Coherent Exciton-Phonon Coupling in CdSe/ZnS Nanocrystals Studied by Two-Dimensional Electronic Spectroscopy

Coherent exciton-phonon coupling in CdSe/ZnS nanocrystals have been investigated by temperature-dependent two-dimensional electronic spectroscopy (2DES) measurements. Benefiting from the ability of 2DES to dissect assembles in nanocrystal films, we have clearly identified experimental evidences of coherent coupling between exciton and phonon in CdSe/ZnS nanocrystals. In time domain, 2DES signals of excitonic transitions beat at a frequency resonant to a longitudinal optical phonon mode; in energy domain, phonon side bands are distinct at both Stokes and anti-Stokes sides. When temperature increases, phonon-induced exciton dephasing is observed with dramatic broadening of homogeneous linewidth. The results suggest exciton-phonon coupling is essential in elucidating the quantum dynamics of excitonic transitions in semiconductor nanocrystals.     

Effect of static and dynamic disorder on exciton mobility in oligothiophenes

The polarized optical absorption spectra of different quaterthiophene single crystals in the energy region of the exciton bands originating from the first molecular transition are reported as measured in the temperatures ranging from 7 to 140 K. The intrinsic higher mobility of the b-polarized 0-0 a(u) exciton both with respect to its replicas and to the a-polarized structures is demonstrated in high quality crystals at the lowest temperatures. The influence of structural disorder on mobility is discussed considering, for the different samples, the measured lineshape and linewidth of the absorption peaks, and the relative lineshift and intensity ratio between the 0-0 a(u) line and its first replica at the lowest temperature. The influence of dynamic disorder is discussed considering the lineshape and linewidth of the measured peaks as a function of temperature for both polarizations in the framework of the exciton-phonon coupling theory.     

Interaction between excitons determines the non-linear response of nanocrystals

The non-linear response of semiconductor quantum dots is investigated using three-pulse photon echo peak shift (3PEPS) experiments and simulations. The third-order non-linear response is modeled by a three-level system, utilizing Brownian oscillators to model the line-broadening functions. Our results show that biexciton formation and exciton–exciton scattering significantly influence the non-linear response of quantum dots. The exciton to biexciton excited state absorption pathways are also investigated for quantum dots with different crystal structures. Our calculations suggest that the probability of excited state absorption to the biexcitonic state is higher for zinc-blende structured nanocrystals.     

Temperature- and field-dependent energy transfer in CdSe nanocrystal aggregates studied by magneto-photoluminescence spectroscopy

The influence of temperature and applied magnetic fields on photoluminescence (PL) emission and electronic energy transfer (ET) of both isolated and aggregated CdSe nanocrystals was investigated. Following 400-nm excitation, temperature-dependent, intensity-integrated and energy-resolved PL measurements were used to quantify the emission wavelength and amplitude of isolated CdSe nanocrystals. The results indicated an approximately three-fold increase in PL intensity upon decreasing the temperature from 300 K to 6 K; this was attributed to a reduction of charge carrier access to nanocrystal surface trap states and suppression of thermal loss channels. Temperature-dependent PL measurements of aggregated CdSe nanocrystals, which included both energy-donating and -accepting particles, were analyzed using a modified version of F?rster theory. Temperature-dependent ET efficiency increased from 0.55 to 0.75 upon decreasing the sample temperature from 225 K to 6 K, and the ET data contained the same trend observed for the PL of isolated nanoclusters. The application of magnetic fields to increase nanocrystal ET efficiency was studied using magneto-photoluminescence measurements recorded at a sample temperature of 1.6 K. We demonstrated that the exciton fine structure population of the donor was varied using applied magnetic fields, which in turn dictated the PL yield and the resultant ET efficiency of the CdSe nanocrystal aggregate system. The experimental data indicated an ET efficiency enhancement of approximately 7%, which was limited by the random orientation of the spherical nanocrystals in the thin film.     

Measuring electronic coupling in the reaction center of purple photosynthetic bacteria by two-color, three-pulse photon echo peak shift spectroscopy

One- and two-color, three-pulse photon echo peak shift spectroscopy (1C and 2C3PEPS) was used to estimate the electronic coupling between the accessory bacteriochlorophyll (B) and the bacteriopheophytin (H) in the reaction center of the purple photosynthetic bacterium Rhodobacter sphaeroides as approximately 170 +/- 30 cm-1. This is the first direct experimental determination of this parameter; it is within the range of values found in previously published calculations. The 1C3PEPS signal of the Qy band of the bacteriochlorophyll B shows that it is weakly coupled to nuclear motions of the bath, whereas the 1C3PEPS signal of the Qy band of the bacteriopheophytin, H, shows that it is more strongly coupled to the bath, but has minimal inhomogeneous broadening. Our simulations capture the major features of the data with the theoretical framework developed in our group to separately calculate the response functions and population dynamics.     

Effect of methyl as the simplest C–H side group on the significant variation of physical properties of biodegradable poly(ethylene succinate)

Through a melt polycondensation, novel poly(ethylene succinate-co-1,2-propylene succinate) (PEPS) copolymers were synthesized in this work. The thermal behavior, crystal structure, morphology, and mechanical and rheological properties of PEPS copolymers and poly(ethylene succinate) (PES) homopolymer were extensively investigated and compared with each other. Relative to PES, an increase in 1,2-propylene succinate (PS) units content slightly increased the glass transition temperature and apparently decreased the melting point and equilibrium melting point of PEPS copolymers. PEPS copolymers had the similar high thermal stability as PES. The introduction of PS units did not change the crystal structure of PES. As the content of PS units increased, the nucleation density and growth rate of PEPS spherulites both decreased. The Young's modulus of PEPS copolymers gradually decreased, while the elongation at break increased significantly with increasing the content of PS units. In addition, the rheological behavior study illustrated that the complex viscosities, storage modulus and loss modulus of PES and its copolymers first increased and then decreased with increasing the content of PS units. In brief, the minor change in the chemical structure may bring the significant variation of physical properties between PEPS copolymers and PES.     

Low-temperature dynamics of weakly localized Frenkel excitons in disordered linear chains

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible nonequilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or nonresonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive nonmonotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intraband relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes.     

Communication: Systematic shifts of the lowest unoccupied molecular orbital peak in x-ray absorption for a series of 3d metal porphyrins

Porphyrins are widely used as dye molecules in solar cells. Knowing the energies of their frontier orbitals is crucial for optimizing the energy level structure of solar cells. We use near edge x-ray absorption fine structure (NEXAFS) spectroscopy to obtain the energy of the lowest unoccupied molecular orbital (LUMO) with respect to the N(1s) core level of the molecule. A systematic energy shift of the N(1s) to LUMO transition is found along a series of 3d metal octaethylporphyrins and explained by density functional theory. It is mainly due to a shift of the N(1s) level rather than a shift of the LUMO or a change in the electron-hole interaction of the core exciton.     

The source of excess linewidth in EPR of triplet excitons in molecular crystals

It is shown that the EPR linewidth spectrum of triplet excitons in molecular crystals can be severely distorted by weak orientational disorder. The demonstration employs a one-parameter correction procedure based on the assumption that the angular dependence of excess width is directly proportional to the angular gradient of the resonant field. Application to the discrepant data of Haarer and Wolf brings them into agreement with theory and with more recent experiments. A phenomenological model of the disorder is used to interpret the distortion parameter. These results suggest the potential value of exciton EPR as a probe of structural imperfections in molecular crystals.     

Exciton self-trapping in tetrafluoro-dimethyl-aminoacridine single crystals

The UV-visible optical spectra of 1,2,3,4-tetrafluoro-7-(N,N)dimethyl-amino-acridine single crystals are reported. The results are discussed on the basis of the molecular transitions and crystal packing in the framework of the theory of molecular excitons under a fluctuating potential field due to dynamic disorder. A strong local geometry distortion is demonstrated by applying the Urbach rule to the absorption tails, which is the amplitude of the local potential fluctuation being larger than the intermolecular transfer energy. The lineshape and linewidth of the emission band and its temperature dependence give further evidence of exciton self-trapping.     

Multiple exciton generation in nanocrystal quantum dots--controversy, current status and future prospects

Multiple exciton generation is a process that can occur in quantum dots by which the energy of an absorbed photon in excess of the bandgap can be used to create one or more additional excitons instead of being wasted as heat. This effect has received considerable interest because it has the potential to significantly enhance the performance of solar cells, nanocrystal lasers, high speed electronic devices and photocatalysts. However, measuring the efficiency of multiple exciton generation is experimentally challenging and the results of these measurements have been the subject of some controversy. This Perspective describes the techniques used to determine the quantum yield of multiexcitons in nanocrystals and also details the experimental artefacts that can confuse these measurements and have been the source of much of the recent debate. The greater understanding of these artefacts that has emerged recently and the experimental techniques developed to eliminate their effects on quantum yield measurements will also be described. The efficiency of multiple exciton generation currently obtainable from nanocrystals and its potential impact on solar cell performance is assessed in the light of this improved experimental understanding. Whilst it is found the quantum yields thus far reported are insufficient to result in more than a modest increase in solar cell efficiency, an analysis of the expected performance of a nanocrystal engineered to maximise multiple exciton generation indicates that a significant improvement in solar cell performance is possible. Moreover, a nanocrystal design is proposed for optimised efficiency of multiple exciton generation which would allow its potential benefit to solar power production to be realised.     

Wideline NMR studies of oriented nylon 66 under tensile stress

NMR linewidth studies of nylon 66 as a function of temperature and applied tensile stress have been conducted. The principal motional transition temperature was found to be shifted to higher temperatures with stress application by 9°C./g./den. At any given temperature, increased stress resulted in an increased linewidth. An attempt was made to correlate the shift in the motional transition temperature with the concept that a segment experiencing motion must do work against the applied tensile stress.     

Exciton interactions in CdS nanocrystal aggregates in reverse micelle

Here we report the formation and spectroscopic properties of cadmium sulfide (CdS) nanocrystal systems: individual nanocrystal and CdS aggregates. The optical absorption and luminescence spectra of the aggregated CdS nanocrystals and individual nanocrystal show exciton aggregate and individual exciton characteristics. Although it is not Bose-Einstein condensation, such aggregated quantum dots (QDs) seem to supply us opportunity to study the interactions and condensation of excitons in multi-QDs system, not in the separated QDs system.     

Photoluminescence and Raman scattering of ZnO nanorods

Zinc oxide (ZnO) nanorods were synthesized by a simple microemulsion method. The photoluminescence (PL) spectra at room temperature were measured. The strong UV excitonic emission indicates the good optical properties, and the weak deep-level emission reveals very limited structural defects in the crystals. The multiple peaks in the PL spectrum obtained at 15 K are assigned to the donor-bound exciton (DBE), free to bound transition (FB) and FB–LO phonon replicas. The temperature dependence of energy, intensity, and linewidth of each emission band confirms the effect of thermal ionization progress of excitons and nonradiative recombination activated thermally. The nonresonant Raman scattering spectra at room temperature were excited by He–Ne laser (wavelength ～632.8 nm). The perfect wurtzite structure in ZnO nanorods has been verified by the intense E₂ modes, which include low and high frequency vibrations. The possible reasons for the red shift and broadening of vibration modes were studied by the resonant Raman scattering spectra at room temperature. The power-dependence of Raman shift and FWHM shows the laser irradiation effect on the vibrational modes.     

Aqueous solvation dynamics at metal oxide surfaces

Broadband transient absorption (TA) spectroscopy, three-pulse photon echo peak shift (3PEPS), and anisotropy decay measurements were used to study the solvation dynamics in bulk water and interfacial water at ZrO(2) surfaces, using Eosin Y as a probe. The 3PEPS results show a multiexponential behavior with two subpicosecond components that are similar in bulk and interfacial water, while a third component of several picoseconds is significantly lengthened at the interface. The bandwidth correlation function from TA spectra exhibits the same behavior, and the TA spectra are well reproduced using the doorway-window picture with the time constants from PEPS. Our results suggest that interfacial water is restricted to a thickness of less than 5 A. Also the high-frequency collective dynamics of water does not seem to be affected by the interface. On the other hand, the increase of the third component may point to a slowing down of diffusional motion at the interface, although other effects, may play a role, which are discussed.     

The EPR spectrum of triplet excitions in crystalline pyrene

The room temperature EPR spectrum of triplet excitions in crystalline pyrene has been studied. Spin hamiltonian parameters have been obtained and from these the molecular fine structure constants are deduced assuming only monomeric sites. The correspondence between the latter and the directly measured values is taken to imply the absence of an excimeric contribution to the triplet exciton state.     

Temperature-dependent relaxation of excitons in tubular molecular aggregates: Fluorescence decay and stokes shift

We report temperature-dependent steady-state and time-resolved fluorescence studies to probe the exciton dynamics in double-wall tubular J-aggregates formed by self-assembly of the dye 3,3'-bis(3-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine. We focus on the lowest energy fluorescence band, originating from the inner cylindrical wall. At low temperatures, the experiments reveal a nonexponential decay of the fluorescence, with a typical time scale that depends on the emission wavelength. At these temperatures we also find a dynamic Stokes shift of the fluorescence spectrum and its nonmonotonic dependence on temperature under steady-state conditions. All these data indicate that below about 20 K the excitons in the lowest fluorescence band do not reach thermal equilibrium before emission occurs, while above about 60 K thermalization on this time scale is complete. By comparing the two lowest fluorescence bands, we also find indications for fast energy transfer from the outer to the inner wall. We show that the Frenkel exciton model with diagonal disorder, which previously has been proposed to explain the absorption and linear dichroism spectra of these aggregates, yields a quantitative explanation to the observed dynamics. To this end, we extend the model to account for weak phonon-induced scattering of the localized exciton states; the spectral dynamics are then described by solving a Pauli master equation for the exciton populations.     

Broadband femtosecond sum-frequency spectroscopy of CO on Ru1010 in the frequency and time domains

CO on Ru[1010] was investigated by broadband femtosecond sum-frequency spectroscopy at 200 K. Approximately half of the frequency shift of 71 cm(-1) over the coverage range from 0.15 to 1.22 monolayers is shown to originate from dipole-dipole coupling, with the remainder due to a chemical shift. Despite low adlayer-surface registration at the highest coverages, the linewidth of the C-O stretch is comparatively low, and is described by homogeneous broadening according to sum-frequency free-induction decay measurements in the time domain. This can be explained by the dominance of the CO dipole coupling strength over the static disorder present in a coincidence structure. As the coverage decreases below 0.3 monolayer, the linewidth increases considerably, indicative of inhomogeneous broadening. Supported by a concomitant frequency change we suggest that at low coverages CO molecules form chains of irregular length in the [0001] direction, as has been shown for other surfaces with similar symmetry.     

Colloidal gallium indium oxide nanocrystals: a multifunctional light-emitting phosphor broadly tunable by alloy composition

We demonstrate compositionally tunable photoluminescence in complex transparent conducting oxide nanocrystals. Alloyed gallium indium oxide (GIO) nanocrystals with variable crystal structures are prepared by a colloidal method throughout the full composition range and studied by different structural and spectroscopic methods, including photoluminescence and X-ray absorption. The structures and sizes of the GIO nanocrystals can be simultaneously controlled, owing to the difference in the growth kinetics of In(2)O(3) and Ga(2)O(3) nanocrystals and the polymorphic nature of both materials. Using the synthesized nanocrystal series, we demonstrate the structural and compositional dependences of the photoluminescence of GIO nanocrystals. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, are used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor-acceptor pair recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In(3+) occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga(2)O(3) nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition. The oxygen vacancy defects, responsible for the observed photoluminescence properties, are also implicated in other functional properties, particularly conductivity, enabling the application of colloidal GIO nanocrystals as integrated optoelectronic materials.