# DNP-NMR of surface hydrogen on silicon microparticles

## TL;DR

This study uses DNP-enhanced NMR to investigate surface hydrogen environments on silicon microparticles, revealing two distinct proton populations with different chemical shifts and stability, indicating diverse surface chemistries.

## Contribution

It demonstrates the application of DNP-NMR to detect and characterize surface hydrogen in silicon microparticles, highlighting the presence of two distinct proton environments.

## Key findings

- Two proton peaks observed at 10 ppm and -19 to -37 ppm.
- Homogeneous linewidths of 150-300 Hz indicate sparse proton distribution.
- The upfield peak varies significantly, suggesting a weakly-bound species.

## Abstract

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 $^{1}$H 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 $\sim 150-300$ Hz for both peaks, indicative of a sparse distribution of protons in both environments. The downfield 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 upfield peak was found to vary in position between -19 and -37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The upfield 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.