Missing row structure

Surface reconstructions have been observed by STM in many systems, and the teclmique has, indeed, been used to confmn the missing row structure in the 1 x 2 reconstruction of Au(l 10) [28]. As the temperature was increased within 10 K of the transition to the disordered 1 1 phase (700 K), a drastic reduction in domain size to -20-40 A (i.e. less than the coherence width of LEED) was observed. In this way, the STM has been used to help explain and extend many observations previously made by diffraction methods. 

A series of LEED intensity studies, together with ion-scattering spectroscopy, established that a missing row structure was the correct model for the (1 x 2) phase,14 with some small subsurface relaxation and reconstruction.10... 

Atom resolved studies were first reported by Ertl s group15 in the early 1990s for Cu(110)-K, indicating the development of (1 x 3) and (1 x 2) structures depending on the surface coverage. They were missing row structures with... 

Initial adsorption of potassium (0.15-0.25 ML) at room temperature does not change the (1 x 2) missing row structure of the Au(110) surface. The adsorbed... 

A sequence of STM images were obtained (Figure 7.2) of the Cu(l 10) surface before hydrogen exposure (A), at an ambient H2 pressure of 1 bar (B) and finally after evacuation under UHV conditions (C). It is clear that in the presence of H2 the surface reconstructs into the well-known (1x2) missing row structure and that an evacuation the surface reconstruction is lifted with the (2 x 1) structure observed. AES established that no impurities were present at the Cu(110) surface. 

Figure 1.4. Plan view of Cu(l 00)(x 2 /2)R45°-0 surface reconstruction. The outermost layer Cu atoms are shown more lightly shaded than those of the underlying substrate to show more clearly the missing-row structure of this outermost layer. The full lines show the surface unit mesh.

Oil] and the [Oll] direction (Fig. 3.2e). Analysis of the Pt3Ni(110) (Fig. 3.2f) indicates that this surface may exhibit a mixture of (1 x 1) and (1x2) periodicities, the latter being known as the (1 x 2) missing row structure [24]. From the LEIS spectra in Fig. 3.1b, it is apparent after a final anneal, the surface atomic layer of all three Pt3Ni(M/) crystals is pure Pt they all form the Pt-skin structures. 

The open surface of bcc W(100) allows the penetration of O into the hollow site to bond to W atoms within the second W layer. The disordered O overlayer at 120K reconstructs the top W layer such that the four adjacent W atoms move towards the adatom. At elevated temperatures the p(2xl) phase can be prepared and is found to be a missing row structure in which the adatom adsorption geometry is similar to that of the disordered phase. 

Pt(110) is known to be reconstructed under UHV conditions [107,108]. This (1x2) missing-row reconstruction was reported to be quite stable within the double-layer charging region, that is, when oxide formation was avoided [109,110]. It has been also reported that the missing-row structure was enhanced after the immersion of the flame-annealed electrode into the electrolyte. However, in that study, the Pt(110) crystal was cooled in air, which is known to induce a high density of surface defects [111], and above all, the electrode was quenched in the electrolyte at elevated temperatures. Such quenching is known to destroy the crystal structure due to the temperature shock [112]. 

The difference in the atomic density in the topmost layer becomes even more pronounced with the Pt(l 10) surface, which in the clean stable state exhibits a 1 x 2 missing row structure, while CO adsorption causes transformation in the bulk-like 1x1 structure that contains twice as many surface atoms (Fig. 2.22). In this case, at 300K, the transformation is initiated by homogeneous... 

A quite interesting alternative for the formation of an adsorbate -induced surface reconstruction is offered by the O/Cu(l 10) surface. Upon adsorption of O atoms on the clean (nonrecon-structed) 1x1 surface, a (2 x l)-0-Cu(l 10) phase is formed whose structure is depicted in Fig. 2.23c. This new "missing row" structure is, however, formed by condensation of mobile chemisorbed O atoms with Cu adatoms evaporating from steps and diffusing across the terraces of the substrate surface. This nucleation process is illustrated by the STM snapshots in Fig. 2.23a and b leading to the structure of Fig. 2.23c that is more appropriately described as "added row" than "missing row" phase [27]. 

Surfaces of Au(llO) and Pt(llO) in u.h.v. reconstruct to 1 x 2 or a mixture of 1 x 2 and 1x3 missing row structures (Fig. 24). In the electrochemical environment, the reconstructed state is stable for a range of electrode potentials smaller than a positive critical value. For potentials more positive than the critical value, the reconstruction is lifted. Potential variation reversibly changes the structure between the reconstructed and unreconstructed states. We will dwell on the missing row reconstruction in the following text, the easiest example for tutorial purposes, since it was rationalized by a relatively simple theory. 

The third kind of stractural change, faceting, is much more violent and involves a massive lateral transport of matter. Adsorption of alkali metals on some fee (110) surfaces causes the missing row structure of Fig. 33b. This structure is characterized by (111) oriented nanoscale facets. Thus reconstruction can in some cases be viewed as a nucleus to faceting. Facets of pm dimension are often... 

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