Determination of Charge and Size of Rings by Atomic Force Microscopy

Abstract

We consider a charged ring probed by the tip sensor of an Atomic Force Microscope (AFM) and present a method to measure, from the force-vs.-separation curve, the charge and size of the ring. A number of systems of nano- and sub-nanometer dimensions can be modeled as rings of charge. For example, in molecular pumps, rings are driven, based on their charge content, to link amino acids into growing peptides. Knowledge of charge content and size is necessary to apply these pumps to growing other assemblies.1 More generally, charge plays a key role in the structure attained by large molecules when they self-assemble.2 There have also been proposals to use charged rings to physically store qubits for quantum computing.3 These and other examples underscore the relevance of understanding electrostatics measurements at the nanoscale with the AFM, in particular due to rings of charge.