Exciton Recombination Research Articles

Pressure-induced abnormal blue-shift emission enhancement and phase transition of perovskite DMAPbI3

Designing low-dimensional hybrid organic–inorganic perovskites (HOIPs) with enhanced broadband emission under ambient conditions remains a pressing challenge. Here, interesting results of emission blueshift and enhancement were successfully achieved by pressure modulation of the (NH2(CH3)2PbI3 ((NH2(CH3)2 = DMA) structure. The observed rapid blueshifted emission is closely related to the distortion of PbI6 octahedron induced by phase transition from the P63/mc phase to the P1 phase at 0.8 GPa, which has been confirmed by in situ high-pressure PL, UV–vis absorption, synchrotron XRD and Raman spectroscopy. The pressure-induced PL enhancement of DMAPbI3 can be ascribed to the mechanism that the distorted PbI6 octahedrons promote the radiative recombination of self-trapped excitons by increasing the activation barrier energy for detrapping. The PL intensity enhancement was retained at 4 times the initial value after the pressure was released. These findings deepen the understanding of the structure-property relationships of one-dimensional perovskite and reveal the roles of pressure in regulating the optical properties of perovskites.

Enhancing Blue Polymer Light-Emitting Diode Performance by Optimizing the Layer Thickness and the Insertion of a Hole-Transporting Layer.

Polymer light-emitting diodes (PLEDs) hold immense promise for energy-efficient lighting and full-color display technologies. In particular, blue PLEDs play a pivotal role in achieving color balance and reducing energy consumption. The optimization of layer thickness in these devices is critical for enhancing their efficiency. PLED layer thickness control impacts exciton recombination probability, charge transport efficiency, and optical resonance, influencing light emission properties. However, experimental variations in layer thickness are complex and costly. This study employed simulations to explore the impact of layer thickness variations on the optical and electrical properties of blue light-emitting diodes. Comparing the simulation results with experimental data achieves valuable insights for optimizing the device's performance. Our findings revealed that controlling the insertion of a layer that works as a hole-transporting and electron-blocking layer (EBL) could greatly enhance the performance of PLEDs. In addition, changing the active layer thickness could optimize device performance. The obtained results in this work contribute to the development of advanced PLED technology and organic light-emitting diodes (OLEDs).

Cathodoluminescence from interlayer excitons in a 2D semiconductor heterobilayer.

Photoluminescence has widely been used to study excitons in semiconducting transition metal dichalcogenide (MX$_2$) monolayers, demonstrating strong light matter interactions and locked spin and valley degrees of freedom. In heterobilayers composed of overlapping monolayers of two different MX$_2$, an interlayer exciton can form, with the hole localised in one layer and the electron in the other. These interlayer excitons are long-lived, field-tunable, and can be trapped by moir'e patterns formed at small twist angles between the layers. Here we demonstrate that emission from radiative recombination of interlayer excitons can be observed by cathodoluminescence from a WSe$_2$/MoSe$_2$ heterobilayer encapsulated in hexagonal boron nitride. The higher spatial resolution of cathodoluminescence, compared to photoluminescence, allows detailed analysis of sample heterogeneity at the 100s of nm lengthscales over which twist angles tend to vary in dry-transfer fabricated heterostructures.

Emission mechanisms in low-threshold UV random laser based on ZnO microrod array

Despite rather extensive study of the random lasing effect in ZnO structures, the issue of the optical gain mechanisms in microstructured ZnO random lasers remains poorly understood. In this work, the radiative properties of an array of vertically aligned ZnO microrods, synthesized by a modified thermal evaporation method, were studied. The microrods exhibited lengths up to 60 μm and diameters ranging from 1 to 5 μm. Random lasing from a microrod array was observed in the near-UV range (with a laser emission peak wavelength of ∼391 nm) with a threshold down to 40 kW/cm2 under optical excitation. An analysis of the nature of optical gain in the grown structure was conducted at various temperatures. It was found that at room temperature, two-phonon-assisted exciton recombination is the main process leading to light amplification.

Synergistically enhanced photocatalytic properties of Co3O4-G/GO nanocomposites: unravelling their interactions and charge-transfer dynamics using XAS.

Metal oxide composites with graphene/graphene oxide have increasingly gained popularity in enhancing the photocatalytic degradation of several existing harmful dyes. Moreover, identifying the role of carbon networks and their interactions in composite formation would assist in the design and development of photocatalysts. In the present study, we investigated the role of carbon networks in improving photocatalytic properties. Electronic structure analysis of cobalt oxide-graphene (C2)/graphene oxide (C3) nanocomposites using XAS suggested possible charge transfer from cobalt oxide nanoparticles to the carbon network during composite formation. The photocatalytic degradation of C3 towards phenol dye (1×10-3M) was >50% and improved the degradation rate with k = 0.231 h-1.In the quest to understand the mechanism unfolding on its surface, in situ XAS under UV-visible irradiation was performed, which shed light on delayed excitonic recombination in the synthesized nanocomposites. This enabled hydroxy radicals (˙OH) to play a preeminent role in the cleavage of the phenol ring and its intermediaries. Based on these observations, a detailed mechanism for charge transfer occurring during nanocomposite formation and the mechanism involved in the enhanced photocatalytic activity of the nanocomposite photocatalyst towards phenol degradation under the influence of UV-visible irradiation are discussed.

Small Polaron Dynamics in Transition Metal Oxide Photoelectrodes

The transition metal oxides are widely used in many optoelectronic applications, such as working as charge-transporting layers in solar cells and photoelectrodes in light-driven water-splitting. Considering the ionic bonding nature, the lattice of these oxides can be polarized by the charge carriers due to the strong electron-phonon coupling, giving rise to lattice distortion that fosters a local potential well. This lattice distortion together with the trapped carrier is regarded as a quasi-particle, i.e., a polaron. Here, the photocarrier dynamics in transition metal oxides were investigated using the transient absorption and time-resolved terahertz spectroscopies. We identified the spectral features of small polarons, self-trapped excitons, and free carriers and extracted their dynamics independently. We proposed a molecular model that could successfully explain the non-barrier self-trapping and quantitatively describe the temperature-dependent recombination of self-trapped excitons and the dopant effect on the photocarrier dynamics. Our model also suggested that the recombination of self-trapped excitons at room temperature was dominated by nuclear quantum tunneling. This model also predicted the micro-heterogeneity of the self-trapped species, and thus be able to quantitatively describe the Auger annihilation dynamics of self-trapped excitons.

(Invited) Photoluminescence Dynamics in PbS/CdS Core/Shell Nanocrystal Films: Carrier Recombination and Energy Transfer

Colloidal semiconductor nanocrystals (NCs) exhibit highly controllable size-dependent optical properties due to the quantum confinement effect. Among other semiconductor NCs, the lead chalcogenides, especially PbS, are one of the most studied and promising NC systems. They are the material of choice for studies of fundamental phenomena as well as for the development of solution processed infrared optoelectronic devices including LEDs, lasers, solar cells, and photodetectors. Compared to organically passivated NCs, the core-shell approach (more specifically PbS/CdS core-shell NCs) offers not only the improved stability and higher photoluminescence (PL) quantum efficiency but also an opportunity of the core/shell interface nano-engineering resulting in suppression of Auger recombination, reduction of the exciton radiative lifetime and an improved lasing threshold. However, despite more than a decade of detailed studies there are conflicting reports on PL dynamics in PbS/CdS core-shell NCs, and there is no accepted model explaining the observed PL lifetimes ranging from hundred nanoseconds to tens of microseconds. In this work we use time-resolved PL measurements and demonstrate that low-temperature PL dynamics in PbS/CdS core shell NCs show strong dependence on the PL detection wavelengths. The presented experimental results allow us to distinguish between processes of energy transfer and exciton recombination. We propose a model, which considers energy transfer between NCs and various mechanisms of exciton recombination (non-radiative, radiative, spin-flip, diffusion limited, etc.).

Influence of Substrate-Induced Charge Doping on Defect-Related Excitonic Emission in Monolayer MoS2.

Many applications of transition metal dichalcogenides (TMDs) involve transfer to functional substrates that can strongly impact their optical and electronic properties. We investigate the impact that substrate interactions have on free carrier densities and defect-related excitonic (XD) emission from MoS2 monolayers grown by metal-organic chemical vapor deposition. C-plane sapphire substrates mimic common hydroxyl-terminated substrates. We demonstrate that transferring MoS2 monolayers to pristine c-plane sapphire dramatically increases the free electron density within MoS2 layers, quenches XD emission, and accelerates exciton recombination at the optical band edge. In contrast, transferring MoS2 monolayers onto inert hexagonal boron nitride (h-BN) has no measurable influence on these properties. Our findings demonstrate the promise of utilizing substrate engineering to control charge doping interactions and to quench broad XD background emission features that can influence the purity of single photon emitters in TMDs being developed for quantum photonic applications.

Competition among recombination pathways in single FAPbBr3 nanocrystals.

Single particle level microscopy of immobilized FAPbBr3 nanocrystals (NCs) has elucidated the involvement of different processes in their photoluminescence (PL) intermittency. Four different blinking patterns are observed in the data from more than 100 NCs. The dependence of PL decays on PL intensities brought out in fluorescence lifetime intensity distribution (FLID) plots is rationalized by the interplay of exciton- and trion-mediated recombinations along with hot carrier (HC) trapping. The high intensity-long lifetime component is attributed to neutral exciton recombination, the low intensity-short lifetime component is attributed to trion assisted recombination, and the low intensity-long lifetime component is attributed to hot carrier recombination. Change-point analysis (CPA) of the PL blinking data reveals the involvement of multiple intermediate states. Truncated power law distribution is found to be more appropriate than power law and lognormal distribution for on and off events. Probability distributions of PL trajectories of single NCs are obtained for two different excitation fluences and wavelengths (λex = 400, 440nm). Trapping rate (kT) prevails at higher power densities for both excitation wavelengths. From a careful analysis of the FLID and probability distributions, it is concluded that there is competition between the HC and trion assisted blinking pathways and that the contribution of these mechanisms varies with excitation wavelength as well as fluence.

Composition‐Graded Perovskite Microwire Toward Broad Wavelength Tunable Single‐Mode Lasing

AbstractContinuously manipulating the resonant wavelength of lasing modes within a large spectral range is of great significance for expanding device functionality. Here, to synthesize a single CsPbClxBr3‐x perovskite microwire with the energy bandgap gradient spanning from 2.33 to 2.83 eV along the length direction defined as the axis by using the vapor‐phase anion exchange method is proposed. A high‐quality (≈103), wide range (≈60 nm), and continuously tunable single‐mode laser is achieved in as‐prepared perovskite microwire alloy, which provides both gain media and microresonator. Simultaneously, the exciton recombination dynamics and atomic‐scale interdiffusion mechanisms at different components are clarified through time‐resolved photoluminescence (PL) spectra and theoretical calculations. The vacancy defects have a significant impact on the interdiffusion of halogen anions, excitonic recombination lifetime, and fluorescence quantum efficiency. The work provides a new strategy for the construction of new‐type broadband tunable lasers and high‐precision microspectrometers.

Improving the operating performance of organic solar cells through a novel cathode interface layer

In the classical system of organic PSCs (PTB7-Th: PC71BM), the presence of a cathode interface layer has the function of improving the device performance by lowering the interfacial barrier between the active layer and the electrode, increasing the charge selectivity, regulating the morphology of the active layer, and regulating the absorption of sunlight by the active layer. Therefore, a novel alcohol-soluble electron transport material is synthesized in this paper, which can cause a more prominent self-doping effect by enhancing intramolecular conjugation, and the results show that in terms of exciton dissociation and recombination, it is found that bimolecular recombination and trap-assisted recombination are obviously inhibited, so that the battery using this interface obtains more than 69% of the fill factor (FF) and has a superior energy conversion efficiency (PCE).

Weakly Confined Semiconductor Nanocrystals Excel in Photochemical and Optoelectronic Properties: Evidence from Single-Dot Studies.

Single-molecule spectroscopy offers state-resolved measurements on charge-transfer reactions of single semiconductor nanocrystals, leading to the discovery of up to six single-charge transfer reactions with seven transient states for single CdSe/CdS core/shell nanocrystals with water (or oxygen) as the hole (or electron) acceptors. Kinetic rates of three photoinduced single-hole transfer reactions decrease significantly upon increasing the number of excess electrons in a nanocrystal, mainly due to efficient Auger nonradiative recombination of the charged single excitons. Conversely, the kinetic rates of three single-electron transfer reactions of an unexcited nanocrystal increase proportionally to the number of excess electrons in it. Results here reveal that charge-transfer reactions of nanocrystals, at the center of nearly all their functions, could only be deciphered at a state-resolved level on a single nanocrystal. Size-dependent studies validate the weakly confined semiconductor nanocrystals, instead of strongly confined ones (quantum dots), as optimal candidates for photochemical and optoelectronic applications.

Evaluation of the effect of oxygen addition on charge carrier transport properties in single-crystal diamond growth

The effects of oxygen addition during single-crystal diamond growth by microwave plasma CVD method on crystal surface morphology and charge carrier transport properties were evaluated. Diamond single-crystals were homoepitaxially grown on (001) surface of high-pressure and high-temperature (HPHT) synthesized type IIa single-crystal diamond substrates with a fixed methane concentration of 1 % and oxygen concentrations of 0–0.8 %. Inverted pyramid-shaped pits were generated as the oxygen concentration increased. The free exciton recombination emission intensity in the cathodoluminescence spectrum reached a maximum at oxygen concentration of 0.5 %. The samples with oxygen concentrations of 0.5 % and 0.8 % showed excellent charge collection efficiency and energy resolution. On the other hand, the mobility-lifetime (μτ) product was only (7.1 ± 1.7) × 10−5 cm2/V, which is approximately 1/4 of that of the sample with 0.2 % methane and no oxygen addition.

Interlinking the Fe doping concentration, optoelectronic properties, and photocatalytic performance of ZnO nanostructures

Doping is one of the effective strategies to modulate the optoelectronic properties and photocatalytic activity of photocatalysts. In this study, the effect of Fe doping (0–10 %) on morphology, optical and electronic properties, and photocatalytic activity of ZnO nanostructures is studied. The X-ray diffraction analysis shows that >5 % Fe doping, ZnFe2O4 is segregated as a secondary phase. The crystalline size decreases from 50.8 nm to 21.4 nm and the micro-strain increases with increasing the Fe concentration. The Fe doping-induced electronic restructuring facilitates visible light absorption through the O 2p → Fe 3d transition and the suppression of charge recombination by efficiently trapping conduction band electrons. Density functional theory (DFT) calculations are employed to unravel the underlying electronic changes induced by Fe doping in ZnO. The formation of shallow donor levels below the conduction band originates from the Fe 3d state. Photoluminescence spectra of pristine and Fe-doped ZnO nanostructures show characteristic emission peaks at approximately 384 nm and 570 nm, indicating the recombination of free excitons and oxygen interstitial defects, respectively. The results of the photocatalytic activity tests confirm that the 1 % Fe-doped ZnO nanostructures can exhibit the highest efficiency compared to the heavily doped ZnO nanostructures. The high efficiency in photocatalytic activity of 1 % Fe-doped ZnO nanostructures is ascribed to the modulated electronic structure and defect density. The adsorption affinity of methylene blue and water molecules to the surfaces of pristine and Fe-doped ZnO is simulated using the Monte-Carlo method. This study emphasizes the importance of controlling the dopant concentration to enhance the photocatalytic activity of various photocatalysts.

Light-induced Kondo-like exciton-spin interaction in neodymium(II) doped hybrid perovskite

Tuning the properties of a pair of entangled electron and hole in a light-induced exciton is a fundamentally intriguing inquiry for quantum science. Here, using semiconducting hybrid perovskite as an exploratory platform, we discover that Nd2+-doped CH3NH3PbI3 (MAPbI3) perovskite exhibits a Kondo-like exciton-spin interaction under cryogenic and photoexcitation conditions. The feedback to such interaction between excitons in perovskite and the localized spins in Nd2+ is observed as notably prolonged carrier lifetimes measured by time-resolved photoluminescence, ~10 times to that of pristine MAPbI3 without Nd2+ dopant. From a mechanistic standpoint, such extended charge separation states are the consequence of the trap state enabled by the antiferromagnetic exchange interaction between the light-induced exciton and the localized 4 f spins of the Nd2+ in the proximity. Importantly, this Kondo-like exciton-spin interaction can be modulated by either increasing Nd2+ doping concentration that enhances the coupling between the exciton and Nd2+ 4 f spins as evidenced by elongated carrier lifetime, or by using an external magnetic field that can nullify the spin-dependent exchange interaction therein due to the unified orientations of Nd2+ spin angular momentum, thereby leading to exciton recombination at the dynamics comparable to pristine MAPbI3.

Spectator Exciton Effects in Nanocrystals III: Unveiling the Stimulated Emission Cross Section in Quantum Confined CsPbBr3 Nanocrystals.

Quantifying stimulated emission in semiconductor nanocrystals (NCs) remains challenging due to masking of its effects on pump-probe spectra by excited state absorption and ground state bleaching signals. The absence of this defining photophysical parameter in turn impedes assignment of band edge electronic structure in many of these important fluorophores. Here we employ a generally applicable 3-pulse ultrafast spectroscopic method coined the "Spectator Exciton" (SX) approach to measure stimulated-emission efficiency in quantum confined inorganic perovskite CsPbBr3 NCs, the band edge electronic structure of which is the subject of lively ongoing debate. Our results show that in 5-6 nm CsPbBr3 NCs, a single exciton bleaches more than half of the intense band edge absorption band, while the cross section for stimulated emission from the same state is nearly 6 times weaker. Discussion of these findings in light of several recent electronic structure models for this material proves them unable to simultaneously explain both measures, proving the importance of this new input to resolving this debate. Along with femtosecond time-resolved photoluminescence measurements on the same sample, SX results also verify that biexciton interaction energy is intensely attractive with a magnitude of ∼80 meV. In light of this observation, our previous suggestion that biexciton interaction is repulsive is reassigned to hot phonon induced slowdown of carrier relaxation leading to direct Auger recombination from an excited state. The mechanism behind the extreme slowing of carrier cooling after several stages of exciton recombination remains to be determined.

Photothermal-boosted S-scheme heterojunction of α-Fe2O3@NiOx for high-selective reduction of CO2 to CO

Self-heating photothermal catalysis, combining light promotion and thermal activation, offers unique advantages for CO2 conversion. The meticulous design of active sites and light-absorbing materials is crucial for high-performance photothermal catalysts. Herein, a S-Scheme heterostructure composed of NiOx subunits with abundant oxygen vacancies (Vo) bonded with α-Fe2O3 nanoplatelets has been fabricated for solar-driven photothermal catalytic CO2 reduction. Under continuous full-spectrum light irradiation (0.3 W·cm−2), the α-Fe2O3@NiOx hybrids exhibited a CO yield rate of 199.3 μmol∙g−1·h−1 in a gas–solid reaction models with near-unity selectivity. Additionally, the apparent quantum yield for CO production at 420 nm reaches 1.56 %. The prominent activity is attributed to three key factors: (1) The Vo-rich NiOx facilitates the accumulation of photogenerated electrons and activation of chemisorbed CO2. (2) The implemented S-Scheme heterostructures boost interfacial charge-transfer and provide spatially separated catalytic sites for efficient overall CO2 reduction. (3) The substantial photothermal effect of NiOx, which arises from nonradiative exciton recombination, induces local heating of the catalyst, thereby enhancing reaction kinetics. This work provides valuable insights into the utilization of a photothermal-photocatalytic S-Scheme heterojunction system for the photothermal catalytic CO2 reduction.

Twist-angle-dependent optical behaviors of excitons in twisted bilayer MoS2 at low temperature

Twisted bilayers of two-dimensional materials provide a powerful and attractive platform to control band structure and tailor optical properties since the twist angle θ between the bilayer has been shown to be a critical parameter in engineering the moiré potential. Here, we demonstrate the θ-driven effects on excitons in twisted bilayer MoS2 (tBLMs) by measuring photoluminescence (PL) spectra. The localized state exciton between 1.70 eV and 1.80 eV is observed at 7 K with properties periodically modulated by θ. The θ is also a crucial factor affecting direct excitons. Furthermore, the phonon around 410 cm−1 has a θ dependence similar to excitons by measuring Raman spectra. The localized stacking patterns, the recombination of excitons, and the phonon-exciton interactions are confirmed in tBLMs. Our work provides the help to investigate the new quantum phenomena of tBLMs by optical methods and provides possibilities to manipulate the emission of excitons in tBLMs.

Enhanced visible light harvesting in dye-sensitized solar cells through incorporation of solution-processable silver plasmons and anthracite-derived graphene quantum dots

The major setback for the enhanced performance of DSSC is the narrow absorption window and the interfacial exciton recombination. Therefore, in this work, the photovoltaic performance of dye-sensitized solar cells has been improved by the synergistic effect of anthracite-derived graphene quantum dots and silver plasmons. GQD and Ag coupled photoanodes were fabricated by a facile solution processable process under room temperature. The as-fabricated DSSC TiO2/Ag/GQD (TAG) exhibited an enhanced power conversion efficiency of 10.5 % with a current density of 22.40 mAcm−2 measured under solar irradiation of 100 mWcm−2 with AM 1.5G. An enhancement surpassing 30.5 % was obtained for the champion cell when compared to the pristine TiO2 based DSSC. Furthermore, this study emphasizes developing a cutting-edge approach for the high-quality use of fossil fuel-derived graphene quantum dots in energy conversion systems, thereby encouraging the green conversion of fossil fuels and broadening the potential of anthracite coal's utilization in energy conversion applications.

Ligand compensation enabling efficient and stable exciton recombination in perovskite QDs for high-performance QLEDs

Ligand compensation enabling efficient and stable exciton recombination in perovskite QDs for high-performance QLEDs