Endogenous Stable Radicals for Characterization of Thermally Carbonized Porous Silicon by Solid-State Dynamic Nuclear Polarization ¹³C NMR

Abstract

As with all nanomaterials, characterization of the surface chemistry of mesoporous silicon (PSi) is crucial for the development in its diverse applications. Nuclear magnetic resonance (NMR) is one of the most powerful methods to study the chemistry of nanomaterials, but it is currently underutilized with PSi due to low signal-to-noise ratios achieved with this material which lead to very long measurement times. Here we show that endogenous radicals exist in thermally carbonized PSi and demonstrate the feasibility of solid-state dynamic nuclear polarization (DNP) NMR without addition of organic radicals. Use of DNP NMR is demonstrated to highly improve the signal-to-noise ratio while significantly reducing the measurement times. This technique opens new possibilities for the use of more advanced NMR techniques allowing the detailed characterization of complex materials such as PSi. Furthermore, the chemical structure of thermally carbonized PSi is studied by complementary techniques, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy.

Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning

“…These results, obtained using a magnetic field strength of 10 T, are of primary importance for the development of MAS-DNP at higher magnetic fields and should allow extending significantly the range of polarizing agents that can be employed efficiently (biradicals, 19–21 organic radicals with narrow EPR resonances, 22 metal ions, 23 intrinsic paramagnetic defects 24 etc. ).…”

Thermally Carbonized Porous Silicon and Its Recent Applications

“…This leads to the formation of a hydrocarbon‐terminated surface (THCPSi). At higher temperatures, usually above 750 °C, all of the hydrogen atoms are desorbing and the surface forms a nonstoichiometric SiC layer (TCPSi), which upon exposure to ambient conditions becomes covered by a thin oxide layer …”

“…X‐ray photoelectron spectroscopy (XPS) is another very good tool for surface characterization . However, it also suffers from some deficiencies related to carbon and oxygen, similar to FTIR.…”

Persistent Radicals of Self‐assembled Benzophenone bis‐Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid‐state DNP MAS Spectroscopy

“…4 Recent work suggests that high field DNP enhancement may also be observed with endogenous radicals. 5 This manuscript probes the structure of an unusually persistent endogenous radical in bis -urea macrocycle 1 and its use as a polarizing agent for DNP enhancement. The photogenerated radical was first noted when investigating the applications of benzophenone bis -urea macrocycle 1 to facilitate selective photooxidations.…”

“…4, 12 Few studies have examined DNP using endogenous radicals in the solid-state or single crystals. 5, 9, 10 …”