Raman spectroscopy of graphene and graphite: Disorder, electron phonon coupling, doping and nonadiabatic effects

Microstructures and properties of high-entropy alloys

Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.

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Anomalous lattice vibrations of single- and few-layer MoS2.

Most electronic devices exploit the electric charge of electrons, but it is also possible to build devices that rely on other properties of electrons. Spintronic devices, for example, make use of the spin of electrons. Valleytronics is a more recent development that relies on the fact that the conduction bands of some materials have two or more minima at equal energies but at different positions in momentum space. To make a valleytronic device it is necessary to control the number of electrons in these valleys, thereby producing a valley polarization. Single-layer MoS(2) is a promising material for valleytronics because both the conduction and valence band edges have two energy-degenerate valleys at the corners of the first Brillouin zone. Here, we demonstrate that optical pumping with circularly polarized light can achieve a valley polarization of 30% in pristine monolayer MoS(2). Our results, and similar results by Mak et al., demonstrate the viability of optical valley control and valley-based electronic and optoelectronic applications in MoS(2) monolayers.

Valley polarization in MoS2 monolayers by optical pumping

A broad review of recent research work on the preparation and the remarkable properties of intercalation compounds of graphite, covering a wide range of topics from the basic chemistry, physics and materials science to engineering applications.

Intercalation compounds of graphite

The frequencies of certain normal modes of vibration of the graphite lattice have been studied on samples of high-quality pyrolytic graphite by coherent, inelastic-neutron-scattering techniques. Some of the data are not compatible with certain restrictions imposed by the valence-bond model as presented by Young and Koppel. Therefore, the data have been analyzed in terms of a simple axially symmetric, Born-von K\'arm\'an force-constant model. The results show that appreciable interactions exist between third nearest neighbors in the basal plane. The force model has been used to calculate a frequency distributio function and the lattice specific heat of graphite. These calculations are in excellent agreement with the specific heat measured for natural graphite in the temperature range 1.5-300\ifmmode^\circ\else\textdegree\fi{}K.

Lattice Dynamics of Pyrolytic Graphite

The complete phonon dispersion relations for gold in the high-symmetry directions have been measured at room temperature by the coherent inelastic scattering of neutrons. It is found that the forces in gold are not homologous with the other noble metals, the frequencies of gold lying appreciably higher than those "scaled" from copper and silver. An analysis of the data in terms of different force-constant models reveals that a general tensor force is required for the first-neighbor interaction, whereas for neighbors beyond the first either general tensor or axially symmetric forces give an excellent fit to the data. The axially symmetric model alone does not adequately describe the data even when forces extending to ninth-nearest neighbors are included in the fit. In addition, simple screened-pseudopoential models were fit to the data and these results also indicate the need for the first-neighbor interaction to be general. Frequency distribution functions and related thermodynamic quantities were calculated from the various force-constant models. The Debye temperature ${\ensuremath{\Theta}}_{C}$ versus temperature curves obtained show an anomaly at low temperatures consistent with the ${\ensuremath{\Theta}}_{C}(T)$ obtained from specific-heat measurements. The relation between this anomaly and the character of the dispersion curves is given.

Lattice Dynamics of Gold

The phonon dispersion relation for rutile (Ti${\mathrm{O}}_{2}$) has been measured by the coherent inelastic scattering of thermal neutrons along principal symmetry directions of the Brillouin zone. Theoretical models based on rigid-ion and shell models, with either axially symmetric or tensor first- and second-neighbor forces, have been fitted to the measured dispersion relation. Only the shell model with tensor forces for all interactions except the second-neighbor oxygen-oxygen interaction was able to give acceptable qualitative agreement with the data, and that agreement is good for only some modes. A frequency distribution and Debye temperature spectrum are presented for that model. The temperature dependence of the frequency of the ${\ensuremath{\Gamma}}_{1}^{\ensuremath{-}}({A}_{2u})$ transverse optic mode is measured from 4 to 300\ifmmode^\circ\else\textdegree\fi{}K, and the behavior of the square of the frequency is in good agreement with that predicted by the static dielectric constant measurements of Parker.

Lattice Dynamics of Rutile

Phonon-dispersion curves for the hexagonal-layered compound Mo${\mathrm{S}}_{2}$ have been measured at room temperature by inelastic-neutron-scattering techniques for the [100] and [001] directions. The results have been analyzed on the basis of a model which includes valence forces between atoms in a layer and axially symmetric forces between atoms in neighboring layers. While the model reproduces the essential over-all features of the data, the present analysis indicates the need for a more sophisticated treatment of valence forces and/or for the inclusion of the atomic polarizabilities. The temperature dependence of the specific heat and the Debye temperature have been calculated from the model.

Lattice dynamics of hexagonal Mo S 2 studied by neutron scattering

We have measured the phonon-dispersion relations in symmetry directions for $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$ at 90 \ifmmode^\circ\else\textdegree\fi{}K and InSb at 300 \ifmmode^\circ\else\textdegree\fi{}K. A limited number of measurements were made at 240 \ifmmode^\circ\else\textdegree\fi{}K in $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$ to look for effects associated with the phase transition, but none were found. The two dispersion relations are qualitatively similar, the main differences arising from the lower symmetry and a slight general drop in frequency in going from $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$ to InSb. Systematic trends in the comparison with other group-IV and III-V semiconductors can be seen. We have found second-neighbor shell models which give a reasonable description of the dispersion relations but in which the parameters are poorly defined and apparently devoid of physical meaning. The microscopic implications of this fact are discussed.

R. M. Nicklow

Papers

Lattice Dynamics of Pyrolytic Graphite

Lattice Dynamics of Gold

Lattice Dynamics of Rutile

Lattice dynamics of hexagonal Mo S 2 studied by neutron scattering

Lattice Dynamics of Grey Tin and Indium Antimonide