Organometallic-Derived ZnO Quantum Dots for UV Photodetectors: A Comparative Study of Ligand-Free and Zwitterion-Coated Systems

Abstract

Surface chemistry is a key factor governing the performance of solution-processed ZnO quantum dot (QD)-based optoelectronic devices. Conventional low-temperature synthetic approaches frequently yield nanostructures containing persistent surface ligands, residual impurities, and a high density of structural defects, limiting charge transport and device efficiency. Here, we demonstrate that an organometallic synthetic strategy based on weakly coordinated ligands provides a simple route toward effectively ligand-free ZnO QDs with improved optoelectronic performance. ZnO QDs were prepared using dimethyl sulfoxide (DMSO) or betaine as model capping agents, followed by low-temperature spin-coating to fabricate UV photodetectors. Structural and spectroscopic characterization confirmed the formation of phase-pure wurtzite ZnO nanocrystals with comparable sizes and optical properties for both synthetic routes. Importantly, direct comparison of effectively ligand-free DMSO-derived and zwitterion-coated betaine-derived ZnO QD isolates the role of surface chemistry in governing thin-film formation and device performance. The DMSO-derived device exhibited superior photodetection characteristics, including an on/off ratio of 3050, rise/fall times of 1.9/4.3 s, a photoresponsivity of 3.55 × 10−3 A W−1, and a detectivity of 4.18 × 1010 Jones under 373 nm irradiation at a 1 V bias. The enhanced performance is attributed to the weak coordination of DMSO, which facilitates ligand removal, improves interparticle electronic coupling, and promotes more efficient charge transport. More broadly, these findings establish weakly coordinated organometallic precursors as a versatile platform for molecular engineering of ZnO QD surfaces, providing a general strategy for improving solution-processed optoelectronic devices.