BUFFALO, N.Y. — A University at Buffalo-led team has found a way to help chiral semiconductors, electronic materials whose structures are left- or right-handed like many of life's building blocks, absorb visible light.
In a study published in Nature Communications , researchers chemically combined a chiral semiconducting material with a non-chiral molecule that more readily absorbs visible light. The result is a new material system that can both absorb visible light and distinguish between left- and right-handed light waves, opening new possibilities for optoelectronic technologies.
"We were able to transfer the properties of chirality to a non-chiral molecule," says Wanyi Nie, PhD, associate professor in the UB Department of Physics and the study's corresponding author. "The resulting material retains the handedness that makes chiral semiconductors promising building blocks for next-generation electronics, while adding the ability to respond to visible light."
The study, published online last week following an earlier early-access release, was supported by the National Science Foundation. Collaborators include Los Alamos National Laboratory, Brookhaven National Laboratory, the University of California, Berkeley, and National Taiwan University.
Taking a cue from DNA
Chiral molecules have structures that cannot be superimposed on their mirror image, like left and right hands. Many biological molecules are chiral and exist in either the left- or right-handed forms; DNA and its famed double-helix structure are right-handed.
Crucially, this handedness changes how they interact with other molecules that are either left- or right-handed.
Similarly, a semiconductor with a chiral crystal structure can distinguish between left- and right-circularly polarized light — the left- and right-handed versions of light waves — and respond differently to each.
"This allows for more complex ways of detecting, processing and transmitting information using light, with potential applications in advanced polarized light sensors, optical communications systems and photocatalysis," says co-author Dave (Hsinhan) Tsai, PhD, assistant professor in the UB Department of Chemical and Biological Engineering.
However, chiral semiconductors have been limited because most do not absorb visible light efficiently. Light absorption occurs when photons have enough energy to excite electrons to a higher energy state. Many chiral semiconductors have a large bandgap — the energy range where electrons cannot exist — requiring more energy for electrons to make that jump.
"Visible light does not carry enough energy to interact with chiral materials, so these materials primarily absorb higher-energy UV light," Nie says.
The art of the assist
To address this, the researchers combined a chiral semiconductor made from perovskite with what's known as a dopant molecule — an organic compound called F4TCNQ that readily accepts electrons.
They then exposed the material to visible light. The chiral semiconductor responded differently to left- and right-handed light waves, with electrons moving from the chiral host to the dopant molecule's higher energy states. This charge transfer state enabled visible light absorption.
"The core physics is associated to the electron transfer that carries chirality from the chiral perovskite host to the non-chiral dopant molecule," Nie says.
The partnership between the two molecules is like an assist in basketball, Tsai adds.
"The chiral molecule is the guard and the dopant molecule is the forward. Guards read the play and then pass the ball to the forward," he says.
The researchers' next step is to dive deeper into the physical mechanisms by which the semiconductor's chiral properties are transferred to the dopant molecule.
"We see that the ability to tell left- from right-handed light is being passed from one material to another, but we don't yet fully understand how electrons carry that information across, and what governs this process." Nie says.
Other UB co-authors include Reshna Shrestha, a PhD student in Nie's lab, and Cunyi Wei, a former graduate student in the Department of Chemical and Biological Engineering.
