Phase- and Substoichiometry-Dependent Magneto-Optical Responses of Plasmonic Cu–Sn–S Ternary Semiconductor Nanodots

Abstract Cu–Sn–S (copper–tin–sulfide; CTS) ternary semiconducting chalcogenides are composed of nontoxic and earth-abundant elements, so there has been a great deal of interest in future photovoltaic devices. In this article, we investigate optical and magneto-optical (MO) properties of CTS nanodots with different phases, Cu3SnS4 and Cu2SnS3, to better understand the influence of elemental deficiencies on their composition-dependent performance. Both Cu3SnS4 and Cu2SnS3 nanodots are successfully obtained via direct synthesis using multiple cation precursors in the presence of thiourea and found to possess isostructural orthorhombic phases. The nanodots of Cu3SnS4 exhibit optical absorption attributed to localized surface plasmon resonance (LSPR) with a broad peak at 1500–2000 nm, while those of Cu2SnS3 at > ∼3000 nm, depending on their substoichiometry. The p-type LSPR mode of the Cu3SnS4 nanodots is rationalized by magnetic circular dichroism (MCD), and the surface Cu-deficiency rather than the donor defects (S-deficiency and/or Sn-excess) significantly contributes to the free carrier (hole) generation. The value of effective hole mass is estimated by the MCD spectral simulation. As for the Cu2SnS3 nanodots, in addition to the LSPR band in the IR region, two distinct optical transitions near the bandgap region (∼500–800 nm) are unveiled by strong MCD responses with the same signs, stemming from two noninteracting Faraday B-terms, and then the origin of these transitions is discussed.