Isotopically pure C13 layer as a stress sensor in a diamond anvil cell

Isotopically pure ${}^{13}\mathrm{C}$ homoepitaxial diamond layer of $20\pm 5\text{microns}$ thickness was grown on top of a brilliant cut diamond anvil by a microwave plasma chemical vapor deposition process for application as a stress sensor. This isotopically pure diamond tip was then used in conjunction with a natural abundance ${}^{12}\mathrm{C}$ diamond anvil to generate high pressure on a soft sample containing carbon nanotubes to $100\mathrm{GPa}$ . The ${}^{13}\mathrm{C}$ diamond Raman signal remained a distinct single peak showing that the mean normal stress experienced by this thin layer is uniform to the highest pressure. The stress-induced shift of the ${}^{13}\mathrm{C}$ layer is related to the quasihydrostatic pressure in the sample chamber using a ruby pressure sensor and shows a remarkably linear behavior to extreme pressures.