Plasmon-enhanced nonradiative energy transfer is demonstrated in two inorganic semiconductor systems. The first is
comprised of colloidal nanocrystal CdTe donor and acceptor quantum dots, while the second is a hybrid InGaN quantum
well-CdSe/ZnS quantum dot donor-acceptor system. Both structures are in a planar geometry. In the first case a
monolayer of Au nanospheres is sandwiched between donor and acceptor quantum dot monolayers. The largest energy
transfer efficiency is seen when the donor is ~3 nm from the Au nanopshere. A plasmon-enhanced energy transfer
efficiency of ~ 40% has been achieved for a separation of 3 nm between the Au nanopshere monolayer and the acceptor
monolayer. Despite the increased energy transfer efficiency these conditions result in strong quenching of the acceptor
QD emission. By tuning the Au nanosphere concentration and Au nanosphere-acceptor QD separation the acceptor QD
emission can be increased by a factor of ~2.8. The plasmon-enhanced nonradiative energy transfer is observed to extend
over larger distances than conventional Forster resonance energy transfer. Under the experimental conditions reported
herein, it can be described by the same d-4 dependence but with a larger characteristic distance. Using a Ag nanobox
array plasmonic component plasmon-enhanced nonradiative energy transfer has also demonstrated from an InGaN
quantum well to a ~80 nm thick layer of CdSe/ZnS colloidal quantum dots. An efficiency of ~27% is achieved, with an
overall increase in the QD emission by ~70%.
Networks of silver nanowires (AgNWs) are promising candidates for transparent conducting electrodes in organic photovoltaics (OPV), as they achieve similar performance as the commonly used indium tin oxide (ITO) at lower cost and increased flexibility. The initial sheet resistance (Rs) of AgNW electrodes typically needs to be reduced by a post-annealing step (90 min@200 °C), being detrimental for processing on polymeric substrates.
We present novel low temperature-based methods to integrate AgNWs in organic small molecule-based photovoltaics, either as transparent and highly conductive bottom-electrode or, for the first time, as spray-coated AgNW top-electrode. The bottom-electrodes are prepared by organic matrix assisted low-temperature fusing. Here, selected polymers are coated below the AgNWs to increase the interaction between NWs and substrate. In comparison to networks without these polymeric sublayers, the Rs is reduced by two orders of magnitude.
AgNW top-electrodes are realized by dispersing modified high-quality AgNWs in inert solvents, which do not damage small molecule layers. Accordingly, our AgNW dispersion can be spray-coated onto all kind of OPV devices. Both bottom- and top-electrodes show a Rs of <11 Ω/ at >87 % transparency directly after spray-coating at very low substrate temperatures of <80 °C. We also demonstrate the implementation of our AgNW electrodes in organic solar cells. The corresponding devices show almost identical performance compared to organic solar cells exploiting ITO as bottom or thermally evaporated thin-metal as top-electrode.
Further improvement of infrared single photon sources is a major challenge for future implementations of quantum information and quantum communication applications. In this paper, we give further insight into a recently presented, conceptually novel method for the generation of single photons.1 The method is of particular interest for spectral domains where stable room temperature single photon sources are not available. For example, this is the presently the case for the near-infrared. This wavelength regime is important for data transfer over long distances where optical losses in fibers are minimal. The presented method is based on the following idea. The fundamental key requirement for single photon generation is the generation of a single excitation in an optically active system. It is not the presence of a single quantum system. The presented method is applied to realize a stable, non-blinking, room temperature infrared single photon source by converting visible single photons from a defect center in diamond to the near infrared. For the presented implementation, the theoretical conversion efficiency was estimated to be 26 %. In a first prove of principle experiment, a conversion efficiency of 0.1 % was achieved.
A general synthetic strategy for the synthesis of nanocrystals of both visible and near infrared emitting materials is
introduced. Further, the potential for these materials to be employed in a wide variety of applications is discussed.
In this work, we demonstrate optical functionalities obtained with CdTe nanocrystals embedded in polystyrene.
These functionalities are based on our experimentally observed large, saturable, and controlled nonlinear optical
properties of CdTe nanocrystals, in the case of strong quantum confinement and near resonant interaction with the
excitation light. Our investigation considers the optical limiting functionality, presenting experimental proof of concepts.
These types of functionalities are of special interest for integrated optical quantum dots devices with applications in
imaging and telecommunications.
We present a detailed study of the localized coupled-cavity modes in a photonic molecule formed from two dielectric
spherical microcavities with CdTe nanocrystals. A layer-by-layer deposition technique provides controllable coating of
the microspheres with a shell of close-packed nanocrystals of approximately 4 nm in diameter. The observed spectral
structures originate from the coupling of the electronic transitions in nanocrystals and the photon states of interacting
microspheres and, in analogy to the formation of molecular electronic orbits, can be assigned to bonding and antibonding
photon localized states.
The synthesis of strongly luminescing semiconductor nanocrystals reported is based on a wet-chemical approach. The
use of aqueous solutions and the avoiding of dangerous and unstable precursors make this synthetic approach to be easily
up-scaleable. The reaction yields are approaching gram amounts of the dried product and are limited solely by the
laboratory facilities and thus may be further increased by using industrial equipments. The nanocrystals obtained by this
route are strongly luminescing and suitable for various assembling procedures allowing the creation of core-shell
spherical and thin-film composites, that are promising for photonic and optoelectronic applications. New assembly
procedures allowing template-based formations of different types of metal-dielectric and porous metal nanostructures are
presented.
We describe how two different kinds of ordered bimetallic nanostructures (Au/Pt, Au/Ag) with hierarchical porosity, such as macroporous nanostructures and nanostructures constructed from hollow spheres, can be selectively and conveniently fabricated by a general template technique on silicon wafers and on glass substrates, and how such nanostructures can find application in catalyst or surface-enhanced Raman scattering (SERS) substrates.
A 5-fold enhancement in the luminescence of CdTe nanocrystal quantum dots (QDs) is observed when they are placed in proximity to a nanostructured Au film deposited by pulsed laser deposition technique. No enhancement is observed with a nanostructured Ag film. The enhancement is due to the interaction of the QDs excitons with the localized surface plasmons (LSP). The Au surface plasmon (SP) frequency is closer to the QDs emission frequency than Ag LSP frequency and this accounts for the differences in observed behavior. As the SP-QD interaction strongly depends on the geometric structure and shape of the metal nanoparticles, a comparison with QDs deposited on a film of Au colloidal nanoparticles is presented. In the case of QDs placed directly on the Au colloids the luminescence quenching is much stronger and with a spacer layer a 3.5-fold enhancement over the bare QDs luminescence is observed.
We present a detailed study of the localized coupled-cavity modes in a photonic molecule formed from two dielectric spherical microcavities with CdTe nanocrystals, which show a multi-peak narrowband modal structure resulting from lifting of the mode degeneracy with respect to the azimuthal quantum number. The waveguiding through the coupled microcavities and wavelength switching effect is demonstrated. The feasibility of photonic molecules as the basis for a multi-channel, wavelength-tunable optical delay device is analysed.
We present a detailed study of the localized coupled-cavity modes in a photonic molecule formed from two dielectric spherical microcavities with CdTe nanocrystals, which show a multi-peak narrowband modal structure resulting from lifting of the mode degeneracy with respect to the azimuthal quantum number. The feasibility of photonic molecules as the basis for a multi-channel, wavelength-tunable optical delay device is analysed.
Spherical microcavities consisting of a dielectric material show unique optical characteristics as resonators in combination with semiconductor nanoparticles. A high quality factor results in a very narrow bandwidth of the resonant modes (whispering-gallery modes) inside the microcavity. The polystyrene microspheres are coated with one monolayer of CdTe nanocrystals which offer a high photostability and a high quantum yield at room temperature. Due to strong confinement of the electrons in all three dimensions, excitation from the quantum dots is highly size-dependent and tuneable over almost the whole visible spectrum. The deposition of the nanocrystals on the sphere surface allows efficient coupling of the light of the CdTe quantum dots into the microcavity. Photoluminescence and Raman spectra were taken with a Renishaw Raman system. The setup is equipped with an Ar+-laser and a HeNe-laser to excite the nanocrystals. Raman measurements show a series of very sharp resonant peaks instead of a continuous spectrum. Strong interaction between the electronic states of the nanocrystals and the resonant modes in the microsphere causes a considerable enhancement of the Raman scattering and luminescence from the CdTe quantum dots in Stokes and anti-Stokes region. Furthermore, a linear blue shift of the resonances in the photoluminescence spectrum was observed during continuous excitation for 18 minutes with a HeNe laser.
We present details on the CdTe nanowires formation, which were found to grow in a standard physiological phosphate-buffered solution, including in-situ observation of growth with a confocal microscope. The choice of proper nanocrystals concentration allowed reasonably slow growth rates and thus a controllable formation of nanowires. Once formed in solution, nanowires showed a significant degree of structural rigidity and resistance to externally applied mechanical stress. Luminescence and Raman spectroscopy data evidence on re-crystallization processes during the nanowire formation.
A heterojunction between two 3-dimensional photonic crystals has been realized by interfacing two opal films of different lattice constants. The interface-related transmission minimum has been observed in the frequency range between two directional lowest-order bandgaps of the hetero-opal constituents. The interface transmission minimum has been modelled numerically and tentatively explained by formation of the standing wave across the photonic hetero-crystal due to matching of group velocities of optical modes in both parts at this frequency.
We report highly efficient Forster resonance energy transfer between CdTe nanocrystals and two different dyes, Rhodamine B and Oxazine, where the nanocrystals are mixed with the dyes on top of glass substrates. A faster NC decay curve is observed in the samples containing NCs mixed with dyes than in those containing NCs on their own. For the samples containing nanocrystals mixed with Rhodamine B, room temperature PL measurements are presented as a function of the ratio between the amount of acceptors and the amount of donors, CA/CD. This ratio is varied between 0.03 and 5. The strongest enhancement of the acceptor PL intensity relative to that of the donor PL intensity is reached for 0.2A/CD<5, suggesting that most efficient FRET is also achieved in this region.
We measured the coupling of the florescence from semiconductor nano-crystals and sub-micron sized dye-doped beads to high-Q whispering gallery modes (WGM) of a microsphere resonator. With Q-factors as high as 109 the florescence could be extracted in a controlled way via a prism coupler. We observed nearly 100 % modulation in
the spectrum which reflects the coupling to the WGMs. With the help of a beam scanning confocal microscope we were able to address a single 500 nm sized dye-doped bead on the sphere’s surface and to collect and analyze its florescence in a well defined manner via the prism coupler.
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