Chemical insights into silica growth on nanoscale diamond using multimodal characterization and simulation

Surface chemistry of materials that host quantum bits such as diamond are an important avenue of exploration as quantum computation and quantum sensing platforms mature. Interfacing diamond in general, and nanoscale diamond (ND) in particular with silica is a potential route to integrate the quantum bit into a photonic device, fiber optic, cells or tissues with flexible functionalization chemistry. While silica growth on ND cores has been used successfully for quantum sensing and biolabeling, the surface mechanism to initiate growth was unknown. This report describes the surface chemistry responsible for silica bond formation on diamond and uses X-ray absorption spectroscopy (XAS) to probe the diamond surface chemistry and its electronic structure with increasing silica thickness. A modified Stöber (Cigler) method was used to synthesize 2–35 nm thick shells of SiO2 onto carboxylic acid rich ND cores and the diamond features and surface structure were characterized by overlapping techniques including electron microscopy. Importantly, we discovered that SiO2 growth on carboxylated NDs eliminates the presence of carboxylic acids and that basic ethanolic solutions converts the ND surface to an alcohol-rich surface prior to silica growth. The data supports a mechanism that alcohols on the ND surface generate silyl-ether (ND-O-Si-(OH)3) bonds due to rehydroxylation by ammonium hydroxide in ethanol. Additionally, resonant inelastic X-ray scattering (RIXS) maps produced by the transition edge sensor supports the chemical analysis provided by XAS. The suppression of the diamond electronic structure as a function of SiO2 thickness was observed, and the Auger electron escape depth was modeled using the NIST database for the Simulation of Electron Spectra for Surface Analysis (SESSA) to support our experimental results. Researchers using high-pressure high temperature (HPHT) NDs or any alcohol-terminated material (metal oxides, oxidized silicon carbide or cubic-boron nitride) for quantum sensing applications may exploit these results to design new core-shell quantum sensors with base-catalyzed reactions and metal oxide precursors.