We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in concentrations up to 10 13 per square centimeter and with room-temperature mobilities of ∼10,000 square centimeters per volt-second can be induced by applying gate voltage.

http://science.sciencemag.org/content/sci/306/5696/666.full.pdf

Electric Field Effect in Atomically Thin Carbon Films

THE synthesis of molecular carbon structures in the form of C60 and other fullerenes1 has stimulated intense interest in the structures accessible to graphitic carbon sheets. Here I report the preparation of a new type of finite carbon structure consisting of needle-like tubes. Produced using an arc-discharge evaporation method similar to that used for fullerene synthesis, the needles grow at the negative end of the electrode used for the arc discharge. Electron microscopy reveals that each needle comprises coaxial tubes of graphitic sheets, ranging in number from 2 up to about 50. On each tube the carbon-atom hexagons are arranged in a helical fashion about the needle axis. The helical pitch varies from needle to needle and from tube to tube within a single needle. It appears that this helical structure may aid the growth process. The formation of these needles, ranging from a few to a few tens of nanometres in diameter, suggests that engineering of carbon structures should be possible on scales considerably greater than those relevant to the fullerenes. On 7 November 1991, Sumio Iijima announced in Nature the preparation of nanometre-size, needle-like tubes of carbon — now familiar as 'nanotubes'. Used in microelectronic circuitry and microscopy, and as a tool to test quantum mechanics and model biological systems, nanotubes seem to have unlimited potential.

Helical microtubules of graphitic carbon

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

Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

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

/pdf/global-views-on-advancing-renewable-energies-5a6uqmm9wb.pdf

Global Views on Advancing Renewable Energies

Shubnikov\char22{}de Haas (SdH) oscillations are reported for well-characterized (single stage) encapsulated potassium (stages $n=4,5,8$) and rubidium ($n=2,3,5,8$) graphite intercalation compounds. Shapes of the Fermi surface (FS) are deduced from the dependence of the FS cross sections on the angle between the $c$-axis of the sample and $\stackrel{\ensuremath{\rightarrow}}{H}$. The temperature dependence ($1.4lT\ensuremath{\le}25$ K) of the amplitudes of the SdH oscillations has been studied to find cyclotron effective masses for specific FS cross sections. A simple phenomenological energy-band model, based on the $\ensuremath{\pi}$ bands of pristine graphite and $c$-axis zone folding, is used to calculate SdH frequencies as a function of Fermi energy and is applied to interpret the stage- and intercalant-dependent experimental SdH frequencies and effective masses. The good agreement between the observed and predicted effective masses and the FS cross sections confirms the general validity of this model.

Shubnikov—de Haas measurements in alkali-metal—graphite intercalation compounds

We present a resonant Raman study of the tangential G-band in metallic SWNTs. By measuring the Raman spectra for isolated SWNTs, we show that the two different lineshapes observed for semiconducting and metallic SWNTs in bundles also occur for isolated SWNTs. A lineshape analysis of the tangential G-band feature for metallic SWNT bundles is presented, showing that only two components are present, the higher frequency component having a Lorentzian lineshape, and the lower one having a Breit–Wigner–Fano (BWF) line-shape. Through comparisons of the Raman tangential G-band spectra from three different diameter distributions of carbon nanotubes, we find that both the frequency and linewidth of the BWF component are diameter dependent.

Breit-Wigner-Fano Lineshape Analysis of the angential G -band of Metallic Nanotubes

Systematic investigations were performed of the thickness dependences of the thermoelectric properties of PbSe thin films, freshly prepared and exposed to air at room temperature. It is shown that oxidation leads to a sharp change in the thermoelectric properties of the PbSe films including a change in the sign of the dominant carrier type from n-type to p- type at d ≤ 80 nm. Using a two carrier model for thin films (d 50 nm) allows us to give a satisfactory qualitative interpretation of the observed experimental dependences of the thermoelectric properties on the film thickness.

Thermoelectric Properties of PbSe Epitaxial Thin Films and PbSe/EuS Heterostructures

The dependences of the thermoelectric properties of (001)KCl/PbTe/SnTe/PbTe three-layer structures on the SnTe layer thickness (dSnTe = 0.5–6.0 nm) at a fixed thickness of PbTe layers were studied. It was established that the thickness dependences of the Seebeck coefficient, the Hall coefficient, electrical conductivity, charge carrier mobility, and the thermoelectric power factor are distinctly non-monotonic. Two possible reasons for this non-monotonic behavior of the thickness dependences of the thermoelectric properties are considered: the size quantization of the energy spectrum in a SnTe quantum well and / or the formation of edge misfit dislocations at the interfaces after reaching the critical thickness, which corresponds to the transition from a pseudomorphic growth to the introduction of misfit dislocations at the interfaces. It is suggested that the observed effect has a general character and should be taken into account when optimizing thermoelectric properties of superlattices.

Mildred S. Dresselhaus

Papers

Global Views on Advancing Renewable Energies

Shubnikov—de Haas measurements in alkali-metal—graphite intercalation compounds

Breit-Wigner-Fano Lineshape Analysis of the angential G -band of Metallic Nanotubes

Thermoelectric Properties of PbSe Epitaxial Thin Films and PbSe/EuS Heterostructures

Thickness Dependences of the Thermoelectric Properties in (001)KCl/PbTe/SnTe/PbTe Heterostructures