Efficient light-electricity conversion: A 2D material’s geometry can be changed to unlock its promise

Optoelectronics materials are capable of converting light energy into electricity and vice versa. They have many promising applications in light-emitting, energy-harvesting, and sensing technologies. These devices could be more efficient and retain significant amounts of helpful energy as heat. New principles for light-electricity conversion are required to break the current efficiency limitations.

Inversion symmetry is a physical property that restricts engineers’ ability to control electrons in materials and limits their design options. A team of materials scientists, engineers, and professors of materials science at Rensselaer Polytechnic Institute used a strain gradient to break the inversion symmetry. This created a new optoelectronic phenomenon for molybdenum disulfide (MoS2), published in Nature Nanotechnology.

The team placed a Vanadium oxide (VO2) copper wire under a MoS2 sheet to break the inversion symmetry. She explained that Molybdenum dioxide is a flexible material, so it was bent to follow the curves of the VO2wire, creating a gradient in its crystal lattice. Imagine what might happen if you put a piece of paper on top of a pencil on a desk. The paper’s varied tension is similar to the strain gradient in the MoS2 lattice.

She explained that the gradient breaks the material’s inversion symmetry and allows electrons to travel within the crystal to be manipulated. A current can flow through the material because of the unique photo-response observed near the strain gradient. This is known as the Flexo-Photovoltaic Effect. It could be used to create novel, high-efficiency optoelectronics.

Shi stated that this is the first time such an effect has been demonstrated in this material. “If we can find a solution that doesn’t produce heat during photon-electricity transformation, then electronic devices and circuits could be improved.”

Vanadium oxide is sensitive to temperature, so the team showed that the flexo photovoltaic effect caused temperature dependence at the location where the MoS2 & VO2 materials meet — altering the lattice’s slope accordingly.

Jie Jiang is a postdoctoral researcher in Shi’s laboratory and the first author of this paper.

Shi stated that the team’s demonstrations here showed great promise for the material and suggested the possibility of using this approach to engineer other materials with optoelectronic characteristics that are affected by inversion symmetry.

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