Hybrid nanorod-polymer solar cells

Charge transfer efficiency in hybrid bulk heterojunction composites

On the basis of recently published electrochemical measurements, the charge transfer efficiency within CdSe nanocrystal/conducting polymer heterojunction composites was investigated by means of luminescence interaction strength. It was found that poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] and poly-9-vinylcarbazole luminescence was not totally quenched by nanocrystals, whereas poly-3-octylthiophene and polyvinylpyrrolidone was completely quenched. In case of poly-3-hexylthiophene, the nanocrystal luminescence was quenched. The results are in complete agreement with the electrochemical findings and thus, the CdSe nanocrystal/Polyvinylpyrrolidone composite should be a promising material for electroluminescent devices.

Colloidal nanocrystal heterostructures with linear and branched topology

The development of colloidal quantum dots has led to practical applications of quantum confinement, such as in solution-processed solar cells, lasers and as biological labels. Further scientific and technological advances should be achievable if these colloidal quantum systems could be electronically coupled in a general way. For example, this was the case when it became possible to couple solid-state embedded quantum dots into quantum dot molecules. Similarly, the preparation of nanowires with linear alternating compositions--another form of coupled quantum dots--has led to the rapid development of single-nanowire light-emitting diodes and single-electron transistors. Current strategies to connect colloidal quantum dots use organic coupling agents, which suffer from limited control over coupling parameters and over the geometry and complexity of assemblies. Here we demonstrate a general approach for fabricating inorganically coupled colloidal quantum dots and rods, connected epitaxially at branched and linear junctions within single nanocrystals. We achieve control over branching and composition throughout the growth of nanocrystal heterostructures to independently tune the properties of each component and the nature of their interactions. Distinct dots and rods are coupled through potential barriers of tuneable height and width, and arranged in three-dimensional space at well-defined angles and distances. Such control allows investigation of potential applications ranging from quantum information processing to artificial photosynthesis.

A General Solution-Phase Approach to Oriented Nanostructured Films of Metal Chalcogenides on Metal Foils: The Case of Nickel Sulfide

Oriented films of nickel sulfide nanostructures, ranging from hierarchical dendrites to nanobelts and nanorods, were hydrothermally grown on Ni foils. This approach has proven to be a general method for preparing nanostructured metal chalcogenides films on corresponding metal foils.

Surface Passivation of Luminescent Colloidal Quantum Dots with Poly(Dimethylaminoethyl methacrylate) through a Ligand Exchange Process

Polydimethylaminoethyl methacrylate (PDMAEMA) was used as a multidentate ligand to modify the surface of CdSe/ZnS core-shell colloidal quantum dots in toluene with trioctylphosphine oxide (TOPO) as the surface ligand. Adsorption of PDMAEMA was accompanied by release of TOPO. The process is free of agglomeration, and the modified nanocrystals become soluble in methanol. The photoluminescence properties are well-preserved in either toluene or methanol.

Polarization surface-charge density of single semiconductor quantum rods

Electrostatic force microscopy was used to determine that single CdSe quantum rods (QRs) have a permanent polarization surface-charge density, an unexpected observation for supposedly well-shaped particles. The surface charge results from a slight angle between the QR sides and the direction of internal electric polarization. By contrast, despite the large dipole moment expected for CdSe QRs, none was observed. The unavoidable presence of permanently charged surfaces on CdSe QRs has the potential to impede the development of novel devices incorporating these materials.

High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion

We demonstrate for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than 3 times the NC band gap energy. The generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) time scale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells.

Employing End-Functional Polythiophene To Control the Morphology of Nanocrystal−Polymer Composites in Hybrid Solar Cells

A poly(3-hexylthiophene) containing an interacting amino chain end enhances the performance of P3HT/CdSe solar cells by increasing the dispersion of CdSe nanocrystals and improving the morphology of the nanocomposite without introducing insulating surfactants.

Effect of monomer geometry on the fractal structure of colloidal rod aggregates

The fractal structure of clusters formed by diffusion-limited aggregation of rodlike particles is characterized over three decades of the scattering vector q, and displays an unexpected dependence on the aspect ratio of the constituent monomers. Monte Carlo simulations of aggregating Brownian rods corroborate the experimental finding that the measured fractal dimension is an increasing function of the monomer aspect ratio. Moreover, increasing the rod aspect ratio eliminates the structural distinction between diffusion- and reaction-limited cluster aggregation that is observed for spheres.

Density-functional study of organic–inorganic hybrid single crystal ZnSe(C2H8N2)1/2

Unusual properties (i.e., strong band dispersion, high carrier mobility, wide absorption-energy window, and sharp band-edge transition) that are desirable for hybrid-material electronics and for solar electric energy conversion are predicted to exist in the organic-inorganic chalcogenide single crystal ZnSe(C2H8N2)(1/2) by using density-functional calculations. A simple mechanism, namely that the band-edge electronic states of the hybrid composite is predominantly determined by the inorganic constituent, is revealed to be responsible for governing these properties. Suggestions for further engineering hybrid semiconductors are also provided

Growth of InP Nanostructures via Reaction of Indium Droplets with Phosphide Ions: Synthesis of InP Quantum Rods and InP−TiO2 Composites

InP quantum rods were synthesized via the reaction of monodispersed colloidal indium droplets with phosphide ions. In(0) droplets, which do not act as a catalyst but rather a reactant, are completely consumed. The excess electrons that are produced in this reaction are most likely transferred to an oxide layer at the indium surface. For the synthesis of InP quantum rods with a narrow size distribution, a narrow size distribution of In(0) particles is also required because each indium droplet serves as a template to strictly limit the lateral growth of individual InP nanocrystals. Free-standing quantum rods, 60, 120, or 150 A in diameter, with aspect ratios of 1.6-3.5, and without the residual metallic catalyst at the rod tip, were synthesized from the diluted transparent solution of metallic indium particles. The same approach was used to synthesize InAs quantum rods. A photoactive InP-TiO(2) composite was also prepared by the same chemical procedure; InP nanocrystals grow as well-defined spherical or slightly elongated shapes on the TiO(2) surface.

Excitonic coupling in polythiophenes: Comparison of different calculation methods

In conjugated polymers the optical excitation energy transfer is usually described as Forster-type hopping between so-called spectroscopic units. In the simplest approach using the point-dipole approximation the transfer rate is calculated based on the interaction between the transition dipoles of two spectroscopic units. In the present work we compare this approach with three others: The line-dipole approximation, the Coulomb integral between the transition densities, and a quantum-chemical calculation of the interacting dimer as entity. The latter two approaches are based on the semiempirical method ZINDO. The line-dipole approximation is an attractive compromise between computational effort and precision for calculations of the excitonic coupling in extended conjugated polymers.

A new approach to the synthesis of conjugated polymer–nanocrystal composites for heterojunction optoelectronics

We report a simple one pot process for the preparation of lead sulfide (PbS) nanocrystals in the conjugated polymer poly (2-methoxy-5-(2'ethyl-hexyloxy)-p-phenylene vinylene)(MEH-PPV), and we demonstrate electronic coupling between the two components.

Fluorescence Resonance Energy Transfer Between Quantum Dot Donors and Dye-Labeled Protein Acceptors

We used luminescent CdSe-ZnS core-shell quantum dots (QDs) as energy donors in fluorescent resonance energy transfer (FRET) assays. Engineered maltose binding protein (MBP) appended with an oligohistidine tail and labeled with an acceptor dye (Cy3) was immobilized on the nanocrystals via a noncovalent self-assembly scheme. This configuration allowed accurate control of the donor-acceptor separation distance to a range smaller than 100 A and provided a good model system to explore FRET phenomena in QD-protein-dye conjugates. This QD-MBP conjugate presents two advantages: (1) it permits one to tune the degree of spectral overlap between donor and acceptor and (2) provides a unique configuration where a single donor can interact with several acceptors simultaneously. The FRET signal was measured for these complexes as a function of both degree of spectral overlap and fraction of dye-labeled proteins in the QD conjugate. Data showed that substantial acceptor signals were measured upon conjugate formation, indicating efficient nonradiative exciton transfer between QD donors and dye-labeled protein acceptors. FRET efficiency can be controlled either by tuning the QD photoemission or by adjusting the number of dye-labeled proteins immobilized on the QD center. Results showed a clear dependence of the efficiency on the spectral overlap between the QD donor and dye acceptor. Apparent donor-acceptor distances were determined from efficiency measurements and corresponding Förster distances, and these results agreed with QD bioconjugate dimensions extracted from structural data and core size variations among QD populations.

The use of surface-active agents in the preparation and assembly of quantum-sized nanoparticles

This overview summarises the use of surface-active agents for preparation of nanoparticles. It starts with description of the nanotechnology vision. Nanoparticle products are exemplified by using solid-state processes, e.g. for the production of ceramics or catalysts, as well as colloidal processes whereby the use of surface-active agents plays a major role. Several examples are given in the review, e.g. use of coordination ligands and nanoparticle self-assembly. The process of control of nanoparticle shape and its modulation is briefly described and examples are given to demonstrate such control.

Quaternary Self-Organization of Porphyrin and Fullerene Units by Clusterization with Gold Nanoparticles on SnO2 Electrodes for Organic Solar Cells

Novel organic solar cells prepared using quaternary self-organization of porphyrin (donor) and fullerene (acceptor) dye units by clusterization with gold nanoparticles on SnO2 electrodes exhibit the remarkable enhancement of the photoelectrochemical properties relative to the reference systems.

Shape Effects on Electronic States of Nanocrystals

High-performance supercomputing and high-fidelity atomistic methods are used to study the shape effects on the single-particle electronic states of nanocrystals. We found that the shape can be used as an efficient way to control the electronic structures of the nanocrystals. Changing the shape is more flexible and provides more variety of electronic states than simply changing the size of the system. The special features of the electronic states achieved by different shapes of the nanocrystals can be used in various device applications. Simple rules are summarized to predict the electronic structure shape effects on similar nanocrystals.

In situ quantum dot growth on multiwalled carbon nanotubes

The generation of nanoscale interconnects and supramolecular, hierarchical assemblies enables the development of a number of novel nanoscale applications. A rational approach toward engineering a robust system is through chemical recognition. Here, we show the in situ mineralization of crystalline CdTe quantum dots on the surfaces of oxidized multiwalled carbon nanotubes (MWNTs). We coordinate metallic precursors of quantum dots directly onto nanotubes and then proceed with in situ growth. The resulting network of molecular-scale "fused" nanotube-nanocrystal heterojunctions demonstrates a controlled synthetic route to the synthesis of complex nanoscale heterostructures. Extensive characterization of these heterostructures has been performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-visible spectroscopy, and X-ray diffraction (XRD).