Weak Exciton-phonon Coupling Research Articles

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.

Elastic Phonon Scattering Dominates Dephasing in Weakly Confined Cesium Lead Bromide Nanocrystals at Cryogenic Temperatures.

Cesium lead halide perovskite nanocrystals (PNCs) have emerged as a potential next-generation single quantum emitter (QE) material for quantum optics and quantum information science. Optical dephasing processes at cryogenic temperatures are critical to the quality of a QE, making a mechanistic understanding of coherence losses of fundamental interest. We use photon-correlation Fourier spectroscopy (PCFS) to obtain a lower bound to the optical coherence times of single PNCs as a function of temperature. We find that 20 nm CsPbBr3 PNCs emit nearly exclusively into a narrow zero-phonon line from 4 to 13 K. Remarkably, no spectral diffusion is observed at time scales of 10 μs to 5 ms. Our results suggest that exciton dephasing in this temperature range is dominated by elastic scattering from phonon modes with characteristic frequencies of 1-3 meV, while inelastic scattering is minimal due to weak exciton-phonon coupling.

Temperature-dependent photoluminescence properties of C(carbon)-aided ZnO nanorod arrays on (1 0 0) Si substrate

We investigate the photoluminescence (PL) spectra of ZnO nanorod (NR) and C(carbon)-aided NR arrays from 10 to 300 K. The activation energy of the donor-bound exciton of the ZnO NRs was determined to be 11.3 meV ranging over 10–300 K. The decreasing behavior of the PL intensity for the C(carbon)-aided ZnO NRs exhibited two activation energies, i.e., 3.2 and 65.8 meV at T < 70 K and T > 70 K, respectively. The activation energy for the donor-bound exciton decreased because of stronger quantum confinement in the C(carbon)-aided ZnO NRs than that of the ZnO NRs. We attribute these results to lower lattice mismatch, a larger surface-to-volume ratio, and the dense structure of the vertically orientated hexagonal pillars with flat faceted surfaces in the C(carbon)-aided ZnO NRs. A considerable exciton binding energy of 65.8 meV from the C(carbon)-aided ZnO NR arrays produced luminescence stably at 300 K. In addition, PL spectra demonstrated that the luminescence intensities of the C(carbon)-aided ZnO NRs were higher than that of the ZnO NRs because of weak exciton-phonon coupling. The higher PL intensities of the C(carbon)-aided ZnO NRs suggest that these structures might feature improved performance in optoelectronic nano-devices manufactured from ZnO NRs.

Weak Exciton–Phonon Coupling in CdSe Nanoplatelets from Quantitative Resonance Raman Intensity Analysis

Resonance Raman spectra, cross sections, and depolarization ratios have been measured for 4.5 monolayer thick CdSe nanoplatelets dispersed in chloroform. Five excitation wavelengths between 514.5 and 476.5 nm were employed. The resonance Raman spectra are dominated by the longitudinal optical (LO) phonon near 201 cm–1 and its overtone, as in CdSe quantum dots. The absolute scattering intensity is much higher for excitation on resonance with the sharp, lowest-energy heavy-hole to conduction band transition than with higher energy transitions, decreasing by about a factor of 30 between 514.5 and 496.5 nm excitation. The LO phonon overtone is weak directly on resonance with the heavy-hole transition but much stronger at higher excitation energies, a result that is reproduced by simulations of the spectra using standard resonance Raman intensity theory. The absolute Raman cross sections imply a Huang–Rhys parameter for the LO phonon of about 0.08 on resonance with the lowest heavy-hole transition. This is a f...

Size- and morphology-dependent optical properties of ZnS:Al one-dimensional structures.

Typical morphology substrates can improve the efficiency of surface-enhanced Raman scattering; the need for SERS substrates of controlled morphology requires an extensive study. In this paper, one-dimensional ZnS:Al nanostructures with the width of approximately 300 nm and the length of tens um, and micro-scale structures with the width of several um and the length of tens um were synthesized via thermal evaporation on Au-coated silicon substrates and were used to study their size effects on Raman scattering and photoluminescent spectra. The photoluminescence spectra reveal the strongest green emission at a 5 at% Al source, which originates from the Al-dopant emission. The Raman spectra reveal that the size and morphology of the ZnS:Al nanowires greatly influences the Raman scattering, whereas the Al-dopant concentration has a lesser effect on the Raman scattering. The observed Raman scattering intensity of the saw-like ZnS:Al nanowires with the width of tens nm was eight times larger than that of the bulk sample. The enhanced Raman scattering can be regarded as multiple scattering and weak exciton—phonon coupling. The branched one-dimensional nanostructure can be used as an ideal substrate to enhance Raman scattering.

Engineering Intrinsic and Extrinsic Quantum Interference in Electronic States of Carbon Nanotubes Measured By Resonant Raman Scattering

Examination of Raman resonant excitation profiles (REPs) of the radial breathing mode (RBM) and G-band phonons has provided a wealth of new insights into the coupling between electrons/excitons and phonons in single-wall carbon nanotubes. Very recently, new measurements of the REPs of the G-band phonon on highly pure, single semiconducting species SWCNT samples displayed a high degree of asymmetry when comparing the Raman intensity at the incident and scattered Raman resonances for either the first or second optical transitions. The REP asymmetry is due to a self-interference between Condon and non-Condon terms in Raman polarizability, where the effect of the non-Condon term becomes visible in the weak exciton-phonon coupling regime. This results in the observation that the Raman intensity at the scattered resonance is always lower in intensity than that of the incident resonance, a consequence of the failure of the Franck-Condon approximation. This is contrary to the symmetrical G-band REP expected by theory and observed for RBM REPs. At even higher energy optical transitions, quantum interference can occur between a pair of closely spaced electronic transitions, such as the third and fourth transitions in certain semiconducting species, where the scattered resonance of the lower energy optical transition interferes constructively or destructively with the incident resonance of the higher energy optical transition. This is on top of the already underlying self-interference effects stemming from the non-Condon contribution, creating a highly complex and rich energy structure for SWCNT Raman measurements of high energy phonon modes.Here, we examine the origin of these deviations from the expected symmetrical REPs for exciton-mediated, one-phonon Raman processes in SWCNTs, by expanding the previous measurements on the RBM and G-band phonons to include additional pure SWCNT species as a function of chiral angle, diameter, and electronic type. We measured the intrinsic non-Condon-induced self-interference of the G-band in the “metallic” family of SWCNTs, consisting of the so-called armchair and narrow-gap semiconductor (NGS)-type nanotube species. Specifically, we examined the diameter dependence of REP asymmetry of armchairs ranging from (5,5) to (8,8), as well as the NGS-species (7,4). The addition of these particular nanotube structures allows the examination of the effect of electronic type on REP behavior, where the unique linear band structure of armchair and quasi-linear band structure of NGS species produces additional electron-phonon coupling resonances through the so-called Kohn anomaly, further enhancing the observed REP asymmetry due to non-Condon-based self-interference. Preliminary data already suggests that armchair species exhibit the largest degree of G-band REP asymmetry relative to other species. Furthermore, NGS-type SWCNT species, such as the (7,4), possess the additional trigonal warping splitting term in their band structure, which causes each optical transition to split into two, allowing for additional quantum interference between upper and lower branch transitions.To further explore and control the degree of quantum interference between transitions possible, we applied mechanical uniaxial strain to single-chirality SWCNTs embedded in stretchable polymer films to further induce and enhance quantum interference for chiralities such as the (12,4), (8,6) and (7,4) and measured their RBM and G-band REPs as a function of strain. Theoretical calculations predict that the application of a few percent uniaxial strain will cause the third and fourth optical transitions in certain type-I semiconductor species and the trigonal-warping split transitions in NGS-type tubes to move toward one another in energy, thereby inducing quantum interference. In this situation, interference can be manipulated and, in some cases where quantum interference did not previously exist, created artificially, allowing a broader range of REP asymmetries to be examined.

Electronic excitation dynamics in multichromophoric systems described via a polaron-representation master equation

We derive a many-site version of the non-Markovian time-convolutionless polaron master equation [Jang et al., J. Chem Phys. 129, 101104 (2008)] to describe electronic excitation dynamics in multichromophoric systems. By treating electronic and vibrational degrees of freedom in a combined frame (polaron frame), this theory is capable of interpolating between weak and strong exciton-phonon coupling and is able to account for initial non-equilibrium bath states and spatially correlated environments. Besides outlining a general expression for the expected value of any electronic system observable in the original frame, we also discuss implications of the Markovian and Secular approximations highlighting that they need not hold in the untransformed frame despite being strictly satisfied in the polaron frame. The key features of the theory are illustrated using as an example a four-site subsystem of the Fenna-Mathews-Olson light-harvesting complex. For a spectral density including a localised mode, we show that oscillations of site populations may only be observed when non-equilibrium bath effects are taken into account. Furthermore, we illustrate how this formalism allows us to identify the electronic and vibrational components of the oscillatory dynamics.

Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...

Quantum dot Rabi rotations beyond the weak exciton–phonon coupling regime

We study the excitonic dynamics of a driven quantum dot under the influence of a phonon environment, going beyond the weak exciton–phonon coupling approximation. By combining the polaron transform and time-local projection operator techniques, we develop a master equation that can be valid over a much larger range of exciton–phonon coupling strengths and temperatures than in the case of the standard weak-coupling approach. For the experimentally relevant parameters considered here, we find that the weak-coupling and polaron theories give very similar predictions for low temperatures (below 30 K), while at higher temperatures we begin to see discrepancies between the two. This is because, unlike the polaron approach, the weak-coupling theory is incapable of capturing multiphonon effects, while it also does not properly account for phonon-induced renormalization of the driving frequency. In particular, we find that the weak-coupling theory often overestimates the damping rate when compared to the polaron theory. Finally, we extend our theory to include non-Markovian effects and find that, for the parameters considered here, they have little bearing on the excitonic Rabi rotations when plotted as a function of pulse area.

Exciton Dynamics of Molecular Aggregate Systems Composed of Triangular Lattice Units: Structural Dependence of Exciton Migration and Recurrence

The exciton dynamics in molecular aggregate models composed of triangular lattice units have been investigated using the quantum master equation approach including relaxation terms originating in weak exciton-phonon coupling. We observe directed migration and recurrence motion of exciton after the irradiation of a laser field. It is found that the triangular lattice aggregates tend to suppress the decoherence behavior and thus to exhibit more distinguished longer-term exciton recurrence than the similar size dendritic aggregates, although the former show less efficient exciton migration, that is, light-harvesting behavior, than the latter. These dynamical features of exciton in the triangular lattice aggregate models turned out to be characterized by the strong interactions among spatially close-packed monomers through the branching points, originating in the equilateral triangular structure, as well as by the fractal, tournament-like, architecture.

Quantum dynamics of exciton recurrence motion in dendritic molecular aggregates

The quantum master equation approach involving weak exciton–phonon coupling has been applied to the investigation of the exciton recurrence motion in molecular aggregate models with dendritic structures. Each monomer in the models is assumed to be a dipole unit (a two-state monomer) coupled with each other by the dipole–dipole interaction. Two types of the dendritic molecular aggregates, which have mutually different interactions through the branching points, are examined. A generation of one-exciton states by an electric field and its subsequent exciton dynamics with and without relaxation terms have been investigated. It is found that the configuration of each dipole unit significantly affects the coherent motions of the exciton population. We also suggest the possibility of controlling the coherent exciton motions by tuning the frequency of an external electric field.

Monte Carlo wavefunction approach to the exciton dynamics of molecular aggregates with exciton–phonon coupling

We develop the second-order Monte Carlo wavefunction (MCWF) approach to the exciton dynamics of molecular aggregate systems composed of dipole-coupled two-state monomers. The explicit form of Lindblad operator, which is indispensable for applying the MCWF approach, for population relaxation among exciton states is derived based on the quantum master equation involving weak exciton–phonon coupling. The exciton migration behaviors obtained by the MCWF approach are turned out to coincide with those by the conventional master equation approach, indicating the high potential of the MCWF approach to the dissipative exciton dynamics of extended molecular aggregates or supermolecules.

Excitonic and vibronic structure of absorption spectra of Me-PTCDI and PTCDA crystals

The excitonic and vibronic spectra (exciton + one quantum of intramolecular vibration) of Me-PTCDI and PTCDA crystals are studied in the case of strong mixing of a Frenkel exciton (FE) and charge-transfer excitons (CTEs). The linear optical susceptibility is calculated in the framework of dynamical theory of vibronic spectra. The absorption spectra of both crystals have been modelled. The positions, maximal values and integral intensity of the absorption peaks which correspond to the bound exciton–phonon states and to unbound (many-particle) states have been calculated using FE and CTEs’s parameters of Me-PTCDI and PTCDA crystals. The calculated spectra show: (i) the possible recovering of excitonic and vibronic regions; (ii) bigger integral intensity of many-particle states in the case of weak exciton–phonon coupling; (iii) the dominant role of the bound states in the case of intermediate and strong exciton–phonon coupling.

Real-time spectroscopy of pseudoisocyanine J-aggregates with sub-5 fs lasers

The oscillations of the absorbance change due to molecular vibrations in pseudoisocyanine bromide J-aggregates were real-time resolved with the sub-5-fs pulses. In spite of weak exciton–phonon coupling in the Frenkel exciton system, it was possible to observe the modulation of difference absorbance because of the enhanced transition moment induced by the break down of the Condon approximation. The non-Condon-type exciton–phonon coupling was invoked to explain the oscillatory signals and the modulation of transition-dipole moment were evaluated. Together with the intramolecular molecular vibrational modes, two lower frequency modes than 100cm−1 were observed. The band at 38cm−1 was assigned to an optical phonon mode. A band at 152cm−1 considered to be low intramolecular vibrational mode was also observed. Because of low frequency this mode had not been previously detected in Raman scattering.

Exciton dynamics in nanostar dendritic systems using a quantum master equation approach: core monomer effects and possibility of energy transport control

The directional energy transport, i.e. exciton migration, in nanostar dendritic systems composed of two-state monomer units is studied using a quantum master equation approach. We examine the effects of the variation in the excitation energy of the monomer in the core region (core monomer) on the multistep exciton migration from the periphery to the core based on the relaxation factors among exciton states originating in weak exciton–phonon coupling. It turns out that when the core monomer possesses both an excitation energy slightly lower than that of the first generation and a partial exciton overlap with the first generation, more efficient and rapid exciton migration to the core is expected as compared with other core monomer cases with the energy level closer to or much lower than that of the first generation.

Sub-5-fs two-dimensional spectroscopy of pseudoisocyanine J-aggregates

The oscillations of the absorbance change due to molecular vibrations in pseudoisocyanine bromide J-aggregates were real-time resolved with sub-5-fs laser pulses. In spite of weak exciton–phonon coupling in the Frenkel exciton system, it was possible to observe the modulation of difference absorbance because of the enhanced transition moment. The non-Condon-type exciton–phonon coupling was invoked to explain the oscillatory signals and the transition dipole modulations were evaluated. The band at 38 cm −1 was assigned to optical phonon mode, and the band at 152 cm −1, which had never been detected in Raman scattering, was also real-time resolved.

EXCITON MIGRATION IN DENDRITIC AGGREGATE SYSTEMS USING THE QUANTUM MASTER EQUATION APPROACH INVOLVING WEAK EXCITON-PHONON COUPLING

The quantum master equation approach involving a weak exciton-phonon coupling is applied to the exciton migration dynamics of dendritic molecular aggregates modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of efficient exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton-phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by step-by-step transfer from the periphery to the core via multi-step exciton states. The same calculation method is applied to the exciton dynamics of a larger dendritic aggregate model, D127, modeled after the largest synthesized phenylacetylene dendrimer D127 in order to examine the dependencies of the exciton migration on the strength of the intermolecular interaction and the temperature of phonon bath. In the case of the relatively weak dipole-dipole coupling, exciton is not observed to migrate efficiently from the periphery to the core, while the largest exciton population is found to remain in the intermediate generations. This is ascribed to the fact that the thermal excitation to the higher exciton states significantly contributes to the exciton distribution in the equilibrium state when the weak intermolecular interaction reduces the energy difference between exciton states. This indicates that the intermolecular interaction is important not only for the overlaps of the exciton distributions between the exciton states, but also the relation between the exciton energy differences and thermal excitations, which spoil the distinct concentration of exciton distribution in the core generation. In the case of a low temperature, even if the intermolecular interaction is weak, the exciton population in the core region is found to be the largest in all the generations. This suggests that exciton tends to efficiently migrate from the periphery to the core when the temperature is sufficiently low. Implications of these theoretical results are discussed in relation to design of magneto-optical materials and other technological applications.

Theoretical study on the structural dependency of the exciton migration of a denritic molecular aggregate

We investigate an exciton migration of a dendritic molecular aggregate D25 involving two species of chromophores in order to elucidate the structural dependency of the light-harvesting property. We perform the exciton dynamics of D25 after the irradiation of an electric field using the master equation approach involving a weak exciton-phonon coupling. It is found that the feature of exciton migration of D25 is closely related to not only the energy differences between exciton states but also the spatial structure of a molecular aggregate.

Mechanism of exciton migration of dendritic molecular aggregate: a master equation approach including weak exciton–phonon coupling

A master equation approach including weak exciton–phonon coupling is applied to the exciton dynamics of a dendritic molecular aggregate modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton–phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by stepwise transfer from the periphery to the core via multi-step exciton states.

Exciton–exciton annihilation in linear molecular aggregates at low temperature

Exciton–exciton annihilation is observable in linear molecular aggregates at low temperature as the pump intensity rises in spite of the fact that, due to weak exciton–phonon coupling, the exciton mobility at low temperatures is expected to be slow. Under the circumstances outlined, the applicability of the bimolecular theory, which implies the approach of two excitons before they annihilate, seems to be questionable. We propose an alternative channel of excitonic annihilation expected to be an appropriate candidate in treating this effect and in estimating the corresponding rate. It appears to be inversely proportional to the cube of the localization length and reasonably fits the experimental data.