True Atomic-Resolution Imaging of (101̅4) Calcite in Aqueous Solution by Frequency Modulation Atomic Force Microscopy

Abstract

Calcite (CaCO3) is one of the most abundant minerals on earth and plays an important role in a wide range of different fields including, for example, biomineralization and environmental geochemistry. Consequently, surface processes and reactions such as dissolution and growth as well as (macro)molecule adsorption are of greatest interest for both applied as well as fundamental research. An in-depth understanding of these processes requires knowledge about the detailed surface structure in its natural state which is quite often a liquid environment. We have studied the most stable cleavage plane of calcite under liquid conditions using frequency modulation atomic force microscopy. Using this technique, we achieved true atomic-resolution imaging, demonstrating the high-resolution capability of frequency modulation atomic force microscopy in liquids. We could reproduce contrast features reported before using contact mode atomic force microscopy, originating from the protruding oxygen atom of the carbonate groups. Besides this contrast, however, our results, indeed, indicate that we obtain more detailed structural information, revealing the calcium sublattice of the (1014) cleavage plane.

High resolution imaging of the dolomite (104) cleavage surface by atomic force microscopy

“…scan sizes and angles, integral and propor tional gains, and filters) are very similar to those reported for calcite (104) surfaces by Rachlin et al [19], and more recently by Rode et al [20]. This allowed us to relate features of the high resolution images of the dolomite (104) surfaces to structural characteristics shared by the calcite and dolomite surfaces.…”

“…4 doubling the surface unit cell in the b dimension) that has been ascribed to a slight rotation of some surface carbonate groups. While the (2 xl) superstructure on calcite (104) face matches with low energy electron diffraction 122] and computer modelling studies [23,24], it has not been observed by Rode et al [20] using frequency modulation AFM.…”

“…This makes especially difficult to image the two CaH and Mg2+ sublattices on the dolomite (104) surface. In fact, none of the AFM images analysed revealed any significant difference in the cationic positions when compared with the high resolution images of calcite (104) surfaces [20). Along the [421] direction we have not found changes in the shape or intensity of spots that could be clearly attributed to the alternating CaH and MgH and/or local differences in hydration on dolomite (104) surfaces.…”

“…Rode et al [20] observed a zigzag sequence of bright spots along the [421] direction on calcite (104) surface that can be clearly attributed to the protruding oxygens of the carbonate groups. On the dolomite (104) surfaces we have observed an identical sequence of spots along the same direction.…”

Advanced Instrumentation of Frequency Modulation AFM for Subnanometer-Scale 2D/3D Measurements at Solid-Liquid Interfaces

“…Examples of the early applications include minerals [12,[14][15][16] and biological systems [14,[17][18][19][20][21][22]. While these results highlighted the high spatial resolution and wide applicability of liquid-environment FM-AFM, they also clarified its fundamental limitation.…”