PL Decay Time Research Articles

Microstructures and properties of Pr,Ce:Gd2O2S ceramics fabricated from powders synthesized at different initial water bath temperatures

Crystallized precursor with stacked layered was synthesized in hot water bath using oxide powders and concentrated sulfuric acid as raw materials. Gd2O2S powders with high phase purity were obtained by reducing the precursor under flowing hydrogen atmosphere. The influence of initial bath temperature of Gd2O3 and H2SO4 on the microstructure of Gd2O2S powders was investigated. The Gd2O2S:Pr,Ce powders showed a stacked layered structure and as the initial water bath temperature increases, the particle size of the flaky powders also increases. Using the synthesized powders, Gd2O2S:Pr,Ce scintillation ceramics were fabricated through pre-sintering in vacuum followed by HIP post-treatment in argon atmosphere. The Gd2O2S:Pr,Ce ceramics fabricated from the powders synthesized at lower initial water bath temperature show a rapid densification rate during pre-sintering. The Gd2O2S:Pr,Ce ceramic sample fabricated from powders synthesized at 7°C showed the highest optical transmittance, XEL intensity, and light yield value of 25,800 ph/MeV @ 662 keV γ rays. The PL decay time of Pr3+3P0→3H4 transition was measured to be ∼2.87 μs for all ceramic samples. The effect of the initial water bath temperature on optical transmittance and light yield of Gd2O2S:Pr,Ce ceramics was also discussed.

Size Dependence of Ultrafast Electron Transfer from Didodecyl Dimethylammonium Bromide-Modified CsPbBr3 Nanocrystals to Electron Acceptors.

Charge transfer efficiencies in all-inorganic lead halide perovskite nanocrystals (NCs) are crucial for applications in photovoltaics and photocatalysis. Herein, CsPbBr3 NCs with different sizes are synthesized by varying the ligand contents of didodecyl dimethylammonium bromide at room temperature. Adding benzoquinone (BQ) molecules leads to a decrease in the PL intensities and PL decay times in NCs. The electron transfer (ET) efficiency (ηET) increases with NC size in complexes of CsPbBr3 NCs and BQ molecules (NC-BQ complexes), when the same concentration of BQ is maintained, as investigated by transient photobleaching and photoluminescence spectroscopies. Controlling the same number of attached BQ acceptor molecules per NC induces the same ηET in NC-BQ complexes even though with different NC sizes. Our findings provide new insights into ultrafast charge transfer behaviors in perovskite NCs, which is important for designing efficient light energy conversion devices.

Luminescence studies of high color purity red-emitting [formula omitted] phosphor prepared by microwave-assisted synthesis technique

A set of red-emitting phosphor materials (Ca1−xAl4O7:Eux3+, x = 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.11, 0.13) were synthesized through microwave-assisted combustion synthesis for the first time. The crystal structures and phase purity of Eu3+ ions-activated calcium dialuminate (CaAl4O7) phosphor were investigated by XRD study and Rietveld refinement analysis were performed. The prepared phosphors exhibit monoclinic crystal structure having C2/c space group. From the radius difference percentage calculation, the occupancy of Eu3+ ions in Ca2+ sites was confirmed which is in better harmony with the refinement result. Morphological analysis was investigated through SEM study. Structural characteristics and the presence of the functional groups were analysed though FTIR study. The optical band gap (Eg) was determined to be 5.46 eV by studying the UV–VIS reflectance spectra. PL measurements were carried out for the prepared material to study the luminescent behaviour. The presence of charge transfer band was observed in the excitation spectra. It is found that the prepared phosphor materials exhibits emission rich in red at 611 nm with fixed excitation wavelength at 393 nm. The concentration quenching and energy transfer kinetics were analysed by employing Blasse and Dexter’s formula that attributed the non-radiative energy transfer to the dipole-dipole (d-d) interactions. Local site symmetry was analysed to study the symmetry of the environment in the neighbourhood of Eu3+ ions in the host matrix. To analyse the colorimetric performance of the phosphor materials, the correlated color temperature (CCT), color purity were calculated by using the CIE chromaticity co-ordinates (CIE 1931). The calculated CCT value shows the appearance of the emitted color of the phosphor in the warmer region. CaAl4O7:Eu3+ phosphor manifests high color purity which was found to be 97.39 %. The PL decay time measurement were performed for Ca1−xAl4O7:Eux3+ (x = 0.07) phosphor to obtain the decay lifetime. The estimated decay time (1.44 ms) indicates that the prepared phosphor material has a good potency to be utilized as a red-emitting phosphor for wLED, display technology and other applications.

Effect of Mg doping on carrier recombination in GaN

Time-resolved photoluminescence measurements have been performed on Mg-doped GaN for Mg concentrations in the low- to mid-1019 cm−3. As-grown and annealed (600–675 °C) samples were studied. In the as-grown samples, the nonradiative carrier lifetime was found to be about 200 ps and nearly independent of the Mg concentration. Upon annealing, the carrier lifetimes shorten to ∼150 ps but, again, show little dependence on the annealing temperature. The analysis of possible Shockley–Read–Hall recombination centers and their behavior during doping and annealing suggests that the main nonradiative recombination center is the Mg–nitrogen vacancy complex. The weak dependence of the PL decay times on temperature indicates that carrier capture into this center has a very low potential barrier, and the nonradiative recombination dominates even at low temperatures.

A2Bn-1PbnI3n+1 (A = BA, PEA; B = MA; n = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators.

Quantum-well (QW) hybrid organic-inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications.

Understanding FRET in Upconversion Nanoparticle Nucleic Acid Biosensors.

Upconversion nanoparticles (UCNPs) have been frequently applied in Förster resonance energy transfer (FRET) bioanalysis. However, the understanding of how surface coatings, bioconjugation, and dye-surface distance influence FRET biosensing performance has not significantly advanced. Here, we investigated UCNP-to-dye FRET DNA-hybridization assays in H2O and D2O using ∼24 nm large NaYF4:Yb3+,Er3+ UCNPs coated with thin layers of silica (SiO2) or poly(acrylic acid) (PAA). FRET resulted in strong distance-dependent PL intensity changes. However, the PL decay times were not significantly altered because of continuous Yb3+-to-Er3+ energy migration during Er3+-to-dye FRET. Direct bioconjugation of DNA to the thin PAA coating combined with the closest possible dye-surface distance resulted in optimal FRET performance with minor influence from competitive quenching by H2O. The better comprehension of UCNP-to-dye FRET was successfully translated into a microRNA (miR-20a) FRET assay with a limit of detection of 100 fmol in a 80 μL sample volume.

Photo‐Carrier Lifetime in Binary and Ternary Heterostructures of Transition Metal Dichalcogenides

Binary and ternary transition metal dichalcogenide (TMD) heterostructures, MSe2 and MSeS (where M = Mo or W), are investigated as future building blocks for high‐performance device applications with a tailorable electronic bandgap. For light‐emitting device applications, investigating the decay time of the material is a prerequisite. Herein, time‐resolved photoluminescence (TRPL) of binary and ternary TMD heterostructures is measured. A tri‐exponential function is adopted to fit the TRPL data. Short, medium, and long decay times are &lt;50 ps, ≈400 ps, and &gt;400 ps, respectively. Except for monolayer MoSe2 and MoSeS, only two PL decay time components are observed with a large contribution from short decay time photocarriers (&gt;80%). Bilayer TMDs exhibit a much shorter PL decay time, which could be attributed to the indirect nature of excitons and different dielectric environments, compared with that of monolayer TMDs. Using the center‐of‐mass (CM) method, a PL decay time map is successfully obtained for binary and ternary TMD heterostructures. This allows the qualitative visualization of the decay time distribution within the heterostructure or among other materials. This technique could be applied to assess the decay time of materials used in high‐performance light‐emitting devices.

Influence of calcium doping concentration on the performance of Ce,Ca:LuAG scintillation ceramics

Ce,Ca:LuAG scintillation ceramics with different Ca2+ co-doping concentrations were prepared by the solid-state reaction method. The concentration of Ce3+ was fixed at 0.3 at% and the concentration of Ca2+ ranged from 0 to 1.2 at%. We systematically studied how the Ca2+ concentration affects the optical quality of Ce,Ca:LuAG ceramics by influencing the microstructure in the vacuum sintering and HIP post-treatment. Good optical transmittance could be obtained with Ca2+ concentrations between 0.05 and 0.8 at%, which reached 76.0–81.9 % at 520 nm. The PL and scintillation decay times decrease with increasing Ca2+ concentration up to 0.6 at% with no clear trend above this value. The light yield (LY) values at different shaping times decrease with increasing Ca2+ concentration but the fast scintillation component (LY0.5 μs/ LY3.0 μs) increases significantly from 79 % to 97 %. The co-doping of Ca2+ also reduces the afterglow level by more than one order of magnitude.

Energy Transfer between Size-Controlled CsPbI3 Quantum Dots for Light-Emitting Diode Application.

Perovskite quantum dots (PQDs) are applicable in light-emitting diodes (LEDs) owing to their color tunability, high color purity, and excellent photoluminescence quantum yield (PLQY) in the solution state. However, a PQD film obtained through nonradiative recombination by concentration quenching and the formation of surface defects exhibited a low PLQY. In this study, we focused on the energy transfer between PQDs with different energy gaps (Eg) to reduce nonradiative recombination in the film state and consequently achieve high device performance. We prepared size-controlled PQDs measuring 10.7 nm (large-size QD; LQD) and 7.9 nm (small-size QD; SQD) with different Eg values and observed a spectral overlap between SQD emission and LQD absorption. To investigate the Förster resonance energy transfer (FRET) from SQDs to LQDs, we prepared SQD-LQD mixed QDs (MQDs). The MQD film enhanced LQD emission and exhibited a higher PLQY (52%) with a longer PL decay time (7.4 ns) than those exhibited by the neat LQD film (38% and 6.2 ns). This energy transfer was determined to be FRET by photoluminescence excitation and PL decay times. Moreover, the external quantum efficiency of an MQD-based LED increased to 15%, indicating that the FRET process can enhance the PLQY of the film and LED efficiency.

Comparison of a bottom-up and a top-down approach for the creation of contact openings in a multi-stack oxide layer at the front interface of Cu(In,Ga)Se2

The implementation of an oxide layer with contact openings to the Cu(In,Ga)Se2 (CIGS) interfaces has become especially popular to reduce the recombination losses, resulting in a higher open-circuit voltage. While implementing this approach on the back surface is straightforward, it is more complicated for the front surface due to the roughness of the absorber as well as material selection constraints imposed by further processing steps. In this contribution, an AlOx/HfOx multi-stack oxide layer with contact openings is applied between the CIGS and CdS layers. Two different approaches to create contact openings in multi-stack oxide layer are presented, and their advantages and disadvantages are investigated. In the bottom-up approach (BU), a NaCl salt pattern is created on the CIGS absorber surface, while in the top-down approach (TD), the pattern is created on the AlOx layer surface. Time-resolved photoluminescence (TR-PL) shows that the multi-stack design improves the PL decay time regardless of the approach, but when the CdS layer is deposited, the PL decay time decreases. A more severe decrease is observed in case of the BU approach. Capacitance-Voltage measurements show that there is no impact on doping level when the NaCl pattern is created on an AlOx surface. This work shows that any oxide material, even if not chemically resistant to the chemical bath deposition, can be applied at the CIGS/buffer layer interface, and additionally, any template can be used to create contact openings while preventing interaction between the material used for patterning and the CIGS, using the TD approach.

Optical and scintillation properties of LuGd2Al2Ga3O12:Ce, Lu2GdAl2Ga3O12:Ce, and Lu2YAl2Ga3O12:Ce single crystals: A comparative study

Optical and scintillation properties of LuGd2Al2Ga3O12:Ce, Lu2GdAl2Ga3O12:Ce, and Lu2YAl2Ga3O12:Ce single crystals are compared. The blue-shift of 5d1→ 4f photoluminescence emission (PL) band is observed upon partial substitution of Gd3+ with a smaller Lu3+ (or Y3+) ion. At 662 keV γ-rays, the light yield of 35,400 ± 1,700 ph/MeV obtained for LuGd2Al2Ga3O12:Ce is higher than that of 29,200 ± 1,500 ph/MeV and 20,200 ± 1,000 ph/MeV for Lu2GdAl2Ga3O12:Ce and Lu2YAl2Ga3O12:Ce, respectively. Scintillation decays are approximated by sum of exponentials with the fast component decay time and relative intensity of 78 ns (42%) for LuGd2Al2Ga3O12:Ce, 48 ns (71%) for Lu2GdAl2Ga3O12:Ce and 45 ns (88%) for Lu2YAl2Ga3O12:Ce, which leads to a superior coincidence time resolution of Lu2YAl2Ga3O12:Ce. PL decay time at 110 K and 290 K, afterglow characteristics, and mass attenuation coefficient at 59.5 keV and 662 keV γ - rays are also presented.

An Optically Reconfigurable Förster Resonance Energy Transfer Process for Broadband Switchable Organic Single-Mode Microlasers

An Optically Reconfigurable Förster Resonance Energy Transfer Process for Broadband Switchable Organic Single-Mode Microlasers

Correlation of structural and optical properties of AlGaN films grown on SiN-treated sapphire by MOVPE

We investigated the structural and optical properties of AlGaN films grown on SiN-treated sapphire substrates, without and with GaN-template, by atmospheric pressure metal organic vapor phase epitaxy. The samples were characterized using high-resolution X-ray diffraction (HR-XRD), time-resolved photoluminescence (TR-PL), and photoreflectance (PR) spectroscopies. Furthermore, the carrier mobility was determined from Hall-effect measurements. When the AlGaN (GaN-template) layer thickness increases up to 0.6 µm (1.3 µm), an increase in the PL decay times is observed and correlated with the transition from 3D to 2D growth mode resulting in a decrease in the dislocations density as obtained from the HR-XRD measurements. Beyond the aforementioned layer thicknesses, we observed a deterioration in the PL transient corresponds to an increase in the density of VAl-related complexes during the relaxation process, which act as non-radiative recombination centers. Our observations strongly suggest that this type of defects influences the carrier transport and carrier recombination process in the AlGaN layers. Furthermore, our results reveal a phenomenological linear relationship between the internal electric field, obtained from the PR measurements, and the dislocations density. This finding predicts an increase in the GaN internal electric field by about 147 KV/cm when the Al content is increased to 7% in the AlGaN layers. We attribute this increase to a rise in the polarization-induced electric field due to Al incorporation in the AlGaN layer. Based on the obtained correlation between the internal electric field and the dislocation density, we propose an experimental approach that can be utilized to determine the internal electric field, at zero dislocation density, which is very important for designing high-efficient electronic and optoelectronic devices.

Photoluminescence modulation in the graphene oxide dispersed 4-n-octyl-4’-cyanobiphenyl molecular system

Herein, we investigate the mechanism of photoluminescence modulation (PLM) in the 4-n-octyl-4’-cyanobiphenyl liquid crystal (8CB LC) in the presence of various doping concentrations (0.02 to 0.4 wt%) of 2D-GO (two-dimensional graphene oxide) nanoflakes. The role of intermolecular interactions on PLM and the subsequent effect on the decay lifetime is also examined via time-resolved PL measurements. The presence of intermolecular interactions is additionally confirmed by means of differential scanning calorimetry and polarized optical microscopy. The successive addition of GO nanoflakes sharpens the absorption band of pure 8CB LC; consequently, it is blue shifted. The PL properties of the 8CB LC enhance due to the addition of GO nanoflakes which might be attributed to electron–phonon interaction between 8CB molecules and GO nanoflakes. The PL enhancement has been further supported by the time-resolved PL decay time measurement. The longest and shortest fluorescence lifetimes have been recorded for the 0.1 and 0.4 wt% GO/8CB composites, respectively. The investigated composites have shown their potential applications for fluorescent optical devices; however, the quantum efficiency needs to be further improved.

Preparation of highly emissive and reproducible Cu–In–S/ZnS core/shell quantum dots with a mid-gap emission character

Copper indium sulfide (CIS) quantum dots (QDs) are one of the newest types of luminescent semiconductors with low-toxicity and earth-abundant features. The present work reports the successful aqueous synthesis of CIS/ZnS core/shell QDs using dual-stabilizing agents of N-acetyl-l-cysteine and trisodium citrate. Off-stoichiometric QDs with In-rich compositions were found to be very small and highly emissive after coating by a shell of wide bandgap ZnS. The effect of various experimental parameters was evaluated to achieve highly reproducible QDs with bright reddish emission. Results showed a significant contribution of mid-gap defect states in the recombination processes (based on the gradual increase in absorbance recorded for samples, relatively high Urbach energy, large Stokes shift, large FWHM value in PL spectra, as well as the long-lived PL decay time). In addition, the chemical stability of samples was investigated using highly oxidant H2O2 agent and results demonstrate their superior stability. The combination of low-toxicity, intense and stable emission, along with synthetic advantages demonstrates that the present aqueous-soluble and emissive QDs can be considered as an excellent bio-photonic structure suitable for different fields of biological imaging and diagnostics.

Characterizations of Pr:Ca2Al2SiO7 single crystal scintillator for α-ray detection

Undoped and Pr-doped Ca2Al2SiO7 single crystals were synthesized by the floating zone method. The X-ray diffraction patterns confirmed that all the crystals have no impurity phases. Pr-doped crystals show photoluminescence with broad emission bands from 250 to 400 nm and sharp emission lines from 500 to 800 nm due to the 5d-4f and 4f-4f transitions of Pr3+, respectively. Under VUV photon excitation using a synchrotron source, the undoped crystal shows broad emission bands around 380 and 600 nm considered to be due to self-trapped excitons (STEs). The PL decay time constants of the Pr-doped crystals are 15–19 ns and 23–56 μs. The scintillation spectra of Pr-doped crystals are different compared with PL spectra, and the emissions consist of emission peaks due to Pr3+ as well as the STEs. From the pulse height spectra measured using the Pr-doped crystals under 241Am 5.5 MeV α-rays, all the crystals exhibit a clear peak of full-energy absorption, and the 3.0% Pr-doped crystal shows the highest scintillation light yield of 2,600 ph/5.5 MeV-α.

Eu2+: A suitable substituent for Pb2+ in CsPbX3 perovskite nanocrystals?

Eu2+ is used to replace toxic Pb2+ in metal halide perovskite nanocrystals (NCs). The synthesis implies injection of cesium oleate into a solution of europium (ii) bromide at an experimentally determined optimum temperature of 130 °C and a reaction time of 60 s. Structural analysis indicates the formation of spherical CsEuBr3 nanoparticles with a mean size of 43 ± 7 nm. Using EuI2 instead of EuBr2 leads to the formation of 18-nm CsI nanoparticles, while EuCl2 does not show any reaction with cesium oleate forming 80-nm EuCl2 nanoparticles. The obtained CsEuBr3 NCs exhibit bright blue emission at 413 nm (FWHM 30 nm) with a room temperature photoluminescence quantum yield of 39%. The emission originates from the Laporte-allowed 4f7-4f65d1 transition of Eu2+ and shows a PL decay time of 263 ns. The long-term stability of the optical properties is observed, making inorganic lead-free CsEuBr3 NCs promising deep blue emitters for optoelectronics.

Scintillation and dosimetric properties of Sn-doped ZnO-P2O5-SiO2 glasses

We have investigated scintillation and dosimetric properties of Sn-doped ZnO-P2O5-SiO2 glasses. The Sn-doped ZnO-P2O5-SiO2 glasses were prepared by using a conventional melt-quenching method. From the scintillation spectra of the samples, emission peaks due to the 5p → 5s transition of Sn2+ ions were observed. The PL decay times of the samples were from 4.43 to 4.62 μs that are consistent with typical decay times for the 5p → 5s transition of Sn2+ ions. The thermoluminescence (TL) glow curves of the Sn-doped ZnO-P2O5-SiO2 glasses each showed a broad glow peak at 150 °C. From the result of the dose–response curve measurement, the lower limit of detection for the 0.5% Sn-doped sample was 1 mGy.

High-speed growth of CsSr1-xEuxI3 (x = 003, 005, 007) single crystals by the edge-defined film-fed growth method

CsSrI3:3%Eu (Φ10×55mm), CsSrI3:5%Eu (Φ10×50mm) and CsSrI3:7%Eu (Φ10×45mm) single crystals have been successfully grown by the edge-defined film-fed growth method for the first time, with the growth rate reaching 10-20mm/h. We designed a crystal growth device that achieved the first growth of this binary scintillation crystal by the EFG method. The raw material purification, temperature gradient of experimental device and growth rate, which are the effect factors of crystal quality, were systematically investigated. Moreover, the effect of Eu2+ concentration on optical properties were studied. The Eu2+ 5d-4f emission band was observed at 450-455nm, and the PL decay time was determined as 1.32µs for CsSrI3:3%Eu, 1.35µs for CsSrI3:5%Eu and 0.73µs for CsSrI3:7%Eu.

Structure and photoluminescence properties of Dy3+ doping in Sr-Si-O-N frameworks for highly efficient white light and optical information storage applications

A series of Dy3+ doped Sr-Si-O-N frameworks with single phase were successfully developed by a two-step synthetic method. Under excitation of NUV light, Dy3+ singly-doped SrSi2O2N2 (a typical Sr-Si-O-N framework) phosphors exhibited two emission peaks at 478 nm and 571 nm, corresponding to blue (4F9-2→6H15-2) and yellow (4F9-2→6H13-2) emissions of Dy3+ ions. Compared with the traditional solid state reaction, the two-step method facilitated the formation of SrSi2O2N2: Dy3+ phosphor with pure phase and greatly enhanced the photoluminescence emission intensity (~4 times). The emission intensities of SrSi2O2N2: xDy3+ phosphors reached the maximum when x = 0.06 due to the concentration quenching, which was confirmed to be electric dipole-electric dipole interaction. The SrSi2O2N2: Dy3+ phosphors with an excellent thermal stability can be regarded as the potential single phase white-light phosphors when excited by NUV light. The optimal chromaticity coordinate of white light is (0.3347, 0.3966) for SrSi2O2N2: 0.06Dy3+ phosphor. Interestingly, the single green emission band at 525 nm was observed and a unique thermo-stimulated luminescence property was demonstrated for Dy3+/Eu2+ co-doped SrSi2O2N2 phosphors. A specific photographic pattern (for example pentagon pattern) was obtained by covered the mask on the surface of SrSi2O2N2: Eu2+, Dy3+ pellets under excitation of UV light. After UV lights turn off, the pentagon patterns were reproduced by heating the SrSi2O2N2: Eu2+, Dy3+ pellets. It was validated from the decrease of PL emission intensity and decay time with the increasing Dy3+ content that Dy3+ ions were acted as the deep trap centers. Similar thermal-stimulated luminescence was also found for Eu2+ singly-doped SrSi2O2N2 phosphor due to the oxygen vacancy in host lattice. Furthermore, the mechanism of thermal-stimulated luminescence has been proposed and Dy3+/Eu2+ co-doped SrSi2O2N2 phosphors with deep traps are suitable to be the good candidates for optical information storage applications.