We use dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) at liquid helium temperatures to directly detect hydrogen attached to the surface of silicon microparticles. The proton 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 attached to the silicon surface. These protons are within a few atomic monolayers of the silicon surface. The high sensitivity NMR detection of surface protons from low surface area (0.26 − 1.3 m 2 /g) particles is enabled by an overall signal enhancement of 4150 over the room temperature NMR signal at the same field. When the particles were suspended in a solvent with 80% H 2 O and 20% D 2 O, the narrow peak was observed to grow in intensity over time, indicating growth of the sparse surface proton layer. However, when the particles were suspended in a solvent with 20% H 2 O and 80% D 2 O, the narrow bound-proton peak was observed to shrink due to exchange between * To whom correspondence should be addressed arXiv:1610.06544v1 [cond-mat.mtrl-sci] 20 Oct 2016 the surface protons and the deuterium in solution. This decrease was accompanied by a concomitant growth in the intensity of the frozen solvent peak, as the relative proton concentration of the solvent increased. When the particles were suspended in the organic solvent hexane, the proton NMR spectra remained unchanged over time. These results are consistent with the known chemisorption of water on the silicon surface resulting in the formation of hydride and hydroxyl species. Low-temperature DNP NMR can thus be used as a non-destructive probe of surface corrosion for silicon in aqueous environments. This is important in the context of using silicon MEMS and bioMEMS devices in such environments, for silicon micro-and nano-particle MRI imaging agents, and the use of nanosilicon for splitting water in fuel cells.

Magnetic-field and microwave-frequency modulated DNP experiments have been shown to yield improved enhancements over conventional DNP techniques, and even to shorten polarization build-up times. The resulting increase in signal-to-noise ratios can lead to significantly shorter acquisition times in signal-limited multi-dimensional NMR experiments and pave the way to the study of even smaller sample volumes. In this paper we describe the design and performance of a broadband system for microwave frequency-and amplitude-modulated DNP that has been engineered to minimize both microwave and thermal losses during operation at liquid helium temperatures. The system incorporates a flexible source that can generate arbitrary waveforms at 94 GHz with a bandwidth greater than 1 GHz, as well as a probe that efficiently transmits the millimeter waves from room temperature outside the magnet to a cryogenic environment inside the magnet. Using a thin-walled brass tube as an overmoded waveguide to transmit a hybrid HE11 mode, it is possible to limit the losses to 1 dB across a 2 GHz bandwidth. The loss is dominated by the presence of a quartz window used to isolate the waveguide pipe. This performance is comparable to systems with corrugated waveguide or quasi-optical components. The overall excitation bandwidth of the probe is seen to be primarily determined by the final antenna or resonator used to excite the sample and its coupling to the NMR RF coil. Understanding the instrumental limitations imposed on any modulation scheme is key to understanding the observed DNP results and potentially identifying the underlying mechanisms. We demonstrate the utility of our design with a set of triangular frequency-modulated DNP experiments.

Electrically-detected magnetic resonance (EDMR) provides a highly sensitive method for reading out the state of donor spins in silicon. The technique relies on a spin-dependent recombination (SDR) process involving dopant spins that are coupled to interfacial defect spins near the Si/SiO2 interface. To prevent ionization of the donors, the experiments are performed at cryogenic temperatures and the mobile charge carriers needed are generated via optical excitation. The influence of this optical excitation on the SDR process and the resulting EDMR signal is still not well understood. Here, we use EDMR to characterize changes to both phosphorus and defect spin readout as a function of optical excitation using: a 980 nm laser with energy just above the silicon band edge at cryogenic temperatures; a 405 nm laser to generate hot surface-carriers; and a broadband white light source. EDMR signals are observed from the phosphorus donor and two distinct defect species in all the experiments. With near-infrared excitation, we find that the EDMR signal primarily arises from donor-defect pairs, while at higher photon energies there are significant additional contributions from defect-defect pairs. The optical penetration depth into silicon is also known to be strongly wavelength dependent at cryogenic temperatures. The energy of the optical excitation is observed to strongly modulate the kinetics of the SDR process. Careful tuning of the optical photon energy could therefore be used to control both the subset of spin pairs contributing to the EDMR signal as well as the dynamics of the SDR process.