9.1 Introduction
Similar to hBN, TMDCs also have a binary composition and their 2H phase possesses 3-fold symmetry (D3H). As discussed in previous Chapters, the alignment of a 2D material on a substrate is dominated by the interplay between their symmetries. Therefore, the similarity in the symmetries between hBN and 2H phase TMDCs may make the alignment of 2H phase TMDCs on a high-index low symmetric substrate to be similar to that of hBN. Up to now, the substrates used for the growth of TMDC are mostly inert ones including amorphous SiO2/Si,12,368-370 and high symmetric Al2O3(0001)371-
373 and GaN(0001) substrates,172,374,375 where the grown TMDC islands usually present multiple orientations and thus polycrystalline TMDC films are formed.
Recent experimental studies show that Au surfaces are promising substrates for the growth of various TMDCs, including MoS2, MoSe2,WSe2 and WS2, etc.,190,376-378 because Au can decrease the sulfurization barrier of TM precursors and increase the growth rate.379 By reducing the nucleation density through pretreatment of the substrate, both WSe2 and WS2 single crystals with a millimeter size have been realized on reusable Au substrates,167,380 which is 2~3 orders larger than those grown on insert substrates. Moreover, the synthesis of unidirectional WS2 islands has been achieved on vicinal Au(111) substrate and WSSC WS2 films have been obtained by the seamless stitching method.179
In this chapter, we chose WS2 as an representative of the TMDC materials to explore their alignment on Au{111}-based high-index low symmetric substrates with straight step edges.
9.2 Calculation methods
The same as all previous studies of graphene and hBN on the TM substrate surfaces, DFT-D3 calculations are carried out in this chapter to investigate the alignment of WS2 islands on Au{111}- based low symmetric substrates. The exchange-correlation functionals are treated by GGA,348 and the interaction between valence electrons and ion cores is described by the PAW method.349 A criteria of force on each atom to be less than 0.01 eV/Å and an energy convergence of 10-4 eV is used for the structural optimization.
9.3 Results and Discussions
Since the structural reconstruction of the Au(111) surface has been extensively observed with the adsorption of S adatoms,381-383 the Au(111) surface reconstruction should be considered. Here, a
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commonly observed Au(√3 × √3) R30º honeycomb lattice384,385 is used as the representative of reconstructed Au(111) surfaces and the growth of WS2 on the Au(111) surface with and without S- termination is investigated. Figure 9.1(a) shows the atomic configuration of the (√3 × √3) R30º S- terminated Au(111) surface.
Figure 9.1 Comparison of WS2 film on the pristine Au(111) surface and S-terminated Au(111) surface.
(a) Atomic configuration of the (√3 × √3) R30º S-terminated Au(111) surface. (b) Formation energy of WS2 film on the pristine Au(111) surface and on the (√3 × √3) R30º S-terminated Au(111) surface as a function of the chemical potential of sulfur. (c) Atomic configurations of WS2 film on the (√3 × √3) R30º S-terminated Au(111) surface with relative angles of 0º and 30º. (d) Atomic configurations of WS2
film on the pristine Au(111) surface with relative angles of 0º and 30º.
We first investigated the stabilities of a WS2 film on the Au(111) surface and on the (√3 × √3) R30º S-terminated Au(111) surface by calculating their formation energies, which are defined by:
𝐸𝑓 = (𝐸𝑇− 𝜀𝐴𝑢× 𝑁𝐴𝑢− 𝜇𝑆× 𝑁𝑆− (𝜇𝑊+ 2 × 𝜇𝑆) × 𝑁𝑊)/𝑁𝑊 (9.1) where 𝐸𝑇 is the total energy of the WS2 film on a substrate, 𝜀𝐴𝑢 and 𝑁𝐴𝑢 denote the energy of a Au atom in the Au(111) substrate and the number of Au atom in the substrate, 𝜇𝑆 and 𝑁𝑆 denote the chemical potential of S and the number of S adatoms in the (√3 × √3) R30º S-terminated Au(111) surface, 𝜇𝑊+
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2 × 𝜇𝑆 = 𝜇𝑊2𝑆 and it denotes the energy of a WS2 unit in the WS2 film, 𝜇𝑊 and 𝑁𝑊 are the chemical potential of W and the number of W atoms in the WS2 film. As 𝜇𝑊2𝑆 is a constant, the formation energy is only related to the chemical potential of S, and in Figure 9.1(b), the formation energy profiles of a WS2 film on the Au(111) surface and on the (√3 × √3) R30º S-terminated Au(111) surface with relative rotation angles of 0º and 30º (see Figure 9.1(c-d)) are given. Because 𝜇𝑆 is equal to -4.32 eV or -5.47 eV by using S bulk or W bulk as a reference, it is clear that the growth of WS2 film on the pristine Au(111) surface is always energetically preferred in the 𝜇𝑆 range between the two values, indicating that the Au(111) surface can remain its pristine configuration with the cover of the WS2 film. Therefore, the pristine Au(111) surface is used as the substrate in the following exploration for the alignment of WS2 islands.
To explore the orientation of WS2 islands on a low symmetric substrate, vicinal Au(111) surface with <110> and <211> step edges are chosen as the substrate and a square-shape WS2 cluster with two AC edges, one ZZW and one ZZS edge is chosen as the WS2 island, so that the interaction between straight WS2 edges and straight Au step edges can be investigated. Different with one-atom thick 2D materials, the edges of the three-atom thick TMDC is preferred to be self-terminated, and it has been reported that the reconstructed edges are highly dependent of the ambient condition and various self- terminated edges have been explored by both experimental and theoretical studies.318,339,386-388 To make it simple, we suppose a S-rich condition and thus the reconstructed AC and ZZW edges of the WS2
cluster with S-termination are energetically preferred, as shown in Figure 9.2. The total energies of such a WS2 cluster with its different edges attached to the Au<110> and Au<211> step edges are compared.
Because of the square shape of the WS2 cluster and the equal number of atoms for all these models, a lower total energy implies a stronger interfacial binding energy between the WS2 edge and the Au step edge.
Along the Au<110> step edge, ZZS and ZZW edges are more superior than the AC edge, but there is no obvious binding energy difference for the interfaces of the Au<110> step edge attached by ZZS and ZZW edges because both of the WS2 ZZ edges have been well passivated by S atoms.
However, the different compositions of the ZZS and such S-terminated ZZW edge must result in different formation energies of the ZZS and such S-terminated ZZW edge, which will lead to only one preferential orientation of the WS2 island along the Au<110> step edge. Along the Au<211> step edge, AC edge is more superior than the two ZZ edges, as a result, two orientations of WS2 island with a misangle of 60º will form because the absence of the mirror symmetry for WS2 respect to its ZZ direction, which is similar with the cases (II) and (III) in Figure 7.3 showing the hBN AC edge attached to the Cu<211> step edge. Although there is a large lattice mismatch between WS2 and Au(111), the results of WS2 edges attaching to Au<110> and Au<211> step edges are quite similar to those of hBN on Cu{111}-based low symmetric surfaces that have very close lattice constants.
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Figure 9.2 Total energies of different edges of a WS2 cluster attaching (a) Au<110> step edge and (b) Au<211> step edge, the unit of the total energy is eV.
It should be noted that, because of the unbalanced stoichiometry, the edge formation energies of the WS2 edges as well as the interfacial formation energies between these edges and the Au step edges highly depends on the ambient condition. Consequently, TMDC islands can show different orientations under different ambient conditions, which means that achieving unidirectionally aligned TMDC islands on a low symmetric TM substrate is possible by tuning the ambient condition. Further explorations are required to investigate the alignment of TMDCs on low symmetric substrates under different ambient conditions. In addition, the interfaces between various reconstructed TMDC edges and various TM step edges also need to be studied in the future.
9.4 Summary
In this Chapter, we simply investigated the alignment of TMDC islands on low symmetric TM substrates with straight step edges by using WS2/Au{111}-based low symmetric surfaces as an example.
The results are quite similar with those obtained in the section 7.3.1, confirming the ability of the low symmetric Au surfaces in templating the growth of unidirectionally aligned WS2 islands. However, the study in this Chapter only provides preliminary results and a systematical research on the alignment of
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TMDC islands on low symmetric TM surfaces needs further study, and moreover, the exploration on the alignment of TMDC on low symmetric inorganic substrates, such as Al2O3, is also needed as they are also commonly used substrates in CVD growth of TMDC materials.
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