Photoluminescence Peak Position Research Articles

Enhancement of photodetector performance of aluminum-doped zinc oxide thin films fabricated via SILAR method: Structural, optical, and electrical analysis

This study presents a comprehensive analysis of the structural, optical, electrical, and photosensing characteristics of aluminum-doped zinc oxide (Al:ZnO) thin films, fabricated via successive ionic layer adsorption and reaction (SILAR) techniques. The Raman and X-ray diffraction (XRD) analyses validated the hexagonal wurtzite structure and revealed that Al doping enhances crystallinity and crystal growth of ZnO. The average crystallite size increased from 21.6 nm in undoped sample to 23.8 nm in 5.0 wt% Al-doped sample, while strain effects decreased from 4.67 × 10-3 to 4.36 × 10-3. Scanning electron microscopy (SEM) images revealed improved film morphology and more defined geometries with increased Al doping, while energy-dispersive X-ray analysis (EDAX) confirmed successful Al integration into the ZnO lattice. Optical analyses indicate a narrowing of the ZnO bandgap to 2.81 eV in 5.0 wt% Al:ZnO from 3.02 eV in the undoped variant, while photoluminescence (PL) peak positions remains unchanged. Time-resolved fluorescence (TRF) indicated a reduction in electron lifetime with higher Al doping. Furthermore, Hall effect assessments reveal a decrease in carrier mobility and electrical resistivity, alongside a rise in carrier concentration with higher Al content. The photodetection efficacy was substantially improved with Al doping, particularly at 2.5 wt% Al, achieving a responsivity of 42.5 × 10-2 A/W, external quantum efficiency of 99.3 %, and detectivity of 1.8 × 1011 Jones. These findings exhibit that Al-doping substantially enhances the photodetection properties of ZnO thin films.

Charged Exciton Generation by Curvature-Induced Band Gap Fluctuations in Structurally Disordered Two-Dimensional Semiconductors.

Curvature is a general factor for various two-dimensional (2D) materials due to their flexibility, which is not yet fully unveiled to control their physical properties. In particular, the effect of structural disorder with random curvature formation on excitons in 2D semiconductors is not fully understood. Here, the correlation between structural disorder and exciton formation in monolayer MoS2 on SiO2 was investigated by using photoluminescence (PL) and Raman spectroscopy. We found that the curvature-induced charge localization along with band gap fluctuations aid the formation of the localized charged excitons (such as trions). In the substrate-supported region, the trion population is enhanced by a localized charge due to the microscopic random bending strain, while the trion is suppressed in the suspended region which exhibits negligible bending strain, anomalously even though the dielectric screening effect is lower than that of the supported region. The redistribution of each exciton by the bending strain leads to a huge variation (∼100-fold) in PL intensity between the supported and suspended regions, which cannot be fully comprehended by external potential disorders such as a random distribution of charged impurities. The peak position of PL in MoS2/SiO2 is inversely proportional to the Raman peak position of E12g, indicating that the bending strain is correlated with PL. The supported regions exhibit an indirect portion that was not shown in the suspended regions or atomically flat substrates. The understanding of the structural disorder effect on excitons provides a fundamental path for optoelectronics and strain engineering of 2D semiconductors.

Exploring the Influence of Solvents on Electrochemically Etched Porous Silicon Based on Photoluminescence and Surface Morphology Analysis.

Porous silicon (PSi) has promising applications in optoelectronic devices due to its efficient photoluminescence (PL). This study systematically investigates the effects of various organic solvents and their concentrations during electrochemical etching on the resulting PL and surface morphology of PSi. Ethanol, n-butanol, ethylene glycol (EG) and N,N-dimethylformamide (DMF) were employed as solvents in hydrofluoric acid (HF)-based silicon etching. The PL peak position exhibited progressive blue-shifting with increasing ethanol and EG concentrations, accompanied by reductions in the secondary peak intensity and emission linewidth. Comparatively, changes in n-butanol concentration only slightly impacted the main PL peak position. Additionally, distinct morphological transitions were observed for different solvents, with ethanol and n-butanol facilitating uniform single-layer porous structures at higher concentrations in contrast to the excessive etching caused by EG and DMF resulting in PL quenching. These results highlight the complex interdependencies between solvent parameters such as polarity, volatility and viscosity in modulating PSi properties through their influence on surface wetting, diffusion and etching kinetics. The findings provide meaningful guidelines for selecting suitable solvent conditions to tune PSi characteristics for optimized device performance.

Photoluminescence enhancement of chemical vapor-deposited MoSe2 monolayers

The transition metal dichalcogenide (TMDC) materials have attracted a great interest owing to their superior features. The chemical vapor deposition (CVD) is very feasible to synthesis of TMDC materials. The thermal expansion coefficient difference between TMDC materials and substrate in the CVD synthesis can cause a strain, resulting in non-radiative recombination and PL decrement for these materials. Therefore, in this work, we studied acetone and isopropanol (IPA) treatment on the photoluminescence (PL) properties of CVD-grown MoSe2 monolayers. Firstly, monolayer MoSe2 flakes on SiO2/Si substrates were synthesized via the CVD method by optimizing synthesis parameters. The Raman and PL measurements were taken after acetone and IPA treatments were applied to grown samples. The results indicated an important PL enhancement was seen for acetone treatment. For IPA treatment, there was a PL peak position shifting with decreasing intensity due to its possible structural damage. The acetone, IPA, and transfer processes caused the releasing strain on MoSe2 by breaking the strong interaction between MoSe2 and substrate. For the whole treatment, PL peaks shift to blue with about 80 meV. As a result of the present study, acetone treatment was found as an easy and quick way to enhance radiative emission of CVD-grown MoSe2.

Mid-infrared detectors for space electronics based on InAs-core/InP-shell nanowires

The Mid-Infrared (MIR) spectral range is most important for free-space communications and astronomy. It contains radiation emitted by astrophysical objects during evolution of the planets, stars, galaxies and in particular by prebiosignature molecules on exoplanets. Nanowires (NWs) are expected to improve various optoelectronic devices, including IR photodetector technology. The bandgap of the catalystless InAs NWs can be tuned by introducing mechanical strain due to lattice mismatches in the core/shell NWs structures. Passivation with wider bandgap InP shell provide highly tunable functionality for future electronic devices. Temperature dependence of photoluminescence (PL) spectra of NWs InAs and InAs-core/InP-shell was acquired using Fourier-spectrometer. The position of the high-energy PL peak between calculated values of Eg for wurtzite and sphalerite structures confirms the formation of NWs into a combined polytype. Low energy PL peak is connected with parasitic bulk islands. Surface passivation successfully eliminates surface states and provides nontrivial temperature dependence of high-energy PL peak due to tensile and compressive strain in InAs core. Thus, detectors based on NWs InAs and InAs-core/InP-shell structures can operate in the MIR range of the spectrum with a wavelength from 2 to 5 μm.

Uniaxial Strain Tuning of Upconversion Photoluminescence in Monolayer WSe2

Strain engineering is one of the leading mechanical ways to tune the optical properties of monolayer transition metal dichalcogenides among different techniques. Here, uniaxial strain is applied on exfoliated 1L‐WSe2 flakes transferred on flexible polycarbonate substrates to study the strain tuning of upconversion photoluminescence. It is demonstrated that the peak position of upconversion photoluminescence is redshifted by around 20 nm as the applied uniaxial strain increases from 0% to 1.17%, while the intensity of upconversion photoluminescence increases exponentially for the upconversion energy difference ranging from −155 to −32 meV. The linear and sublinear power dependence of upconversion photoluminescence is observed for different excitation wavelengths with and without uniaxial strain, suggesting the multiphonon‐assisted mechanism in one‐photon regime for the upconversion process. These results offer the potential to advance 2D material‐based optical upconversion applications in night vision, strain‐tunable infrared detection, and flexible optoelectronics.

Mechanochromic and Solvomechanochromic Fluorescent Photonic Crystals for Dual-Mode Modulating Fluorescence and Multilevel Anticounterfeiting.

Fluorescent photonic crystals (FPCs) are ideal candidates for regulating dyes' fluorescence through their unique photonic band gaps (PBGs). However, challenges, including the lack of dynamic regulation of fluorescence, dye release in solvents, and instability, dramatically limit their practical applications. Here, we report mechanochromic and solvomechanochromic rhodamine B (RhB)-based FPCs with dynamic regulation of photoluminescence (PL) by stretching and swelling, brilliant fluorescent and structural colors, and no release of the RhB in solvents. The FPCs with force/solvent-responsive nonclose-packing structures were fabricated by (1) preparing RhB-silica particles by combining click chemistry and cohydrolysis processes and (2) self-assembling these particles in poly(ethylene glycol) phenyl ether acrylate followed by a photopolymerization. Maximal PL inhibition (37%, stretching strain of 6.8%) and enhancement (150%, swelling time of 8 min) were gained when PBGs and their blue edges are precisely adjusted to the PL peak position, respectively. Compared with stretching, PL regulation is more efficient by swelling. These characteristics benefit from the rational design and combination of unique compositions, chemical bonds, nonclosely packed micro/nanostructures, and solvents for swelling. Moreover, these FPCs have been used to encrypt photonic patterns, which display background/strain/angle/UV-dependent color contrasts, showing their potential applications in multilevel anticounterfeiting, optical devices, wireless sensors, etc.

Effects of the excitation wavelength and temperature on the zero-phonon emission lineofCr:YAG.

In this work, a detailed study was conducted of the temperature and excitation wavelength-dependent photoluminescence (PL) spectra of the chromium-doped yttrium aluminum garnet (Cr:YAG) transparent ceramic. Focusing on the two sets of zero-phonon lines (ZPLs) of the 2 E→4 A 2 transition in this material, the PL spectra are discovered to evolve significantly with respect to temperature and be highly dependent on the excitation wavelength. Compared to the continuous variation behavior with temperature, an increase in the excitation wavelength leads to a blueshift of the peak position within the regions of 450nm to 465nm, 465nm to 490nm, and 490nm to 500nm, and a sharp change in the PL position at the excitation wavelengths of 465nm and 490nm. The electron-phonon coupling (EPC) effect is believed to be more sensitive to the excitation wavelength. Different excitation wavelengths involve different electronic levels participating in the light emission processes, which explains the evolution behavior of the PL peak position with respect to the excitation wavelength. Moreover, the emergence of weak peaks next to the ZPLs at particular temperatures and excitation wavelengths is also observed. This work compares the influence of the temperature and excitation wavelength to the PL properties of the Cr:YAG transparent ceramic, which promotes an advanced understanding of the luminescence behavior of the Cr:YAG transparent ceramics.

Disentangling doping and strain effects at defects of grown MoS2 monolayers with nano-optical spectroscopy.

The role of defects in two-dimensional semiconductors and how they affect the intrinsic properties of these materials have been a widely researched topic over the past few decades. Optical characterization techniques such as photoluminescence and Raman spectroscopies are important tools to probe the physical properties of semiconductors and the impact of defects. However, confocal optical techniques present a spatial resolution limitation lying in a μm-scale, which can be overcome by the use of near-field optical measurements. Here, we use tip-enhanced photoluminescence and Raman spectroscopies to unveil the nanoscale optical properties of grown MoS2 monolayers, revealing that the impact of doping and strain can be disentangled by the combination of both techniques. A noticeable enhancement of the exciton peak intensity corresponding to trion emission quenching is observed at narrow regions down to a width of 47 nm at grain boundaries related to doping effects. Besides, localized strain fields inside the sample lead to non-uniformities in the intensity and energy position of photoluminescence peaks. Finally, two distinct MoS2 samples present different nano-optical responses at their edges associated with opposite strains. The edge of the first sample shows a photoluminescence intensity enhancement and energy blueshift corresponding to a frequency blueshift for E2g and 2LA Raman modes. In contrast, the other sample displays a photoluminescence energy redshift and frequency red shifts for E2g and 2LA Raman modes at their edges. Our work highlights the potential of combining tip-enhanced photoluminescence and Raman spectroscopies to probe localized strain fields and doping effects related to defects in two-dimensional materials.

Controlling photoluminescence of silicon quantum dots using pristine-nanostates formation

This study investigates the effective control of photoluminescence (PL) of silicon nanocrystals (Si–NCs) embedded in a SiO2 matrix, with a specific focus on tuning the luminescent wavelength profile and intensity for silicon-based optoelectronic applications. By manipulating the composition ratio (x) of SiOx, the well-known simultaneous changes in PL peak position and intensity were observed after annealing. To preserve the desired wavelength characteristics and enhance the luminesce intensity, we propose a novel approach to generate pristine nanostates (PNs) for Si-NC formation using proton irradiation (PI). The application of proton irradiation was found to be successful in significantly increasing PL intensity while strongly suppressing the peak position shifts. Through extensive spectroscopic analysis including PL, X-ray photoelectron spectroscopy, and absorption spectroscopy, we investigate specific defect states in the SiOx matrix, and identified as PNs. We interpret that these PNs consist of self-trapped exciton, E′ center, non-bridging oxygen hole center, and oxygen-deficiency center states. In the as-grown sample, the PNs promote appropriate phase separation and play a significant role in stabilizing the structure during the subsequent annealing processes. This study elucidates the Si-NC formation mechanism by enhancing O-diffusion-induced phase separation, a process accelerated in SiOx with controlled PNs. The proposed PI-induced PNs offer a novel pathway for developing Si-based optoelectronic devices with desired characteristics in the operating wavelength range. These findings open up promising possibilities for advanced silicon-based optoelectronic applications.

Multi-colour emission from ZnO[SO4]:Eu2+ nanophosphors: Effects of SO42- and Eu2+ doping concentration

Blue-emitting ZnO-SO4:Eu2+ powder phosphors were synthesized via solid state reaction method. Based on the X-ray diffraction (XRD) data, hexagonal wurtzite structures of ZnO were formed. Further structural elucidation via Raman spectroscopy showed a peak at 439 cm−1 associated with E2hgh optical mode that is a distinctive characteristic band of hexagonal wurtzite ZnO phase. The ZnO powders were composed of distorted spherical nanoparticles, which were agglomerated together as confirmed from scanning electron microscopy images. After incorporation of SO42- and Eu2+ into the ZnO lattices sites, the particle shapes became irregular. When excited at 351 nm with Xenon lamp, our samples emitted UV and blue-green visible light associated, respectively, with excitonic recombination and intrinsic structural lattice defects in ZnO. Both the photoluminescence (PL) peak intensity and positions of these emissions were influenced by SO42- and Eu2+ ions. The blue emission was dependent on the Eu2+ content with the largest intensity measured from the sample doped with 1.0 mol% of Eu2+ ions. Mechanisms of energy transfer and concentration quenching effect are discussed.

Investigating the effect of H+-ion irradiation on layered α-MoO3 flakes by defect engineering

Ion irradiation is a versatile and convenient tool for modifying the optical, electrical, and catalytic properties of two-dimensional (2D) materials through controlled induction of impurities and defects. The behavior of 2D materials under ion irradiation is interesting, which needs to be explored in the contest of their optoelectronic applications. In the present work, we have reported the effect of H+-ion irradiation on layered α-MoO3 flakes by defect engineering. Initially, the α-MoO3 crystals were synthesized using the physical vapor deposition technique followed by mechanical exfoliation of an as-grown crystal to obtain α-MoO3 flakes of different thicknesses. Then, the exfoliated flakes were exposed to H+-ion/proton irradiation with a fluence of 1 × 1016 ions/cm2 using a 30 keV source. After irradiation, new photoluminescence (PL) emission peaks were observed at different positions in the range of ∼2.4–1.9 eV, which was found to be absent in pristine flakes. Raman studies revealed non-uniform oxygen vacancy distribution in H+-ion irradiated α-MoO3 flakes, which affected the PL peak positions. Additionally, first-principle calculations and Bader charge analysis were performed to identify the origin of the new PL peaks. Our findings indicate that oxygen vacancies positioning at different locations of the α-MoO3 lead to the emergence of new absorption peaks within the range of ∼2.2–1.25 eV, which is consistent with our experimental findings. The present study gives insight into exploring the use of ion-irradiated α-MoO3 in optoelectronics applications with tunable properties.

Bandgap and Photoluminescence Tunability of Lead‐Free Cs3Bi2(Br,I)9 Solid Solution Compounds

Herein, lead‐free, mixed‐halide perovskites of cesium bismuth bromide/iodide are synthesized by a hydrothermal method, and their structures and optical properties of the solid solution compounds are examined. The synthesized compounds exhibit X‐ray diffraction patterns similar to that of trigonal Cs3Bi2Br9 (CBB), whose reflections are shifted to lower angles depending on the experimental iodine content. Very interestingly, even with the excessive introduction of iodine content in nominal, the symmetry of the crystals is kept to be trigonal, indicating that bromide CBB and iodide Cs3Bi2I9 (CBI) are crystallographically mixed in a facile way, although the stable crystal form of CBI is hexagonal. The optical properties of bandgap energy Eg and photoluminescence (PL) for the synthesized crystals are also examined. It is found that Eg decreases and PL peak position is correspondingly shifted to a longer wavelength with an increase in the experimental iodine content due to their band‐to‐band transitions. Moreover, first‐principles calculation suggests the reduction of Eg with iodine content and the varied nature of band structure from direct (trigonal CBB) to indirect (trigonal CBI) transition. These novel findings can make the proposed strategy successful for developing lead‐free, mixed‐halide Bi‐based perovskite crystals with the tunability of their optical properties.

Analysis of Temperature-Dependent Photoluminescence Spectra in Mid-Wavelength Infrared InAs/InAsSb Type-II Superlattice

Photoluminescence (PL) is one of the commonly used methods to determine the energy gap ({E}_{mathrm{g}}) of semiconductors. In order to use it correctly, however, the shape of the PL peak must be properly analyzed; otherwise, the value of {E}_{mathrm{g}} is burdened with a large error. {E}_{mathrm{g}} is often mistakenly attributed to the PL peak position, which in type-II superlattices (T2SLs) exhibits typical “S-shaped” behavior as a function of temperature, significantly different from the Varshni model used to define the energy gap of III-V compounds. The position peak of the PL relative to the real {E}_{mathrm{g}} in T2SLs is red-shifted because of the carrier localization at low temperatures and blue-shifted because of the free carrier emission at high temperatures. To correctly determine {E}_{mathrm{g}}, the shape of the PL peak should be analyzed using the theoretical PL line shape model that takes into account both localized (below the bandgap) and free carriers (above the bandgap) emissions. This work shows that the use of such a model to analyze the shape of the PL signal gives the correct results of determining {E}_{mathrm{g}} for mid-wave infrared InAs/InAsSb T2SL, which showed a significant contribution of localized states in optical transitions and characteristic “S-shaped” PL peak behavior. This allowed us to determine the correct values of the Varshni coefficients for a given T2SL. The result also agrees with the theoretical calculations of {E}_{mathrm{g}} made using the k·p method.

Influence of the Arsenic Pressure during Rapid Overgrowth of InAs/GaAs Quantum Dots on Their Photoluminescence Properties

In this paper, for the first time, we report a strong effect of the arsenic pressure used for the high-rate GaAs capping of self-assembled InAs quantum dots on their optical properties. A 140 nm red shift of the photoluminescence peak position is observed when the overgrowth arsenic pressure increases threefold. We explain this behavior in terms of different intensities of quantum dot decomposition, which occurs during the overgrowth under different conditions. When the arsenic pressure is sufficiently high, a GaAs capping layer is formed by deposited species with a low impact on initial quantum dots. At a low arsenic pressure, arsenic deficiency leads to the intensive intermixing caused both by the enhanced Ga/In atom exchange and by the consumption of arsenic atoms belonging to quantum dots for the GaAs capping layer formation. As a result of the overgrowth, quantum dots are divided into families with a large (high pressure) and a small (low pressure) average size, yielding long-wave (1.23 µm) and short-wave (1.09 µm) photoluminescence peaks, respectively. Thus, a significant influence of the overgrowth arsenic pressure on the characteristics of InAs quantum dots is evidenced in this study.

Probing Sulfur Vacancies in CVD-Grown Monolayer MoS2 on SiO2/Si in the Temperature Range 750–900°C

This work reports morphologically alike, high-quality monolayer MoS2 flakes with a similar strain at various growth temperatures (750–900°C) achieved by adjusting sulfur temperature. The growth dynamics of MoS2 are correlated with changes in the photoluminescence (PL) and Raman peak positions. Monolayer MoS2 crystals are synthesized at different growth temperatures from 750°C to 900°C using chemical vapor deposition (CVD). We examined the structural quality and aimed to extract the recombination mechanisms in MoS2 using low-temperature, variable, and low-laser-intensity PL measurements. Our studies of the defect-associated bound exciton emission are well correlated with the blueshift in the A1g mode of Raman spectra, blueshift in PL spectra, and x-ray photoelectron spectroscopy results for crystal grown at 900°C. Our research findings not only shed light on a thorough, non-intrusive method for modifying growth parameters to enhance optical performance, but they also suggest a way to modify the optical characteristics of MoS2 while maintaining the morphology.

Photoluminescence properties of Cu-poor Cu2Sn1− x Ge x S3 thin films with varying Ge/(Ge+Sn) ratio

This study investigates the photoluminescence (PL) spectra of Cu2Sn1−x Ge x S3 (CTGS) thin films, which are currently the most suitable composition ratio for high-efficiency absorbers through low temperature-PL measurements to reveal the effects of the x ratio on defect properties of CTGS thin films. The PL spectrum of Cu2SnS3 (CTS) thin films with x= 0.00 exhibits five peaks at 0.782, 0.832, 0.862, 0.885, and 0.933 eV. Moreover, all PL peak positions in the CTGS thin films shift to higher energies with increasing x ratios because the defect levels in the films changed with an increase in the x ratio. Moreover, we obtain the estimated activation energy (E a) values of the CTS thin films with x = 0.00 ranging from 6 to 20 meV. The E a values of CTGS are similar to those of the CTGS thin films, even at x ratios of up to 0.19 in CTGS thin films. The increasing x ratio in CTGS thin films does not influence the acceptor in CTGS. Therefore, the CTGS is advantageous as an absorption layer in solar cells rather than a CTS because E g can be large while maintaining a shallow acceptor. Hence, CTGS can be expected to be increasingly used like CuIn1−x Ga x Se2 and Cu2ZnSnS4 as next-generation absorption materials.

Transformation of carrier recombination mechanism as increasing the Germanium content in Cu2ZnGexSn1-xS4 single crystal prepared by molten salt method

Return to the physics origin of Cu2ZnSnS4 type (CZTS-type, include CZGS, CZGTS, CZTSSe, etc.) material system, studying the influence of replacement elements on the optoelectronic properties by preparing high-quality single crystal materials to clarify the relationship between the key defect states of materials and carrier dynamics is the only way which must be passed to find efficient paths to improve the existing performance of CZTS-type optoelectronic devices. Herein, the optical behavior of high-quality Cu2ZnGexSn1-xS4 (CZGTS) single crystal prepared by the molten salt method and its relationship with Ge content were systematically studied using a photoluminescence (PL) and absorption tests at room temperature. Combined with the simple electrical tests, the transformation of carrier recombination mechanism as increasing the Germanium content in CZGTS single crystal before and after phase transition were reported for the first time. The transmission electron microscopy images, X-ray diffraction spectra and Raman scattering spectra of CZGTS were adopted to reveal the evolution mechanism of single crystal growth and phase transition caused by the change of Ge composition. The results show that the tunable blue-shift of PL peak emerges with the increase of Ge content and the excitation power dependence has a saturation phenomenon in the CZGTS single crystal particles. In consideration of the optical bandgap, PL peak position, PL intensity, carrier lifetime and the macroscopic resistivity, it is confirmed that the intrinsic PL behavior could be effectively tailored by different Ge amounts. Compared with the recombination PL of interband carrier (Ep > Eg) in Cu2ZnSnS4, the band tailed carriers (Ep < Eg) dominated by defect states would play a powerful role in CZGTS single crystal. Our work provides a strategy for tailoring the bandgap of CZGTS materials and a new perspective for understanding the influence of defect states on carrier dynamics.

Abnormal Negative Thermal Quenching of Photoluminescence (PL) in Laser-Induced Exfoliated Black Phosphorus Quantum Dots for Applications as a Semiconductor PL Nanomaterial

Black phosphorus quantum dots (BPQDs) are promising candidates for semiconductor photoluminescence (PL) nanomaterials with the desired blue-violet emission and size-tuned optical response. Here, three prominent PL peaks were observed in the BPQDs with uniform nanosize, which were fabricated by pulsed-laser ablation of black phosphorus crystals. A sharp increase in the peak intensity was observed in the narrow temperature range of 225–250 K. This anomalous temperature dependence of the PL intensity is derived from negative thermal quenching, in which more electrons that are restricted in the intermediate states are thermally activated and excited to the lowest unoccupied molecular orbital (LUMO) with increasing temperature. The intermediate states were formed due to the defects induced by laser ablation. Consequently, the radiative transitions from LUMO to the σinner, σouter, and surface states dominate. By contrast, in the temperature ranges of 200–225 and 250–320 K, the PL intensity decreases because of normal thermal quenching, which usually occurs in conventional PL materials. The fitting result shows a nonmonotonicity similar to that in the negative thermal quenching semiconductors, reflecting the competition between normal and negative thermal quenching with changing temperature. Furthermore, the PL peak positions are blue-shifted with increasing temperature, which is associated with the stronger electron–phonon coupling and the lattice thermal expansion caused by transverse optical vibrations. The peak full width at half-maximum values broadened with temperature, which is attributed to the exciton–phonon interaction. This work is of great significance for not only a comprehensive understanding of the PL mechanism of the BPQDs but also the design of BPQDs-based optoelectrical nanodevices.

Effects of concertation and size of aqueous QDs on the detection of ionizing radiation based on changes in optical properties

Colloidal quantum dots (QDs) have recently been used in many applications. In particular, semiconductor and luminescent QDs are suitable candidate for use in optoelectronic devices and optical sensors. The optical properties of aqueous CdTe QDs with high efficiency photoluminescence (PL) make them good candidates for new dosimetry applications. Therefore, comprehensive studies are required of the effects of ionizing radiation on the optical properties of CdTe QDs. In the present study, we investigated the properties of aqueous CdTe QDs by a gamma 60Co source at different doses. For the first time, we determined the effects of the concentration and size of QDs, which are key factors in a gamma dosimeter. The results demonstrated the concentration-dependent photobleaching property of QDs, which grater changes in the optical properties occurred. The initial size of the QDs affected their optical properties, where red-shifting of PL peak position increased with smaller sizes. Analysis of the effect of gamma irradiation on the thin film QDs indicated that the PL intensity decrease as the dose increased. with increasing dose. X-ray diffraction analysis found no changes in the crystal structure. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy indicated decomposition of the capping agent thioglycolic acid on CdTe QDs after gamma irradiation.