Low-voltage high-resolution scanning electron microscope imaging of the uncoated and Cr-coated zeolite Beta

Abstract

Fine surface features of the uncoated, as-synthesized zeolite Beta and the effect of Cr sputter coating on the appearance of these features were examined using high-resolution (lateral resolution ∼ 2.5 nm) field emission scanning electron microscopy (FE-SEM). The uncoated particles exhibited charging effects despite a reduced accelerating voltage (2 kV). Thus, only rapid scanning at “TV rates”, where an average of multiple frames is acquired, produced useful FE-SEM images. Pyramidal irregularities, resulting in the “saw teeth-like” surface features less than ∼20–30 nm in size, were observed on the uncoated particles. These images did not reveal any texture over flat areas of the surface. Deposition of thin Cr films with a nominal thickness of 1–2 nm reduced/eliminated sample charging so that a slow scanning speed could be used. The resulting images showed better topographic contrast than the images of the uncoated particles. The appearance of fine “teeth” was not affected by sputtering with thin 1–2 nm Cr films. However, the development of a grainy surface texture in the form of nodules smaller than ∼5 nm over flat areas of the surface, which appeared to grow in number density and size (to more than ∼5 nm) with the increasing nominal (up to 10 nm) Cr film thickness, could be observed. This is hypothesized to be the result of Cr particle formation. Sputtering used to deposit thin Cr films did not appear to damage crystal surfaces. Thus, thin (1–2 nm) Cr coatings improve image quality with little or no loss in surface texture resolution.