Certainly nanomaterials aren’t new in human history, but we now have the tools to understand their reactivities, control their functions, and manipulate matter at the atomic level. We are also producing these products at an increasingly rapid rate without really knowing their environmental (or health) costs.

What do you think? Do you think people tend to overreact when it comes to regulating nano products? Do you think governments should be doing more? How concerned are you with this stuff getting into your water or air or food?

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Atom interferometers have been made before, but the NIST technique introduces some new twists. The researchers trap about 20,000 ultracold rubidium atoms with optical lattices, a lacework of light formed by three pairs of infrared laser beams that sets up an array of energy “wells,” shaped like an egg carton, that trap the atoms. The lasers are arranged to create two horizontal lattices overlapping like two mesh screens, one twice as fine as the other in one dimension. If one atom is placed in each site of the wider lattice, and those lasers are turned off while the finer lattice is activated, then each site is split into two wells, about 400 nanometers apart. Under the rules of the quantum world, the atom doesn’t choose between the two sites but rather assumes a “superposition,” located in both places simultaneously. Images reveal a characteristic pattern as the two parts of the single superpositioned atom interfere with each other.
Everything changes when two atoms are placed in each site of the wider lattice, and those sites are split in two. The original atom pair is now in a superposition of three possible arrangements: both atoms on one site, both on the other, and one on each. In the two cases when both atoms are on a single site, they interact with each other, altering the interference pattern—an effect that does not occur with light. The imbalance among the three arrangements creates a strobe-like effect. Depending on how long the atoms are held in the lattice before being released to interfere, the interference pattern flickers on (with stripes) and off (no stripes). A similar “collapse and revival” of an interference pattern was seen in similar experiments done earlier in Germany, but that work did not confine a pair of atoms to a single pair of sites. The NIST experiments allowed researchers to measure the degree to which they had exactly one or exactly two atoms in a single site, and to controllably make exactly two atoms interact. These are important capabilities for making a quantum computer that stores information in individual neutral atoms.

[1] J. Sebby-Strabley, B.L. Brown, M. Anderlini, P.J. Lee, W.D. Phillips, J.V. Porto and P.R. Johnson. 2007. Preparing and probing atomic number states with an atom interferometer. Physical Review Letters 98, 200405 (2007).

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“Our nanowires were designed to provide this feature, along with a superior electrical conductivity,” said NREL materials scientist Yong Zhang, the study’s corresponding researcher, to PhysOrg.com. “Both of these properties are critical in order for renewable energy devices to reach their ultimate efficiency limits.”

Conventional coaxial cables consist of a central copper wire symmetrically surrounded by a braided copper conductor, with an insulating spacer material between the two. The braid serves as a return route for electrons that have already passed down the core wire; it can equally be viewed as a channel for holes moving in the opposite direction. The insulator separates the charge passing through the wire and braid.

Mimicking this structure, the group designed a nanoscale version consisting of a central wire, the “core,” surrounded by a shell (the shell is not cylindrical like conventional cables, but rather is hexagonal). The researchers used two semiconducting materials: gallium nitride (GaN) and gallium phosphide (GaP). They made two samples, one with a GaN core and GaP shell, and another with a GaP core and GaN shell. Both wires are approximately four nanometers in diameter (according to Zhang, this particular size was chosen by considering the computational effort needed to analyze the wires’ properties, because larger wires, while easier to make, require considerably more computing power and time to model. Similar success, he says, could be achieved with nanowires up to 10-15 nanometers in diameter). In neither sample is an insulating spacer required. This phenomenon is the result of the specific semiconducting behaviors of GaN and GaP.
GaN and GaP, like all semiconductors, are classified by “band gap” – how much energy is required for electrons in the material to jump from the top of the “valence band,” a range of energies for which they don’t participate in conduction, to the bottom of the “conduction band,” a range for which they do participate. When GaN and GaP are combined into a wire, the structure as a whole assumes its own band gap, which is very different from that of either component but much more appropriate for solar energy applications.

Besides providing efficient charge separation, the design may be able to remedy several shortcomings of solar-energy applications. For example, they could help widen the coverage of the solar spectrum and minimize energy loss associated with electron-hole recombination.

Yong Zhang, Lin-Wang Wang, and Angelo Mascarenhas, “’Quantum Coaxial Cables’ for Solar Energy Harvesting.” Nano Lett. ASAP Article, DOI: 10.1021/nl070066t

http://www.nature.com/nnano/journal/
vaop/ncurrent/abs/nnano.2007.106.html

http://www.scielo.br/scielo.php?script=sci_abstract&pid=S1517-70762006000400002&lng=pt&nrm=iso&tlng=en