Exciton Energy Research Articles

Exciton states in molecular superlattices

The structure of molecular superlattices consists of van der Waals heterostructures of two-dimensional atomic crystals (ultrathin films) connected to each other with regular and periodic patterns. Inspired by the increasing importance of excitons for optical spectroscopy and the engineering of photonic materials in recent years, we have calculated the energy spectra of excitons in superlattices in this theoretical study. The number of film layers and the ratio between the exciton energy transfer within each motif (film) and the exciton energy transfer between neighboring motifs (between films) are important parameters that determine the behavior in an external electromagnetic field.

Local tuning of Rydberg exciton energies in nanofabricated Cu2O pillars

Rydberg excitons in Cu2O feature giant optical nonlinearities. To exploit these nonlinearities for quantum applications, the confinement must match the Rydberg blockade size, which in Cu2O could be as large as a few microns. Here, in a top-down approach, we show how exciton confinement can be realised by focused-ion-beam etching of a polished bulk Cu2O crystal without noticeable degradation of the excitonic properties. The etching of the crystal to micron sizes allows for tuning the energies of Rydberg excitons locally, and precisely, by optically induced temperature change. These results pave the way for exploiting the large nonlinearities of Rydberg excitons in micropillars for making non-classical light sources, while the precise tuning of their emission energy opens up a viable pathway for realising a scalable photonic quantum simulation platform.

High-enhancement photoluminescence of monolayer MoS2 in hybrid plasmonic systems.

Monolayer molybdenum disulfide (M o S 2) has a weak light-matter interaction due to ultrathin thickness, which limits its potential application in lasing action. In this study, we propose a hybrid structure consisting of a nanocavity and Au nanoparticles to enhance the photon emission efficiency of monolayer M o S 2. Numerical simulations show that photoluminescence (PL) emission is significantly enhanced by introducing localized surface plasmon resonance (LSPR) to the proposed structure. Furthermore, an exciton energy band system is proposed to elucidate the physical mechanism of the PL process. By optimizing the spacer thickness, a high Purcell enhancement factor of 95 can be achieved. The results provided by this work pave the way to improve the PL efficiency of two-dimensional (2D) material, which constitutes a significant step towards the development of nanodevices such as nanolasers and sensors.

Solvation effects on the excitation dynamics, structural and photorelaxation, quantum chemical investigation, and photophysical properties of 2D p-expanded quinoidal terthiophene (2DQTTs): Outlook from Ab-initio calculations

All dyes have different degrees of absorption. The present study delves into the intricate interplay between solvation and the photophysical behaviors of 2D p-expanded quinoidal terthiophene (2DQTT) dye compounds represented as A3, A4, and A5. In this study, a comprehensive investigation is presented, elucidating how solvation impacts excitation dynamics, ground and excited state geometry, photophysical characteristics. and photorelaxation dynamics of 2DQTTs. The energy gap data shows that the energies of the compounds increased in polar mediums and were greater for compound A3 which had ΔE values of 3.5227 eV in water, and 3.5225 eV in DMSO while compound A5 had the highest ΔE in all phases, with a ΔE value of 3.5453 eV in EtOH. Relaxation of the compounds leads to changes in the bonds and structural conformation of the 2D P-expanded quinoidal terthiophene (2DQTT) compounds as the compounds de-excite back to a lower energy level, and the photon causes a change in structural bonding (Stokes shift) in the heteroatoms which was taken into consideration. This manuscript not only advances our fundamental understanding of the interactions between solvation and molecular structure but also provides valuable insights into optimizing the performance of 2DQTT in optoelectronic devices as the compounds display a high absorption band in all phases, especially in water medial with compound A3, A4 and A5 recording absorbances of 785.32 nm, 787.13 nm, and 786.78 nm respectively, a high light harvesting efficiency LHE, great electron exciton energy and an injecting capacity (ΔGinject) greater than 0.20 eV in all phases. This work contributes significantly to the design and application of novel materials for next-generation optoelectronic technologies hence we hope that experimental research tests the studied compound towards the mentioned application.

Fine-Tuning of the Excitonic Response in Monolayer WS2 Domes via Coupled Pressure and Strain Variation.

We present a spectroscopic investigation of the vibrational and optoelectronic properties of WS2 domes in the 0-0.65 GPa range. The pressure evolution of the system morphology, deduced by the combined analysis of Raman and photoluminescence spectra, revealed a significant variation in the dome's aspect ratio. The modification of the dome shape caused major changes in the mechanical properties of the system resulting in a sizable increase of the out-of-plane compressive strain while keeping the in-plane tensile strain unchanged. The variation of the strain gradients drives a nonlinear behavior in both the exciton energy and radiative recombination intensity, interpreted as the consequence of a hybridization mechanism between the electronic states of two distinct minima in the conduction band. Our results indicate that pressure and strain can be efficiently combined in low dimensional systems with unconventional morphology to obtain modulations of the electronic band structure not achievable in planar crystals.

Extended Conjugation Strategy Enabling Red-Shifted and Efficient Emission of Orange-Red Thermally Activated Delayed Fluorescence Materials.

In account of the energy gap law, the development of efficient narrow-band gap thermally activated delayed fluorescence (TADF) materials remains a major challenge for the application of organic light-emitting diodes (OLEDs). The orange-red TADF materials are commonly designed with either large π-conjugated systems or strong intramolecular donor-acceptor (D-A) interactions for red-shift emission and small singlet-triplet energy gap (ΔEST). There are rare reports on the simultaneous incorporation of these two strategies on the same material systems. Herein, two orange-red emitters named 1P2D-BP and 2P2D-DQ have been designed by extending the conjugation degree of the center acceptor DQ and increasing the number distribution of the peripheral donor PXZ units, respectively. The emission peak of 1P2D-BP is red-shifted to 615 nm compared to 580 nm for 2P2D-DQ, revealing the pronounced effect of the conjugation extension on the emission band gap. In addition, the distorted molecular structure yields a small ΔEST of 0.02 eV, favoring the acquisition of a high exciton utilization through an efficient reverse intersystem crossing process. As a result, orange-red OLEDs with both 1P2D-BP and 2P2D-DQ have achieved an external quantum efficiency (EQE) of more than 17%. In addition, the efficient white OLED based on 1P2D-BP is realized through precise exciton assignment and energy transport modulation, showing an EQE of 23.6% and a color rendering index of 82. The present work provides an important reference for the design of high-efficiency narrow-band gap materials in the field of solid-state lighting.

Excitonic Evolution in WS2/MoS2 van der Waals Heterostructures Turned by Out-of-Plane Localized Pressure

In this study, we explore the exciton dynamics in a WS2/MoS2 van der Waals (vdW) heterostructure under varying pressures by integrating a laser-confocal photoluminescence (PL) spectroscope and an atomic force microscope (AFM). For the WS2/MoS2 heterostructure, the exciton emission belonging to MoS2 is too weak to be distinguished from the PL spectra. However, upon contact with a Si probe, the emission intensity of WS2 excitons significantly decreases from 34,234 to 6560, thereby matching the intensity level of MoS2. This alteration substantially facilitates the exploration of interlayer excitonic properties within the heterostructures using PL spectroscopy. Furthermore, the Si probe can apply out-of-plane localized pressure to the heterostructure. With increasing pressure, the emission intensity of the WS2 trions decreases at a rate twice that of other excitons, and the exciton energy increases at a rate of 0.1 meV nN−1. These results elucidate that the WS2 trions are particularly sensitive to the out-of-plane pressure within a WS2/MoS2 vdW heterostructure.

A-Site Cation Influence on the Structural and Optical Evolution of Ultrathin Lead Halide Perovskite Nanoplatelets.

Imposing quantum confinement has the potential to significantly modulate both the structural and optical parameters of interest in many material systems. In this work, we investigate strongly confined ultrathin perovskite nanoplatelets APbBr3. We compare the all-inorganic and hybrid compositions with the A-sites cesium and formamidinium, respectively. Compared to each other and their bulk counterparts, the materials show significant differences in variable-temperature structural and optical evolution. We quantify and correlate structural asymmetry with the excitonic transition energy, spectral purity, and emission rate. Negative thermal expansion, structural and photoluminescence asymmetry, photoluminescence full width at half-maximum, and splitting between bright and dark excitonic levels are found to be reduced in the hybrid composition. This work provides composition- and structure-based mechanisms for engineering of the excitons in these materials.

Inter-subunit energy transfer processes in a minimal plant photosystem II supercomplex.

Photosystem II (PSII) is an integral part of the photosynthesis machinery, in which several light-harvesting complexes rely on inter-complex excitonic energy transfer (EET) processes to channel energy to the reaction center. In this paper, we report on a direct observation of the inter-complex EET in a minimal PSII supercomplex from plants, containing the trimeric light-harvesting complex II (LHCII), the monomeric light-harvesting complex CP26, and the monomeric PSII core complex. Using two-dimensional (2D) electronic spectroscopy, we measure an inter-complex EET timescale of 50 picoseconds for excitations from the LHCII-CP26 peripheral antenna to the PSII core. The 2D electronic spectra also reveal that the transfer timescale is nearly constant over the pump spectrum of 600 to 700 nanometers. Structure-based calculations reveal the contribution of each antenna complex to the measured inter-complex EET time. These results provide a step in elucidating the full inter-complex energy transfer network of the PSII machinery.

Enhancing energy transfer via inhibition defect density of coupled quasi-2D perovskite layers for efficient sky-blue light-emitting diodes

Metal halide perovskite materials have emerged as unique and promising candidates for full-color display and solid-state lighting applications. While substantial progress has been made in developing green, red and near-infrared perovskite light-emitting diodes (PeLEDs) with external quantum efficiency (EQE) approaching their theoretical upper limit, the performance of blue PeLEDs is still lagging far behind. Here, synergistic long and short hydrobromide chains are strategically introduced into the perovskite to regulate composition and dimensional engineering for promoting coupled quasi-two-dimensional perovskite thin films with enhanced photoluminescence quantum yield (PLQY) and reduced trap state density. The inclusion of the hydrobromide can also simultaneously passivate the perovskite defects by reducing nonradiative recombination loss and increasing the formation of n >1 phases for efficient exciton energy transfer. Encouraged by these findings, we finally demonstrate the most stable sky-blue PeLEDs to date with a half-lifetime of 515 min at high external quantum efficiency (EQE＞10 %), emitting at 486 nm with a peak EQE of up to 15.6 %.

Strongly Bound Frenkel Excitons on TiO2 Nanoparticles: An Evolutionary and DFT Approach

An evolutionary algorithm was employed to locate the global minimum of TiO2n nanoparticles with n=2–20. More than 61,000 structures were calculated with a semiempirical method and reoptimized using density functional theory. The exciton binding energy of TiO2 nanoparticles was determined through the fundamental and optical band gap. Frenkel exciton energy scales as EB eV=8.07/n0.85, resulting in strongly bound excitons of 0.132–1.2 eV for about 1.4 nm nanoparticles. Although the exciton energy decreases with the system size, these tightly bound Frenkel excitons inhibit the separation of photogenerated charge carriers, making their application in photocatalysis and photovoltaic devices difficult, and imposing a minimum particle size. In contrast, the exciton binding energy of rutile is 4 meV, where the Wannier exciton energy scales as EB eV=13.61 μ/ε2. Moreover, the Wannier excitons in bulk TiO2 are delocalized according to the Bohr radii: 3.9 nm for anatase and 7.7 nm for rutile.

Coherent transient exciton transport in disordered polaritonic wires

Abstract Excitation energy transport can be significantly enhanced by strong light–matter interactions. In the present work, we explore intriguing features of coherent transient exciton wave packet dynamics on a lossless disordered polaritonic wire. Our main results can be understood in terms of the effective exciton group velocity, a new quantity we obtain from the polariton dispersion. Under weak and moderate disorder, we find that the early wave packet spread velocity is controlled by the overlap of the initial exciton momentum distribution and its effective group velocity. Conversely, when disorder is stronger, the initial state is nearly irrelevant, and red-shifted cavities support excitons with greater mobility. Our findings provide guiding principles for optimizing ultrafast coherent exciton transport based on the magnitude of disorder and the polariton dispersion. The presented perspectives may be valuable for understanding and designing new polaritonic platforms for enhanced exciton energy transport.

Population and Energy Transfer Dynamics in an Open Excitonic Quantum Battery.

In a previous study, we proposed an open quantum network model of a quantum battery (QB) that possesses dark states owing to its structural exchange symmetries. While in a dark state, the QB is capable of storing an exciton without any environment-induced population losses. However, when the structural exchange symmetry is broken, the QB begins to discharge the exciton towards its exit site. In this article, we start by demonstrating that this QB is not only loss-free with respect to exciton population during the storage phase, but also with respect to the QB energy. We then explore the exciton population and energy transfer dynamics of the QB during the discharge phase over a wide range of site energies, bath temperatures, and bath reorganization energies. Our results shed light on how to optimize the QB's population and energy transfer dynamics for different purposes.

Dynamics of Electron–Hole Coulomb Attractive Energy and Dipole Moment of Hot Excitons in Donor–Acceptor Polymers

Dynamics of Electron–Hole Coulomb Attractive Energy and Dipole Moment of Hot Excitons in Donor–Acceptor Polymers

Auger‐Assisted Secondary Hot Carrier Transfer in a Type I MoS2/PtSe2 Heterostructure

AbstractCharge transfer is vital in determining the optoelectronic properties of atomically thin materials, yet remains elusive in type I heterostructures. Here, distinct two‐step charge transfer processes in a type I MoS2/PtSe2 heterostructure are reported. By exclusively exciting the smaller bandgap PtSe2, strong exciton photobleaching peaks of the larger bandgap MoS2 are observed, indicating primary hot carrier transfer from PtSe2 to MoS2 within 70 fs. More importantly, the amplitude of the exciton peaks shows a secondary increase after the initial rapid decay. These dynamics are distinctly different from the monotonic decrease in monolayer MoS2 and indicate a secondary charge transfer process that is attributed to hot carriers re‐generated in PtSe2 by intralayer Auger recombination. Concurrently, the exciton energy blue shifts within 100 ps, probing the dynamic buildup of a charge‐transfer induced electric field across the heterostructure interface, which displaces electron and hole wavefunctions of MoS2 excitons and reduces the exciton binding energy. The results are corroborated by carrier dynamics and transient absorption spectra simulations by considering the two‐step charge transfer processes. The work reveals Auger‐assisted hot carrier transfer processes in type I heterostructures and suggests the possibility for optoelectronic and photocatalytic applications by optical sub‐bandgap excitation.

Key requirements for ultraefficient sensitization in hyperfluorescence organic light-emitting diodes

Blue organic light-emitting diode (OLED) technology requires further advancements, and hyperfluorescent (HF) OLEDs have emerged as a promising solution to address stability and colour-purity concerns. A key factor influencing the performance of HF-OLEDs is Förster resonance energy transfer (FRET). Here we investigate the FRET mechanism in blue HF-OLEDs using contrasting thermally activated delayed fluorescence (TADF) sensitizers. We demonstrate that the molecular structure of the sensitizer profoundly impacts the FRET efficiency, exemplified by the spiro-linked TADF molecule ACRSA, which suppresses the dihedral-angle inhomogeneity and any lower-energy conformers that exhibit minimal FRET to the terminal emitter. Consequently, the FRET efficiency can be optimized to nearly 100%. Further, we demonstrate how the properties of a near-ideal sensitizer diverge from ideal TADF emitters. As a result, blue HF-OLEDs utilizing a greenish sensitizer exhibit a remarkable tripling of external quantum efficiency (~30%) compared with non-HF devices. This new understanding opens avenues for sensitizer design, indicating that green sensitizers can efficiently pump blue terminal emitters, thereby reducing device exciton energies and improving blue OLED stability.

Optical characterizations of GaN/MoS2 van der Waals heterojunctions with different band alignments

The integration of gallium nitride (GaN) with 2D molybdenum disulfide (MoS2) to form GaN/MoS2 semiconductor heterojunctions will have high potential applications in novel electronics and optoelectronics. In this study, GaN thin films were grown on pulse-laser-deposited MoS2 layer by plasma-assisted MBE at different substrate temperatures (500 °C, 600 °C and 700 °C, respectively). The energy-band alignments of GaN/MoS2 semiconductor heterostructures were analyzed by X-ray photoelectron spectroscopy. The epitaxial growth conditions of GaN films influenced the band alignment of GaN/MoS2 heterojunction. Type-I heterostructure, a straddling relation between narrow-bandgap MoS2 and wide-bandgap GaN, was observed at the optimized growth temperature of 600 °C. At the same time, photoluminescence (PL) and photoreflectance spectroscopies were employed to analyze the optical properties of MoS2 and GaN/MoS2 heterostructures. The PL and exciton energy transition of 2D MoS2 layer can be enhanced by the capping layer GaN, especially for type-I band alignment structure.

Deterministic reflection contrast ellipsometry for thick multilayer two-dimensional heterostructures

Abstract Optical spectroscopy is a powerful tool for characterizing the properties of two-dimensional (2D) heterostructures. However, extracting the permittivity information of each 2D layer in optically thick heterostructures is challenging because of interference. To accurately measure the optical permittivity of each 2D layer in a heterostructure or on a substrate with a thick insulating spacer, such as oxides, we propose deterministic reflection contrast ellipsometry (DRCE). Our DRCE method has two advantages over conventional techniques. It deterministically measures the optical permittivity of 2D materials using only the measured reflection spectra of the heterostructure, rather than dispersion fitting as in spectroscopic ellipsometry. Additionally, the DRCE is free of excitonic energy errors in reflection-contrast spectroscopy. We believe that DRCE will enable accurate and rapid characterization of 2D materials.

Simultaneously Regulated Highly Polarized and Long-Lived Valley Excitons in WSe2/GaN Heterostructures.

Interlayer excitons, with prolonged lifetimes and tunability, hold potential for advanced optoelectronics. Previous research on the interlayer excitons has been dominated by two-dimensional heterostructures. Here, we construct WSe2/GaN composite heterostructures, in which the doping concentration of GaN and the twist angle of bilayer WSe2 are employed as two ingredients for the manipulation of exciton behaviors and polarizations. The exciton energies in monolayer WSe2/GaN can be regulated continuously by the doping levels of the GaN substrate, and a remarkable increase in the valley polarizations is achieved. Especially in a heterostructure with 4°-twisted bilayer WSe2, a maximum polarization of 38.9% with a long lifetime is achieved for the interlayer exciton. Theoretical calculations reveal that the large polarization and long lifetime are attributed to the high exciton binding energy and large spin flipping energy during depolarization in bilayer WSe2/GaN. This work introduces a distinctive member of the interlayer exciton with a high degree of polarization and a long lifetime.

Energy Gain and Loss Processes in OLEDs with Spacer‐Separated Electron–Hole Pairs Investigated by Magnetic Field Effects

AbstractInterfacial or diluted exciplex organic light emitting diodes (OLEDs) with spacer‐separated electron–hole pairs have emerged as a promising approach for precise tuning the energy level of charge‐transfer (CT) excitons and the improvement in device performance. However, their microscopic energy gain and loss mechanisms are not well understood. In this study, magnetic field effects (MFEs) including magneto‐electroluminescence (MEL), magneto‐conductance (MC), and magneto‐efficiency (Mη) in two series of donor‐spacer‐acceptor type exciplex‐based OLEDs, which exhibited opposite distance‐dependent device performances, are studied to explore the microscopic dynamics of spacer‐separated electron–hole pairs. Intersystem crossing (ISC) between CT excitons and between polaron‐pairs, reverse intersystem crossing (RISC) between CT excitons, triplet–triplet annihilation (TTA), scattering and dissociation channels of triplet‐charge annihilation (TQA), and additional Dexter energy transfer (DET) processes are identified. The decreased external quantum efficiency (EQE) and MEL with increasing thickness of space layer in Configuration II devices are attributed to an activation of the potential DET process from the lowest triplet of spacer‐separated CT excitons to the lowest triplet of Bphen caused by their smaller energy gaps. The study offers in‐depth insights into the exciton utilization and energy loss, which may allow better design and optimization of space‐modulated OLEDs.