Shimon, Daphna, Kipp J. van Schooten, Subhradip Paul, Zaili Peng, Susumu Takahashi, Walter Köckenberger, and Chandrasekhar Ramanathan. “DNP-NMR of Surface Hydrogen on Silicon Microparticles.” Solid State Nuclear Magnetic Resonance 101 (September 2019): 68–75.
Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) offers a promising route to studying local atomic environments at the surface of both crystalline and amorphous materials. We take advantage of unpaired electrons due to defects close to the surface of the silicon microparticles to hyperpolarize adjacent 1H nuclei. At 3.3 T and 4.2 K, we observe the presence of two proton peaks, each with a linewidth on the order of 5 kHz. Echo experiments indicate a homogeneous linewidth of $ 150 À 300 Hz for both peaks, indicative of a sparse distribution of protons in both environments. The high frequency peak at 10 ppm lies within the typical chemical shift range for proton NMR, and was found to be relatively stable over repeated measurements. The low frequency peak was found to vary in position between À19 and À37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The low frequency peak was also found to vary signiﬁcantly in intensity across different experimental runs, suggesting a weakly-bound species. These results suggest that the hydrogen is located in two distinct microscopic environments on the surface of these Si particles.