Article pubs.acs.org/cm
Copper Vacancies and Heavy Holes in the Two-Dimensional Semiconductor KCu3−xSe2 Alexander J. E. Rettie,† Mihai Sturza,‡ Christos D. Malliakas,†,§ Antia S. Botana,† Duck Young Chung,† and Mercouri G. Kanatzidis*,†,§ †
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Leibniz Institute for Solid State and Materials Research (IFW), Dresden 01069, Germany § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡
S Supporting Information *
ABSTRACT: The two-dimensional material KCu3−xSe2 was synthesized using both a K2Se3 flux and directly from the elements. It crystallizes in the CsAg3S2 structure (monoclinic space group C2/m with a = 15.417(3) Å, b = 4.0742(8) Å, c = 8.3190(17) Å, and β = 112.94(3)°), and single-crystal refinement revealed infinite copper-deficient [Cu3−xSe2]− layers separated by K+ ions. Thermal analysis indicated that KCu3−xSe2 melts congruently at ∼755 °C. UV−vis spectroscopy showed an optical band gap of ∼1.35 eV that is direct in nature, as confirmed by electronic structure calculations. Electronic transport measurements on single crystals yielded an in-plane resistivity of ∼6 × 10−1 Ω cm at 300 K that has a complex temperature dependence. The results of Seebeck coefficient measurements were consistent with a doped p-type semiconductor (S = +214 μV K−1 at 300 K), with doping being attributed to copper vacancies. Transport is dominated by low-mobility (on the order of 1 cm2 V−1 s−1) holes caused by relatively flat valence bands with substantial Cu 3d character and a significant concentration of Cu ion vacancy defects (p ∼ 1019 cm−3) in this material. Electronic band structure calculations showed that electrons should be significantly more mobile in this structure type.
■
INTRODUCTION Binary copper chalcogenides are host to a bevy of emergent phenomena such as superconductivity,1,2 superionic conductivity,3 and structural4,5 or order−disorder6 phase transitions. By the introduction of alkali-metal ions into these materials, their dimensionality and electronic structures can be tuned. Many copper-chalcogenide motifs are known in the A/Cu/Q system (A = alkali metal; Q = S, Se, Te), ranging from one-dimensional (1D) columns in A3Cu4S4 (A = Na, K),7,8 to two-dimensional (2D) layers in KCu3Q2 (Q = S, Te)9,10 and A3Cu8Q6 (A = K, Rb, Cs; Q = S, Se)11,12 and the three-dimensional (3D) networks in ACu7−xS4 (A = K, Rb, Tl).13 While the valence states of these ions are not strictly formal because of the appreciable covalency of Cu−Q bonds, a formal framework is useful for description. Most known alkali-metal copper chalcogenides contain only Cu+, with any mixed valence implied by stoichiometry falling on the chalcogenide anions as combinations of Q22− and Q2−. The mixed-valent members tend to exhibit metallic conduction, whereas the valence-precise compounds are semiconductors, as exemplified by our recent reports on the layered p-type compounds NaCu6Se414 and NaCu4Se315 as well as NaBa2Cu3S516 which was found to be a 2D degenerate semiconductor. Further complexity is added by easily formed, stable copper vacancies, which are often produced during synthesis of these materials. For example, © 2017 American Chemical Society
both Cu2−xSe and KCu7−xS4 can host a significant number of Cu vacancies (x values up to ∼0.05917 and 0.3418 at room temperature, respectively) and are p-type semiconductors. These defects are critical in determining the electronic properties and can influence or drive phase transitions such as vacancy ordering.6,19 Low-melting fluxes are powerful tools in materials discovery,20,21 often yielding kinetically stable phases that can decompose at the elevated temperatures employed in solidstate reactions. In this way, the high yields of traditional solidstate techniques can be combined with the structural diversity and rational design that characterize organic syntheses.22 Recently, molten polysulfide fluxes containing copper and tin were probed using in situ synchrotron X-ray diffraction, resulting in several new ternary phases and underlining the influence of the melt composition on their formation.8 With regard to our interest in layered compounds with physical properties related to the AxFe2−xSe2 (A = alkali metal) superconductors, we discovered the ternary phase KCu3−xSe2. Although its crystal structure was unknown, Jacyna-Onyszkiewicz et al. briefly reported its temperature-dependent Received: May 23, 2017 Revised: June 21, 2017 Published: June 21, 2017 6114
DOI: 10.1021/acs.chemmater.7b02117 Chem. Mater. 2017, 29, 6114−6121
Article
Chemistry of Materials resistivity.23,24 These data contained unexplained anomalies that bore resemblance to charge-density-wave formation (e.g., as in TiSe225) and thus warranted further study. Here we present the synthesis, crystal structure, and optical and electronic properties of KCu3−xSe2. We show that KCu3−xSe2 is a layered semiconductor with a direct band gap of ∼1.35 eV. Copper deficiency in crystals grown in excess K2Se3 flux is directly observed by single-crystal X-ray diffraction. The complex temperature dependence of the resistivity is explained by the presence of copper vacancies, which act as shallow acceptor defects and dope the material in a p-type manner. Density functional theory (DFT) calculations show a relatively flat valence band and hence holes with high effective masses. This, in combination with scattering processes from Cu vacancy defects, explains the low hole mobility (on the order of 1 cm2 V−1 s−1) determined from resistivity and Seebeck coefficient measurements. The electronic band structure indicates light, mobile electrons within the layers of KCu3−xSe2 and related materials.
■
Table 1. Crystal Data and Structure Refinement for KCu3−xSe2 at 293(2) K empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions volume Z density (calculated) absorption coefficient F(000) crystal size θ range for data collection index ranges reflections collected independent reflections completeness to θ = 25.240° refinement method data/restraints/ parameters goodness of fit final R indicesa [I > 2σ(I)] R indicesa [all data] largest diff. peak and hole
EXPERIMENTAL SECTION
Reagents. The following reagents were used as received: potassium metal (99%, Sigma-Aldrich) copper metal powder (99.9%, Cerac), and selenium beads (99.999%, Plasmaterials Inc.). Synthesis. All of the chemical manipulations were conducted inside an argon-filled glovebox (MBraun) with oxygen and moisture levels below 0.1 ppm. Phase-pure KCu3−xSe2 was synthesized from a near-stoichiometric mixture of the elements: K, Cu, and Se in 1.05:3:2 molar ratio, with a 5% molar excess of K metal added to account for evaporation. K (0.105 g, 2.7 mmol), Cu (0.489 g, 7.7 mmol), and Se (0.405 g, 5.13 mmol) were loaded into an alumina crucible and covered with an alumina cap before being flame-sealed in a 15 mm O.D. × 13 mm I.D. fused-silica tube under a pressure of
