Carbon Vacancies Research Articles

Low‐temperature reactive sintering of carbon vacant high‐entropy carbide ceramics with in‐situ formed silicon carbide

AbstractIt is thought that the sintering of high‐entropy (HE) ceramics is generally more difficult when compared to that of the corresponding single‐component ceramics. In this paper, we report a novel approach to densify the HE carbide ceramics at relatively low temperatures with a small amount of silicon. Reactive spark plasma sintering (SPS) was used to densify the ceramics using powders of HE carbide and silicon as starting materials. Dense ceramics can be obtained at 1600 ‐1700°C. X‐ray diffraction analysis reveals that only non‐stoichiometric HE carbide phase with carbon vacancy and SiC phase exist in the obtained ceramics. The in‐situ formed SiC phase inherits the morphology of the starting silicon powder owing to the slower diffusion of silicon atoms compared to that of the carbon atoms in HE carbide phase. The mechanical properties of the prepared ceramics were preliminarily studied.

Assessing the effect of hydrogen on the electronic properties of 4H-SiC

As a common impurity in 4H silicon carbide (4H-SiC), hydrogen (H) may play a role in tuning the electronic properties of 4H-SiC. In this work, we systemically explore the effect of H on the electronic properties of both n-type and p-type 4H-SiC. The passivation of H on intrinsic defects such as carbon vacancies (VC) and silicon vacancies (VSi) in 4H-SiC is also evaluated. We find that interstitial H at the bonding center of the Si–C bond ( H i bc ) and interstitial H at the tetrahedral center of Si ( H i Si − te ) dominate the defect configurations of H in p-type and n-type 4H-SiC, respectively. In n-type 4H-SiC, the compensation of H i Si − te is found to pin the Fermi energy and hinder the increase of the electron concentration for highly N-doped 4H-SiC. The compensation of H i bc is negligible compared to that of VC on the p-type doping of Al-doped 4H-SiC. We further examine whether H can passivate VC and improve the carrier lifetime in 4H-SiC. It turns out that nonequilibrium passivation of VC by H is effective to eliminate the defect states of VC, which enhances the carrier lifetime of moderately doped 4H-SiC. Regarding the quantum-qubit applications of 4H-SiC, we find that H can readily passivate VSi during the creation of VSi centers. Thermal annealing is needed to decompose the resulting VSi–nH (n = 1–4) complexes and promote the uniformity of the photoluminescence of VSi arrays in 4H-SiC. The current work may inspire the impurity engineering of H in 4H-SiC.

Investigation of deep electronic levels in n‐type and p‐type 3C‐SiC using photoluminescence

Among the various SiC polytypes, cubic 3C‐SiC is much more difficult to grow in high crystalline quality than the commercially introduced hexagonal 6H‐SiC and 4H‐SiC counterparts. Besides some benefits of 3C‐SiC for transistor applications related to a greater electron mobility and a lower metal‐oxide‐semiconductor interface trap density compared to 4H‐SiC, new potential optoelectronic applications have been introduced very recently. Boron doped 3C‐SiC may act as an ideal candidate for an intermediate band (IB) solar cell material. Aluminum doped p‐type 3C‐SiC could lead to the development of efficient optoelectrochemical water splitting cells. Finally, 3C‐SiC with its various intrinsic point defects has been considered as a suitable candidate for future spintronic‐applications. All these applications will critically depend on further understanding defect behaviour on atomic level. In our study we investigated free standing n‐type and p‐type 3C‐SiC material grown in our lab. Temperature dependent photoluminescence measurements revealed the presence of carbon vacancy related VC and VC-CSi defect transitions in the p‐type materials but not in the n‐type materials. This observation present in as grown 3C-SiC is believed to have significant impact on the optoelectronic applications. Copyright © 2017 VBRI Press.

Synthesis of nonstoichiometric high entropy Nb-added carbides

A novel class of nonstoichiometric high-entropy carbide (HECx) materials, namely, Nb/TiC/TaC, Nb/TiC/TaC/VC and Nb/TiC/TaC/VC/WC, were produced by mechanically milled and spark plasma sintering (SPS) from Nb and carbides. XRD, SEM-EDS and S/TEM-EDS were used to characterize the phase constitution, microstructure and compositional distribution of samples, respectively. HECx exhibits a single-phase rock-salt crystal structure with a relatively uniform elemental distribution. Among the three different HECx materials, Nb/TiC/TaC/VC/WC with an average grain size of 2.15 μm sintered at 1600 °C shows an enhanced fracture toughness of 5.1 ± 0.1 MPa m1/2 compared with transition metal carbides. The mechanically mixed and low sintering temperature lead to the formation of finer grains. The higher fracture toughness can be attributed to atomic relaxation resulting from carbon vacancies and solid solution strengthening.

M-Center in Neutron-Irradiated 4H-SiC

We report on the metastable defects introduced in the n-type 4H-SiC material by epithermal and fast neutron irradiation. The epithermal and fast neutron irradiation defects in 4H-SiC are much less explored compared to electron or proton irradiation-induced defects. In addition to the carbon vacancy (Vc), silicon vacancy (Vsi) and carbon antisite-carbon vacancy (CAV) complex, the neutron irradiation has introduced four deep-level defects, all arising from the metastable defect, the M-center. The metastable deep-level defects were investigated by deep level transient spectroscopy (DLTS), high-resolution Laplace DLTS (L-DLTS) and isothermal DLTS. The existence of the fourth deep-level defect, M4, recently observed in ion-implanted 4H-SiC, has been additionally confirmed in neutron-irradiated samples. The isothermal DLTS technique has been proven as a useful tool for studying the metastable defects.

Directly Confirming the Z1/2 Center as the Electron Trap in SiC Through Accessing the Nonradiative Recombination

SiC is an important wide‐bandgap semiconductor for high‐power electronics and high‐temperature applications. Its Z 1/2 center, recognized as a carrier lifetime killer, has been extensively studied as it is a key issue impeding many applications of SiC. It is well established that the Z 1/2 center originates from carbon vacancies (VC), but direct access to the microscopic mechanism underlying its nonradiative recombination process is lacking. Herein, to consolidate such identification, the multiphonon‐assisted nonradiative recombination rates of previously proposed different candidates of the Z 1/2 center are evaluated by performing first‐principles calculations. The calculated electron transition cross sections of the VC ( at the k‐site and at the h‐site) are in accordance with the experimental values [from to ] for the Z 1/2 center. However, all of the other candidates are orders of magnitudes smaller in transition cross sections. This provides further evidence that the carbon vacancy (VC) is the Z 1/2 center from the electron transition cross sections. However, it is also found that the hole transition cross sections of the VC are very small. Various hypotheses are provided to consolidate this fact with the conclusion that Z 1/2 is the carrier lifetime killer.

Surface modification with oxygen doping of g-C3N4 nanoparticles by carbon vacancy for efficient dehydrogenation of sodium borohydride in methanol

g-C3N4, which is used in different catalytic activities, was applied to the H2 field from NaBH4 methanolysis for the first time in this study. The development of g-C3N4 is necessary to make it a viable alternative to the heterogeneous catalysts currently used in this field. The g-C3N4 used in this study contains both carbon vacancies and oxygen doping atoms. Compared to the g-C3N4 catalyst, the O doped g-C3N4 catalyst obtained showed an approximately 3.4-fold improvement in H2 production. The HGR value obtained with O doped g-C3N4 (5 mg) was 10800 ml min−1 g−1. The structural properties of the obtained catalysts were characterized by XPS, SEM-EDX, TEM, FTIR, Raman, BET and XRD analyses. These analyses showed significant changes for O doped g-C3N4. The activation energy obtained for O doped g-C3N4 was approximately 36.13 kJ mol−1.

The self-powered artificial synapse mechanotactile sensing system by integrating triboelectric plasma and gas-ionic-gated graphene transistor

The emulation of biological nerves to develop artificial synapse tactile sensing system has great application potentials in the fields of Internet of Things and artificial intelligence. Novel sensing strategies to achieve low power consumption, low cost, low complexity and high efficiency still face challenges. Here, a self-powered tactile sensing system has been developed by integrating a triboelectric plasma and a gas-ions-gated (GIG) graphene transistor, in which the GIG transistor is served as the artificial synapse, and the triboelectric plasma is served as both a tactile sensor and the driving signals of the GIG transistor. The N2+ ions in the triboelectric plasma are directly adsorbed on the graphene surface, acting as a floating gate of the GIG transistor to regulate its electrical transport characteristics. The adsorption density of N2+ ions reach up to 3.96 × 1012 cm−2 with a measured desorption energy of 196 meV. The theoretical simulation shows that the N2+ ion is adsorbed at the site of carbon vacancy on the graphene surface. By regulating the number, frequency and polarization of the discharge pulse, various synaptic behaviors are achieved, such as short-term depression, long-term depression, long-term potentiation, paired-pulse facilitation, etc. Also, the neural functions of learning and temporal decoding have been demonstrated in experiments. By combining triboelectric plasma and GIG transistor, a facile experimental scheme for a self-powered, integrated, and simple structured intelligent tactile sensing system has been proposed, which is highly expected to promote the development of intelligent sensing fields in the future.

Carbon defective carbon nitride with large specific surface area by hot oxygen etching for promoting photocatalytic performance

Defect engineering is an extremely powerful strategy for carbon nitride materials. In our paper, carbon defective-carbon nitride (V-g-C3N4) with a large specific surface area (555.3 m2 g−1) was prepared by multiple thermal and oxygen etching using urea as a precursor. The photocatalytic performance of the materials was evaluated by synthetic ammonia under visible light and full spectrum. Under visible light, V-g-C3N4 can produce 3.60 mmol/h/g of NH3, which is approximately ~ 3.24 times higher than that of g-C3N4. Under the sunlight, V-g-C3N4 can produce 21.35 mmol/h/g of NH3. Particularly, positron annihilation lifetime spectroscopy (PALS) was used to confirm the type of vacancies. The experimental results show that the method of multiple calcinations not only can successfully introduce the carbon vacancy into the polymerization network structure of carbon nitride, resulting in a large number of active sites are added for the reaction of ammonia synthesis, but also can expand the bandgap from 2.82 eV to 3.03 eV. In addition, After g-C3N4 was exposed to high temperature and weak oxygen three times, its structure became loose and porous, which also provides more active sites for the reaction. Herein, these findings provide a novel concept of introducing carbon vacancies to promote the efficiency of photocatalytic ammonia synthesis.

Defect Configurations and Ionization Energies of Carbon Vacancies, Hydrogen, Boron and Their Complexes in Diamond

Defect Configurations and Ionization Energies of Carbon Vacancies, Hydrogen, Boron and Their Complexes in Diamond

Atomic study of hydrogen behavior in different vanadium carbides

In this study, the behavior of hydrogen (H) is investigated in different bulk vanadium carbides (VC, V 8 C 7 , V 6 C 5 , V 4 C 3 and V 2 C), which are potential candidates for use as a hydrogen permeation barrier during radiation exposure. Both carbon (C) vacancy and structural symmetry can influence the stability of VxCy phases. The H diffusion barrier properties are distinctly improved due to a large endothermic detrapping energy from C vacancies. However, too many C vacancies (during the phase transition from V 8 C 7 to V 4 C 3 ) may result in decreased energy barriers for H detrapping from C vacancies. Energetic, electronic and quantum effects on the H behavior are thoroughly discussed. In general, the large negative H formation energy, low solution numbers in the C vacancies and the high vacancy-induced diffusion barriers all indicate that vacancy-ordered V 4 C 3 can be a very promising phase as a hydrogen permeation barrier.

Enhanced visible-light H2 evolution performance of nitrogen vacancy carbon nitride by improving crystallinity

A new kind of improved crystallinity nitrogen vacancy carbon nitride (CNCN) photocatalyst was successful prepared. This material has better light separation, meanwhile, improved crystallinity urges CNCN exhibiting narrower inter-layer distance and might promote the charge transfer among various layers, conducting to the photoinduced charges separation. When it was used to photocatalysis splitting water to hydrogen evolution, the hydrogen produced rate of CNCN was 13.6 folds that of bulk g-C 3 N 4 and 3.5 folds that of nitrogen vacancy carbon nitride. Obviously, it provides an effective and feasible strategy to gain an effective photocatalyst. • Improved crystallinity nitrogen vacancy carbon nitride (CNCN) photocatalyst was successful prepared. • Resultant CNCN possesses the improved separation rate of photoinduced charge. • H 2 evolution rate over resultant CNCN is obviously increased.

Complete removal of Tetracycline by sonophotocatalysis using ultrasound-assisted hierarchical graphitic carbon nitride nanorods with carbon vacancies

Tuning a graphitic carbon nitride (CN) structure is an effective strategy to advance its physicochemical and electronic properties. Herein, hierarchical CN nanorods with carbon vacancy were synthesized via ultrasound-assisted thermal polycondensation method wherein melamine-HONH2·HCl complex acts as a template. The hierarchical CN nanorods can facilitate multiple light scattering, provide large specific surface area with extensive reactive sites and endow abundant mass-transport channels for charge migration. The existence of carbon vacancies can serve as shallow charge trapping sites and prompt charge separation. Consequently, hierarchical CN nanorod possessed excellent sonophotodegradation efficiency of ∼100% towards Tetracycline (TC) antibiotic within 60 min under ultrasonic irradiation and visible light illumination. Moreover, the sonophotocatalytic degradation was higher than the sum of sonocatalytic and photocatalytic TC degradation using hierarchical CN nanorods due to its synergistic performance. A plausible sonophotocatalytic mechanism and TC degradation pathway using hierarchical CN nanorod were proposed. Lastly, hierarchical CN nanorod is durable and stable which can withstand the sonophotocatalytic condition even after the fifth run. This work offers an insight into hierarchical CN nanorod to advance sonophotocatalytic degradation performance for highly efficient removal of various recalcitrant pollutants.

Sequential ion-electron irradiation of zirconium carbide ceramics: Microstructural analysis

Zirconium Carbide (ZrCx) was irradiated with 10 MeV Au3+ ions to a dose of 10 displacements per atoms (dpa) and subsequently with 100 and 300 keV electrons in a transmission electron microscope (TEM). After ion irradiation, dislocation loops were observed in the microstructure and an increase in the number of carbon vacancies was revealed by Raman spectroscopy. Grazing incidence X-ray diffraction (GIXRD) analysis showed that neither amorphization nor oxidation occurred during ion irradiation of the specimen. Subsequent electron irradiation of the pre-implanted ZrCx foil led to formation of nanosized tetragonal ZrO2 precipitates (5−10 nm diameter) on the surface of the TEM lamella. The formation of the new oxide phase was not related to the electron beam-induced heating of the specimen, but to electron stimulated oxidation caused by the residual oxygen inside the transmission electron microscope. Changes in size and density of ZrO2 crystallites were observed between the pristine and ion irradiated ZrCx regions following electron irradiation, suggesting that the initial microstructure of the ZrCx substrate played a key role in the nucleation and growth of the oxide islands. The obtained results provide insights into the microstructural response of ZrCx to different types of radiation and the inadvertent effects of the electron beam during TEM analysis of in-situ and ex-situ ion irradiated ZrCx. Additionally, the findings of this work suggest a method to prepare local ZrO2 nanoprecipitates within ZrCx grains by selective electron beam irradiation.

Vacancy-modified g-C3N4 nanosheets via one-step thermal polymerization of thiosemicarbazide precursor for visible-light-driven photocatalytic activity

The graphitic carbon nitride (g-C3N4) with nanosheets structure or carbon vacancies is considered as the promising photocatalysts duo to their fascinating properties. Herein the carbon vacancy modified g-C3N4 nanosheet (TCN-600) was synthesized via one-step thermal polymerization of thiosemicarbazide precursor without auxiliary agent and extra gas protection. The higher calcination temperature and self-generating gas atmosphere (N2H4, CS2 and H2S) during the precursor polymerization impel introduction of carbon vacancies and simultaneous formation of nanosheet structures. The as-prepared TCN-600 exhibits excellent photocatalytic activity for tetracycline (TC) degradation and hydrogen production. TCN-600 shows average hydrogen production rate of 1.86 mmol g−1 h−1, which is 11.6 times higher than that of bulk g-C3N4. Meanwhile, TCN-600 displays high TC degradation efficiency of 83.3% within 90 min. The photodegradation rate of tetracycline by TCN-600 is 5.6 times higher than that by bulk g-C3N4. The significant improvement in photocatalytic activity is mainly attributed to its huge specific surface areas (144.5 m2/g), extended visible-light absorption, negatively shifted conduction band position, longer charge carries lifetime and faster migration of the photo-induced electron-holes. This study provides a new strategy to synthesize g-C3N4 with carbon vacancies and simultaneous nanosheet structure.

Calculating the hyperfine tensors for group-IV impurity-vacancy centers in diamond using hybrid density functional theory

The hyperfine interaction is an important probe for understanding the structure and symmetry of defects in a semiconductor. Density-functional theory has shown that it can provide useful first-principles predictions for both the hyperfine tensor and the hyperfine constants that arise from it. Recently there has been great interest in using group-IV impurity-vacancy color centers X$V^-$ (where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy) for important applications in quantum computing and quantum information science. In this paper, we have calculated the hyperfine tensors for these X$V^-$ color centers using the HSE06 screened Hartree-Fock hybrid exchange-correlation functional with the inclusion of core electron spin polarization. We have compared our results to calculations which only use the PBE exchange-correlation functional without the inclusion of core electron spin polarization and we have found our results are in very good agreement with available experimental results. Finally, we have theoretically shown that these X$V^-$ color centers exhibit a Jahn-Teller distortion which explains the observed anisotropic distribution of the hyperfine constants among the neighboring $^{13}$C nuclear spins.

Carbon vacancy control in p+-n silicon carbide diodes for high voltage bipolar applications

Controlling the carbon vacancy (VC) in silicon carbide (SiC) is one of the major remaining bottleneck in manufacturing of high voltage SiC bipolar devices, because VC provokes recombination levels in the bandgap, offensively reducing the charge carrier lifetime. In literature, prominent VC evolutions have been measured by capacitance spectroscopy employing Schottky diodes, however the trade-offs occurring in the p+-n diodes received much less attention. In the present work, applying similar methodology, we showed that VC is re-generated to its unacceptably high equilibrium level at ∼2 VC cm−3 by 1800 °C anneals required for the implanted acceptor activation in the p+-n components. Nevertheless, we have also demonstrated that the VC eliminating by thermodynamic equilibrium anneals at 1500 °C employing carbon-cap can be readily integrated into the p+-n components fabrication resulting in VC cm−3, potentially paving the way towards the realization of the high voltage SiC bipolar devices.

Effective Photocatalytic Hydrogen Evolution Using Covalent Triazine Framework-Derived Carbon Nitride Nanofiber Containing Carbon Vacancies for Visible-Light-Driven

In this study, a novel fibrous carbon nitride (FCN) was prepared from laminated covalent triazine framework (CTF) via pyrolysis, using functionalized 2,5-thiophenedicarboxylic acid and melamine as the precursors. A carbon vacancy was produced by two-step calcination in argon and air atmospheres. These carbon vacancies further exposed the edges and diffusion channels of the FCN nanofibers, which accelerated photogenerated charge transfer and provided more active sites. The FCN was characterized using various techniques and used for H2 evolution under visible-light irradiation. The as-synthesized FCN exhibited excellent stability, and its photocatalytic activity for H2 evolution under visible-light irradiation was 66 times higher than that of bare C3N4 (BCN), attaining a maximum H2 evolution rate of 102.63 μmol in 6 h. The FCN remained stable following visible-light irradiation at the end of 10 cycles. The FCN benefited from the absorption of solar energy and a large number of active sites. These advantages facilitated the efficient separation of photoexcited electron-hole pairs to hinder charge recombination. This work generates new insights into the preparation of highly effective FCN photocatalysts that may be put to various applications, especially in the fields of energy and environment.

Atomic-scale observation of hydrogen trap sites in bainite–austenite dual-phase steel by APT

It was reported by the thermal desorption analysis (TDA) of hydrogen that bainite–austenite dual-phase steels showed two activation energies of ~30 kJ/mol and ~ 47 kJ/mol under low- and high-fugacity conditions, respectively, and the latter energy corresponded to hydrogen diffusion in austenite; however, the hydrogen trap sites of the former energy were unclear (Sekine et al., 2019, 2018 [ 1 , 2 ]). The hydrogen trap sites under the low-fugacity condition in the steel were investigated by directly observing deuterium-charged specimens using atom probe tomography (APT). The charged deuterium atoms were distributed in carbon-rich regions of the fine irregular structure in the bainite region of the steel. The carbon concentrations in these region were in the range of 12–20 at.% without enrichment of other alloying elements. The diffraction analysis with transmission electron microscopy (TEM) indicated the presence of a substantial amount of fine precipitates of epsilon carbide in the bainite region of the steel. Hence, the hydrogen trap sites were considered to be associated with fine precipitates of epsilon carbide (ε-carbide) with carbon compositions marginally lower than the stoichiometric composition. First principles calculations suggested that the hydrogen trap sites in the steel originated from the carbon vacancies in the surface region of ε-carbide. • Hydrogen trap sites in bainite–austenite dual-phase steel were investigated by APT. • Charged deuterium atoms were observed in carbon-enriched regions in the bainite. • TEM analysis indicated the presence of a large amount of fine ε-carbide precipitates. • The trap energies in ε-carbide were investigated by first principles calculation. • Carbon vacancies in the surface region of ε-carbide are probably the trap sites.

Vacancy-cluster and off-lattice metal-atom diffusion mechanisms in transition metal carbides

Using ab initio simulations, we report potential metal atom diffusion mechanisms in the group IVB and VB transition metal carbides. By computing the metal vacancy formation energies of vacancy clusters, we find that a metal vacancy surrounded by six carbon vacancies is the lowest metal vacancy formation energy structure for the group IVB carbides while the lowest energy for the group VB carbides is a metal vacancy surrounded by two carbon vacancies. The vacancy cluster mechanisms reveal activation energies that are consistent with experiments in both TiC (group IVB) and TaC (group VB). We also report that an off-lattice diffusion mechanism, that is only energetically favorable in the group IVB transition metal carbides, has a lower formation energy than the regular vacancy cluster mechanism. This new mechanism shows lower formation energies for a given carbon vacancy concentration which indicates this off-lattice mechanism might be the most dominant metal-atom diffusion mechanism among the existing mechanisms proposed for the group IVB metal carbides.