Atomic and electronic structures of carbon nanotubes on Si(001) stepped surfaces

We report first-principles total-energy calculations that provide energetics and electronic structures of adsorbed carbon nanotubes (CNTs) on stepped Si(001) surfaces. We find that adsorption energies strongly depend on the directions of CNTs, and that there are several metastable adsorption sites both on terraces and near step edges. We also find that the electronic structure of adsorbed metallic CNTs becomes semiconducting or remains metallic, depending on the adsorption site. Charge redistribution upon adsorption is prominent mainly at the CNT-surface interface.     

Theoretical Studies on Interaction Between Methanol and Functionalized Single-Walled Carbon Nanotubes

We study the adsorption of a methanol molecule on single-walled carbon nanotubes (SWCNTs) with various diameters and chiral angles by using the density functional theory based calculations. We find that methanol prefers to be adsorbed physically on the exterior surface of chiral nanotubes in comparison to the armchair and zigzag tubes with binding energy of about-2.76 kcal/mol, which is consistent with recent experimental andtheoretical investigation results. We further consider the adsorption of methanol on the exterior surface and edge site of functionalized SWCNTs. The obtained results indicate that the binding energy of methanol is significantly increased for adsorption on the sidewall of functionalized nanotubes. It is also found that the adsorption of methanol at the edge site of both functionalizedand pristine SWCNT is remarkably different (chemisoption process) incomparison to the exterior sidewall of the tubes. Furthermore, the electronic structures and Mulliken charge population of the considered complexes at their ground state are discussed within the context.     

Interaction between methanol and single-walled carbon nanotubes: Density functional theory study

Density functional calculations have been performed to investigate the dependence of methanol interaction with the side walls of single-walled carbon nanotubes (SWCNTs) on the nanotube's type, curvature and chirality. The author's results show that methanol prefers to be physically adsorbed on semiconducting CNTs in comparison with the metallic one. It was found that the binding energy of methanol is increased for adsorption on larger-diameter nanotubes. Furthermore, we find that when a methanol molecule was adsorbed on higher chiral angle nanotubes the binding energy was increased. The study of the electronic structures and Mulliken analysis indicate that the methanol and CNT are interacting rather weakly, consistent with recent experimental observation.     

5d过渡金属原子吸附氮化硼纳米管的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法, 研究了氮化硼纳米管六元环中心吸附5d过渡金属原子后体系的几何结构, 电子结构和磁性性质. 研究发现, 吸附原子向一个氮原子或硼原子偏移; 吸附体系在费米能级附近出现明显的杂质能级; 各个体系的总磁矩随原子序数出现规律性变化, 局域磁矩主要分布在吸附原子上.     

Semiconducting electronic property of graphene adsorbed on (0001) surfaces of SiO2

First-principles total energy calculations are performed to investigate the energetics and electronic structures of graphene adsorbed on both an oxygen-terminated SiO2 (0001) surface and a fully hydroxylated SiO2 (0001) surface. We find that there are several stable adsorption sites for graphene on both O-terminated and hydroxylated SiO2 surfaces. The binding energy in the most stable geometry is found to be 15 meV per C atom, indicating a weak interaction between graphene and SiO2 (0001) surfaces. We also find that the graphene adsorbed on SiO2 is a semiconductor irrespective of the adsorption arrangement due to the variation of on-site energy induced by the SiO2 substrate.     

Enhanced charge transfer in a monolayer of the organic charge transfer complex TTF-TNAP on Au(111)

Electronic doping is a key concept for tuning the properties of organic materials. In bulk structures, the charge transfer between donor and acceptor is mainly given by the respective ionization potential and electron affinity. In contrast, monolayers of charge transfer complexes in contact with a metal are affected by an intriguing interplay of hybridization and screening at the metallic interface, determining the resulting charge state. Using scanning tunneling microscopy and spectroscopy, we characterize the electronic properties of the organic acceptor molecule 11,11,12,12-tetracyanonaptho-2,6-quinodimethane (TNAP) adsorbed on a Au(111) surface. The ordered islands remain in a weakly physisorbed state with no charge transfer interaction with the substrate. When the electron donor tetrathiafulvalene (TTF) is added, ordered arrays of alternating TNAP and TTF rows are assembled. In these structures, we find the lowest unoccupied molecular orbital (LUMO) of the free TNAP molecule shifted well below the Fermi level of the substrate. The TNAP is thus charged with more than one electron.     

Alkali-metal-affected adsorption of molecules on metal surfaces

The study of coadsorption of alkali metals and simple molecules on transition metal surfaces has been a favored topic of research ever since the pioneering work by Langmuir in 1923. The main reasons for this continued interest are both of fundamental and applied nature. There are a number of interesting physical effects, such as work function changes, charge transfer, two-dimensional ordering, bond energy and molecule orientational changes, and altered surface reactive properties, which have been investigated by a large number of different surface analytical techniques. From an applied point of view, alkali metal covered surfaces are important in the areas of electron and ion emission and heterogenous catalysis, for example.In this paper we will give a short review of the adsorption of alkali metals on well-defined transition metal surfaces. The interaction of these adsorbed alkali metals with subsequently adsorbed atomic or molecular species will be treated more extensively. The emphasis will be on recent experiments dealing with well-characterized surfaces. In particular we will consider questions of adsorption energetics and kinetics, but also review in detail the vibrational, electronic, structural and reactive properties of the coadsorbed complex. Based on a wealth of experimental data, several models of the coadsorbed alkali metal-molecule complex will be introduced and discussed.     

用Xα-DV方法研究CO,NO在Ⅷ族过渡金属表面的化学吸附(Ⅱ)——NO分子在Pd(111)表面的化学吸附

本文用X_α-DV方法计算了NO在Pd(111)表面化学吸附问题。得到了它的电子结构,包括分子丛轨道能量本征值谱、态密度、电荷转移等等结果。在计算中特别考虑了NO之间的相互作用,所得总态密度与实验UPS十分相符,从而支持了LEED所示的几何结构,决定了NO的吸附高度为1.27?,并得知吸附于Pd表面的NO分子之间的相互作用十分重要。从理论上探讨了NO分子在Pd表面吸附时的活化作用。计算了NO分子各个轨道上的占有数,发现其电荷转移情况与CO在过渡金属表面吸附的情况相似。另外,还发现NO的吸附对Pd的价电子能带无重大影响。 关键词：     

第一性原理研究气体小分子吸附的石墨烯负载金属原子体系

石墨烯负载的单个金属原子体系(M-gra)具有高的结构稳定性,显正电性的金属原子可作为活性位用在气敏器件和催化材料.本文采用基于密度泛函理论的第一性原理方法研究单个有毒气体小分子(NO和CO)在M-gra表面的吸附特性.研究结果表明:单个NO分子吸附的稳定性高于CO分子,由于其能够从反应衬底获得更多的转移电荷,因此,M-gra衬底对NO分子表现出高的灵敏度.此外,不同小分子吸附能够改变M-gra体系的电荷密度和自旋电荷分布,进而使得气体分子吸附体系表现出不同大小的磁矩.通过对比气体分子吸附前后M-gra体系的磁矩变化,能够有效判断吸附分子和反应衬底的类型.     

Curvature-induced metallization of double-walled semiconducting zigzag carbon nanotubes

We report total-energy electronic-structure calculations that provide energetics and electronic structures of double-walled carbon nanotubes consisting of semiconducting (n,0) nanotubes. We find that optimum spacing between the walls of the nanotubes is slightly larger than the interlayer spacing of the graphite. We also find that the electronic structures of the double-walled nanotubes with the inner (7,0) nanotube are metallic with multicarrier characters in which electrons and holes exist on inner and outer nanotubes, respectively. Interwall spacing and curvature difference are found to be essential for the electron states around the Fermi level.     

Adsorption on the carbon nanotubes

Adsorption on single walled carbon nanotubes (SWCNTs) is a subject of growing experimental and theoretical interest. The possible adsorbed patterns of atoms and molecules on the single-walled carbon nanotubes vary with the diameters and chirality of the tubes due to the confinement. The curvature of the carbon nanotube surface enlarges the distance of the adsorbate atoms and thus enhances the stability of high coverage structures of adsorbate. There exist two novel high-coverage stable structures of potassium adsorbed on SWCNTs, which are not stable on graphite. The electronic properties of SWCNTs can be modified by adsorbate atoms and metal-semiconductor and semiconductor-semi-conductor transitions can be achieved by the doping of alkali atoms.     

Correlation between charge transfer and stick-slip friction at a metal-insulator interface

Techniques have been developed that facilitate the measurement and imaging of the charge exchanged between metal-insulator surfaces in relative motion. In the regime where the forces of friction lead to stick-slip motion, we find that the charge transfer accompanying the slip events is proportional to the force jumps and is bunched at the stick locations. The constant of proportionality is measured in electron volts per angstrom and has a small variance over a large range of slip sizes, suggesting that in these experiments macroscopic friction originates from and scales to the intrinsic electronic interactions that form between metal and insulator surfaces.     

气体分子在类石墨烯材料负载单个金属原子表面的吸附特性

采用密度泛函理论方法研究了不同种类和数量的气体小分子在类石墨烯材料(graphenylene)衬底支撑的金属原子(M=Co, Mo和Pd, gra-M)表面的吸附特性,系统地分析了吸附不同数量的NO和CO分子的稳定构型,吸附能,电荷转移量以及引起的体系电子结构和磁性变化.研究结果表明:1) NO、CO气体小分子的稳定吸附位在金属原子顶位,吸附物与衬底间的电荷转移量表明负载不同的金属原子能够有效地调制类石墨烯材料的气敏特性;2)单个和两个气体分子吸附能够引起gra-M体系的自旋电荷密度分布发生变化,进而使得气体吸附体系表现出不同磁矩大小.     

剪切形变对硼氮掺杂碳纳米管超晶格电子结构和光学性能的影响

碳纳米管作为最先进的纳米材料之一, 在电子和光学器件领域有潜在的应用前景, 因此引起了广泛关注. 掺杂、变形及形成超晶格为调制纳米管电子、光学性质提供了有效途径. 为了理解相关机理, 利用第一性原理方法研究了不同剪切形变下扶手椅型硼氮交替环状掺杂碳纳米管超晶格的空间结构、电子结构和光学性质. 研究发现, 剪切形变会改变碳纳米管的几何结构, 当剪切形变大于12%后, 其几何结构有较大畸变. 结合能计算表明, 剪切形变改变了掺杂碳纳米管超晶格的稳定性, 剪切形变越大, 稳定性越低. 电荷布居分析表明, 硼氮掺杂碳纳米管超晶格中离子键和共价键共存. 能带和态密度分析发现硼氮交替环状掺杂使碳纳米管超晶格从金属转变为半导体. 随着剪切形变加剧, 纳米管超晶格能隙逐渐减小, 当剪切形变大于12%后, 碳纳米管又从半导体变为金属. 在光学性能中, 剪切形变的硼氮掺杂碳纳米管超晶格的光吸收系数及反射率峰值较未受剪切形变的均减小, 且均出现了红移.     

Energetics and electronic structures of encapsulated C60 in a carbon nanotube

We report total-energy electronic structure calculations that provide energetics of encapsulation of C60 in the carbon nanotube and electronic structures of the resulting carbon peapods. We find that the encapsulating process is exothermic for the (10,10) nanotube, whereas the processes are endothermic for the (8,8) and (9,9) nanotubes, indicative that the minimum radius of the nanotube for the encapsulation is 6.4 A. We also find that the C(60)@(10,10) is a metal with multicarriers each of which distributes either along the nanotube or on the C60 chain. This unusual feature is due to the nearly free electron state that is inherent to hierarchical solids with sufficient space inside.     

碳纳米管负载矾的第一性原理研究

本文基于密度泛函第一性原理研究了原始和带有缺陷的(Stone-Wales缺陷和单空位缺陷)碳纳米管负载金属V的稳定构型.对于V吸附在原始碳纳米管(CNT)上时,V在内表面的吸附比外表面的吸附有更强的相互作用力,且六元环内表面结构最稳定.当V与Stone-Wales缺陷碳纳米管相互作用时,V原子易吸附在管外七元环C-C键的外表面和内表面处,这说明缺陷位置的有效结合使之局域化加强.而以单空位缺陷碳纳米管为载体时V最易吸附在外缺陷处,相当于碳纳米管的一个C被金属V原子取代,形成了3个V-C_(sur)键,这进一步表明SV管外吸附比管内吸附更容易.我们从上述三种构型载体中发现,金属V吸附在缺陷碳纳米管时的稳定性要优于原始碳纳米管,且SV缺陷对金属V的固定效果最好.     

单向压力对碳纳米管(6, 6)晶体电子结构的影响

通过第一性原理计算研究了垂直于碳纳米管轴向的单向压力对碳纳米管(6,6)晶体电子结构特性的影响.计算研究发现：由碳纳米管(6,6)组成的四方结构晶体(t相)具有金属特性,电子可以沿碳纳米管管壁运动;在单向压力作用下,t相发生结构相变形成非成键相,随着压力的进一步增大,碳纳米管间产生键合,形成了成键相;单向压力对碳纳米管(6,6)晶体的能带结构影响主要表现在π能带和π^*能带,伴随着单向压力的增加,碳纳米管晶体的电学性质经历从金属到半导体再到活泼金属的转变;非成键相的电子被局域在碳纳米管附近使晶体具有半导体特性,而成键相的电子不仅可以沿着碳纳米管管壁运动,还可以在碳纳米管之间(即成键方向)运动,从而使成健相晶体具有活泼的金属特性. 关键词： 碳纳米管晶体 第一性原理计算 金属—半导体转变     

Surface interactions of Au(I) cyclo-trimer with Au(111) and Al(111) surfaces: A computational study

A plane-wave density functional theory (DFT) study on surface interactions of a cyclo-[Au(μ-Pz)]₃ monolayer (denoted as T), Pz = pyrazolate, with Au(111) and Al(111) surfaces (denoted as M′) has been performed. Structural and electronic properties at the M′–T interfaces are determined from individually optimized structures of M′, T and M′–T. Results show that the gold pyrazolate trimer (T) binds more strongly on the Au(111) surface than on Al(111). Charge redistribution has been observed at both M′–T interfaces, where charge is “pushed” back towards the Au(111) surface from the trimer monolayer in Au(111)–T system, while the opposite happens in the Al(111)–T system where the charge is being pushed toward the trimer monolayer from the Al(111) surface. Considerable changes to the work function of Au(111) and Al(111) surfaces upon the trimer adsorption which arise from monolayer vacuum level shifts and dipole formation at the interfaces are calculated. The interaction between cyclo-[Au(μ-Pz)]₃ with metal surfaces causes band broadening of the gold pyrazolate trimer in M′–T systems. The present study aids better understanding of the role of intermolecular interactions, bond dipoles, energy-level alignment and electronic coupling at the interface of metal electrodes and organometallic semiconductor to help design metal–organic field effect transistors (MOFETs) and other organometallic electronic devices.     

Band structure of Au layers on the Ru(0001) and graphene/Ru(0001) surfaces

The electronic structures of Au monolayers on the Ru(0001) and graphene-coated Ru(0001) surfaces have been calculated by DFT method using the supercell (repeated-slab) approach. The local densities of states (LDOS) and band structures of the monolayer and bilayer Au films adsorbed on the graphene/Ru(0001) and those of free hexagonal Au layers are found to be very similar. This result indicates that the monolayer graphene almost completely screens the Au layers from the Ru(0001) substrate surface, so that electronic properties of Au films adsorbed on graphene are determined predominantly by the electronic structure of the Au adlayers, essentially independent on the electronic structure of the substrate surface.     

Low energy electron diffraction and auger electron spectroscopy studies of the structure of adsorbed gases on solid surfaces

Low energy electron diffraction (LEED) studies of the structure of adsorbed molecules on crystal surfaces revealed that ordered surface structures predominate under most conditions of the experiments. In the absence of chemical reactions with the substrate, the degree of ordering depends on the heats of adsorption, ΔH_ads, and the activation energies for surface diffusion, ΔE_D1. Since ΔH_ads is usually markedly larger than ΔE_D1, small changes of substrate temperature facilitate ordering without appreciable increase in desorption rates. The surface structures of adsorbed gases that have been reported so far have been tabulated. For molecules whose size is compatible with the interatomic distance of the substrate, rules of ordering can be proposed that permit prediction of the structure of the adsorbed layer that is likely to form. These rules indicate close packing due to attractive interactions in the adsorbed layer, and that the rotational multiplicity of the substrate is likely to be maintained by the adsorbate structure. When molecules whose dimensions are larger than the substrate interatomic distance are adsorbed, the conditions that control ordering are more complex and simple rules may not be readily applicable.The surface structures of adsorbed gases have also been studied on high Miller Index substrate surfaces. These surfaces are characterized by ordered steps separated by terraces of low index surface orientation. Many gases have different ordering characteristics on stepped surfaces than on low index crystal faces due to the stronger substrate-adsorbate interactions in these surfaces. The dissociation of diatomic molecules at steps induces the formation of new types of surface structures (frequently one-dimensional) and the dehydrogenation of hydrocarbons at steps induces the formation of ordered carbonaceous surface structures that would not nucleate on low index substrate planes.So far, mostly work function changes upon adsorption gave indication of the magnitude of charge transfer upon adsorption and on forming of new surface chemical bonds. Most recently, chemical shifts of the Auger transitions of the substrate atoms and of the adsorbed molecules upon chemisorption, have been found to provide additional information on charge redistribution during adsorption.