Two-dimensional islands

Hwang R Q, Schroder J, Gunther C and Behm R J 1991 Fractal growth of two-dimensional islands Au on Ru(OOOI) Rhys. Rev. Lett. 67 3279... 

In the process of MBE, the surface structure can be investigated by reflected high energy electron diffraction (RHEED). During MBE growth, one often observes an oscillation in the intensity of the specular reflected beam as a function of time. This is interpreted to be due to the layer-by-layer growth of a two-dimensional island. 

Figure 2.13 Activation energies and prefactors for the desorption of Ag from Ru(001) as determined with the complete analysis. The desorption parameters become essentially constant for coverages above 0.15 ML, indicative of zero-order kinetics. This suggests that Ag atoms desorb from the edges of relatively large two-dimensional islands (data from Niemantsverdriet et al. [18]). 

Ill) surface, one can evaluate both step energies which are 17.0 and 19.6 meV/A for the [110](111) and [110](100) steps, respectively, atT = 473 K. The more open step has the higher energy. The step energy ratio is thus 0.87. This ratio can be compared with 0.935 calculated for Al(lll) [40], with 0.89 calculated for Pt(lll) [41] and 0.87 found experimentally for Pt(lll) from the shape of small two-dimensional islands at 625 K by Michely and Comsa [42]. It is remarkable that such veiy similar ratios are found by these different approaches. 

Catalyst particle nucleation in the initial stages and their subsequent growth play an important role in catalytic mechanisms. In a model Pt/alumina catalyst, the general view is that the formation of particles is a stepwise process incorporating the following steps (Wynblatt and Gjostein 1975, Cottrell 1971) individual metal atoms (called monomers) transform to two-dimensional islands, which subsequently transform to three-dimensional clusters. These clusters eventually transform into finite-sized particles. 

Figure 6.2-4 Underpotential phenomena during Al reduction in acidic [BMIMJ Cr/AICIs on Au(l 11). At -h950 mV vs. AI/AICI3, two-dimensional islands with a height of 250 20 pm form (a). At -h400 mV (b), two-dimensional Al islands with an averaged height of 230 20 pm are reversibly deposited. In (c) the islands are shown with a higher resolution, while (d) shows a typical height profile.

Figure 6.2-12 Cyclic voltammogram of 0.1 - 1 mmol dm Geb on gold in dry [BMIMj PFg , starting at-500 mV towards cathodic (a) and anodic (b) regime. Two quasireversible (E, and E2) and two apparently irreversible (E4 and E5) diffusion-controlled processes are observed. E3 is correlated with the growth of two-dimensional islands on the surface, E4 and E5 with the electrodeposition of germanium, Ej with gold step oxidation, and E, probably with the iodine/iodide couple. Surface area 0.5 cm (picture from [59] - with permission of the Peep owner societes).

Figure 6.2-13 UPD phenomena of Ge on Au(l 11) in dry [BMIM] PFe" two-dimensional islands with an average height of 250 + 20 pm start growing upon a potential step from the open circuit potential to -500 mV vs. the Pt quasi reference electrode (picture from [59] - with permission of the Peep owner soeietes).

Fig. 4.37 (a) On the Ir (001) surface, for an island of fewer than 6 Ir atoms, a two-dimensional structure is mctastable, whereas the linear chain structure is stable. (/) An island of 6 Ir atoms equilibrated at 460 K ( 7) one atom is field evaporated from the 6-atom island (Hi) upon heating to 450 K, the two-dimensional five-atom island reconfigures to a linear chain, (b) For an island of 6 or more Ir atoms, the linear chair is mctastable whereas the two-dimensional island is stable. (/) 6 Ir atoms in a linear chain, equilibrated at 315 K (if) upon heating to 450 K, the island transforms into a two-dimensional structure. From Schwoebcl ... 

Fig. 6. 3 The series of STM pictures shows the UPD of Ge on Au(l 11) in GeCl4/[BMIM]PF6. At +1200 mV vs. Ge (OCP) a typical Au(lll) structure with step height 250pm is observed (a). When the electrode potential is reduced to +950 mV small two-dimensional islands with heights between 100 and 150 pm appear (b). Be-...

Conclusions. Submonolayer deposits of titania grow on the surface of Rh in the form of two-dimensional islands until a coverage of nearly a monolayer is achieved, at which point some three-dimensional growth of the islands is observed. The titania islands exclude CO chemisorption on Rh sites covered by the titania. The Ti + ions in the overlayer are readily reduced to TP+. This process begins at the perimeter of the islands and extends inwards as reduction proceeds. Titania promotion of Rh enhances the rate of CO hydrogenation by up to a factor of three and increases the selectivity to C2+ hydrocarbons. By contrast, the activity of Rh for the hydrogenolysis of ethane decreases monotonically with increasing titania promotion. 

A half-order desorption process is consistent with a kinetic model in which desorption occurs from the edges of two-dimensional islands. Half-order desorption processes have been... 

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