Exponential Photoluminescence Decay Research Articles

Eu3+ doped (Y0.75Sc0.25)2O3 red-emitting ceramics with excellent photoluminescence properties for LEDs

Developing narrow-band red-emitting ceramic phosphors with high quantum yield and low thermal quenching is vital for mid to high powder solid state lighting. In this study, a series of (Y0.75Sc0.25)2O3:Eu3+ ceramics were fabricated and their photoluminescence properties were studied. The spectral measurement suggests that the optimal doping concentration of Eu3+ is as high as 14%. The heavily doped samples show strong absorption in the NUV-blue region (particularly 360–420 nm and 460–475 nm), bright red emission dominated by the sharp peak located at 611 nm, CIE chromaticity coordinates close to the NTSC standard value of red emission, and single exponential photoluminescence decay without afterglow. In particular, ((Y0.75Sc0.25)0.86Eu0.14)2O3 ceramic exhibits high QY of 86.9%, high color purity of 96.9%, and low thermal quenching loss of 16% as the temperature increases from 300 to 570 K. The excellent photoluminescence properties highlight the great potential of Eu3+ heavily doped (Y0.75Sc0.25)2O3 red-emitting ceramics as color converters for solid state lighting, which is demonstrated by the outstanding electroluminescence performance of the fabricated red and white LEDs.

Concentration dependent neodymium doped oxy fluoroborate glasses for 1.08 μm laser applications

Different concentrations of neodymium-doped (Nd3+ ions) Oxy Fluoroborate (OFB) glasses were fabricated and systematically analyzed by employing spectroscopic techniques such as optical absorption, photoluminescence (PL) and PL decay spectral measurements to understand the NIR lasing potentialities. The information pertaining to absorption spectral features were subjected to Judd-Ofelt (J-O) analysis to understand the radiative properties such as transition probability (AR), radiative lifetime (τR) and branching ratios (βR) possessed by the as prepared Nd3+ ions doped OFB glasses. By correlating the radiative properties with the emission and PL decay spectral profiles, the emission cross-sections (σse) and quantum efficiency values are evaluated for the prominent fluorescent levels of Nd3+ ions in the as prepared glasses. Under 808 nm diode laser excitation, all the OFB glasses under investigation showcased two prominent NIR emissions at 1085 nm (4F3/2 → 4I11/2) and 1328 nm (4F3/2 → 4I13/2). The intensities of both the emission peaks escalates with increases in Nd3+ ions concentration up to 1 mol% and beyond decreases showcasing concentration quenching. The experimental lifetime values estimated from the single exponential PL decay profiles of 4F3/2 level are decreasing monotonously with increase in Nd3+ ions concentration in the as prepared OFB glasses. Relatively higher branching ratios, emission cross-sections and quantum efficiency values estimated for Nd3+ ions doped OFB glasses indicates the potential usage of the as prepared glasses for 1.08 μm laser applications.

On the choice of proper average lifetime formula for an ensemble of emitters showing non-single exponential photoluminescence decay

In this paper, we investigate non-single exponential photoluminescence decays in various disordered condensed-matter systems. For such materials, two formulas for the average lifetime of system’s excited state are commonly used in the analysis of experimental data. In many cases, the choice of formula is arbitrary and lacks a clear physical justification. For this reason, our main goal is to show that the choice of correct mathematical formula should be based on the interpretation of measured photoluminescence decay curve. It is shown that depending on the investigated system, after appropriate normalization, photoluminescence decay curve can represent either a survival probability function or a probability density function of lifetime and for this reason two different formulas for the average lifetime are required. It is also shown that, depending on luminescence quantum yield, some information on the probability density function of lifetime can be lost in the process of measurement, which results in underestimated values of average lifetime. Finally, we provide an interpretation of total decay rate distributions which are frequently obtained by phenomenological modeling of non-single exponential photoluminescence decays.

Identification of the photoluminescence response in the frequency domain modulated infrared radiometry signal of ZnTe:Cr bulk crystal

In this work we investigated the photoluminescence response in the frequency domain modulated infrared radiometry signal observed of ZnTe:Cr bulk crystal. In mid-infrared range, three characteristic phenomena are observed in ZnTe:Cr crystal: absorption and emission of IR photons (2–3μm) and the free carrier absorption. This implies that the modulated infrared radiometry signal yields information about the effective infrared absorption coefficient (photothermal response) as well about the recombination lifetime of carriers related with the infrared photoluminescence emission. In this paper, the frequency equivalence of the two-term independent exponential photoluminescence decay model in order to explain the measured frequency characteristics is proposed. The measured recombination lifetimes (2.3μs for two exponential decay model and 1.5μs for one exponential decay model) are in good agreement with the values given by other authors (about 2.5–3.0μs). Moreover, we found that the photothermal response is uncorrelated with the photoluminescence one, in contrast, to the photocarrier response.

Modeling the Non-Single Exponential Photoluminescence Decay Using the Boubaker Polynomial Expansion Scheme

Many experimental and theoretical questions have not been answered as regards the non-single exponential photoluminescence decay. One of the questions is the possibility of obtaining the probability density function with respect to the photoluminescence decay and time. The Boubaker polynomial expansion scheme was used to quantitatively describe the extent at which a material exhibit the non single exponential photoluminescence decay.

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Optical properties and luminescence dynamics ofEu3 + -doped terbium orthophosphate nanophosphors

The development of luminescent inorganic nanocrystals (NCs) doped withrare-earth (RE) ions has attracted increasing interest owing to their distinctoptical properties and versatile applications in time-resolved bioassays,multiplex biodetection, DNA hybridization and bioimaging. HexagonalTbPO4:Eu3 + NCs (10–30 nm) were synthesized via a facile hydrothermal method assisted with oleic acid (OA)surfactants, which exhibit tunable emissions from green to red by varying the concentration ofEu3 + . TheTb3 + -to-Eu3 + energy transfer efficiency observed reaches up to 94%. Different from theirbulk counterparts, a new interface-state band (316 nm) in addition to thecommonly observed spin-forbidden 4f–5d transition band (265 nm) ofTb3 + wasfound to be dominant in the excitation spectrum of NCs due presumably to the formation of surfaceTbPO4/OA complexes,which provides an additional excitation antenna in practical utilization. Two kinds of luminescence sitesof Eu3 + inTbPO4 NCs, with the sitesymmetry of C2 or C1, were identified based on the emission spectra at 10 K and roomtemperature. Furthermore, the photoluminescence (PL) dynamics ofTb3 + ions inpure TbPO4 NCs have been revealed. Compared to the exponential PL decay in bulk counterpartsinduced by very fast energy migration, the non-exponential decay from 5D4 ofTb3 + inTbPO4 NCsis mainly attributed to the diffusion-limited energy migration due to more rapid energy transfer fromTb3 + to defects than theenergy migration among Tb3 + .

Characterization of nanometer scale compositionally inhomogeneous AlGaN active regions on bulk AlN substrates

The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions.

Hydrothermal Yb3+-Doped NaGd(WO4)2 Nano- and Micrometer-Sized Crystals with Preserved Photoluminescence Properties

Mild hydrothermal preparations using nitrate and chloride reagents as Gd3+ and Yb3+ sources lead to the synthesis of NaGd1−xYbx(WO4)2 (0.001≤ x ≤ 0.5) particles with tetragonal scheelite-like structure phase. Nearly neutral pH ∼7.5 conditions ensure the stability of this crystalline phase over a wide range of reaction times. Synthetical routes with both kind of reagents yield basically the same particle morphology sequences, although using Gd(Yb)-chlorides the presence of nanorods is more evident, whereas using Gd(Yb)-nitrates the faster growth rate favors well-defined micrometer-sized octahedral particles. The spectroscopic properties of Yb3+ in NaGd1−xYbx(WO4)2 synthesized micrometer-sized octahedra are equivalent to those obtained in bulk single crystals, showing a single exponential photoluminescence decay and 2F5/2 lifetime τ ≈ 330 μs for 0.001 ≤ x ≤ 0.005 doped samples. The nanoparticles and nanorods formed with Gd(Yb)-nitrates and pH < 7 by short time annealing as well as in Gd(Yb)-chloride prepara...

Luminescence decay of broadband emission from CdS quantum dots

The stretched exponential photoluminescence decay of the energy-resolved broadband emission of purified and unpurified CdS quantum dots (QDs) made in reverse micelles is characterized as a function of photolysis time and thiol addition. Photolysis is found to proportionately increase both the lifetime and quantum yield of these QDs. This proportionality is consistent with a simple parallel channel model of the decay of the excited states. The ultimate QY of the purified sample is found to be as high as 24%, which is twice that previously reported for this preparation. At −70 °C both the QY and the lifetime increase by more than a factor of two, indicating that thermal quenching limits the QY at room temperature. Finally, the addition of alkanethiols is shown to red-shift and quench the emission while only modestly altering the lifetime. These thiolated QDs show an extremely large temperature dependence of QY, demonstrating stronger thermal quenching than the unfunctionalized QDs.

Acceptors in undoped GaN studied by transient photoluminescence

Acceptors responsible for the 2.2, 2.9 and 3.27 eV photoluminescence (PL) bands in undoped GaN were studied by transient PL. A non-exponential decay of PL intensity for all the three bands is observed at low temperature consistent with donor–acceptor pair recombination. The transitions are attributed to recombination involving a shallow donor and three different acceptors. At temperatures in excess of about 100 K a new recombination process emerges as evident from the nearly exponential PL decay. We attribute this to free-to-bound transitions involving free electrons and acceptors. Electron-capture cross-sections of 3×10 −19, 2×10 −20 and 3×10 −21 cm 2 are calculated for the acceptors responsible for the 3.27, 2.9 and 2.2 eV bands. These acceptors are attributed to multiply charged defects involving a gallium vacancy.

Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO2

Time resolved photoluminescence (PL) decays have been measured for Si nanocrystals embedded in silicon dioxide. The nanocrystals were formed by implanting 40 keV Si ions into a 1000 Å thick film of thermally grown SiO2, followed by thermal annealing at 1000–1200 °C. The observed luminescence, peaking at 700–850 nm, is compared to similar measurements performed on porous Si emitting in the same wavelength range. The results show that the PL from the nanocrystals exhibits a stretched exponential decay with characteristic decay time τ in the range 10–150 μs and dispersion factor β in the range 0.7–0.8. Both parameters are, however, higher for the nanocrystals compared to those of porous Si indicating superior passivation of the nanocrystals in the SiO2 matrix. Evidence is also presented for a single exponential behavior at the decay end suggesting a remaining fraction of excitons in isolated nanocrystals. We attribute the highly nonlinear dose dependence of the PL yield to a nucleation process for the nanocrystals and a more curved decay line shape for higher ion doses to a higher crystal density, promoting excitonic migration to nearby nanocrystals. These observations provide strong evidence that the origin of the stretched exponential line shape of the PL decay results from migration and trapping of excitons in a system of randomly distributed and interconnected nanocrystals with a dispersion in size.

Defect quenching of conjugated polymer luminescence.

Transient photoluminescence measurements in poly($p$-phenylene vinylene) illustrate that exciton diffusion to quenching centers explains the nonexponential decay dynamics. The structure of these defects is studied by vibrational spectroscopy and the reason they act as quenchers is explained. In their absence, we observe nearly exponential photoluminescence decay with lifetime g 1 ns. The new picture of photoexcitations which emerges and its implications for polymer electroluminescent devices are discussed.