Atomistic Mechanisms of Sliding in Few-Layer and Bulk Doped MoS2_2

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$_2$, 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$_2$, 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 aa-Si and aa-Si:H by cavitation at low density

Use of amorphous silicon ($a$-Si) and hydrogenated amorphous silicon ($a$-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 $a$-Si and $a$-Si:H that maintain 4-coordination. We find a smooth evolution in bond lengths, bond angles, and bond angle deviations $\Delta \theta$ as the density is changed around the equilibrium value of $4.9\times10^{22}\$atoms/cm$^3$. However, a significant change occurs at densities below $4.3\times10^{22}\$atoms/cm$^3$, where voids begin to form to relieve tensile stress, akin to a cavitation process in liquids. We find both small voids (radius $\sim$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 $\Delta\theta$. 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 (CH$_3$NH$_3$PbI$_3$), and identify the vibrational modes most favorable for local strain mapping (86 cm$^{-1}$, 97 cm$^{-1}$, 1457 cm$^{-1}$, and 1537 cm$^{-1}$) 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

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 $\epsilon$ and found peak shifts $\Delta \omega = \left( -460 \pm 10\ \mathrm{cm}^{-1} \right) {\rm Tr}\ \epsilon$. 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 $-510 \pm 120\ \mathrm{cm}^{-1}$. 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

Basis set effects on the hyperpolarizability of CHCl_3: Gaussian-type orbitals, numerical basis sets and real-space grids

Calculations of the hyperpolarizability are typically much more difficult to converge with basis set size than the linear polarizability. In order to understand these convergence issues and hence obtain accurate ab initio values, we compare calculations of the static hyperpolarizability of the gas-phase chloroform molecule (CHCl_3) using three different kinds of basis sets: Gaussian-type orbitals, numerical basis sets, and real-space grids. Although all of these methods can yield similar results, surprisingly large, diffuse basis sets are needed to achieve convergence to comparable values. These results are interpreted in terms of local polarizability and hyperpolarizability densities. We find that the hyperpolarizability is very sensitive to the molecular structure, and we also assess the significance of vibrational contributions and frequency dispersion

Orbital magneto-optical response of periodic insulators from first principles

Magneto-optical response, i.e. optical response in the presence of a magnetic field, is commonly used for characterization of materials and in optical communications. However, quantum mechanical description of electric and magnetic fields in crystals is not straightforward as the position operator is ill defined. We present a reformulation of the density matrix perturbation theory for time-dependent electromagnetic fields under periodic boundary conditions, which allows us to treat the orbital magneto-optical response of solids at the \textit{ab initio} level. The efficiency of the computational scheme proposed is comparable to standard linear-response calculations of absorption spectra and the results of tests for molecules and solids agree with the available experimental data. A clear signature of the valley Zeeman effect is revealed in the continuum magneto-optical spectrum of a single layer of hexagonal boron nitride. The present formalism opens the path towards the study of magneto-optical effects in strongly driven low-dimensional systems.Comment: 12 pages, 3 figure