Room Temperature DNP of Diamond Powder Using Frequency Modulation

Abstract: Dynamic nuclear polarization (DNP) is a method of enhancing NMR signals via the transfer of polarization from electron spins to nuclear spins using on-resonance microwave (MW) irradiation. In most cases, monochromatic continuous-wave (MCW) MW irradiation is used. Recently, several groups have shown that the use of 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 $^{13}$C-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 the enhancement obtained via the two most important parameters for frequency modulation 1) Modulation frequency, $f_{m}$ (how fast the modulation frequency is varied) and 2) Modulation amplitude, $\Delta\omega$ (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.

• The paper investigates how frequency modulation influences dynamic nuclear polarization (DNP) mechanisms in diamond powders at room temperature, extending analysis beyond traditional methods.
• Key findings show that modulation frequency and amplitude significantly impact DNP enhancement and the behavior of mechanisms like SE, CE, OE, and tCE.
• The research demonstrates the potential to control and selectively enhance specific DNP mechanisms through careful selection of frequency modulation parameters, improving DNP efficiency in complex systems.

The paper "Room Temperature DNP of Diamond Powder Using Frequency Modulation" presents an investigation into the effects of frequency modulation on dynamic nuclear polarization (DNP) mechanisms in diamond powders featuring $^{13}$C nuclei. The study extends beyond the traditional use of monochromatic continuous-wave (MCW) microwave irradiation which primarily assesses the solid effect (SE) and the cross effect (CE), by incorporating frequency modulation to additionally scrutinize the Overhauser effect (OE) and the truncated cross effect (tCE) at room temperature.

This research capitalizes on room-temperature DNP experiments utilizing diamond powders enriched with P1 centers, a type of nitrogen defect. The P1 centers serve as electron spin sources which facilitate the polarization of $^{13}$C nuclei under the influence of microwave irradiation at a magnetic field of 3.34 T. The study builds on previous understandings that diamond powders can exhibit multiple simultaneous DNP mechanisms due to varying environments within the diamond crystallites.

Key findings of this work include:

• Impact of Frequency Modulation Parameters: Two critical parameters, modulation frequency ($f_{m}$), which dictates the rate of change, and modulation amplitude ($\Delta\omega$), which defines the bandwidth of frequency variation, play significant roles in influencing DNP enhancement. The paper systematically explores these parameters across a range of values, illustrating that frequency modulation can enhance DNP signals by potentially saturating a larger number of spin packets, adding flexibility in targeting specific DNP mechanisms.
• Behavior of DNP Mechanisms: The behavior of SE and CE under modulation is expanded to include OE and tCE. While SE and CE behaviors are consistent with previous low-temperature studies using radicals like TEMPOL, extended studies at room temperature reveal that the OE and tCE also react meaningfully to modulation. The SE showed sensitivity to low $f_{m}$ and large $\Delta\omega$ combinations, while the CE displayed optimal enhancement under certain conditions of these parameters. OE and tCE demonstrate sensitivity to similar modulation conditions, underscoring the potential to control mechanism activity via frequency modulation.
• Numerical Simulations: The results are corroborated with numerical simulations for both small and large spin systems. These simulations account for complex phenomena such as electron spectral diffusion (eSD) which interacts with electron relaxation times to either enhance or degrade nuclear polarization indirectly. Simulations for large spin systems, considering many interacting electron packets, underscore the importance of understanding eSD and relaxation dynamics in tailoring DNP enhancements through frequency modulation.
• Mechanism Control: A major insight from the study is the ability to selectively enhance certain DNP mechanisms while suppressing others through careful choice of modulation parameters. This provides a potential method for optimizing DNP processes in complex systems by influencing the relative contributions of competing mechanisms.

The paper makes significant strides in understanding the interplay between different DNP mechanisms at room temperature and illustrates the potential for frequency modulation to act as a tool to enhance nuclear polarization. These findings have implications in the application of DNP-enhanced NMR spectroscopy, particularly in systems like diamond powders where multi-mechanism DNP processes are inherent due to system heterogeneity and electron spin interactions.