Quantum Confinement and Carrier Transport in π-SnS Colloidal Quantum Dot Solids

Colloidal quantum dot (QD) solids are materials that exploit the quantum confinement properties of the constituent nanocrystals. Among the manifestation of the quantum confinement effect is the energy bandgap value variations by size that make them suitable for spectral matching in optoelectronic applications, such as photodetectors and photovoltaics. Significant progress in QD optoelectronic devices has been made mainly containing compounds with concerning high degree of toxicity for practical applications. Sn-chalcogenides are among the possible environmentally benign alternatives which their bulks have demonstrated intriguing properties for energy devices. Nanostructuring these Sn-chalcogenides may create variations in their crystal structures and properties. So far, all approaches to synthesizing Sn-chalcogenide NCs have difficulties producing a small nanoparticle diameter close to the limit where quantum confinement can occur. It hampers further investigation and exploration of the Sn-chalcogenide NCs despite the prospects of the materials.<br/><br/>Here we demonstrate robust quantum confinement effect in small-diameter cubic SnS colloidal nanocrystals and their uses as optoelectronic materials in the form of a high-performing photodetector device. Well-controlled production of cubic SnS colloidal NCs with a 4-10 nm diameter is established by developing a one-pot synthesis procedure inside an inert condition that allows Sn complexation with only a single type of ligand molecule. The x-ray diffractogram and transmission electron micrograph analysis revealed that the structure of the NCs is a so-called p-type chiral and cubic structure rather than the orthorhombic structure found in most of the bulk SnS. These NCs behave as quantum dots, exhibiting a quantum confinement effect with strong variations of energy bandgap by size. The cubic structure of the QDs eases the device fabrication, analogous to the established approaches in rocksalt QDs (e.g., PbS). The photodetectors of the SnS NCs exhibit high responsivity comparable to the other established QD systems, despite that the channels are made only from a single or few-layer(s). This new compound will open new pathways for establishing environmentally safe and high-performing QD optoelectronic, photovoltaic, and photocatalysis devices.<br/>References:<br/>[1] S.Z. Bisri, R. Miranti, N. Matsushita, Y. Iwasa, Jpn Pat. Appl. (2021)<br/>[2] R. Miranti, S.Z. Bisri et al. <i>submitted</i>