Shallow ripples with giant wavelengths observed by atomic force microscopy: Real effects and the report of a new artifact

Abstract

The (210) surface of ammonium perchlorate has been studied with an atomic force microscope. Unexpected surface ripples with shallow amplitudes (typically less than 3 nm) and giant wavelengths (typically between 0.5 and 1.3 μm) have been discovered. The dominant type shows a wave vector along the [001] direction, and a second type forms a pattern in which two wave vectors make an angle of ∼60° with each other. The amplitude and wavelength of the ripples are clearly correlated. Their size depends on the local surface structure, but their direction is usually keyed to the main crystalline directions. Unfortunately we have also discovered a new artifact that can produce similar images. We discuss the evidence we have gathered thus far to support the existence of the ripples in addition to that of the artifact.