Trapping Centers Research Articles

Simulation of the Thermoluminescence Isothermal Decay Curves in Al2O3–BeO Ceramics Taking into Account the Energy Distribution of Trapping Centers

Simulation of the Thermoluminescence Isothermal Decay Curves in Al2O3–BeO Ceramics Taking into Account the Energy Distribution of Trapping Centers

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.

Opto‐Mechano‐Thermo‐Sensitive Allochroic Luminescence Based on Solution‐Grown Halide Double Perovskite Crystal

AbstractMultimode luminescence generally exhibits tunable photon emission through multidimensional stimuli, demonstrating high‐throughput and facile identification for anticounterfeiting and encryption. Nevertheless, producing multimodal luminescent materials through low‐temperature treatment remains a challenge. Herein, the Mn2+/Er3+ co‐doped Cs2Ag0.8Na0.2InCl6 perovskite material (reaction temperature of 180 °C) achieves long‐persistent luminescence, multicolor luminescence, and upconversion emission by responding to several types of excitations ranging from UV to NIR and mechanical force. The adjustable energy transfer process between self‐trapped excitons (STE) and Mn2+ ions achieves multicolor luminescence with temperatures ranging from 97 to 397 K. Theoretical calculation reveals that the important role of Mn2+ dopants and Na+ dopants in the modulation of local electronic environments and the afterglow emission comes from carriers escaping from the trap center (primarily caused by Cl vacancy and interstitial Na) to the local emission center (Mn2+ ions). This integrated multimodal luminescent material provides a novel direction for the development of multifunctional optical materials and devices.

Surface treatment of GaN nanowires for enhanced photoelectrochemical water-splitting

High-efficiency hydrogen production through photoelectrochemical (PEC) water splitting has emerged as a promising solution to address current global energy challenges. III-nitride semiconductor photoelectrodes with nanostructures have demonstrated great potential in the near future due to their high light absorption, tunable direct band gap, and strong physicochemical stability. However, several issues, including surface trapping centers, surface Fermi level pinning, and surface band bending, need to be addressed. In this work, enhanced photovoltaic properties have been achieved using gallium nitride (GaN) nanowires (NWs) photoelectrodes by adopting an alkaline solution surface treatment method to reduce the surface states. It was found that surface oxides on NWs can be removed by an alkaline solution treatment without changing the surface morphology through X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and other characterization methods. These findings provide new insights to the development of high-efficiency photoelectrodes for new energy source applications.

Improved high-temperature energy storage of polyetherimide by energy level structure regulation

Polyetherimide (PEI) for high-temperature energy storage still face the critical problem of low discharged energy density. The dramatic increase in leakage current is the basic reason for the deterioration of energy storage characteristics under elevated temperatures. Herein, a molecular engineering strategy is presented to suppress electrical conduction by introducing a high electron-affinity dianhydride structure into the main chain of PEI. The energy band structure of PEI is markedly affected by the dianhydride, which is illustrated by experimental research and density functional theory (DFT) calculations. Structures with high electron-affinity act as trap centers to capture carriers, significantly inhibiting conduction at elevated temperatures. Consequently, the PEI hybrid film exhibits a discharged energy density of 4.01 J/cm3 and a charge-discharge efficiency of 91% at 150 °C. The high throughput and easy processing of the PEI hybrid film makes it a potential choice for energy storage under harsh conditions. This work represents a route for preparing polymer dielectrics with outstanding energy storage characteristics capable of operating in high temperature environments.

Employing defect luminescence of LiGaO2 to realize dynamic readout and multiplexing of optical information

Optical data storage (ODS) materials have attracted much attention due to their unparalleled advantages, including outstanding low cost, high speed, and high storage density. The dynamic readout of high-resolution optical information in a single medium has always been a hotspot in the design of ODS materials. This work successfully developed a multi-mode dynamic luminescent material LiGaO2. By adjusting the defect states in different synthetic atmospheres, LiGaO2 materials with different luminescence properties were obtained, it appears green glow in air and nitrogen atmospheres and orange-red glow in reductive atmospheres. LiGaO2 material synthesized in a reduction atmosphere (R–LiGaO2) not only has the characteristics of rapid filling, but also has more abundant trap distribution, so it has a good optical information storage capacity, which is very suitable for the field of ODS. Through the analysis and characterization of R–LiGaO2 material in thermally-stimulated luminescence (TSL) and photostimulated luminescence (PSL), it is found that the dynamic optical information readout based on intensity-color-wavelength multiplexing in undoped single ODS materials is realized through the synergistic effect of trap centers and luminescence centers. This work provides a new strategy for the design and application of ODS materials.

Intrinsic emissions and formation of electron-hole trapping centres in excited Li2SO4

Vacuum ultraviolet and thermal activation spectroscopies were used to study the nature of intrinsic emission and the mechanisms of the formation of electron and hole trapping centres in irradiated Li2SO4 powders and crystals. The obtained results along with excitation spectra showed that recombination emission at 3.8–4.2 eV and 4.5–5.5 eV occurs when an electron passes from the second and third sub-bands of the valence band formed from the 2p state of oxygen to the conduction band. It has been determined that Li2SO4 electron and hole trapping centres are created when electrons are trapped in the SO42– anionic sites. The holes are localised in the form of SO4 – radicals. The spectral position and intensity of individual emission bands are dependent on the crystallographic arrangement of localised holes.

Pyridine group incorporated mixed electron transporting layers for high-performance perovskite light-emitting diodes

The unsaturated Pb2+ ions, located at the perovskite/charge transport layer (CTL) interfaces, are one of the main trap centers that result in non-radiative recombination of perovskite light-emitting diodes (PeLEDs), thus compromising device efficiency. Therefore, interface engineering and modulation are of significance in passivating the surface defects of the perovskite film as well as modifying the perovskite/CTL interface properties. In this work, a high-mobility pyridine-based electron transport material, 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB), is co-evaporated with 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) to form efficient electron-transport layers (ETLs) for PeLEDs. The interaction of the pyridine group of TmPyPB with the uncoordinated Pb2+ ions can effectively passivate the perovskite defects. Furthermore, the mixed ETL can also enhance electron mobility by tuning the mixing ratios between the TmPyPB and TPBi, thus boosting the device efficiency. As a result, PeLEDs with enhanced electron mobility and reduced defects are prepared, in which the device exhibits a maximum current density (CE) of 74 cd/A and an external quantum efficiency (EQE) of 25.4%. This EQE is nearly two fold higher than that of the pure TPBi-based PeLEDs.

White afterglow observed in Ce3+, Eu2+ and Dy3+ Co-activated Sr3Al2O5Cl2 phosphor

A white phosphor, Sr3Al2O5Cl2: Ce3+, Eu2+ and Dy3+, was synthesized successfully through high temperature solid state process. XRD refinement revealed that all the doping cations replaced the original Sr2+ site in the phosphor lattice. Upon ultraviolet excitation, the phosphor displayed a blue emission from the 5 d-4f Ce3+ transition centered at 441 nm, as well as a broad orange emission from the 5 d-4f Eu2+ transition centered at 636 nm. By adjusting the concentrations of Eu2+, a white emission was achieved with the formula Sr3Al2O5Cl2: 0.005Eu2+, 0.02Dy3+, 0.005Ce3+. After the removal of excitation, a persistent white afterglow was observed due to the presence of Dy3+ electron traps in the lattice with optimal trap depth. The mechanism proposed suggests that the recombination between thermally released electrons originating from the trap center and existing holes at the emission centers of Ce3+ and Eu2+ drives the observed white afterglow phenomenon. These findings highlight the potential of Sr3Al2O5Cl2: Ce3+, Eu2+ and Dy3+ phosphors in solid-state lighting applications.

Amorphous FeOOH-mediated directional charges transfer in light-switchable oxygen vacancies BiOBr for boosting photocatalytic oxygen evolution

Directing the transfer of photoinduced charges to various reaction sites through catalyst design plays crucial roles in enhancing photocatalytic oxygen evolution. Herein, we used a facile alcoholysis method to synthesize amorphous FeOOH modified light-switchable oxygen vacancies BiOBr, which exhibited outstanding photocatalytic oxygen evolution activity. Density functional theory calculations, photoelectronchemical tests and kelvin probe force microscopy revealed that FeOOH overlayer acts as the hole storage layer, while oxygen vacancies (OVs) provided a number of electron trap centers. Benefiting from this synergistic effect, the photogenerated charges showed an outstanding directional transfer performance, which greatly prolong the lifetime of the photo-induced charge carriers. Moreover, as a cocatalyst for water oxidation, FeOOH can effectively lower the kinetics barriers and provide more active sites for oxygen evolution reaction. Therefore, under simulated sunlight irradiation, the 2%FeOOH/BiOBr photocatalyst demonstrated remarkable photocatalytic activity towards oxygen evolution, achieving a rate of 284.34 μmol·g−1·h−1, which is around 1.71 folds than pure BiOBr. Finally, the possible reaction mechanism has also been discussed. This work has provided valuable insights and inspiration for the effective design and optimization of BiOBr-based catalysts, which play a critical role in facilitating efficient directional transfer of photogenerated charges.

Fabrication and characterization of SiC-based transparent passivating contacts for enhanced photovoltaic performance

Silicon Carbide (SiC) passivating contacts are thought to be an emerging solution to bridge the existing gap in the silicon-based solar market. Passivating contacts directly tackle the intrinsic metal-semiconductor interfaces issues like recombination losses and Fermi-level pinning. Considering this approach, SiC-based Transparent Passivating Contacts (TPC) are fabricated on n-Si <100> using RF magnetron sputtering at multiple RF powers for enhanced solar cell performance. All said contacts were convolved through a variety of optical, and electrical techniques. It is confirmed that the passivating contacts grown by higher RF power have an enhanced provision for both transmission and contact passivation. Additionally, it has been found that a sufficient amount of oxygen vacancies (VO) are located at the SiC/Si interface and are mainly responsible for limiting of the contact passivation. This issue might be resolved by opting for larger RF power during the SiC growth that may suppress VO impact. Further, purely optical and electro-optical parameters and their dependence on VO have been readily studied. Most importantly the extent of contact passivation has been readily measured by the transient of photo voltages and Q-DLTS techniques and cross-verified by current kinetics studies. It is found by the current-voltage and energy band picture that the SiC grown at higher RF would provide better contact passivation with surface resistance of ∼109 Ω and positive trap centers, respectively.

Design, Synthesis, and Characterization of a Novel Blue-Green Long Afterglow BaYAl3O7:Eu2+, Nd3+ Phosphor and Its Anti-Counterfeiting Application.

Herein, a series of novel long afterglow nanophosphors BaYAl3O7:Eu2+, Nd3+ was synthesized by the combustion method. The investigation encompassed the characterization of X-ray diffraction, morphology, chemical valence, elemental composition, and photoluminescence behavior of BaYAl3O7:Eu2+ and BaYAl3O7:Eu2+, Nd3+ nanoparticles. Under 365 nm excitation, BaYAl3O7:Eu2+ and BaYAl3O7:Eu2+, Nd3+ show emission bands centered at 497 nm and 492 nm, which are attributed to the 4f65d→4f7 transition of Eu2+ ions. The optimal samples of BaYAl3O7:0.03Eu2+ and BaYAl3O7:0.03Eu2+, 0.02Nd3+ have average fluorescence lifetimes of 850 ns and 1149 ns, respectively. The co-doping of Nd3+ ions as the trap centers produced long afterglow luminescence properties, and the afterglow time could reach up to 8 min. Furthermore, the fluorescent powder can be mixed with polyacrylic acid to prepare anti-counterfeiting inks; a clover pattern and snowflake pattern have been successfully printed using screen printing technology, proving its potential application in the field of anti-counterfeiting.

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.

Scalable trapping of single nanosized extracellular vesicles using plasmonics

Heterogeneous nanoscale extracellular vesicles (EVs) are of significant interest for disease detection, monitoring, and therapeutics. However, trapping these nano-sized EVs using optical tweezers has been challenging due to their small size. Plasmon-enhanced optical trapping offers a solution. Nevertheless, existing plasmonic tweezers have limited throughput and can take tens of minutes for trapping for low particle concentrations. Here, we present an innovative approach called geometry-induced electrohydrodynamic tweezers (GET) that overcomes these limitations. GET generates multiple electrohydrodynamic potentials, allowing parallel transport and trapping of single EVs within seconds. By integrating nanoscale plasmonic cavities at the center of each GET trap, single EVs can be placed near plasmonic cavities, enabling instant plasmon-enhanced optical trapping upon laser illumination without detrimental heating effects. These non-invasive scalable hybrid nanotweezers open new horizons for high-throughput tether-free plasmon-enhanced single EV trapping and spectroscopy. Other potential areas of impact include nanoplastics characterization, and scalable hybrid integration for quantum photonics.

Over- and Undercoordinated Atoms as a Source of Electron and Hole Traps in Amorphous Silicon Nitride (a-Si3N4).

Silicon nitride films are widely used as the charge storage layer of charge trap flash (CTF) devices due to their high charge trap densities. The nature of the charge trapping sites in these materials responsible for the memory effect in CTF devices is still unclear. Most prominently, the Si dangling bond or K-center has been identified as an amphoteric trap center. Nevertheless, experiments have shown that these dangling bonds only make up a small portion of the total density of electrical active defects, motivating the search for other charge trapping sites. Here, we use a machine-learned force field to create model structures of amorphous Si3N4 by simulating a melt-and-quench procedure with a molecular dynamics algorithm. Subsequently, we employ density functional theory in conjunction with a hybrid functional to investigate the structural properties and electronic states of our model structures. We show that electrons and holes can localize near over- and under-coordinated atoms, thereby introducing defect states in the band gap after structural relaxation. We analyze these trapping sites within a nonradiative multi-phonon model by calculating relaxation energies and thermodynamic charge transition levels. The resulting defect parameters are used to model the potential energy curves of the defect systems in different charge states and to extract the classical energy barrier for charge transfer. The high energy barriers for charge emission compared to the vanishing barriers for charge capture at the defect sites show that intrinsic electron traps can contribute to the memory effect in charge trap flash devices.

Thermal Cycling Induced Aging on Polypropylene/Elastomer Blends for DC Cable Insulation

This paper reports on the ageing phenomenon of the polypropylene (PP) /elastomer blends that are used as DC cable insulation. The elastomers employed are propylene-based elastomer (PBE) and ethylene-octene copolymer elastomer (EOC), which are blended into the PP to form the test samples. The samples go through thermal cycling treatment with temperatures from -30 to 150 °C, after which the trap distribution and the DC breakdown strength are measured. Structural changes in the samples are estimated through Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and differential scanning calorimetry (DSC) measurements. The test results indicate that, with the growth of the thermal cycling number, the trap center decreases whereas the trap density increases, and the DC breakdown strength tends to be reduced. The appearance of oxidation product, loose structure and decreased crystallinity with the thermal cycling ageing should be responsible for the encouragement in charge transport and the reduction in the breakdown strength. The degradation of PP/EOC is more obvious than that of PP/PBE due to its worse compatibility, the PP/PBE exhibits better resistance to the thermal cycling ageing.

Integration of a high finesse cryogenic build-up cavity with an ion trap

We report on the realization of a hemispherical optical cavity with a finesse of F = 13 000 and sustaining inter-cavity powers of 10 kW, which we operate in a closed-cycle cryostat vacuum system close to 4 K. This was designed and built with an integrated radio-frequency Paul trap in order to combine optical and radio-frequency trapping. The cavity provides a power build-up factor of 2300. We describe a number of aspects of the system's design and operation, including low-vibration mounting and locking and thermal effects at high powers. Thermal self-locking in the high intracavity power regime was observed to enhance passive stability below 1 kHz. Observations made over repeated cool-downs over the course of a year show a repeatable shift between the ion trap center and the cavity mode.

The effect of boron co-doping on thermoluminescence and optically stimulated luminescence properties of LiMgPO4:Tb,Sm

The effect of boron co-doping on optically stimulated luminescence (OSL) and thermoluminescence (TL) properties of LiMgPO4:Tb,Sm,B prepared by solid-state reaction was studied. The structure of the prepared materials was confirmed by X-ray diffraction. The TL glow curves of samples irradiated with different doses of β rays were tested. No matter how much B is doped, the peaks at 190 or 280 °C are for a first-order kinetic glow curve. The activation energy and frequency factor remained almost unchanged with the increase of boron doping concentration, which indicated the trap centers of LiMgPO4:Tb,Sm,B were independent of boron doping concentration. The E at 190 and 280 °C are around 1.16 eV and 1.55 eV. Photo-ionization cross-section showed significant changes and the sensitivity of OSL reached a maximum for a concentration of 4 mol% with increasing of boron doping concentration. B-doping enhanced the OSL-signal with a factor 2.6 when B concentration is 4 mol%. The repeatability of the powders for OSL were also studied and the relative standard deviation was less than 1.5%. The minimal detectable dose (MDD) of LiMgPO4:Tb,Sm,B with a boron doping concentration of 4 mol% was 8.2 μGy. This material has a fast decay time, good repeatability, low MDD and has further research value.

High Performance Pd/4H-SiC Epitaxial Schottky Barrier Radiation Detectors for Harsh Environment Applications.

Although many refractory metals have been investigated as the choice of contact metal in 4H-SiC devices, palladium (Pd) as a Schottky barrier contact for 4H-SiC radiation detectors for harsh environment applications has not been investigated adequately. Pd is a refractory metal with high material weight-to-thickness ratio and a work function as high as nickel, one of the conventional metal contacts for high performing 4H-SiC Schottky barrier detectors (SBDs). In this article, Pd/4H-SiC epitaxial SBDs have been demonstrated for the first time as a superior self-biased (0 V applied bias) radiation detector when compared to benchmark Ni/4H-SiC SBDs. The Pd/4H-SiC SBD radiation detectors showed a very high energy resolution of 1.9% and 0.49% under self- and optimized bias, respectively, for 5486 keV alpha particles. The SBDs demonstrated a built-in voltage (Vbi) of 2.03 V and a hole diffusion length (Ld) of 30.8 µm. Such high Vbi and Ld led to an excellent charge collection efficiency of 76% in the self-biased mode. Capacitance mode deep level transient spectroscopy (DLTS) results revealed that the "lifetime-killer" Z1/2 trap centers were present in the 4H-SiC epilayer. Another deep level trap was located at 1.09 eV below the conduction band minimum and resembles the EH5 trap with a concentration of 1.98 × 1011 cm-3 and capture cross-section 1.7 × 10-17 cm-2; however, the detector performance was found to be limited by charge trapping in the Z1/2 center. The results presented in this article revealed the unexplored potential of a wide bandgap semiconductor, SiC, as high-efficiency self-biased radiation detectors. Such high performance self-biased radiation detectors are poised to address the longstanding problem of designing self-powered sensor devices for harsh environment applications e.g., advanced nuclear reactors and deep space missions.

Influence of White Light on the Photoelectric Characteristics of UV Detectors Based on β-Ga2O3

In this report, we consider the effect of radiation with a radiation energy below the β-Ga2O3 band-gap on the photoelectric characteristics of ultraviolet (UV) radiation detectors based on gallium oxide thin films. The gallium oxide film was deposited by the radio frequency magnetron sputtering. Interdigital Ti/V electrodes were formed with an inter electrode distance d = 30 μm. The structures exhibit high value of photocurrent under irradiation with wavelength of λ = 254 nm and intensity of 780 μW/cm2. The detectors exhibit persistent photoconductivity after exposure to UV radiation. The presence of persistent conductivity is explained by the high concentration of traps in the Ga2O3 film and their recharging under UV irradiation. Radiation with an energy quantum below the the β-Ga2O3 band-gap changes the charge state of trap centers in a gallium oxide film. This effect leads to an increase in photocurrent under UV exposure and increases the stability of detectors.