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Departmental Colloquium

Photophysical properties and applications of CdSe quantum dots.  
Guest Speaker
Prof. Ho-Soon Yang  
Guest Affiliation
Department of Physics, Pusan National University, Busan, Korea  
Bill Dennis  
Thursday, March 20, 2014 4:00 pm - 5:00 pm  
Physics Auditorium (Rm. 202)  

Colloidal semiconductor nanocrystals have been of interest in aspects of fundamental understandings and applications such as light emitting diodes, lasers, photovoltaic cells, biomedical tags and solar cells. The optical and electronic properties of CdSe quantum dots (QDs) show a strong size and shape dependence. QDs have a high ratio of surface to volume and the surface of QDs prepared with a chemical method has a complex condition due to organic ligands. Therefore, the complex surface condition of colloidal QDs plays an important role in determining quantum yield (QY), carrier relaxation, and recombination process. The asymmetric crystal field and electron-hole exchange interaction in CdSe splits the lowest exciton state, (1se-1sh2/3), into five levels which includes the ‘dark’ and ‘bright’ exciton states. CdSe QDs are synthesized in wurtzite structure using the chemical colloidal method in this study. The optical modal gain enhancement of CdSe QDs is observed in a cylindrical glass-waveguide by using a variable stripe length method. CdSe QDs are prepared as having various surface conditions, which induces QY to be changed. The time resolved spectra show two different energy relaxation processes, a fast process of electron hole combination and a slow process involved with surface states. The ratio of slow to fast processes increases as the QY increases, which reveals that the surface related emission contributes to high QY of QDs. The photoluminescence and time resolved spectra are obtained over a broad temperature range (4 ~ 300K). The dark and bright exciton states are observed at a low temperature regime.