Photophysical behavior of ensembles of single semiconductor quantum dots
- PDF / 67,826,175 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 38 Downloads / 227 Views
Photophysical behavior of ensembles of single semiconductor quantum dots Kira D. Patty1 and S. M. Sadeghi1,2 1 Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899, U.S.A. 2 Nano and Micro Device Center, university of Alabama in Huntsville, AL35899, U.S.A. ABSTRACT In this contribution we explored the impact of irradiation intensity on the fluorescence of single quantum dots and ensembles of single quantum dots over time scales much greater than the blinking of the individual quantum dots. We also investigated how blinking and photo enhancement are influenced by photo-oxidation when the intensity of the irradiating laser was increased. Analysis of the emission photographs shows the presence of large time scale individual and ensemble blinking behavior. Analysis of the data indicates that photoinduced fluorescence enhancement occurs even when the inter-dot spacing of the quantum dots precludes the formation of a Coulomb blockade. Additionally, irradiation of ensembles of CdSe/ZnS SQDs at higher source intensities lead to a more accelerated suppression of quantum dots emission via photo-oxidation than for ensembles of closely packed quantum dots. INTRODUCTION Just as the properties of a semiconductor can differ greatly for the bulk material and quantum dots of that same material, the photophysical behavior of the quantum dots can be significantly different for single quantum dots (SQDs) versus an ensemble of the quantum dots (QDs) in close proximity [1-2]. Colloidal quantum dots are easily manufactured and tailored to provide a desired emission peak. Other research with CdSE/ZnS (peak emission near 639nm) has explored photoinduced fluorescence enhancement (PFE) in samples where the concentration of QDs ensures close proximity of neighbors [3]. That research showed that when the QDs were excited with a low intensity coherent source that PFE results in a steady increase of the total emission intensity of the sample over long time scales. At high source intensities above, photooxidation interferes with PFE and the QDs emissions rapidly maximize then decrease over time. Such results prompt the question of what will occur in samples containing ensembles of isolated SQDs. The main difference between such samples compared to those consisting of an ensemble of close packed QDs is that in the later the interdot interaction can influence the photophysics of the QDs, leading to PFE. The reason behind this is that when the QDs are closed to each other, photoionization of one QD can set up an electrostatic field that can suppress photoionization of other QDs in its vicinity. Such a Coulomb process is caused by ejection of electrons into deep trap states (slow photoionization) with long neutralization times. In SQDs the photoionization process is seen as one of the main reasons for long time scale blinking. One does not, however, expect that the Coulomb blockade plays any major role in the case of ensembles of SQDs, where the distance between the QDs is large [1, 3]. In this paper we study the emission b
