Toward Performance and Applications of Large Area Optical Thermometry Based on the Luminescence of Germanium‐Vacancy Defects in Diamond Nanocrystals

Herein optical properties of aggregates of hydrophobic nanodiamonds (NDs) with germanium‐vacancy (GeV) defects distributed on substrates with different hydrophilicities (bare Si or TiO2 (anatase) film on Si substrate) are investigated with an aim to achieve millimeter‐scale, nearly homogeneous spatial distribution of ND aggregates. ND aggregates are spread by drop casting of a water colloid. The luminescence spectra of GeV defects are studied in a wide temperature range (85–400 K). The possibility of optical thermometry operating in a wide temperature range also covering biotemperatures is demonstrated using a separate microaggregate of NDs on the anatase film. Millimeter‐scale large area optical thermometry is designed using substrates with different hydrophilicities. This finding opens a perspective to use luminescence spectra of germanium‐vacancy (GeV) defects for large‐scale optical thermometry over a wide temperature range (85–400 K) where the spatial resolution is determined by the size of the nanodiamond aggregates.