Category Archives: silicon

DNP-NMR of surface hydrogen on silicon microparticles #DNPNMR

Shimon, Daphna, Kipp J. van Schooten, Subhradip Paul, Zaili Peng, Susumu Takahashi, Walter Köckenberger, and Chandrasekhar Ramanathan. “DNP-NMR of Surface Hydrogen on Silicon Microparticles.” Solid State Nuclear Magnetic Resonance 101 (September 2019): 68–75.

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 1H 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 $ 150 À 300 Hz for both peaks, indicative of a sparse distribution of protons in both environments. The high frequency 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 low frequency peak was found to vary in position between À19 and À37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The low frequency 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.

Hyperpolarization of Frozen Hydrocarbon Gases by Dynamic Nuclear Polarization at 1.2 K #DNPNMR

Vuichoud, B., et al., Hyperpolarization of Frozen Hydrocarbon Gases by Dynamic Nuclear Polarization at 1.2 K. J Phys Chem Lett, 2016. 7(16): p. 3235-9.

We report a simple and general method for the hyperpolarization of condensed gases by dynamic nuclear polarization (DNP). The gases are adsorbed in the pores of structured mesoporous silica matrices known as HYPSOs (HYper Polarizing SOlids) that have paramagnetic polarizing agents covalently bound to the surface of the mesopores. DNP is performed at low temperatures and moderate magnetic fields (T = 1.2 K and B0 = 6.7 T). Frequency-modulated microwave irradiation is applied close to the electron spin resonance frequency (f = 188.3 GHz), and the electron spin polarization of the polarizing agents of HYPSO is transferred to the nuclear spins of the frozen gas. A proton polarization as high as P((1)H) = 70% can be obtained, which can be subsequently transferred to (13)C in natural abundance by cross-polarization, yielding up to P((13)C) = 27% for ethylene.

Cubic three-dimensional hybrid silica solids for nuclear hyperpolarization

Baudouin, D., et al., Cubic three-dimensional hybrid silica solids for nuclear hyperpolarization. Chemical Science, 2016.

Hyperpolarization of metabolites by dissolution dynamic nuclear polarization (D-DNP) for MRI applications often requires fast and efficient removal of the radicals (polarizing agents). Ordered mesoporous SBA-15 silica materials containing homogeneously dispersed radicals, referred to as HYperPolarizing SOlids (HYPSOs), enable high polarization – P(1H) = 50% at 1.2 K – and straightforward separation of the polarizing HYPSO material from the hyperpolarized solution by filtration. However, the one-dimensional tubular pores of SBA-15 type materials are not ideal for nuclear spin diffusion, which may limit efficient polarization. Here, we develop a generation of hyperpolarizing solids based on a SBA-16 structure with a network of pores interconnected in three dimensions, which allows a significant increase of polarization, i.e. P(1H) = 63% at 1.2 K. This result illustrates how one can improve materials by combining a control of the incorporation of radicals with a better design of the porous network structures.

Radical-free dynamic nuclear polarization using electronic defects in silicon

Cassidy, M.C., et al., Radical-free dynamic nuclear polarization using electronic defects in silicon. Physical Review B, 2013. 87(16): p. 161306.

Direct dynamic nuclear polarization of 1H 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.

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