Influence of crystallization temperature on fluorescence of n-diamond quantum dots

Nanodiamonds are popular biological labels because of their superior mechanical and optical properties. Their surfaces bridging the core and surrounding medium play a key role in determining their bio-linkage and photophysical properties. n-diamond is a mysterious carbon allotrope whose crystal structure remains debated. We study the influence of the crystallization temperature on the fluorescence properties of the colloidal n-diamond quantum dots (n-DQDs) with sizes of several nanometers. They exhibit multiband fluorescence across the whole visible region which depends sensitively on the crystallization temperature. Their surfaces turn from hydrophobic ones rich of sp²-bonded carbon into hydrophilic ones rich of carboxyl derivatives and hydroxyl groups as the crystallization temperature increases. The different surface states correlated with the surface structures account for the distinct fluorescence properties of the n-DQDs crystallized at different temperatures. These high-purity ultrasmall n-DQDs with tunable surface chemistry and fluorescence properties are promising multicolor biomarkers and lighting sources.