Mildred S. Dresselhaus

Bio: Mildred S. Dresselhaus is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Carbon nanotube & Raman spectroscopy. The author has an hindex of 136, co-authored 762 publications receiving 112525 citations. Previous affiliations of Mildred S. Dresselhaus include University of California, Los Angeles & Universidade Federal de Minas Gerais.

Intensity of the resonance Raman excitation spectra of single-wall carbon nanotubes

Abstract: The electron-phonon matrix elements are calculated for the radial breathing mode (RBM) and the $G$-band $A$ symmetry mode of single-wall carbon nanotubes. The RBM intensity decreases with increasing nanotube diameter and chiral angle. The RBM intensity at van Hove singular $k$ points is larger outside the two-dimensional Brillouin zone around the $K$ point than inside the Brillouin zone. For the $G$ band $A$ symmetry mode, the matrix element shows that all semiconducting nanotubes have nonzero LO mode intensity, and the LO mode generally has a larger intensity than the TO mode, while the ratio of the intensity of the LO mode to that of the TO mode decreases with increasing chiral angle. In particular, zigzag nanotubes have zero intensity for the TO mode, and armchair nanotubes have zero intensity for the LO mode. Using the matrix elements thus obtained, the resonance Raman excitation profiles are calculated for nanotube samples under different broadening factor $\ensuremath{\gamma}$ regimes. For semiconducting nanotubes, the excitation profiles for the RBM are consistent with experiments. For metallic nanotubes, a quantum interference effect in the Raman intensity is found for both the RBM and LO modes. For the RBM and LO modes, different kinds of excitation profiles are discussed for nanotube samples in the large and small $\ensuremath{\gamma}$ regimes by considering the electron-phonon matrix element and the trigonal warping effect. For nanotube samples in the large $\ensuremath{\gamma}$ regime, a shift in the energy of the peak in the RBM intensity relative to the corresponding peak in the joint density of states is found.

Location of Electron and Hole Carriers in Graphite from Laser Magnetoreflection Data

Abstract: Electron and hole carriers location in pyrolytic graphite from magnetoreflection data, using circularly polarized radiation from IR gas laser source

Stable planar single-layer hexagonal silicene under tensile strain and its anomalous Poisson's ratio

Abstract: Here, we report the structural and mechanical properties of several two-dimensional (2-D) materials by using first-principles density functional theory calculations. We find that the buckled single-layer silicene could transit to planar hexagonal silicene at a critical tensile strain of 0.20. Phonon dispersion analysis suggests that the planar hexagonal silicene under tension is stable. The Poisson's ratio of silicene and MoS2 shows strong anisotropy: it increases while stretched in the zigzag direction, but decreases when strained in the armchair direction. When stretched in the zigzag direction, the Poisson's ratio of silicene could reach 0.62.

Role of Molecular Sieves in the CVD Synthesis of Large-Area 2D MoTe2

Abstract: The synthesis of high-quality 2D MoTe2 with a desired phase on SiO2/Si substrate is crucial to its diverse applications. A side reaction of Te with the substrate Si leading to SiTe and Si2Te3 tends to happen during growth, resulting in the failure to obtain MoTe2. It has been found that molecular sieves can adsorb the silicon telluride byproducts and eliminate the influence of the side reaction during the chemical vapor deposition synthesis of MoTe2. With the help of molecular sieves, few-layer 1T′ MoTe2 can be grown from the MoOx precursor. Pure 1T′ MoTe2 and 2H MoTe2 regions in centimeter-sized areas synthesized on the same piece of SiO2/Si substrate can be obtained by using an overlapped geometry. The strategy provides a new method to controllably synthesize MoTe2 with desired phases and can be generalizable to the synthesis of other tellurium-based layered materials.

Electrical properties of ion‐implanted poly(p‐phenylene sulfide)

Abstract: Ion implantation of impurities into thin films of poly(p-phenylene sulfide) (PPS) is found to increase the conductivity of the material by up to 12 orders of magnitude. The increase is stable under exposure to ambient conditions, in contrast to the instability of the conductivity increases in PPS produced by chemical doping with AsF5. PPS films 0.1–0.2 μm thick are spin cast from solution onto interdigitated electrodes patterned on an oxidized silicon substrate. The room-temperature interelectrode resistance is measured as a function of implantation fluence. An estimate of film conductivity is obtained from this resistance with a simple model for the electrode and film geometry. A first experiment yielded similar conductivity increases for implantation of either arsenic or krypton. At a fluence of 1 × 1016cm−;2, which corresponds to an average impurity concentration of 2.5 × 1021cm−3, the conductivity reaches an apparently saturated value of 1.5 × 10−5 (Ω cm)−1. Infrared spectra of the films before and after implantation suggest that crosslinking may be present in the implanted films, and Auger studies show stoichiometric changes throughout the implanted layer. These results suggest that the observed conductivity changes are the result of molecular rearrangements produced by the implantation rather than the result of specific chemical doping. Specific chemical doping may, however, explain the results of a second experiment in which implantation of bromine resulted in substantially larger conductivities found to increase at an approximate linear rate from a value of 1.0 × 10−4 (Ω cm)−1 at a fluence of 1 × 1016 cm−2 to a value of 4.0 × 10−4 (Ω cm)−1 at a fluence of 3.16 × 1016 cm−2.

Electric Field Effect in Atomically Thin Carbon Films

Abstract: 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.

Helical microtubules of graphitic carbon

Abstract: 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.

The rise of graphene

Abstract: 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.

Fast parallel algorithms for short-range molecular dynamics

Abstract: 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.

The electronic properties of graphene

Abstract: 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.