Shallow Nitrogen-vacancy Centers Research Articles

Diamond Surfaces with Lateral Gradients for Systematic Optimization of Surface Chemistry for Relaxometry - a Low-Pressure Plasma-Based Approach.

Diamond is increasingly popular because of its unique material properties. Diamond defects called nitrogen vacancy (NV) centers allow for measurements with unprecedented sensitivity. However, to achieve ideal sensing performance, NV centers need to be within nanometers from the surface and are thus strongly dependent on the local surface chemistry. Several attempts have been made to compare diamond surfaces. However, due to the high price of diamond crystals with shallow NV centers, a limited number of chemical modifications have been studied. Here, we developed a systematic method to investigate the continuity of different local environments with varying densities and natures of surface groups in a single experiment on a single diamond plate. To achieve this goal, we used diamonds with a shallow ensemble of NV centers and introduced a chemical gradient across the surface. More specifically, we used air and hydrogen plasma. The gradients were formed by a low-pressure plasma treatment after masking with a right-angled triangular prism shield. As a result, the surface contained gradually more oxygen/hydrogen toward the open end of the shield. We then performed wide-field relaxometry to determine the effect of surface chemistry on the sensing performance. As expected, relaxation times and thus sensing performance indeed vary along the gradient.

Charge and Spin Dynamics and Destabilization of Shallow Nitrogen-Vacancy Centers under UV and Blue Excitation.

Shallow nitrogen-vacancy (NV) centers in diamond offer opportunities to study photochemical reactions, including photogeneration of radical pairs, at the single-molecule regime. A prerequisite is a detailed understanding of charge and spin dynamics of NVs exposed to the short-wavelength light required to excite chemical species. Here, we investigate the charge and spin dynamics of shallow NVs under 445 and 375 nm illumination. With blue excitation, charge-state preparation is power-dependent, and modest spin initialization fidelity is observed. Under UV excitation, charge-state preparation is power-independent and no spin polarization is observed. Aging of NVs under prolonged UV exposure manifests in a reduced charge stability and spin contrast. We attribute this aging to modified local charge environments of near-surface NVs and identify distinct electronic traps only accessible at short wavelengths. Finally, we evaluate the prospects of NVs to probe photogenerated radical pairs based on measured sensitivities and outline possible sensing schemes.

Multidimensional Spectroscopy of Nuclear Spin Clusters in Diamond.

Optically active spin defects in solids offer promising platforms to investigate nuclear spin clusters with high sensitivity and atomic-site resolution. To leverage near-surface defects for molecular structure analysis in chemical and biological contexts using nuclear magnetic resonance (NMR), further advances in spectroscopic characterization of nuclear environments are essential. Here, we report Fourier spectroscopy techniques to improve localization and mapping of the test bed ^{13}C nuclear spin environment of individual, shallow nitrogen-vacancy centers at room temperature. We use multidimensional spectroscopy, well-known from classical NMR, in combination with weak measurements of single-nuclear-spin precession. We demonstrate two examples of multidimensional NMR: (i)improved nuclear spin localization by separate encoding of the two hyperfine components along spectral dimensions and (ii)spectral editing of nuclear-spin pairs, including measurement of internuclear coupling constants. Our work adds important tools for the spectroscopic analysis of molecular structures by single-spin probes.

Diamond surface functionalization via visible light–driven C–H activation for nanoscale quantum sensing

Nitrogen-vacancy (NV) centers in diamond are a promising platform for nanoscale NMR sensing. Despite significant progress toward using NV centers to detect and localize nuclear spins down to the single spin level, NV-based spectroscopy of individual, intact, arbitrary target molecules remains elusive. Such sensing requires that target molecules are immobilized within nanometers of NV centers with long spin coherence. The inert nature of diamond typically requires harsh functionalization techniques such as thermal annealing or plasma processing, limiting the scope of functional groups that can be attached to the surface. Solution-phase chemical methods can be readily generalized to install diverse functional groups, but they have not been widely explored for single-crystal diamond surfaces. Moreover, realizing shallow NV centers with long spin coherence times requires highly ordered single-crystal surfaces, and solution-phase functionalization has not yet been shown with such demanding conditions. In this work, we report a versatile strategy to directly functionalize C-H bonds on single-crystal diamond surfaces under ambient conditions using visible light, forming C-F, C-Cl, C-S, and C-N bonds at the surface. This method is compatible with NV centers within 10 nm of the surface with spin coherence times comparable to the state of the art. As a proof-of-principle demonstration, we use shallow ensembles of NV centers to detect nuclear spins from surface-bound functional groups. Our approach to surface functionalization opens the door to deploying NV centers as a tool for chemical sensing and single-molecule spectroscopy.

Spin-State Control of Shallow Single NV Centers in Hydrogen-Terminated Diamond.

The ability to control the charge and spin states of nitrogen-vacancy (NV) centers near the diamond surface is of pivotal importance for quantum applications. Hydrogen-terminated diamond is promising for long spin coherence times and ease of controlling the charge states due to the low density of surface defects. However, it has so far been challenging to create negatively charged single NV centers with controllable spin states beneath a hydrogen-terminated surface because atmospheric adsorbates that act as acceptors induce surface holes. In this study, we optically detected the magnetic resonance of shallow single NV centers in hydrogen-terminated diamond through precise control of the nitrogen implantation fluence. Furthermore, we found that the probability of detecting the resonance was enhanced by reducing the surface acceptor density through passivation of the hydrogen-terminated surface with hexagonal boron nitride without air exposure. This control method opens up new opportunities for using NV centers in quantum applications.

Charge Stability and Charge-State-Based Spin Readout of Shallow Nitrogen-Vacancy Centers in Diamond.

Spin-based applications of the negatively charged nitrogen-vacancy (NV) center in diamonds require an efficient spin readout. One approach is the spin-to-charge conversion (SCC), relying on mapping the spin states onto the neutral (NV0) and negative (NV-) charge states followed by a subsequent charge readout. With high charge-state stability, SCC enables extended measurement times, increasing precision and minimizing noise in the readout compared to the commonly used fluorescence detection. Nanoscale sensing applications, however, require shallow NV centers within a few nanometers distance from the surface where surface related effects might degrade the NV charge state. In this article, we investigate the charge state initialization and stability of single NV centers implanted ≈5 nm below the surface of a flat diamond plate. We demonstrate the SCC protocol on four shallow NV centers suitable for nanoscale sensing, obtaining a reduced readout noise of 5-6 times the spin-projection noise limit. We investigate the general applicability of the SCC for shallow NV centers and observe a correlation between the NV charge-state stability and readout noise. Coating the diamond with glycerol improves both the charge initialization and stability. Our results reveal the influence of the surface-related charge environment on the NV charge properties and motivate further investigations to functionalize the diamond surface with glycerol or other materials for charge-state stabilization and efficient spin-state readout of shallow NV centers suitable for nanoscale sensing.

Creation of shallow nitrogen vacancy centers in HPHT diamond surface via catalytic etching of transition metal

The nitrogen vacancy (NV) color centers are a solid-state quantum system with the ability to maintain long coherence times at room temperatures. NV centers have potential applications in quantum detection, which necessitates their positioning at minimal distances from surfaces. However, the existing methods for fabricating NV centers are cumbersome, expensive, and difficult to control. This study investigates the etching mechanism of transition metals on diamond under plasma conditions and develops a method for producing shallow NV centers on the surface of high pressure and high temperature diamond. Our proposed method enables the production of shallow NV centers below the surface in the longitudinal direction, while simultaneously allowing precise control over the lateral distribution. Moreover, the operating costs associated with our technique are significantly lower than those of ion implantation and electron irradiation. This research contributes to the advancement of iron-etching diamond processing technology, and the proposed method facilitates the more portable preparation of shallow color centers in specific regions, enabling the realization of locally controllable and scalable coupled quantum systems.

Hydrogen and oxygen mixed-termination for nitrogen-vacancy quantum sensors in hexagonal diamond

Controlling the chemical termination of hexagonal diamond surface opens up new possibilities for shallow nitrogen-vacancy (NV) centers in quantum sensing applications. In this work, the H/O/OH mixed terminations on hexagonal diamond (100) surfaces to host NV centers are investigated by first-principles calculations. Results indicate that H–O, O–OH, and H–O–OH mixed surfaces produce positive electron affinity with neither spin noise nor surface-related state. It is concluded that the surface-connected ether bonds should be maintained, and unconnected ether bonds should be avoided, preserving the shallow NV centers unaffected. Based on the surface stability analysis, we further confirm that the H–O surface has a higher possibility to be fabricated. Moreover, we make an intensive study by placing an NV center ∼12 Å from the hexagonal (100) surface, and the results elucidate that the H–O surface has no obvious effect on the defect states of the NV center. Therefore, we identify a combination of surface terminators that is a prospective candidate for NV-based quantum sensors.

Fluorine and oxygen terminated hexagonal diamond (100) surfaces for nitrogen-vacancy based quantum sensors

Chemical modification of hexagonal diamond surface is a new and important issue for the negatively charged nitrogen-vacancy (NV) center in quantum sensors. In this work, six types of terminated hexagonal diamond (100) surfaces for shallow NV centers are proposed, and structural and electronic properties are investigated by structure prediction method combined with first-principles calculation. In these structures, only fluorine and oxygen terminated surfaces (F-hex and α-O-hex) have positive electron affinity of 3.00 and 2.85 eV, respectively, no surface-related states and electron spin noise, indicating the F-hex and α-O-hex surfaces could be a promising host of NV center. We make further inspections by placing an NV center ~12 Å below the surfaces, and the results confirm that the surfaces have little effect on the defect states of NV center. Consequently, the F-hex and α-O-hex surfaces are prospective candidates for NV-based quantum sensors.

Enhancing photon collection from single shallow nitrogen-vacancy centers in diamond nanopillars for quantum heterodyne measurements

The developments in quantum sensing protocols and nano-photonic waveguides are merged to improve the performance of single nitrogen-vancancy (NV) centers in nuclear magnetic resonance (NMR) sensing. Nanopillars are designed with NV centers placed approximately 5 nm below the top facet and fabricated through a simple procedure, suitable for mass production. Fluorescence intensities from these nanopillars are 3.5 times greater than that of single shallow NV centers embedded in unstructured flat diamond. Quantum heterodyne measurements of an alternating magnetic field are performed with these nanopillars and evidence of improved peak clarity in the frequency spectrum is shown.

Advances in Stabilization and Enrichment of Shallow Nitrogen-Vacancy Centers in Diamond for Biosensing and Spin-Polarization Transfer.

Negatively charged nitrogen-vacancy (NV-) centers in diamond have unique magneto-optical properties, such as high fluorescence, single-photon generation, millisecond-long coherence times, and the ability to initialize and read the spin state using purely optical means. This makes NV- centers a powerful sensing tool for a range of applications, including magnetometry, electrometry, and thermometry. Biocompatible NV-rich nanodiamonds find application in cellular microscopy, nanoscopy, and in vivo imaging. NV- centers can also detect electron spins, paramagnetic agents, and nuclear spins. Techniques have been developed to hyperpolarize 14N, 15N, and 13C nuclear spins, which could open up new perspectives in NMR and MRI. However, defects on the diamond surface, such as hydrogen, vacancies, and trapping states, can reduce the stability of NV- in favor of the neutral form (NV0), which lacks the same properties. Laser irradiation can also lead to charge-state switching and a reduction in the number of NV- centers. Efforts have been made to improve stability through diamond substrate doping, proper annealing and surface termination, laser irradiation, and electric or electrochemical tuning of the surface potential. This article discusses advances in the stabilization and enrichment of shallow NV- ensembles, describing strategies for improving the quality of diamond devices for sensing and spin-polarization transfer applications. Selected applications in the field of biosensing are discussed in more depth.

Detecting nuclear spins in an organosilane monolayer using nitrogen-vacancy centers for analysis of precursor self-assembly on diamond surface

We demonstrated the correlation spectroscopy of organosilane monolayers using an ensemble of shallow nitrogen-vacancy centers as the quantum sensor. Several types of organosilane monolayers were grown directly on the diamond surface by exposing the surface to a silane precursor vapor. The feasibility of detecting 1H and 19F in the monolayer was examined by correlation spectroscopy measurements. The effect of the magnetic dipole–dipole interaction on the peak width was also discussed by comparing the spectrum of the monolayer with that of surface-attached 1H and that of immersion oil. The results highlight the feasibility of nitrogen-vacancy centers as the spin probe for physicochemical analyses of monolayers grown on the diamond surface.

Charge stabilization of shallow nitrogen-vacancy centers using graphene/diamond junctions

We studied the charge-state stabilization of shallow nitrogen-vacancy (NV) centers in (111) diamond using graphene/diamond junctions. Measurement of the fluorescence stability and evaluation of the charge-state stability were conducted on the NV centers at the graphene and the graphene-free region. The results revealed that about half of the total NV centers (NV0 + NV−) at the graphene-free region were unstable, while over 90% of the measured NV centers at the graphene region were stabilized as NV− centers. Graphene/diamond junctions contribute significantly to charge-state stabilization of shallow NV− centers in (111) diamond.

Reporter-Spin-Assisted T1 Relaxometry

The detection of fluctuating electromagnetic fields through single-spin relaxometry affords important insight into the dynamics of solid-state systems and chemical processes, but its sensitivity is often limited by the proximity of the sensor spin to the target. This study proposes the use of an auxiliary reporter spin to improve sensitivity by as much as a factor of 100, and experimentally verifies the method with a single shallow nitrogen-vacancy center in diamond. This work motivates the development of engineered spin systems as relaxation sensors without the need for optical initialization or readout.

Radio-Frequency Electric Field Sensing Based on a Single Solid-State Spin

Nanoscale amplitude and phase sensing of a rf electromagnetic field can benefit broad frontiers in science and technology. The imaging applications in these research areas demand a nanoscale sensor able to detect rf electric fields over a wide frequency range under ambient conditions, which remains a challenge. Here, we present a rf electrometry based on the forbidden magnetic dipole transition of a single shallow nitrogen-vacancy center in diamond and experimentally demonstrate the detection of rf electric fields at frequencies ranging from 13.85 MHz to 2.02 GHz. A sensitivity of $265\phantom{\rule{0.2em}{0ex}}{\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$ for amplitude measurement and a standard deviation of ${0.2}^{\ensuremath{\circ}}$ for phase measurement are achieved. The potential applications of nanoscale sensing and imaging of electromagnetic field are discussed.

Robust magnetometry with single nitrogen-vacancy centers via two-step optimization

Shallow Nitrogen-Vacancy (NV) centers are promising candidates for high-precision sensing applications; these defects, when positioned a few nanometers below the surface, provide an atomic-scale resolution along with substantial sensitivity. However, the dangling bonds and impurities on the diamond surface result in a complex environment which reduces the sensitivity and is unique to each shallow NV center. To avoid the environment's detrimental effect, we apply feedback-based quantum optimal control. We first show how a direct search can improve the initialization/readout process. In a second step, we optimize microwave pulses for pulsed Optically Detected Magnetic Resonance (ODMR) and Ramsey measurements. Throughout the sensitivity optimizations, we focus on robustness against errors in the control field amplitude. This feature not only protects the protocols' sensitivity from drifts but also enlarges the sensing volume. The resulting ODMR measurements produce sensitivities below 1$\mu$T\,Hz$^{-\frac{1}{2}}$ for an 83\% decrease in control power, increasing the robustness by approximately one third. The optimized Ramsey measurements produce sensitivities below 100\,nT\,Hz$^{-\frac{1}{2}}$ giving a two-fold sensitivity improvement. Being on par with typical sensitivities obtained via single NV magnetometry, the complementing robustness of the presented optimization strategy may provide an advantage for other NV-based applications.

Fluorine-terminated diamond (110) surfaces for nitrogen-vacancy quantum sensors

Diamond (110) surface is one of the low-index diamond faces but its effects on nitrogen-vacancy (NV) based quantum sensor remain unclear. The fluorine, hydrogen, nitrogen, and oxygen-terminated diamond (110) surfaces used for the NV centers are proposed here, and their electronic properties are investigated based on first-principles calculations. The oxygen-terminated diamond (110) surface has inter-bandgap states and surface electron spins, the nitrogen-terminated (110) surface has inter-band gap states, and the hydrogen-terminated (110) surface has negative electron affinity. Thus, these three surfaces may not be suitable for shallow NV centers. The fluorine-terminated diamond (110) surface has positive electron affinity, no surface related inter bandgap states, and no surface electron spins, so it is a promising candidate for NV-based quantum sensors.

Zero-field magnetometry using hyperfine-biased nitrogen-vacancy centers near diamond surfaces

Shallow nitrogen-vacancy (NV) centers in diamond are promising for nanomagnetometry, for they can be placed proximate to targets. To study the intrinsic magnetic properties, zero-field magnetometry is desirable. However, for shallow NV centers under zero field, the strain near diamond surfaces would cause level anticrossing between the spin states, leading to clock transitions whose frequencies are insensitive to magnetic signals. Furthermore, the charge noises from the surfaces would induce extra spin decoherence and hence reduce the magnetic sensitivity. Here, we demonstrate that the relatively strong hyperfine coupling (130 MHz) from a first-shell $^{13}\mathrm{C}$ nuclear spin can provide an effective bias field to an NV center spin so that the clock-transition condition is broken and the charge noises are suppressed. The hyperfine bias enhances the dc magnetic sensitivity by a factor of 22 in our setup. With the charge noises suppressed by the strong hyperfine field, the ac magnetometry under zero field also reaches the limit set by decoherence due to the nuclear spin bath. In addition, the 130 MHz splitting of the NV center spin transitions allows relaxometry of magnetic noises simultaneously at two well-separated frequencies (\ensuremath{\sim}2.870 \ifmmode\pm\else\textpm\fi{} 0.065 GHz), providing (low-resolution) spectral information of high-frequency noises under zero field. The hyperfine-bias-enhanced zero-field magnetometry can be combined with dynamical decoupling to enhance single-molecule magnetic resonance spectroscopy and to improve the frequency resolution in nanoscale magnetic resonance imaging.

Generation of shallow nitrogen-vacancy centers in diamond with carbon ion implantation

The shallow nitrogen-vacancy center of diamond exhibits excellent sensitivity and resolution in the magnetic detection and quantum sensing areas. Compared with other methods, low-energy carbon ion implantation does not need high-purity diamond nor introduce new impurity atoms, but the formation mechanism of nitrogen-vacancy center is not clear. In this work, shallow nitrogen-vacancy centers are created in the diamond by low energy carbon ion implantation and vacuum annealing, and the transformation mechanism of nitrogen-vacancy centers in diamond is studied by Raman spectroscopy, X-ray photoelectron spectroscopy, and positron annihilation analysis. The results show that shallow nitrogen-vacancy centers can be obtained by carbon ion implantation combined with vacuum annealing. After implantation, superficial layer of diamond shows the damage zone including lattice distortion and amorphous carbon, and carbon-vacancy cluster defects (carbon atoms are surrounded by vacancy clusters) are generated. In the vacuum annealing process, the damaged area gradually transforms into the diamond structure through the recovery of the distortion area and the solid-phase epitaxy of the amorphous carbon area, accompanied by the continuous dissociation of carbon-vacancy cluster defects. When samples are annealed at 850 and 900 ℃, the structure of the damaged area is partially repaired. While annealing at 950 ℃, not only the damaged layer is basically recovered, but also nitrogen atoms capture the single vacancy obtained by the dissociation of carbon vacancy clusters, forming the nitrogen-vacancy centers.

Nanoscale Vector Electric Field Imaging Using a Single Electron Spin.

The ability to perform nanoscale electric field imaging of elementary charges at ambient temperatures will have diverse interdisciplinary applications. While the nitrogen-vacancy (NV) center in diamond is capable of high-sensitivity electrometry, demonstrations have so far been limited to macroscopic field features or detection of single charges internal to the diamond itself. In this work, we greatly extend these capabilities by using a shallow NV center to image the electric field of a charged atomic force microscope tip with nanoscale resolution. This is achieved by measuring Stark shifts in the NV spin-resonance due to AC electric fields. We demonstrate a near single-charge sensitivity of ηe = 5.3 charges/√Hz and subelementary charge detection (0.68e). This proof-of-concept experiment provides the motivation for further sensing and imaging of electric fields using NV centers in diamond.