Dynamic Nuclear Polarization Enhanced NMR of Surface Hydrogen on Silicon Microparticles

Dynamic nuclear polarization (DNP) is a driven process that transfers higher electron spin polarization to surrounding nuclear spins via microwave irradiation near the electron Larmor frequency. We use frequency-modulated 94 GHz excitation of the endogenous dangling-bond electron spins present at the silicon surface, to acquire DNP-enhanced NMR spectra of the surface hydrogen . The high sensitivity DNP NMR signal detection at 3.34 T and 4 K is enabled by an overall signal enhancement of 4150 over the room temperature NMR signal at the same field. The NMR spectrum from a dry sample of polycrystalline silicon powder (1-5 μm) shows a distinctively narrow Lorentzian-shaped resonance with a width of 6.2 kHz, indicative of a very sparse distribution of protons within a few atomic layers of the silicon surface. The narrow proton spectrum measured from the silicon powder is similar to one of the spectral components observed in previous NMR studies of hydrogenated-amorphous silicon films in device-quality wafers. We also characterize the growth and exchange of these surface protons when the silicon particles were immersed in different solvents.