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
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.
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