空位缺陷对金属基外延石墨烯态密度和电导率的影响

考虑到空位缺陷的存在和原子非简谐振动,以铜、镍基外延石墨烯为例, 研究了金属基外延石墨烯空位缺陷浓度和态密度以及电导率随温度的变化规律,探讨了空位缺陷的影响。结果表明：(1) 空位缺陷浓度随温度升高而非线性增大,外延石墨烯的空位缺陷浓度及其随温度的变化率均大于石墨烯; (2) 与石墨烯相同,金属基外延石墨烯的态密度变化曲线对电子能量为0为对称,但空位缺陷的存在使态密度在电子能量为零时的值不为零,空位缺陷对导带态密度的影响大于价带;态密度随空位缺陷浓度的增大而线性减小,但减小幅度不大,而温度对石墨烯态密度几乎无影响;（3）金属基外延石墨烯的电导率近似等于电子声子相互作用贡献的电导率,并随温度升高而非线性减小;空位缺陷的存在使电导率有所减小,但只在较高温度下才明显。原子非简谐振动情况的电导率稍大于简谐近似的电导率,温度愈高,两者电导率的差愈大,即非简谐效应愈显著。     

Engineering and metrology of epitaxial graphene

Here we review the concepts and technologies, in particular photochemical gating, which contributed to the recent progress in quantum Hall resistance metrology based on large scale epitaxial graphene on silicon carbide.     

Growth of epitaxial graphene: Theory and experiment

A detailed review of the literature for the last 5–10 years on epitaxial growth of graphene is presented. Both experimental and theoretical aspects related to growth on transition metals and on silicon carbide are thoroughly reviewed. Thermodynamic and kinetic aspects of growth on all these materials, where possible, are discussed. To make this text useful for a wider audience, a range of important experimental techniques that have been used over the last decade to grow (e.g. CVD, TPG and segregation) and characterize (STM, LEEM, etc.) graphene are reviewed, and a critical survey of most important theoretical techniques is given. Finally, we critically discuss various unsolved problems related to growth and its mechanism which we believe require proper attention in future research.     

Anomalous increase of thermopower in epitaxial graphene

Thermoelectric effect in epitaxial graphene formed on the surface of a semiconductor is considered in the context of the Davydov model. The approach based on the Kubo formula for the conductivity and differential thermopower is used. It is shown that near the edges of the semiconductor bandgap, the thermopower of epitaxial graphene increases by more than four times as compared to the thermopower near the Dirac point. A possible explanation of this effect is given.     

Angle-resolved mapping of the fermi velocity in a quasi-two-dimensional organic conductor

We demonstrate a new method for determining the Fermi velocity in quasi-two-dimensional (Q2D) conductors. Application of a magnetic field parallel to the conducting layers results in periodic open orbit quasiparticle trajectories along the Q2D Fermi surface. Averaging of this motion over the Fermi surface leads to a resonance in the interlayer microwave conductivity. The resonance frequency is simply related to the extremal value of the Fermi velocity perpendicular to the applied field. Thus, angle dependent microwave studies enable a complete mapping of the in-plane Fermi velocity. We illustrate the applicability of this method for the highly 2D organic conductor kappa-(BEDT-TTF)2I3.     

Role of graphene waviness on the thermal conductivity of graphene composites

The waviness effect of graphene nanoplates (GNPs) on the thermal conductivity of GNP-based composites is investigated. Two types of wrinkled GNPs (w-GNPs) and flat GNPs (f-GNPs) are used to fabricate GNP/epoxy composites. Thermal conductivity enhancement is observed in both types of composites. However, under the same processing, f-GNPs exhibit a higher thermal conductivity enhancement than w-GNPs. We finally introduce a concept, the waviness factor, to theoretically analyze the thermal conductivity considering the waviness effect of GNPs. The theoretical predictions are found to show good agreement with experimental observations.     

Space-time dispersion of graphene conductivity

We present an analytic calculation of the conductivity of pure graphene as a function of frequency ω, wave-vector k, and temperature for the range where the energies related to all these parameters are small in comparison with the band parameter γ≃3 eV, but much larger than the collision rate τ^-1. The simple asymptotic expressions are given in various limiting cases. For instance, the conductivity for kv₀≪ T≪ω is equal to σ(ω,k)=e²/4ħ and independent of the band structure parameters γ and v₀. Our results are also used to explain the known dependence of the graphite conductivity on temperature and pressure.     

Optical conductivity of multilayer graphene

The optical conductivity of graphene monolayer and multilayers are theoretically studied taking into account of the full dispersion, yielding results that are valid up to the ultraviolet region. Compared to the optical conductivity of monolayer graphene, extra peak structures appear in both the infrared and ultraviolet frequency regions for the multilayer case, which is understood as a combined effect of van Hove singularities in the energy band structures and optical transition selection rules. The number of ultraviolet peaks is equal to the number of layers. The number of infrared peaks in an even-layered graphene is equal to one half of the layer number. In odd-layered graphene, due to the existence of a monolayer-like subband, the number of infrared peaks is equal to that in the corresponding even-layered graphene with one layer less.     

On the specific heat of fermi liquids

The spin fluctuation contribution to theT ³ logT term in the specific heat of nearly ferromagnetic Fermi liquids is evaluted again. A result different from^1,2 is found, which in the case of liquid Helium agrees better with experiment.     

Substrate-induced bandgap in the spectrum of an epitaxial graphene layer

It has been shown that a bandgap can appear in the spectrum of a graphene bilayer formed on the surface of a semiconductor. Bandgap widths have been estimated for various SiC polytypes. The predicted effect is important for the practical applications of graphene.     

Effect of the Coulomb scattering on graphene conductivity

The effect of the Coulomb scattering on graphene conductivity in field-effect transistor structures is discussed. Interparticle scattering (electron-electron, hole-hole, and electron-hole) and scattering on charged defects are taken into account in a wide range of gate voltages. It is shown that an intrinsic conductivity of graphene (purely ambipolar system, where both electron and hole densities exactly coincide) is defined by a strong electron-hole scattering. It has a universal value independent of the temperature. We give an explicit derivation based on the scaling theory. When there is even a small discrepancy in the electron and hole densities caused by the applied gate voltage, the conductivity is determined by both a strong electron-hole scattering and a weak external scattering: on the defects or phonons. We suggest that the density of the charged defects (occupancy of defects) depends on the Fermi energy to explain the sublinear dependence of conductivity on a fairly high gate voltage observed in the experiments. We also eliminate the contradictions between the experimental data obtained in the deposited and suspended graphene structures regarding the graphene conductivity. The text was submitted by the authors in English.     

Optical conductivity of ABA-stacked trilayer graphene

The optical conductivity of a trilayer graphene is studied using the Kubo-Greenwood formula. We calculate the real part of the diagonal optical conductivity of an ABA-stacked trilayer graphene with different Fermi energies. The optical conductivity arises from interband matrix elements of the electric current operator involving the transitions from the occupied states to the unoccupied ones. We study the dependence of the real part of the diagonal optical conductivity on the photon energy, and the role of the transitions.     

Carrier relaxation in epitaxial graphene photoexcited near the Dirac point

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.     

To the theory of adsorption on epitaxial graphene: Model approach

A model of adsorption on epitaxial graphene has been constructed in two stages: first, the density of states of a graphene monolayer adsorbed on a solid substrate has been found and then an adsorbed atom has been placed on the epitaxial graphene thus formed. Metallic and semiconductor substrates have been considered. Charge transfer between the adatom and epitaxial graphene has been calculated. The roles of the substrate and graphene layer in the formation of the electronic state of adatoms have been estimated.     

Synthesis of epitaxial graphene on rhodium from 3-pentanone

The synthesis of high quality single layer graphene on rhodium, g/Rh(111), is reported. The graphene layers are grown at 1060 K by low pressure chemical vapor deposition (CVD) using 3-pentanone as a precursor molecule. The presented growth technique shows an easy high quality production method for epitaxial graphene monolayers. The chemical composition and structural properties of such self-assembled monolayers were characterized by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Scanning Tunneling Microscopy (STM) confirms the formation of a 3 nm super cell and a unique surface morphology which establishes the potential of g/Rh(111) as a template for molecules.     

Exciton spectra and electrical conductivity of epitaxial silicon-doped GaN layers

Optical spectra and electrical conductivity of silicon-doped epitaxial gallium nitride layers with uncompensated donor concentrations N _D — N _A up to 4.8 × 10¹⁹ cm^?3 at T ≈ 5 K have been studied. As follows from the current-voltage characteristics, at a doping level of ～3 × 10¹⁸ cm^?3 an impurity band is formed and an increase of donor concentration by one more order of magnitude leads to the merging of the impurity band with the conduction band. The transformation of exciton reflection spectra suggests that the formation of the impurity band triggers effective exciton screening at low temperatures. In a sample with N _D — N _A = 3.4 × 10¹⁸ cm^?3, luminescence spectra are still produced by radiation of free and bound excitons. In a sample with N _D — N _A = 4.8 × 10¹⁹ cm^?3, Coulomb interaction is already completely suppressed, with the luminescence spectrum consisting of bands deriving from impurity-band-valence band and conduction-band-valence band radiative transitions.     

Chemical approach to the realization of electronic devices in epitaxial graphene

We have shown that nitrophenyl groups may be added to the surface of few‐layer epitaxial graphene (EG) by the formation of covalent carbon–carbon bonds thereby changing the electronic structure and transport properties of EG from near‐metallic to semiconducting. In the present Letter we discuss the opportunities afforded by such chemical processes to engineer device functionality in graphene by modification of the electronic properties without physical patterning.

     

High frequency noise of epitaxial graphene grown on sapphire

Microwave noise technique is applied to study in‐plane electronic properties of epitaxial graphene grown on sapphire by chemical vapor deposition and subjected to high electric field applied in the plane. The noise spectrum is measured in the field direction at room temperature. While a 1/f^1.25‐type dependence is observed in the 200 MHz–2.5 GHz band, a shot noise contribution is resolved at 10 GHz. The shot noise is possibly associated with hole jumps across the potential barriers located in the graphene layer. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)