95 research outputs found
A DFT study of structural, dynamical properties and quasiparticle band structure of solid nitromethane
We report a detailed theoretical study of the structural, vibrational, and
optical properties of solid nitromethane using first principles density
functional calculations. The ground state properties were calculated using a
plane wave pseudopotential code with either the local density approximation
(LDA), the generalized gradient approximation (GGA), or with a correction to
include van derWaals interactions. Our calculated equilibrium lattice
parameters and volume using a dispersion correction are found to be in
reasonable agreement with the experimental results. Also, our calculations
reproduce the experimental trends in the structural properties at high
pressure. It was found to be a discontinuity in the bond length, bond angles
and also a weaking of hydrogen bond strength in the pressure range from 10 to
12 GPa, picturing the structural transition from phase I to Phase II. Moreover,
we predict the elastic constants of solid nitromethane and found that the
corresponding bulk modulus is in good agreement with experiments. The
calculated elastic constants are showing an order of C11> C22 > C33, indicating
that the material is more compressible along the c-axis. We also calculated the
zone center vibrational frequencies and discuss the internal and external modes
of this material under pressure. From this, we found the softing of lattice
modes around 8 to 12 GPa. We have also attempt the quasiparticle band structure
of solid nitromethane with the G0W0 approximation and found that nitromethane
is an indirect band gap insulator with a value of the band gap of about 7.8 eV
with G0W0 approximation. Finally, the optical properties of this material,
namely the absorptive and dispersive part of the dielectric function, and the
refractive index and absorption spectra are calculated and the contribution of
different transition peaks of the absorption spectra are analyzed.Comment: 12 pages, 9 figure
Phase Stability and Thermoelectric Properties of the Mineral FeS2: An Ab Initio Study
First principles calculations were carried out to study the phase stability
and thermoelectric properties of the naturally occurring marcasite phase of
FeS at ambient condition as well as under pressure. Two distinct density
functional approaches has been used to investigate the above mentioned
properties. The plane wave pseudopotential approach was used to study the phase
stability and structural, elastic, and vibrational properties. The full
potential linear augment plane wave method has been used to study the
electronic structure and thermoelectric properties. From the total energy
calculations, it is clearly seen that marcasite FeS is stable at ambient
conditions, and it undergoes a first order phase transition to pyrite FeS
at around 3.7 GPa with a volume collapse of about 3. The calculated ground
state properties such as lattice parameters, bond lengths and bulk modulus of
marcasite FeS agree quite well with the experiment. Apart from the above
studies, phonon dispersion curves unambiguously indicate that marcasite phase
is stable under ambient conditions. Further, we do not observe any phonon
softening across the marcasite to pyrite transition and the possible reason
driving the transition is also analyzed in the present study, which has not
been attempted earlier. In addition, we have also calculated the electronic
structure and thermoelectric properties of the both marcasite and pyrite
FeS. We find a high thermopower for both the phases, especially with p-type
doping, which enables us to predict that FeS might find promising
applications as good thermoelectric materials.Comment: 10 Figure
Modeling Interlayer Interactions and Phonon Thermal Transport in Silicene Bilayer
We develop an accurate interlayer pairwise potential derived from the
\textit{ab-initio} calculations and investigate the thermal transport of
silicene bilayers within the framework of equilibrium molecular dynamics
simulations. The electronic properties are found to be sensitive to the
temperature with the opening of the band gap in the M
direction. The calculated phonon thermal conductivity of bilayer silicene is
surprisingly higher than that of monolayer silicene, contrary to the trends
reported for other classes of 2D materials like graphene and hBN bilayers. This
counterintuitive behavior of the bilayer silicene is attributed to the
interlayer interaction effects and inherent buckling, which lead to a higher
group velocity in the LA/LA phonon modes. The thermal conductivity of
both the mono- and bilayer silicene decreases with temperature as because of the strong correlations between the characteristic
timescales of heat current autocorrelation function and temperature (). The mechanisms underlying phonon thermal transport in silicene
bilayers are further established by analyzing the temperature induced changes
in acoustic group velocity.Comment: To appear in Phys. Rev.
Predicting the reactivity of energetic materials : an ab initio multi-phonon approach
The ease with which an energetic material (explosives, propellants, and pyrotechnics) can be initiated is a critical parameter to as-sess their safety and application. Impact sensitivity parameters are traditionally derived experimentally, at great cost and risk to safety. In this work we explore a fully ab initio approach based on concepts of vibrational energy transfer to predict impact sensi-tivities for a series of chemically, structurally and energetically diverse molecular materials. The quality of DFT calculations is as-sessed for a subset of the materials by comparison with experimental inelastic neutron scattering spectra (INS). A variety of mod-els are considered, including both qualitative and quantitative analysis of the vibrational spectra. Excellent agreement against ex-perimental impact sensitivity is achieved by consideration of a multi-phonon ladder-type up-pumping mechanism that includes both overtone and combination pathways, and is improved further by the added consideration of temperature. This fully ab initio approach not only permits ranking of energetic materials in terms of their impact sensitivity but also provides a tool to guide the targeted design of advanced energetic compounds with tailored properties
Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics
Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement
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