Controllable Ultrathin Nickel Nanoislands With Dense Discrete Space Charge Regions: Steering Hole Extraction for High‐Performance Underwater Multispectral Weak‐Light Photodetection

Abstract Undersea optical communication (UOC) is vital for ocean exploration and military applications. In the dim‐light underwater environment, photodetectors must maximize photon utilization by minimizing optical losses and carrier recombination. This can be achieved by integrating ultrathin metal nanostructures with photocatalysts to form Schottky junctions, which enhance charge separation and injection while mitigating metal‐induced light shading. The strategic design of discrete metal nanostructures providing numerous high‐depth space charge regions (SCRs) without overlap offers a promising approach to optimize hole transport paths and further suppress recombination. Here, a facile phase‐separation lithography technique is explored to fabricate tunable ultrathin Ni nanoislands atop n‐Si, yielding high‐performance photoelectrochemical photodetectors (PEC PDs) tailored for underwater weak‐light environments. This results indicate that key determinant of hole extraction behavior is the relationship between the spacing distance of adjacent Ni nanostructures (d_s) and twice the SCR radius (W_s). PEC PDs with optimized 8 nm ultrathin Ni nanostructures featuring closely but non‐overlapping SCRs, exhibit a 55‐fold increase in photoresponsivity (2.2 mA W⁻¹) and a 128‐fold enhancement in detection sensitivity (3.2 × 10¹¹ Jones) at 0 V over Ni film, revealing the exceptional stability. Furthermore, this approach enables effective detection across UV–vis‐near infrared spectrum, supporting reliable multispectral UOC and underwater imaging capabilities.