Surface Structures Of AU Single Crystals Biology Essay


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Gold crystallizes as a face-centered cubic (fcc) structure and three low Miller indexed surfaces, (100), (110), and (111) (see Figure 2.1), are usually considered. The coordination number of atoms at the surface for these three different facets is 7 [Au(100)], 11 [Au(110)], and 9 [Au(111)]. The surface energies of gold are 0.08 eV/Å2, 0.100 eV/Å2, and 0.05 eV/Å2 for the clean (100),1 (110),2 and (111)3 surfaces, respectively. The classical arrangement of the atoms associated with each of the facets is not necessarily the most stable (e.g., minimum surface free energy), therefore, all three surfaces undergo reconstruction.

Figure 2.1. The (100), (110), and (111) fcc crystal surfaces.

Figure 2.2. (a) STM image of the reconstructed Au (111) surface. Inset shows atomic resolution of the edge dislocations (depressions) which are present at the elbows of the herringbone reconstruction; (b) In-plane structure of the Au(111) reconstructed surface. The circle and crosses correspond to atoms in the first and second surface layers, respectively.4

The (111) facet is the lowest energy surface of gold, as reflected in the tendency for gold thin film growth to propagate in the [111] direction. Gold has the highest ductility and malleability of any element.5 It is the only metal for which the close-packed (111) surface of an fcc crystal has been observed to reconstruct,5 which is attributed to relativistic effects in the large electronic core of gold.6 The reconstruction of Au(111) has been investigated by low energy electron diffraction (LEED),7 helium atom scattering,5 scanning tunneling microscopy (STM),4, 8, 9 density functional theory (DFT) calculations.10 Reconstructed Au(111) has a 22 x √3 herringbone structure (e.g., a unit cell whose sides are 22 and √3 times the nearest neighbor Au atom distance in the unreconstructed surface, respectively) (Figure 2.2a). This surface structure has been considered to be the result of a balance between two opposing tendencies:11 the surface layer undergoes contraction in order to compensate for its reduced coordination, whereas opposing this contraction the underlying substrate potential favors a commensurate surface layer.4 The misfit between the surface layer and the substrate leads to the formation of a periodic array of pairs of partial dislocations (domain walls), leading to the separation of alternating domains in which surface atoms occupy fcc and hexagonal close-packed (hcp) sites (Figure 2.2b). The surface atoms at the dislocation lines occupy bridge instead of hollow sites, and in STM images of the surface the dislocation lines appear as ridges with a height of -0.1 to 0.2 Å.4 The unit cell of this reconstruction (22 times the unit length) contains 23 surface atoms instead of 22, thus allowing a 4.4% contraction along the [10] direction in the surface layer.12 Because of the small energy difference between fcc and hcp sites, the widths of fcc and hcp domains are different; the wider domain is presumed to have the fcc structure, which is energetically more favored.

Figure 2.3. Schematic missing row reconstructed Au(110) (1 x 2) surface with (a) top view;13 (b) cross-section;13 (c) STM image of Au(110)-(1 x 2) with an atomic-scale spatial resolution.14

Reconstruction of the Au(110) surface into the (1 x 2) pattern is formed by a "missing row" along the {10} direction in the surface layer, which was first observed by Fedak et al. employing LEED15 and has been intensively studied subsequently7,12,15-22 [similar superstructures have also been observed on the (110) faces of platinum16 and iridium17]. A schematic of the missing row reconstructed Au(110) (1 x 2) surface along with an atomic resolution STM image are shown in Figure 2.3. The "missing row" reconstruction gives rise to surface atoms with three different coordination numbers: on top of row atoms, side of row atoms, and trench atoms, and this structure is strongly related to the adsorptive behavior of the surface.18 The sides of the rows in this reconstructed surface are similar in structure to the fcc(111) surface and are sometimes referred to as (111) microfacets.19 The atomic arrangement in the surface causes some distortion in the underlying layers. A variation of the first two layer spacings and a row pairing in the second layer have usually been considered.20 Moritz and Wolf reported that the top layer spacing was contracted by about 20% in the vertical direction, while the second layer exhibited a lateral pairing displacement of 0.07 Å and the third layer is buckled by 0.24 Å.20 Additionally, Gritsch et al. found that structural defects such as steps interacted with the reconstruction.18 The stabilization of the (111) microfacets, at step edges, was generally found to govern the step terrace topography of these surfaces. The presence of trace impurities was correlated with an increase to (1 x 3) structural units along the [10] direction, leading at first to a decrease in (1 x 2) domain size and, at higher densities, to structures such as (1 x 7), (1 x 5), and finally a (1 x 3). This result exhibits periodic arrangements of (1 x 3) units as the common building principle.18 For the defect-free surface, the order-disorder transition of the (1 x 2) reconstruction appears at a critical temperature of (670 ± 20) K.21

Figure 2.4. High resolution STM image of the reconstructed Au(100) (5 x 20) surface (size 5.3 x 5.3 nm2).22

Compared to the other two low indexed surfaces, the reconstruction of the Au(100) surface is somewhat controversial. A (1 x 5) reconstruction was first observed by Fedak and Gjostein15 and soon after by Mattera et al.23 However, later in 1967, Fedak and Gjostein resolved a splitting in the LEED spots for Au, leading to a (5 x 20) rather than (1 x 5) superstructure and first proposed a hexagonal overlayer on the square substrate mesh as a model for the surface rearrangement.24 Since then, various models have been put forward including a (5 x 20) with rotation,25 a c(26 x 68),7 (24 x 48) with rotation,26 and a hexagonal (5 x 28)R0.6°.27 A (5 x 20)-based reconstruction is now widely accepted and the surface can be described as a quasi-hexagonal top-layer, in which interatomic distances are shortened by about 4% with respect to the bulk fcc values.26,32 An atomic resolution STM image of the characteristic (5 x 20) reconstruction is shown in Figure 2.4.

The reconstructed surfaces experience various combinations of the above-mentioned structures depending on the step density and surface temperature.22,33 The reconstruction is generally considered to be limited to the first layer, indicating that the more compact surface arrangement is favored to a degree where the energy cost due to a lack of commensuration with the layer underneath can be overcome.19 However, based on electron density calculations and He diffraction, Rieder et al. showed that the complex LEED patterns observed for Au(100) was due to the secondary structural features such as minor atomic displacements in both topmost and lower layers.28 Marks and Smith proposed that the reconstruction could be the result of surface Shockley partial dislocations.29 Their high resolution electron microscope (HREM) images indicated that small displacements could occur on unreconstructed Au(100) based on an observation that one whole column of atoms shifted both laterally and away from the surface. They concluded that the large degree of mass transfer taking place for the reconstruction occurs through a mechanism involving the observed dislocations.29 Using first-principles calculations, Takeuchi et al. interpreted the reconstruction of Au(100) from the viewpoint of a relativistic effect manifested by the high degree of d orbital participation in bonding,30 as is the case for Au(111).1 Mochrie and co-workers studied the reconstruction of Au(100) as a function of temperature employing X-ray scattering and found that above 1170 K the surface undergoes a structural phase transition [i.e., (1 x 1) superstructure] marked by a reduced asymmetry in the specular reflectivity.31

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