Abstract
Terminating the tip of an atomic force microscope with a CO molecule allows data to be acquired with a well-known and inert apex. Previous studies have shown conflicting results regarding the electrostatic interaction, indicating in some cases that the negative charge at the apex of the CO dominates, whereas in other cases the positive charge at the end of the metal tip dominates. To clarify this, we investigated hbox {CaF}_{2}(111). hbox {CaF}_{2} is an ionic crystal and the (111) surface does not possess charge inversion symmetry. Far from the surface, the interaction is dominated by electrostatics via the negative charge at the apex. Closer to the surface, Pauli repulsion and CO bending dominate, which leads to an unexpected appearance of the complex 3-atom unit cell. We compare simulated data in which the electrostatics are modeled by point particles versus a charge density calculated by DFT. We also compare modeling Pauli repulsion via individual Lennard–Jones potentials versus a total charge density overlap. In doing so, we determine forcefield parameters useful for future investigations of biochemical processes.
Highlights
Terminating the tip of an atomic force microscope with a CO molecule allows data to be acquired with a well-known and inert apex
When comparing the images obtained with a single-atom metal tip in Ref. 39 to the images obtained with the CO-terminated tips (CO tips) presented here, we find in agreement to previous studies an inversion of the atomic force microscopy (AFM) contrast in the electrostatic imaging regime[12,27]
Metal tips present a positive pole at the tip apex due to the Smoluchovski e ffect[27,47,48,49], while the negative charge density in front of the O atom at the CO tip apex is relevant when imaging an ionic lattice[12,13]
Summary
Terminating the tip of an atomic force microscope with a CO molecule allows data to be acquired with a well-known and inert apex. Ellner et al considered AFM images of Cl vacancies in NaCl thin films with a CO tip and showed that for such a charged feature the interaction with the strong background metal tip dipole dominates the electrostatic tipsample interaction[12], to the case when imaging the atomic lattice of hexagonal boron nitride[13]. The strongly spatially localized negative charge density at the tip apex is relevant when imaging the flat NaCl lattice[12,27]. These findings raise the question as to whether the atomic-scale electrostatic AFM contrast measured with CO tips on bulk insulators can be generally explained with a negative tip apex. In previous AFM studies atomic identification of sample atoms required indirect theoretical c haracterization[28,29], ochraardgseoirnbveedrsmioanrksyermmmoeltercyualensdotnhethseursfuarcfeacaeto30m,31i.cIlnayceornctoranssti,stthsesothlerleyeofof lFd− -siyomnsm, wetitrhictCheaFs2e(c1o1n1d) surface layer of lacks Ca2+
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