Photoinduced electron transfer in composites of conjugated polymers and dendrimers with branched colloidal nanoparticles.

Charge generation and separation dynamics in donor:acceptor systems based on composites of branched CdSe nanoparticles with a phenyl-cored thiophene-containing dendrimer (4G1-3S), or a low-bandgap conjugated polymer (PCPDTBT) are reported upon exclusive excitation of the donor or the acceptor. Time-resolved microwave conductivity is used to study the dynamics of either transfer of holes from the nanoparticle to dendrimer, or conversely the transfer of electrons from the polymer to the nanoparticle. Higher photoconductance signals and longer decay-times are correlated with device efficiencies, where composites with higher nanoparticle concentration exhibit higher solar photovoltaic power conversion efficiencies and an increase in external quantum efficiencies. This work evaluates the contribution of both components to device performance, but specifically the role of photoexcited nanoparticles.

Controlled assembly of hydrogenase-CdTe nanocrystal hybrids for solar hydrogen production.

We present a study of the self-assembly, charge-transfer kinetics, and catalytic properties of hybrid complexes of CdTe nanocrystals (nc-CdTe) and Clostridium acetobutylicum [FeFe]-hydrogenase I (H(2)ase). Molecular assembly of nc-CdTe and H(2)ase was mediated by electrostatic interactions and resulted in stable, enzymatically active complexes. The assembly kinetics was monitored by nc-CdTe photoluminescence (PL) spectroscopy and exhibited first-order Langmuir adsorption behavior. PL was also used to monitor the transfer of photogenerated electrons from nc-CdTe to H(2)ase. The extent to which the intramolecular electron transfer (ET) contributed to the relaxation of photoexcited nc-CdTe relative to the intrinsic radiative and nonradiative (heat dissipation and surface trapping) recombination pathways was shown by steady-state PL spectroscopy to be a function of the nc-CdTe/H(2)ase molar ratio. When the H(2)ase concentration was lower than the nc-CdTe concentration during assembly, the resulting contribution of ET to PL bleaching was enhanced, which resulted in maximal rates of H(2) photoproduction. Photoproduction of H(2) was also a function of the nc-CdTe PL quantum efficiency (PLQE), with higher-PLQE nanocrystals producing higher levels of H(2), suggesting that photogenerated electrons are transferred to H(2)ase directly from core nanocrystal states rather than from surface-trap states. The duration of H(2) photoproduction was limited by the stability of nc-CdTe under the reactions conditions. A first approach to optimization with ascorbic acid present as a sacrificial donor resulted in photon-to-H(2) efficiencies of 9% under monochromatic light and 1.8% under AM 1.5 white light. In summary, nc-CdTe and H(2)ase spontaneously assemble into complexes that upon illumination transfer photogenerated electrons from core nc-CdTe states to H(2)ase, with low H(2)ase coverages promoting optimal orientations for intramolecular ET and solar H(2) production.

Photovoltaic devices with a low band gap polymer and CdSe nanostructures exceeding 3% efficiency.

We report the fabrication and measurement of solar cells approaching a power conversion efficiency of 3.2% using a low band gap conjugated polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] and CdSe nanoparticles. These devices exhibit an external quantum efficiency (EQE) of >30% in a broad range of 350-800 nm with a maximum EQE of 55% in a range of 630-720 nm. We also present certified device efficiencies of 3.13% under AM 1.5 illumination.

Direct synthesis of CdSe nanoparticles in poly(3-hexylthiophene).

A new approach was developed for the synthesis of nearly monodisperse CdSe nanoparticles directly in a polymer-containing solution in the absence of any other surface capping molecules. The comparatively high synthesis temperature reaction produces good quality crystalline CdSe nanoparticles. Time-resolved microwave conductivity measurements show that photoinduced charge separation occurs at the interface between the CdSe quantum dots and the polymer. This method can be extended to the synthesis of other II-VI semiconductor nanomaterials directly in a polymer-containing solution.

One- and two-photon induced QD-based energy transfer and the influence of multiple QD excitations.

Due to the increased use of quantum dots (QDs) in diverse laser microscopies, it is interesting to study the excitation pump power and excitation wavelength dependence of QD-based energy transfer (ET) processes. The ET in QD conjugates with phthalocyanines (Pcs) was studied with femtosecond time-resolved pump-probe spectroscopy upon one- and two-photon excitation. At the used excitation wavelengths only the QDs are excited and become the energy donors. Due to the matched spectral overlap of QD photoluminescence and Pc absorption, the ET occurs on a picosecond time scale. The ET process shows strong pump power dependence whereby an increase in excitation power results in multiple QD excitations and in shorter excited state lifetimes on the QDs due to Auger relaxation. As a result, high excitation pump power leads also to an accelerated ET to the acceptor molecules from the initially multiply excited states of the QDs. Excited state quenching studies as function of pump power suggest that ET occurs mainly from the lowest one-exciton state (n = 1) and only to a minor extent from the multiply excited states (n > 1). For the short-lived, multiply excited states the ET competes inefficiently with Auger recombinations and energy transfer efficiencies of phi(ET)(n=1>) approximately 20%, phi(ET)(n=2>) approximately 7%, phi(ET)(n=3>) < or = 2% were obtained. Also after two-photon excitation the ET efficiency is highest from the one-exciton state. The experimentally determined ET efficiencies were compared with theoretical ET efficiencies upon multiple excitations. In both cases the ET efficiency decreases with the increase in excitation pump power.

Effect of the functionalization of the axial phthalocyanine ligands on the energy transfer in QD-based donor-acceptor pairs.

This study examines the electronic coupling between quantum dots (QDs) and molecules on their surfaces as a function of the modality of their interaction. As a probe, the energy transfer (ET) between CdSe QDs and phthalocyanines (Pcs) was monitored and evaluated with regard to the functionalization of the axial phthalocyanine ligand, bulkiness of the functional group bridging the QD donor and Pc acceptor, and the number of the functionalized axial ligands. New silicon PCs and their conjugates with CdSe QDs were synthesized. The ET efficiency and kinetics were studied by steady state and femtosecond time-resolved absorption spectroscopy. We observed a decrease in ET efficiency with the increase in functional group bulkiness, which could be explained by increasing steric hindrance between the ET pair. In addition, a higher ET efficiency was observed for amino and thiol functionalized Pcs compared to Pcs without functional group on the axial alkyl chain.

Surface effects on quantum dot-based energy transfer.

CdSe quantum dot (QD)-phthalocyanine (Pc) conjugates were prepared as energy transfer donor-acceptor pairs, and the efficiency of the energy transfer process in this system was investigated as a function of QD size and under different surface chemistry conditions. The kinetics and efficiency of the energy transfer process were studied by femtosecond time-resolved laser spectroscopy. We observed that the energy transfer efficiency does not follow a linear dependence on spectral overlap integrals as predicted by the Förster theory for molecules. This observation is found to be due to the involvement of QD surface states in the energy transfer process from the photoexcited QDs to the molecular energy acceptor.

Observation of non-Förster-type energy-transfer behavior in quantum dot-phthalocyanine conjugates.

The effect of linker chain length on the energy transfer from CdSe quantum dots (QDs) to silicon phthalocyanine (Pc) photodynamic therapy agents was investigated by steady-state and femtosecond time-resolved spectroscopy with 500 nm light for the specific excitation of the QD energy donor. The conjugation between the QD and the Pc was achieved with linker chains varying from 4 to 9 bond lengths by incorporating 1-6 methylene groups into the axial ligand of the Pc. With increasing chain length, the energy-transfer efficiency increased, which appears to be opposed to a purely Förster-type resonance energy-transfer behavior that is commonly discussed for the energy transfer in QD conjugates. The obtained results provide strong evidence for a capping-layer-mediated energy transfer in the QD-based donor-acceptor conjugates.

Quantum dot-based energy transfer: perspectives and potential for applications in photodynamic therapy.

Quantum dots have emerged as an important class of material that offers great promise to a diverse range of applications ranging from energy conversion to biomedicine. Here, we review the potential of using quantum dots and quantum dot conjugates as sensitizers for photodynamic therapy (PDT). The photophysics of singlet oxygen generation in relation to quantum dot-based energy transfer is discussed and the possibility of using quantum dots as photosensitizer in PDT is assessed, including their current limitations to applications in biological systems. The biggest advantage of quantum dots over molecular photosensitizers that comes into perspective is their tunable optical properties and surface chemistries. Recent developments in the preparation and photophysical characterization of quantum dot energy transfer processes are also presented in this review, to provide insights on the future direction of quantum dot-based photosensitization studies from the viewpoint of our ongoing research.

Doped semiconductor nanomaterials.

The development and properties of doped nanomaterials including doped titanium dioxide, doped silicon, and doped cadmium telluride are reviewed, as well as their ultrafast dynamics. Doping nanomaterials provides a flexible way to tune to the properties of the materials while maintaining their high surface areas. The electronic, optical, photochemical, photoelectrochemical, photocatalytic and photoexcited relaxation properties can be tuned towards the desired direction by doping different elements. The materials can be engineered towards specific applications through careful selection of the dopants.