Nuclear Magnetic Resonance (NMR) spectroscopy and imaging techniques have found application across a very broad range of subjects. Dynamic nuclear polarization (DNP) utilizes microwave irradiation of a coupled electron-nuclear spin system, in either a liquid or solid, to facilitate a transfer of polarization from the electron to the nuclear spin. As the magnetic moment of an electron spin is around three orders of magnitude larger than that of a nuclear spin (a factor of 660 for protons and 2640 for carbon-13), this results in a significant enhancement of the nuclear spin polarization. This increased signal can be used to measure nuclear spins present at surfaces and interfaces – something that has traditionally been very difficult. Our group has studied DNP in both silicon and diamond, though we are now primarily focused on exploring room-temperature DNP in diamond. We currently have one DNP setup at 94 GHz and are in the process of putting together a second one at 196 GHz.
Diamond
The most well known defect in diamond is likely the nitrogen-vacancy (NV) defect, where a substitutional nitrogen (a nitrogen that substitutes a carbon) is next to a vacancy in the carbon lattice. NV centers have received a great deal of attention in many fields of science, including NMR and dynamic nuclear polarization (DNP) because the electron (spin 1) is optically-active, and because the diamonds exhibit long coherence and relaxation times, making room temperature DNP a possibility. Another lesser studied defect is the substitutional nitrogen (or P1) center. P1 electrons (spin 1/2) are not optically-active, but also exhibit long coherence and relaxation times. These P1 centers are typically present at significantly larger concentrations (about an order magnitude larger) than NVs and are well suited for DNP applications at room temperature. Several groups have shown that P1 centers are good sources for DNP of 13C nuclei within the diamonds themselves.
We recently showed that multiple DNP mechanisms can coexist in the same diamond crystal due to clustering of P1 centers. This work was selected as one of the covers for the Journal of Physical Chemistry C. We also explored how microwave modulation influences the relative contributions of different mechanisms.
We are currently exploring if it is possible to optically hyperpolarize the P1 centers via the NV centers to further improve DNP enhancement.