We present a comprehensive DFT study of size-dependent atomic and electronic properties of antimony selenide ( Sb 2 Se 3 ) nanowires in three main crystallographic directions. Our calculations show a significant enhancement in the band gap of wires oriented in [100] and [010] directions due to confinement effects, however the band gap of [001] oriented wires is reduced with respect to bulk. We attribute this anomaly in band gap reduction to the surface reconstructions in these nanostructures. These surface reconstructions are similar to the polyhedral distortions observed in bulk Sb 2 Se 3 under high pressure leading to the insulator-metal transition related to the topological insulating states and then at lower temperature (8K) to superconductivity.

We study the aging and Mn doping effect on third generation lead based relaxor single crystals. We measured the polarization (PE) and strain with applied field on two perpendicular orientations of the rhombohedral pseudocubic [001] poled crystal. To understand these effects along the average dipoles/defect dipoles direction, we adopt a simple model with direction cosine and sine of polarization and strain. We found that when the PE measurement is perpendicular to average defect dipoles, a double loop is observed, and when it is parallel an asymmetric response is observed. We propose that the varied response found in PE measurements depend on the relative direction of average dipoles/defect dipoles to the measurement direction.

For decades, numerous attempts have been made to produce polar oxynitride perovskites, where some of the oxygen is replaced by nitrogen, but a polar ordered oxynitride has never been demonstrated. Caracas and Cohen [Appl. Phys. Lett. 91, 092902 (2007)] studied possible ordered polar oxynitrides within density-functional theory (DFT) and found a few candidates that were predicted to be insulating and at least metastable. YSiO2N stood out with huge predicted polarization and nonlinear optic coefficients. In this study, we demonstrate the synthesis of perovskite-structured YSiO2N by using a combination of a diamond-anvil cell and in situ laser-heating techniques. Subsequent in situ x-ray diffraction, second-harmonic generation, and Raman-scattering measurements confirm that it is polar and a strong nonlinear optical material, with structure and properties similar to those predicted by DFT.

Magnetocrystalline anisotropy at Heusler $\mathrm{alloy}|\mathrm{MgO}$ interfaces has been studied using first-principles calculations. It has been found that Co-terminated ${\mathrm{Co}}_{2}\mathrm{FeAl}|\mathrm{MgO}$ interfaces show perpendicular magnetic anisotropy up to 1.31 mJ/${\mathrm{m}}^{2}$, while those with FeAl termination exhibit in-plane magnetic anisotropy. Atomic layer-resolved analysis indicates that the origin of perpendicular magnetic anisotropy in ${\mathrm{Co}}_{2}\mathrm{FeAl}|\mathrm{MgO}$ interfaces can be attributed to the out-of-plane orbital contributions of interfacial Co atoms. At the same time, ${\mathrm{Co}}_{2}\mathrm{MnGe}$ and ${\mathrm{Co}}_{2}\mathrm{MnSi}$ interfaced with MgO tend to favor in-plane magnetic anisotropy for all terminations.

Submitted for the MAR10 Meeting of The American Physical Society Novel solid-state nuclear detectors based on defect-free crystalline α-HgI2 nanowires synthesized at diffusion controlled conditions1 EDGAR MOSQUERA-VARGAS2, RAJASEKARAKUMAR VADAPOO, University of Puerto Rico-Rio Piedras, CARLOS MARIN, University of Puerto RicoMayaguez — Solid-state detectors are based on semiconductors materials that directly convert X-Ray or Gamma-Ray photons in hole-electron pair with sufficient mobility to produce electric current. HgI2 is a very large band-gap semiconductor material able to operate at room-temperature (RT) under ideal conditions provide by perfect crystallinity. Crystals of α-HgI2 were proposed as the perfect detector material due to its large seminsulating band-gap and large stopping power. Although HgI2 crystals of good quality and large size have been grown, their commercial use is reduced because the crystalline quality degrades during the processes for fabrication of devices. Trapping defects are created and no fabrication method has been found to circumvent the problem in a systematic and reproducible manner. Based on our capability to synthesized defect-free crystalline HgI2 nanoneedles inside porous matrix we are proposing to fabricate detectors that will not require manipulation of the HgI2 crystals and, therefore, will not suffer degradation and the associated lack of performance. Fundamental understanding, control and application for the fabrication of these detectors will be studied in the context of preparation and synthesis of HgI2 nanostructures in the porous matrix. 1NSF-EPSCoR IFN (Grant 0701525) 2Institute for Functional Nanomaterials Edgar Mosquera-Vargas University of Puerto Rico Date submitted: 28 Oct 2009 Electronic form version 1.4

One of the main challenges for the future hydrogen economy is finding a safe and efficient way to store hydrogen. Materials with large surface areas, like carbon nanotubes and their analogues boron-nitride nanotubes, are being studied as potential candidates for this purpose. We perform density functional theory (DFT) and dispersion-corrected DFT (DFT-D) calculations of the adsorption of molecular hydrogen on graphene and boron-nitride sheets and compare the results against Moller-Plesset perturbation theory (MP2 and MP2.5). Our results indicate that DFT underestimates the binding energies, while DFT-D gives a very good agreement with the higher-order theory. Within DFT-D, we show that the binding energy of molecular hydrogen to the outer walls of carbon nanotubes is more than 40% larger than that of boron-nitride nanotubes.

Sub-10 nm semiconducting nanostructures are crucial for the realization of nanoscale devices. Fabrication of nanostructures at this scale with homogeneous properties is challenging. Using ab initio calculations, we show that self-standing ribbons of antimony selenide and antimony sulfide of width 1.1 nm exhibit well-defined bandgaps of 1.66 and 2.16 eV, respectively. Molecular dynamics studies show that these ribbons are stable at 500 K. The one-dimensional (1D) heterostructure of these nanoribbons (Sb2Se3/Sb2S3) along the [001] direction shows a straddling type behavior.

Antimony selenide (Sb2Se3) has been proposed as an alternative material for a wide range of applications; however, the electronic structure of the Sb2Se3 lattice is not clearly known yet. As a consequence, there are abundant contradictory interpretations of experimental results leading to incoherent determinations of its energy band gap and the type of optical transitions. Moreover, Sb2Se3 is recently being synthesized in different types of nanostructures; therefore, detailed knowledge of the bulk electronic structure is necessary to evaluate deviations due to confinement or surface effects. In this paper, we study the electronic band structure of antimony selenide using density functional theory (DFT) within the generalized gradient approximation (GGA) with GW corrections. Our calculations show that Sb2Se3 has an indirect energy band gap of 1.21 eV; however, a direct transition only 0.01 eV higher than the band gap (1.22 eV) is also possible. The calculated density of states agrees well with the experiments reporting photoemission spectra. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The article by R. Vadapoo and coworkers (pp. 700–705) reports the electronic structure of the Sb2Se3 crystal using first-principles calculations. The material is receiving increasing attention due to its ability to get synthesized in different types of nanostructures and to form nanosize heterostructures with other similar lattices. The authors find that the density of states is packed with van Hove singularities typical of confinements in one dimension. The results are not surprising considering the Sb2Se3 structure that can be described as an array of weakly bonded 1D nanoribbons. The calculations detailed in this article will certainly be of much help to many researchers looking for designs of engineered nanostructures based on lattices of the Sb2Se3 type.

Pristine armchair single wall carbon nanotubes (SWCNTs) are metallic; however, they undergo a transition to semiconductor under certain external perturbations. The effect of the bi-site perturbation created by selenium adsorption on electronic properties of armchair SWCNTs for a range of diameters are studied using first principles calculations. We find that the established rules of periodicity for the relative position of the adsorbates are not sufficient to predict a metal to semiconductor transition. By investigating the structural deformation induced in the nanotubes by the adsorbates, we show that the effects of all possible bi-site perturbations are not equivalent. We also show that this factor becomes significant to predict metallic to semiconductor transition on bi-site perturbed Se adsorbed armchair SWCNTs.

Using ab initio calculations, we show that atomic selenium (Se) strongly adsorbs to the outer surface of single wall carbon nanotubes (SWCNTs), converting the semiconductor nanotubes to metallic and enhancing the conductance of the metallic and semimetallic ones. Considering the ease of vaporizing and depositing selenium, due to its volatile nature and low melting point, this system could be potentially viable. Thus, selenium adsorption offers a single process for enhancing the conductance of SWCNTs regardless of their chirality making them a promising candidate for nanoscale interconnects applications.

In this article we report for the first time the synthesis of Sb2Se3 nanowires using a physical vapor-liquid-solid (VLS) process. We used microcrystals of Antimony as solid catalytic material and molten Selenium to generate the vapor source. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images show that as-obtained Sb2Se3 nanowires have diameters in the range between 20 nm and 2 microm and lengths up to 30 microm. Fringes in TEM imaging reveals that Sb2Se3 nanowires are oriented along the [010] crystallographic direction. This orientation is being reported for the first time.

Nanowires of alpha-monoclinic Selenium have been synthesized using a physical vapor-liquid-solid (VLS) process for first time. We used micron sized particles of crystalline Antimony Selenide as catalyst and molten Selenium to generate the vapor source. The synthesized nanowires have diameters in the range between 20 nm and 1 microm and lengths up to 30 microm. Nanowires with diameters of approximately 50 nm are the most predominantly produced. Transmission Electron Microscopy (TEM), Micro-Raman Spectroscopy, Absorption Spectroscopy, Energy Dispersive X-ray Analysis (EDAX), High-Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED) were used to characterize their morphology, composition and crystallographic structure. It is found that the nanowires grow perpendicular to the (053) plane and exhibit an energy band-gap of 2.2 eV (a 0.18 eV increase compared to bulk).