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Band-Gap Engineering in High-Temperature Boron-Rich

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Band-Gap Engineering in High-Temperature Boron-Rich Article Cite This: J. Phys. Chem. C 2019, 123, 12505−12513 pubs.acs.org/JPCC Band-Gap Engineering in High-Temperature Boron-Rich Icosahedral Compounds Hongwei Wang and Qi An* Department of Chemical and Materials Engineering, University of Nevada-Reno, Reno, Nevada 89557, United States *S Supporting Information ABSTRACT: Band-gap engineering is essential for boron-rich icosahedral compounds for high-temperature applications such as β-voltaic devices and thermoelectrics, and the extreme complex chemical bonding in these icosahedral compounds leads to intriguing electronic properties. Here, we first employed quantum mechanics simulations to determine the electronic states that control the valence band maximum and conduction band minimum of these icosahedral compounds, as well as the detailed band structures. Then, we examined the band-gap modulation by applying twin boundary, chemical substitution, structure arrangement, and hydrostatic pressure, with which the band gaps of these icosahedral compounds can be engineered wider or narrower to a large extent. Particularly, we find that some icosahedral compounds show an increased band gap with the applied hydrostatic pressure, an unusual phenomenon compared to the conventional inorganic semiconductors. Our study provides a fundamental understanding of the electronic properties of icosahedral solids and an effective way to tune their band gaps for designing promising optical, thermoelectric, and β-voltaic devices at extremely high temperature and radiation conditions. 1. INTRODUCTION conductors and perform band-gap engineering to optimize Icosahedral boron and boron-rich compounds are mainly their engineering performance. The boron-rich compounds α boron (α-B ), boron carbide composed of 12-atom boron clusters in which boron atoms 12 (B C), boron suboxide (B O ), and boron subphosphide occupy the 12 vertices of icosahedra,1 displaying promising 4 12 2 (B P ) have similar crystal structures in which the doped properties such as super hardness, high melting temperature, 12 2 2 elements C, O, and P form the multiatomic chains linked to and radioresistance. The ability of icosahedral boron-rich adjacent icosahedra. The different types of bonding inter- compounds to survive in extreme environments makes them actions between the chain and icosahedral atoms in these 3 − useful for various advanced device applications. One primary boron-rich systems result in unusual electronic properties.8 10 Downloaded via UNIV OF NEVADA RENO on July 26, 2019 at 02:46:57 (UTC). β use is -voltaic cells, which utilize energy from a radioactive Boron carbides, which have attracted more attention in β 2,3 β source of particles to generate electricity and heat. The - contrast to other boron-rich compounds, possess a wide voltaic devices made with conventional semiconductors tend See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. range of energy gaps between 0.48 and 2.5 eV reported to undergo very serious radiation damage, which can be experimentally.9,11,12 Moreover, boron carbide is also found to circumvented by replacing them with icosahedral boron-rich display a pressure-dependent energy gap,13 which is opaque at semiconductors. The long-life and low-power characteristics of the ambient condition but becomes transparent at high these icosahedral compounds are particularly well suited for pressure. The abundant electronic behaviors in boron carbides military and spacecraft applications. Another important are due to their complex structure configurations and various application of icosahedral boron-rich compounds is in stoichiometries. Ektarawong et al. found that one B atom of 4 thermoelectric energy conversion, generating electrical the icosahedron in B4C boron carbide tends to be substituted power from solar heat or recycling waste thermal energy.5 by one C atom able to be randomly distributed over all polar Some boron-rich semiconductors have been found to possess sites of the icosahedron.14 In addition, the complex large Seebeck coefficients,6 high electrical conductivities, and intericosahedral arrangements such as CBC, CBB, and B□B very low thermal conductivities.7 These are essential (□ represents a vacancy) may coexist in the actual boron prerequisites to achieve high-efficiency thermoelectric energy carbide structures,15,16 which has been investigated by Werheit conversion.5 Since the energy conversion through radiation and thermoelectricity highly depends on the systematic Received: March 10, 2019 electronic structures, it is essential to examine the band Revised: April 15, 2019 structures and band gaps of icosahedral boron-rich semi- Published: April 24, 2019 © 2019 American Chemical Society 12505 DOI: 10.1021/acs.jpcc.9b02254 J. Phys. Chem. C 2019, 123, 12505−12513 The Journal of Physical Chemistry C Article α Figure 1. Atomic structures, band structures, and partial charge density isosurfaces of (a) -B12, (b) B12O2, (c) B12P2, and (d) B4C. The blue and yellow isosurfaces represent the VBM and CBM electronic states. α α Figure 2. (a) Schematic view of the B12 icosahedral unit in -B12. (b) Three-center intericosahedral bonds formed by equatorial B atoms in -B12. α (c) Two-center intericosahedral bonds formed by polar B atoms in -B12. (d, e) Schematic illustration of the B12 icosahedral unit and intericosahedral chain in B12P2. (f, g) Schematic view of the B12 icosahedral unit and intericosahedral chain in polar B4C. and his co-workers. The mixture of ∼10% B□B arrangements, transport through interface heterojunction,22,23 introducing a − − ∼ 19,24,25 <2% C B B chains, and 90% CBC chains with 100% B11C gap state with composition substitution, and band-edge icosahedron13,15 is responsible for the experimentally observed modulation by mechanical-strain-induced structure distor- 18,26 B4.3C structural formula, the carbon-rich limit composition of tion, have been utilized to tailor electronic properties in boron carbide compounds.16 The electron transition levels the traditional semiconductors and novelly discovered two- introduced by the above configurational disorder and chain- dimensional materials.27 Within these methods, their electronic related defects in B4.3C compound, accounting for the wide- conductivities, light-emitting properties, and visible-light- range band gap, have been systematically studied by Werheit induced photocatalytic efficiencies have gained significant with theoretical analysis recently.11 Furthermore, Hushur et al. improvements.28 However, the research on how to tune proposed that the optical transition in B4.3C compounds results electronic properties in boron-rich icosahedral compounds has from the reduced structural defects by high pressure,13 which not been considered so far because of the intriguing structure enable the stabilization of the defect-free B4C boron carbide and bonding characteristics in these materials. Hence, it is with a wide energy gap. necessary to explore the band-gap engineering in these boron- Although the electronic performances of the boron-rich rich systems to design more advanced devices, in combination compounds have been investigated both theoretically and with exciting electrical, thermal, and mechanical performances. experimentally before, the intrinsic electronic properties of In this study, we focus on the electronic structures of α 1,291,29 30 semiconductors are usually not attractable for practical icosahedral boron-rich compounds -B12, B12O2, 31,32 9,33 9 applications. For example, their band gaps need to be reduced B12P2, B4C, and B14C1, which are investigated by to raise the usage efficiency of sunlight for the photo- means of the first-principles method. The effects of twin − catalysis17 20 or increased for short-wavelength optical boundary (TB), high pressure, chemical substitution, and applications, ultraviolet photodetectors, and resisting radiation structure rearrangement on electronic properties are consid- damages.21 Several effective approaches, such as carrier ered in this work. As for the boron carbide, we consider only 12506 DOI: 10.1021/acs.jpcc.9b02254 J. Phys. Chem. C 2019, 123, 12505−12513 The Journal of Physical Chemistry C Article a Table 1. Lattice Parameters, Volume, Structure Symmetry, and Band Gaps of the Boron-Rich Icosahedral Compounds a (Å) b (Å) c (Å) volume (Å3) space group band gap (eV) ̅ indirect B12 5.026 85.744 R3m 2.18 5.064Expt. 87.834Expt. τ indirect -B12 4.870 8.798 16.019 686.376 Cmcm 2.21 ̅ indirect p-B12 4.853 76.872 R3m 1.58 ̅ direct B12O2 5.124 101.625 R3m 2.89 5.146Expt. 102.484Expt. τ indirect -B12O2 5.359 8.732 8.684 406.400 Cmcm 2.89 ̅ direct p-B12O2 4.928 91.551 R3m 3.22 ̅ indirect B12S2 5.324 123.325 R3m 2.15 ̅ indirect B12P2 5.221 120.800 R3m 3.40 5.256Expt. 123.222Expt. τ indirect -B12P2 5.965 8.583 18.899 967.497 Cmcm 3.80 indirect polar-B11C-CBC 5.182 5.182 5.031 107.381 Cm 3.84 ̅ direct chain-B12-CCC 5.160 109.681 R3m 2.53 5.163Expt. 109.419Expt. indirect equatorial-B11C-CBC 5.178 5.178 5.145 107.781 Cm 3.51 indirect kink-B14C1 5.062 5.111 5.184 108.945 P1 2.01 indirect p-kink-B14C1 4.829 4.917 4.920 95.938 P1 2.46 indirect linear-B14C1 5.103 107.785 R3m 1.59 indirect p-linear-B14C1 4.893 95.960 R3m 2.08 aτ and p represent the twinned and high-pressure structures, and the superscript index Expt. stands for the experimental results. the major constituent (B11C)CBC in B4.3C compounds, and two-atom or three-atom intericosahedral chain, which links the chain-related defects are not considered in our work. We with three neighboring icosahedra through the equatorial select three reprehensive structures of the (B11C)CBC system atoms. The more detailed structural information is shown in 14 fi studied by Ektarawong to investigate the con guration Figure 2. The 2-atom or 3-atom chains in both B12P2 and B4C ff fi fi disorder e ect on electronic structures.
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