Ab Initio Study of Structural and Electronic Properties of Hexagonal BC2N

We investigate hexagonal BC2N in graphite unit cells using the first-principles method and calculate the total energies, lattice parameters, and electronic band structures after full relaxation. It is shown that stable hexagonal BC2N should be stacked sequentially with one graphite layer and one h-BN layer. The density of states indicates that this structure should have metallicity.     

Highly-angle-resolved ultraviolet photoelectron spectroscopy of C8Cs

Highly-angle-resolved ultraviolet photoelectron spectroscopy was carried out for a C₈Cs single crystal to study the electronic charge transfer in alkali metal graphite intercalation compounds. The dispersive π¹-band at the K̃ point in the Brillouin zone was observed for the first time. The electron occupation in the π¹-band was estimated to be 0.45±0.05 unit electronic charge. This strongly suggests that a substantial part of an interlayer band exists below the Fermi level at the γ point, forming a spherical Fermi surface on the center of the Brillouin zone.     

Observation of unfilled electron states in alkali graphite intercalation compounds by secondary electron spectroscopy

The energy position of distinct σ-electron energy bands above the Fermi level has been measured in pure graphite, in a variety of stage 1 alkali intercalation compounds and in several stages of C_xK. Changes of the σ-band gap between occupied and unoccupied states near the Λ-point by a nonuniform shift of the valence- and conduction-bands are small for the heavy alkali graphite intercalation compounds, whereas a change of 1 eV is observed for C₆Li.     

Low energy excitations in graphite: The role of dimensionality and lattice defects

In this paper, we present a high resolution angle resolved photoemission spectroscopy (ARPES) study of the electronic properties of graphite. We found that the nature of the low energy excitations in graphite is particularly sensitive to interlayer coupling as well as lattice disorder. As a consequence of the interlayer coupling, we observed for the first time the splitting of the π bands by ≈0.7 eV near the Brillouin zone corner K. At low binding energy, we observed signatures of massless Dirac fermions with linear dispersion (as in the case of graphene), coexisting with quasiparticles characterized by parabolic dispersion and finite effective mass. We also report the first ARPES signatures of electron-phonon interaction in graphite: a kink in the dispersion and a sudden increase in the scattering rate. Moreover, the lattice disorder strongly affects the low energy excitations, giving rise to new localized states near the Fermi level. These results provide new insights on the unusual nature of the electronic and transport properties of graphite.     

Moiré superstructures of silicene on hexagonal boron nitride: A first-principles study

Using first-principles calculations, we predicted hexagonal boron nitride (h-BN) with flat surface is an ideal substrate for silicene. Van der Waals interactions hold silicene and h-BN together, forming silicene/BN moiré superstructures. The moiré superstructures open a band gap of about 30 meV at the Dirac point of silicene at equilibrium distance. The band gap is almost independent of the rotation angle between the two lattices, but can be effectively tuned by changing the interlayer spacing. The high Fermi velocity of silicene is well preserved in these superstructures. These features are helpful in achieving applications of silicene in nanoscale electronic devices.     

铜箔上生长的六角氮化硼薄膜的扫描隧道显微镜研究

利用扫描隧道显微镜研究了采用化学气相沉积法在铜箔表面生长出的高质量的六角氮化硼薄膜. 大范围的扫描隧道显微镜图像显示出该薄膜具有原子级平整的表面, 而扫描隧道谱则显示, 扫描隧道显微镜图像反映出的是该薄膜样品的隧穿势垒空间分布. 极低偏压的扫描隧道显微镜图像呈现了氮化硼薄膜表面的六角蜂窝周期性原子排列, 而高偏压的扫描隧道显微镜图像则呈现出无序和有序排列区域共存的电子调制图案. 该调制图案并非源于氮化硼薄膜和铜箔衬底的面内晶格失配, 而极有可能来源于两者界面处的氢、硼和/或氮原子在铜箔表面的吸附所导致的隧穿势垒的局域空间分布.     

The charge neutrality level and the fermi level pinning in A<Superscript>3</Superscript>N (BN,AlN, GaN,InN) nitrides

The electronic band structure and position of the charge neutrality level (CNL) in BN, AlN, GaN, and InN compounds with cubic and hexagonal lattices are calculated within the density functional theory (DFT-GGA). It is shown that the charge neutrality level is shifted from the middle of the BN and AlN forbidden band to the upper half of the GaN forbidden band and to the allowed energy region in the InN conduction band as the cation atomic weight increases. This determines semiinsulating properties of BN and AlN, n-type conductivity of GaN, and n ⁺-type conductivity of InN upon saturation of these materials by intrinsic lattice defects due to hard radiation. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 24–31, December, 2008.     

Magnetoreflection study of graphite intercalated with bromine

We report here the first observation of interband Landau level transitions in graphite intercalation compounds. Magnetoreflection measurements are reported for residue compounds of graphite intercalated with up to 1.2 at.% Br. The observed magnetoreflection resonances show evidence for domains in which the electronic band structure and Fermi energy are only slightly dependent on bromine concentration. These conclusions are in general agreement with recently reported de Haas-van Alphen results for these compounds.     

Electronic structure of graphite-like and rhombohedral boron nitride

The electronic spectra of the valence and conduction bands of the hexagonal graphite-like h-BN and rhombohedral r-BN modifications of boron nitride are presented. The electronic structures are calculated by the pseudopotential technique with an auxiliary parameter introduced which takes into account anisotropy of the crystal pseudopotential; the parameter is chosen to ensure agreement with the optical data for h-BN. The electronic structures of both modifications are qualitatively similar but differ slightly (up to 0.2 eV) with respect to interband energies. Both modifications are indirect-band dielectrics with minimal indirect and direct band gaps of 4.65 and 5.27 eV, respectively, for h-BN amd 4.8 and 5.5 eV for r-BN. The anisotropy of the electronic structure and its possible alterations on intercalation of the BN lattice with carbon are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 27–32, February, 1992.     

The electronic structure of Ca-intercalated superconducting graphite CaC6

We have measured Ca-intercalated graphite superconductor CaC₆ (T_c = 11.2 K) by soft X-ray photoemission spectroscopy in order to understand the electronic structure. For the valence band, we observed several structures that correspond to those of calculated density of states with the partial density of states of Ca 3d at the Fermi level (E_F). We also observed core level spectra that are a very large asymmetric Ca 2p and asymmetric C 1s for CaC₆, suggesting the existence of conduction electrons derived from Ca 3d and a charge transfer from Ca to graphene layer. These results provide spectroscopic evidence for PDOS of Ca 3d at E_F. From a comparison of electronic structure of CaC₆ and other graphite intercalation compounds (GICs), we found the difference between CaC₆ and other superconducting GICs, which provides deeper understanding of the superconductivity of CaC₆.     

Formation of quasi-free graphene on the Ni(111) surface with intercalated Cu,Ag, and Au layers

This paper reports on a study by angle-resolved photoelectron and low-energy electron energy loss spectroscopy of graphene monolayers, which are produced by propylene cracking on the Ni(111) surface, followed by intercalation of Cu, Ag, and Au atoms between the graphene monolayer and the substrate, for various thicknesses of deposited metal layers and annealing temperatures. It has been shown that the spectra of valence-band π states and of phonon vibrational modes measured after intercalation become similar to those characteristic of single-crystal graphite with weak interlayer coupling. Despite the strong coupling of the graphene monolayer to the substrate becoming suppressed by intercalation of Cu and Ag atoms, the π state branch does not reach at the K point of the Brillouin zone the Fermi level, with the graphene coating itself breaking up partially to form graphene domains. At the same time after intercalation of Au atoms, the electronic band structure approaches the closest to that of isolated graphene, with linear π-state dispersion near the K point of the Brillouin zone, and the point of crossing of the filled, (π), with empty, (π*), states lying in the region of the Fermi level, which makes this system a promising experimental model of the quasi-free graphene monolayer.     

Electronic structure of layered compounds

The electronic structure of the intercalated graphite compounds XC₆ (X = Ca, Sr, Ba, Yb, and La) has been studied using the linearized augmented plane-wave method. It has been found that the electronic structure of the carbon layers in these compounds is qualitatively different from a two-dimensional graphite structure. A lower critical superconducting-transition temperature in YbC₆, as compared with that in CaC₆, at a higher electron density in the carbon layers can be explained by the strong hybridization of the p states of carbon and the d states of ytterbium near the Fermi level. An increase in the critical temperature would be expected in the compounds XC₆ with Group III metals, for example, in LaC₆.     

Li掺杂少层MoS2的电荷分布及与石墨和氮化硼片的比较

基于第一性原理计算, 研究了Li掺杂的少层(1-3层) MoS₂的电荷分布, 并与石墨片和BN片的电荷分布特征进行了比较. 与石墨片和BN片相同的是: 电荷转移的大部分只发生在Li与最靠近Li的第一层MoS₂之间. 然而, 第二层和第三层MoS₂也能获得10%的转移电荷, 而石墨片和BN片的第二层和第三层得不到2%的电荷. 结合静电能和功函数的分析可知, MoS₂、石墨片和BN片的电荷分布主要由层间的静电相互作用和功函数来决定. 这些研究结果对于揭示具有多层结构的电荷分布特征及其电子器件的设计提供了理论支持.     

Electronic structure of superconducting KC8 and nonsuperconducting LiC6 graphite intercalation compounds: evidence for a graphene-sheet-driven superconducting state

We have performed photoemission studies of the electronic structure in LiC(6) and KC(8), a nonsuperconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC(8) than in LiC(6), opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and superconductivity monotonically increase with the filling of graphene π(*) states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.     

Conduction electron spin resonance in acceptor-type graphite intercalation compounds

An initial survey of the conduction electron spin resonance is presented for a series of graphite compounds intercalated with acceptor molecules: stages 1–3 AsF₅, stages 2–5 HNO₃, and stage 2 Br₂ and ICl. The g-values and lineshapes were studied as functions of temperature and concentration. The results suggest metallic behavior but with very small density of states at the Fermi energy: N(E_F) ～10²⁰/cm³ eV. The temperature dependence of the linewidth is dominated by an order-disorder transition of the intercalant layers, implying that the conduction electrons are not entirely confined to the graphite portion of the crystal. The decrease in g-value anisotropy upon intercalation can be understood in terms of Elliott's theory.     

First principle study of Li-intercalated (5, 5) ZnO nanotube bundles

We have investigated the geometric and electronic structure of Li-intercalated (5, 5) zinc oxide nanotube (ZnONT) bundles via density functional theory as implemented in the code WIEN2k. Our results show that the geometrical structures are changed because of intercalation of lithium. The effect of Li intercalation on the density of state and electronic band structure is a shift of the Fermi energy due to the charge transfer from lithium to the ZnONTs. Although, the bundle of clean (5, 5) ZnONTs is semiconductor, all the Li-intercalated (5, 5) ZnONT bundles are found to be metallic. Both inside of the nanotube and the interstitial spaces are susceptible for intercalation.     

In-situ photoelectron spectroscopy study of a TiS2 thin film cathode in an operating Na intercalation electrochemical cell

TiS₂ thin films were prepared and intercalated in UHV either chemically or electrochemically and investigated by photoelectron spectroscopy. The chemical reaction was induced by Na evaporation. For the electrochemical reaction, the film was deposited on a Na solid electrolyte and the voltage between the TiS₂ and a graphite layer on the back of the plate was controlled during PES investigations. With both methods the same effects on the substrate are observed. The Na Auger peak appears and increases at a kinetic energy typical for intercalated Na. Due to the electron transfer to the conduction band an increase of the electron density at the Fermi level is clearly observed. The progressive filling of higher energy states shifts the Fermi level as reference for the PES spectra, and as a result the S 2p core levels and valence bands are shifted to higher binding energies. A shoulder appears at the lower binding energy side of the Ti 2p peak, indicating a higher negative charge density on the Ti atoms. It is also shown how the in-situ electrochemical intercalation allows experimentally to extrapolate the variation of the ionic contribution to the battery voltage. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Phenomenological electronic energy bands in graphite intercalation compounds

The electronic energy bands near the Fermi level for both donor and acceptor graphite intercalation compounds are modelled using a k_z-axis zone folded form of the SWMcC bands for pristine graphite. The effect of intercalation is included through terms for the intercalant and for the interaction between intercalant and graphite layers. Graphitic π-bands appropriate to both donor and acceptor compounds are presented.     

Modification of the electronic structure of graphene by intercalation of iron and silicon atoms

The ab initio calculations of the electronic structure of low-dimensional graphene–iron–nickel and graphene–silicon–iron systems were carried out using the density functional theory. For the graphene–Fe–Ni(111) system, band structures for different spin projections and total densities of valence electrons are determined. The energy position of the Dirac cone caused by the p _z states of graphene depends weakly on the number of iron layers intercalated into the interlayer gap between nickel and graphene. For the graphene–Si–Fe(111) system, the most advantageous positions of silicon atoms on iron are determined. The intercalation of silicon under graphene leads to a sharp decrease in the interaction of carbon atoms with the substrate and largely restores the electronic properties of free graphene.