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Energy and Charge Transfer in Nanoscale Hybrid Materials

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Abstract

Hybrid materials composed of colloidal semiconductor quantum dots and π-conjugated organic molecules and polymers have attracted continuous interest in recent years, because they may find applications in bio-sensing, photodetection, and photovoltaics. Fundamental processes occurring in these nanohybrids are light absorption and emission as well as energy and/or charge transfer between the components. For future applications it is mandatory to understand, control, and optimize the wide parameter space with respect to chemical assembly and the desired photophysical properties. Accordingly, different approaches to tackle this issue are described here. Simple organic dye molecules (Dye)/quantum dot (QD) conjugates are studied with stationary and time-resolved spectroscopy to address the dynamics of energy and ultra-fast charge transfer. Micellar as well as lamellar nanostructures derived from diblock copolymers are employed to fine-tune the energy transfer efficiency of QD donor/dye acceptor couples. Finally, the transport of charges through organic components coupled to the quantum dot surface is discussed with an emphasis on functional devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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The combination of semiconductor nanocrystals (NCs) and molecules for efficient electronic excitation energy transfer is expected to be a promising ingredient of novel hybrid photovoltaic devices. Here energy transfer from a CdSe NC to the tetrapyrrole-type Pheophorbide-a molecule (Pheo) is studied theoretically. The rate expression accounts for the correct NC–Pheo transfer coupling, for the multitude of NC single exciton levels as well as their thermal distribution, and for the electron-vibrational Pheo states. A spherical Cd1159Se1450 NC is compared with a similar large NC of pyramidal and hemisphere shape. Because of the different exciton energies and wave functions, the transfer rates differ somewhat. For all three types of NC, however, the Coulomb correlation essentially determines the magnitude of the transfer coupling and the exciton energy. In any case, the energy-transfer coupling is below 1 meV, excluding hybrid-state formation.
Article
This article reviews the structural and electronic features of colloidal quantum dot (QD)–organic complexes that influence the rate of photoinduced charge separation (PCS) across the interface between the inorganic core of the QD and its organic surface ligands. While Marcus theory can be used to describe the rate of PCS in QD–organic complexes, uncertainties in the exact atomic configuration of the inorganic–organic interface and heterogeneities in this interfacial structure within an ensemble of QDs complicate the determination of the most fundamental Marcus parameters—electronic coupling, reorganization energy, and driving force. This article discusses strategies for accounting for uncertainties and heterogeneities when using Marcus theory to interpret rates of PCS in QD–organic complexes and highlights how measurement of PCS rates can provide information about the interfacial structure of the QD surface. Recent progress in the application of mechanistic knowledge of PCS to harvest multiple charge carriers from QDs containing multiple excitons and extend the lifetime of the charge-separated state is also discussed.
Article
This paper describes the surface composition-dependent binding of the dichloride salt of methyl viologen (MV(2+)) to CdS quantum dots (QDs) enriched, to various degrees, with either Cd or S at the surface. The degree of enrichment is controlled synthetically and by post-synthetic dilution of the QDs in their solvent, THF. (1)H NMR shows that the Cd-enriched QDs are coated by oleate at a density of 2.8 ligands/nm(2), and that the S-enriched QDs are coated in both oleate and octadecene at a density of 1.0 ligands/nm(2). Electron transfer-mediated photoluminescence quenching of the QDs by MV(2+) serves as a probe for the binding affinity of MV(2+) for the surfaces of the QDs. Diluting Cd-enriched QDs removes Cd-oleate from the surface, exposing the stoichiometric CdS surface beneath, and increasing the quenching efficiency of MV(2+), whereas diluting S-enriched QD does not change their surface chemistry or the efficiency with which they are quenched by MV(2+). The photoluminescence quenching data for all of the surface chemistries we studied fit well to a Langmuir model that accounts for binding of MV(2+)+ through two reaction mechanisms: (i) direct adsorption of MV(2+) to exposed stoichiometric CdS surfaces (with an equilibrium adsorption constant of 1.5×10(5) M(-1)), and (ii) adsorption of MV(2+) to stoichiometric CdS surfaces upon displacement of weakly bound Cd-oleate complexes (with an equilibrium displacement constant of 3.5×10(3) M(-1)). Ab initio calculations of the binding energy for adsorption of the dichloride salt of MV(2+) on Cd- and S-terminated surfaces reveal a substantial preference of MV(2+) for S-terminated lattices due to alignment of the positively charged nitrogens on MV(2+) with the negatively charged sulfur. These findings suggest a strategy to maximize the adsorption of redox-active molecules in electron transfer-active geometries through synthetic and post-synthetic manipulation of the inorganic surface.
Article
Semiconductor quantum dots are inorganic nanocrystals which, because of their unique size-dependent electronic properties, are of high potential interest for the development of light-responsive nanodevices. Their surface can be chemically modified, by either covalent or non-covalent approaches, in order to interface them with molecular units endowed with specific physico-chemical properties. Photoinduced electron- and energy-transfer processes between quantum dots and attached molecular species offer versatile strategies to implement functionalities such as photosensitized processes, and luminescence sensing and switching. In this review we will discuss the strategies underlying the rational construction of this kind of multicomponent species, and we will illustrate a few examples taken from our own research.
Article
Herein, we report a simple fabrication of hybrid nanowires (NWs) composed of a p-type conjugated polymer (CP) and n-type inorganic quantum dots (QDs) by exploiting the crystallization-driven solution assembly of poly(3-hexylthiophene)-b-poly(2-vinylpyridine) (P3HT-b-P2VP) rod-coil amphiphiles. The visualization of the crystallization-driven growth evolution of hybrid NWs through systematic transmission electron microscopy (TEM) experiments showed that discrete dimeric CdSe QDs bridged by P3HT-b-P2VP polymers were generated during the initial state of crystallization. These, in turn, assemble into elongated fibrils, forming the coaxial P3HT-b-P2VP/QDs hybrid NWs. In particular, the location of the QD arrays within the single strand of P3HT-b-P2VP can be controlled precisely by manipulating the regioregularity (RR) values of P3HT block and the relative lengths of P2VP block. The degree of coaxiality of the QD arrays was shown to depend on the coplanarity of the thiophene rings of P3HT block, which can be controlled by the RR value of P3HT block. In addition, the location of QDs could be regulated at the specific-local site of P3HT-b-P2VP NW according to the surface characteristics of QDs. As an example, the comparison of two different QDs coated with hydrophobic alkyl-terminated and hydroxyl-terminated molecules, respectively, is used to elucidate the effect of the surface properties of QDs on their nano-location in the NW.
Article
Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron-hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO2, and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.
Article
Hybrid nanowires (NWs) of light-emitting poly(3-hexylthiophene) (P3HT) blended with gold nanoparticles (Au-NPs) were fabricated by a wetting method. The functionalized CdSe/ZnS quantum dots (QDs) were attached to the surfaces of P3HT/Au-NPs NWs. The nanoscale photoluminescence (PL) characteristics of the P3HT, P3HT/Au-NPs, and QDs/P3HT/Au-NPs single NWs were investigated using a high resolution laser confocal microscope (LCM). For a P3HT/Au-NPs single NW, the LCM PL intensity of the P3HT NW decreased due to the luminescence quenching effect by the blending with Au-NPs. However, the LCM PL intensity of the P3HT/Au-NPs NW drastically increased when the QDs were attached to the surface of the NW. The PL enhancement of the P3HT NW part in the hybrid QDs/P3HT/Au-NPs single NW originated from the Förster resonance energy transfer (FRET) effect between the QDs and the P3HT NW, which was assisted by the surface plasmon (SP) coupling of Au-NPs with the QDs. Based on the analysis of time-resolved PL spectra, the exciton lifetimes of the QDs for the QDs/P3HT/Au-NPs NW were found to decrease considerably in comparison with those of the QDs/P3HT NWs without Au-NPs. We also found that the energy transfer rate of the QDs/P3HT NW increased from 0.76 to 0.93 with the Au-NPs. These observations support the notion of SP assisted FRET effect in hybrid nanosystems.
Article
Doped and de-doped poly(3-methylthiophene) (P3MT) nanowires (NWs) were synthesized through electrochemical polymerization and functionalized CdSe/ZnS quantum dots (QDs) with blue and green emissions were homogeneously attached to the NW surfaces. The photoluminescence (PL) spectra of the QDs/P3MT single NWs indicated a drastic enhancement of PL intensity for both doped and de-doped NWs owing to the energy transfer effect from QDs to NWs. The QDs/de-doped P3MT NWs exhibited higher energy transfer efficiency because of a reduced screening effect for energy transfer, which was confirmed by a decrease in the exciton lifetime of the QDs, determined from time-resolved PL decay curves.
Article
We demonstrate that the optical and photoresponsive electrical properties of a single nanowire (NW) consisting of a p-type poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were changed by hybridization with functionalized CdSe/ZnS QDs. Surface decorating and bulky infiltrating methods were employed for the hybridization of the QDs with the NWs. For the QD-infiltrated P3HT/PCBM single NW, the current density in both dark and light conditions was clearly enhanced and the nanoscale photoluminescence (PL) intensity was reduced due to the charge transfer effect. For the P3HT/PCBM NWs decorated with QDs, the current density was not changed much, however, the PL characteristics of both the QDs and the NWs were simultaneously changed by the energy transfer effect. From time-resolved PL spectra, the exciton lifetimes of the QDs in the hybrid NWs drastically decreased through the hybridization with the NWs, supporting the charge and/or energy transfer effects.
Article
Hybrid nanosystems comprising functionalized CdSe/ZnS core–shell quantum dots (QDs) on the surface of light-emitting poly(3-hexylthiophene) (P3HT), metallic copper (Cu), and insulating polystyrene (PS) nanowires (NWs) are fabricated. Using high-resolution scanning transmission electron microscopy, we observe that the QDs are attached to the surface of the NWs. The nanoscale photoluminescence (PL) characteristics of the hybrid QD/P3HT, QD/Cu, and QD/PS single NWs are investigated using laser confocal microscopy (LCM) with high spatial resolution. For the hybrid QD/P3HT single NW, the LCM PL intensity from the P3HT NW increases considerably, while that of the QDs decreases due to Förster resonance energy transfer. Hybridization affects the nanoscale PL characteristics of both the P3HT NW and the QDs. The LCM PL intensity of the hybrid QD/Cu NW is three times higher than that of the QD/PS NW, because of surface plasmon resonance coupling energy transfer between the QDs and the Cu NW. Time-resolved PL spectra reveal that the exciton lifetimes of the QDs drastically decrease after the hybridization with P3HT or Cu NWs, due to energy transfer effects. The nanoscale PL efficiency of the QDs can be controlled by hybridization with NWs having distinct properties.
Article
Polymeric micelles and polymersomes self-assembled from amphiphilic and double hydrophilic block copolymers (DHBCs) offer great promise as smart nanocarriers of chemotherapeutic drugs, genes, and imaging or contrast agents. Specifically, fluorescent polymeric assemblies and nanoparticles render possible the in situ tracking of intracellular transport, in vivo circulation characteristics, and biodistribution of drug-loaded or conjugated nanocarriers. The introduction of fluorescence resonance energy transfer (FRET) processes into polymeric assemblies and nanoparticles can further enhance relevant functions due to the fact that the tracking can be conducted in a ratiometric manner to effectively exclude background interference and enhance spatiotemporal detection resolution. Moreover, if the FRET efficiency is responsive to the variation of solution conditions such as pH value, temperature, metal ions, glucose, and tissue-specific enzymes by taking advantage of the responsiveness of polymeric matrix, sensing of the external microenvironment will also be achieved. This article highlights recent developments of fluorescent polymers, polymeric assemblies and nanoparticles involving FRET events, in which the modulation of FRET efficiency can be achieved by tuning the spatial distribution of fluorescent donors and acceptors.
Article
The understanding of underlying phenomena of localized surface plasmon resonance of metal nanoparticles (NPs) enables the unique control on the intrinsic properties of fluorophores in the proximity of metal NPs. While other parameters have to be considered, the near-field interactions between metal NPs and fluorophores are strongly interconnected with their nanoscale organization. In this perspective, many researchers are struggling to find a better assembling method to engineer NP–fluorophore interactions and also to discover a new opportunity in many fields of science. In this study, we applied the self-segregating property of block copolymer micelles to organize the spatial location of fluorophores in the proximity of metal NPs. The strong correlation of micellar structures with surface-plasmon-coupled fluorescence has been discussed with an emphasis on the strategy for fluorescence enhancement.
Article
Organic and hybrid organic–inorganic photovoltaics are among the most promising options for low-cost and highly scalable renewable energy. In order to fully realize the potential of these technologies, power conversion efficiencies and stability will both have to be improved beyond the current state-of-the-art. The morphology of the active layer is of paramount importance in the photon to electron conversion process in organic and hybrid solar cells, with all length scales, from molecular ordering to intradevice composition variability, playing key roles. Given the central influence of morphology, characterizing the structure of these surprisingly complex material systems at multiple length scales is one of the grand challenges in the field. This review addresses the techniques, some of which have only recently been applied to organic and hybrid photovoltaics, available to scientists and engineers working to understand—and ultimately improve—the operation of these fascinating devices.
Article
Waves on the surface of a fluid provide a powerful tool for studying the fluid itself and the surrounding physical environment. For example, the wave speed is determined by the force per unit mass at the surface, and by the depth of the fluid: the decreasing speed of ocean waves as they approach the shore reveals the changing depth of the sea and the strength of gravity. Other examples include propagating waves in neutron-star oceans and on the surface of levitating liquid drops. Although gravity is a common restoring force, others exist, including the electrostatic force which causes a thin liquid film to adhere to a solid. Usually surface waves cannot occur on such thin films because viscosity inhibits their motion. However, in the special case of thin films of superfluid 4He, surface waves do exist and are called `third sound'. Here we report the detection of similar surface waves in thin films of superfluid 3He. We describe studies of the speed of these waves, the properties of the surface force, and the film's superfluid density.
Article
In a previous paper [Chem. Phys. Lett. 33, 289 (1975)] we treated the kinetics of quenching of luminescent probes in micellar systems, assuming that the distribution of solubilized molecules among the micelles obeys Poisson statistics. In this paper we extend our treatment to a more general case where there is a limit to the number of solubilized molecules in any one micelle. Mechanisms for migration of solubilized molecules between micelles are also discussed.
Article
The chapter examines some general salient features of resonance energy transfer by stressing the kinetic competition of the Förster resonance energy transfer (FRET) pathway with all other pathways of de-excitation. This approach emphasizes many of the biotechnological and biophysical uses of FRET, along with emphasizing the important competing processes and biological functions of FRET in photosynthesis. The chapter focuses on a few critical essentials concerning the fundamentals of energy transfer and the methods of measurement. These basic aspects of FRET are helpful for understanding the important features and interpretations of energy transfer measurements. As various macromolecular systems are ideally suited for FRET applications, FRET has received so much interest in biotechnology and medicine as well as in biophysics. Applications for FRET extend from more traditional cuvette spectroscopic measurements on larger volumes to FRET imaging experiments in the fluorescence microscope and single molecule experiments. The recent applications of FRET in the optical microscope have become very popular because of its interpretive power on the molecular scale with regard to statically and dynamically associating molecular systems in cellular biology.
Article
Since fluorescence resonance energy transfer (FRET) between fluorophores strongly depends on the distance between and position of donors and acceptors at the nanometer scale, the accurate organization of multiple fluorophores in a specific arrangement plays a critical role in controlling their energy-transferring processes. Herein, we highlight our recent development on the utilization of nanostructures of diblock copolymer micelles for nanoscale arrangement of multiple fluorophores including quantum dots (QDs) to adjust FRET for tuning emissions from a single emitting layer.
Article
The nanoscale arrangement of quantum dots (QD) donors and fluorescent dye acceptors in a single-layered films of diblock copolymer micelles to control the fluorescence resonance energy transfer (FRET) was demonstrated. Single-layered films of micelles were produced by spin-coating on a quartz plate or a freshly cleaved mica substrate from micellar solutions of polystyrene-poly(4- vinylpyridine) (PS-PVP) copolymers. The sulforhodamine 101 (S101) dye are not stable and remain as powders in toluene micelles and are efficiently loaded into the solid film with the assistance of the micellar structure. The acceptor S101 dyes when incorporated in the core of the micelles shows that the energy transfer between Qds and S101 can be controlled by changing the nanometer-sized PS gap with micelles of different molecular weights.
Article
Controlled light emission is generated from thin films of fluorophores that are encapsulated in diblock copolymer micelles. The mutual distance between the two fluorophores is regulated by utilization of the nanometer-sized micellar structure, which enables or restricts energy transfer between the fluorophores. This encapsulation method results in single or simultaneous light emission (see figure).
Article
Recently, a new spotlight has been focused on block copolymers, thoroughly studied for nearly half a century, because of their potential use in numerous nanotechnologies. This renewed interest is a consequence of the self-assembled microdomains characteristic of these materials. The size, shape, and arrangement of these nanoscopic structures are all tunable through synthetic chemistry of the constituent molecules. Capturing the vast technological potential of block copolymers will, in many cases, require precise control over the orientation and alignment of the microdomains. This review summarizes extant applications and alignment techniques and provides an outlook toward the future. In an effort to provide a practical resource for researchers, the article is structured to identify the reported alignment approaches for a given polymer morphology rather than sorting by alignment technique. Specific materials have also been deemphasized because the alignment methods, with few exceptions, are general to a specific morphology or set of morphologies. In addition to a detailed summary of traditional methodologies, some very recent results such as optical alignment of liquid crystalline block copolymers, lithographic chemical patterning, and alignment in pores are highlighted.
Article
In 2004, we reported single-pair fluorescence resonance energy transfer (spFRET), based on a perylene diimide (PDI) and terrylene diimide (TDI) dyad (1) that was bridged by a rigid substituted para-terphenyl spacer. Since then, several further single-molecule-level investigations on this specific compound have been performed. Herein, we focus on the synthesis of this dyad and the different approaches that can be employed. An optimized reaction pathway was chosen, considering the solubilities, reactivities, and accessibilities of the building blocks for each individual reaction whilst still using established synthetic techniques, including imidization, Suzuki coupling, and cyclization reactions. The key differentiating consideration in this approach to the synthesis of dyad 1 is the introduction of functional groups in a nonsymmetrical manner onto either the perylene diimide or the terrylene diimide by using imidization reactions. Combined with well-defined purification conditions, this modified approach allows dyad 1 to be obtained in reasonable quantities in good yield.
Article
Template wetting is a simple, solution based nanofabrication method that has been shown effective for a wide range of polymers. Like other solution based polymer processing methods, it is reasonable to expect that the choice of solvent will have a significant impact on the chain orientation in the final solid structure. Here we examine the impact of wetting solvent on the properties of 100nm diameter poly(3-hexylthiophene) (P3HT) nanotubules made via template wetting. The degree of alignment of the P3HT backbone with the nanotubule axis as observed through dichroism in the FTIR spectrum was observed to depend on the strength of polymer–solvent interaction forces, observed experimentally through thermogravimetric analysis experiments. This solvent effect was not observed in other properties as neither the UV–Vis absorbance nor the hole mobility was observed to depend significantly on the wetting solvent. It is believed that the rigid rod structure and large side chain limited the degree of increase in the effective conjugation length and preventing even the aligned chains from being more tightly packed as would be necessary for an increase in inter-chain π-bond interactions sufficient to impact these performance characteristics of the material. [Copyright &y& Elsevier]
Article
Recent reports of multiexciton generation (MEG), a process by which one absorbed photon generates multiple excitons, in lead chalcogenide nanocrystals (NCs) have intensified research interest in using this phenomenon to improve the efficiency of solar energy conversion. Practical implementation of MEG processes in solar cells and solar-to-fuel conversion devices requires the development of materials with higher MEG efficiencies and lower excitation thresholds than are currently available, as well as schemes for efficient multiexciton extraction before the ultrafast exciton-exciton annihilation occurs. This Account focuses on the extraction of multiexcitons by interfacial electron transfer in model NC-molecular acceptor complexes. We provide an overview of multiexciton annihilation and multiexciton dissociation (MED) processes in NC-acceptor complexes of (i) CdSe quantum dots (QDs), (ii) CdSe/CdS quasi-type II core/shell QDs, (iii) CdSe quantum confined nanorods (QRs), and (iv) PbS QDs. We show that ultrafast electron transfer to adsorbed molecular acceptors can efficiently dissociate multiexcitons generated by absorption of multiple photons in (i), (ii), and (iii). Compared to core-only CdSe QDs, the electron hole distributions in CdSe/CdS quasi-type II QDs and CdSe QRs significantly improve their MED efficiencies by simultaneously retarding Auger recombination and facilitating interfacial electron transfer. Finally, in PbS-methylene blue (MB(+)) complexes, we show that the presence of electron acceptors does not affect the MEG efficiency and electron transfer to MB(+) efficiently dissociates the multiple excitons generated in PbS QDs. Our findings demonstrate that ultrafast interfacial charge transfer can be an efficient approach for extracting multiexcitons, and wavefunction engineering in quantum confined NCs can further improve MED efficiency.
Article
SEMICONDUCTOR nanocrystals offer the opportunity to study the evolution of bulk materials properties as the size of a system increases from the molecular scale1,2. In addition, their strongly size-dependent optical properties render them attractive candidates as tunable light absorbers and emitters in optoelectronic devices such as light-emitting diodes3,4 and quantum-dot lasers5,6, and as optical probes of biological systems7. Here we show that light emission from single fluorescing nanocrystals of cadmium selenide under continuous excitation turns on and off intermittently with a characteristic timescale of about 0.5 seconds. This intermittency is not apparent from ensemble measurements on many nanocrystals. The dependence on excitation intensity and the change in on/off times when a passivating, high-bandgap shell of zinc sulphide encapsulates the nanocrystal8,9 suggests that the abrupt turning off of luminescence is caused by photo-ionization of the nanocrystal. Thus spectroscopic measurements on single nanocrystals can reveal hitherto unknown aspects of their photophysics.
Article
Specific binding of biotinilated bovine serum albumin (bBSA) and tetramethylrhodamine-labeled streptavidin (SAv−TMR) was observed by conjugating bBSA to CdSe−ZnS core−shell quantum dots (QDs) and observing enhanced TMR fluorescence caused by fluorescence resonance energy transfer (FRET) from the QD donors to the TMR acceptors. Because of the broad absorption spectrum of the QDs, efficient donor excitation could occur at a wavelength that was well resolved from the absorption spectrum of the acceptor, thereby minimizing direct acceptor excitation. Appreciable overlap of the donor emission and acceptor absorption spectra was achieved by size-tuning the QD emission spectrum into resonance with the acceptor absorption spectrum, and cross-talk between the donor and acceptor emission was minimized because of the narrow, symmetrically shaped QD emission spectrum. Evidence for an additional, nonspecific QD−TMR energy transfer mechanism that caused quenching of the QD emission without a corresponding TMR fluorescence enhancement was also observed.
Article
Semiconductor sensitized solar cells have attracted growing interest in the past few years. Starting from quite low conversion efficiencies, these have grown very rapidly to values of around 4−5%. This Perspective analyzes the optimization of three aspects toward an increase in cell performance, (i) materials, including not only light-absorbing material but also electron and hole conductors and counter electrodes, (ii) control of recombination and band alignment by surface treatments, and (iii) development of absorbing nanocomposites with enhanced light-harvesting and -collecting properties. We argue that these key topics could promote major breakthroughs in the design and development of semiconductor-sensitized solar cells.
Article
We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 Å. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the growth of ZnS on CdSe cores as locally epitaxial and determine how the structure of the ZnS shell influences the photoluminescence properties.
Article
We propose a stochastic model for the kinetics of energy transfer from quantum dots or rods of CdS to Nile Red dye molecules. We assume that the distribution of the dye molecules around quantum dots or rods follows Poisson statistics. By analyzing time-resolved fluorescence decay curves of quantum dots or rods, we obtained the average number of dye molecules attached to the surface of quantum dots or rods as a function of the concentration of dyes and that of quantum dots or rods. The equilibrium constants for attachment/detachment of dye molecules to/from the surface of quantum dots or rods were evaluated. The quenching rate constants per dye molecule attached to the surface of quantum dots or rods were also estimated.
Article
The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future research efforts toward utilization of tailored nanostructures will be an important challenge for the development of next generation solar cells.
Article
Exciton separation dynamics in the electron transfer system containing highly photostable CdSe/CdS core/shell nanocrystal quantum dots and adsorbed methylviologen was investigated by means of femtosecond absorption spectroscopy. The experiments revealed that electron extraction from the photoexcited core is possible, and the rate of the ET reaction strongly depends on the CdS shell thickness. A CdS associated exponential decay constant β of 0.33 Å−1 was obtained reflecting the electronic barrier effect of the shell. These findings show that core/shell structures are well suited for the design of optimized QD-based solar cells.
Article
In the present study, we found a pronounced effect on the PL and shortening of the overall lifetime of rhodamine 6G (R6G) when interacting with the spherical, shaped, and capped Au NPs, but there are no measured effects on radiative rate of the dye. The observed Förster distances (R0) are 105.75, 170, and 124 Å for spherical, shaped, and capped Au nanoparticles, respectively and the distances between the donor and acceptor are 103.56, 194.77, and 134.43 Å. However, the distances between the donor and acceptor are 78.26, 99.04, and 88.43 Å for spherical, shaped, and capped Au nanoparticles, respectively, using the efficiency of surface energy transfer which follows a 1/d4 distance dependence between donor and acceptor. On the basis of these findings, we may suggest that surface energy transfer process has a more reasonable agreement with experimental finding. The anisotropy decay of R6G with spherical Au nanoparticles is single exponential with correlation time constant of 240 ps. However, the anisotropy decay of R6G in shaped nanoparticles is biexponential with correlation time constants of 347 ps and 2.46 ns.
Article
The synthesis of epitaxially grown, wurtzite CdSe/CdS core/shell nanocrystals is reported. Shells of up to three monolayers in thickness were grown on cores ranging in diameter from 23 to 39 Å. Shell growth was controllable to within a tenth of a monolayer and was consistently accompanied by a red shift of the absorption spectrum, an increase of the room temperature photoluminescence quantum yield (up to at least 50%), and an increase in the photostability. Shell growth was shown to be uniform and epitaxial by the use of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and optical spectroscopy. The experimental results indicate that in the excited state the hole is confined to the core and the electron is delocalized throughout the entire structure. The photostability can be explained by the confinement of the hole, while the delocalization of the electron results in a degree of electronic accessibility that makes these nanocrystals attractive for use in optoelectronic devices.
Article
For concurrent emission of multiple fluorophores from a single emitting layer with a highly efficient energy transfer from antenna molecules to emitting molecules by fluorescence resonance energy transfer (FRET), a paradoxical requirement in an emitting layer is necessary, that is, close placement of an emitting fluorophore and a harvesting molecule, and isolation of emitting fluorophores. Here we demonstrate how to overcome this paradox by full utilization of a micellar nanostructure consisting of a core and a corona, that is, the core is used as a place for FRET between light-collecting donors and light-emitting fluorophores, and the corona is used as a barrier for FRET between light-emitting fluorophores. Enhancement of light emission from fluorophores was achieved by locating emitting fluorophores and light-harvesting molecules at the same core of micelles. Moreover, with the same micellar nanostructure, concurrent emission of multiple fluorophores with enhanced intensity was induced by isolating them in independent micelles, the corona structure of which worked as an effective blockade for FRET.
Article
In pursuit of a better understanding of how electronic excitation migrates within complex structures, the concept of resonance energy transfer is being extended and deployed in a wide range of applications. Utilizing knowledge of the quantum interactions that operate in natural photosynthetic systems, wide-ranging molecular and solid-state materials are explored in the cause of more efficient solar energy harvesting, while advances in theory are paving the way for the development and application of fundamentally new mechanisms. In this review, an introduction to the underlying processes that cause singlet-singlet and triplet-triplet energy transfer leads into a discussion of how a new conception of these fundamental processes has emerged over recent years. Illustrative examples relevant to laser science and photonics are described, including photosynthetic light-harvesting, light-activated sensors, processes of cooperative and accretive energy pooling and quantum cutting in rare earth-doped crystals, and incoherent triplet-triplet energy upconversion in molecular solutions.
Article
a b s t r a c t A review is presented of recent work on (1) the origin of the concept of enhanced multiple electron–hole pair (i.e. exciton) production in semiconductor quantum dots (QDs), (2) various experiments based on time-resolved fs to ns spectroscopy (transient IR absorption, transient visible to near-IR bleaching due to state filling, terahertz spectroscopy, and time-resolved photoluminescence) that support the occur-rence of highly efficient multiple exciton generation (MEG) in QDs, (3) thermodynamic analyses of the theoretical enhancement of the conversion efficiency in solar cells that are based on MEG in QDs, (4) MEG in QD arrays that can be used in QD solar cells, (5) theoretical models to explain MEG, and (6) some recent controversy about the evidence for MEG.
Article
We have investigated the sensitization of nanoporous titanium dioxide by previously synthesized CdSe quantum dots (QDs) protected with trioctylphosphine. Covering the nanoporous TiO 2 films with QDs has been achieved using two strategies: (i) direct adsorption from dichoromethane dispersions and (ii) anchoring the QDs through a molecular linker, concretely, mercaptopropionic acid (MPA). In contrast with MPA-mediated adsorption, direct adsorption leads to a high degree of QD aggregation, as revealed by atomic force microscopy (AFM) images obtained with both TiO 2 nanoporous films and monocrystalline surfaces. Importantly, at saturation, only 14% of the real surface area of a 5-µm thick P25 TiO 2 layer is covered for both attachment modes. For MPA attachment, the incident photon-to-current efficiency (IPCE) increases with the loading, whereas a maximum (close to 40% at the QD excitonic peak) is defined for intermediate coverages in the case of QD direct adsorption. In addition, for equivalent QD loading, IPCE values are larger in the case of direct adsorption.