Hole Trapping Centers Research Articles

Electrolytic coloration below 100 °C and spectral properties of potassium alum crystals

Potassium alum crystals are colored electrolytically below 100°C and under various voltages using a pointed cathode and a flat anode. SO3−, SO2−, O3−, O2−, O− hole-trapped centers and O0, O2−, H+ radicals are produced in colored potassium alum crystals. No obvious characteristic absorption band in ultraviolet and visible wavelength regions is observed in absorption spectrum of uncolored potassium alum crystal. Characteristic absorption bands of SO3−, SO2− and O3− hole-trapped centers are observed in the absorption spectra of the colored potassium alum crystals. The hole-trapped centers and radicals come from electric- and thermal-induced decomposition of SO42− radicals and crystalline water molecules. Current–time curve for electrolytic coloration of potassium alum crystal is given. Electron exchanges from electrons and small charged radicals to electrodes induce complete current.

Radiation-induced defects in LiAlO2 crystals: Holes trapped by lithium vacancies and their role in thermoluminescence

Electron paramagnetic resonance (EPR) is used to identify the primary hole trap in undoped lithium aluminate (LiAlO2) crystals. Our interest in this material arises because it is a candidate for radiation detection applications involving either optically stimulated luminescence (OSL) or thermoluminescence (TL). During an x-ray irradiation at room temperature, holes are trapped at oxygen ions adjacent to lithium vacancies. Large concentrations of these lithium vacancies are introduced into the crystal during growth. With the magnetic field along the [001] direction, the EPR spectrum from these trapped-hole centers consists of eleven lines, evenly spaced but with varying intensities, caused by nearly equal hyperfine interactions with two 27Al nuclei (I=5/2, 100% abundant). The g matrix is determined from the angular dependence of the EPR spectrum and has principal values of 2.0130, 2.0675, and 2.0015. These g shifts strongly support the model of a hole in a p orbital on an oxygen ion. The adjacent lithium vacancy stabilizes the hole on the oxygen ion. A sequence of pulsed thermal anneals above room temperature shows that the EPR spectrum from the holes trapped adjacent to the lithium vacancies disappears in the 90–120°C range. The thermal decay of these hole centers directly correlates with an intense TL peak near 105°C. Signals at lower magnetic field in the 9.4GHz EPR spectra suggest that the electron trap associated with this TL peak at 105°C may be a transition-metal-ion impurity, most likely Fe, located at a cation site. Additional less intense TL peaks are observed near 138, 176, and 278°C.

Radiation induced color centers in cerium-doped and cerium-free multicomponent silicate glasses

The effect of doped cerium on the radiation-resistance behavior of silicate glass was investigated in our work. The ultraviolet-visible absorption spectra and electron paramagnetic resonance (EPR) spectra were obtained after the cerium-rich and cerium-free multicomponent silicate glasses (K509 and K9) were irradiated by gamma rays with a dose range from 10 to 1000 kGy. The results showed that E' center, oxygen deficient center (ODC) and non-bridging oxygen hole center (HC1 and HC2) were induced in K9 and K509 glasses after radiation. The concentrations of all color centers presented an exponential growth with the increase of the gamma dose. Moreover, the concentration of HC1 and HC2 in cerium-doped K509 glass was much lower than that in cerium-free K9 glass at the same dose of radiation, which could be attributed to the following mechanism: Ce3+ ions capturing holes then forming Ce3++ centers inhibited the formation of hole trapped color centers (HC1 and HC2) and Ce4+ ions capturing electrons to form Ce3+ centers suppressed the formation of electron trapped color centers like E' center.

Simulation of sublinear dose dependence of thermoluminescence with the inclusion of the competitive interaction of trapping centers

The sublinearity of dose characteristics of the thermoluminescence has been calculated in terms of the model of competitive interaction of electron and hole trapping centers. It has been found that the sub-linearity is caused by the nonradiative electron recombination with hole deep traps during irradiation and recording of the thermoluminescence upon linear heating of a dosimetric phosphor. It has been shown that the proposed sublinearity mechanism can be used to explain the dose characteristics of the thermoluminescence of anion-defective aluminum oxide.

Comprehensive Studies on the Carrier Transporting Property and Photo-Bias Instability of Sputtered Zinc Tin Oxide Thin Film Transistors

This study examined the effects of the chamber pressure, radio frequency power and oxygen flow ratio during channel deposition on the performance and photobias stability of zinc tin oxide (ZTO) thin film transistors (TFTs). The densification of the ZTO thin film allowed the improvement in the field-effect mobility and the suppression in the negative bias illumination stress (NBIS) instability of the resulting TFTs simultaneously, irrespective of the specific process condition. The porosity in the ZTO channel layer was shown to prevent the effective intercalation of the Sn 5s orbital and, thus, deteriorate the field-effect mobility. Furthermore, the increased effective surface area in the porous ZTO film adversely affected the NBIS stability of the resulting TFTs because the porosity-related surface states and oxygen vacancy defects provide the hole trapping centers and the delocalized electron free carrier, respectively. Therefore, the densification of ZTO channel layer is a key factor for the high mobility and good photobias stability of the TFTs. This concept can be applicable for any metal-oxide-TFTs.

Electron paramagnetic resonance and thermoluminescence study of Ag2+ ions in Li2B4O7 crystals

Electron paramagnetic resonance (EPR) is used to investigate the effects of ionizing radiation on Ag-doped lithium tetraborate (Li2B4O7) crystals. Two similar, yet distinct, trapped-hole centers (Ag2+ ions substituting for Li+ ions) are produced by 60kV x rays. One Ag2+ ion, labeled Center A, has no nearby defects and the other Ag2+ ion, labeled Center B, has a neighboring impurity which is most likely a Ag+ ion substituting for a Li+ ion. The production and thermal decay properties of the two Ag2+ ions are described and their g matrices and 107Ag and 109Ag hyperfine matrices are obtained from the EPR angular dependences. The principal values of the g matrices are similar for the two centers, but the hyperfine principal values differ significantly (Center B has smaller values than Center A). There are also differences in the directions of the principal axes for the two centers. Together, these results imply (1) that the unpaired spin is less localized for Center B and (2) that the ground-state positions of the neighboring oxygen ions are different for Centers A and B. This explains why the peaks of the Ag2+ charge-transfer photoluminescence bands associated with Centers A and B occur at different wavelengths (502 and 725nm, respectively). An isochronal pulsed thermal anneal shows that these radiation-induced Ag2+ ions serve as the recombination site for the intense thermoluminescence peak observed near 152°C.

Enhanced photoactivity with nanocluster-grafted titanium dioxide photocatalysts.

Titanium dioxide (TiO2), as an excellent photocatalyst, has been intensively investigated and widely used in environmental purification. However, the wide band gap of TiO2 and rapid recombination of photogenerated charge carriers significantly limit its overall photocatalytic efficiency. Here, efficient visible-light-active photocatalysts were developed on the basis of TiO2 modified with two ubiquitous nanoclusters. In this photocatalytic system, amorphous Ti(IV) oxide nanoclusters were demonstrated to act as hole-trapping centers on the surface of TiO2 to efficiently oxidize organic contaminants, while amorphous Fe(III) or Cu(II) oxide nanoclusters mediate the reduction of oxygen molecules. Ti(IV) and Fe(III) nanoclusters-modified TiO2 exhibited the highest quantum efficiency (QE = 92.2%) and reaction rate (0.69 μmol/h) for 2-propanol decomposition among previously reported photocatalysts, even under visible-light irradiation (420-530 nm). The desirable properties of efficient photocatalytic performance with high stability under visible light with safe and ubiquitous elements composition enable these catalysts feasible for large-scale practical applications.

Radiation-induced defects in manganese-doped lithium tetraborate phosphor.

Lithium tetraborate doped with manganese synthesised by solid-state sintering technique exhibits a dosimetric peak at 280°C. The high-temperature glow curve results in no fading for three months. The sensitivity of Li2B4O7:Mn is determined to be 0.9 times that of TLD-100. The infrared spectrum of this phosphor indicates the presence of bond vibrations corresponding to BO4 tetrahedral and BO3 triangles. The mechanism for thermoluminescence in this phosphor was proposed based on the thermoluminescence (TL) emission spectra, kinetic analysis of TL glow curves and electron paramagnetic resonance (EPR) measurements on non-irradiated and gamma-irradiated phosphors. It was identified that oxygen vacancies and Boron oxygen hole centre (BOHC) are the electron and hole trap centres for TL in this phosphor. When the phosphor is heated, the electrons are released from the electron trap and recombine with the trapped holes. The excitation energy during the recombination is transferred to the nearby Mn(2+) ions, which emit light at 580 nm.

Photoluminescence from Ag2+ ions in lithium tetraborate (Li2B4O7) crystals

Two broad photoluminescence (PL) bands peaking near 502 and 725nm are observed at room temperature in Ag-doped Li2B4O7 crystals pre-irradiated at room temperature with X-rays. Their respective excitation bands peak near 297 and 325nm. The requirement that the crystal be pre-irradiated establishes that these emissions are not related to Ag+ ions. Electron paramagnetic resonance (EPR) helps assign models to the PL bands. EPR spectra show that two substitutional Ag2+ trapped-hole centers and one interstitial Ag0 trapped-electron center are produced at room temperature by the X-rays. A majority of the Ag2+ ions have no neighboring defects (and are referred to as Center A) while the remaining Ag2+ ions have a nearby defect (and are referred to as Center B). The PL bands are assigned to these two Ag2+ ions (the 502nm emission to Center A and the 725nm emission to Center B). A charge-transfer mechanism is responsible for the observed luminescence, i.e., an electron from a neighboring oxygen ion, upon excitation, moves to the Ag2+ ion and produces a Ag+ ion with an adjacent O− ion. This excited-state complex quickly decays radiatively and the original Ag2+ and O2− ions are restored. Optical absorption related to Ag2+ and Ag0 defects are observed in the ultraviolet and visible after the Ag-doped crystals are X-ray irradiated at room temperature.

Coloration of Ce-doped Multicomponent Silicate Glasses by Electron Irradiation

The coloration of Ce-doped multicomponent silicate glass (type K509) irradiated by 10 MeV electrons was investigated by absorption spectra, electron paramagnetic resonance and photoluminescence spectra. The results show that non-bridging oxygen hole centers (HC1 and HC2) are induced in K509 glasses after electron irradiation, leading to significant degradation of visible transmission. The concentration of color center grows in an exponential law with the increase of radiation dose at the same dose rate, and decreases in exponential decay law with radiation dose rate at the same total dose. According to photoluminescence spectra, the concentration of Ce ions has a negative correlation with radiation dose, and has a positive correlation with dose rate, which proves the mechanism of radiation-hardness in cerium-doped glasses. Ce ions capturing holes induced by electron irradiation to form Ce inhibits the formation of hole trapped color centers, HC1 and HC2, which avoids additional absorption in visible light range. The energy level diagram of Ce in K509 glasses is also obtained by Gaussian resolution of the broad asymmetric emission spectra of Ce.

Correlated defects and the transformation of energy in irradiated oxyanioniс crystals

Using methods of thermally stimulated luminescence and vacuum-ultraviolet spectroscopy the nature of the recombination luminescence which arises in the correlated electron–hole trapping centers was investigated. It is shown that the nature of a broad emission band in the irradiated LiKSO4, Li2SO4 and Na2SO4 with several maxima is due to the existence of correlated defects of different energy states, corresponding to radicals located in different crystallographic directions.

Trapping Center Parameters in N-Implanted Tl2Ga2S3Se Single Crystals by Thermally Stimulated Currents Measurements

As-grown Tl2Ga2S3Se crystals have been doped by ion implantation technique. The samples were bombarded at room temperature in the direction perpendicular to the layer by N ion beam of about 120 keV having dose of 1 × 10 ions/cm. The e ect of N implantation with annealing at 300 ◦C was studied by using thermally stimulated current measurements. The investigations were performed in temperatures ranging from 10 to 290 K. The experimental evidence was found for presence of one deep hole trapping center with activation energy of 392 meV. The capture cross-section was calculated as 3.9 × 10−20 cm. Also the concentration of the traps was estimated to be 8.0× 10 cm−3.

Persistent luminescence in Bi3+ doped CaWO4 matrix

A series of persistent phosphors CaWO4:Bi3+ were prepared via a traditional solid state method at 1000 °C in air atmosphere. The photoluminescence (PL) and long afterglow (LAG) properties were investigated systematically. The optimal Bi3+ ions concentration was experimentally determined to be 0.15 mol% and the performance of afterglow was greatly improved with the incorporation of equivalent amount of Li+ ions. Both WO42− and Bi3+ ions can act as luminescent centers. The emission of blue long afterglow is observed. The origin of this phenomenon is confirmed to the transition from 3P1 to 1S0 in Bi3+ ions. Defects BiCa· serve as electron traps and Ca2+ vacancies mainly act as new hole trapping centers. In order to study the trapping process and explain the mechanism of long afterglow, a schematic model is also proposed and discussed in detail.

EPR identification of defects responsible for thermoluminescence in Cu-doped lithium tetraborate (Li2B4O7) crystals

Electron paramagnetic resonance (EPR) is used to identify the electron and hole traps responsible for thermoluminescence (TL) peaks occurring near 100 and 200°C in copper-doped lithium tetraborate (Li2B4O7) crystals. As-grown crystals have Cu+ and Cu2+ ions substituting for lithium and have Cu+ ions at interstitial sites. All of the substitutional Cu2+ ions in the as-grown crystals have an adjacent lithium vacancy and give rise to a distinct EPR spectrum. Exposure to ionizing radiation at room temperature produces a second and different Cu2+ EPR spectrum when a hole is trapped by substitutional Cu+ ions that have no nearby defects. These two Cu2+ trapped-hole centers are referred to as Cu2+-VLi and Cuactive2+, respectively. Also during the irradiation, two trapped-electron centers in the form of interstitial Cu0 atoms are produced when interstitial Cu+ ions trap electrons. They are observed with EPR and are labeled CuA0 and CuB0. When an irradiated crystal is warmed from 25 to 150°C, the Cuactive2+ centers have a partial decay step that correlates with the TL peak near 100°C. The concentrations of CuA0 and CuB0 centers, however, increase as the crystal is heated through this range. As the crystal is further warmed between 150 and 250°C, the EPR signals from the Cuactive2+ hole centers and CuA0 and CuB0 electron centers decay simultaneously. This decay step correlates with the intense TL peak near 200°C.

Hole traps and thermoluminescence in Li2B4O7:Be

A trapped-hole centre with a high thermal stability is studied in Li2B4O7:Be. EPR spectrum demonstrates a hyperfine splitting due to interaction of the trapped hole with a 9Be nucleus. The hole is trapped at the common vertex (bridging oxygen) of two adjacent BO4 tetrahedra when Be2+ substitutes for B3+ in one of them. The centre is stable up to ≈500 K. The high-temperature thermoluminescence peak at 510–575 K occurs when the released holes recombine with trapped electrons. This peak shows an ultraviolet exciton-like luminescence band and also luminescence bands of casual impurities.

Role of manganese in red long-lasting phosphorescence of manganese-doped diopside for in vivo imaging

Materials with red long-lasting phosphorescence, such as MnII-doped diopsides, can be used for small animal in vivo imaging. CaMgSi2O6:Mn powders with various amounts of Mn were prepared by sol–gel to investigate their long-lasting phosphorescence mechanism. X-ray diffraction, X-ray absorption fine and near-edge structure and electron paramagnetic resonance showed that manganese is quantitatively introduced in the structure as MnII. Most of the Mn doping ions substitute Mg and possess a highly elongated octahedral environment. While photoluminescence and X-ray excited optical luminescence spectra show both orange (585nm) and red (685nm) 4T1 (4G)→6A1 (6S) emission of MnIICa and MnIIMg, respectively, MnIIMg red emission dominates long-lasting phosphorescence and thermally stimulated luminescence spectra. These results point to MnIIMg as the preferential hole trap and recombination center in the long-lasting phosphorescence mechanism. An intense persistent red emission suitable for in vivo imaging probes is obtained for the highest nominal Mn content (7.5%).

Pt and Au/TiO2 photocatalysts for methanol reforming: Role of metal nanoparticles in tuning charge trapping properties and photoefficiency

Metal-loaded TiO2 is, by far, one of the most important class of photocatalysts in hydrogen production through photoreforming of organics and water photosplitting. In this study anatase loaded with Au and Pt nanoparticles (Au/TiO2 and Pt/TiO2) by an impregnation-reduction method was investigated as for morphological, electronic (XPS) and photocatalytic properties in hydrogen production by methanol photoreforming. The electron and hole trapping centers, Ti3+ and O−, respectively, formed under UV–vis irradiation of the photocatalysts, were studied by in situ electron spin resonance (ESR) spectroscopy. The nature of the loaded metal affected both the H2 evolution rate and the distribution of the methanol oxidation products. The better performance of Pt/TiO2 is attributable to the greater ability of Pt with respect to Au to act as electron sink, slowering the recombination of photoproduced electron–hole couples. Direct evidence of this effect was obtained by ESR analysis, showing that the amount of Ti3+ active sites follows the order TiO2>Au/TiO2≫Pt/TiO2, thus confirming easier electron transfer from Ti3+ to Pt, where the H+ reduction to H2 occurs.

The synthesis and afterglow luminescence properties of a novel red afterglow phosphor: SnO2:Sm3+,Zr4+

The novel SnO2:Sm3+,Zr4+ afterglow phosphor is synthesized by solid state reaction. The SnO2:Sm3+ doesn’t show afterglow. But, after co-doping Zr4+ in SnO2:Sm3+, the red afterglow luminescence of Sm3+ is firstly observed and the important role of Zr4+ on the afterglow is investigated. The photoluminescence shows that the doping of Zr4+ increases the emission lines of Sm3+ and decreases the emission band due to trapped hole center. The thermoluminescence reveals that the co-doping of Zr4+ induces the increase of shallow traps corresponding to the thermoluminescence peak at 48 °C. The depth of the traps is calculated to be 0.61 eV. The positron annihilation indicates that the traps are due to negatively charged tin vacancies and they play the important role on the afterglow of Sm3+ as hole traps.

Mechanoluminescence and thermoluminescence of BaFCl:Sm 2+ and BaFBr:Sm 2+ crystals

The alkaline-earth fluorohalide crystals MFX, where M=Ca, Sr, Ba, Pb and X=Cl, Br, I, form an important class of materials crystallizing in the PbFCl-type tetragonal structure which is also called the matlockite structure. These compounds have long been of interest because of the various defect species which can be detected by spin resonance and associated techniques. The crystals were prepared by slow cooling of the melt of a stoichiometric mixture of BaF 2 and the corresponding chloride or bromide under 0.2 bar of ultrapure argon (5N5), often slightly fluorinated. We have studied the mechanoluminescence (ML) of BaFBr:Sm 2+ and BaFCl:Sm 2+ crystals. It is seen that after the impact of a moving piston, initially the ML intensity increases with time, attains a maximum value and then it decreases with time up to a particular minimum value, and then it increases again, attaining a peak value and finally disappears. The first peak lies in the deformation region and the second peak lies in the post-deformation region. The ML intensity of the BaFCl:Sm 2+ crystal is much higher than the ML intensity of the BaFBr:Sm 2+ crystal. For different impact velocities, the ML intensity increases with velocity; and the total ML intensity attains a saturation value for higher impact velocities. The total ML intensity increases with the increase in the applied load. It is suggested that the moving dislocation produced during deformation of crystals captures holes from hole-trapped centers (like H centers), and the subsequent radiative recombination of the dislocation holes with electron gives rise to ML. Thermoluminescence (TL) of BaFBr:Sm 2+ and BaFCl:Sm 2+ crystals was studied after exposure to ultraviolet rays with the help of a TLD reader. The peak of TL for the BaFBr:Sm 2+ crystal is found at ∼247°C and for BaFCl:Sm 2+ crystals at 283°C. The TL intensity initially increases with increase in the UV radiation and then it attains saturation for higher values of UV exposure. The absorption spectrum was recorded with the help of a UV–visible spectrophotometer (Shimadzu). The band found at 275 nm was attributed to H centers.

Effect of Femtosecond-laser Irradiation on Magnetic and Optical Properties in Au- and Fe-doped Transparent Glass

We have investigated an effect of near-infrared femtosecond- (fs-) laser irradiation on magnetic and optical properties in Au- and Fe-doped transparent glasses. A fs-laser beam with a wavelength of 775 nm was focused 1 mm below the surfaces of glass samples. This produces absorption peaks due to the formation of hole-trap centers in the irradiated region, which changes into different peaks originated from localized surface plasmon absorption after annealing at 450°C. It is presumed that precipitation of Au nanoparticles occurs as results of photoreduction of Au ions and agglomeration by the thermal atomic diffusion. In addition, enhancement of magnetization in the localized area inside the glass was realized after laser irradiation and subsequent annealing. Ferrimagnetic behaviors were increased in the M-H curve even at room temperature, whereas the diamagnetic component dominated in the M-H curve of the as-prepared glass sample. The irradiated and annealed glass sample also exhibited superparamagnetic blocking in the temperature dependence of the magnetization with a blocking temperature higher than room temperature. These changes in magnetic properties are explainable in terms of the local crystallization of ferrimagnetic nanoparticles, such as magnetite, induced by fs-laser irradiation and annealing. Transmission-electron-microscopy observations support our considerations that Au nanoparticles and ferrimagnetic spinel-phase nanoparticles were simultaneously and separately crystallized in the focused region of the fs-laser beam.