[MOS] April 7, 2009

[Zina Queen]

Seminar on

Modern Optics and Spectroscopy

Many-body processes in colloidal semiconductor nanocrystals

Gautham Nair, MIT

Tuesday, April 7, 2009


12:00 - 1:00 p.m.


To help guide applications, the fundamental optical properties of 
semiconductor nanocrystals (NCs) have been studied extensively, and 
most relevant aspects of the single exciton state structure and 
fluorescence are well understood. A handful of potential 
applications, however, rely heavily on less understood multiexcitonic 
and many-body effects. The first part of the talk will focus on 
carrier multiplication (CM), the generation of more than one electron 
and hole in a semiconductor after absorption of one photon. Interest 
in CM, a desirable process in photovoltaic applications, has renewed 
in recent years because of reports suggesting greatly increased 
efficiencies in nanocrystalline materials compared to the bulk form. 
We will describe our experimental assessments of CM yields in 
semiconductor NCs by careful study of exciton and biexciton 
signatures in transient photoluminescence decays. The CM yields we 
determine are much lower than most of the reports in the literature. 
We discuss the implications of our findings within a general 
theoretical framework. The second  part of the talk examines the role 
of many-body processes in the fluorescence intermittency (blinking) 
of NCs. The prevailing hypothesis for fluorescence quenching in an 
off NC involves a three-body Coulomb-mediated relaxation process that 
is also though to play a dominant role in the relaxation of 
multiexcitons in NCs. We have studied this connection by spectrally 
resolving exciton and multiexciton emission from single NCs and 
measuring the correlationof their intermittency.


Grier Room, MIT Bldg 34-401
Refreshments served after the lecture
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