Can Point Defects in Surfaces in Solution be Atomically Resolved by Atomic Force Microscopy?

Abstract

While the atomic force microscope (AFM) is able to image mineral surfaces in solution with atomic resolution, so far, it has been a matter of debate whether imaging point defects is also possible under these conditions. The difficulties stem from the limited knowledge of what types of defects may be stable in the presence of an AFM tip, as well as from the complicated imaging mechanism involving interactions between hydration layers over the surface and around the tip apex. Here, we present atomistic molecular dynamics and free energy calculations of the AFM imaging of vacancies and ionic substitutions in the calcite (101[over ¯]4) surface in water, using a new silica AFM tip model. Our results indicate that both calcium and carbonate vacancies, as well as a magnesium substitution, could be resolved in an AFM experiment, albeit with different imaging mechanisms.

Resolving Point Defects in the Hydration Structure of Calcite (10.4) with Three-Dimensional Atomic Force Microscopy

“…Observing the same deviation from the otherwise periodic calcite surface structure in consecutive images makes it highly unlikely that the deviation is an imaging artifact and, thus, confirms our assignment as a defect. Our observation is consistent with the MD simulations by Reischl et al [5], which indicate that calcium and carbonate vacancies, as well as magnesium substitutions, on calcite (10.4) are stable, even in the presence of an AFM tip. Moreover, the presented data show that defects can be observed at least within a time that is similar to that taken to record a typical 3D dataset (which is on the order of 2 min).…”

“…The magnitude of these perturbations is in good agreement with predictions from previous atomistic simulations of calcite surfaces with Ca and CO 3 vacancies and Mg 2þ (or Fe 2þ ) substitutions [5] and additional simulations of two charge-neutral defects performed in this work, using the same methodology: a substitution of Ca and with Fe 3þ , charge compensated by a OH − ion from solution. Details regarding the simulations are described in the first section of the Supplemental Material and Fig.…”

“…All images are trace-down images, the ½421 direction points to the lower right corner. data and in the MD simulation data, the lateral extent fits remarkably well with the MD simulation results [5].…”

“…The authors have simulated both calcium and carbonate vacancies as well as the substitution of a single Ca atom with a Mg atom. Two major results have been obtained [5]. First, the defects were found to be stable over the time scale of the simulationseven though the vacancies carried a local charge of AE2e.…”

“…A recent molecular dynamics (MD) study by Reischl et al [5] has considered bulk water instead of a single water molecule as well as the presence of a nanoscopic atomic force microscopy (AFM) tip. The authors have simulated both calcium and carbonate vacancies as well as the substitution of a single Ca atom with a Mg atom.…”