Electrostatic force microscopy study of photoionization kinetics of cadmium selenide/cadmium sulfide core/shell nanoparticles
Abstract
Luminescence blinking and spectral diffusion in colloidal quantum dots in some way reflect charge redistribution and/or photoionization. The observation of inverse power law kinetics over many orders of magnitude in time, even in the core/shell nanocrystals synthesized by best current methods, implies that a wide range of trap states are weakly coupled, with fluctuating matrix elements, to the optically excited internal state. This implies that the particle interacts with external states, at least as far as 1--2 nm away. The excitation wavelength-dependent photoionization of CdSe/CdS core-shell nanocrystals is studied by Electrostatic Force Microscopy (EFM) under N 2 atmosphere, on N and P-type silicon substrates with 21 and 14 A thermal SiO2 layers as well as on graphite. The particles' photoionization dynamics are strongly affected by their electrostatic environment. Effects of substrate doping, photexcitation wavelength, and oxide thickness on photoionization and relaxation kinetics are investigated. A quantitative model is developed to describe the electrostatic interactions between the EFM probe and the sample. Calculations based on this model confirm that upon photoexcitation the particles lose multiple elementary charges. A kinetic model is developed to derive the rate constants for the photoionization and reneutralization processes under various experimental conditions. The results are discussed in light of a 1D tunneling model.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2003
- Bibcode:
- 2003PhDT.......123C