Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

/pdf/electronics-and-optoelectronics-of-two-dimensional-2jbqr1unvw.pdf

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/p85.pdf

Advances in the science and technology of carbon nanotubes and their composites: a review

The one phonon Raman spectrum in microcrystalline silicon

In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanismsproton transfer, structural reorganization and diffusional motion of extended moietiesare discussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density of electronegative species (e.g., oxygen, nitrogen) and self-localization effects due to solvent interactions which allows for significant proton diffusivities only when assisted by the dynamics of the proton environment in Grotthuss and vehicle type mechanisms. In systems with high proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton transfers along extended hydrogen-bond chains as is frequently suggested in textbooks of physical chemistry. There is no indication for significant proton tunneling in fast proton conduction phenomena for which almost barrierless proton transfer is suggest...

Proton Conductivity: Materials and Applications

The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors

Analytical expressions for the velocities of the longitudinal and the torsional sound waves in single-walled carbon nanotubes are derived using Born's perturbation technique within a lattice-dynamical model. These expressions are compared to the formulas for the velocities of the sound waves in an elastic hollow cylinder from the theory of elasticity to obtain analytical expressions for the Young's and shear moduli of nanotubes. The calculated elastic moduli for different chiral and achiral ~armchair and zigzag! nanotubes using force constants of the valence force field type are compared to the existing experimental and theoretical data. vibration amplitude of several isolated MWNT's was ana- lyzed in a transmission electron microscope to eventually obtain 1.8 TPa for the average Young's modulus. Later on, this technique was applied to measure Young's modulus of isolated SWNT's in the diameter range 1.0 21.5 nm and an average value ^Y&51.2520.35/10.45 TPa was derived. 7 In another experimental approach 8 the MWNT's were pinned to a substrate by conventional lithography and the force was measured at different distances from the pinned point by atomic force microscope ~ATM!. The average Young's modulus for different MWNT's with diameters from 26 to 76 nm was found to be 1.2860.59 TPa. Recently, Young's and shear moduli of ropes of SWNT's were measured by sus- pending the ropes over the pores of a membrane and using ATM to determine directly the resulting deflection of the rope. 9 The theoretical estimation of the elastic moduli was accomplished exclusively by numerical second derivatives of the energy of the strained nanotubes. In the calculation of the elastic moduli of various SWNT's within a simple force- constant model 10 it was found that the moduli were insensi- tive to tube size and helicity and had the average values of ^Y&50.97 TPa and ^G&50.45 TPa. In several works, molecular-dynamics ~MD! simulation algorithms using the Tersoff-Brenner potential for the carbon-carbon interactions were implemented to relax the strained nanotubes and calcu- late their energy. 11-13 For tubes of diameter of 1 nm values for Y of 5.5 TPa ~Ref. 12! and 0.8 TPa ~Ref. 13! were ob- tained. A non-orthogonal tight-binding ~TB! scheme was ap- plied to calculate Young's modulus of several chiral and achiral SWNT's yielding an average value of 1.24 TPa. 14 Recently, the second derivative of the strain energy with re- spect to the axial strain, calculated with a pseudopotential density-functional-theory ~DFT! model for a number of SWNT's, 15 was found to vary slightly with the tube type and to have the average value of 56 eV. In this paper, we choose a different approach to the cal- culation of the elastic properties of SWNT's. Namely, we derive analytical expressions for the elastic ~Young's and shear! moduli of SWNT's using a perturbation technique due to Born 16 within a lattice-dynamical model for nanotubes. 17 This scheme has the advantage that the elastic moduli are consistent with the lattice dynamics of the nanotubes and that each of these moduli is obtained in one calculational step only. The essential features of a model of the lattice dynamics of SWNT's based on the explicit accounting for the helical symmetry of the tubes 17 are summarized in Sec. II A. This model is applied to study of the long-wavelength vibrations in nanotubes using Born's perturbation technique 16 and to obtain analytical expressions for the velocities of the longi- tudinal and the torsional sound waves in SWNT's ~see Sec. II B!. The comparison of these expressions with the formulas from the theory of elasticity for the velocities of these waves in an elastic hollow cylinder allows one to determine the Young's and shear moduli of the nanotubes. The calculated phonon dispersion of a (10,10) nanotube and elastic moduli for various chiral and achiral ~armchair and zigzag! nano- tubes using force constants of the valence force field ~VFF! type 18 are presented in Sec. III and discussed in comparison with the existing experimental and theoretical data.

/pdf/elastic-properties-of-single-walled-carbon-nanotubes-4u466rpfw4.pdf

Elastic properties of single-walled carbon nanotubes

Elastic properties of crystals of single-walled carbon nanotubes

The electronic band structure of the semiconducting \ensuremath{\beta} and \ensuremath{\gamma} polytypes of InSe is calculated from first principles, with spin-orbit effects included. The two polytypes differ by the lateral stacking arrangement of four-atomic-plane building blocks, composed of Se-In-In-Se planes. These building blocks are terminated by Se lone-pair orbitals, which form the electronic states at the valence-band maximum. There is very little bonding between these four-atomic-plane units, and so the band structures of the \ensuremath{\gamma} and \ensuremath{\beta} polytypes are nearly identical, being related simply by zone folding. We find that both materials have direct band gaps (which occur at the \ensuremath{\Gamma} and Z points in the Brillouin zone for the \ensuremath{\beta} and \ensuremath{\gamma} polytypes, respectively), and that there is a large dipole matrix element (i.e., a strong oscillator strength) between the valence maximum and conduction minimum states. These materials are thus potentially significant for light emitting and absorbing devices. In order to study the optical absorption at energies above the band gap, the imaginary part of the dielectric function ${\mathrm{\ensuremath{\epsilon}}}_{2}$(\ensuremath{\omega}) is calculated for \ensuremath{\omega}\ensuremath{\le}12 eV. The effective masses are also calculated: the out-of-plane masses are extremely light, and are equal in magnitude at the valence maximum and conduction minimum states (${\mathit{m}}_{\mathrm{\ensuremath{\perp}}}^{\mathit{c}}$=${\mathit{m}}_{\mathrm{\ensuremath{\perp}}}^{\mathit{v}}$\ensuremath{\approxeq}0.03${\mathit{m}}_{0}$), while the in-plane masses are light for the electrons (${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}^{\mathit{c}}$\ensuremath{\approxeq}0.1${\mathit{m}}_{0}$) and heavy for the holes (${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}^{\mathit{v}}$\ensuremath{\approxeq}3${\mathit{m}}_{0}$). The ``anomalous'' mass character (${\mathit{m}}_{\mathrm{\ensuremath{\perp}}}$${\mathit{m}}_{\mathrm{\ensuremath{\parallel}}}$) is explained in terms of intralayer bonding and the lack of lateral bonding between the Se lone-pair orbitals that consitute the valence maximum states.

First-principles study of the electronic structure of γ-InSe and β-InSe

An experimental and theoretical study of the lattice dynamics of MP${X}_{3}$ layered compounds (M=Mn,Fe,Ni,Zn; X=S,Se) has been carried out. We present room-temperature infrared and Raman spectra and the phonon dispersion curves calculated in the framework of an axially symmetric force-constant model. The parameters of the dynamical model have been fitted on the Raman and ir data, consistently with the conditions of static equilibrium and of vanishing stress. A satisfactory assignment of the vibrational bands and a qualitative explanation of the dynamical differences among the materials are obtained. Moreover, the theoretical results support the hypothesis of phonon branches' folding effects in the antiferromagnetic phase of ${\mathrm{FePS}}_{3}$ and provide the basis for an interpretive scheme of the vibrational spectra of lithium intercalated phases.

Lattice dynamics of layered MPX

Universite´Pierre et Marie Curie, 4 Place Jussieu, F-75252 Paris Cedex 05, France~Received 14 May 1998!In this paper, the theory of lattice dynamics of single-walled carbon nanotubes is presented The screwsymmetry of the system is used to reduce the rank of the dynamical matrix to six, independent of the numberof atoms in the unit cell Calculations of the lattice dynamics are carried out within a valence force ﬁeld modeland of the Raman intensity—within a bond-polarizability model It is found that the breathing mode frequencyis inversely proportional to the radius of the tube that can be used for the characterization of the samples on thebasis of Raman-scattering data The results for the Raman intensity are compared to available Raman spectra@S0163-1829~98!06434-0#

/pdf/lattice-dynamics-of-single-walled-carbon-nanotubes-q6rnly60tn.pdf

M. Balkanski

Papers

Elastic properties of single-walled carbon nanotubes

Elastic properties of crystals of single-walled carbon nanotubes

First-principles study of the electronic structure of γ-InSe and β-InSe

Lattice dynamics of layered MPX

Lattice dynamics of single-walled carbon nanotubes