Power/Performance Bits: April 22

Plasmonics could improve solar performance, data storage
According to researchers at Purdue University, plasmonic metamaterials that operate at high temperatures could significantly improve solar cell performance and make advanced computer data storage technology possible that uses heat to record information on a magnetic disk.

These materials could make it possible to harness clouds of electrons called surface plasmons to manipulate and control light, although some of the plasmonic components under development rely on the use of metals such as gold and silver, which cannot withstand high temperatures. They also are not compatible with CMOS manufacturing but the researchers are working to replace silver and gold with titanium nitride and zirconium nitride. These materials remain stable at the high operational temperatures required for high efficiency and performance.

Metamaterials have engineered surfaces that contain features, patterns or elements, such as tiny antennas or alternating layers of nitrides that enable unprecedented control of light. Under development for about 15 years, the metamaterials owe their unusual potential to precision design on the scale of nanometers.

The Purdue researchers have discovered that a new class of plasmonic technologies might use high temperatures to achieve superior efficiency. One obstacle, however, is that the operational temperature required for high-efficiency devices is estimated to be around 1,500 degrees Celsius (about 2,700 Fahrenheit). Titanium nitride and zirconium nitride are said to be refractory, meaning they have a high melting point and chemical stability at temperatures above 2,000 Celsius (about 3,600 degrees Fahrenheit).

This diagram shows the respective properties of plasmonic and refractory materials for applications in high-temperature plasmonics, which could radically improve solar cell performance and bring advanced computer data storage technology that uses heat to record information on a magnetic disk. (Source: Birck Nanotechnology Center/Purdue University)

In solar thermophotovoltaics, an extremely thin layer of “metamaterial” depicted here uses plasmonic nanoantennas to absorb and emit light, potentially resulting in high-efficiency solar cells. (Source: Birck Nanotechnology Center/Purdue University)

The materials might be used for solar thermophotovoltaics, in which an ultrathin layer of plasmonic metamaterials could dramatically improve solar cell efficiency: Whereas today’s solar cells have an efficiency of about 15 percent, in theory the efficiency might be improved to as high as 85 percent with solar thermophotovoltaics. The plasmonic layer acts as a thin “intermediate spectral converter” that absorbs the entire spectrum of sunlight and then illuminates the solar cell.

Light, breathable nano-membrane
A new nano-membrane made by researchers at ETH Zurich out of “super material” graphene is extremely light and breathable and is expected to enable a new generation of functional waterproof clothing, as well as ultra-rapid filtration.

Artist’s rendering of the two-layered graphene membrane (grey honeycomb structure) with molecules (blue) being able – as a function of their size – to pass the pores. (Source: ETH Zurich)

The researchers have produced a stable porous membrane that is thinner than a nanometer, consisting of two layers of the so called ”super material” graphene, a two-dimensional film made of carbon atoms, on which the team of researchers etched tiny pores of a precisely defined size.

The membrane can thus permeate tiny molecules. Larger molecules or particles, on the other hand, can pass only slowly or not at all. With a thickness of just two carbon atoms, this is the thinnest porous membrane that is technologically possible to make.

The ultra-thin graphene membrane may one day be used for a range of different purposes, including waterproof clothing. The membrane could also potentially be used to separate gaseous mixtures into their constituent parts or to filter impurities from fluids. The researchers were able to demonstrate for the first time that graphene membranes could be suitable for water filtration. The researchers also see a potential use for the membrane in devices used for the accurate measurement of gas and fluid flow rates that are crucial to unveiling the physics around mass transfer at nanoscales and separation of chemical mixtures.

Part of a graphene membrane with a multiplicity of pores (black) of precisely defined size (in this case with a diameter of 50 nanometres; photomicrograph). (Source: ETH Zurich)