Friday, September 13, 2013

Ionic liquids and gating - how much is chemistry?

I've written before (here, here and here) about the use of ionic liquids in condensed matter physics investigations.  These remarkable liquid salts, with small organic molecules playing the roles of both positive and negative ions, can be used in electrochemical applications to generate extremely large surface charge densities near electrode interfaces.  Many experiments have been published in the last few years in which ionic liquids are meant to induce (via capacitive coupling) large densities of mobile charge carriers within interesting solids at the solid/ionic liquid interface. 

One concern in these experiments has been the role of surface chemistry.  While the molecular ions themselves are intended to be stable over a large range of electrochemical conditions, the ionic liquids can dissolve more reactive species (like water).  Likewise, recent experiments by Stuart Parkin of IBM Research have shown that in some systems (vanadium oxide in particular), under certain electrochemical conditions it would appear that ionic liquids can favor the formation of oxygen vacancies in the adjacent solid.  Since oxygen vacancy defects in many oxide materials can act as dopants, changing the concentration of charge carriers, one must be extremely careful that any measured changes in electronic properties are really from electrostatics rather than effective chemical doping. 

These concerns can only be ratcheted higher by the simultaneous online publication of two more papers from the Parkin lab, this one in Nano Letters (on SrTiO3) and this one in ACS Nano (on TiO2).  In both systems, the authors again find evidence that changes in oxygen stoichiometry (rather than pure electrostatic charging) can be extremely important in generating apparently metallic 2d surface layers.  

This is a very subtle issue, and the gating experiments remain of great interest.  Unraveling the physics and chemistry at work in all the relevant systems is going to be a big job, with a strong need for in situ characterization of buried solid-liquid interfaces.  Fun, challenging stuff that shows how tricky this area can be.

1 comment:

Tatvachintan said...

Truly clever point of view. I agree with all your arguments on Ionic liquid & the rest. Will share it with my chemo friends too.