Chapter 1 Research background on two-dimensional (2D) materials growth
1.1 Chemical vapor deposition (CVD) growth of single crystalline graphene on a substrate
1.1.2 Graphene growth via seamless stitching on single-crystal substrates
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Figure 1.6 Synthesis of single-crystal metal substrate by thin film growth technique and as-grown graphene. (a) Cross-sectional SEM image of the Ge(110) film on the Si(110) wafer.68 (b) SEM image of graphene islands on the Ge(110) surface. (c) Optical image of graphene grown on a 5.08 cm Ge/Si wafer. (d) High-resolution TEM image of the single-crystal graphene film. (e) Optical image of a 4- inchs single-crystal Cu(111) thin film on sapphire.77 (f) STEM image of the interface regions of Cu/sapphire. (g) Optical image of graphene grown on the Cu/sapphire substrate. (h) Optical image showing the same orientation of graphene islands.
Except for the deposition methods discussed above, single crystalline Cu substrates have also been obtained through annealing of polycrystalline Cu foils. In 2015, Nguyen et al. obtained a 6×3 cm2 single crystalline Cu foils by repetitively annealing and chemical-mechanical polishing a polycrystalline Cu foil and found that ~98% graphene islands grown on the obtained Cu(111) foil were unidirectionally aligned, as shown in Figure 1.7(a-b).78 Figure 1.7(c) displayed the coalesced two misaligned (left) and aligned (right) graphene islands after UV irradiation treatment. Two grain boundary lines can be clearly observed for the coalesced two misaligned graphene islands and each grain boundary corresponds to a sharp concave corner. On the contrary, no grain boundary was observed in the coalesced aligned graphene islands and the concave corner between the two graphene islands become dull during growth. It is worth noting that such dull concave corner has been proved to be an indicator of the seamless stitching of well aligned 2D materials.79
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Figure 1.7 Synthesis of single-crystal Cu(111) substrate by annealing methods and as-grown graphene.
(a) Optical image of 6×3 cm2 single-crystal Cu foil with aligned graphene islands grown on it.78 (b) Optical image of unidirectional graphene islands grown at point 4 in (a). (c) Optical microscopy images of a polycrystalline graphene island and a single crystalline graphene island formed through coalescence after UV treatment. (d) Schematic of the continuous production of single-crystal Cu(111) foil.69 (e) Cu(111) foils with various graphene coverages. (f) Optical image of graphene covered Cu(111) foil marked as 2 in (e). (g) Schematic of a Cu foil suspended by a quartz holder. (h) Photograph of the obtained single-crystal Cu(111) foil. (i) SEM image of graphene islands on the obtained Cu(111) foil.67
In 2017, Xu et al. reported a temperature-gradient-driven annealing strategy to transform a polycrystalline Cu foil into a single crystalline Cu(111) foil, as demonstrated in Figure 1.7(d).69 The Cu polycrystalline foil was fed into the heating zone that is located at the central area of the furnace tube continuously by two rollers, and as a result, a temperature gradient was induced at the Cu foil near the heating area. Molecular dynamics (MD) simulations proved that grain boundaries in the Cu foil can be driven towards the high temperature zone. Using this continuous annealing method, single crystalline Cu(111) foils with a size up to (5×50) cm2 were achieved successfully. Furthermore, the obtained single crystalline Cu(111) foils have been used as substrates to synthesize meter-sized single crystalline
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graphene films by the seamless coalescence of unidirectionally aligned graphene islands on them.
Figure 1.7(e) shows the obtained Cu(111) foils with different graphene coverages. To demonstrate the alignment of graphene islands on it, the border of a graphene film (marked as 2 in Figure 1.7(e)) is shown in Figure 1.7(f). One year later, Ruoff’s group also realized the production of single crystalline Cu(111) foils with a size of 32 cm2 by the contact-free annealing of commercial polycrystalline Cu foils, and similarly graphene islands grown on the obtained Cu(111) foils were also proved to be unidirectional, as shown in Figure 1.7(g-i).67
In 2019, high-index transition metal (TM) surfaces were found to show a great potential for the growth of unidirectionally aligned 2D material islands.80-84 The synthesis of TM foils with high-index surfaces attracts great attention in the field of epitaxial growth of 2D materials. By a seeded growth technique, Wu et al. realized the production of more than 30 kinds of single crystalline high-index Cu surfaces with a size of about (30×20) cm2, and Figure 1.8(a) shows several representative samples.85 The possibility of using such high-index surfaces as substrates for the epitaxial growth of graphene was also explored, as shown in Figure 1.8(b), graphene islands grown on Cu(112), Cu(113), Cu(133) and Cu(223) surfaces all showed unidirectional alignment. Besides, this seeded abnormal grain growth method was also proved to be suitable for the growth of single crystalline Ni foils with high-index surfaces, which may be able to serve as substrates for the growth of wafer-scale multi-layer graphene films with a high crystallinity in the future. Right after that, Li et al. also successfully transformed commercial decimeter-sized polycrystalline Cu foils into a series of high-index Cu single crystals through a strain-engineered anomalous grain growth technique (see Figure 1.8(c-d)), and the growth of unidirectionally aligned graphene islands on high-index Cu surfaces was also realized (see Figure 1.8(e)).86
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Figure 1.8 Synthesis of single-crystal high-index Cu substrates and as-grown graphene. (a) Optical image of the eight representative high-index single-crystal Cu foils with a size of 35 × 21 cm2 after mild oxidation.85 (b) SEM images of unidirectional graphene islands on four representative high-index single-crystal Cu foils. (c) Photograph of the high-index Cu foils synthesized by strain-engineered annealing.86 (d) Pie chart of the proportions of obtained high-index single-crystal Cu foils in 133 pieces.
(e) EBSD map of the obtained Cu(311) foil and SEM image of well-aligned graphene islands grown on it.