Quantitative measurement of graphitic sp2 on single nanodiamonds with sub-monolayer sensitivity using differential interference contrast and photo-thermal microscopy

The surface of nanodiamonds is prone to be decorated with a graphitic layer due to the thermodynamic instability of diamond versus graphite. Notably, this surface layer affects the zeta-potential of nanodiamonds controlling their colloidal stability, modulates the charging of embedded nitrogen vacancy centres used for sensing and quantum technology, and introduces an optical absorption which leads to heating and destruction of nanodiamonds under intense laser irradiation. Physical and chemical treatments to reduce the graphitic carbon are available, but a sensitive method to measure this graphitic layer on single nanodiamonds is lacking. Here, we demonstrate a non-destructive method to quantitatively determine the graphitic carbon correlatively with the nanodiamond size, on individual nanodiamonds. The method allows determining the fraction of graphitic sp2 to diamond sp3 bonded atoms, and estimate the sp2 surface layer thickness. We investigated milled nanodiamonds, both untreated and chemically treated to remove sp2. We found that untreated diamonds were covered by the equivalent of a single sp2 monolayer, which was reduced to 10% of a monolayer for treated nanodiamonds. In both cases, significant variations between individual nanodiamonds were observed. The minimum number of sp2 bonded atoms detectable under the measurement conditions reported here was found to be 2×104.