Total internal reflection fluorescence microscopy—a powerful tool to study single quantum dots

We present total internal reflection fluorescence microscopy (TIRFM) measurements of colloidal CdSe quantum dots (QDs). This technique provides the advantage of recording the time-resolved fluorescent signal of many single QDs simultaneously, thus allowing data with high statistical confidence to be obtained in a relatively short time. We discuss methods of taking and extracting the data from the collected images, and in particular, investigate the on/off intermittent fluorescence behavior from single QDs over a range of excitation intensities.We find that on/off-times are independent of the excitation power at intensities above 0.2 kW/cm2, while at lower intensities, there is a pronounced dependence. We discuss the observed behavior in terms of current physical models of QD ionization and neutralization, which are often used to explain the on and off blinking. # 2004 Elsevier B.V. All rights reserved