Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.

The rise of graphene

This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.

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The electronic properties of graphene

抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m(-1)) and -690 Nm(-1), respectively. The breaking strength is 42 N m(-1) and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of sigma(int) = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.

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Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene

Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

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Graphene: Status and Prospects

Photoconductivity in unoriented Shirakawa-type polyacetylene as well as in highly oriented Durham-Graz-type polyacetylene has been measured. Both cw and transient experiments on different time scales have been carried out. The existence and the behavior of a fast and a slow component of the photoconductivity have been investigated in detail and the relationship between the two components has been elucidated. For the fast component we have observed a relaxation time of ${\ensuremath{\tau}}_{1/2}$\ensuremath{\approxeq}100 ps, a migration distance of the carriers up to 400 A\r{}, and a mobility of \ensuremath{\mu}\ensuremath{\approxeq}2 ${\mathrm{cm}}^{2}$/V s. The mobility of the slow component is 2 orders of magnitude lower. In the oriented samples two kinds of anisotropy of the photocurrent have been found, one with respect to the direction of the applied electric field with a value of ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{\parallel}}}$/${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{\perp}}}$=50, and the other with respect to the polarization of the incident light. From a comparison of these data with those of photoinduced absorption measurements, we attribute the fast component mainly to polarons and the slow component to a hopping transport of charged solitonlike defects.

Photoconductivity in trans -polyacetylene: Transport and recombination of photogenerated charged excitations

In order to decide definitely on the dependence of the intensity of the Breit-Wigner-Fano (BWF) component with the size of the bundle, we have measured the radial breathing modes and tangential modes (TMs) of well defined metallic individual single-wall carbon nanotubes (SWCNTs) and individual SWCNT bundles. In this aim, a complete procedure including the preparation of the substrates, the sample preparation, atomic-force-microscopy imaging and Raman spectroscopy has been developed. From this procedure, we show unambiguously that the BWF component vanishes in isolated metallic SWCNTs. In other words, the observation of a BWF component in the TM bunch is an intrinsic feature of the metallic SWCNT bundle.

Vanishing of the Breit-Wigner-Fano component in individual single-wall carbon nanotubes.

We show how the main features of the electronic transport properties of single-wall carbon nanotube (SWNT) networks can be understood in terms of a simple model involving metallic conduction interrupted by thin tunnelling barriers, backscattering by zone-boundary phonons, and variable range hopping. Within this framework we examine the effect of reducing the thickness of the SWNT networks, chemical treatments, and ion irradiation. The conduction mechanism can be tuned from variable-range hopping between localized states (for the thinnest networks), to metallic conduction interrupted by thin barriers through which conduction is by tunnelling (for thick freestanding films). Chemical treatment of the thick films by different molecules leads to retention of metallic character but changes (increases or decreases) the charge carrier density. For ion-irradiated thick films we find two competing effects: a change to hopping-type conduction in the direct impact layer that lowers conductivity, and annealing effects extending deeper than the ion penetration depth that increase conductivity and lead to a peak in conductivity as a function of irradiation dose. We briefly discuss current–voltage characteristics and possible Luttinger liquid effects.

Modelling conduction in carbon nanotube networks with different thickness, chemical treatment and irradiation

The microwave and DC conductivities of several samples of trans- and cis-polyacetylene, doped with iodine to sub-metallic levels, were measured in the temperature range 8 to 300K. The DC conductivity shows Mott's T-14/-law at low temperatures where the microwave conductivity is up to two orders of magnitude higher. An energy-dependent hopping model is therefore employed in an analysis of the data. Care is taken to identify correctly the regime in which the T-14/-law may be applied. A qualitative description of the density of states is suggested by an analysis.

http://iopscience.iop.org/article/10.1088/0022-3719/17/21/009/pdf

S. Roth

Papers

Photoconductivity in trans -polyacetylene: Transport and recombination of photogenerated charged excitations

Vanishing of the Breit-Wigner-Fano component in individual single-wall carbon nanotubes.

Modelling conduction in carbon nanotube networks with different thickness, chemical treatment and irradiation

DC and microwave conductivity of iodine-doped polyacetylene

129I-Mössbauer study of iodine-doped polyacetylene