Category Archives: integrated solid-effect

Carbon-13 dynamic nuclear polarization in diamond via a microwave-free integrated cross effect #DNPNMR

Henshaw, Jacob, Daniela Pagliero, Pablo R. Zangara, María B. Franzoni, Ashok Ajoy, Rodolfo H. Acosta, Jeffrey A. Reimer, Alexander Pines, and Carlos A. Meriles. “Carbon-13 Dynamic Nuclear Polarization in Diamond via a Microwave-Free Integrated Cross Effect.” Proceedings of the National Academy of Sciences 116, no. 37 (September 10, 2019): 18334–40.

https://doi.org/10.1073/pnas.1908780116

Color-center–hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center–assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition—where the P1 Zeeman splitting matches one of the NV spin transitions—we demonstrate efficient microwave-free 13C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal.

Frequency-Swept Integrated and Stretched Solid Effect Dynamic Nuclear Polarization #DNPNMR

Can, T. V., J. E. McKay, R. T. Weber, C. Yang, T. Dubroca, J. van Tol, S. Hill, and R. G. Griffin. “Frequency-Swept Integrated and Stretched Solid Effect Dynamic Nuclear Polarization.” The Journal of Physical Chemistry Letters 9, no. 12 (June 21, 2018): 3187–92. 

https://doi.org/10.1021/acs.jpclett.8b01002

We investigate a new time domain approach to dynamic nuclear polarization (DNP), the frequency-swept integrated solid effect (FS-ISE), utilizing a high power, broadband 94 GHz (3.35 T) pulse EPR spectrometer. The bandwidth of the spectrometer enabled measurement of the DNP Zeeman frequency/field profile that revealed two dominant polarization mechanisms, the expected ISE, and a recently observed mechanism, the stretched solid effect (S2E). At 94 GHz, despite the limitations in the microwave chirp pulse length (10 μs) and the repetition rate (2 kHz), we obtained signal enhancements up to ∼70 for the S2E and ∼50 for the ISE. The results successfully demonstrate the viability of the FS-ISE and S2E DNP at a frequency 10 times higher than previous studies. Our results also suggest that these approaches are candidates for implementation at higher magnetic fields.

Frequency-Swept Integrated Solid Effect #DNPNMR

Can, T.V., et al., Frequency-Swept Integrated Solid Effect. Angew Chem Int Ed Engl, 2017. 56(24): p. 6744-6748.

https://www.ncbi.nlm.nih.gov/pubmed/28497528

The efficiency of continuous wave dynamic nuclear polarization (DNP) experiments decreases at the high magnetic fields used in contemporary high-resolution NMR applications. To recover the expected signal enhancements from DNP, we explored time domain experiments such as NOVEL which matches the electron Rabi frequency to the nuclear Larmor frequency to mediate polarization transfer. However, satisfying this matching condition at high frequencies is technically demanding. As an alternative we report here frequency-swept integrated solid effect (FS-ISE) experiments that allow low power sweeps of the exciting microwave frequencies to constructively integrate the negative and positive polarizations of the solid effect, thereby producing a polarization efficiency comparable to (+/-10 % difference) NOVEL. Finally, the microwave frequency modulation results in field profiles that exhibit new features that we coin the “stretched” solid effect.

Dynamic nuclear polarisation via the integrated solid effect II: experiments on naphthalene-h8doped with pentacene-d14 #DNPNMR

Eichhorn, T.R., et al., Dynamic nuclear polarisation via the integrated solid effect II: experiments on naphthalene-h8doped with pentacene-d14. Mol. Phys., 2013. 112(13): p. 1773-1782.

https://doi.org/10.1080/00268976.2013.863405

In dynamic nuclear polarisation (DNP), also called hyperpolarisation, a small amount of unpaired electron spins is added to the sample containing the nuclear spins, and the polarisation of these unpaired electron spins is transferred to the nuclear spins by means of a microwave field. Traditional DNP polarises the electron spin of stable paramagnetic centres by cooling down to low temperature and applying a strong magnetic field. Then weak continuous wave microwave fields are used to induce the polarisation transfer. Complicated cryogenic equipment and strong magnets can be avoided using short-lived photo-excited triplet states that are strongly aligned in the optical excitation process. However, a much faster transfer of the electron spin polarisation is needed and pulsed DNP methods like nuclear orientation via electron spin locking (NOVEL) and the integrated solid effect (ISE) are used. To describe the polarisation transfer with the strong microwave fields in NOVEL and ISE, the usual perturbation methods cannot be used anymore. In the previous paper, we presented a theoretical approach to calculate the polarisation transfer in ISE. In the present paper, the theory is applied to the system naphthalene-h8 doped with pentacene-d14 yielding the photo-excited triplet states and compared with experimental results.

Dynamic nuclear polarisation via the integrated solid effect I: theory #DNPNMR

Henstra, A. and W.T. Wenckebach, Dynamic nuclear polarisation via the integrated solid effect I: theory. Mol. Phys., 2013. 112(13): p. 1761-1772.

https://doi.org/10.1080/00268976.2013.861936

In the hyperpolarisation method known as dynamic nuclear polarisation (DNP), a small amount of unpaired electron spins is added to the sample containing the nuclear spins and the polarisation of these unpaired electron spins is transferred to the nuclear spins by means of a microwave field. Traditional DNP uses weak continuous wave (CW) microwave fields, so perturbation methods can be used to calculate the polarisation transfer. A much faster transfer of the electron spin polarisation is obtained with the integrated solid effect (ISE) which uses strong pulsed microwave fields. As in nuclear orientation via electron spin locking, the polarisation transfer is coherent, similar to the coherence transfer between nuclear spins. This paper presents a theoretical approach to calculate this polarisation transfer. ISE is successfully used for a fast polarisation transfer from short-lived photo-excited triplet states to the surrounding nuclear spins in molecular crystals. These triplet states are strongly aligned in the photo-excitation process and do not require the low temperatures and strong magnetic fields needed to polarise the electron spins in traditional DNP. In the following paper, the theory is applied to the system naphthalene-h8 doped with pentacene-d14 which provides the photo-excited triplet states, and compared with experimental results.

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