Michael Y. Odoi

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.

Single Molecule Studies of a 2,7-Bis-(Phenylethenyl)fluorenone: Implications for Green-Emission Bands in Fluorene-Based OLEDs

The color purity of blue polyfluorene fluorescence emission is often adversely contaminated by green emission bands. The origin of this green emission has been attributed to various factors including on-chain oxidation of fluorene leading to monomeric fluorenone emission and inter/intra-chain interaction between segments of fluorenone units leading to excimer formation on the polyfluorene backbone. We report here emission properties at the bulk and single molecule level of a molecular fluorene derivative and compare them to those of its oxidative fluorenone derivative. Whereas the bulk fluorescence emission of 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl- 9H -fluorene (OFPV) exhibits a blue to blue-green emission at about 480 nm, 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl- 9H -fluorenone (OFOPV) shows red luminescence with a peak centered at about 630 nm. However, the peak position for OFOPV shifts to higher energies (∼540 nm) upon dilution or dispersion in polymer matrices like PMMA or Zeonex. Single molecule measurements of OFOPV show fluorescence spectra dominated by peaks around 540 nm, with a small minority at longer wavelengths that are attributed to emission from dimers or higher aggregates. This distribution indicates that monomeric emission of OFOPV is green, consistent with green emission bands seen in polyfluorenes.

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.