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Low Energy Electron Diffraction

Since the observation of  the first electron diffraction pattern by Davisson and Germer in 1927, Low Energy Electron Diffraction has been extensively used in a number of surface crystallography studies. The two scientists interpreted their results in terms of the wave nature of the electrons: this unequivocal proof of the wave-particle dualism of electrons was indeed a milestone in the history of modern Physics.
Low energy electrons are an ideal probe for surface structural studies, mainly for two reasons:
(a) their short mean free path in solids makes for a high surface sensitivity of all techniques based on low energy electrons;
(b) the de Broglie wavelength of such electrons, λ = h/p is comparable with the typical lattice parameters and  interlayer distances encountered in crystals.
On these grounds, diffraction phenomena are to be expected when a monochromatic beam of low energy electrons impinges on a solid crystalline surface.
A conventional LEED system consists of two main components: (i) an electron gun to

LEED pattern from an ultra-thin alumina layer grown on Ni3Al(111).

LEED pattern from a graphene layer grown on Ir(111).

LEED pattern from an AlNiCo quasi-crystal surface.

generate monochromatic electrons and (ii) a detector system which is selective only towards the elastically scattered electrons.
Since in LEED diffraction occurs at Bragg  planes of the crystal, a long-range order of the system is required (differently from X-ray Photoelectron Diffraction , which can be performed also on short-range ordered structures).
On the other hand, a quantitative structural analysis on the basis of electron-based techniques poses a series of difficulties:  low energy electrons, in fact, interact with matter much more strongly than other probes likes X-rays.
There are two main applications of LEED. The first one is the simple qualitative analysis of surface diffraction patterns: a quick inspection of the raw data, in fact, already provides information on the degree of surface ordering and cleanliness.  When the surface is reconstructed or covered with adsorbates,  LEED patterns  yield insight into the symmetry and periodicity of both the substrate and the possible superstructures formed by adsorbed species.

A more quantitative application of LEED is represented by LEED I-V, which allows investigating the surface structure by studying the intensity (I) modulations of a diffracted signal as a function of the electron energy (V). The measured I-V curves are  successively compared with complex multiple scattering simulations for a model system. This model structure must be changed until a good agreement between calculations and experimental data is achieved.
Despite the high demands of this kind of analysis, LEED still represents one of the primary tool for the quantitative surface structure determination.


External links:

Low Energy Electron Diffraction


 

Last Updated on Saturday, 02 March 2013 17:16