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Showing papers on "Exciton published in 2010"


Journal ArticleDOI
Frank C. Spano1
TL;DR: This Account shows that the aggregation-induced deviations from the Poissonian distribution of vibronic peak intensities take on two extremes identified with ideal H- and J-aggregates, and reveals several more distinguishing traits between the two aggregate types.
Abstract: Electronic excitations in small aggregates, thin films, and crystals of conjugated organic molecules play a fundamental role in the operation of a wide array of organic-based devices including solar cells, transistors, and light-emitting diodes. Such excitations, or excitons, are generally spread out over several molecules: a balance between the delocalizing influence of resonant intermolecular coupling and the localizing influence of static and dynamic disorder determines the coherence range of the exciton. Because of the "soft" nature of organic materials, significant nuclear relaxation in the participating molecules also accompanies the electronic excitations. To properly understand energy or charge transport, one must treat intermolecular (excitonic) coupling, electron-vibrational coupling, and disorder on equal footing. In this Account, we review the key elements of a theoretical approach based on a multiparticle representation that describes electronic excitations in organic materials as vibronic excitations surrounded by a field of vibrational excitations. Such composite excitations are appropriately called Frenkel excitonic polarons. For many conjugated molecules, the bulk of the nuclear reorganization energy following electronic excitation arises from the elongation of a symmetric vinyl stretching mode with energy approximately 1400 cm(-1). To appreciate the impact of aggregation, we study how the vibronic progression of this mode, which dominates the isolated (solvated) molecule absorption and emission spectra, is distorted when molecules are close enough to interact with each other. As we demonstrate in this Account, the nature of the distortion provides a wealth of information about how the molecules are packed, the strength of the excitonic interactions between molecules, the number of molecules that are coherently coupled, and the nature of the disorder. We show that the aggregation-induced deviations from the Poissonian distribution of vibronic peak intensities take on two extremes identified with ideal H- and J-aggregates. The sign of the nearest neighbor electronic coupling, positive for H and negative for J, distinguishes the two basic aggregate forms. For several decades, researchers have known that H-aggregates exhibit blue-shifted absorption spectra and are subradiant while J-aggregates exhibit the opposite behavior (red-shifted absorption and superradiance). However, the exact inclusion of exciton-vibrational coupling reveals several more distinguishing traits between the two aggregate types: in H(J)-aggregates the ratio of the first two vibronic peak intensities in the absorption spectrum decreases (increases) with increasing excitonic coupling, while the ratio of the 0-0 to 0-1 emission intensities increases (decreases) with disorder and increases (decreases) with increasing temperature. These two extreme behaviors provide the framework for understanding absorption and emission in more complex morphologies, such as herringbone packing in oligo(phenylene vinylene)s, oligothiophenes and polyacene crystals, as well as the polymorphic packing arrangements observed in carotenoids.

1,307 citations


Journal ArticleDOI
01 Oct 2010-Science
TL;DR: A photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO2 single crystals is used to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon, which has implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.
Abstract: Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO 2 single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO 2 facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.

770 citations


Journal ArticleDOI
TL;DR: Polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors is reported in this paper.
Abstract: The optical properties of organic semiconductors are almost exclusively described using the Frenkel exciton picture1. In this description, the strong Coulombic interaction between an excited electron and the charged vacancy it leaves behind (a hole) is automatically taken into account. If, in an optical microcavity, the exciton–photon interaction is strong compared to the excitonic and photonic decay rates, a second quasiparticle, the microcavity polariton, must be introduced to properly account for this coupling2. Coherent, laser-like emission from polaritons has been predicted to occur when the ground-state occupancy of polaritons 〈ngs〉, reaches 1 (ref. 3). This process, known as polariton lasing, can occur at thresholds much lower than required for conventional lasing. Polaritons in organic semiconductors are highly stable at room temperature, but to our knowledge, there has as yet been no report of nonlinear emission from these structures. Here, we demonstrate polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors. Polaritons in organic semiconductors are highly stable at room temperature, but so far nonlinear emission from these structures has not been demonstrated. Here, polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors is reported.

735 citations


Journal ArticleDOI
TL;DR: In this article, the optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasiparticle spectrum to an optical absorption of such a system.
Abstract: We demonstrate that optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasiparticle spectrum to an optical absorption of such a system. These results are applied to graphene deposited on a surface of oxidized silicon for which we measure ellipsometric spectra, extract optical constants of a graphene layer and reconstruct the electronic dispersion relation near the K point using optical transmission spectra. We also present spectroscopic ellipsometry analysis of graphene placed on amorphous quartz substrates and report a pronounced peak in ultraviolet absorption at 4.6 eV because of a van Hove singularity in graphene's density of states. The peak is asymmetric and downshifted by 0.5 eV probably due to excitonic effects.

528 citations


Journal ArticleDOI
Hikmet Najafov1, Bumsu Lee1, Q. Zhou1, Leonard C. Feldman1, Vitaly Podzorov1 
TL;DR: The findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors and suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials.
Abstract: Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10-50 nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2-8 μm), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.

453 citations


Book
01 Jan 2010
TL;DR: In this paper, the authors present a detailed analysis of linear optical properties close to the fundamental Absorption Edge of the ZnO crystal structure and the influence of external fields.
Abstract: Crystal Structure, Chemical Binding, and Lattice Properties.- Growth.- Band Structure.- Electrical Conductivity and Doping.- Intrinsic Linear Optical Properties Close to the Fundamental Absorption Edge.- Bound Exciton Complexes.- Influence of External Fields.- Deep Centres in ZnO.- Magnetic Properties.- Nonlinear Optics, High Density Effects and Stimulated Emission.- Dynamic Processes.- Past, Present and Future Applications.- Conclusion and Outlook.

357 citations


Journal ArticleDOI
TL;DR: The internal quantum yield of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states, which is consistent with negligible impact from hot exciton dissociation.
Abstract: We examine the significance of hot exciton dissociation in two archetypical polymer−fullerene blend solar cells. Rather than evolving through a bound charge transfer state, hot processes are proposed to convert excitons directly into free charges. But we find that the internal quantum yields of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states. The internal quantum yield, together with the temperature dependence of the current−voltage characteristics, is consistent with negligible impact from hot exciton dissociation.

353 citations


Journal ArticleDOI
TL;DR: It is demonstrated that the fundamental unit of energy required to produce each electron-hole pair in a given QD is the band gap energy, and the research challenges associated with achieving the maximum benefit of MEG in solar energy conversion are discussed since the threshold and efficiency are mathematically related.
Abstract: Multiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron-hole pair per absorbed photon. We derive expressions for the proper way to compare MEG in QDs with II in bulk semiconductors and argue that there are important differences in the photophysics between bulk semiconductors and QDs. Our analysis demonstrates that the fundamental unit of energy required to produce each electron-hole pair in a given QD is the band gap energy. We find that the efficiency of the multiplication process increases by at least 2 in PbSe QDs compared to bulk PbSe, while the competition between cooling and multiplication favors multiplication by a factor of 3 in QDs. We also demonstrate that power conversion efficiencies in QD solar cells exhibiting MEG can greatly exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD band gap energy, which requires a further increase in the MEG efficiency. Finally, we discuss the research challenges associated with achieving the maximum benefit of MEG in solar energy conversion since we show the threshold and efficiency are mathematically related.

343 citations


Journal ArticleDOI
TL;DR: Insight is provided into the control and ultimately the tunability of the exciton diffusion length inorganic systems, which is crucial for the management of energy transport in a wide range of important organic electronic devices.
Abstract: One of the most fundamental properties of both organic and inorganic semiconductors is charge mobility. It has been unambiguously shown that the mobility in both of these materials systems is strongly linked to the degree of long range order—thatis,moreextendedcrystallinityleadstoalargercharge mobility, which ultimately determines such extrinsic properties as seriesresistance andresponse tocurrentand optical pulses. An equally fundamental property for organic semiconductors is the molecular excited state-, or exciton-, diffusion length which characterizes energy transport within these more correlated solids. While it has been predicted that exciton transport should also be linked to the extent of crystalline order, to our knowledge nosuchdependencehasyetbeenestablished.Here,weaccurately measure the exciton diffusion length of the archetypal organic semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and clearly show its relationship to thin-film crystal morphology. As in the case of charge mobility, we show that the exciton transport diffusion length is a monotonic function of the extent of crystalline order. This study provides insight into the control and ultimately the tunability of the exciton diffusion lengthinorganic systems, whichiscrucial forthemanagementof energy transport in a wide range of important organic electronic devices.

314 citations


Journal ArticleDOI
TL;DR: It is shown that charge generation primarily occurs 2-10 ns after photoexcitation, which supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction.
Abstract: Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

311 citations


Journal ArticleDOI
TL;DR: In this paper, an all-optical spin switch based on exciton-polaritons in a semiconductor microcavity is demonstrated, which may lead to small and fast spin-based on-chip logic devices.
Abstract: An all-optical spin switch based on exciton–polaritons in a semiconductor microcavity is demonstrated. These results may lead to small and fast spin-based on-chip logic devices.

Journal ArticleDOI
TL;DR: The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films.
Abstract: The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20–100 ns as opposed to 2 μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are ...

Journal ArticleDOI
TL;DR: An experimental demonstration of strong coupling between a surface plasmon propagating on a planar silver thin film and the lowest excited state of CdSe nanocrystals, akin to those studied in cavity quantum electrodynamics, offering the possibility to study the regime of strong light-matter coupling in semiconductor nanocry crystals under easily accessible experimental conditions.
Abstract: We present an experimental demonstration of strong coupling between a surface plasmon propagating on a planar silver thin film and the lowest excited state of CdSe nanocrystals Attenuated total reflection measurements demonstrate the formation of plasmon−exciton mixed states, characterized by a Rabi splitting of ∼112 meV at room temperature Such a coherent interaction has the potential for the development of nonlinear plasmonic devices, and furthermore, this system is akin to those studied in cavity quantum electrodynamics, thus offering the possibility to study the regime of strong light−matter coupling in semiconductor nanocrystals under easily accessible experimental conditions

Journal ArticleDOI
TL;DR: It is shown, through time-resolved terahertz spectroscopy, that low mobility in polycrystalline TiO(2) nanotubes is not due to scattering from grain boundaries or disorder-induced localization as in other nanomaterials, but instead results from a single sharp resonance arising from exciton-like trap states.
Abstract: Nanoparticle films have become a promising low-cost, high-surface-area electrode material for solar cells and solar fuel production. Compared to sintered nanoparticle films, oriented polycrystalline titania nanotubes offer the advantage of directed electron transport, and are expected to have higher electron mobility. However, macroscopic measurements have revealed their electron mobility to be as low as that of nanoparticle films. Here, we show, through time-resolved terahertz spectroscopy, that low mobility in polycrystalline TiO(2) nanotubes is not due to scattering from grain boundaries or disorder-induced localization as in other nanomaterials, but instead results from a single sharp resonance arising from exciton-like trap states. If the number of these states can be lowered, this could lead to improved electron transport in titania nanotubes and significantly better solar cell performance.

Journal ArticleDOI
TL;DR: Density matrix theory is used to examine how various molecular properties such as orbital energies and electronic couplings affect singlet fission yield in the regime of fast, coherent electron transfer, applicable when the initial excited state is localized.
Abstract: In traditional solar cells one photon absorbed can lead to at most one electron of current. Singlet fission, a process in which one singlet exciton is converted to two triplet excitons, provides a potential improvement by producing two electrons from each photon of sufficient energy. The literature contains several reports of singlet fission in various systems, but the mechanism of this process is poorly understood. In this paper we examine a two-step mechanism with a charge transfer state intermediate, applicable when the initial excited state is localized. Density matrix theory is used to examine how various molecular properties such as orbital energies and electronic couplings affect singlet fission yield in the regime of fast, coherent electron transfer. Several promising chromophores are discussed and density functional theory is used to predict fission yield for each in the context of this mechanism. Finally, implications for chromophore design are discussed, and future experiments are suggested.

Journal ArticleDOI
Jier Huang1, Zhuangqun Huang1, Ye Yang1, Haiming Zhu1, Tianquan Lian1 
TL;DR: It is shown that excitons in QDs dissociate by ultrafast electron transfer to MB(+) with an average time constant of approximately 2 ps, demonstrating that ultrafast interfacial charge separation can effectively compete with exciton-exciton annihilation.
Abstract: Multiexciton generation in quantum dots (QDs) may provide a new approach for improving the solar-to-electric power conversion efficiency in QD-based solar cells. However, it remains unclear how to extract these excitons before the ultrafast exciton−exciton annihilation process. In this study we investigate multiexciton dissociation dynamics in CdSe QDs adsorbed with methylene blue (MB+) molecules by transient absorption spectroscopy. We show that excitons in QDs dissociate by ultrafast electron transfer to MB+ with an average time constant of ∼2 ps. The charge separated state is long-lived (>1 ns), and the charge recombination rate increases with the number of dissociated excitons. Up to three MB+ molecules per QD can be reduced by exciton dissociation. Our result demonstrates that ultrafast interfacial charge separation can effectively compete with exciton−exciton annihilation, providing a viable approach for utilizing short-lived multiple excitons in QDs.

Journal ArticleDOI
TL;DR: The time resolved population and phase dynamics reveal the analogy with the ac Josephson effect, and a theoretical two-mode model describes the observed effects, explaining how the different realizations of the pulsed experiment can be in phase.
Abstract: We report on the observation of spontaneous coherent oscillations in a microcavity polariton bosonic Josephson junction. Condensation of exciton polaritons here takes place under incoherent excitation in a double potential well naturally formed in the disorder. Coherent oscillations set on at an excitation power well above the condensation threshold. The time resolved population and phase dynamics reveal the analogy with the ac Josephson effect. A theoretical two-mode model describes the observed effects, explaining how the different realizations of the pulsed experiment can be in phase.

Journal ArticleDOI
TL;DR: It is shown that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy.
Abstract: Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices.

Journal ArticleDOI
TL;DR: It is pointed out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PC DTBT:fullerene blends, and alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton are discussed.
Abstract: The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband π−π* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes ...

Journal ArticleDOI
TL;DR: Close examination of exciton PL intensity time traces of single CdSe(CdZnS) core(shell) nanocrystals reveals that the dark state PL quantum yield can be 10 times less than the biexciton PL quantum yields, which directly contradict the predictions of the charging model.
Abstract: Semiconductor nanocrystals emit light intermittently; i.e., they “blink,” under steady illumination. The dark periods have been widely assumed to be due to photoluminescence (PL) quenching by an Auger-like process involving a single additional charge present in the nanocrystal. Our results challenge this long-standing assumption. Close examination of exciton PL intensity time traces of single CdSe(CdZnS) core (shell) nanocrystals reveals that the dark state PL quantum yield can be 10 times less than the biexciton PL quantum yield. In addition, we observe spectrally resolved multiexciton emission and find that it also blinks with an on/off ratio greater than 10:1. These results directly contradict the predictions of the charging model.

Journal ArticleDOI
TL;DR: In this paper, a technique to control the energetic splitting of two quantum states using a vertical electric field, facilitating the observation of coherent coupling between them, was described, leading to the generation of entangled photon pairs.
Abstract: The signature of coherent coupling between two quantum states is an anticrossing in their energies as one is swept through the other. In single semiconductor quantum dots containing an electron–hole pair the eigenstates form a two-level system that can be used to demonstrate quantum effects in the solid state, but in all previous work these states were independent. Here we describe a technique to control the energetic splitting of these states using a vertical electric field, facilitating the observation of coherent coupling between them. Near the minimum splitting the eigenstates rotate in the plane of the sample, being orientated at 45° when the splitting is smallest. Using this system we show direct control over the exciton states in one quantum dot, leading to the generation of entangled photon pairs.

Journal ArticleDOI
TL;DR: It is demonstrated that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb shift.
Abstract: We study optically driven Rabi rotations of a quantum dot exciton transition between 5 and 50 K, and for pulse areas of up to 14 pi In a high driving field regime, the decay of the Rabi rotations is nonmonotonic, and the period decreases with pulse area and increases with temperature By comparing the experiments to a weak-coupling model of the exciton-phonon interaction, we demonstrate that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb shift

Journal ArticleDOI
TL;DR: In this article, the Bethe-Salpeter equation was used to calculate the dielectric function for both the rutile and anatase structures of a frozen crystal lattice.
Abstract: Quasiparticle excitation energies and optical properties of ${\text{TiO}}_{2}$ in the rutile and anatase structures are calculated using many-body perturbation-theory methods. Calculations are performed for a frozen crystal lattice; electron-phonon coupling is not explicitly considered. In the GW method, several approximations are compared and it is found that inclusion of the full frequency dependence as well as explicit treatment of the Ti semicore states are essential for accurate calculation of the quasiparticle energy-band gap. The calculated quasiparticle energies are in good agreement with available photoemission and inverse photoemission experiments. The results of the GW calculations, together with the calculated static screened Coulomb interaction, are utilized in the Bethe-Salpeter equation to calculate the dielectric function ${ϵ}_{2}(\ensuremath{\omega})$ for both the rutile and anatase structures. The results are in good agreement with experimental observations, particularly the onset of the main absorption features around 4 eV. For comparison to low-temperature optical-absorption measurements that resolve individual excitonic transitions in rutile, the low-lying discrete excitonic energy levels are calculated with electronic screening only. The lowest energy exciton found in the energy gap of rutile has a binding energy of 0.13 eV. In agreement with experiment, it is not dipole allowed but the calculated exciton energy exceeds that measured in absorption experiments by about 0.22 eV and the scale of the exciton binding energy is also too large. The quasiparticle energy alignment of rutile is calculated for nonpolar (110) surfaces. In the GW approximation, the valence-band maximum is 7.8 eV below the vacuum level, showing a small shift from density-functional theory results.

Journal ArticleDOI
24 Nov 2010-ACS Nano
TL;DR: The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.
Abstract: Photoluminescence quantum yields and nonradiative decay of the excitonic S(1) state in length fractionated (6,5) single-wall carbon nanotubes (SWNTs) are studied by continuous wave and time-resolved fluorescence spectroscopy. The experimental data are modeled by diffusion limited contact quenching of excitons at stationary quenching sites including tube ends. A combined analysis of the time-resolved photoluminescence decay and the length dependence of photoluminescence quantum yields (PL QYs) from SWNTs in sodium cholate suspensions allows to determine the exciton diffusion coefficient D = 10.7 ± 0.4 cm(2)s(-1) and lifetime τ(PL) for long tubes of 20 ± 1 ps. PL quantum yields Φ(PL) are found to scale with the inverse diffusion coefficient and the square of the mean quenching site distance, here l(d) = 120 ± 25 nm. The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.

Journal ArticleDOI
26 May 2010-ACS Nano
TL;DR: Density functional theory calculations indicate that the highest occupied molecular orbital of PTC is near resonant with that of the QD, and that the two have correct symmetry to exchange electron density, and it is proposed that the relaxation of exciton confinement occurs through delocalization of the photoexcited hole of theQD into the ligand shell.
Abstract: Coordination of phenyldithiocarbamate (PTC) ligands to solution-phase colloidal CdSe quantum dots (QDs) decreases the optical band gap, Eg, of the QDs by up to 220 meV. These values of ΔEg are the largest shifts achieved by chemical modification of the surfaces of solution-phase CdSe QDs and are—by more than an order of magnitude in energy—the largest bathochromic shifts achieved for QDs in either the solution or solid phases. Measured values of ΔEg upon coordination to PTC correspond to an apparent increase in the excitonic radius of 0.26 ± 0.03 nm; this excitonic delocalization is independent of the size of the QD for radii, R = 1.1−1.9 nm. Density functional theory calculations indicate that the highest occupied molecular orbital of PTC is near resonant with that of the QD, and that the two have correct symmetry to exchange electron density (PTC is a π-donor, and the photoexcited QD is a π-acceptor). We therefore propose that the relaxation of exciton confinement occurs through delocalization of the ph...

Journal ArticleDOI
03 Dec 2010-Science
TL;DR: Single-particle light-harvesting action spectroscopy is used to probe the impact of particle morphology on energy transfer and carrier relaxation across a heterojunction, suggesting that non-uniform geometries favor efficient charge separation for light harvesting.
Abstract: Nanoscale semiconductor heterostructures such as tetrapods can be used to mimic light-harvesting processes. We used single-particle light-harvesting action spectroscopy to probe the impact of particle morphology on energy transfer and carrier relaxation across a heterojunction. The generic form of an action spectrum [in our experiments, photoluminescence excitation (PLE) under absorption in CdS and emission from CdSe in nanocrystal tetrapods, rods, and spheres] was controlled by the physical shape and resulting morphological variation in the quantum confinement parameters of the nanoparticle. A correlation between single-particle PLE and physical shape as determined by scanning electron microscopy was demonstrated. Such an analysis links local structural non-uniformities such as CdS bulbs forming around the CdSe core in CdSe/CdS nanorods to a lower probability of manifesting excitation energy-dependent emission spectra, which in turn is probably related to band alignment and electron delocalization at the heterojunction interface.

Journal ArticleDOI
TL;DR: Using first principles many-body theory methods (GW+Bethe-Salpeter equation) it is demonstrated that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium.
Abstract: Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound excitons opens the path towards an excitonic Bose-Einstein condensate in graphane that can be observed experimentally.

Patent
21 Jul 2010
TL;DR: In this paper, a light-emitting device with a light emitting layer composed of a monomolecular film of quantum dots, which is enhanced in luminance and luminous efficiency, is described.
Abstract: Disclosed is a light-emitting device having a light-emitting layer composed of a monomolecular film of quantum dots, which is enhanced in luminance and luminous efficiency. Specifically disclosed is a light-emitting device (1) comprising at least an anode (3), a hole transporting light-emitting layer (5) composed of a hole transporting material and a quantum dot (11), an electron transporting layer (7) and a cathode (4) in this order. The hole mobility of the electron transporting layer (7) is lower than that of tris(8-quinolinolato)aluminum complex (Alq3), and the hole transporting light-emitting layer (5) is so formed as to emit light when excitons generated in the electron transporting layer (7) are transferred into the light-emitting layer.

Journal ArticleDOI
TL;DR: This work studies the effect of an external biaxial stress on the light emission of single InGaAs/GaAs(001) quantum dots placed onto piezoelectric actuators to provide a robust method to achieve color coincidence in the emission of X and XX, which is a prerequisite for the possible generation of entangled photon pairs via the recently proposed "time reordering" scheme.
Abstract: We study the effect of an external biaxial stress on the light emission of single InGaAs/GaAs(001) quantum dots placed onto piezoelectric actuators. With increasing compression, the emission blueshifts and the binding energies of the positive trion (X+) and biexciton (XX) relative to the neutral exciton (X) show a monotonic increase. This phenomenon is mainly ascribed to changes in electron and hole localization and it provides a robust method to achieve color coincidence in the emission of X and XX, which is a prerequisite for the possible generation of entangled photon pairs via the recently proposed “time reordering” scheme.

Journal ArticleDOI
26 Aug 2010-Nature
TL;DR: The measurement methods open a new window into high-order many-body interactions in materials and molecules, and the present results should guide ongoing work on first-principles calculations of electronic interactions in semiconductor nanostructures.
Abstract: Long-range correlations between charge particles (electrons and 'holes') in semiconductors lead to many-body effects, which are of fundamental interest and also of importance in optoelectronic applications. The exciton state, in which an electron and hole are paired, has been extensively studied, but the properties of multiple exciton states involving three or more charge particles are largely unknown as they are challenging to observe experimentally. Daniel Turner and Keith Nelson have extended a spectroscopy technique called multidimensional Fourier transform optical spectroscopy, and demonstrate its ability to generate and characterize bi-excitons, tri-excitons and other unbound correlations in a gallium arsenide nanostructure. This experiment involves controlling the geometry, temporal delays and optical phases of up to seven light fields simultaneously. The findings are of particular interest as it was previously not known whether tri-excitons — involving correlations between six particles — could exist at all. The authors also present clear evidence that four-exciton states do not exist, indicating an upper limit for many-body correlations in this type of semiconductor system. The exciton state in semiconductors, where an electron and hole are paired, has been studied extensively, but the properties of exciton states involving three or more charged particles are largely unknown. These authors use a challenging spectroscopy technique to generate and characterize biexcitons, triexcitons and other, unbound, correlations in a gallium arsenide nanostructure. It was previously unknown whether triexcitons, which involve correlations between six particles, can exist at all. Strong, long-range Coulomb interactions can lead to correlated motions of multiple charged particles, which can induce important many-body effects in semiconductors. The exciton states formed from correlated electron–hole pairs have been studied extensively1,2, but basic properties of multiple-exciton correlations—such as coherence times, population lifetimes, binding energies and the number of particles that can be correlated—are largely unknown because they are not spectroscopically accessible from the ground state. Here we present direct observations of high-order coherences in gallium arsenide quantum wells, achieved using two-dimensional multiple-quantum spectroscopy methods in which up to seven successive light fields were used. The measurements were made possible by the combination of a reconfigurable spatial beam-shaper that formed multiple beams in specified geometries and a spatiotemporal pulse-shaper that controlled the relative optical phases and temporal delays among pulses in all the beams. The results reveal triexciton coherences (correlations of three excitons or six particles), whose existence was not obvious because the third exciton spin is unpaired, and the values of their coherence times and binding energies. Rephasing of biexcitons, triexcitons and unbound two-exciton coherences was demonstrated. We also determined that there are no significant unbound correlations of three excitons and no bound or unbound four-exciton (eight-particle) correlations. Thus, the limits, as well as the properties, of many-body correlations in this system were revealed. The measurement methods open a new window into high-order many-body interactions in materials and molecules3, and the present results should guide ongoing work on first-principles calculations of electronic interactions in semiconductor nanostructures4.