X-ray absorption spectroscopy or XAS is a powerful tool to examine the electronic structure of solids and adsorbed molecules on surfaces and is especially powerful in examining low-Z elements such as carbon. Using XAS to measure shell thickness of a core-shell nanostructures is rare and we demonstrate it here with nanoscale diamond with HRTEM as a supporting technique. Silica shells on high-pressure high-temperature (HPHT) nanodiamond (ND) cores provides a versatile platform to engineer their surface structure and functionality for biolabeling applications based on the fluorescent nitrogen vacancy center (NVC), yet in- depth studies of their electronic structure and mechanism for silica growth. Fluorescent NVC-NDs are an emergent biolabeling tool due to being non-cytotoxic, photostable, and capable of detecting magnetic and electric fields via spin-readout optical microscopy. Here we use a modified Stöber method to synthesize 2-20 nm thick shells of SiO₂ onto carboxylic acid rich ND cores (20-40 nm) and study their electronic structure via X-ray absorption spectroscopy. We observe the suppression of the diamond core-hole exciton as a function of dielectric thickness (SiO₂) for the first time and the observed signal based on total electron yield is consistent with the photoelectron escape model (Auger and direct photoemission). Silica growth on carboxylated NDs was found to reduce or eliminate the presence of carboxylic acids These finding support our mechanistic theory that silyl-ether (C-O-Si) bond formation is the main driver for SiO₂ formation and that surface reconstruction plays a role. The SiO₂ shells allow for the grafting of moieties that provide colloidal stability (PEG) in biological environments while amine, sulfyl-hydryl and fluorine moieties act as anchors for bioconjugation. Surface structure analysis was confirmed via FTIR, X-ray absorption (XAS) and X-ray photoelectron spectroscopy. Total electron yield XAS allows for observation of both the SiO₂ shell and ND core electronic structure simultaneously at SiO₂ thicknesses less then 5 nm. These observations help clarify the mechanistic and kinetic details of chemically activating high pressure high temperature nanodiamond surfaces and will advance the utility of fluorescent NDs in biological and quantum sensing applications.

Nawal Sugal

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