π-SnS Colloidal Quantum Dots for Field-Effect Transistors

Significant progress in QD optoelectronic devices and energy devices has been made mainly by Pb-based, Hg-based, and Cd-based binary compounds, or the involvement of rare noble metal alloys. While some have reached commercial markets, their high degree of toxicity is the primary concern for practical applications. We recently developed a rapid one-pot method to synthesize a novel phase of tin monosulfide quantum dots (QDs), <i>π-</i>SnS¹. This cubic QD phase exhibits a larger band gap bandgap (1.53–1.69 eV) than the other SnS phase (the orthorhombic <i>α-</i>SnS), and more thermodynamically stable. This method yields QD radii below 10 nm, allowing us to explore the quantum confinement effect, which can be utilized for high-performance photodetector devices. Despite these prospects, obtaining the scalable synthesis of this SnS QD phase with good monodispersity is still challenging. On the other hand, the electronic transport properties of this material have never been investigated, which will directly impact the design of its practical uses for electronic or optoelectronic device applications.<br/>Here we report a controllable synthesis of monodispersed <i>π-</i>SnS QDs and demonstrate the field-effect transistors of colloidal <i>π-</i>SnS QD assemblies. We can synthesize the SnS QDs with diameters ranging from 5 nm to 8 nm and better air stability through synthesis protocol optimizations than the previously reported method. Furthermore, by performing the integration of synthesis and optical absorption measurement, we could fine-tune the synthesis methods so that we can obtain the synthesis condition boundary to obtain either <i>π-</i>SnS QDs or <i>α-</i>SnS. Synthesis temperature is found to become the critical factor that can lead to different end results. The <i>π-</i>SnS phase was generated at a specific injection temperature range.<br/>We are able to fabricate field-effect transistors of the <i>π-</i>SnS by adapting the methods of performing ligand exchange in the solid-state phase during the layer-by-layer deposition of the QD monolayers.^[2,3] The oxide-gated <i>π-</i>SnS QD FETs exhibit p-type characteristics with charge carrier mobility values comparable to the early development of many other metal chalcogenides QDs. This investigation will provide insights into the factors influencing the formation of different phases of SnS QDs. Furthermore, the first success of demonstrating transistor operation of <i>π-</i>SnS QD assemblies presents new opportunities to investigate their charge carrier transport for developing environmentally friendly QD electronics.<br/><br/>Refs.: [1] R. Miranti, R.D. Septianto et.al. J. Phys. Chem. C, 126, 11 5323 (2022) [2] R.D. Septianto, L. Liu et.al. NPG Asia Mater, 12, 33 (2020). [3] R.D. Septianto, R. Miranti et.al. Nat. Commun. 14, 2670 (2023).