Exciton Interaction Research Articles

Spectroscopic Characterization of Thiacarbocyanine Dye Molecules Adsorbed on Hexagonal Boron Nitride: a Time-Resolved Study.

Physisorption on hexagonal boron nitride (hBN) gained interest over the years thanks to its properties (chemically and thermally stable, insulating properties, etc.) and similarities to the well-known graphene. A recent study showed flat-on adsorption of several cationic thiacarbocyanine dyes on hBN with a tendency to form weakly coupled H- or I-type aggregates, while a zwitterionic thiacarbocyanine dye rather led to a tilted adsorption. With this in-depth time-resolved study using the TC-SPC technique, we confirm the results proven by adsorption isotherms, atomic force microscopy, and stationary state spectroscopy combined with molecular mechanics simulations and estimation of the corresponding exciton interaction. The absence of a systematic trend for the dependence of the decay times, normalized amplitudes of the decay components, and contribution of different components to the stationary emission spectra upon the emission wavelength observed for all studied dyes and coverages suggests the occurrence of a single emitting species. At low coverage levels, the non-mono-exponential character of the decays was attributed to adsorption on different sites characterized by different intramolecular rotational freedom or energy transfer to nonfluorescent traps or a combination of both. The difference between the decay rates of the four dyes reflects a different density of the nonfluorescent traps. Although the decay time of the unquenched dyes was in the order of magnitude of that of dye monomers in a rigid environment, it is also compatible with weakly coupled aggregates such as proposed earlier based on the stationary spectra. Hence, the adsorption leads to a rigid environment of the dyes, blocking internal conversion. Increasing the concentration of the dye solution from which the adsorption on hBN occurs increases not only the coverage of the hBN surface but also the extent of energy transfer to nonfluorescent traps. For TDC (5,5-dichloro-3-3'-diethyl-9-ethyl-thiacarbocyanine) and TD2 (3-3'-diethyl-9-ethyl-thiacarbocyanine), besides direct energy transfer to traps, exciton hopping between dye dimers followed by energy transfer to these traps occurs, which resulted in a decreasing decay time of the longest decaying component. For all dyes, it was also possible to analyze the fluorescence decays as a stretched exponential as would be expected for energy transfer to randomly distributed traps in a two-dimensional (2D) geometry. This analysis yielded a fluorescence decay time of the unquenched dyes similar to the longest decay time obtained by analysis of the fluorescence decays as a sum of three of four exponentials.

Unraveling the role of thermal fluctuations on the exciton structure of the cryptophyte PC612 and PC645 photosynthetic antenna complexes.

Protein scaffolds play a crucial role in tuning the light harvesting properties of photosynthetic pigment-protein complexes, influencing pigment-protein and pigment-pigment excitonic interactions. Here, we investigate the influence of thermal dynamic effects on the protein tuning mechanisms of phycocyanin PC645 and PC612 antenna complexes of cryptophyte algae, featuring closed or open quaternary structures. We employ a dual molecular dynamics (MD) strategy that combines extensive classical MD simulations with multiple short Born-Oppenheimer quantum/molecular mechanical (QM/MM) simulations to accurately account for both static and dynamic disorder effects. Additionally, we compare the results with an alternative protocol based on multiple QM/MM geometry optimizations of the pigments. Subsequently, we employ polarizable QM/MM calculations using time-dependent density functional theory (TD-DFT) to compute the excited states, and we adopt the full cumulant expansion (FCE) formalism to describe the absorption and circular dichroism spectra. Our findings indicate that thermal effects have only minor impacts on the energy ladder in PC612, despite its remarkable flexibility owing to an open quaternary structure. In striking contrast, thermal effects significantly influence the properties of PC645 due to the absence of a hydrogen bond controlling the twist of ring D in PCB β82 bilins, as well as the larger impact of fluctuations on the excited states of MBV pigments, which possess a higher conjugation length compared to other bilin types. Overall, the dual MD protocol combined with the FCE formalism yields excellent spectral properties for PC612 and PC645, and the resultant excitonic Hamiltonians pave the way for future investigations concerning the implications of open and closed quaternary structures on phycocyanin light harvesting properties.

Nonlinear self-action of ultrashort guided exciton–polariton pulses in dielectric slab coupled to 2D semiconductor

Recently reported large values of exciton–polariton nonlinearity of transition metal dichalcogenide (TMD) monolayers coupled to optically resonant structures approach the values characteristic for GaAs-based systems in the regime of strong light-matter coupling. Contrary to the latter, TMD-based polaritonic devices remain operational at ambient conditions and therefore have greater potential for practical nanophotonic applications. Here, we present the study of the nonlinear properties of Ta2O5 slab waveguide coupled to a WSe2 monolayer. We confirm that the hybridization between the waveguide mode and the exciton resonance in WSe2 gives rise to the formation of guided exciton–polaritons with Rabi splitting of 36 meV. By measuring transmission of ultrashort optical pulses through this TMD-based polaritonic waveguide, we demonstrate the strong nonlinear dependence of the output spectrum on the input pulse energy. We develop a theoretical model that shows agreement with the experimental results and gives insights into the dominating microscopic processes which determine the nonlinear pulse self-action: Coulomb exciton–exciton interaction and scattering to an incoherent excitonic reservoir. Based on the numerical simulation of nonlinear phenomena in our polariton system, we conclude that it may support a quasi-stationary solitonic regime of pulse propagation at intermediate pump energies. Our results provide an important step for the development of nonlinear on-chip polaritonic devices based on 2D semiconductors.

Greek Cross (+) Aggregate

The independent discovery of dye aggregate by Jelley and Scheibe marked the beginning of a spectacular development in the field of aggregate photophysics.1 Subsequent research warranted an exceptional model for defining the exciton interactions in aggregates, proposed by Kasha and Davydov independently, which was later modified by incorporating the short-range excitonic coupling.2-3 Apart from the well-studied H- and J-aggregate, achieving the orthogonally cross-stacked assembly possessing null excitonic interactions remained elusive. The first crystalline evidence for the chromophoric Greek cross (+) assembly exhibiting monomer-like optical properties was reported in 2018.5 The emergent photophysical properties of the crafted Greek cross (+) architectures were explored using both experimental and computational techniques for understanding the aggregate properties beyond Kasha’s model.6-7 The manifestation of selective hole transfer coupling in the edge-to-edge arranged Greek cross (+) perylenediimide (PDI) dimer resulted in ultrafast dissociation of null exciton and evolution of the charge-separated state in a polar solvent, providing a way forward in the fundamental understanding of null aggregates.8 References 1 (a) Jelley, E. E.; Nature, 1936, 138, 1009– 1010; (b) Scheibe, G.; Angew. Chem., 1937, 50, 212– 219; (c) Ramakrishnan, R.; Niyas, M. A.; Lijina, M. P.; Hariharan, M.; Acc. Chem. Res., 2019, 52, 3075−3086. 2 (a) Kasha, M.; Rawls, H. R.; El-Bayoumi, M.; Pure Appl. Chem., 1965, 11, 371–392; (b) Davydov, A. S.; Sov. Phys. Uspekhi; 1964, 7, 145– 178, 3 Hestand, N. J.; Spano, F. C.; J. Chem. Phys. 2015, 143, 244707. 4 Sebastian, E.; Philip, A. M.; Benny, A.; Hariharan, M.; Angew. Chem. Int. Ed., 2018, 57,15696- 15701. 5 Lijina, M. P.; Benny, A.; Ramakrishnan, R.; Nair, N. G.; Hariharan, M.; J. Am. Chem. Soc., 2020, 142, 17393-17402. 7 Benny, A.; Ramakrishnan, R.; Hariharan, M., Chem. Sci., 2021, 12, 5064– 5072. 8 Sebastian, E.; Hariharan, M.; J. Am. Chem. Soc., 2021, 143, 13769–13781; (b) Sebastian, E.; Hariharan, M.; ACS Energy Lett., 2022 , 7, 696−711. Figure 1

Boron Ketoimine Fluorophores with Unique Optical Properties: Multicolor Tuning, Efficient Dual‐State Emission, and Reversible Acid‐Base Response

AbstractOrganic luminophores with high emission efficiency in solution as well as solid‐state have received increasing attention due to their promising applications in biosensors, biological imaging, and optoelectronic devices. Herein, four triphenylamine (TPA)‐substituted boron ketoimine molecules (TPA‐BKI a‐d) are designed and synthesized by combining a donor‐acceptor (D‐A) electronic system and highly twisted block. The asymmetry of molecular and the planarity of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) sides in the excited state endow TPA‐BKI a‐d with rigidified conformations, allowing bright emission in dilute solution. Additionally, their highly twisted structures avoid close contact between molecules and detrimental exciton interactions, leading to efficient emission in the solid state. Meanwhile, the emission colors are modulated by the incorporation of different substituents through the amino‐yne click reaction. Furthermore, because of the protonation and deprotonation of the amino substituents, TPA‐BKI‐d exhibits reversible alterations of optical properties in both solution and solid states.

Influence of vibronic interaction of charge transfer excitons in PTB7/BTA-based nonfullerene organic solar cells.

We studied photoinduced charge transfer (CT) states and their dissociation processes at the donor/acceptor (D/A) interface of PTB7/BTAx (x = 1 and 3) nonfullerene organic thin-film solar cells using density functional theory (DFT) and time-dependent DFT calculations. We focused on the CT distances and electron coupling in the CT state generated by photoexcitation and the Huang-Rhys (HR) factors that describe the nonadiabatic processes associated with vibronic interactions. The PTB7/BTA3 system with a large short-circuit current density (JSC) exhibited a large charge CT distance and electronic coupling. Contrastingly, the PTB7/BTA1 system with a low JSC has a large HR factor because of the low-wavenumber vibrational modes in the CT state of the D/A complex and is prone to nonadiabatic relaxation to the ground state. Systematic theoretical analysis of the excitonic states in the D/A complex has provided insight into the control of CT exciton dynamics, namely JSC and electron-hole recombination.

Subtle Structural Change in the Exciton-Forming Dye Drives Plasmon–Exciton Interaction Involving Gold Nanorods into the Strong Coupling Regime: Implications for Enhanced Optical Processes

Subtle Structural Change in the Exciton-Forming Dye Drives Plasmon–Exciton Interaction Involving Gold Nanorods into the Strong Coupling Regime: Implications for Enhanced Optical Processes

High-temperature phonon-assisted upconversion photoluminescence of monolayer WSe2

Phonon-assisted upconversion photoluminescence (UPL) is an anti-Stokes process emitting photons of energy higher than the excitation photons, with upconversion energy gain provided by optical phonons. Atomically thin transition metal dichalcogenides provide a promising platform for exploring the phonon-assisted UPL process due to their strong phonon–exciton interactions. Here, high-temperature phonon-assisted UPL process in monolayer WSe2 is investigated, aiming to understand the role of phonon population and the number of phonons involved in the UPL process at elevated temperatures. It is demonstrated that the integrated intensity of UPL emission significantly increases by two orders of magnitude as the temperature rises from room temperature of 295 to 476 K, which is distinguished from the photoluminescence emission usually suffering from thermal quenching. The observed growth of UPL emission intensity is attributed to both the increased phonon population and the reduced number of phonons required at elevated temperatures. Our study paves the way toward near-infrared light detection, anti-Stokes energy harvesting, optical refrigeration, and temperature sensing.

Interspecies exciton interactions lead to enhanced nonlinearity of dipolar excitons and polaritons in MoS2 homobilayers

Nonlinear interactions between excitons strongly coupled to light are key for accessing quantum many-body phenomena in polariton systems. Atomically-thin two-dimensional semiconductors provide an attractive platform for strong light-matter coupling owing to many controllable excitonic degrees of freedom. Among these, the recently emerged exciton hybridization opens access to unexplored excitonic species, with a promise of enhanced interactions. Here, we employ hybridized interlayer excitons (hIX) in bilayer MoS2 to achieve highly nonlinear excitonic and polaritonic effects. Such interlayer excitons possess an out-of-plane electric dipole as well as an unusually large oscillator strength allowing observation of dipolar polaritons (dipolaritons) in bilayers in optical microcavities. Compared to excitons and polaritons in MoS2 monolayers, both hIX and dipolaritons exhibit ≈ 8 times higher nonlinearity, which is further strongly enhanced when hIX and intralayer excitons, sharing the same valence band, are excited simultaneously. This provides access to an unusual nonlinear regime which we describe theoretically as a mixed effect of Pauli exclusion and exciton-exciton interactions enabled through charge tunnelling. The presented insight into many-body interactions provides new tools for accessing few-polariton quantum correlations.

Vliyanie tolshchiny inkapsuliruyushchikh sloev na kachestvo geterostruktur na osnove MoSe2

The dependence of the width of exciton and trion photoluminescence lines in MoSe2 monolayers on the thickness of hexagonal boron nitride encapsulating layers has been investigated. The possibility of variation of the exciton photoluminescence linewidth due to the interaction of excitons with the modes of resonator cavities made up of a silicon substrate and a boron nitride top layer has been checked. This interaction may significantly change the photoluminescence linewidth owing to the Parcell effect. Measurements taken of samples with different thicknesses of the bottom and top boron nitride layers have not revealed any influence of the Parcell effect on the linewidth. It has turned out however that the linewidth narrows by several times with increasing boron nitride bottom layer thickness from 10 to 100 nm and reaches 2 meV at a thickness of 100 nm. Supposedly, such narrowing of the photoluminescence line is associated with a decrease in the density of submicron bubbles, which takes place at longitudinal stress relaxation in the thicker layer of boron nitride.

Observation of an Exciton-Plasma Transition in a Molecular Semiconductor.

The nonfullerene electron acceptors (NFAs) for organic solar cells are attracting intense research efforts due to their impressive performance. Understanding the temporal evolution of the excited states in NFAs is essential to gain insights into the working mechanism of these state-of-the-art devices. Here we characterized the photoconductivities of a neat Y6 film and a Y6:PM6 blend film using time-resolved terahertz spectroscopy. Three different types of excited states were identified based on their distinct terahertz responses, i.e., plasma-like carriers, weakly bound excitons, and spatially separated carriers. Under high-intensity excitation, the many-body interaction of excitons in the Y6 film leads to the plasma-like state, giving rise to a terahertz response characteristic for a dispersive charge transport. This transient state decays quickly into exciton gas due to fast Auger annihilation. Under low-intensity excitation, only isolated excitons are created and the plasma state is absent.

Plasmon–Exciton Strong Coupling Effects of the Chiral Hybrid Nanostructures Based on the Plexcitonic Born–Kuhn Model

A deep understanding of the optical properties of the chiral plexcitonic systems not only adds a new dimension to the exploration of the strong plasmon–exciton interaction but also provides a new method to design chiroptical devices. We develop a plexcitonic Born–Kuhn model to explore the strong coupling dynamics of the chiral hybrid nanostructures made of corner-stacked nanorod dimers and molecules from the chiroptical perspective. As a consequence of strong interaction between plasmons and excitons, double normal mode splittings/Rabi splittings and an anti-crossing phenomenon appear in circular dichroism (CD) spectroscopy. The character of dual plexciton (symmetric and anti-symmetric plexciton) and associated chiroptical properties have been demonstrated. The interplay between mirror symmetry breaking and plasmon–plasmon/plasmon–exciton interaction leads to an optimal configuration with the maximal chiroptical response. Moreover, chirality (of incident light) selective excitation of plexcitonic modes has been achieved by structural design. Finally, we verify our theory experimentally by synthesizing chiral plasmonic nanodimers and coating these structures with J-aggregate dye molecules.

Electrical control of hybrid exciton transport in a van der Waals heterostructure

Interactions between out-of-plane dipoles in bosonic gases enable the long-range propagation of excitons. The lack of direct control over collective dipolar properties has so far limited the degrees of tunability and the microscopic understanding of exciton transport. In this work we modulate the layer hybridization and interplay between many-body interactions of excitons in a van der Waals heterostructure with an applied vertical electric field. By performing spatiotemporally resolved measurements supported by microscopic theory, we uncover the dipole-dependent properties and transport of excitons with different degrees of hybridization. Moreover, we find constant emission quantum yields of the transporting species as a function of excitation power with radiative decay mechanisms dominating over nonradiative ones, a fundamental requirement for efficient excitonic devices. Our findings provide a complete picture of the many-body effects in the transport of dilute exciton gases, and have crucial implications for studying emerging states of matter such as Bose–Einstein condensation and optoelectronic applications based on exciton propagation.

CdS/CdSe/CdS Spherical Quantum Wells with Near-Unity Biexciton Quantum Yield for Light-Emitting-Device Applications

Compared to zero-dimensional (0D) semiconductor quantum dots, 2D semiconductor nanoplatelets (NPLs) offer a spectrally narrow luminescence and superior absorption coefficients, which makes this geometry an attractive candidate for optoelectronic applications. However, optical devices based on NPLs still suffer from nonradiative Auger decay of multiple excitons (MX), which limits the efficiency of the processes, including MX luminescence, electroluminescence, and optical gain. Here, we demonstrate that Auger recombination is strongly suppressed in spherically shaped nanoplatelets, called quantum shells (QSs), where a relaxed confinement of charges leads to diminished exciton–exciton interactions. In particular, we use single photon counting and photon correlation spectroscopy to show that two-dimensional CdS/CdSe/CdS core/shell/shell spherical QSs reach near-unity biexciton emission yield. The Auger suppression was found most prominent in QSs with the largest shell diameter. A combination of ultralong (>15 ns) biexciton (BX) emission lifetimes and strong exciton–exciton repulsion in these QS samples allowed demonstrating low-threshold amplified spontaneous emission (ASE), large modal gain, and microcavity lasing featuring sharp emission modes at the BX and MX transitions. Finally, by introducing QSs within perovskite matrices, we achieved a strong electroluminescence enhancement in light-emitting devices, yielding devices as bright as 213 W/m2 and a 2.3-fold enhancement of the maximum external quantum efficiency. These results represent a major step toward realizing solution-processed colloidal lasers and LEDs.

PEG effects on excitonic properties of LH2 from Rhodobacter sphaeroides 2.4.1 in different environments

The effects of Polyethylene glycol (PEG) on B800 and B850 excitonic properties of the Light-Harvesting complex 2 (LH2) from Rhodobacter (Rba.) sphaeroides 2.4.1 in micellar solutions of detergents and phospholipid bilayers were investigated by a series of spectroscopies. Compared to the micellar solution, in the lipid bilayer, the lateral force applied on LH2 changed the excitonic structure of B850 and weakened the excitonic interaction between B800 molecules. In micelles, PEG addition causes LH2 aggregation, resulting in the enhancement of B850 excitonic coupling inter-complexes and conformational change of the bound carotenoid (Car). In liposomes, PEG addition causes ∼37 % fluorescence quenching of B850 excitons in the embedded LH2s by changing the surrounding hydration state.

Separating single- from multi-particle dynamics in nonlinear spectroscopy.

Quantum states depend on the coordinates of all their constituent particles, with essential multi-particle correlations. Time-resolved laser spectroscopy1 is widely used to probe the energies and dynamics of excited particles and quasiparticles such as electrons and holes2,3, excitons4-6, plasmons7, polaritons8 or phonons9. However, nonlinear signals from single- and multiple-particle excitations are all present simultaneously and cannot be disentangled without a priori knowledge of the system4,10. Here, we show that transient absorption-the most commonly used nonlinear spectroscopy-with N prescribed excitation intensities allows separation of the dynamics into N increasingly nonlinear contributions; in systems well-described by discrete excitations, these N contributions systematically report on zero to N excitations. We obtain clean single-particle dynamics even at high excitation intensities and can systematically increase the number of interacting particles, infer their interaction energies and reconstruct their dynamics, which are not measurable via conventional means. We extract single- and multiple-exciton dynamics in squaraine polymers11,12 and, contrary to common assumption6,13, we find that the excitons, on average, meet several times before annihilating. This surprising ability of excitons to survive encounters is important for efficient organic photovoltaics14,15. As we demonstrate on five diverse systems, our procedure is general, independent of the measured system or type of observed (quasi)particle and straightforward to implement. We envision future applicability in the probing of (quasi)particle interactions in such diverse areas as plasmonics7, Auger recombination2 and exciton correlations in quantum dots5,16,17, singlet fission18, exciton interactions in two-dimensional materials19 and in molecules20,21, carrier multiplication22, multiphonon scattering9 or polariton-polariton interaction8.

Bovine serum albumin prevents human hemoglobin aggregation and retains its chaperone-like activity

This study investigates the ability of bovine serum albumin (BSA) to act as an extracellular chaperone (EC) on human hemoglobin (Hb) at a pH of 7.4. The best temperature for studying this behavior was determined by analyzing Hb’s aggregation kinetics at multiple temperatures. 55 °C was chosen as the optimal temperature for forming Hb amyloids. BSA was then tested at various concentrations (20–100 μM) to assess its chaperone-like activity on Hb at 55 °C. At a concentration of 100 μM, BSA exhibits chaperone-like activity with a client protein:BSA ratio of 1:10. The high ratio implies that the chaperone activity of BSA is favored by the effects of macromolecular crowding. The results showed that BSA has the potential to inhibit Hb’s dissociation into alpha and beta subunits and protein aggregation by inhibiting secondary nucleation. BSA also causes the depolymerization of fibrils over time. The results were validated using molecular docking and all-atom molecular dynamics simulations. MD analysis such as RMSD, RMSF, Rg, SASA, Hydrogen bond, PCA, Free energy landscape (FEL) revealed that the stability of hemoglobin is greater when it is bound to BSA compared to unbound state. The study suggests that BSA can potentially bind to Hb dimers and reduce excitonic interactions, which reduces Hb aggregation. These results are consistent with the aggregation kinetics experiments. Communicated by Ramaswamy H. Sarma

Organic quantum materials: A review

AbstractInterests in organic quantum materials (OQMs) have been explosively growing in the field of condensed physics of matter due to their rich chemistry and unique quantum properties. They are strongly correlated systems and show novel electromagnetic performance such as high‐temperature superconducting, quantum sensing, spin electronics, quantum dots, topological insulating, quantum Hall effects, spin liquids, qubits, and so forth, which exhibit promising prospects in information communication and thus facilitate the construction of a modern intelligent society. This article reviews recent developments in the research on the electromagnetic characteristics of OQMs. We mainly give an overview on the progress of superconductors and quantum spin liquids based on organic materials and describe their possible mechanisms. Numerous experimental findings exhibit new exciton interactions and provide insights into exotic electronic properties. Finally, their association and strategies for realizing multiple quantum states in one system are discussed.

Lateral quantum confinement regulates charge carrier transfer and biexciton interaction in CdSe/CdSeS core/crown nanoplatelets

Charge carrier dynamics essentially determine the performance of various optoelectronic applications of colloidal semiconductor nanocrystals. Among them, two-dimensional nanoplatelets provide new adjustment freedom for their unique core/crown heterostructure. Herein, we demonstrate that by fine-tuning the core size and the lateral quantum confinement, the charge carrier transfer rate from the crown to the core can be varied by one order of magnitude in CdSe/CdSeS core/alloy-crown nanoplatelets. In addition, the transfer can be affected by a carrier blocking mechanism, i.e., the filled carriers hinder further possible transfer. Furthermore, we found that the biexciton interaction is oppositely affected by quantum confinement and electron delocalization, resulting in a non-monotonic variation of the biexciton binding energy with the emission wavelength. This work provides new observations and insights into the charge carrier transfer dynamics and exciton interactions in colloidal nanoplatelets and will promote their further applications in lasing, display, sensing, etc.

AB Bilayer graphene in the external magnetic field

This paper is dedicated to studying the magnetic field effects on the physical properties of the AB bilayer graphene. We consider the generalized Hubbard model to study the physical properties in the AB staked bilayer graphene exposed to the external magnetic field and applied electric field potential. We calculated a series of physical parameters in the system numerically as a function of the magnetic field and electric field potential. Different site-filling regimes have been considered and their influence on the behavior of the calculated physical parameters has been studied. We show that at the half-filling and high magnetic field values the charge neutrality is attained in the bilayer graphene, while at the partial-filling the system gets only charge-stabilized. Moreover, for both filling regimes, we have found a narrow region of the magnetic field parameter, where the excitonic pairing interaction survives.