Photoluminescence Peak Wavelength Research Articles

Effect of Cd precursors on luminescence of CdTe quantum dots and their luminescent temperature action

Highly luminescent CdTe quantum dots (QDs) is a typical luminescent material with great application potential. The photoluminescence (PL) and stability of the QDs depended strongly on Cd precursors, ligand (e.g. thioglycolic acid TGA) and preparation conditions. In this paper, CdTe QDs were prepared using Cd precursors with various molar ratios of TGA/Cd at different conditions. The composition of Cd-precursors depend strongly on the molar ratios of TGA/Cd and storage time due to the formation of Cd-TGA clusters which affects strongly the surface states of the QDs. The effect of the preparation conditions of Cd-precursors (e.g. molar ratios of TGA/Cd) on the PL peak wavelength, the full width at half maximum of PL spectra, PL quantum yields (PLQYs), fluorescence lifetime, and stability were tested in detailed. Interestingly, the PLQYs of the QDs prepared using a large molar ratio of TGA/Cd remarkably increased after Cd precursor stored in dark conditions for long time. This confirms that Cd-TGA clusters in the precursors drastically affected the synthesis of the QDs. The mechanism was discussed in detailed. These aqueous CdTe QDs with high PLQYs were obtained using a molar ratio of TGA/Cd of 1.25. In addition, the PL peak position of the QDs was red-shifted with increasing temperature, but is back to the initial value after cooling down to room temperature. Cycle test indicates red-emitting QDs reveal high stability.

Tuning the photoluminescence of large Ti3C2Tx MXene flakes

Ti3C2Tx MXene has been reported to be a metallic two-dimensional (2D) material with high conductivity, whereas its photoluminescence (PL) mechanism is still under debate. Herein, we demonstrate that large Ti3C2Tx MXene flakes exhibit tunable PL under ambient conditions. The as-prepared Ti3C2Tx MXene flakes emit blue, yellow-green and red light under different excitation wavelengths. Their PL emission wavelengths redshift as the excitation wavelength changes from violet to red light. Surface modification of the MXenes can further tune the PL peak wavelength into the near infrared region. Using density function theory (DFT) calculations, this excitation wavelength-dependent PL can be correlated to TiO2 defects that exist on the surface of Ti3C2Tx. Our study expounds on the optical properties of Ti3C2Tx MXene and is helpful for comprehensively understanding this novel material.

InAs/InP quantum dots stacking: Impact of spacer layer on optical properties

The optical properties of a vertical multilayer stack of semiconductor self-assembled quantum dots (QDs) depend on the coupling status. We present an improved modeling method for analysis of QD stacks, consisting of several separate steps in the elastic strain analysis, to reproduce the realistic unidirectional strain accumulation along the QD growth direction from lower and upper dots. Based on this modeling method, by varying the spacer thickness, we systematically study a large variety of QD stack structures, for better understanding the influence of strain and electronic coupling mechanisms. A “quasi continuum band” or highly degenerate excited state (ES) is found in the closely multi-stacked QDs, probably accounting for the occurrence of distinct blueshifting of the photoluminescence peak wavelength for the decreasing spacer thickness. The enhanced ES emission from this type of stacking QD modeled by using rate equations may pave a way for designing potential high-performance laser devices operating on ES.

Growth mechanisms for InAs/GaAs QDs with and without Bi surfactants

We report systematic study of growth of self-assembled InAs quantum dots (QDs) on GaAs substrate at various temperatures with and without exposure of bismuth surfactants. Results show that the coalescence amongst InAs QDs is considerably inhibited by the exposure of bismuth flux during growth in the temperature range from 475 to 500 °C, leading to improved dot uniformity and a modified dot density. The mechanism of the suppression effect by bismuth surfactants on the strain-induced islanding through inhibiting the indium adatom mobility and the evaporation rate on the surface kinetically is thus clarified for the growth of InAs QDs. The photoluminescence peak wavelength for the InAs QDs with Bi exposure red shifted slightly due to the suppression of Bi atoms on the QD dissolution during the capping process at higher temperatures. Moreover, by investigating the temperature-dependent quenching processes with and without Bi exposure, it is observed that the the weak carrier confinement occurred in QDs with the presence of Bi caused broadness in the linewidths.

Nanothick aluminate long-afterglow phosphors using inherited hydrothermal deriving

A novel strategy of the inherited hydrothermal method was used to prepare SrAl2O4: 0.01Eu2+, 0.01Dy3+ (SAO) without boron (ISAO) and boron-doping (BSAO) via NaAlO2 as an aluminum source and mineralizer at 200 °C for 6–24 h. The investigated optical properties was correlated to phase transformation and phosphor morphology. The main constituting phases of Al2(OOH)2 and SrCO3 transformed to monoclinic SrAl4O7 and mixed monoclinic/hexagonal SrAl2O4 then pure monoclinic SrAl2O4 during different hydrothermal period. The hydrothermally prepared samples exhibited flower-like morphology. The annealed plate phosphors with nanothick single crystals inherited from hydrothermal feature. The shift of photoluminescence (PL) peak wavelength corresponded to the phase transformation from SrAl4O7 to SrAl2O4 with hydrothermal time for ISAO and BSAO. The PL intensity increased with hydrothermal time due to achieving the pure SrAl2O4 phase with preferred orientation geometry and further enhanced by boron-doping. The afterglow of BSAO was significantly enhanced than ISAO, solid-state prepared MSAO and commercial SAO at the initial 100 s after excitation cut-off. The boron doped in the BSAO was scarcely on the grain surface as to small grain size. Thus, the afterglow enhancement should be attributed to the boron in the lattice inducing defect traps. Overall, the inherited hydrothermal and boron-doping derived nanothick SAOs with large surface preferred orientation enhanced the PL and afterglow properties.

Temperature and excitation wavelength-dependent photoluminescence of CH3NH3PbBr3 crystal.

We have investigated temperature and excitation wavelength-dependent luminescent properties of CH3NH3PbBr3 (MAPbBr3) perovskite crystal using steady-state and time-resolved photoluminescence (PL) spectroscopy. Optical spectroscopic results indicate that the PL intensity, peak wavelength, and full width at half maximum are jumped due to the phase transition from orthorhombic to tetragonal and cubic. As temperature increases, the peaks of above-bandgap and intrinsic PL have blueshift and redshift, respectively. The above-bandgap PL emerges under one-photon laser excitation and vanishes under the excitation of near-infrared femtosecond laser due to reabsorption. The initial redshift of PL peak and lifetime change at various wavelengths reveals the existence of a trap-assisted excitonic state. It is found that upconversion PL in all phases is composed of a photoinduced thermal-assisted and an intrinsic excitonic state, which produces the observation of biexponential dynamics. Our results can facilitate the understanding of the thermal effect of exciton recombination in hybrid perovskite crystal.

Photoluminescence Spectroscopy of Rhodamine 800 Aqueous Solution and Dye-Doped Polymer Thin-Film: Concentration and Solvent Effects

This paper investigates solvent and concentration effects on photoluminescence (PL) or fluorescence properties of Rhodamine 800 (Rho800) dyes formed in aqueous solution and polymer thin-film. Various commonly used organic solvents including ethanol, methanol and cyclopentanol were studied at a constant dye concentration. There were small changes in the PL spectra for the different solvents in terms of PL intensity and peak wavelength. The highest PL intensity was observed for cyclopentanol and the lowest for ethanol. The longest peak wavelength was found in cyclopentanol (716 nm) and the shortest in methanol (708 nm). Dissolving the dye powder in the methanol solvent and varying the dye concentration in aqueous solution from the high concentrated solution to highly dilute states, the wavelength tunability was observed between about 700 nm in the dilute state and 730 nm at high concentration. Such a large shift may be attributed to the formation of dye aggregates. Rho800 dye-doped polyvinyl alcohol (PVA) polymer thin-film was further investigated. The PL intensity of the dye in the form of thin-film is lower than that of the aqueous solution form whereas the peak wavelength is redshifted due to the presence of PVA. This paper, to our best knowledge, reports the first study of spectroscopic properties of Rho800 dyes in various forms and provides useful guidelines for production of controllable organic luminescence sources.

Pick a Color MARIA: Adaptive Sampling Enables the Rapid Identification of Complex Perovskite Nanocrystal Compositions with Defined Emission Characteristics.

Recent advances in the development of hybrid organic-inorganic lead halide perovskite (LHP) nanocrystals (NCs) have demonstrated their versatility and potential application in photovoltaics and as light sources through compositional tuning of optical properties. That said, due to their compositional complexity, the targeted synthesis of mixed-cation and/or mixed-halide LHP NCs still represents an immense challenge for traditional batch-scale chemistry. To address this limitation, we herein report the integration of a high-throughput segmented-flow microfluidic reactor and a self-optimizing algorithm for the synthesis of NCs with defined emission properties. The algorithm, named Multiparametric Automated Regression Kriging Interpolation and Adaptive Sampling (MARIA), iteratively computes optimal sampling points at each stage of an experimental sequence to reach a target emission peak wavelength based on spectroscopic measurements. We demonstrate the efficacy of the method through the synthesis of multinary LHP NCs, (Cs/FA)Pb(I/Br)3 (FA = formamidinium) and (Rb/Cs/FA)Pb(I/Br)3 NCs, using MARIA to rapidly identify reagent concentrations that yield user-defined photoluminescence peak wavelengths in the green-red spectral region. The procedure returns a robust model around a target output in far fewer measurements than systematic screening of parametric space and additionally enables the prediction of other spectral properties, such as, full-width at half-maximum and intensity, for conditions yielding NCs with similar emission peak wavelength.

Micro-photoluminescence mapping of surface plasmon enhanced light emissions from InGaN/GaN quantum wells

We conducted photoluminescence (PL) mapping to elucidate the detailed mechanism of PL enhancement based on the energy transfer from excitons to surface plasmon polaritons (SPPs) in silver-coated InGaN quantum wells (QWs). The PL mapping for bare InGaN QWs showed positive or negative correlations between the PL peak intensity and wavelength. These correlations are normally caused by exciton localization and the quantum confined Stark effect, respectively; however, they did not appear in the silver-coated region of the InGaN QWs, and the wavelength distribution shifted into shorter wavelengths due to the SP-induced PL enhancement. These results suggest that the energy transfer from the excitons to the SPPs should be much faster than that in the exciton localization and charge screening processes of the piezoelectric field in QWs.

Temperature sensing property of hollow-core photonic bandgap fiber filled with CdSe/ZnS quantum dots in an UV curing adhesive

Abstract A temperature sensor based on the hollow-core photonic bandgap fiber filled with the CdSe/ZnS QDs dissolved in an ultraviolet (UV) curing adhesive is reported. The sensor shows a linear variation of the photoluminescence (PL) peak wavelength for a temperature range from 40 °C to 140 °C, with a correlation factor of 0.99263 and a sensitivity of 0.05744 nm/°C. Although the peak intensity of emission spectrum increased exponentially with the temperature, a linear temperature-dependence result with a correlation factor of 0.99917 and a slope of 2.04 × 10 −3 °C −1 can be obtained with a self-reference spectral intensity method. The linear variation characteristics of the peak wavelength and the self-reference intensity of PL spectrum indicates the designed fiber temperature sensor is feasible in the practical application.

Fluorescence Temperature Sensor Based on GQDs Solution Encapsulated in Hollow Core Fiber

Graphene quantum dots (GQDs), a smart functional nano-sensing material, present excellent thermo-optic properties. Here a novel fiber-optic temperature sensor based on GQDs solution encapsulated in hollow core fiber (HCF) is first proposed. GQDs with the small size less than 20 nm are successfully filled in the inner part of HCF by capillarity, and the compact sensing probe is length of 6 mm. The relationships of the photoluminescence (PL) peak, intensity, and full-width at half-maximum of the PL spectra are investigated with temperature changing from 10 °C to 80 °C. The photoluminescence peak wavelength and the self-referenced intensity all present good sensitivities, which are 123.7 pm/°C and – 0.01375/°C, respectively.

Synthesis and photoluminescence characteristics of blue‐green luminescent aluminum oxynitride

AbstractAluminum oxynitride (AlON), which can be regarded a nitrogen‐stabilized cubic γ‐Al2O3, has attracted attention in terms of its good mechanical, chemical, and optical stability. Because of its optical inertness, however, photoluminescence (PL) emission from nominally pure AlON has not been carefully investigated and evaluated. In this work, we prepared visibly luminescent AlON by nitridation of γ‐Al2O3 under N2 atmosphere without adding aluminum nitride (AlN) using a high‐frequency induction heating unit. The resulting AlON exhibits a broad PL emission in the blue/green spectral region under excitation with light of ~260 nm. In the luminescent AlON sample, the excitation and emission events will occur at different sites; the electron transfer from the excitation site to the emission site is preceded by the radiative recombination process. It has also been found that the PL peak wavelength shows an anomalous blue shift by ~50 nm with increasing temperature from 78 to 500 K. The observed temperature dependent PL characteristics are governed by thermalization among multiple emitting levels. Aluminum vacancies and oxygen vacancies, both of which are introduced into the crystalline lattice during nitridation without the presence of AlN, are very likely candidates for the excitation and emission centers, respectively. Hence, the present direct nitridation method provides a simple and effective way to add an additional optical functionality to otherwise optically inactive AlON.

Emission wavelength selection for InGaAs quantum dots by anodic-aluminum-oxide membrane

Anodic-aluminum-oxide (AAO) membranes are fabricated and bonded on the surface of self-assembled InGaAs quantum dots (QDs) samples. The photoluminescence peak wavelengths for the AAO-bonded QDs samples are fixed despite by the different QDs structures of the samples, and the optical intensity is enhanced from the photonic-crystal radiation modes of the AAO quasi-periodic air-hole array.

GaAs1\u2212xBix/GaNyAs1\u2212y type-II quantum wells: novel strain-balanced heterostructures for GaAs-based near- and mid-infrared photonics

The potential to extend the emission wavelength of photonic devices further into the near- and mid-infrared via pseudomorphic growth on conventional GaAs substrates is appealing for a number of communications and sensing applications. We present a new class of GaAs-based quantum well (QW) heterostructure that exploits the unusual impact of Bi and N on the GaAs band structure to produce type-II QWs having long emission wavelengths with little or no net strain relative to GaAs, while also providing control over important laser loss processes. We theoretically and experimentally demonstrate the potential of GaAs1−xBix/GaNyAs1−y type-II QWs on GaAs and show that this approach offers optical emission and absorption at wavelengths up to ~3 µm utilising strain-balanced structures, a first for GaAs-based QWs. Experimental measurements on a prototype GaAs0.967Bi0.033/GaN0.062As0.938 structure, grown via metal-organic vapour phase epitaxy, indicate good structural quality and exhibit both photoluminescence and absorption at room temperature. The measured photoluminescence peak wavelength of 1.72 μm is in good agreement with theoretical calculations and is one of the longest emission wavelengths achieved on GaAs to date using a pseudomorphically grown heterostructure. These results demonstrate the significant potential of this new class of III-V heterostructure for long-wavelength applications.

Minimization of material inter-diffusion for thermally stable quaternary-capped InAs quantum dot via strain modification

In this study, a theoretical model is developed for investigating the effect of thermal annealing on a single-layer quaternary-capped (In0.21Al0.21Ga0.58As) InAs quantum dot heterostructure (sample A) and compared to a conventional GaAs-capped sample (sample B). Strain, an interfacial property, aids in dot formation; however, it hinders interdiffusion (up to 650 °C), rendering thermal stability to heterostructures. Three diffusing species In/Al/Ga intermix because of the concentration gradient and temperature variation, which is modeled by Fick’s law of diffusion. Ground-state energy for both carriers (electron and holes) is calculated by the Schrodinger equation at different annealing temperatures, incorporating strain computed by the concentration-dependent model. Change in activation energy due to strain decreases particle movement, thereby resulting in thermally stable structures at low annealing temperatures. At low temperature, the conduction band near the dot edge slightly decreases, attributed to the comparatively high strain. Calculated results are consistent with the experimental blue-shift i.e. towards lower wavelength of photoluminescence peak on the same sample with increasing annealing temperatures. Cross-sectional transmission microscopy (TEM) images substantiate the existence of dot till 800 °C for sample (A). With increasing annealing temperature, interdiffusion and dot sublimation are observed in XTEM images of samples A and B. Strain calculated from high-resolution X-ray diffraction (HRXRD) peaks and its decline with increasing temperature are in agreement with that calculated by the model. For highlighting the benefits of quaternary capping, InAlGaAs capping is theoretically and experimentally compared to GaAs capping. Concentration-dependent strain energy is calculated at every point and is further used for computing material interdiffusion, band profiles, and photoluminescence peak wavelength, which can provide better insights into strain energy behavior with temperature and help in the better understanding of thermal annealing.

Dismantling the "Red Wall" of Colloidal Perovskites: Highly Luminescent Formamidinium and Formamidinium-Cesium Lead Iodide Nanocrystals.

Colloidal nanocrystals (NCs) of APbX3-type lead halide perovskites [A = Cs+, CH3NH3+ (methylammonium or MA+) or CH(NH2)2+ (formamidinium or FA+); X = Cl–, Br–, I–] have recently emerged as highly versatile photonic sources for applications ranging from simple photoluminescence down-conversion (e.g., for display backlighting) to light-emitting diodes. From the perspective of spectral coverage, a formidable challenge facing the use of these materials is how to obtain stable emissions in the red and infrared spectral regions covered by the iodide-based compositions. So far, red-emissive CsPbI3 NCs have been shown to suffer from a delayed phase transformation into a nonluminescent, wide-band-gap 1D polymorph, and MAPbI3 exhibits very limited chemical durability. In this work, we report a facile colloidal synthesis method for obtaining FAPbI3 and FA-doped CsPbI3 NCs that are uniform in size (10–15 nm) and nearly cubic in shape and exhibit drastically higher robustness than their MA- or Cs-only cousins with similar sizes and morphologies. Detailed structural analysis indicated that the FAPbI3 NCs had a cubic crystal structure, while the FA0.1Cs0.9PbI3 NCs had a 3D orthorhombic structure that was isostructural to the structure of CsPbBr3 NCs. Bright photoluminescence (PL) with high quantum yield (QY > 70%) spanning red (690 nm, FA0.1Cs0.9PbI3 NCs) and near-infrared (near-IR, ca. 780 nm, FAPbI3 NCs) regions was sustained for several months or more in both the colloidal state and in films. The peak PL wavelengths can be fine-tuned by using postsynthetic cation- and anion-exchange reactions. Amplified spontaneous emissions with low thresholds of 28 and 7.5 μJ cm–2 were obtained from the films deposited from FA0.1Cs0.9PbI3 and FAPbI3 NCs, respectively. Furthermore, light-emitting diodes with a high external quantum efficiency of 2.3% were obtained by using FAPbI3 NCs.

Modification of degenerative photoluminescence in aged monolayer WS2 by PC61BM surface processing.

Owing to their unique physical properties, monolayer transition metal dichalcogenides (TMDCs) have been widely used in applications of light-emitting diodes (LEDs). However, monolayers of TMDCs undergo dramatic aging effects, including intense degradation in optical behavior, extensive cracking, and severe quenching of the direct gap photoluminescence (PL), seriously limiting the device performance. In this work, we show that monolayer WS2 stored for three months even in the glovebox exhibits obvious degenerative PL with changed peak position that can be attributed to the presence of a large number of trions induced by the aging effect. PC61BM surface processing was used to modify the surface of the aged monolayer WS2. As expected, higher uniformity in the PL spectrum was obtained. Besides, the PL peak wavelength was modified to be the same as that of the nonaged one and did not change even at higher excitation power. This strategy is shown to successfully suppress the formation of the trion by transferring the excess electrons from WS2 to PC61BM. The results demonstrate the feasibility of applying PC61BM surface modification to improve the performance of the LED based on monolayer WS2.

Scanning near-field microscopy of carrier lifetimes in m-plane InGaN quantum wells

Time-resolved scanning near-field photoluminescence (PL) spectroscopy was applied to map carrier lifetimes in wide m-plane InGaN/GaN quantum wells grown on on-axis and miscut substrates. Both radiative and nonradiative lifetimes were found to be spatially nonuniform. Lifetime variations were smaller for quantum wells grown on miscut, as compared to on-axis substrates. Correlation with surface topography showed that largest deviations of recombination times occur at +c planes of pyramidal hillocks of the on-axis sample. Observed correlation between radiative lifetimes and PL peak wavelength was assigned to a partial electron localization.

One-Pot Aqueous Synthesization of Near-Infrared Quantum Dots for Bioimaging and Photodynamic Therapy of Gliomas.

As the early detection and total destruction of gliomas are essential for longer survival, we attempted to synthesize a quantum dot (QD) that is capable of recognizing glioma cells for imaging and photodynamic therapy. Using a one-pot aqueous approach, near infrared-emitting CdTe was produced. After detection of its physicochemical characteriistics, it was conjugated with RGD. The emission images were observed with confocal microscopy. To test its toxicity, CdTe-RGD at various concentrations was separately added to a human glioma cell line (U251) and a mouse embryo fibroblast cell line (3T3) (control) for incubation in dark conditions. To test its photodynamic effect, the U251 and 3T3 cells were then irradiated for 5-60 min, using a 632.8-nm laser. This QD (Φ = 3.75 nm, photoluminescence (PL) peak wavelength = 700 nm, photoluminescence quantum yield (PLQY) = 20 %), was a spherical crystal with excellent monodispersity. Under a confocal microscope, U251 cells were visualized, but not the 3T3 cells. In dark conditions, the survival rates of both U251 and 3T3 cells were above 85 %. After laser irradiation, the survival rate of U251 cells decreased to 37 ± 1.6 % as the irradiation time and the CdTe-RGD concentration were increased. With good physicochemical characteriistics and low toxicity, this QD-RGD has broad prospects for use in the biomedical imaging and photodynamic therapy of gliomas.

Surface morphology and optical properties of InGaN/GaN multiple quantum wells grown on freestanding GaN (0001) substrates

Surface morphology and optical properties of InGaN-based multiple quantum wells grown on c-plane freestanding GaN substrates by metalorganic vapor phase epitaxy was investigated. The surface shows a dependence on the substrate crystallographic orientation, presenting wavy and mirror-like feature with the substrate off-cut towards the [112¯0] direction and the [11¯00] direction, respectively. To get sharp hetero-interfaces, InGaN multiple-quantum-wells-based blue laser diode wafers were grown on substrates with off-cut towards the [11¯00] direction. The photoluminescence peak wavelength of the blue laser wafers shows a linearly blue shift as the substrate off-cut angle increases. A minimum spectra line width of 90 meV@450 nm can be obtained with a substrate off-cut angle of about 0.5°. These surface and optical properties were ascribed to the step growth kinetics of the wurtzite type materials.