Friday, August 29, 2008


First, following up on my earlier post about the field effect.... I'd seen some of this before but had some really good conversations at the workshop in Japan last week about electrochemical gating. The field effect, as I'd said, is a great way of tuning the density of charge carriers at a surface without the disorder associated with chemical doping. By cranking up some gate voltage you can in some sense just rely on the attraction of opposite charges to accumulate carriers, for example. One major limitation to this technique, though, is the amount of charge that you can really get in there using reasonable dielectrics between the gate and the surface of interest. A given insulator can only take a certain amount of electric field across it before leakage current (and eventual breakdown due to damage from "hot" electrons) starts. Calling that limiting field Emax, you can find the maximum gated charge density to be \epsilon_0 \kappa Emax, where \epsilon_0 is the permittivity of free space in SI units (8.85 x 10-12 Farad/m) and \kappa is the (unitless) relative dielectric constant. If you've got a really good oxide you can get this product up near 1013 carriers per square cm. Usually there is a tradeoff - materials with a big \kappa have a smaller breakdown field. One way around this is to use electrochemical gating. Instead of a dielectric, use either a polymer electrolyte or an ionic liquid. If you don't care about speed of response, this is a great idea because you can get a layer of counterions right next to the surface of interest. As a result, you can accumulate carrier densities exceeding 1014/cm2. That's a huge density that can let you do some fun things even in strongly correlated materials, where you're now talking about adding or removing more than one carrier per unit cell.

A very brief follow-up to my
post about the weather: Aww, not this crap again. Hurricane Gustav is really starting to look like a potential annoyance. Here's hoping that it hits neither New Orleans nor Houston.

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