Disordered Carbon Shells Enable Shape Dependent Fluorescence Enhancement in Nanodiamond Quantum Sensors Revealed by Correlative TEM–Fluorescence Microscopy

ABSTRACT Fluorescent nanodiamonds (FNDs) containing nitrogen–vacancy (NV) centers are powerful quantum sensors capable of nanoscale optical and spin‐based detection of temperature and electric or magnetic fields. Improving their fluorescence brightness is critical for enhancing sensitivity, yet the underlying mechanisms remain poorly understood. Here, we show that treatment with strongly oxidizing molten produces an ultrathin (2 nm) disordered‐carbon shell on FND that significantly enhances NV emission. Using correlative transmission electron microscopy and photoluminescence (TEMPL), we directly link single‐particle morphology to optical performance and uncover previously inaccessible shape‐dependent behavior. For particles with aspect ratios exceeding six, volume‐normalized brightness increases by more than twofold compared with conventional air‐oxidized FNDs. Such enhancement does not arise from altered surface chemistry but from constructive sub‐wavelength interference at the diamond–carbon interface, enabled by the shell's lower refractive index (1.8–2.2). Moreover, despite the presence of disordered carbon, spin properties, including ODMR contrast and times remain comparable to those of air‐oxidized FNDs. These results reveal the physical origin of brightness enhancement in ‐treated FNDs, establish low‐index interfacial shells as an effective strategy for enhancing NV emission, and provide a simple route to brighter FND quantum sensors.