Prospects for nuclear spin hyperpolarisation of molecular samples using nitrogen-vacancy centres in diamond

Abstract

After initial proof-of-principle demonstrations, optically pumped nitrogen-vacancy (NV) centres in diamond have been proposed as a non-invasive platform to achieve hyperpolarisation of nuclear spins in molecular samples over macroscopic volumes and enhance the sensitivity in nuclear magnetic resonance (NMR) experiments. In this work, we model the process of polarisation of external samples by NV centres and theoretically evaluate their performance in a range of scenarios. We find that average nuclear spin polarisations exceeding 10% can in principle be generated over macroscopic sample volumes ( µL) with a careful engineering of the system's geometry to maximise the diamond-sample contact area. The fabrication requirements and other practical challenges are discussed. We then explore the possibility of exploiting local polarisation enhancements in nano/micro-NMR experiments based on NV centres. For micro-NMR, we find that modest signal enhancements over thermal polarisation (by 1-2 orders of magnitude) can in essence be achieved with existing technology, with larger enhancements achievable via micro-structuring of the sample/substrate interface. However, there is generally no benefit for nano-NMR where the detection of statistical polarisation provides the largest signal-to-noise ratio. This work will guide future experimental efforts to integrate NV-based hyperpolarisation to NMR systems.

Maximising Dynamic Nuclear Polarisation via Selective Hyperfine Tuning

“…In pioneering work by Jacques, et al 28 , this effect was demonstrated under 532 nm laser illumination by facilitating a cross-relaxation process between the polarised electron spin and its intrinsic 15 N nuclear spin in the electronic excited state, due to the excited state level anticrossing (ESLAC) at ∼ 500 G. The extent of polarisation was inferred by using optically detected magnetic resonance (ODMR, 29 ) to monitor the intensity of its electron paramagnetic resonance (EPR) lines, and was found to be in excess of 99% for the 15 N nucleus, and in excess of 90% when similarly applied to a strongly coupled 13 C nucleus in the first coordination shell of the NV. This effect was independently verified in ref.…”

“…For example, in the case of a 14 NV coupled to proton spins, only the after-GSLAC interaction is observed due to the avoided crossing before the GSLAC, however both transitions are observed for nuclei with smaller gyromagnetic ratios. In the case of 15 NV, no transition frequency exists below roughly 3 MHz, making it suitable for NV-DNP of 1,3 H and 19 F spins, but little else (detailed discussions of these considerations may be found in refs. 39,41).…”

“…Of these systems, the nitrogen-vacancy (NV) centre in diamond 5 is particularly noteworthy; owing to its convenient room-temperature operation and efficient optical spin polarisation of the electronic ground state, which remove the need for strong magnetic fields and cryogenic operation [6][7][8][9][10] . As such, the motivation for employing this system is two-fold: not only does the NV provide an ideal test-bed for this investigation, it may also provide the ideal technology platform for the realisation of compact, cost effective on-chip DNP devices [10][11][12][13][14][15] .…”