Simple synthesis of ultra-small nanodiamonds with tunable size and photoluminescence

We report on a simple method for fabricating ultra-small nanodiamonds (NDs) with size tunability and uniformity by femtosecond laser ablation in solution. Photoluminescence (PL) is tailored in a broad range from ultra-violet (UV) to red region through adjusting the carbon sources or solutions. The NDs also emit multiphoton luminescence. Comprehensive discussions based on the anomalous PL behavior and well-defined absorption and excitation peaks suggest that UV emission originates from radiative recombination through intrinsic direct transitions between highly localized π states and blue or green emission originates from radiative recombination through defect energy trapping states.