1. Through the Looking Glass at the Atomic Scale
MRSEC DMR
2016
Cornell Center for Materials Research: A NSF MRSEC
In Through the Looking Glass, Alice steps through a mirror into a world in which everything is its mirror image. Realizing that writing in books is reversed, Alice wonders what has happened on the atomic scale. “Perhaps Looking-glass milk isn't good to drink?,” she says to her cat.
Using a material only two atoms thick, researchers at Cornell University and colleagues from Instituto de Física UNAM, Mexico have confirmed Alice’s suspicion that mirror-image materials are different. The researchers stacked two sheets of carbon atoms, each a single atom thick, with a precise left- handed twist. They then stacked two more sheets with a precise right-handed twist. As shown in the figure, these two stacks are mirror images of one another. The researchers showed that light behaves differently when passing through the two materials; the films showed remarkably large “circular dichroism.” In the future, these handed or “chiral” thin films may enable the production of ultrathin devices with advanced chiral functionalities.
Chiral materials possess left- and right-handed counterparts linked by mirror symmetry. These materials are useful for advanced applications in polarization optics, stereochemistry and spintronics. In particular, the realization of spatially uniform chiral films with atomic-scale control of their handedness could provide a powerful means for developing nanodevices with novel chiral properties. However, previous approaches based on natural or grown films or arrays of fabricated building blocks, could not offer a direct means to program intrinsic chiral properties of the film on the atomic scale. Here, we report a chiral stacking approach, where two-dimensional materials are positioned layer-by-layer with precise control of the interlayer rotation (θ) and polarity, resulting in tunable chiral properties of the final stack. Using this method, we produce left- and right-handed bilayer graphene, that is, a two-atom-thick chiral film. The film displays one of the highest intrinsic ellipticity values (6.5 deg μm–1 ) ever reported, and a remarkably strong circular dichroism (CD) with the peak energy and sign tuned by θ and polarity. We show that these chiral properties originate from the large in-plane magnetic moment associated with the interlayer optical transition. Furthermore, we show that we can program the chiral properties of atomically thin films layer-by-layer by producing three-layer graphene films with structurally controlled CD spectra.
(Foreground) Mirror image materials created by stacking single-atom-thick films. (Background) Artist’s rendering of (top) right-handed and (bottom) left-handed films at the atomic scale.
C. J. Kim, et al. Nat. Nanotechnol. 11, 520–524 (2016).