Funneling the sun’s energy

The quest to harness a broader spectrum of sunlight's energy to produce electricity has taken a radically new turn, with the proposal of a ''solar energy funnel'' that takes advantage of materials under elastic strain.

''We're trying to use elastic strains to produce unprecedented properties,'' says Ju Li, an MIT professor and corresponding author of a paper describing the new solar-funnel concept.

In this case, the ''funnel'' is a metaphor: electrons and their counterparts, holes - which are split off from atoms by the energy of photons - are driven to the centre of the structure by electronic forces, not by gravity as in a household funnel. And yet, as it happens, the material actually does assume the shape of a funnel: It is a stretched sheet of vanishingly thin material, poked down at its centre by a microscopic needle that indents the surface and produces a curved, funnel-like shape.

The pressure exerted by the needle imparts elastic strain, which increases toward the sheet's centre. The varying strain changes the atomic structure just enough to ''tune'' different sections to different wavelengths of light - including not just visible light, but also some of the invisible spectrum, which accounts for much of sunlight's energy.

Li, who holds joint appointments as the Battelle Energy Alliance Professor of Nuclear Science and Engineering and as a professor of materials science and engineering, sees the manipulation of strain in materials as opening a whole new field of research.

Strain - defined as the pushing or pulling of a material into a different shape - can be either elastic or inelastic. Xiaofeng Qian, a post-doc in MIT's Department of Nuclear Science and Engineering who was a co-author of the paper, explains that elastic strain corresponds to stretched atomic bonds, while inelastic, or plastic, strain corresponds to broken or switched atomic bonds.