Dependent Photoluminescence Spectra Research Articles

Optical and EPR study of Mn4+ ions in different crystal environments in Mn, Li co-doped MgO

The influence of Li ion on the symmetry of Mn4+ center and its photoluminescence (PL) in MgO:0,01%Mn, 3% Li ceramics is studied by the X-ray diffraction, electron paramagnetic resonance (EPR), PL and optical absorption methods. It is shown that Li co-doping stimulates conversion of Mn2+ on Mg site into Mn4+ and can decrease symmetry of Mn4+ in a case of their specific close arrangement in the crystal lattice. Two kinds of Mn4+ centers are identified in the EPR and PL spectra: cubic Mn4+ and tetragonal complex Mn4+-Li+. The parameters of corresponding EPR centers are estimated from the experimental spectra (gcub=1.994, |A|cub=70.9x104 cm-1; g1tetr=g2tetr=1.994, g3tetr=1.993, |A|tetr=71x104 cm-1, |D|tetr=220x104 cm-1). The PL lines observed in the low temperature PL spectra are assigned to the specific Mn4+ related centers by using a complementary study of PL excitation spectra and temperature dependent PL spectra. The lines at 654 nm and 671 nm are ascribed to R-line of Mn4+ in a site of cubic symmetry and its phonon overtone, correspondingly, as well as the lines at 666 nm and 681 nm are attributed to R2 line of Mn4+ in site with tetragonal symmetry and to its phonon overtone, correspondingly. It is shown that in Mn doped MgO ceramics made without intentional Li co-doping, the cubic Mn4+ center can be present and contribute to the PL spectra.

Wavelength-tunable photoluminescence of CsPbBr3 microcrystal via in situ halogen doping

Lead halide perovskite semiconductors have received significant attention in recent years as promising candidates for optoelectronic applications. Their controllable bandgap in nanostructures (nanocrystals, nanowires) has been widely and successfully investigated. Intentionally manipulating the atomic behavior with dopants in their macroscopic structure with high quality and stability is still challenging. We studied the crystal structure and photoluminescence (PL) behavior of the anion (Cl) doped CsPbBr3 microcrystals. The doped microcrystal with a low doping concentration has shown a large wavelength-tunable PL emission from 535 nm to 457 nm. Two separate emission peaks (blue and green) are carefully studied in the excitation power dependent PL spectra. The doping and excitation endow the perovskite microcrystal adjustable PL emission band and peak intensities, which may serve as a new degree of manipulation freedom in the color-tunable display and functional optoelectronics.

Photoluminescence Enhancement in Silica-Confined Ligand-Free Perovskite Nanocrystals by Suppression of Silanol-Induced Traps and Phase Impurities.

The detriments of intrinsic silanol groups in mesoporous silica to the photoluminescence (PL) of lead halide perovskite nanocrystals (LHP NCs) confined in the template have never been determined and clearly elucidated. Here, we disclose that silanol-induced Cs+ and Br- deficiencies prompt the generation of traps and CsPb2Br5 impurities. The temperature-dependent PL spectra verify the higher energetic barrier of trap states in CsPbBr3 NCs confined in silanol-rich mesoporous silica. Femtosecond transient absorption spectra reveal the trapped state mediates a broadband photoinduced absorption and long-lived decay pathway of CsPbBr3 NCs in silanol-rich templates. A remarkable improvement (up to 160-fold) in PL quantum yields is realized by simple silanol elimination. This work demonstrates the detrimental effects of silanol sites on the PL properties of LHP NCs impregnated in mesoporous silica and provides a new perspective for the ligand-free synthesis of high-quality LHP NCs in mesoporous templates by facile impregnation for practical applications.

Exploring potential of quantum dots for high power solid-state lighting protected by transparent ceramics

When quantum dot diffusion plates (QDDPs) are used as phosphor conversion materials for high-power lighting, transparent ceramics (TCs) can be used to prevent thermal damage of QDDPs. The spectral, suitable for diverse lighting environments, can be precisely calibrated by modifying their ratio. The temperature dependent photoluminescence spectra, fluorescence lifetime, Huang Rhys factor and activation energy of TCs were studied in detail. It was evident from the results that the intensity of TCs remained above 89% even at 423 K, while fluorescence lifetime remained stable, nearly 64 ns. Huang-Rhys factor and activation energy were consistently measured at 1.81 and 0.13 eV, respectively. The extraordinary luminescent stability of TCs revealed their integral role in providing reliable protection for QDDPs. The spectra of LEDs/LDs were successfully predicted using Monte Carlo ray tracing method. The service condition under high-energy laser was studied using finite element method. White light with Commission Internationale de l'Eclairage (CIE) of (0.38, 0.39) and (0.31, 0.34) and luminous efficiency of 58 and 150 lm/W were prepared by combining QDDPs/TCs with LEDs/LDs, and the Color Rendering Index (CRI) reached a remarkable 88.4 and 78.8. A detailed exploration of the proficient thermal management was accomplished using finite element analysis, with the simulated outcomes matching closely with the experimental results.

Photoluminescence Enhancement in Silica‐Confined Ligand‐Free Perovskite Nanocrystals by Suppression of Silanol‐Induced Traps and Phase Impurities

AbstractThe detriments of intrinsic silanol groups in mesoporous silica to the photoluminescence (PL) of lead halide perovskite nanocrystals (LHP NCs) confined in the template have never been determined and clearly elucidated. Here, we disclose that silanol‐induced Cs+ and Br− deficiencies prompt the generation of traps and CsPb2Br5 impurities. The temperature‐dependent PL spectra verify the higher energetic barrier of trap states in CsPbBr3 NCs confined in silanol‐rich mesoporous silica. Femtosecond transient absorption spectra reveal the trapped state mediates a broadband photoinduced absorption and long‐lived decay pathway of CsPbBr3 NCs in silanol‐rich templates. A remarkable improvement (up to 160‐fold) in PL quantum yields is realized by simple silanol elimination. This work demonstrates the detrimental effects of silanol sites on the PL properties of LHP NCs impregnated in mesoporous silica and provides a new perspective for the ligand‐free synthesis of high‐quality LHP NCs in mesoporous templates by facile impregnation for practical applications.

Green Emission of Erbium Doped SYW Phosphors for Optical Thermometry And Solid‐State Lighting

AbstractA series of Er3+ ions doped Sr9Y2W4O24 (SYW) phosphors have been synthesised using solid‐state reaction technique. For optical thermometry and solid state lighting applications, the crystal structure, morphological, and luminescence features of the SYW : Er3+ phosphors were explored. X‐ray diffraction (XRD) reveal that synthesized phosphors having single phase with a tetragonal crystal structure and I41/a space group. Scanning electron microscopy (SEM) was used to examine the morphological behaviour and presence of composition elements. The optical bandgap of the phosphor was evaluated using UV‐Vis diffuse reflection spectra (DRS). The photoluminescence (PL) emission spectra were captured under 380 nm excitation, and the peak with the maximum intensity was identified at 563 nm ascribed to transition 4S3/2→4I15/2, that emits green color in visible region. The temperature dependent PL (TD‐PL) spectra show that SYW phosphor is substantially more thermally stable, having thermal activation energy of about 0.247 eV. Additionally, the fluorescence intensity ratio (FIR) was used to study the optical sensing characteristics of the thermal quenching transitions (2H11/2, 4S3/2) with maximum relative sensitivity (SR) and an absolute sensitivity (SA) being 1.33 % K−1and 0.307 % K−1, respectively. All of the aforementioned findings demonstrate that SYW : Er3+ ions doped phosphors have potential for optical thermometry and other photonic devices.

Optical and luminescent characteristics of thermally stable new Eu3+ doped potassium tungstate tellurite glasses for epoxy-free luminescent devices

Transparent, Eu3+ doped potassium tungstate tellurite (TKWZBiEu) glass matrices were successfully synthesized via employing the traditional melt quenching method and their thermal, structural and photoluminescent characteristics were thoroughly investigated. To estimate the aggregate weight loss, glass transition temperature (Tg) and thermal stability factor (ΔT) of the prepared host glass matrix, thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) were utilized. The non-crystalline character of the prepared TKWZBiEu glass was studied via XRD profile. Various vibrational functional groups were revealed via employing Fourier transform infrared (FT-IR) spectroscopy. The optical bandgap (Eopt) values for all prepared TKWZBiEu glasses have been evaluated by employing the absorption spectra. Under n-UV and blue excitations, all the prepared TKWZBiEu glasses are demonstrating reddish emission at 614 nm ascribed to the 5D0 → 7F2 transition, in which the intensity is increasing continuously with Eu3+ ion content up to 5.0 mol%. The experimental lifetime (τ) profiles demonstrate the single-exponential nature of prepared TKWZBiEu glasses under n-UV excitation. Furthermore, temperature dependent photoluminescence (TDPL) spectra indicate excellent thermal stability of the TKWZBiEu glass matrix with the highest value of activation energy (ΔE). The prototype organic epoxy resin/binder-free device has been developed using the 5.0 mol% Eu3+ doped with TKWZBi glass matrix and n-UV LED chip. All the aforementioned findings validate that the optimized TKWZBiEu glass is an auspicious candidate for the red component to fabricate organic epoxy-free w-LEDs.

Luminescence properties of Mn-doped 2D organic-inorganic hybrid perovskites: Insights from (PPA)2PbBr4 perovskite

The incorporation of Mn2+ ions has been widely used to modulate optical properties of two-dimensional (2D) organic-inorganic hybrid perovskites (OIHPs). However, the luminescence mechanism of Mn2+ ions that is related to the A-site cations remains largely unexplored. Herein, we report the synthesis of a series of Mn-doped A2PbBr4 (A = C6H5(CH2)nNH3, n = 1,2,3,4,) 2D perovskites. The (C6H5(CH2)3NH3)2PbBr4 ((PPA)2PbBr4) 2D perovskite exhibited a maximum photoluminescence (PL) quantum yield of 67 %, a prolonged PL lifetime of 0.75 ms, and the highest energy transfer efficiency of 78.07 %. Furthermore, we systematically investigated the luminescence properties of Mn-doped (PPA)2PbBr4 perovskite with different Mn2+ concentrations. The temperature dependent PL spectra indicate a thermalized de-trap process of self-trapped exciton at high temperatures and the presence of permanently shallow defect states in highly doped samples with lower activation energy.

The realization of the nanoscale bar-codes based on CdS branched nanostructure

The nanostructures with controllable emission colors have shown great attention for achieving high throughput sensing, anti-counterfeiting, and information security. Here, a branched CdS nanostructure with Sn nanoparticles implanted in the junctions is proposed to construct lateral photonic barcodes in a two dimensional domain. Due to the encapsulated Sn nanoparticles, the plasmon enhanced photoluminescence is practically observed. However, with the generation of the high-energy electrons under illumination of the electron beam, the transfer process occurring between the CdS and Sn nanoparticles, result in cathodoluminescence quenching at the junctions. In addition, both temperature coefficients and Debye temperatures of the CdS branched nanostructures are determined to be 5.56 × 10−4 eV/K and 97 K, respectively, based on the temperature dependent photoluminescence spectrum. Also, the emission intensity of the band edge and the defect related emission can be well tuned by the excitation power. As the laser power increases, the emission color changes from red to the green. Therefore, the coding capacity of the CdS branched barcodes can be well modulated by the temperature and pump laser power. Our studies demonstrate the possibility of using these branched nanostructures in multifunctional optical devices.

Investigations on photoluminescence and energy transfer studies of Sm3+ and Eu3+ ions doped Sr9Y2W4O24 red emitting phosphors with high color purity for w-LEDs

Sm3+/Eu3+ co-doped Sr9Y2W4O24 phosphors were prepared using the solid-state reaction process. The structure and micro-morphological analysis of the as-prepared phosphors were explored via x-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Optical & spectroscopic characteristics were investigated by the mean of diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) techniques. The Sr9Y2W4O24 phosphor is tetragonal in structure and belongs to the I41/a space group. The excitation spectra recorded demonstrates that near-ultraviolet and blue LED chips can effectively excite the titled Sm3+-Eu3+ co-doped phosphors. The emission intensity pertaining to Eu3+ ions increases at the expense of decrease in intensity of Sm3+ ions with increase in the concentration of Eu3+ ion in titled Sm-Eu co-doped Sr9Y2W4O24 phosphors. The Reisfeld's approximation applied to the emission spectra of the titled Sm3+-Eu3+ co-doped Sr9Y2W4O24 phosphor reveal quadrupole-quadrupole as the mechanism responsible for energy transfer (ET) from Sm3+ to Eu3+ ions. The PL decay lifetimes values recorded for 603 nm emission of Sm3+ ions under 407 nm excitation wavelength were found to be decreasing with increase in Eu3+ ions concentration. This clearly reveal the ET from Sm3+ to Eu3+ ions in the titled phosphors. With increase in Eu3+ ion concentration the calculated CIE parameters are gradually shifting towards the deep red region of the visible spectrum under UV excitation. The temperature dependent PL (TD-PL) spectra demonstrate that the Sr9Y2W4O24: Sm3+/Eu3+ phosphor has relatively better thermally stablity with its thermal activation energy at around 0.256 eV. The results obtained in the present investigation allows us to explore the utility of Sm3+/Eu3+ co-doped Sr9Y2W4O24 phosphors as an effective deep red emitter needed to fabricate white LEDs.

Optical temperature sensing of four modes by using CsPb(Cl/Br)3 quantum dots and Tb3+ ions co-doping glass

We have fabricated CsPb(Cl/Br)3 quantum dots (QDs) and Tb3+ co-doping glass and comparatively investigated four-mode temperature sensing of fluorescence intensity ratio (FIR), full width at half maximum (FWHM), lifetime and chromatic coordinate. Temperature dependent photoluminescence (PL) spectra are measured. The Tb3+ ions and CsPb(Cl/Br)3 QDs have independent luminescence and FIR of Tb3+ to QDs has obvious decrease with temperature, which is used for FIR temperature sensing. The PL and PL decay of QDs are used for FWHM and lifetime temperature sensing. The change of FIR with temperature leads to the change of luminous color and chromatic coordinate with temperature. A new chromatic temperature sensing method is established, which utilizes linear relation of chromatic coordinate and temperature. Temperature dependence for FIR, FWHM, lifetime and chromatic coordinate has been fitted. Absolute sensitivity, relative sensitivity, temperature accuracy and repeatability have been evaluated. Among the four sensing modes, the FIR and chromatic coordinate modes have large sensitivity and small accuracy. The FIR mode is better than chromatic coordinate mode in absolute sensitivity and relative sensitivity, whereas the chromatic coordinate mode is better in its good linear relation. The four sensing modes have excellent repeatability due to the stability of QDs-Tb3+ co-doping glass.

High efficiency mid-infrared interband cascade light emitting diodes with immersion lens

We report on ten-stage interband cascade light-emitting diodes (ICLEDs) using an InAs/GaAsSb superlattices active region with a peak emission wavelength of 4.9 μm at the temperature of 80 K. The ICLED devices integrated with an immersion lens achieve a wall-plug quantum efficiency of 6.6% and an emittance of 1.9 W/cm2 under 80 K and 7.7 A/cm2, which is seven times larger than the basic device without the immersion lens. We present a detailed analysis of the recombination rates and their relationship with the quantum efficiency. The Shockley–Read–Hall and Auger recombination rates were measured using carrier-density dependent time-resolved photoluminescence spectra. The band structure of InAs/GaAsSb superlattices is calculated to study their relationship with the Auger recombination rates.

Influence of co-doping alkali metal ions Li+/Na+/K+ on the luminescence enhancement properties of the blue emission from Pb2+/Bi3+ activated di-barium calcium di-zinc hexa-silicate based phosphors for solid state light applications

The non rare earth element Pb2+/Bi3+ activated and alkali metal Li+/Na+/K+ ions codoped with the silicate based Ba2CaZn2Si6O17 (hereafter mentioned as BCZSO) blue-emitting phosphors were prepared through a simple solid state reaction route and characterized. The phase identification of the as prepared samples was done by taking the powder X-ray diffraction (PXRD). The Rietveld refinement of the as prepared phosphor was analyzed. The morphological analysis was performed using the scanning electron microscope (SEM). The photoluminescence (PL) and PL excitation (PLE) properties of the phosphors were investigated. Exciting the BC1-xZSO:xPb2+ phosphor with 295 nm light yielded a wide blue emission centered at 438 nm. Also, when the BC1-xZSO:xBi3+ phosphor was excited with 317 nm, a bright blue emission at 465 nm was observed. The Stokes shift values of these phosphors were calculated to be 11,067, and 10,040 cm−1 in the same order. The optimum concentrations for the Pb2+, and the Bi3+ ions in the BCZSO host were found to be 3 and 7 mol % respectively. The critical distance (Rc) values were calculated to be 29.32 Å and 23.05 Å for the Pb2+ and the Bi3+ activated BCZSO phosphors respectively. When the alkali metal ions were co-doped in the blue emitting phosphors, a PL enhancement was observed. The decay lifetime was calculated for the BCZSO: M (M:Pb2+/Bi3+) phosphors, and found to decline with increasing concentration of the activators. The CIE color coordinates, and the quantum efficiency of these silicate phosphors were calculated. These values for the BC1-xZSO:xPb2+ phosphor were closer to the commercial blue BaMgAl10O17:Eu2+ (BAM) phosphor. In addition, the temperature dependent photoluminescence (TDPL) spectra were recorded to examine the thermal stability of this phosphor. These results suggest that the BCZSO:M (M:Pb2+/Bi3+) and the BC1-x-yZSO:xPb2+/Bi3+, yLi+/Na+/K+, blue emitting phosphors can serve as promising candidates for white light emitting diodes (WLEDs).

Tri-chromatic luminescence of CsPb(Cl/Br)3 quantum dots-Tb3+-Mn2+ co-doped single component glasses for white LED application

Recently, CsPbX3 quantum dots (QDs) doping Mn2+ have been researched for application in white LED, but the main problem is low color render index (Ra) due to deficient green light. We have fabricated novel CsPb(Cl/Br)3 QDs-Tb3+-Mn2+ trichromatic glasses for white LED application. The interaction mechanism has been investigated by using QDs, Mn, Tb, QDs-Mn, QDs-Tb, Mn–Tb and QDs-Mn-Tb samples. Wide tri-chromatic photoluminescence (PL) spectra have been obtained by optimizing PL peak of the QDs, concentrations of QDs, Tb3+ and Mn2+, and heat treatment conditions. The PL peak of QDs is 442 nm. The molar concentrations are 2%, 0.36–0.60% and 3% for CsPb(Cl/Br)3 QDs, Tb3+ and Mn2+ respectively. The heat treatment conditions are 420 for 3 h and 450–470 °C for another 3 h. Color coordinates are adjusted in the warm white light range. Correlated color temperature (CCT) is adjustable in the range of 3136–4474 K. Ra is in the suitable range of 66–86. Based on temperature dependent PL spectra in the range of 25–80 °C, the color coordinates, CCT and Ra are calculated and they are still in the warm light range and suitable for white LED application. The trichromatic glasses excited at 378 nm have potential application in white LED lighting.

Lower threshold current density of GaN-based blue laser diodes by suppressing the nonradiative recombination in a multiple quantum well.

The influence of the nonradiative recombination in a multiple quantum well of GaN-based blue laser diodes (LDs) has been are studied experimentally and theoretically by analyzing the optical and electrical properties of LDs with various thickness and indium content of quantum wells (QWs). It is found that when keeping the LD emission wavelength nearly unchanged, the LD device performance with thinner QW and higher indium content of InGaN QWs is much better than the LD with thicker QW and lower indium content, having smaller threshold current density, higher output optical power and larger slope efficiency. Typically, the threshold current density is as low as 0.69 kA/cm2, and the corresponding threshold current is only 250 mA. The lifetime is more than 10,000 hours at a fixed injection current of 1.2 A under a room-temperature continuous-wave operation. Characteristics of photoluminescence (PL) microscopy images, temperature dependent PL spectra, time-resolved PL and electroluminescence spectra demonstrate that a reduction of the nonradiative recombination centers and an improvement of homogeneity in QWs are the main reason for the performance improvement of GaN-based LD using thinner QW layers with a higher indium content in a certain range. Moreover, theoretical calculation results demonstrate that using a thinner quantum well is also helpful for improving the device performance if the change of alloy material quality is considered during the calculation.

Cs4CuSb2Cl12−x I x (x = 0–10) nanocrystals for visible light photodetection

Lead-free layered double perovskite nanocrystals (NCs) with tunable visible range emission, high carrier mobility and low trap density are the need of the hour to make them applicable for optoelectronic and photovoltaic devices. Introduction of Cu2+ in the high band gap Cs3Sb2Cl9 lattice transforms it to the monoclinic Cs4CuSb2Cl12 (CCSC) NCs having a direct band gap of 1.96 eV. The replacement of 50% Cl− by I− ions generates <5 nm Cs4CuSb2Cl6I6 (C6I6) monodispersed NCs with an unchanged crystal system but with further lowering of the band gap to 1.92 eV. The p-type C6I6 NCs exhibit emission spectra, lower trap density, appreciable hole mobility and most importantly a lower exciton binding energy of only 50.8 ± 1.3 meV. The temperature dependent photoluminescence (PL) spectra of the C6I6 NCs show a decrease in non-radiative recombination from 300 K down to 78 K. When applied as the photoactive layer in out-of-plane photodetector devices, C6I6 NC devices exhibit an appreciable responsivity of 0.67 A W−1 at 5 V, detectivity of 4.55 × 108 Jones (2.5 V), and fast photoresponse with rise and fall time of 126 and 94 ms, respectively. On the other hand, higher I- substitution in Cs4CuSb2Cl2I10 NCs (C2I10) degrades the lattice into a mixture of monoclinic and trigonal crystal phases, which also lowers the device performance.

A comprehensive study on structure, opto-nonlinear and photoluminescence properties of Co3O4 nanostructured thin films: An effect of Gd doping concentrations

Cobalt oxide (Co3O4) thin films with 0, 1.0, 3.0 and 5.0 wt% Gd doping concentrations were fabricated on glass substrates by spray pyrolysis method. X-ray diffraction pattern (XRD) of as-deposited thin films confirm spinel structure growth oriented along (311) plane. Crystallite sizes of the grown films decreases with the increase in Gd doping concentrations and found in range of ∼17.7–7.2 nm. Raman investigation of films further approves spinel growth of cobalt oxide films. Surface morphology of pure and Gd: Co3O4 thin films were analyzed using field emission scanning electron microscopy (FESEM) with round shape particles at nanoscale. Chemical compositions, elements, and their stoichiometry were observed by energy dispersive spectra (EDX). Tauc's plots of films clearly shows two direct optical band gaps, (1) in the range of 1.49–1.52 eV and (2) in the range of 2.01 eV–2.58 eV respectively. A significant impact of Gd doping was observed on second optical band gaps, which increases with Gd contents. Room temperature (RT) dependent photoluminescence (PL) spectra of the deposited films showed that emission peaks at 363, and 753 nm under excitation wavelength of 240 and 500 nm respectively. The third order nonlinear optical susceptibility χ(3) and nonlinear refractive n(2) which were calculated and found in range 4.46 × 10−13- 2.25 × 10−10 esu and 1.01 × 10−10-2.39 × 10−9 esu., respectively.

Improved two-step photon absorption current by Cl-doping in ZnTeO-based intermediate band solar cells with n-ZnS layer

We report the improved photocurrent induced by two-step photon absorption (TSPA) in the intermediate band solar cells (IBSCs) with a n-ZnS/ZnTe/ZnTeO/ZnTe/p-ZnTe structure using Cl-doped ZnTeO. In the IBSC with an undoped ZnTeO, the TSPA current is not observed at room temperature but it is significantly improved by using Cl-doped ZnTeO. Temperature and excitation intensity dependent photoluminescence (PL) spectra reveal the high activation energy of Cl-donor, and the observed activation temperature by PL measurements is consistent with the temperature where the TSPA current starts to increase. The temperature dependence of the open circuit voltage of the IBSC using Cl-doped ZnTeO also supports that the Cl doping in the ZnTeO layer introduces electrons into the intermediate band.

Carbon Dots from Turnip Juice: Synthesis, Characterization and Investigation of pH-Dependent Optical Properties

In this study, carbon dots (CDs) were synthesized using a simple one-pot hydrothermal method by using turnip juice as carbon source. The water-soluble nontoxic carbon dots were obtained after reaction. The structural and optical properties of as synthesized CDs were elucidated by X-ray diffraction (XRD), Raman, Fourier Transmission Infrared (FTIR), UV–Vis absorption and Photoluminescence (PL) spectroscopy. A contour plot of the excitation dependent PL spectra of the turnip juice derived CDs and PL emission spectra in 3D color map were plotted. The maximum PL emission was found at 434 nm when excited at 364. The pH dependent luminescence properties of the CDs were investigated from pH 2-12 range in phosphate buffer solution (PBS). The resulting CDs can be evaluated in a variety of application areas, from anti-counterfeiting to bioimaging.

Hot carrier redistribution, electron-phonon interaction, and their role in carrier relaxation in thin film metal-halide perovskites

Temperature dependent (4K-300K) photoluminescence and transmission spectra are analyzed to study the effect of changing the different components of a perovskite compound, be it A, B, or X. Four different films are compared: FAMAPbSnI3, FAPbI3, FAMAPbI3, and FAPbBr3. The low temperature results highlight the changes that occur especially, underlying ones that are easily masked at room temperature. The overall Stokes shift is of similar magnitude at room temperature for the 3 Pb only based samples. This is governed by the interaction strength GammaLO, phonon energy ELO, and the exciton binding energy Eex. One exception to this behavior is the Sn-based FAMAPbSnI3 film which shows a lack of Stokes shift between the absorption and photoluminescence. However,the strong absorption (more than 100 meV) below the band gap is indicative of an excitonic feature that has a large density of states. Transient absorption measurements confirm the trends observed in continuous wave (CW) measurements, the 3 Pb only films all show the convolution of an excitonic feature within 20 meV of the band gap as a contributing factor to the photobleach along with a region of high energy PIA. However, the behavior for the Sn-based film is notably different (just as in the CW measurements) with an unusual low energy PIA and a lack of high energy PIA. The large low energy PIA is attributed to the large sub band gap absorption observed in the CW transmission/absorption measurements. Notably regardless of interchanging components, the slow cooling of carriers in metal-halide perovsites shows little effect of GammaLO, ELO, and Eex. As such, here it is proposed: while the initial cooling of carriers is attributed to LO phonons, the overall cooling of carriers is dominated by the intrinsic low thermal conductivity of all metal-halide perovskites which limits the dissipation of acoustic phonons in these systems.