Photoluminescence Energy Research Articles

Polariton-condensation effects on photoluminescence dynamics in a CuBr microcavity

Temporal profiles of photoluminescence (PL) from the cavity polariton at the ground state in a CuBr microcavity at 10K have been investigated from the viewpoint of the cavity-polariton condensation. We measured angle-resolved reflectance spectra to characterize the cavity-polariton dispersions, cavity-photon dispersion, and quality factor from which the intrinsic cavity-polariton lifetime was estimated. The threshold-like changes of the PL intensity, band width, and energy of the lower polariton as a function of excitation power are consistent with previously reported experimental results for the cavity-polariton condensation. It was found that the PL rise and decay times are markedly decreased by the cavity-polariton condensation, which can be attributed to the appearance of the bosonic final state stimulation and intrinsic lifetime of polaritons in the relaxation and decay processes, respectively.

Thick-Shell CuInS2/ZnS Quantum Dots with Suppressed "Blinking" and Narrow Single-Particle Emission Line Widths.

Quantum dots (QDs) of ternary I-III-VI2 compounds such as CuInS2 and CuInSe2 have been actively investigated as heavy-metal-free alternatives to cadmium- and lead-containing semiconductor nanomaterials. One serious limitation of these nanostructures, however, is a large photoluminescence (PL) line width (typically >300 meV), the origin of which is still not fully understood. It remains even unclear whether the observed broadening results from considerable sample heterogeneities (due, e.g., to size polydispersity) or is an unavoidable intrinsic property of individual QDs. Here, we answer this question by conducting single-particle measurements on a new type of CuInS2 (CIS) QDs with an especially thick ZnS shell. These QDs show a greatly enhanced photostability compared to core-only or thin-shell samples and, importantly, exhibit a strongly suppressed PL blinking at the single-dot level. Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than that of the ensemble samples. To explain this distinction, we invoke a model wherein PL from CIS QDs arises from radiative recombination of a delocalized band-edge electron and a localized hole residing on a Cu-related defect and also account for the effects of electron-hole Coulomb coupling. We show that random positioning of the emitting center in the QD can lead to more than 300 meV variation in the PL energy, which represents at least one of the reasons for large PL broadening of the ensemble samples. These results suggest that in addition to narrowing size dispersion, future efforts on tightening the emission spectra of these QDs might also attempt decreasing the "positional" heterogeneity of the emitting centers.

Evaluation of photoluminescence quantum yield of monolayer WSe2 using reference dye of 3‐borylbithiophene derivative

We studied an evaluation method of photoluminescence (PL) quantum yield of monolayer transition metal chalcogenides (TMDs) on a transparent substrate using a reference dye. A 3‐borylbithiophene derivative (C58H49BN2S4) reference dye shows a high PL quantum yield of 36% even in the solid form and the high quantum yield is kept in a Poly(methyl methacrylate) (PMMA) matrix. Moreover, the photon energy of PL is similar to those of monolayer TMDs. These PL characteristics of the reference dye are useful for a evaluation of PL quantum yield in monolayer TMDs. The PL quantum yield of 0.18 ± 0.03% was evaluated for mechanically exfoliated monolayer WSe2 on glass substrate at room temperature.

The temperature behavior and mechanism of exciton luminescence in quantum dots.

The processes of direct and indirect optical excitation of spatially confined excitons in quantum dots (QDs) embedded in a silica thin-film matrix have been reported and discussed. A generalized scheme for the electronic transitions is employed for a detailed description of luminescence temperature behavior using various excitation methods. This scheme considers three different models of exciton relaxation and substantiates the occupation of the triplet radiative states as a result of singlet-triplet intersystem crossing or excitation energy transfer from free excitons of the matrix. Analytical expressions describing five types of different temperature curves were derived. It is established that their shapes are exactly defined by the excitation mechanism and the parameters involved in the numerical model. The conditions allowing the estimation of the energy and kinetic characteristics of QD photoluminescence are formulated. We have shown that the confinement effect causes a decrease in the thermal activation barriers and frequency characteristics for non-radiative transitions. An application of the developed concepts allows predicting and estimating the temperature dependences for direct and indirect optically excited QD luminescence employing silicon nanoclusters in a silica thin-film matrix as an example.

Photoluminescence properties and energy transfer of Ce3+–Tb3+ co-doped SrAlF5 nanorods by a hydrothermal method

SrAlF5:Ce3+,Tb3+ nanorods were synthesized via a mild hydrothermal process and the energy transfer from Ce3+ to Tb3+ was analyzed.

Carrier localization in In0.21Ga0.79As/GaAs multiple quantum wells: A modified Pässler model for the S-shaped temperature dependence of photoluminescence energy

The optical properties of In0.21Ga0.79As/GaAs MQWs, with triple unequal layer thickness NW (3 nm), MW (6 nm) and WW (9 nm) grown on (001) and (113) GaAs substrates, is studied by using continuous wave photoluminescence (cw-PL) spectroscopy. A comparative study has been performed to demonstrate the influence of electric field and QW thickness on the exciton localization. An S-shaped form in temperature-dependent PL peak energy has been observed in polar middle QW (MW (113)) but not seen in non-polar ones (MW (001)). This behavior is linked to carrier localization in triangular potential and polarity. We have observed also this atypical evolution in non-polar wide QW (WW (001)) but not in non-polar middle QW (MW (001)), which is attributed to potential fluctuation in larger ones. With the aid of modified Pässler model for including the effect of localized states, we can persuasively reproduce the S-shaped temperature dependence of PL band gap energy and contribute to the estimated value of exciton localization energy. The values of σ are obtained from adjustment of experimental points, which indicate the degree of localization in QW layer.

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.

Tunable excitonic transitions in strained GaAs ultra-thin quantum disk

Simultaneous influences of hydrostatic pressure and temperature combined to the size effect on the behaviour of the exciton in 2D AlAs/GaAs/AlAs ultra thin quantum disk are investigated. Our approach is performed in the framework of effective mass theory and adiabatic approximation by using a variational method with a robust trial wave function and by taking into account the dependence of the size, the dielectric constant and the effective masses on the pressure and temperature. Variations of the excitonic binding energy, photoluminescence energy and oscillator strength are determined according to hydrostatic pressure and temperature for different confinement regimes. The results of our numerical calculations show that the applied pressure favours the electron-hole attraction while the temperature tends to decrease the exciton binding energy. Another interesting result is the possibility of transforming a thin quantum disk into a large-gap material by strain effect. The opposing effects caused by temperature and pressure reveal a big practical interest and offer an alternative way to the tuning of the excitonic transition in optoelectronic devices.

Competition Between Resonant Plasmonic Coupling and Electrostatic Interaction in Reduced Graphene Oxide Quantum Dots.

The light emission from reduced graphene oxide quantum dots (rGO-QDs) exhibit a significant enhancement in photoluminescence (PL) due to localized surface plasmon (LSP) interactions. Silver and gold nanoparticles (NPs) coupled to rGO nanoparticles exhibit the effect of resonant LSP coupling on the emission processes. Enhancement of the radiative recombination rate in the presence of Ag-NPs induced LSP tuned to the emission energy results in a four-fold increase in PL intensity. The localized field due to the resonantly coupled LSP modes induces n-π* transitions that are not observed in the absence of the resonant interaction of the plasmons with the excitons. An increase in the density of the Ag-NPs result in a detuning of the LSP energy from the emission energy of the nanoparticles. The detuning is due to the cumulative effect of the red-shift in the LSP energy and the electrostatic field induced blue shift in the PL energy of the rGO-QDs. The detuning quenches the PL emission from rGO-QDs at higher concentration of Ag NPs due to non-dissipative effects unlike plasmon induced Joule heating that occurs under resonance conditions. An increase in Au nanoparticles concentration results in an enhancement of PL emission due to electrostatic image charge effect.

Single-dot spectroscopy of boron and phosphorus codoped silicon quantum dots

Boron (B) and phosphorous (P) codoped silicon quantum dots (Si QDs) are dispersible in polar solvents without organic ligands, and exhibit size controllable photoluminescence (PL) from 0.85 to 1.85 eV due to the electronic transitions between the donor and the acceptor states. We study the PL spectra of the codoped Si QDs at room temperature and at 77 K. We show that the broad PL band of codoped colloidal Si QDs (full width at half maximum is over 400 meV) is composed of narrower PL bands of individual QDs with different PL energies. We also show that the PL linewidth of individual codoped Si QDs is almost twice as large as those of undoped Si QDs. In contrast to the significant narrowing of the PL linewidth of undoped Si QDs at low temperatures, that of codoped Si QDs is almost independent of the temperature except for a few very small QDs. These results suggest that a large number of B and P are doped in a QD and there are a number of non-identical luminescence centers in each QD.

Synthesis and photoluminescence properties of KBaY(BO3)2:Eu2+ bluish-green phosphor

The bluish-green KBaY(BO3)2:Eu2+ phosphors were prepared by high-temperature solid state reaction method. The photoluminescence properties, concentration quenching and energy transfer as well as the luminescence decays were investigated. The emission spectrum showed a single intensive band centered at 500 nm, which correspond to the 4f 65d 1 → 4f 7 transition of Eu2+. The optimal concentration for the luminescence of Eu2+ in KBaY(BO3)2:Eu2+ was determined to be 0.03 mol. The critical energy transfer distance was calculated as 21.34 A. The corresponding concentration quenching mechanism was verified to be a dipole–dipole interaction. And the blue-shift behavior of KBaY(BO3)2:Eu2+ with increasing temperature was analyzed by the configurational coordinate diagram. The results indicate that the KBaY(BO3)2:Eu2+ phosphors have potential applications as an ultraviolet-convertible phosphor due to its effective excitation in the ultraviolet rang.

Photoluminescence Properties and Energy Transfer in Color-Tunable Garnet Phosphor for High Quality LED Lighting

Recently, the performance of phosphor-converted white light-emitting diodes (WLED) has been improved mainly by diversifying the components, encapsulation of the device, phosphor synthesis, as well as designing new strategies for the development of phosphors. Nowadays, single-composition white-emitting phosphors which are excited by near UV or blue LED have been developed to prevent the reabsorption, instability of color temperature, and cost issues. We report a garnet phosphor with partially displaced Tb3+ and Pr3+ as green and red components respectively. A green-emitting and white-emitting phosphor was synthesized by a solid-state reaction, exhibiting an efficient green emission and red emission at 543 nm and 609 nm under blue excitation. The structural and optical properties of the phosphor were studied using powder X-ray diffraction and photoluminescence analysis. The compound crystalizes in cubic phase with space group Ia d. The partial substitution of Tb3+ and Pr3+ in garnet phosphor led to a considerably broad emission spectrum. The WLED was fabricated by using phosphor-in-glass with a blue LED chip and exhibited a warm white light with an excellent color rendering index of 92. The garnet phosphor with Ce3+, Tb3+, and Pr3+ is thus a potential green and white-emitting phosphor for LED lighting. Figure 1. PL excitation and emission spectrum of garnet phosphor with (a) Ce3+, (b) Ce3+, Tb3+, and (c) Ce3+, Pr3+. Figure 1

Silicon Nanoparticles with Surface Nitrogen: 90% Quantum Yield with Narrow Luminescence Bandwidth and the Ligand Structure Based Energy Law.

Silicon nanoparticles (NPs) have been widely accepted as an alternative material for typical quantum dots and commercial organic dyes in light-emitting and bioimaging applications owing to silicon's intrinsic merits of least toxicity, low cost, and high abundance. However, to date, how to improve Si nanoparticle photoluminescence (PL) performance (such as ultrahigh quantum yield, sharp emission peak, high stability) is still a major issue. Herein, we report surface nitrogen-capped Si NPs with PL quantum yield up to 90% and narrow PL bandwidth (full width at half-maximum (fwhm) ≈ 40 nm), which can compete with commercial dyes and typical quantum dots. Comprehensive studies have been conducted to unveil the influence of particle size, structure, and amount of surface ligand on the PL of Si NPs. Especially, a general ligand-structure-based PL energy law for surface nitrogen-capped Si NPs is identified in both experimental and theoretical analyses, and the underlying PL mechanisms are further discussed.

Photoluminescence enhancement and energy transfer mechanism of Bismuth added LaGaO3: Eu nanophosphor for display applications

A systematic investigations were carried out on pure LaGaO3, Bi3+ doped-LaGaO3, Eu3+doped-LaGaO3, Bi3+ and Eu3+ co-doped LaGaO3 (LaGaO3:Bi3+, Eu3+) nanophosphors synthesized by low temperature polyol technique for enhanced luminescent properties. Powder x-ray diffraction (PXRD), UV⿿vis spectroscopy, FTIR, and FE-SEM with EDAX measurements were done for the phase identification, energy band gaps, functional group analysis and structural morphology and elemental presence respectively. Photoluminescence (PL) spectra of prepared nanophosphors under near-UV light exhibited a red luminescence corresponding to the electric dipole transition 5D0-7F2 at 616nm. The enhanced PL intensity was observed in LaGaO3:Bi3+, Eu3+ nanophosphor due to strong absorption of excitation energy and effective energy transfer mechanism in between the dopants. The corresponding decay curve was observed with a long life time of 2.13ms. The obtained CIE data indicates these nanophoshors serve as potential candidates for display device industry.

Exciton recombination at crystal-phase quantum rings in GaAs/InxGa1−xAs core/multishell nanowires

We study the optical properties of coaxial GaAs/InxGa1−xAs core/multishell nanowires with x between 0.2 and 0.4 at 10 K. The evolution of the photoluminescence energy of the InxGa1−xAs quantum well shell with x and shell thickness agrees with the result of 8-band k·p calculations, demonstrating that the shell growth is pseudomorphic. At low excitation power, the photoluminescence from the shell is dominated by the recombination of exciton states deeply localized within the shell. We show that these states are associated with crystal-phase quantum rings that form at polytype segments of the InxGa1−xAs quantum well shell.

Time-resolved analysis of the white photoluminescence from chemically synthesized SiCxOy thin films and nanowires

The study reported herein presents results on the room-temperature photoluminescence (PL) dynamics of chemically synthesized SiCxOy≤1.6 (0.19 < x < 0.6) thin films and corresponding nanowire (NW) arrays. The PL decay transients of the SiCxOy films/NWs are characterized by fast luminescence decay lifetimes that span in the range of 350–950 ps, as determined from their deconvoluted PL decay spectra and their stretched-exponential recombination behavior. Complementary steady-state PL emission peak position studies for SiCxOy thin films with varying C content showed similar characteristics pertaining to the variation of their emission peak position with respect to the excitation photon energy. A nearly monotonic increase in the PL energy emission peak, before reaching an energy plateau, was observed with increasing excitation energy. This behavior suggests that band-tail states, related to C-Si/Si-O-C bonding, play a prominent role in the recombination of photo-generated carriers in SiCxOy. Furthermore, the PL lifetime behavior of the SiCxOy thin films and their NWs was analyzed with respect to their luminescence emission energy. An emission-energy-dependent lifetime was observed, as a result of the modulation of their band-tail states statistics with varying C content and with the reduced dimensionality of the NWs.

Photoluminescence properties and energy transfer between activators at different crystallographic sites in Ce3+ doped Sr2MgAl22O36

In this paper, a series of Ce3+ doped Sr2MgAl22O36 (SMA) phosphors have been prepared by high temperature solid-state reaction method. The phase structure of prepared samples was checked by the powder X-ray diffraction (XRD). The morphology of the samples was inspected using a field-emission scanning electron microscope (SEM). Under different UV radiation, this phosphor exhibits different emission bands due to the Ce3+ ions located at different lattice sites. The corresponding luminescence and energy transfer mechanisms have been proposed in detail. The phosphor exhibits different concentration quenching mechanisms because the Ce3+ ions substitute two different crystallographic sites in the host. Moreover, the temperature dependent emission properties of SMA:Ce3+ were conducted from 30°C to 200°C, as much as 72.96% of the room-temperature emission intensity is retained at 150°C. The SMA:Ce3+ phosphor exhibits bright blue emission with CIE coordinates (x=0.16, y=0.12) under UV excitation. The results indicate that SMA:Ce3+ phosphor has great potential applications in UV-pumped light emitting diodes.

Anomalous Light Emission and Wide Photoluminescence Spectra in Graphene Quantum Dot: Quantum Confinement from Edge Microstructure.

The physical origin of the observed anomalous photoluminescence (PL) behavior, that is, the large-size graphene quantum dots (GQDs) exhibiting higher PL energy than the small ones and the broadening PL spectra from deep ultraviolet to near-infrared, has been debated for many years. Obviously, it is in conflict with the well-accepted quantum confinement. Here we shed new light on these two notable debates by state-of-the-art first-principles calculations based on many-body perturbation theory. We find that quantum confinement is significant in GQDs with remarkable size-dependent exciton absorption/emission. The edge environment from alkaline to acidic conditions causes a blue shift of the PL peak. Furthermore, carbon vacancies are inclined to assemble at the GQD edge and form the tiny edge microstructures. The bound excitons, localized inside these edge microstructures, determine the anomalous PL behavior (blue and UV emission) of large-size GQDs. The bound excitons confined in the whole GQD lead to the low-energy transition.

Color-Tunable Resonant Photoluminescence and Cavity-Mediated Multistep Energy Transfer Cascade.

Color-tunable resonant photoluminescence (PL) was attained from polystyrene microspheres doped with a single polymorphic fluorescent dye, boron-dipyrrin (BODIPY) 1. The color of the resonant PL depends on the assembling morphology of 1 in the microspheres, which can be selectively controlled from green to red by the initial concentration of 1 in the preparation process of the microspheres. Studies on intersphere PL propagation with multicoupled microspheres, prepared by micromanipulation technique, revealed that multistep photon transfer takes place through the microspheres, accompanying energy transfer cascade with stepwise PL color change. The intersphere energy transfer cascade is direction selective, where energy donor-to-acceptor down conversion direction is only allowed. Such cavity-mediated long-distance and multistep energy transfer will be advantageous for polymer photonics device application.

Photoluminescence of gallium ion irradiated hexagonal and cubic GaN quantum dots

We report on ion implantation into GaN QDs and investigate their radiation hardness. The experimental study is carried out by photoluminescence (PL) measurements on molecular beam epitaxy-grown GaN quantum dots after ion implantation. Both quantum dots grown in the hexagonal (H) and the cubic (C) crystal structure were subjected to gallium ions with an energy of 400kV (H) and 75kV (C) with fluences ranging from 5×1010cm−2 to 1×1014cm−2 (H) and to 1×1015cm−2 (C), respectively. Low-temperature PL measurements reveal a PL quenching for which a quantitative model as a function of the ion fluence is developed. A high degradation resistance is concluded. A non-radiative trap with one main activation energy is found for all QD structures by temperature-dependent PL measurements. Further analysis of fluence-dependent PL energy shifts shows ion-induced intermixing and strain effects. Particular for the hexagonal quantum dots, a strong influence of the quantum confined Stark effect is present.