Impact of Photo-induced Physical and Chemical Processes on Lateral Transport of Excitons in Quantum Dot Thin Films

Abstract

We study how lateral transport of excitons (exciton diffusion) in quantum dot thin films is influenced by the photo-physical and photo-chemical properties of the quantum dots. For this, thin layers of colloidal quantum dots on glass substrates are exposed to laser pulses with different total average powers. Spatial mappings of the lifetimes of the quantum dots within the diffraction-limited spot sizes of the laser beam show that their photo-induced properties can be influenced by the excitation intensity mode profiles significantly, leading to three distinct regimes of exciton diffusion across the films. These include the cases wherein the lateral energy transport (i) is linearly influenced by the laser intensity mode profiles, (ii) is convoluted by variations of laser intensity and photo-physical and photo-chemical properties of the quantum dots, and (iii) is significantly influenced by the formation of oxidation hole burning. The third regime, oxidation hole burning effect, occurs at the center of the excited regions of the quantum dot thin films, suppressing their emission and transport of energy considerably. We will discuss variations of the diffusivity as a function of the laser intensity mode profiles and time. The results shed light on the impact of photo-chemical and photo-physical processes on transport of energy in quantum dot thin films, highlight the regimes of efficient exciton diffusion, and demonstrate the prominent impact of the intensity mode profiles of the light used to excite quantum dot thin films.

Excitation-Dependent Exciton Diffusion in Semiconductor Quantum Dot Thin Films with Spatially Patterned Lifetimes

“…We suggest that the increase in diffusion rate might be caused by a phenomenon known as superradiance. − It is already known that superradiance increases the radiative rate of exciton decay, and we have even observed the phenomenon in a previous report (Figure ). , Figure shows that exciton decay rates in the center of an exciton distribution increase steadily with incident power. An increased incident power corresponds to an increased density and number of incident photons, and therefore an increase in the number of generated excitons in the QD thin film.…”

“…Since the radiative rate enhancement due to superradiance has already been shown to be proportional to the number of interacting excited fluorophores, − we conclude that the rate enhancement in Figure may be due to supperradiant collective emission. Furthermore, these measurements were taken within a 2 s period of the sample being exposed to prevent photoinduced processes, and the center of the exciton distribution was chosen so that there would be no convolution with diffusion effects (eq ). Because of these experimental methods, the only cause for the faster exciton decay is the incident power.…”

“…Assuming that no photoinduced processes occur during the measurement, and that the radiative/nonradiative decay rates of the excitons do not depend on position, it is possible to use this data to observe exciton diffusion. Photoinduced processes include photoinduced enhancement and oxidation, both of which result in a lifetime that is spatially dependent. − Since the center of a diffraction-limited spot is far more intense than the edges, various degrees of photooxidation will occur as a function of radial position . For this paper, we chose excitation powers below the photooxidation threshold by first experimentally determining how our specific batch of QDs responded to various input powers.…”

“…While the implementation of the OHB is simple, the mechanisms are poorly understood. In the current paper, we utilized OHB to create micron-scale patterns of darkened lines to further relate the results of this paper to our previous study …”

“…OHB technique utilizes a relatively high-power pulsed laser to reduce the photoluminescence efficiency of QDs via generation of defect sites on the surface of QDs. OHB also reduces the lifetime of excitons . While the implementation of the OHB is simple, the mechanisms are poorly understood.…”