Room-temperature hyperpolarization of polycrystalline samples with optically polarized triplet electrons: pentacene or nitrogen-vacancy center in diamond?

Abstract

We demonstrate room-temperature C-13 hyperpolarization by dynamic nuclear polarization (DNP) using optically polarized triplet electron spins in two polycrystalline systems: pentacene-doped [carboxyl-C-13] benzoic acid and microdiamonds containing nitrogen-vacancy (NV(-)) centers. For both samples, the integrated solid effect (ISE) is used to polarize the C-13 spin system in magnetic fields of 350–400 mT. In the benzoic acid sample, the C-13 spin polarization is enhanced by up to 0.12 % through direct electron-to-C-13 polarization transfer without performing dynamic H-1 polarization followed by H-1-C-13 cross-polarization. In addition, the ISE has been successfully applied to polarize naturally abundant C-13 spins in a microdiamond sample to 0.01 %. To characterize the buildup of the C-13 polarization, we discuss the efficiencies of direct polarization transfer between the electron and C-13 spins as well as that of C-13-C-13 spin diffusion, examining various parameters which are beneficial or detrimental for successful bulk dynamic C-13 polarization.