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Surface double layer on (001) surfaces of alkali halide crystals: a scanning force microscopy study
Clemens Barth1, Claude R Henry
1CRMCN-CNRS, Campus de Luminy, case 913, 13288 Marseille Cedex 09, France. barth@crmcn.univ-mrs.fr
Physical Review Letters
|May 16, 2007
Summary
Surface charge on alkali halide crystals is negative due to cation vacancies at kinks. Dynamic scanning force microscopy and Kelvin probe force microscopy revealed charged patches at step sites.
Area of Science:
- Surface Science
- Condensed Matter Physics
- Materials Science
Background:
- Alkali halide crystals exhibit surface charge phenomena.
- The role of divalent impurity ions in surface charge is not fully understood.
- Understanding surface double layers is crucial for materials properties.
Purpose of the Study:
- To investigate the surface double layer on alkali halide (001) surfaces.
- To characterize the charge distribution at surface steps and kinks.
- To correlate experimental findings with theoretical models of surface charge.
Main Methods:
- Ultra-high vacuum (UHV) cleavage and annealing of alkali halide crystals (KCl, NaCl).
- Dynamic scanning force microscopy (DSFM) for atomic resolution imaging.
- Kelvin probe force microscopy (KPFM) for surface potential mapping.
Main Results:
- Kelvin probe images revealed bright, round patches at corner sites and steps.
- Atomic resolution images consistently showed kinks at step sites.
- Charged patches were localized at these kinks, indicating specific defect sites.
Conclusions:
- Experimental evidence supports the presence of a net negative surface charge on alkali halide crystals.
- This negative charge originates from cation vacancies located at kinks on surface steps.
- Divalent impurity ions are identified as the source of these vacancies and the resulting surface charge.
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