Quantum Dots - hopeful but how??

News Article: June 2006
Depending on the quantum dot's composition and size, more than 1 electron-hole pair (exciton) can be created by an incident photon. The practical potential is huge, but the fundamental mechanism is not clear.. hmm!!

Points to ponder about:

• High energy photons cause impact ionization..
• QDs have higher bandgaps than bulk semiconductors, hence absorb high freq light
• Since the QD is much smaller than the wavelength of an electron inside it, the dot immobilizes the electron.
• Nozik figured that a dot's grip on an electron would nullify the motion-related subtleties that squelched the impact ionization process at larger scales.
• multiple excitons appear so quickly—within less than 50 femtoseconds (fs).. however, impact ionization proceeds sequentially.. so is it impact ionization?
  
• Alexander L. Efros has instead invoked quantum theory to propose that a photon hitting a quantum dot instantaneously creates a novel quantum object that's simultaneously both one and many excitons.
• Vladimir M. Agranovich suggests that a so-called virtual exciton springs into existence for a moment after the photon hits. Armed briefly with more energy than physics ordinarily permits, it spawns the multiple excitons simultaneously.
• Alex Zunger's team's calculations indicate that impact ionization can account for the experimental findings.. debatable!!
  

Experiments on 8 nm dia lead selenide quantum dots have given the best results so far: Ultraviolet-light photons—albeit at a wavelength found sparingly in sunlight—released seven electrons apiece!!
Relevant details/references in: http://www.sciencenews.org/articles/20060603/bob8.asp