Evidence for Potassium Carbonate Crystallites on Air-Cleaved Mica Surfaces

Abstract

Air-cleaved mica surfaces exhibit a high density of nanometer or micrometer size particles that have been ascribed to potassium carbonate formed as a reaction product of carbonaceous gases with potassium ions. Unambiguous evidence for this assignment has, however, never been presented. We study air-cleaved mica surfaces by high-resolution noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum to reveal the detailed structure of such precipitates on the surface. Among a large number of irregularly shaped surface structures, we find f lat, hexagonally shaped islands exhibiting two different patterns on their surfaces, namely a rectangular atomic corrugation pattern and a hexagonal moir e structure. The unit cell of the rectangular pattern corresponds to the dimensions of the potassium carbonate bulk structure and is found on high crystallites. The moir e structure solely appears on very f lat islands and is caused by the interference of the potassium carbonate lattice periodicity and the lattice periodicity of the underlying mica substrate. Both results strongly point to the presence of potassium carbonate crystallites on air-cleaved mica surfaces.

Alkali Metal Doped Organic Molecules on Insulators: Charge Impact on the Optical Properties

“…Substrate, fi lm deposition, and doping : After cleavage in air, the mica sheets were rinsed with deionized water for several minutes to either substitute or neutralize charges that originated from the substrate, e.g., adherent potassium atoms, by dissociated water ions [45][46][47][48] . The substrates were then resistively heated in ultrahigh vacuum (UHV) (250 ° C for about 30 min) to remove adsorbates.…”

The nature of the air-cleaved mica surface

“…In a later study, the crystallites on air-cleaved mica samples were imaged at higher resolution by non-contact AFM in UHV [142]. Anhydrous K2CO3, which normally forms at temperatures below 38 C [143], is monoclinic with unit cell dimensions a = 0.564 nm, b = 0.980 nm, c = 0.688 nm and  = 98.8° [144], and high-resolution AFM scans showed row periodicities that were close to the a and b unit cell dimensions, indicating that the c-axis would be perpendicular to the surface.…”

“…Potassium bicarbonate is also monoclinic but with very different lattice parameters (a = 1.52 nm, b = 0.56 nm and c = 0.37 nm,  = 104.9°) [145] so this cannot explain the observations. The sesquihydrate of potassium carbonate is again monoclinic [146; 147], but its lattice parameters of a = 1.19 nm, b = 1.38 nm, c = 0.71 nm and  = 120.6° are not consistent with these AFM data [142]. It is very likely that the mica surface is able to impose a structure different from that of the bulk crystal with these very thin crystals that are only about 3 unit cells thick.…”

The influence of potassium on the growth of ultra-thin films of para-hexaphenyl on muscovite mica(001)

“…It was argued that this is the result of the adsorption of carbonaceous gases, CO, CO 2 , CH 4 , and their reactions with H 2 O. [15][16][17][18] Nevertheless, this type of surface leads to the pronounced needlelike island growth of rod-like molecules, as long as the mica substrate is immediately installed into the vacuum chamber after cleaving. Here we will demonstrate that by evaporation of potassium under ultra-high vacuum (UHV) conditions a full monolayer of potassium can be created on the mica surface, which is stable up to 1000 K. On such a surface, the layer growth of 6P again changes from needle-like islands, composed of lying molecules, to weakly dendritic islands composed of standing molecules.…”