Understanding the synthetic pathway of a single-phase quarternary semiconductor using surface-enhanced Raman scattering: A case of wurtzite Cu₂ZnSnS₄ nanoparticles

Single-phase Cu₂ZnSnS₄ (CZTS) is an essential prerequisite toward a high-efficiency thin-film solar cell device. Herein, the selective phase formation of single-phase CZTS nanoparticles by ligand control is reported. Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time as a characterization tool for nanoparticles to differentiate the mixed compositional phase (e.g., CZTS, CTS, and ZnS), which cannot be distinguished by X-ray diffraction. Due to the superior selectivity and sensitivity of SERS, the growth mechanism of CZTS nanoparticle formation by hot injection is revealed to involve three growth steps. First, it starts with nucleation of Cu_2-xS nanoparticles, followed by diffusion of Sn ⁴⁺ into Cu_2-xS nanoparticles to form the Cu ₃SnS₄ (CTS) phase and diffusion of Zn²⁺ into CTS nanoparticles to form the CZTS phase. In addition, it is revealed that single-phase CZTS nanoparticles can be obtained via balancing the rate of CTS phase formation and diffusion of Zn²⁺ into the CTS phase. We demonstrate that this balance can be achieved by 1 mL of thiol with Cu(OAc) ₂, Sn(OAc)₄, and Zn(acac)₂ metal salts to synthesize the CZTS phase without the presence of a detectable binary/ternary phase with SERS.