Soluble Semiconductors AAsSe_2 (A = Li, Na) with a Direct-Band-Gap and Strong Second Harmonic Generation: A Combined Experimental and Theoretical Study

摘要

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/∞)[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_xAsSe_2 stoichoimetry (α-LiAsSe_2, β-LiAsSe_2, γ-NaAsSe_2, and δ-NaAsSe_2). Single-crystal X-ray diffraction studies showed an average cubic NaCI-type structure for α-LiAsSe_2, which was further demonstrated to be locally distorted by pair distribution function (PDF) analysis. The β and γ forms have polar structures built of different (1/∞)[AsSe_2~-] chain conformations, whereas the δ form has nonpolar packing. A wide range of direct band gaps are observed, depending on composition: namely, 1.11 eV for α-LiAsSe_2,1.60 eV for LiAsS_2,1.75 eV for γ-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 γ-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 γ-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.