Exciton Radiation Research Articles

(Invited) Thermal Exciton Radiation of Macroscopic Semiconducting Carbon Nanotube Membranes

Individual semiconducting carbon nanotubes at high temperature more than ~1000 K show narrowband near infrared thermal radiation owing to exciton resonances [1], which may be useful in thermophotovoltaic energy harvesting. However, it still remains to be clarified whether macroscopic membranes consisting of semiconducting carbon nanotubes can also show narrowband thermal exciton radiation. Our recent progress on the study of thermal radiation properties of macroscopic membranes of chirality-separated [2] semiconducting carbon nanotubes will be discussed. [1] T. Nishihara, A. Takakura, Y. Miyauchi, and K. Itami, Nat. Commun. 9, 3144 (2018). [2] H. Liu, D. Nishide, T. Tanaka, and H. Kataura, Nat. Commun. 2, 309 (2011).

Efficiency improvement in solution-processed multilayered phosphorescent white organic light emitting diodes by silica coated silver nanocubes

Localized surface plasmon resonance (LSPR) effect of metal nanoparticles (MNs) has been widely applied in organic light-emitting diodes (OLEDs) to improve the radiation of excitons. The LSPR wavelength and intensity of MNs and the coupling between MNs and excitons greatly affect the LSPR effect on exciton radiation. In this work, silica coated silver nanocubes (Ag@SiO<sub>2</sub> NCs) were doped in the electron transport layer (ETL) of a solution-processed multilayered white OLED (WOLED). Due to the sharp edges and corners, Ag NCs have strong LSPR effect and can effectively enhance the radiation of nearby excitons. With an appropriate concentration of Ag@SiO<sub>2</sub> NCs, the WOLED achieved two fold improvement in the current efficiency comparing with the control device without Ag@SiO<sub>2</sub> NCs incorporated. The working mechanism of the Ag@SiO<sub>2</sub> NCs based WOLED was investigated in detail. For the solution-processed OLED, excitons usually form and recombine near the interface of emission layer and electron transport layer (EML/ETL) because the commonly used host material (such as polyvinylcarbazole, PVK) has the unipolar hole transport property. So the Ag@SiO<sub>2</sub> NCs in ETL greatly enhanced the radiation of the excitons located near the EML/ETL interface, which mostly contributed to the performance enhancement of the Ag@SiO<sub>2</sub> NCs based WOLED. Study on a group of devices with Ag@SiO<sub>2</sub> NCs doped in different locations indicated that Ag@SiO<sub>2</sub> NCs in ETL showed more effective LSPR effect than those in hole injection layer. Electroluminescence and photoluminescence spectra of the WOLEDs declared that the Ag@SiO<sub>2</sub> NCs simultaneously improved the radiation intensities of the blue and yellow excitons and helped the WOLED maintain the good chromaticity stability, which was mainly attributed to the wide LSPR wavelength range (450–650 nm) of the Ag@SiO<sub>2</sub> NCs. The SiO<sub>2</sub> coating layer of the Ag@SiO<sub>2</sub> NCs played the important role in the LSPR enhanced emission. On the one hand, it formed an appropriated distance between the Ag NCs and the extions, helping to generate the strong coupling between them. On the other hand, it suppressed the effect of Ag NCs on charge trapping, keeping the stability of the carrier transport in the device. Our research demonstrate MNs can effectively improve the performance of OLEDs by carefully designing the device structure.

Strong Coupling of Excitons in Hexagonal GaN Microcavities

The GaN planar hexagonal microcavities are grown by the selective vapor-phase epitaxy technique. The spectra are measured by the low-temperature cathodoluminescence method using a scanning electron microscope. The obtained spectra show a huge Rabi splitting (~100 meV). Numerical simulation of the spatial distribution of the intensities of modes in a hexagonal cavity is carried out. Certain modes can have a high spatial localization leading to strong coupling with the exciton and huge Rabi splitting. The fraction of excitons in polariton modes, which correlates with the intensity of exciton radiation associated with these modes, is theoretically calculated for hexagonal-shaped microcavities. Thus, the form of the dependence of the radiation probability on the eigenfrequencies of the structure is obtained.

Surface/Deep Defects manipulated Fluorescence Properties and LED application of Copper Nanoclusters

The contribution of metal defects to the emission enhancement of Cu nanoclusters has been proposed. But the insufficient understanding of the defects luminescence mechanism limits its further effective adjustment and control to the emission color. In this work, the surface metal defects of the assembled fluorescent materials are deliberately engineered. Metal defects have effects on luminescent energy levels, which are relative to the dynamic process of exciton radiation transition and regulate the photophysical properties of the materials. Further studies demonstrate the surface metal defects possess shallow energy level and multi sublevels, broaden the emission colors. Moreover, the continuous alteration of emission color is achieved by simply adjusting the relative ratio of the surface and deep metal defects. Based on the full-color emission of copper clusters, the white light emitting diodes (WLEDs) with different color temperatures (warm, cold, and pure white) are prepared for illumination application.

Сильная связь экситонов в микрорезонаторах GaN гексагональной формы

The GaN planar hexagonal microcavities were grown by a selective vapor phase epitaxy. The spectra were measured by the low-temperature cathodoluminescence method in-situ scanning electron microscope. The obtained spectra show a huge Rabi splitting (in order of 100 meV). Numerical simulation of the spatial distribution of the modes’ intensities of a hexagonal resonator is shown. Some modes can have strong spatial localization, leading to strong coupling with the exciton and huge Rabi splitting. Theoretically, we calculated the fraction of excitons in polariton modes, which correlates with the intensity of exciton radiation associated with these modes, for microcavities of a hexagonal shape. Thus, we have obtained the form of the dependence of the radiation probability on the eigenfrequencies of the structure.

Temperature features of ZnO exciton luminescence

The study shows exciton-phonon luminescence (EPL) of free excitons in ZnO crystals within temperatures 77-300 K. It’s been found that low resistivity samples with temperatures above 150 K demonstrate temperature anomalies in the EPL of free excitons, which is supposed to be the result of the exciton – electron interaction. The EPL temperature dynamics is closely related to the nature of a defective crystal structure. This allows that we should propose a method of quality non-destructive operation with zinc oxide crystals. The paper describes conditions for recording the specific luminescence spectrum of ZnO, which can be the result of inelastic scattering of free excitons and which is accompanied by radiation of excited excitons and exciton-phonon complexes.

The interaction of an exciton and several optical modes in layer of GaAs

Theoretically, we calculated exciton fraction in polariton modes, which correlates with the intensity of the exciton radiation associated with these modes for several microcavities. Thus, we obtained the form of the dependence of the radiation probability on the eigenfrequencies of the structure.

Ultrahigh-performance blue organic light-emitting diodes based on SiO2 coated Ag nanocubes and its working mechanism

Abstract Ag nanocubes coated with SiO2 shell were synthesized and incorporated into the emission layer of blue organic light-emitting diodes to improve the device performance. The thickness of SiO2 shell was changed to adjust the distance between Ag NCs and excitons to investigate the coupling of the localized surface plasmon resonance (LSPR) of Ag NCs and the radiation of excitons. In addition, the host materials with different carrier transporting properties are used to study the correlation between the LSPR enhanced exciton radiation and the exciton recombination zone in emission layer. Research findings demonstrate the thickness of SiO2 shell and the exciton recombination zone together affect the coupling of LSPR and exciton radiation. Using the Ag NCs coated with 15 nm SiO2 shell and the bipolar host material of 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), the blue OLED achieved an very high improvement in the current efficiency. The maximum peak current efficiency of the blue OLED with Ag NCs reached to 97 cd/A, being nearly 4 times of that of the device without Ag NCs.

Exciton Localization and Radiative Lifetimes in CdSe Nanoplatelets

The spatial extents of CdSe nanoplatelet (NPL) excitons are investigated using transient absorption (TA) spectroscopy. The bleach magnitudes of a series of NPLs with varying lateral dimensions are ...

Emission of diamonds, leucosapphire, and KU-1 quartz in the range of 200–800 nm excited by electron beams with a pulse duration of 0.5 and 12 ns

The paper presents the results of Cherenkov radiation (CR) studies in various crystals excited by nanosecond and subnanosecond electron beams with an electron energy of up to 400 and 200 keV, respectively. These studies are motivated by the need to develop and create calibrated sensors of runaway electrons for tokamak-type installations. The spectral and amplitude-time characteristics of the radiation of different specimens of diamonds, leucosapphire, and KU-1 quartz excited by the electron beams were obtained and compared. These crystals were chosen on the basis of their transparency in the ultraviolet region of the spectrum. CR was observed with a spectrometer. An increase in the radiation energy with decreasing wavelength was observed in synthetic diamonds of IIa type, leucosapphire, and KU-1 quartz. The amplitude-time characteristics of the radiation of crystals, including CR, were measured with a photodiode having a pulse response time of up to ≈80 ps. In chemical vapor deposited high purity diamonds excited by both nanosecond and subnanosecond beams, an exciton radiation with a maximum at 235 nm was observed. Results show that leucosapphire and quartz can be used to create detectors of electrons with energies above 200 keV.

(Invited) Thermo-Excitonic Properties of Semiconducting Single-Walled Carbon Nanotubes

Because of the large binding energy of one-dimensional excitons on the order of 0.5 eV, excitons are stable in semiconducting single-walled carbon nanotubes (SWNTs) even at high temperatures more than 1000 K. This unique exciton property, with the very high intrinsic thermal stability of the materials, enables thermal generation of excitons in semiconducting SWNTs; this results in thermal radiation with very narrow spectral bandwidth even at 1000-2000 K [1]. We will report our recent progress in the study on the thermal exciton radiation from SWNTs. [1] T. Nishihara, A. Takakura, Y. Miyauchi, and K. Itami, Nat. Commun. 9, 3144 (2018).

Bipolar Blue Host Emitter with Unity Quantum Yield Allows Full Exciton Radiation in Single-Emissive-Layer Hybrid White Organic Light-Emitting Diodes.

Phosphorescence/fluorescence hybrid white organic light-emitting diodes (OLEDs) are highly appealing for solid-state lighting. One major challenge is how to fully utilize the electrically generated excitons for light output. Herein, an efficient strategy to realize full exciton radiation is successfully revealed by a judicious molecular design and suitable device engineering. A blue host emitter TP-PPI is designed and synthesized, exhibiting a near 100% photoluminescence quantum yield and a high triplet energy level, enabling high-performance blue fluorescence and sensitization of a yellow phosphorescent dopant. Full exciton radiation in hybrid white OLEDs is demonstrated with a single emitting layer formed by doping a yellow phosphor (PO-01) into TP-PPI. Near 100% exciton utilization and state-of-the-art external quantum efficiency of 27.5% are achieved with the high-efficiency blue-emitting host and an electron-trap engineered device architecture.

Ultra-narrow-band near-infrared thermal exciton radiation in intrinsic one-dimensional semiconductors

Thermal radiation is the most primitive light emission phenomenon of materials. Broadband radiation from red-hot materials is well known as the kick-starter phenomenon of modern quantum physics in the early twentieth century; even nowadays, its artificial control plays a central role in modern science and technology. Herein, we report the fundamental thermal radiation properties of intrinsic one-dimensional semiconductors and metals, which have not been elucidated because of significant technical challenges. We observed narrow-band near-infrared radiation from semiconducting single-walled carbon nanotubes at 1000–2000 K in contrast to its broadband metallic counterpart. We confirm that the ultra-narrow-band radiation is enabled by the thermal generation of excitons that are hydrogen-like neutral exotic atoms comprising mutually bound electrons and holes. Our findings uncover the robust quantum correlations in intrinsic one-dimensional semiconductors even at 2000 K; additionally, the findings provide an opportunity for excitonic optothermal engineering toward the realization of efficient thermophotovoltaic energy harvesting.

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.

Highly Efficient Visible Colloidal Lead-Halide Perovskite Nanocrystal Light-Emitting Diodes.

Lead-halide perovskites have been attracting attention for potential use in solid-state lighting. Following the footsteps of solar cells, the field of perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their application prospects in lighting, however, remain still uncertain due to a variety of shortcomings in device performance including their limited levels of luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs based on colloidal perovskite nanocrystals (PeNCs) synthesized at room temperature possessing dominant first-order excitonic radiation (enabling a photoluminescence quantum yield of 71% in solid film), unlike in the case of bulk perovskites with slow electron-hole bimolecular radiative recombination (a second-order process). In these PeLEDs, by reaching charge balance in the recombination zone, we find that the Auger nonradiative recombination, with its significant role in emission quenching, is effectively suppressed in low driving current density range. In consequence, these devices reach a maximum external quantum efficiency of 12.9% and a power efficiency of 30.3 lm W-1 at luminance levels above 1000 cd m-2 as required for various applications. These findings suggest that, with feasible levels of device performance, the PeNCs hold great promise for their use in LED lighting and displays.

Photoinduced Effects in the ZnO Luminescence Spectra

The effect of intense UV irradiation on the photoluminescence (PL) spectra of ZnO powders and nanocrystalline films obtained by atomic layer deposition (ALD) was investigated. At room temperature, the behavior of the spectra under continuous UV irradiation in multiple vacuum-atmosphere cycles was studied. The changes in the intensities of exciton radiation and radiation in the “green” band region, associated with the phenomena of oxygen photodesorption and photoadsorption, are discussed. In the temperature range of 5–300 K, the effect of strong UV irradiation on the near-edge luminescence spectrum of ZnO films was studied. The nature of a new line arising in the photoluminescence spectra of an irradiated film in the region of emission of bound excitons is discussed.

Фотоиндуцированные эффекты в спектрах люминесценции ZnO

AbstractThe effect of intense UV irradiation on the photoluminescence (PL) spectra of ZnO powders and nanocrystalline films obtained by atomic layer deposition (ALD) was investigated. At room temperature, the behavior of the spectra under continuous UV irradiation in multiple vacuum-atmosphere cycles was studied. The changes in the intensities of exciton radiation and radiation in the “green” band region, associated with the phenomena of oxygen photodesorption and photoadsorption, are discussed. In the temperature range of 5–300 K, the effect of strong UV irradiation on the near-edge luminescence spectrum of ZnO films was studied. The nature of a new line arising in the photoluminescence spectra of an irradiated film in the region of emission of bound excitons is discussed.

Radiative coupling of A and B excitons in ZnO

Radiation-induced coupling between A and B excitons in ZnO is theoretically studied. Considering the center-of-mass motion of excitons in bulk and thin film structures, we reveal the eigenmodes of an exciton--radiation coupled system and the ratio of each excitonic component, which is determined from diagonalization of the self-consistent equation between the polarization and the Maxwell electric field. In particular, in a nano-to-bulk crossover size regime, the large interaction volume between multipole-type excitonic waves and radiation waves causes radiative coupling between excitons from different valence bands, which leads to an enhancement of the radiative correction. The results presented in this study are in striking contrast with the conventional view of the optical response of excitons in ZnO, where A and B excitons are independently assigned to their respective spectral structures. It is also clarified that the density of each excitonic component is of a significant value for attribution of nonlinear optical signals.

α-ZnSe HETEROLAYERS FOR SENSORS

The optical and luminescent properties of hexagonal α-ZnSe heterolayers, obtained by isovalent substitution for α-CdSe have been investigated. For the first time defined the parameters of the band structure – Δcr = 0,07 еV і Δso = 0,37 еV. It was found that α-ZnSe luminescence is high temperature up to 550 K, determined by the dominant radiation of excitons bounded on Cd isovalent impurity and interband recombination of free charge carriers. The high quantum efficiency η=8-10 % of heterolayers α-ZnSe luminescence allows to use them as a source of stable radiation, as well as sensors that registered the temperature changes.

Impurity–defect emission from undoped Cd1–x Zn x Te single crystals near the fundamental absorption edge

Shallow impurity–defect states in undoped Cd1–xZnxTe (x ∼ 3–6%) single crystals have been studied using low-temperature photoluminescence measurements. It has been found that the effect exerted by zinc is mainly reduced to a rigid shift of all the specific features associated with the exciton radiation, which made it possible, with a high (∼0.3 meV) accuracy, to measure the band gap and the zinc concentration in solid solutions. Hydrogen-like donors with the ground-state energy of ∼14 meV and four types of acceptors with average activation energies of 59.3 ± 0.6 meV, 69.6 ± 1.5 meV, 155.8 ± 2.0 meV, and 52.3 ± 0.6 meV have been identified in all the crystals studied. Based on a comparison with the results of the analysis of the impurity background and the data available in the literature on impurity–defect emission in undoped CdTe, the first three acceptors can be assigned to the substitutional impurities NaCd, PTe, and CuCd, respectively. The most shallow acceptor (52.3 ± 0.6 meV) is a complex defect in which there is a nonstandard excited level separated by only 7 meV from the ground level. This level is formed apparently due to the removal of degeneracy, which is characteristic of TD acceptors, by the low-symmetry potential of the complex defect.