LAYER-BY-LAYER ETCHING OF 2-DIMENSIONAL METAL CHALCOGENIDES WITH THE ATOMIC FORCE MICROSCOPE

The atomic force microscope (AFM) is used to etch off individual layers from the surface of two-dimensional metal dichalcogenide crystals, extending our earlier work on this phenomena with the scanning tunneling microscope (J. Am. Chem. Soc. 1990, 112, 7498). The etching proceeds via a nucleation and growth of holes, where the nucleation sites are provided by missing chalcogenide atoms from the surface of the crystal. Some substrates, such as NbSe2, show faceted triangular etch pits which are observed to be rotated 180-degrees on alternate layers, indicative of the polytype of the material. Counting nucleation sites over a given area provides a new method for determining extremely small deviations from stoichiometry in layered materials. The rate of the etching process was proportional to the force applied to the AFM tip. The mechanism for etching appears to be related to direct bonding interactions between tip atoms and substrate atoms with dangling bonds. The etching process could be simulated with a computer model relating the probability of removal of a given atom with its nearest neighbor environment. The implications of the strong interactions between AFM and STM tips and substrates probed in air are discussed. The nanofabrication of several small device-like structures is demonstrated.