Symposium EQ03—Spin-Based Sensing at the Nanoscale and Hyperpolarization with NV-Diamond and Beyond

Single nitrogen-vacancy (NV) color centers in diamond currently have sufficient sensitivity for detecting single external nuclear spins and resolve their position within a few angstroms. The ability to bring the sensor close to biomolecules by implantation of single NV centers and attachment of proteins to the surface of diamond enabled the first proof of principle demonstration of label-free detection of the signal from a single protein. Single-molecule nuclear magnetic resonance experiments open the way towards unraveling dynamics and structure of single biomolecules. However, for that purpose, NV magnetometers must reach spectral resolutions comparable to that of conventional solution state NMR. New techniques for this purpose will be discussed. We will also show first experiments towards hyperpolarisation of extrneal nuclear spins using shallow NV centers..

We report on new experiments that demonstrate the potential of hyperpolarized nuclear spins to serve as sensitive magnetometers and sensors of their local environment. This methodology leverages two important features, (1) the ability of sensor nuclei to be hyperpolarized to >3% levels through optical pumping, and (2) their ability to be put into long-lived “Floquet prethermal” states that can be rendered highly sensitive to external magnetic fields. We demonstrate this with 13C nuclei in diamond that are optically hyperpolarized through interactions with lattice Nitrogen-Vacancy (NV) centers. We leverage the exquisite transverse spin lifetimes possible in 13C nuclei under RF driving, wherein we observe lifetimes in excess of T2’=90s at room temperature (see Fig. 1). These long-lived states constitute an extension of >60,000 over conventional FID decay times T2*~1.5ms in this system. Simultaneously, hyperpolarization and continuous spin readout enable significant gains in signal to noise, and forms the key basis for ultimately making these nuclei serve as quantum sensors. These experiments suggest interesting new opportunities for high-field spin sensors possible via hyperpolarized, low-gamma, nuclei.

Quantum two-level systems offer attractive opportunities for sensing and imaging at the nanoscale. Since its inception [1], this idea has advanced from proof of concept [2] to a mature quantum technology [3], with broad applications in condensed matter physics, materials science and engineering, and a demonstrated economic impact [4].

In this talk, I will present our activities in quantum sensing of mesoscopic, solid-state systems using scanning Nitrogen-Vacancy (NV) centre magnetometry in cryogenic environments. I will on one hand discuss engineering challenges we addressed in photonic engineering of all-diamond scanning probes for quantum sensing [5], and on the other hand highlight recent achievements in nanoscale imaging of two-dimensional, “van der Waals” magnetic systems [6][7].

In the second part of my talk, I will focus on recent studies we conducted in systematically addressing the photo-physics of individual NV centres under cryogenic conditions [8], with the goal to further advancing sensing performance of NV centres at cryogenic temperatures. We observed significant variations of optical spin readout and initialisation efficiencies with magnetic field, that we connect to excited state level anti-crossing in the NVs orbital excited state. The understanding and control of these features is key for NV magnetometry in that it enabled improvements of magnetic field sensitivities by up to an order of magnitude over established baseline values at cryogenic temperatures

I will conclude with an outlook on how these insights could be used to offer fundamentally new sensing modalities and how we will further push our single spin magnetometer developments into the range of extreme environmental conditions, such as high magnetic fields, or millikelvin temperatures.


[1] B. Chernobrod and G. Berman, J. of Applied Physics 97, 014903 (2004)
[2] G. Balasubmaranian et al., Nature 455, 644 (2008)
[3] P. Appel et al., Review of Scientific Instruments 87, 063703 (2016)
[4] www.qnami.ch
[5] N. Hedrich et al. Phys. Rev. Applied 14, 064007 (2020)
[6] M. Gibertini et al., Nature Nanotechnology 14, 408 (2019)
[7] L. Thiel et al., Science 364, 973 (2019)
[8] J. Happacher et al., arXiv:2105.08075 (2021)

Hyperpolarization of nuclear spins by Dynamic Nuclear Polarization (DNP) is significantly higher compare to the thermal polarization. As the result, many Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) applications become possible. Optical DNP requires high-power laser irradiation for polarizing electron spins of negatively charged nitrogen vacancy (NV^-) centres and a microwave irradiation for inducing the polarization transfer to ¹³C nuclear spins. The initial goal of our research is hyperpolarization of diamond powder and investigating ways to improve the polarization. However here we present our DNP measurements obtained on bulk diamonds at a low magnetic field of 18 mT. We have obtained triple DNP profile, ¹³C polarization by multi-MW frequencies, dependences of ¹³C polarization over different parameters and NV^- concentrations. This data will be helpful for maximizing the ¹³C polarization in diamond powder.

We will discuss some recent results on zero-field and microwave-free magnetometry, nuclear-spin polarization and readout, demonstration of robust rotation sensing, and dual-modality magnetic imaging/magneto-optical Kerr effect.

The surface of diamond plays a crucial role in the control of its electronic properties. Not only the electron affinity and thus the ability to emit electrons into the surrounding media depends on the actual surface termination, but also the interaction with components from the environment such as serum proteins etc. It is thus of great importance to develop methods for the functionalization of the surface in a highly controlled manner that include the termination itself but also the grafting of more complex units for dedicated purposes.
Here we report on different methods for the selective establishment of different oxygenated surface groups as well as an efficient fluorination using wetchemical methods. The influence on the materials properties will be discussed. Additionally, methods for the grafting of protein repelling moieties based on zwitterionic motifs are discussed as an essential tool for bioapplications. Besides, diamond nanoparticles functionalized with organic fluoroionophores for sensing applications will be presented.
This research has received funding from the European Commission’s Horizon 2020 program under contract 665085 (DIACAT) and the Volkswagen Foundation under contract 88393.

Magnetic microscopy based on nitrogen vacancy (NV) centers in diamond has become a multifunctional tool to study condensed matter phenomena at the nanoscale [1]. Unlike current magnetic imaging techniques, the NV microscopy platform works at a wide range of temperatures, frequencies, magnetic fields, and it does not perturb the target magnetic sample. We present recent results where NV microscopy is used in both far field and near field (scanning probe) geometries to map spin textures and magnetic excitation (spin-wave) modes in transition metal magnetic nanostructures. Understanding their static and dynamic spin textures can help in exploiting their useful properties such as high surface-to-volume ratio and tailorable surface chemistry for applications in biosensing, catalysis, spintronics, and ultra-high-density magnetic recording. At sizes below 10 nm, surface and edge effects play a major role in determining their magnetic moment and also effect significantly their spin-wave modes. Accurate measurements of spin textures of individual magnetic nanostructures would therefore shed light on their critical properties for tailored applications. Here we report ongoing experiments using NV magnetic microscopy to measure the static and dynamic magnetic properties of individual Co, FeCo (size range 4-15 nm), and CoFeB (size range 20-100 nm) nanostructures. Histograms of important parameters such as magnetization saturation, magnetic domain size, magnetic anisotropy, spin relaxation, and dynamic magnetic excitation modes are extracted and correlated with morphology data taken by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. We acknowledge support from NSF-DMR award #1809800 and NSF-EPSCoR RII Track-1: Emergent Quantum Materials and Technologies (EQUATE), Award OIA-2044049.
[1] F. Casola, T. van der Sar, and A. Yacoby, Nat. Rev. Mat. 3, 17088 (2018).

Antiferromagnetic (AF) materials are interesting for future generation memory and logic applications due to their sub-picosecond spin dynamics and potential for high density. α-Fe₂O₃ is an AF insulator with canted spin-order that leads to a weak magnetic moment oriented perpendicular to the Néel vector at room temperature. Recent studies have shown that the magnetic order of α-Fe₂O₃ can be manipulated using applied currents and read out using spin Hall magnetoresistance. The origin of current-induced switching is still under debate – with spin-orbit torque or thermomagnetoelastic effects due to Joule heating as the two leading mechanisms. To better understand the magnetism of α-Fe₂O₃ and how it is manipulated, here we report magnetic imaging of α-Fe₂O₃ thin film devices with a diamond nitrogen-vacancy (NV) center scanning microscope. We image at 110 nm above a 60 nm thick α-Fe₂O₃ thin film capped with 6 nm of Pt patterned into a Hall cross. The local magnetic texture switches after passing a current pulse through the Pt layer. The difference images between the magnetic field images before and after the current pulse show the switched regions and their switching directions. Our results show that the amount of switching depends on the current pulse duration and the current amplitude. Pulses shorter than 1 µs do not generate switching even with current density up to 8 x 10¹² A/m². This indicates that spin-orbit torque may not be the most important mechanism for the current-induced switching. Meanwhile, we also observe switching outside the current path, where there is no spin-orbit torque. These results suggest that Joule heating from the current and/or heat-induced strain is sufficient to produce magnetic switching. The images also indicate that the current-induced switching can be reversed by an orthogonal current pulse. In addition, we perform autocorrelation analysis of the magnetic images to identify preferred directions of Néel order, which provides more information about the spin domains in thin-film α-Fe₂O₃. The high resolution, high sensitivity magnetic images obtained with scanning NV microscopy provides a means to further investigate the magnetization and switching dynamics of thin film AF insulators.

Antiferromagnetic thin films attract significant interest for future low-power spintronic devices [1]. Multiferroics, such as bismuth ferrite BiFeO₃, in which antiferromagnetism and ferroelectricity coexist at room temperature, appears as a unique platform for spintronic [2] and magnonic devices [3]. The nanoscale structure of its ferroelectric domains has been widely investigated with piezoresponse force microscopy (PFM), revealing unique domain structures and domain wall functionalities [4]. However, the nanoscale magnetic textures present in BiFeO₃ and their potential for spin-based technology remain concealed. In this report, we present two different antiferromagnetic spin textures in multiferroic BiFeO₃ thin films with different epitaxial strains, using a commercial non-invasive scanning Nitrogen-Vacancy (NV) magnetometer based on a single NV defect in diamond, with a calibrated NV flying height of ~60 nm and a proven DC field sensitivity of ~1 μT/√Hz. Two BiFeO₃ samples were grown on DyScO₃ (110) and SmScO₃ (110) substrates (later mentioned as BFO/DSO and BFO/SSO, respectively) using pulsed laser deposition. The striped ferroelectric domains in both samples are first observed by the in-plane PFM. The scanning NV magnetometry (SNVM) confirms the existence of the spin cycloid texture, with zig-zag wiggling angles of 90° and 127°, and propagation wavelength of λ_DSO=64 nm and λ_SSO =103 nm, respectively. At the local scale, the combination of PFM and SNVM allows to identify the relative orientation of the ferroelectric polarization and cycloid propagation directions on both sides of a domain wall. For the BFO/DSO sample, the 90-degree in-plane rotation of the ferroelectric polarization imprints the 90-degree in-plane rotation of the cycloidal propagation direction along k1=[-1 1 0], corresponding to the type-I cycloid. On the contrary, in the BFO/SSO sample, the propagation vectors are found to be along k1'=[-2 1 1] and k2'=[1 -2 1] directions in the neighbouring domains separated by the 71° domain wall. It is worth to mentioned that in the previous report [5], BFO/SSO, prepared in another growth chamber, showed G-type antiferromagnetic textures, compared to the observed type-II cycloid here. Our results here shed the light on future potential for reconfigurable nanoscale spin textures on multiferroic systems by strain engineering.

[1] Baltz V., et al. (2018) Antiferromagnetic spintronics. Rev. Mod. Phys. 90, 015005.
[2] Heron J. et al. (2014). Deterministic switching of ferromagnetism at room temperature using an electric field. Nature 516, 370–373.
[3] Rovillain P., et al. (2010). Electric-field control of spin waves at room temperature in multiferroic BiFeO3. Nat. Mater. 9, 975–979
[4] Balke N., et al. (2011). Enhanced electric conductivity at ferroelectric vortex cores in BiFeO3. Nat. Phys. 8, 81–88.
[5] A. Haykal, et al., (2020). Antiferromagnetic textures in BiFeO3 controlled by strain and electric field. Nat. Commun. 11, 1704.

Free radicals play a key role in many biological processes including cell communication, immune responses, metabolism or cell development. But they are also involved whenever something is wrong in a cell and are thus important in many diseases including cardiovascular diseases, cancer or bacterial and viral infection. Unfortunately, they are very reactive and short lived and thus difficult to detect for the state of the art. We have used diamond magnetometry to achieve this. We make use of nanodiamonds which we bring into cells. We then use of NV centers in diamonds to perform relaxometry measurements. These are sensitive to spin noise (in this case from radicals) and deliver signals that are equivalent to T1 in conventional MRI but from nanoscale voxels. Using this method, we are able to quantify free radical generation with nanoscale resolution in the nanomole range1. In our recent work we were able to detect free radical generation in single mitochondria (the energy factories of the cell) in isolated form as well as in their cellular environment2.
1 Perona Martiinez, F., Nusantara, A.C., Chipaux, M., Padamati, S.K. and Schirhagl, R., 2020. Nanodiamond Relaxometry-Based Detection of Free-Radical Species When Produced in Chemical Reactions in Biologically Relevant Conditions. ACS Sensors.
2 Nie, L., Nusantara, A.C., Damle, V.G., Sharmin, R., Evans, E.P.P., Hemelaar, S.R., van der Laan, K.J., Li, R., Martinez, F.P., Vedelaar, T. and Chipaux, M., Schirhagl, R., 2021. Quantum monitoring of cellular metabolic activities in single mitochondria. Science Advances, 7(21), p.eabf0573.

Magnetocardiography (MCG) is a contactless technique that remotely measures the stray magnetic fields produced by cardiac currents in the heart. The spatial resolution of MCG deteriorates significantly as the standoff distance between the target and the sensor increases. Commercialized MCG devices usually provide centimeter-scale spatial resolution. As an alternative approach, nitrogen-vacancy (NV) centres in diamond have gained interest as a magnetometor for biological experiments [1,2]. In this talk, I will present MCG of mammalian animals using an ensemble of NV centers [3]. A millimeter proximity from the sensor to heart surface enhances the cardiac magnetic field to greater than nanoteslas and allows the mapping of these signals with intra-cardiac resolution. I will also discuss the possiblity of further improvement in sensitivity and resolution as well as applying this technique to studying the origin and progression of myriad cardiac arrhythmias including flutter, fibrillation, and tachycardia.

This work is supported by the MEXT Quantum Leap Flagship Program (MEXT Q LEAP ), Grant Numbers JPMXS0118067395 and JPMXS0118068379, and JST PRESTO, Grant Number JPMJPR20B1.

[1] D. LeSage et al., Optical magnetic imaging of living cells, Nature 496, 486-489 (2013).
[2] J. Barry et al., Optical magnetic detection of single neuron action potentials using quantum defects in diamond, PNAS 113 14133 (2016).
[3] K. A rai et al., Millimetre-scale magnetocardiography of living rats, arXiv2105.11676 (2021).

Nitrogen vacancy (NV) centers in diamond have been used as ultrasensitive sensors for performing nuclear magnetic resonance (NMR) spectroscopy of molecules external to the diamond at nanometer to micrometer length scales. However, the spectral linewidth is typically limited to the kHz level on the nanoscale, both by the NV sensor coherence time and by rapid molecular diffusion of the nuclei through the detection volume.
In addition, NV centers offers promising opportunities for nanoscale hyperpolarization by combining dynamic nuclear polarization methods with optical hyperpolarization. We have previously shown that hyperpolarized signals could enable high spectral resolution on the nanoscale, not limited by molecular diffusion rates.

In this talk we will detail the schemes and challenges for nanoscale polarization using NV centers and show how these could be overcome. We will demonstrate the first unambiguous signal of NMR detection from NV-hyperpolarized external molecules and discuss the potential outlook and future applications.

Dynamic nuclear polarization (DNP), a technique to transfer spin polarization from electrons to nuclei, has been studied since its early discovery [1] and has opened the way for high sensitive nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging. In "conventional" DNP, which uses thermally-polarized unpaired electrons as the source of polarization, the DNP enhancement factor is limited to γ_e/γ_n, where γ_e(n) are the gyromagnetic ratios of the electron (nuclear) spins. In DNP using the paramagnetic electrons in thermal equilibrium, experiments need to be performed at cryogenic temperature to increase the electron-spin polarization as much as possible. Conversely, the spins of optically created electrons in the triplet state can have much higher polarization than their thermal equilibrium value, DNP using the triplet state, referred to as triplet DNP, can lead to nuclear hyperpolarization beyond the limit of the conventional DNP using thermal electron polarization. Furthermore, experiments can be carried out at room temperature, by irradiating the sample with laser light. Recently, DNP of an ensemble of ¹³C nuclear spins using negatively charged nitrogen-vacancy (NV⁻) color centers in a bulk diamond single crystal has been demonstrated at room temperature and the ¹³C polarization of 6 % has been achieved via the combination of the thermal mixing and the solid effect [2]. DNP using NV⁻ in powdered microdiamonds has been reported by Ajoy et al., who took advantage of the reduced width of the anisotropic electron spin resonance powder pattern of the NV⁻ centers at the magnetic field of ca. 30 mT [3]. However, triplet DNP is much older and achieved a ¹H polarization of 34 % at room temperature and a magnetic field of 0.4 T using pentacene in p-terphenyl crystal [4]. Even though triplet DNP in both systems, NV^- centers and pentacene, relies on the transfer of spin polarization from optically hyperpolarized triplet electrons to nuclei , there are important differences. While the NV^- center has an electronic triplet ground state and is therefore paramagnetic, pentacene has an electronic singlet ground state, is diamagnetic, and only becomes paramagnetic through optical excitation into a triplet state. On the other hand, the zero-field splitting parameter D for pentacene is only half as large as in NV^- centers, which is an advantage when disordered powder is used as a sample.
In this work, we compare triplet-DNP of NV⁻ centers in diamond and pentacene doped in [carboxyl-¹³C] benzoic acid (PBA) in polycrystalline samples at room temperature [5]. In the DNP experiments, the integrated solid effect (ISE) was used to transfer the polarization from electrons to nuclei. The ISE employs microwave irradiation and external magnetic-field sweep, so that the Hartmann–Hahn matching is implemented between the electron spins in the rotating frame and the nuclear spins in the laboratory frame. We study the behavior of the ¹³C polarization buildup in terms of the polarization efficiency of the transfer from the electron to nuclei, exchange rate, and the ¹³C spin diffusion. As a result, we obtained the ¹³C polarization of 0.01 % in the microdiamonds, and of 0.12 % in PBA at room temperature in a magnetic field of 0.4 T by using the integrated solid effect and the obtained exchange rate was 0.87 % for microdiamonds and 3.5 % for PBA. The ¹³C polarization enhancements for the diamond and the PBA were 300 and 3600 compared to the thermal NMR polarization. Besides the initial polarization transfer from the triplet electron to the nuclei, we also shed light on the process of nuclear spin-spin diffusion, which distributes the hyperpolarization within the sample.

References
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[2] J. P. King et al., Nat. Commun., 6, 8965, (2015).
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[5] K. Miyanishi et al., Magn. Reson. 2, 1 (2021)

The cell membrane is a nanoscale 2D fluid crystalline assembly with sub-compartment domains that are critical for cellular functions, including transport of molecules, communications, and metabolic properties with its external medium [1]. These domains are distinguished by different phases of lipid membranes, and the fluidity of the lipid bilayer, described by the 2D translational diffusion of lipid molecules [2], determines the most fundamental property of lipids in different phases and therefore domains. Nanoscale NMR and correlation spectroscopy using a NV center has emerged as a quantum measurement platform that allows for label-free diffusion measurement with nuclear spin from a small detection volume of ~ (6 nm) ³ [3].
In this study, we investigate nanoscale phase change detection of lipid bilayers utilizing ensemble-averaged nuclear spin detection from small volume ~ (6 nm)³, which is determined by the depth of the NV center. Analysis of nanoscale NMR signal confirms thickness of lipid bilayer to be 6.2 nm ± 3.4 nm with proton density of 65 proton/nm3 on top of diamond sample. The result of the correlation spectroscopy was compared with the 2D molecular diffusion model constructed by Monte Carlo simulation combined with results from molecular dynamics (MD) simulation. There is a change in diffusion constant from 1.5 ± 0.25 nm²/μsec to 3.0 ± 0.5 nm²/μsec when the temperature changes from 26.5 °C to 36.0 °C [4]. Our results demonstrate that label-free detection of changes in translational diffusion of lipid molecules is possible using nanoscale diamond magnetometry. Direct observation of changes in the diffusion constant paves the way for the label-free identification of domains that are formed in the cell membrane to understand the relationship between cell membrane dynamics and cell function [5].

This study was supported by JST PRESTO Grant number JPMJPR17G1 and in part by the MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.

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[5] M. A. Pavel, E. N. Petersen, H. Wang, R. A. Lerner, S. B. Hansen, Proc. Natl. Acad. Sci. USA. 2020, 117, 13757-13766.

Imaging of weight introduces a new modality for the study of biological systems. Existing techniques either have high weight sensitivity but can measure only a single cell or offer a large field of view with limited weight sensitivity[1,2]. Thus, a key challenge in weight imaging is to provide a capability to measure various biological systems extending over multiple length scales from single cells to biological tissues. Such a high dynamic range in length scale can be realized by a hybrid sensor based on piezoactive magnetic material[3] and nitrogen-vacancy (NV) centers in diamond[4]. In the hybrid sensor, due to the inverse magnetostrictive effect, weight is converted into the rotation of magnetization and domain wall motion in piezoactive magnetic material. This rotation and domain wall motion induce a change in the stray magnetic field, which can be detected by NV centers via optically-detected magnetic resonance (ODMR). For the realization of the hybrid sensor, accurate imaging of weight distribution is essential. Here, we demonstrate weight imaging using a hybrid system composed of piezoactive magnetic material and the NV center and calibrated weight.
We fabricated a hybrid sensor by gluing a piezoactive magnetic material of a SmFe₂ thin film, deposited on an ultra-thin glass substrate, and NV centers in a diamond film deposited on a type-Ib diamond (111) substrate. We applied weight onto the system in an area of 3.1 mm² by putting on a rivet with a total weight of 9 g. As a comparison, we performed the measurement without applying weight. Imaging of the magnetic field distribution was performed by continuous-wave ODMR while applying a bias magnetic field of ~2 mT to break the degeneracy of the NV center spin levels.
We observed a different response between the stressed and unstressed areas. The change of the magnetic field was more significant in the stressed area. On the other hand, the shape of the magnetic field distribution that is thought to originate from the magnetic domain did not change. These results imply that the rotation of the magnetization was dominant compared to domain wall motion. We can understand the weight response of SmFe₂ by a simple single domain model, including magnetization, intrinsic magnetic anisotropy, and strain-induced anisotropy. The observed magnetic field difference of about ±15 µT is comparable with the value estimated by the single domain model. We established essential techniques for constructing the hybrid system through imaging weight response of piezo active magnetic material by NV center.
This study was supported in part by MEXT Q-LEAP JPMXS0118067395.

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In this talk I will outline reasoning as to why we should not expect entanglement to improve the performance of quantum sensors. I will consider the specific sensing example of estimating the frequency of an unknown field, and provide additional arguments for general sensing. The talk will also discuss published literature and why the experimental evidence is in agreement with this talk's central thesis.

Quantum sensing has a bright future for many applications given its potential for high sensitivity and resolution. Sensing with nitrogen-vacancy (NV) centres is a much explored option given its room-temperature operation, making it a feasible choice for biology applications. One example is measuring nuclear magnetic resonance (NMR) spectra of molecules. Given the potential for arbitrary resolution [1,2], Fourier-based algorithms have been implemented for measuring such NMR signals [3]. However, these are limited for use at high frequencies (over ~1 kHz) given the usage of dynamical decoupling sequences. On the other hand, low-frequency signals are important for chemical structure analysis [4] and for searching new particles beyond the standard model [5]. Here, we demonstrate a technique to measure low-frequency fields which sensitivity is frequency independent. Moreover, we apply it to measure low-frequency NMR signals with a remote NMR system, giving a line width for the free nuclear precession of water of 1.6 Hz.

[1] J. M. Boss et al., Science 356, 837–840 (2017).
[2] S. Schmitt et al., Science 356, 832–837 (2017).
[3] D. R. Glenn et al., Nature 555, 351–354 (2018).
[4] D. A. Barskiy et al., Nat. Commun. 10, 3002 (2019).
[5] T. Wu et al., Phys. Rev. Lett. 122, 191302 (2019).

The authors acknowledge the financial support from JST OPERA (No. JPMJOP1841), KAKENHI (No. 21H04653) and the Collaborative Research Program of ICR, Kyoto University (2021-114).

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) of bulk quantum materials has provided significant insight into magnetic phenomena such as quantum phase criticality, magnetism, and superconductivity. With the emergence of nanoscale 2-D materials with magnetic phenomena, traditional, inductively detected NMR is not sufficiently sensitive to detect the small nuclear density in these materials. The nitrogen-vacancy center in diamond (NV) has shown great promise in bringing the spectroscopic capabilities of NQR to the nanoscale and has been demonstrated utilizing single NV spins to detect 11B NQR in a monolayer of hBN¹. Single NV spins provide 30nm resolution but can be unstable and maybe less sensitive to NMR than NV ensembles. However, NV NMR detection of 2D materials has yet to be demonstrated with NV ensembles. Furthermore, the optimal NV layer for NMR detection will require an optimized depth calibration which trades off sample-NV standoff (closer to the diamond surface) and higher activation/better spin properties (further from diamond surface).
In this work, we prepare a range of near-surface NV ensembles, determine their depths by 19F NMR, determine their NMR sensitivities, and use our most sensitive NV ensemble for detection of 11B NQR in hBN exfoliated onto the diamond surface. To prepare our near surface NV ensembles, we implant approximately 100ppm nitrogen at energies from 1.5 to 7 keV and activate NVs by ultra-high vacuum annealing. By applying fomblin oil on the surface of these diamonds and detecting 19F NMR signal intensity, we find effective NV ensemble depths to be from 3 nm to 12 nm. The 19F NMR signal intensity also serves as an experimental determination of the nuclear spin sensitivity for our NV ensembles, and we find that our 3 keV (7.5 nm deep) NV ensemble is the most sensitive to 19F on the diamond surface. We then exfoliated 100nm thick flakes of hBN onto the surface of these diamonds and measure the NQR signal intensity of 11B by both CPMGXY8-N and correlation spectroscopy. The intensity of 11B NQR spectra are also found to be optimal for the 3keV NV implant, reenforcing the results of our 19F signal optimization. As a next step, with our optimized NV ensemble, we will measure the spatial dependence of the NQR frequency on our hBN flakes to determine the spatial variations of strain and electric fields.
Sandia National Laboratories is a multi-mission laboratory managed and operated by the National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the DOE’s National Nuclear Security Administration under Contract No. DE-NA0003525. This work was funded by the Laboratory Directed Research and Development Program and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science.

1. Lovchinsky, I. et al. Magnetic resonance spectroscopy of an atomically thin material using a single-spin qubit. Science (80-. ). 355, 503–507 (2017).

Nitrogen-vacancy (NV) quantum defects in diamond can detect nuclear magnetic resonance (NMR) signals with high-spectral resolution from micron-scale sample volumes. However, a key challenge for NV-NMR spectroscopy is detecting samples at millimolar molecular concentrations to enable analysis of complex micron-scale systems such as individual cells. Here we describe methods to increase NV-NMR concentration sensitivity by several orders of magnitude using hyperpolarization of target proton spins and a quantum memory readout. To date, these methods have allowed micron-scale NMR spectroscopy of small-molecule sample concentrations as low as 1 millimolar in picoliter volumes, with further sensitivity improvements expected.