Chemical trends in adsorption on semiconductor surfaces: Results from local density theory

In this paper we briefly review some chemical trends in structural and electronic properties of monolayers of group IV to group VII adatoms adsorbed on (001) surfaces of homopolar semiconductors. Particular emphasis is put on adsorption of Si, As, Se and Cl at the Si (001) surface. We discuss results from our local density Green function calculations for semi-infinite adsorption systems. The calculated optimal structures can be interpreted in a simple picture of the surface chemical bond and they are in excellent agreement with experimental data where they are available. The calculated electronic structure agrees very good with ARPES data for those systems for which well-ordered monolayer adsorption on the substrate surface has been observed experimentally.     

Gradient corrections in density functional theory calculations for surfaces: Co on Pd{110}

Ab initio total energy calculations have been performed for CO chemisorption on Pd{110}. Local density approximation (LDA) calculations yield chemisorption energies which are significantly higher than experimental values but inclusion of the generalised gradient approximation (GGA) gives better agreement. In general, sites with higher coordination of the adsorbate to surface atoms lead to a larger degree of overbinding with LDA, and give larger corrections with GGA. The reason is discussed using a first-order perturbation approximation. It is concluded that this may be a general failure of LDA for chemisorption energy calculations. This conclusion may be extended to many surface calculations, such as potential energy surfaces for diffusion.     

Synergy between theory and experiment in structure resolution of low-dimensional oxides

In this paper, I review recent progress in joint theoretical and experimental studies aiming at atomic structure determination of low-dimensional metal oxides. Low-dimensional systems can be generally defined as materials of unusual structure that extend to less than three dimensions. In recent years low-dimensional systems have attracted increasing attention of physicists and chemists, and the interest is expected to rise in the near future. Two- and one-dimensional structures in form of thin oxide films or elongated oxide chains have many potential applications including model supports for heterogeneous catalysts and insulating layers in semiconductor industry. The interest in zero-dimensional gas-phase oxide clusters ranges from astrophysics to studies of elementary steps in catalysis. The key prerequisite for understanding physical and chemical properties of low-dimensional systems is a detailed knowledge of their atomic structures. However, such systems frequently present complex structures to solve. Only in a few cases experimental data can provide some information about possible arrangement of atoms, but data interpretation relies to a large extent on intuition. Therefore, in the recent years quantum chemical calculations became an indispensable tool in structure identification of low-dimensional systems, yet the accuracy of theoretical tools is often limited. The results reviewed here demonstrate that often the only way of an unambiguous atomic structure determination of low-dimensional systems are experimental studies combined with theoretical calculations. Particularly the global optimization methods such as genetic algorithm in combination with the density functional theory prove very useful in automatic structure determination of the observed surface structures and gas-phase clusters.     

Using semiclassical surface hopping for coupled partial wave calculations on systems with non-spherically symmetric potentials

It is shown that a semiclassical surface hopping (SH) approach provides a simple and efficient method for scattering calculations with non-spherically symmetric potentials. The calculations are performed by expanding the wave function in an angular momentum state basis. Since the potential is not spherically symmetric, the different angular states are coupled. The semiclassical SH method, which is typically used for problems with coupled electronic states, can, in principle, be employed for any coupled state problem. The particular SH method employed is known to provide highly accurate results for coupled electronic state problems. The method is tested on model two angular state problems using potential surfaces and couplings arising from a non-spherically symmetric scattering problem. The results for these model problems are in excellent agreement with exact quantum calculations. Full calculations, which are converged with regard to the number of angular basis states, are also performed for the non-spherically symmetric problem. It is shown that an approximation to the surface hopping amplitudes that simplifies the numerical implementation of the method provides results in excellent agreement with the full surface hopping calculation.     

Attachment of styrene and phenylacetylene on Si(111)-7 x 7: the influence of substitution groups on the reaction mechanism and formation of pi-conjugated skeletons

The interactions of styrene and phenylacetylene and their isotope substitutions with a Si(111)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of conjugated pi-electron systems to di-radical-like silicon dangling bonds of the adjacent adatom-rest atom pair. Vibrational studies show that styrene mainly binds to the surface through a diradical reaction involving both the external C=C and its conjugated internal C=C of the phenyl ring with an adjacent adatom-rest atom pair, forming a 5-ethylidene-1,3-cyclohexadiene-like skeleton. On the other hand, phenylacetylene was shown to be covalently attached to Si(111)-7 x 7 through the external C[triple bond]C, forming a styrene-like conjugation system. These experimental results are consistent with density functional theory calculations. The different binding mechanisms for styrene and phenylacetylene clearly demonstrate that reaction channels for multifunctional organic molecules are strongly dependent on the chemical and physical properties of the functional groups. The resulting pi-electron conjugation structures may possibly be employed as intermediates for further organic syntheses and fabrication of multilayer organic films on semiconductor surfaces.     

Charge heterogeneity of surfaces: mapping and effects on surface forces

The DLVO theory treats the total interaction force between two surfaces in a liquid medium as an arithmetic sum of two components: Lifshitz–van der Waals and electric double layer forces. Despite the success of the DLVO model developed for homogeneous surfaces, a vast majority of surfaces of particles and materials in technological systems are of a heterogeneous nature with a mosaic structure composed of microscopic and sub-microscopic domains of different surface characteristics. In such systems, the heterogeneity of the surface can be more important than the average surface character. Attractions can be stronger, by orders of magnitude, than would be expected from the classical mean-field DLVO model when area-averaged surface charge or potential is employed. Heterogeneity also introduces anisotropy of interactions into colloidal systems, vastly ignored in the past. To detect surface heterogeneities, analytical tools which provide accurate and spatially resolved information about material surface chemistry and potential — particularly at microscopic and sub-microscopic resolutions — are needed.Atomic force microscopy (AFM) offers the opportunity to locally probe not only changes in material surface characteristic but also charges of heterogeneous surfaces through measurements of force–distance curves in electrolyte solutions. Both diffuse-layer charge densities and potentials can be calculated by fitting the experimental data with a DLVO theoretical model. The surface charge characteristics of the heterogeneous substrate as recorded by AFM allow the charge variation to be mapped. Based on the obtained information, computer modeling and simulation can be performed to study the interactions among an ensemble of heterogeneous particles and their collective motions. In this paper, the diffuse-layer charge mapping by the AFM technique is briefly reviewed, and a new Diffuse Interface Field Approach to colloid modeling and simulation is briefly discussed.     

一种超硬新材料BeP2N4的电子结构和力学性质及本征硬度

基于密度泛函理论,采用局域密度近似(LDA)和广义梯度近似(GGA)泛函研究了硅铍石、尖晶石结构的 BeP₂N₄ 材料的晶格参数、能带结构、态密度、分态密度、Mulliken布居值和弹性性质, 计算结果与已有的实验值和理论值符合很好. 能带结构和态密度表明两种结构的BeP₂N₄材料是宽的直接带隙的绝缘体材料. 尖晶石结构BeP₂N₄的体弹性模量、剪切模量和弹性模量比硅铍石结构的相应的力学量大得多. 利用Sung等提出的硬度经验判据和Gao等提出的基于Mulliken轨道重叠布居数的共价固体本征硬度计算方法, 预测了两种结构的本征硬度值. 计算结果表明硅铍石结构BeP₂N₄虽然体弹模量小, 但是它并不是一种软的材料, 而是一种易脆的硬度较硬的材料, 随着压力增加硅铍石结构BeP₂N₄的脆性逐渐过渡到延性. 尖晶石结构BeP₂N₄是一种易脆的超硬材料. 采用GGA计算得到的硅铍石BeP₂N₄向尖晶石相转变压力为14 GPa, 与理论预测值(24 GPa)相比偏小.     

A systematic study of the structure and bonding of halogens on low-index transition metal surfaces

The structure and bonding of halogens on various transition metal low-index surfaces has been studied by means of density functional theory (DFT) calculations using periodic slabs to model the surface. This approach is shown to be capable of reproducing available experimental data of naked and halogen-covered surfaces. Periodic trends are discerned and discussed for several properties, including metal-halogen bond distances and vibrational frequencies, adsorption energies, and bond ionicities, which have been evaluated by a Bader population analysis of the corresponding density. A simple correlation is discerned, relating the bond ionicity to the metal work function, so that higher work function surfaces are associated with more covalent bonding. Periodic trends in bond ionicities and metal-halogen vibrational frequencies are in harmony with corresponding data derived in an electrochemical environment, indicating that the metal-halogen bonding in vacuum share some features with the electrode metal surface-halogen bonding.     

Analytic density-functional self-consistent-field theory of diblock copolymers near patterned surfaces

Analytical solutions are derived for the density profiles and the free energies of compressible diblock copolymer melts (or incompressible copolymer solutions) near patterned surfaces. The density-functional self-consistent-field theory is employed along with a Gaussian chain model for bonding constraints and a random mixing approximation for nonbonded interactions. An analytical solution is rendered possible by expanding the chain distribution function around an inhomogeneous reference state with a nontrivial analytical solution, by retaining the linear terms, and by requiring consistency with the homopolymer limit. The density profiles are determined by both real and complex roots of a sixth-degree polynomial that may easily be obtained by solving a generalized eigenvalue problem. This analytical formulation enables one to efficiently explore the large nine-dimensional parameter space and can serve as a first approximation to computationally intensive studies with more detailed models. Illustrative computations are provided for uniform and patterned surfaces above the order-disorder transition. The results are consistent with the previous self-consistent-field calculations in that lamellar ordering appears near the surface above the order-disorder transition and the lamella order perpendicular or parallel to the surface depending on the commensurability between the periods of the surface pattern and the density oscillations.     

Six-Dimensional State-to-State Quantum Dynamics of H2/D2 Scattering from Cu(100): Validity of Site-Averaging Model

Six-dimensional quantum dynamics calculations for the state-to-state scattering of H\begin{document}$ _2 $\end{document}/D\begin{document}$ _2 $\end{document} on the rigid Cu(100) surface have been carried out using a time-dependent wave packet approach, based on an accurate neural network potential energy surface fit for thousands of density functional theory data computed with the optPBE-vdW density functional. The present results are compared with previous theoretical and experimental ones regarding to the rovibrationally (in)elastic scattering of H\begin{document}$ _2 $\end{document} and D\begin{document}$ _2 $\end{document} from Cu(100). In particular, we test the validity of the site-averaging approximation in this system by which the six-dimensional (in)elastic scattering probabilities are compared with the weighted average of four-dimensional results over fifteen fixed sites. Specifically, the site-averaging model reproduces vibrationally elastic scattering probabilities quite well, though less well for vibrationally inelastic results at high energies. These results support the use of the site-averaging model to reduce computational costs in future investigations on the state-to-state scattering dynamics of heavy diatomic or polyatomic molecules from metal surfaces, where full-dimensional calculations are too expensive.     

不同覆盖度下Li原子在Si(001)表面上的吸附构型和电子结构

采用基于赝势平面波基组的密度泛函理论, 对不同Li原子覆盖度下Li/Si(001)体系的吸附构型、电子结构以及吸附Li原子对表面性质的影响进行了系统研究. 计算结果表明, 在所考察的覆盖度范围内, Li原子倾向于吸附在相邻两个Si-Si二聚体之间各种对称性较高的空穴位, 其中覆盖度为0.75 ML(monolayer)时具有最小的平均吸附能. 由能带结构分析结果可知, 随着覆盖度的增大, Si(001)表面存在由半导体→导体→半导体的变化过程. 在覆盖度为1.00 ML时, 由于表层二聚体均受到显著破坏, 使得体系带隙明显增大. 吸附后, 有较多电子从Li原子转移到底物, 导致Si(001)表面功函显著下降, 并随着覆盖度的增加表面功函呈现振荡变化. 此外, 从热力学稳定性角度上看, 覆盖度为0.75 ML的Li/Si(001)表面较难形成.     

Computational studies of semiconductor quantum dots

Light-absorption and luminescence processes in nano-sized materials can be modelled either by using computational approaches developed for quantum chemical calculations or by applying computational methods in the effective mass approximation (EMA) originally intended for solid-state theory studies. An overview of the theory and implementation of an ab initio correlation EMA method for studies of luminescence properties of embedded semiconductor quantum dots is presented. The applicability of the method and the importance of correlation effects are demonstrated by calculations on InGaAs/GaAs quantum-dot and quantum-ring samples. Ab initio and density functional theory (DFT) quantum chemical studies of optical transitions in freestanding silicon nanoclusters are also discussed. The accuracy of the optical gaps and oscillator strengths for silicon nanoclusters obtained using different computational methods is addressed. Changes in the cluster structures, excitation energies and band strengths upon excitation are reported. The role of the surface termination and functional groups on the silicon nanocluster surfaces is discussed.     

基于氨基与表面乙烯砜基反应动力学调控配基表面密度

配基表面密度可控为定量研究生物分子相互作用提供了精准的分子基础。然而,经典混合自组装的方法控制配基密度对于不同自组装体系不具有普适性。本文报道了一种基于表面乙烯砜基反应动力学的配基表面密度调控方法。以Nα,Nα-二（羧甲基）-L-赖氨酸（ab-NTA）为生物配基模型,对该表面反应进行了催化剂筛选并利用X射线光电子能谱（XPS）和表面膜电位对该表面反应进行了表征。采用静态水接触角的方法对表面反应的动力学进行了定量表征,计算得到反应速率常数为0.0012 min^-1。采用表面等离子体共振（SPR）分析了该生物功能表面结合组氨酸标签蛋白（SA-6His）的能力,结果表明该表面比传统NHS-NTA表面具有更高的蛋白结合量和结合强度。通过控制反应时间和催化剂种类制备了四种配基密度不同的生物功能表面,并利用SPR对四种表面进行了蛋白质静态吸附实验。实验结果表明通过控制反应时间和催化剂类型均能够实现配基表面密度的调控,并且可以实现表面多价态的调控。     

钛铁矿型六方相ZnTiO3的电子结构和光学性质

分别采用基于密度泛函理论(DFT)的局域密度近似(LDA)和广义梯度近似(GGA)方法对钛铁矿型六方相ZnTiO₃的电子结构进行了第一性原理计算, 并在局域密度近似下计算了六方相ZnTiO₃的光学性质, 并将计算结果与实验数据进行了对比. 结果表明, 在局域密度近似下计算得到的结构参数更接近实验数据. 理论预测六方相ZnTiO₃属于直接带隙半导体材料, 其禁带宽度(布里渊区Z 点)为3.11 eV. 电子态密度和Mulliken 电荷布居分析表明Zn―O键是典型的离子键而Ti―O键是类似于钙钛矿型ATiO₃ (A=Sr, Pb, Ba)的Ti―O共价键. 在50 eV的能量范围内研究了ZnTiO₃的介电函数、吸收光谱和折射率等光学性质, 并基于电子能带结构和态密度对光学性质进行了解释.     

Unilaterally Fluorinated Acenes: Synthesis and Solid‐State Properties

The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14‐hexafluoropentacene ( 1 ) and the related 1,2,9,10,11‐pentafluorotetracene ( 2 ). Quantum chemical DFT calculations in combination with optical and X‐ray absorption spectroscopy data indicate that the single‐molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct F???H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid‐state properties such as the exciton binding energy and the sublimation enthalpy.     

Structural, thermodynamic and optical properties of MgF2 studied from first-principles theory

A detailed theoretical study of structural, electronic, elastic, thermodynamic and optical properties of rutile type MgF₂ has been carried out by means of first-principles Density Functional Theory (DFT) calculations using plane wave pseudo-potentials within the local density approximation and generalized-gradient approximation for the exchange and correlation functionals. The calculated ground state properties and elastic constants agree quite well with experimental values. From the calculated elastic constants we conclude that MgF₂ is relatively hard when compared to other alkaline-earth fluorides and ductile in nature. The thermodynamic properties such as heat capacity, entropy, free energy, phonon density of states and Debye temperatures are calculated at various temperatures from the lattice dynamical data obtained through the quasi-harmonic Debye model. From free energy and entropy it is found that the system is thermodynamically stable up to 1200 K. The imaginary part of the calculated dielectric function ε₂(ω) could reproduce the six prominent peaks which are observed in experiment. From the calculated ε(ω), other optical properties such as refractive index, reflectivity and electron energy-loss spectrum are obtained up to the photon energy range of 30 eV.     

Chemisorption and charge transfer at ionic semiconductor surfaces: Implications in designing gas sensors

A detailed atomistic understanding of charge transfer reactions between semiconductor surfaces and adsorbing particles is essential for designing gas sensors or metal-oxide catalysts.This will be demonstrated in a discussion of thermodynamically or kinetically controlled solid/gas interactions at extensively investigated “prototype surfaces”, such as ZnO (1010) and TiO₂ (110). Interaction steps discussed are physisorption, chemisorption, surface and volume reactions of small molecules. The discussion is based upon results from (PAR) UPS, XPS, BELS, LEED, AES, EPR, TDS and measurements of conductivities and work functions.Chemisorption steps and reactions involving surface as well as bulk defects of the substrate are of particular importance for sensor applications. Both types of interaction generally involve localized charge redistribution in the valence-band range and delocalized charge tranfer of electrons in the conduction band. In this context, quantum chemical cluster calculations are particularly useful in interpreting and generalizing experimental data.     

The AM05 density functional applied to solids

We show that the AM05 functional [Armiento and Mattsson, Phys. Rev. B 72, 085108 (2005)] has the same excellent performance for solids as the hybrid density functionals tested in Paier et al. [J. Chem. Phys. 124, 154709 (2006); 125, 249901 (2006)]. This confirms the original finding that AM05 performs exceptionally well for solids and surfaces. Hartree-Fock hybrid calculations are typically an order of magnitude slower than local or semilocal density functionals such as AM05, which is of a regular semilocal generalized gradient approximation form. The performance of AM05 is on average found to be superior to selecting the best of local density approximation and PBE for each solid. By comparing data from several different electronic-structure codes, we have determined that the numerical errors in this study are equal to or smaller than the corresponding experimental uncertainties.     

Ab initio theoretical study of substituted dicarboxylic acids adsorbed on GaAs surfaces: correlation between microscopic properties and observed electrical behavior

Five substituted tartaric acid derivatives are studied using density functional theory, both isolated and adsorbed onto an oxidized GaAs cluster, to model molecular layers on semiconductor surfaces. The structures, energies, and electronic properties are computed to clarify the interactions responsible for the electric behavior of the modified surfaces, used in semiconductor/metal junction devices. The chemical structure of the molecule/GaAs adducts is optimized ab initio and discussed for the first time. A strong binding scheme is found, providing useful insights about the microscopic structure of the molecular layer. A widely used model based on molecular dipole layers is discussed and verified, by computing the dipole moment for the isolated systems and estimating the charge separation in the adducts; moreover the molecular orbitals energies are analyzed and correlated to the experimental measures of the modified surface electron affinity.     

DFT Investigations About Pyrazine Molecules on Si(100)-2×1 Surface

It is important to understand the interface of aromatic molecules on semiconductor surfaces because of the rich functionality of such molecules on semiconductor surfaces. The chemisorption of pyrazine molecules on the Si( 100)-2×1 surface has been investigated using the B3LYP density functional theory with Si9H12 one-dimer and Si15H16 twodimer cluster models. The calculated results predict that N-dative bonded-state, C2= C5 [ 4 2 ] and the tightbridge1, 2, 5,6 products may coexist on the Si(100)-2×1 surface.