The stacking of atomically thin, 2D materials into multilayered heterostructures has revealed a wealth of exotic physical phenomena, resulting in an explosion of research on this topic and fueling advances in areas ranging from quantum materials to functional devices such as batteries and solar cells. A big hurdle to overcome, however, is the production of 2D materials with the exact properties needed for a given application.
In a recent paper published in JACS, researchers reported on the successful fabrication of 2D polymers, made to order using powerful computer models and built from the bottom up using synthetic chemistry methods.
“Using this approach, you can arrange molecules into 2D lattices connected by covalent chemical bonds,” said Halleh Balch, a recent PhD from the UC Berkeley Physics Department and the paper’s first author. “Your choice of the constituent molecules and type of chemical bond changes the properties of the 2D material—like the colors of light it absorbs and emits, and how conductive it is. By being smart about what molecules you start with, you can, in principle, embed specific physical properties within this new 2D material.”
The researchers verified that the synthesized material—a layered 2D polymer—was true to design by using an array of techniques, including optical spectroscopy, transmission electron microscopy, atomic force microscopy, and grazing-incidence wide-angle x-ray scattering (GIWAXS) at ALS Beamline 7.3.3.
The tests showed that the 2D polymer was highly crystalline, with the requisite semiconducting band gap and ability to electronically couple to monolayer transition metal dichalcogenides, like molybdenum disulfide (MoS2).
Dramatic optical and electronic changes occurred when the polymer thickness was reduced: the polymer’s efficiency at re-radiating absorbed light increased by more than a factor of 100. More surprisingly, the rate at which the layers transfer energy between each other was also affected. This unanticipated result underscores the potential for the discovery of emergent phenomena in studies of hybrid heterostructures.
H. Balch, R. Dasari, H. Li, R. Li, S. Thomas, D. Wang, R. Bisbey, K. Slicker, I. Castano, S. Xun, L. Jiang, C. Zhu, N. Gianneschi, D. Ralph, J.‑L. Bredas, S. Marder, W. Dichtel, and F. Wang, “Electronically Coupled 2D Polymer/MoS2 Heterostructures,” JACS 142, 21131 (2020), doi:10.1021/jacs.0c10151.