Marcus Jones

Enhancement of Multiphoton Emission from Single CdSe Quantum Dots Coupled to Gold Films

Single molecule time-resolved fluorescence spectroscopy of CdSe/ZnS core-shell quantum dots (QDs) localized near a rough gold thin film demonstrates significant enhancement of multiphoton emission while at the same time showing a decrease in single photon emission. A rigorous analysis of time-resolved photon correlation spectroscopy and fluorescence lifetime data on single quantum dots at room temperature reveals an increase in radiative recombination rate of multiexcitons that is much higher than expected and, perhaps more significantly, is not correlated with concomitant increases in single exciton recombination rates. We believe that these results confirm a stronger coupling of multiexcitons to plasmon modes via a coupling to plasmon multipole modes.

Comparison of blended versus layered structures for poly"p-phenylene vinylene…-based polymer photovoltaics

electron- and hole-transporting polymers, respectively. We find that both blended and bilayered structures have substantially improved current densities 3m A/cm 2 and power efficiencies 1% under white light over neat films. Improved exciton dissociation at multiple interfaces and reduced recombination due to energy and charge transfers increases the charge-carrier collection in both types of heterojunction devices, but low electron mobilities in the polymers lead to low fill factors and reduced quantum efficiency 20% that limit the power efficiency. Time-resolved photoluminescence reveals that for blended structures both the hole and electron-transporting polymers undergo efficient quenching with the exciton decay being dominated by the existence of two fast decay channels of 0.12 and 0.78 ns that are assigned to interspecies charge transfer and account for the increased short-circuit current observed. For layers, these components are not as prevalent. This result indicates that greater exciton generation at the dissociating interface and more efficient charge collection in the thin layers is primarily responsible for the improved short-circuit current, a conclusion that is further supported by numerical simulations of the exciton generation rate and charge collection. We also report evidence for an intermediate exciplex state in both types of structures with the greatest yield for blends with 50 wt % of CN-ether-PPV. Overall, the improved performance is due to different processes in the two structures; efficient bulk exciton quenching and charge transfer in blends and enhanced exciton generation and charge collection in layers. The optimization of each photovoltaic heterostructured device relies on this understanding of the mechanisms by which each material architecture achieves high power efficiencies.

Near-infrared Fourier transform photoluminescence spectrometer with tunable excitation for the study of single-walled carbon nanotubes

A fast, sensitive, automated Fourier transform (FT) photoluminescence (PL) spectrometer with tunable excitation has been developed for analyzing carbon nanotube suspensions over a wide spectral range. A commercially available spectrometer was modified by the addition of a tunable excitation source, custom collection optics, and computer software to provide control and automated data collection. The apparatus enables excitation from 400to1100nm and detection from 825to1700nm, permitting the analysis of virtually all semiconducting single-walled nanotubes (SWNTs), including those produced by the high pressure carbon monoxide conversion and laser processes. The FT approach provides an excellent combination of high sensitivity and fast measurement. The speed advantage exists because the entire emission spectrum is collected simultaneously, while the sensitivity advantage stems from the high optical throughput. The high sensitivity is demonstrated in the measurement of very dilute SWNT suspensions and the obse...

Fluorescence modulation in single CdSe quantum dots by moderate applied electric fields

Single molecule time-resolved fluorescence spectroscopy of CdSe/ZnS core-shell quantum dots (QDs) under the influence of moderate applied electric fields reveals distributed emission from states which are neither fully on nor off and pronounced changes in the excited state decay. The data suggest that a 54 kV/cm applied electric field causes small perturbations to the QD surface charge distribution, effectively increasing the surface trapping probability and resulting in the appearance of gray states. We present simultaneous blinking and fluorescence decay results for two sets of QDs, with and without an applied electric field. Further kinetic modeling analysis suggests that a single trapped charged cannot be responsible for a blinking off event.

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

Quantum Dots Stabilized By Genetically Engineered Proteins

Colloidal quantum dots (QDs) exhibit unique optical and electronic properties due to their physical size and the semi-conductor material from which they are made [1,2]. While the isolated properties of these nanoparticles offer a number of potential applications, such as bioimaging, their future use in novel electronic devices will require large arrays of known order. The assembling of these arrays is non-trivial and requires a monodisperse sample of quantum dots in order to facilitate the self-assembling process [3,4]. We propose first to validate the concept of using natural and/or recombined (genetic engineered) proteins to build stable, water soluble QD-protein conjugates [5]. Eventually, strategies to design these structures with highly controlled patterns will emerge.