Thursday, May 24, 2007

This week in cond-mat

Only one quick blurb for now - there have been a number of neat looking papers on the arxiv lately, but I just haven't had time to read them. I am actually making some progress on my book, though.

arxiv:0705.2180 - Martin et al., Observation of electron-hole puddles in graphene using a scanning single electron transistor
A single-electron transistor (SET) consists of an "island" (in this case, a patch of aluminum film) weakly connected by tunnel barriers (in this case, aluminum oxide) to source and drain electrodes (also aluminum films here). Defining the total capacitance of the island to be C, the Coulomb energy cost of adding another electron to the island is E_c ~ e^2/C. If E_c >> kT, the thermal energy scale, and the tunneling resistances of the barriers are >~ h/e^2 (~ 26 kOhms), then the number of electrons on the island is fixed to be an integer. By varying the voltage on a nearby gate electrode coupled capacitively to the island, it is possible to change the average population of the island by one electron at a time. When the gate is set such that the island is just on the cusp of going from an electronic population of n to n+1, the source-island-drain conductance of the device has a peak and is very strongly dependent on that gate voltage. Instead of using a gate electrode, one could use the local electronic environment near the island to modulate the island potential (and hence the conductance). SETs are incredibly good electrometers, able to sense tiny fractions of an electronic charge nearby. Now consider sticking such an SET electrometer on the end of a scanned probe tip (in this case, fabricate it directly on the end of a tapered optical fiber). This is the scanning SET, a wonderful imaging tool developed and refined originally at Bell Labs by people like Harald Hess, Ted Fulton, Bob Willett, Mike Yoo, Amir Yacoby, and Nicolai Zhitenev.

In this paper Amir and colleagues (von Klitzing and company) use the scanning SET to look at graphene near the charge neutrality point as well as in the quantum Hall regime. They can see how the system breaks up into puddles of electron-rich and hole-rich regions with ~ 100 nm spatial resolution. This is a nice application of the S-SET technique, which can be extremely arduous - meeting the temperature requirement for good charge sensitivity requires working at very low temperatures (at least 3He fridge); the SET itself is very fragile and static sensitive; and the scanned probe setup is easy to crash into the sample surface. All in all, a tour de force tool that is unlikely to make its way into common usage any time soon.


Kun Gao said...

Yeah,I addmit the single-electron technique has its great potential application but I really want to know if or not it can be put into practice in near future and create true industrial value.Just like the Carbon nanotube, we know its many good properties for many years, but until now we have not yet seen its any practical applications,which is only fascinating in the theoretical research.

Douglas Natelson said...

I think I can safely say that the scanning SET electrometer is faaar from industrial application, primarily because of the pain associated with achieving the low temperatures required. Single-electron devices in other forms are very likely to have industrial applications. There is at least one company bringing sensors to market based on related physics. Doing logic with SETs at room temperature is still a long way off - there are many associated problems, most especially related to size control, reproducibility of tunnel barriers, and control of offset charge. SETs for implementation and readout of potential quantum computers is not crazy to think about - that's one application where people are probably going to be willing to deal with the temperature issue.
Bear in mind that next-generation flash memory may well run into single-electron physics at some level, too.

Regarding nanotubes, applications are coming, but they are most likely to be initially in structural composites and as electrode materials (supercapacitors, fuel cells, batteries, photovoltaics, ground planes for displays). Practical devices based on individual nanotubes are a long way off, until someone figures out either how to grow specific tube types, or reliably separate tube types on an industrial scale. Individual semiconductor nanowire devices are likely to be applied sooner on a large scale, IMO.

Kun Gao said...

en,I know this situation.Not only the practical application but exploration of fundamental physics are missions of great physicists.

Kai-ge Zhou said...

Graphene is a fantastic materials. The ultra-high high mobility will give us a alternative of carbon nanotubes in construction of nanodevices as far as I can see. But the question is the orientation effects. So making graphene properly will be a work for the chemists.