Optical studies of colloidal quantum dots : optical trapping with plasmonic nanoapertures and thermal recovery from photoinduced dimming

Abstract

This doctoral research has been defined by two main goals. The first has been to develop single colloidal quantum dot (QD) absorption as a new spectroscopic tool for investigating single QD electronic properties, dynamics, and inhomogeneities. In an important step towards achieving this goal, QDs were introduced into the field of optical trapping. Silica coated QDs were optically trapped using bowtie apertures in a thin silver film with low incident flux of 1.56 MW/cm 2 at 1064 nm. Additionally, QDs emitted upon trapping via two-photon excitation from the trapping laser due to strong field enhancement inside the aperture. The second goal of this research has been to investigate processes involved in single QD fluorescence intermittency, or blinking. Specifically, the transition from a nonemissive QD to an emissive QD was investigated using controlled amounts of thermal energy to drive recovery from photoinduced dimming in QD ensembles. Nonlinear thermal recovery was well described by a stretched exponential function, and further analysis yielded an underlying probability distribution of rate constants. Casting the rate constants as a collection of first-order activated processes provided an activation barrier probability distribution with significant density at room temperature thermal energy that peaks at 200 meV before decaying to zero. Progress towards single QD absorption using alternative nanoscale structures, including slot waveguides and circular apertures in silver film, is also discussed. Lastly, self-assembled cyanine-dye nanotubes were monitored during flash dilution with absorption spectroscopy at a high frame rate to separate spectroscopic contributions of the outer layer in double walled and bundled nanotubes.