Low-energy electron diffraction
One of the most informative techniques for determining the arrangement of the atoms close to the surface is low energy electron diffraction (LEED). This technique is like X-ray diffraction but uses the wave character of electrons, and the sample is now the surface of a solid. The use of low energy electrons (with energies in the range 10–200 eV, corresponding to wavelengths in the range 100–400 pm) ensures that the diffraction is caused only by atoms on and close to the surface. The experimental arrangement is shown in Fig. 25.11, and typical LEED patterns, obtained by photographing the fluorescent screen through the viewing port, are shown in Fig. 25.12. A LEED pattern portrays the two-dimensional structure of the surface. By studying how the diffraction intensities depend on the energy of the electron beam it is also possible to infer some details about the vertical location of the atoms and to measure the thickness of the surface layer, but the interpretation of LEED data is much more complicated than the interpretation of bulk X-ray data. The pattern is sharp if the sur face is well-ordered for distances long compared with the wavelength of the incident electrons. In practice, sharp patterns are obtained for surfaces ordered to depths of about 20 nm and more. Diffuse patterns indicate either a poorly ordered surface or the presence of impurities. If the LEED pattern does not correspond to the pattern
Fig. 25.11 A schematic diagram of the apparatus used for a LEED experiment. The electrons diffracted by the surface layers are detected by the fluorescence they cause on the phosphor screen.
Fig. 25.12 LEED photographs of (a) a clean platinum surface and (b) after its exposure to propyne, CH3C.CH. (Photographs provided by Professor G.A. Somorjai.)
Fig. 25.13 The structure of a surface close to the point of attachment of CH3C- to the (110) surface of rhodium at 300 K and the changes in positions of the metal atoms that accompany chemisorption.
expected by extrapolation of the bulk surface to the surface, then either a reconstruction of the surface has occurred or there is order in the arrangement of an adsorbed layer. The results of LEED experiments show that the surface of a crystal rarely has exactly the same form as a slice through the bulk. As a general rule, it is found that metal sur faces are simply truncations of the bulk lattice, but the distance between the top layer of atoms and the one below is contracted by around 5 per cent. Semiconductors generally have surfaces reconstructed to a depth of several layers. Reconstruction occurs in ionic solids. For example, in lithium fluoride the Li+ and F− ions close to the surface apparently lie on slightly different planes. An actual example of the detail that can now be obtained from refined LEED techniques is shown in Fig. 25.13 for CH3C-adsorbed on a (111) plane of rhodium. The presence of terraces, steps, and kinks in a surface shows up in LEED patterns, and their surface density (the number of defects in a region divided by the area of the region) can be estimated. The importance of this type of measurement will emerge later. Three examples of how steps and kinks affect the pattern are shown in Fig. 25.14. The samples used were obtained by cleaving a crystal at different angles to a plane of atoms. Only terraces are produced when the cut is parallel to the plane, and the density of steps increases as the angle of the cut increases. The observation of additional structure in the LEED patterns, rather than blurring, shows that the steps are arrayed regularly.
Fig. 25.14 LEED patterns may be used to assess the defect density of a surface. The photographs correspond to a platinum surface with (top) low defect density, (middle) regular steps separated by about six atoms, and (bottom) regular steps with kinks. (Photographs provided by Professor G.A. Samorjai.)
