SPA-LEED

Few other methods allow to study in situ the nucleation and growth of metal clusters on oxide surfaces. Modern diffraction methods are able to follow the mean distance between neighbouring clusters and the mean cluster size. If the clusters are well distributed on the surface small diffraction peaks appear close to the specular. The reciprocal distance between these two peaks gives the average distance between the clusters. The width of the specular peak gives the mean cluster size. By GISAXS it has been possible to follow the nucleation and growth of metal on MgO(l 00) [29]. By SPA-LEED the same information can be also obtained but with less accuracy [67]. 

The results on the NiAl(llO) faee are not yet published, and hence only preliminary results are available [143]. A 6 A-thick oxide was formed, as the result of a 30 min.-long annealing at 950°C under 2T0 Pa of O2. This thickness was determined by X-ray reflectivity measurements. Two different incommensurate struetures were observed, one of which had been previously observed by SPA-LEED. The other, whieh had much stronger reflections, had never been reported previously, and was indeed invisible on the LEED patterns. This new stmcture is still under analysis. 

In the technique of spot-profile analysis LEED (SPA-LEED) [29, 30] intensity variations across LEED diffraction spots are measured. The technique provides information on both periodic and non-periodic arrangements of superstructure domains, terraces or facets, and strained regions. The interpretation of spot profiles is simplified due to the validity of the kinematic approximation, which is not the case for LEED I-V analysis. Deviations from simple structures resulting from defects produce characteristic modifications of the spot profile, so that a quantitative evaluation is possible. SPA-LEED has proven itself to be useful in the study of dynamic phenomena on surfaces such as phase transitions. Several examples of this capability are discussed in section 3 for Pb on Cu. 

Investigations of Pb on the stepped surfaces of Cu provide a systematic way to study the influence of defects on the statics and d5mamics of overlayer formation. Stepped surfaces are also interesting in that the observed phenomena (self-assembly, phase transitions, etc.) have a one-dimensional character to them. Although studied less extensively than the flat surfaces, LEED/AES and SPA-LEED studies have been reported for the adsorption of Pb on the (211), (311), (511), (711) (510), and (10,10) planes of Cu. 

In 1991 Wollschlager et al. [100] used SPA-LEED and AES to investigate the epitaxial growth and thermal behaviour of Pb on Cu(311). They found that... 

The interplay of the two phases on the PtaSnCl 11) surface has been object of an extensive study carried out by Ceelen et al. [18] who used mainly a combination of LEIS and SPA-LEED, also carrying the sample at higher temperatures than those attained in the previous studies. A wealth of temperature dependent phenomena was observed in this study concerning bulk-surface chemical equilibrium, domain size variation and phase transitions. The main conclusions that can be drawn from these combined structural and compositional studies is that the ( /3 x /3) R30° reconstruction is stabilized by the depletion of tin in the subsurface layers and that this depletion is caused by a the combination of sputtering and high temperature annealing (Fig. 3). 

Fig. 3 SPA-LEED results for NiAl(lOO) Intensity variation of the normalized (00) spot (full circles) and the background intensity (open squares) with temperature recorded for in-phase conditions (Eq = 95 eV). The full lines are guidelines for the eye the dashed line indicates a result for the situation of surface roughening of the Kosterlitz-Thouless type, (from ref [37]).

SPA-LEED Spot Profde Analysis Low Energy Electron Diffraction... 

SPA-LEED Spot profile analysis - low energy electron difiraction... 

With increasing oxygen exposure of the Rh(OOl) surface, three outstanding phases form sequentially, as identified using LEED [151-153], STM, SPA-LEED and PES [154, 155]. The disordered p(2 x 2)-0 phase ( = 1/4 ML) forms first, then the c(2 X 2)-20 (0 = 1/2 ML) radial reconstruction, and, finally, the c(2 X 2)p4g-20 (0 = 1/2 ML) clockwise and anticlockwise rotation of the unit cells. These reconstruction patterns are the same as those occurring on the Ni(OOl) surfaces with chemisorbed carbon and nitrogen. 

Figure 9.15 High-resolution LEED pattern of Si(OOl) taken by SPA-LEED [19]. Top pattern recorded at room temperature. Superposition of (2x1) and (1x2) reconstructed areas. Bottom pattern recorded at 150 K. Superposition of c(4x2) and c(2x4) reconstructed areas. The intensity is plotted in a logarithmic scale.

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