Solution-Cast Monolayers of Cobalt Crown Ether Phthalocyanine\non Highly Ordered Pyrolytic Graphite

Abstract

15-Crown-5-ether-substituted cobalt­(II)\nphthalocyanine (CoCrPc)\nis an atomically thin and flat-laying, electrically insulating molecule\nthat can solvate ions; these properties are desirable for nanoelectronic\ndevices. A simple, solution-phase deposition method is demonstrated\nto produce a monolayer of CoCrPc on highly ordered pyrolytic graphite\n(HOPG). A uniform and continuous CoCrPc layer is obtained on freshly\ncleaved HOPG by solution drop casting, followed by thermal annealing\nunder ambient pressure in Ar in the temperature range of 150–210\n°C. While the quality of the monolayer is independent of annealing\ntime, the composition of the annealing atmosphere is critical; exposure\nto ambient air degrades the quality of the monolayer over the time\nscale of minutes. Using ultrahigh vacuum scanning tunneling microscopy,\na highly ordered and flat CoCrPc layer with hexagonal symmetry and\naverage spacing of 4.09 ± 0.2 nm is observed. The band gap of\nthe CoCrPc, measured by scanning tunneling spectroscopy, is 1.34 ±\n0.07 eV. The ability to prepare uniform, ordered, and conformal monolayers\nof CoCrPc molecules on HOPG represents the first step toward using\nthese materials to seed dielectric growth on 2D crystals and provide\na 2D electrolyte for the electrostatic gating of semiconductors at\nthe ultimate limit of scaling.