Exciton-phonon Coupling Constant Research Articles

Excitonic luminescence in the self assembled organic–inorganic quantum well crystal: (C4H3SC2H4–NH3)2[PbI4

In this article we focus on the optical properties of a luminescent two-dimensional organic–inorganic material (thiopheneethyl ammonium lead iodide) with the formula (C4H3SC2H4NH3)2[PbI4]. This compound exhibits a self-assembled nano-layered structure, forming a multi-quantum wells (QWs) structure. X-ray diffraction patterns revealed that thin films deposited by a spin coating method were highly oriented with the c-axis perpendicular to the substrate surface. The optical properties were investigated by, absorption, ellipsometry and photoluminescence spectroscopy. At room temperature a strong photoluminescence, due to exciton confined in the PbI quantum well, is found around 2.32eV. The Stokes shift was determined and compared with those found for homologous materials. The exciton-phonon interaction was also investigated by studying the temperature dependence of photoluminescence. The exciton-phonon coupling constants for both acoustic and optical phonons were extracted using a phenomenological model are more than one order of magnitude larger than in conventional QWs.

THE CHECKOUT AND VERIFICATION OF THEORY OF BIO-ENERGY TRANSPORT IN THE PROTEIN MOLECULES

The phosphorylation and de-phosphorylation reactions in the cell, through which the bio-energy is released from ATP hydrolysis in biological systems, are described in this paper. Firstly, the bio-energy is accepted by the vibrational amides in protein molecules in virtue of resonant mechanism of frequency or energy, and can transport along the protein molecules in soliton, which is formed by self-trapping of vibrational quantum (or exciton), by means of dipole–dipole interaction among the neighboring peptide groups (or amino acids). Theory and properties of bio-energy transport were proposed and described by many researchers. We here reviewed mainly the theories and features of Davydov's and Pang's models. However these theoretical models including Davydov's and Pang's model were all established based on a periodic and uniform proteins, which are different from practically biological proteins molecules. Therefore, it is necessary to inspect and verify the validity of the theory of bio-energy transport in real biological protein molecules. These problems were extensively studied by a lot of researchers using different methods in past thirty years, a considerable number of research results were obtained. I review here the situation and progresses of study on this problem, in which we reviewed the correctness of the theory of bio-energy transport including Davydov's and Pang's model and its investigated progresses under influences of structural non-uniformity and disorder, side groups and imported impurities of protein chains as well as the thermal perturbation and damping of medium arising from the biological temperature of the systems. The structural non-uniformity arises from the disorder distribution of sequence of masses of amino acid residues, side groups and imported impurities, which results in the changes and fluctuations of the spring constant, dipole–dipole interaction, exciton–phonon coupling constant, diagonal disorder or ground state energy and chain–chain interaction of the molecular channels in the dynamic equations in different models. The influences of structural non-uniformity, side groups and imported impurities as well as the thermal perturbation and damping of medium on the bio-energy transport in the proteins with single chain and three chains were studied by different numerical simulation techniques and methods including the average Hamiltonian way of thermal perturbation, fourth-order Runge–Kutta method, Monte Carlo method, quantum perturbed way and thermodynamic and statistical method, and so on. In this review, the numerical simulation results of bio-energy transport in uniform protein molecules, the influence of structural non-uniformity on the bio-energy transport, the effects of system temperature on the bio-energy transport and the simultaneous effects of structural non-uniformity, damping and thermal perturbation of proteins on the bio-energy transport in single chains and α-helical molecules were included and studied, respectively. The results obtained from these studies and reviews suggest that Davydov's soliton is unstable, but Pang's soliton is stable at physiologic temperature 300 K under influences of structural non-uniformity and disorder, side groups, imported impurities and damping of medium. Thus we can conclude that the soliton in Pang's model is exactly a carrier of the bio-energy transport and Pang's theory is appropriate to α-helical protein molecules. Finally we provided a few of experimental evidences for real existence of the soliton and validity of the theory of bio-energy transport in proteins and stated further the applications of the theory in living systems.

Unraveling exciton dynamics in amorphous silicon dioxide: Interpretation of the optical features from 8 to 11 eV

We introduce a model comprehensively describing the optical features of amorphous silicon dioxide ($a$-SiO${}_{2}$) in the spectral range from $~$8 up to $~$11 eV. Our model is grounded on the critical analysis of the temperature dependence of Kramers-Kronig-derived absorption spectra in the range from $~$8 up to $~$17.5 eV, together with the features of the Urbach absorption tail and of self-trapped exciton emission. In a paper we recently published [Phys. Rev. Lett. 105, 116401 (2010)] we showed the 10.4-eV resonance in the absorption spectra to feature a close Lorentzian line shape, thus implying a delocalized nature for excitons in $a$-SiO${}_{2}$. Here we provide estimations of the main parameters ruling exciton dynamics in SiO${}_{2}$, such as the energy of the mean lattice vibrational mode coupled to excitons ($\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=0.083$ eV), the half width of the excitonic energy band ($B~2$ eV), the root-mean-square amplitude of site-to-site energy fluctuations of exciton energy ($D~0.7$ eV), and the exciton-phonon coupling constant ($g~2.1$). The quantum yield of excitonic emission ($\ensuremath{\eta}~10{}^{\ensuremath{-}3}$) at $T=10$ K in $a$-SiO${}_{2}$ is determined as well. Our model suggests that $a$-SiO${}_{2}$ features an indirect gap near $~$9 eV and a direct one near $~$11 eV, and allows a coherent description of the properties of the intrinsic Urbach absorption tail. The latter results are satisfactorily explained as arising from the momentary self-trapping of the 10.4-eV exciton. As far as near-edge absorption properties are concerned, our model places SiO${}_{2}$ in the wider context of wide-band-gap solids, such as LiF or NaF, where excitons are weakly scattered, but strongly coupled to phonons. On the whole, the present study shows that exciton dynamics accounts for all optical properties of $a$-SiO${}_{2}$ from $~$8 up to $~$11 eV.

Difference in optical properties of anthracene molecule aggregates in fluorene and phenanthrene crystals

AbstractOptical properties of anthracene molecules in phenanthrene crystals with anthracene concentration of 0.03% to 8.4% were investigated and compared with those in fluorene crystals. In contrast to fluorene system where two kinds of dimers, one of which is accompanied by lattice relaxation, are observed, it was found that only one kind of dimers without lattice relaxation are observed in phenanthrene system. Also in aggregates of anthracene in higher concentration samples, the luminescence is a ‘free exciton’‐type in phenanthrene crystals in contrast to a ‘self‐trapped exciton’‐type in fluorene crystals. With reference to the variation of the magnitude of exciton–phonon coupling constant on nearest neighbour distance of molecules known for bulk crystals of aromatic molecules, the difference is attributed to longer inter molecular distance between anthracene molecules in phenanthrene matrix in comparison to those in fluorene matrix (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DEPENDENCE OF THE DAVYDOV'S SOLITON BEHAVIOR IN THE LINEAR POLYMER CHAIN ON THE EXCITON–PHONON COUPLING

The Davydov's soliton propagation in the linear polymer chain is analyzed numerically by solving the discrete equations of motion for exciton and phonon amplitudes. The main difference with respect to the results of continual approximation is that two exciton–phonon coupling constants influence separately the soliton behavior. Their influence is studied both in the ideal chain and the chain with single impurity.

The dynamics of a coherently driven localized exciton in a semiconducting single-wall carbon nanotube

The dynamics of a coherently driven localized exciton in a semiconducting single-wall carbon nanotube (SWNT) is investigated theoretically in terms of the perturbation treatment based on a unitary transformation. The decoherence rate of the exciton is obtained analytically and is compared with the one obtained with Born–Markov approximation. The results indicate that the decoherence strongly depends on the exciton–phonon coupling constant and the Rabi frequency of the coherently driving field. Furthermore, since the exciton–phonon coupling for SWNT is relatively strong and Born–Markov approximation becomes increasingly unreliable, we do need the numerically precise approach to treat the dynamics of the exciton in SWNT.

Influence of structure disorders and temperatures of systems on the bio-energy transport in protein molecules (II)

Influence of structure disorders and temperatures of systems on the bio-energy transport in protein molecules (II)

COMBINATION EFFECTS OF STRUCTURE NONUNIFORMITY OF PROTEINS ON THE SOLITON TRANSPORTED BIO-ENERGY

Combination effects of structure disorder of protein molecules containing the fluctuations of spring constant, dipole-dipole interaction constant and exciton-phonon coupling constant and diagonal disorder, resulting from nonuniform distribution of masses of amino acid residues and impurities, on the soliton transported the bio-energy in the proteins have been numerically simulated by fourth-order Runge–Kutta method in the improved model. The results obtained show that these structure disorders can change the states of solitons but as the solitons are quite robust against these disorder effects, they can only be dispersed or disrupted in the cases of quite large structure disorders. From these results and the properties of molecular structure of biological proteins we can conclude that the new soliton in the improved model is quite stable in normal conditions. Thus the soliton is possibly a carrier of bio-energy transport in the protein molecules.

Nonperturbative approach to the calculation of multiphonon Raman scattering in semiconductor quantum dots: Polaron effect

Based on the polaron concept, a general nonperturbative approach to the calculation of the multiphonon Raman scattering cross section for semiconductor quantum dots (QDs) is proposed. Within this concept, the Raman scattering process consists of two virtual transitions in the exciton-polaron spectrum, one with absorption and the other with emission of a photon, which change the QD ground state (exciton vacuum with different number of phonons in the beginning and in the end of the process). By applying the formalism to a set of resonantly excited CdSe spherical QDs, it is found that the overall scattering spectra differ from the perturbation theory predictions, especially in what concerns the relative intensities of scattering processes of different order. The Raman spectra calculated in the framework of the polaron picture, without using any fitting parameters, show a better agreement with previously published experimental results, compared to those obtained within the perturbation theory approach. These results suggest that, even for materials with relatively small exciton-phonon coupling constants, the polaron effects can play an important role in the multiphonon Raman scattering of light by QDs.

Excitonic dynamics in a single quantum dot within a microcavity

The excitonic dynamics in a single quantum dot within a microcavity is investigated theoretically in terms of the perturbation treatment based on a unitary transformation. The excitonic Rabi oscillations with a damping rate are obtained explicitly. It is shown that the damping rate depends on exciton-phonon coupling constant and the vacuum Rabi frequency. Even at absolute zero temperature and in the vacuum cavity, the exciton-phonon interaction and exciton-photon interaction still cause excitonic decoherence. We also predict that under strong control fields the collapse and revival of excitonic Rabi oscillation may be observed for the quantum dots with small exciton-phonon coupling in the $T\ensuremath{\rightarrow}0$ limit.

Is polaron effect important for resonant Raman scattering in self‐assembled quantum dots?

While the diagonal (or intra-level) interaction of a confined exciton with optical phonons in self-assembled quantum dots (SAQD's) is rather weak, the non-diagonal one can lead to a considerable change of the exciton spectrum and the formation of a polaron. An impact of this effect on resonant inelastic light scattering is studied theoretically. The polaron spectrum is obtained by numerical diagonalisation of the exciton–phonon interaction Hamiltonian in a truncated Hilbert space of the non-interacting excitons and phonons. Based on this spectrum, the probability of the multi-phonon Raman scattering is calculated, which is compared to that obtained within the standard perturbation theory approach (where phonon emission and absorption are irreversible). It is shown that there are two major effects of the polaron formation: (i) the intensity of the two-phonon (2 LO) peak, relative to that of the fundamental 1 LO one is strongly increased and (ii) the resonant behaviour of the 1 LO peak differs considerably from the perturbation theory predictions. With the correct theoretical interpretation, resonant Raman scattering in SAQD's opens the possibility of accessing the (renormalised) exciton spectrum and exciton–phonon coupling constants. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transport of excitation energy in a molecular aggregate. VIII. Numerical simulation of exciton processes in thylakoid membrane

We investigate the exciton dynamics in a molecular crystal. The dynamics is based on the exciton Hamiltonian, the phonon Hamiltonian, and the exciton–phonon interaction linear in phonon coordinates. Using the interaction picture, expressions were previously obtained for the rate of coherent migration of the exciton clothed by phonons, and the rate of incoherent or hopping motion. In this work we derive an expression for the clothed exciton propagator, which permits an explicit calculation of the hopping rate. Thus, the rates of exciton generation, coherent transfer, and hopping are determined completely from theory. These velocity constants, along with the experimental fluorescence decay constants and reaction rate constants for the excited molecules, are used to write a generalized master equation that describes the rate of change of exciton population at each site. The master equation can be numerically solved by using a time step of the order of a few femtoseconds, while the excited-state reactions and exciton transfer occur in the picosecond scale. Exciton dynamics is numerically simulated for a simple model of thylakoid membrane in green plants. The model is based on the known characteristics of thylakoid architecture. The membrane is divided into 97 zones, each zone in the bulk containing 1,442 chlorophylls, one P700, and one P680. The latter two pigments are randomly placed in each zone, while keeping their distance between 55 and 60 A. This accounts for the randomness in orientation. The disorder of the chlorophyll molecules within the domains of photosystems is neglected. Our findings are as follows. On average, about 6 million photons within the range of 655–681 nm pass through a membrane in 1 s. About 2.3% of incident photons are absorbed by the membrane chlorophyll molecules. For an excitation bandwidth of 70–175 cm−1, the coherent transfer rate between two adjacent molecules is 1.138 ± 0.488 ps−1. Because the Debye frequency is expected to be much smaller, the slow phonon limit applies. The exciton–phonon coupling constant that can be determined from the transition dipole–transition dipole interaction model leads to a hopping rate of 0.088 ps−1 at 300 K. A steady state is effectively achieved for each zone in the bulk in ∼5 ns. After a period of 50 ns, P700 and P680 in average trap at the rate of 650–702 and 629–679 excitons per second, respectively. The average rate of charge separation and subsequent reactions of each excited photosystem varies as 592–639 per second for PSI and 630–680 per second for PSII. These numbers lead to a reasonable estimate for the amount of glucose produced in each square meter of leaf area in a day. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

The dynamics of coherently driven exciton in a single quantum dot

The dynamics of coherently driven exciton dressing with phonons in a single semiconductor quantum dot is investigated theoretically in terms of the perturbation treatment based on a unitary transformation. The decoherence rate of the exciton is obtained analytically. The results show that the decoherence due to phonons strongly depends on the exciton–phonon coupling constant and the Rabi frequency of the coherently driving field and is a typical non-Markovian process.

Specific features in the temperature dependence of photoluminescence from CdTe/ZnTe size-quantized islands and ultrathin quantum wells

A study has been carried out of the temperature dependences of luminescence spectra on a large number of CdTe/ZnTe structures differing in average thickness, 〈L z〉=0.25–4 monolayers (ML), and CdTe layer geometry (continuous, island type). The influence of geometric features in the structure of ultrathin layers on linewidth, the extent of lateral localization of excitons, their binding energy, and exciton-phonon coupling is discussed. It is shown that in island structures there is practically no lateral exciton migration. The exciton-phonon coupling constant in a submonolayer structure has been determined, Γph=53 meV, and it is shown that in structures with larger average thicknesses Γph is considerably smaller. Substantial lateral exciton migration was observed to occur in a quantum well with 〈L z〉=4 ML, and interaction with acoustic phonons was found to play a noticeable part in transport processes. It has been established that the depth of the exciton level in a quantum well and structural features of an ultrathin layer significantly affect the temperature dependences of integrated photoluminescence intensity.

Quantum confinement induced collapse of a large to a small radius self-trapped exciton in a permethylated polysilane chain

Low-temperature absorption and fluorescence spectra of a series of permethylated oligosilanes SinMe2n+2 in their all-anti form permits the identification of Si7Me16 as the shortest chain capable of supporting a large-radius exciton in its relaxed lowest singlet excited state, while Si6Me14 and shorter chains only carry small-radius self-trapped excitons. The following approximate one-dimensional semiconductor characteristics are derived from the data: exciton band half-width, ∼3.0 eV; lattice relaxation energy, ∼1.8 eV; and exciton–phonon coupling constant in the infinite chain, ∼0.6. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 361–367, 1999

Influence of the fluctuations of polarization in molecular chains

We analyze the influence of the fluctuations of polarization on the soliton transfer in one-dimensional biexciton molecular chains. Important modifications appear on the model Hamiltonian with different exciton-phonon coupling constants. The parameters of the solitary excitations (length and energy) are calculated. We show that the fluctuations of dipoles will increase the soliton width and its rest energy. Finally, a brief study of the statistical properties (partition function and specific heat) is done.

Exciton scattering, κ selection rule, exciton bandwidth in pyrene microcrystallites, and lattice relaxation energy for the origin of V luminescence

The lowest 1L b singlet exciton band of pyrene crystal is studied. Analyzing excitation spectra of pyrene microcrystallites recorded at 2 K, we find that the exciton bandwidth increases superlinearly with microcrystallite size toward the maximum value 330 cm −1, which is interpreted as the bandwidth of the 1L b band in bulk crystal. Using the obtained bandwidth for bulk pyrene crystal and the exciton-phonon coupling constant, the lattice relaxation energy for the 1L b exciton is estimated to be 180 cm −1 and the self-trap depth for the V state 15 cm −1. The κ selection rule for the exciton creation is lost in small size microcrystallites, but it is recovered in microcrystallites larger than 50 Å. Comparing the exciton bandwidth data with those for anthracene microcrystallites, it is found that the exciton surface scattering, which occurs strongly in anthracene microcrystallites, does not play a significant role in pyrene microcrystallites. The surface scattering layer thickness in anthracene microcrystallites is found to be 30 Å.

Vibronic coupling with low-frequency vibrations in quasi-forbidden Frenkel exciton states

A simple dimer model is used to demonstrate that in weakly allowed Frenkel exciton states, where the factor-group splitting is largely due to charge transfer interactions, the exciton-phonon coupling constant may be considerably different for the two Davydov components. The model is applied to the S 1 transition of the naphthalene crystal and suggests a smaller Davydov splitting than normally assumed (60 cm −1 rather than 150 cm −1).

Exciton—phonon coupling and pressure-induced structural phase changes in coronene crystals

The exciton—phonon coupling constant in coronene crystals is determined to be 1.39 for the a-axis polarized band and 1.22 for the b-axis polarized lowest absorption band. The self-trap depth is estimated to be 250 cm −1. The pressure-induced structural phase change and temperature-induced phase change are studied from the changes in the luminescence spectral shapes and spectral shifts, which are also substantiated by X-ray diffraction patterns and microscopic observation of the crystal. The crystal structure above 1 GPa is assigned tentatively to be I-orthorhombic while it is monoclinic at ambient pressure.

Perturbation of the nonlinear optical response of a conjugated polymer by an adsorbate-induced electronic state

The coupling between ground state vibrations and excited electronic states in the polydiacetylene PTS has been investigated using stimulated inverse Raman scattering. Both Raman resonances and optical Stark effects have been measured and the interference between them has been used to determine several exciton—phonon coupling constants. An unexpected narrow (7.5 meV half-width) electronic features at ≈ 2.23 eV in PTS is observed. An even parity exciton in this spectral region has been postulated before by others. Our work demonstrates that this new electronic feature is resident on the polymer backbone, the same as the odd-parity exciton. This new feature must also possess some odd-parity character. The cause of this odd-parity contribution is likely a complex between molecular oxygen and the diacetylenic backbone.