Nitrogen-vacancy Defect Research Articles

Enhancement of n-type conductivity of hexagonal boron nitride films by in-situ co-doping of silicon and oxygen

Effective doping of ultra-wide band gap semiconductors is of crucial importance, yet, remains challenging. Here, we report the enhancement of n-type conductivity of nanocrystalline hexagonal boron nitride (h-BN) films with simultaneous incorporation of Si and O while deposition by radio frequency magnetron sputtering method. The resultant h-BN films are of ∼50 nm in thickness, containing nitrogen vacancy (VN) defects. Incorporation of O together with Si results in effective healing of VN defects and significantly reduces electric resistivity in h-BN thin films. X-ray photoelectron spectroscopy results reveal that under B-rich condition, the substitutional O in VN bonding with B leads to the formation of Si–N, which thus plays an important role to the n-type conductivity in h-BN films. The temperature dependent electrical resistivity measurements of the Si/O co-doped h-BN films reveal two donor levels of 130 and 520 meV at room temperature and higher temperatures, respectively. The n-h-BN/p-Si heterojunctions demonstrate apparent rectification characteristics at room temperature, where the tunneling behavior dominates throughout the injection regimes due to the effective carrier doping. This work proposes an effective approach to enhance the n-type conductivity of h-BN thin films for future applications in electronics, optoelectronics and photovoltaics.

Sub-second temporal magnetic field microscopy using quantum defects in diamond

Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in it’s current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This experimental technique allows magnetic field imaging of sub-second varying microscale currents in planar microcoils with imaging frame rates in the range of 50–200 frames per s (fps). Our work demonstrates that widefield per-pixel lock-in detection of frequency modulated NV ODMR enables dynamic magnetic field microscopy.

Endohedral σ-Diradical Nitrogen-Vacancy Diamond Nanoclusters with a Confined Magnetic Space and Strong Electronic Spin Couplings.

The electronic properties and their modulations for the nitrogen-vacancy (NV) centers in various nanoscale diamonds are of profound current interest because of their potential applications. However, although the NV centers as chromophores in diamond are the most widely studied, surprisingly, little is known about their magnetic spin coupling properties up to now. Here, we for the first time show, using the spin-polarized DFT calculations, that the NV centers can act as unique endohedral σ-diradical magnets in diamond nanoclusters and exhibit quite strong ferromagnetic (FM) or antiferromagnetic (AFM) spin coupling characteristics due to their unique endotetrahedral structures with favorable radical-radical contacts. Although the neutral NV center (NV0) in its doublet ground state exhibits quite strong AFM spin coupling among three radical C-sites (i.e., an AFM triradical center), interestingly, excess electron injection can convert it to a FM diradical magnet (i.e., the triplet ground state NV-) with almost unchanged J-coupling magnitude, and the J-coupling of the nanocluster can be noticeably enhanced by F-termination of the surface due to triradical spin delocalization mediated by excess electron. However, interior modification (one C in the endotetrahedron core is substituted by N or B or is hydrogenated) can assign the nanocluster perfect AFM diradical character. The spin coupling strength presents a quasilinear correlation with the distance between the two C radicals in the NV core for the same size of the clusters and a high linear correlation with the energy difference between two singly occupied molecular orbitals. Clearly, the FM and AFM couplings as well as their switching behavior in such NV defect diamond nanoclusters featuring the endohedral σ-diradicals are a novel type of promising magnetic material motifs. These findings open up promising spintronic application prospects of the NV diamonds and provide helpful information for the design of inorganic magnetic materials and logic devices.

Multidefect N-Nb2 O5- x @CNTs Incorporated into Capillary Transport Framework for Li+ /Na+ Storage.

As an ion-embedded material with small strain and low transport energy barrier, the limited ion transport rate and conductivity of niobium pentaoxide (Nb2 O5 ) are the main factors limiting its application in lithium/sodium storage systems. In this work, the microsphere composites (N-Nb2 O5- x @CNTs) are prepared by combining Nb2 O5 , rich in nitrogen doping and vacancy defects, with carbon nanotubes (CNTs) penetrating the bulk phase. With the capillary effect, CNTs can enable the rapid electrolyte infiltration into the microspheres, thus shorting the Li+ /Na+ transport path. In addition, CNTs also hinder the direct contact between the electrolyte and Nb2 O5 , and inhibit the irreversible reaction. Meanwhile, nitrogen doping and oxygen vacancy defects reduce the energy barrier of Li+ /Na+ transport, and improve their transport rate, proved by density functional theory. Highly conductive CNTs and unpaired electrons from defects also ameliorate the insulation property of Nb2 O5 . Therefore, N-Nb2 O5- x @CNTs display good electrochemical performance in both Li/Na half-cell and Li/Na hybrid capacitors. Interestingly, kilogram-scale microsphere composites can be produced in laboratory conditions by using industrial grade raw materials, implying its potential for practical application.

Nitrogen-vacancy defects in germanium

While nitrogen doping has been investigated extensively in silicon, there is only limited information on its interaction with vacancies in germanium, despite most point defect processes in germanium being vacancy controlled. Thus, spin polarized density functional theory calculations are used to examine the association of nitrogen with lattice vacancies in germanium and for comparison in silicon. The results demonstrate significant charge transfer to nitrogen from the nearest neighbor Ge and strong N–Ge bond formation. The presence of vacancies results in a change in nitrogen coordination (from tetrahedral to trigonal planar) though the total charge transfer to N is maintained. A variety of nitrogen vacancy clusters are considered, all of which demonstrated strong binding energies. Substitutional nitrogen remains an effective trap for vacancies even if it has already trapped one vacancy.

Influence of (N,H)-terminated surfaces on stability, hyperfine structure, and zero-field splitting of NV centers in diamond

We present a density functional theory analysis of the negatively charged nitrogen-vacancy (${\mathrm{NV}}^{\ensuremath{-}}$) defect complex in diamond located in the vicinity of (111)- or (100)-oriented surfaces with mixed (N,H)-terminations. We assess the stability and electronic properties of the ${\mathrm{NV}}^{\ensuremath{-}}$ center, and we study their dependence on the H:N ratio of the surface termination. The formation energy, the electronic density of states, the hyperfine structure, and the zero-field-splitting parameters of an ${\mathrm{NV}}^{\ensuremath{-}}$ center are analyzed as a function of its distance and orientation to the surface. We find stable ${\mathrm{NV}}^{\ensuremath{-}}$ centers with bulklike properties at distances of at least $\ensuremath{\sim}8$ \AA{} from the surface provided that the surface termination consists of at least 25% substitutional nitrogen atoms. The studied surface terminations have a minor effect on the ground-state properties, whereas the NV orientation has major effects. Our results indicate that axial NV centers near a flat 100% N-terminated (111) surface are the optimal choice for NV-based quantum sensing applications as they are the least influenced by the proximity of the surface.

Theoretical investigation of nitrogen-vacancy defects in silicon

Nitrogen-vacancy defects are important for the material properties of silicon and for the performance of silicon-based devices. Here, we employ spin polarized density functional theory to calculate the minimum energy structures of the vacancy-nitrogen substitutional, vacancy-dinitrogen substitutionals, and divacancy-dinitrogen substitutionals. The present simulation technique enabled us to gain insight into the defect structures and charge distribution around the doped N atom and the nearest neighboring Si atoms. Using the dipole–dipole interaction method, we predict the local vibration mode frequencies of the defects and discuss the results with the available experimental data.

Single-particle T1 relaxation measurements on diffusing nanodiamonds for intracellular reactive oxygen species detection

The fluorescent nitrogen-vacancy (NV) defect in diamond has the great advantage of offering long-term single-particle tracking and high sensitivity nanosensing. Such capabilities originate from the spin-dependent fluorescence of the NV centre, which is also free from photobleaching. As a result, NV centres in diamond nanoparticles have been used to detect different fluctuations in the nanoenvironment (e.g., magnetic field or pH). Because diamond is biocompatible and easy to functionalise, nanodiamonds (NDs) have also been used for intracellular nanosensing in living cells.

Explainable prediction of N-V-related defects in nanodiamond using neural networks and Shapley values

Although the negatively charged nitrogen-vacancy (N-V − ) defect in nanodiamonds is desirable for a variety of biomedical applications, a range of other defect complexes involving nitrogen and/or vacancies can also exist, depending on their relative stability. Using machine learning, a re-usable model is developed to predict the likelihood of a particular defect complex being stable at a given depth below reconstructed or hydrogen-passivated surfaces. A neural network is used to generate a system of equations that can be easily implemented in any workflow, and explainable artificial intelligence (XAI) methods are used to provide insights into which structural features and defect configurations are most responsible for the model prediction. It is found that, although the number of nitrogen atoms present in the defect is the most important feature determining the defect likelihood, the most influential data instances are the unlikely defects, providing a type of baseline for comparison. • An accurate neural network predicts the likelihood of N-V defects in nanodiamond • Explainable AI identifies important features and influential defect configurations Although the negatively charged nitrogen-vacancy defect in nanodiamonds is useful, a range of other defect complexes involving nitrogen and/or vacancies can also exist. Here, Barnard uses machine learning to develop a re-usable model to predict the likelihood of a particular defect complex being stable at a given depth below a given surface.

Observation of novel coexistence of Kondo effect and room temperature magnetism in AlN/Al/AlN trilayer thin film

In this work for the first time, we are reporting the unusual observation of the Kondo effect with the coexistence of room temperature ferromagnetism in AlN/Al/AlN trilayer thin film. The grown film shows resistivity minimum at a temperature of ∼48K, which shifts to the lower temperature on the application of magnetic fields. After considering various possibilities for an upturn in resistivity, we found that the Kondo scattering is responsible for upturn at low temperature. The simultaneous presence of ferromagnetism and Kondo scattering is explained by spatial variation of nitrogen vacancy defects from the film surface to the Al sandwich layer. Furthermore, magneto-transport properties of the film measured at different temperature exhibits both negative and positive components described by localized magnetic moment model for the spin scattering of carriers and two-band model, respectively. This work provides insight into the novel co-existence of ferromagnetism and Kondo effect in crystalline AlN.

Cathodoluminescence of Diamond: Features of Visualization

Zonal and sectorial heterogeneities in natural diamonds provide information on the growth conditions and post-growth changes. Cathodoluminescence (CL) microscopy revealed these heterogeneities in a very detailed manner with high spatial resolution. In this study, factors affecting the CL images of two natural diamonds were analyzed and the results of cathodoluminescence studies in steady-state (SS-CL) and scanning modes were compared. SS-CL was observed using an optical microscope, and scanning mode was evaluated via SEM (SEM-CL). It was demonstrated that the relative brightness of the <111> and <100> growth sectors in diamond crystals depends on the nature of defects in them and on the method of image detection (steady-state/scanning versus color/panchromatic). The differences between SS-CL and SEM-CL images can be attributed to the kinetics of luminescence and spectral sensitivity of the detectors. It was established that the nature of lattice defects around small inclusions can be changed (e.g., the intensity of blue luminescence from nitrogen-vacancy defects (N3V) decreases due to their transformation into nitrogen–hydrogen defects (N3VH). The hydrogen disproportion between the sectors is caused by different growth mechanisms. Hydrogen atoms in the diamond matrix can affect the kinetics of transformation of the defects by transforming a part of N3V to N3VH.

Diffusion in hypo-stoichiometric uranium mononitride

Uranium nitride is a nuclear fuel of interest as it offers enhanced accident tolerance, owing to its intrinsic properties. Before UN can be deployed commercially it is essential to understand its properties and how they evolve during operation. Therefore, molecular dynamics has been employed to study the thermal expansion and diffusivity of the intrinsic species in UN and how these change with stoichiometry. The introduction of hypostoichiometry as either nitrogen antisites or nitrogen vacancy defects is predicted to lead to an increase in nitrogen diffusivity and a concomitant decrease in the activation energy. The activation energies predicted for nitrogen diffusion in hypostoichiometry samples containing antisite defects are shown to offer a very close agreement with experimental observations.

Low-field microwave-mediated optical hyperpolarization in optically pumped diamond

The emergence of a new class of optically polarizable electronic spins in diamond, nitrogen vacancy (NV) defect centers, has opened interesting new avenues for dynamic nuclear polarization. Here we review methods for the room-temperature hyperpolarization of lattice 13C nuclei using optically pumped NV centers, focusing particular attention to a polarization transfer via rotating-frame level anti-crossings. We describe special features of this optical dynamic nuclear polarization (DNP) mechanism at low-field, in particular, its deployability to randomly oriented diamond nanoparticles. In addition, we detail methods for indirectly obtaining high-resolution NV ESR spectra via hyperpolarization readout. These mechanistic features provide perspectives for interesting new applications exploiting the optically generated 13C hyperpolarization.

Imaging phonon-mediated hydrodynamic flow in WTe2

In the presence of interactions, electrons in condensed-matter systems can behave hydrodynamically, exhibiting phenomena associated with classical fluids, such as vortices and Poiseuille flow. In most conductors, electron-electron interactions are minimized by screening effects, hindering the search for hydrodynamic materials; however, recently, a class of semimetals has been reported to exhibit prominent interactions. Here we study the current flow in the layered semimetal tungsten ditelluride by imaging the local magnetic field using a nitrogen-vacancy defect in a diamond. We image the spatial current profile within three-dimensional tungsten ditelluride and find that it exhibits non-uniform current density, indicating hydrodynamic flow. Our temperature-resolve current profile measurements reveal a non-monotonic temperature dependence, with the strongest hydrodynamic effects at approximately 20 K. We also report ab initio calculations showing that electron-electron interactions are not explained by the Coulomb interaction alone, but are predominantly mediated by phonons. This provides a promising avenue in the search for hydrodynamic flow and prominent electron interactions in high-carrier-density materials.

Screened configuration interaction method for open-shell excited states applied to NV centers

We present a computational approach, based on density functional theory and screened configuration interaction, able to accurately treat highly correlated electron spins localized around semiconductor defects, typically occurring in the context of qubit implementations. The method is computationally not more demanding than a usual density functional theory calculation, which makes it suitable for the calculation of isolated defects, or defect complexes, typically requiring large simulation cells. We illustrate the approach by applying it to the three different charge states of the nitrogen vacancy defect in diamond and obtain very good agreement with experiment.

Relaxation of a single defect spin by the low-frequency gyrotropic mode of a magnetic vortex

We excite the gyrotropic mode of a magnetic vortex and observe the resulting effect on the spin state of a nearby nitrogen-vacancy (NV) defect in diamond. Thin permalloy disks fabricated on a diamond sample are magnetized in a vortex state in which the magnetization curls around a central core. The magnetization dynamics of this configuration are described by a discrete spectrum of confined magnon modes as well as a low-frequency gyrotropic mode in which the vortex core precesses about its equilibrium position. Despite the spin transition frequencies being far-detuned from the modes of the ferromagnet, we observe enhanced relaxation of the NV spin when driving the gyrotropic mode. Moreover, we map the spatial dependence of the interaction between the vortex and the spin by translating the vortex core within the disk with an applied magnetic field, resulting in steplike motion as the vortex is pinned and de-pinned. Strong spin relaxation is observed when the vortex core is within approximately 250 nm of the NV center defect. We attribute this effect to the higher frequencies in the spectrum of the magnetic fringe field arising from the soliton-like nature of the gyrotropic mode when driven with sufficiently large amplitude.

Acid-assisted synthesis of nitrogen-deficient mesoporous graphitic carbon nitride for hydrogen storage

Defect engineering is an important strategy to tune the surface physical and chemical properties of carbon nitride materials. Herein, a nitrogen-deficient mesoporous graphitic carbon nitride (ND-MCN) with a high specific surface area of 62.6 m2 g−1 was synthesized via hydrochloric acid-assisted thermal polycondensation of melamine and its potential application in hydrogen storage was investigated. Hydrochloric acid acting as activation agent can increase the specific surface area of carbon nitride, and introduce nitrogen-vacancy defects into the framework, leading to a stronger interaction between ND-MCN and hydrogen molecules. As a result, the ND-MCN shows a high hydrogen storage density of 6.39 × 10−3 wt%·g m−2 at 298 K under 20 bar.

Synergistic effects of surface Lewis Base/Acid and nitrogen defect in MgAl layered double Oxides/Carbon nitride heterojunction for efficient photoreduction of carbon dioxide

Conversion of CO2 to CO from solar energy on semiconductors is a promising route to renewable energy, but their practicality is limited by the poor adsorption and activation of CO2 on the semiconductor surface and the low utilization of photovoltaic carriers. Here, we construct a MgAl layered double oxide/nitrogen-deficient carbon nitride (MgAl LDO/Nv-CN) nanosheets hybrid by in situ deposition of MgAl layered double hydroxide (MgAl LDH) on Nv-CN nanosheets followed by subsequent calcination. The MgAl LDO serves as the surface-rich Lewis basic sites to effectively enhance adsorption of CO2 and H2O, while the nitrogen defects can enhance the optical absorption of g-C3N4 and improve the charge carrier separation. The optimal MgAl LDO/Nv-CN has high photocatalytic activity for CO2 reduction with a CO generation rate of 20.47 µmol·g−1·h−1 under visible light illumination, which is 6 times higher than that of bulk g-C3N4. Our work highlights the significance of synergy of MgAl LDO Lewis base/acid sites and nitrogen vacancy defects in the design of efficient photocatalysts for CO2 reduction.

Mixed type Ib-IIa diamond from a Fe–Mg2Si3O8·5H2O–C system under high pressure and high temperature

The composition of the crystalline medium has an essential effect on the formation and properties of diamonds. Large single-crystal diamonds grown in a Fe–Mg2Si3O8·5H2O–C system by the temperature gradient method (TGM) at 5.8–6.3 GPa and 1400–1480 °C were investigated. It was found that as the Mg2Si3O8·5H2O content increased from 0 to 4 wt%, the synthesis pressure and temperature both increased, and more nitrogen-vacancy (NV) defect centers appeared in the diamond crystal. The nitrogen impurity concentration in type Ib diamonds synthesized in the pure FeC system was about 90 ppm, and the distribution of nitrogen impurities was uniform from core to rim. Furthermore, with a Mg2Si3O8·5H2O content of approximately 4.0 wt%, type Ib-IIa diamond was successfully synthesized. The infrared spectra (IR) results showed that the nitrogen content of the type Ib-IIa diamond was higher in the central zone of the crystal (79 ppm), decreased in the intermediate zone (42 ppm), and decreased further at the periphery (< 1 ppm). Thus, the type Ib-IIa diamond had significant heterogeneity in the distribution of nitrogen.

Quantitative study of the response of a single NV defect in diamond to magnetic noise

The nitrogen-vacancy (NV) defect in diamond is an efficient quantum sensor of randomly fluctuating signals via relaxometry measurements. In particular, the longitudinal spin relaxation of the NV defect accelerates in the presence of magnetic noise with a spectral component at its electron spin resonance frequency. We look into this effect quantitatively by applying a calibrated and tunable magnetic noise on a single NV defect. We show that an increase of the longitudinal spin relaxation rate translates into a reduction of the photoluminescence (PL) signal emitted under continuous optical illumination, which can be explained using a simplified three-level model of the NV defect. This PL quenching mechanism offers a simple, all-optical method to detect magnetic noise sources at the nanoscale.