Temperature Dependence Of Photoluminescence Spectra Research Articles

Temperature dependence of photoluminescence spectrum of single lead halide perovskite nanocrystals: Effect of size on the phase transition temperature.

Photoluminescence (PL) spectroscopy of individual semiconductor nanocrystals (NCs) is a powerful method for understanding the intrinsic optical properties of these materials. Here, we report the temperature dependence of the PL spectra of single perovskite FAPbBr3 and CsPbBr3 NCs [FA = HC(NH2)2]. The temperature dependences of the PL linewidths were mainly determined by the Fröhlich interaction between excitons and longitudinal optical phonons. For FAPbBr3 NCs, a redshift in the PL peak energy appeared between 100 and 150K, which was because of the orthorhombic-to-tetragonal phase transition. We found that the phase transition temperature of FAPbBr3 NCs decreases with decreasing NC size.

CdTe quantum dots film by electrospinning as bearing temperature sensor

Temperature monitoring of bearing rotating elements is crucial for bearing condition assessment and early warning. Using CdTe quantum dots (QDs) to prepare sensors requires other membrane material as the substrate, which affects the accuracy of the test. Meanwhile, CdTe QDs exposed to air for long time will lead to oxidation, which affects the storage life of the sensors. Hence, based on excellent film forming properties of polyvinyl alcohol (PVA), the CdTe/PVA polymeric nanofiber was fabricated by electrospinning method. For comparison, the CdTe/PVA composite film by drop-casting method was also prepared. Thermal properties show that there are excellent linear relationships between peak wavelength and temperature of the CdTe/PVA composite film and the CdTe/PVA polymeric nanofiber respectively. However, the uniformity of the CdTe/PVA polymeric nanofiber obviously better than the CdTe/PVA composite film. Outstanding temperature dependence of photoluminescence spectra and uniform film formation demonstrating the potential value of the obtained CdTe/PVA polymeric nanofiber as temperature sensor for bearing rotating elements.

Effect of Temperature and High Pressure on Luminescence Properties of Mn3+ Ions in Ca3Ga2Ge3O12 Single Crystals

In this work, the temperature and high-pressure behaviors of Mn3+-doped garnet-type Ca3Ga2Ge3O12 single crystals have been investigated by means of photoluminescence and Raman spectroscopy, respectively. The Jahn–Teller stabilization energy in the 5E ground state was found to be 1630 cm–1, being as much as 6 times greater than that in the 5T2 excited state, that is, 237 cm–1. The room-temperature emission spectrum is dominated by the spin-allowed 5T2 → 5E transition at 670 nm upon 532 nm excitation. The temperature dependences of photoluminescence spectra and time decays reveal strong thermalization between 5T2 and 1T2 levels. Cooling to 50 K empties the 5T2 level by virtue of multiphonon transition to the lower-lying 1T2 level. As a result, the 1T2 → 3T1/5E electric-dipole transitions are induced by coupling to odd-parity phonons. The existence of a small amount of Mn4+ can be ascertained by lowering the temperature or applying high pressure. Upon compression up to 100 kbar, the 5T2 → 5E transition of Mn3+ undergoes a blue shift at a rate of ∼10.5 cm–1/kbar. Since both 5T2 and 1T2 have inverse crystal field dependences, the quenching process from the 5T2 level becomes stronger under high pressure. Cryogenic luminescence ratiometric thermometry based on the diverse thermal quenching behaviors of Mn3+ and Mn4+ was explored. Furthermore, theoretical calculations employing the exchange-charge model of crystal field for Mn3+ and Mn4+ ions in Ga3+ octahedral sites (C3i) in garnet-type Ca3Ga2Ge3O12 remain in perfect agreement with the experimental data.

Temperature dependence of photoluminescence spectra from a suspended single-walled carbon nanotube with water adsorption layer

On a single-walled carbon nanotube (SWCNT) surface, water forms a peculiar adsorption layer comprising two monolayers, and the physical properties of this adsorption layer remain unclear. We studied the changes that occurred in this water adsorption layer from room temperature down to 140 K using photoluminescence (PL) spectroscopy of suspended SWCNTs. The PL emission energy exhibited complex changes with temperature depending on the chirality of the SWCNTs. These changes were described quantitatively on the basis of changes in the bandgap, the dielectric constant of the adsorption layer, and the strain imposed by the adsorption layer. The results suggested that the adsorption layer might be a two-dimensional disordered solid rather than a liquid.

(Invited) Momentum-Resolved Dielectric Response of Free Standing Black Phosphorus Down to the Monolayer

Atomic layers of Black Phosphorus (BP) have been recently isolated, ten years after graphene. BP stands out in the 2D-materials panorama by its unique semiconducting properties: direct bandgap which can be tuned by the layer number in a wide range of wavelengths from visible (monolayer) to midinfrared (bulk) [1]. This tunability combined the anisotropy of the structure therefore offers promising perspectives in various fields such as electronics and photonics. However, the fast photooxidation in ambient condition, coupled to a high sensitivity to quantum confinement and dielectric environment in ultrathin BP, make very difficult the investigations on its intrinsic optical properties [2]. Further, as screening effects may strongly affect electronic and spectroscopic properties of 2D materials, it is highly desirable to investigate intrinsic properties of free-standing layers as well the ones of the bulk material which remain poorly known.To start with, we have investigated photoluminescence (PL) and absorption properties of high quality BP single crystals at 2K [FTIR]. The PL intensity appears comparable to high quality InAs crystals confirming the suitability of BP for infrared applications. Two peaks are evidenced at 0.275 eV and 0.26 eV. The highest energy peak is particularly narrow (FWHM = 3.8meV) and is tentatively attributed to exciton recombination, thanks to the temperature dependence of PL spectra from 2K to 300K. Further, we have measured the gap related energy shift of the PL spectra as a function of the BP thickness in a series of mechanically exfoliated samples with thicknesses down to 10 nm, complementary to the previous measures done on thinner samples [3].As far as the thinnest layers are concerned, which cannot manipulated in air, we have shown that Angular resolved Electron energy loss spectroscopy implemented in Transmission Electron Microscopy (Ar-EELS-TEM) offers a unique way to investigate dielectric response of free-standing layers related to valence band and plasmon excitations with the advantage to get access to their q dispersion and their symmetry properties [4]. By combining this technique with suitable ab initio calculations, we have studied the dielectric response of free-standing BP layers as a function of the number of layers. We found optical bandgap values of 1.9 eV, 1.4 eV and 1.1 eV for the mono- bi- and trilayer respectively. Moreover, by combining our results with a simple variational model, we correlate the exciton energy with the dielectric screening. We hence demonstrate that the variations of the electronic gap are sizeably larger than the variations of the binding energy. Finally, we probe and analyze the volume and surface plasmons dispersion as a function of momentum for the 1-3 BP layers and bulk and highlight a deviation and linearization of the parabolic dispersion with strong anisotropic fingerprints [5].[1] G. Zhang et al., Nat. Com., 8, 14071, (2017)[2] A. Favron, E. Gaufres et al Nature Mat. 14 (2015) 826.[3] C. Chen et al., NanoLett., (2019)[4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017)[5] E. Gaufres et al, Nanoletters (2019); DOI: 10.1021/acs.nanolett.9b03928

Luminescence and Thermal-Quenching Properties of Red-Emitting Ca2Al2SiO7:Sm3+ Phosphors

Ca2Al2SiO7:x.Sm3+ (x = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.5 mol.%) (CAS:x.Sm3+) phosphors were synthesized by a solid-state reaction technique. The structure, photoluminescence properties and thermal stability of phosphors were investigated in detail. Results of X-ray diffraction show that CAS:x.Sm3+ materials have a single-phased tetragonal structure, and an expansion of the unit cell relates to the increasing of Sm3+ concentration. Photoluminescence study displayed that the CAS:x.Sm3+ phosphors reach the highest emission intensity at 1.5 mol.% Sm3+ and achieved the luminescence quenching phenomenon a higher concentration. The dominant interaction mechanism of the concentration quenching process is determined due to the dipole–dipole interaction, and the critical transfer distance (Rc) is 26.7 A. The temperature dependence of photoluminescence spectra indicates that the Ca2Al2SiO7:Sm3+ (1.5 mol.%) phosphor possess good thermal stability, and that the activation energy is around 0.12 eV (968 cm−1). Several characteristic vibrations in the 200–1000-cm−1 region were observed by Raman spectra, and the color chromaticity coordinates of the samples were also calculated and discussed.

Towards a white-emitting phosphor Ca10V6O25 based material

To achieve a stable material capable of white-light emission, we fabricated Ca10V6O25 by both co-precipitation (CP) and microwave-assisted hydrothermal (MAH) methods at different temperatures. The effects of structural disorder at a short-range (Raman spectra), medium-range (UV–vis spectra), and long-range (X-ray diffraction) on the material structure and its effect on optical properties were investigated. Both CP and the MAH methods were efficient for obtaining pure Ca10V6O25 material with different morphologies. Significant changes in morphology and degree of structural order and disorder were found to affect the photoluminescence (PL) properties. All samples exhibited a broadband PL spectrum, characteristic associated to charge transfer processes of the [VO4]3- group, as well as transitions that occurred in the energy levels located within the band gap. The samples obtained by the MAH method at temperatures of 100 °C and 120 °C exhibited a lower structural disorder (at long- and short-ranges), the CIE chromaticity coordinates were located in the white region, MAH100 (x = 0.311 and y = 0.348) and MAH120 (x = 0.350 and y = 0.368). Furthermore, we conducted a detailed study of temperature dependence of PL spectra. All samples displayed decreased luminescence intensity as temperatures increased and low values of activation energy (approximately 2 meV). Finally, the as-synthetized Ca10V6O25 material has interesting properties for possible applications as a white-emitting phosphor under ultraviolet excitation.

Photoluminescence Spectra of the 580-nm Center in Irradiated Diamonds

The formation mechanisms of the zero-phonon line optical center at 580 nm (H19 center) in photoluminescence spectra of irradiated natural diamonds and those deposited from the vapor phase were studied after their high-temperature vacuum annealing. The photoluminescence band intensity of the H19 center was shown to increase exponentially as the annealing temperature increased. Temperature dependences of photoluminescence spectra and local mechanical stress effects on the position and full width at half-height of the 580-nm zero-phonon line optical peak led to the conclusion that the H19 optical center was a complex intrinsic vacancy defect.

Defect-induced excitonic recombination in TixZn1−xO thin films grown by DC-unbalanced magnetron sputtering

We study the effects of Ti doping on the near-band-edge emission (NBE) and defect-related deep-level emission (DLE) of ZnO thin films grown by DC unbalanced magnetron sputtering. DLE in pure ZnO is contributed by zinc and oxygen vacancies (VZn+VO), as revealed by photoluminescence (PL) spectroscopy, current–voltage (I–V) characteristic measurement, and spectroscopic ellipsometry. The reduction in the number of VZn states is clearly observed upon Ti doping, resulting in the enhancement of green emission from VO. Interestingly, the thin film with a Ti concentration of 1 at. % shows a higher excitonic emission. Furthermore, the temperature dependence of PL spectra shows that the enhanced excitonic emission originates from the donor-bound exciton promoted by the Ti dopant and native VO. This study shows an important role of the defects in controlling the optical and electronic properties of ZnO films for future optoelectronic applications.

Anomalous temperature dependence of photoluminescence spectra from InAs/GaAs quantum dots grown by formation–dissolution–regrowth method**Project supported by the National Basic Research Program of China (Grant No. 2013CB632104) and the National Key Research and Development Program of China (Grant No. 2016YFB0402302).

Two kinds of InAs/GaAs quantum dot (QD) structures are grown by molecular beam epitaxy in formation–dissolution–regrowth method with different in-situ annealing and regrowth processes. The densities and sizes of quantum dots are different for the two samples. The variation tendencies of PL peak energy, integrated intensity, and full width at half maximum versus temperature for the two samples are analyzed, respectively. We find the anomalous temperature dependence of the InAs/GaAs quantum dots and compare it with other previous reports. We propose a new energy band model to explain the phenomenon. We obtain the activation energy of the carrier through the linear fitting of the Arrhenius curve in a high temperature range. It is found that the GaAs barrier layer is the major quenching channel if there is no defect in the material. Otherwise, the defects become the major quenching channel when some defects exist around the QDs.

Temperature and excitation power dependence of photoluminescence of ZnO nanorods synthesized by pattern assisted hydrothermal method

Abstract Pattern assisted ZnO nanorods (NRs) growth are performed using electron beam lithography (EBL) and hydrothermal method. The patterns of lines, circles and squares are prepared on polycrystalline ZnO seed layer using PMMA as photoresist. It is found that the ZnO NRs can only grow on the area with ZnO seed layer, not on the PMMA surface. The TEM results show that the ZnO NRs have single crystal structures with good orientation along the c-axis. The excitation power dependence of photoluminescence (PL) spectra of ZnO NRs shows that with excitation power increasing, the near band edge emission increases rapidly than the defects related visible emissions. The temperature dependence of PL spectra was measured in the temperature range of 13–300 K. The dependence of peak position and peak energy of exciton bound (D0X) emission of ZnO NRs on temperature were analyzed. Two non-radiative recombination channels for the D0X emission are discussed in detail.

Investigation of defects in indium doped TiO2 thin films using electrical and optical techniques

Existence of defect levels into the band gap of titanium oxide (TiO2) due to indium (In) doping was investigated by Deep level transition spectroscopy (DLTS), Raman Spectroscopy and photoluminescence (PL). Particularly, two distinct e-beam grown TiO2 thin film (TF) samples on Si substrates were doped using In films with thicknesses of 5 nm and 50 nm instantaneous source. It was observed that the increasing in In doping concentration has changed the TiO2 crystal structure from anatase to rutile phase. In addition, the low doped Ti/Au/5 nm In/TiO2 TF samples showed at −5 V reverse-bias lower leakage current (3.0 × 10−7 A) as compared to the highly doped Ti/Au/50 nm In/TiO2 TF devices (7.0 × 10−5 A). The free carrier concentration was increased from about 1014 cm−3 to 1015 cm−3 for 5 nm In/TiO2 TF to 50 nm In/TiO2 TF devices, respectively. DLTS results have revealed a unique behaviour where a substantial reduction in deep trap concentration was observed in the samples having larger In doping. A PL band around 2.4 eV and 1.9 eV was observed for 5 nm and 50 nm In/TiO2 TF samples, respectively A blue shift of photoluminescence (PL) energy peak with the increase of temperature was also observed for both samples and was associated to defect related emissions. Finally, the shallow activation energy was determined from the temperature dependence of PL spectra. It was observed that the activation energy increased from 25 meV for the low In-doped TiO2 TF samples to 65 meV for the highly In-doped TiO2 TFs.

Temperature dependence of photoluminescence spectra and dynamics of the red-emitting K2SiF6:Mn4+ phosphor

Investigating and understanding temperature-dependent photoluminescence (PL) properties of inorganic phosphors are of vital importance to optimize their performance and expand their application. Recently, K2SiF4:Mn4+ phosphors have attracted intense attention as a red phosphor candidate for white LED applications because of their extremely narrow emission peaks in the red spectral region. Here, temperature-dependent (20–200 °C) diffuse reflection spectra and steady and transient PL spectra of K2SiF4:Mn4+ were systematically investigated. Both the emission intensities and spectral characteristics of the phosphor showed excellent thermal stability at temperatures from 20 to 150 °C, ensuring its great potential for practical white LED applications. An abnormal PL enhancement was found at elevated temperatures that differed from the thermal quenching behavior of common phosphors and could be attributed to enhanced phonon-assisted radiative transitions at higher temperatures. We further successfully tested and verified that the intensity ratio of anti-Stokes and Stokes emission peaks of Mn4+ ions was sensitive to temperature and followed a Boltzmann-type distribution function very well, making the K2SiF4:Mn4+ phosphor a good candidate for temperature sensing, especially for investigating the thermal distribution of white LEDs.

Temperature-dependent photoluminescence spectra of GaN epitaxial layer grown on Si (111) substrate**Project supported by the National Natural Science Foundation of China (Grant Nos. 61474110, 61377020, 61376089, 61223005, and 61176126) and the National Science Fund for Distinguished Young Scholars of China (Grant No. 60925017).

In this paper, the temperature-dependent photoluminescence (PL) properties of GaN grown on Si (111) substrate are studied. The main emission peaks of GaN films grown on Si (111) are investigated and compared with those grown on sapphire substrates. The positions of free and bound exciton luminescence peaks, i.e., FXA and D0X peaks, of GaN films grown on Si (111) substrates undergo red shifts compared with those grown on sapphire. This is attributed to the fact that the GaN films grown on sapphire are under the action of compressive stress, while those grown on Si (111) substrate are subjected to tensile stress. Furthermore, the positions of these peaks may be additionally shifted due to different stress conditions in the real sample growth. The emission peaks due to stacking faults are found in GaN films grown on Si (111) and an S-shaped temperature dependence of PL spectra can be observed, owing to the influence of the quantum well (QW) emission by the localized states near the conduction band gap edge and the temperature-dependent distribution of the photo-generated carriers.

Temperature dependence of photoluminescence spectra for green light emission from InGaN/GaN multiple wells.

Three green light emitting InGaN/GaN multiple quantum well (MQW) structures with different In composition grown by metal-organic chemical vapor deposition are investigated by the X-ray diffraction and the temperature-dependent photoluminescence (PL) measurements. It is found that when the In composition increases in the InGaN/GaN MQWs, the PL spectral bandwidth may anomalously decrease with increasing temperature. The reduction of PL spectral bandwidth may be ascribed to the enhanced non-radiative recombination process which may lower the light emission efficiency of the localized luminescent centers with shallow localization energy in the high-In-content InGaN quantum wells and also cause a reduction of integrated PL intensity.

Temperature dependence of photoluminescence spectra of bilayer two-dimensional electron gases in LaAlO3/SrTiO3 superlattices: coexistence of Auger recombination and single-carrier trapping

We report emerging photoluminescence (PL) of bilayer two-dimensional electron gases (2DEG) in LaAlO3/SrTiO3 (LAO/STO) systems. A strong blue PL emerges in bilayer-2DEGs in LAO/STO/LAO/STO which doesn’t show in LAO/STO. PL band in bilayer-2DEGs includes both nearly temperature independent Auger recombination and temperature dependent free electron trapping while it crossovers from Auger recombination to single carrier trapping in LAO/STO. The PL signal of free electron trapping appears at high temperatures and it is much stronger than Auger recombination in the conducting channel in bilayer 2DEGs. This observation shows that high mobility carriers dominate the carrier dynamics in bilayer-2DEGs in LAO/STO superlattices.

Light Emitting Properties of Si-Rich-Si3N4 Films Grown By PECVD Method

Bandgap engineering of Si-based materials through the control of the distribution of Si nanoclusters’ (Si-ncs) offered future applications of Si-based nanostructured materials in optoelectronics as low-cost, miniaturized, and CMOS-compatible, light-emitting and photovoltaic devices. In the past, most efforts were concentrated on the development of luminescent Si-ncs embedded in SiO2 host. However, its insulating nature remains a barrier for the production of future electrically pumped LEDs and efficient photovoltaic cells. This detrimental aspect can be coped with a medium of higher conductivity and lower band gap, for instance, Si nitride. Tunable room temperature visible light emission from Si-ncs embedded in Si nitride matrices has been recently demonstrated. Present work deals with the Si-rich Silicon nitride films were grown by PECVD technique on silicon substrates. The film stoichiometry was controlled via variation of NH3/SiH4 ratio from R=0.45 up to 1.0. Thermal treatment was performed at 1100°C for 30min in nitrogen flow to form Si-ncs. To control structural and light emitting properties of the films Raman scattering, X-ray diffraction (XRD), Transmission electron microscopy (TEM) and photoluminescence (PL) methods were used. The evolution of PL spectra with the temperature of measurements from 20 to 300 K was studied also aiming at the determination of the types of optical transitions. The PL spectra were found to be complex and the shape and magnitude of PL spectra depends on silicon nitride stoichiometry. The increase of gas ratio from R=0.63 to R=1 results in the shift of PL peak position from 1.6 eV up to 2.7 eV. Analysis of the temperature dependence of PL spectra revealed the presence of several PL components with the maxima at 2.9-3.0 eV, 2.5-2.7 eV, 1.9-2.2 eV and 1.8-1.9 eV. The peak position of the former three PL components unchanged with the decrease of temperature of the measurements. This allows describing all these components to the defects of silicon nitride host. At the same time, PL band peaked at 1.8-1.9 eV showed high-energy shift with sample cooling and can be ascribed to the exciton recombination inside Si-ncs. The presence of these latter was confirmed by Raman scattering spectra and TEM images. The nature of light emitting defects in silicon nitride, the mechanism of photoluminescence and the way for the optimization of light emitting properties are discussed.

Effect of the stoichiometry of Si-rich silicon nitride thin films on their photoluminescence and structural properties

Si-rich Silicon nitride films were grown on silicon substrates by plasma enhanced chemical vapor deposition. The film stoichiometry was controlled via the variation of NH3/SiH4 ratio from 0.45 up to 1.0. Thermal annealing at 1100°C for 30min in the nitrogen flow was applied to form the Si nanocrystals in the films that have been investigated by means of photoluminescence and Raman scattering methods, as well as transmission electron microscopy. Several emission bands have been detected with the peak positions at: 2.8–3.0eV, 2.5–2.7eV, 2.10–2.25eV, and 1.75–1.98eV. The temperature dependences of photoluminescence spectra were studied with the aim to confirm the types of optical transitions and the nature of light emitting defects in silicon nitride. The former three bands were assigned to the defects in silicon nitride, whereas the last one (1.75–1.98eV) was attributed to the exciton recombination inside of Si nanocrystals. The photoluminescence mechanism is discussed.

Carrier Hopping and Relaxation in InAs/GaAs Quantum Dot Heterostructures

We investigate the effect of carrier dynamics on the temperature dependence of photoluminescence spectra from InAs/GaAs quantum dot heterostructures with different dot size uniformity. Intersublevel relaxation lifetimes and carrier transferring mechanisms are simulated using a model based on carriers relaxing and thermal emission of each discrete energy level in the quantum dot system. Calculated relaxation lifetimes are decreasing with temperature and have larger values for sample with lower dot size uniformity. In the quantitative discussion of carrier dynamics, the influence of thermal redistribution on carriers relaxing process of quantum dot system is demonstrated by our model.

Temperature dependence of photoluminescence spectra in hole-doped single-walled carbon nanotubes: Implications of trion localization

We studied the dimensionality of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes (SWNTs) by means of photoluminescence (PL) spectroscopy. We found that the temperature ($T$) dependence of the PL intensity of the trions and excitons is significantly different, reflecting their dimensionality. The PL intensity of the trions was almost unchanged for 5--300 K, whereas clear ${T}^{\ensuremath{-}1/2}$ dependence of the exciton PL intensity was observed, which reflects the characteristic radiative decay rate of one-dimensional excitons. The results suggest that the radiative decay rate of the trions is independent of the temperature, which is the manifestation of the localization of the trions in hole-doped SWNTs.