Scanning tunnelling microscopy : design and tunnelling characteristics
Scanning Tunnelling Microscopy (STM) is a newly developed technique providing, for the first time, atomic resolution images of conducting surfaces. In its short lifetime it has not only solved a number of long standing surface science problems, for instance simultaneously elucidating both the atomic and electronic structure of the Si(lll) surface, but also earned its inventors, G.Binnig and H.Rohrer, the Nobel Prize. This dissertation describes the construction of one of the earliest STMs and its continued development into a generally available machine. This design differs significantly from that of Binnig and Rohrer both in terms of the all important vibration isolation techniques used and the mechanics of the STM itself. There are now a number of machines built along these lines elsewhere. One of the advantages of this approach is the comparative ease with which it can be modified to operate at low temperatures, and a cryogenic STM running in Ultra High Vacuum has been built The STM operates by holding a sharp metal point (the 'tip') sufficiently close to the surface under investigation that electrons may tunnel directly from one metal to the other. Observation of the tunnel current/separation relationship whilst the tip is scanned in a raster pattern across the surface provides the information from which the images are formed. The tunnelling process itself is of considerable interest, and theory and data are presented relating to the low voltage parabolic conductance, the effect of the image potential on tunnelling and the effect of tip curvature at high applied voltages. During this investigation a number of surfaces have been studied, and atomic resolution images of Pt(llO), Si (100) and graphite are presented. Moreover, images of local molecular ordering in a Langmuir-Blodgett monolayer have been obtained, as well as data showing the effect of protein molecules. These exemplify the surprising range of applications of this technique.