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Wurtzite CuInS₂ and CuIn_xGa_1−xS₂ nanoribbons: synthesis, optical and photoelectrical properties

Single crystalline wurtzite ternary and quaternary semiconductor nanoribbons (CuInS₂, CuIn_xGa_1−xS₂) were synthesized through a solution-based method. The structure and composition of the nanoribbons were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), the corresponding fast Fourier transform (FFT) and nanoscale-resolved elemental mapping. Detailed investigation of the growth mechanism by monitoring the structures and morphologies of the nanoribbons during the growth indicates that Cu_1.75S nanocrystals are formed first and act as a catalyst for the further growth of the nanoribbons. The high mobility of Cu⁺ promotes the generation of Cu⁺ vacancies in Cu_1.75S, which will facilitate the diffusion of Cu, In or Ga species from solution into Cu_1.75S to reach supersaturated states. The supersaturated species in the Cu_1.75S catalyst, Cu–In–S and Cu–In–Ga–S species, start to condense and crystallize to form wurtzite CuInS₂ or CuIn_xGa_1−xS₂ phases, firstly resulting in two-sided nanoparticles. Successive crystallizations gradually impel the Cu_1.75S catalyst head forward and prolong the length of the CuInS₂ or CuIn_xGa_1−xS₂ body, forming heterostructured nanorods and thus nanoribbons. The optical band gaps of CuIn_xGa_1−xS₂ nanoribbons can be continuously adjusted between 1.44 eV and 1.91 eV, depending on the Ga concentration in nanoribbons. The successful preparation of those ternary and quaternary semiconductor nanoribbons provide us an opportunity to study their photovoltaic properties. The primary photoresponsive current measurements demonstrate that wurtzite CuIn_xGa_1−xS₂ nanoribbons are excellent photoactive materials. Furthermore, this facile method could open a new way to synthesize other various nano-structured ternary and quaternary semiconductors, such as CuInSe₂ and CuIn_xGa_1−xSe₂, for applications in solar cells and other fields.