Silver Sulfide Nanocrystals and Their Photodetector\nApplications

ConspectusSilver sulfide\nnanocrystals (Ag<sub>2</sub>S NCs) exhibit unique\ninfrared (IR) absorption and emission capabilities, drawing great\ninterest for their broad applicability. These NCs are considered \nenvironmentally friendly alternatives to heavy metals such as lead\n(Pb), mercury (Hg), and cadmium (Cd) chalcogenides. This Account provides\na comprehensive overview based on our research on Ag<sub>2</sub>S\nNCs. We investigated their synthesis over size and shape, surface\nstoichiometry control, postsynthetic surface composition change, and\noptoelectronic application. The work began with developing a synthesis\nprotocol for the Ag<sub>2</sub>S NCs. Size-tunable and nearly monodisperse\nNCs were obtained through the precise control of precursor ratio.\nThe ability to manipulate the size of the NCs enabled us to explore\nand adjust their optical properties. Another important aspect of the\nresearch focused on the mechanism of shape transformation. The evolution\nof the NCs from their initial spherical structure to more complex\nshapes such as rods and cubes was observed. Through rigorous investigations\nusing a transmission electron microscope (TEM), we studied the relationship\nbetween the morphological changes and crystal facets. Investigations\nwere also extended to surface chemistry, where methods were developed\nto tune the surface stoichiometry of Ag<sub>2</sub>S NCs. Perfectly\nstoichiometric-surfaced Ag<sub>2</sub>S NCs synthesized through ion-pair\nligand-assisted surface reactions exhibited significantly increased\nphotoluminescence (PL) and an enhanced epitaxial ZnS growth rate.\nFinally, we explored the cation exchange reactions of Ag<sub>2</sub>S NCs. The cation exchange reaction with indium (In) ions yielded\nAgInS<sub>2</sub> NCs with size-dependent crystal structures: tetragonal\nfor small NCs and orthorhombic for large NCs. A critical size at around\n4.3 nm was observed, representing a trade-off between a thermodynamically\nmore stable tetragonal structure and an orthorhombic structure that\npreserves the anionic framework. Throughout this Account, we address\nthe challenges for the application of Ag<sub>2</sub>S NCs and propose\nfuture directions including advancements in synthesis techniques,\nsurface chemistry, and their applications. Ag<sub>2</sub>S NCs typically\nshow limitations such as low chemical and electrical stability, which\nmay originate from the low lattice energy and high concentration of\ncation vacancies. However, such unique features can be advantageous\nfor some applications, for example, acceptor materials in photomultiplication\n(PM)-type photodiodes. PM-type photodiodes were developed by combining\npolymeric semiconductors and Ag<sub>2</sub>S NCs. These photodiodes\ncan amplify signals by trapping electrons within Ag<sub>2</sub>S\nNCs. These NCs efficiently trap multiple charge carriers from donor\nmaterials, in which their typical disadvantage is reinterpreted as\na beneficial attribute for advanced device applications. In order\nto enhance the electron trapping efficiency, we synthesized Ag<sub>2</sub>S NCs with a cation-rich surface were synthesized. This electron\ntrapping property resulted in an optimized PM-type photodiode with\na high EQE of over 170,000% and a specific detectivity of 3 ×\n10<sup>13</sup> Jones. We anticipate that this Account will provide\ncomprehensive insights into the chemistry and optoelectronic applications\nof the Ag<sub>2</sub>S NCs.