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

Topological insulators are new states of quantum matter which cannot be adiabatically connected to conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time-reversal symmetry. These topological materials have been theoretically predicted and experimentally observed in a variety of systems, including HgTe quantum wells, BiSb alloys, and Bi2Te3 and Bi2Se3 crystals. Theoretical models, materials properties, and experimental results on two-dimensional and three-dimensional topological insulators are reviewed, and both the topological band theory and the topological field theory are discussed. Topological superconductors have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. The theory of topological superconductors is reviewed, in close analogy to the theory of topological insulators.

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Topological insulators and superconductors

CONJUGATED polymers are organic semiconductors, the semiconducting behaviour being associated with the π molecular orbitals delocalized along the polymer chain. Their main advantage over non-polymeric organic semiconductors is the possibility of processing the polymer to form useful and robust structures. The response of the system to electronic excitation is nonlinear—the injection of an electron and a hole on the conjugated chain can lead to a self-localized excited state which can then decay radiatively, suggesting the possibility of using these materials in electroluminescent devices. We demonstrate here that poly(p-phenylene vinylene), prepared by way of a solution-processable precursor, can be used as the active element in a large-area light-emitting diode. The combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green–yellow part of the spectrum with reasonably high efficiency, suggest that the polymer can be used for the development of large-area light-emitting displays.

Light-emitting diodes based on conjugated polymers

The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.

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Polymer photovoltaic cells : enhanced efficiencies via a network of internal donor-acceptor heterojunctions

We present electron spin resonance studies of the magnetic defect in undoped polyacetylene. Temperature dependences of the absorption spectra of (CD)x and (CH)x over the range 2–295 K have been obtained. The properties of the magnetic defect in fully isomerized trans samples and in cis‐rich samples are compared. The data are consistent with a description in terms of a mobile neutral magnetic defect in the trans polymer and a stationary one in the cis‐rich polymer. The spin resonance linewidths are determined by unresolved hyperfine splittings resulting from interaction of the electron spin with many nuclear spins within the envelope of the spatially extended magnetic defect. The results of analyses of the detailed line shapes and linewidths are discussed in terms of the bond alternation domain wall (soliton) model of the magnetic defect.

Electron spin resonance studies of magnetic soliton defects in polyacetylene

Polymer light-emitting devices have been divided into two general types: polymer light-emitting diodes (PLEDs) and polymer light-emitting electrochemical cells (PLECs). The advantages of PLEDs include a fast response and relatively long operating lifetime (with proper packaging). However, low-work-function cathodes and/or thin interfacial layers (e.g., LiF) between the metal and the emitting polymer layer are required. In contrast, PLECs have relatively low turn-on voltages (approximately equal to the bandgap of the luminescent semiconducting polymer), and low work-function metals are not required. One of the serious disadvantages of PLECs, however, is the slow response time (the time required for the mobile ions to diffuse during junction formation). A solution to this problem is to “freeze” the junction after ion redistribution. A frozen junction system that operates at room temperature is necessary for practical use. A second limiting disadvantage of PLECs has been the short operating lifetimes compared with those of PLEDs. We report here the results of an initial study of light emission from a luminescent polymer blended with a dilute concentration of an ionic liquid. Even with an aluminum cathode, the devices turn on at low voltage (approximately equal to the bandgap of the luminescent semiconducting polymer). These ionic-liquid-containing LECs were operated continuously in the glove-box (without packaging) for several days without significant degradation in brightness. After sealing with epoxy and a glass cover slide, the ionic-liquid-containing LECs were operated continuously in air for several weeks. The major difference between PLEDs and PLECs is that the latter possess mobile ions inside the polymer; therefore, the selection of the mobile ions is one of the keys to fabricating high-performance PLECs. Previously, the mobile-ion systems that have been used fall into three categories. The first is polyethylene oxide (PEO) containing Li salts. Crown ethers (and derivatives) and other organic salts have also been used in combination with metal salts. Finally, polymers with ionic side chains (polyelectrolyte conjugated polymers) and appropriate mobile counterions have been used. For almost all PLECs, the additives comprise at least 5 wt %. More importantly, these systems involve two-component phase separation with the emitting polymer in one phase and the mobile ions (e.g., dissolved in PEO) in a second phase. To create the p-type–intrinsic–n-type (p-i-n) junction of the LEC, ions must move from one phase into the other; for example, from the PEO into the luminescent polymer. This phase separation appears to degrade the device performance, especially the lifetime. The phase separation results from the relatively poor compatibility of the ionic materials (hydrophilic) with the host light-emitting polymers (hydrophobic). In order to reduce the phase separation, surfactants or bifunctional additives were introduced into the emitting layer and better performance was reported. Single-component PLEC polymers have been fabricated using luminescent polymers with ionic side chains (polyelectrolyte conjugated polymers), but the electroluminescence (EL) was weak and the operating lifetimes were poor. In the devices described here, we utilized the simplest sandwich structure for the device configuration with poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonate) (PEDOT– PSS) coated indium tin oxide (ITO) glass as the anode and aluminum as the cathode. The well-known soluble phenylsubstituted poly(para-phenylene vinylene) (PPV) copolymer (“superyellow” from Merck/Covion) was selected as our host light-emitting polymer and an organic ionic liquid, methyltrioctylammonium trifluoromethanesulfonate (MATS), was used to introduce a dilute concentration of mobile ions into the emitting polymer layer. The molecular structure of MATS is shown in Figure 1a. The merits of MATS include its good solubility in common organic solvents, such as toluene, hexane, and acetonitrile, and its relatively high decomposition temperature (approximately 220 °C). Because MATS has a melting temperature of approximately 56 °C, frozen-junction devices can be prepared for operation at room temperature. For device fabrication, the materials were used as received without further purification. MATS and superyellow were both dissolved into toluene at a weight ratio of 1:50 in a nitrogen-filled glove-box (oxygen level under 3 ppm); the total concentration of the solution was 6 mg mL. Solid thin films were prepared by spin-casting from this solution in the glovebox. Superyellow and superyellow with 2 wt % MATS show strong photoluminescence (PL) both in solution and in the solid state with almost identical spectra. Figure 2 shows the C O M M U N IC A IO N

Long‐Lifetime Polymer Light‐Emitting Electrochemical Cells

The p-i-n junction in a polymer light-emitting electrochemical cell (LEC) is stabilized by cooling the device below the glass transition temperature of the ion-transport polymer. LECs with frozen p-i-n junctions exhibit typical light emitting diode (LED) behavior including diode rectification, unipolar light emission (same polarity as that used for generating the junction), and fast response. The freezeout of ion motion allows polymer LECs to be driven at bias voltages well beyond the electrochemical stability window, and thereby extends the potential applications of polymer LECs to high pixel density, column-row addressable displays.

Polymer light-emitting electrochemical cells with frozen p-i-n junction

Polyaniline with surfactant counterions : conducting polymer materials which are processible in the conducting form

The device performance of light‐emitting electrochemical cells is improved by adding a bifunctional liquid additive into the light‐emitting layer. Because of the surfactant‐like character of the additive, the light‐emitting layer exhibits a high surface area bicontinuous three‐dimensional network morphology. The semiconducting polymer forms a continuous network phase enabling electronic transport of injected electron and holes: the electrolyte forms a continuous network phase enabling fast ion transport; the nm length scale of the phase separated network enables rapid, effective transport of the ions into the conducting polymer during electrochemical doping.

Efficient, fast response light‐emitting electrochemical cells: Electroluminescent and solid electrolyte polymers with interpenetrating network morphology

Alan J. Heeger

Papers

Electron spin resonance studies of magnetic soliton defects in polyacetylene

Long‐Lifetime Polymer Light‐Emitting Electrochemical Cells

Polymer light-emitting electrochemical cells with frozen p-i-n junction

Polyaniline with surfactant counterions : conducting polymer materials which are processible in the conducting form

Efficient, fast response light‐emitting electrochemical cells: Electroluminescent and solid electrolyte polymers with interpenetrating network morphology