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.     

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.     

First principles study of hafnium intercalation between graphene and Ir(111) substrate

The intercalation of heteroatoms between graphene and metal substrates is a promising method for integrating epitaxial graphene with functional materials. Various elements and their oxides have been successfully intercalated into graphene/metal interfaces to form graphene-based heterostructures, showing potential applications in electronic devices. Here we theoretically investigate the hafnium intercalation between graphene and Ir(111). It is found that the penetration barrier of Hf atom is significantly large due to its large atomic radius, which suggests that hafnium intercalation should be carried out with low deposition doses of Hf atoms and high annealing temperatures. Our results show the different intercalation behaviors of a large-size atom and provide guidance for the integration of graphene and hafnium oxide in device applications.     

Mechanical exfoliation of epitaxial graphene on Ir(111) enabled by Br2 intercalation

We show here that Br(2) intercalation is an efficient method to enable exfoliation of epitaxial graphene on metals by adhesive tape. We exemplify this method for high-quality graphene of macroscopic extension on Ir(111). The sample quality and the transfer process are monitored using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), scanning electron microscopy (SEM) and Raman spectroscopy. The developed process provides an opportunity for preparing graphene of strictly monatomic thickness and well-defined orientation including the transfer to poly(ethylene terephthalate) (PET) foil.     

Interaction of two symmetric monovacancy defects in graphene

We investigate the interactions between two symmetric monovacancy defects in graphene grown on Ru(0001) after silicon intercalation by combining first-principles calculations with scanning tunneling microscopy(STM). First-principles calculations based on free-standing graphene show that the interaction is weak and no scattering pattern is observed when the two vacancies are located in the same sublattice of graphene, no matter how close they are, except that they are next to each other. For the two vacancies in different sublattices of graphene, the interaction strongly influences the scattering and new patterns' emerge, which are determined by the distance between two vacancies. Further experiments on silicon intercalated graphene epitaxially grown on Ru(0001) shows that the experiment results are consistent with the simulated STM images based on free-standing graphene, suggesting that a single layer of silicon is good enough to decouple the strong interaction between graphene and the Ru(0001) substrate.     

Scalable synthesis of graphene on single crystal Ir(111) films

We have investigated single crystal Ir(111) films grown heteroepitaxially on Si(111) wafers with yttria-stabilized zirconia (YSZ) buffer layers as possible substrates for an up-scalable synthesis of graphene. Graphene was grown by chemical vapor deposition (CVD) of ethylene. As surface analytical techniques we have used scanning tunneling microscopy (STM), low-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The mosaic spread of the metal films was below 0.2° similar to or even below that of standard Ir bulk single crystals, and the films were basically twin-free. The film surfaces could be improved by annealing so that they attained the perfection of bulk single crystals. Depending on the CVD conditions a lattice-aligned graphene layer or a film consisting of different rotational domains were obtained. STM data of the non-rotated phase and of the phases rotated by 14° and 19° were acquired. The quality of the graphene was comparable to graphene grown on bulk Ir(111) single crystals.     

Scanning Tunneling Spectroscopy of Lithium-Decorated Graphene

Lithium decoration of graphene on SiC(0001) is achieved in a surface science approach by intercalation and adsorption of the alkali metal. Spectroscopy of the differential conductance with a scanning tunneling microscope at the Li-decorated graphene surfaces does not give rise to a pairing gap at the Fermi energy, which may be expected because of the previously predicted superconducting phase [Profeta et al., Nat. Phys. 2012 , 8, 131]. Rather, pronounced gaps in the spectroscopic data of intercalated samples reflect the excitation of graphene phonons. Rationales that possibly explain this discrepancy between experimental findings and theoretical predictions are suggested.     

Intercalation of Iron Atoms under Graphene Formed on Silicon Carbide

The intercalation of iron under a graphene monolayer grown on 4H-SiC(0001) is studied. The experiments have been carried out in situ under conditions of ultrahigh vacuum by low-energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and near carbon K-edge X-ray absorption spectroscopy. The deposited iron film thicknesses have been varied within 0.1–2 nm and the sample temperatures from room temperature to 700°C. It is shown that the intercalation process begins at temperatures higher than ~350°C. In this case, it is found that intercalated iron atoms are localized not only between graphene and a buffer layer coating SiC, but also under the buffer layer itself. The optimal conditions of the intercalation are realized in the range 400–500°C, because, at higher temperatures, the system becomes unstable due to the chemical interaction of the intercalated iron with silicon carbide. The inertness of the intercalated films to action of oxygen is demonstrated.

     

三层石墨烯及其n型和p型插层化合物的制备和拉曼光谱表征

我们利用微机械剥离方法制备了三层石墨烯.在此基础上,利用两室气体传输法,以三氯化铁和钾为化学掺杂剂,成功合成了三层石墨烯的一阶p型和n型插层化合物.三层石墨烯的高分辨率拉曼光谱具有独特的2D谱峰线形,该线形可以用作指纹来鉴别三层石墨烯.三层石墨烯一阶插层化合物的拉曼光谱表明,三氯化铁和钾的插层掺杂使得三层石墨烯的层间耦...     

Ultrahigh-vacuum STM studies of the structure of a graphene layer on the Ir(111) surface

The atomic structure of a two-dimensional graphite film formed on the Ir(111) surface is studied. In order to weaken the interaction of the graphite film with the metal surface, cesium atoms are intercalated beneath the film. The studies by scanning tunneling microscopy in ultrahigh vacuum provide support for the formation of a continuous graphite layer, with the regular arrangement of carbon atoms at extended surface regions. From comparison of the roughness of the film and that of the substrate, it follows that the spacing between the film and metal surface varies within the limits of 1 nm. Characteristic structural defects of configuration (5, 7) are detected in the film.     

The adsorption behaviours of Pt adatom on pristine and defective bilayer graphene

The structural stability and electronic property of metal Pt atom anchors on two typical substrates (including the pristine and defective bilayer graphene, PBG and DBG) are studied using the first-principles calculations. For the PBG sheets, the Pt atom at the bridge site of bottom layer has only one stable adsorption, which is more stable than other sites of the top layer. For the DBG sheets, the doped Pt below defective site has the larger adsorption energy than that of the upper one. Compared to the isolated graphene films, the Pt(111) substrate-supported graphene systems have effect on the adsorption energies of Pt adatom to some extent, but it does not affect the most preferable configurations. Moreover, the diffusion pathways and energy barriers of Pt adatom on PBG and DBG substrates are comparatively investigated. For the DBG sheets, the Pt dopant has smaller diffusion barrier on upper layer than that of the intercalation process through the defective site. Therefore, the Pt dopant prefers to diffuse on the top layer and then forms the metal impurity. This work provides valuable information on understanding the formation process and intercalation mechanism of metal adatom on graphene sheets.     

Comparative study on graphene growth mechanism using Ni films,Ni/Mo sheets,and Pt substrates

We demonstrate a comparative study on graphene growth mechanism using various catalytic metal substrates such as Ni thin films, Ni-deposited Mo (Ni/Mo) sheets, and Pt sheets during chemical vapor deposition (CVD). Depending on the substrates, two kinds of graphene growth mechanisms that involve either precipitation or surface adsorption of carbon have been reported. We synthesized graphene, focusing especially on the initial growth stage during CVD, by varying synthesis parameters such as synthesis time, amount of feedstock, and cooling rate after synthesis. We concluded that precipitation-driven synthesis is dominant in the case of Ni substrates whereas adsorption-driven growth is dominant in the Ni/Mo system. In the case of the Pt substrate, which is generally believed to grow by carbon precipitation, graphene growth by adsorption was found to be dominant. We believe that our results will contribute to a clearer understanding of the graphene synthesis mechanism, and development of manufacturing routes for controllable synthesis of high-quality graphenes.     

Wave-function mapping of graphene quantum dots with soft confinement

Using low-temperature scanning tunneling spectroscopy, we map the local density of states of graphene quantum dots supported on Ir(111). Because of a band gap in the projected Ir band structure around the graphene K point, the electronic properties of the QDs are dominantly graphenelike. Indeed, we compare the results favorably with tight binding calculations on the honeycomb lattice based on parameters derived from density functional theory. We find that the interaction with the substrate near the edge of the island gradually opens a gap in the Dirac cone, which implies soft-wall confinement. Interestingly, this confinement results in highly symmetric wave functions. Further influences of the substrate are given by the known moiré potential and a 10% penetration of an Ir surface resonance into the graphene layer.     

Synthesis and characterization of SWNT-heavy alkali metal intercalation compounds,effect of host SWNTs materials

Singlewall carbon nanotubes (SWNTs) produced by electric-arc and laser ablation methods were characterized by X-ray diffraction before and after the reaction with alkali metals (M=K, Rb, and Cs). Reaction with annealed SWNTs gave MC₈ composition at saturation. The alkali metal lattice showed short range order incommensurate with graphene cylinders of SWNTs. X-ray diffractogram simulations have enabled the study of the influence of SWNTs structure on that of intercalation compounds. Chemically-purified bundles, constituted of open SWNTs, can be intercalated inside and between the tubes forming disordered structures. Annealed or pristine bundles were intercalated only between the tubes leading to short or long range ordered structure depending on host crystallinity and alkali metal (K, Rb or Cs). The expansion of the 2D SWNTs lattice after intercalation is comparable to graphite intercalation compounds. Some 2D arrangements of SWNTs and K atoms are proposed and discussed to reproduce XRD results. ¹³C NMR and ESR studies of annealed doped SWNTs emphasize the fact that the intercalation compounds of SWNTs are metallic.     

On the charge transfer in the “single-sheet graphene-intercalated metal layer-SiC substrate” system

The proposed scheme for the consideration of charge transfer in the three-layer Gr/Me/SiC system (where Gr is a single-sheet graphene, Me is an intercalated metal layer, and SiC is a substrate) contains three stages. At the first stage, a metal monolayer adsorbed on silicon carbide is considered and the charge of adatoms in this monolayer is calculated. At the second stage, the shift of the Dirac point of free-standing single-layer graphene in an electrostatic field induced by charged adatoms of the monolayer is estimated. At the third stage, a weak interaction between Me/SiC and free-standing graphene is included, which allows electrons to tunnel but does not significantly distort the density of states of free-standing graphene. Estimations are performed for n- and p-type 6H-SiC(0001) substrates and Cu, Ag, and Au layers. The charge state of the graphene sheet and the shift of the Dirac point with respect to the Fermi level of the system are calculated. A comparison with the available experimental and theoretical results shows that the proposed scheme works quite satisfactorily.     

Structural,electronic,and magnetic behaviors of 5d transition metal atom substituted divacancy graphene:A first-principles study

Structural, electronic, and magnetic behaviors of 5d transition metal(TM) atom substituted divacancy(DV) graphene are investigated using first-principles calculations. Different 5d TM atoms(Hf, Ta, W, Re, Os, Ir, and Pt) are embedded in graphene, these impurity atoms replace 2 carbon atoms in the graphene sheet. It is revealed that the charge transfer occurs from 5d TM atoms to the graphene layer. Hf, Ta, and W substituted graphene structures exhibit a finite band gap at high symmetric K-point in their spin up and spin down channels with 0.783 μB, 1.65 μB, and 1.78 μB magnetic moments,respectively. Ir and Pt substituted graphene structures display indirect band gap semiconductor behavior. Interestingly, Os substituted graphene shows direct band gap semiconductor behavior having a band gap of approximately 0.4 e V in their spin up channel with 1.5 μB magnetic moment. Through density of states(DOS) analysis, we can predict that d orbitals of 5d TM atoms could be responsible for introducing ferromagnetism in the graphene layer. We believe that our obtained results provide a new route for potential applications of dilute magnetic semiconductors and half-metals in spintronic devices by employing 5d transition metal atom-doped graphene complexes.     

Oxidation of graphene on Ru(0 0 0 1) studied by scanning tunneling microscopy

The oxidation of graphene layer on Ru(0 0 0 1) has been investigated by means of scanning tunneling microscopy. Graphene overlayer can be formed by decomposing ethyne on Ru(0 0 0 1) at a temperature of about 1000 K. The lattice mismatch between the graphene overlayer and the substrate causes a moiré pattern with a superstructure in a periodicity of about 30 Å. The oxidation of graphene/Ru(0 0 0 1) was performed by exposure the sample to O₂ gas at 823 K. The results showed that, at the initial stage, the oxygen intercalation between the graphene and the Ru(0 0 0 1) substrate takes place at step edges, and extends on the lower steps. The oxygen intercalation decouples the graphene layer from the Ru(0 0 0 1) substrate. More oxygen intercalation yields wrinkled bumps on the graphene surface. The oxidation of graphene, or the removal of carbon atoms can be attributed to a process of the combination of the carbon atoms with atomic oxygen to form volatile reaction products. Finally, the Ru(0 0 0 1)-(2 × 1)O phase was observed after the graphene layer is fully removed by oxidation.     

First-principles study of lithium intercalated bilayer graphene

Lithium intercalated bilayer graphene has been investigated using first-principles density functional theory calculations. Results show that there exist AB and AA stacking sequences for bilayer graphene in which the latter is more favorable for the Li storage and the former will evolve into the latter with the intercalation of Li ions. The relationship between the interlayer distance of two graphene sheets and the intercalated capacity of Li ions is discussed. It is found that structural defect is identified to store Li ions more favorably than pristine bilayer graphene and an isolated C atom vacancy in bilayer graphene can capture three Li ions between two graphene sheets.     

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.     

Two-dimensional Ir cluster lattice on a graphene moiré on Ir(111)

Lattices of Ir clusters have been grown by vapor phase deposition on graphene moirés on Ir(111). The clusters are highly ordered, and spatially and thermally stable below 500 K. Their narrow size distribution is tunable from 4 to about 130 atoms. A model for cluster binding to the graphene is presented based on scanning tunneling microscopy and density functional theory. The proposed binding mechanism suggests that similar cluster lattices might be grown of materials other than Ir.