173 research outputs found
Phonon transport and thermal conductivity in two-dimensional materials
Two-dimensional materials, such as graphene, boron nitride and transition
metal dichalcogenides, have attracted increased interest due to their potential
applications in electronics and optoelectronics. Thermal transport in
two-dimensional materials could be quite different from three-dimensional bulk
materials. This article reviews the progress on experimental measurements and
theoretical modeling of phonon transport and thermal conductivity in
two-dimensional materials. We focus our review on a few typical two-dimensional
materials, including graphene, boron nitride, silicene, transition metal
dichalcogenides, and black phosphorus. The effects of different physical
factors, such as sample size, strain and defects, on thermal transport in
Two-dimensional materials are summarized. We also discuss the environmental
effect on the thermal transport of two-dimensional materials, such as substrate
and when two-dimensional materials are presented in heterostructures and
intercalated with inorganic components or organic molecules.Comment: 67 pages, 18 figures. Submitted to Annual Review of Heat Transfe
Temperature Effect on Phonon Dispersion Stability of Zirconium by Machine Learning-driven Atomistic Simulations
It is well known that conventional harmonic lattice dynamics cannot be
applied to energetically unstable crystals at 0 K, such as high temperature
body centered cubic (BCC) phase of crystalline Zr. Predicting phonon spectra at
finite temperature requires the calculation of force constants to the third,
fourth and even higher orders, however, it remains challenging to determine to
which order the Taylor expansion of the potential energy surface for different
materials should be cut off. Molecular dynamics, on the other hand,
intrinsically includes arbitrary orders of phonon anharmonicity, however, its
accuracy is severely limited by the empirical potential field used. Using
machine learning method, we developed an inter-atomic potential for Zirconium
crystals for both the hexagonal closed-packed (HCP) phase and the body centered
cubic phase. The developed potential field accurately captures energy-volume
relationship, elastic constants and phonon dispersions. The instability of BCC
structure is found to originate from the double-well shape of the potential
energy surface where the local maxima is located in an unstable equilibrium
position. The stabilization of the BCC phase at high temperature is due to the
dynamical average of the low-symmetry minima of the double well due to atomic
vibrations. Molecular dynamics simulations are then performed to stochastically
sample the potential energy surface and to calculate the phonon dispersion at
elevated temperature. The phonon renormalization in BCC-Zr is successfully
captured by the molecular dynamics simulation at 1188 K
Phonon Transport in Single-Layer Transition Metal Dichalcogenides: a First-Principles Study
Two-dimensional transition metal dichalcogenides (TMDCs) are finding
promising electronic and optical applications due to their unique properties.
In this letter, we systematically study the phonon transport and thermal
conductivity of eight semiconducting single-layer TMDCs, MX2 (M=Mo, W, Zr and
Hf, X=S and Se), by using the first-principles-driven phonon Boltzmann
transport equation approach. The validity of the single-mode relaxation time
approximation to predict the thermal conductivity of TMDCs is assessed by
comparing the results with the iterative solution of the phonon Boltzmann
transport equation. We find that the phononic thermal conductivities of 2H-type
TMDCs are above 50 W/mK at room temperature while the thermal conductivity
values of the 1T-type TMDCs are much lower, when the size of the sample is 1
{\mu}m. A very high thermal conductivity value of 142 W/mK was found in
single-layer WS2. The large atomic weight difference between W and S leads to a
very large phonon bandgap which in turn forbids the scattering between acoustic
and optical phonon modes and thus resulting in very long phonon relaxation
time.Comment: 21 pages, 7 figure
Tutorial: Time-domain thermoreflectance (TDTR) for thermal property characterization of bulk and thin film materials
Measuring thermal properties of materials is not only of fundamental
importance in understanding the transport processes of energy carriers
(electrons and phonons) but also of practical interest in developing novel
materials with desired thermal conductivity for applications in energy,
electronics, and photonic systems. Over the past two decades, ultrafast
laser-based time-domain thermoreflectance (TDTR) has emerged and evolved as a
reliable, powerful, and versatile technique to measure the thermal properties
of a wide range of bulk and thin film materials and their interfaces. This
tutorial discusses the basics as well as the recent advances of the TDTR
technique and its applications in the thermal characterization of a variety of
materials. The tutorial begins with the fundamentals of the TDTR technique,
serving as a guideline for understanding the basic principles of this
technique. A diverse set of TDTR configurations that have been developed to
meet different measurement conditions are then presented, followed by several
variations of the TDTR technique that function similarly as the standard TDTR
but with their own unique features. This tutorial closes with a summary that
discusses the current limitations and proposes some directions for future
development.Comment: 82 pages, 23 figures, invited tutorial submitted to Journal of
Applied Physic
Lattice Thermal Conductivity of Organic-Inorganic Hybrid Perovskite CH3NH3PbI3
Great success has been achieved in improving the photovoltaic energy
conversion efficiency of the organic-inorganic perovskite-based solar cells,
but with very limited knowledge on the thermal transport in hybrid perovskites,
which would affect the device lifetime and stability. Based on the potential
developed from the density functional theory calculations, we studied the
lattice thermal conductivity of the hybrid halide perovskite CH3NH3PbI3 using
equilibrium molecular dynamics simulations. Temperature-dependent thermal
conductivity is reported from 160 K to 400 K, which covers the tetragonal phase
(160-330 K) and the pseudocubic phase (>330K). A very low thermal conductivity
(0.50 W/mK) is found in the tetragonal phase at room temperature, whereas a
much higher thermal conductivity is found in the pseudocubic phase (1.80 W/mK
at 330 K). The low group velocity of acoustic phonons and the strong
anharmonicity are found responsible for the relatively low thermal conductivity
of the tetragonal CH3NH3PbI3
Three-Dimensional Anisotropic Thermal Conductivity Tensor of Single Crystalline \b{eta}-Ga2O3
\b{eta}-Ga2O3 has attracted considerable interest in recent years for high
power electronics, where thermal properties of \b{eta}-Ga2O3 play a critical
role. The thermal conductivity of \b{eta}-Ga2O3 is expected to be
three-dimensionally (3D) anisotropic due to the monoclinic lattice structure.
In this work, the 3D anisotropic thermal conductivity tensor of a
(010)-oriented \b{eta}-Ga2O3 single crystal was measured by using a novel
time-domain thermoreflectance (TDTR) method with a highly elliptical pump beam.
Our measured results suggest that at room temperature, the highest in-plane
thermal conductivity is along a direction between [001] and [102], with a value
of 13.3+/-1.8 W/mK, and the lowest in-plane thermal conductivity is close to
the [100] direction, with a value of 9.5+/-1.8 W/mK. The through-plane thermal
conductivity, which is along the [010] direction, has the highest value of
22+/-2.5 W/mK among all the directions. Temperature-dependent thermal
conductivity of \b{eta}-Ga2O3 was also measured and compared with a modified
Callaway model calculation to understand the temperature dependence and the
role of impurity scattering.Comment: 14 pages, 4 figure
Super-stretchable borophene and its stability under straining
Recent success in synthesizing two-dimensional borophene on silver substrate
attracts strong interest in exploring its possible extraordinary physical
properties. By using the density functional theory calculations, we show that
borophene is highly stretchable along the transverse direction. The
strain-to-failure in the transverse direction is nearly twice as that along the
longitudinal direction. The straining induced flattening and subsequent stretch
of the flat borophene are accounted for the large strain-to-failure for tension
in the transverse direction. The mechanical properties in the other two
directions exhibit strong anisotropy. Phonon dispersions of the strained
borophene monolayers suggest that negative frequencies are presented, which
indicates the instability of free-standing borophene even under high tensile
stress.Comment: 11 pages, 4 figure
First-Principles Prediction of Phononic Thermal Conductivity of Silicene: a Comparison with Graphene
There has been great interest in two-dimensional materials, beyond graphene,
for both fundamental sciences and technological applications. Silicene, a
silicon counterpart of graphene, has been shown to possess some better
electronic properties than graphene. However, its thermal transport properties
have not been fully studied. In this paper, we apply the first-principles-based
phonon Boltzmann transport equation to investigate the thermal conductivity of
silicene as well as the phonon scattering mechanisms. Although both graphene
and silicene are two-dimensional crystals with similar crystal structure, we
find that phonon transport in silicene is quite different from that in
graphene. The thermal conductivity of silicene shows a logarithmic increase
with respect to the sample size due to the small scattering rates of acoustic
in-plane phonon modes, while that of graphene is finite. Detailed analysis of
phonon scattering channels shows that the linear dispersion of the acoustic
out-of-plane (ZA) phonon modes, which is induced by the buckled structure,
makes the long-wavelength longitudinal acoustic (LA) phonon modes in silicene
not as efficiently scattered as that in graphene. Compared with graphene, where
most of the heat is carried by the acoustic out-of-plane (ZA) phonon modes, the
ZA phonon modes in silicene only have ~10% contribution to the total thermal
conductivity, which can also be attributed to the buckled structure. This
systematic comparison of phonon transport and thermal conductivity of silicene
and graphene using the first-principle-based calculations shed some light on
other two-dimensional materials, such as two-dimensional transition metal
dichalcogenides.Comment: To appear in J. Appl. Phys. (2015) Vol.117 Issue 3. 50 pages, 11
figure
Mechanics and Tunable Bandgap by Straining in Single-Layer Hexagonal Boron-Nitride
Current interest in two-dimensional materials extends from graphene to others
systems like single-layer hexagonal boron-nitride (h-BN), for the possibility
of making heterogeneous structures to achieve exceptional properties that
cannot be realized in graphene.The electrically insulating h-BN and semi-metal
graphene may open good opportunities to realize a semiconductor by manipulating
the morphology and composition of such heterogeneous structures.Here we report
the mechanical properties of h-BN and its band structures tuned by mechanical
straining by using the density functional theory calculations.The elastic
properties, both the Young's modulus and bending rigidity for h-BN, are
isotropic.We reveal that there is a bi-linear dependence of band gap on the
applied tensile strains in h-BN. Mechanical strain can tune single-layer h-BN
from an insulator to a semiconductor, with a band gap in the 4.7eV to 1.5eV
range.Comment: 16 pages, 5 figure
Phonon transport in single-layer Mo1-xWxS2 alloy embedded with WS2 nanodomains
Two-dimensional (2-D) transition metal dichalcogenides (TMDs) have shown
numerous interesting physical and chemical properties, making them promising
materials for electronic, optoelectronic, and energy applications. Tuning
thermal conductivity of two-dimensional (2-D) materials could expand their
applicability in many of these fields. In this paper, we propose a strategy of
using alloying and nanodomains to suppress the thermal conductivity of 2-D
materials. To predict the thermal conductivity of 2-D alloy embedded with
nanodomains, we employ the Green's function approach to assess the phonon
scattering strength due to alloying and nanodomain embedding. Our
first-principles-driven phonon Boltzmann transport equation calculations show
that the thermal conductivity of single-layer MoS2 can be reduced to less than
one-tenth of its intrinsic thermal conductivity after alloying with W and
introducing nanodomains due to the strong scattering for both high- and
low-frequency phonons. The strategies to further reduce the thermal
conductivity are also discussed.Comment: 20 pages, 6 figure
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