Etching and surface termination of K2Cr2O7 {001} faces observed using in situ atomic force microscopy

The etching behaviour of the double-layered (001) faces of potassium bichro mate crystals was studied by using in situ atomic force microscopy (AFM). T hese crystals were cleaved in air or ethanol along (001), and the opposite cleavage faces (001) and (001) were then etched in ethanol or a water/ethan ol mixture. We found that in air an etch resistant layer is formed. Whether this layer can be removed depends on the exposure time to air, on the wate r content of the etch mixture and on the amount of scanning by the AFM. Und er specific conditions this allows us to do lithography on the surface. On the air-cleaved faces, only at some positions expanding etch pits developed , in which layer-by-layer etching took place at freshly etched parts on the flat bottom. Most etch steps were of height d(001) as expected for the dou ble-layered structure of K-2 Cr-2 O-7. However, sometimes split steps of ha lf height d(002) were encountered. Both the shapes of the 2D nucleation etc h pits and the molecular-resolution images were similar on the (001) and th e (001) faces, which suggests PT symmetry. Therefore on this molecular scal e, the hypomorphism (P1 symmetry in this case) visible on macroscopic K2Cr2 O7 crystals appears to be absent. From the orientation of the d(002) steps inside the etch pits the stable and the unstable (001) half layers A and B could be identified with respect to the crystal structure. During etching, A was nearly always on top of the crystal surface, proving that this is the most stable layer. (C) 2000 Elsevier Science B.V. All rights reserved.