82 research outputs found
Temperature-dependent thermal conductivity in nanoporous materials studied by the Boltzmann Transport Equation
Nanostructured materials exhibit low thermal conductivity because of the
additional scattering due to phonon-boundary interactions. As these
interactions are highly sensitive to the mean free path (MFP) of a given phonon
mode, MFP distributions in nanostructures can be dramatically distorted
relative to bulk. Here we calculate the MFP distribution in periodic nanoporous
Si for different temperatures, using the recently developed MFP-dependent
Boltzmann Transport Equation. After analyzing the relative contribution of each
phonon branch to thermal transport in nanoporous Si, we find that at room
temperature optical phonons contribute 18 % to heat transport, compared to 5%
in bulk Si. Interestingly, we observe a steady thermal conductivity in the
nanoporous materials over a temperature range 200 K < T < 300 K, which we
attribute to the ballistic transport of acoustic phonons with long intrinsic
MFP. These results, which are also consistent with a recent experimental study,
shed light on the origin of the reduction of thermal conductivity in
nanostructured materials, and could contribute to multiscale heat transport
engineering, in which the bulk material and geometry are optimized
concurrently
Energy Gap from Step Structure of the Analytically Inverted Non-Additive Kinetic Potential
The bandgap constitutes a challenging problem in density functional theory
(DFT) methodologies. It is known that the energy gap values calculated by
common DFT approaches are underestimated. The bandgap was also found to be
related to the derivative discontinuity (DD) of the exchange-correlation
potential in the Kohn-Sham formulation of DFT. Several reports have shown that
DD appears as a step on the potential curve. The step structure is a mandatory
structure for aligning the KS energy levels in the ionization potentials in a
dissociated molecule in both fragments and is a function of electron
localisation. Reproducing the step in the DFT framework gives the charge
transfer process and the correct energy gap and describes the source of
dissociation. This step phenomenon has not yet been studied in the non-additive
kinetic potential , a key quantity
used in embedding theories. While
is known to be difficult to approximate, in this work, we explain how an
accurate energy gap can be produced from the analytically inverted
, even if we use the input densities
calculated by the local and semi-local functionals. We used the precisely
calculated reported in our previous
publication [Phys. Rev. A 106, 042812 (2022)] to produce the energy gap for
some model systems and report in this work the promising accuracy of our
results through the comparison with the results obtained from one of the most
accurate calculations, OEP theory with the KLI local approximation.Comment: 33 pages, 18 figure
Atomistic Mechanisms of Sliding in Few-Layer and Bulk Doped MoS
Sliding of two-dimensional materials is critical for their application as
solid lubricants for space, and also relevant for strain engineering and device
fabrication. Dopants such as Ni surprisingly improve lubrication in MoS,
despite formation of interlayer bonds by intercalated Ni, and the mechanism has
remained unclear. While sliding on the atomistic level has been theoretically
investigated in pristine 2D materials, there has been little work on doped
forms, especially for the complicated case of intercalation. We use density
functional theory to study sliding of Ni-doped MoS, considering Mo/S
substitution and octahedral/tetrahedral intercalation. We find that bulk and
trilayers are well described by pairwise bilayer interactions. Tetrahedral
intercalation between layers dramatically increases their sliding barrier, but
minimally affects sliding between adjacent undoped layers, thus preserving
effective lubrication. We provide an atomistic view of how sliding occurs in
doped transition-metal dichalcogenides, and a general methodology to analyze
doped sliding.Comment: 19 pages, 5 figure
Computational generation of voids in -Si and -Si:H by cavitation at low density
Use of amorphous silicon (-Si) and hydrogenated amorphous silicon
(-Si:H) in photovoltaics has been limited by light-induced degradation (the
Staebler-Wronski effect) and low hole mobilities, and voids have been
implicated in both problems. Accurately modeling the void microstructure is
critical to theoretically understanding the cause of these issues. Previous
methods of modeling voids have involved removing atoms according to an {\it a
priori} idea of void structure and/or using computationally expensive molecular
dynamics. We propose a new fast and unbiased approach based on the established
and efficient Wooten-Winer-Weaire (WWW) Monte Carlo method, by using a range of
fixed densities to generate equilibrium structures of -Si and -Si:H that
maintain 4-coordination. We find a smooth evolution in bond lengths, bond
angles, and bond angle deviations as the density is changed
around the equilibrium value of atoms/cm. However, a
significant change occurs at densities below atoms/cm,
where voids begin to form to relieve tensile stress, akin to a cavitation
process in liquids. We find both small voids (radius 3 \AA) and larger
ones (up to 7 \AA), which compare well with available experimental data. The
voids have an influence on atomic structure up to 4 \AA beyond the void surface
and are associated with decreasing structural order, measured by
. We also observe an increasing medium-range dihedral order with
increasing density. Our method allows fast generation of statistical ensembles,
resembles a physical process during experimental deposition, and provides a set
of void structures for further studies of their effects on degradation, hole
mobility, two-level systems, thermal transport, and elastic properties.Comment: 23 pages, 8 figure
Stress effects on vibrational spectra of a cubic hybrid perovskite: A probe of local strain
Inhomogeneous strain may develop in hybrid organic metal-halide perovskite
thin films due to thermal expansion mismatch with a fabrication substrate,
polycrystallinity or even light soaking. Measuring these spatially varying
strains is difficult but of prime importance for understanding the effects on
carrier mobility, non-radiative recombination, degradation and other
optoelectronic properties. Local strain can be mapped using the shifts in
vibrational frequencies using Raman or infrared microscopy. We use density
functional theory to investigate the effect of uniaxial strain on the
vibrations of pseudo-cubic methylammonium lead iodide (CHNHPbI),
and identify the vibrational modes most favorable for local strain mapping (86
cm, 97 cm, 1457 cm, and 1537 cm) and provide
calibration curves. We explain the origin of the frequency changes with strain
using dynamical matrix and mode eigenvector analysis and study strain-induced
structural changes. We also calculate mode Gr\"uneisen parameters, giving
information about anharmonicity and anisotropic negative thermal expansion as
recently reported for other phases. Our results provide a basis for strain
mapping in hybrid perovskites to further the understanding and control of
strain, and improve stability and photovoltaic performance.Comment: 41 pages, 10 figure
Thermodynamic limits to energy conversion in solar thermal fuels
Solar thermal fuels (STFs) are an unconventional paradigm for solar energy
conversion and storage which is attracting renewed attention. In this concept,
a material absorbs sunlight and stores the energy chemically via an induced
structural change, which can later be reversed to release the energy as heat.
An example is the azobenzene molecule which has a cis-trans photoisomerization
with these properties, and can be tuned by chemical substitution and attachment
to templates such as carbon nanotubes, small molecules, or polymers. By analogy
to the Shockley-Queisser limit for photovoltaics, we analyze the maximum
attainable efficiency for STFs from fundamental thermodynamic considerations.
Microscopic reversibility provides a bound on the quantum yield of
photoisomerization due to fluorescence, regardless of details of
photochemistry. We emphasize the importance of analyzing the free energy, not
just enthalpy, of the metastable molecules, and find an efficiency limit for
conversion to stored chemical energy equal to the Shockley-Queisser limit. STF
candidates from a recent high-throughput search are analyzed in light of the
efficiency limit.Comment: 16 pages, 4 figure
Enhanced interlayer interactions in Ni-doped MoS, and structural and electronic signatures of doping site
The crystal structure of MoS with strong covalent bonds in plane and weak
Van der Waals interactions out of plane gives rise to interesting properties
for applications such as solid lubrication, optoelectronics, and catalysis,
which can be enhanced by transition-metal doping. However, the mechanisms for
improvement and even the structure of the doped material can be unclear, which
we address with theoretical calculations. Building on our previous work on
Ni-doping of the bulk 2H phase, now we compare to polytypes (1H monolayer and
3R bulk), to determine favorable sites for Ni and the doping effect on
structure, electronic properties, and the layer dissociation energy. The most
favorable intercalation/adatom sites are tetrahedral intercalation for 3R (like
2H) and Mo-atop for 1H. The relative energies indicate a possibility of phase
change from 2H to 3R with substitution of Mo or S. We find structural and
electronic properties that can be used to identify the doping sites, including
metallic behavior in Mo-substituted 3R and 2H, and in-gap states for Mo- and
S-substituted 1H, which could have interesting optoelectronic applications. We
observe a large enhancement in the interlayer interactions of Ni-doped MoS,
opposite to the effect of other transition metals. For lubrication
applications, this increased layer dissociation energy could be the mechanism
of low wear. Our systematic study shows the effect of doping concentration and
we extrapolate to the low-doping limit. This work gives insight into the
previously unclear structure of Ni-doped MoS and how it can be detected
experimentally, the relation of energy and structures of doped monolayers and
bulk systems, the electronic properties under doping, and the effect of doping
on interlayer interactions.Comment: 31 pages, 8 figure
Stress effects on the Raman spectrum of an amorphous material: theory and experiment on a-Si:H
Strain in a material induces shifts in vibrational frequencies, which is a
probe of the nature of the vibrations and interatomic potentials, and can be
used to map local stress/strain distributions via Raman microscopy. This method
is standard for crystalline silicon devices, but due to lack of calibration
relations, it has not been applied to amorphous materials such as hydrogenated
amorphous silicon (a-Si:H), a widely studied material for thin-film
photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H
\ab initio under different strains and found peak shifts . This
proportionality to the trace of the strain is the general form for isotropic
amorphous vibrational modes, as we show by symmetry analysis and explicit
computation. We also performed Raman measurements under strain and found a
consistent coefficient of . These results
demonstrate that a reliable calibration for the Raman/strain relation can be
achieved even for the broad peaks of an amorphous material, with similar
accuracy and precision as for crystalline materials.Comment: 12 pages, 3 figures + supplementary 8 pages, 4 figure
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Approaching periodic systems in ensemble density functional theory via finite one-dimensional models
Abstract:
Ensemble Density Functional Theory (EDFT) is a generalization of ground-state Density Functional Theory (GS DFT), which is based on an exact formal theory of finite collections of a system's ground and excited states. EDFT in various forms has been shown to improve the accuracy of calculated energy level differences in isolated model systems, atoms, and molecules, but it is not yet clear how EDFT could be used to calculate band gaps for periodic systems. We extend the application of EDFT toward periodic systems by estimating the thermodynamic limit with increasingly large finite one-dimensional "particle in a box" systems, which approach the uniform electron gas (UEG). Using ensemble-generalized Hartree and Local Spin Density Approximation (LSDA) exchange-correlation functionals, we find that corrections go to zero in the infinite limit, as expected for a metallic system. However, there is a correction to the effective mass, with results comparable to other calculations on 1D, 2D, and 3D UEGs, which indicates promise for non-trivial results from EDFT on periodic systems
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