Kevin T. Early

Polarization-Driven Stark Shifts in Quantum Dot Luminescence from Single CdSe/oligo-PPV Nanoparticles

We demonstrate polarization-induced spectral shifts and associated linearly polarized absorption and emission in single CdSe/oligo-(phenylene vinylene) (CdSe/OPV) nanoparticles. A mechanism for these observations is presented in which charge separation from photoexcited ligands results in a significant Stark distortion of the quantum dot electron/hole wavefunctions. This distortion results in an induced linear polarization and an associated red shift in band-edge photoluminescence. These studies suggest the use of single quantum dots as local charge mobility probes.

Disentangling the role of linear transition dipole in band-edge emission from single CdSe/ZnS quantum dots: Combined linear anisotropy and defocused emission pattern imaging

Combined linear anisotropy and defocused wide-field fluorescence imaging of individual CdSe/ZnS quantum dots reveal an unambiguous contribution of a linear transition dipole polarized along the hexagonal (c-) symmetry axis of the nanocrystal which grows with increasing tilt angle of the c-axis with respect to the surface normal. These results offer some insights into quantum dot luminescence polarization dynamics, and provide a basis for unambiguously assigning the two Euler angles describing the c-axis orientation from defocused emission patterns for multidipole systems.

Linear Dipole Behavior in Single CdSe-Oligo(phenylene vinylene) Nanostructures

We report on linearly polarized absorption and emission from individual (4.3 nm) CdSe quantum dots whose surfaces are coordinated with monodisperse oligo-phenylene vinylene ligands. Shown previously to suppress quantum dot blinking, we demonstrate here that the electronic interaction of photoexcited ligands with the quantum dot core is manifested as a strong polarization anisotropy in absorption (M = 0.5), as well as distinct linear dipole emission patterns from the quantum dot core. Further, there is a correlation between the quantum dot emission moment and polarization orientation corresponding to the absorption maxima that is manifested as fluctuations in emission moment orientation in the X-Y plane. The observed polarization effects can be switched off by tuning the excitation away from the ligand absorption band. We propose a mechanism based on exciton dissociation from the photoexcited ligand, followed by the pinning of electrons at the quantum dot surface. The resulting Stark interaction is sufficiently strong to break the 2D degeneracy of the emission moment within the dot, and may therefore account for the linear dipole emission character.

Directional Absorption and Emission in Hybrid Polymer-Quantum Dot Nanostructures Studied by Scanning Probe and Fluorescence Microscopy

The optical properties of hybrid organic/quantum dot nanostructures, probed at the single molecule limit by scanning probe and fluorescence microscopy, reveal novel and highly directional absorption and emission characteristics, ideal for polarization-based switching applications.

Diffusive Coordinate Model for Blinking Suppression and Intensity Fluctuations in CdSe-OPV Nanocrystals

We describe here numerical simulation of a modified Frantsuzov-Marcus diffusive coordinate (DC) model[1] which yields blinking suppression and low frequency fluctuations as observed[2] in Oligo-(phenylene vinylene) (OPV) coated CdSe quantum dots.

Periodic Intensity Fluctuations in Functionalized Semiconductor Quantum Dots: Correlation with Ligand Coverage

Fluorescence microscopy has been used in conjunction with atomic force microscopy to study size-correlated emission properties of single oligo-phenylene vinylene-functionalized CdSe nanocrystals, which reveals size-dependent intensity fluctuations on time scales of 10-60 seconds.

Modification of Blinking Statistics in Solid State Quantum Dot/Conjugated Organic Polymer Composite Nanostructures

Fluorescence intermittency, or “blinking” in quantum dot systems has been the subject of great interest since the first observation of this phenomenon nearly 10 years ago. The stability of quantum dot fluorescence emission is especially important in the context of photovoltaic, optoelectronic, and biological applications, where device performance, or the ability to track labeled particles, is affected adversely by fluorescence intermittency. Single-molecule spectroscopy combined with atomic force microscopy measurements reveal that CdSe quantum dots functionalized with oligo(phenylene vinylene), OPV, ligands exhibit modified optical properties such as suppression of blinking when compared to conventional TOPO covered or ZnS-capped CdSe quantum dots. The blinking suppression is shown to be highly sensitive to the degree of ligand coverage on the quantum dot surface and this effect is interpreted as resulting from charge transport from photoexcited OPV into vacant trap sites on the quantum dot surface. This direct surface derivatization of quantum dots with organic ligands also enables a “tunable” quantum dot surface that allows dispersion of quantum dots in a variety of polymer supported thin films without phase segregation. This facilitates straightforward inclusion of these new hybrid materials into solid state formats and suggests exciting new applications of composite quantum dot/organic systems in optoelectronic systems.

Suppression of Blinking in Solid State Quantum Dot/ Conjugated Organic Polymer Composite Nanostructures

Single-molecule spectroscopy combined with AFM measurements reveal that CdSe quantum dots functionalized with oligo-phenylenevinylene ligands exhibit enhanced optical properties such as reduced blinking. The degree of polymer coverage is found to control this effect.