XPS measurements, a bias voltage is applied to the rear semiconductor surface with respect to the ∼3 nm-thick front platinum layer connected to the ground, and the bias voltage changes the occupation of interface states. Charges accumulated in the interface states shift semiconductor core levels at the interface, and thus the analysis of the bias-induced shifts of the semiconductor core levels measured as a function of the bias voltage gives energy distribution of interface states. In the case of Si-based MOS diodes, the energy distribution and density of interface states strongly depend on the atomic density of silicon dioxide (SiO₂) layers and the interfacial roughness, respectively. All the observed interface state spectra possess peaked-structures, indicating that they are due to defect states. An interface state peak near the Si midgap is attributable to isolated Si dangling bonds at the interface, while those above and below the midgap to Si dangling bonds interacting weakly with Si or oxygen atoms in the SiO₂ layers. A method of the elimination of interface states and defect states in Si using cyanide solutions has been developed. The cyanide method simply involves the immersion of Si in KCN solutions. Due to the high Si-CN bond energy of ∼4.5 eV, the bonds are not ruptured at 800 °C and upon irradiation. The cyanide treatment results in the improvement of the electrical characteristics of MOS diodes and solar cells.     

Demonstration of sixth-order coulomb correlations in a semiconductor single quantum well

Spectroscopic magnetization-induced optical second harmonic generation (MSHG) measurements from a clean Ni(110) surface reveal strong resonance effects near 2.7 eV that can be attributed to the presence of an empty surface state. The good agreement with model calculations shows the potential of MSHG to probe spin-polarized interface band structures.     

Towards understanding the carbon trapping mechanism in copper by investigating the carbon-vacancy interaction

We propose a vacancy trapping mechanism for carbon-vacancy (C-V) complex formation in copper (Cu) according to the first-principles calculations of the energetics and kinetics of C-V interaction. Vacancy reduces charge density in its vicinity to induce C nucleation. A monovacancy is capable of trapping as many as four C atoms to form CnV (n=1,2,3,4) complexes. A single C atom prefers to interact with neighboring Cu at a vacancy with a trapping energy of 0.21 eV. With multiple C atoms added, they are preferred to bind with each other to form covalent-like bonds despite of the metallic Cu environment. For the CnV complexes, C2V is the major one due to its lowest average trapping energy (1.31 eV). Kinetically, the formation of the CnV complexes can be ascribed to the vacancy mechanism due to the lower activation energy barrier and the larger diffusion coefficient of vacancy than those of the interstitial C.     

CO在Pu(100)表面吸附的研究

采用密度泛函理论(DFT)研究了CO分子在Pu (100)面上的吸附. 计算结果表明：CO在Pu (100)表面的C端吸附比O端吸附更为有利，属于强化学吸附. CO吸附态的稳定性为穴位倾斜>穴位垂直>桥位>顶位. CO分子与表面Pu原子的相互作用主要源于CO分子的杂化轨道和Pu原子的杂化轨道的贡献. 穴位倾斜吸附的CO分子的离解能垒较小(0.280eV)，表明在较低温度下，CO分子在Pu (100)表面会发生离解吸附，离解的C，O原子将占据能量最低的穴位.     

CO在Pu(100)表面吸附的研究

采用密度泛函理论(DFT)研究了CO分子在Pu (100)面上的吸附. 计算结果表明：CO在Pu (100)表面的C端吸附比O端吸附更为有利,属于强化学吸附. CO吸附态的稳定性为穴位倾斜>穴位垂直>桥位>顶位. CO分子与表面Pu原子的相互作用主要源于CO分子的杂化轨道和Pu原子的杂化轨道的贡献. 穴位倾斜吸附的CO分子的离解能垒较小(0.280eV),表明在较低温度下,CO分子在Pu (100)表面会发生离解吸附,离解的C,O原子将占据能量最低的穴位. 关键词： 密度泛函理论 Pu (100) CO 分子和离解吸附     

有机分子PTCDA在P型Si单晶(100)晶面的生长机理

对PTCDA的分子结构及其化学键的形成进行了分析,并讨论了晶面指数（100）Si单晶的晶格结构。在此基础上,评述了PTCDA分子在P-Si单晶（100）晶面上生长的机理,并制备了样品PTCDA/P-Si（100）。利用XRD对样品测试得出,在P-Si（100）晶面上沉积的PTCDA薄膜中仅存在α物相。利用XPS对样品测试得出,在其界面层中PTCDA酸酐中的4个羟基O原子与C原子结合,其结合能为532.4 eV;苝核基团外围的8个C、H原子以共价键结合,其结合能为289.0 eV;在界面处,悬挂键上的Si原子与PTCDA酸酐中的C、O原子结合,形成C-Si-O键及C-Si键,构成了界面层的稳定结构。     

Band alignment of SiO2/Si structure formed with nitric acid vapor below 500 °C

A relatively thick (i.e., ∼9 nm) SiO₂ layer can be formed by oxidation of Si with nitric acid (HNO₃) vapor below 500 °C. In spite of the low temperature formation, the leakage current density flowing through the SiO₂ layer is considerably low, and it follows the Fowler-Nordheim mechanism. From the Fowler-Nordheim plots, the conduction band offset energy at the SiO₂/Si interface is determined to be 2.57 and 2.21 eV for HNO₃ vapor oxidation at 500 and 350 °C, respectively. From X-ray photoelectron spectroscopy measurements, the valence band offset energy is estimated to be 4.80 and 4.48 eV, respectively, for 500 and 350 °C oxidation. The band-gap energy of the SiO₂ layer formed at 500 °C (8.39 eV) is 0.68 eV larger than that formed at 350 °C. The higher band-gap energy for 500 °C oxidation is mainly attributable to the higher atomic density of the SiO₂ layer of 2.46 × 10²²/cm³. Another reason may be the absence of SiO₂ trap-states.     

苯并咪唑苝与金属Ag的界面电子结构研究

利用同步辐射光电子能谱实验技术考察了苯并咪唑苝(BZP)和Ag的界面形成过程与电子结构.单层覆盖度以下时，BZP分子与Ag有弱相互作用，在有机分子禁带中出现明显界面反应态，结合能位于0.9eV.单层铺满后，BZP分子呈现三维岛式生长，且与Ag的相互作用逐渐减弱，同时最高占据分子轨道由于终态效应逐渐向高结合能方向位移至体相结合能位置(2.3eV). Ag衬底上BZP分子的生长导致样品表面功函数减小，表明形成了表面偶极势(Δ=0.3eV)，且电子从有机分子向金属Ag偏移.最后，考察了BZP/Ag 关键词： 有机-金属界面 电子结构 光电子能谱 同步辐射     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.