Layered heavy metal iodides examined by atomic force microscopy

Abstract

Atomic force microscopy was used to characterize the surface of semi-insulating bismuth tri-iodide, lead iodide, and mercuric iodide crystals. These heavy metal iodides belong to a unique class of materials that provide a wide range of interesting problems that can be studied by atomic force microscopy. They have relatively high vapor pressures and cannot be examined noninvasively in vacuum conditions, such as by electron microscopy. Mercuric, bismuth, and lead iodide are layered materials with van der Waals bonding along the c axis and therefore, like mica, can be easily cleaved to provide fresh atomically flat crystalline surfaces to study. These crystals are soft with a high degree of plasticity and, as a result, the surfaces are easily modified by the cantilever tip. They demonstrate reactivity in ambient air at the surfaces as well as within the van der Waals gap. Freshly cleaved surfaces were examined and it was found that single and multiple atomic layers were easily displaced on the cleaved surfaces in all of the crystals when scanned in both contact and modulated contact mode. The effects of cantilever induced modifications of the surfaces of mercuric, lead, and bismuth iodide were studied by atomic force microscopy and their properties are discussed.