Dynamic nuclear polarization in silicon

We report on our DNP experiments on silicon wafers and nanoparticles. The DNP mechanism in n-doped wafers depends on the doping concentration, and a broad range of DNP enhancements have been observed. The largest enhancements we have recorded (over 200) were for wafers with donor concentrations in the range of $1-3 \times 10^{17}$ cm$^{-3}$, where the DNP is mediated by exchange-coupled clusters of donors. The silicon polarization obtained was over 10\%. We have also characterized the DNP of silicon nanoparticles suspended in a frozen solution. Off-resonant microwave irradiation of paramagnetic electron defects at the silicon-silicon dioxide interface in the nanoparticles results in a hyperpolarization of both the silicon spins of the particle as well as the water protons at the surface of the particle.