Electron Trap Depth Research Articles

Probing Energy-Funneling Kinetics in Nanocrystal Sublattices for Superior X-Ray Imaging.

Long-lasting radioluminescence scintillators have recently attracted substantial attention from both research and industrial communities, primarily due to their distinctive capabilities of converting and storing X-ray energy. However, determination of energy-conversion kinetics in these nanocrystals remains unexplored. Here we present a strategy to probe and unveil energy-funneling kinetics in NaLuF4:Mn2+/Gd3+ nanocrystal sublattices through Gd3+-driven microenvironment engineering and Mn2+-mediated radioluminescence profiling. Our photophysical studies reveal effective control of energy-funneling kinetics and demonstrate the tunability of electron trap depth ranging from 0.66 to 0.96 eV, with the corresponding trap density varying between 2.38×105 and 1.34×107 cm-3. This enables controlled release of captured electrons over durations spanning from seconds to 30 days. It allows tailorable emission wavelength within the range of 520-580 nm and fine-tuning of thermally-stimulated temperature between 313-403 K. We further utilize these scintillators to fabricate high-density, large-area scintillation screens that exhibit a 6-fold improvement in X-ray sensitivity, 22 lp/mm high-resolution X-ray imaging, and a 30-day-long optical memory. This enables high-contrast imaging of injured mice through fast thermally-stimulated radioluminescence readout. These findings offer new insights into the correlation of radioluminescence dynamics with energy-funneling kinetics, thereby contributing to the advancement of high-energy nanophotonic applications.

Probing Energy‐Funneling Kinetics in Nanocrystal Sublattices for Superior X‐Ray Imaging

AbstractLong‐lasting radioluminescence scintillators have recently attracted substantial attention from both research and industrial communities, primarily due to their distinctive capabilities of converting and storing X‐ray energy. However, determination of energy‐conversion kinetics in these nanocrystals remains unexplored. Here we present a strategy to probe and unveil energy‐funneling kinetics in NaLuF4:Mn2+/Gd3+ nanocrystal sublattices through Gd3+‐driven microenvironment engineering and Mn2+‐mediated radioluminescence profiling. Our photophysical studies reveal effective control of energy‐funneling kinetics and demonstrate the tunability of electron trap depth ranging from 0.66 to 0.96 eV, with the corresponding trap density varying between 2.38×105 and 1.34×107 cm−3. This enables controlled release of captured electrons over durations spanning from seconds to 30 days. It allows tailorable emission wavelength within the range of 520–580 nm and fine‐tuning of thermally‐stimulated temperature between 313–403 K. We further utilize these scintillators to fabricate high‐density, large‐area scintillation screens that exhibit a 6‐fold improvement in X‐ray sensitivity, 22 lp/mm high‐resolution X‐ray imaging, and a 30‐day‐long optical memory. This enables high‐contrast imaging of injured mice through fast thermally‐stimulated radioluminescence readout. These findings offer new insights into the correlation of radioluminescence dynamics with energy‐funneling kinetics, thereby contributing to the advancement of high‐energy nanophotonic applications.

Comparing thermoluminescence data on lanthanides in 36 compounds with predictions from vacuum referred binding energy diagrams

Thermoluminescence (TL) often involves the liberation of a charge carrier (an electron or a hole) from a charge carrier trapping centre into the conduction band (CB) or the valence band (VB) with subsequent recombination with a counter charge carrier at a luminescence centre. TL glow peak analysis can provide the energy ΔEt needed to liberate such charge carrier which then defines the location of the charge transition levels (CTL) of the carrier trapping centres below the CB-bottom or above the VB-top. The temperature at the maximum of the TL glow peak changes 3–4 K per 0.01 eV change in ΔEt thus providing an extremely sensitive probe of energy changes in CTLs. This work collects and reviews data on glow peaks due to electron or hole release from lanthanide dopants in 36 different inorganic compounds. To compare results from different literature sources, data were always re-analysed using the same method that is solely based on the temperature at the maximum of the glow peak. The changes in ΔEt along the lanthanides series provides insight at the sub 0.1 eV level on the changes in CTL energies. We will use a compound-dependent parameter to account for the nephelauxetic effect and a compound dependent parameter to account for lattice relaxation around the lanthanide. Together with information from lanthanide luminescence spectroscopy, the vacuum referred binding energy (VRBE) diagram will be constructed for each compound. The lanthanide electron or hole trap depth read from the VRBE scheme will be compared with that derived from the TL glow peak. Surprisingly good agreement will be demonstrated.

Tuning scintillation properties of Lu2SiO5 by Ce and Ca codoping

The light-yield enhancement and decay-time shortening by Ca codoped with ${\mathrm{Lu}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$:Ce have been observed in experiment. However, the underlying mechanisms remain enigmatic. First-principles calculations are performed to gain insight into the effect of Ca codoping. We show that first, the formation of ${\mathrm{Ca}}_{\mathrm{Lu}}\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}{V}_{\mathrm{O}}$ complex defects is favored, which causes the dissociation of ${\mathrm{Ce}}_{\mathrm{Lu}}\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}{V}_{\mathrm{O}}$; therefore, the nonradiative decay of Ce via ${V}_{\mathrm{O}}$ is suppressed, leading to light-yield enhancement. Secondly, the downward shift of the Fermi level by the codoping effect causes the observation of ${\mathrm{Ce}}^{4+}$ ions, and the electron trap depth of ${V}_{\mathrm{O}}$ decreases by the combination with Ca; thus, the trapping/detrapping rates increase, leading to decay-time shortening. Hence, our study has demonstrated an effective way to gain insight into the mechanisms for tuning properties of optical materials by codoping. The principles of the obtained mechanisms provide guidance for designing and optimizing a broad range of functional materials.

Evolution of Charge Trapping and Insulating Performances of Epoxy Materials Containing Hydrolyzable Chlorine During Hygrothermal Aging

Hydrolyzable chlorine is a well-known residue issued from the synthesis of epoxy resin, which may impart specific dielectric properties. In this work, the changes in distributed energy levels and charge trap depths of the molecule containing such defects are investigated using quantum chemical calculations. These changes are analyzed from the microscopic perspectives of electron energy structure and electron cloud offset. The charge transport and charge injection behavior in epoxy materials before and after the hydrolysis are predicted using the analysis results. The volume resistivities and space charges of epoxy materials with three different hydrolyzable chlorine contents are tested at different hygrothermal aging times, which are consistent with the previous predictions. These results are used to explain the ac breakdown strength of these epoxy materials at different hygrothermal aging times. It is indicated that the structural changes (electronegativity and atomic position changes) of the molecule containing hydrolyzable chlorine before and after hydrolysis change the spatial distribution of electron cloud density between and on valence bonds, and in turn, change the energy levels of different molecular segments. This results in a large increase in the overall electron trap depth and a small decrease in the overall hole trap depth of the molecule after hydrolysis.

Introducing Chlorine Into Epoxy Resin to Modulate Charge Trap Depth in the Material

The epoxy insulators in dc gas-insulated transmission lines (GIL) tend to accumulate surface charges, which causes insulation flashover. Increasing the surface conductivity of epoxy resin, which can restrain the accumulation of surface charges on the epoxy insulator, is a potential method to improve the insulation performance of dc GIL insulators. The conductivity of polymer dielectric is strongly influenced by the charge trap characteristics of the polymer. In this work, we introduce chlorine with strong electronegativity and a larger atomic radius into the epoxy group segment of epoxy resin to improve the distributed energy level structure, which in turn reduces the electron trap depth, to increase the surface conductivity of epoxy insulating material without affecting its intrinsic dielectric strength. Based on the results of quantum chemistry calculation, the modulation laws of introduced chlorine (including the chlorine in the form of a hydrolyzable chlorine atom and a nonhydrolyzable chlorine atom) on distributed energy levels of epoxy resin molecule are anticipated. These laws are explained from the microscopic perspectives of electron energy structure and electron cloud offset. Both the inductive effect of the chlorine atom and the conjugation effect of the 2p electron orbital of the oxygen atom in the epoxy group impact the distributed energy levels by changing the spatial distribution of electron cloud density between and on valence bonds.

Thermoluminescence study of Pink Himalayan salt from Khewra mines, Pakistan

In this study thermoluminescence characteristics of the pink Himalayan salt has been studied. For this purpose, pellets of pink Himalayan salt weighing 40 mg were prepared. The ICP-OES and EDX analytical techniques were used to find the elemental composition of the sample. It confirms the presence of Al, Mg, Ca, Sr, S, Zn, K, Mn and Fe etc. along with the main matrix. The XRD analysis shows that the salt has face centered cubic (FCC) structure with average crystallite size of 70.6 nm. The scanning electron microscope (SEM) was used for surface morphology. To investigate the thermoluminescence properties, the pellets were irradiated with 137Cs source in the dose range of 1–5000 mGy. The glow curve indicated the presence of three peaks namely P1, P2 & P3 with two very prominent peaks occurring at 135 °C and 260 °C. The Tmax-Tstop procedure revealed the coinciding peaks in the glow curve and their relative positions. The trapping/kinetic parameters, such as electron trap depth (E), order of kinetic (b) and frequency factor (s) were explored by employing CGCD technique. The values of figures of merit (FOM) calculated at different doses (500 mGy, 1 Gy, 2 Gy & 5 Gy) were found to be 1.14%, 1.24%, 1.11% and 1.13% respectively. This indicates excellent agreement between the experimental and theoretical fitting. The post irradiation fading of the sample pellets were studied at dose of 500 mGy for different time intervals up to 8 week and 30% loss in TL signal was observed within first 24 h which becomes stable after 2 week. On the basis of these results it can be concluded that pink Himalayan salt has a potential to be used in radiation dosimetry.

Unraveling electron liberation from Bi2+ for designing Bi3+-based afterglow phosphor for anti-counterfeiting and flexible X-ray imaging

It is challenging to rational design persistent luminescence and storage phosphors with high storage capacity of electrons and holes after X-ray charging. Such phosphors have potential applications in anti-counterfeiting and X-ray imaging. Here we have combined vacuum referred binding energy diagram (VRBE) construction, photoluminescence spectroscopy, and thermoluminescence to study the trapping processes of charge carriers in NaYGeO4. In NaYGeO4:0.004Bi3+ and NaYGeO4:0.004Bi3+,0.005Ln3+ (Ln = Tb or Pr), Bi3+ appears to act as a shallow electron trap, while Bi3+ and Ln3+ act as deep hole trapping and recombination centres. We will show how to experimentally determine the VRBE in the Bi2+ 2P1/2 ground state in NaYGeO4 and NaLuGeO4 by thermoluminescence study. The electron trap depth produced by Bi3+ codopant in NaLu1-xYxGeO4:0.003Bi3+,0.008 Tb3+ can be adjusted, by increasing x, resulting in conduction band engineering. By combining Bi3+ as an electron trap and Bi3+ and Tb3+ as the hole traps, excellent X-ray charged afterglow phosphors were developed. The integrated TL intensity of the optimized NaYGeO4:0.004Bi3+ and NaYGeO4:0.003Bi3+,0.008Tb3+ after exposure to X-rays is about 4.5 and 1.1 times higher than that of the state-of-the-art BaFBr(I):Eu2+ storage phosphor. Intense initial Tb3+ 4f → 4f afterglow appears in NaYGeO4:0.003Bi3+,0.008Tb3+ and more than 40 h afterglow is measurable in NaYGeO4:0.004Bi3+ and NaYGeO4:0.003Bi3+, 0.008 Tb3+ after X-ray charging. We will show proof-of-concept anti-counterfeiting and X-ray imaging applications by using the developed afterglow phosphors and CsPbI3 quantum dots. This work not only provides experimental evidence on the VRBE in the Bi2+ 2P1/2 ground state in NaYGeO4, but also shows how to design and develop good afterglow phosphors for anti-counterfeiting and X-ray imaging by deeply studying and controlling the trapping processes of charge carriers in bismuth and/or lanthanides doped inorganic compounds.

Understanding the defect levels and photoluminescence in a series of bismuth-doped perovskite oxides: First-principles study

First-principles calculations using the hybrid density functional are carried out to study the defect levels and photoluminescence of bismuth doped in perovskites $R{\mathrm{AlO}}_{3}$ ($R=\mathrm{Y}$, Gd, La) and $\mathrm{La}B{\mathrm{O}}_{3}$ ($B=\mathrm{Al}$, Ga, In). Bismuth dopants are confirmed to be predominantly ${\mathrm{Bi}}^{3+}$ occupying the $R$ site, i.e., ${\mathrm{Bi}}_{R}^{0}$ ($R=\mathrm{Y}$, Gd, La). The variation of the electron trap depth ${\mathrm{Bi}}_{R}(0/\ensuremath{-}1)$ is shown mainly due to the shift of the conduction band, while the hole trap level ${\mathrm{Bi}}_{R}(+1/0)$ shows the correlation with the shortest $R\text{--}\mathrm{O}$ bond length in hosts. Based on the defect level diagram from the first-principles calculation, the transition types of excitation and emission are predicted. Among the systems considered, the lowest excited level that produces the photoluminescence is the $^{3}P_{0,1}$ levels of the $6{s}^{1}6{p}^{1}$ configuration of ${\mathrm{Bi}}^{3+}$, except for ${\mathrm{Bi}}^{3+}$-doped ${\mathrm{LaAlO}}_{3}$, which is the valence band to ${\mathrm{Bi}}^{3+}$ charge transfer state. The photoluminescence involving $^{3}P_{0,1}$ states shows similar Stokes shift in the series, and the excitation and emission energies exhibit almost linear correlation with the band gaps of hosts of a slope $\ensuremath{\sim}0.22$. Furthermore, our calculations show that there is no tendency of forming excessive ${\mathrm{Bi}}^{3+}$ pairs in all the five $RB{\mathrm{O}}_{3}$:${\mathrm{Bi}}^{3+}$, while the ${\mathrm{Gd}}^{3+}\text{\ensuremath{-}}{\mathrm{Gd}}^{3+}$ coupling in ${\mathrm{GdAlO}}_{3}$ provides a ``bridge'' for energy migration from an excited isolated ${\mathrm{Bi}}^{3+}$ ion to the ${\mathrm{Bi}}^{3+}$ pairs that are naturally present due to random distribution, leading to the 495-nm emission being observed uniquely in bismuth-doped ${\mathrm{GdAlO}}_{3}$ among the series of systems considered. The results lay the basis for manipulating the trap levels and excitation and emission wavelengths in ${\mathrm{Bi}}^{3+}$-doped perovskite oxides.

Enhanced breakdown strength of multilayer polypropylene film with structured interface

Extensive research has been carried out to improve the breakdown strength (BDS) of polymer materials used in high-voltage power equipment. However, almost all the modifications, including nanocomposites, polymer blends, and multilayer films have enhanced the BDS by introducing alien material, which inevitably resulted in an increase in the dielectric losses. Therefore, we propose a multilayer structured film simplex composed of polypropylene (PP) to simultaneously achieve high BDS and low losses. By including virgin and fluorinated bi-axially oriented PP (BOPP) films, three different interface structures were constructed. All the multilayer PP films showed DC BDSs that were improved by varying degrees, compared to the single-layer film. In particular, owing to the enhanced electron trap depth and density caused by fluorination, the DC BDSs of multilayer films with fluorinated BOPP reached 458.7 kV mm−1, which was 20% higher than that of a single-layer PP film of the same thickness, while their dielectric loss was even lower than that of the single-layer film.

Study of Electronic and Steric Effects of Different Substituents in Donor–Acceptor Molecules on Multilevel Organic Memory Data Storage Performance

AbstractTo study the influence of different types of substituent moieties onto the molecular backbones of conjugated donor–acceptor (D–A) molecules on the thin‐film morphology and performance of their memory devices, three new molecules X‐TBT were synthesized, which consist of the same backbone of two triphenylamine (T) groups and benzothiadiazole (BT) group, but have different substituents (X) with different electronic effects, that is, cyano group (CN), tert‐butyl group (tBu), and methoxy group (OMe). Nonvolatile ternary write‐once‐read‐many‐times (WORM) data storage behavior is achieved for the CN‐TBT and tBu‐TBT based devices as compared to the binary memory characteristic of TBT (X = H). In contrast, OMe‐TBT based device still maintains binary WORM behavior due to its unfavorable molecular packing motif and weak intermolecular charge transfer effect, but exhibits the lowest operating voltage (1.4 V) as a result of the lowest energy barrier between electrode and active layer. Notably, the tBu‐TBT based device displays the highest ION2/ION1/IOFF ratio of 107:103:1. Altering the substituents in D–A molecules can adjust the molecular packing, thin film morphology, and electron trap depth of the active layer, which then significantly influence the memory performance.

Towards information storage by designing both electron and hole detrapping processes in bismuth and lanthanide-doped LiRE(Si,Ge)O4 (RE = Y, Lu) with high charge carrier storage capacity

Guided by vacuum referred binding energy (VRBE) diagrams, both the trapping and detrapping processes of electrons and holes are explored in the bismuth and lanthanide-doped LiRE(Si,Ge)O4 (RE = Y, Lu) family of compounds. The Tm3+ electron trap has been combined with the deep hole traps of Ln3+ (Ln = Ce, Tb, or Pr) or Bi3+ in LiLuSiO4. During the thermoluminescence readout, the electrons released from Tm2+ recombine with holes at Ln4+ and Bi4+ to produce typical Ln3+ 4f-4f or 5d-4f emission and Bi3+ A-band emission. The electron trap depth of lanthanide ions can be tuned by the choice of Ln3+ (Ln = Tm or Sm), and for fixed pair of Ln3+ and/or Bi3+ dopants like in LiLu1−xYxSiO4:0.01Ce3+,0.01Ln3+ and LiLu1−xYxSiO4:0.01Bi3+,0.01Sm3+ solid solutions, by adjusting x, resulting in the engineering of the VRBE at the conduction band bottom. The thermoluminescence (TL) intensity of the optimized LiLu0.5Y0.5SiO4:0.01Ce3+, 0.005Sm3+ is about 8.5 times higher than that of the commercial X-ray BaFBr(I):Eu2+ storage phosphor. By combining deep Eu3+ or Bi3+ electron traps with Ln3+ (Ln = Tb or Pr) or Bi3+, Ln3+ and Bi3+ appear to act as less deep hole capturing centres in LiLuSiO4. Here the recombination is achieved through hole liberation rather than the more commonly reported electron liberation. The holes are released from Ln4+ and Bi4+ to recombine with electrons at Eu2+ or Bi2+ to give characteristic Eu3+ 4f-4f and Bi3+ A-band emissions. The tailoring of Ln3+ and Bi3+ hole trap depths by crystal composition modulation is discussed in LiLu1−xYxSiO4 and LiLu0.25Y0.75Si1−yGeyO4:0.01Bi3+ solid solutions. The TL intensity of the optimized LiLu0.25Y0.75SiO4:0.01Bi3+ is ~4.4 times higher than that of the commercial BaFBr(I):Eu2+. Proof-of-concept information storage will be demonstrated with X-ray or UV-light charged LiLu0.5Y0.5SiO4:0.01Ce3+,0.01Sm3+ and LiLu0.25Y0.75SiO4:0.01Bi3+ phosphors dispersed in silicone gel imaging plates.

Pressure-induced variation of persistent luminescence characteristics in Y3Al5-xGaxO12:Ce3+-M3+ (M = Yb, and Cr) phosphors: opposite trend of trap depth for 4f and 3d metal ions.

Changing the electronic structure of materials by pressure and the accompanying changes in optical properties have attracted scientific interest. We have reported that the energy position of the conduction band (CB) bottom and the crystal field splitting of the Ce3+:5d excited level in Y3Al5-xGaxO12:Ce3+ are changed by applying pressure, which results in the red shifting of the Ce3+:5d → 4f luminescence and the increase of the quenching temperature. We also reported dramatic improvement of the persistent luminescence performance by either Cr3+ or Yb3+ codoping into the Y3Al5-xGaxO12:Ce3+ phosphors. The different trap depths formed by Cr3+ and Yb3+ affect the initial persistent luminescence intensity and the persistent luminescence duration. In this study, the effect of pressure on the persistent luminescence performance was investigated. For the Y3AlGa4O12:Ce3+-Yb3+ phosphor, the slope of persistent luminescence decay curve becomes more gentle with increasing pressure, while for the Y3AlGa4O12:Ce3+-Cr3+ phosphor the slope becomes steeper. These results indicate that the trap depth of Yb3+ becomes deeper and that of Cr3+ becomes shallower with increasing pressure. Based on the pressure-dependence of the luminescence quenching and the trap depth change estimated from the decay slopes, the relative electronic energies of the CB bottom and the Yb2+ (4f14) or Cr2+ (3d4) levels are discussed. The CB bottom energy is increased relative to the ground 1S0 state of Yb2+ with increasing pressure, which results in deepening of the electron trap depth of the Yb2+ state. The opposite tendency of the Cr3+ codoped sample was described by a decreasing tendency of the energy gap between the CB bottom and the Cr2+:eg level, the relative energy level of which is increased by the increase of the crystal field with increasing pressure in the garnet host material, where the electron-trapping Cr2+ ions take the high spin state (t32ge1g) rather than the low-spin state (t42g).

Efficient Red Phosphorescent Polymers with Trap-Assisted Charge Balance: Molecular Design, Synthesis, and Electroluminescent Properties

Three classes of red phosphorescent polymers (PF-H- x, PF-DPO- x, and PF-DPA- x, where x denotes the mole content of Ir complex) have been designed and synthesized, where the C∧N ligand of the tethered dopant bis(2,4-diphenylquinolyl)iridium(acetylacetonate) is substituted by hydrogen (H), diphenylphosphine oxide (DPO), and diphenylamine (DPA), respectively. It is found that the electron-withdrawing DPO group can lower the lowest unoccupied molecular orbital (LUMO) level of the phosphor, whereas the electron-donating DPA group leads to an upshifted highest occupied molecular orbital (HOMO) level of the phosphor. Following a sequence of PF-DPA- x, PF-H- x, and PF-DPO- x, the electron trap depth between dopant and host is gradually up from 0.43 to 1.01 eV, and the hole trap depth is correspondingly down from 0.74 to 0.46 eV. As a result, PF-DPO- x achieves the most balanced charge transport in the emitting layer among these polymers, revealing a record-high luminous efficiency (LE) of 10.3 cd/A and Commission Internationale de L'Eclairage (CIE) coordinates of (0.62, 0.33) on the basis of the simple single-layer device structure. Compared with PF-H- x (3.8 cd/A) and PF-DPA- x (1.2 cd/A) containing the same Ir content, the significantly improved performance indicates that trap-assisted charge balance is a promising strategy to optimize the device efficiency of red phosphorescent polymers.

Investigation of luminescence quenching and persistent luminescence in Ce3+ doped (Gd,Y)3(Al,Ga)5O12 garnet using vacuum referred binding energy diagram

Optical properties of Ce3+:5d−4f luminescence in garnet compounds are strongly related to the energy locations of the excited 5d levels and the host conduction band (CB). By constructing the vacuum referred binding energy (VRBE) diagram including information on these energy location for GdyY3-yAl5-xGaxO12 garnet (GYAGG) from the measured spectroscopic data, the luminescence quenching and persistent luminescence properties of GYAGG:Ce3+(-Cr3+) were discussed. The quantum yield (QY) of Ce3+:5d−4f in GYAGG:Ce3+ is clearly related to the energy gap, ΔE5d1−CB, between the lowest 5d level and the bottom of conduction band in the constructed VRBE diagram. This result indicates that the luminescence quenching of GYAGG:Ce3+ is caused by the thermal ionization. For GYAGG:Ce3+-Cr3+ persistent phosphors, the depth of the electron trap formed by Cr3+ was remarkably changed from 0.27 to 0.69 eV by varying the Gd and Ga contents. The estimated VRBE value of Cr2+ in GYAGG can be used as effective value to predict the electron trap depth by Cr3+ in other oxide compounds as well as garnet hosts.

Role of lanthanum in thermoluminescence properties of La2xLu2(1−x)SiO5:Ce crystals

The influence of lanthanum content on thermally stimulated luminescence properties of La2xLu2(1−x)SiO5:Ce (x = 0, 0.08, 0.18 and 1.50 at.%) crystals was investigated. Trapping parameters such as electron trap depth Et and electron traps content n 0 were fitted with general order kinetic function. According to the results of the VUV transmittance spectra and band gap calculation which based on the density functional theory with the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA–PBE), band structure of the crystals and recombination mechanisms of released electrons were further studied. Results showed that there were mainly two kinds of electron traps, namely shallow (Et ≈ 0.2 eV) and deep (Et ≈ 1.0 eV) traps. With the increasing of La content, the concentration of deep traps was obviously depressed, and the depth of deep traps decreased at the same time, which was explained by suggesting the downward shift of the bottom of conduction band.

Improved performance of photoconductive gain hybrid UV detector by trap state engineering of ZnO nanoparticles

With the purpose of examining the impact of donor polymer on the performance of nanocomposite photodetectors (PDs) and to better understand the underlying physics, different wide-bandgap semiconducting polymers, poly(N-vinylcarbazole), poly(9, 9-di-n-octylfluorenyl-2, 7-diyl) , and [9,9′-dioctyl-fluorene-2,7-diyl]-copoly[diphenyl-p-tolyl-amine-4,4′-diyl] (BFE), are mixed with ZnO nanoparticles (NPs) to fabricate hybrid UV PDs. Three different polymer matrix nanocomposites were investigated that differ in the electron-trap depth in the nanocomposite and also the carrier tunneling energy at the interface. All the fabricated PDs exhibit strong photoconductive gain characteristics which can be attributed to trapped electron accumulation and band bending at the cathode interface. Experimental results show that the manipulation of the photoactive nanocomposite improves the PD properties simultaneously, namely, the external quantum efficiency (EQE, ∼104%), the maximum detectivity (D*, ∼1013 Jones), and the linear dynamic range (LDR, ∼85 dB). In addition, the gain bandwidth product of the device improves more than 50 times. Furthermore, the effect of the photogenerated carrier profile within the active layer is investigated experimentally by changing the direction of the incident light using a transparent cathode. Interestingly, under illumination through the Al cathode, faster photocurrent response, wider spectral range toward the deep UV region, and higher EQE in relatively low voltages are observed. These considerations might provide a general strategy to fabricate low-cost photoconductive PDs with a reasonably good combination of gain, response speed, LDR, and selectivity.

Vacuum referred binding energy of 3d transition metal ions for persistent and photostimulated luminescence phosphors of cerium-doped garnets

The Y3Al5−xGaxO12(YAGG):Ce3+-Cr3+ persistent phosphor is one of the materials in which Cr3+ ions act as electron traps. The possibility of electron traps by other transition metal ions (TM3+, TM = Sc, Ti, V, Cr, Fe) was investigated and those electron trap depth was compared with each other. In the thermoluminescence (TL) glow curves, the YAGG:Ce3+ samples co-doped with different TM3+ ions show different TL glow peak temperatures (i.e. different electron trap depth). The estimated vacuum referred binding energy of TM2+ from the trap depth shows a zig-zag curve, which is found to be originated from the different 3d electron energies affected by a number of d electron, nuclear charge and crystal field splitting. Utilizing the obtained TM2+ zig-zag curve, a new persistent phosphor of Y3Al2Ga3O12:Ce3+-Sc3+ and a photostimulated phosphor of Y3Al2Ga3O12:Ce3+-V3+ were successfully developed.

Development of computer code for deconvolution of thermoluminescence glow curve and DFT simulation

Phosphor material shows wide applications in the field of radiation dosimetry, archaeological and geological dating. When the irradiated phosphor material is studied as a function of temperature, it generates thermoluminescence (TL) glow curve. The glow curve gives information about kinetic parameters like electron trap depth (E), order of kinetics (b) and frequency factor (s). These parameters are helpful in determining the dose absorbed by phosphor material. In the present paper, a computer program compiled in Fortran language for the deconvolution of thermoluminescence glow curve was developed. Initially, the behaviour of TL glow curve with respect to change in E and change in b was analysed separately. Using these results, a method was developed to simplify the process of getting the value of E and b of the glow curve. Self consistency, no initial guess, less time consumption and user friendly nature are the key points of the presented computer program. Performance of the program was tested by deconvolution of standard reference glow curves of GLOCANIN project and better Figure of Merit (FOM) and kinetic parameter were obtained as compared to reported data. Moreover, the correctness of the program was checked with the help of results obtained by Density Functional Theory (DFT) simulation. Activation energy of LiF material was evaluated by DFT simulation using Quantum ESPRESSO and it found to be 0.89eV. Besides, activation energy of standard LiF material was also determined by deconvolution of the TL glow curve and found to be 0.879eV which matched very well with the activation energy estimated by DFT simulation.

Morphology-sensitive trapping states of photogenerated charge carriers on SrTiO3 particles studied by time-resolved visible to Mid-IR absorption spectroscopy: The effects of molten salt flux treatments

Abstract The effects of the morphology-change of SrTiO3 particles on the behavior of photogenerated charge carriers are studied by time-resolved absorption (TA) spectroscopy from the visible to mid-IR region. In the case of as-purchased defect-rich commercial SrTiO3 particles, most of the charge carriers are deeply trapped, showing a transient absorption peak at 11,000 cm−1. Scanning electron microscopy reveals that the irregular-shaped primary particles are heavily aggregated and that the photocatalytic activity for the water splitting reaction is negligibly small. However, when this powder is flux-treated by SrCl2, fine cubic SrTiO3 crystals exposing well-defined surfaces are formed and the photocatalytic activity is greatly improved. TA measurements show that the spectral shape is changed dramatically: the number of deeply trapped electrons is reduced, and that of shallowly trapped electrons producing the absorption band at 2500 cm−1 is increased. The change in electron trap depth, observed upon flux treatment, is due to the decrease in the number of defects. We also found that further flux treatment in an Al2O3 crucible (i) induces Al doping into SrTiO3, (ii) enhances the photocatalytic activity, (iii) changes the spectral shape, and (iv) prolongs the lifetime of shallowly trapped electrons. The increase in photocatalytic activity is presumably due to the change in lifetime.