Non-Contact Mode AFM
samples are fragile (low friction between graphene and hBN). T erefore, minimized interaction between the tip and sample is necessary to maintain the conditions of the sample during the characterization. T ere is no mechanical interaction, and the imaging is performed in the ambient atmosphere without the need for a vacuum. T us, non-contact mode imaging, as a benign characterization technique, can play a key role for characterization of devices fabricated by epitaxial growth of graphene on hBN and other two-dimensional materials. Preserving tip sharpness is also another advantage of using non-contact mode imaging. In addition to reducing the tip cost, a reliably sharp tip preserves the image quality and improves measurement repeatability. Although fi nding the correct tip-sample distance for true non-contact mode imaging is challenging and could depend heavily on user experience, using automated soſt ware enables these measurements to be performed with minimal user interaction. It also improves repeatability, productivity, and measurement throughput. A standard non-contact mode probe (PPP-NCHR) was used for this measurement, not the special or super-sharp tip usually required for this sort of high-resolution imaging. In non-contact mode, van der Waals interaction between tip and sample is used to image the moiré superlattice with high resolution and repeat- ability, even though the moiré pattern lattice constant (15 nm) is almost twice the nominal tip radius (7 nm).
Conclusion T e moiré superlattice of epitaxial graphene grown on hBN has been imaged in non-contact mode using a recently
developed automated AFM. Images were collected using a standard silicon probe with nominal tip radius of 7 nm. Images of the moiré superlattice revealed a lattice constant of ~15 nm, which was verifi ed against simulation values. T e newly developed automated AFM improves repeatability, productivity, and throughput. Automated non-contact mode imaging is therefore an effi cient characterization technique for quality control of devices fabricated by epitaxial growth, such as graphene/hBN-based devices.
Acknowledgement T e authors are immensely grateful to David Goldhaber- Gordon and Patrick Gallagher from Stanford University and Guangyu Zhang from Chinese Academy of Sciences for providing the graphene/hBN sample and fruitful discussions.
References [1] W Yang et al ., Nature Materials 12 ( 2013 ) 792 – 7 . [2] S Tang et al ., Scientifi c Reports 3 ( 2666 ) 2013 . [3] B Sachs et al ., Phys Rev B 84 ( 2011 ) 195414 – 195422 . [4] CR Woods et al ., Nature Physics 10 ( 2014 ) 451 – 6 . [5] GT Smith , Industrial Metrology: Surfaces and Roundness . Springer , New York , 2002 .
[6] G Binnig et al ., Phys Rev Lett 56 ( 9 ) 1986 ) 930 – 3 . [7] P Gallagher et al ., arXiv:1504.05253v1 [
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http://arxiv.org/pdf/1504.05253.pdf .
[8] A Zandiatashbar , NanoScientifi c , Winter ( 2014 ) 14 – 16 . [9] R Garcia and R Perez , Surf Sci Rep 47 ( 6 ) 2002 ) 197 – 301 .
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