AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS2-TuA

Paper SS2-TuA10
Alkali Halide Nanocrystal Growth and Etching Studied by AFM and Modeled MD Simulation

Tuesday, November 5, 2002, 5:00 pm, Room C-110

Session: Atmospheric Surface Chemistry
Presenter: S. Garcia-Manyes, University of Barcelona, Spain
Authors: S. Garcia-Manyes, University of Barcelona, Spain
A. Verdaguer, University of Barcelona, Spain
P. Gorostiza, University of California at Berkeley
F. Sanz, University of Barcelona, Spain
Correspondent: Click to Email
Adsorption of water on alkali halides single crystals plays a key role in many fields ranging from interstellar dust grains, biochemistry, industrial applications and environmental processes, such as nucleation of clouds or sea salt spray. To understand the atomistic details of the wetting and dissolution processes that take place in, we combined Atomic Force Microscopy and Molecular Dynamics to study the adsorption of water vapor on a stepped (100)-oriented crystals surface prepared by cleavage. The AFMicroscope, placed in a humidity-controlled chamber is used to induce local step nanostructures forming hillocks. A few seconds of contact between the cantilever tip and the crystal surface are enough to create a hillock of a few monatomic steps due to the water neck formed on account of capillary forces. The shape and distribution of the monatomic steps in the formed hillock follows the minimization of the interface free energy established between the aqueous ionic solution and the air phase in order to reduce surface tension. After hillock creation the AFM tip is retracted and the hillock starts to dissolve and ions migrates until the hillock disappears. Terraces in the hillock disappear one by one, from the upper one to the lowest in order to minimize again the free energy of the whole system. We have studied creation and free evolution of hillocks for alkali halides single crystals by AFM under chosen humidity conditions. Imaging during experiments is performed in tapping mode so as to reduce the surface perturbation. We performed molecular dynamics simulations of the interface between the single crystal and water using a hillock-like surface model and results are compared with experimental data in order to understand the dynamics of the system. Ionic migration, steps mobility and dissolution directly calculated by the simulations have been used to modeling the hillock evolution according to experimental data.