NAN/PHY/MSE 546 (Venables) Sect 3.1

Notes for NAN/PHYMSE 546 Sect 3.1 (Venables)

© Arizona Board of Regents for Arizona State University and John A. Venables

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Lecture notes by John A. Venables. Notes revised for Spring 2005. Latest version 26 August 2012, reformatted.

3. Techniques for Examining Surfaces

Refs: Woodruff and Delchar's book is entirely on 'Modern Techniques of Surface Science'. Prutton largely classifies his book in terms of techniques, with Chap. 2 on Chemical Composition, Chap. 3 on Surface Structure and Chap 4. on Surface Electronics. Luth has several 'Panels' on individual techniques, while devoting the whole of Chap. 4 to scattering theory and scattering-based techniques. Zangwill sprinkles techniques and acronyms in the text, although no one part of the book is specifically about techniques. There are many books which are just about one technique, such as L.J. Clarke 'Surface Crystallography: An Introduction to Low Energy Electron Diffraction' (1985), or J.B. Pendry 'Low Energy Electron Diffraction' (1974). So it is not possible for us to be comprehensive here. What we can do is to discuss simple ways of classifying the large number of techniques which exist, and to study some of the most widely used. It will then be an assignment to look in more detail into a technique which is of some relevance to your own work, or you are interested in. I will ask you to present a description to the class in seminar form, followed by discussion. We will work these presentations into class time after Spring Break.

3.1 Classification of Surface and Microscopy Techniques

  • Surface Techniques as Scattering Experiments
  • Most physics techniques can be classified as scattering expts: a particle is incident on the sample, and another particle (not necessarily the same) is detected after the interaction with the sample. Surface Physics is no exception: we can think of an incident Probe X and a Response Y, as indicated schematically in diagram 61. The probe will be formed from a particular type of particle, and typically will have a well defined energy E0, and often a well defined wave-vector k0, or equivalently momentum p0. The response may be either the same or a different particle, and, depending on the detection system, its energy E and/or its wave vector (momentum) k(p), and maybe other attributes, may be measured. If we understand the nature/ physics of the scattering, then we can interpret the experiment and deduce the corresponding characteristics of the sample.

    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.

    Table 3.1: Particle Scattering Techniques

    Probe Response
    Electrons (E0,k0,..spin s)
    Electrons (E,k,..spin s')
    Radiation (ω0,k0,..polarization)
    Radiation (ω,k,..polarization)
    Atoms (E0,k0,Z)
    Atoms (E,k,Z')
    Ions (E0,k0,±Z)
    Ions (E,k,±Z')

  • Reasons for Surface Sensitivity
  • The next question is which techniques are actually useful for studying surfaces. There are two cases: either

    (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.

  • Microscopic Examination of Surfaces
  • Refs: P. Buseck, J.M. Cowley and L. Eyring (Eds), High Resolution Transmission Electron Microscopy and Associated Techniques (1988), especially Chap 13 on Surfaces (K. Yagi). Of the many books on STM/ SPM, look at C.J. Chen, Introduction to Scanning Tunneling Microscopy (1993). There are several multi-author texts, including H.J. Guntherodt and R. Weisendanger, vols 1-3 (1991 onwards). Review and specialist articles referred to in the text.

    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 80ís, 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.

  • Acronyms
  • Then there is the question of Acronyms. Much of modern life is beset with them, and they are defined at various places in the books. You could consider constructing an updateable list. So far we have met:

  • General Vacuum terms: UHV, QMS, TSP, SNR;
  • Surface and Crystal Growth terms: ML, TLK, BCF, CVD and MBE;
  • Diffraction Techniques: LEED, RHEED, THEED;
  • Chemical Analysis and Ion Scattering Techniques: AES,SIMS,ICISS;
  • Microscopy Techniques: TEM, REM, LEEM, PEEM, SEM, STEM, STM, AFM, NSOM.

    Now is a good time to check you know what these stand for, since we will be adding many more to the list as we start to study the individual surface techniques in more detail. We emphasise a common case, where electrons are both the probe and the detected (reponse) particle. The other cases are then suitable for individual seminars or mini-projects.

    Continue to section 3.2

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