(546c) Hot Electron Transfer From Semiconductor Nanocrystals

In conventional
semiconductor solar cells, absorption of photons with energies greater than the
semiconductor band gap generate ?hot? charge carriers that quickly ?cool?
before all of their energy can be captured ? a process that limits device
efficiency. Semiconductor nanocrystals (or quantum dots) have been touted as
promising materials for photovoltaics because discretization of their
electronic energy levels can slow down this cooling process, which might enable
the extraction of photogenerated charge carriers before their excess energy is
converted to heat.

In this talk, I will
demonstrate sub-50 fs electron transfer from hot energy levels of PbSe
nanocrystals to delocalized conduction band sates of TiO₂. In order
to make these measurements, we developed the use of optical second harmonic
generation for femtosecond time-resolved studies of interfacial charge
separation. I will discuss the information we obtain from this technique as
well as the effect of temperature, nanocrystal size, and surface chemistry.
Additionally, I will show how ultrafast electron transfer excites coherent
vibration of the first layer of TiO₂ surface atoms, whose collective
atomic motions can be followed in real time.