It is easy to see from the following table how the number of techniques, and the corresponding acronyms, can be very large, especially once one realises that any probe particle can give rise to any repsonse, and that we may have different names for essentially the same technique used at different energy, and different wavevector, momentum or angular regimes.
(i) the incident particle or the probe particle has a short mean free path, lambda. This leads to a useful 'single surface' technique. Examples are AES, where the emerging electrons in the energy range 100-2000 eV have lambda for inelastic scattering in solids, of order 1nm or less. Using an energy analyser to measure only those Auger electrons which have not lost energy, attenuates the signal from subsurface layers strongly (see dashed line in diagram 61). A cruder form of energy discrimination is used in observing LEED patterns, where both the incident and the emergent particles have short mean free paths for energy loss processes. In SIMS (Secondary Ion Mass Spectrometry), the emergent ions have a very high probability of being neutralised if they do not originate very near the surface. ICISS (Impact Collision Ion Scattering Spectroscopy) is surface sensitive because the incident ion will be neutralised, and are thereby not detected if the probe particle penetrates the solid. Most surface techniques fall into this class;
or,(ii) The sample has a large surface to volume ratio. This condition allows us to extract surface information from techniques which are not particularly surface sensitive. We can use powdered/exfoliated samples, and perform heat capacity or other thermodynamic measurements, X-ray or neutron scattering. Here we need to know the signal from the bulk, and maybe subtract it in a differential measurement. Much of physical chemistry work on surfaces has been done this way. Or we can use thin film samples, and concentrate on the surface-related contribution. For example, THEED and the corresponding microscopy TEM or STEM are done on thin films around 10-100 nm thick. There is much information related to surfaces in such diffraction patterns and images, especially when combined with UHV technology.
There are now a whole range of techniques available for studying surfaces on a microscopic scale: study of these techniques and their applications would take a whole course in itself. Some of these are suitable topics for seminars, but here I wish to mention generally relevant points, in the context of surface studies. What can one say to help you on your way?
Microscopy can be categorised into Fixed beam, Scanned beam and Scanned probe Techniques. A typical fixed beam technique is Transmission Electron Microscopy (TEM); this instrument can also be used for Reflection Electron Microscopy (REM). Examples will be given later which show that it is not essential to have these instruments operating at UHV in order to produce useful surface related information: a UHV experiment followed by ex-situ examination can be very informative. A few groups have converted their instuments to, or constructed instruments for, UHV operation, and in-situ experiments. These instruments, which can also be used for the corresponding diffraction techniques (THEED and RHEED), have produced highly valuable information on surface studies, as reviewed, for example by Yagi. More recently Low Energy Electron Microscopy (LEEM) has been developed, which can be combined with LEED, is making a major contribution (Bauer, Surface Sci 299/300 (1994) 102, see also Professor Bauer's webpage for tutorials and references). This instrument can also be used for Photoemission Microscopy (PEEM), which has been developed in several different versions. A specialist form of microscopy with a venerable history is Field Ion Microscopy (FIM), which is especially useful for studying individual atomic events such as diffusion and cluster formation.
The great virtue of fixed beam techniques is that the information from each picture element (pixel) is recorded at the same time, in parallel. This leads to relatively rapid data aquisition, and the ability to study dynamic events, often in real time, e.g. via video recording. In contrast, data in a scanned beam technique, such as Scanning Electron Microscopy (SEM) or Scanning Transmission Electron Microscopy (STEM), is collected serially, point by point. This makes the instrument ideally adapted for computer control and computer- based data collection, but can have a corresponding disadvantage; the need to concentrate a very high current density into a small spot means that not all forms of information can be obtained rapidly, that there will be substantial signal to noise ratio (SNR) problems, and that the beam can cause damage to sensitive specimens. Nonetheless SEM and STEM form the basis of a class of very useful techniques, and UHV-SEM has been developed in several laboratories, and UHV-STEM especially at ASU. We look at some particular developments in section 3.5.
The above techniques have been available for several decades, and have been substantially developed in an evolutionary sense, year by year. By contrast, the scanned probe techniques burst upon the scene in the early 80’s, in the revolutionary development of first Scanning Tunneling Microscopy (STM), followed in quick succession by Atomic Force Microscopy (AFM), Near-field Scanning Optical Microscopy (NSOM), and related spectroscopies (see e.g. Chen, Chapter 14, pages 295-312 and R.M. Feenstra, Surface Sci 299/300 (1994) 965). Several of you have opted for talks on these topics.