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Ultra‐wide bandgap (UWBG) materials are poised to play an important role in the future of power electronics. Devices made from UWBG materials are expected to operate at higher voltages, frequencies, and temperatures than current silicon and silicon‐carbide‐based devices and can even lead to significant miniaturization of such devices. In the UWBG field, aluminum nitride and boron nitride have attracted great interest; however, the BxAl1−xN alloys are much less studied. In this work, using first‐principles simulations combining density‐functional theory and the cluster expansion method, the crystal structure of BxAl1−xN alloys is predicted. Seventeen ground state structures of BxAl1−xN with formation energies between 0.11 and 0.25 eV atom‐1 are found. All of these structures are found to be dynamically stable. The BxAl1−xN structures are found to have predominantly a tetrahedral bonding environment; however, some structures exhibit sp2 bonds similar to hexagonal BN. This work expands knowledge of the structures, energies, and bonding in BxAl1−xN which aids their synthesis, the innovation of lateral or vertical devices, and discovery of compatible dielectric and Ohmic contact materials. [ABSTRACT FROM AUTHOR]

We present a first-principles-based formalism to provide a quantitative measure of the thermodynamic instability and propensity for electrochemical stabilization, passivation, or corrosion of metastable materials in aqueous media. We demonstrate that this formalism can assess the relative Gibbs free energy of candidate materials in aqueous media as well as their decomposition products, combining solid and aqueous phases, as a function of pH and potential. On the basis of benchmarking against 20 stable as well as metastable materials reported in the literature and also our experimental characterization of metastable triclinic-FeVO4, we present quantitative estimates for the relative Gibbs free energy and corresponding aqueous regimes where these materials are most likely to be stable, form inert passivating films, or steadily corrode to aqueous species. Furthermore, we show that the structure and composition of the passivating films formed on triclinic-FeVO4 are also in excellent agreement with the Point Defect Model, as proposed by the corrosion community. [ABSTRACT FROM AUTHOR]

Impurity doping in two-dimensional (2D) materials can provide a route to tuning electronic properties, so it is important to be able to determine the distribution of dopant atoms within and between layers. Here we report the tomographic mapping of dopants in layered 2D materials with atomic sensitivity and subnanometer spatial resolution using atom probe tomography (APT). APT analysis shows that Ag dopes both Bi2Se3 and PbSe layers in (PbSe)5(Bi2Se3)3, and correlations in the position of Ag atoms suggest a pairing across neighboring Bi2Se3 and PbSe layers. Density functional theory (DFT) calculations confirm the favorability of substitutional doping for both Pb and Bi and provide insights into the observed spatial correlations in dopant locations. [ABSTRACT FROM AUTHOR]

The article presents a study focusing on the use of antimony selenide nanowires in flexible polyimide substrates for broadband photodetectors (PDs). Topics discussed include the nanostructures and crystal structure of antimony selenide, the use of X-ray photoelectron spectroscopy (XPS) in the study, and the electronic band structure of bulk antimony selenide.

Sapphire (α-Al2O3) is a common substrate for the growth of single- to few-layer MoS2 films, and amorphous aluminium oxide serves as a high-κ dielectric gate oxide for MoS2 based transistors. Using density-functional theory calculations with a van der Waals functional, we investigate the structural, energetic, and electronic properties of n-layer MoS2 (n = 1and 3) on the α-Al2O3 (0001) surface. Our results show that the sapphire stabilizes single-layer and tri-layer MoS2, while having a negligible effect on the structure, band gap, and electron effective masses of MoS2. This combination of a strong energetic stabilization and weak perturbation of the electronic properties shows that α-Al2O3 can serve as an ideal substrate for depositing ultra-thin MoS2 layers and can also serve as a passivation or gate-oxide layer for MoS2 based devices. [ABSTRACT FROM AUTHOR]

Sapphire (α-Al2O3) is a common substrate for the growth of single- to few-layer MoS2 films, and amorphous aluminium oxide serves as a high-κ dielectric gate oxide for MoS2 based transistors. Using density-functional theory calculations with a van der Waals functional, we investigate the structural, energetic, and electronic properties of n-layer MoS2 (n = 1and 3) on the α-Al2O3 (0001) surface. Our results show that the sapphire stabilizes single-layer and tri-layer MoS2, while having a negligible effect on the structure, band gap, and electron effective masses of MoS2. This combination of a strong energetic stabilization and weak perturbation of the electronic properties shows that α-Al2O3 can serve as an ideal substrate for depositing ultra-thin MoS2 layers and can also serve as a passivation or gate-oxide layer for MoS2 based devices. [ABSTRACT FROM AUTHOR]

Sapphire (α-Al2O3) is a common substrate for the growth of single- to few-layer MoS2 films, and amorphous aluminium oxide serves as a high- dielectric gate oxide for MoS2 based transistors. Using density-functional theory calculations with a van der Waals functional, we investigate the structural, energetic, and electronic properties of n-layer MoS2 (n = 1and 3) on the α-Al2O3 (0001) surface. Our results show that the sapphire stabilizes single-layer and tri-layer MoS2, while having a negligible effect on the structure, band gap, and electron effective masses of MoS2. This combination of a strong energetic stabilization and weak perturbation of the electronic properties shows that α-Al2O3 can serve as an ideal substrate for depositing ultra-thin MoS2 layers and can also serve as a passivation or gate-oxide layer for MoS2 based devices. [ABSTRACT FROM AUTHOR]

Sapphire (α-Al2O3) is a common substrate for the growth of single- to few-layer MoS2 films, and amorphous aluminium oxide serves as a high- dielectric gate oxide for MoS2 based transistors. Using density-functional theory calculations with a van der Waals functional, we investigate the structural, energetic, and electronic properties of n-layer MoS2 (n = 1and 3) on the α-Al2O3 (0001) surface. Our results show that the sapphire stabilizes single-layer and tri-layer MoS2, while having a negligible effect on the structure, band gap, and electron effective masses of MoS2. This combination of a strong energetic stabilization and weak perturbation of the electronic properties shows that α-Al2O3 can serve as an ideal substrate for depositing ultra-thin MoS2 layers and can also serve as a passivation or gate-oxide layer for MoS2 based devices. [ABSTRACT FROM AUTHOR]

Ordered assemblies of inorganic nanocrystals coated with organiclinkers present interesting scientific challenges in hard and softmatter physics. We demonstrate that a nanocrystal superlattice undercompression serves as a nanoscopic pressure cell to enable studiesof molecular linkers under uniaxial compression. We developed a methodto uniaxially compress the bifunctional organic linker by attachingboth ends of aliphatic chains to neighboring PbS nanocrystals in asuperlattice. Pressurizing the nanocrystal superlattice in a diamondanvil cell thus results in compression of the molecular linkers alongtheir chain direction. Small-angle and wide-angle X-ray scatteringduring the compression provide insights into the structure of thesuperlattice and nanocrystal cores under compression, respectively.We compare density functional theory calculations of the molecularlinkers as basic Hookean springs to the experimental force–distancerelationship. We determine the density of linkers on the nanocrystalsurfaces. We demonstrate our method to probe the elastic force ofsingle molecule as a function of chain length. The methodology introducedin this paper opens doors to investigate molecular interactions withinorganic molecules compressed within a nanocrystal superlattice. [ABSTRACT FROM AUTHOR]

The synthesis of single-layer materials relies on suitable substrates. In this paper, we identify suitable substrates for the stabilization and growth of single-layer GaN and characterize the effect of the substrate on the electronic structure of single-layer GaN. We identify two classes of epitaxial substrates, refractory metal diborides and transition-metal dichalcogenides. We find that the refractory diborides provide epitaxial stabilization for the growth and functionalization of single layer GaN. We show that chemical interactions of single layer GaN with the diboride substrates result in n-type doping of the single-layer GaN. Transition-metal dichalcogenides, on the other hand, although epitaxially matched, cannot provide sufficient thermodynamic stabilization for the growth of single layer GaN. Nonetheless, energy band alignments of GaN/metal chalcogenides show that they make good candidates for heterostructures. [ABSTRACT FROM AUTHOR]

Density functional calculations determine the structure, stability, and electronic properties of two-dimensional materials in the family of group-IV monochalcogenides, MX (M = Ge, Sn, Pb; X = O, S, Se, Te). Calculations with a van der Waals functional show that the two-dimensional IV-VI compounds are most stable in either a highly distorted NaCl-type structure or a single-layer litharge type tetragonal structure. Their formation energies are comparable to single-layer MoS2, indicating the ease of mechanical exfoliation from their layered bulk structures. The phonon spectra confirm their dynamical stability. Using the hybrid HSE06 functional, we find that these materials are semiconductors with bandgaps that are generally larger than for their bulk counterparts due to quantum confinement. The band edge alignments of monolayer group IV-VI materials reveal several type-I and type-II heterostructures, suited for optoelectronics and solar energy conversion. [ABSTRACT FROM AUTHOR]

Single-layer materials open up tremendous opportunities for nanoelectronic devices. Using a first-principles design approach we identify a previously unrecognized family of single-layer III-V materials. We determine their energetic and dynamical stability, identify a surprising reconstruction, and calculate their electronic properties using a hybrid density functional and the G0W0 method. Finally, we find that metal substrates stabilize these as-yet hypothetical materials. Our results provide guidance for experimental synthesis efforts and future searches of single-layer materials suitable for device applications. [ABSTRACT FROM AUTHOR]

Ripples in graphene can occur due to strain or thermal fluctuations and stabilize the two-dimensional material. Molecular dynamics simulations show that thermally induced ripples in graphene lead to angular deviations of the surface normal that agree with previous electron diffraction experiments [ J. C. Meyer et al. Nature (London) 446 60 (2007)]. We discover scaling relationships for the average angular deviations as a function of size of the graphene sheet L and averaging radius R. The average angle scales as exp{-c(R/L)α} with a scaling exponent α=1 for single layer, 5/4 for bilayer, and 5/3 for trilayer graphene, respectively. We show how these angular deviations depend on temperature, strain, and layer numbers. The scaling relations can provide guidance to the optimization of properties that are sensitive to out-of-plane distortions. [ABSTRACT FROM AUTHOR]

The photocatalytic conversion of the greenhouse gas CO2 to chemical fuels such as hydrocarbons and alcohols continues to be a promising technology for renewable generation of energy. Major advancements have been made in improving the efficiencies and product selectiveness of currently known CO2 reduction electrocatalysts, nonetheless, materials discovery is needed to enable economically viable, industrial-scale CO2 reduction. We report here the largest CO2 photocathode search to date, starting with 68860 candidate materials, using a rational first-principles computation-based screening strategy to evaluate synthesizability, corrosion resistance, visible-light absorption, and compatibility of the electronic structure with fuel synthesis. The results confirm the observation of the literature that few materials meet the stringent CO2 photocathode requirements, with only 52 materials meeting all requirements. The results are well validated with respect to the literature, with 9 of these materials having been studied for CO2 reduction, and the remaining 43 materials are discoveries from our pipeline that merit further investigation. While the conversion of greenhouse CO2 to chemical fuels offers a promising renewable energy technology, there is a dire need for new materials. Here, authors report the largest CO2 photocathode search using a first-principles approach to identify both known and unreported candidate photocatalysts. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials present a new class of materials whose structures and properties can differ from their bulk counterparts. We perform a genetic algorithm structure search using density-functional theory to identify low-energy structures of 2D group-IV dioxides AO2 (A=Si, Ge, Sn, Pb). We find that 2D SiO2 is most stable in the experimentally determined bi-tetrahedral structure, while 2D SnO2 and PbO2 are most stable in the 1T structure. For 2D GeO2, the genetic algorithm finds a new low-energy 2D structure with monoclinic symmetry. Each system exhibits 2D structures with formation energies ranging from 26 to 151 meV/atom, below those of certain already synthesized 2D materials. The phonon spectra confirm their dynamic stability. Using the HSE06 hybrid functional, we determine that the 2D dioxides are insulators or semiconductors, with a direct band gap of 7.2 eV at Γ for 2D SiO2, and indirect band gaps of 4.8-2.7 eV for the other dioxides. To guide future applications of these 2D materials in nanoelectronic devices, we determine their band-edge alignment with graphene, phosphorene, and single-layer BN and MoS2. An assessment of the dielectric properties and electrochemical stability of the 2D group-IV dioxides shows that 2D GeO2 and SnO2 are particularly promising candidates for gate oxides and 2D SnO2 also as a protective layer in heterostructure nanoelectronic devices. [ABSTRACT FROM AUTHOR]

The discovery of a novel material requires the identification of the material's composition as well as of suitable synthesis conditions. We present a data-mining approach to identify suitable substrates for the growth of two-dimensional materials and apply the method to the recently predicted two-dimensional III-V compounds. We identify several lattice-matched substrates for their epitaxial growth, stabilization, and functionalization. Density-functional calculations show that these substrates sufficiently reduce the formation energies of the metastable two-dimensional materials to make them thermodynamically stable. We show that chemical interactions of the two-dimensional materials with the substrates shift the Fermi level of these materials, resulting in doping. The large adsorption energies and strong doping indicate that these metals should provide good electrical contact to enable transport measurements and electronic applications. [ABSTRACT FROM AUTHOR]

The electronic properties of single-layer MoS2 make it an ideal two-dimensional (2D) material for application in electronic devices. Experiments show that MoS2 can undergo structural phase transitions. Applications of single-layer MoS2 will require firm laboratory control over the phase formation. Here we compare the stability and electronic structure of the three experimentally observed single-layer MoS2 phases, 2H, 1T, and 1T', and an in-plane metal/semiconductor heterostructure. We reveal by density-functional theory calculations that charge doping can induce the phase transition of single-layer MoS2 from the 2H to the 1T structure. Further, the 1T structure undergoes a second phase transition due to the occurrence of a charge-density wave (CDW). By comparing the energies of several possible resulting CDW structures, we find that the 1T' orthorhombic structure is the most stable one, consistent with experimental observations and previous theoretical studies. We show that the underlying CDW transition mechanism is not due to Fermi surface nesting, but nonetheless, can be controlled by charge doping. In addition, the stability landscape is highly sensitive to charge doping, which can be used as a practical phase selector. We also provide a prescription for obtaining the 1T' structure via growth or deposition of MoS2 on a Hf substrate, which transfers electrons uniformly and with minimal structural distortion. Finally, we show that lateral heterostructures formed by the 2H and 1T' structures exhibit a low interfacial energy of 0.17 eV/Å, a small Schottky barrier of 0.3 eV for holes, and a large barrier of 1.6 eV for electrons. [ABSTRACT FROM AUTHOR]

In this work the influence of film-substrate misorientation on the strain-induced ordering of graphene films on various metallic surfaces is examined using a mesoscopic continuum model and first-principles atomistic calculations. The periodicity and free energy of the Moiré patterns that emerge are studied as a function of film-substrate adhesion strength for misfit strains far from and close to an incommensurate-commensurate phase transition. Interestingly the lowest energy states are found to be at small but finite misorientation even though these states have a higher domain wall density than the zero-misorientation states. First-principles density functional theory calculations are used to connect the results with experimental findings in graphene epitaxy. This combination of mesoscopic and atomistic approaches can be applied to the study of a wide range of strained 2D material systems including the III-nitride monolayer systems. [ABSTRACT FROM AUTHOR]