The picture tells the story: Using surface morphology to probe chemical etching reactions

Highly anisotropic etchants, which only attack specific crystallographic planes of a material, are prized by industry, but the inherent defect selectivity of these etchants has stymied investigations into their chemical mechanism. Because of their chemical specificity, these etchants produce characteristic nanoscale morphological features, such as triangular etch pits and jagged atomic steps, which reflect the site-specific chemical reactions that control the etching. Using a combination of scanning tunnelling microscopy and kinetic Monte Carlo simulations, these site-specific rates can be directly quantified. By correlating these measurements with the structure of the etched sites, information on the reaction mechanism can be obtained. These techniques are illustrated with examples drawn from aqueous silicon etching reactions. For example, the production of atomically flat Si(111) surfaces by NH 4 F etching can be explained by the existence of a pentacoordinate transition state to OH - attack. Sites held in a rigid tetrahedral geometry, such as the Si(111) terrace site, are relatively resistant to attack, while defect sites, such as steps and kinks, are rapidly etched.