Non-Contact Mode AFM
fi lter has been applied to the images. T e moiré superlattice of epitaxial graphene is easily distinguishable in all images in the form of a hexagonal pattern. T e hexagonal pattern was confi rmed by fast Fourier transform (FFT) of AFM images (insets). Grains of a second layer of hBN can be observed in the 500 nm image as bright islands. To characterize the height of the secondary grains, a line profi le of the 500 nm image was selected as illustrated in Figure 3d . T e height of the secondary grain is ~0.38 nm, which is similar to the thickness of mono-layer graphene. T e moiré pattern with similar lattice constant is observable on the secondary grains as well. It is consistent with the height measurement and indicates growth of graphene on the secondary grains.
Discussion T e moiré superlattice is charac-
Figure 3 : Non-contact mode images of graphene/hBN samples with (a) 500 nm, (b) 250 nm, and (c) 125 nm scan sizes. No fi lter has been applied to the images. The insets indicate Fourier transformation of the image taken over the entire image. (d) The profi le of the dashed line in the 500 nm image.
Figure 4 : The moiré pattern after an FFT fi lter was applied to one of the 250 nm images in Figure 3b . Line profi les for the red and green lines in the image are shown on the right side. The lattice constant of moiré pattern is measured to be ~15 nm.
was detected by automated soſt ware and followed by navigating the sample via light optical microscopy and automated imaging. T e images were collected in 500 nm, 250 nm, and 125 nm square sizes. First, lower magnifi cation images were collected, and then image size was decreased for higher magnifi cations. Figures 3 a– 3 c show non-contact mode images of graphene/ hBN samples with 500 nm, 250 nm, and 125 nm scan widths. No
2015 November •
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terized in Figure 4 by using one of the 250 nm images. A selective Fourier fi lter is applied to eliminate additional signal and enable easier character- ization of the pattern ( Figure 4 ). For this purpose, only the hexagonal peaks from the FFT of the image are kept, and other signals are eliminated. T e lattice constant of the moiré pattern was measured to be ~15 nm. T is is consistent with the reported simulation results of ~14 nm [4], which is two orders of magnitude larger than lattice constants of graphene and hBN. T e red and green lines indicate the axes of the superlattice and are used for measuring periodicity in either direction. T e peak-to-valley value for each line is below 0.07 nm. T e continuity of moiré pattern over the surface indicates successful growth of epitaxial graphene over hBN. T e continuity of moiré pattern over the secondary grains also indicates growth of graphene on hBN irrespective of the number of hBN layers, as seen in the 500 nm image in Figure 3a . It is consistent with the height measurement over the secondary
grain. T e height measurement of the secondary grains indicates a thickness equivalent to mono-layer graphene. T e presence of moiré pattern and thickness of ~0.38 nm confi rms that graphene is present on both primary and secondary grains. To the knowledge of the authors, imaging the moiré pattern of epitaxial graphene in non-contact mode has not until now been performed. T is is of great importance because the
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