Low Energy Electron Diffraction
Low energy electron diffraction (LEED) is one of the most widely-used tools for the investigation of surface structure. The technique is based on the fact that electrons can be described as wave-like and therefore exhibit characteristics such as diffraction. For energies in the region of 10 to 500 eV, the electron wavelength is comparable to typical lattice spacings in crystalline structures and hence diffraction occurs.
The structure and periodicity of a surface can be determined using LEED. The sharpness and background intensity of the pattern acts as a gauge of the quality of the surface, with a high background or diffuse spots indicating some degree of disorder. As LEED shows the periodicity in reciprocal space, features in between the integer order spots are due to real space periodicities which are larger than the (1×1) surface unit mesh. Analysis of such patterns must include the possibility of rotated domains on the surface (e.g. (2×1) and (1×2)), as the observed LEED pattern is simply a combination of the various periodic structures which exist in different regions of the surface.
The surface specificity of LEED arises from the limited penetration depth of low energy electrons (< 100 eV). In most materials this is limited to around 2 nm at low energies. The short mean free path means that electrons cannot escape from deep in the crystal without undergoing one or more inelastic scattering events, making the technique ideal for surface structure determination.
(a) The (1x1) reconstruction of a clean Ni(110) surface. (b) The (3x1) reconstruction formed during oxygen adsorption on to the Ni(110) surface.
