Exciton Recombination Processes Research Articles

Grain size control in quasi-two-dimensional perovskite thin film via intermediate phase engineering for efficient bound exciton generation

AbstractQuasi-two dimensional (2D) perovskites have emerged as a promising class of materials due to their remarkable photoluminescence efficiency, which stems from their exceptionally high exciton binding energies. The spatial confinement of excitons within smaller grain sizes could enhance the formation of biexcitons leading to higher radiative recombination efficiency. However, the synthesis of high-quality quasi-2D perovskite thin films with controllable grain sizes remains a challenging task. In this study, we present a facile method for achieving quasi-2D perovskite thin films with controllable grain sizes ranging from 500 to 900 nm. This is accomplished by intermediate phase engineering during the film fabrication process. Our results demonstrate that quasi-2D perovskite films with smaller grain sizes exhibit more efficient bound exciton generation and a reduced stimulated emission threshold down to 15.89 µJ cm−2. Furthermore, femtosecond transient absorption measurements reveal that the decay time of bound excitons is shorter in quasi-2D perovskites with smaller grain sizes compared to that of those with larger grains at the same pump density, which is 230.5 ps. This observation suggests a more efficient exciton recombination process in the smaller grain size regime. Our findings would offer a promising approach for the development of efficient bound exciton lasers.

Modulating optical and electrical properties of chemically synthesized ZnMn2O4 nanoparticles through crystallinity: Integrating theoretical and experimental insights

Theoretical and experimental analyses of zinc manganite (ZnMn2O4) nanoparticles were carefully conducted for in-depth exploration of their electronic and optical properties. Nanosized particles, approximately 11–15 nm in size, were synthesized using a post-sintering assisted co-precipitation method. Theoretical calculations using Rietveld refinement were conducted to accurately predict crystal parameters and simulate XRD patterns consistent with experimental XRD observations. UV–visible absorption spectra and Tauc plot analysis indicated a bandgap ranging from 0.65 to 0.79 eV, while photoluminescence spectra revealed the most intense peak at 466 nm, possibly arising from exciton recombination processes or defect-related emissions. Density function theory (DFT) calculations were employed to compute the band structure and density of states of ZnMn2O4, resulting in a calculated bandgap of 0.73 eV, which aligned closely with the experimental findings. Moreover, DFT calculation identified atomic-level transitions responsible for absorption peaks in the material. Comprehensive evaluations of the mechanical and optical properties, integrating theoretical insights with experimental data, provided a holistic understanding of the intricate electronic and optical characteristics of ZnMn2O4. Furthermore, frequency and temperature-dependent dielectric properties demonstrated the MWS-type polarization effect. Besides, the activation energy of 0.582–0.553 eV, calculated using the Arrhenius plot, was good in accordance with our dielectric analysis, solidifying the potential of this nanomaterial for advanced optoelectronic applications.

Transient Nanoscopy of Exciton Dynamics in 2D Transition Metal Dichalcogenides.

The electronic and optical properties of 2D transition metal dichalcogenides are dominated by strong excitonic resonances. Exciton dynamics plays a critical role in the functionality and performance of many miniaturized 2D optoelectronic devices; however, the measurement of nanoscale excitonic behaviors remains challenging. Here, a near-field transient nanoscopy is reported to probe exciton dynamics beyond the diffraction limit. Exciton recombination and exciton-exciton annihilation processes in monolayer and bilayer MoS2 are studied as the proof-of-concept demonstration. Moreover, with the capability to access local sites, intriguing exciton dynamics near the monolayer-bilayer interface and at the MoS2 nano-wrinkles are resolved. Such nanoscale resolution highlights the potential of this transient nanoscopy for fundamental investigation of exciton physics and further optimization of functional devices.

Computational Characterization of Quantum‐Dot Light‐Emitting Diodes by Combinatorial Exciton Recombination Parameters and Photon Extraction Efficiency

AbstractQuantum‐dot light‐emitting diodes (QD‐LEDs) have gained significant attention for next‐generation display and lighting systems owing to their superior color selectivity and color purity. To maximize the efficiency of QD‐LED devices, it is of great importance to identify the key factors that govern their electro–optical properties. The efficiency of QD‐LED devices is strongly influenced by combinatorial processes, represented by the Shockley‐Read‐Hall rate A, Langevin strength B, and Auger probability C (ABC parameters) of quantum‐dots (QDs), along with photon extraction efficiency of QD‐LED devices. In this study, an integrated computational framework is proposed to accurately analyze the electro–optical properties of QD‐LED devices. The experimental device properties are characterized by ABC and photon extraction efficiency parameters through an innovative numerical data‐fitting procedure. Utilizing these parameters, a parametric analysis is performed based on a complete computational charge transport simulation model to explore the influence of the combinatorial exciton recombination processes. This computational framework aligns excellently with experimental results, showcasing its remarkable reliability and effectiveness in both quantitatively characterizing QD nanoparticles and in the detailed analysis of the electro–optical properties of QD‐LED devices.

Photophysical properties of nitrogen-doped carbon quantum dots synthesized by graphite

Here, nitrogen-doped carbon quantum dots (N-CQDs) were successfully synthesized by the solvothermal method using graphite as the carbon source and N,N-dimethylformamide as the nitrogen source. We characterized the structure and chemical constitution of N-CQDs using X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. We investigated the pump- and temperature-dependent photoluminescence (PL) properties and the dynamic exciton recombination processes of N-CQDs, using both steady-state and time-resolved PL techniques. The spectral results show that the PL emission peak located at 518 nm at room temperature, mainly originates from the n-π∗ transition on the surface of N-CQDs. The pump fluence and PL integral intensity were analyzed to demonstrate the existence of single-photon excitation under the 405 nm laser excitation. As the temperature increases, the non-radiative transition gradually increases, which decreases the PL intensity, the full width at half maxima first narrows and then widens and the PL lifetime gradually decreases. Furthermore, we combined the N-CQDs with chip to prepare light-emitting diode (LED). The resulting chromaticity coordinate was obtained to be (0.29, 0.40). This study offers a comprehensive understanding of the luminescence mechanism in N-doped CQDs and introduces a novel approach for the quickly fabrication of full-color display LEDs.

(Invited) Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects

Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a new class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7,5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions additional, gate-dependent emission lines appear which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6,5) nanotubes. We also compare functionalized (7,5) electroluminescence data with photoluminescence of pristine and functionalized (7,5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.Reference: Min-Ken Li et al., ACS Nano 2022 (doi: 10.1021/acsnano.2c03083) Figure 1

Theoretical study on organic photovoltaic heterojunction FTAZ/IDCIC

Understanding organic photovoltaic (OPV) work principles and the materials’ optoelectronic properties is fundamental for developing novel heterojunction materials with the aim of improving power conversion efficiency (PCE) of organic solar cells. Here, in order to understand the PCE performance (>13%) of OPV device composed of the non-fullerene acceptor fusing naphtho[1,2-b:5,6-b′]dithiophene with two thieno[3,2-b]thiophene (IDCIC) and the polymer donor fluorobenzotriazole (FTAZ), with the aid of extensive quantum chemistry calculations, we investigated the geometries, molecular orbitals, excitations, electrostatic potentials, transferred charges and charge transfer distances of FTAZ, IDCIC and their complexes with face-on configurations, which was constructed as heterojunction interface model. The results indicate that, the prominent OPV performance of FTAZ:IDCIC heterojunction is caused by co-planarity between the donor and acceptor fragments in IDCIC, the the charge transfer (CT) and hybrid excitations of FTAZ and IDCIC, the complementary optical absorptions in visible region, and the large electrostatic potential difference between FTAZ and IDCIC. The electronic structures and excitations of FTAZ/IDCIC complexes suggest that exciton dissociation can fulfill through the decay of local excitation exciton in acceptor by means of hole transfer, which is quite different from the OPVs based on fullerenes acceptor. The rates of exciton dissociation, charge recombination and CT processes, which were evaluated by Marcus theory, support the efficient exciton dissociation that is also responsible for good photovoltaic performance.

Controlling Kinetic Quenching Depth Toward High‐Performance and Photo‐Stable Organic Solar Cells Printed from a Non‐Halogenated Solvent

AbstractThe investigation of drying dynamics and kinetic quenching depth related degradation of high‐performance photoactive materials with scalable coating techniques demands significant research attention. Herein, film formation kinetics regulated crystallinity, preferential orientation and vertical phase separation of the active layers is revealed, which then further affects exciton diffusion, dissociation, charge‐transport and recombination processes. By suppressing over aggregation/crystallization in slow drying process and overcoming quenching of disordered liquid phase in fast‐dried films, the optimized PM6:BTP‐eC9‐based organic solar cells obtain power conversion efficiencies up to 16.81%. In addition, photoluminescence lifetime distribution is found to be an alternative probe for kinetic quenching depth that governs the degradation rate. This study provides valuable insights into the control of thin film formation kinetics during scalable processing and develops an effective way to associate the kinetic quenching depth with morphological degradation for mechanistic understanding of long‐term stability.

Highly Thermally Sensitive Cascaded Wannier-Mott Exciton Ionization/Carrier Localization in Manganese-Doped Perovskite Nanocrystals.

Transition-metal doping in perovskite nanocrystals strongly alters the photophysical properties of these nanocrystals. However, the details of the underlying thermal and optical processes within such an intriguing symmetry-breaking nanosystem are far from clear. Herein, we study the sensitively temperature-dependent and highly competent delocalized exciton and transition-metal ion-captured carrier recombination processes in manganese-doped CsPbBr0.6Cl2.4 nanocrystals. The combined experimental and theoretical studies reveal that both the exciton ionization and capture of the band-edge carriers by the manganese ions play the dominant roles in determining the proportion of the manganese ions-dominated recombination process. A density functional theory calculation of the temporal fluctuation of the manganese ions-accommodated localized orbitals further confirms that the thermally enhanced nonadiabatic electron-phonon coupling promotes the probability of the carrier localization. These findings reveal the respective crucial roles of the exciton ionization and carrier capture in the localized recombination process in the transition-metal-doped semiconductor nanocrystals.

Impact of the organic cation on the band-edge emission of two-dimensional lead-bromide perovskites.

Organic-inorganic low-dimensional layered metal-halide perovskites are semiconductors in which the optoelectronic properties can be tuned by the material composition and the design of the layered architecture. While the electronic band structure is mainly determined by the inorganic octahedra lattice, the binding and conformation of the organic cations induces related lattice distortions that can break the symmetry and lead to the splitting of the exciton energy levels, and influence the dielectric confinement. Furthermore, organic-induced lattice deformations lead to offsets in k-space (where k is the wavevector) that go along with the exciton energy level splitting. Hence, the electronic transitions between these levels require the momentum contribution of phonons, and contributions of phonons in the exciton recombination dynamics result in thermal broadening of the emission linewidth. In this work, we investigate the band-edge emission of two-dimensional Ruddlesden-Popper lead-bromide perovskites synthesized with different organic cations that vary in their binding head group and their alkyl chain length. We find several peaks in the low-temperature photoluminescence spectra, and the number of peaks in the band-edge emission and their decay dynamics depend strongly on the type of organic cation in the material, which we relate to the difference in the inorganic lattice distortions that the cations induce. For two-dimensional layered perovskites with mainly in-plane distortions, induced by short primary ammonium molecules, we find a two-fold splitting of the band edge emission at low temperatures. If also out-of-plane distortions are present, as for the long-chain primary ammoniums, a three-fold splitting is observed. Interestingly, the low-energy peaks of the split series merge into the highest energy peak with increasing temperature. Thermal broadening analysis of the temperature-dependent photoluminescence linewidth in the structures with out-of-plane distortions yields energies that are larger than those reported for the inorganic lattice phonons. This indicates the involvement of either high-frequency oscillations involving the organic cations, or the broadening might be related to higher order phonon scattering processes in the excitonic recombination process. The strong directionality of the phonon modes in the octahedral lattice could promote the involvement of multiple electron-phonon scattering processes in the exciton relaxation dynamics, for example involving modes with orthogonal directionality.

Plasmon-exciton couplings in the MoS2/AuNP plasmonic hybrid structure

The understanding and engineering of the plasmon-exciton coupling are necessary to control the innovative optoelectronic device platform. In this study, we investigated the intertwined mechanism of each plasmon-exciton couplings in monolayer molybdenum disulfide (MoS2) and plasmonic hybrid structure. The results of absorption, simulation, electrostatics, and emission spectra show that interaction between photoexcited carrier and exciton modes are successfully coupled by energy transfer and exciton recombination processes. Especially, neutral exciton, trion, and biexciton can be selectively enhanced by designing the plasmonic hybrid platform. All of these results imply that there is another degree of freedom to control the individual enhancement of each exciton mode in the development of nano optoelectronic devices.

Study on charge dynamics and recombination process via transient electroluminescence analysis in blue fluorescence organic light-emitting diodes

Fluorescent organic light-emitting diodes (FOLEDs) exhibit substantially different efficiency-lifetime characteristics depending on dopant materials even in the same host material. Thus, obviously charge and exciton dynamics and recombination process may vary depending on the dopants. However, an accurate understanding of charge and exciton dynamics with dopants in FOLEDS is lacking so far. Herin, triplet–triplet-fusion-based FOLEDs with two blue dopants, 2,5,8,11-tetra-tert-butylperylene (TBPe) and 2,12-di-tert-butyl-5,9-bis(4-(tert-butyl)phenyl)-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de] anthracene (t-DABNA) were investigated via transient electroluminescence (TrEL). Interestingly, the TrEL of the blue FOLEDs elucidated different charge profiles and recombination processes within the emission layers. The TrEL spikes after pulse-off and the characteristic times of EL intensity after pulse-on revealed the accumulated charge profile concerning the charge balance. In temperature-dependent TrEL, the delayed fluorescence, exciton decay time, and EL spikes after pulse-on reflected the emission mechanisms as well. The results showed that the electrical properties and the device performances of blue FOLEDs can be significantly impacted by the alignment of the energy levels of hosts and dopants, even with a low doping concentration. Furthermore, depending on the charge trap conditions, it was found that efficiency roll-off can be attributed to Dexter energy transfer from host to dopant and singlet–triplet annihilation by direct recombination on dopant molecules.

Conversion of the non-luminous lead-free inorganic halide perovskite variant CsNiCl3 nanocrystals into photoluminescent materials by Cu+ and In3+ doping

All-inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) as promising optoelectronic materials have attracted tremendous research interest because of their intense photoluminescence, broad absorption, and tunable bandgap. Even with these fascinating characteristics, their practical applications are still suffering from the presence of toxic lead and inadequate stability. Herein, we successfully synthesize CsNiCl3 NCs in a comparatively low-temperature route for the first time. By Cu+ and In3+ doping, CsNiCl3 NCs can be converted from non-luminous into photoluminescent materials. The broad green fluorescence emission at 530 nm of CsNiCl3 NCs with high luminescence intensity and quantum yield (42.2%) is achieved. Broad band emission with a large Stokes shift can be attributed to the recombination process of self-trapped excitons (STEs). The transient fluorescence spectra of the doped CsNiCl3 NCs show two ultra-long lifetimes of about 50 and 120 µs, which are assigned to the STE recombination, indicating that the nonradiative recombination process is suppressed after doping. Our research provides an innovative method for developing environmentally friendly lead-free metal halide NCs.

Ultrafast Exciton and Trion Dynamics in High‐Quality Encapsulated MoS2Monolayers

The extreme confinement and reduced screening in monolayer transition metal dichalcogenides (TMDs) leads to the appearance of tightly bound excitons which can also couple to free charges, forming trions, owing to strong Coulomb interactions. Low temperatures and encapsulation in hexagonal boron nitride (hBN) can narrow the excitonic linewidth, approaching the regime of homogeneous broadening, mostly dominated by the radiative decay. Ultrafast spectroscopy is a perfect tool to study exciton formation and relaxation dynamics in TMD monolayers. However, high‐quality hBN‐encapsulated structures have usually lateral sizes of the order of a few micrometers, calling for the combination of high spatial and temporal resolution in pump–probe experiments. Herein, a custom broadband pump–probe optical microscope is used to measure the ultrafast dynamics of neutral and charged excitons in high‐quality hBN‐encapsulated monolayer MoS2at 8 K. Neutral excitons exhibit a narrow linewidth of 7.5 meV, approaching the homogeneous limit, which is related to the fast recombination time of ≈130 fs measured in pump–probe. Moreover, markedly different dynamics of the trions over the neutral ones are observed. The results provide novel insights on the exciton recombination processes in TMD monolayers, paving the way for exploring the ultrafast behavior of excitons and their many‐body complexes in TMD heterostructures.

Optical characteristics of bilayer decoupling MoS2 grown by the CVD method.

Study of exciton recombination process is of great significance for the optoelectronic device applications of two-dimensional transition metal chalcogenides (TMDCs). This research investigated the decoupling MoS2 structures by photoluminescence (PL) measurements. First, PL intensity of the bilayer MoS2 (BLM) is about twice of that of the single layer MoS2 (SLM) at low temperature, indicating no transition from direct bandgap to indirect bandgap for BLM due to the decrease of interlayer coupling which can be shown by Raman spectra. Then, the localized exciton emission appears for SLM at 7 K but none for BLM, showing different exciton localization characteristics. The PL evolution with respect to the excitation intensity and the temperature further reveal the filling, interaction, and the redistribution among free exciton states and localized exciton states. These results provide very useful information for understanding the localized states and carrier dynamics in BLM and SLM.

Transient Dynamics of Charges and Excitons in Quantum Dot Light-Emitting Diodes.

Wide interest in quantum dot (QD) light-emitting diodes (QLEDs) for potential application to display devices and light sources has led to their rapid advancement in device performance. Despite such progress, detailed operation mechanisms of QLEDs, which are necessary for the fundamental understanding and further improvements, have been still uncertain because of the intricate interaction between charges and excitons in electrical operation. In this work, the transient electroluminescence (TREL) signals of dichromatic QLEDs which are purposely designed to consist of two different color-emitting QD layers are analyzed. As a result, not only can the charge injection and exciton recombination processes be visualized but the electron mobility of the QD layer can also be estimated. Furthermore, the effects of Förster resonant energy transfer between two QDs and exciton quenching near the QD layer are quantitatively measured in QLED operation. The authors believe that their results based on TREL analyses will contribute to the understanding and development of high-performance QLEDs.

Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects.

Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions, additional gate-dependent emission lines appear, which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6, 5) nanotubes. We also compare functionalized (7, 5) electroluminescence data with photoluminescence of pristine and functionalized (7, 5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.

Fabrication of a near-infrared fluorescence-emitting SiO2-AuZnFeSeS quantum dots-molecularly imprinted polymer nanocomposite for the ultrasensitive fluorescence detection of levamisole

Levamisole is an anthelminthic drug that is usually used as a cutting agent to lace illicit cocaine samples and has been classified as a dangerous substance. In this work, we report on the fabrication of a novel fluorescence sensor for levamisole using AuZnFeSeS alloyed quantum dots (QDs)-molecularly imprinted polymer (MIP) nanocomposite. Firstly, organic-phased AuZnFeSeS QDs were synthesized via the organometallic hot-injection pyrolysis of metal precursors, surfactants, and organic ligands. Silica (SiO2) was then encapsulated on the QDs surface to render it compact, stable, and biocompatible. Via a free radical polymerization approach, a robust MIP shell templated with levamisole was coated around the SiO2-AuZnFeSeS QDs surface to form a AuZnFeSeS QDs@MIP core/shell nanocomposite. Passivation of the SiO2-AuZnFeSeS QDs core surface with the MIP shell led to radiative exciton recombination process, whereby the QDs fluorescence was enhanced. Interestingly, the SiO2-AuZnFeSeS QDs was characterized by rod-shaped and spherical-shaped particle morphology. Under optimum conditions, the fluorescence of the AuZnFeSeS QDs@MIP nanocomposite was quenched in the presence of levamisole and was selective against other tested drugs. Quantitative detection was achieved in the concentration range of 0.5 – 100 μM with a limit of detection of 0.05 μM (47.4 nM). Validation of the MIP-levamisole affinity was demonstrated by the superior sensitivity of the AuZnFeSeS QDs@MIP over the AuZnFeSeS QDs@NIP (non-templated material). Levamisole was successfully detected in a mixed drug sample containing cocaine with satisfactory analytical recoveries. This work is the first reported use of a cadmium-free quintenary alloyed QDs@MIP nanocomposite fluorescence detection system for levamisole.

High resolution and time resolved photoemission spectroscopy for developing more efficient materials to reduce energy consumption and increase renewable energy production

Due to the increase of energy consumption and the resulting ecological challenge, a collective awareness leads to the development of renewable energies and more efficient materials to increase the green energy production. Development of efficient photovoltaic materials is very closely related to their chemical and electronic properties. A better knowledge of these imbricated properties is needed, in addition to a better comprehension of their interplay with charge transport mechanisms. Exciton creation and recombination processes, charge transfer and charge collection processes take place at the surface and interface of the photoactive materials. Photoemission spectroscopy as chemical specific and surface sensitive spectroscopic technique is a method of choice on the study of physical phenomena at the origin of photoconversion efficiency. Time resolved photoemission spectroscopy has been recently renewed interest covering time scale from fs to more than seconds. It permits to probe the dynamics of relaxation of photoexcited charges and determine their lifetime. It finds application in various materials used in solar photovoltaics. In this paper, we define the physical and chemical properties determined by the combination of high resolution and time resolved photoemission spectroscopy. We show examples dealing with the development of renewable energy and energy consumption reduction in agreement with the current ecological trend for a better future.

Extremely high internal quantum efficiency of AlGaN-based quantum wells on face-to-face annealed sputter-deposited AlN templates

This study investigated the influence of high-quality AlN templates on the internal quantum efficiency (IQE) of AlGaN-based multiple quantum wells (MQWs) using photoluminescence spectroscopy. An extremely high IQE of 90% at room temperature was obtained from MQWs on face-to-face annealed sputter-deposited AlN/sapphire templates. The dependence of efficiency curves on temperature indicated that nonradiative recombination centers were almost fully saturated under higher excitation power densities even at 400 K. Moreover, analysis of the efficiency curves using a rate equation model based on exciton recombination processes suggested that radiative recombination was the dominant recombination process even at 400 K.