Tuesday, July 31, 2007

Recent ACS + cond-mat

A couple of interesting recent results - a busy summer has really cut into my non-essential paper-reading, unfortunately.

One sideline that has popped up with the recent graphene feeding frenzy is trying to understand its optical properties. I don't mean anything terribly exotic - I mean just trying to get a good understanding of why it is possible, in a simple optical microscope, to see any optical contrast from atomically thin single layers of graphene. Papers that have looked at this include:
arxiv:0705.0259 - Blake et al., Making graphene visible
arxiv:0706.0029 - Jung et al., Simple approach for high-contrast optical imaging and characterization of graphene-based sheets
doi:10.1021/nl071254m (Nano Lett., in press) - Ni et al., Graphene thickness determination using reflection and contrast spectroscopy
UPDATE: Here's another one:
doi:10.1021/nl071158l (Nano Lett., in press) - Roddaro et al., The optical visibility of graphene: interference colors of ultrathin graphite on SiO2
It all comes down to the dielectric function of graphene sheets, how that evolves with thickness, and how that ultrathin dielectric layer interacts optically with the oxide coating on the substrate.

Another paper that looks important at a quick read is:
doi: 10.1021/nl071486l (Nano Lett., in press) - Beard et al., Multiple exciton generation in colloidal silicon nanocrystals
To excite the charge carriers in a (direct gap) semiconductor optically typically requires a photon with an energy exceeding the band gap, Eg, between the top of the valence band and the bottom of the conduction band. If an incident photon has excess energy, say 2Eg, what ordinarily happens is that a single electron-hole pair is produced, but that pair has excess kinetic energy. It's been shown recently that in certain direct-gap semiconductor nanocrystals, it's possible to generate multiple e-h pairs with single photons. That is, a photon with energy 3Eg might be able to make three e-h pairs. That's potentially big news for photovoltaics. In this new paper, Beard and coauthors have demonstrated the same sort of effect in Si nanocrystals. This is even more remarkable because bulk Si is an indirect gap semiconductor (this means that the because of the crystal structure of Si, taking an electron from the top of the valence band to the bottom of the conduction band requires more momentum than can be provided by just a photon with energy Eg). At a quick read, I don't quite get how this works in this material, but the data are pretty exciting.

2 comments:

Cindy Margott said...

If an incident photon has excess energy, say 2Eg, what ordinarily happens is that a single electron-hole pair is produced, but that pair has excess kinetic energy.

Jackson said...

Thank you..