Nitrogen-vacancy Complexes Research Articles

Photoexcitation and recombination processes of the neutral nitrogen-vacancy center in diamond from first principles

Nitrogen-vacancy (NV) complex in diamond is one of the most prominent solid state defects as the negatively charged NV defect (NV−) is a leading contender for quantum technologies. In quantum information processing applications, NV− is photoexcited that often leads to photoionization to neutral NV defect, NV0, and re-ionization back to NV− should occur to control the S=1 spin of NV−. As a consequence, understanding the photophysics of NV0 is crucial for controlling NV−. Furthermore, recent studies have shown that the S=1/2 electron spin of NV0 can also be initialized and read out at certain conditions that turns single NV0 a potential quantum bit. Quantum optics protocols rest on detailed knowledge on the electronic structure of the given system, which is obviously missing for NV0 in diamond. In this study, we combine the group theory and density functional theory calculations toward exploring the nature of the ground and excited states of NV0. We show that the effective three-electron system of NV0 leads to high correlation effects that make this system very challenging for ab initio simulations.

Modification of Diamond Surface by Femtosecond Laser Pulses

The basic mechanisms of laser interaction with synthetic diamond are reviewed. The characteristics of the main regimes of diamond surface etching are considered. In addition to the well-known graphitization and ablation processes, nanoablation and accumulative graphitization, which have attracted relatively recent attention, are described in detail. The focus is on femtosecond (fs) laser exposure, which allows for the formation of a dense cold electron–hole plasma in the focal zone and minimal overheating in the surrounding area. This potentially opens the way to the development of unique laser-based technologies that combine physical and chemical processes for precise surface treatment and functionalization. The physical limitations that determine how precisely the diamond surface can be treated by short-pulsed laser radiation and possible ways to overcome them with the ultimate goal of removing ultrathin layers of the material are discussed. Special attention is paid to the novel possibility of inducing the local formation of point active defects—nitrogen vacancy (NV) complexes in the laser-irradiated zone. Such defects have been at the forefront of solid-state physics for the past thirty years due to continuous attempts to exploit their unique properties in quantum optics, quantum computing, magnetometry, probing, and other fields. Both regimes of NV center formation with and without graphitization of the diamond lattice are considered. Thus, it is shown that intense pulsed laser irradiation is a perfect tool for the processing of synthetic diamonds at the micro-, nano-, and even at the atomic level, which can be well controlled and managed.

Effect of Mg doping on carrier recombination in GaN

Time-resolved photoluminescence measurements have been performed on Mg-doped GaN for Mg concentrations in the low- to mid-1019 cm−3. As-grown and annealed (600–675 °C) samples were studied. In the as-grown samples, the nonradiative carrier lifetime was found to be about 200 ps and nearly independent of the Mg concentration. Upon annealing, the carrier lifetimes shorten to ∼150 ps but, again, show little dependence on the annealing temperature. The analysis of possible Shockley–Read–Hall recombination centers and their behavior during doping and annealing suggests that the main nonradiative recombination center is the Mg–nitrogen vacancy complex. The weak dependence of the PL decay times on temperature indicates that carrier capture into this center has a very low potential barrier, and the nonradiative recombination dominates even at low temperatures.

Deep Levels and Electron Paramagnetic Resonance Parameters of Substitutional Nitrogen in Silicon from First Principles.

Nitrogen is commonly implanted in silicon to suppress the diffusion of self-interstitials and the formation of voids through the creation of nitrogen-vacancy complexes and nitrogen-nitrogen pairs. Yet, identifying a specific N-related defect via spectroscopic means has proven to be non-trivial. Activation energies obtained from deep-level transient spectroscopy are often assigned to a subset of possible defects that include non-equivalent atomic structures, such as the substitutional nitrogen and the nitrogen-vacancy complex. Paramagnetic N-related defects were the object of several electron paramagnetic spectroscopy investigations which assigned the so-called SL5 signal to the presence of substitutional nitrogen (NSi). Nevertheless, its behaviour at finite temperatures has been imprecisely linked to the metastability of the NSi center. In this work, we build upon the robust identification of the SL5 signature and we establish a theoretical picture of the substitutional nitrogen. Through an understanding of its symmetry-breaking mechanism, we provide a model of its fundamental physical properties (e.g., its energy landscape) based on ab initio calculations. Moreover by including more refined density functional theory-based approaches, we calculate EPR parameters (↔g and ↔A tensors), elucidating the debate on the metastability of NSi. Finally, by computing thermodynamic charge transition levels within the GW method, we present reference values for the donor and acceptor levels of NSi.

Reductions of implantation induced defects and leakage current by annealing in NH3/N2 atmosphere for Mg- and N-implanted GaN

We demonstrate the advantage of post-implantation annealing (PIA) in NH3/N2 for a p-n diode (PND) fabricated by the implantation of Mg and N ions into an n-type GaN layer by comparison with that annealed in N2. The leakage current for the PND with a reverse bias was lower in the case of NH3/N2 annealing. The cathodoluminescence spectrum measured for NH3/N2 annealing indicated a reduction in the densities of non-radiative recombination centers and nitrogen vacancy complexes. PIA in NH3/N2 is thus effective to suppress the density of implantation induced defects as leakage current sources.

Simulation of Nitrogen Indiffusion and Its Impact on Silicon Wafer Strength

Herein, the concentration profiles of nitrogen after nitriding rapid thermal annealing (RTA) of Si wafer surfaces are simulated. A model describing the fundamental partial differential equations for diffusion and interaction of nitrogen, vacancies, interstitials, and nitrogen vacancy (NV) complexes is used for this thermal process and the equations are solved simultaneously. In addition, thermal stress tests via intentionally induced stress due to the temperature gradient in an RTA process are conducted. By these wafer strength tests, which apply a controlled stress level, the relative slip robustness of wafers with nitrogen‐indiffused RTA and polished wafers are compared quantitatively.

Density functional theory studies of transition metal doped Ti3N2 MXene monolayer

Spin-polarized density functional theory calculations with van der Waals correction were applied to investigate the presence of transition metal dopant atoms (Fe, Co, Ni, Pd and Pt) and their respective nitrogen vacancy complexes in the Ti3N2 MXene monolayer as candidate material for hydrogen evolution reaction (HER). We found that the transition metal dopant atoms as well as transition metal vacancy will feasibly form. However, the cation transition metal dopant atoms with nitrogen vacancy complexes and nitrogen vacancy are endothermic except for the PtTi + VN complex, thus would require additional energy to be realized. To evaluate for the HER activity, the Gibbs free energy and adsorption energetics were calculated. We found that the H atom stably bind to the considered surface configurations. However, the Gibbs free energy is far from optimal for the dopant complexes and the pristine Ti3N2 MXene monolayer. All the considered configurations have magnetic metallic ground state with possible application as 2D magnetic system.

Orientation Dependence of Cathodoluminescence and Photoluminescence Spectroscopy of Defects in Chemical-Vapor-Deposited Diamond Microcrystal.

Point defects, impurities, and defect–impurity complexes in diamond microcrystals were studied with the cathodoluminescence (CL) spectroscopy in the scanning electron microscope, photoluminescence (PL), and Raman spectroscopy (RS). Such defects can influence the directions that microcrystals are grown. Micro-diamonds were obtained by a hot-filament chemical vapor deposition (HF CVD) technique from the methane–hydrogen gas mixture. The CL spectra of diamond microcrystals taken from (100) and (111) crystallographic planes were compared to the CL spectrum of a (100) oriented Element Six diamond monocrystal. The following color centers were identified: 2.52, 2.156, 2.055 eV attributed to a nitrogen–vacancy complex and a violet-emitting center (A-band) observed at 2.82 eV associated with dislocation line defects, whose atomic structure is still under discussion. The Raman studies showed that the planes (111) are more defective in comparison to (100) planes. What is reflected in the CL spectra as (111) shows a strong band in the UV region (2.815 eV) which is not observed in the case of the (100) plane.

Single-Photon Emission from Point Defects in Aluminum Nitride Films.

Quantum technologies require robust and photostable single-photon emitters. Here, room temperature operated single-photon emissions from isolated defects in aluminum nitride (AlN) films are reported. AlN films were grown on nanopatterned sapphire substrates by metal organic chemical vapor deposition. The observed emission lines range from visible to near-infrared, with highly linear polarization characteristics. The temperature-dependent line width increase shows T3 or single-exponential behavior. First-principle calculations based on density functional theory show that point defect species, such as antisite nitrogen vacancy complex (NAlVN) and divacancy (VAlVN) complexes, are considered to be an important physical origin of observed emission lines ranging from approximately 550 to 1000 nm. The results provide a new platform for on-chip quantum sources.

Atomic structures and scanning tunnelling microscopy of nitrogen-doped carbon nanotubes

We report on atomic structures, energetics, and scanning tunnelling microscopy images of nitrogen defects in carbon nanotubes (CNTs) based on first-principles density-functional study. As defect configurations in CNTs, not only the substitutional nitrogen defect but also the nitrogen-vacancy complex defects (pyridine-type defects) are considered. The substitutional nitrogen defect is found to be energetically the most stable defect. On the other hand, for the pyridine-type defects, the tetramerized and the trimerized configurations are found to be the possible configurations in energy. The scanning tunnelling microscopy (STM) images for defect configurations in N-doped CNTs are studied, and the various N defects in CNTs are expected to be identified by STM experiments.

Enhanced photocatalytic hydrogen production on GaN–ZnO oxynitride by introduction of strain-induced nitrogen vacancy complexes

Oxynitrides are considered as new potential candidates for photocatalysis due to their lower bandgap compared with traditional oxide photocatalysts. However, the formation of native nitrogen monovacancies during the synthesis of oxynitrides reduces their photocatalytic activity due to the vacancy-induced electron/hole recombination. This study shows that a transition from the native nitrogen monovacancies to the nitrogen-based vacancy complexes in a GaN–ZnO oxynitride not only diminishes the recombination but also enhances the photocatalytic hydrogen production. Here, the vacancy complexes are introduced by mechanical straining via the high-pressure torsion (HPT) method and it is shown that the vacancy complexes reduce the bandgap and increase the over-potential for hydrogen production on the conduction band. The current results introduce a simple but effective approach to turn the nitrogen vacancies to favorable defects for photocatalysis.

Effect of lonsdaleite on the optical properties of impact diamonds

The special features of impact diamonds are the orientation of the nanosized grains relative to each other, the presence of hexagonal diamond (lonsdaleite, L) in a large part of the samples and the increased wear resistance. Using Raman spectroscopy and XRD, two groups of translucent samples of Popigai impact diamonds (PIDs) were selected: with and without lonsdaleite and the effect of lonsdaleite on the optical properties of the samples was studied. In all L-containing PIDs there is a strong absorption band of about 1230 cm-1 in the one-phonon region, in the mid-IR. The absorption edge is blurred and described by the Urbach rule. The estimated value of Eg ~4 eV for L is consistent with the first principles calculations. Impurity nitrogen is found only in L-free PIDs: There are signals from nitrogen-vacancy complexes in the photoluminescence (PL) spectra. Variations in the number of nitrogen atoms (N = 1 to 4) in the structure of these centers indicate significant variations in the parameters of PID annealing. L-containing PIDs are characterized by large strains in the lattice and, as a consequence, there are problems with the defect diffusion. The narrow lines in PL spectra, uncommon for diamond, can be the result of several orders of magnitude higher concentrations of impurities in PIDs formed during the solid-phase transition. The broadened peaks of 180, 278 and 383 K are distinguishable in the curves of thermostimulated luminescence (TSL) for L-free PIDs, but in the presence of L the TSL glow becomes continuous as in natural IaA-type diamonds with platelets. In general, lonsdaleite deteriorates the optical properties of impact diamonds and makes it difficult to create certain types of impurity-vacancy complexes for different applications.

Annealing-Induced Changes in the Nature of Point Defects in Sublimation-Grown Cubic Silicon Carbide.

In recent years, cubic silicon carbide (3C-SiC) has gained increasing interest as semiconductor material for energy saving and optoelectronic applications, such as intermediate-band solar cells, photoelectrochemical water splitting, and quantum key distribution, just to name a few. All these applications critically depend on further understanding of defect behavior at the atomic level and the possibility to actively control distinct defects. In this work, dopants as well as intrinsic defects were introduced into the 3C-SiC material in situ during sublimation growth. A series of isochronal temperature treatments were performed in order to investigate the temperature-dependent annealing behavior of point defects. The material was analyzed by temperature-dependent photoluminescence (PL) measurements. In our study, we found a variation in the overall PL intensity which can be considered as an indication of annealing-induced changes in structure, composition or concentration of point defects. Moreover, a number of dopant-related as well as intrinsic defects were identified. Among these defects, there were strong indications for the presence of the negatively charged nitrogen vacancy complex (NC–VSi)−, which is considered a promising candidate for spin qubits.

Study of nitrogen content in HPHT diamond by nuclear reaction analysis

Nuclear reaction analysis (NRA) has been used for the determination of nitrogen content in diamond by monitoring the 14N(d,p0)15N nuclear reaction from a 1.4 MeV deuteron beam incident on the diamond samples. Results obtained in bulk diamond are compared to those inferred from infrared optical transmission measurements. Furthermore, nitrogen content has also been measured by NRA in diamond nanoparticles and in the light of these results we evaluate the conversion of nitrogen to nitrogen-vacancy (NV) complexes in both bulk diamond and diamond nanoparticles after a 2.4 MeV-proton irradiation and subsequent thermal annealing.

Optical defects in milky type IaB diamonds

The optical features of milky type IaB diamonds were studied systematically by non-destructive approaches including FTIR, photoluminescence (PL), and cathodoluminescence (CL) spectroscopy. From 97 type IaB diamonds ranging from 0.2 ct to ~100 ct submitted to GIA's New York laboratory for screening, we found that all the milky type IaB diamonds consistently displayed the hydrogen-related defect with an absorption line at 3107 cm−1, and ~96% of them were accompanied by a weaker line at 3085.4 cm−1, which is undetectable in most non-milky diamonds. Most of the diamond samples display no platelet defect or a very tiny residual platelet peak with a position at larger wavenumber in milky diamonds than in non-milky counterparts. “Amber center” with a weak but sharp absorption line at 4168.8 cm−1 has been observed in ~73% of the milky diamonds and ~24% of the non-milky ones. Photoluminescence (PL) results reveal that several defects with ZPLs at 490.7, 536, 575.9 and 612.4 nm are more common in milky type IaB diamonds than non-milky ones. A zero-phonon line (ZPL) at 536 nm has been confirmed by PL mapping and CL spectra as a product of plastic deformation, and it might be linked with the H4 center (N4V2 defect). A ZPL at 490.7 nm could be related to a nitrogen-vacancy complex. The defects present more often in milky IaB diamonds are generally associated with plastic deformation. The presence of a hydrogen-related peak at 3085.4 cm−1 and a 536 nm center would help effectively distinguish IaB diamonds with subtle milky areas from their non-milky counterparts.

Infrared defect dynamics—Nitrogen-vacancy complexes in float zone grown silicon introduced by electron irradiation

The interaction of nitrogen and intrinsic point defects, vacancy (V) and self-interstitial (I), was examined by infrared absorption spectroscopy on the electron irradiated and post-annealed nitrogen doped float zone (FZ) silicon crystal. Various absorption lines were observed, at 551 cm−1 in as-grown samples, at 726 and 778 cm−1 in as-irradiated samples (Ir group), at 689 cm−1 after post-annealing at 400 °C and above (400 °C group), at 762 and 951 cm−1 after annealing at 600 °C (600 °C group), and at 714 cm−1 up to 800 °C (800 °C group). By irradiation, a part of N2 was changed into the Ir group. VN2 is the candidate for the origin of the Ir group. By the post annealing at 400 and 600 °C, a part of N2 and the Ir group were changed into the 400 °C group, to less extent at 600 °C. V2N2 is the candidate for the origin of the 400 °C group. By annealing at 600 °C, most of the Ir group turned into 400 °C and 600 °C groups. By annealing at 800 °C, N2 recovered almost completely, and most other complexes were not observed. Recently, lifetime degradation has been observed in the nitrogen doped FZ Si annealed at between 450 and 800 °C. The N-V interaction in the same temperature range revealed here will help to understand the lifetime degradation mechanism. The behavior of the 689 cm−1 line corresponded well to the lifetime degradation.

Optical properties of CVD single crystal diamonds before and after different post-growth treatments

Single crystal diamonds grown by chemical vapor deposition (CVD) before and after different post-growth treatments were studied using optical spectroscopy. The most pronounced changes in color were observed after irradiation with subsequent annealing at 800 °C whereas a weakening of gray tint took place after the high-pressure high-temperature annealing. Low-pressure high-temperature annealing in microwave plasmas of different composition and by electric arc discharge did not produce a noticeable effect on the diamond properties. Signals from nitrogen-vacancy complexes were detected in the absorption and luminescence spectra of CVD diamonds, but the pink tint in the irradiated and annealed diamonds is due to SiV complexes, not NV. The peaks at 150 and 262 K associated with boron were observed in thermoluminescence (TL) curve of original CVD diamonds. Combined irradiation and annealing allowed us to create deep traps of charge carriers responsible for TL peaks in the 430 to 470 K range, which made CVD diamonds promising for TL dosimetry.

Quantum-optical characterization of single-photon emitters created by MeV proton irradiation of HPHT diamond nanocrystals

Deterministic single-photon sources are a fundamental tool for several emerging applications in quantum sensing and quantum metrology. In these fields, the exploitation of individual quantum systems could significantly improve the current measuring capabilities and define a new generation of standard measure units. In addition, the availability of nanoscale-sized single-photon emitters is also of considerable interest, as it enables a further integration with external structures or biological samples.In this work, we investigate the role of MeV proton irradiation in the production of single-photon emitters based on the nitrogen-vacancy complex (NV center) in nanodiamonds (NDs). Powders of nitrogen-containing type Ib diamond nanocrystals with a median size distribution of 80 nm were irradiated with 2 MeV protons at 5 × 1013 cm−2 fluence with the purpose of creating vacancies and thus promote the formation of NV centers upon a subsequent thermal annealing. Following a suitable chemical processing, the ND powders were characterized in their opto-physical properties by means of a single-photon-sensitive confocal photoluminescence microscopy setup equipped with a “Hanbury Brown & Twiss” interferometer, enabling the measurement of the second-order autocorrelation function characterizing the PL emission. A comparison with two reference batches of unirradiated NDs powder, which only underwent the chemical, and the chemical and thermal treatment, respectively, enabled to assess the role of ion implantation in the production of single-photon sources.

Nanodiamonds-induced effects on neuronal firing of mouse hippocampal microcircuits

Fluorescent nanodiamonds (FND) are carbon-based nanomaterials that can efficiently incorporate optically active photoluminescent centers such as the nitrogen-vacancy complex, thus making them promising candidates as optical biolabels and drug-delivery agents. FNDs exhibit bright fluorescence without photobleaching combined with high uptake rate and low cytotoxicity. Focusing on FNDs interference with neuronal function, here we examined their effect on cultured hippocampal neurons, monitoring the whole network development as well as the electrophysiological properties of single neurons. We observed that FNDs drastically decreased the frequency of inhibitory (from 1.81 Hz to 0.86 Hz) and excitatory (from 1.61 to 0.68 Hz) miniature postsynaptic currents, and consistently reduced action potential (AP) firing frequency (by 36%), as measured by microelectrode arrays. On the contrary, bursts synchronization was preserved, as well as the amplitude of spontaneous inhibitory and excitatory events. Current-clamp recordings revealed that the ratio of neurons responding with AP trains of high-frequency (fast-spiking) versus neurons responding with trains of low-frequency (slow-spiking) was unaltered, suggesting that FNDs exerted a comparable action on neuronal subpopulations. At the single cell level, rapid onset of the somatic AP (“kink”) was drastically reduced in FND-treated neurons, suggesting a reduced contribution of axonal and dendritic components while preserving neuronal excitability.

Optical characterization of single-crystal diamond grown by DC arc plasma jet CVD

Optical centers of single-crystal diamond grown by DC arc plasma jet chemical vapor deposition (CVD) were examined using a low-temperature photoluminescence (PL) technique. The results show that most of the nitrogen-vacancy (NV) complexes are present as NV− centers, although some H2 and H3 centers and B-aggregates are also present in the single-crystal diamond because of nitrogen aggregation resulting from high N2 incorporation and the high mobility of vacancies under growth temperatures of 950–1000°C. Furthermore, emissions of radiation-induced defects were also detected at 389, 467.5, 550, and 588.6 nm in the PL spectra. The reason for the formation of these radiation-induced defects is not clear. Although a Ni-based alloy was used during the diamond growth, Ni-related emissions were not detected in the PL spectra. In addition, the silicon-vacancy (Si-V)-related emission line at 737 nm, which has been observed in the spectra of many previously reported microwave plasma chemical vapor deposition (MPCVD) synthetic diamonds, was absent in the PL spectra of the single-crystal diamond prepared in this work. The high density of NV− centers, along with the absence of Ni-related defects and Si-V centers, makes the single-crystal diamond grown by DC arc plasma jet CVD a promising material for applications in quantum computing.