Chapter 1 Research background on two-dimensional (2D) materials growth
1.4 Graphene moiré superstructures on a substrate
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Figure 1.17 The growth of WSSC MoS2 film on a vicinal Au(111) surface.179 (a) Illustration of the vicinal Au(111) surface formation and MoS2 growth. (b) Photography of a MoS2 monolayer on the Au(111)/W substrate. (c) SEM images showing MoS2 growth process at∼720 °C. (d) STM images of a continuous MoS2 monolayer over single atomic steps of the vicinal Au(111). (e) SEM images of two type films composed by misaligned (left) and aligned (right) MoS2 domains after water oxidation.
In this section, the experimental progresses on synthesizing WSSC TMDC monolayers are presented. The nucleation density of TMDCs during CVD is typically over thousands per mm2 and to synthesize WSSC TMDC monolayers via route (i) seems formidable until now. On the contrary, the progresses on the seamless stitching of well-aligned TMDC islands is more promising, and centimeter scale single crystalline MoS2 films have been achieved on a vicinal Au(111) surface, implying the critical roles of high-index substrates in controlling the alignment of TMDC islands.
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the 2D material and the substrate, and therefore are generally used to identify the qualities of the grown 2D materials, as we inferred in Figures 1.12(e-f) and 1.13(f).
Depending on the lattice relationships between graphene and the underlying TM substrates, graphene moiré patterns present different shapes and sizes. On the Cu(100) and Ni(100) substrate, graphene moiré patterns show either striped or rhombic shapes, as shown in Figure 1.18,193-196 while on a (111) surface of a faced centered cubic (FCC) crystal or a (0001) surface of a hexagonal close packed (HCP) crystal, they always show a regular rhombic shape.197-202 In this thesis, the structures, properties and applications of graphene moiré superstructures on various TM surfaces are also explored.
Figure 1.18 Graphene moiré patterns on FCC(100) surfaces. (a) STM topographic images of graphene on the Cu(100) surface showing stripe pattern and the rhombic pattern.194 (b) STM topographic images of graphene on the Ni(100) surface showing either stripe pattern or rhombic network depending on the relative angle between graphene and the substrate, with the corresponding Fourier transforms shown in inserts.195
1.4.1 Structures of graphene moiré superstructures
Graphene moiré pattern is not only a visual effect for graphene/TM systems, especially on active TM surfaces, like Ru(0001),203,204 Rh(111),205,206 Pb(111),207 Pt(111)208,209 and Ir(111),210,211 the graphene lattice shows regular height variation in the periodicity of the moiré pattern. Figure 1.19 presents atomically resolved STM images of graphene on Ru(0001), Rh(111) and Ir(111) surfaces.
Obviously, the different contrasts indicate different height of graphene lattice on the substrate, and here we call such corrugated graphene layers as graphene moiré superstructures.
Besides, many experimental and theoretical studies revealed that the corrugation degree (the height variation) of graphene moiré superstructures highly depends on the rotation angle of graphene
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on the substrate.212-215 On the Pt(111) surface, graphene films present a corrugation degree of 0.5~0.8 Å at small rotation angles of 2º ~ 6º, while become ultra-flat at large rotation angles of 14º ~30 º.208 On the Ir(111) substrate, graphene layer with a smaller rotation angle corresponds a larger graphene corrugation, and that with zero rotation angle shows the largest corrugation degree.216
Figure 1.19 Atomically resolved STM images of graphene moiré superstructures on (a) Ru(0001),204 (b) Rh(111),205 (c) Ir(111)197 surfaces.
1.4.2 Applications of graphene moiré superstructures
Except for identifying the quality of the grown graphene as introduced in above, the unique corrugated graphene moiré superstructures lead to inhomogeneous properties of the graphene layers, which brings about potential applications in electronic devices.198,217,218 The inserts of Figure 1.20 illustrate the height profiles of graphene monolayer on different TM substrate. Compared with the weakly corrugated graphene layers on the Pt(111) and Ir(111) surfaces, the C 1s photoelectron spectrums of the strongly corrugated graphene on the Rh(111) and Ru(0001) surfaces exhibit a split double-peak, indicating two distinct types of bondings in graphene.219 Besides, it was revealed that the highly corrugated graphene moiré superstructure on the Ru(0001) surface presents quantum-dot-like behaviors with confined effects in both lateral and vertical directions, and the hump region of the superstructure has a higher local work function than other regions.220,221
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Figure 1.20 C 1s photoelectron spectrums of different graphene/TM superstructures and that of the highly oriented pyrolytic graphite (HOPG) is used for comparison, with the side views of graphene/TM superstructures are illustrated. MG means monolayer graphene.219
Moreover, it is found that the adsorption abilities of graphene/TM superstructures are site- dependent, making them good templates for the synthesis of metal clusters and the assembly of organic molecules.222-230 According to the lattice relationship between graphene and the underlying TM substrate, a unit cell of graphene/TM superstructure can be divided into four regions, namely ATOP, FCC and HCP and Bridge sites,197,231 as illustrated in Figure 1.21.
Figure 1.21 Illustration of lattice relationships of graphene on the four high-symmetric sites of (a) an FCC(111) and (b) an HCP(0001) surfaces.
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Interestingly, the adsorption ability of a graphene/TM superstructure for different TM metals are distinguishable.224,228,232,233 For instance, on the graphene/Ru(0001) template, Pt, Ru, Ir, Ti and Rh atoms prefer to form small dispersed nanoclusters at FCC sites, while Pd, Au, Ag, Cu and Co prefer to form large islands by covering different regions with similar coverages, here the formed Pt cluster and Pd islands are displayed in Figure 1.22(a-d).234,235 Graphene/TM superstructures have also been used in templating the assembly of organic molecules,236-239 it was revealed that Fe phthalocyanines (Pc) molecules can duplicate the lattice of the graphene/Ru(0001) superstructure and form a Kagome lattice that is composed of four neighbored lattice points located in interlaced triangles (see Figure 1.22(e- f)),240 whereas CoPc molecules self-assemble into a nearly squire lattice on the graphene/Ir(111) surface despite of the hexagonal corrugated template (see Figure 1.22(g-h)).241
Figure 1.22 Synthesis of TM clusters and assembly of Kagome lattices templated by graphene/TM superstructure. (a-d) SEM images of (a-b) Pt clusters and (c-d) Pd clusters on the graphene/Ru(0001) substrate.234 (e) STM image of Kagome lattice of FePc molecules on the graphene/Ru(0001) substrate with a marked unit cell.240 (f) Atomic structure of the Kagome lattice in (e). (g) STM image of squire lattice of CoPc molecules on the graphene/Ir(111) substrate.241 (h) Atomic structure of the squire lattice in (g).
In summary, modulated by TM substrates, graphene displays corrugated moiré superstructures and inhomogeneous surface property. Depending on the type of the substrate and the rotation angle on the substrate, graphene moiré superstructures are distinct. The versatile moiré superstructures in graphene make it a good candidate in templating the synthesis of metal clusters and organic molecular
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frameworks. In addition, the inhomogeneity in the electronic structure of graphene induced by the moiré superstructures also hold promising applications in nanoelectronics.