Suchergebnisse

Monolayers of transition-metal dichalcogenide semiconductors present spin-valley locked electronic bands, a property with applications in valleytronics and spintronics that is usually believed to be absent in their centrosymmetric (as the bilayer or bulk) counterparts. Here we show that bulk 2H-MoS2 hides a spin-polarized nature of states determining its direct band gap, with the spin sequence of valence and conduction bands expected for its single layer. This relevant finding is attained by investigating the behavior of the binding energy of A and B excitons under high pressure, by means of absorption measurements and density-functional-theory calculations. These results raise an unusual situation in which bright and dark exciton degeneracy is naturally broken in a centrosymmetric material. Additionally, the phonon-assisted scattering process of excitons has been studied by analyzing the pressure dependence of the linewidth of discrete excitons observed at the absorption coefficient edge of 2H-MoS2. Also, the pressure dependence of the indirect optical transitions of bulk 2H-MoS2 has been analyzed by absorption measurements and density-functional-theory calculations. These results reflect a progressive closure of the indirect band gap as pressure increases, indicating that metallization of bulk MoS2 may occur at pressures higher than 26 GPa. ; Work inValenciawas supported by the SpanishGovernment (Grants No. TEC2014-53727-C2-1-R and No. MAT2016- 75586-C4-1-P) and the Generalitat Valenciana (Grant No. PROMETEOII/2014/059). Work in Bellaterra was supported by Spanish MINECO (Grants No. FIS2015-64886-C5-3-P and No. FIS2015-64886-C5-4-P), and the Severo Ochoa Centers of Excellence Program (Grants No. SEV-2013-0295 and No. SEV-2015-0496), Generalitat de Catalunya (Grant No. 2017SGR1506 and the CERCA Programme) and by the European Union H2020-EINFRA-5-2015 MaX Center of Excellence (Grant No. 676598). M.B.G. acknowledges Fellowship No.UVINV-PREDOC13-110538 under the program "Atracció de Talent, VLC-CAMPUS" of the University of Valencia. ; Peer reviewed

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic Chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://doi.org/10.1021/acs.inorgchem.8b00778 ; [EN] alpha(R)-In2Se3 has been experimentally and theoretically studied under compression at room temperature by means of X-ray diffraction and Raman scattering measurements as well as by ab initio total-energy and lattice-dynamics calculations. Our study has confirmed the alpha (R3m) -> beta' (C2/m) ? beta (R (3) over barm) sequence of pressure-induced phase transitions and has allowed us to understand the mechanism of the monoclinic C2/m to rhombohedral R (3) over barm phase transition. The monoclinic C2/m phase enhances its symmetry gradually until a complete transformation to the rhombohedral R (3) over barm structure is attained above 10-12 GPa. The second-order character of this transition is the reason for the discordance in previous measurements. The comparison of Raman measurements and lattice-dynamics calculations has allowed us to tentatively assign most of the Raman-active modes of the three phases. The comparison of experimental results and simulations has helped to distinguish between the different phases of In2Se3 and resolve current controversies. ; The authors acknowledge financial support from Spanish government MINECO, the Spanish Agencia Estatal de Investigacion (AEI), and Fondo Europeo de Desarrollo Regional (FEDER) under Grants No. MAT2016-75586-C4-1/2/3-P and MAT2015-71070-REDC. ; Vilaplana Cerda, RI.; Gallego-Parra, S.; Jorge-Montero, A.; Rodríguez-Hernández, P.; Muñoz, A.; Errandonea, D.; Segura, A. (2018). Experimental and Theoretical Studies on alfa-In2Se3 at High Pressure. Inorganic Chemistry. 57:8241-8252. https://doi.org/10.1021/acs.inorgchem.8b00778 ; S ; 8241 ; 8252 ; 57

High-pressure optical absorption measurements have been performed in defect chalcopyrite HgGa2Se4 to investigate the influence of pressure on the bandgap energy and its relation with the pressure-induced order-disorder processes that occur in this ordered-vacancy compound. Two different experiments have been carried out in which the sample undergoes either a partial or a total pressure-induced disorder process at 15.4 and 30.8GPa, respectively. It has been found that the direct bandgap energies of the recovered samples at 1GPa were around 0.15 and 0.23eV smaller than that of the original sample, respectively, and that both recovered samples have different pressure coefficients of the direct bandgap than the original sample. A comprehensive explanation for these results on the basis of pressure-induced order-disorder processes is provided. ; This study was supported by the Spanish government MEC under Grants No: MAT2010-21270-C04-01/03/04 and MAT2013-46649-C4-1/2/3-P, by MALTA Consolider Ingenio 2010 project (CSD2007-00045), by Generalitat Valenciana (GVA-ACOMP-2013-1012 and GVA-ACOMP-2014-243), and by the Vicerrectorado de Investigacion y Desarrollo of the Universitat Politecnica de Valencia (UPV2011-0914 PAID-05-11 and UPV2011-0966 PAID-06-11). E. P.-G., J. L.-S., P. R.-H, and A. M. acknowledge computing time provided by Red Espanola de Supercomputacion (RES) and MALTA-Cluster. J.R.-F. thanks the Alexander von Humboldt foundation for a postdoctoral fellowship. ; Gomis, O.; Vilaplana Cerda, RI.; Manjón Herrera, FJ.; Ruiz-Fuertes, J.; Pérez-González, E.; López-Solano, J.; Bandiello, E. (2015). HgGa2Se4 under high pressure: an optical absorption study. physica status solidi (b). 252(9):2043-2051. https://doi.org/10.1002/pssb.201451714 ; S ; 2043 ; 2051 ; 252 ; 9 ; Bernard, J. E., & Zunger, A. (1988). Ordered-vacancy-compound semiconductors: PseudocubicCdIn2Se4. Physical Review B, 37(12), 6835-6856. doi:10.1103/physrevb.37.6835 ; Jiang, X., & Lambrecht, W. R. L. (2004). Electronic band structure of ordered ...