Temperature Dependence Of Photoluminescence Intensity Research Articles

Effect of confinement on photoluminescence of MeLPPP/SBA-15 nanocomposite

Photoluminescence spectra and kinetics of methyl-substituted ladder-type poly(para-phenylene) (MeLPPP)/SBA-15 mesoporous silica (SBA-15) nanocomposite have been studied and compared with photoluminescence properties of the MELPPP film and solution. Relaxation processes of excited singlet states in the closed geometry of the nanocomposite and in the diluted solution compared to the film based on the temperature dependence of photoluminescence (5–220 K) were studied. It was shown that the nanocomposite has twice the photoluminescence lifetime and three times weaker temperature dependence of photoluminescence intensity in comparison with the film. These peculiarities are due to a decrease in the number of polymer chains in a low-dimensional structure and a reduced role of exciton diffusion.

Unveiling the phonon effect on the narrow-band deep-red emission from solution-combustion synthesized Mn4+ doped CaYAlO4 microcrystals

In this study, we report a unique solution-combustion synthesis of Mn4+ doped CaYAlO4 microcrystals and demonstrate a narrow-band deep-red emission. Through the simulation of low-temperature photoluminescence (PL) spectroscopy and theoretical fitting on temperature-dependent PL intensity at T < 400 K, we obtained a dominant phonon mode with energy of 40 meV during the vibronic coupling process, with a small Huang-Rhys factor of S= 0.68. From the analysis of the measured PL lifetime evolution of Mn4 + doped CaYAlO4, it is found that luminescence thermal quenching effect was mainly attributed to four-dominant phonon interaction with Mn4+ electronic states. More interestingly, the deviation of luminescence decay curve from the single-exponential mode at T > 400 K suggests that the occurrence of two radiative transitions is a consequence of ionically thermal excitation to the second excited state of 4T2. These findings provide not only a general approach to identify the predominant phonon vibration, but also deep insights into the effect of weak electron-phonon coupling on PL properties of solid-state luminescent.

Photoluminescence spectroscopy of Mn2+ in zero-thermal expansion Zn4B6O13 green-emitting phosphor

Investigating temperature dependence of phosphor properties is not only of scientific interest, but also technological importance. Here, we investigate the temperature dependence of photoluminescence intensity, IPL, for the Zn4B6O13:Mn2+ green-emitting phosphor. Interestingly, Zn4B6O13 crystal has been reported to exhibit zero-thermal expansion (zero-TE) behavior along with its unique phosphor properties, called zero-thermal quenching behavior of IPL, up to T = 523 K. It has been shown that such unique temperature dependence of IPL is not related to the zero-TE behavior of Zn4B6O13, but is to be due to an increased phonon number that leads to an increased parity-forbidden 4T1 → 6A1 transition probability with increasing T. No or a very little change in the excited-state zero-phonon line transition energy, E(4T1)ZPL, of the Zn4B6O13:Mn2+ phosphor from T = 20–523 K can be well understood by considering the Tanabe−Sugano energy-level diagram with taking into account the zero-TE behavior, i.e., no large change of the metal−ligand distance RMn−O (or, equivalently, no large change of the crystal-field splitting energy Dq) against T in such zero-TE phosphor of Zn4B6O13:Mn2+.

Photoluminescence Spectroscopy of the InAsSb-Based p-i-n Heterostructure.

Photoluminescence in a double heterostructure based on a ternary InAsSb solid solution was observed in the mid-infrared range of 2.5–4 μm. A range of compositions of the InAs1−ySby ternary solid solution has been established, where the energy resonance between the band gap and the splitting-off band in the valence band of the semiconductor can be achieved. Due to the impact of nonradiative Auger recombination processes, different temperature dependence of photoluminescence intensity was found for the barrier layer and the narrow-gap active region, respectively. It was shown that efficient high-temperature photoluminescence can be achieved by suppressing the nonradiative Auger recombination (CHHS) process. Increased temperature, for which the energy gap is lower than the split-off band energy, leads to violation of the resonance condition in narrow gap antimonide compounds, which explains the observed phenomenon. This finding might influence future application of the investigated material systems in mid-infrared emitters used for, e.g., optical gas sensing.

Wide gamut white LED device using green CsPbBr3 quantum dots glass and red K2SiF6: Mn4+ phosphor

Wide gamut white LED device is fabricated by combining green CsPbBr3 quantum dots glass (QDG) and red K2SiF6: Mn4+ phosphor with blue LED. The green CsPbBr3 QDG shows steep absorption spectra, narrow and symmetrical photoluminescence (PL) spectra, broad band excitation spectra and bi-exponential fluorescence decay. PL peak, full width at half maximum and average lifetime are 515 nm, 18 nm and 94.9 ns respectively. The red K2SiF6: Mn4+ phosphor shows narrow red emission peaks of Mn4+, broad excitation peaks, and single exponential fluorescence decay with lifetime of 8.5 ms. The green CsPbBr3 QDG has strong temperature dependence of PL intensity. The red K2SiF6: Mn4+ phosphor has negligible temperature dependence. The white LED device has wide color gamut of 134% NTSC and high color purity of 98%, 89% and 99% for the blue, green and red lights. The white light of white LED device has been realized by controlling thickness of the green CsPbBr3 QDG and content of the red K2SiF6: Mn4+ phosphor. We have evaluated the chromatic coordinate stability at different operating powers and times, and the maximum chromatic changes are as small as (0.004, 0.012) and (0.009, 0.026) for (x, y) respectively in CIE 1931 (Commission International de l′Eclairage 1931) chromatic coordinate.

External quenching of luminescence in ZnSe crystals

The luminescence of the 630 nm band in ZnSe single crystals was studied under intracenter excitation (hvex < Eg). We obtained luminescence spectra at different temperatures, temperature dependences of photoluminescence intensity and photoconductivity, lux-ampere and lux-luminescence dependences both at low temperatures, when there is no thermal quenching, and at 270 K, when thermal quenching is observed. Comparing of the photoluminescence spectrum with the X-ray luminescence spectrum has showed that only one component of the 630 nm band with a hole luminescence mechanism is observed in the photoluminescence. Temperature dependences of photoluminescence intensity and photoconductivity indicate that at T > 250 K external quenching of photoluminescence is observed under intracenter excitation. Lux-ampere and lux-luminescent dependences also confirm this conclusion.

Mechanisms of Thermal Quenching of Defect‐Related Luminescence in Semiconductors

The intensity of defect‐related photoluminescence (PL) in semiconductors changes with temperature, and it usually decreases exponentially above some critical temperature, a process called the PL quenching. Herein, main mechanisms of PL quenching are reviewed. Most examples are given for defects in GaN as the most studied modern semiconductor, which has important applications in technology. Peculiarities of defect‐related PL in I–VII, II–VI, and III–V compounds are also reviewed. Three basic mechanisms of PL quenching are distinguished. Most examples of PL quenching can be explained by the Schön–Klasens mechanism, whereas very few or even no confirmed cases can be found in support of the Seitz–Mott mechanism. Third mechanism, the abrupt and tunable quenching, is common for high‐resistivity semiconductors. Temperature dependence of capture coefficients and a number of other reasons may affect the temperature dependence of PL intensity. The “negative quenching” or a significant rise in PL intensity with temperature is explained by a competition between recombination channels for minority carriers.

Temperature dependence of nonradiative recombination processes in UV-B AlGaN quantum well revealed by below-gap excitation light

Nonradiative recombination (NRR) processes through defect states and their temperature dependence in UV-B AlGaN MQW sample on sapphire substrate grown by MOCVD technique have been studied by photoluminescence (PL) spectroscopy. We detected NRR centers by adding a below-gap excitation light with photon energies from 0.93 eV to 1.46 eV on an above-gap excitation light of 4.66 eV. All the BGE energies decreased PL intensity at 25 K, and the most-distinct quenching is observed by 1.27 eV BGE at the same BGE photon number density. The temperature-dependent PL intensity for the BGE energy of 1.27 eV is interpreted by three NRR centers. The one-level model dominates over that of two-level model in the temperature range 58 K < T < 88 K. The two-level model prevails in other region of temperature. The combination of one-level and two-level models is consistent with the spectral peak-energy shift as a function of temperature.

Quantum Size Effects in Germanium Nanocrystals and Amorphous Nanoclusters in GeSixOy Films

Films of nonstoichiometric germanium–silicon glasses of two types—GeOx[SiO](1 – x) and GeOx[SiO2](1 – x)—are deposited onto cold Si (001) substrates by evaporating GeO2 and SiO (or SiO2) powders simultaneously in high vacuum. Film samples in their initial (as-deposited) state and after being annealed at 550 and 650°C for 1 h are investigated using IR and Raman spectroscopies and electron microscopy, and their photoluminescence (PL) is studied as well. Raman spectroscopy shows that, in contrast to the initial GeO[SiO2] film, the initial GeO[SiO] one contains clusters of amorphous germanium, their size being ~3 nm, as found by electron microscopy. The presence of Si–O, Ge–O, and Si–O–Ge bonds in the films is established by IR spectroscopy. Clusters of amorphous germanium are found in both films after annealing at 550°C, while germanium nanocrystals are formed in the films subjected to annealing at 650°C. For the initial films, a broad band with a maximum at 1050 nm is registered in their low-temperature PL spectra, which may originate from such defects as oxygen vacancies and overstoichiometric germanium atoms. Annealing causes structural changes in the films and affects their PL behavior. The films containing germanium nanoclusters give rise to PL with a maximum at 1400–1600 nm, with the defect-related signal being diminished. The temperature dependence of PL intensity exhibits a decreasing behavior, but PL is observed to temperatures as high as 200 K. The contribution of germanium nanocrystals formed at the annealing stage to PL is discussed.

Luminescence properties of Ca2Sn2Al2O9: Mn as a long afterglow and field-emission displays material with high yellow color purity

In this work, a series of rare earth free orange phosphors Ca2Sn2Al2O9: Mn (CSA: Mn) are synthesized successfully by two-steps high temperature solid-state reaction. Their crystal structure and luminescence properties are studied detail. CSA is a typical orthorhombic crystal system and belonging to P b c n (60) space group. There is only one kind Ca2+ site with seven coordinations in the structure for Mn2+ occupying. This phosphor possesses self-emitting and self-reduction due to the defects carrying negative charge which can provide electrons for Sn4+ and Mn4+. When Sn4+ and Mn4+ get electrons, they will generate their characteristic low valence electron transfer (Sn2+ and Mn2+) and emit relative radiation transition. The host emission yielding from Sn2+ and the broad orange emission from Mn2+ has an apparent energy transfer because of their overlap of the luminescence spectra. Furthermore, CSA: Mn2+ possesses orange PL emission with long afterglow about 7 h after 10 min UV excitation due to the defects’ trap energy level. And, Mn2+’s temperature dependent photoluminescence intensity appeared anomalous phenomena that the intensity increases firstly and then decreases, which is due to the traps energy level’s contribution of electron’s transition. Besides, CSA: Mn2+ can emit bright orange light under cathode ray and it has very stable CL properties after suffering long time electron beam bombarding. CSA: Mn2+ is a kind potential application value material in long afterglow and FED.

Novel hypophosphite hybrid perovskites of [CH3NH2NH2][Mn(H2POO)3] and [CH3NH2NH2][Mn(H2POO)2.83(HCOO)0.17] exhibiting antiferromagnetic order and red photoluminescence.

Hybrid perovskites based on hypophosphite ligands constitute an emerging family of compounds exhibiting unusual structures and offering a platform for construction of novel functional materials. We report the synthesis, crystal structure, and magnetic and optical properties of novel undoped and HCOO−-doped manganese hypophosphite frameworks templated by methylhydrazinium cations. The undoped compound crystallizes in a three-dimensional perovskite-like orthorhombic structure, space group Pnma, with ordered organic cations located in windows between the perovskite cages expanding along the a-direction. Both conventional anti-phase tilting, unconventional in-phase tilting and columnar shifts in the a-direction are present. Doping with HCOO− ions has a insignificant influence on the crystal structure but leads to a decrease of the unit cell volume. Magnetic studies indicate that these compounds order antiferromagnetically at TN = 6.5 K. Optical studies indicate that they exhibit red photoluminescence under 266 nm excitation with the activation energy for thermal quenching of 98 and 65 meV for the undoped and doped sample, respectively. For the undoped sample, the emission lifetime reaches 5.05 ms at 77 K but it decreases to 62.26 μs at 300 K. The low value of the activation energy and huge temperature dependence of photoluminescence intensity suggest a high potential of these hypophosphites for non-contact temperature sensing.

Theoretical model of excitonic luminescence and its application to the study of fine structure and exciton dynamics in ZnO

By solving the continuity equation of excitons under steady excitation, a theoretical model for the excitonic luminescence of semiconductors was developed taking into account the exciton diffusion and surface recombination. The theoretical model was used to analyze the photoluminescence (PL) spectra of ZnO obtained from the bulk single-crystal samples with and without surface passivation, showing that the nonradiative recombination on the surface is an important channel of losing excitons, thus substantially reducing the PL quantum efficiency of excitons at room temperature. In addition, the surface recombination was found to have impacts on the fine structure of excitonic luminescence at low temperature. Using the theoretical model, the diffusion length of excitons at room temperature was estimated and found to be different from sample to sample, strongly depending on the sample processing. The theoretical model was demonstrated to be capable of accurately fitting the temperature-dependent PL intensity of passivated samples and showed that the exciton diffusion has significant impacts on the dynamics of excitonic luminescence at high temperature.

Momentum-forbidden dark excitons in hBN-encapsulated monolayer MoS2

Encapsulation by hexagonal boron nitride (hBN) has been widely used to address intrinsic properties of two-dimensional (2D) materials. The hBN encapsulation, however, can alter properties of 2D materials through interlayer orbital hybridization. In this paper, we present measurements of temperature dependence of photoluminescence intensity from monolayer MoS2 encapsulated by hBN flakes. The obtained temperature dependence shows an opposite trend to that of previously observed in a monolayer MoS2 on a SiO2 substrate. This is caused by the existence of stable momentum-forbidden dark excitons in the hBN-encapsulated MoS2. Ab-initio band-structure calculations have shown that orbital hybridization between MoS2 and hBN leads to upward shift of Γ-valley of MoS2, which results in lowering of energy of the momentum-forbidden dark excitons. This work shows an important implication that the hBN-encapsulated structures used to address intrinsic properties of two-dimensional crystals can alter basic properties of encapsulated materials.

Low Temperature Photoluminescence Spectroscopy of Defect and Interband Transitions in CdSexTe1-x Thin Films

We present the defect analysis by photoluminescence (PL) spectroscopy of CdSexTe1-x thin films, grown with varying Se content by a co-sputtered deposition method. We observe a peak at 1.203 eV in the CdSexTe1-x film for x = 0.21, which shifts towards higher energies with increase in laser power. This peak was assigned to a donor-to-acceptor (DAP) transition, with a measured j-shift of ∼4.7 meV/decade. Temperature dependent PL intensity measurements confirm that the observed DAP peak involves a shallow defect state of binding energy ∼34.7 meV. In contrast, a free-to-bound (FB) peak at 1.294 eV involving a shallow defect of binding energy ∼18.3 meV was observed in the CdSexTe1-x film for x = 0.14. Additionally, we observe band edge emission at 1.452 eV and 1.448 eV in CdSexTe1-x films for x = 0.14 and x = 0.21 respectively. Our analysis shows that the Se concentration not only changes the band gap energy of the resulting CdSexTe1-x alloy thin film, but also modifies the nature of the dominant observed defect emission.

Color-tunable Ca10Na(PO4)7:Ce3+/Tb3+/Mn2+ phosphor via energy transfer

A series of Ca10Na(PO4)7:Ce3+/Tb3+/Mn2+ (CNPO:Ce3+/Tb3+/Mn2+) phosphors with high brightness were synthesized by high-temperature solid-state method. X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectra (DRS), photoluminescence (PL) spectra, luminescence decay curves and thermally stability were performed to characterize the as-prepared samples. For Ce3+-doped samples, an intense and broad band emission is present under 265 nm excitation. When Ce3+ and Tb3+ are codoped, energy transfer (ET) process from Ce3+ to Tb3+ is demonstrated with electric dipole–dipole interaction. The internal and external quantum efficiencies (QEs) of CNPO:0.15Ce3+, 0.04Tb3+, 0.005Mn2+ are measured to 76.79% and 54.11% under 265 nm excitation and temperature-dependent PL intensity can remain 51.78% at 150 °C of its initial intensity at 25 °C. It is indicated that single-phased white light-emitting CNPO:Ce3+/Tb3+/Mn2+ phosphor can serve as a promising phosphor for illumination devices.

Synthesis and properties of Rb2GeF6:Mn4+ red-emitting phosphors

Rb2GeF6:Mn4+ red-emitting phosphors were synthesized by coprecipitation and their structural and optical properties were investigated by laser microscopy observation, X-ray diffraction (XRD) analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and PL decay measurement. Single-crystalline ingots in the form of a hexagonal pyramid were prepared with a basal plane diameter of ∼2 mm. The XRD analysis suggested that Rb2GeF6 crystallizes in the hexagonal structure () with a = 0.5955 nm and c = 0.9672 nm. The phosphor exhibited the strong Mn4+-related zero-phonon line (ZPL) emission peak typically observed in host crystals with piezoelectrically active lattices such as a hexagonal lattice. The quantum efficiencies of the bulk ingot and powdered samples were 87 and 74%, respectively, with nearly the same luminescence decay time of ∼6 ms. The exact ZPL energies and related crystal-field and Racah parameters were obtained from the PL and PLE spectra by Franck–Condon analysis. Temperature-dependent PL intensities were analyzed from T = 20 to 500 K using a thermal quenching model by considering Bose–Einstein phonon statistics. A comparative discussion on the phosphor properties of Rb2GeF6:Mn4+ and Rb2MF6:Mn4+ with M = Si and Ti was also given.

Photoluminescence of ZnTe/ZnMgTe multiple quantum well structures grown on ZnTe substrates by molecular beam epitaxy

Photoluminescence (PL) properties of ZnTe/ZnMgTe quantum well (QW) structures grown by molecular beam epitaxy (MBE) were investigated systematically with respect to well widths and Mg contents. Observed PL peak energies were consistent well with the calculated emission energies of the QWs considering a lattice distortion in the ZnTe well. From the temperature dependence of PL intensity, it was found that a suppression of a carrier escape from QW is crucial to obtain a PL at higher temperature in the ZnTe/ZnMgTe QW. Based on the results, multiple quantum well structures were designed and fabricated, which exhibited a green PL at room temperature.

Temperature-dependent photoluminescence of size-tunable ZnAgInSe quaternary quantum dots**Project supported by the National High Technology Research and Development Program of China (Grant No. 2013AA014201), the National Key Foundation for Exploring Scientific Instrument of China (Grant No. 2014YQ120351), and the Natural Science Foundation of Tianjin (Grant No. 11JCYBJC00300, 4JCZDJC31200, 15JCYBJC16700, and 15JCYBJC16800).

Colloidal ZnAgInSe (ZAISe) quantum dots (QDs) with different particle sizes were obtained by accommodating the reaction time. In the previous research, photoluminescence (PL) of ZAISe QDs only could be tuned by changing the composition. In this work the size-tunable photoluminescence was observed successfully. The red shift in the photoluminescence spectra was caused by the quantum confinement effect. The time-resolved photoluminescence indicated that the luminescence mechanisms of the ZAISe QDs were contributed by three recombination processes. Furthermore, the temperature-dependent PL spectra were investigated. We verified the regular change of temperature-dependent PL intensity, peak energy, and the emission linewidth of broadening for ZAISe QDs. According to these fitting data, the activation energy () of ZAISe QDs with different nanocrystal sizes was obtained and the stability of luminescence was discussed.

Ab Initio Site Occupancy and Far-Red Emission of Mn4+ in Cubic-Phase La(MgTi)1/2O3 for Plant Cultivation.

Mn4+-activated oxide phosphors La(MgTi)1/2O3 (LMT) with far-red emitting were prepared via a sol-gel route. The structures of samples were determined by X-ray diffraction (XRD) and Reitveld refinement. The occupied sites of Mn4+ (d3 electronic configuration) in host La(MgTi)1/2O3 were confirmed by ab initio calculations in which the system has the lower formation energy, stable lattice structure, and strong bonding state as Mn4+ enters into Ti site. The luminescent properties of Mn4+-doped samples were investigated; the samples emit far-red light centered at 708 nm with ultraviolet light (345 nm) or blue light (487 nm) excitation. According to the photoluminescence (PL) and excitation (PLE) spectra, the crystal field strength of the Mn4+-occupied environment was estimated. The thermal stability of phosphor was also evaluated through temperature-dependent PL intensity in a heating and cooling cycle process. The emission band is well-matched with the absorption band of phytochrome PFR under the excitation of light in near-ultraviolet to blue, which suggests that the LMT: Mn4+ phosphor has great potential applications in light-emitting diodes (LEDs) for modulating plant growth.

Investigation of carrier dynamics in InAs/GaAsSb quantum dots with different silicon delta-doping levels

The optical properties of InAs quantum dots (QDs) embedded in a GaAsSb matrix with different delta (δ)-doping levels of 0, 2, 4, and 6 electrons per dot (e−/dot), incorporated to control the occupation of QD electronic states, are studied by photoluminescence (PL) spectroscopy. The time-resolved PL data taken at 10 K reveal that the increase of δ-doping density from 2 to 6 e−/dot decreases the recombination lifetime of carriers at ground states of the QDs from 996 ± 36 to 792 ± 19 ps, respectively. Furthermore, the carrier lifetime of the sample with 4 e−/dot is found to increase at a slower rate than that of the undoped sample as temperature increases above 70 K. An Arrhenius plot of the temperature dependent PL intensity indicates that the thermal activation energy of electrons in the QDs, required for carrier escape from the dot ground state to continuum state, is increased when the δ-doping density is high enough (>4 e−/dot). These results are attributed to the enhanced Coulomb interaction of electrons provided by the δ-doping, leading to reduced thermal quenching of the PL.