Radical-free dynamic nuclear polarization using electronic defects in silicon

Direct dynamic nuclear polarization of ${}^1$H nuclei in frozen water and water-ethanol mixtures is demonstrated using silicon nanoparticles as the polarizing agent. Electron spins at dangling-bond sites near the silicon surface are identified as the source of the nuclear hyperpolarization. This polarization method opens avenues for the fabrication of surface engineered nanostructures to create high nuclear spin polarized solutions without introducing contaminating radicals, and for the study of molecules adsorbed onto surfaces.

Figure 1

Microwave frequency dependence of the (a) ${}^1$H NMR signal (solid circles) and (b) ${}^{29}$Si NMR signal from a sample of natural isotopic abundance SiNP suspended in frozen water, measured at 4.2 K. No change in the ${}^1$H NMR signal was observed for the same sample without the SiNP (open circles), and no ${}^1$H signal was observed from a dry sample of SiNP. Upper insets in (a) and (b): ${}^1$H and ${}^{29}$Si spectra for $\mu$W irradiation frequency of 65.875 GHz. Lower inset in (b): Room temperature ESR signal of natural abundance SiNP.

Figure 2

(a) Schematic model showing the location of DNP-active electron spins at and near the silicon-silicon oxide interface. Arrows indicate the directions of nuclear polarization flow away from the surface of the particle. (b) Thermodynamic model (see text) for the polarization process of both the ${}^1$H and ${}^{29}$Si nuclear spin baths. ${}^{29}$Si and ${}^1$H nuclei near the defect electrons are polarized directly through a dipolar interaction on time scales ${\tau }_{\text{Si}1}$ and ${\tau }_{\text{H}1}$. Nuclei far away from the surface are then polarized by nuclear spin diffusion on time scales ${\tau }_{\text{Si}2}$ and ${\tau }_{\text{H}2}$ that are dependent on the spatial separation of the nuclear spins.

Figure 3

Time evolution of the ${}^1$H nuclear polarization with (solid shapes) and without (open shapes) applied $\mu$W irradiation for frozen solutions of SiNP:H${}_2$O (4.7$\%$ ${}^{29}$Si) and SiNP:EtOH:H${}_2$O (4.7$\%$ ${}^{29}$Si, 91.4$\%$ ${}^{29}$Si). The enhancements are scaled relative to the 4.2 K equilibrium polarization. Fits are exponential (dashed lines) or biexponential (solid lines) (see text). Error bars are shown when greater than the data symbol.