Abstract
Two-dimensional (2D) metal–organic frameworks (MOFs) have been recently proposed as a flexible material platform for realizing exotic quantum phases including topological and anomalous quantum Hall insulators. Experimentally, direct synthesis of 2D MOFs has been essentially confined to metal substrates, where the strong interaction with the substrate masks the intrinsic electronic properties of the MOF. In addition to electronic decoupling from the underlying metal support, synthesis on weakly interacting substrates (e.g., graphene) would enable direct realization of heterostructures of 2D MOFs with inorganic 2D materials. Here, we demonstrate synthesis of 2D honeycomb MOFs on epitaxial graphene substrate. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by density-functional theory (DFT) calculations, we show the formation of a 2D band structure in the MOF decoupled from the substrate. These results open the experimental path toward MOF-based designer electronic materials with complex, engineered electronic structures.
Highlights
Metal−organic frameworks (MOFs) are an important class of materials that present intriguing opportunities in the fields of sensing, gas storage, catalysis, and optoelectronics.[1−4] While there are a tremendous number of examples of three-dimensional, bulk metal−organic frameworks (MOFs), synthesis strategies for two-dimensional (2D), monolayer thick MOFs ( referred by various names[5] such as metal−organic coordination networks (MOCNs),[6] surface-confined metal−organic networks (SMONs),[7] metal−organic materials (MOMs), and metal−organic graphene analogues (MOGs)8) are more limited
Direct synthesis of 2D MOFs has been essentially confined to metal substrates, where the strong interaction with the substrate masks the intrinsic electronic properties of the MOF
Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by density-functional theory (DFT) calculations, we show the formation of a 2D band structure in the MOF decoupled from the substrate
Summary
Chemical reactions are a virtual terra incognita on weakly interacting, noncatalytic substrates[10,36−40] and the experimental observation of the intrinsic electronic properties of 2D MOFs has been elusive. If there was (integer) charge transfer to DCBP or DCA molecules in the respective complexes, the LUMO peak would split to singly occupied/unoccupied molecular orbital (SOMO/SUMO) peaks at negative and positive bias.[51,52] dI/dV spectra recorded on Co center of the complex shows an additional shoulder at the onset of the peak. We attribute this shoulder to the metalstate as the metal center becomes brighter in the STM images at sample bias beyond 0.7 V (see Supporting Information Figure S4).
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