Figure 1.6: The graphite structure which can be depressed by a tip consisting of an atom on its top part (•) at different positions. [16].
Figure 1.7: The enhanced ”Zig-zag” and ”arm-chair-configuration” [16].
1.3 Reconstruction of Au(111)
Nucleation control is one of the crucial issues for nanostructure synthesis since it August 14, 2009
1.3. Reconstruction of Au(111) 8 is the dominant factor for how the atoms/molecules form self-assembled nanostruc-tures on the surface [20]. Naturally formed nanostuctured-templates, such as square islands of N/Cu(100) [21] and dislocation network of Ag/Pt(111) [22, 23], are exten-sively studied to provide the well-organized nucleation patterns. These techniques and knowledge are promising due to their capabilities to assemble the deposited materials into various nanostuctures which are of the importance for applications, especially in catalysis and nanomagetism [24–27]. However, each nanostructured template is mostly limited to only one nucleation pattern, which restricts the further application to more complicated catalytic process or to tune the magnetic coupling.
So far the study for creating the variety of nucleation patterns on the same template is crucial, but still lacking. Au(111) is the lowest energy surface of gold, as reflected in the tendency of thin-film growth to propagate in the [111] direction [28, 29]. In recent technological applications, AuCnX are more likely to rely on thin film Au, rather than single-crystal Au.
Figure 1.8: (A)with the process as depositing decanethiol monolayer at 300 K and annealed to 325 K for 5 min in vacuum, the resulting surface exhibits c(4 × 2) domains, domain boundary network, and vacancy islands. (B) Monolayer as in A, annealed to 375 K for 10 min invacuum, the Surface exhibits large c(4 × 2) domains and large domains of the lower density (p×√
3) phase. (C) Monolayer as in B, anneal Au to 575 K for 10 min in vacuum, the urface exhibits herringbone reconstruction characteristic of clean Au-(111) [28, 41].
Au(111) herringbone reconstruction surface is a high-lighted template for
fabrica-1.3. Reconstruction of Au(111) 9 tion of Fe, Co, Ni, and Mn well-ordered nanoisland arrays [30–39]. The herringbone surface is constituted of zigzag domains of hexagonal close-packed (hcp) and face center cubic (fcc) crystalline structures. Both of the fcc and hcp kinks, are favored nucleation sites for the 3dtransition metals. Up to now, no clear differences between fcc and hcp kinks are observed. The Au(111) herringbone template is always limited in only using the regularly spaced kinks as preferred nucleation sites. Actually, by tuning the mobility or the diffusion behaviors of deposit atoms, one can very possi-bly control the nucleation patterns for the fabrication of various nanostructures on the same template [40].
The other interesting case is the Fe on Au(788) vicinal surface, with different annealing temperature, one can obtain morphologies from nanodots to step flow [47]
Figure 1.9: (a),(b)and(c) are smaller islands under 250K;note that in (a)44K and (b)58 K the islands locate randomly, (c)at 70 K one can se that there doublets appear.From(d)200 K and (e)300 K,doublets start merging at someposition and step flow growth begins. [47].
The STM image with a shematic picture of structure in figure 1.10 shows a step-flow gwoth; the linprofile in the left shows the size of terrace and three types of the Fe growth on Au(788) vicinal surface.
The nucleation on Au(111) herringbone kinks are usually observed on larger lat-August 14, 2009
1.3. Reconstruction of Au(111) 10
Figure 1.10: The structure of Au(788)vincinal surface and step growth begin at fcc site when annealing temperature increasing near 300 K [47].
tice misfit systems, such as Fe, Co, Ni on Au(111) but not intensitive on small lattice misfit systems, such as Al, Ag, Au on Au(111)systems. [42–45]. The topic for us to study of iron on gold(111) and its vicinal surface is that would the nucleation lead to special, not only epitaxial growth but also give some phenomenon in magnetism?
Chapter 2
Basic Concepts
2.1 Growth of thin film and islands
The forces rearrange the growing surface, a dominant role is played by the free energy γ on the surface and interface, which determing the growth modes in thermal equilibrium.
Figure 2.1: Surface energies γ for magnetic and non-magnetic materials [46].
From Figure 2.1 we can see that magnetic material exhibit a relatively high surface energy. In equilibrium growth, there are usually three growth modes: layer by layer growth, island growth and Stranski-Krastanov growth.
The morphology depends on the balance between free surface of substrate,
over-11
2.1. Growth of thin film and islands 12
Figure 2.2: Schematic display of three growth modes.
layer and interface. The relations are :
γsubstrate < γoverlayer+ γinterf ace (2.1)
γsubstrate > γoverlayer+ γinterf ace (2.2) In case (1), the first atomic layer wants to coat the whole surface to provide optimum energy reduction, which is called layer-by-layer growth; in case (2), the over-layer has a tendancy to nucleate three-dimensional island, and leaves the low-energy substrate exposed, this is island growth. The situation usually occurs when growing and magnetic materials on top of an inert substrate, such as noble metal or an oxide.
By the ”rules” (1) and (2), if material A grows in material B layer by layer, then B on A will grow in islands, since the surface energies are reversed, this is Murphy’s law of epitaxy growth when A and B are with substrate surface energy difference.
To overcome this, one would try non-equilibrium growth.
The thermal-dynamics laws are very restrictive for growing desired nano-structure, especially in equilibrium growth. Therefore non-equilibrium growth, such as at low-temperature or high deposition rate are applied. Figure 2b shows the general result in non-equilibrium growth. At low-temperature, or higher rate or lower step den-sities, the arriving atoms do nat have enough energy (or diffusion time) to find the nearest step edge and to nucleate into islands. Additionally, if the microscopic kinetic processes taking place at the surface is taken into account, the substrate
2.2. Magnetic hysteresis loop 13