Date/Time: October 13, 12.30 p.m.

Coffee and rolls available from 12.15h

Location: Lectureroom A, DelftChemTech, Julianalaan 136

Semiconductor nanocrystals fabricated by chemical routes are remarkable objects which are often considered as artificial atoms hose properties can be tailored by changing their shape, their size and their composition. They are subject of intense research and therefore their optical properties like optical absorption and luminescence spectra are now well understood.

In this seminar, we will address more complex properties of semiconductor nanocrystals concerning the dynamics of excitons after optical excitation at energies well above the band gap. These properties presently receive growing interest, in particular in the context of solar energy production based on nanotechnologies. We will discuss resonant energy transfers induced by electron-electron interactions in these systems.  

We will review experimental studies engaged at the international level and we will summarize our theoretical works on the following problems:

1. Auger effect. What is its experimental signature? What is its efficiency?

2. Resonant energy transfer between neighbor nanocrystals. Can we apply the Förster’s model well-known for energy transfers between molecules? [1]

3. Multiple exciton generation. Many experimental studies suggested that it is possible to generate several excitons after absorption of a single photon in nanocrystals, up to seven excitons in PbSe nanocrystals [2], which can be very interesting to enhance the efficiency of solar cells, but these results are highly debated and the mechanisms are not understood. Using theoretical simulations, we discuss mechanisms of multiple exciton generation like impact ionization and direct generation [3]. We also consider the possible role of defects in these problems [4].

[1] G. Allan and C. Delerue, Phys. Rev. B 75, 195311 (2007).

[2] R. Schaller and V. Klimov, Phys. Rev. Lett. 92, 186601 (2004).

[3] G. Allan and C. Delerue, Phys. Rev. B 77, 125340 (2008).

[4] G. Allan and C. Delerue, submitted.