Cs2Hg3S4: A Low-Dimensional Direct Bandgap Semiconductor

• Published in: Chemistry of materials Vol. 27; no. 1; pp. 370 - 378
• Main Authors: Islam, Saiful M, Vanishri, S, Li, Hao, Stoumpos, Constantinos C, Peters, John. A, Sebastian, Maria, Liu, Zhifu, Wang, Shichao, Haynes, Alyssa S, Im, Jino, Freeman, Arthur J, Wessels, Bruce, Kanatzidis, Mercouri G
• Format: Journal Article
• Language: English
• Published: American Chemical Society 13.01.2015
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm504089r

Cs2Hg3S4 was synthesized by slowly cooling a melted stoichiometric mixture of Hg and Cs2S4. Cs2Hg3S4 crystallizes in the Ibam spacegroup with a = 6.278(1) Å, b = 11.601(2) Å, and c = 14.431(3)­Å; d calc = 6.29 g/cm3. Its crystal structure consists of straight chains of [Hg3S4] n 2n– that engage in side-by-side weak bonding interactions forming layers and are charge balanced by Cs+ cations. The thermal stability of this compound was investigated with differential thermal analysis and temperature dependent in situ synchrotron powder diffraction. The thermal expansion coefficients of the a, b, and c axes were assessed at 1.56 × 10–5, 2.79 × 10–5, and 3.04 × 10–5 K–1, respectively. Large single-crystals up to ∼5 cm in length and ∼1 cm in diameter were grown using a vertical Bridgman method. Electrical conductivity and photoconductivity measurements on naturally cleaved crystals of Cs2Hg3S4 gave resistivity ρ of ≥108 Ω·cm and carrier mobility-lifetime (μτ) products of 4.2 × 10–4 and 5.82 × 10–5 cm2 V–1 for electrons and holes, respectively. Cs2Hg3S4 is a semiconductor with a bandgap E g ∼ 2.8 eV and exhibits photoluminescence (PL) at low temperature. Electronic band structure calculations within the density functional theory (DFT) framework employing the nonlocal hybrid functional within Heyd–Scuseria–Ernzerhof (HSE) formalism indicate a direct bandgap of 2.81 eV at Γ. The theoretical calculations show that the conduction band minimum has a highly dispersive and relatively isotropic mercury-based s-orbital-like character while the valence band maximum features a much less dispersive and more anisotropic sulfur orbital-based band.