Article pubs.acs.org/cm
CsHgInS3: a New Quaternary Semiconductor for γ‑ray Detection Hao Li,† Christos D. Malliakas,† Zhifu Liu,‡ John A. Peters,‡ Hosub Jin,∥ Collin D. Morris,† Lidong Zhao,† Bruce W. Wessels,‡,§ Arthur J. Freeman,∥ and Mercouri G. Kanatzidis*,† †
Department of Chemistry, ‡Department of Materials Science and Engineering, §Department of Electrical Engineering, and Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United States
∥
S Supporting Information *
ABSTRACT: The new layered compound CsHgInS3 was synthesized using solid state and flux synthesis techniques. The compound is a semiconductor and shows promising properties for X-ray and γray detection. It features a layered structure that crystallizes in the monoclinic space group C2/c with cell parameters: a = 11.2499(7) Ǻ , b = 11.2565(6) Ǻ , c = 22.146(1) Ǻ , β = 97.30(5)°, V = 2781.8(4) Ǻ 3, and Z = 8. CsHgInS3 is isostructural to Rb2Cu2Sn2S6, where the Hg, In, and Cs atoms occupy the Cu, Sn, and Rb sites, respectively. Large single crystals with dimension up to 5 mm were grown with a vertical Bridgman method as well as a horizontal traveling heater method. CsHgInS3 has a γ-ray attenuation length comparable to commercial Cd1−xZnxTe and a band gap value of 2.30 eV. The electrical resistivity of CsHgInS3 is anisotropic with values of 98 GΩ cm and 0.33 GΩ cm perpendicular and parallel to the (001) plane, respectively. The mobility-lifetime product (μτ) of electrons and holes estimated from photoconductivity measurements on the as-grown crystals were (μτ)e = 3.6 × 10−5 cm2 V−1 and (μτ)h = 2.9 × 10−5 cm2 V−1, respectively. Electronic structure calculations at the Density Functional Theory level were performed based on the refined crystal structure of CsHgInS3 and show a direct gap with the conduction band near the Fermi level being highly dispersive, suggesting a relatively small carrier effective mass for electrons. KEYWORDS: X-ray detection, chalcogenide, semiconductors, crystal growth
■
INTRODUCTION The evolution from simple binary chalcogenides to more complex ternary and quaternary ones is a proven path to increasing diversity in structure and function. For example, the focus on quaternary metal chalcogenides in recent years has resulted in the discovery of many materials with broad functionality, structural diversity, and unique chemical and physical properties. The higher compositional complexity offers more types of atomic sites and can allow for more possibilities to tune properties which range from solar energy conversion,1−5 thermoelectric energy conversion,6−11 photodetection,12−15 photocatalysis,16−18 phase change memory,19−21 enhanced optical nonlinearity,22−29 to topological insulator behavior.30−32 One way to create complexity in the class of chalcogenides is to introduce alkali metals A (Li, Na, K, Rb, Cs) into known binary or ternary systems. When alkali metal chalcogenides (A2Q) react with metal chalcogenides (MQ) to produce [MxQy]n− frameworks, the resulting structural dimensionality is reduced relative to the parent MQ, and the energy gap increases dramatically. Some notable examples include A2Hg6Q7, A2Hg3Q4, A2Cd3Q4, NaCu4S4, and A(Zn/Cd/ Hg)4Ga5Q12 (A = K, Rb; Q = S, Se, Te).33−42 Most of these have been synthesized by the molten A2Qx flux method.8,22,43 Recently, we suggested that the concept of “dimensional © 2012 American Chemical Society
reduction” could be used to identify new candidate materials for γ-ray detectors. Such materials must fulfill several strict requirements for γ-ray detection, such as larger band gap, high atomic number (Z) elements, high density, and high product of carrier mobility (μ) and lifetime (τ), which is the most important figure of merit.44,45 For example, we reported that Cs2Hg6S7, Cs2Cd3Te4, and Cs2Hg3Se4,44 as well as TlGaSe246 and Tl2Hg3Q4(Q = S, Se, Te),45 are in fact promising for X-ray and γ-ray detection applications. This is indicated by their high mobility-carrier lifetime product, μτ, which is required for application in detectors. In this report we describe the new compound semiconductor CsHgInS3, which is composed of high Z elements and has large optical band gap, high specific density, and promising transport properties for X-ray and γ-ray detection. We present the synthesis, crystal structure, electronic band structure calculations, and spectroscopic properties of CsHgInS3 along with results on the growth of large single crystals using both the Bridgman and the Traveling Heater method (THM). Finally, we report resistivity and photoconductivity measurements on Received: September 3, 2012 Revised: October 24, 2012 Published: November 12, 2012 4434
dx.doi.org/10.1021/cm302838v | Chem. Mater. 2012, 24, 4434−4441
Chemistry of Materials
Article
the grown crystals, which indicate a promising mobility-carrier lifetime product.
■
Table 1. Crystal Data and Structure Refinement for Cs2Hg2In2S6 at 140(2) K empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions
EXPERIMENTAL SECTION
The following reagents were used as received: (i) Indium metal, 99.99%, Cerac, Milwaukee, WI; (ii) Mercury metal, 99.999%, Alfa Aesar, Ward Hill, MA; (iii) Sulfur shot, 99.99%, 5N Plus Inc., StLaurent, QC, Canada; (iv) Cesium metal, 99.9+%, Strem Chemicals, Newburyport, MA; (v) N,N-dimethylformamide (DMF), analytical reagent; diethyl ether, anhydrous, Mallinckrodt Baker, Inc., Phillipsburg, NJ. HgS was prepared by the method described previously.45 Cs2S was synthesized by a stoichiometric reaction in liquid ammonia. All manipulations were performed in a nitrogen filled glovebox. CsHgInS3 was synthesized from a mixture of Cs2S (0.298 g, 1 mmol), HgS (0.465 g, 2 mmol), In (0.229 g, 2 mmol), and S (0.096 g, 3 mmol). The starting materials were loaded into a fused silica tube and flame-sealed under a pressure of
