Exciton Luminescence Intensity Research Articles

Effect of Intrinsic Defects on the Photoluminescence of Pristine and Doped CsPbBr3 Perovskite

We report the experimental results demonstrating the effect of either photoinduced or doping-induced intrinsic defects on excitonic photoluminescence in both pristine and doped CsPbBr3 halide perovskite. It is shown that an increase in the number of intrinsic defects leads to stronger photoluminescence quenching, which is confirmed by the direct negative correlations between the absorption of the intrinsic defects and excitonic luminescence intensity. The proposed mechanism indicates that the excitonic luminescence quenching originates from the exciton decay at the intrinsic defect sites. The efficiency of the exciton decay depends on the type and charge of the cations located near the defect sites and increases in the order Ag+ < Pb2+ < Bi3+.

Photon upconversion of all-inorganic CsPbX3 quantum dots based on fluorescence resonance energy transfer in hetero-structured perovskite/upconversion nanocomposites

Photo upconversion that converts low-energy light into high-energy photons holds great promise for boosting the energy conversion efficiencies particularly for photovoltaic devices based on all-inorganic CsPbX3 (X = Cl, Br, and I) perovskite quantum dots. However, the realization of nonlinear upconversion luminescence (UCL) from these all-inorganic CsPbX3 perovskites has been met with very limited success until now. Herein, we propose a rational perovskite/upconversion hetero-structured design strategy to achieve excitonic UCL of all-inorganic CsPbX3 perovskites based on fluorescence resonance energy transfer (FRET), where all-inorganic CsPbX3 perovskites and upconverting CaF2:Ln (Ln = Yb3+/Er3+, Yb3+/Ho3+, and Yb3+/Tm3+) nanocrystals are closely integrated with each other as a whole by using a modified two-step epitaxial growth approach, forming the unique upconverting CsPbX3/CaF2:Ln hetero-structured nanocomposites (HSNCs). Benefitting from the extremely high FRET efficiency of ∼99.7%, excitonic UCL from CsPbX3 perovskites can be realized upon low-power density 980-nm diode laser irradiation. More importantly, all the HSNCs are found to outperform their pure CsPbX3 counterparts in terms of air-stability, thermal-stability, and excitonic downconversion luminescence intensity, clearly demonstrating the overwhelming advantages of upconverting HSNCs we prepared over the pure CsPbX3 perovskites for high-performance optoelectronic and photovoltaic applications.

Optical properties of CdTe/ZnTe structures with thin CdTe layers

The optical properties of series of CdTe/ZnTe heterostructures with two built-in planar CdTe inclusions of thickness 4.0 and 1.5 monolayers are investigated. The temperature behavior of the exciton luminescence intensity at above-barrier and under-barrier excitation depends on the ZnTe barrier thickness which varies from 15 to 55 monolayers. It occurs that the shape of exciton luminescence bands changes strongly under the increasing optical excitation. The excitation luminescence spectra are studied in the temperature range 5 – 80 K.

Structural, Optical, and Luminescent Properties of ZnO:Ga and ZnO:In Ceramics

Zinc oxide optical ceramics were prepared by the hot uniaxial pressing technique. Nominally undoped and gallium- and indium-doped ceramics were studied. Undoped ceramics with a thickness of 0.5 mm had a total transmittance of about 65% in the visible range and exhibited mainly deep level luminescence at 600 nm. Doping of the ceramics resulted in the appearance of free charge carriers, suppression of the defect emission band, and enhancement of the excitonic luminescence intensity. The typical concentration of donor centers in doped ceramics was on the order of 1019 cm−1. Admixture of the dopants also decreased the average grain size of the ceramics and, in the case of indium-doped ceramics, affected their morphology. With respect to the excitonic emission intensity, the optimal concentration of dopants was 0.028 at% of Ga3+ and 0.045 at% of In3+. The maximum intensity of excitonic luminescence in indium-doped ceramics was about 1.5 times higher than that in gallium-doped ones. Luminescence decay curves of the ceramics with suppressed defect emission had a main decay time constant of 0.7 ns.

Magnetic field influence on the intensity of ZnO random lasing and exciton luminescence

The effect of the magnetic field on the exciton radiation of ZnO films formed by randomly packed nanorods and covered with Ag nanoparticles has been discovered and studied at room temperature. Photoexcitation was performed by third and fourth harmonics of Nd:YAG laser. The radiation of the investigated samples consisted of exciton luminescence and random lasing in the near UV range. Several percent enhancement of the exciton radiation intensity was observed at some areas of the films subjected to the magnetic field H ∼ 1 T. The enhancement depends slightly on the excitation level. At the same time, in some other areas of the same films, the effect was negligible or absent. It is supposed that the observed effect is related to magnetoexcitons, which are possibly formed due to increasing of the exciton momentum nearby the Ag nanoparticles. The probability of magnetoexciton recombination increases significantly even in a weak magnetic field due to strong stretching of its wave function.

Intrinsic luminescence of SrF2 nanoparticles

The influence of SrF2 nanoparticle size on their luminescence and kinetic properties under photo- and X-ray excitation was studied. The self-trapped exciton luminescence intensity was shown to reveal dependence on both the nanoparticle size and the range of excitation energies. The lowest sensitivity to the nanoparticle size is observed in the case of excitation in the region of optical exciton formation. The recombination of electrons with surface defects and non-radiative decay of excitons due to diffusion to the surface is found to be the determining factors of the luminescence quenching in the case of X-ray excitation.

Luminescence of CdSe quantum dots near a layer of silver nanoparticles ion-synthesized in sapphire

We study the characteristics of the luminescence of composite films based on polymethyl methacrylate with CdSe quantum dots deposited from solution onto the surface of a sapphire substrate containing a preliminarily formed layer with ion-synthesized silver nanoparticles. The sapphire layer with silver nanoparticles exhibits selective plasmon absorption in the visible spectral range with a peak at 463 nm. Enhancement in the exciton luminescence intensity of quantum dots with a peak at 590 nm is observed upon excitation at wavelengths lying in the region of plasmon resonance of metal nanoparticles, as well as luminescence quenching for quantum dots located in the vicinity of silver nanoparticles.

Manifestation of oxygen desorption in photoluminescence spectra of ZnO

The influence of oxygen desorption on the photoluminescence of zinc oxide has been investigated. Photodesorption and thermal desorption of oxygen have been controlled using mass spectrometry. It has been found that the removal of oxygen from the ZnO surface leads to a manifold increase in the exciton photoluminescence intensity. The process has a reversible character. The mechanism of increase in the exciton luminescence intensity of ZnO during the oxygen desorption has been discussed.

Influence of annealing on the scintillation properties of zinc oxide powders and ceramics

It was shown that annealing ZnO and ZnO:Ga initial powders and ceramics in different atmospheres significantly changes the characteristics of the studied samples. Two main luminescence bands of different origins were observed in powders at 540 nm and 580 nm. Annealing either in vacuum or in Ar:H2 atmosphere increased intensity of green luminescence with peak at 540 nm whereas annealing in air enhanced a luminescence band with peak at 580 nm in the powders. Corresponding changes in luminescence kinetic properties were observed. Annealing of the ceramics in vacuum and air did not affect the luminescence properties, while annealing them in Ar:H2 atmosphere increased green luminescence intensity of undoped ceramics and excitonic luminescence intensity of doped ones. Comparison of the X-ray, gamma-ray and cathode-ray excited luminescence lead to conclusion that the enhancement of luminescence intensity took place in surface layer of about 100 μm thickness.

Fabrication of highly efficient ZnO nanoscintillators

Photo-induced synthesis of high-efficiency ultrafast nanoparticle scintillators of ZnO was demonstrated. Controlled doping with Ga(III) and La(III) ions together with the optimized method of ZnO synthesis and subsequent two-step annealing in air and under reducing atmosphere allow to achieve very high intensity of UV exciton luminescence, up to 750% of BGO intensity magnitude. Fabricated nanoparticles feature extremely short sub-nanosecond photoluminescence decay times. Temperature dependence of the photoluminescence spectrum within 8–340K range was investigated and shows the absence of visible defect-related emission within all temperature intervals.

Influence of Surface Plasmon Resonance on ZnO Films Photoluminescence. Role of Excitation Level

In the course of luminescence enhancement investigation of ZnO films coated with Ag the authors discovered that enhancement by surface plasmon resonance takes place only under low enough pumping level while under a high pumping level only quenching is observed. For interpretationof this effect simplified mathematical model describing influence of surface plasmon resonance on near band edge luminescence in ZnO is proposed. Proposed model is presented by set of rate equations considering particularly increasing of exciton recombination probability and losses depended on pumping level. These losses most likely should be of Joule’s type. Experimental dependences of excitonic luminescence intensity of ZnO films coated with Ag on pumping level appear to be close to simplified theoretical ones based on this model.

Observation of long-lived interlayer excitons in monolayer MoSe2-WSe2 heterostructures.

Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ~1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

Temperature‐dependent Energy Transfer in Mn‐doped CdS/ZnS Nanocrystals#

Temperature dependence of the exciton and luminescence intensities in Mn‐doped CdS/ZnS quantum dots (QDs) emitting both exciton and dopant luminescence simultaneously was studied in the temperature range 77–320 K. With increasing temperature, exciton luminescence intensity decreased as a result of the increased charge carrier trapping, similar to the usual undoped QDs. In contrast, the sensitized Mn luminescence intensity increased with increasing temperature despite the decrease in the exciton population available for the sensitization. The observed opposite temperature dependence of the exciton and Mn luminescence indicates that the exciton–Mn energy transfer rate should increase with temperature significantly more rapidly than the charge carrier trapping. Temperature shift of the bandgap of the host QD and the energy of accepting d–d transition, resulting in the variation of the donor–acceptor spectral overlap, is considered responsible for the large temperature dependence of the energy transfer rate in Mn‐doped QDs.

The luminescence of BaF2 nanoparticles upon high-energy excitation

The dependence of X-ray excited luminescence intensity on BaF2 nanoparticle size was studied. A sharp decrease of self-trapped exciton luminescence intensity was observed when the nanoparticle size is less than 80 nm. The main mechanism of the luminescence quenching is caused by the escape of electrons from the nanoparticles. Escape of electrons from nanoparticles is confirmed by the considerable increase of luminescence intensity of the polystyrene scintillator with embedded BaF2 nanoparticles comparing with pure polystyrene scintillator.

Self-trapped exciton and core-valence luminescence in BaF2 nanoparticles

The influence of the BaF2 nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (hν ≤ Eg) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

Photoluminescence and exciton resonances over the scattered light in multiphonon spectra of resonant scattering in the CdTe/ZnTe superlattices with narrow quantum wells

Photoluminescence and Raman scattering spectra in CdTe/ZnTe heterostructures and superlattices with narrow quantum wells (4.8–9.2 A) in a temperature range of 5–300 K have been measured. The temperature dependences of the intensity of exciton luminescence in isolated quantum wells have been studied, and the thermal activation energies associated with the effective barriers for electrons and holes have been determined. In CdTe/ZnTe heterostructures, the binding energies of an exciton with a heavy hole have been determined as functions of the quantum well width. The multiphonon Raman spectra that exhibit distinctive features, such as the weak intensity of nLO phonon lines of ZnTe (n 2), and the multiple participation in scattering of acoustic LA phonons with large wave vector, have been investigated. The results have been explained based on the concept of the relaxation of hot excitons over the exciton band.

GROWTH, PROPERTIES AND SENSOR APPLICATIONS OF LOW TEMPERATURE GROWN ZnO NANORODS

ZnO nanorods with the average diameter of about 60–300 nm have been grown by the low temperature aqueous chemical growth technique. The impact of growth time and reactant concentration on the morphology on ZnO structures was tested by scanning electron microscopy and photoluminescence spectroscopy. High optical quality of obtained ZnO nanorod films was evidenced by dominating free exciton transitions. The reduction of surface to volume ratio by increasing the diameter of ZnO nanorods resulted in a linear increase of excitonic luminescence intensity with respect to the defect related emission. Excellent photoluminescence properties and high surface area of ZnO nanorods can be used for the detection of organic contaminants dissolved in water. The quenching of both excitonic and defect luminescence was observed upon exposure of ZnO nanorod films to various organic contaminants.

电场调控下不对称耦合量子点中激子的发光特性

We theoretically analyzed the bonding and anti-bonding energy states in the GaAs/Al0.35Ga0.65As asymmetric coupled quantum dots under a series of electric fields.The carrier tunneling effect strongly influences the exciton energy and oscillator strength.We find that energy splitting of anti-crossing occurs under an critical electric field for hole bounding and anti-bonding states,and both the energy gap and the critical field decrease with increasing the barrier distance.Meanwhile,the exciton luminescence intensity changes from bright(dark) to dark(bright) under external fields.

Growth, properties and sensor applications of low temperature grown ZnO nanorods

ZnO nanorods with the average diameter of about 60–300 nm have been grown by the low temperature aqueous chemical growth technique. The impact of growth time and reactant concentration on the morphology on ZnO structures was tested by scanning electron microscopy and photoluminescence spectroscopy. High optical quality of obtained ZnO nanorod films was evidenced by dominating free exciton transitions. The reduction of surface to volume ratio by increasing the diameter of ZnO nanorods resulted in a linear increase of excitonic luminescence intensity with respect to the defect related emission. Excellent photoluminescence properties and high surface area of ZnO nanorods can be used for the detection of organic contaminants dissolved in water. The quenching of both excitonic and defect luminescence was observed upon exposure of ZnO nanorod films to various organic contaminants.

Ferromagnetic Fe-Doped GaN Nanowires Grown by Chemical Vapor Deposition

We report the first synthesis and characterization of Fe-doped GaN nanowires. The Fe-doped nanowires were grown by chemical vapor deposition process, using pretreated iron oxide as dopant precursors. The morphological and structural analysis showed that the obtained nanowires were triangular cross section and single crystalline wurtzite GaN structure. It was proved that the pretreatment could induce doping effectively and higher substrate temperature could produce higher dopant concentration. The highest concentration of Fe ions in GaN was 0.12% in our condition. Fe 2p core-level X-ray photoelectron spectroscopy (XPS) spectra showed that the charge state of Fe transformed from +3 to +2 with the increase of Fe concentration. Moreover, the change in the binding energy of Ga 3p in the XPS spectra could be attributed to position change of Fermi level governed by Fe concentration. In the photoluminescence (PL) spectra, the relative intensity of exciton luminescence increased with Fe concentration. The magnetic measurement revealed the Fe-doped GaN nanowires were ferromagnetic at room temperature. The XPS results provided the evidence for the charge state of Fe dopant, indicating the intrinsic nature of room-temperature ferromagnetism of Fe-doped GaN nanowires.