Amorphous and crystalline copper sulfides, CuS
Copper sulfides CuS have been synthesized by a reaction between [Cu(en)2]2+(en = ethylenediamine) and thiourea in alkaline solutions at different temperatures. The product obtained at 10 °C (CuS-10) was amorphous and showed semiconductive properties with an electrical conductivity of 0.1 S cm–1 at 298 K and 3.6 × 10–3 S cm–1 at 108 K, whereas the product at 30°C (CuS-30) was a crystalline CuS (covellite) that exhibited a metal-like temperature dependence with a conductivity of 92 S cm –1 at 304 K and 101 S cm–1 at 101 K. CuS-10 was changed to a crystalline CuS when annealed at 100-150°C. The resulting CuS had a higher crystallinity than that of CuS-30, and the electrical conductivity showed a metal-like temperature dependence: 27 S cm–1 at 299 K and 31 S cm –1 at 98 K. When the reactions were conducted with an immersed polymer film in the solution, CuS films were formed on the substrate surface. The CuS-30 film and the annealed CuS-10 film were electroconductive and transparent in the visible spectral region. The synthesis of amorphous CuS followed by thermal treatment is a new method for the fabrication of metal sulfide films of high quality.