The synthesis of a single-layer covalent organic framework (COF) with spatially modulated internal potentials provides new opportunities for manipulating the electronic structure of molecularly defined materials. Here, the fabrication and electronic characterization of COF-420: a single-layer porphyrin-based square-lattice COF containing a periodic array of oriented, type II electronic heterojunctions is reported. In contrast to previous donor-acceptor COFs, COF-420 is constructed from building blocks that yield identical cores upon reticulation, but that are bridged by electrically asymmetric linkers supporting oriented electronic dipoles. Scanning tunneling spectroscopy reveals staggered gap (type II) band alignment between adjacent molecular cores in COF-420, in agreement with first-principles calculations. Hirshfeld charge analysis indicates that dipole fields from oriented imine linkages within COF-420 are the main cause of the staggered electronic structure in this square grid of atomically-precise heterojunctions.
|Original language||English (US)|
|State||Published - Nov 28 2018|
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: T.J., C.C., H.L., and C.S.D. contributed equally to this work. This research was supported by the Army Research Office Multidisciplinary University Research Initiative (MURI) program under grant no. W911NF-15-1-0447 (STM spectroscopy), by the Army Research Office grant no. W911NF-17-1-0339 to Georgia Tech (DFT calculations), by the U.S. Department of Energy, Office of Basic Energy Sciences Nanomachine Program under contract no. DEAC02-05CH11231 (COF sample preparation), and by the joint KACST-UC Berkeley Center for Nanomaterials and Clean Energy (molecular synthesis). The KAUST IT Research Computing Team and the KAUST Supercomputing Laboratory are gratefully acknowledged for providing generous computational resources for part of our theoretical work. T.J. acknowledges support from the National Science Foundation (NSF) Graduate Research Fellowship Program under grant no. DGE 1106400. C.S.D. acknowledges support from a Kavli ENSI Philomathia Graduate Student Fellowship. P.J.W. acknowledges the Berkeley Center for Green Chemistry and NSF for support through a Systems Approach to Green Energy Integrative Graduate Education and Research Traineeship (1144885). G.W. acknowledges fellowship support from the National Natural Science Foundation of China under grant no. 61622116, the Strategic Priority Research Program of Chinese Academy of Sciences under grant no. XDB28010200, and the International Partnership Program of Chinese Academy of Sciences under grant no. 112111KYSB20160061.