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.

Thursday, July 26, 2007

Texas and education

Governor Perry, why did you have to go and ruin my week? It's bad enough that the Texas Republican Party platform explicitly declares that "America is a Christian nation" - so much for not establishing a preferred religion. Now our governor has gone and appointed a creationist anti-intellectual to be the head of the state board of education. Frankly I don't care what his personal religious beliefs are, but I am extremely bothered that the governor has appointed a man who believes that education and intellectualism are essentially useless ("The belief seems to be spreading that intellectuals are no wiser as mentors, or worthier as exemplars, than the witch doctors or priests of old. I share that scepticism.") to run the state educational system. Great move, Governor. Ever wonder why it's hard to convince high tech industry to create jobs here?

Wednesday, July 25, 2007

Ob: Potter

This is the obligatory Harry Potter post. Yes, I read the 7th book, and while it's got a few narrative problems (characters sometimes behaving in deliberately obtuse ways for dramatic necessity - like nearly every episode of Lost), on the whole it was a satisfying wrap-up of the series. If you don't care about spoilers, here is a great parody of the whole thing (via Chad Orzel).

Thursday, July 19, 2007

This week in cond-mat

It's been a busy summer, hence the sparseness of my recent postings. Here are a couple of papers that caught my eye this past week.

arxiv:0707.1923 - Hogele et al., Quantum light from a carbon nanotube
Here the authors do careful time-resolved photoluminescence experiments on individual single-walled carbon nanotubes. By studying the time distribution of photon production, they can get insights into the exciton (bound electron-hole) dynamics that lead to light emission. They find evidence that photons are produced one-at-a-time in these structures, and that multiphoton processes are strongly suppressed. Perhaps nanotubes could be useful as sources of single photons, strongly desired for quantum cryptography applications.

arxiv:0707.2091 - Quek et al., Amine-gold linked single-molecule junctions: experiment and theory
This is a nice example of a mixed experiment/calculation paper in molecular electronics that actually has an interesting point. Very pretty experimental work by Venkataraman et al. at Columbia has shown that NH2-terminated molecules form better-defined contacts with Au electrodes than the conventional thiol (sulfur)-based chemistry. For example, looking at huge data sets from thousands of junction configurations, benzene diamine glommed into a Au break junction has a well-defined most likely conductance of around 0.0064 x 2e^2/h. Now theory collaborators have done a detailed examination via density functional theory of more than a dozen likely contact geometries and configurations for comparison. The calculations do show a well-defined junction conductance that's robust - however, the calculations overestimate the conductance by a factor of seven compared to experiment. The authors say that this shows that DFT likely misses important electronic correlation effects. Hmmm. It's a neat result, and now that they mention it, the almost every non-resonant molecular conduction calculation I've ever seen based on DFT overestimates the conduction by nearly an order of magnitude. The only underestimates of molecular conduction that come to mind are in the case of Kondo-based mechanisms, which can strongly boost conductance and are always missed by ordinary DFT.

Friday, July 13, 2007

This is just silly.

I got an email about an audio conference about faculty recruiting titled "How to Recruit Gen X Faculty Members". I shouldn't pre-judge, and I should be glad that anyone is trying to improve the faculty recruiting process, but it's sad that anyone needs to be told this stuff. The premise is this:
The era when colleges and universities could rely on prestige and a little cash to recruit top academic talent is gone. Increasingly, up-and-coming faculty talent is from Generation X, the much derided and little understood generation that is much more than the Gap-employee stereotype you heard about a decade ago. This generation has a different set of work priorities, and colleges that understand these priorities stand a better chance of landing the best candidates and keeping them.
Riiiggght. It must be because of their generational culture, not the fact that two income families are vastly more common now, and there are many more women faculty candidates then forty years ago, etc. The topics to be covered include:
  • Why prestige and tenure may not matter as much to this generation as previous generations, and what that means for recruiting.

  • The importance of being "family friendly" and how job candidates judge that now that all colleges are claiming that they are.

  • How Gen X professors view hierarchy and what that means in the context of departments.

  • The importance of transparency and collegiality.
  • So, basically we can sum this up in a few words that generalize beyond the university setting: People don't want to work at places where they will be treated poorly. People may want to actually have lives outside of their jobs, and like to work at places that understand that. Smart, educated people don't like being told what to do by people who are clueless just because the clueless have seniority. People don't like it when their employers are rude or have obscure, byzantine policies. My goodness, those Gen X slackers are totally unreasonable.

    Tuesday, July 10, 2007

    Organic Microelectronics workshop

    I just spent two days at the 3rd Annual Organic Microelectronics Workshop, meeting this year in Seattle. The workshop, sponsored jointly by the ACS, MRS, IEEE, and APS, was really very good - about 90 participants, and most of the big movers in the field. The talks were a great mix from the very applied (e.g. trying to optimize solvent conditions to avoid the coffee ring problem when inkjet or gravure printing solution-processable organic semiconductors) to the basic physics and chemistry of these materials. Among the things that I learned:
    • Among the single-crystal organic semiconductors, rubrene is truly special in a number of ways. The most important point from the perspective of understanding electronic transport is that it can be made particularly pure, and oxidation in this material is reversible, unlike, e.g., pentacene.
    • With polymer electrolytes, it is possible to make field-effect devices with gated surface charge densities exceeding 10^14 carriers/cm^2. I'd seen a couple of papers on this, and it's looking very impressive as a technique.
    • Clever phase separation tricks can produce self-assembling organic devices that encapsulate themselves within a protective coating.
    • RFID tags from Si are very very cheap.
    • When developing a manufacturing process, "'Good enough' is good enough, and 'better' is not necessarily better."

    Wednesday, July 04, 2007

    This ought to be fun.

    Looks like those folks at Steorn are going to do a 'demo' of their alleged free energy machine. I think I can safely predict (a) Steorn will claim success; (b) the reporting will generally give them the benefit of the doubt and "report the controversy"; and (c) we will not cure all the world's energy needs with magnet-based machines that violate the first law of thermodynamics.

    UPDATE: Wow - it turns out that I'd overestimated Steorn. They couldn't get their demo to work. Apparently they'd decided to ignore back-ups, rehearsals, and contingency planning in addition to the laws of physics. So, was this self-deception, the long con, a postmodern publicity stunt designed to show how effectively they could market vaporware, or something else?

    Tuesday, July 03, 2007

    four interesting ACS journal articles

    Here are four recent articles ACS journals, two from Nano Letters and two from JACS, that made an impression on me.

    Dattoli et al., Fully transparent thin-film transistor devices based on SnO2 nanowires
    The authors of this paper have made fully functional n-type FETs based on lightly doped tin oxide nanowires with indium tin oxide source, drain, and gate electrodes, and the performance of these FETs is reasonable when compared with the ones currently driving the pixels in your flat panel display. Since the entire FET structure is very transparent in the visible, this could have some significant applications in display technologies.

    Angus et al., Gate-defined quantum dots in intrinsic silicon
    People have been making Coulomb blockade devices out of puddles of gate-confined two-dimensional electron gas for nearly two decades now. Mostly this has been done at the GaAs/AlGaAs interface, and more recently it's been achieved in nanotubes, semiconductor nanowires, and SiGe heterostructures. The authors of this work have managed to do this nicely at the Si/SiO2 interface in a MOSFET. What this really shows is how well the interface states at that junction are passivated, how nicely the authors can make gates without messing up the surrounding material, and that properly made Ohmic contacts in Si FETs can operate well down to cryogenic temperatures. This could be a very important paper if one can build on it to manipulating electron spins in these dots - unlike GaAs structures, there should be many fewer nuclear spins to worry about for effects like hyperfine-induced decoherence of electron spins.

    Albrecht et al., Intrinsic multistate switching of gold clusters through electrochemical gating
    Lots of people in the molecular electronics community have pointed out the similarities and differences between three-terminal (electrostatically gated) molecular devices and solution-based electrochemical oxidation/reduction experiments in electron transfer. These authors are some of the only experimentalists out there that I have seen really delving into this, trying to unravel how the electrochemical case really works. This experiment is analogous to the Coulomb blockade experiment of the preceding paper, but performed using an STM in an electrochemical medium, with ligand-protected gold clusters playing the role of the quantum dot.

    Shim et al., Control and measurement of the phase behavior of aqueous solutions using microfluidics
    This isn't particularly deep, but it sure is cool. Microfluidics has come a long way, and the extremely nice properties of polydimethylsiloxane (PDMS) have been a big help. That's the transparent silicone rubber used for many microfluidic applications, as well as being related to the silicone used for soft contact lenses and breast implants. The authors here have carefully used the water and gas permeability of thin PDMS layers to control the concentrations of solutes in water-based solutions, allowing them to do things like gently make supersaturated conditions to control crystallization of proteins. We're just at the leading edge of the potential applications for these kinds of systems.