Hole Trapping Centers Research Articles

Kinetic simulation of the optical absorption dose response of LiF:Mg,Ti (TLD-100) incorporating spatially correlated electron and hole trapping centers

A conduction band/valence band (CB/VB) model incorporating a spatially-correlated trapping center/luminescent center (TC/LC) complex is described which is associated with composite glow peak 5 in the thermoluminescence (TL) glow curve of LiF:Mg,Ti (TLD-100). The evidence for such a complex observed in the TL characteristics of TLD-100 is reviewed. Additional experimental evidence from the optical absorption (OA) energy spectrum and dose response of the individual bands has recently been reported in which the 4.0eV band is interpreted to arise from two sub-bands at 3.84eV and 4.3eV. The 3.84eV band demonstrates the commonly observed linear/exponentially saturating dose response and is interpreted to arise from the e-only TC which is not spatially correlated with an LC. The 4.3eV band is interpreted to arise from two sub-bands which are unresolved in the OA spectrum but which show two very different dose-response characteristics. The first arising from the TC/LC which has captured only an electron (resulting again in a linear/exponentially saturating dose response) and the other from an electron-hole captured configuration which continues to increase in concentration up to the highest levels of dose investigated. The CB/VB model is used to simulate the kinetics of charge carrier transport and capture leading to the dose response of all the observed OA bands.

Bi quantum dots on rutile TiO2 as hole trapping centers for efficient photocatalytic bromate reduction under visible light illumination

Bi quantum dots were deposited onto rutile TiO2 nanoparticles by a one-pot, solvent-thermal process to create the Bi/TiO2 (rutile) heterojunction photocatalyst. Due to the specific semimetal property of Bi, a metal to semiconductor transition occurred for Bi quantum dots, which endowed them with the hole trapping capability to enhance the charge carrier separation in rutile TiO2 and eliminate the need of sacrificial agents for the consumption of photogenerated holes in photocatalytic reduction process. The Bi/TiO2 (rutile) heterojunction photocatalyst demonstrated an efficient photocatalytic bromate reduction under visible light illumination without the addition of sacrificial agents in the reaction solution, and it could be easily regenerated for reuse. Different with previously reported noble/transition metal modifications as the electron trapping center, this study demonstrated a novel material design strategy of the introduction of hole trapping centers to create photocatalysts with strong photocatalytic reduction capabilities, which could be readily adopted for a broad range of technical applications.

Adsorption behavior of formaldehyde on ZnO [formula omitted] surface: A first principles study

In a first principles study of the formaldehyde adsorption on ZnO surface, we found a novel chain adsorption structure on ZnO (101¯0) plane. This adsorption structure results from the electrostatic interactions between those adsorbed formaldehyde molecules and the unique arrangement of Zn-O surface dimers on (101¯0) plane. This adsorption mechanism has the potential to extend to other wurtzite materials’ (101¯0) plane and other similar cases. As the physical adsorption configurations are unstable, the chemical adsorption has to happen. The electronic properties show that the CO double bond in CH2O turns into CO single bond and the highest occupied molecule orbital (HOMO) of formaldehyde is lifted into ZnO band gap becoming the hole trapping center. These results may be meaningful for formaldehyde degradation and detection.

Solution processed bilayer junction of silk fibroin and semiconductor quantum dots as multilevel memristor devices

Silkworm protein is integrated with semiconductor quantum dots (CdSe) to form bi-layer electrical memory devices. These films are individually spin coated on ITO surface to form ITO-silk fibroin:CdSe-metal junctions. These materials are characterized by DSC, TGA, FTIR, UV–visible absorbance, X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) and TEM to understand their physical properties. The charge transport is carried out on the above devices by cyclic voltage scan in the sequence zero-positive-zero-negative-zero to study their electrical memory switching behavior. Devices exhibit multilevel electrical switching both in the forward and in the reverse regions that are quite symmetric, the ON/OFF ratios are well resolved. These junctions posses a high repeatability with a good endurance and hence can be important interfaces for electrical information storage applications. The observed results of the above memristors are explained in terms of their energy band diagram, it is observed that in these interfaces monopolar charge transport of holes is the dominant mechanism that is further supported by multiple hole trapping centers both in CdSe and silk fibroin energy gaps.

Charge Carrier Trapping Processes in RE2O2S (RE = La, Gd, Y, and Lu).

Two different charge carrier trapping processes have been investigated in RE2O2S:Ln3+ (RE = La, Gd, Y, and Lu; Ln = Ce, Pr, and Tb) and RE2O2S:M (M = Ti4+ and Eu3+). Cerium, praseodymium and terbium act as recombination centers and hole trapping centers while host intrinsic defects provide the electron trap. The captured electrons released from the intrinsic defects recombine at Ce4+, Pr4+, or Tb4+ via the conduction band. On the other hand, Ti4+ and Eu3+ act as recombination centers and electron trapping centers while host intrinsic defects act as hole trapping centers. For these codopants we find evidence that recombination is by means of hole release instead of electron release. The released holes recombine with the trapped electrons on Ti3+ or Eu2+ and yield broad Ti4+ yellow-red charge transfer (CT) emission or characteristic Eu3+ 4f–4f emission. We will conclude that the afterglow in Y2O2S:Ti4+, Eu3+ is due to hole release instead of more common electron release.

Kinetic simulations of optical absorption in LiF:Mg,Ti (TLD-100) following irradiation by 137Cs gamma rays and bleaching at photon energies of 3.1 eV, 4.0 eV and 5.08 eV

The kinetic simulation of optical absorption (OA) dose response of LiF:Mg,Ti (TLD-100) following 137Cs gamma irradiation and the effect of bleaching at photon energies of 3.1eV, 4eV and 5.08eV on the OA energy spectrum is discussed. The simulations are limited to charge carrier migration in the conduction band and the valence bands leading to electron/hole capture in the various electron and hole trapping centers. An exhaustive search for identical values of the parameters of the model which result in agreement with certain combinations of the sets of experimental data has proven difficult. The reasons most susceptible to further investigation are, in the opinion of the authors, the incorporation of localized capture and recombination in the kinetic simulations which take place in multi-defect and spatially correlated complexes in parallel with the participation of delocalized charge carrier migration in the conduction and valence bands. Other possibilities, however, cannot currently be ruled out and should also be the subject of future investigations.

Intrinsic luminescence centers in γ- and θ-alumina nanoparticles

In this study, we investigate the photoluminescence (PL) properties of γ and θ-alumina nanoparticles synthesized by the chemical wet method followed by annealing. The obtained experimental results indicate the presence of some favorable near ultraviolet (NUV)-orange luminescent centers for usage in various luminescence applications, such as oxygen vacancies (F, , , and F2 centers), OH related defects, cation interstitial centers, and some new luminescence bands attributed to trapped-hole centers or donor–acceptor centers. The energy states of each defect are discussed in detail. The defects mentioned could alter the electronic structure by producing some energy states in the band gap that result in the optical absorption in the middle ultraviolet (MUV) region. Spectra show that photoionazation of F and F2 centers plays a crucial role in providing either free electrons for the conduction band, or the photoconversions of aggregated oxygen vacancies into each other, or mobile electrons for electrons-holes recombination process by the Shockley–Read–Hall (SRH) mechanism.

Suppression mechanism of radiation-induced darkening by Ce doping in Al/Yb/Ce-doped silica glasses: Evidence from optical spectroscopy, EPR and XPS analyses

Yb3+/Al3+ co-doped silica glasses with different Ce2O3 contents were prepared using the sol–gel method combined with high-temperature sintering. Changes in refractive index, absorption, emission and fluorescence lifetime of these glasses caused by X-ray irradiation were recorded and analyzed systematically. It is found that co-doping with certain amount of Ce could greatly improve the radiation resistance without evident negative effects on the basic optical properties of the Yb3+ ions in the near-infrared region. The nature of the radiation-induced color centres and the mechanism by which Ce prevented the formation of these centres were studied using optical absorption, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) methods. Direct evidence confirmed that trapped electron centres (Yb2+/Si-E′/Al-E′) and trapped hole centres (Al-OHCs) were effectively inhibited by Ce doping, which was correlated to the coexistence of the redox couple Ce3+/Ce4+ in the glasses. These results are helpful to understand the micro-structural origin and the suppression mechanism by Ce co-doping of the photodarkening effect in Yb3+-doped silica fibers.

Application of CdSe–PVK Nanocomposite as a Hole Transport Layer for OLEDs

An organic light‐emitting diode was fabricated using cadmium selenide (CdSe)/poly(N‐vinylcarbazole) nanocomposite as the hole transport layer (HTL). The CdSe nanoparticles (NPs) with a mean crystallite size of 6.2 nm were prepared by high‐energy ball milling. Based on the current–voltage curves, the threshold voltage (V th) of the composite diode was found to be ~1.3 ± 0.1 V lower than that of the diode without CdSe, with a significant increase in the current density for the composite diode. Moreover, the electroluminescence (EL) properties (luminous flux, emittance, and intensity) of the diode were found to be enhanced by ~16% with respect to those of the diode without CdSe. The decrease of the threshold voltage and the increase of the current density and the EL were due to the CdSe NPs that operate as hole trap centers in the HTL.

Environmental Effects on Hysteresis of Transfer Characteristics in Molybdenum Disulfide Field-Effect Transistors.

Molybdenum disulfide (MoS2) has recently received much attention for nanoscale electronic and photonic applications. To explore the intrinsic properties and enhance the performance of MoS2-based field-effect transistors, thorough understanding of extrinsic effects such as environmental gas and contact resistance of the electrodes is required. Here, we report the effects of environmental gases on the transport properties of back-gated multilayered MoS2 field-effect transistors. Comparisons between different gases (oxygen, nitrogen, and air and nitrogen with varying relative humidities) revealed that water molecules acting as charge-trapping centers are the main cause of hysteresis in the transfer characteristics. While the hysteresis persisted even after pumping out the environmental gas for longer than 10 h at room temperature, it disappeared when the device was cooled to 240 K, suggesting a considerable increase in the time constant of the charge trapping/detrapping at these modestly low temperatures. The suppression of the hysteresis or instability in the easily attainable temperature range without surface passivation is highly advantageous for the device application of this system. The humidity dependence of the threshold voltages in the transfer curves indicates that the water molecules dominantly act as hole-trapping centers. A strong dependence of the on-state current on oxygen pressure was also observed.

Formation of silver nanoparticles in silicate glass using excimer laser radiation: Structural characterization by HRTEM, XRD, EXAFS and optical absorption spectra

Plasmonic silver nanostructures in surfaces of soda-lime silicate glasses were generated using Ag+ ↔ Na+ ion exchange and UV laser irradiation (ArF laser, 193 nm) with different number of ns laser pulses (from 2 to 5000). To identify the correlations between the optical properties (surface plasmon resonance (SPR) parameters) and atomic structure of silver nanoparticles and their agglomerations, characterization of the samples was performed by HRTEM, XRD, optical absorption in visible range and Ag K-edge EXAFS spectra. Analysis of the optical spectra was performed using a Mie theory approach, accounting for the most plausible defect centers in silicate glass like hole trap centers and non-bridging oxygen hole centers. Processing of Ag K-edge EXAFS yielded values of Ag-Ag and Ag-O interactions averaged over ionic and neutral states of silver. The consistent treatment of HRTEM and XRD data, the behavior of features in optical spectra and the obtained dependence of Ag-Ag and Ag-O structural parameters upon the number of laser pulses enabled to suggest a mechanism of plasmonic Ag nanoparticles formation in silicate glass under UV laser irradiation.

Controlled Electron–Hole Trapping and Detrapping Process in GdAlO3 by Valence Band Engineering

Two different trapping and detrapping processes of charge carriers have been investigated in GdAlO3:Ce3+,Ln3+ (Ln = Pr, Er, Nd, Ho, Dy, Tm, Eu, and Yb) and GdAlO3:Ln3+,RE3+ (Ln = Sm, Eu, and Yb; RE = Ce, Pr, and Tb). Cerium is the recombination center and lanthanide codopants act as electron-trapping centers in GdAlO3:Ce3+,Ln3+. Different lanthanide codopants generate different trap depths. The captured electrons released from the lanthanide recombine at cerium via the conduction band, eventually producing the broad 5d–4f emission centered at ∼360 nm from Ce3+. On the other hand, Sm3+, Eu3+, and Yb3+ act as recombination centers, while Ce3+, Pr3+, and Tb3+ act as hole-trapping centers in GdAlO3: Ln3+,RE3+. In this situation, we find evidence that recombination is by means of hole release instead of the more commonly reported electron release. The trapped holes are released from Pr4+ or Tb4+ and recombine with the trapped electrons on Sm2+, Eu2+, or Yb2+ and yield characteristic trivalent emission from Sm3...

Role of surface water molecules in stabilizing trapped hole centres in titanium dioxide (anatase) as monitored by electron paramagnetic resonance

Abstract A key factor affecting the photo-efficiency of TiO2 is strictly related to the fate of charge carriers, electrons (e−) and holes (h+), generated upon band gap excitation. In the present paper we point our attention to the nature of the hole trapping sites in the anatase polymorph monitored coupling the conventional continuous wave EPR (CW-EPR) technique with pulse electron nuclear double resonance (ENDOR) experiments. The attention is focused on the role of surface adsorbed water (both in molecular and in dissociated form) in the stabilization of photogenerated hole centres. CW-EPR results indicate that two distinct O − hole centres can be identified in Anatase (O −surf. and O −subsurf.) and that the quantitative ratio (measured in terms of spectral intensity) of these two species is markedly conditioned by the presence of surface physisorbed water. For the first time a h+-proton distance, evaluated via ENDOR measurement, is reported.

Persistent Luminescence Hole-Type Materials by Design: Transition-Metal-Doped Carbon Allotrope and Carbides.

Electron traps play a crucial role in a wide variety of compounds of persistent luminescence (PL) materials. However, little attention has been placed on the hole-trap-type PL materials. In this study, a novel hole-dominated persistent luminescence (PL) mechanism is predicted. The mechanism is validated in the night pearl diamond (NPD) composed of lonsdaleite with ultralong persistent luminescence (PL) (more than 72 h). The computed band structures suggest that the Fe ion dopant in lonsdaleite is responsible for the luminescence of NPD due to the desired defect levels within the band gap for electronic transition. Other possible impurity defects in lonsdaleite, such as K, Ca, Mg, Zn, or Tl dopants, or C vacancy can also serve as the hole-trap centers to enhance the PL. Among other 3d transition-metal-ion dopants considered, Cr and Mn ions are predicted to give rise to PL property. The predicted PL mechanism via transition-metal doping of lonsdaleite offers an exciting opportunity for engineering new PL materials by design.

Probing on green long persistent Eu2+/Dy3+ doped Sr3SiAl4O11 emerging phosphor for security applications

Herein, a novel green emitting long-persistent Sr3SiAl4O11:Eu2+/Dy3+ phosphor was synthesized in a single phase form using facile solid state reaction method under the reducing atmosphere of 10% H2 and 90% N2. The resulting phosphor exhibits hyper-sensitive strong broad green emission, peaking at 510 nm upon 340 nm excitation wavelength, which is attributed to the 4f65d1-4f7 transitions of emission center of europium (Eu2+) ions. Moreover, the incorporation of dysprosium (Dy3+) ions, which act as effective hole trap centers with appropriate depth, largely enhances the photoluminescence characteristics and greatly improves the persistent intense luminescence behavior of Sr3SiAl4O11:Eu2+/Dy3+ phosphor under ultraviolet (UV) excitation. In addition, with the optimum doping concentration and sufficient UV excitation time period, the as-synthesized phosphor can be persisted afterglow for time duration ∼4 h with maximum luminescence intensity. Thus, these results suggest that this phosphor could be expected as an ultimate choice for next generation advanced luminescent materials in security applications such as latent finger-marks detection, photo-masking induced phosphorescent images, and security code detection.

Channel length influenced contribution of hole and electron trapping effect to threshold voltage stability in organic field effect transistors

Effect of channel length on hysteresis and threshold voltage shift in copper phthalocyanine (CuPc) based organic field effect transistors was studied. Contrary to expectation, longer channel length devices exhibited minimum threshold voltage shift. Influence of channel length on the contribution of hole and electron trapping to threshold voltage stability was determined. Shortest channel length devices exhibited highest electron trapping effect while longest channel devices exhibited minimum hole as well as electron trapping. Lower hole trap effect for longer channel length devices was suggested to be due to reduced longitudinal field between source and drain electrodes while minimum electron trapping was attributed to suppression of drain current by increased hole trap centres.

Electrolytic coloration below 0 °C and spectral properties of ammonium alum crystals using a pointed anode

Ammonium alum crystals are colored electrolytically below 0°C and under various voltages using a pointed anode and a flat cathode. SO2−, O3−, O2−, O− hole-trapped centers and O0, O2−, H+ radicals are produced in colored ammonium alum crystals. The absorption bands of the SO2− and O3− hole-trapped centers are observed in the absorption spectra of the colored ammonium alum crystals. The hole-trapped centers and radicals come mainly from the electric-field-induced decomposition of the SO42− radicals and crystalline water molecules. Current is measured during electrolytic coloration. Electron exchanges from electrons and small charged radicals to electrodes induce complete current.

Identification of defects responsible for optically stimulated luminescence (OSL) from copper-diffused LiAlO2 crystals

A large optically stimulated luminescence (CW-OSL) response has been observed from bulk lithium aluminate (LiAlO2) crystals that have been copper-diffused. After introducing the copper, an optical absorption band peaking at 277nm is present and is assigned to Cu+ (3d10) ions at Li+ sites. Photoluminescence (PL and PLE) from copper-diffused samples shows an emission band near 359nm and an excitation band near 273nm. These PL bands are also assigned to the Cu+ ions. After an irradiation at room temperature with x-rays, the copper-diffused crystals give an intense OSL signal when stimulated with light in the 425–500nm region. The OSL emitted light has a peak near 357nm and thus is directly associated with recombination at a Cu ion. Electron paramagnetic resonance (EPR) shows that the x-rays produce two distinct Cu2+ (3d9) centers, both at Li+ sites. One of the trapped-hole centers is a Cu2+ with no nearby defects and the other is a Cu2+ with a neighboring Li+ vacancy. Monitoring the EPR spectra before and after OSL shows that only the isolated Cu2+ ions (i.e., those with no nearby defect) participate in the OSL process. The electron traps participating in the OSL are shown with EPR to be transition-metal ions that are unintentionally present. Specifically, Fe2+ ions replacing Li+ ions, possibly with a nearby lithium vacancy, convert to Fe+ (3d7) ions when they trap an electron during irradiation. These electrons are optically released by the OSL exciting light and migrate to the holes trapped as isolated Cu2+ ions, where electron–hole recombination gives the characteristic Cu+ emission. By combining OSL, PL, and EPR, we establish the general OSL mechanism in copper-diffused LiAlO2 crystals.

Low temperature thermally stimulated current measurements in N-implanted TlGaSeS layered single crystals

As-grown TlGaSeS crystals have been implanted by a ion implantation technique. The samples were bombarded at room temperature in the direction perpendicular to the layer by nitrogen ion beam of about 120keV having dose of 1×1016ions/cm2. The effect of N implantation and annealing at 300°C was studied by using thermally stimulated current measurements. The measurements were performed in temperatures ranging from 10K to 250K. The experimental evidences were found for the presence of one shallow electron trapping center and one deep hole trapping center with activation energies of 6meV and 796meV, respectively. The capture cross-sections (3.0×10−24cm2 and 2.3×10−15cm2) and concentrations (5.6×1011cm−3 and 2.1×1013cm−3) were determined for the electron and hole trapping centers, respectively.

Properties of solid solutions, doped film, and nanocomposite structures based on zinc oxide

A study of the properties of materials based on the wide bandgap zinc oxide semiconductor, which are promising for application in optoelectronics, photovoltaics and nanoplasmonics. The structural and optical properties of solid solution Zn1−xCdxO films with different cadmium content, are studied. The samples are grown using magnetron sputtering on sapphire backing. Low-temperature photoluminescence spectra revealed emission peaks associated with radiative recombination processes in those areas of the film that have varying amounts of cadmium. X-ray phase analysis showed the presence of a cadmium oxide cubic phase in these films. Theoretical studies of the solid solution thermodynamic properties allowed for a qualitative interpretation of the observed experimental phenomena. It is established that the growth of the homogeneous solid solution film is possible only at high temperatures, whereas regions of inhomogeneous composition can be narrowed through elastic deformation, caused by the mismatch of the film-backing lattice constants. The driving forces of the spinodal decomposition of the Zn1−xCdxO system are identified. Fullerene-like clusters of Znn−xCdxOn are used to calculate the bandgap and the cohesive energy of ZnCdO solid solutions. The properties of transparent conductive ZnO films, doped with Group III donor impurities (Al, Ga, In), are examined. It is shown that oxygen vacancies are responsible for the hole trap centers in the zinc oxide photoconductivity process. We also examine the photoluminescence properties of metal-ZnO nanocomposite structures, caused by surface plasmons.