Room temperature DNP of diamond powder using frequency modulation

Daphna Shimon; Kelly Cantwell; Linta Joseph; Chandrasekhar Ramanathan

doi:10.1016/j.ssnmr.2022.101833

Room temperature DNP of diamond powder using frequency modulation

Solid State Nucl Magn Reson. 2022 Dec:122:101833. doi: 10.1016/j.ssnmr.2022.101833. Epub 2022 Sep 27.

Authors

Daphna Shimon¹, Kelly Cantwell², Linta Joseph², Chandrasekhar Ramanathan²

Affiliations

¹ Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra, Givat Ram, Jerusalem, 9190401, Israel. Electronic address: daphna.shimon@mail.huji.ac.il.
² Department of Physics and Astronomy, Dartmouth College, Hanover, NH, 03755, USA.

PMID: 36209552
DOI: 10.1016/j.ssnmr.2022.101833

Abstract

Dynamic nuclear polarization (DNP) is a method of enhancing NMR signals via the transfer of polarization from electron spins to nuclear spins using microwave (MW) irradiation. In most cases, monochromatic continuous-wave (MCW) MW irradiation is used. Recently, several groups have shown that frequency modulation of the MW irradiation can result in an additional increase in DNP enhancement above that obtained with MCW. The effect of frequency modulation on the solid effect (SE) and the cross effect (CE) has previously been studied using the stable organic radical 4-hydroxy TEMPO (TEMPOL) at temperatures under 20 K. Here, in addition to the SE and CE, we discuss the effect of frequency modulation on the Overhauser effect (OE) and the truncated CE (tCE) in the room-temperature ¹³C-DNP of diamond powders. We recently showed that diamond powders can exhibit multiple DNP mechanisms simultaneously due to the heterogeneity of P1 (substitutional nitrogen) environments within diamond crystallites. We explore how the two parameters that define the frequency modulation: (i) the Modulation frequency, f_m (how fast the microwave frequency is varied) and (ii) the Modulation amplitude, Δω (the magnitude of the change in microwave frequency) influence the enhancement obtained via each mechanism. Frequency modulation during DNP not only allows us to improve DNP enhancement, but also gives us a way to control which DNP mechanism is most active. By choosing the appropriate modulation parameters, we can selectively enhance some mechanisms while simultaneously suppressing others.