Dynamics of epitaxial graphene growth and adsorptions of cobalt 1

1 Introduction & Research Objectives There are four main objectives in this thesis i.e. (i) revisit the precursor phase prior to formation of graphene on silicon carbide, 6H-SiC(0001) substrate i.e. (63x63)R30o; (ii) to probe the dynamics and kinetics occurring during the evolution from 63x63)R30o to graphene, (iii) to investigate the growth dynamics of Co at room temperature and comparing the similarities and differences between Co on graphene and Co on graphite; and (iv) to investigate the change in the growth dynamic when Co is deposited on graphene at elevated temperatures. This chapter is organised as follows: (a) background and motivation leading to this thesis; (b) literature reviews; (c) research objectives; and (d) the organisation of this thesis. Introduction and Research Objectives 1.1 Introduction and Motivation The discovery of graphene has opened up a new paradigm in nanoelectronics that could surpass the performance of conventional semiconductor devices owing to its superior mobility that near the speed of light [1,2], anomalous half-integer quantum Hall effect [1,3] and quasi-relativistic Klein tunneling [4]. Research on graphene, ranging from fundamental interests to exploration of its potential as a new platform for nanoelectronics, have been vigorously pursued since the first discovery in 2004 [5]. Graphene is essentially an isolated single sheet of graphite i.e. a two dimensional sp2–bonded honeycomb lattice of carbon atoms (Fig. 1.1a). The peculiarity of its transport properties arises from its unusual valence and conduction band. Having four valence orbitals (2s2 2p2) for each carbon atom, three of the orbitals (one s and two p orbitals) hybridised to form three sp2 bonding orbitals. The free lone p orbitals which oriented perpendicular to the graphene plane hybridise to form the  (valence) and * (conduction) bands of graphene. At the K and K’ point of the graphene Brillouin zone (Fig. 1.1b), these  and * bands meet at the Fermi level, forming a zero gap. Instead of showing the typical parabolic dependence, these bands change linearly with the momentum and therefore the electron transport of graphene is essentially obeying the Dirac’s (relativistic) equation (Fig. 1.1c). Owing to this, the electrons behave like massless Dirac fermion* and gives rise to many interesting Dirac-properties mentioned above. With the emergence of graphene, there is thus a tremendous interest to integrate graphene, carbon nanotubes and fullerenes together as a platform for fully carbon-based nanoelectronics. Because  and * band touching each other, the Dirac point also coincides with the Fermi level, hence the name Dirac fermion. * Chapter (a) (b) ky M 1.42Å K’ a1 a2  K Ek ky (c) K K’ 2.46Å kx K K’  * K  kx K’ Fig. 1.1 (a) A graphene sheet with atomic arrangement of carbon atoms in a 2-dimensional honeycomb lattice. The sp2 bonding gives shorter (1.42 Å) C-C bonding. The unit cell of graphene with lattice parameter of 2.46 Å is also drawn. Grey and red spheres represent the two different sublattice of graphene. The basic vectors a1 and a2 of the lattice are shown; (b) the first Brillouin zone of the honeycomb lattice and (c) the linear dependence of conduction () and valence (*) bands around K and K’ points. Fabrication of graphene-based devices requires reliable and convenient process for mass production. Mechanical exfoliation, which was the first method employed to produce ‘free-standing’ graphene in the laboratories [1,3,5], involves lifting off several layers of graphene sheets from a graphite crystal using adhesive tape and transfer onto the SiO2 as supporting substrate. This method produces high quality flakes of graphene but it is time consuming and impractical for large-scale production as it lacks consistency in reproducing flakes of the same sizes. Apart from this method of preparation, there are also several avenues to produce graphene on a larger scale. These includes epitaxial growth via solid-state decomposition of silicon carbide (SiC) at above 1200oC [6,7,8,9] and cracking of a carbon carrying gas on a metallic surface [10,11,12]. Of all these methods, the solid-state decomposition of SiC is perhaps the most direct route to prepare Introduction and Research Objectives graphene. Besides being able to produce graphene epitaxially, it is also compatible and easier to integrate with current Si-based CMOS technology. It has been reported that the thicknesses ranging from single to multiple layers graphene can now be prepared depending on the growth parameters. Recent reported works have also showed the possibility to tune (opening) the band gap of graphene via substrate effects from SiC, thus bring this material a step closer to realise carbon-based nanoelectronics [13,14,15,16].* Some of the unique properties of exfoliated graphene, however, fail to replicate themselves in SiC supported graphene (hereafter epitaxial graphene). Despite having identical atomic structure with the exfoliated graphene and having demonstrated that the  and * bands obey Dirac’s equation [14,16,17], epitaxial graphene exhibits some dissimilarities to those exfoliated graphene [18]. These includes for example, (i) the absence of the quantum Hall effect and this has been attributed to the weak localisation imparted from coupling with underlying substrate [18], (ii) a reduced carrier mobility by two orders of magnitude even after formation of wide terraces (which should aid in reducing electrons scattering) [19] and (iii) instead of forming zero band gap, a small gap is observed between  and * [13,14,15]. At present, it has also been suggested that the first graphene layer interacts weakly with underlying substrate where the unshifted * states and a pinned Fermi level at ~3 eV observed for graphene are similar to graphite characteristics [7,17]. This observation does not explain the disparity of properties between exfoliated and epitaxial grown graphene supported on SiC substrates. One of the main challenges in resolving these discrepancies is because epitaxial * The biggest hurdle to utilise graphene as an electronic material is its zero gap property at Dirac point, which makes the switch-off mode in a transistor becomes impossible. Chapter graphene is supported on a rather complex interfacial structure whose electronic and structural details are still not well understood. The growth of epitaxial graphene from the Si-terminated SiC(0001) is known to occur when the surface becomes increasingly carbon (C) rich due to gradual depletion of Si during thermal annealing. This SiC(0001) surface undergoes a series of surface reconstruction which is increasingly deficient in Si and the formation of graphene takes place when a C-rich phase known as (63x63)R30o (hereafter 63 for short) has formed and annealed to temperatures above 1200oC [7]. Despite being first discovered in 1975 [20], the structural details of this 63 phase are still in debate. There are several different and contrasting structural models reported in the literature. A more thorough review of these various models will be given in Section 1.3 (Literature Reviews). The problem becomes more complicated when it was found later that the decomposition of this initial surface to form graphene also resulted in the formation of a new 63-like phase at the interface between the graphene surface and the SiC substrate [15,21,22]. Clearly, the structure and the nature of interactions (covalent vs van der Waals) at the graphene/interface and together with defects/impurities/lattice strains are anticipated to cause scattering of charge carriers and change the transport properties of graphene. For these reasons, it is of interest to revisit this structure and elucidate the salient features leading to formation of epitaxial graphene on the SiC substrates. Providing an insight into the structure of 63 will allow better understanding of the interface-graphene interaction (covalent vs. van der Waals), growth mode and properties of epitaxial films produced. Knowledge of the mechanism leading to the formation of graphene monolayer will also allow the fabrication process to be tailored to control the thickness of graphene and also improve the quality and properties of graphene. Introduction and Research Objectives Being a platform for nanoelectronic, it is unavoidable that graphene will come in contact with metal to form electrical contact. As the technology node always continue to miniaturise, understanding and probing the interaction of graphene with metal at atomic level becomes indispensable. Another added-value for using graphene as host for metal studies is its “inertness” which may prove to be an excellent supporting surface for formation of magnetic dots/ clusters of a few hundred atoms. Technology exploitation of cluster with properties superior than the bulk requires them to be supported on surfaces or embedded in matrices. However, the superior cluster size-dependant magnetism which comes from additional orbital moments [23], often altered or disappeared when these orbitals interacts with or perturbed by the support or the medium. For example, nonmagnetic Ru has shown the importance of this aspect when its supported clusters display 4d ferromagnetism on inert graphite [24] but appear non-magnetic on noble metals such as Ag and Au [25]. For the film materials, Co appears to be an ideal candidate due to its ferromagnetism. Free Co clusters have been reported to be more magnetic than bulk for sizes up to 400 atoms [23]. In addition, recent computation by Xiao et al. shows that Co dimer adsorbed on a C(0001) surface displays the highest perpendicular magnetic anisotropy energy (MAE) (~100 meV per atom) [26]. This reported value is about an order higher than the next giant MAE reported from Co adatoms on Pt(111) [27,28]. For this reason, Co/graphene system will be vital for future molecular magnetic storage [29]. Given that properties at the nanoscale are sensitive to the interaction at interface and also the morphology (island sizes, densities and distributions), there is therefore a need to Chapter understand and control the growth kinetics and dynamics involved in producing these atomic clusters. The importance of this is further spurred by recent controversies involving Co/C(0001) system. At ambient temperature, we have found that Co nucleate as metallic 3-dimensional clusters that physisorbed on graphite surface [ 30 ]. However, results from computational work has been contradictory with some suggested strong binding between Co and graphene sheet [31,32] while other proposed weak binding i.e. physisorption [ 33 ]. Up to now, there is no experimental study reported for Co on graphene. It is also fundamentally interesting to probe the dynamics of a system involving two elements of very different surface energy. Similar to graphite, graphene is expected to possess very low surface energy. This has been recently confirmed via the liquid droplet test [34]. In this instance, the surface energy of Co (2709 mJ/m2) [35,36] is almost fifty times higher than graphene monolayer (46.7 mJ/m2) or graphite flakes (54.8 mJ/m2) [34]. Hence the dynamics of Co on graphene at elevated temperature may totally draw a very different picture. When the system is energetically allowed, such a huge difference in surface energy will vigorously drives the system to minimise the total surface energy. This is normally achieved by either (i) Ostwald ripening to form bigger islands on the surface, or (ii) adsorbates are embedded inside the substrate. The second scenario is particular interesting where this mechanism can be use to form embedded magnetic dots or to form layered structure. Although the second scenario is not often reported in comparison to surface coalescence, surface burrowing of gold (Au) by (Bi) surface [37] and Co cluster by Au surface [38] have been observed. For the case of metal deposited on graphite or graphene surface, embedding of adsorbate will result in formation of graphite (or graphene)-intercalated structure. In this case, Co may insert into graphene as Introduction and Research Objectives individual atoms or clusters. A 2D magnetism of Co is also possible when Co is sandwiched in between two layers of graphene epitaxially grown from SiC. Thus far surface intercalation between graphene and underlying SiC substrate are reported for Ni [39] and more recently Au [40]. The dynamic and energetics for surface intercalation has yet to be fully understood as the study of metal intercalation underneath graphene is still relatively new. More rigorous studies are required since this structure may give rise to properties similar to the graphite-intercalated-compounds (GICs). In summary, there are four main goals in this work. They are (i) revisit the (63x63)R30o precursor phase structure prior to formation of graphene on 6H-SiC(0001) substrate; (ii) to probe the dynamics and kinetics occurring during the evolution from 63x63)R30o to graphene, (iii) to probe and compare the similarity and difference between the growth dynamics of Co on graphene and Co on highly oriented pyrolytic graphite (HOPG) at room temperature and (iv) the dynamics of Co on graphene at elevated temperatures where surface burrowing of Co is observed in this work. In the next section, the controversies involving the precursor phase of graphene i.e. 63 phase will be reviewed. Various structural models have been proposed for the 63 phase. The significance and short falls of these models will be compared. The current status of the graphene growth dynamic on SiC is also discussed in this section. The section ends with review on metal adsorption on graphene and graphite, which will provide the background to the third and fourth objective of this thesis. Chapter 1.2 Literature Reviews 1.2.1 (63x63)R30o: the precursor phase prior to graphene/6H-SiC(0001) The structure of hexagonal SiC(0001) and various phases leading to epitaxial graphene will be discussed prior to review of the 63 phase. Emphasis is given to 6HSiC(0001) since our preliminary studies found this polytype produces better quality of epitaxial graphene than 4H-SiC(0001). (i) Introduction to crystal structure of 6H-SiC(0001) Figure 1.2a shows a single unit of Si-C structure that consists of a Si atom bonded to four nearest C atoms (or vice versa) via sp3 bonding network with tetrahedral angle of 109.5o. This single unit of Si-C structure is the building block for all the polytypes of SiC. For hexagonal polytypes, repeating this unit in two directions as shown in Fig. 1.2b gives rise to a (0001) plane of the hexagonal SiC. A SiC bilayer (BL) is defined as a layer consists of Si alternating with the nearest C layer along a direction, in this case the [0001] direction. In a hexagonal symmetry, there are three positions (A, B or C) which the layers can be stacked along the [0001] with each position rotated 60o from one another (Fig. 1.2c). A change in the stacking sequence resulted in change of the unit cell or a new polytype. Introduction and Research Objectives (a) (c) Side view (b) C Si Layer BL [0001] Layer Top view (d) 60o A B off-bond (0.63 Å) A 15.12 Å C (e) on-bond C 7.56 Å (1.89 Å) [0001] B [1100] A 3.08 Å Top view Top view Fig. 1.2 (a) A single unit of Si-C structure; (b) translation of this unit structure along two directions perpendicular to [0001] direction creates the (0001) plane of hexagonal SiC; (c) the orientation of next layer can be the same or rotated by 60o. This rotation creates many polytypes of SiC; (d) cross section view of a 6H polytype as projected on the (11 0) plane. The switch of stacking sequence occurs after the third SiC bilayer (from top) and (e) half of a unit cell of 6HSiC(0001). The top views i.e. (0001) plane for both full and half unit cell in (c) and (d) are also provided at the bottom. The top views show that due to switch in stacking sequence by 60o, the unit cell (solid line diamond) is rotated 60o when a 6H unit cell is terminated at half a unit cell. 10 Chapter protrusions observed in STM (Figs. 1.4b-c). Akin to STM observation by Riedl et al., their simulation of the surface shows hexagonal rings of different sizes and shape. However the simulated images not match the trimers observed in Fig. 1.4a and 1.4d(i). The vast amount of sp3 type of C-C components observed by photoemission studies of Mårtesson et al. [67] are also not found in their model. Similar to earlier thought, both work rebuff the idea of Si clusters being present on this 63 surface. For comparison and convenience, all of the abovementioned models are tabulated in Table 1.1. They are arranged in an order ascending the year they were reported. It is clear that despite the continuous effort in resolving the 63 surface, there is still no consensus achieved for this 63 surface. Clearly resolving the structure will be useful in aiding the understanding of its role in the growth of graphene and the resulting properties. 19 Introduction and Research Objectives Table 1.1 List of structural models proposed for (63x63)R30o surface. Proposed (63x63)R30o structure Model Graphene overlayer Descriptions of the structure Year The SiC-1x1 is terminated with a graphene overlayer. This model was proposed based on LEED pattern where 63 diffraction pattern was observed. In this instance, the length of a (63x63) SiC cell matches with 13 times the length of a (1x1) graphene unit cell. 1975 [20] Based on inverse photoemission studies and LEED pattern. Authors suggested a graphene monolayer rests on top of a 3 surface that consists of Si adatoms. The graphene was oriented 30o from bulk-1x1. The four Si adatoms (marked by ) at the corner of 13 x 13 graphene cell coincide with the troughs of the honeycomb lattice and thus give rise to the four nodes of the (63x63)R30o unit cell. 1998 [7 ] The 63 surface is assumed terminated by a 6x6 structure based on the STM images which shows 6x6 honeycomb structure. Model proposed based on STM images, XPS intensities and the binding energy of various C 1s components. The surface is terminated with sp3 carbon clusters of two different sizes and they self-organised to form 6x6 honeycomb. They are also surrounded by small graphene islands. 2005 [63] Unit cell of SiC(0001) Unit cell of graphene layer Model Si-terminated 6H-SiC(0001) bulk 3 Si adatoms Graphene honeycomb lattice Model sp3 C clusters Graphene islands C SiC bulk 20 Chapter Proposed (63x63)R30o structure Model Si atoms 3.8Å Model Si tetramer T4 Si adatom STM image Model Descriptions of the structure Year Model proposed based on STM images, clusters size measurement and XPS results. Similar to Ref. 63 , the 63 surface is observed terminated by 6x6 honeycomb. However, unlike other models which suggested a fully Cterminated surface, they proposed the surface still have remnants of Si clusters that left over from the 3 surface, as suggested by detection of elemental Si signals (XPS). They arrived at this structure by systematically remove n unit of Si4 tetramer starting from a 3x3 surface. 2006 [64] Based on their STM imaging of graphitised 4H-SiC(0001), they observed pyramidal structure similar to Si tetramer at the interface with 63 periodicity. In their final structure, the 63 structure consists of Si tetramer and T4 Si adatoms. 2007 [21] No structural model is put forward but the authors claimed that they observed the 63 periodicity from the STM image which link to the 63 pattern from LEED. The 63 periodicity comes from rings of two different sizes (marked by hexagonal dark gray and pentagonal light gray shaded area). 2007 [53] 21 Introduction and Research Objectives Proposed (63x63)R30o structure Model 63 unit cell Bonded to substrate C atoms (gray balls) Model Bonded to substrate C atoms (void) Unbonded C atoms (-bonding- linked using black lines) Unbonded C atoms (-bonding) Descriptions of the structure Year The 63 surface is terminated with a graphene layer bonded to the top Si layer of the bulk SiC. This graphene layer is rotated 30o from the SiC bulk so that the (63x63)R30o of SiC(0001) coincidence with 13x13 of the graphene mesh. Due to mismatch between the SiC(1x1) and graphene(1x1), not all C atoms are bonded to the Si adatoms. The bonded C atoms creates a 3 periodicty while the unbonded C atoms form  bonds with the neighbouring unbonded C atoms (black lines in bottom image). The arrangement of bonded and unbonded C atoms gives rise to 6x6 quasi-periodic arrangement. The bonded (unbonded) C atoms are attracted towards (retracted from) the surface and responsible for the hole (bright protrusions) of 6x6 honeycomb structure observed by STM. 2008 [72] Same discussion as Model 7. 2008 [74 ] 3 63 6x6 22 Chapter 1.2.2 Growth of epitaxial graphene on SiC substrates The study of graphene growth from SiC has been lacking prior to discovery of graphene. Currently this interest is renewed since epitaxial growth on 6H-SiC(0001) offers large scale production and more importantly epitaxial graphene produced this way shows some Dirac properties similar to those free standing graphene [14,16,17]. While it is known that graphene evolves from the C-rich 63 surface, there are not many work that address the dynamics and kinetics by which this transformation occurs. Below are the summary of the current understanding of epitaxial growth of graphene on SiC(0001): (i) using low-energy electron microscopy (LEEM) technique, Hannon et al. provided a real time in-situ observation of the formation of graphene [75]. In their work, they observed graphene growth begins with pit formation on the surface due to pinning of the precursor phase, 63. They also observed that three BL of the 6H-SiC(0001) surface collapsed during the formation of 63 reconstruction prior to graphene formation. This prompt them to suggest that the 63 layer consists carbon equivalent to BL of SiC which by coincidence fulfil the carbon ratio needed to form a monolayer of graphene; (ii) via ex-situ transmission electron microscopy (TEM), Borysiuk et al. observed graphene monolayer draping across the atomic steps of SiC, giving rise a carpet-like structure [76]. However because their observations were made ex-situ, they could not predict the growth mechanism adopted by the surface as it transforms from 63 to graphene; 23 Introduction and Research Objectives (iii) recent reported work involved annealing the surface gradually between the temperature regime of graphitisation (1200oC to 1400oC) and investigating the surface morphology in particular the step densities as their surface is progressively heated [77,78]. Both of this work observed atomic steps as the main source for releasing the Si atoms and graphene formation begins at these borders. Their observations are in agreement with our work reported earlier [30]. Using electron channeling contrast imaging, Ferralis et al. observed that the film thickness is mainly determined by growth temperature [79]. In addition they found that increasing the annealing time beyond minutes at constant temperature leads to pinning of graphene films at step edges, resulting in compressively stressed films at room temperature which may be responsible for the blue shift observed for the optical phonon of epitaxial graphene, and (iv) recent STM imaging of graphitised SiC(0001) surface revealed that the epitaxial graphene rest on an interface that mimic the same filled-state images of 63 phase. These studies show that as the initial 63 surface converts to graphene, a new 63-like layer also forms concurrently at the interface between graphene and the SiC bulk [22,30,80]. The interface with 63-like structure is consistently recorded over a wide range of tunneling conditions due to the “partial transparency” of graphene layer above it. This has made the study of the evolution of graphene from the 63 surface challenging as it is difficult to distinguish between areas that are covered with graphene and 63. For these reasons, the mechanism leading towards the graphene formation has not been investigated in detail. 24 Chapter 1.2.3 Metals on graphene (i) Co adsorption studies Adsorption of metal on graphene has been actively studied, mostly using computational method due to huge interest in utilising graphene as platform for carbon based nanoelectronics [33,81,82,83]. Most of the computational work reported strong interaction between Co (covalent bonding) and single layer graphene [31,32]. However recent work shows otherwise [33] where interaction of physisorption nature is reported. All of these reports predict Co preferentially adsorb on top of the hole of the honeycomb graphene lattice. Table 1.2 summarised the adsorption energy and bond strength of Co on graphene calculated using the density functional theory (DFT). Magnetic moment and charge transfer from Co to graphene are also provided as reference. Table 1.2 The preferred adsorption site, adsorption energy, bond length, vertical distance of Co from graphene, magnetic moment and charge transfer from Co adatom to graphene as predicted by calculation using density functional theory. Preferred adsorption sites Adsorption energy (eV) AdatomC bond length (Å) Vertical distance of adatom from graphene plane (Å) Magnetic moment (B) Adatom to graphene charge transfer (e) Reference Hole 1.27 2.12 - 1.31) - 81 Hole 1.32 2.09 1.56 1.1 1.1 83 Hole 0.97 - 1.51 1.0 0.6 33 Hole 2.4 - 1.52 1.0 0.2 31 Hole 1.58 2.10 1.49 1.0 - 32 *E(adsorption) = E(adatom+graphene) − E(graphene) − E(adatom) 25 Introduction and Research Objectives Experimental studies of metal adsorption on epitaxial graphene are lacking. In particular, adsorption of Co metals on graphene has not been reported. Adsorption studies (which includes interaction, nucleation and growth) of metals on graphite are more common for various reasons; ranging from using it as a model for physisorption system [84] to employing graphite as inert host for supporting clusters with unique properties [85]. Depending on the choice of metals, various morphologies of islands have been observed. These include triangular Pd islands [86,87], spherical Cu islands [88] and dendritic Au and Sb4 [89,90] islands. Interestingly, when island morphology, growth mode and interfacial interactions involving a same metal-graphite system were examined, contradicting and sometimes inconsistent results were observed [91-98]. The growth mode of Ru/graphite at 300K for example, was reported by Pfandzelter et al. to be in a layer-by-layer mode for the first monolayer [92]. This result, which was deduced based on change of Auger signal with coverage, contradicts with the 3D growth mode observed by Binns et al. using photoemission and reflectivity measurements [91]. The morphology of Ag on graphite is another example. Irregular and ramified 3D Ag structures were found to grow on graphite surface by Ganz et al. [93] whereas spherical clusters were observed by Binns et al. [94]. 3D islands are generally reported to be physisorbed on graphite. Nonetheless carbides formation are also observed for Ti [95] and Al [96]. These results were later disputed by Ma and Rosenberg, who claimed that the carbides not form on clean graphite but are catalysed by surface contaminants such as oxygen [ 97 , 98 ]. Presence of surface contaminants may alter the surface morphology and growth mode, especially in the submonolayer regime. Clearly, work reported for metal on graphite in this respect have been lacking although adsorptions of metals on graphite have been widely studied. In view of this, there is a need to verify the influence by contaminants on interfacial reactions as well as the corresponding morphology. 26 Chapter (ii) Thermal effect on growth dynamics Surface burrowing only occurs if the surface energy of adsorbates is much higher than the surface energy of the host. Such huge energy difference creates a capillary force that induces embedding of adatoms underneath the host surface. This reason has been put forward to account for Au burrowing into bismuth surface [37]. Surface burrowing of metal atoms beneath graphene monolayer has only been reported recently. Shikin et al. reported intercalation of gold beneath a graphene monolayer that was pre-grown on Ni(111) surface [99]. In this instance, Au was deposited onto a graphene/Ni(111) surface at room temperature. Surface intercalation occurred when the sample was annealed, resulting in a graphene/Au/Ni(111) structure. Based on their spectroscopy studies, they also predicted that the Au formed 2D-islands at the interface between graphene and Ni(111). A recent work using STM and STS by Bremlal et al. reported similar intercalation behaviour by Au between graphene and SiC(0001) [40]. Similar to Shikin et al. [99], Au was first deposited on graphene/SiC(0001) at room temperature and then annealed to 727oC. The annealing appears to drive the penetration of Au through the graphene lattice. They also reported that there was no interaction between Au and graphene. For Co, it has a surface energy of 2709 mJ/m2 [35,36], which is almost fifty times higher than graphene monolayer (46.7 mJ/m2) or graphite flakes (54.8 mJ/m2) [34]. Thus driven by the system to minimise surface energy, it is anticipated that embedding of Co underneath a graphene monolayer may occur at high growth temperatures. 27 Introduction and Research Objectives 1.3 Research objectives The research objectives of this thesis are motivated by the problems associated with 63 surface and graphene growth and also the fundamental interest in studying the dynamics of Co adsorptions on graphene surfaces. There are thus four main objectives in this thesis and the means of realising the objectives are discussed as follows: (i) Revisit the (63x63)R30o structure on 6H-SiC(0001) The structure of C-rich 63 surface is studied. Using STM, we will show that under different sample biases, two very different structures exist on the 63 surface i.e. a groups of three atomic clusters bonded as trimer and individual clusters scattering around these trimer. The size and height correlation between these clusters will be measured from STM images and their periodicity (unit cell and orientation) with respect to the bulk SiC1x1 reveals that they have 6x6 and 3 periodicities that can be correlated to explain the LEED patterned observed by others. Angled-resolved photoemission will also be carried out to identify the bulk- and surface- related chemical states for C and Si. We will show that the surface is composed of Si rich and C rich regions. The 6H-SiC(0001), one of the many polytypes of SiC, is chosen as substrate to grow epitaxial graphene for the following reasons. Hexagonal polytype will provide better epitaxial quality than cubic SiC since it shares the same symmetry as graphene. A (63x63)R30o unit cell of SiC is a direct match to a mesh of (13x13) cells of graphene. The 6H-SiC(0001) is chosen over 4H-SiC(0001) mainly because our preliminary studies 28 Chapter shows that 6H produce better graphene film with large terraces. In addition, half of the 6H unit cell i.e. three bilayers of SiC provide the exact amount of carbon needed to form a monolayer of graphene. This make the 6H-SiC(0001) a more interesting substrate to grow epitaxial graphene. (ii) Probing the mechanism leading to formation of epitaxial graphene The growth and evolution of graphene from the initial 63 surface is studied as a function of temperature and time using the STM. The aim is to investigate where the initial nucleation occurs, and how it leads to formation of epitaxial graphene and the formation of the 63 interface structure beneath the surface. As discerning the graphene from the initial 63 surface is made difficult due to the co-existence of 63-like interface and the “transparency” of graphene under certain STM biasing, direct observation using STM alone is rather challenging especially on a large scale. In this respect, we have established in this work an indirect method to probe the transformation process at the global scale using Co decoration. We found that Co display a rather different morphology when adsorbed on 63 surface and on graphene at room temperature. High density but smaller Co clusters nucleate on the 63 surface versus the low density but bigger Co clusters on graphene. As such, we were able to examine the formation and evolution of partially to fully graphitised surface as a function of time and temperature at both atomic and global scale. The formation of graphene is observed to begin from the step edges as Si desorption occurs and the growth process continues akin to a step-flow growth mode and is accompanied by considerable step rearrangements. Analysis of step heights evolution at various stages of graphitisation shows that as the initial 63 surface converts 29 Introduction and Research Objectives to graphene, three Si-C bilayers beneath collapses to regenerate C-rich structure with also a 63 periodicity at the interface between graphene and the bulk SiC. Based on these observations, a structural mechanism for growth of mono- and multilayer graphene is proposed. In addition, we also examined the rate at which the initial 63 surface converts to graphene as a function of temperature and time. Kinetic analysis reveals the transformation occurs with an activation energy of 3.0 ± 0.4 eV, a value close to the breaking of a Si-C bond. (iii) Probing the growth dynamics of Co on epitaxial graphene at room temperature This third objective includes probing and understanding adsorption, nucleation and growth of Co on epitaxial graphene at room temperature. The influence of underlying interface will also be investigated here by carrying out the same study using HOPG and 63 as surface. Growth studies will be carried out using a controllable atomic flux of Co. The growth mode, nucleation and growth will be probed using in-situ STM while the interaction of Co with the host surface will be probed using spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS) and photoemission spectroscopy (PES). Nucleation is an important aspect to be understood since it is the starting point for all the subsequent physical processes that control morphology and size distribution. To understand the kinetic of nucleation and growth, STM technique becomes indispensable. It provides atomic resolution which allows us to extract information such as cluster density, size, shape and structure. However it cannot directly tell us microscopic terms 30 Chapter such as the critical nucleus size, i* for stable cluster nucleation and the surface energetics which includes diffusion and nucleation barriers. In this instance, atomistic nucleation theory will be used to relate the measurements of cluster density and cluster size to these microscopic terms. In particular, we will employ a scaling function developed from this model to deduce i* for Co/graphene, Co/graphite and Co/63. We begin the investigation with the nucleation and adsorption studies of Co on graphite where highly oriented pyrolytic graphite will be used. The influence of surface contaminants on interaction and growth of Co on graphite will also be investigated. The formation of Co clusters on 63 surface will also be studied under the similar conditions used for HOPG. This part of work will be mainly focus on the difference in terms of bonding, growth mode and morphology of Co when Co is deposited on sp3-rich carbon surface (the 63 surface is rich with sp3-type of C-C bonding). Adsorption studies in similar manner will be continued with Co on epitaxial graphene that supported on a 63like interface. Knowledge learnt from Co/graphite and Co/63 system will be used as a framework to elucidate the influence of underlying 63 interface on the nucleation and growth of Co on epitaxial graphene. On all the three surfaces, Co was found to adopt the most common growth mode for physisorption system i.e. 3-dimensional (3D) or Volmer-Weber growth mode and it can be attributed to Co-substrate interaction being weaker than Co-Co. They form domeshaped clusters with round base on all three surfaces. Co clusters of similar dimension and density are found on the Co/HOPG and Co/graphene. Smaller but higher density of Co clusters are found on 63 due to impedance of Co diffusion on the corrugated 63 surface. Despite similarities of atomic structure between graphite and graphene and the 31 Introduction and Research Objectives morphology of Co on these two surfaces, scaling performed onto the distributions of cluster volume reveals very different critical nucleus size, i* between them. i* of are found for Co/HOPG while i* of are found for Co/graphene. The difference can be attributed to effect of charge transfer to the graphene overlayer from the 63 interface. For Co/63, i* of is deduced with preferential nucleation on certain adsorption site on 63 surface is observed. (iv) Probing the growth dynamics of Co on epitaxial graphene at elevated temperature As the surface energy of Co and graphene is very different, it is interesting to study how the growth dynamic and kinetics of Co on graphene can be modified by controlling the substrate temperature. Both STM and XPS experiments similar to part (iii) will again be employed to investigate the growth, interaction and chemical states of the Co structure grown at various temperatures. We will show that growth of Co at elevated temperatures induces crystal ordering in Co on graphene/6H-SiC(0001) surface. Flat top islands with thicknesses of just a few atomic layers and spreading as wide as 50 nm are seen along with two other cluster-like structures (dim clusters (~ Å) and bright clusters (1 to Å)) on the surface. Post-growth annealing experiments further reveal that the dim clusters mediated the flat islands formation. Scaling analysis performed on the flat islands reveals that a single dim cluster can transforms into a flat island i.e. i*= 0. More interestingly, these Co structures (flat islands, dim and bright clusters) are found burrowed beneath graphene surface leading to formation of metal intercalation. As these features are found beneath graphene, they are stable against ambient oxidation. 32 Chapter 1.4 Thesis organisation This thesis is divided into three parts described as follows: (i) Part 1: Introduction, motivations, research objectives, concepts and methodology This section covers (i) the discussion on current issues and research interests that shape the objectives of this thesis (Chapter 1), (ii) the fundamental concepts related cluster nucleation and growth (Chapter 2). These concepts will be used to understand the dynamics of Co adsorption; and (iii) descriptions on the procedures used for preparing clean surfaces, experimental methods and also introduction on characterisation techniques used in this programme such as XPS and STM (Chapter 3); (ii) Part 2: All the result chapters (a) Chapter The STM study of polycrystalline pyrolytic graphite (HOPG), carbon-rich(63x63)R30o phase and epitaxial graphene will be discussed here. The chapter begins with STM imaging of HOPG and followed by the 63 surface. A structural model is proposed for the 63 surface. The mechanism leading to formation of epitaxial graphene on 6H-SiC(0001) and its relation with the 63 interface is the main focus of this chapter; (b) Chapter Adsorption of Co on these three carbon surfaces at room temperature will be discussed and compared here. Important observations of this chapter include 33 Introduction and Research Objectives influence of underlying interface that gives rise to different morphology and critical nucleus size, i* for Co/graphite, Co/63 and Co/graphene. (c) Chapter This chapter is devoted to growth of Co on graphene at elevated temperature. Co/graphene system is shown to switch from 3D growth mode at room temperature to multilayer crystalline 2D Co islands at elevated temperatures. Further investigations reveal these multilayer islands are burrowed underneath graphene which is interestingly not observed for annealing of Co clusters pregrown at room temperature. The top graphene layer prevents ambient oxidation of Co, which may provide natural shielding of just one atomic layer thick. (iii) Part 3: Conclusions and Future Work Summary of key results, their significance and outlook for this research programme will be given in Chapter 7. 34 [...]... Equation (1. 1) shows the surface transition: +Si, ~850oC ~950oC ~10 70oC ~12 00oC o o Graphene SiC-1x1 (63x63)R30 3x3 (3x3)R30 - Si - Si - Si - SiOx (1. 1) 12 Chapter 1 1x1 Å 3x3 9 2 ~850oC 5Å 3x3 Silicon rich 3x3 1x1 Si SiC bulk (3x3)R30o 5.3 Å 3 x 3 ~950oC Si 5Å SiC bulk (63x63)R30o 63 x 63 ? ~10 70oC 32.0 Å Carbon rich 5Å Graphene Å 2.5 1x1 ~12 00oC graphene 63 (?) graphene Fig 1. 3 Left panel... 0.97 - 1. 51 1.0 0.6 33 Hole 2.4 - 1. 52 1. 0 0.2 31 Hole 1. 58 2 .10 1. 49 1. 0 - 32 *E(adsorption) = E(adatom +graphene) − E (graphene) − E(adatom) 25 Introduction and Research Objectives Experimental studies of metal adsorption on epitaxial graphene are lacking In particular, adsorption of Co metals on graphene has not been reported Adsorption studies (which includes interaction, nucleation and growth) of metals... length of a (63x63) SiC cell matches with 13 times the length of a (1x1) graphene unit cell 19 75 [20] Based on inverse photoemission studies and LEED pattern Authors suggested a graphene monolayer rests on top of a 3 surface that consists of Si adatoms The graphene was oriented 30o from bulk-1x1 The four Si adatoms (marked by ) at the corner of 13 x 13 graphene cell coincide with the troughs of the... distance of Co from graphene, magnetic moment and charge transfer from Co adatom to graphene as predicted by calculation using density functional theory Preferred adsorption sites Adsorption energy (eV) AdatomC bond length (Å) Vertical distance of adatom from graphene plane (Å) Magnetic moment (B) Adatom to graphene charge transfer (e) Reference Hole 1. 27 2 .12 - 1. 31) - 81 Hole 1. 32 2.09 1. 56 1. 1 1. 1 83... structure will be useful in aiding the understanding of its role in the growth of graphene and the resulting properties 19 Introduction and Research Objectives Table 1. 1 List of structural models proposed for (63x63)R30o surface Proposed (63x63)R30o structure Model 1 Graphene overlayer Descriptions of the structure Year The SiC-1x1 is terminated with a graphene overlayer This model was proposed based... prior to discovery of graphene Currently this interest is renewed since epitaxial growth on 6H-SiC(00 01) offers large scale production and more importantly epitaxial graphene produced this way shows some Dirac properties similar to those free standing graphene [14 ,16 ,17 ] While it is known that graphene evolves from the C-rich 63 surface, there are not many work that address the dynamics and kinetics by... in a graphene/ Au/Ni (11 1) structure Based on their spectroscopy studies, they also predicted that the Au formed 2D-islands at the interface between graphene and Ni (11 1) A recent work using STM and STS by Bremlal et al reported similar intercalation behaviour by Au between graphene and SiC(00 01) [40] Similar to Shikin et al [99], Au was first deposited on graphene/ SiC(00 01) at room temperature and then... addition, half of the 6H unit cell i.e three bilayers of SiC provide the exact amount of carbon needed to form a monolayer of graphene This make the 6H-SiC(00 01) a more interesting substrate to grow epitaxial graphene (ii) Probing the mechanism leading to formation of epitaxial graphene The growth and evolution of graphene from the initial 63 surface is studied as a function of temperature and time using... graphitised 6H-SiC(00 01) showing 6x6 honeycomb beneath the graphene monolayer) As the graphene layers grow thicker, both of the diffraction from SiC-1x1 and 63 patterns become diffuse and disappear with a new 1x1 diffraction originated from graphene layers emerged [7] This new 1x1 pattern rotated 30o from SiC(1x1) and consistent with STM imaging where the graphene layer rotated 30o from SiC More recently,... growth dynamics of Co on epitaxial graphene at room temperature This third objective includes probing and understanding adsorption, nucleation and growth of Co on epitaxial graphene at room temperature The influence of underlying interface will also be investigated here by carrying out the same study using HOPG and 63 as surface Growth studies will be carried out using a controllable atomic flux of . (c) Top view (d) (e) off-bond on-bond 3.08 Å (1. 89 Å) [00 01] [11 00] [00 01] [11 00] [11 00] 15 .12 Å (0.63 Å) 7.56 Å A B C A C B A Si C 1 BL Layer 1 Layer 2 [00 01] Top view Chapter 1 11 The most common. to the graphene plane hybridise to form the  (valence) and  * (conduction) bands of graphene. At the K and K’ point of the graphene Brillouin zone (Fig. 1. 1b), these  and  * bands meet. rich ~950 o C ~10 70 o C Si SiC bulk Si SiC bulk Si SiC bulk Si SiC bulk 3x3 5.3 Å 5 Å 3x3 1x1 3x3 1x1 63x63 5 Å 32.0 Å 3x3 9. 2 Å 5 Å 1 x 1 3x3 9. 2 Å 5 Å 1 x 1 ~850 o C ~12 00 o C graphene 63 (?) graphene 2 . 5