Silver Sulfide Nanocrystals and Their Photodetector Applications

Silver sulfide nanocrystals (Ag₂S NCs) exhibit unique infrared (IR) absorption and emission capabilities, drawing great interest for their broad applicability. These NCs are considered environmentally friendly alternatives to heavy metals such as lead (Pb), mercury (Hg), and cadmium (Cd) chalcogenides. This Account provides a comprehensive overview based on our research on Ag₂S NCs. We investigated their synthesis over size and shape, surface stoichiometry control, postsynthetic surface composition change, and optoelectronic application. The work began with developing a synthesis protocol for the Ag₂S NCs. Size-tunable and nearly monodisperse NCs were obtained through the precise control of precursor ratio. The ability to manipulate the size of the NCs enabled us to explore and adjust their optical properties. Another important aspect of the research focused on the mechanism of shape transformation. The evolution of the NCs from their initial spherical structure to more complex shapes such as rods and cubes was observed. Through rigorous investigations using a transmission electron microscope (TEM), we studied the relationship between the morphological changes and crystal facets. Investigations were also extended to surface chemistry, where methods were developed to tune the surface stoichiometry of Ag₂S NCs. Perfectly stoichiometric-surfaced Ag₂S NCs synthesized through ion-pair ligand-assisted surface reactions exhibited significantly increased photoluminescence (PL) and an enhanced epitaxial ZnS growth rate. Finally, we explored the cation exchange reactions of Ag₂S NCs. The cation exchange reaction with indium (In) ions yielded AgInS₂ NCs with size-dependent crystal structures: tetragonal for small NCs and orthorhombic for large NCs. A critical size at around 4.3 nm was observed, representing a trade-off between a thermodynamically more stable tetragonal structure and an orthorhombic structure that preserves the anionic framework. Throughout this Account, we address the challenges for the application of Ag₂S NCs and propose future directions including advancements in synthesis techniques, surface chemistry, and their applications. Ag₂S NCs typically show limitations such as low chemical and electrical stability, which may originate from the low lattice energy and high concentration of cation vacancies. However, such unique features can be advantageous for some applications, for example, acceptor materials in photomultiplication (PM)-type photodiodes. PM-type photodiodes were developed by combining polymeric semiconductors and Ag₂S NCs. These photodiodes can amplify signals by trapping electrons within Ag₂S NCs. These NCs efficiently trap multiple charge carriers from donor materials, in which their typical disadvantage is reinterpreted as a beneficial attribute for advanced device applications. In order to enhance the electron trapping efficiency, we synthesized Ag₂S NCs with a cation-rich surface were synthesized. This electron trapping property resulted in an optimized PM-type photodiode with a high EQE of over 170,000% and a specific detectivity of 3 × 10¹³ Jones. We anticipate that this Account will provide comprehensive insights into the chemistry and optoelectronic applications of the Ag₂S NCs.