石墨烯-六方氮化硼面内异质结构的扫描隧道显微学研究

石墨烯-六方氮化硼面内异质结构因可调控石墨烯的能带结构而受到广泛关注. 本文介绍了在超高真空体系内, 利用两步生长法在两类对石墨烯分别有强和弱电子掺杂的基底, 即Rh(111)和Ir(111)上制备石墨烯-六方氮化硼单原子层异质结构. 通过扫描隧道显微镜及扫描隧道谱对这两种材料的形貌和电子结构进行研究发现: 石墨烯和六方氮化硼倾向于拼接生长形成单层的异质结构, 而非形成各自分立的畴区; 在拼接边界处, 石墨烯和六方氮化硼原子结构连续无缺陷; 拼接边界多为锯齿形型, 该实验结果与密度泛函理论计算结果相符合; 拼接界面处的石墨烯和六方氮化硼分别具有各自本征的电子结构, 六方氮化硼对石墨烯未产生电子掺杂效应.     

Effect of strain on geometric and electronic structures of graphene on Ru(0001) surface

The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations. Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately. Considering the strain induced by the lattice mismatch, we optimize the atomic structures and investigate the electronic properties of the graphene. Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N×N superlattice and the substrate. The strain does not induce significant changes in electronic structure. Furthermore, the results calculated in the local density approximation (LDA) are compared with those obtained in the generalized gradient approximation (GGA), showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.     

Intercalation of metals and silicon at the interface of epitaxial graphene and its substrates

Intercalations of metals and silicon between epitaxial graphene and its substrates are reviewed. For metal intercala- tion, seven different metals have been successfully intercalated at the interface of graphene/Ru（O001） and form different intercalated structures. Meanwhile, graphene maintains its original high quality after the intercalation and shows features of weakened interaction with the substrate. For silicon intercalation, two systems, graphene on Ru（O001） and on Ir（l I 1）, have been investigated. In both cases, graphene preserves its high quality and regains its original superlative properties after the silicon intercalation. More importantly, we demonstrate that thicker silicon layers can be intercalated at the interface, which allows the atomic control of the distance between graphene and the metal substrates. These results show the great potential of the intercalation method as a non-damaging approach to decouple epitaxial graphene from its substrates and even form a dielectric layer for future electronic applications.     

Deposition of metal clusters on single-layer graphene/Ru(0001): Factors that govern cluster growth

Fabrication of nanoclusters on a substrate is of great interest in studies of model catalysts. The key factors that govern the growth and distribution of metal on graphene have been studied by scanning tunneling microscopy (STM) based on different behaviors of five transition metals, namely Pt, Rh, Pd, Co, and Au supported on the template of a graphene moiré pattern formed on Ru(0001). Our experimental findings show that Pt and Rh form finely dispersed small clusters located at fcc sites on graphene while Pd and Co form large clusters at similar coverages. These results, coupled with previous findings that Ir forms the best finely dispersed clusters, suggest that both metal–carbon (M–C) bond strength and metal cohesive energies play significant roles in the cluster formation process and that the M–C bond strength is the most important factor that affects the morphology of clusters at the initial stages of growth. Furthermore, experimental results show Au behaves differently and forms a single-layer film on graphene, indicating other factors such as the effect of substrate metals and lattice match should also be considered. In addition, the effect of annealing Rh on graphene has been studied and its high thermal stability is rationalized in terms of a strong interaction between Rh and graphene as well as sintering via Ostwald ripening.     

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.     

Electronic structures and vibrational properties of coronene on Ru(0001): first-principles study

We calculate the configurations, electronic structures, vibrational properties at the coronene/Ru(0001) interface, and adsorption of a single Pt atom on coronene/Ru(0001) based on density functional theory calculations. The geometric structures and electronic structures of the coronene on Ru(0001) are compared with those of the graphene/Ru(0001). The results show that the coronene/Ru(0001) can be a simplified model system used to describe the interaction between graphene and ruthenium. Further calculations of the vibrational properties of coronene molecule adsorbed on Ru(0001) suggest that the phonon properties of differently corrugated regions of graphene on Ru(0001) are different. This model system is also used to investigate the selective adsorption of Pt atoms on graphene/Ru(0001). The configurations of Pt on coronene/Ru(0001) with the lowest binding energy give clues to explain the experimental observation that a Pt cluster selectively adsorbs on the second highest regions of graphene/Ru(0001). This work provides a simple model for understanding the adsorption properties and vibrational properties of graphene on Ru(0001) substrate.     

Graphene on Ru(0001): a 25 x 25 supercell

The structure of a single layer of graphene on Ru(0001) has been studied using surface x-ray diffraction. A surprising superstructure containing 1250 carbon atoms has been determined, whereby 25 x 25 graphene unit cells lie on 23 x 23 unit cells of Ru. Each supercell contains 2 x 2 crystallographically inequivalent subcells caused by corrugation. Strong intensity oscillations in the superstructure rods demonstrate that the Ru substrate is also significantly corrugated down to several monolayers and that the bonding between graphene and Ru is strong and cannot be caused by van der Waals bonds. Charge transfer from the Ru substrate to the graphene expands and weakens the C-C bonds, which helps accommodate the in-plane tensile stress. The elucidation of this superstructure provides important information in the potential application of graphene as a template for nanocluster arrays.     

Phonon dispersion of quasi-freestanding graphene on Pt(111)

High-resolution electron energy loss spectroscopy has been used to probe phonon dispersion in quasi-freestanding graphene epitaxially grown on Pt(111). Loss spectra clearly show different dispersing features related to both acoustic and optical phonons. The present results have been compared with graphene systems which strongly interact with the substrate, i.e. the nearly-flat monolayer graphene (MLG)/Ni(111) and the corrugated MLG/Ru(0001). We found that the phonon dispersion of graphene/Pt(111) reproduces well the behavior of pristine graphite. This could be taken as an indication of the negligible interaction between the graphene sheet and the underlying Pt substrate. The softening of out-of-plane modes observed for interacting graphene/metal interfaces does not occur for the nearly-free-standing graphene/Pt(111).     

Graphene-graphite phase transition at the surface of a carbonized metal

A phase transition leading to the transformation of a graphene layer into a multilayer graphite film at the surface of a carbonized metal has been experimentally studied on the atomic level under ultrahigh-vacuum conditions. It has been shown that this process is governed by dynamic equilibrium between edge atoms of graphene islands and a chemisorbed carbon phase, two-dimensional carbon “gas,” and is observed in the temperature range of 1000–1800 K. The features of the phase transition at the surfaces Ni(111), Rh(111), and Re(10-10) are similar, although the specific kinetic characteristics of the process depend on the properties of the substrate. It has been shown that change in the emissivity of the substrate after the formation of a multilayer graphite film increases the rate of the phase transition and leads to a temperature hysteresis.     

Graphene domain boundaries on Pt(111) as nucleation sites for Pt nanocluster formation

The growth of Pt nanoclusters on a graphene layer on Pt(111) was studied with ultra high vacuum scanning tunneling microscopy. Different periodicities in the moiré patterns of the graphene layer are observed corresponding to different orientations with respect to the Pt(111) lattice. Various graphene orientations are possible because of a relatively weak graphene–Pt interaction. Following Pt deposition onto the graphene-covered surface, small Pt nanoclusters are observed to preferentially form along the moiré domain boundaries. The weak interaction of graphene with Pt(111) leads to a weak corrugation in the superlattice compared to other transition metals, such as Ru, but it is found even this weak corrugation is sufficient to serve as a template for the formation of mono-dispersed one-dimensional Pt nanocluster chains. These Pt nanoclusters are relatively stable and only undergo agglomeration at annealing temperatures above 600 K.     

Electronic structures and coronene on Ru（0001）： vibrational properties of first-principles study

We calculate the configurations,electronic structures,vibrational properties at the coronene/Ru(0001) interface,and adsorption of a single Pt atom on coronene/Ru(0001) based on density functional theory calculations.The geometric structures and electronic structures of the coronene on Ru(0001) are compared with those of the graphene/Ru(0001).The results show that the coronene/Ru(0001) can be a simplified model system used to describe the interaction between graphene and ruthenium.Further calculations of the vibrational properties of coronene molecule adsorbed on Ru(0001) suggest that the phonon properties of differently corrugated regions of graphene on Ru(0001) are different.This model system is also used to investigate the selective adsorption of Pt atoms on graphene/Ru(0001).The configurations of Pt on coronene/Ru(0001) with the lowest binding energy give clues to explain the experimental observation that a Pt cluster selectively adsorbs on the second highest regions of graphene/Ru(0001).This work provides a simple model for understanding the adsorption properties and vibrational properties of graphene on Ru(0001) substrate.     

Graphene/Rh(111) Structure Studied Using In-Situ Scanning Tunneling Microscopy

Scanning tunnel microscopy(STM)is performed to verify if an Rh 'nails' structure is formed accompanying the graphene growing during chemical vapor deposition.A structure of a graphene island in an Rh vacancy island is used as the start.While the graphene island is removed by oxygenation,the variations of the Rh vacancy island are imaged with an in-situ high-temperature STM.By fitting with our model and calculations,we conclude that the best fit is obtained for 0%Rh,i.e.,for the complete absence of nails below graphene on Rh(111).That is,when graphene is formed on Rh(111),the substrate remains flat and does not develop a supporting nail structure.     

金衬底调控单层二硫化钼电子性能的第一性原理研究

基于密度泛函的第一性原理研究了金衬底对单层二硫化钼电子性能的调控作用. 从结合能、能带结构、电子态密度和差分电荷密度四个方面进行了深入研究. 结合能计算确定了硫原子层在界面的排布方式, 并指出这种吸附结构并不稳定. 能带结构分析证实了金衬底与单层二硫化钼形成肖特基接触, 并出现钉扎效应. 电子态密度分析表明金衬底并没有影响硫原子和钼原子之间的共价键, 而是通过调控单层二硫化钼的电子态密度增加其导电率. 差分电荷密度分析表明单层二硫化钼的导电通道可能在界面处产生. 研究结果可对单层二硫化钼晶体管的建模和实验制备提供指导.     

Influence of the substrate structure on the bonding of chemisorbed acetylene to transition metal surfaces

The electronic spectrum of acetylene adsorbed on various transition metals has been measured by ultraviolet photoemission spectroscopy in various laboratories. At low temperatures (T < 150 K), all measurements concur in finding an electron spectrum that differs only moderately from the gas phase spectrum of acetylene. At room temperature, the electron spectrum of acetylene is reported to be similar to the low-temperature form on Ir(100) and Pt(100), but acetylene is reported to form an olefinic surface complex on Pd(111) and Pt(111) surfaces. In order to examine whether the surface structure of the substrate is responsible for the difference, we have measured the electronic spectrum of acetylene adsorbed on the Pd(100) and Ru(0001) surfaces. At 120 K, the spectrum of adsorbed acetylene is again a distorted gas phase spectrum on both surfaces. At 330 K, we find the acetylenic form (with a splitting of 2.5 eV of the σ-orbitals) on Pd(100) and an olefinic form on the basal plane of Ru. We conclude that the olefinic complex is proper to the threefold symmetry of the (111) and (0001) surfaces and the gas-like form is favored on the (100) surfaces of the fcc crystals.     

金属衬底上高质量大面积石墨烯的插层及其机制

石墨烯作为一种新型二维材料,因其优异的性质,在科学和应用领域具有非常重要的意义.而其超高的载流子迁移率、室温量子霍尔效应等,使其在信息器件领域备受关注.如何获得高质量并且与当代硅基工艺兼容的石墨烯功能器件,是未来将石墨烯应用于电子学领域的关键.近年来,研究人员发展了一种在外延石墨烯和金属衬底之间实现硅插层的技术,将金属表面外延石墨烯高质量、大面积的特点与当代硅基工艺结合起来,实现了无需转移且无损地将高质量石墨烯置于半导体之上.通过系统的实验研究并结合理论计算,揭示了插层过程包含四个主要阶段:诱导产生缺陷、异质原子插层、石墨烯自我修复和异质原子扩散成膜,并证实了这一插层机制的普适性.拉曼和角分辨光电子能谱实验结果表明,插层后的石墨烯恢复了本征特性,接近自由状态.此外,还实现了多种单质元素的插层.不同种类的原子形成不同的插层结构,从而构成了多种石墨烯/插层异质结.这为调控石墨烯的性质提供了实验基础,也展现了该插层技术的普适性.     

Corrugated single layer templates for molecules: From <Emphasis Type="Italic">h</Emphasis>-BN nanomesh to graphene based quantum dot arrays

Functional nano-templates enable self-assembly of otherwise impossible arrangements of molecules. A particular class of such templates is that of sp ² hybridized single layers of hexagonal boron nitride or carbon (graphene) on metal supports. If the substrate and the single layer have a lattice mismatch, superstructures are formed. On substrates like rhodium or ruthenium these superstructures have unit cells with ～3-nm lattice constant. They are corrugated and contain sub-units, which behave like traps for molecules or quantum dots, which are small enough to become operational at room temperature. For graphene on Rh(111) we emphasize a new structural element of small extra hills within the corrugation landscape. For the case of molecules like water it is shown that new phases assemble on such templates, and that they can be used as “nano-laboratories” where many individual processes are studied in parallel. Furthermore, it is shown that the h-BN/Rh(111) nanomesh displays a strong scanning tunneling microscopy-induced luminescence contrast within the 3 nm unit cell which is a way to address trapped molecules and/or quantum dots.     

Electronic States and Schottky Barrier Height at Metal/MgO(100) Interfaces

Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.     

Electronic structure of Ge-GaAs(111) and (111) interfaces

The interface states of Ge-GaAs(111) and ($\text{111}$) heterojunctions are calculated by applying extended Hückel theory to a superlattice with alternating Ge and GaAs atomic layers. The band-edge discontinuity, interface bands, and local densities of states are presented. It is found that no interface states are revealed in the fundamental gaps of Ge and GaAs and that there is an appreciable difference in electronic structure between both kinds of interface.     

Investigation of the transport mechanism in graphene-induced metal-oxide-semiconductor capacitors (MOSCAP)

Graphene is the most promising contender for the future generation of electronic and photonic devices, based on its extraordinary properties. The effect of the metal interface with graphene, however, which completely alters its properties, is of great importance. The effects of the substrate supporting the graphene matrix, the graphene/metal contact resistance and the overall metal oxide semiconductor capacitors (MOSCAP) for possible CMOS circuitry have been thoroughly investigated in this research work. We have fabricated a structure with pertinent deposition techniques and performed a detailed electrical analysis to obtain the transport characteristics. Nickel (Ni) is chosen as the transition metal which makes the chemisorption bonding with graphene while qualifying as an interface. We present an analysis of the metal contacts, a study of the metal resistivity at various planes, a study of the graphene (carbon) atom's resistance at the atomistic scale, the graphene based MOSCAP leakages, the necessary charge accumulation at the metal–graphene interface and the charge inversion just beneath the oxide layer.     

Time-resolved two-photon photoemission of unoccupied electronic states of periodically rippled graphene on Ru(0001)

The unoccupied electronic states of epitaxially grown graphene on Ru(0001) have been explored by time- and angle-resolved two-photon photoemission. We identify a Ru derived resonance and a Ru/graphene interface state at 0.91 and 2.58?eV above the Fermi level, as well as three image-potential derived states close to the vacuum level. The most strongly bound, short-lived, and least dispersing image-potential state is suggested to have some quantum-well character with a large amplitude below the graphene hills. The two other image-potential states are attributed to a series of slightly decoupled states. Their lifetimes and dispersions are indicative of electrons moving almost freely above the valley areas of the moiré superstructure of graphene.