Nitrogen Vacancies Research Articles

Synthesis of ultrathin porous g-C3N4 nanofilm via template-free method for photocatalytic degradation of tetracycline

Constructing high-performance visible-light responsive semiconductor photocatalysts for removing organic contamination from sewage has gained considerable attention recently. Herein, an ultrathin porous g-C3N4 nanofilm is fabricated via a facile template-free strategy and the large specific area, abundant nitrogen vacancies and ultrathin porous structure endow it with broad visible-light-responsive range, narrow bandgap as well as sufficient separation of photoinduced carriers. In consequence, the ultrathin porous g-C3N4 nanofilm exhibits prominent photocatalytic tetracycline (TC) degradation performance and high reaction rate constant under the visible light, which is upto 8.57 folds efficiency improvement over the bulk g-C3N4. Meanwhile, it also exhibits excellent performance with the irradiation of UV–vis light and actual sunlight. This work delivers a brand-new template-free approach to prepare ultrathin porous g-C3N4 nanofilm, and provides a deeper understanding of photocatalytic TC degradation for practical applications.

The construction of a vanadium-doped polysulfide iron bimetallic active center for enhanced photocatalytic nitrogen fixation

Inspired by natural nitrogen fixation, we constructed a metal–S mimetic biocatalyst using vanadium-doped iron polysulfide as the active center combined with g-C3N5, which is rich in nitrogen vacancies.

Symmetry or asymmetry: which one is the platform of nitrogen vacancies for alkaline hydrogen evolution.

Conventional nitrogen vacancies with a symmetric coordination of metal cations (i.e., M1-Nv-M1) play a crucial role in tuning the local environment of the metal sites in metal nitrides and improving their electrochemical activity in the hydrogen evolution reaction (HER). However, the symmetric Nv sites, which feature a uniform charge distribution on adjacent metal sites, suffer from sluggish water dissociation kinetics and a poor capability for hydrogen desorption. Here, we fabricated Cr-doped and Nv-rich Co4N nanorods grown on a Ni foam (Cr-Co4N-Nv/NF) with asymmetric Cr-Nv-Co sites to effectively catalyze hydrogen evolution under alkaline conditions, with a low overpotential of 33 mV at a current density of 10 mA cm-2 and a small Tafel slope of 37 mV dec-1. The experimental characterizations and theoretical simulations collectively reveal that the construction of asymmetric Cr-Nv-Co sites gives rise to the upshift of the d-band center, thus promoting water adsorption and activation. Moreover, asymmetric Nv sites allow a balance between hydrogen adsorption and desorption, which avoids the limited desorption process over the symmetric Co-Nv-Co sites.

Strategy for reducing the carriers transfer antagonistic effect between heterojunction and plasmonic effect and weakening photocorrosion of Cu2O for excellent photocatalytic bacteriostasis.

Heterogeneous catalysis composed of plasmonic metal and semiconductor has been utilized to tune local surface electron density in MOA (Molecular oxygen activation). However, there is a severe antagonistic effect between Schottky junction carriers and SPR (Surface Plasmon Resonance) induced hot carriers transfer routers when metal and semiconductor are both excited to dramatically reduce carriers separation efficiency. Hence, a highly effective photocatalytic antifoulant obtained by V-CN (carbon nitride with nitrogen vacancies) in-situ loading Cu2O and Ag nanoparticles (Cu2O/Ag/V-CN) was introduced to promote MOA to assist the metal ions sterilization. The DFT calculations (Density Functional Theory) and FEM calculations (Finite Element Method) intuitively proved the photocatalytic antifoulant belonged to a ternary Z-scheme heterojunction and could visibly weaken the antagonistic effect of hot carriers and Schottky carriers transport routes. The delocalized electron structure caused by V-CN and the effective electron mediator of Ag were the key to the formation of Z-scheme interfacial heterojunctions. These conclusions were also supported by experimental data, like more ∙O2- production capacity, efficient carriers separation, and higher carriers lifetime (27% higher than Cu2O and Cu2O/V-CN) as well as the weakened Cu2O photocorrosion tendency (Cu2O turning into CuO). Additionally, except for increasing nearly-three times adsorption energy of O2 for rapid activation, Cu2O/Ag/V-CN with abundant nitrogen vacancies can more significantly slow metal ions release (less about 97% to pure Cu2O and at least 22% higher than reported systems), which can observably save the amount of catalyst and heavy metals content. Therefore, Cu2O/Ag/V-CN has great potential for practical antifouling applications.

ENGINEERING GEOMETRIC PHASE AND CORRELATION DYNAMICS OF NITROGEN VACANCIES IN DIAMOND INTERACTING WITH TWO NANOCAVITIES

In this paper, we study the interaction of Nitrogen Vacancies in Diamond (NVD) with quantized cavity field. The system is explored analytically and the effect of the system parameters is analyzed. The stability of a quantum system with influencing factors is investigated using the Mandal Parameter. The generated correlation between the NVD and the quantized cavity field is quantified using the negativity. This study also investigates geometric phase and its dependence on the system parameters. The results show that this system holds great potential applications in quantum computation and quantum memory. Additionally, the features of the system can be controlled by the system parameters.

Fabricating transfer channel and charge trap in carbon nitride ultrathin nanosheet for enhanced photocatalytic hydrogen evolution

Fabricating environmental and stable graphitic carbon nitride (g-C3N4) photocatalyst with highly efficient visible light response and charge separation is still a challenging work. Herein, g-C3N4 ultrathin nanosheets are synthesized via oxidation etching method. The as-synthesized samples show improved visible-light response and photogenerated charge separation, which is revealed to be brought from the synergy of hierarchical porous structure and nitrogen vacancies. On one hand, macropores can provide light transport channels for enhanced visible light absorption, meanwhile the photogenerated charge favors to be migrated on two-dimensional ultrathin planes and trapped in mesoporous structure to avoid the recombination. Also, macropore and mesopore promote the transport and adsorption of substrates. On the other hand, DFT calculations verify that nitrogen vacancies are conductive to the rearrangement of charge density distribution and the formation of defect levels in the band gap of g-C3N4, leading to the improvement of carrier mobility and separation. Thus, nitrogen vacancies promote more active sites generation. Consequently, the g-C3N4 ultrathin nanosheet presents a promising photocatalytic hydrogen evolution rate of 1.77 mmol .g−1 .h−1 in a long time run under visible light, which is far higher than that of comparison samples (0.18 and 0.29 mmol .g−1.h−1 respectively).

Construction of g-C3N4 with three coordinated nitrogen (N3C) vacancies for excellent photocatalytic activities of N2 fixation and H2O2 production

Nitrogen defect engineering has been confirmed to be an effective strategy to improve photocatalytic performance, and nitrogen vacancies at different positions show different effects on photocatalytic performance. Here, we prepare g-C3N4 with three-coordinate nitrogen (N3C) vacancies (g-C3N4-N3C) by a simple in situ copyrolysis method. The introduction of N3C vacancies helps to narrow the band gap, enhance the light-harvesting efficiency, accelerate the separation and transfer of photogenerated carriers, and provide more active centers. Interestingly, g-C3N4-N3C exhibits photocatalytic performance for the production of NH3 and H2O2. The optimized g-C3N4-N3C-0.3 exhibits the best photocatalytic N2 fixation rate (1915 μmol h−1 g−1) and photocatalytic H2O2 production rate (1098 μmol h−1 g−1), with corresponding apparent quantum efficiency (AQE) of 7.79 % and 12.43 % at λ = 370 nm, respectively, which are superior to most known photocatalysts. Meanwhile, density functional theory (DFT) calculations indicate that g-C3N4-N3C makes the activation and further reduction of *N2 and *O2 more thermodynamically favorable than pure g-C3N4. This work deepens the understanding of the role of nitrogen defect engineering in enhancing the photocatalytic performance of g-C3N4, and provides a new way for the design and preparation of efficient and stable photocatalysts.

Preclinical PET and MR Evaluation of 89Zr- and 68Ga-Labeled Nanodiamonds in Mice over Different Time Scales.

Nanodiamonds (NDs) have high potential as a drug carrier and in combination with nitrogen vacancies (NV centers) for highly sensitive MR-imaging after hyperpolarization. However, little remains known about their physiological properties in vivo. PET imaging allows further evaluation due to its quantitative properties and high sensitivity. Thus, we aimed to create a preclinical platform for PET and MR evaluation of surface-modified NDs by radiolabeling with both short- and long-lived radiotracers. Serum albumin coated NDs, functionalized with PEG groups and the chelator deferoxamine, were labeled either with zirconium-89 or gallium-68. Their biodistribution was assessed in two different mouse strains. PET scans were performed at various time points up to 7 d after i.v. injection. Anatomical correlation was provided by additional MRI in a subset of animals. PET results were validated by ex vivo quantification of the excised organs using a gamma counter. Radiolabeled NDs accumulated rapidly in the liver and spleen with a slight increase over time, while rapid washout from the blood pool was observed. Significant differences between the investigated radionuclides were only observed for the spleen (1 h). In summary, we successfully created a preclinical PET and MR imaging platform for the evaluation of the biodistribution of NDs over different time scales.

Surface engineering of phase controlled defective 1 T-MoS2 QDs@g-C3Nx material for significantly enhanced hydrogen evolution under visible-light irradiation

Design and construction of non-precious metal photocatalysts are necessary to achieve excellent hydrogen evolution reaction (HER) performance and good stability in photocatalytic water splitting systems. Herein, the phase control and defect engineering are applied to fabricate metallic 1 T-phase molybdenum disulfide quantum dots (1 T-MoS2 QDs) decorated 2D g-C3Nx nanosheets with the feature of nitrogen vacancies (denoted as 1 T-MoS2 QDs@g-C3Nx) for photocatalytic HER. The introduction of N defects facilitates to design electronic and energy band structures of g-C3N4. MoS2 QDs is beneficial to improve the charge carrier transport performances in the composite system. And the synergistic effect of phase control and defect engineering can optimize the reaction dynamics of photocatalytic HER. The resultant 1 T-MoS2 QDs@g-C3Nx (15 wt%) shows excellent properties for photocatalytic H2 evolution, with highest H2 generation rate reaching 4.08 mmol g-1h−1. Significantly, the superior H2 production performance of 1 T-MoS2 QDs@g-C3Nx (15 wt%) are achieved, outperforming the benchmark photocatalyst Pt-loading g-C3Nx (2.69 mmol g-1h−1). In addition, 1 T-MoS2 QDs@g-C3Nx (15 wt%) also exhibits high and stable cycling and durability with no significant decay of photocatalytic H2 evolution under continuous 25 h of visible light irradiation (λ > 420 nm). Meanwhile, the possible HER mechanism is explained based on the electronic and energy band structures effects of the g-C3Nx and the 1 T-MoS2 QDs cocatalyst. This work presents an efficient strategy for tuning the optical properties of nitrogen-based photocatalyst to enhance solar energy capture and conversion.

Threshold voltage instability by charge trapping effects in the gate region of p-GaN HEMTs

In this work, threshold voltage instability of normally off p-GaN high electron mobility transistors has been investigated by monitoring the gate current density during a device on-state. The origin of gate current variations under stress has been ascribed to charge trapping occurring at different interfaces in the metal/p-GaN/AlGaN/GaN system. In particular, depending on the stress bias level, electrons (VG < 6 V) or holes (VG > 6 V) are trapped, causing a positive or negative threshold voltage shift ΔVTH, respectively. By monitoring the gate current variations at different temperatures, activation energies associated with the electrons and holes trapping could be determined and correlated with the presence of nitrogen (electron traps) or gallium (hole traps) vacancies. Moreover, the electrical measurements suggested the generation of a new electron-trap upon long-time bias stress, associated with the creation of crystallographic dislocation-like defects extending across different interfaces (p-GaN/AlGaN/GaN) of the gate stack.

Post-synthetic modification of graphitic carbon nitride with PCl3 and POCl3 for enhanced photocatalytic degradation of organic compounds

Graphitic carbon nitride (g-C 3 N 4 ) was synthesised from melamine at 550 °C in the air for a period of 4 h. As such prepared g-C 3 N 4 was dispersed in PCl 3 and POCl 3 with and without pyridine at an ambient temperature for us to dope g-C 3 N 4 with phosphorus. The bulk structural properties of g-C 3 N 4 examined by X-ray diffraction (XRD) and Fourier transformed infrared (FTIR) spectroscopy were not changed. On the contrary, a surface modification in terms of pore size distribution studied using physisorption of nitrogen and electron microscopy was observed. Using PCl 3 (in the presence of pyridine), nitrogen vacancies were filled with phosphorus and phosphoramidate groups were formed (with and without pyridine). When POCl 3 was used nitrogen vacancies were removed and the surface structure was rearranged, but no phosphorus was doped in g-C 3 N 4 . The band gap energies varied from 2.69 to 2.73 eV and specific surface areas varied from 8 to 11 m 2 g −1 . The g-C 3 N 4 surface structure rearrangement was associated with altered electronic properties which led to higher photocatalytic activity observed by the degradation of Ofloxacin, Amoxicillin and Rhodamine B (RhB) under LED irradiation of 420 nm. A degradation efficiency decreased in the order: Ofloxacin > RhB > Amoxicillin. Superoxide radicals were found to be able to react with all the organic compounds, but holes could react only with Ofloxacin and RhB. All the modified materials were more active than the pristine g-C 3 N 4 and the best photocatalyst was prepared through the reaction with PCl 3 in the presence of pyridine. • g-C 3 N 4 was synthetized from melamine at 550 °C in the air for 4 h. • Then, g-C 3 N 4 was modified with PCl 3 and POCl 3 . • Using PCl 3 , the surface of g-C 3 N 4 was doped with phosphorus. • In the case of POCl 3 , g-C 3 N 4 was undoped but its surface structure was changed. • These modifications led to the enhanced photocatalytic activity of g-C 3 N 4 .

Reinforced photocatalytic H2 generation behavior of S-scheme NiO/g-C3N4 heterojunction photocatalysts with enriched nitrogen vacancies

In view of limited light response capacity and fast recombination of photon-excited charge pairs (e−/h+), photocatalytic efficiency of graphite carbon nitride (g-C3N4) is inferior, giving rise to the confined utility of the single g-C3N4 (GCN). In this paper, S-scheme NiO/g-C3N4 (NiO/CN) heterojunction photocatalysts were acquired via a facile impregnating approach and characterized through a sequence of measurements. Based on the characterization results of XPS and ESR, abundant nitrogen defects exist in the prepared NiO/g-C3N4 sample. Nitrogen vacancies (NVs) serve as the electron trapping site, effectively inhibiting the recombination of photo-induced charge pairs, dramatically boosting the photocatalytic hydrogen production efficiency. Under solar light irradiation, 2% NiO/CN composite presents the highest photocatalytic H2 generation performance (169.5 μmol h−1 g−1) without any co-catalyst, realizing 140.3 folds higher than that of GCN (1.2 μmol h−1 g−1). S-scheme NiO/g-C3N4 heterojunctions and the defect level formed by nitrogen vacancies (NVs) accelerate the photo-induced carrier separation and transfer. This approach is demonstrated to be a potential measure to remarkably ameliorate the photocatalytic H2 production behavior of g-C3N4 via utilizing the transition metal oxides to effectively separate and migrate photo-excited charge pairs under solar light irradiation.

Oxygen doping facilitated N-vacancies in g-C3N4 regulates electronic band gap structure for trimethoprim and Cr (VI) mitigation: Simulation studies and photocatalytic degradation pathways

The strategic designing of graphitic carbon nitride (CN)by elemental doping is likely to generate nitrogen vacancies which benefit by serving as electron trapping sites or by introducing additional energy levels (mid-gap states). In this work, uracil-assisted oxygen doped graphitic carbon nitride (OCNv-Ux) was synthesized by the hydrothermal method. The OCNv-U40 exhibited vigorous photocatalytic efficiency for 98% trimethoprim (TMP) antibiotic degradation and 90.8% Cr (VI) reduction under visible light irradiations. A combined inference from experimental results and simulation studies indicates band gap reduction from 2.7 to 2.34 eV for extended visible light absorption up to 620 nm. In particular, the density of states comprehends band structure regulation by the electron redistribution resulting in valence band up shifting and downshift of the conduction band, as validated by Mott-Schottky spectra. The synergistic outcomes of nitrogen vacancies and oxygen doping sites restrain the recombination of electron-hole pairs by mid-gap state formation and also promote high oxygen molecule adsorption on the nitrogen vacancies. The pioneering advantage of highly adsorbed oxygen molecule leads to the generation of plentiful superoxide anion radicals along with indirect production of hydroxyl radicals via in-situ produced H2O2, evident by radical scavenging and electron paramagnetic resonance experiments. The effect of chloride and nitrate anions showed inhibitory effect on the TMP degradation efficiency. The identification of formed intermediates and TMP degradation mechanism involving oxidation, hydroxylation, and cleavage was proposed, as investigated by TMP DFT studies. This research accomplishes significant enhancement in pristine CN photocatalytic activity by the virtue of increased specific surface area, fully exposed N-vacancies active sites, optimized electronic, structure and visible light active band gap.

Classical molecular dynamics study of structural properties of pristine and defective mono-layer 2D h-BNNSs

The structural, stability and vacancy properties of both defect-free and defected 2D h-BNNSs were studied using the classical molecular dynamics (MD) approach. The calculations were performed in the canonical (NVT Evans) and isothermal-isobaric (NPT Hoover) ensembles using the Tersoff potentials with the Verlet leapfrog algorithm to obtain reliable structural properties and energies for defect-free, boron (B) and nitrogen (N) vacancies. B and N defect energies were calculated relative to the bulk defect-free total energies, and the results suggest that N vacancy is the most stable vacancy as compared to the B vacancy. The radial distribution functions (RDF) and structure factors were used to predict the most probable structural form. Mean square displacement (MSD) suggests that the mobility of B and N atoms in the system increases with an increase in the surface area of the nanosheets. Results obtained were compared with the bulk defect-free h-BNNSs.

Boosting Photocatalytic N2 Fixation on N‐Defect g‐C3N4/WO3: the Synergistic Effects of Nitrogen Vacancy and Z‐Scheme Heterojunction

AbstractPhotocatalytic nitrogen fixation is a promising strategy for ammonia synthesis under mild conditions by using solar energy, but N2 activation remains a great challenge. Herein, we demonstrate that a Z‐scheme g‐C3N4/WO3 heterojunction possessing abundant nitrogen vacancies exhibits the highly enhanced activity for photocatalytic N2 fixation through efficient nitrogen molecular activation compared with pristine NVs‐g‐C3N4 and WO3 photocatalysts. The construction of the internal electric field induced by Z‐scheme NVs‐g‐C3N4/WO3 heterojunction allows a rapid charge carrier separation and simultaneously maintains the powerful redox ability of photogenerated charge carriers, defined by the covalent CO bond. Moreover, nitrogen vacancy plays a crucial role in the adsorption/activation of N2, which substantially facilitates the hydrogenation to generate NH3. According to experimental and theoretical investigations, photocatalytic N2 fixation on NVs‐g‐C3N4/WO3 composites is proposed to be energetically favorable in the alternating pathway. This study offers an alternative way for the design of efficient photocatalysts for photocatalytic N2 fixation.

Synergistic Effects of B-F/B-S and Nitrogen Vacancy Co-Doping on g-C3N4 and Photocatalytic CO2 Reduction Mechanisms: A DFT Study.

Nonmetallic co-doping and surface hole construction are simple and efficient strategies for improving the photocatalytic activity and regulating the electronic structure of g-C3N4. Here, the g-C3N4 catalysts with B-F or B-S co-doping combined with nitrogen vacancies (Nv) are designed. Compared to the pristine g-C3N4, the direction of the excited electron orbit for the B-F-co-doped system is more matching (N2pz→C2pz), facilitating the separation of electrons and holes. Simultaneously, the introduced nitrogen vacancy can further reduce the bandgap by generating impurity states, thus improving the utilization rate of visible light. The doped S atoms can also narrow the bandgap of the B-S-Nv-co-doped g-C3N4, which originates from the p-orbital hybridization between C, N, and S atoms, and the impurity states are generated by the introduction of N vacancies. The doping of B-F-Nv and B-S-Nv exhibits a better CO2 reduction activity with a reduced barrier for the rate-determining step of around 0.2 eV compared to g-C3N4. By changing F to S, the origin of the rate-determining step varies from *CO2→*COOH to *HCHO→*OCH3, which eventually leads to different products of CH3OH and CH4, respectively.

The Synthesis and Characterisation of the High-Hardness Magnetic Material Mn2N0.86.

High-quality P6322 Mn2N0.86 samples were synthesised using a high-pressure metathesis reaction, and the properties of the material were investigated. The measurements revealed that the Vickers hardness was 7.47 GPa, which is less than that predicted by commonly used theoretical models. At low air pressure, Mn2N0.86 and MnO coexist at 500 to 600 °C, and by excluding air, we succeeded in producing Mn4N by heating Mn2N0.86 in nitrogen atmosphere; we carefully studied this process with thermogravimetry and differential scanning calorimetry (TG-DSC). This gives a hint that to control temperature, air pressure and gas concentration might be an effective way to prepare fine Mn-N-O catalysis. Magnetic measurements indicated that ferromagnetism and antiferromagnetism coexist within Mn2N0.86 at room temperature and that these magnetic properties are induced by nitrogen vacancies. Ab intio simulation was used to probe the nature of the magnetism in greater detail. The research contributes to the available data and the understanding of Mn2N0.86 and suggests ways to control the formation of materials based on Mn2N0.86.

Realizing a strong visible-light absorption band in piezoelectric 2D carbon nitride sheets for enhanced piezocatalysis

Piezoelectric photocatalytic water splitting is a viable approach to generate renewable energy and carbon nitride sheets (CNs) has emerged as a promising material. However, the narrow solar absorption range of CNs due to its wide-band-gap hinders its further development and practical applications. In this study, CNs with a strong visible-light absorption band and a sharp absorption edge beyond 574 nm are successfully prepared by developing a fluorination and defluorination strategy. Experimental results show that introducing cyano groups into the framework of CNs as well as the accompanied nitrogen vacancies at the edges maintains high piezoelectricity and that the separation of photo-generated electron-hole pairs by the piezo-polarization charges is enhanced in the modulated CNs. Under simultaneous visible-light exposure and the ultrasonic irradiation, the modulated CNs loaded with Pt cocatalyst as a photocatalyst show a superior photocatalytic hydrogen evolution rate of 13.53 mmol g−1 h−1, which is one of the highest record reported so far for carbon nitride-based materials. Theoretical calculations reveal that the introduction of cyano into CNs leads to a suitable band gap for visible light, which dramatically improves the photocatalytic ability. This study advances the investigation of piezoelectric photocatalytic water splitting, and is also an enabling technology for piezoelectric semiconductors for their practical applications.

Probing the activity origin of the enhanced methanol electrooxidation on Ni-induced PdNix(OH)y-TaN/C catalyst with nitrogen vacancies

Engineering high-performance inexpensive TaN-modified Pd-based nanostructures for alkaline methanol oxidation reaction(MOR) has attracted extensive attention, yet the related enhancement mechanism of TaN for MOR remains lacked. The core-shell like Pd3Ni1-TaN/C catalyst decorated with Ni(OH)2/NiOOH that contains highly dispersed Ni sites and nitrogen defects has been fabricated for the first time. The unsaturated coordination environment and core-shell like morphology of Pd3Ni1-TaN/C bring fruitful active sites and a down-shifted d-band center, making it a highly-efficient electrocatalyst for alkaline MOR (17.3-fold that of commercial Pd/C). Besides the electronic effects and surface defect sites of Pd3Ni1-TaN/C, TaN also helps MOR proceed via HCOOad pathway (non-CO) accompanied by enhanced OH adsorption induced by the addition of Ni. DFT(Density Functional Theory) calculations also further confirm the energy barrier of the formation of HCOOad intermediate and desorption energy of CO on Pd3Ni1-TaN/C are reduced. The introduction of Ni species and nitrogen vacancies enhances the CO tolerance evidently.

Indium Tin Oxide (ITO) based Ohmic Contacts on Bulk n-GaN Substrate

A transparent indium tin oxide (ITO) contact to bulk n-GaN and n-GaN thin film on c-face sapphire with a specific contact resistivity of 8.06 × 10−4 Ω.cm2 and 3.71 × 10−4 Ω.cm2 was measured, respectively. Our studies relied on an RF sputtering system for ITO deposition. We have investigated the formation of the ITO-based contacts on untreated and plasma treated samples. A nonlinear I–V curve was observed for ITO deposited on untreated samples. On the other hand, an I–V curve with linear behavior was observed for plasma-treated samples, indicating the formation of ohmic contacts. From the C-V measurements, it was observed that there was also an increase in the carrier concentration in plasma treated samples compared to untreated samples. This can be attributed to the removal of surface oxide layer present on the GaN surface, and increase in nitrogen vacancies after SiCl4 plasma treatment. In addition, the increase in nitrogen vacancies at the GaN surface can also enhance localized surface/sub-surface carriers, thereby reducing the contact resistance further.