Friday, October 19, 2007

Three papers and a video.

Three interesting papers on ASAP at Nano Letters at the moment:

http://dx.doi.org/10.1021/nl0717715 and http://dx.doi.org/10.1021/nl072090c are both papers where people have taken graphite flakes, oxidized them to make graphite oxide, and then suspended the graphene oxide sheets in solvent. They then deposit the sheets onto substrates and made electronic devices out of them after trying to reduce the graphene oxide back to just graphene. There are a couple of people here at Rice trying similar things from the chemistry side. Interesting that a number of groups are all working on this at about the same time. That's one reason why it can be dangerous to try to jump into a rapidly evolving hot topic - it's easy to get scooped.

This one is a cute paper titled "Carbon nanotube radio". The science is nicely done, though not exactly surprising. AM radio works by taking an rf carrier signal and demodulating it to get back just the envelope of that carrier signal. Back in the early 20th century (or more recently, if you bought an old kit somewhere), people used to do the demodulating using a diode made semi-reliably by jamming a metal needle (a "cat's whisker") into a lead sulfide crystal - hence the term "crystal radio". It's simple trig math to see that a nonlinear IV curve (one with a nonzero d^2I/dV^2) can rectify an ac signal of amplitude V0 to give a dc signal of (1/4)(d^2I/dV^2)V0^2. Well, in this case the nonlinear element is a nanotube device. Cute, though I have to admit that I found the media hype a bit much. Wilson Ho did the same essential thing very nicely with an STM, but didn't talk about atomic-scale radio receivers....

Lastly, via Scott Aaronson, a link to a fantastic math presentation. Watch the whole thing - this really is a model of clarity and public outreach. On a bitter-sweet note, in the credits at the end I realized that one of the people responsible for this was an acquaintance from college who has since passed away. Small world.

2 comments:

hypnose said...

Hi Doug,
I have a few questions. Zero bias anomaly in Ho's paper is presumably Fano resonance, right? If so, what is the reason of the ballistic transport in this system?

Doug Natelson said...

I'm not certain exactly what you mean. In STM measurements that have Fano resonances, as I understand it these result from interference between "direct" tunneling (electrons going directly between delocalized states in the sample - surface or bulk electronic states - and delocalized states in the tip) and tunneling via a local resonant level between the same initial and final states. If I understand your question correctly, I believe the "ballistic transport" that you're talking about is the ordinary direct tunneling.

My point was that one can get rectification of microwave frequencies via the nonlinearity in STM tunneling characteristics. As Ho et al. explain in a later JPCB paper at some length, it's been a long slog actually demonstrating this unambiguously because of various artifacts that can creep into the picture.