Negative Photoconductivity in Liquid-Metal-Derived Ultrathin 2D Ceria-Rich Oxide Nanosheets for Broadband Photodetection

Engineering two-dimensional (2D) ultrathin oxides through in situ control during growth enables functional materials to be realized from systems whose bulk crystals are nonlayered and unsuitable for conventional exfoliation. Liquid-metal chemistry provides such a route by using liquid gallium as a solvent and growth medium, allowing added metal atoms to participate directly in surface-oxide formation, while yielding continuous and smooth nanoscale 2D sheets with preserved morphological integrity. Cerium oxide, despite its broad relevance in sensing, catalysis, and optoelectronics, has remained largely unexplored as a stratified 2D oxide because its parent phases are nonlayered and difficult to form as stable ultrathin nanosheets via conventional routes. Here, we show that dissolving Ce metal into liquid gallium and oxidizing at optimized temperatures at the metal and ambient air interface produce mixed ultrathin 2D oxides with a surface-enriched CeO2 character. The progressive evolution of Ce-dominated surface oxides as a function of temperature during growth functionalizes the surface-delaminated nanoscale oxides, giving rise to devices that exhibit negative photodetection spanning from the ultraviolet (UV, 365 nm) to the near-infrared (NIR, 1050 nm) spectral range. Density functional theory (DFT) calculations predict the formation of midgap electronic states, which facilitate broadband UV–NIR negative photodetection. This response originates from surface-engineered ultrathin 2D oxides, where nanoscale thickness and high surface-to-volume ratio play a critical role in the observed optoelectronic behavior.