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Wiebke Albrecht,1 Philipp Sippel,2 Dariusz Mitoraj,1 Thomas Hannappel,2 Daniel Vanmaekelbergh1
1Utrecht University, The Netherlands 2Helmholtz Zentrum, Berlin Investigation the of time-evolution of hot electrons in colloidal nanocrystalline quantum dots is of high fundamental interest as well as for applications of quantum dots as photovoltaic cells, lasers and photon up-convertors. Decay dynamics of hot excitons were studied by monitoring the transient absorption1 or terahertz-domain spectroscopy2. Herein, we report on Two-Photon-Photoemission Spectroscopy (2-PPE) studies of electronic intraband relaxation in CdSe solid nanocrystals of different sizes. The technique allows us to follow the (de)population dynamics of the lowest (1S) and first excited (1P) electron states with femtosecond time resolution. A unique feature with respect to other ultra-fast methods is the measurement of absolute energies. Our data indicate that for oleic acid capped CdSe NC's the 1P* -to-1S* relaxation is characterized by a time constant of few hundred femtoseconds, whereas for 1,6-hexanedithiol capped CdSe NC's the time constant is one order of magnitude higher. Energy donation from the electron to the hole and subsequent fast relaxation of the hole (Auger cooling) is proposed to explain the fast electron cooling. 1. Pandey, A. et Guyot-Sionnest P. Science 322, 2008, 929-932 2. Hendry, E. et al. Phys Rev. Lett. 96 2006, 057408 3. Albrecht, W. et al. in preparation |
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