To date, field effect transistors based on graphene have all introduced a band gap into the graphene beneath the gate. Various treatments like oxidation, hydrogenation, etc. are difficult to control and furthermore adversely impact the novel properties of pristine graphene which one would like to retain. In the May 28 issue of PNAS, a joint work between researchers at KAIST and KIAS in Korea and at CalTech in southern California proposes using pristine graphene with a saw-toothed gate field instead of the ordinary rectangular gate field. Since gate geometry is something we know how to engineer, this proposal may lead to more readily manufacturable digital graphene devices.
Nanoparticle transport through nanotubes
Two recent articles show very surprising mechanisms for transporting nanoparticles through nanotubes. In the first, Xu and Chen at Columbia embed a single water molecule inside a fullerene cage, which was placed inside a carbon nanotube. An applied electrical field parallel to the tube causes the fullerene to migrate, even though the cage and its entrapped water molecule are electrically neutral. This phenomenon seems to be due to conservation of energy and momentum: the initially randomly oriented water molecule largely aligns with the electrical field. Leftover momentum provides the kick needed to move the whole cage forward. There’s a nice summary here.
In the second, the Cohen group at Berkeley show that an iron nanoparticle migrates through a carbon nanotube with an applied electrical field. In this case, electromigration within the nanoparticle induces something like a plastic flow of the particle through the tube.
Femtosecond electron pulse source
Hoffrogge et al. recently posted a pre-print to the arXiv describing a laser-heated field emission source demonstrating sub-100 femtosecond pulsed electron emission. There have been previous disclosures of laser-heated field emission cathodes (see, for example US 6,828,996 and references therein), but none have been designed for pulsed emission.
The new source should find application in time-resolved electron diffraction. The coherence of the source has apparently not yet been established. The implementation shown operates at 30 keV.
Another application comes to mind – pump-probe spectroscopy. In the usual scheme, a femto-second laser pulse is used to excite a target molecule, while a second, time-delayed pulse measures the decay product. One can envision replacing the pump laser pulse with a pump electron beam pulse. Since the selection rules for energy absorption are entirely different for photons and electrons, the excited states and resultant decay channels will also differ. Comparing photo-pumped and electron-pumped systems could, for example, clarify why EUV resists are generally not particularly good electron-beam resists and vice versa.
A recent advance in laser pump-probe spectroscopy, involving a third photon pulse, would be very useful here as well, since we know that electron impact with resist yields a vast number of excited states.
Wide angle, high coherence field emission
Built up field emission electron sources, while bright, have limited angle over which they are spatially coherent. For purposes such as electron holography and low-energy electron point source (LEEPS) microscopy, this angular limitation has long been a fundamental stumbling block. In a major advance, Mutus et al. demonstrate a tip which opens the spatial coherence angle to ~14 degrees. The tip is formed by field assisted nitrogen etch of tungsten to expose a single tungsten atom, while enveloping the remainder of the tip in a TiN sheath. The authors claim that the tip is extra-ordinarily stable.
The paper may be found on the arXiv. For an interesting application, see this paper (also on the arXiv).
Stability limits in electron microscopy
In a paper coming out of Kamerlingh Onnes Laboratorium, Schramm et al. remind us that instability is inherent in aberration corrected electron microscopes. They propose that the system designer use a stability budget to trade off stability and resolution. The stability budget is somewhat orthogonal to the usual error budget. They further point out that real-time control is required to further advance TEM (and SEM, LEEM, etc. for that matter), but that suitable measurement parameters from which to derive feedback are presently unknown.
The paper is on the arXiv. It would appear that there’s an entrepreneurial opportunity here.
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