High Defect Concentration Research Articles

Realizing efficient photocatalytic water splitting over Mg-modified BaNbO2N under visible light illumination

With a light absorption up to 740 nm, BaNbO2N is promising for solar water splitting but normally owns a low activity due to a high defect concentration. Here, Mg is adopted as a dopant to modify the BaNbO2N. The presence of Mg not only reduces the defect concentration and enhances surface hydrophilicity but also tunes the bandgap as well as the band edge positions. These modifications greatly promote the charge separation and transfer of BaNbO2N, boosting the photocatalytic activities for O2-evolution reactions. An apparent quantum efficiency of 1.65 % at 420 ± 20 nm has been attained for Mg modified BaNbO2N. Overall water splitting at a stable gas-evolution rate (∼ 2.0 μmol·h−1 for H2 and ∼ 1.0 μmol·h−1 for O2) has been realized by integrating Mg-modified BaNbO2N into a Z-scheme system. These findings justify Mg as an effective dopant to modulate the photocatalytic behavior of Nb-based perovskite oxynitrides.

Influence of the Surface State on the Interfacial Bonding Strength of the Cold-Rolled Brass/Carbon Steel Composite.

To improve the interfacial bonding of dissimilar composites, the interaction mechanism between the surface state and severe plastic deformation to strengthen the interfacial bonding strength was revealed. In this study, the different surface states of the steel strip were designed by louver blade grinding (LBG) and diamond bowl grinding (DBG), and the cold-rolled composite method was developed to prepare the brass/carbon steel composite strips. The results show that the steel surface after DBG has a large roughness of 9.79 μm, a hard hardening layer of 6.2 GPa, and high cleanliness of 1.34 atomic % oxygen content, while that after LBG has a roughness of 1.31 μm, a hardening layer of 4.2 GPa, and an oxygen content of 2.37 atomic %. The large roughness promotes the breaking of the hardening layer; the hardening layer is beneficial to obtain sufficient interfacial stress to expose the fresh metal; and the high cleanliness reduces the barrier to the fresh metal and contributes to the bonding of the fresh metal. The interface of the cold-rolled brass/carbon steel composite strip after LBG and DBG is mechanical bonding and metallurgical bonding, respectively. In the process of the cold-rolling composite, large shear deformation occurs at the interface of brass and steel, resulting in a high concentration of vacancy and dislocation defects, which provides a channel for interdiffusion of atoms at the interface. Under the diffusion driving force provided by the cold-rolling shear deformation heat, a nanodiffusion layer with a thickness of 60 nm and high interfacial bond strength was formed.

Improved thermolytic dehydrogenation of LiBH4 nanoconfined in few-layer graphene with different functionalities

In this work, lithium borohydride (LiBH4) was loaded into plasma-activated nanoporous few-layer graphene (FLG) powders with different specific surface areas (∼400–800 m2/g) and functional groups (carboxyl and amine) to investigate the effect of LiBH4@FLG nanoconfinement on the dehydrogenation properties. It was observed that the dehydrogenation temperature dropped significantly from 463 °C for pure LiBH4 to ∼120 °C for all LiBH4@FLG nanocomposites. This was attributed to the nano-sized pores of the FLG materials that can constrain LiBH4 by nanoconfinement and thus decrease the dehydrogenation temperature. The highest dehydrogenation yield of 83% occurred in LiBH4@FLG with 400 m2/g surface area and amine groups, possibly due to Lewis basic amino groups and better graphitic structure. Moreover, it was found that both the surface area and the graphitic defects on the FLG host materials have an influence on the dehydrogenation kinetics. LiBH4@FLG with 800 m2/g surface area and carboxyl groups possesses the lowest activation energy due to its high surface area and high concentration of graphitic defects.

Triggering Thermal Healing of Large Area MOCVD Grown TMDs at the Trion Dissociation

AbstractOvercoming the thermal sensitivity of the van‐der Waals layered material, an inevitable high defect concentration in a large area (2 × 2 cm2) vapor phase grown few‐layer MoS2 (2.3 nm thick) is uniformly repaired over the entire area through thermal process. The defect concentration of the healed sample decreased by 3.8 times, and both the PL intensity and field‐effect mobility over the entire area increased by more than two times. Through observing in situ photoluminescence (PL) spectra with raising temperature, it is confirmed that the oxygen‐substitutional healing proceeds radically from the starting point at which trion dissociation energy is zero, inducing e–h plasma. At this time, for achieving successful healing of the entire area, immediate cooling is required to prevent accelerated oxidation when reaching the critical temperature. Considering the laboratory‐dependent crystalline quality of vapor‐phase grown transition metal dichalcogenides (TMDs), the methodology to find the optimal thermal healing temperature through in situ PL monitoring will be quite useful.

Two-Dimensional Pb Square Nets from Bulk (RO)nPb (R = Rare Earth Metals, n = 1,2).

All two-dimensional (2D) materials of group IV elements from Si to Pb are stabilized by carrier doping and interface bonding from substrates except graphene which can be free-standing. The involvement of strong hybrid of bonds, adsorption of exotic atomic species, and the high concentration of crystalline defects are often unavoidable, complicating the measurement of the intrinsic properties. In this work, we report the discovery of seven kinds of hitherto unreported bulk compounds (RO)nPb (R = rare earth metals, n = 1,2), which consist of quasi-2D Pb square nets that are spatially and electronically detached from the [RO]δ+ blocking layers. The band structures of these compounds near Fermi levels are relatively clean and dominantly contributed by Pb, resembling the electron-doped free-standing Pb monolayer. The R2O2Pb compounds are metallic at ambient pressure and become superconductors under high pressures with much enhanced critical fields. In particular, Gd2O2Pb (9.1 μB/Gd) exhibits an interesting bulk response of lattice distortion in conjunction with the emergence of superconductivity and magnetic anomalies at a critical pressure of 10 GPa. Our findings reveal the unexpected facets of 2D Pb sheets that are considerably different from their bulk counterparts and provide an alternative route for exploring 2D properties in bulk materials.

Numerical investigation of thermal conductivity of Cu/graphene/Cu interface

The purpose of this work is to investigate the contribution of in-plane and out-of-plane phonon modes to interface thermal conductivities (ITC) of the Cu/graphene/Cu interface through nonequilibrium molecular dynamics simulations. The proportions of the ITC of the in-plane and out-of-plane phonon modes in the pristine ITC are 1.1% and 99.3%, respectively. Defect engineering can change the coupling strength between in-plane and out-of-plane phonon modes. There is a strong coupling between the in-plane and the out-of-plane phonon mode when the defect concentration is lower than 3%. Phonon coupling has been transformed into weak interaction when the defect concentration is higher than 3%. The high defect concentration can suppress the coupling between in-plane and out-of-plane phonon modes. The results of the phonon density of states show that the out-of-plane phonons are mainly concentrated at low frequencies, and the in-plane phonons are mainly concentrated at high frequencies. This work helps to understand the mechanism of heat transfer of the graphene-based interface and provides theoretical guidance for the application of graphene-based interface nanodevices.

Engineering Interface Defects and Interdiffusion at the Degenerate Conductive In2O3/Al2O3 Interface for Stable Electrodes in a Saline Solution.

A low-temperature Al2O3 deposition process provides a simplified method to form a conductive two-dimensional electron gas (2DEG) at the metal oxide/Al2O3 heterointerface. However, the impact of key factors of the interface defects and cation interdiffusion on the interface is still not well understood. Furthermore, there is still a blank space in terms of applications that go beyond the understanding of the interface's electrical conductivity. In this work, we carried out a systematic experimental study by oxygen plasma pretreatment and thermal annealing post-treatment to study the impact of interface defects and cation interdiffusion at the In2O3/Al2O3 interface on the electrical conductance, respectively. Combining the trends in electrical conductance with the structural characteristics, we found that building a sharp interface with a high concentration of interface defects provides a reliable approach to producing such a conductive interface. After applying this conductive interface as electrodes for fabricating a field-effect transistor (FET) device, we found that this interface electrode exhibited ultrastability in phosphate-buffered saline (PBS), a commonly used biological saline solution. This study provides new insights into the formation of conductive 2DEGs at metal oxide/Al2O3 interfaces and lays the foundation for further applications as electrodes in bioelectronic devices.

Efficient Generation of Large Collections of Metal-Organic Framework Structures Containing Well-Defined Point Defects.

High-throughput molecular simulations of metal-organic frameworks (MOFs) are a useful complement to experiments to identify candidates for chemical separation and storage. All previous efforts of this kind have used simulations in which MOFs are approximated as defect-free. We introduce a tool to readily generate missing-linker defects in MOFs and demonstrate this tool with a collection of 507 defective MOFs. We introduce the concept of the maximum possible defect concentration; at higher defect concentrations, deviations from the defect-free crystal structure would be readily evident experimentally. We studied the impact of defects on molecular adsorption as a function of defect concentrations. Defects have a slightly negative or negligible influence on adsorption at low pressures for ethene, ethane, and CO2 but a strong positive influence for methanol due to hydrogen bonding with defects. Defective structures tend to have loadings slightly higher than those of defect-free structures for all adsorbates at elevated pressures.

Enhanced solar-driven water splitting performance using oxygen vacancy rich ZnO photoanodes

This work reports a facile two-step method of producing highly efficient ZnO photoanodes for photoelectrochemical (PEC) water splitting under solar light conditions and describes the role of surface oxygen vacancies (VO) in their enhanced PEC performance. The photoanode fabrication involves post-growth oxidation of a metallic Zn layer, which produces a nanostructured ZnO film consisting of ∼50 nm diameter nanorods containing a high concentration of VO defects. The PEC activity of the ZnO films is investigated by studying water oxidation in an aqueous electrolyte under simulated solar illumination. The relationship of PEC and charge transfer characteristics of the ZnO photoanodes with ionized surface VO defects is established using cathodoluminescence, X-ray photoemission, voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The combined results show that the photoanode fabricated in this work possesses a high surface density of ionized VO states that facilitate the effective transportation of holes for water oxidation. It is found that the photoanode exhibits an exceptional photocurrent density of 1.14 mA/cm2 at 1.23 VRHE, being one of the best performances reported in the literature for ZnO-based photoelectrodes so far. Our results demonstrate a simple, low-cost method for fabricating highly efficient VO rich ZnO-based PEC photoanodes that is suitable for large scale production.

Stabilization of metallic phase in nanostructured hollow V2O3 spheres prepared by ultrasonic spray pyrolysis

In the development of nanostructured V2O3 materials, the topical issue is to elucidate the factors that affect the temperature of a metal-insulator transition (MIT). In this work, we study the structural, magnetic and electrical properties of V2O3 thin-walled microsphere prepared by spray-pyrolysis (SP). Low-temperature XRD and TEM revealed that SP-microspheres contain the anisotropic (50×20 nm) nanocrystallites with a preferred (001)-orientation along the sphere surface and show moderate biaxial compressive stresses. SP-aggregates exhibit a high concentration of intercrystallite boundaries and inner pores, which induce high lattice microstrains. Structural, magnetic and electrical properties showed no evidence of MIT down to 100 K. NMR investigations indicated that antiferromagnetic areas appear at temperatures below 115 K. Comparative studies were performed on V2O3 50 nm nanoparticles prepared by thermolysis using the same parent vanadyl solution. The findings suggest that a high defect concentration and microstrains rather than compressive stresses lead to the suppression of MIT.

Thermal stimulated structural transformation of cubic Zn0.78Cd0.22S nanoparticles to ZnO nano-hexagons: Tailoring of optical band gap and emission spectra for optoelectronic implementations

Ternary alloys of Zn0.78Cd0.22S nanoparticles (NPs) are synthesized via the facile co-precipitation technique. The as-synthesized sample exhibits a zincblende-type cubic phase with an average crystalline size of 2 nm. Thermal annealing and UV irradiation are utilized as post-treatment processes for tailoring the optical properties of Zn0.78Cd0.22S NPs. The as-synthesized sample exhibits a stable cubic phase up to 400 °C, during which partial phase transformation to a hexagonal structure is observed at a higher temperature of 500 °C. The oxidation of Zn0.78Cd0.22S NPs to mixed oxide phases with the majority of ZnO begins at 600 °C which induces morphology transformation to a relatively large nano-hexagon with a single crystalline domain size. The increase of the annealing temperature is accompanied by a decrease of Zn0.78Cd0.22S NPs optical band gap (Eg) due to the weakness of size confinement as well as the formation of localized states below the mobility band edges. The brief UV irradiation results in the increase of Eg whereas a further increase in exposure time is accompanied by a reduction of Eg. The photoluminescence (PL) spectrum of the as-synthesized sample covers a wide spectral range from UV to visible. Thermal annealing has a slight effect on the PL emission at high excitation energy, whereas low excitation energy reveals higher sensitivity to deep-state emission. Thermal oxidation incorporates a high concentration of oxygen-related defects that induce strong enhancement in the green emission at the expense of UV emission. This indicates the thermal-induced bleaching of shallow trapping sites and the evolution of other deep trapping or recombination sites.

Temperature-dependent microwave dielectric permittivity of gallium oxide: A deep potential molecular dynamics study

The microwave (MW) dielectric permittivity of gallium oxide (β-Ga2O3) fundamentally determines its interaction with an electromagnetic wave in bulk power. Yet, there is a lack of experimental data due to limitations of high-temperature MW dielectric measurements and the large uncertainty under variable-temperature conditions. Herein, we develop a deep potential (DP) based on density functional theory (DFT) results and apply deep potential molecular dynamics (DPMD) for accurately predicting temperature-dependent MW dielectric permittivity of β-Ga2O3. The predicted energies and forces by DP demonstrate excellent agreement with DFT results, and DPMD successfully simulates systems up to 1280 atoms with quantum precision over nanosecond scales. Overall, the real part of the MW dielectric permittivity decreases with rising frequency, but the dielectric loss increases. The MW dielectric permittivity gradually increases as the temperature increases, which is closely related to the reduced dielectric relaxation time and increased static and high-frequency dielectric constants. Besides, the oxygen vacancy defects significantly reduce the relaxation time; however, augmenting the defect concentration will cause a slight rise in relaxation time. The electron localization function analysis reveals that more free electrons and low localization of electrons produced by high defect concentrations facilitate the increased relaxation time. This study provides an alternative route to investigate the temperature-dependent MW permittivity of β-Ga2O3, which attains prime importance for its potential applications in RF and power electronics.

Effect of stirring on characteristics of electrochemically exfoliated graphene

Electrochemical exfoliation of graphite is a promising technique for the synthesis of graphene at a commercial scale. Electrochemical synthesis of graphene at high voltage (∼10 V) is widely reported. However, at high voltage, multilayer graphene platelets with high defect concentration are produced. In this study, high quality graphene nanoplatelets consisting of 1–5 layers were produced at low voltage (3–4 V) using KOH solution as electrolyte. The effect of electrolyte stirring on the characteristics of graphene was studied. The stirring of electrolyte during electrolysis showed profound effect on the characteristics of graphene. The scanning electron microscopic (SEM) analysis showed that the stirring of electrolyte increased the average size of graphene nanoplatelets to 360 nm from 253 nm (without stirring). Raman spectroscopy indicated the presence of 1–5 layers of carbon atoms and the same was confirmed by transmission electron microscopy (TEM). A decrease in the defect concentration was noted with the increase in number of layers of carbon atoms in graphene produced with stirring.

Selective hydrogenation of crotonaldehyde over Ir/TiO2 catalysts: Unraveling the metal-support interface related reaction mechanism

For the supported catalysts for the selective hydrogenation of α, β - unsaturated aldehyde with H2, tailoring of metal - support interfacial effect to improve reaction performance is a significant but challenging subject. In this work, we regulated both the size regimes of the Ir deposits (single atom, nanocluster and nanoparticle) and the crystal plane of anatase TiO2 (TiO2(101), TiO2(100) and TiO2(001)) so as to alter the Ir-TiOx interfacial properties, in order to unravel reaction mechanism of selective hydrogenation of crotonaldehyde over the Ir/TiO2 catalysts. Both the activity and selectivity to crotyl alcohol increased as the Ir size grew from single atom to nanoparticle, ascribed to high Ir0 content, high concentration of surface defects and prominent H-spillover effect on the Ir nanoparticles. Moreover, the in situ Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations demonstrated that an end-on CO adsorption mode for crotonaldehyde molecule was favored at the Ir-TiOx interface particularly at high H coverage, which thus facilitated the hydrogenation of CO bond to form crotyl alcohol. Moreover, the highest reactivity of the Ir nanoparticles was evidenced by the lowest reaction barrier. These results explicitly demonstrate that larger metal nanoparticles and adsorption geometry of the reactant at the metal - support interface are vital for improved catalytic performance, which would shed light on the ingenious catalyst design.

Micro-Photoluminescence spectroscopy of two distinct types of WS2 monolayers using fs-Ti: Sapphire laser under vacuum and ambient conditions and multivariate calibration models

We utilized Micro-Photoluminescence (μPL) and time-resolved photoluminescence (TRPL) spectroscopies with a femtosecond laser (440 nm) to assess the optical properties of two types of WS2 monolayers (MLs) obtained through mechanical exfoliation of different crystals. Based on cryogenic μPL spectra (4 K), the monolayers were classified as low-defect-concentration (LDC) and high-defect-concentration (HDC). By subjecting the MLs to laser excitation at various photon fluxes under ambient and vacuum conditions at room temperature, we characterized their optical properties. The spectra exhibited two main regions corresponding to optical transitions of neutral excitons (X) and trions (T). Notably, two distinct regimes were observed for quasi-particle decay: X-X annihilation at low photon fluxes and Mott transition at high photon fluxes. The photoluminescence quantum yield (PLQY) was measured under ambient and vacuum conditions, with the presence of physisorbed O2 on the ML surface influencing the results. Additionally, we employed a novel analytical approach combining PL and TRPL spectra with chemometric methods for the regression analysis of the two types of WS2 MLs. Utilizing partial least squares regression and principal component regression models, we estimated the decay times of X and T quasi-particles based on the spectral datasets.

IR Spectroscopy of Vacancy Clusters (Amber Centers) in CVD Diamonds Nanostructured by Fast Neutron Irradiation

We investigated the IR absorption spectra of CVD diamond damaged by fast neutrons (>0.1 MeV) with high fluences ranging from 1 × 1018 to 2 × 1019 cm−2 and annealed at temperatures of 200 °C to 1680 °C. After annealing above 1000 °C, the formation of “amber-centers” (ACs), associated with multivacancy clusters, is detected as deduced from the appearance of a strong absorption line at 4100 cm−1. Moreover, the concentration of the ACs in the irradiated diamond can be an order of magnitude higher than that observed previously in the darkest brown natural diamonds. A number of other absorption lines, including the H1b center at 4936 cm−1 (0.612 eV) and new lines at ~5700 cm−1 (0.706 eV) and 9320 cm−1 (1.155 eV) not reported before in the literature, are observed, and their intensity evolutions at annealing temperatures are documented. At the highest fluences, all the lines show reduced intensities and broadening and spectral shifts due to a very high defect concentration and partial amorphization. The obtained experimental data can be used for the analysis of defect generation, transformations and healing in irradiated synthetic and natural diamonds.

Ultrafast Mechanism of Material Removal in the Femtosecond Laser Ablation of WS2 and Its Diode Rectification Characteristics

The study investigates the two different underlying ablation mechanisms of WS2 processed by femtosecond (fs) laser with different fluences. With increasing fluence, the saturable expansion of craters and the transformation of three distinct crater morphologies are found. The material response and the transfer and deposition of laser energy are tracked by using a plasma model based on the classical single rate equation model and the Drude model. The results of the numerical simulation and time-resolved transient reflectivity reveal the two different ablation mechanisms, which are coulomb explosion and phase explosion. The mechanism of material removal is distinguished by the critical threshold of 0.85 J/cm2. In addition, the internal ablation region exhibits a high concentration of defects and WO3 according to the results of Raman spectra, X-ray photoelectron spectra, and morphology-dependent photoluminescence mapping. Due to the high concentration with high fluence, the device of WS2/Si p-n junction exhibits a 2.6 times enhancement on the current under forward bias. The findings would be of value to engineer structures to tailor the optoelectronic response of WS2 and to develop potential future optoelectronic devices.

High-capacity anode materials for lithium-ion batteries: perovskite strontium vanadate with crystalline-amorphous dual-phase structure

Breaking the capacity barrier in intercalation-type anode materials for lithium-ion batteries while preserving other essential electrochemical properties remains a significant challenge. Here, using perovskite SrVO3 as a model, we demonstrate the feasibility of employing defect engineering to provide extra active sites for electrochemical reactions. Defective SrxVO3-δ samples with a unique amorphous-crystalline dual-phase structure are successfully prepared by regulating the nonstoichiometric Sr/V ratio and using a non-equilibrium solution combustion strategy to alleviate nucleation. The dual-phase SrxVO3-δ electrodes, which take advantage of both the high defect concentration advantage of amorphous and the high electrical/ionic conductivity of crystalline, give a high specific capacity of 508 mAh/g at a safe potential of 1.0 V vs. Li/Li+, showing a 40% increase over SrVO3 electrodes and outperforming most state-of-the-art intercalation-based anodes. In addition, when coupling with LiFePO4 cathode, the SrxVO3-δ electrodes can operate over 10,000 cycles with 91% capacity retention, overcoming the limited structural stability exhibited in typical defective electrodes. The study demonstrates the viability of modulating the crystallinity in intercalation-based anodes to enhance electrochemical performance and paves the way for the advancement of Li-ion battery technology.

A new approach to self-regulation: A controlled trial on preventive health care including a vibro shearing-and-breathing methodology

Breaking the capacity barrier in intercalation-type anode materials for lithium-ion batteries while preserving other essential electrochemical properties remains a significant challenge. Here, using perovskite SrVO3 as a model, we demonstrate the feasibility of employing defect engineering to provide extra active sites for electrochemical reactions. Defective SrxVO3-δ samples with a unique amorphous-crystalline dual-phase structure are successfully prepared by regulating the nonstoichiometric Sr/V ratio and using a non-equilibrium solution combustion strategy to alleviate nucleation. The dual-phase SrxVO3-δ electrodes, which take advantage of both the high defect concentration advantage of amorphous and the high electrical/ionic conductivity of crystalline, give a high specific capacity of 508 mAh/g at a safe potential of 1.0 V vs. Li/Li+, showing a 40% increase over SrVO3 electrodes and outperforming most state-of-the-art intercalation-based anodes. In addition, when coupling with LiFePO4 cathode, the SrxVO3-δ electrodes can operate over 10,000 cycles with 91% capacity retention, overcoming the limited structural stability exhibited in typical defective electrodes. The study demonstrates the viability of modulating the crystallinity in intercalation-based anodes to enhance electrochemical performance and paves the way for the advancement of Li-ion battery technology.

Verletzungen im Pferdesport – Verletzungsmuster und Präventionsmöglichkeiten

Breaking the capacity barrier in intercalation-type anode materials for lithium-ion batteries while preserving other essential electrochemical properties remains a significant challenge. Here, using perovskite SrVO3 as a model, we demonstrate the feasibility of employing defect engineering to provide extra active sites for electrochemical reactions. Defective SrxVO3-δ samples with a unique amorphous-crystalline dual-phase structure are successfully prepared by regulating the nonstoichiometric Sr/V ratio and using a non-equilibrium solution combustion strategy to alleviate nucleation. The dual-phase SrxVO3-δ electrodes, which take advantage of both the high defect concentration advantage of amorphous and the high electrical/ionic conductivity of crystalline, give a high specific capacity of 508 mAh/g at a safe potential of 1.0 V vs. Li/Li+, showing a 40% increase over SrVO3 electrodes and outperforming most state-of-the-art intercalation-based anodes. In addition, when coupling with LiFePO4 cathode, the SrxVO3-δ electrodes can operate over 10,000 cycles with 91% capacity retention, overcoming the limited structural stability exhibited in typical defective electrodes. The study demonstrates the viability of modulating the crystallinity in intercalation-based anodes to enhance electrochemical performance and paves the way for the advancement of Li-ion battery technology.