The local strain distribution in bilayer materials: a multiscale study.

Recent studies show that small geometric changes can result in dramatic changes in physical properties and need to be carefully evaluated. In this regard, we calculate the distribution of local strains in bilayer graphene and two configurations of hexagonal BN (h-BN), which is different from previous studies that focus on homogeneous strains in such materials. We consider a mismatch of one lattice parameter and calculate how strain distributes without external stresses.

Excitation Energies of Localized Correlated Defects via Quantum Monte Carlo: A Case Study of Mn-Doped Phosphors.

Accurate excitation energies of localized defects have been a long-standing problem for electronic structure calculation methods. Using Mn-doped solids as our proof of principle, we show that diffusion quantum Monte Carlo (DMC) is able to predict phosphorescence emission energies within statistical error. To demonstrate the generality of our DMC approach for other possible localized defects, we conduct charge density analyses using DMC and density functional theory (DFT).

Real-Time Observation of Order-Disorder Transformation of Organic Cations Induced Phase Transition and Anomalous Photoluminescence in Hybrid Perovskites.

A fundamental understanding of the interplay between the microscopic structure and macroscopic optoelectronic properties of organic-inorganic hybrid perovskite materials is essential to design new materials and improve device performance. However, how exactly the organic cations affect the structural phase transition and optoelectronic properties of the materials is not well understood. Here, real-time, in situ temperature-dependent neutron/X-ray diffraction and photoluminescence (PL) measurements reveal a transformation of the organic cation CH NH from order to disorder with increasing temperature in CH NH PbBr perovskites.

Bulk assembly of organic metal halide nanotubes.

The organic metal halide hybrids welcome a new member with a one-dimensional (1D) tubular structure. Herein we report the synthesis and characterization of a single crystalline bulk assembly of organic metal halide nanotubes, (CHN)PbBr. In a metal halide nanotube, six face-sharing metal halide dimers (PbBr) connect at the corners to form rings that extend in one dimension, of which the inside and outside surfaces are coated with protonated hexamethylenetetramine (HMTA) cations (CHN).

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CHNHPbI: Implications on Solar Cell Degradation and Choice of Electrode.

Solar cells based on methylammonium lead triiodide (MAPbI) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long-term stability of MAPbI-based solar cells has yet to be achieved. Besides the well-known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current-voltage hysteresis and photoinduced phase segregation.

Synthesis, Crystal and Electronic Structures, and Optical Properties of (CHNH)CdX (X = Cl, Br, I).

We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MACdX (MA = CHNH; X = Cl, Br, I). MACdI is a new compound, whereas, for MACdCl and MACdBr, structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MACdX solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals.

Observation of Nanoscale Morphological and Structural Degradation in Perovskite Solar Cells by in Situ TEM.

High-resolution in situ transmission electron microscopy (TEM) and electron energy loss spectroscopy were applied to systematically investigate morphological and structural degradation behaviors in perovskite films during different environmental exposure treatments. In situ TEM experiment indicates that vacuum itself is not likely to cause degradation in perovskites. In addition, these materials were found to degrade significantly when they were heated to ∼50-60 °C (i.

First-Principles Prediction of Thermodynamically Stable Two-Dimensional Electrides.

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. Experimentally, however, 2D electrides have been only found in a couple of layered nitrides and carbides. Here, we report new thermodynamically stable alkaline-earth based 2D electrides by using a first-principles global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation.

Formation of Ideal Rashba States on Layered Semiconductor Surfaces Steered by Strain Engineering.

Spin splitting of Rashba states in two-dimensional electron system provides a promising mechanism of spin manipulation for spintronics applications. However, Rashba states realized experimentally to date are often outnumbered by spin-degenerated substrate states at the same energy range, hindering their practical applications. Here, by density functional theory calculation, we show that Au one monolayer film deposition on a layered semiconductor surface β-InSe(0001) can possess "ideal" Rashba states with large spin splitting, which are completely situated inside the large band gap of the substrate.

sd(2) Graphene: Kagome band in a hexagonal lattice.

Graphene, made of sp^{2} hybridized carbon, is characterized with a Dirac band, representative of its underlying 2D hexagonal lattice. The fundamental understanding of graphene has recently spurred a surge in the search for 2D topological quantum phases in solid-state materials. Here, we propose a new form of 2D material, consisting of sd^{2} hybridized transition metal atoms in hexagonal lattice, called sd^{2} "graphene.

Quantum size effect on dielectric function of ultrathin metal film: a first-principles study of Al(1 1 1).

Using first-principles calculations, we show manifestations of the quantum size effect in the dielectric function ε(2) of free-standing Al(1 1 1) ultrathin films of 1 monolayer to 20 monolayers, taking into account size dependent contributions from both interband and intraband electronic transitions. Overall the in-plane components (interband transition) of ε(2) increase with film thickness at all frequencies, converging towards a constant value. However, the out-of-plane components of ε(2) show a more complex behavior, and, only at frequencies less than 0.

Formation of quantum spin Hall state on Si surface and energy gap scaling with strength of spin orbit coupling.

For potential applications in spintronics and quantum computing, it is desirable to place a quantum spin Hall insulator [i.e., a 2D topological insulator (TI)] on a substrate while maintaining a large energy gap.

Epitaxial growth of large-gap quantum spin Hall insulator on semiconductor surface.

Formation of topological quantum phase on a conventional semiconductor surface is of both scientific and technological interest. Here, we demonstrate epitaxial growth of 2D topological insulator, i.e.

Quasiparticle dynamics in reshaped helical Dirac cone of topological insulators.

Topological insulators and graphene present two unique classes of materials, which are characterized by spin-polarized (helical) and nonpolarized Dirac cone band structures, respectively. The importance of many-body interactions that renormalize the linear bands near Dirac point in graphene has been well recognized and attracted much recent attention. However, renormalization of the helical Dirac point has not been observed in topological insulators.

Creation of helical Dirac fermions by interfacing two gapped systems of ordinary fermions.

Topological insulators are a unique class of materials characterized by a Dirac cone state of helical Dirac fermions in the middle of a bulk gap. When the thickness of a three-dimensional topological insulator is reduced, however, the interaction between opposing surface states opens a gap that removes the helical Dirac cone, converting the material back to a normal system of ordinary fermions. Here we demonstrate, using density function theory calculations and experiments, that it is possible to create helical Dirac fermion state by interfacing two gapped films-a single bilayer Bi grown on a single quintuple layer Bi(2)Se(3) or Bi(2)Te(3).

First-principles study of the electronic, vibrational, electron-phonon interaction and thermodynamics properties of ZrNi(2)Ga.

Using first-principles calculation, we investigate systematically the properties of ZrNi(2)Ga with fcc L 2(1) Heusler structure, including the electronic structure, phonon dispersion, electron-phonon interaction and thermodynamics. The calculated electron-phonon coupling constant λ and the logarithmically averaged frequency [Formula: see text] are 0.747 and 68.