CHAPTER Chapter 7: 7.1 C o n clu s i o n Summary In this section, we will provide a summary of our key findings which will address the research objectives in the following the work; 1) Si magic clusters on 6H-SiC(0001) 2) Si magic clusters on Si(111) 3) Co-Si magic clusters on Si(111) Following which, we will also briefly address the future work related to this work. 338 CHAPTER 7.1.1 Si magic clusters on 6H-SiC(0001) In this work, we have established the occurrence of Si magic clusters on 6HSiC(0001). This structure exists as one of the phases as the 6H-SiC(0001) surface undergoes phase transformation with progressive annealing from (3x3) → (6x6) clusters → (6x6) rings. This process is facilitated by the formation of Si magic clusters which first occurs with Si popping out from the (3x3) surface as tetra-clusters (size~5.0±0.5Å), driven by the breaking of highly strained co-planar Si-Si bonds. These tetra-clusters agglomerate to form larger clusters of uniform shape and size (~14.3±0.5Å), which self assembly to form (6x6) unit cells at higher temperatures. This process is motivated by the minimization of dangling bonds and a consequent lowering of surface energy. We propose a hexagonal cluster structure model consisting of tetra-clusters or 32 Si adatoms to account for the observed cluster shape, size and (6x6) periodicity. We also show the formation of (6x6) rings from the (6x6) clusters at higher temperatures and consequently establish the absence of the (6√3x6√3) R30º reconstruction from line profile and auto co-relation analysis. Although our XPS data shows that Si is lost from the surface during annealing, however the ring surface is still Si-rich and not graphitized. STM data shows that smaller type “A” clusters (tetraclusters, size~5.0±0.5Å) form from type “B” clusters (size~14.3ű0.5) and leads to the formation of (6x6) ring structure (size~15.5 Å ±0.5) consisting of type “C” Si adatoms (size~3.0ű0.5) when the surface is heated beyond 1000°C. This shows that type “A” tetra-clusters participate in the phase transformations by mediating the surface structural 339 CHAPTER transformation. We propose a model to account for this surface evolution through the selective removal of M x Si atoms (Where M=number of clusters) at various annealing temperatures. By removing M=19 per unit cell, we obtain a structural model of the ring structure consisting of locally organized type “C” Si adatoms arranged into hexagonal D1 and D2 formations, which accounts for the experimental observations. 340 CHAPTER 7.1.2 Si magic clusters on Si(111) From our results, Si magic clusters can be grown via methods; spontaneous nucleation from high temperature heating/quenching of Si(111) surface or selective growth from Si adatoms deposited on Si(111) by Si atom source. (I) Heating /quenching of Si(111) In the first method, in addition to being able to form Si magic clusters from heating/quenching, we have also established the origin, formation and structure of Si magic clusters formed spontaneously on Si(111)-(7x7) in UHV. STM data shows that uniformly shaped Si magic clusters of size ~ 14.0 ± 0.5Å are formed during the “1x1”→ (7x7) phase transformation. Real time data shows that these clusters form from excess Si adatoms which are expelled from the surface. This is attributed to the “1x1” phase having a greater atomic density of 45 more Si atoms per (7x7) unit cell than the (7x7) phase. Thus, Si magic clusters originate from the excess Si adatoms which have to be removed from the “1x1” structure in order to facilitate the structural change to (7x7). Si magic clusters are also shown to be mobile as evidenced by the cluster trails coexisting with the “1x1” and (7x7) domains. Hence diffusion of Si on Si(111) occurs via Si magic clusters which facilitates the transport of excess Si from the “1x1” regions to the step edges and consequently propagating the (7x7) domains. Formation of Si magic clusters is therefore critical to the “1x1” to (7x7) transition as they accommodate the excess atoms during structural change and facilitate mass transport during (7x7) domain nucleation. From the 341 CHAPTER dual biasing data, Si magic clusters of average size ~ 14.0 ± 0.5Å are found to comprise of sub-unit features of size ~4.5±0.5Å each, arranged in a isosceles triangular configuration with an average separation of ~7.5 ±0.5Å, ~5.7 ±0.5Å and ~5.7 ±0.5Å. (II) Growth of Si magic clusters from atom source In the second part of this work, we have addressed how to fabricate stable monodisperse magic clusters from Si adatoms deposited on (7x7)-Si(111) by; (i) Demonstrating the self assembly of Si magic cluster via the use of an atom source instead of a cluster source. In doing so, we avoid growth issues related to inconsistent cluster size and shape distribution typically attributed to the use of cluster source techniques. (ii) Demonstrating the stability and self assembly phenomena of Si magic clusters to form spatially well–ordered cluster arrays at low temperatures (7 increase in occurrence at 490oC. 345 CHAPTER At 530oC, there are even less single or paired clusters while more clusters now 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. 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. Growth of cluster structures is translated via diffusion, attachment and self alignment of a magic clusters instead of adatoms. Hence (√7x√7) structure is not a surface reconstruction but is due to self assembly of Co-Si magic clusters. 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) 346 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). 347 CHAPTER 7.2 Future Work From this work, we propose the following areas of interest for future work. 1) In the elucidation of the respective structures of Si magic clusters on 6H-SiC(0001) and on Si(111) as well as Co-Si magic clusters on Si(111), the ball and stick models proposed are a result of the consideration of dangling bonds and comparison of sizes of clusters observed from STM measurements. However the experimental observations are confined within the constraints of the STM measurements, as the existence of artifacts or distortion due to electronic effects could affect the accuracy of our size measurement. Nevertheless, we used a slow scan speed through a large bias range to eliminate drift and electronic effects in ensuring accurate topographical representations of the surface features. However, a first principle calculation of the structures would be useful as part of future work in confirming the cluster structure. Considering the large of number of atoms and different possible types of bonding configurations, a theoretical approach similar to that taken by Que et al [1-2] may be necessary in order to solve this structure efficiently. 2) In the formation of (6x6) rings from the (6x6) clusters at higher temperatures on 6HSiC(0001), XPS shows that although Si is lost from the surface during annealing, the ensuing ring surface is still Si-rich and not graphitized. As we have demonstrated that large areas of the Si-rich (6x6) ring structures may be fabricated, it would be of interest to probe if this structure would eventually lead to the formation of graphene via further heating of the surface and inducing graphitization of SiC. 348 CHAPTER Thus far graphene has been generated typically from the micro-mechanical cleaving of graphite [3]. However, this approach has been plagued by inconsistent film quality, thus affecting the fabrication of graphene for practical electronic applications [45]. Our proposed method of controlled graphitization of SiC to form large areas of graphene would be significant in overcoming the growth issues associated with typical mechanical approaches to graphene formation and thus allow us to tap its much feted electronic properties for further applications. As it is unclear as to how each layer of the SiC substrate develops from a Si-rich to a graphitic surface according to high temperature treatment, our future work will address the nature and formation of this phase using STM and XPS. In particular, we will study if the progressive loss of Si from the heating of the SiC surface results in surface structural change which follows the empirical rule of removing M x Si atoms (where M=number of clusters). As most of our work is done on 6H-SiC(0001) substrates, it would also be interesting to extend this to 4H substrates, where a different atomic stacking sequence exists. This would allow us to see if this mechanism still operates under the same conditions. At the same time we will probe if the introduction of Hydrogen gas would further enhance the formation of wide graphene terraces from our approach of SiC graphitization. In order to develop this area of work further, we would also probe the growth of binary magic clusters on the graphene surface. As graphene, being non-reactive in nature, is not expected to alter the properties of the clusters, it would thus be interesting for us to probe 349 CHAPTER the magnetic and electronic properties of Co-Si and other metallic/binary magic clusters on graphene surfaces for possible applications in quantum computing or memory devices. 3) In the progressive formation of Si magic clusters from Si adatoms on Si(111)- (7x7), Si4 tetra-clusters were involved as building block units. In the same work, similar tetra-clusters have also been observed separately on 6H-SiC(0001) surface [6-7]. Grass et al [8] also reported the existence of Si tetra-clusters consisting of Si atoms on HOPG. The Si4 clusters in his work were deposited from a cluster source, and the tetra-clusters were shown to be stable against coalescence from UPS, at room temperature. It will therefore be interesting to see if similar growth process leading to the assembly of Si magic clusters can also occur on an inert HOPG surface as part of future work. This would be useful as this would allow us to further exploit the unique properties arising from Si magic clusters on an inert surface as opposed to Si magic clusters on a reactive Si substrate. In particular, this would be useful in relation to Si nano-structured solar photovoltaics, as this would open a new route to the manufacturing of mono-disperse Si nano-crystals controllably and reproducibly. 350 CHAPTER 7.3 References [1] J.Z. Que, M.W. Radny, P.V. Smith, Surf. Sci. 444 (2000) 123 [2] J.Z. Que, M.W. Radny, P.V. Smith, Surf. Sci., 444, (2000) 140 [3] K.S. Novoselov, A.K. Geim, S.V. Morozov, V.I. Fal’ko, M. Katsnelson, I.V. Grigorieva, S.V. Dubonos and A.A. Firsov, Nature 438, (2005) 197 [4] U. Starke, Silicon Carbide – Major Advances, Springer, Berlin (2004) pp 281-316. [5] W. Chen, H. Xu, L. liu, X. Gao, D. Qi, G. Peng, S.C. Tan, Y. Feng, K.P. Loh and A.T.S. Wee, Surf,. Sci. 596, (2005) 176 [6] E.S. Tok, W.J. Ong, H. Xu and A.T.S. Wee, Surf. Sci. 558, (2004) 145 [7] W.J. Ong and E.S.Tok, Phys. Rev. B 73, (2006) 045330 [8] M. Grass, D. Fischer, M. Mathes, G. Gantefor and P. Nielaba, Appl. Phy. Lett. 81, (2002) 3810 351 [...]...CHAPTER 7 7.2 Future Work From this work, we propose the following areas of interest for future work 1) In the elucidation of the respective structures of Si magic clusters on 6H-SiC(0001) and on Si(111) as well as Co-Si magic clusters on Si(111), the ball and stick models proposed are a result of the consideration of dangling bonds and comparison of sizes of clusters observed from STM... applications in quantum computing or memory devices 3) In the progressive formation of Si magic clusters from Si adatoms on Si(111)- (7x7), Si4 tetra -clusters were involved as building block units In the same work, similar tetra -clusters have also been observed separately on 6H-SiC(0001) surface [6 -7] Grass et al [8] also reported the existence of Si tetra -clusters consisting of 4 Si atoms on HOPG The Si4 clusters. .. graphitization In order to develop this area of work further, we would also probe the growth of binary magic clusters on the graphene surface As graphene, being non-reactive in nature, is not expected to alter the properties of the clusters, it would thus be interesting for us to probe 349 CHAPTER 7 the magnetic and electronic properties of Co-Si and other metallic/binary magic clusters on graphene surfaces. .. properties arising from Si magic clusters on an inert surface as opposed to Si magic clusters on a reactive Si substrate In particular, this would be useful in relation to Si nano-structured solar photovoltaics, as this would open a new route to the manufacturing of mono-disperse Si nano-crystals controllably and reproducibly 350 CHAPTER 7 7.3 References [1] J.Z Que, M.W Radny, P.V Smith, Surf Sci 444 (2000)... Fal’ko, M Katsnelson, I.V Grigorieva, S.V Dubonos and A.A Firsov, Nature 438, (2005) 1 97 [4] U Starke, Silicon Carbide – Major Advances, Springer, Berlin (2004) pp 281-316 [5] W Chen, H Xu, L liu, X Gao, D Qi, G Peng, S.C Tan, Y Feng, K.P Loh and A.T.S Wee, Surf, Sci 596, (2005) 176 [6] E.S Tok, W.J Ong, H Xu and A.T.S Wee, Surf Sci 558, (2004) 145 [7] W.J Ong and E.S.Tok, Phys Rev B 73 , (2006) 045330... principle calculation of the structures would be useful as part of future work in confirming the cluster structure Considering the large of number of atoms and different possible types of bonding configurations, a theoretical approach similar to that taken by Que et al [1-2] may be necessary in order to solve this structure efficiently 2) In the formation of (6x6) rings from the (6x6) clusters at higher... of the surface and inducing graphitization of SiC 348 CHAPTER 7 Thus far graphene has been generated typically from the micro-mechanical cleaving of graphite [3] However, this approach has been plagued by inconsistent film quality, thus affecting the fabrication of graphene for practical electronic applications [45] Our proposed method of controlled graphitization of SiC to form large areas of graphene... and the tetra -clusters were shown to be stable against coalescence from UPS, at room temperature It will therefore be interesting to see if similar growth process leading to the assembly of Si magic clusters can also occur on an inert HOPG surface as part of future work This would be useful as this would allow us to further exploit the unique properties arising from Si magic clusters on an inert surface... However the experimental observations are confined within the constraints of the STM measurements, as the existence of artifacts or distortion due to electronic effects could affect the accuracy of our size measurement Nevertheless, we used a slow scan speed through a large bias range to eliminate drift and electronic effects in ensuring accurate topographical representations of the surface features However,... (where M=number of clusters) As most of our work is done on 6H-SiC(0001) substrates, it would also be interesting to extend this to 4H substrates, where a different atomic stacking sequence exists This would allow us to see if this mechanism still operates under the same conditions At the same time we will probe if the introduction of Hydrogen gas would further enhance the formation of wide graphene . will address the research objectives in the following the work; 1) Si magic clusters on 6H-SiC(0001) 2) Si magic clusters on Si(111) 3) Co-Si magic clusters on Si(111) Following which, we. C C H H A A P P T T E E R R 7 7 339 7. 1.1 Si magic clusters on 6H-SiC(0001) In this work, we have established the occurrence of Si magic clusters on 6H- SiC(0001). This structure exists as one of the. to (7x7). Si magic clusters are also shown to be mobile as evidenced by the cluster trails coexisting with the “1x1” and (7x7) domains. Hence diffusion of Si on Si(111) occurs via Si magic clusters