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Abstract:

The surface of the misfit-layer compounds (LaSe)xNbSe2 and (PbSe)xNbSe2 is studied by means of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in air and nitrogen, in the range of 0.5 to 10 μm². The cleaved surface of (LaSe)xNbSe2 presents flat non-stepped surfaces which crack and react. Inner layers do not appear to crack. The dynamic process taking place in the superficial layers produces pieces which arrange in a self-similar way and eventually gives rise to discrete islands which self-organize periodically in one direction. On the other hand, the cleaved surface of (PbSe)xNbSe2 is stepped and shows no reactivity. The influence of the cleavage, the strain, the strength of the interlayer and intralayer bonding, and the misfit of the compounds are discussed as reasons for the mesoscopic behaviour of these materials.

Abstract:

Atomic images of the misfit layer compounds (LaSe)xNbSe2 and (PbSe)xNbSe2 were obtained with a scanning tunneling microscope (STM) operating in constant height mode in air. It was possible to record pictures of only the NbSe2 layers of both compounds, the LaSe and PbSe layers could not be observed. Formation of stage-2 portions embedded in the stage-1 crystal and instability of the LaSe and PbSe layers under the scanning conditions are discussed as possible causes. Comparing with the NbSe2 crystal, the lattice of the NbSe2 layers of the misfit layer compounds appear deformed. Nonperiodic Moiré-like structures have been observed in the (LaSe)xNbSe2 surface. We consider that this feature is caused by the STM tip pushing down the surface where it is softer. The nonperiodicity of the patterns might be due to the strain conditions of the crystal growth that would give rise to dislocations and defects.