Charge Trapping Centers Research Articles

Hyperdoping: doping TiO2 beyond thermodynamic limits using Flash sintering

For the first time, it has been demonstrated that TiO2 optical properties can be improved by doping Fe using flash sintering. Hyperdoped TiO2 (doped with 10 at% Fe) fabricated using flash sintering, exhibited remarkable improvements in optical characteristics. Hyperdoping (exceeding the equilibrium solubility limit), induces charge trap centers or inhibits charge recombination in semiconductors. By employing XPS and UV-Vis spectroscopy, surface species alterations including oxygen vacancies, Ti4+, Ti3+, and optical characteristics were investigated. With varying holding periods, flash sintering unveiled substantial enhancements in optical absorbance and absorption regions by generating Ti3+ and oxygen vacancies. Notably, a 10-second hold time during flash sintering displayed the highest redshift among the investigated samples, indicating superior optical improvement. Oxygen vacancies and Ti3+ increased with decreased holding time during flash, suggesting the influence of flash sintering parameters on forming a metastable phase, signifying the pivotal role of flash sintering's time parameter in modulating material properties.

Distinguishing the Charge Trapping Centers in CaF2-Based 2D Material MOSFETs.

Crystalline calcium fluoride (CaF2) is drawing significant attention due to its great potential of being the gate dielectric of two-dimensional (2D) material MOSFETs. It is deemed to be superior to boron nitride and traditional silicon dioxide (SiO2) because of its larger dielectric constant, wider band gap, and lower defect density. Nevertheless, the CaF2-based MOSFETs fabricated in the experiment still present notable reliability issues, and the underlying reason remains unclear. Here, we studied the various intrinsic defects and adsorbates in CaF2/molybdenum disulfide (MoS2) and CaF2/molybdenum disilicon tetranitride (MoSi2N4) interface systems to reveal the most active charge-trapping centers in CaF2-based 2D material MOSFETs. An elaborate Table comparing the importance of different defects in both n-type and p-type devices is provided. Most impressively, the oxygen molecules (O2) adsorbed at the interface or surface, which are inevitable in experiments, are as active as the intrinsic defects in channel materials, and they can even change the MoSi2N4 to p-type spontaneously. These results mean that it is necessary to develop a high-vacuum packaging process, as well as prepare high-quality 2D materials for better device performance.

X-ray assisted point defects creation in micron-size Zn, Mo oxide particles at liquid nitrogen temperature

Charge trapping processes induced by the X-ray irradiation in the heavy Mo doped ZnO and MoO3 micropowders synthesized by the hydrothermal growth method were investigated in detail. Electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) were applied in a correlated manner to discover the role of the Mo doping in the charge trapping processes in ZnO. Thermally unstable oxygen- and molybdenum-related charge trapping centers were studied. Molybdenum and oxygen created electron-hole trapping pairs in some cases were observed. Some part of the hole trapping centers seemed to be directly connected with the creation of Mo5+. The correlation between EPR and TSL data was found.

Unveiling the potential of MXene-fabricated catalysts: an effective approach for H2 generation from water splitting.

Hydrogen has enough potential and can be successfully used as an alternative to the conventional fuel. It can be successfully produced from water that is not only a sustainable source but exists everywhere on earth. Additionally, its combustion releases water that is quite safe and environment friendly. The current project was designed to generate hydrogen from catalytic water splitting on TiO2@Ti3C2T x catalysts. To obtain the required catalytic characteristics, titania was engineered on Ti3C2T x surfaces in situ using an ethanol-assisted solvothermal approach. After careful recovery, the catalysts were characterized and assessed for the photoreaction. All photoreactions were performed in a quartz reactor (150 mL), where hydrogen evolution activities were monitored on GC-TCD (Shimadzu-JP). The comparative activities indicated that TiO2@C and TiO2@Ti3C2T x catalysts deliver 9.37 and 18.57 mmol g-1 h-1 of hydrogen, respectively. The higher activities of TiO2@Ti3C2T x were attributed to the existence of higher active sites (charge trapping centres) on the multilayer MXene that progressively promote and facilitate redox reactions. Reason is that existence of titania on MXene interfaces develops heterojunctions that rectify the charge transfer; hence reduce the charge recombination (i.e., back reaction). On the basis of encouraging activities, it has been concluded that the aforementioned approach holds promise to replace the costly and conventional hydrogen generation technologies.

Investigating versatile capabilities of organic field-effect transistors incorporated with vacuum-deposited metal nanoparticles

Optically programmable organic field-effect transistors, developed using vacuum-deposited metal nanoparticles, enhance photocarrier generation and act as charge trapping centers, modulating charge retention.

High-performance flexible resistive random-access memory based on SnS2 quantum dots with a charge trapping/de-trapping effect.

The application of resistive random-access memory (RRAM) in storage and neuromorphic computing has attracted widespread attention. Benefitting from the quantum effect, transition metal dichalcogenides (TMD) quantum dots (QDs) exhibit distinctive optical and electronic properties, which make them promising candidates for emerging RRAM. Here, we show a high-performance forming-free flexible RRAM based on high-quality tin disulfide (SnS2) QDs prepared by a facile liquid phase method. The RRAM device demonstrates high flexibility with a large on/off ratio of ∼106 and a long retention time of over 3 × 104 s. The excellent switching behavior of the memristor is elucidated by a charge trapping/de-trapping mechanism where the SnS2 QDs act as charge trapping centers. This study is of significance for the understanding and development of TMD QD-based flexible memristors.

CeO2 nanorods with bifunctional oxygen vacancies for promoting low-pressure photothermocatalytic CO2 conversion with CH3OH to dimethyl carbonate

The direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol is a kind of highly-attractive route for CO2 conversion to high value-added chemicals. However, a key core problem is how to more effectively activate CO2. Here, we adopt simple template-free hydrothermal method to prepare CeO2 nanorods with oxygen vacancies and introduce light energy into the thermal catalysis system for promoting low-pressure photothermalcatalytic CO2 with methanol direct conversion to DMC. The SEM, HRTEM, XRD, XPS, N2 adsorption-desorption isotherm, CO2-TPD, EPR, Raman, photoluminescence spectra and UV-Vis-IR DRS characterizations are carried out to analyze the microscopical properties including the crystal phases, chemical states, structural features, optical properties for building up the intrinsic relationship with macroscopic performance. Our findings reveal that oxygen vacancies can not only act as photo-produced charge trapping center to prevent the recombination of photoinduced electrons and holes, but also promote CO2 adsorption and activation abilities through Lewis acid-base interaction. Finally, the detailed action mechanism for photothermalcatalytic CO2 conversion with methanol to DMC is proposed. This work should provide new ideas and insights for achieving the CO2 conversion into high value-added long-chain chemicals.

Dangling Bonds as Possible Contributors to Charge Noise in Silicon and Silicon-Germanium Quantum Dot Qubits.

Spin qubits based on Si and Si1-xGex quantum dot architectures exhibit among the best coherence times of competing quantum computing technologies, yet they still suffer from charge noise that limit their qubit gate fidelities. Identifying the origins of these charge fluctuations is therefore a critical step toward improving Si quantum-dot-based qubits. Here, we use hybrid functional calculations to investigate possible atomistic sources of charge noise, focusing on charge trapping at Si and Ge dangling bonds (DBs). We evaluate the role of global and local environment in the defect levels associated with DBs in Si, Ge, and Si1-xGex alloys, and consider their trapping and excitation energies within the framework of configuration coordinate diagrams. We additionally consider the influence of strain and oxidation in charge-trapping energetics by analyzing Si and GeSi DBs in SiO2 and strained Si layers in typical Si1-xGex quantum dot heterostructures. Our results identify that Ge dangling bonds are more problematic charge-trapping centers both in typical Si1-xGex alloys and associated oxidation layers, and they may be exacerbated by compositional inhomogeneities. These results suggest the importance of alloy homogeneity and possible passivation schemes for DBs in Si-based quantum dot qubits and are of general relevance to mitigating possible trap levels in other Si, Ge, and Si1-xGex-based metal-oxide-semiconductor stacks and related devices.

Correlated EPR and optical study of charge trapping phenomena in tellurite glasses. The role of barium oxide

Tellurite glasses of the compositions TeO2(60%)-ZnO(40%) and TeO2(60%)-ZnO(20%)-BaO(20%) were prepared in the bulk form with following grinding into powders. The samples were X-ray irradiated. The created charge trapping centers were studied in both types of samples using electron paramagnetic resonance (EPR) spectroscopy. The O− defect and two types of F+ centers were discovered. One of the F+ center remained unchanged whereas the other one as well as the O− defect were affected by the presence of BaO. Moreover, photo-, radio- and thermoluminescence (PL, RL, TL, respectively) methods were applied as well. The discovered charge trapping centers have been proven to influence luminescence properties as well – improving them in the TeO2(60%)-ZnO(40%) and vice versa degrading them in the TeO2(60%)-ZnO(20%)-BaO(20%) glasses. The effect was even stronger in the grinded samples as compared to the bulk ones. The role of the BaO has been disclosed as supporting oxygen leakage from the host.

Stochastic theory of charge dynamics and recombination in defect clusters in bulk silicon

Various types of defect clusters are generated in bulk Si-based high-energy particle detectors. They become either recombination centres or charge trapping centres. Populated trapping centres create internal fields which may affect the dynamics and recombination of remaining free charges. In the semiclassical regime, the charge dynamics can be described by the Boltzmann equation. In this paper, the stochastic description is presented as an alternative to a direct solution of the Boltzmann equation approach. It is demonstrated that the hole dynamics can be described in the overdamped regime in both light-hole and heavy-hole cases. Electrons have to be described by including ballistic components. The theory allows an efficient simulation of the electron and hole dynamics in the vicinity of a defect cluster and demonstrates that local trapping centres are the major components enabling fast charge recombinations. The dipolar type internal fields of permanently trapped charges only weakly influence the charge recombination kinetics.

In situ oxidation of ethylene glycol coupled with Bi2O3 epitaxial growth to prepare Bi2O3/BiOCOOH heterojunctions with oxygen vacancies for efficient photocatalytic lignin degradation

Papermaking wastewater has large output and high chemical oxygen demand (COD), which seriously affects the water environment. As a by-product of papermaking industry, lignin is the main contaminant that causes excessive COD value. Bismuth oxide is a potential catalyst for photocatalytic degradation of lignin-contained wastewater because of its high charge conduction performance and light corrosion resistance. However, the high electron-hole recombination rate and poor surface chemical state of pure Bi2O3 seriously hinder the degradation efficiency. Herein, we designed an OV-Bi2O3/BiOCOOH heterojunction with oxygen vacancies and heterojunction coexisting for photocatalytic degradation of lignin. Temperature induced the oxidation of ethylene glycol into formic acid, resulting in the epitaxial growth of Bi2O3 and forming a Bi2O3/BiOCOOH heterojunction. The heterojunction presents a tight interface by sharing the Bi-O tetrahedron, and provides a highway for the charge transfer. Meanwhile, the oxidation of ethylene glycol induces oxygen vacancies (OVs) on Bi2O3/BiOCOOH surface, forming charge trapping centers to promote electron transport and surface reactions. The degradation efficiency of OV-Bi2O3/BiOCOOH is approximately 3 and 16 times that of Bi2O3 and BiOCOOH, respectively. The improvement of photocatalytic performance can be attributed to the synergy of heterojunction and oxygen vacancy, and the stability is attributed to covalent bonds tightly connected.

Fabrication and characterization of high-resolution 4H-SiC epitaxial radiation detectors for challenging reactor dosimetry environments

Reactor dosimetry environments require radiation detectors that are capable of operating at high temperatures in extremely high neutron and gamma-ray dose rates. Silicon carbide (SiC) is one of the most promising wide bandgap semiconductors (3.27 eV) for harsh environment applications due to its radiation hardness, high breakdown voltage, high electron saturation velocity, and high thermal conductivity. In this paper, we summarize the prospect of Schottky barrier radiation detectors, fabricated on highly crystalline low-defect detector-grade n-type 4H-SiC epitaxial layers with thickness ranging from 20 to 250 lm, for harsh environment applications. A comprehensive discussion on the characterization of the parameters that influence the energy resolution has been included. The usage of electrical and radiation spectroscopic measurements for characterizing the junction and rectification properties, minority carrier diffusion lengths, and energy resolution has been elaborated. Characterization of crucial factors that limit the energy resolution of the detectors such as charge trap centers using thermally stimulated transient techniques is summarized. Finally, the effect of neutron fluence on the performance of the 4H-SiC detectors is discussed.

Charge traps in Zn- and Mo-based oxide microstructures. The role of Mo

Peculiarities of the point defects creation under the X-ray irradiation and the influence of Mo doping in ZnO: Mo (10 and 30%) and MoO3 micro-powders fabricated by the hydrothermal growth method were studied in detail. It has been found that some new phases other than ZnO are created upon the heavy molybdenum doping. They were tentatively ascribed to complex zinc molybdates existing in the form of platelet flakes. The MoO3 sample contained both hydrated and anhydrous MoO3 phases in the form of microneedles. The molybdenum charge state was Mo6+ before X-ray irradiation in all the materials. X-ray irradiation resulted in creation of molybdenum- and oxygen-based charge trapping centres, i.e., Mo5+ and O− or . Part of them is expected to exist in the pure ZnO and MoO3 phases whereas the rest appears in the complex zinc molybdates.

First-Principles Study on the Origin of Tribo-Negative Nature of Polytetrafluoroethylene

Triboelectric nanogenerator (TENG) is an energy harvesting device that converts mechanical energy into electricity based on the coupling of contact electrification and electrostatic induction. Fluorocarbon polymers, such as polytetrafluoroethylene (PTFE) and perfluoroalkoxy alkane (PFA) are the widely used polymeric triboelectric materials, however, their tribo-negative nature has been poorly understood. Here, using first-principles calculations, we have systematically investigated the electronic structure of PTFE and PFA. It is revealed that the defects associated with carbon dangling bonds act as charge trapping centers during contact electrification. Furthermore, the simulation on the contact model of PTFE and nylon confirms that the charge transfer occurs when the distance between them is as near as the atomic orbitals overlap. In addition, the calculated work functions of polymeric materials are consistent with the triboelectric series, giving a powerful method to predict the triboelectric property of materials. We expect our study provides fundamental understandings on the origin of the triboelectric property of materials and the atomistic phenomena during contact electrification. Our results also give insight into triboelectric material designs with a strong electron-withdrawing property.

Effect of high-radiation-dose-induced structural modifications of HfSiO4/n-Si on electrical characteristics

High-k/n-Si structures were formed with HfSiO4 films annealed at the temperature range of RT (room temperature)-900 °C and radiation-induced structural modifications were determined by XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy) techniques in the study. The effect of oxygen-deficient bond contents on the electrical characteristics of HfSiO4 pMOS (n type Metal Oxide Semiconductor) capacitors whose radiation response was investigated in the 0–50 kGy dose range was investigated. While no XRD peak was observed before and after irradiation at RT and 500 °C-HfSiO4/n-Si, crystallization started with irradiation at 900 °C. The dielectric constant of the film was found in the range of 16–23. It was determined that Hf–Hf oxygen-deficient bonds act as negative charge trapping centers, while Hf–Si and Si–Si oxygen-defective bonds act as positive charge trapping centers. The direction of the C–V (Capacitance-Voltage) curve changed continuously with increasing radiation dose. The change in the interface trap charge density was found to be higher than the change in the oxide trap charge density for all doses in RT-HfSiO4 pMOS capacitor and for 1 kGy at 500 °C-HfSiO4 pMOS capacitor. Oxygen defective bond content and Hf-O-Si/Si–O–Si ratios were sufficient to establish a link between structural analyses and electrical characteristics at some doses. In some cases, the frequency-dependent charges had a more dominant effect on the radiation response of the device compared to the oxide trap charges.

The origin of bright cyan persistent luminescence in Ca2SnO4:La3+

The origin of bright cyan persistent luminescence in La3+-doped Ca2SnO4 is investigated by optical and electron paramagnetic resonance spectroscopy methods. The incorporation of La3+ ions in the material stimulates the formation of Sn2+ emission centers and charge trap centers. After exposure of the material to UV radiation, charge transfer processes to the trap centers result in the creation of two types of paramagnetic defects. Both defects are spin 1/2 systems with rhombic symmetry g-factor values, which correspond to electron and hole centers respectively. The role of the identified defects in the optical processes of the material is evidenced by a correlation of defect decay and persistent luminescence kinetics. Due to appropriate activation energies of the defects, light emission of Sn2+ centers is efficient and long-lasting. In this article, optimal lanthanum ion content is determined, the identity of the defects is discussed and a model of persistent luminescence in Ca2SnO4:La3+ is proposed.

Interfacial charge and energy transfer in van der Waals heterojunctions

AbstractVan der Waals heterojunctions are fast‐emerging quantum structures fabricated by the controlled stacking of two‐dimensional (2D) materials. Owing to the atomically thin thickness, their carrier properties are not only determined by the host material itself, but also defined by the interlayer interactions, including dielectric environment, charge trapping centers, and stacking angles. The abundant constituents without the limitation of lattice constant matching enable fascinating electrical, optical, and magnetic properties in van der Waals heterojunctions toward next‐generation devices in photonics, optoelectronics, and information sciences. This review focuses on the charge and energy transfer processes and their dynamics in transition metal dichalcogenides (TMDCs), a family of quantum materials with strong excitonic effects and unique valley properties, and other related 2D materials such as graphene and hexagonal‐boron nitride. In the first part, we summarize the ultrafast charge transfer processes in van der Waals heterojunctions, including its experimental evidence and theoretical understanding, the interlayer excitons at the TMDC interfaces, and the hot carrier injection at the graphene/TMDCs interface. In the second part, the energy transfer, including both Förster and Dexter types, are reviewed from both experimental and theoretical perspectives. Finally, we highlight the typical charge and energy transfer applications in photodetectors and summarize the challenges and opportunities for future development in this field.

Charge trapping processes in hydrothermally grown Er-doped ZnO

Influence of X-ray irradiation on luminescence properties and defects creation was studied in the erbium doped hydrothermally grown ZnO by using the combination of photo- and radioluminescence (PL and RL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR). The photoluminescence properties of the ZnO:Er appeared affected by the X-ray irradiation – the intensity of the zinc vacancy-related red band and exciton-related ultraviolet band were weakened. The irradiation also evoked charge trapping and new centers creation whose type and number depended on Er doping. The X-ray induced defects were oxygen-based hole and electron trapping centers, O− and O2−, respectively. The X-ray irradiation had also influence on shallow donors in the bulk and at the surface of the ZnO nano- and microrods. TSL glow curves consisted of one broad peak with the maximum at about 110 K, its intensity was varying with the changing Er content. The glow peak was correlated with the thermal destruction of the unstable O2− electron trapping centers. Zinc vacancies-based defects participated in the thermally stimulated luminescence as recombination centers for the electrons released from the O2−.

Effect of Calcination Temperature on the Photocatalytic H2 Evolution of Bronze Phase Monoclinic TiO2(B) Nanosheets

In this work, we have successfully fabricated bronze phase monoclinic TiO2 (B) nanosheets from TiCl3 under hydrothermal conditions. The fabricated samples were calcined at different annealing temperature (100, 200, 300, 400 and 500 oC) to select optimum temperature under UV light irradiation for the efficient photocatalytic water splitting. Different techniques were used for the characterization of fabricated photocatalysts. Interestingly, the sample calcined at 400 oC delivered optimum H2 evolution from water which is attributed to the relatively high surface area and effective charge separation. The characteristic anisotropic nature of TiO2(B) plays very crucial role in charge separation which is evident from photoluminescence spectra, steady-state surface photovoltage spectra, and produced hydroxyl radical amount. It has been concluded that optimum annealing temperature generally introduces charge trapping centres which help in the separation of excite charges for improved photocatalytic activity. However, high temperature results in particles aggregation to reduce the surface area and hence retards the photocatalytic efficiency. This work will direct future research to fabricate materials at optimized temperature for the improved photocatalytic activities.

Visible-Light Driven Enhanced Photocatalytic Properties of rGO/Mn3O4/MoO3 Ternary Hybrid Nanocomposite

In the present study, the nanostructured ternary hybrid composite of rGO/Mn3O4/MoO3 has been synthesized for photocatalytic studies investigated in visible light. The graphene oxide, rGO/Mn3O4, and rGO/Mn3O4/MoO3 nanocomposites were prepared by using the modified Hummers’ method, co-precipitation method, and hydrothermal method respectively. Organic dyes such as methylene blue and methylene violet were used for the degradation process. In rGO/Mn3O4/MoO3 hybrid, graphene oxide (GO) acts as an excellent electron conductor and manganese oxide (Mn3O4) decomposes the dye molecules within a short time. Molybdenum trioxide (MoO3) shows good photocatalytic activity towards organic pollutants as well as promotes oxygen evolution reaction. The physiochemical characterizations such as X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX), FT-IR, and UV–vis spectroscopy analysis were carried out. Furthermore, the photocatalytic efficiency of the as-synthesized ternary composite was recorded as 95% which is much higher than the efficiency of individual components due to the synergetic effect of rGO. Two-dimensional structured GO increases the absorption sites which act as charge trapping centers which help to reduce electron-hole recombination effect. Photodegradation mechanism of methylene blue and methylene violet dyes for wastewater treatment application under ambient conditions has been discussed.