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“An NV center is created when two adjacent carbon atoms in the lattice of a pure diamond crystal are removed from the lattice leaving two gaps, one of which is filled with a nitrogen atom, and one of which remains vacant,” Pines explains. “This leaves unbound electrons in the center between the nitrogen atom and a vacancy that give rise to unique and well-defined electron spin polarization states.”
In earlier studies, Pines and his group demonstrated that a low-strength magnetic field could be used to transfer NV center electron spin polarization to nearby carbon-13 nuclei, resulting in hyperpolarized nuclei. This spin transference process – called dynamic nuclear polarization – had been used before to enhance NMR signals, but always in the presence of high-strength magnetic fields and cryogenic temperatures. Pines and his group eliminated these requirements by placing a permanent magnet near the diamond.
“In our new study we’re using microwaves to match the energy between electrons and carbon-13 nuclei rather than a magnetic field, which removes some difficult restrictions on the strength and alignment of the magnetic field and makes our technique more easy to use,” says King. “Also, in our previous studies, we inferred the presence of nuclear polarization indirectly through optical measurements because we weren’t able to test if the bulk sample was polarized or just the nuclei that were very close to the NV centers. By eliminating the need for even a weak magnetic field, we’re now able to make direct measurements of the bulk sample with NMR.”
In their Nature Communications paper, Pines, King and the other co-authors say that hyperpolarized diamonds, which can be efficiently integrated into existing fabrication techniques to create high surface area diamond devices, should provide a general platform for polarization transfer.
“We envision highly enhanced NMR of liquids and solids using existing polarization transfer techniques, such as cross-polarization in solids and cross-relaxation in liquids, or direct dynamic nuclear polarization to outside nuclei from NV centers,” King says, noting that such transfer of polarization to solid surface and liquids had been previously demonstrated by the Pines group using laser polarized Xe-129. “Our hyperpolarization technique based on optically polarized NV centers is far more robust and efficient and should be applicable to arbitrary target molecules, including biological systems that must be maintained at near ambient conditions.”
This research was supported by the DOE Office of Science.
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