Heat treatment of nanodiamond particles with controlled powder layer depth

A method of making luminescent diamond consisting of:

• subjecting a volume of natural precursor diamond grains with an average grain size of from 10 μm to 50 μm to high-pressure/high-temperature conditions in a pressure cell and thereby forming diamond grains with luminescent centers, wherein the high-pressure/high-temperature conditions are from about 1300° C. to about 2500° C. and from about 3.0 GPa to about 10 GPa;
• forming a suspension by separating the diamond grains into a powder and suspending the diamond grains in water;
• adhering the suspension to a glass substrate and controlling a thickness of the suspension on the substrate to be from 5 μm to 15 μm, wherein the diamond grains have a concentration of from 0.1% to 10% by weight in the suspension;
• heat treating the suspension on the substrate at a temperature in a range between 450° C. and 550° C. for 60 minutes, thereby changing at least one of a color of the suspension or oxygen terminations on diamond particles of the suspension and increasing a photoluminescence intensity of the diamond grains;
• and
• removing the diamond particles from the substrate and separating the removed diamond particles by grain size,
• wherein the pressure cell is configured to impose a differential or asymmetric pressure on the precursor diamond grains in order to plastically deform the precursor diamond grains and produce the luminescent centers in the diamond grains.