Chapter 1 Research background on two-dimensional (2D) materials growth
1.2 CVD growth of single-crystal hexagonal boron nitride (hBN) on a substrate
1.2.2 HBN growth via seamless stitching
The synthesis of WSSC hBN films via the route (ii), i.e., seamless coalescence of unidirectionally aligned hBN islands, is also challenging because no low-index TM surfaces can template the growth of unidirectionally aligned hBN islands. In general, there are two antiparallel alignments of triangular hBN islands on both the Cu(111) and Cu(110) surfaces and four dominant orientations of hBN on the Cu(100) surface.113,119-122 Until 2016, Li et al. for the first time observed the growth of hBN islands with only orientation on both Cu(102) and Cu(103) surfaces, paving a promising route for the synthesis of WSSC hBN films.80 After that, great progress has been made for unidirectional hBN islands growth together with WSSC hBN synthesis on high-index TM surfaces.
In 2019, Wang et al. reported the fabrication of 100 cm2 single crystalline Cu foils that show a tilted angle of about 1º deviating from the ideal Cu(110) surface, and further realized the growth of WSSC hBN by seamless coalescence of unidirectional hBN islands on the obtained vicinal Cu(110) surfaces, as shown in Figure 1.12(a-b).84 The coalescence of two aligned hBN islands was examined by different techniques. The polarized second-harmonic generation (SHG) mapping in Figure 1.12(c) confirms the seamless stitching of the two aligned hBN domains, as there is no boundary line in the coalescence area. On the contrary, in Figure 1.12(d), two dark lines representing grain boundaries are
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presented in the coalescence area of two misaligned hBN islands. To characterize the quality of the stitching area between the two aligned hBN domains at atomic scale, the grown hBN was transferred onto a single-crystal graphene TEM grid. It is known that moiré patterns caused by overlapping of two periodic lattices are very sensitive to the rotation angle of the two lattices, therefore they are widely used to identify the orientation of the two lattices. From the high-resolution TEM image in Figure 1.12(f) that was taken from the concave corner of the joint area between two aligned hBN domains, a consistent moiré pattern is observed, implying the exact same orientation of the two hBN domains. Besides, the seamless stitching of two aligned hBN islands was also confirmed by etching and UV treatment in this work. In the next year, this research team successfully synthesized WSSC Cu foils with various high- index surfaces, as introduced in Figure 1.8, and the growth of unidirectional hBN domains was verified on some of the high-index Cu surfaces.
Figure 1.12 The growth of WSSC hBN on a vicinal Cu(110) surface.84 (a) Optical image of the obtained Cu foil after oxidation showing the vicinal Cu(110) single crystal has an area of 100 cm2. (b) SEM image of unidirectional hBN domains on the obtained substrate. (c, d) Polarized SHG mapping of two aligned (left) and misaligned (right) hBN domains. (e) Low-magnification TEM image of the concave corner in the joint area of two aligned hBN domains on a monolayer single-crystal graphene film. (f) Representative HRTEM image of a uniform hBN/graphene moiré pattern at the concave corner. The orange and blue in the insert represent the diffraction pattern of hBN and graphene, respectively.
Right after Wang’s study, a statistical study about the alignment of hBN islands on more than 100 different high-index Cu surfaces was reported, and it turned out that on 30 vicinal Cu surfaces, hBN
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islands show only one orientation,123 further confirming the great potential of vicinal Cu surfaces for templating unidirectional hBN growth.
The epitaxial growth of WSSC hBN monolayer was also successfully realized on a 2-inch Cu(111) wafer with step edges (see Figure 1.13(a-b)).124 By annealing a 500-nm-thick polycrystalline Cu film on the c-plane of a sapphire wafer at a temperature range of 1040 ~ 1070 °C, a single crystalline Cu(111) film with atomic step edges on surface was produced (see Figure 1.13(d-e)). The grown hBN islands on the obtained Cu(111) surface were found to be unidirectionally aligned with an orientation consistency ratio of more than 99.6% (see Figure 1.13(c)). STM image in Figure 1.13(f) shows that the consistent moiré patterns even exist over the Cu step edges, and atomic-scale STM image in Figure 1.13(g) shows that the lattice constant of the obtained hBN is 0.25 nm, which agrees well with the theoretical value.125
Figure 1.13 The growth of WSSC hBN on a highly stepped Cu(111) surface.124 (a-b) Schematic and photography of as-grown two-inch hBN film on a Cu (111)/sapphire wafer. (c) Optical image of unidirectional hBN islands on the obtained Cu(111) surface. (d-e) STM images of hBN/Cu showing the obtained Cu(111) surfaces are highly stepped. (f) STM image showing moiré pattern of hBN on the obtained Cu(111) surface. (g) Atomic-scale STM image of the grown hBN film.
Except for solid single crystalline TM substrates, the growth of WSSC hBN film by seamless stitching has also been achieved on liquid Au substrates (see Figure 1.14(a-e)).126 It was proposed that the electrostatic interaction between B and N atoms enables the rotation and alignment of the hBN islands and then results in the seamless stitching (Figure 1.14(f)). As inferred from Dong’s study, the grain boundary energy of graphene is a critical factor for the coalescence of graphene islands on liquid
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Cu surface,90 the grain boundary energy of hBN may also affect the coalescence behavior of hBN on liquid Au surface. Compared to graphene, the grain boundary energies of hBN are distinctively higher.127,128 From Figure 1.14(g), we see that there is only one deep local minimum in the grain boundary energy profile for hBN, indicating that hBN islands on liquid Au surface tend to seamlessly coalesce into a single crystal from the viewpoint of minimizing the grain boundary energy.120 Moreover, it should be noted that the hBN flakes on the liquid Au surface show a round shape, which is different from the triangular hBN islands grown on solid TM substrates. Actually, graphene islands with a circular shape have also been observed on liquid Cu surface,129,130 whereas the mechanism of the formation of circular 2D islands remains unclear until now.
Figure 1.14 WSSC hBN grown on a liquid Au substrate. (a) Photograph of a WSSC hBN film transferred on a SiO2-Si wafer.126 (b-e) SEM images showing the process of hBN film growth on the liquid Au surface. (f) Illustration for the growth of single crystalline hBN film by self-collimated grains.
(g) Formation energy profile of grain boundaries of hBN as a function of the misaligned angles of hBN domains.120
In summary, the synthesis of WSSC hBN films via the route (i) is still a great challenge, while for the route (ii), choosing a proper substrate is of critical importance. To date, high-index TM and liquid Au surfaces show great potentials in synthesizing WSSC hBN films.
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