The pressure exerted by the needle imparts elastic strain, which increases toward the sheet’s center. 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.

We’re trying to use elastic strains to produce unprecedented properties.

—Ju Li, corresponding author

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 at MIT, and with Xi’an Jiaotong University in China, sees the manipulation of strain in materials as opening a whole new field of research.

Elastic strain has the potential to be used to achieve rapid and reversible tuning of the bandgap. However, as a result of plasticity or fracture, conventional materials cannot sustain a high enough elastic strain to create sufficient changes in their physical properties. Recently, an emergent class of materials—named ‘ultrastrength materials’—have been shown to avoid inelastic relaxation up to a significant fraction of their ideal strength. Here, we illustrate theoretically and computationally that elastic strain is a viable agent for creating a continuously varying bandgap profile in an initially homogeneous, atomically thin membrane.

—Feng et al.

Strain—defined as the pushing or pulling of a material into a different shape—can be either elastic or inelastic. Xiaofeng Qian, a postdoc 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. A spring that is stretched and released is an example of elastic strain, whereas a piece of crumpled tinfoil is a case of plastic strain.

The new solar-funnel work uses precisely controlled elastic strain to govern electrons’ potential in the material. The MIT team used computer modeling to determine the effects of the strain on the thin layer of molybdenum disulfide (MoS₂), a material that can form a film just a single molecule thick.

The elastic strain, and therefore the change that is induced in electrons’ potential energy, changes with their distance from the funnel’s center—much like the electron in a hydrogen atom, except this “artificial atom” is much larger in size and is two-dimensional. In the future, the researchers hope to carry out laboratory experiments to confirm the effect.

Unlike graphene, another prominent thin-film material, MoS₂ is a natural semiconductor: It has a crucial characteristic (a bandgap), that allows it to be made into solar cells or integrated circuits. But unlike silicon, now used in most solar cells, placing the film under strain in the “solar energy funnel” configuration causes its bandgap to vary across the surface, so that different parts of it respond to different colors of light.

In an organic solar cell, the electron-hole pair (exciton), moves randomly through the material after being generated by photons, limiting the capacity for energy production. “It’s a diffusion process,” Qian says, “and it’s very inefficient.” But in the solar funnel, he adds, the electronic characteristics of the material “leads them to the collection site [at the film’s center], which should be more efficient for charge collection.”

The convergence of four trends, Li says, has opened up the field of elastic strain engineering:

• the development of nanostructured materials, such as carbon nanotubes and MoS₂, that are capable of retaining large amounts of elastic strain indefinitely;

• the development of the atomic force microscope and next-generation nano-mechanical instruments, which impose force in a controlled manner;

• electron microscopy and synchrotron facilities, needed to directly measure the elastic strain field; and

• electronic-structure calculation methods for predicting the effects of elastic strain on a material’s physical and chemical properties.

The work was done with Ji Feng of Peking University and Cheng-Wei Huang, and was supported by the US National Science Foundation, the US Air Force Office of Scientific Research, and the National Natural Science Foundation of China.

Resources

• Ji Feng, Xiaofeng Qian, Cheng-Wei Huang & Ju Li (2012) Strain-engineered artificial atom as a broad-spectrum solar energy funnel. Nature Photonics doi: 10.1038/nphoton.2012.285