title: Soluble semiconductors AAsSe2 (A = Li, Na) with a direct-band-gap and strong second harmonic generation: a combined experimental and theoretical study.
abstract: AAsSe(2) (A = Li, Na) have been identified as a new class of polar direct-band gap semiconductors. These I-V-VI(2) ternary alkali-metal chalcoarsenates have infinite single chains of (1/infinity)[AsQ(2)(-)] derived from corner-sharing pyramidal AsQ(3) units with stereochemically active lone pairs of electrons on arsenic. The conformations and packing of the chains depend on the structure-directing alkali metals. This results in at least four different structural types for the Li(1-x)Na(x)AsSe(2) stoichiometry (alpha-LiAsSe(2), beta-LiAsSe(2), gamma-NaAsSe(2), and delta-NaAsSe(2)). Single-crystal X-ray diffraction studies showed an average cubic NaCl-type structure for alpha-LiAsSe(2), which was further demonstrated to be locally distorted by pair distribution function (PDF) analysis. The beta and gamma forms have polar structures built of different (1/infinity)[AsSe(2)(-)] chain conformations, whereas the delta form has nonpolar packing. A wide range of direct band gaps are observed, depending on composition: namely, 1.11 eV for alpha-LiAsSe(2), 1.60 eV for LiAsS(2), 1.75 eV for gamma-NaAsSe(2), 2.23 eV for NaAsS(2). The AAsQ(2) materials are soluble in common solvents such as methanol, which makes them promising candidates for solution processing. Band structure calculations performed with the highly precise screened-exchange sX-LDA FLAPW method confirm the direct-gap nature and agree well with experiment. The polar gamma-NaAsSe(2) shows very large nonlinear optical (NLO) second harmonic generation (SHG) response in the wavelength range of 600-950 nm. The theoretical studies confirm the experimental results and show that gamma-NaAsSe(2) has the highest static SHG coefficient known to date, 337.9 pm/V, among materials with band gaps larger than 1.0 eV.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
authors.forename: Tarun K
authors.initials: TK
authors.lastname: Bera
authors.forename: Joon I
authors.initials: JI
authors.lastname: Jang
authors.forename: Jung-Hwan
authors.initials: JH
authors.lastname: Song
authors.forename: Christos D
authors.initials: CD
authors.lastname: Malliakas
authors.forename: Arthur J
authors.initials: AJ
authors.lastname: Freeman
authors.forename: John B
authors.initials: JB
authors.lastname: Ketterson
authors.forename: Mercouri G
authors.initials: MG
authors.lastname: Kanatzidis
mesh_terms: D001149:Arsenates / Q000737:chemistry*; D018360:Crystallography, X-Ray; D018020:Lithium Compounds / Q000737:chemistry*; D018036:Selenium Compounds / Q000737:chemistry*; D012666:Semiconductors; D017670:Sodium Compounds / Q000737:chemistry*; D012995:Solubility; D013053:Spectrophotometry / Q000379:methods; D013059:Spectrum Analysis, Raman; D013816:Thermodynamics
chemical_list: D001149:Arsenates; D018020:Lithium Compounds; D018036:Selenium Compounds; D017670:Sodium Compounds