LEED and RHEED

Low-energy and reflection high-energy electron diffraction (LEED and RHEED)44 

An electrode can be characterized by LEED before and after the electrochemical experiment. Differences give information on adsorption and eventual movements of atoms over the surface. The great practical difficulty is the necessity of locating the electrode in exactly the same position for the two diffraction experiments. 

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Alternatives to XRD include transmission electron microscopy (TEM) and diffraction, Low-Energy and Reflection High-Energy Electron Diffraction (LEED and RHEED), extended X-ray Absorption Fine Structure (EXAFS), and neutron diffraction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivity in neutron diffraction is quite different from XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron diffraction is a good alternative for low-Z materials and for materials where the magnetic structure is of interest. 

Based on electrochemical experiments combined with ex situ analysis by AES, LEED, and RHEED, Wang et al. (2001) suggested the formation of a (2 x 2) (2CO + O) adlayer on Ru(OOOl) at = 0.2 V in CO-samrated HCIO4, similar to the phase formed in UHV after CO adsorption on a (2 x 2)0-covered surface [Schiffer et al., 1997]. Erom the total charge density transferred after a potential step to 1.05 V in a CO-free electrolyte, they concluded that only 60% of the CO content in such an adlayer can be oxidized under these conditions [Wang et al., 2001]. 

Emersion resulting in substantial amounts of electrolyte remaining on the (hydrophilic) electrode is much more commonly observed. When the solvent evaporates from the surface, the electrolyte is left behind as small crystallites. These can distort LEED and RHEED patterns and, more importantly, can render quantitative evaluation of the surface cation and anion concentrations by ESCA impossible. 

Figure 42. (a) Cyclic voltammogram at 1 mV s on Au( 111) in 0.05M H2SO4 + 1 mM CUSO4. (b) Electrochemically derived Cu underpotential deposition coverage as a function of potential, determined by potential steps in the positive direction. The Cu underpotentia) deposition structures obtained from LEED and RHEED are also shown. (From Ref. 142.)... 

Au(100) offers another interesting case for reconstruction [156]. Previous studies on this electrode surface with LEED and RHEED have shown that the reconstructed Au(100)-(5x20) surface is stable in a potential range where no anion adsorption is present [104]. In the presence of anions, the reconstruction is presumed to be lifted to a (lxl) structure. However, at negative potentials, the (5x20) structure is regenerated. The authors observed a difference in the SH rotational anisotropy at these two potentials and attributed it to the reconstruction and lifting of the reconstruction [156]. 

The alternative electron diffraction technique is referred to as reflection electron diffraction and uses a comparatively high energy primary beam (typically 10—50 keV) at a grazing incidence of 1—3°. There is, however, very little difference between LEED and RHEED in the depth of material probed, since, for example, a 50 keV electron at 3° incidence angle will have approximately the same momentum perpendicular to the surface as a normally incident 150 eV electron. To obtain the complete space group symmetry with RHEED, however, it is necessary to use at least two primary beam directions. 

The cyclic voltammogram of Cu UPD on Au(lll) shows two well-defined pairs of current peaks A1/A2 and BxjBi corresponding to energetically different adsorption/desorption processes (Fig. 15) [320-322, 337]. In the first step (peak Ai), the transition between randomly adsorbed copper and (hydrogen) sulfate ions into an ordered layer of copper atoms (electrosorption valency y 1.8 [339, 340]) and coadsorbed sulfate ions takes place. The resulting ( 3 X 3)R30° structure was first observed by ex situ LEED and RHEED experiments [354], and later confirmed by in situ SXS [350], STM [341-343], and AFM [344]. QCM [352, 353], chrono-coulometric [339, 340], and FTIR-... 

Both LEED and RHEED are probes of surface structure that depend on the existence of long-range order of periodic structure in the surface. They do not provide compositional information and are always used along with other techniques such as AES that can analyze for composition. 

Fig. 6. LEED and RHEED patterns of as-received samples (a) and (c) HVPEl, and (b) and (d) HVPE2 samples, respectively. Note that the 00 beam in LEED was not at the screen center. The incident electron direction was [1100] in RHEED. Ep in LEED was 95 eV. In RHEED,...

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