Day: February 10, 2013

A ratcheting socket wrench happily turns one way, but resists rotation in the opposite direction. A magnetic quantum ratchet allows flow of electrons one way, but not the other.

Emilian Robert Vicol

In a common type of mechanical ratchet, back and forth motion provided by a human arm gets converted to rotating motion that acts to tighten a screw or bolt. The role of the ratchet is to convert a force that changes direction into a torque acting in one direction only. That principle is generalized in many other systems that convert fluctuations (some of which may be random) into usable work. Many types of ratchets exist, in mechanical, quantum, and biological systems.

Researchers have now fabricated a magnetic quantum ratchet out of graphene, a two-dimensional hexagonal lattice of carbon atoms. C. Drexler and colleagues introduced asymmetries in the electronic structure by disrupting graphene’s structure with hydrogen and modifying the substrate on which the carbon sat. When they exposed the modified graphene to an alternating electric current and a strong magnetic field, its electrons preferentially moved in one direction, setting up a directed current. So the modified graphene acted as an AC/DC converter. Although it’s not practically useful, the behavior may tell us more about the rules that govern graphene-like materials.

Under ordinary circumstances, graphene is a symmetrical hexagonal lattice of carbon atoms. When exposed to an alternating electric current, the electrons oscillate, producing no direct current on average. Similarly, imposing a steady magnetic field in the presence of the alternating current alters the electronic properties of the graphene slightly, but doesn’t tend to make the electrons move preferentially in one direction.

Electron micrograph of indium phosphide (InP) nanowires. Each is 180 nanometers in diameter; this diameter allows them to capture more light, making them effective in a photovoltaic solar cell.

Wallentin et al.

In the continuing quest to create solar cells, researchers seek new materials, use clever techniques, and look for novel physical phenomena to extract the maximum electricity out of sunlight for the lowest cost. One method of extracting more power at a lower cost relies on creating arrays of nanowires that stand vertically on inexpensive substrates. In contrast to the material in ordinary solar cells, nanowires use less material, can potentially be built with less costly materials, and in principle trap more light thanks to the geometry of the arrays. However, most nanowire solar cells are currently outperformed by their conventional counterparts.

A new effort used indium phosphide (InP) nanowires with diameters smaller than the wavelength of the light they were trapping. That trick enabled Jesper Wallentin and colleagues to reach comparable efficiencies and slightly higher voltage than a conventional InP solar cell. While the wires only covered 12 percent of the surface area, they exploited a principle known as resonant trapping to extract over half as much current as a full planar cell of InP. This approach could lead to even greater efficiency at lower cost for solar cells.

Many candidates for the next generation of photovoltaic (PV) solar cells are being investigated. Research in this area has two goals that don’t always overlap: maximizing the efficiency of converting sunlight into electric current, and reducing cost per unit of electricity. The advantage of nanowire-based cells lies in using a lot less material, since the entire surface need not be covered in PV material. Additionally, the wires themselves can be fabricated from relatively inexpensive semiconductor materials.