Exciton Interaction Research Articles

Synthesis, Photophysical and Electrochemical Properties of Bis-Squaraine Dyes Fused on Isomeric Benzodipyrrole Central Units.

Exciton interactions are not only observed in assembled molecules but also in compounds with multiple chromophores referred to as superchromophores. We have developed isomeric bis-squaraine dyes as superchromophores in which two squaraine chromophores are fused onto the isomeric benzodipyrrole skeleton so as to regulate conformations and to reduce distances between two chromophores. The dyes with benzo[1,2-b:3,4-b']dipyrrole and benzo[1,2-b:5,4-b']dipyrrole moieties exhibited split electronic absorption originated from the intramolecular exciton interaction. The intensity of the split absorption bands varies in correlation with the orientation of chromophores. The isomeric dye with benzo[1,2-b:4,5-b']dipyrrole moiety exhibited a near-infrared absorption associated with the resonance throughout two chromophores. Their electrochemical and spectroelectrochemical properties are distinct from those of monomeric dyes owing to electronic interactions between the two chromophores. Thus, the structural isomerism of the central skeleton significantly affects their optical properties as well as their electrochemical properties.

Synthetic Control of Exciton Dynamics in Bioinspired Cofacial Porphyrin Dimers.

Understanding how the complex interplay among excitonic interactions, vibronic couplings, and reorganization energy determines coherence-enabled transport mechanisms is a grand challenge with both foundational implications and potential payoffs for energy science. We use a combined experimental and theoretical approach to show how a modest change in structure may be used to modify the exciton delocalization, tune electronic and vibrational coherences, and alter the mechanism of exciton transfer in covalently linked cofacial Zn-porphyrin dimers (meso-beta linked ABm-β and meso–meso linked AAm-m). While both ABm-β and AAm-m feature zinc porphyrins linked by a 1,2-phenylene bridge, differences in the interporphyrin connectivity set the lateral shift between macrocycles, reducing electronic coupling in ABm-β and resulting in a localized exciton. Pump–probe experiments show that the exciton dynamics is faster by almost an order of magnitude in the strongly coupled AAm-m dimer, and two-dimensional electronic spectroscopy (2DES) identifies a vibronic coherence that is absent in ABm-β. Theoretical studies indicate how the interchromophore interactions in these structures, and their system-bath couplings, influence excitonic delocalization and vibronic coherence-enabled rapid exciton transport dynamics. Real-time path integral calculations reproduce the exciton transfer kinetics observed experimentally and find that the linking-modulated exciton delocalization strongly enhances the contribution of vibronic coherences to the exciton transfer mechanism, and that this coherence accelerates the exciton transfer dynamics. These benchmark molecular design, 2DES, and theoretical studies provide a foundation for directed explorations of nonclassical effects on exciton dynamics in multiporphyrin assemblies.

An unreported peak of the electroluminescence turn-on transience from OLEDs with electron or hole potential well

An unreported peak was discovered at the transient electroluminescence (TEL) rising edge from pure emission layer (EML) based OLEDs with an energy potential well for electron or hole charge carriers. A series of control experiments indicated that the formation mechanism of this peak is completely different from the already-reported peak originated from exciton interactions such as singlet-triplet and triplet-triplet annihilations. Herein, the peak at the TEL rising edge is generated by the radiative recombination of injected carriers with pre-stored charges in the energy potential well. Surprisingly, for the pure EML-based OLEDs with simultaneous electron and hole potential wells, the peak at the TEL rising edge disappeared, and contrarily, a super-high spike was observed at the TEL falling edge. Such an already-reported spike at the TEL falling edge was considered to only appear in doped-EML systems. This contradiction leads to the reconsideration for the formation mechanisms of the spike at the TEL falling edge. Accordingly, this work not only deepens the physical understanding of the unprecedented peak at the TEL rising edge and the already-reported spike at the TEL falling edge, but also reveals the effects of the energy potential well on the carrier recombination process in OLEDs.

Symmetry-dependent exciton-exciton interaction and intervalley biexciton in monolayer transition metal dichalcogenides

The multivalley band structure of monolayer transition metal dichalcogenides (TMDs) gives rise to intravalley and intervalley excitons. Much knowledge of these excitons has been gained, but fundamental questions remain, such as how to describe them all in a unified picture with their correlations, how are those from different valleys coupled to form the intervalley biexciton? To address the issues, we derive an exciton Hamiltonian from interpair correlations between the constituent carriers-fermions of two excitons. Identifying excitons by irreducible representations of their point symmetry group, we find their pairwise interaction depending on interacting excitons’ symmetry. It is generally repulsive, except for the case excitons from different valleys, which attract each other to form the intervalley biexciton. We establish a semianalytical relationship between the biexciton binding energy with exciton mass and dielectric characteristics of the material and surroundings. Overall, by providing insight into the nature of diverse excitons and their correlations, our theoretical model captures the exciton interaction properties permitting an inclusive description of the structure and energy features of the intervalley biexciton in monolayer TMDs.

FORMATION OF J-AGGREGATES OF THE 21-THIA-5,10,15,20-TETRA-(4-SULFONATOPHENYL)-PORPHYRIN IN WATER SOLUTIONS

The formation of J-aggregates of 21-thia-5,10,15,20-tetra-(4-sulfonatophenyl)-porphyrin in acidified water solutions is herein revealed for the first time and their spectral-luminescent properties are measured. It is shown that the spectral-luminescent properties of J-aggregates are due to the excitonic interactions, and the number of coherent interacting monomeric porphyrin molecules in the aggregate is evaluated. It is found
 that J-aggregates of heteroporphyrin have fluorescence, and the fluorescence quantum yield Фfl is found to be as low as 1.8 . 10–4. The photolability of J-aggregates is found, namely, upon J-aggregate photoexcitation to the absorption band at 503 nm they collapse to the monomeric doubly protonated molecules. The process of photomonomerisation is reversible: upon keeping the solution in the dark, J-aggregates form again.

Interplay between multipole expansion of exchange interaction and Coulomb correlation of exciton in colloidal II–VI quantum dots

We study the effect of Coulomb correlation on the emission properties of the ground state exciton in zincblende CdSe/ZnS core–shell and in wurtzite ZnO quantum dots (QDs). We validate our theory model by comparing results of computed exciton energies of CdSe/ZnS QDs to photoluminescence and scanning near-field optical microscopy measurements. We use that to estimate the diameter of the QDs using a simple model based on infinitely deep quantum well and compare the results with the statistics of the atomic force microscopy scans of CdSe/ZnS dots, obtaining excellent agreement. Thereafter, we compute the energy fine structure of exciton, finding striking difference between properties of zincblende CdSe/ZnS and wurtzite ZnO dots. While in the former the fine structure is dominated by the dipole terms of the multipole expansion of the exchange interaction, in the latter system that is mostly influenced by Coulomb correlation. Furthermore, the correlation sizeably influences also the exciton binding energy and emission radiative rate in ZnO dots.

Theory of exciton-phonon coupling

The effects of the electron-phonon interaction on optical excitations can be understood in terms of exciton-phonon coupling, and require a careful treatment in low-dimensional materials with strongly bound excitons or strong electron-hole interaction in general. Through phonon absorption and emission processes, the optically accessible excitons are scattered into otherwise optically dark finite-momentum exciton states. We derive a practical expression for the phonon-induced term of the exciton self-energy (denoted as the exciton-phonon self-energy) that gives the temperature dependence of the optical transition energies and their lifetime broadening resulting from the exciton's interaction with the phonons. We illustrate this theory on a two-dimensional model, and show that our expression for the exciton-phonon self-energy differs qualitatively from previous expressions found in the literature that neglect the exciton binding or electron-hole correlations.

Optomechanical Lasing and Domain Walls Driven by Exciton-Phonon Interactions

We study theoretically interaction of optically-pumped excitons with acoustic waves in planar semiconductor nanostructures in the strongly nonlinear regime. We start with the multimode optomechanical lasing regime for optical pump frequency {above} the exciton resonance and demonstrate broadband chaotic-like lasing spectra. We also predict formation of propagating optomechanical domain walls driven by optomechanical nonlinearity for the optical pump {below} the exciton resonance. Stability conditions for the domain walls are examined analytically and are in agreement with direct numerical simulations. Our results apply to nonlinear sound propagation in the arrays of quantum wells or in the plane of Bragg semiconductor microcavities hosting excitonic polaritons.

Excitonic theory of doping-dependent optical response in atomically thin semiconductors

The interaction of optically excited excitons in atomically thin semiconductors with residual doping densities leads to many-body effects which are continuously tunable by external gate voltages. Here, we develop a fully microscopic theory to describe the doping-dependent manipulation of the excitonic properties in atomically thin transition metal dichalcogenides. In particular, we establish a diagonalization approach for the Schr\"odinger equation which characterizes the interaction of a virtual exciton with the Fermi sea of dopants. Solving this many-body Schr\"odinger equation provides access to trions as well as a continuum of scattering states. The dynamics of coupled excitons, trions, and scattering continua is subsequently described by Heisenberg equations of motion including mean-field contributions and correlation effects due to the interaction of excitons with trions and scattering continuum states. Our calculations for optical excitation close to the band edge reveal the influence of doping on the exciton resonances in combination with the simultaneous identification of not only ground-, but also excited-, state trion resonances.

3D micro-combs self-assembled from 2D N-doped In2S3 for room-temperature reversible NO2 gas sensing

Two-dimensional (2D) metal sulfides have been an emerging material group for high-performance and power-saving gas sensing. However, relatively little attention is paid to developing doping-driven approaches to improve their gas interaction properties, particularly at room temperature. In this work, we realize room-temperature reversible NO2 gas sensing by utilizing the three-dimensional (3D) micro-combs self-assembled from 2D N-doped In2S3. Specifically, hydrothermal synthesized In2S3 microspheres as the initial material are exfoliated down to 2D single-unit-cell-thick (∼3 nm) nanoflakes in the presence of ultrasonic force, while the N dopants are inserted into the crystal lattices simultaneously. The N dopants significantly modify the electronic band structure of In2S3 to facilitate the hybridization with NO2 molecular orbitals, induce an n- to p-type transition, decrease the surface activation energy, and activate the strong excitonic interaction property. The 3D N-doped In2S3 micro-combs exhibit a response magnitude of 15% for 10 ppm NO2 at room temperature with complete recovery, high selectivity, and excellent repeatability, while incorporating blue light excitation further increases the response magnitude and shortens the response/recovery time, which are rarely seen in 2D metal sulfides. This work demonstrates the possibility of implementing 2D doped metal sulfides and their derivatives for high-performance room temperature gas sensors.

Interaction of excitons with Cherenkov radiation in WSe2 beyond the non-recoil approximation

Cherenkov radiation (CR) from electrons propagating in materials with a high refractive index have applications in particle-detection mechanisms and could be used for high-yield coherent electron beam-driven photon sources. However, the theory of the CR has been treated up to now using the non-recoil approximation, which neglects the effect of electron deceleration in materials. Here, we report on the effect of electron-beam deceleration on the radiated spectrum and exciton-photon interactions in nm-thick WSe2 crystals. The calculation of the CR is performed by simulating the kinetic energy of an electron propagating in a thick sample using the Monto Carlo method combined with the Lienard–Wiechert retarded potential. Using this approach, we numerically investigate the interaction between the excitons and generated photons (CR) beyond the non-recoil approximation and are able to reproduce experimental cathodoluminescence spectra. Our findings pave the way for an accurate design of particle scintillators and detectors, based on the strong-coupling phenomenon.

Exciton interactions in helical crystals of a hydrogen-bonded eumelanin monomer.

Eumelanin, a naturally occurring group of heterogeneous polymers/aggregates providing photoprotection to living organisms, consist of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) building blocks. Despite their prevalence in the animal world, the structure and therefore the mechanism behind the photoprotective broadband absorption and non-radiative decay of eumelanin remain largely unknown. As a small step towards solving the incessant mystery, DHI is crystallized in a non-protic solvent environment to obtain DHI crystals having a helical packing motif. The present approach reflects the solitary directional effect of hydrogen bonds between the DHI chromophores for generating the crystalline assembly and filters out any involvement of the surrounding solvent environment. The DHI single crystals having an atypical chiral packing motif (P212121 Sohncke space group) incorporate enantiomeric zig-zag helical stacks arranged in a herringbone fashion with respect to each other. Each of the zig-zag helical stacks originates from a bifurcated hydrogen bonding interaction between the hydroxyl substituents in adjacent DHI chromophores which act as the backbone structure for the helical assembly. Fragment-based excited state analysis performed on the DHI crystalline assembly demonstrates exciton delocalization along the DHI units that connect each enantiomeric helical stack while, within each stack, the excitons remain localized. Fascinatingly, over the time evolution for generation of single-crystals of the DHI-monomer, mesoscopic double-helical crystals are formed, possibly attributed to the presence of covalently connected DHI trimers in chloroform solution. The oligomeric DHI (in line with the chemical disorder model) along with the characteristic crystalline packing observed for DHI provides insights into the broadband absorption feature exhibited by the chromophore.

Transient Superdiffusion of Energetic Carriers in Transition Metal Dichalcogenides Visualized by Ultrafast Pump-Probe Microscopy

Because of the strong Coulomb interaction and quantum confinement effect, 2-dimensional transition metal dichalcogenides possess a stable excitonic population. To realize excitonic device applications, such as excitonic circuits, switches, and transistors, it is of paramount importance for understanding the optical properties of transition metal dichalcogenides. Furthermore, the strong quantum confinement in 2-dimensional space introduces exotic properties, such as enhanced phonon bottlenecking effect, many-body interaction of excitons, and ultrafast nonequilibrium exciton–exciton annihilation. Exciton diffusion is the primary energy dissipation process and a working horse in excitonic devices. In this work, we investigated time-resolved exciton propagation in monolayer semiconductors of WSe 2 , MoWSe 2 , and MoSe 2 , with a home-built femtosecond pump-probe microscope. We observed ultrafast exciton expansion behavior with an equivalent diffusivity of up to 502 cm 2 s −1 at the initial delay time, followed by a slow linear diffusive regime (20.9 cm 2 s −1 ) in the monolayer WSe 2 . The fast expansion behavior is attributed to energetic carrier-dominated superdiffusive behavior. We found that in the monolayers MoWSe 2 and MoSe 2 , the energetic carrier-induced exciton expansion is much more effective, with diffusivity up to 668 and 2295 cm 2 s −1 , respectively. However, the “cold” exciton transport is trap limited in MoWSe 2 and MoSe 2 , leading to negative diffusion behavior at later time. Our findings are helpful to better understand the ultrafast nonlinear diffusive behavior in strongly quantum-confined systems. It may be harnessed to break the limit of conventional slow diffusion of excitons for advancing more efficient and ultrafast optoelectronic devices.

Plasmon–Exciton Coupling Effect on Plasmon Damping

Plasmon decay via the surface or interface is a critical process for practical energy conversion and plasmonic catalysis. However, the relationship between plasmon damping and the coupling between the plasmon and 2D materials is still unclear. The spectral splitting due to plasmon–exciton interaction impedes the conventional single‐particle method to evaluate the plasmon damping rate by spectral linewidth. The interaction between a single gold nanorod and 2D materials using the single‐particle spectroscopy technique assisted with in situ nanomanipulation is investigated. The approach allows to indisputably identify that the plasmon–exciton coupling would induce plasmon damping in the GNR–WSe2 hybrid. It is confirmed that the resonant energy transfer channel dominates the plasmon decay rather than the charge transfer channel in the GNR–graphene hybrid first. The contribution of the charge transfer channel by using thin hBN layers as an intermediate medium to block the charge transfer is excluded. It is also found that the contact layer between the GNR and 2D materials contributes most of the interfacial plasmon damping. These findings contribute to a deep understanding of interfacial excitonic effects on the plasmon and 2D materials hybrid.

Femtosecond Exciton and Carrier Relaxation Dynamics of Two-Dimensional (2D) and Quasi-2D Tin Perovskites.

The dynamics of exciton and free-carrier relaxation of low-dimensional tin iodide perovskites, BA2FAn-1SnnI3n+1, where n = 1 (N1), 2 (N2), 5 (N5), and 10 (N10), were investigated with femtosecond transient absorption spectra (TAS). The absorption and photoluminescence spectra of N1 and N2 show exciton characteristics due to quantum confinement, whereas N5 and N10 display a free-carrier nature, the same as for bulk three-dimensional (3D) films. The TAS profiles were fitted according to a global kinetic model with three time coefficients representing the interactions of biexcitons, trions, and excitons for N1 and N2 and hot carriers, cold carriers, and shallow trap carriers for N5 and N10. The carrier relaxation dynamics of N5 and N10 were similar to those of 3D FASnI3 except for the absence of surface recombination in the deep-trap states due to passivation of the grain surfaces by the long alkyl chain for these quasi-2D samples (N5/N10 vs 3D).

Optical gain appearance in the CdSe/CdS nanoplatelets colloidal solution

The experimental investigations of the colloidal heterostructured CdSe/CdS nanoplatelets transient absorption in the regime of one-photon stationary excitation of excitons by high-intensity nanosecond laser pulses are presented. A strong dependence of exciton’s transitions behavior on nanoplatelets concentration in colloidal solution and pump intensities was studied for the first time. The revealed nonlinear transmission changes of the nanoplatelets solutions with different optical density were explained by the presence of phase space filling effect, energy exchange between exciton states, reabsorption processes and exciton–exciton interaction. The most vital result of the paper is the identification of conditions for the existence of the optical gain in colloidal solutions of nanoplatelets.

Exciton-Polaritons and Their Bose-Einstein Condensates in Organic Semiconductor Microcavities.

Exciton-polaritons are half-light, half-matter bosonic quasiparticles formed by strong exciton-photon coupling in semiconductor microcavities. These hybrid particles possess the strong nonlinear interactions of excitons and keep most of the characteristics of the underlying photons. As bosons, above a threshold density they can undergo Bose-Einstein condensation to a polariton condensate phase and exhibit a rich variety of exotic macroscopic quantum phenomena in solids. Recently, organic semiconductors have been considered as a promising material platform for these studies due to their room-temperature stability, good processability, and abundant photophysics and photochemistry. Herein, recent advances of exciton-polaritons and their Bose-Einstein condensates in organic semiconductor microcavities are summarized. First, the basic physics is introduced, and then their emerging applications are highlighted. The remaining questions are also discussed and a personal viewpoint about the potential directions for future research is given.

Charting the Exciton–Polariton Landscape of WSe2 Thin Flakes by Cathodoluminescence Spectroscopy

Semiconducting transition‐metal dichalcogenides (TMDCs) provide a fascinating discovery platform for strong light–matter interaction effects in the visible spectrum at ambient conditions. While most of the works have focused on hybridizing excitons with resonant photonic modes of external mirrors, cavities, or nanostructures, intriguingly, TMDC flakes of subwavelength thickness can themselves act as nanocavities. Herein, the optical response of such freestanding planar waveguides of WSe2 by means of cathodoluminescence spectroscopy is determined. Strong exciton–photon interaction effects that foster long‐range propagating exciton–polaritons and enable direct imaging of the energy transfer dynamics originating from cavity‐like Fabry–Pérot resonances are revealed. Furthermore, confinement effects due to discontinuities in the flakes are demonstrated as an efficient means to tailor mode energies, spin–momentum couplings, and the exciton–photon coupling strength, as well as to promote photon‐mediated exciton–exciton interactions. The combined experimental and theoretical results provide a deeper understanding of exciton–photon self‐hybridization in semiconducting TMDCs and may pave the way to optoelectronic nanocircuits exploiting exciton–photon interaction beyond the routinely employed two‐oscillator coupling effects.

Power-Dependent Photoluminescence Efficiency in Manganese-Doped 2D Hybrid Perovskite Nanoplatelets.

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn2+)-doped organic-inorganic hybrid perovskite nanoplatelets (chemical formula: L2[APb1-xMnxBr3]n-1Pb1-xMnxBr4; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb1-xMnxBr64- octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn2+ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm2). Surprisingly, we find that the saturation of long-lived Mn2+ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

Nanoscale \u03c0-conjugated ladders

It is challenging to increase the rigidity of a macromolecule while maintaining solubility. Established strategies rely on templating by dendrons, or by encapsulation in macrocycles, and exploit supramolecular arrangements with limited robustness. Covalently bonded structures have entailed intramolecular coupling of units to resemble the structure of an alternating tread ladder with rungs composed of a covalent bond. We introduce a versatile concept of rigidification in which two rigid-rod polymer chains are repeatedly covalently associated along their contour by stiff molecular connectors. This approach yields almost perfect ladder structures with two well-defined π-conjugated rails and discretely spaced nanoscale rungs, easily visualized by scanning tunnelling microscopy. The enhancement of molecular rigidity is confirmed by the fluorescence depolarization dynamics and complemented by molecular-dynamics simulations. The covalent templating of the rods leads to self-rigidification that gives rise to intramolecular electronic coupling, enhancing excitonic coherence. The molecules are characterized by unprecedented excitonic mobility, giving rise to excitonic interactions on length scales exceeding 100 nm. Such interactions lead to deterministic single-photon emission from these giant rigid macromolecules, with potential implications for energy conversion in optoelectronic devices.