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Register a new userProspects for nuclear spin hyperpolarisation of molecular samples using nitrogen-vacancy centres in diamond
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Added by
Jean-Philippe Tetienne
Publication date
2020
fields
Physics
and research's language is
English
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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 ($gtrsimmu$L) with a careful engineering of the systems 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.
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The nitrogen-vacancy (NV) centre in diamond has emerged as a candidate to non-invasively hyperpolarise nuclear spins in molecular systems to improve the sensitivity of nuclear magnetic resonance (NMR) experiments. Several promising proof of principle experiments have demonstrated small-scale polarisation transfer from single NVs to hydrogen spins outside the diamond. However, the scaling up of these results to the use of a dense NV ensemble, which is a necessary prerequisite for achieving realistic NMR sensitivity enhancement, has not yet been demonstrated. In this work, we present evidence for a polarising interaction between a shallow NV ensemble and external nuclear targets over a micrometre scale, and characterise the challenges in achieving useful polarisation enhancement. In the most favourable example of the interaction with hydrogen in a solid state target, a maximum polarisation transfer rate of $approx 7500$ spins per second per NV is measured, averaged over an area containing order $10^6$ NVs. Reduced levels of polarisation efficiency are found for liquid state targets, where molecular diffusion limits the transfer. Through analysis via a theoretical model, we find that our results suggest implementation of this technique for NMR sensitivity enhancement is feasible following realistic diamond material improvements.
The protocols for the control and readout of Nitrogen Vacancy (NV) centres electron spins in diamond offer an advanced platform for quantum computation, metrology and sensing. These protocols are based on the optical readout of photons emitted from NV centres, which process is limited by the yield of photons collection. Here we report on a novel principle for the detection of NV centres magnetic resonance in diamond by directly monitoring spin-preserving electron transitions through measurement of NV centre related photocurrent. The demonstrated direct detection technique offers a sensitive way for the readout of diamond NV sensors and diamond quantum devices on diamond chips. The Photocurrent Detection of Magnetic Resonance (PDMR) scheme is based on the detection of charge carriers promoted to the conduction band of diamond by the two-photon ionization of NV- centres. Optical detection of magnetic resonance (ODMR) and PDMR are compared, by performing both measurements simultaneously. The minima detected in the measured photocurrent at resonant microwave frequencies are attributed to the spin-dependent occupation probability of the NV- ground state, originating from spin-selective non-radiative transitions.
We present an experimental and theoretical study of electronic spin decoherence in ensembles of nitrogen-vacancy (NV) color centers in bulk high-purity diamond at room temperature. Under appropriate conditions, we find ensemble NV spin coherence times (T_2) comparable to that of single NVs, with T_2 > 600 microseconds for a sample with natural abundance of 13C and paramagnetic impurity density ~10^15 cm^(-3). We also observe a sharp decrease of the coherence time with misalignment of the static magnetic field relative to the NV electronic spin axis, consistent with theoretical modeling of NV coupling to a 13C nuclear spin bath. The long coherence times and increased signal-to-noise provided by room-temperature NV ensembles will aid many applications of NV centers in precision magnetometry and quantum information.
We present a study of the spin properties of dense layers of near-surface nitrogen-vacancy (NV) centres in diamond created by nitrogen ion implantation. The optically detected magnetic resonance contrast and linewidth, spin coherence time, and spin relaxation time, are measured as a function of implantation energy, dose, annealing temperature and surface treatment. To track the presence of damage and surface-related spin defects, we perform in situ electron spin resonance spectroscopy through both double electron-electron resonance and cross-relaxation spectroscopy on the NV centres. We find that, for the energy ($4-30$~keV) and dose ($5times10^{11}-10^{13}$~ions/cm$^2$) ranges considered, the NV spin properties are mainly governed by the dose via residual implantation-induced paramagnetic defects, but that the resulting magnetic sensitivity is essentially independent of both dose and energy. We then show that the magnetic sensitivity is significantly improved by high-temperature annealing at $geq1100^circ$C. Moreover, the spin properties are not significantly affected by oxygen annealing, apart from the spin relaxation time, which is dramatically decreased. Finally, the average NV depth is determined by nuclear magnetic resonance measurements, giving $approx10$-17~nm at 4-6 keV implantation energy. This study sheds light on the optimal conditions to create dense layers of near-surface NV centres for high-sensitivity sensing and imaging applications.
We investigate spin and optical properties of individual nitrogen-vacancy centers located within 1-10 nm from the diamond surface. We observe stable defects with a characteristic optically detected magnetic resonance spectrum down to lowest depth. We also find a small, but systematic spectral broadening for defects shallower than about 2 nm. This broadening is consistent with the presence of a surface paramagnetic impurity layer [Tisler et al., ACS Nano 3, 1959 (2009)] largely decoupled by motional averaging. The observation of stable and well-behaved defects very close to the surface is critical for single-spin sensors and devices requiring nanometer proximity to the target.
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