Development of force fields for novel 2D materials for temperature dependent vibrational properties
Author(s)
Advisor
Date
2019-09Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
A new era of nanodevice engineering has been started after fabricating graphene.
This motivated vast number of researches for predicting, fabricating and utilizing
2D materials. Temperature dependent properties are essential for device applications.
Although rigorous density functional theory based approaches are able
to predict electronic and mechanical properties accurately, but they are mostly
limited to zero temperature and ab initio based molecular dynamics are computationally
very demanding. Classical molecular dynamics is a very powerful
alternative, however its accuracy is basically depend on the interatomic potential
used for describing the considered system and therefore constructing accurate
force fields is always an open problem, especially for the emerging 2D materials
with extra ordinary properties. Single-layer transition metal dichalcogenides
(TMDs) are new class of 2D materials which are shown to be good candidates
for thermoelectric applications, flexible electronic and optoelectronic devices. In
order to investigate thermal properties of TMDs, Stillinger-Weber type potentials
are developed using particle swarm optimization method. These potentials are
validated by comparing the resulted phonon dispersion curves and thermal conductivities
with available first principle and experimental results. In addition, for
understanding the anharmonic effects imposed by the generated force fields the
trends of the shifts of the optical phonon frequencies at point with variation in
the temperature are compared with available experimental data. In all cases, optimized
potentials generate results which are in agreement with the target data.
In the second step, spectral energy density method together with phonon mode
decomposition is used for obtaining temperature dependent phonon frequencies
and lifetimes in entire Brillouin zone. The contribution of each phonon branch
in thermal conductivity is predicted utilizing the obtained phonon lifetimes and
group velocities within the framework of relaxation time approximation.
Eventually, with the aim of constructing transferable potentials for describing 2D and bulk structures, a very fast and reliable optimization method is presented.
Combining local and global optimization methods and utilizing the energy curves
obtained from first principle method, novel Stillinger-Weber type potentials for
graphene, silicene and group III nitrides are developed. The proposed approach
provides a solid framework for parameter selection and investigating the role of
each parameter in the resulted phonon dispersion curves.
Keywords
2D materialsMolecular dynamics
Interatomic potentials
Stillinger- Weber potential
Spectral energy density method
Thermal conductivity