Magic clusters on group IV surfaces 6

CHAPTER Chapter 6: Co-Si magic clusters on Si(111)-(7x7) So far most of the studies have been focused on the formation and behavior of single material magic clusters such as Si clusters on SiC and Si(111). Hence it would be interesting to see if binary magic clusters can be formed and progressively ordered from Co deposited on Si(111). This chapter discusses data from STM and XPS which is used to probe the evolution of Co deposited at θ=2ML, θ=1ML and θ=0.5ML on clean Si(111)-(7x7) (where θ = Co coverage) at room temperature and progressively annealed to higher temperatures. We will first study the global morphology by characterizing the various surface structures such as 3D islands or clusters that could occur at different annealing temperatures as a function of Co flux. We will also determine if there is critical Co coverage where clusters exist exclusively or annealing temperature which promotes ordering and formation of (√7x√7) surface structures. While it is generally assumed that the ordering of clusters occurs spontaneously, we will also investigate the self assembly phenomena of clusters to resolve if the (√7x√7) structure is an intrinsic surface reconstruction or due to self assembly of clusters. It would also be interesting to determine whether a stable critical nuclei exists to promote this self assembly process, analogous to that in thin film growth. We will this through the use of fast scanning STM, to probe directly the process leading to the formation of (√7x√7). Finally, dual biasing STM will be used to analyze the shape and size of the various clusters as a function of tunneling voltage. This information will allow us to elucidate the 284 CHAPTER structure of Co-Si clusters as well as better understand the diffusion mechanisms involved in cluster motion on the Si(111)-(7x7) surface. 285 CHAPTER 6.1 Global surface evolution of Co-Si on Si(111) 6.1.1 Global Morphology: 2.0ML of Co deposited on Si(111)-(7x7) Clean Si(111)-(7x7) was first obtained by flashing the Si(111) sample to 1200oC in-situ, before it is being cooled to room temperature and scanned by STM. Fig. 6.1(a) shows the 100nmx100nm STM image of this surface revealing well ordered (7x7) reconstruction. 2.0ML of Co was then deposited via a solid Co source electron beam evaporator onto this surface at room temperature (RT). The surface is subsequently annealed to 460oC, 610oC, 700oC, 800oC and 900oC for 30min each time. STM and XPS scans of the surface are taken after each annealing cycle and are shown in Fig. 6.1(b) to Fig. 6.1(f) (STM) as well as Fig. 6.2 (XPS) respectively. The STM scan of the surface annealed at 460oC in Fig. 6.1(b) shows the formation of bright hexagonal features which we identify as islands with flat top surfaces possessing an average size and height of ~ 10±5nm and ~ 30±5Ǻ respectively. In order to characterize these 3D islands, we zoom-in on a high resolution 17.5nmx17.5nm STM scan of a typical hexagonal island as shown in Fig. 6.1b(i). The scan reveals that the surface of the island consists of a well ordered array of round protrusions. These features appear smaller than the cluster-like features observed to be surrounding the island. Line [ ] [ ] profile measurements X and Y along the 121 and 211 azimuths show the average separation between adjacent protrusions features is estimated to be ~ 7.6±0.5Å. This is twice the lattice parameter of Si(111)-(1x1) (3.8Ǻ), which indicates that the protrusions 286 CHAPTER possess a (2x2) periodicity. We also estimate the height of the island to be ~ 15.0±0.5Å (Ht) and also find smaller cluster-like features on the surface which appear to be disordered as indicated in the STM image. Fig. 6.1(b)(ii) shows a zoom-in 15.0nmx15.0nm STM image of these cluster-like features. The line profile measurements X and Y, show that each cluster has an average size of ~ 9.0±0.5Å and height of 1.6±0.1Å (Z). The scan also shows that these features are uniform in shape and size. Hence we identify them as Co-Si magic clusters. When the surface is annealed to 610oC for 30min, the STM scan of the surface as shown in Fig. 6.1(c), reveals that the 3D islands retain the same hexagonal shape, but are now larger in size and height (30±5nm and 30±5Ǻ). More interestingly, we now observe the appearance of bright and dark triangular domains coexisting alongside the 3D islands. Zoom-in STM images of the surface as shown in Fig. 6.1(c)(i) show that the bright domains are covered with the same clusters while the darker regions are due to the (7x7) reconstruction. This suggests that the clusters appear to be highly mobile and could have diffused to form triangular domains, thus exposing the underlying (7x7) reconstruction. Further annealing of the surface to 700oC, as shown by STM in Fig. 6.1(d) shows that the hexagonal 3D islands are larger in size and height than before (50±10nm and 50±5Ǻ), whilst its number density appears to be decreasing. The scan also shows that the size of the darker (7x7) triangular domains are now also larger and its number density is also smaller as observed in the zoom-in scan shown in Fig. 6.1(d)(i). The high resolution 287 CHAPTER image also shows that the clusters at 700oC remain similar in size and shape compared to cluster observed after annealing at 460oC. There is no significant change in the surface morphology when the sample is heated to 800oC (Fig. 6.1(e)), however it is interesting to note that the exposed (7x7) domains tend to occur in areas adjacent to the 3D islands. We also observe that the clusters still retain their shape and size in spite of the higher annealing temperatures. When the surface is annealed to 900oC, the cluster-like features are no longer observed. In fact, the 3D islands are now observed to be larger (70±10nm) and appear to be more triangular in shape rather than its original hexagonal shape as previously seen, as observed in Fig. 6.1(f). If we analyze the crystallographic direction of each of the facets of the original [ ][ ][ ][ ][ ] [ ] hexagonal shape of the island are 112 , 121 , 211 , 112 , 121 and 211 , we find that the [112] , [121] and [211] planes appear to be longer than the [112] , [121] and [211] planes. [ ][ ] [ ] [ ][ ] This suggests that the growth speed along 112 , 121 and 211 , is faster than 112 , 121 [ ] and 211 which accounts for the island shape transition from hexagonal to triangular as shown in Fig. 6.3. It is anticipated that during island growth, the island formation will tend to adopt a shape with the lowest energy configuration, thereby exposing the planes with the lowest surface energy. Hence it is not surprising that the stereographic projection of Si (111) in the [111] azimuth show that the lower surface energy facets exist along the [112], [121] and [211] planes respectively as illustrated in Fig. 6.3. 288 CHAPTER The XPS spectra as shown in Fig. 6.2a and Fig. 6.2b corresponds to the Co 2p3/2 and Si 2p core-level spectra taken from the XPS scans of the surface after each annealing cycle. The XPS scan of the surface when Co is deposited at RT shows that the binding energy of Co 2p3/2 (778.5±0.1eV) is close to that of a pure metallic Co (778.3±0.1 eV) rich environment [1]. The peak shape also appears asymmetric and is akin to a metalliclike Co environment. The corresponding binding energy of Si 2p is shown to be 99.2±0.1 eV, which is similar to clean Si substrate (99.2±0.1eV) [1]. When the surface is annealed to 460oC, there is no noticeable energy shift in the Si 2p peak. However, we detect a shift in the binding energy of Co 2p3/2 to 778.9±0.1eV, which is close to the binding energy of Co in a bulk CoSi2 crystal structure (778.8±0.1eV). This indicates the formation of Co-Si when the sample is annealed to 460oC, suggesting that the 3d islands and cluster features observed are Co-Si and not metallic Co. We not observed significant changes in the binding energies of the Si 2p peak (99.2±0.1eV) or the Co 2p3/2 peak (778.9±0.1eV) respectively when the surface is further annealed to 610oC or 700oC. From the global morphological and high resolution STM scans as well as XPS scans, we see that when 2.0ML of Co is deposited onto Si(111) at RT and annealed to 460oC, hexagonal-shaped Co-Si islands (size~ 10±5nm) form immediately. The island surfaces are flat and possess a (2x2) reconstruction. Co-Si magic clusters of average size and height of ~ 9.0±0.5Å and ~ 1.6±0.1Å are also observed to occur in areas adjacent to the islands. At higher temperatures (610oC, 700oC and 800oC), the islands appear to grow larger while the clusters now form into triangular domains coexisting alongside the islands, thus exposing the underlying (7x7) reconstruction. Since the clusters possess 289 CHAPTER uniform size and shape and they not grow larger in size at higher temperatures, hence we identify them as Co-Si magic clusters. When the surface is annealed to 900oC, the 3D islands appear to be more triangular in shape and the clusters are no longer observed. From the experimental evidence, we have demonstrated that binary material CoSi magic clusters can be formed. However these clusters are found to co-exist with 3D islands. Hence it would be of interest to see if we would be able to preferentially form a surface dominated exclusively by magic clusters, devoid of 3D islands. In the next few sections, we attempt to achieve this by lowering the Co coverage used from 2.0ML to 1.0ML and eventually 0.5ML. 290 CHAPTER (b)(i) 460°C X Clusters 1.5 Z[Å] X Y 2.5 Z[Å] Ht Z[Å] (a) RT 1.5 (7x7) 20nm (2x2) Y [121] 3.5nm (b) 460°C 0 X[nm] [211] X[nm] X[nm] Average separation~7.6±0.5Å Height~15.0±0.5Å (b)(ii) 460°C 0.8 Z[Å] 0.6 0.8 0.6 1.5 0.4 X 0.4 0.2 0.2 0 3.0nm 0.5 1.5 2.5 3.5 Y Z 0.5 0.5 X[nm] 1.5 2.5 3.5 X[nm] X[nm] Average size~9.0±0.5Å and height~1.6ű0.1Å 2.5 1.5 6.0nm X Y 0.5 0.5 0 0.5 1.5 2.5 3.5 0 X[nm] Z 1.5 0.5 (7x7) 1.5 Z[Å] Z X Y Clusters 2.5 Z[Å] 3D islands (c)(i) 610°C Clusters (7x7) Z[Å] Clusters Z Z[Å] X Y 2.5 Z[Å] 3D islands 0.5 1.5 2.5 3.5 0.5 X[nm] 1.5 2.5 X[nm] Average size~9.0±0.5Å and height~1.6ű0.1Å (d)(i) 700°C (7x7) Clusters 1.6 1.2 1.4 3D islands Z Z[Å] Y 0.8 Z[Å] X X 0.8 0.6 Clusters Z 0.5 0.2 0.2 6.0nm Y 0.6 0.4 0.4 (7x7) 1.5 1.2 Z[Å] (d) 700°C 0 (c) 610°C 0.5 Ht 0.5 0 0.5 X[nm] 1.5 0 0.2 0.4 0.6 0.8 X[nm] 1.2 1.4 1.6 0.5 1.5 2.5 X[nm] Average size~9.0±0.5Å and height~1.6ű0.1Å (e) 800°C 3D islands at RT (7x7) Clusters (f) 900°C (7x7) [211] [121] [112] [121] 20nm [112] [211] Figure 6.1: 100nmx100nm STM images of (a) clean Si(111)-(7x7) and after 2.0ML of Co deposited at RT and annealed for 30min at (b) 460oC (c) 610oC (d) 700oC (e) 800o (f) 900oC. Figure 6.1(b)(i) shows 17.5nmx17.5nm image of the surface at 460oC and line profiles X and Y across the island surface in the 121 and 211 azimuths show that the average separation between protrusions is ~ 7.6±0.5Å. Island height (Ht) is estimated to be ~ 15.0±0.5Å. Figure 6.1(b)(ii) shows a zoom-in 15.0nmx15.0nm scan of the clusters at 460oC. Line profile X, Y and Z show that the average size and height of each cluster is ~ 9.0±0.5Å and ~ 1.6±0.1Å. Figure 6.1(c)(i) shows a 30nmx30nm zoom-in of the surface at 610oC. Size and height of cluster is ~ 9.0±0.5Å and ~ 1.6±0.1Å. Fig 6.1(d)(i) shows a 30nmx30nm zoom in of surface annealed at 700oC. Size and height of cluster is ~ 9.0±0.5Å and ~ 1.6±0.1Å. [ ] [ ] 291 CHAPTER (a) 300000 Si 2p (99.2eV) Arbitrary Units 250000 200000 150000 800oC 100000 610oC 460oC RT 50000 103 101 99 106 105 104 103 102 101 100 99 98 Binding energy (eV) (b) Co 2p (778.9eV) 97 97 96 Co 2p (778.5eV) 450000 400000 Arbitrary Units 350000 300000 800oC 250000 200000 610oC 150000 460oC 100000 50000 RT 782 781 780 779 778 777 776 775 774 773 772 772 Binding energy (eV) Figure 6.2 shows the XPS signal of (a) Si 2p signal at 99.3eV and (b) Co 2p signal at 778.9eV when Co (θ=2.0ML) is deposited at RT and annealed for 30min to 460oC, 610oC and 800oC . 292 CHAPTER (b) (a) (c) faster [112] [112] [112] − − [211] [121] slower [211] − − [1 21] −− [211] [112] −− − [211] − [1 21] Figure 6.3 shows schematic diagram of (a) hexagonal island with planes and (b) triangular island with planes which are more exposed. (c) illustrates the island growth evolving from hexagonal to triangular shaped islands. 200nm (d) (e) 293 CHAPTER The direct observation of cluster dynamics shown in Fig. 6.11, Fig. 6.12 and Fig. 6.13, also allows us to estimate the lifetime for each respective configurations (i = 4, 5, and 7) prior to cluster detachment. In comparison, the i = configuration seen in Fig. 6.12(e)-(h) clearly has a lifetime that is longer than 20 sec, which is much greater than the lifetime for i = (Fig. 6.11(d)-(f)), i = (Fig. 6.11(g)-(i)) and i = (Fig. 6.13(d)-(f)). Although the present high speed STM scans are unable to resolve the individual lifetimes for i = 6, and 4, these configurations clearly not exist beyond to 10 sec. Using the Frenkel equation for lifetime; τ = τo exp [Eb/kT] (1) where assuming τo = 10-13 s, the binding energy (Eb) of these configurations of clusters are estimated to be approximately between 1.83 and 1.85 eV. At the same time, the observation of individual cluster diffusion on the terrace would also enable us to estimate the diffusion barrier by measuring the mean square displacement of clusters. At 400oC, was determined to be ~ 11.7 x 10-18 m2, as shown in Chapter 6-Appendix 1. Since the mean square displacement and Arrhenius expressions for diffusion co-efficient (D) are related as follows; 322 CHAPTER ~ Dt (2) D = Do exp[-Ed/kbT] (3) Do = γo.ao2 (4) where Ed is the diffusion barrier, t is time taken (5sec), T is the substrate temperature, kb is the Boltzman constant, γo is the pre-factor (1013 s-1) and ao is the lattice parameter (3.8Å), Ed was thus estimated to be ~ 1.6 eV. The binding energy of these cluster configurations, i = 4, and (Eb ~ 1.83 to 1.85 eV), is thus comparable to the diffusion barrier of a single cluster (Ed ~ 1.6 eV) over the Si(111)-(7x7) surface. Hence these configurations are likely to be more susceptible to cluster detachment than i = and even more so at progressively higher temperatures. This is consistent with the histogram data (Fig. 6.10) which shows pre-dominant occurrence of i ≥ compared to the other configurations (i.e. i < 7) at higher temperatures. 323 CHAPTER 6.2.3 Co-Si magic cluster structural elucidation Our data thus far shows that the critical nuclei i* = must first form, before the stable i = configuration can exist to propagate cluster ordering via further individual cluster diffusion and attachment. Hence the cluster ordering with a unit cell of (√7x√7) observed in Fig. 6.5(c) does not occur spontaneously. Individual clusters are observed to diffuse as a whole cluster entity and it appears to glide on the (7x7) surface. This behavior is dissimilar to earlier diffusion mechanism such as sequential displacement of adatoms at cluster edges (eg. Rh/Rh(100) [3]), “leap-frog” diffusion modes (eg. Pt adatom at the end of cluster displaces across the cluster to the front on a Pt(110) surface [4]) and dimer shear motion of compact clusters on metal (100) surfaces [5]. In order to deduce a plausible diffusion mechanism to describe the observed motion of the cluster, a good understanding of the “Co-Si” magic cluster would be required. In this area of work, contrasting models attempting to describe the structure of the Co-Si magic cluster have been proposed thus far [6-7]. Hence we will try to resolve this issue of structural elucidation by probing the cluster features and dimensions as a function of STM tunneling biases. We show 9nmx9nm STM images of an array of clusters arranged in (√7x√7) periodicity (Fig. 6.14) as well as a single isolated cluster (Fig. 6.15), scanned under tunneling biases of (i) V = +2.4, (ii) V = +1.4V, (iii) V = -1.4V and (iv) V = -2.4V. In both sets of data, we show that the clusters are first resolved as a ring-like structures consisting of smaller round protrusions with a dark centre depression when scanned 324 CHAPTER under positive tunneling bias before appearing as single bright protrusions when scanned under negative tunneling bias. In Fig. 6.14, we use a dark circle of diameter 9Å, which is the average measured size of Co-Si magic cluster (Fig. 6.1(b)(ii)), to identify a specific cluster within the cluster array. The STM scan in Fig. 6.14(i) and (ii) where tunneling bias V = +2.4V and V = +1.4V respectively shows smaller round protrusions with a dark centre within the indicated circle. The line profile measurements in Fig. 6.14(i) show that the average separation between the smaller bright protrusions is ~ 4.5±0.5Å. When the tunneling bias of the same scan area is adjusted to Fig. 6.14(iii) V = -1.4V and Fig. 6.14(iv) V = 2.4V, the cluster within the dark circle now appears as a single whole bright protrusion occupying the circle diameter of Å fully. In Fig. 6.15 (i)-(iv), we similarly probe the change in appearance of a Co-Si magic cluster as a function of tunneling bias albeit existing as an isolated cluster in the 9nmx9nm STM images and corresponding zoom-in 6nmx6nm scans of the same area indicated by a dark square line. The same cluster scanned under varying tunneling biases is indicated by a white arrow and is shown to be sitting on a faulted half of a (7x7) unit cell. In Fig. 6.15(i), where tunneling bias V = +2.4V, the cluster is resolved into smaller bright protrusions. The line profile measurements show that the average size of each of the protrusion is ~ 3.0±0.5Å and is similar to the average measured size of a Si adatom (~ 3.0±0.5Å) sited within the (7x7) unit cell. The cluster retains its appearance in Fig. 6.15(ii), where tunneling bias V = +1.4V. The line profile measurements show that the 325 CHAPTER average separation between the smaller bright protrusions is ~ 4.5±0.5Å, similar to Fig. 6.14(i). When the tunneling bias is changed to Fig. 6.15(iii) V = -1.4V and Fig. 6.15(iv) V = -2.4V, the cluster appears as a single whole bright protrusion, consistent with Fig. 6.14(iii) and (iv). Based on the dimensions of the various cluster features obtained from the line profile measurements under different tunneling biases, we propose a ball and stick model to account for the structure of the Co-Si magic cluster as shown in Fig. 6.16. The proposed structure consists of Si adatoms with a uniform separation of 4.5Å sitting on top of Co atoms arranged in a triangular configuration. This cluster structure occupies a circular area of diameter ~9.0Å and sits directly above the rest-atom layer, on the hollow sites of the (7x7) DAS structure. We now compare our findings with the models reported thus far in [6] and [7]. The first “Co-Si” cluster structure was proposed from STM observations [6] and was described as a 7-atom cluster comprising of one Co atom encapsulated within Si atoms (3 Si atoms bridging and Si atoms capping) arranged as a (√7x√7) reconstruction on a (1x1) template. As for the second cluster structure reported in [7], it was constructed using DFT calculations to obtain a relaxed structure of Co atoms on a half (7x7) unit cell of Si atoms. Using STS, they interpreted that the peak in tunneling conductance just below the Fermi level was attributed to occurrence of Co atoms in the second layer of the magic cluster structure. Hence the cluster structure was described as a 9-atom cluster 326 CHAPTER comprising of Si atoms on top of Co atoms which are bonded down to the Si rest atoms below, within a (7x7) half unit cell. We find that the number of atoms, stochiometry of Co to Si atoms as well as geometry of atom arrangement is more akin to the model described in [7] than [6]. The fact that our structure is constructed on a (7x7) template is also more consistent with [7] as opposed to [6], where the model is built on a (1x1) template. Hence our proposed model suggests that the structure in [7] is more accurate, thus resolving the structure of the Co-Si magic cluster. 327 CHAPTER (i) V=+2.4V (ii) V=+1.4V (c) (a) (b) 9Å 1.8nm 9Å 0.35 0.5 0.5 0.3 Z[Å] 0.3 0.2 0.25 0.4 0.2 0.3 Z[Å] 0.4 Z[Å] 1.8nm 0.15 0.2 0.1 0.1 0.1 0.05 0 0.2 0.4 0.6 0.8 1.2 1.4 0 0.2 X[nm] 0.4 0.6 0.8 1.2 X[nm] (a) 4.7±0.5Å (b) 4.5±0.5Å 0.2 0.4 0.6 0.8 1.2 X[nm] (c) 4.6±0.5Å (iv) V=-2.4V (iii) V=-1.4V 1.8nm 9Å 1.8nm 9Å Figure 6.14 shows 9nmx9nm STM images of an array of clusters arranged in (√7x√7) periodicity, scanned under tunneling bias (i) V = +2.4, (ii) V = +1.4V, (iii) V = -1.4V and (iv) V = -2.4V. The same cluster is indicated by the dark circle of diameter 9Å. Line profile in Fig. 6.15(i) shows that the separation between the bright protrusions in each cluster is ~ 4.5±0.5Å. 328 CHAPTER (i) V=+2.4V 0.8 0.7 0.8 0.6 Z[Å] Z[Å] (a) 0.6 0.4 0.4 0.3 0.2 0.2 (b) 0.5 0.1 0 1.2nm 0.2 0.4 0.6 0.8 (a) 3.0±0.5Å 6.0nm 10 X[Å] X[nm] (b) 3.0±0.5Å (ii) V=+1.4V 1.2 1.4 (b) (a) 0.8 1.2 0.8 0.6 0.4 0.8 0.6 0.4 0.2 0 6.0nm 0.4 0.2 0.2 1.2nm Z[Å] Z[Å] Z[Å] (c) 0.6 0.5 1.5 X[nm] (a) 4.7±0.5Å 0 0.5 1.5 X[nm] (b) 4.5±0.5Å 0.5 1.5 X[nm] (c) 4.5±0.5Å (iii) V=-1.4V 1.2nm 6.0nm (iv) V=-2.4V 1.2nm 6.0nm 329 CHAPTER Figure 6.15 shows 30nmx30nm STM scans with 6nmx6nm zoom-in images of the same single cluster as indicated by the arrow, scanned under tunneling bias (i) V = +2.4, (ii) V = +1.4V, (iii) V = -1.4V and (iv) V = -2.4V. Line profile in (i) shows that the size of each of the protrusions (3.0±0.5Å) within a cluster is similar to the size of a Si adatom (3.0±0.5Å). Line profile in (ii) shows that the separation between each of the protrusions in a cluster is ~4.5±0.5Å. 4.5Å 9Å Co-Si magic cluster Co atoms Si adatoms Si rest atoms Si dimer layer atoms Figure 6.16 shows a ball and stick schematic of the proposed model of the Co-Si magic cluster structure comprising of Si adatoms on Co atoms sitting on top of the restatom layer of the (7x7) DAS structure. 330 CHAPTER 6.2.4 Co-Si magic cluster diffusion mechanism In order for the cluster to diffuse, we propose that the process would need to involve (i) exchange of Si atoms between the top layer of the cluster and the underlying adatom layer of the (7x7)-DAS structure and (ii) the sequential breaking and formation of bonds between the base Co atoms of the cluster with the (7x7)-DAS Si rest atoms. We illustrate this process schematically in Fig. 6.17, Fig. 6.18 and Fig. 6.19. In Fig. 6.17(i)-(iv), we identify the Si adatoms within the half unit cell as Si atoms 1, 2, 3, 4, and (indicated as yellow balls), and we illustrate a cluster moving within the faulted half of the (7x7) unit cell. Fig. 6.17(i) shows a Co-Si cluster with Si atoms 2, and sitting on Co atoms indicated as red color balls, which are in turn sitting on the hollow sites in between the rest-atom positions. We show the cluster moving along the unit cell boundary as indicated by the dark arrow, with the cluster now comprising of Si atoms 3, and 5, in Fig. 6.17(ii). In Fig. 6.17(iii), the cluster is shown to move along the direction of the dark arrow while the ensuing Si atom exchange results in the cluster to now consist of Si atoms 2, and 5. We show the cluster moving one lattice space further in the same direction in Fig. 6.17(iv) and the cluster structure to comprise of Si atoms 2, and 6. In Fig. 6.18(i)-(iv), we illustrate a cluster moving along the half unit cell boundary with additional Si atoms 7, and identified in the adjacent (7x7) half unit cell. From its initial position shown in Fig. 6.18(i), the Co-Si cluster comprising of Si atoms 2, and 5, 331 CHAPTER moves across the boundary dissecting both (7x7) unit cell halves as indicated by the dark arrow. The cluster takes up the new position while consisting of Si atoms 1, and as shown in Fig. 6.18(ii). We further illustrate how the same cluster can move along the same unit cell boundary in Fig. 6.18(iii) and (iv) by an exchange of cluster Si atoms and DAS Si adatoms resulting in cluster structures comprising of Si atoms 1,3 and as well as 3, and respectively. In Fig. 6.19(i)-(iv), we show a cluster moving across the unit cell boundary into the adjacent (7x7) unit cell. From the initial position where the cluster consists of Si atoms 2, and (Fig. 6.19(i)), the cluster is shown to move across the unit cell boundary and incorporate Si atoms 1, and in Fig. 6.19(ii). The cluster is shown to move into the adjacent unit cell in Fig. 6.19(iii), where it now consists of Si atoms 3, and 9. We further illustrate the same cluster now moving within the new unit cell in Fig. 6.19(iv) and taking up final position with Si atoms 8, and 10. While we were not able to resolve this diffusion-exchange mechanism in detail, in this present work, the resultant diffusion mechanism proposed would allow the cluster structure to diffuse not only as a single entity in a gliding motion but also to occur without disrupting the underlying (7x7) template, as seen in our STM data. 332 CHAPTER (i) (ii) (iv) (iii) Figure 6.17 (i)-(iv) shows a Co-Si magic cluster diffusing within a half (7x7) unit cell. (ii) (iii) (i) (iv) Figure 6.18 (i)-(iv) shows a Co-Si magic cluster diffusing along the unit cell boundary. 1 (ii) (iii) (i) 10 (iv) Figure 6.19 (i)-(iv) shows a Co-Si magic cluster across the unit cell boundary. 333 CHAPTER 6.3 Summary In conclusion, we have addressed the following; 1) We find that θ=0.5ML is a critical Co coverage where there is no island formation at any annealing temperature. Higher coverages (θ=1.0ML or 2.0ML) lead to island formation at various annealing temperatures. 2) Formation of Co-Si magic clusters occurs after annealing RT deposited Co on Si(111) to 460oC. These clusters are uniformly round in shape, possess an average size and height of 9.0±0.5Å and 1.6±0.1Å, and not increase in size when annealed at higher temperatures. 3) We can selectively grow a full layer of Co-Si magic cluster array with (√7x√7) periodicity on top of Si(111)-(7x7) by annealing θ=1.0ML of Co coverage to 610oC. 4) At low coverages (θ=0.5ML), Co-Si magic clusters group into various 2D cluster configurations of i = 1, 2, 3, 4, 5, and 7, where i is the number of clusters within each formation. In particular, the i=6 cluster configuration consisting of clusters can be seen in U-shaped formation, ring formation or closed packed hexagonal structure. The i=7 configuration are either closed packed hexagonal structures or U-shaped formations. 5) The number density of cluster configuration as a function of annealing temperature (460oC, 490oC and 530oC) shows that i=1 dominate the surface with some i=2, and at 460oC while i=6, i=7 and i>7 increase in occurrence at 490oC. At 530oC, there are even less single or paired clusters while more clusters now 334 CHAPTER exist as i=6, i=7 or i>7. Therefore STM data shows a preferential occurrence of i = and over i = to at higher temperatures. 6) Real time study of the dynamical behavior of these magic clusters shows evidence of individual cluster diffusion and formation of i = 4, 5, and configurations with different lifetimes leading to the self organization and ordering on a Si(111)(7x7) template. The self assembly process does not occur spontaneously and a configuration dependent critical nuclei (i*=6) exists, parallel to the role of a critical nuclei in the nucleation theory of thin film growth [3-5]. The smallest stable configuration is one consisting of seven Co-Si magic clusters arranged in a hexagonal closed packed formation (i = 7). This hexagonal closed packed formation not only maximizes cluster co-ordination but also minimizes surface dangling bonds on an underlying Si(111)-(7x7) template. Finally, in contrast with traditional growth concepts, the “growth” of these cluster structures is translated via diffusion, attachment and self alignment of a magic clusters instead of adatoms. 7) Based on the dimensions of the various cluster features obtained from the line profile measurements under different tunneling biases, we propose the structure of the Co-Si magic cluster to consist of Si adatoms with a uniform separation of 4.5Å sitting on top of Co atoms arranged in a triangular configuration. This cluster structure occupies a circular area of diameter ~9.0Å and sits directly above the rest-atom layer, on the hollow sites of the (7x7) DAS structure. 8) We propose a diffusion mechanism which accounts for the motion of a single CoSi magic cluster gliding over the Si(111) surface without disrupting the (7x7) 335 CHAPTER structure. This mechanism involves (i) exchange of Si atoms between the top layer of the cluster and the underlying adatom layer of the (7x7)-DAS structure and (ii) the sequential breaking and formation of bonds between the base Co atoms of the cluster with the (7x7)-DAS Si rest atoms, to account for the diffusion of Co-Si magic clusters observed on Si(111)-(7x7). 336 CHAPTER 6.4 References [1] J.S. Pan, E.S. Tok, CHA Huan, RS. Liu, JW. Chai, WJ. Ong and KC. Toh, Surface Science 532-535, (2003) 639-644 [2] MH. Tsai, JD. Dow and PA. Bennett, Phys. Rev. B. 48, (1993) 2486 [3] G.L. Kellog, Progress in Surf. Sci 53, (1996) 217 [4] T.R. Linderoth, S. Horch, L. Peterson, E. Laegsgaard, I. Stensgaard and F. Besenbacher, New Journal of Phys. 7, (2005) 13 [5] Z.P. Shi, Z. Zhang, A.K. Swan and J.F. Wendelken, Phys. Rev. Lett. 76, (1996) 4927 [6] M.H. Tsai, J.D. Dow, P.A. Bennett and D.G. Cahill, Phys. Rev. B 48, (1993) 2486 [7] M.A.K. Zilani, Y.Y. Sun, H. Xu, Lei Liu, Y.P. Feng, X-S. Wang and A.T.S. Wee, Phys. Rev. B 72, (2005) 193402 337 [...]... 25sec X Y I =6 configuration (g) 30sec X (h) 35sec X (i) 40sec Y Figure 6. 11 (a)-(i) shows high speed 8nmx8nm STM scans captured in 5 sec frames of the formation and dissociation of a i = 6 configuration at 400oC on a surface 3 16 CHAPTER 6 Fig 6. 12(a) shows the occurrence of 5 clusters with additional cluster detached from the group at time = 0 sec Fig 6. 12(b) subsequently shows the additional cluster... nearest neighboring clusters (ii) closed packed hexagonal structure and (iii) triangular-like formation The i =6 cluster configuration is shown in Fig 6. 9f where 6 clusters can be found in (i) U-shaped formation (ii) ring formation or (ii) closed packed hexagonal structure The final cluster configuration of i=7 is shown in Fig 6. 9g, where the clusters are arranged in closed packed hexagonal structures in... high resolution STM to further probe the self assembly phenomena of Co-Si magic clusters in the following section 308 CHAPTER 6 6.2 Co-Si magic clusters 6. 2.1 Types of cluster configuration and number density as a function of temperature In order to study the different combinations of cluster configurations more closely, we deposit θ=0.5ML of Co onto Si(111)-(7x7) at RT and anneal to 460 oC, before... CHAPTER 6 (f)(i) i =6 3.0nm (g)(i) i=7 No 3.0nm formation island (f)(ii) i =6 (f)(iii) i =6 3.0nm 3.0nm (g)(ii) i=7 (g)(iii) i=7 3.0nm 3.0nm Figure 6. 9 shows zoom in 15nmx15nm STM scans of respective cluster configurations (a) i = 1, (b) i = 2, (c) i = 3, (d) i = 4, (e) i = 5, (f) i = 6 and (g) i = 7 313 CHAPTER 6 Cluster frequency (a) 460 oC 300 294 200 170 153 168 110 100 60 7 0 1 2 3 4 5 6 7 Cluster configuration... which consists of 3 clusters, are observed to be arranged as 3 clusters in a row in Fig 6. 9c(i) and (ii) or in a triangle arrangement with 3-fold symmetry in Fig 6. 9c(iii) We also identify the i=4 cluster configuration in Fig 6. 9d, comprising of 4 clusters arranged in (i) triangular geometry, (ii) in a straight row and (iii) U-shaped configuration Fig 6. 9e shows the i=5 cluster configuration with 5 clusters. .. geometries consisting of different number of clusters, i At 460 oC, individual clusters (i=1), appear to dominate the surface in spite of the existence of other configurations of clusters such as i=2, i=3 or i=4 301 CHAPTER 6 When the surface is annealed to 61 0oC, as shown in Fig 6. 7c, global morphological scans do not show any island formation However, the clusters are observed to have formed new 2D configurations... (7x7) reconstruction exposed These clusters also appeared to be disordered with no sign of island formation The zoom in image seen in Fig 6. 5b is also consistent with the large scale scan However, we also note that some of the Co-Si magic clusters tend to form into localized 2D hexagonal closed packed cluster configurations, as indicated by the dark circle shown in Fig 6. 5b Upon annealing to 61 0oC, as... configuration i 20nm E A D Cluster frequency (b)490oC B 200 150 100 50 0 150 100 60 28 1 2 C 91 90 39 3 4 5 6 7 Cluster configuration i Cluster frequency (c)530oC 150 100 100 50 108 119 76 25 28 1 2 18 0 3 4 5 6 7 Cluster configuration i Figure 6. 10 shows 100nmx100nm STM scans of the surface with corresponding histogram showing the number density of cluster configurations i= 1, 2, 3, 4, 5, 6 and 7 after... 460 oC We avoided island formation and were able to promote ordering amongst the Co-Si magic clusters via annealing at 61 0oC This led to the formation of (√7x√7) ordering of the clusters In fact we also observe from high resolution scans of magic clusters forming localized 2D hexagonal closed packed configurations from previously disordered states This suggests that the clusters are mobile at this temperature... (c)10sec I =6 configuration (e)20sec (f)25sec I =7 configuration (g)30sec (h)35sec Figure 6. 12 (a)-(h) shows high speed 8nmx8nm STM scans of the formation of a stable i = 7 configuration via a critical nuclei i* = 6 at 400oC captured in 5 sec frames 318 CHAPTER 6 We were also able to capture the cluster dynamics of an i = 4 cluster configuration in close proximity to an i = 7 structure Fig 6. 13(a) shows . 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 X[nm] Z[Å] 20nm (a) RT (7x7) (b) 460 °C 3D islands Clusters (b) 460 °C(b) 460 °C 3D islands Clusters (c) 61 0°C Clusters (7x7) 3D islands (c) 61 0°C(c) 61 0°C Clusters (7x7) 3D. islands consists exclusively of the (7x7) reconstruction and the Co-Si magic clusters are not observed. The XPS spectra as shown in Fig. 6. 6a and Fig. 6. 6b corresponds to the Co 2p 3/2 and. (Fig. 6. 5) STM images together with the XPS data (Fig. 6. 6). Fig. 6. 4a and Fig. 6. 5a both show the initial starting Si(111) surface consisting of well ordered (7x7) reconstruction. Fig. 6. 5b