Electron Exciton Research Articles

Vibronic Enhancement of Exciton Sizes and Energy Transport in Photosynthetic Complexes

Transport processes and spectroscopic phenomena in light harvesting proteins depend sensitively on the characteristics of electron-phonon couplings. Decoherence imposed by low-frequency nuclear motion generally suppresses the delocalization of electronic states, whereas the Franck-Condon progressions of high-frequency intramolecular modes underpin a hierarchy of vibronic Coulombic interactions between pigments. This Article investigates the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light-harvesting proteins, allophycocyanin (APC) and C-phycocyanin (CPC). Both APC and CPC possess three pairs of pigments (i.e., dimers) that undergo electronic relaxation on the subpicosecond time scale. Electronic relaxation is ~10 times faster in APC than in CPC despite the nearly identical structures of their pigment dimers. We suggest that the distinct behaviors of these closely related proteins are understood on the same footing only in a basis of joint electronic-nuclear states (i.e., vibronic excitons). A vibronic exciton model predicts well-defined rate enhancements in APC at realistic values of the site reorganization energies, whereas a purely electronic exciton model points to faster dynamics in CPC. Calculated exciton sizes (i.e., participation ratios) show that wave function delocalization underlies the rate enhancement predicted by the vibronic exciton model. Strong vibronic coupling and heterogeneity in the pigment sites are the key ingredients of the vibronic delocalization mechanism. In contrast, commonly employed purely electronic exciton models see heterogeneity as only a localizing influence. This work raises the possibility that similar vibronic effects, which are often neglected, may generally have a significant influence on energy transport in molecular aggregates and photosynthetic complexes.

Structure of host–guest complexes between dibenzo-18-crown-6 and water, ammonia, methanol, and acetylene: Evidence of molecular recognition on the complexation

Complexes of dibenzo-18-crown-6 (DB18C6, host) with water, ammonia, methanol, and acetylene (guest) in supersonic jets have been characterized by laser induced fluorescence (LIF), UV-UV hole-burning (UV-UV HB), and IR-UV double resonance (IR-UV DR) spectroscopy. Firstly, we reinvestigated the conformation of bare DB18C6 (species m1 and m2) and the structure of DB18C6-H(2)O (species a) [R. Kusaka, Y. Inokuchi, T. Ebata, Phys. Chem. Chem. Phys., 2008, 10, 6238] by measuring IR-UV DR spectra in the region of the methylene CH stretching vibrations. The IR spectral feature of the methylene CH stretch of DB18C6-H(2)O is clearly different from those of bare DB18C6 conformers, suggesting that DB18C6 changes its conformation when forming a complex with a water molecule. With the aid of Monte Carlo simulation for extensive conformational search and density functional calculations (M05-2X/6-31+G*), we reassigned species m1 and m2 to conformers having C(1) and C(2) symmetry, respectively. Also, we confirmed the DB18C6 part in species a of DB18C6-H(2)O to be "boat" conformation (C(2v)). Secondly, we identified nine, one, and two species for the DB18C6 complexes with ammonia, methanol, and acetylene, respectively, by the combination of LIF and UV-UV HB spectroscopy. From the IR spectroscopic measurement in the methylene CH stretching region, a similar conformational change was identified in the DB18C6-ammonia complexes, but not in the complexes with methanol or acetylene. The structures of all the complexes were determined by analyzing the electronic transition energies, exciton splitting, and IR spectra in the region of the OH, NH, and CH stretching vibrations. In DB18C6-ammonia complexes, an ammonia molecule is incorporated into the cavity of the boat conformation by forming "bifurcated" and "bidentate" hydrogen-bond (H-bond), similar to the case of the DB18C6-H(2)O complex. On the other hand, in the DB18C6-methanol and -acetylene complexes, methanol and acetylene molecules are simply attached to the C(1) and C(2) conformations, respectively. From the difference of the DB18C6 conformations depending on the type of the guest molecules, it is concluded that DB18C6 distinguishes water and ammonia from methanol and acetylene when it forms complexes, depending on whether guest molecules have an ability to form bidentate H-bonding.

Temperature-dependent study of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals by polarization-dependent piezoreflectance

The temperature dependence of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals were characterized by using polarization-dependent piezoreflectance (PzR) in the temperature range of 10–300 K. The PzR measurements were carried out on the as-grown basal plane with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The PzR spectra revealed polarization-dependent E ⊥ ex and E | | ex features for E⊥ c and Е||c polarization, respectively. Both E ⊥ ex and E | | ex features are associated with the interband excitonic transitions at Γ point and can be explained by crystal-field splitting of valence band. From a detailed lineshape fit to the PzR spectra, the temperature dependence of the transition energies and broadening parameters of the band-edge excitons were determined accurately. The temperature dependence of near band-edge excitonic transition energies were analyzed using Varshni and Bose–Einstein expressions. The temperature dependence of the broadening parameter of excitonic features also has been studied in terms of a Bose–Einstein equation that contains the electron (exciton)–longitudinal optical phonon-coupling constant. The parameters that describe the temperature variation of the excitonic transition energies and broadening parameters were evaluated and discussed.

Spin properties of trions in a dense 2DEG

AbstractReflectivity and photoluminescence spectra from CdTe/CdMgTe modulation‐doped quantum well structures were studied. We have found that in reflectivity spectra the value and the sign of the Zeeman splitting of the trion lines depend on the electron concentration in the quantum well, whereas, the value and sign of the exciton line splitting are constant for all studied electron concentrations. On the other hand, in the photoluminescence spectra the sign and value of the Zeeman splitting are the same for trion and exciton. Such “renormalization” of the trion g‐factor is explained in the model of combined exciton–electron processes.

Electric-field effect of near-band-edge photoluminescence in bulk ZnO

We have studied the effect of electric fields on the near-band-edge (NBE) emissions in bulk zinc oxide (ZnO) by using photoluminescence and photocurrent (PC) spectroscopy simultaneously. The intensity-quenching and peak-shift effects of the free exciton spectra were observed with increasing electric field. From the PC result, we find out that the free excitons are disturbed by the PC carriers of the photo-created electrons and holes. This disturbance reduces the recombination ratio and the lifetime of free excitons. Therefore, the intensity-quenching effect was attributed to the decrease in the recombination of free excitons. Thus, the shift of the free exciton peaks was related to Stark effect induced by electric field. As a result, we have found that these phenomena are caused to the exciton–electron scattering due to a strong interaction between the excitons in the conduction band and the photo-generated electron carriers with increasing the applied electric field.

Spin properties of trions in a dense 2DEG

Reflectivity and photoluminescence spectra from CdTe/CdMgTe modulation doped quantum well structures were studied. We have found that the value and the sign of the trion reflectivity line's Zeeman splitting depends on the electron concentration in the quantum well, whereas the value and sign of the exciton line splitting are absolutely equal for all studied electron concentrations. In the photoluminescence spectra the sign and value of the exciton and trion Zeeman splittings are found to be equal. Such "renormalization" of the trion g-factor is explained in the model of combined exciton electron processes.

Efficiency Improvement of Solution Processed Blue Phosphorescent Devices Using High Triplet Energy Electron Transport Layer

High efficiency solution processed blue phosphorescent organic light emitting diodes (PHOLEDs) were developed using a high triplet energy exciton blocking layer. A spirobifluorene-based phosphine oxide compound was used as the exciton blocking layer for poly(-vinylcarbazole) (PVK):iridium(III) bis[4,6-(difluorophenyl)-pyridinato-] picolinate (FIrpic) emitting layer. A high quantum efficiency of 12.6% and a high current efficiency of 22.9 cd/A were obtained from the PVK:FIrpic PHOLED due to efficient exciton blocking and electron injection.

Electronic Structure and E1 Excitons of CuInS2 Energy-Related Crystals Studied by Temperature-Dependent Thermoreflectance Spectroscopy

Single crystals of were grown by a chemical vapor transport method using as a transport agent. The electronic structure of was experimentally characterized by temperature-dependent thermoreflectance (TR) measurements in a wide energy range of 1.25–6 eV between 20 and 320 K. A pair of excitonic doublets of and near 3.4 eV were observed in the low temperature TR spectrum at 20 K. The temperature– energy shift of the and features identified the origin of the excitonic doublets. The excitons were confirmed to be interband transitions from the nonbonding Cu 3d state in the valence band to the S 3p antibonding state at the conduction band bottom. There were also a lot of derivative-like spectral features observed near the bandedge as well as high lying interband transitions of . The origins of the critical-point transitions of were evaluated. X-ray photoelectron spectroscopic (XPS) measurements were implemented to identify the band blocks below the top of the valence band. Based on the experimental analyses of TR and XPS, together with previous band-structure calculations of , an experimental band structure that describes a fundamental band-edge structure of was then constructed.

S 1 / S 2 excitonic splittings and vibronic coupling in the excited state of the jet-cooled 2-aminopyridine dimer

We analyze the vibronic band structure of the excitonically coupled S(1)<--S(0)/S(2)<--S(0) excitations of the 2-aminopyridine (2AP) self-dimer (2AP)(2), using a linear vibronic coupling model [R. Fulton and M. Gouterman, J. Chem. Phys. 41, 2280 (1964)]. The vibronic spectra of supersonically cooled (2AP)(2) and its (13)C-isotopomer were measured by two-color resonant two-photon ionization and UV/UV-depletion spectroscopies. In the C(2)-symmetric form of (2AP)(2), the S(1)<--S(0) ((1)A<--(1)A) transition is very weak, while the close-lying S(2)<--S(0) ((1)B<--(1)A) transition is fully allowed. A single (12)C/(13)C isotopic substitution breaks the symmetry of the dimer so that the (2AP)(2)-(13)C isotopologue exhibits both S(1) and S(2) electronic origins, which are split by 11 cm(-1). In Fulton-Gouterman-type treatments, the linear vibronic coupling is mediated by intramolecular vibrational modes and couplings to intermolecular vibrations are not considered. For (2AP)(2), a major vibronic coupling contribution arises from the intramolecular 6a(') vibration. However, the low-energy part of the spectrum is dominated by intermolecular shear (chi(')) and stretching (sigma(')) vibrational excitations that also exhibit excitonic splittings; we apply a linear vibronic coupling analysis for these also. The respective excitation transfer integrals V(AB) are 50%-80% of that of the intramolecular 6a(') vibration, highlighting the role of intermolecular vibrations in mediating electronic energy exchange. The S(1)/S(2) electronic energy gap calculated by the approximate second-order coupled-cluster method is approximately 340 cm(-1). This purely electronic exciton splitting is quenched by a factor of 40 by the vibronic couplings to the Franck-Condon active intramolecular vibrations.

Highly efficient organic light-emitting devices beyond theoretical prediction under high current density

We develop a simple method to improve external quantum efficiencies (EQEs) of OLEDs under a wide range of current density. An insulating inorganic ultrathin layer (LiF) was sandwiched between exciton formation layer and electron transporting layer. A maximal EQE of 5.9% in a DCM based fluorescent OLED, which far exceeds the theoretical upper limit of 3.7%, was obtained under the current density of 487 mA/cm(2) with a brightness maximum of 76740 cd/m(2). The similar electroluminescence properties were also obtained in a C545T based green OLED using this method. The overall enhancement of EQE, and the nonlinear enhancement of EQE at high current density in these devices are attributed to the effect of electrical field on excitons.

Measurement of Electron−Electron Interactions and Correlations Using Two-Dimensional Electronic Double-Quantum Coherence Spectroscopy

A two-dimensional (2D) optical coherent spectroscopy that correlates the double excited electronic states to constituent single excited states is described. The technique, termed two-dimensional double-quantum coherence spectroscopy (2D-DQCS), makes use of multiple, time-ordered ultrashort coherent optical pulses to create double and single quantum coherences over the time intervals between the pulses. The resulting 2D electronic spectra map out the energy correlation between the first excited state and two-photon-allowed double-quantum states. Measurements of organic dye molecules show that the near-resonant energy offset for adding a second electronic excitation to the system relative to the first excitation is on the order of tens of millielectronvolts. Simulations of DQC spectra show that vibronic transitions add rich features to the 2D spectra. The results of quantum chemical calculations on model systems provide insight into the many-body origin of the energy shift measured in the experiment. These results demonstrate the potential of 2D-DQCS for elucidating quantitative information about electron-electron interactions, many-electron wave functions, and electron correlation in electronic excited states and excitons.

Exciton−Exciton Correlations Revealed by Two-Quantum, Two-Dimensional Fourier Transform Optical Spectroscopy

The Coulomb correlations between photoexcited charged particles in materials such as photosynthetic complexes, conjugated polymer systems, J-aggregates, and bulk or nanostructured semiconductors produce a hierarchy of collective electronic excitations, for example, excitons, and biexcitons, which may be harnessed for applications in quantum optics, light-harvesting, or quantum information technologies. These excitations represent correlations among successively greater numbers of electrons and holes, and their associated multiple-quantum coherences could reveal detailed information about complex many-body interactions and dynamics. However, unlike single-quantum coherences involving excitons, multiple-quantum coherences do not radiate; consequently, they have largely eluded direct observation and characterization. In this Account, we present a novel optical technique, two-quantum, two-dimensional Fourier transform optical spectroscopy (2Q 2D FTOPT), which allows direct observation of the dynamics of multiple exciton states that reflect the correlations of their constituent electrons and holes. The approach is based on closely analogous methods in NMR, in which multiple phase-coherent fields are used to drive successive transitions such that multiple-quantum coherences can be accessed and probed. In 2Q 2D FTOPT, a spatiotemporal femtosecond pulse-shaping technique has been used to overcome the challenge of control over multiple, noncollinear, phase-coherent optical fields in experimental geometries used to isolate selected signal contributions through wavevector matching. We present results from a prototype GaAs quantum well system, which reveal distinct coherences of biexcitons that are formed from two identical excitons or from two excitons that have holes in different spin sublevels ("heavy-hole" and "light-hole" excitons). The biexciton binding energies and dephasing dynamics are determined, and changes in the dephasing rates as a function of the excitation density are observed, revealing still higher order correlations due to exciton-biexciton interactions. Two-quantum coherences due to four-particle correlations that do not involve bound biexciton states but that influence the exciton properties are also observed and characterized. The 2Q 2D FTOPT technique allows many-body interactions that cannot be treated with a mean-field approximation to be studied in detail; the pulse-shaping approach simplifies greatly what would have otherwise been daunting measurements. This spectroscopic tool might soon offer insight into specific applications, for example, in detailing the interactions that affect how electronic energy moves within the strata of organic photovoltaic cells.

Two-dimensional electronic double-quantum coherence spectroscopy.

The theory of electronic structure of many-electron systems, such as molecules, is extraordinarily complicated. A consideration of how electron density is distributed on average in the average field of the other electrons in the system, that is, mean field theory, is very instructive. However, quantitatively describing chemical bonds, reactions, and spectroscopy requires consideration of the way that electrons avoid each other while moving; this is called electron correlation (or in physics, the many-body problem for fermions). Although great progress has been made in theory, there is a need for incisive experimental tests for large molecular systems in the condensed phase. In this Account, we report a two-dimensional (2D) optical coherent spectroscopy that correlates the double-excited electronic states to constituent single-excited states. The technique, termed 2D double-quantum coherence spectroscopy (2D-DQCS), uses multiple, time-ordered ultrashort coherent optical pulses to create double- and single-quantum coherences over time intervals between the pulses. The resulting 2D electronic spectrum is a map of the energy correlation between the first excited state and two-photon allowed double-quantum states. The underlying principle of the experiment is that when the energy of the double-quantum state, viewed in simple models as a double HOMO-to-LUMO (highest occupied to lowest unoccupied molecular orbital) excitation, equals twice that of a single excitation, then no signal is radiated. However, electron-electron interactions, a combination of exchange interactions and electron correlation, in real systems generates a signal that reveals precisely how the energy of the double-quantum resonance differs from twice the single-quantum resonance. The energy shift measured in this experiment reveals how the second excitation is perturbed by both the presence of the first excitation and the way that the other electrons in the system have responded to the presence of that first excitation. We compare a series of organic dye molecules and find that the energy offset for adding a second electronic excitation to the system relative to the first excitation is on the order of tens of millielectronvolts; it also depends quite sensitively on molecular geometry. These results demonstrate the effectiveness of 2D-DQCS for elucidating quantitative information about electron-electron interactions, many-electron wave functions, and electron correlation in electronic excited states and excitons. Our work helps illuminate the implications of electron correlation on chemical systems. In a broad sense, we are trying to help address the fundamental question "How do we go beyond the orbital representation of electrons in the chemical sciences?"

Electronic States and Exciton Fine Structure in Colloidal CdTe Nanocrystals

We prepared colloidal zinc-blend CdTe nanocrystals (NCs) with sizes ranging from 1.85 to 6.6 nm. Temperature-controlled two-dimensional photoluminescence excitation spectroscopy was applied to inve...

Electron–phonon effects on Stark shifts of excitons in parabolic quantum wells: Fractional-dimension variational approach

Electron–phonon effects on Stark shifts of excitons in parabolic quantum wells are studied theoretically by using a fractional dimension method in combination with a Lee–Low–Pines-like transformation and a perturbation theory. The numerical results for the exciton binding energies and electron–phonon contributions to the binding energies as functions of the well width and the electric field in the Al 0.3Ga 0.7As parabolic quantum well structure are obtained. It is shown that both exciton binding energy and electron–phonon contributions have a maximum with increasing the well width. The binding energy and electron–phonon contribution decrease significantly with increasing the electric-field strength, in special in the wide-well case.

Electronic structure and exciton states in the freestanding ZnO nanorods

The electronic structure and exciton states of cylindrical ZnO nanorods with radius from 2 to 6 nm are investigated based on the framework of the effective-mass theory. Using the adiabatic approximation, the exciton binding energies taking account of the dielectric mismatch are solved exactly when the total angular momentum of the exciton states L=0 and L=±1. We find that the exciton binding energies can be enhanced greatly by the dielectric mismatch and the calculated results are almost consistent with the experimental data. Meanwhile, we obtain the optical transition rule when the small spin-obit splitting Δso of ZnO is neglected. Furthermore, the radiative lifetime and linear optical susceptibilities χ(w) of the exciton states are calculated theoretically. The theoretical results are consistent with the experimental data very well.

The role of different linear and non-linear channels of relaxation in scintillator non-proportionality

The non-proportional dependence of a scintillator's light yield on primary particle energy is believed to be influenced crucially by the interplay of non-linear kinetic terms in the radiative and non-radiative decay of excitations versus locally deposited excitation density. A calculation of energy deposition, − dE/ dx, along the electron track for NaI is presented for an energy range from several electron-volt to 1 MeV. Such results can be used to specify an initial excitation distribution, if diffusion is neglected. An exactly solvable two-channel (exciton and hole(electron)) model containing 1st and 2nd order kinetic terms is constructed and used to illustrate important features seen in non-proportional light-yield curves, including a dependence on pulse shaping (detection gate width).

Coherent Multidimensional Optical Probes for Electron Correlations and Exciton Dynamics: From NMR to X-rays

Over the past 15 years, researchers have extended the multidimensional techniques which originated with NMR in the 1970s to infrared and visible coherent spectroscopy. These advances have dramatically enhanced the temporal resolution from the microsecond to the femtosecond regime. NMR spectroscopists have developed principles for the design of pulse sequences that enhance selected spectral features and reveal desired dynamical events. Extending these principles to the optical regime offers numerous opportunities for narrowing the line shapes in specific directions, unraveling weak cross-peaks from otherwise congested spectra, and controlling the interferences between quantum pathways. We can achieve these enhancements by shaping the spectral and temporal profiles of the pulses. Pulse polarization shaping may lead to unique probes of time-dependent chirality. In this Account, we compare two types of signals. The first, the photon echo, is generated in the direction -k(1) + k(2) + k(3), and the second, double quantum coherence, is detected at +k(1) + k(2) - k(3). Here k(1), k(2), and k(3) are the wave vectors of the three incoming pulses in chronological order. We illustrate the novel information extracted from these signals by simulations of three physical systems. In the first system, spectra of GaAs semiconductor quantum wells provide a direct look at many-body electron correlation effects. We directly observe specific projections of the many-electron wave function, which we can use to test the quality of various levels of computational techniques for electronic structure. Secondly, the spectra of photosynthetic aggregates reveal couplings between chromophores, quantum coherence signatures of chromophore entanglement, and energy-transfer pathways. Using some fundamental symmetries of pulse polarization configurations of nonlinear signals, we can construct superpositions of signals designed to better distinguish among various coherent and incoherent exciton transport pathways and amplify subtle variations among different species of the Fenna-Matthews-Olson (FMO) antenna complex. Both of the first two applications require femtosecond pulses of light in the visible range. The third application demonstrates how resonant core spectroscopy may be used to generate core excitations that are highly localized at selected atoms. Such signals can monitor the motions of valence electron wavepackets in real space with atomic spatial resolution. These future X-ray applications will require attosecond bright X-ray sources, which are currently being developed in several labs. Common principles underlie these techniques in coherent spectroscopy for spins, valence electrons, and core electronic excitations, spanning frequencies from radiowaves to hard X-rays.

THEORETICAL STUDY ON OPEN-SHELL NONLINEAR OPTICAL MOLECULAR SYSTEMS AND A DEVELOPMENT OF A NOVEL COMPUTATIONAL SCHEME OF EXCITON DYNAMICS

This contribution firstly elucidates a structure–property relationship in third-order nonlinear optical molecular systems with singlet diradical characters. It turns out that the second hyperpolarizabilities (γ) of the singlet open-shell molecules with intermediate diradical characters are significantly enhanced as compared with those of closed-shell and pure diradical molecules. The hybrid density functional theory method, i.e. UBHandHLYP, is applied to the calculations of γ of dimer models composed of singlet diradical diphenalenyl molecules, which show a remarkable enhancement of γ per monomer as decreasing the intermolecular distance. The second contribution is concerned with a development of ab initio molecular orbital configuration-interaction-based quantum master equation (QME) approach. This is found to provide both coherent processes, e.g. dynamic polarization and exciton (electron–hole pair) recurrence motion, and incoherent processes, e.g. exciton migration, in molecular systems. Using this approach, the electron/hole dynamics for dynamic polarizabilities α(ω) are examined for several π-conjugated linear chain systems, and the structural dependences of α(ω) are elucidated.

Variations in the Quantum Efficiency of Multiple Exciton Generation for a Series of Chemically Treated PbSe Nanocrystal Films

We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 (+/-0.2) at a photon energy of approximately 4E(g) for films consisting of 3.7 nm NCs and between 1.1 and 1.6 (+/-0.1) at hnu approximately 5E(g) for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry.