The Structure of Surfaces

Abstract

The past several years have seen a virtually explosive development of structural surface science. The techniques of low-energy electron diffraction and electron spectroscopy have found a multitude of applications in investigations of various surface and interfacial phenomena. Studies of the atomic structure of clean solid surfaces of low Miller index by low-energy electron diffraction, LEED, are being expanded to higher Miller index or stepped surfaces, as well as to studies of the structure of adsorbed gases. There are recent LEED studies of catalysts and of electrode surfaces that were examined before and after surface reactions. Among the various techniques of electron spectroscopy, Auger electron spectro­ scopy is the most widely applied to studies of surfaces. The composition of the surface can be determined with a sensitivity of about 1% of a monolayer (about 1013 atoms/cmZ), all elements being detectable this way except hydrogen and helium (1). Auger electron spectroscopy (AES) has also been used to determine the valency of surface atoms, i.e. the oxidation state of the same element in various compounds (2). More recently, other techniques such as ultraviolet photoelectron spectroscopy (3), electron loss spectroscopy (4, 5), and X-ray photoelectron spectroscopy (6) have all been utilized in surface studies to determine surface composition and valency. These techniques are based on one-electron processes and, therefore, the experi­ mental data may be analyzed more easily than in the case of AES, which involves a two-electron process. In this chapter, we review the recent advances made in surface crystallography and in studies of surface structure of solids and of adsorbed layers. This field is developing so rapidly that by the time this review is published, most of the information reported here from the various areas of structural surface science will need updating. We first consider those surfaces for which there have been attempts at obtaining an exact structural analysis from LEED measurements. These are unreconstructed metal surfaces and simple layers of chemisorbed atoms. We go on to review the present state of understanding of more complex surfaces of metals, semiconductors, and insulators where a complete structural analysis is not yet possible.