Behavior Of Excitons Research Articles

Spectroscopic Ellipsometry Study of the Temperature Dependences of the Optical and Exciton Properties of MoS2 and WS2 Monolayers

The optical properties of MoS2 and WS2 monolayers are significantly influenced by fabrication methods, especially with respect to the behavior of excitons at the K−point of the Brillouin zone. Using spectroscopic ellipsometry, we obtain the complex dielectric functions of monolayers of these materials from cryogenic to room temperatures over the energy range 1.5 to 6.0 eV. The excitonic structure of each sample is analyzed meticulously by fitting the data to a standard analytical function to extract the energy positions of the excitons at each temperature. At low temperatures, excitonic structures are blue-shifted and sharpened due to the reduction in phonon noise and lattice distance. The excitons of monolayers fabricated by MOCVD separate into sub-structures at low temperatures, while monolayers grown by LPCVD and APCVD remain a single peak. The origin of these peaks as charged or neutral excitons follows from their temperature dependences.

Exciton dynamics in CsPbBr3 single crystal: LT splitting energy, exciton-polariton dispersion, and biexciton binding energy.

Metal halide perovskite materials (MHPs) are promising for several applications due to their exceptional properties. Understanding excitonic properties is essential for exploiting these materials. For this purpose, we focus on CsPbBr3 single crystals, which have higher crystal quality, are more stable, and have no Rashba effect at low temperatures compared to other 3D MHPs. We have estimated exciton energy positions, longitudinal-transverse splitting energy, and damping energy using low-temperature reflection spectra. Under high excitation intensity, two biexciton emissions (M-emission) and exciton-exciton scattering emission (P-emission) were observed. We assign the two M-emissions to the emission to the states of longitudinal and transverse excitons, i.e., ML and MT emissions. From the energy position of the MT emission, the biexciton binding energy has been estimated to be ∼2meV. By analyzing P-emission obtained from the back side of the sample, we have estimated the exciton binding energy to be 17.8-23.7meV. This estimation minimizes the influence of the wavenumber distribution in the scattering process. In addition, time-resolved transmittance measurements using pulsed white light have revealed the group velocity dispersion. Comparing experimental results with theoretical calculations using the Lorentz model clarifies that exciton dynamics in CsPbBr3 can be described with a simple Lorentz model. These insights enhance the understanding of exciton behavior and support the development of exciton-based devices using MHPs.

High‐Resolution X‐Ray Imaging With 0D Organic–Metal Halide Scintillator Featuring Reversed Exciton Trapping

AbstractScintillators, essential for applications in nuclear medicine, radiation detection, and industrial inspection, convert high‐energy radiation into visible light. Manganese (Mn)‐based inorganic–organic hybrid materials are distinguished by their thermal stability, mechanical strength, and flexibility. However, the effects of temperature on Mn(II)‐based hybrid scintillators have not been clearly analyzed, making the elucidation of their temperature‐dependent luminescence mechanisms particularly important. A notable advancement is the synthesis of Mn‐1 nanocrystals (NCs) using methyltriphenylphosphonium chloride (mtppCl) and MnCl₂. These NCs exhibit distinctive temperature‐dependent photoluminescence luminescence: the intensity decreases from 77 to 150 K but paradoxically increases at higher temperatures due to anomalous thermal exciton behavior in the [MnCl₄]2⁻ tetrahedra. Besides, Mn‐1 NCs achieve a detection limit of 1.01 µGyair/s, surpassing medical diagnostic standards and outperforming commercial scintillators such as Bi₄Ge₃O₁₂ (BGO). Additionally, they show exceptional stability under continuous irradiation and can be incorporated into a flexible scintillating film with a resolution of 11.3 lp/mm at an MTF of 0.2. The current study has further refined the luminescence mechanism of Mn(II)‐based materials and optimizes their properties for a wider range of applications.

Transient Optical Modulation in Vanadium and Selenium Doped MoS2 by Carrier–Carrier and Carrier–Phonon Interactions

AbstractDoping and alloying induce defect states in atomically thin transition metal dichalcogenides (TMDCs), leading to strong carrier–phonon interactions. The robust excitonic behavior of these layered materials can be modified by injecting a high density of charge carriers. However, comprehending the influence of carrier–phonon and carrier–carrier interactions on the optical properties of 2D materials is crucial for their optoelectronic and photonic applications. Here, transient absorption (TA) spectroscopy is employed to demonstrate the modulation of the transient optical behavior of TMDCs through doping and excitation near Mott density. The TA spectra reveal broadening attributed to carrier–carrier and carrier–phonon interactions, with the broadening being particularly pronounced in vanadium (V) doped TMDCs due to the hybridization of defect and exciton transitions. Analysis of TA kinetics suggests the involvement of various carrier species in the carrier dynamics of TMDCs, with the influence of mid‐gap carriers dominating at higher excitation densities. Nonetheless, the presence of strong carrier–phonon coupling in V‐doped TMDCs is demonstrated by temperature‐dependent Raman and photoluminescence spectroscopy. The results reveal that the enhanced coupling between acoustic phonons and carriers can lead to multiphonon emission. The findings of this study hold promise for controlling the optical response of TMDCs in ultrafast optoelectronic applications.

Recent progress in high-performance thermally activated delayed fluorescence exciplexes based on multiple reverse intersystem crossing channels

In the field of organic light-emitting diodes (OLEDs), exciplex materials with thermally activated delayed fluorescence (TADF) characteristics have garnered significant attention in recent years owing to their potential for high fluorescence efficiency, primarily achieved through the reverse intersystem crossing (RISC) process. By converting non-radiative triplet states to radiative singlet states, the RISC process can effectively regulate exciton behavior, resulting in the harvest of triplet excitons and the improvement of luminescence efficiency. Recently, multi-RISC strategies have been developed to further enhance the performance of TADF exciplex materials and devices. In this paper, we review these research progress in this area. We classify molecular design strategies according to the composition of exciplexes, discuss the design principles and energy-harvesting mechanism of high-performance TADF exciplexes based on multi-RISC strategies, and prospect their further research and development. The progressive endeavors summarized in this review may inspire further research on material design and device fabrication, and promote the development of OLED applications.

On-Off Switching of Singlet Self-Trapped Exciton Emission Endows Antimony-Doped Indium Halides with Excitation-Wavelength-Dependent Luminescence.

Excitation-wavelength-dependent (Ex-De) emitters are a fascinating category of luminescent materials whose emission properties vary with the wavelength of the light used for excitation. Antimony (Sb3+)-doped indium (In)-based metal halides are efficient light emitters; however, the peak fluorescence emission of most Sb3+-activated In-halide remains independent of the excitation wavelength. Here, the study introduces a new Sb3+-doped In-halide cluster, (BDPA)2InCl5:Sb (BDPA+ = C15H18N+, benzyldimethylphenylammonium), which demonstrates efficient Ex-De emission originating from the on-off switchable fluorescence behavior of singlet self-trapped exciton (STE) in 5-coordinate Sb3+ dopant. Interestingly, when excited within the range of 240-370nm, photoluminescence (PL) spectra of (BDPA)2InCl5:Sb show both singlet and triplet STE emission. However, under excitation wavelengths of 370 to 420nm, the singlet STE emission is absent, resulting in a noticeable correlated color temperature change from 1700 to 3800 K. The study provides a new approach to designing color-tunable Sb3+-based luminophores, and also presents a novel application scenario for the widely recognized Sb3+ doping strategy.

Multiconfigurational Excitonic Couplings in Homo- and Heterodimer Stacks of Azobenzene-Derived Dyes.

Molecular excitons play a major role within dye aggregates and hold significant potential for (opto)electronic and photovoltaic applications. Numerous studies have documented alterations in the spectral properties of dye homoaggregates, but only limited work has been reported for heteroaggregates. In this article, dimeric dye stacks were constructed from azobenzene-like dyes with identical or distinct structures, and their excitonic features were computationally investigated. Our results show that strong exciton coupling is not limited to identical chromophores, as often assumed, based on a recently made available Frenkel Exciton Hamiltonian and multiconfigurational plus second-order perturbation theory energetics methodology. Heteroaggregate stacks were found to exhibit different absorption features from the corresponding interacting monomers, indicating considerable coupling interactions between units. We analyzed how such coupling may vary according to various aspects, such as the relative positions of the interacting monomers or the differences in their energetics. Such qualitative and semiquantitative analyses allow the evaluation of the excitonic behavior of these dye aggregates to encourage further efforts toward a deeper understanding of the excitonic properties of tailored dye heteroaggregate systems.

Quantum tunneling high-speed nano-excitonic modulator

High-speed electrical control of nano-optoelectronic properties in two-dimensional semiconductors is a building block for the development of excitonic devices, allowing the seamless integration of nano-electronics and -photonics. Here, we demonstrate a high-speed electrical modulation of nanoscale exciton behaviors in a MoS2 monolayer at room temperature through a quantum tunneling nanoplasmonic cavity. Electrical control of tunneling electrons between Au tip and MoS2 monolayer facilitates the dynamic switching of neutral exciton- and trion-dominant states at the nanoscale. Through tip-induced spectroscopic analysis, we locally characterize the modified recombination dynamics, resulting in a significant change in the photoluminescence quantum yield. Furthermore, by obtaining a time-resolved second-order correlation function, we demonstrate that this electrically-driven nanoscale exciton-trion interconversion achieves a modulation frequency of up to 8 MHz. Our approach provides a versatile platform for dynamically manipulating nano-optoelectronic properties in the form of transformable excitonic quasiparticles, including valley polarization, recombination, and transport dynamics.

Robust excitonic light emission in 2D tin halide perovskites by weak excited state polaronic effect

2D perovskites hold immense promise in optoelectronics due to their strongly bound electron-hole pairs (i.e., excitons). While exciton polaron from interplay between exciton and lattice has been established in 2D lead-based perovskites, the exciton nature and behavior in the emerging 2D tin-based perovskites remains unclear. By combining spin-resolved ultrafast spectroscopy and sophisticated theoretical calculations, we reveal 2D tin-based perovskites as genuine excitonic semiconductors with weak polaronic screening effect and persistent Coulomb interaction, thanks to weak exciton-phonon coupling. We determine an excited state exciton binding energy of ~0.18 eV in n = 2 tin iodide perovskites, nearly twice of that in lead counterpart, despite of same large value of ~0.2 eV from steady state measurement. This finding emphasizes the pivotal role of excited state polaronic effect in these materials. The robust excitons in 2D tin-based perovskites exhibit excitation power-insensitive, high-efficiency and color-purity emission, rendering them superior for light-emitting applications.

Observation of anomalous excitonic emission in V-doped WS2 QDs synthesized by hydrothermal method and subsequent mechanical exfoliation

This study focuses on synthesizing vanadium (V) doped 2H phase WS2 via a facile hydrothermal method and subsequent liquid phase mechanical exfoliation of the material into QD-like nanostructures. An increment in dopant percentage from 1 % to 7 % red shifted the band gap of WS2 from 4.15 eV to 3.78 eV. The nanostructures show electronic transition due to B, A, and defect-bound excitons and demonstrate modulation of excitonic behavior with V doping. Low V doping provides the WS2 material with a reduced B/A excitonic peak intensity ratio because of the nonradiative pathways favoring the relaxation of high energy B excitons to A excitons. High V doping results in anomalous emission behavior marked by an increased B/A ratio, attributed to positive trion formation as a result of excess free hole concentration due to the p-type vanadium. The study suggests that V-doped WS2 nanostructures hold potential for technological applications, particularly in spintronics and photonics, emphasizing the importance of engineering exciton dynamics in two-dimensional materials using p-type substitutional dopants.

Manipulation of anisotropic Zhang-Rice exciton in NiPS3 by magnetic field

The effect of external magnetic fields on the behavior of the Zhang-Rice exciton in NiPS3, which captures the physics of spin-orbital entanglement in 2D XY-type antiferromagnets, remains unclear. This study presents systematic study of angle-resolved and polarization-resolved magneto-optical photoluminescence spectra of NiPS3 in the Voigt geometry. We observed highly anisotropic, non-linear Zeeman splitting and polarization rotation of the Zhang-Rice exciton, which depends on the direction and intensity of the magnetic field and can be attributed to the spin-orbital coupling and field-induced spin reorientation. Furthermore, above the critical magnetic field, we detected additional splitting of the exciton peaks, indicating the coexistence of various orientations of Néel vector. This study characterizes orbital change of Zhang-Rice exciton and field-induced spin-reorientation phase transitions in a 2D hexagonal XY-type antiferromagnet, and it further demonstrates the continuous manipulation of the spin and polarization of the Zhang-Rice exciton.

On the optical characteristics of Cd-Doped gadolinium oxide: A DFT based theoretical study

This research investigated the optical characteristics of Cd-doped gadolinium oxide (Gd2O3) by employing density functional theory (DFT) along with the CASTEP simulation package. After the creation of a Gd2O3 supercell at an initial [2x1x1] scale, the study evaluated the altered properties of Gd2O3 at the supercell level to assess the impact of Cd-doping on the optical features of doped one. Optical properties which were explored included the examining the dielectric function, refractive index, conductivity, and loss function. The dielectric function exhibited distinct peaks at certain photon energies that corresponded to various electronic transitions between energy levels of the material. The prominent absorption peaks observed in the energy range of 0.174-1.55 eV in the imaginary part of the dielectric function. This was attributed to energy transitions occurring between specific orbitals for pure and Cd-doped gadolinium oxide. The refractive index exhibited stability at lower energy levels, while the conductivity curves displayed excitonic behavior in response to Cd-doping at the supercellular level. Furthermore, the Cd-doping resulted in an increase in absorption, as indicated by the simulated changes in the loss function

Probing Size-Driven Exciton Behavior in InP Tetrapods via Time-Resolved Spectroscopy

Probing Size-Driven Exciton Behavior in InP Tetrapods via Time-Resolved Spectroscopy

Strong Dipole Moments from Ferrocene Methanol Clusters Boost Exciton Dissociation in Quantum-Confined Perovskite for CO2 Photoreduction.

Perovskite nanocrystals (PCNs) exhibit a significant quantum confinement effect that enhances multiexciton generation, making them promising for photocatalytic CO2 reduction. However, their conversion efficiency is hindered by poor exciton dissociation. To address this, we synthesized ferrocene-methanol-functionalized CsPbBr3 (CPB/FcMeOH) using a ligand engineering approach. By manipulating the electronic coupling between ligands and the PCN surface, facilitated by the increased dipole moment from hydrogen bonding in FcMeOH molecules, we effectively controlled exciton dissociation and interfacial charge transfer. Under 5 h of irradiation, the CO yield of CPB/FcMeOH reached 772.79 μmol g-1, 4.95 times higher than pristine CPB. This high activity is due to the formation of hydrogen-bonded FcMeOH clusters on the CPB surface. The nonpolar disruption and strong dipole moment of FcMeOH molecules enhance electronic coupling between the FcMeOH ligands and the CPB surface, reducing the surface barrier energy. Consequently, exciton dissociation and interfacial charge transfer are promoted, efficiently utilizing multiple excitons in quantum-confined domains. Transient absorption spectroscopy confirms that CPB/FcMeOH exhibits optimized exciton behavior with fast internal relaxation, trapping, and a short recombination time, allowing photogenerated charges to more rapidly participate in CO2 reduction.

Elucidating the Impact of the Dielectric Environment on Excitons and Trions in 2D Semiconductor Electrodes

Transition metal dichalcogenides (TMDs) (MX2; M = Mo, W; X = S, Se) are a promising class of materials for photoelectrochemical solar energy conversion applications due to their optimal bandgaps and natural abundance. In this configuration, TMDs are placed onto a conductive substrate and employed as a photoelectrode. A key step in the solar energy conversion process is to photo-excite the semiconductor and dissociate the photogenerated excitons to drive a reaction of interest (e.g., H2 evolution). Because these materials are so physically thin, the excitons are sensitive to the local environment. In this work, we characterize the exciton and trion formation behavior as a function of solvent dielectric, ionic strength, and chemistry. As the dielectric constant of the solvent decreases, trion formation increases. The lower dielectric solvent causes lower Coulombic screening, increasing the probability that an exciton will encounter another electron to generate a trion. I will discuss how these findings relate to a working photoelectrochemical cell.

Thickness-modulated electronic band structures and exciton behavior of non-van-der-waals 2D Bi2O2Se films

Bi2O2Se, as a non-van-der-waals (non-vdW) interlayer coupling two-dimensional material, its electronic band structures and exciton behavior still merit further in-depth studies, especially in the consideration of thickness tuning degrees of freedom. Here, we reveal the broadband excitonic performance and critical point properties of Bi2O2Se films with different thicknesses (3.54–16.33 nm). Five critical points are determined and assigned to specific optical transitions. Moreover, we observe that electronic bandstructures in the Γ valley are stable with variable thicknesses, while the deeper electron levels received more pronounced thickness modulation. And we first report four excitons in wide spectral range (1.24–6.20 eV) through absorption coefficient fitting. Significantly, due to the strong localization of electrons and holes induced by the intralayer built-in electric field, the exciton binding energy (Eb) value of 3.54 nm-Bi2O2Se (234 meV at Γ position) is apparently larger than typical multi-layer TMDCs, which offering unprecedented opportunities in applications in optoelectronic devices. Furthermore, Eb of two excitons at low energy region (Eb1, Eb2) red-shift with increasing thickness, while Eb of higher-energy excitons (Eb3, Eb4) blue-shift, owing to the dispersion characteristic of the effect dielectric screening effects. Our results are vital to wide-ranging applications of Bi2O2Se from fundamental science to optoelectronic devices.

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.

First-principles calculation to investigate phase transition and associated changes in electronic, excitonic, and optical behavior of RbMgH3 perovskite polymorphs

Motivated by recent experimental and theoretical research on the ability to store and release hydrogen from a hydride perovskite with a large ionic character called RbMgH3, we conducted a study using density functional theory (DFT) to investigate the optoelectronic behavior of RbMgH3 in both Cubic and Hexagonal structures, as well as its phase transition.Our findings reveal that RbMgH3 exhibits an indirect band gap of 2.41 eV in the Hexagonal phase and a direct band gap of 2.15 eV in the Cubic phase. Additionally, we calculated the Gibbs energy to predict the occurrence of the phase transition. It was determined that RbMgH3 undergoes a structural transformation from Cubic to Hexagonal at a pressure of 1.78 GPa. This transition has the potential to enhance the storage kinetics of hydrogen, making RbMgH3 suitable for various applications.

Pyrene‐Alkyne‐Based Conjugated Porous Polymers with Skeleton Distortion‐Mediated ⋅O2− and 1O2 Generation for High‐Selectivity Organic Photosynthesis

AbstractReactive oxygen species (ROS) play a crucial role in determining photocatalytic reaction pathways, intermediate species, and product selectivity. However, research on ROS regulation in polymer photocatalysts is still in its early stages. Herein, we successfully achieved series of modulations to the skeleton of Pyrene‐alkyne‐based (Tetraethynylpyrene (TEPY)) conjugated porous polymers (CPPs) by altering the linkers (1,4‐dibromobenzene (BE), 4,4′‐dibromobiphenyl (IP), and 3,3′‐dibromobiphenyl (BP)). Experiments combined with theoretical calculations indicate that BE‐TEPY exhibits a planar structure with minimal exciton binding energy, which favors exciton dissociation followed by charge transfer with adsorbed O2 to produce ⋅O2−. Thus BE‐TEPY shows optimal photocatalytic activity for phenylboronic acid oxidation and [3+2] cycloaddition. Conversely, the skeleton of BP‐TEPY is significantly distorted. Its planar conjugation decreases, intersystem crossing (ISC) efficiency increases, which makes it more prone for resonance energy transfer to generate 1O2. Therefore, BP‐TEPY displays best photocatalytic activity in [4+2] cycloaddition and thioanisole oxidation. Both above reactant conversion and its product selectivity exceed 99 %. This work systematically reveals the intrinsic structure–activity relationship among the skeleton structure of CPPs, excitonic behavior, and selective generation of ROS, providing new insights for the rational design of highly efficient and selective CPPs photocatalysts.

Pyrene-Alkyne-Based Conjugated Porous Polymers with Skeleton Distortion-Mediated ⋅O2 - and 1O2 Generation for High-Selectivity Organic Photosynthesis.

Reactive oxygen species (ROS) play a crucial role in determining photocatalytic reaction pathways, intermediate species, and product selectivity. However, research on ROS regulation in polymer photocatalysts is still in its early stages. Herein, we successfully achieved series of modulations to the skeleton of Pyrene-alkyne-based (Tetraethynylpyrene (TEPY)) conjugated porous polymers (CPPs) by altering the linkers (1,4-dibromobenzene (BE), 4,4'-dibromobiphenyl (IP), and 3,3'-dibromobiphenyl (BP)). Experiments combined with theoretical calculations indicate that BE-TEPY exhibits a planar structure with minimal exciton binding energy, which favors exciton dissociation followed by charge transfer with adsorbed O2 to produce ⋅O2 -. Thus BE-TEPY shows optimal photocatalytic activity for phenylboronic acid oxidation and [3+2] cycloaddition. Conversely, the skeleton of BP-TEPY is significantly distorted. Its planar conjugation decreases, intersystem crossing (ISC) efficiency increases, which makes it more prone for resonance energy transfer to generate 1O2. Therefore, BP-TEPY displays best photocatalytic activity in [4+2] cycloaddition and thioanisole oxidation. Both above reactant conversion and its product selectivity exceed 99 %. This work systematically reveals the intrinsic structure-activity relationship among the skeleton structure of CPPs, excitonic behavior, and selective generation of ROS, providing new insights for the rational design of highly efficient and selective CPPs photocatalysts.