Missing row

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

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. 

Figure Bl.23.9. Scattering intensity of 4 keV Ne versus azimuthal angle 8 for a Ni 110] surface in the clean (1 X 1), (1 X 2)-H missing row, and (2 x l)-0 missing row phases. The hydrogen atoms are not shown. The oxygen atoms are shown as small open circles. 0-Ni and Ni-Ni denote the directions along which O and Ni atoms, respectively, shadow the Ni scattering centre.

Potentials in Ref. 197 are given vs. Pd(H). They have been converted to SHE by subtracting (57 x pH - 60) mV.197 Reconstructed missing-row surface. cOne-dimensionally ordered surface. 

Figure 1. Image of the Rh(110) surface after treatment with oxygen, producing mainly the c(2x6) reconstruction of the adlayer. The dark lines run parallel to the [011] direcUon and are missing rows of Rh atoms. More details of this structure are given elsewhere [4-7].

Figure 2. Showing the beginning of CO reaction with the surface along the long Rh-0 islands, which are between the missing rows, as bright streaks.

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... 

The authors then chose to examine hydrogen adsorption at Cu(110). This was a well-chosen example in that hydrogen adsorption is activated, being pressure dependent, and also was already known from LEED studies to exhibit a missing row (1 x 2) structure with every second close packed (110) copper row missing at high hydrogen pressures. What, then, was learnt from STM ... 

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 The atomic arrangement in the missing row model of the reconstructed iridium (110) crystal surface. From G.A. Somorjai, Chemistry in Two Dimensions, Cornell University Press, London, 1981, p. 146. Used by permission of Cornell University Press.

The EAM has been used to study the surface structure of other metals and metal alloys. For example. Daw has suggested that a missing row configuration is also the likely structure for the (2 x 1) reconstruction of the Pt(l 10) surface. Studies have also been made of the surface structures of various alloys, where for example surface segregation of one constituent over the other has been observed ° . In addition to studies of specific systems, the EAM formalism is also sufficiently general that it has been used to understand trends in surface reconstructions among various metals . ... 

E.g., figure 9 demonstrates the variation of the phase front along a unit cell of a Wl-PhC-waveguide, which is formed by just one missing row of air holes in a silica embedded silicon film. This for instance is an indication that a direct butt-coupling from and to photonic crystal waveguides would be affected by the relative position of the PhC-interface relative to the unit-cell. 

Pt(l 10) is known experimentally to reconstruct into the so-called missing-row reconstruction. In this reconstruction, alternate rows from the top layer of the surface in a (2 x 1) surface unit cell are missing. Use a supercell defined by a (2 x 1) surface unit cell of Pt(110) to compute the surface energy of the unreconstructed and reconstructed surfaces. Why does this comparison need to be made based on surface energy rather than simply based on the total energy of the supercells Are your results consistent with the experimental observations Use similar calculations to predict whether a similar reconstruction would be expected to exist for Cu(110). 

The (2x1) structure is also known as missing-row reconstruction because one out of two rows of atoms, aligned along the [110] direction, is missing from the surface layer. We stress also that there are two different realizations of these reconstructed states, in which either the even or the odd rows are missing. 

Fig. 1 shows a top and a side view of the missing-row reconstructed structure. Along the [001] direction the surface assumes a hill-and-valley profile, where the sides of the hills are actually (111) microfacets. In the (111) orientation, surface atoms are closely packed, therefore such orientations are energetically favored. 

---