Archive for the ‘Materials’ Category

Marvin Cohen and Alex Zettl have published a small review of boron nitride nanotubes [Physics Today 63 (11), 34-38 (2010)]. Two potential applications loom. The authors point out that these nanotubes (abbreviated BNNT) show field emission that is unusually stable compared to carbon nanotube field emission. Also, BNNT field emission closely follows the Fowler-Nordheim law. The implications to cold field emission device construction are obvious.

Furthermore, the BNNT bandgap can be tuned by application of an electric field transverse to the tube axis. At zero applied voltage, the gap is around 5 eV. At sufficiently large applied voltage, the band gap collapses to 0 eV. Perhaps one may use a BNNT as a transistor channel based on this giant Stark effect.

Other electronic applications may arise from the peculiar spatial localization of the lowest conduction band. It is physically situated in the tube, leading lightly doped BNNTs to act like plumbing for nearly a free electron gas.

The Javey group at UC Berkeley  has developed a novel method for placing III-V compound structures on silicon substrates. The method appeared in Nature 468, 286-9 (11 November 2010) [doi:10.1038/nature09541].

After growing and patterning thin InAs on a suitable III-V substrate, the patterned elements are picked up by a silicon rubber carrier and transferred to an oxidized silicon wafer, where further processing can take place.

Longer blog postings can be found at IEEE Spectrum , and at Technology Review.

Graphene has been touted as the new wonder material for electronics. As we have observed earlier, the excitement is due to the extraordinarily high carrier mobilities, among other interesting properties [1]. Unfortunately, graphene has no band gap, so it must be chemically or physically modified to become a useful semiconductor.

Chemical doping is developing apace, with electron donors (like alkali metals) [3] and hole donors (like bismuth and gold) [2] now experimentally established. Equally interesting is hydrogen “doping” (see references in [1]), realized by reacting graphene with hydrogen. This reaction has generally been carried out globally, but it occurs to me that one might use beam technology to drive localized reactions: proton beam, atomic hydrogen beam, or e-beam induction à la Zeiss MeRiT. All these procedures result in simultaneously splitting the valence and conduction bands and moving the fermi level.

Very recently, Chen’s group at Purdue reports a solid-state reaction under 30 kV e-beam bombardment of graphene on a substrate [4]. This reaction likewise modifies the band structure. (The authors claim that the dose, ~1800 μC / cm^2, is “typical” SEM exposure.)

Physically restructuring graphene also results in changes to its electrical properties, due to edge states (akin to surface states in 3-dimensional materials). Extremely high e-beam doses (~30 C / cm^2 at 200 kV) result in material removal from the beam impact point. Drndić’s group reports [5] creating holes and lines in suspended graphene by this method. The review by Krasheninnikov and Nordlund [6] is a highly valuable resource in regard to such physical restructuring of materials. The shorter review [7], also by Krasheninnikov, focuses on carbon materials.

References:

[1] Berashevich, Chakraborty, “Graphene and graphane: New stars of nanoscale electronics”, arXiv:1003.0044.
[2] Gierz, et al., “Atomic Hole Doping of Graphene”, arXiv:0808.0621.
[3] Ohta, et al., “Controlling the Electronic Structure of Bilayer Graphene”, Science 313, 951 (2006).
[4] Childres, et al., “Effect of electron-beam irradiation on graphene field effect devices”, arXiv:1008.4561.
[5] Fischbein, Drndić, “Electron Beam Nanosculpting of Suspended Graphene Sheets”, arXiv:0808.2974.
[6] Krasheninnikov, Nordlund, “Ion and electron irradiation-induced effects in nanostructured materials”, J Appl Phys 107, 071301 (2010).
[7] Krasheninnikov, Banhart, “Engineering of nanostructured carbon materials with electron or ion beams”, Nature Materials, 6 (2007) 723-33.

Geometric phase (or Pancharatnam-Berry phase) has many surprising effects. What is this variously named “phase”? To quote Wikipedia, “The Berry phase occurs when [two] parameters are changed simultaneously but very slowly (adiabatically), and eventually brought back to the initial configuration.” [1] A light-hearted and easily understood introduction can be found in section 1 of ref. [2]. In what follows, I give a few examples of geometric phase effects that are relevant to nano-technology. (A lengthy review of the Berry phase in electronics can be found in [3].)

• A group at TU Denmark have shown in [4] that the Berry phase effects associated with electrical current flowing through a conductive molecular bridge may induce mechanical vibration sufficiently strong to rupture the bridge. This phenomenon is unrelated to electromigration, Joule heating, or other well-known effects. It is purely a result of the quantum mechanical phase of the electric waves.

• In an interesting experiment [5], Kohmura-san and colleagues at RIKEN demonstrate millimeter distance translation of X-rays by bent silicon crystals. The crystal was bent only 80 nm or so to achieve a beam displacement of 1.5 mm. See [6] for a brief summary.

• The Hasman group at the Technion have constructed optical structures showing technologically applicable geometric phase behavior. In [7] and [8] they display nano-lithographically defined gratings and apertures which can act as optical switches, among other things.

Try the little demonstration in [2]. It will serve as a reminder to keep your eyes open to the geometric phase.

References:

[1] Wikipedia article,  http://en.wikipedia.org/wiki/Berry’s_phase
[2] Robert W. Batterman, “Falling Cats, Parallel Parking, and Polarized Light”, http://philsci-archive.pitt.edu/archive/00000794/00/falling-cats.pdf
[3] Di Xiao, et al., “Berry Phase Effects on Electronic Properties”, Rev. Mod. Phys. 82 (2010) 1959-2007, http://rmp.aps.org/abstract/RMP/v82/i3/p1959_1
[4] Jing-Tao Lu, et al., “Blowing the Fuse: Berry’s Phase and Runaway Vibrations in Molecular Conductors”, Nano Letters 10 (2010) 1657-63, http://pubs.acs.org/doi/abs/10.1021/nl904233u
[5] Yoshiki Kohmura, et al., “Berry-Phase Translation of X Rays by a Deformed Crystal”, Phys. Rev. Lett. 104 (2010) 244801, http://prl.aps.org/abstract/PRL/v104/i24/e244801
[6] Adams, “Geometric phase kicks x rays down a new path”, http://physics.aps.org/articles/v3/50
[7] Erez Hasman, et al., “Polarization beam-splitters and optical switches based on space-variant computer-generated subwavelength quasi-periodic structures”, Optics Communications 209 (2002) 45-54, DOI: 10.1016/S0030-4018(02)01598-5, http://www.sciencedirect.com/science/article/B6TVF-4645DRF-1/2/6f0b7ca5cf17b751e48bbf1cf1e77d03
[8] Yuri Gorodetski, “Observation of Optical Spin Symmetry Breaking in Nanoapertures”, Nano Lett., Vol. 9, No. 8, 2009, http://pubs.acs.org/doi/abs/10.1021/nl901437d