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Low-dimensional nanostructures such as nanotubes, nanoscrolls, and nanofilms have found applications in a wide variety of fields such as photocatalysis, sensing, and drug delivery. Recently, Chu et al. demonstrated that nanoscrolls of Mo and W transition metal oxides, which do not exhibit van der Waals (vdW) layering in their bulk counterparts, can be successfully synthesized using a plasma processing of corresponding layered transition metal dichalcogenides. In this work, we employ data mining, first-principles simulations, and physio-mechanical models to theoretically examine the potential of other dichalcogenide precursors to form oxide nanoscrolls. Through data mining of bulk and two-dimensional materials databases, we first identify dichalcogenides that would be mostly amenable to plasma processing on the basis of their vdW layering and thermodynamic stability. To determine the propensity of forming a nanoscroll, we develop a first-principles simulation-based physio-mechanical model to determine the thermodynamic stability of nanoscrolling as well as the equilibrium structure of the nanoscrolls, i.e. their inner radius, outer radius, and interlayer spacing. We validate this model using the experimental observations of Chu et al.'s study and find an excellent agreement for the equilibrium nanoscroll structure. Furthermore, we demonstrate that the model's energies can be utilized for a generalized quantitative categorization of nanoscroll stability. We apply the model to study the oxide nanoscroll formation in MoS$_2$, WS$_2$, MoSe$_2$, WSe$_2$, PdS$_2$, HfS$_2$ and GeS$_2$, paving the way for a systematic study of oxide nanoscroll formation atop other dichalcogenide substrates.

In recent years, GW-BSE has been proven to be extremely successful in studying the quasiparticle (QP) bandstructures and excitonic effects in the optical properties of materials. However, the massive computational cost associated with such calculations restricts their applicability in high-throughput material discovery studies. Recently, we developed a Python workflow package, $py$GWBSE, to perform high-throughput GW-BSE simulations. In this work, using $py$GWBSE we create a database of various QP properties and excitonic properties of over 350 chemically and structurally diverse materials. Despite the relatively small size of the dataset, we obtain highly accurate supervised machine learning (ML) models via the dataset. The models predict the quasiparticle gap with an RMSE of 0.36 eV, exciton binding energies of materials with an RMSE of 0.29 eV, and classify materials as high or low excitonic binding energy materials with classification accuracy of 90%. We exemplify the application of these ML models in the discovery of 159 visible-light and 203 ultraviolet-light photoabsorber materials utilizing the Materials Project database.

Ultra-wide bandgap (UWBG) materials such as AlN and BN hold great promise for future power electronics due to their exceptional properties. They exhibit large bandgaps, high breakdown fields, high thermal conductivity, and high mechanical strengths. AlN and BN have been extensively researched, however, their alloys, B$_x$Al$_{1-x}$N, are much less studied despite their ability to offer tunable properties by adjusting $x$. In this article, we predict the electronic properties of 17 recently predicted ground states of B$_x$Al$_{1-x}$N in the $x=0-1$ range using first-principles density functional theory and many-body perturbation theory within $GW$ approximation. All the B$_x$Al$_{1-x}$N structures are found to be UWBG materials and have bandgaps that vary linearly from that of wurtzite-phase ($w$) AlN (6.19 eV) to that of $w$-BN (7.47 eV). The bandstructures of B$_x$Al$_{1-x}$N show that a direct-to-indirect bandgap crossover occurs near $x = 0.25$. Furthermore, we find that B$_x$Al$_{1-x}$N alloys have much larger dielectric constants than the constituent bulk materials (AlN=$9.3~\varepsilon_0$ or BN=$7.3~\varepsilon_0$), with values reaching as high as $12.1~\varepsilon_0$. These alloys are found to exhibit large dielectric breakdown fields in the range 9--35 MV/cm with a linear dependence on $x$. This work provides the much needed advancement in the understanding of the properties of B$_x$Al$_{1-x}$N to aid their application in next-generation devices., Comment: 24 pages, 5 figures

Phonon scattering at grain boundaries (GBs) is significant in controlling nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, meV energy resolution is needed with sub-nm spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterised by the presence of five- and seven-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides., Comment: Merged article (8 pages, 3 figures, 1 table) and Supplementary Information (8 pages, 8 figures)

We develop an open-source python workflow package, $py$GWBSE to perform automated first-principles calculations within the GW-BSE (Bethe-Salpeter) framework. GW-BSE is a many body perturbation theory based approach to explore the quasiparticle (QP) and excitonic properties of materials. The GW approximation has proven to be effective in accurately predicting bandgaps of a wide range of materials by overcoming the bandgap underestimation issues of the more widely used density functional theory (DFT). The BSE formalism, in spite of being computationally expensive, produces absorption spectra directly comparable with experimental observations. The $py$GWBSE package achieves complete automation of the entire multi-step GW-BSE computation, including the convergence tests of several parameters that are crucial for the accuracy of these calculations. $py$GWBSE is integrated with $Wannier90$, a program for calculating maximally-localized wannier functions, allowing the generation of QP bandstructures. $py$GWBSE also enables automated creation of databases of metadata and data, including QP and excitonic properties, which can be extremely useful for future material discovery studies in the field of ultra-wide bandgap semiconductors, electronics, photovoltaics, and photocatalysis.

Boron carbide (B$_4$C) has been well studied both theoretically and experimentally in its bulk form due to its exceptional hardness and use as a high temperature thermoelectric. However, the properties of its two-dimensional nanosheets are not well established. In this paper, using van der Waals corrected density-functional theory (DFT) simulations, we show that the bulk B$_4$C can be cleaved along different directions to form B$_4$C nanosheets with low formation energies. We find that there is minimal dependence of the formation energies on the cleavage planes and surface terminations. Whilst the density of states of the bulk B$_4$C indicate that it is a semiconductor, the B$_4$C nanosheets are found to be predominantly metallic. We attribute this metallic behaviour to a redistribution of charges on the surface B-C bonds of the films. The Seebeck coefficients of the the B$_4$C films remain comparable to those of the bulk, and are nearly constant as a function of temperature. Our results provide guidance for experimental synthesis efforts and future application of B$_4$C nanosheets in nanoelectronic and thermoelectric applications.

2D materials find promising applications in next-generation devices, however, large-scale, low-defect, and reproducible synthesis of 2D materials remains a challenging task. To assist in the selection of suitable substrates for the synthesis of as-yet hypothetical 2D materials, we have developed an open-source high-throughput workflow package, $Hetero2d$, that searches for low-lattice mismatched substrate surfaces for any 2D material and determines the stability of these 2D-substrate heterostructures using density functional theory (DFT) simulations. $Hetero2d$ automates the generation of 2D-substrate heterostructures, the creation of DFT input files, the submission and monitoring of computational jobs on supercomputing facilities, and the storage of relevant parameters alongside the post-processed results in a MongoDB database. We demonstrate the capability of $Hetero2d$ in identifying stable 2D-substrate heterostructures for four 2D materials, namely $2H$-MoS$_2$, $1T$- and $2H$-NbO$_2$, and hexagonal-ZnTe, considering 50 cubic elemental substrates. We find Cu, Hf, Mn, Nd, Ni, Pd, Re, Rh, Sc, Ta, Ti, V, W, Y, and Zr substrates sufficiently stabilize the formation energies of these 2D materials, with binding energies in the range of ~0.1 - 0.6 eV/atom. Upon examining the $z$-separation, the charge transfer, and the electronic density of states at the 2D-substrate interface, we find a covalent type bonding at the interface which suggests that these substrates can be used as contact materials for the 2D materials. $Hetero2d$ (https://github.com/cmdlab/Hetero2d) is available on GitHub as an open-source package under the GNU license.

The formation and disassociation of excitons plays a crucial role in any photovoltaic or photocatalytic application. However, excitonic effects are seldom considered in materials discovery studies due to the monumental computational cost associated with the examination of these properties. Here, we study the excitonic properties of nearly 50 photocatalysts using state-of-the-art Bethe-Salpeter formalism. These $\sim$ 50 materials were recently recognized as promising photocatalysts for CO$_2$ reduction through a data-driven screening of 68,860 materials. Here, we propose three screening criteria based on the optical properties of these materials, taking excitonic effects into account, to further down select 6 materials. Remarkably we find a strong correlation between the exciton binding energies obtained from the Bethe-Salpeter formalism and those obtained from the computationally much less-expensive Wannier-Mott model for these chemically diverse $\sim$ 50 materials. This work presents a new paradigm towards the inclusion of excitonic effects in future materials discovery for solar-energy harvesting applications.

Ceria has proven to be an excellent ion-transport and ion-exchange material when used in polycrystalline form and with a high-concentration of aliovalent doped cations. Despite its widespread application, the impact of atomic-scale defects in this material are scarcely studied and poorly understood. In this article, using first-principles simulations, we provide a fundamental understanding of the atomic-structure, thermodynamic stability and electronic properties of undoped grain-boundaries (GBs) and alkaline-earth metal (AEM) doped GBs in ceria. Using density-functional theory simulations, with a GGA+U functional, we find the $\Sigma$3 (111)/[$\bar{1}$01] GB is thermodynamically more stable than the $\Sigma$3 (121)/[$\bar{1}$01] GB due to the larger atomic coherency in the $\Sigma$3 (111)/[$\bar{1}$01] GB plane. We dope the GBs with $\sim$20% [M]$_{GB}$ (M=Be, Mg, Ca, Sr, and Ba) and find that the GB energies have a parabolic dependence on the size of solutes, the interfacial strain and the packing density of the GB. We see a stabilization of the GBs upon Ca, Sr and Ba doping whereas Be and Mg render them thermodynamically unstable. The electronic density of states reveal that no defect states are present in or above the band gap of the AEM doped ceria, which is highly conducive to maintain low electronic mobility in this ionic conductor. The electronic properties, unlike the thermodynamic stability, exhibit complex inter-dependence on the structure and chemistry of the host and the solutes. This work makes advances in the atomic-scale understanding of aliovalent cation doped ceria GBs serving as an anchor to future studies that can focus on understanding and improving ionic-transport., Comment: 23 pages, 4 figures

Novel photoelectrocatalysts that use sunlight to power the CO$_2$ reduction reaction will be crucial for carbon-neutral power and energy-efficient industrial processes. Scalable photoelectrocatalysts must satisfy a stringent set of criteria, such as stability under operating conditions, product selectivity, and efficient light absorption. Two-dimensional materials can offer high specific surface area, tunability, and potential for heterostructuring, providing a fresh landscape of candidate catalysts. From a set of promising bulk CO$_2$ reduction photoelectrocatalysts, we screen for candidate mono-layers of these materials, then study their catalytic feasibility and suitability. For stable monolayer candidates, we verify the presence of visible-light band gaps, check that band edges can support CO$_2$ reduction, determine exciton binding energies, and compute surface reactivity. We find for SiAs, ZnTe, and ZnSe monolayers, visible light absorption is possible, and reaction selectivity biases towards CO production. We thus identify SiAs, ZnTe, and ZnSe monolayers as targets for further investigation, expanding the chemical space for CO$_2$ photoreduction., Comment: 23 Pages, 5 figures

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe$_2$ crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T$^\prime$ semimetallic phase at high temperatures. Alloying MoTe$_2$ with WTe$_2$ reduces the energy barrier between these two phases, while also allowing access to the T$_d$ Weyl semimetal phase. The MoWTe$_2$ alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe$_2$-WTe$_2$ system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study MoWTe$_2$ alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T$^\prime$, and T$_d$ structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe$_2$-WTe$_2$ system, including single-phase 2H, 1T$^\prime$, and T$_d$ regions, as well as a two-phase 1T$^\prime$ + T$_d$ region. Disorder arising from compositional fluctuations in MoWTe$_2$ alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T$^\prime$-MoTe$_2$ mode and the enhancement of a double-resonance Raman process in 2H-MoWTe$_2$ alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in MoWTe$_2$ alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system., Comment: 18 pages, 5 figures, 1 table

A Materials Project based open-source Python tool, MPInterfaces, has been developed to automate the high-throughput computational screening and study of interfacial systems. The framework encompasses creation and manipulation of interface structures for solid/solid hetero-structures, solid/implicit solvents systems, nanoparticle/ligands systems; and the creation of simple system-agnostic workflows for in depth computational analysis using density-functional theory or empirical energy models. The package leverages existing open-source high-throughput tools and extends their capabilities towards the understanding of interfacial systems. We describe the various algorithms and methods implemented in the package. Using several test cases, we demonstrate how the package enables high-throughput computational screening of advanced materials, directly contributing to the Materials Genome Initiative (MGI), which aims to accelerate the discovery, development, and deployment of new materials.

A first principles phase diagram calculation, that included van der Waals interactions, was performed for the system (1-X)$\cdot$MoS$_{2}$-(X)$\cdot$MoTe$_{2}$. Surprisingly, the predicted phase diagram has at least two ordered solid-solution phases, at $X \approx 0.46$, even though all calculated formation energies are positive, in a ground-state analysis that examined all configurations with 16 or fewer anion sites. The lower-temperature {\bf $I$}-phase is predicted to transform to a higher-temperature {\bf $I^{\prime}$}-phase at $T \approx 500K$, and {\bf $I^{\prime}$} disorders at $T \approx 730K$. Both these transitions are predicted to be first-order, and there are broad miscibility gaps on both sides of the ordered regions. Both the {\bf $I$}- and {\bf $I^{\prime}$}-phases are predicted to be incommensurate: {\bf $I$}-phase in three dimensions; and {\bf $I^{\prime}$}-phase in two dimensions., Comment: Probably the first prediction from first-principles of entropy-stabilized incommensurate phases