Copper Vacancies and Heavy Holes in the Two-Dimensional Semiconductor KCu3–xSe2

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.