Characterizing Graphene with Atomic Force Microscopy and PeakForce Tapping

Atomic-force microscopy is a type of scanning probe microscopy with sub-Ångstrom resolution. It is able to gather data on the mechanical and electrical properties of materials and surfaces by ‘feeling’ or ‘touching’ the surface with a cantilevered mechanical probe [1] controlled by Piezoelectric components.

An atomic force microscope (AFM) traditionally operates in one of two modes according to the way the tip moves: contact, or tapping. In contact mode (static mode), the tip is ‘drawn’ across the sample in contact with the surface, which is mapped by recording the deflection of the cantilever directly. If the surface is easily damaged by dragging the tip across it, then tapping mode is preferred as it avoids friction, adhesion and electrostatic forces by oscillating the tip, which then moves on and off the surface to give a high resolution image. Tapping mode is often used for soft or delicate samples (organic thin films or cells). AFM has found particular utility in the field of semiconductor physics for studying changes in local physical or electrical properties from alterations in atomic or molecular arrangement [2] [3]. In recent years, PeakForce Tapping^® has emerged as an alternative imaging mode that combines the advantages of tapping mode with linear force control and rich nanomechanical information.

Graphene is an allotrope of carbon that takes the form of a two-dimensional single layer hexagonal lattice of tightly bonded carbon atoms in a sp2 hybridised arrangement [4]. Graphene has a single atom thickness of around 3.45 Å and has extraordinary electrical and mechanical properties. It is a zero-overlap semi-metal (with both holes and electrons as charge carriers) with very high electrical conductivity, meaning that it has great potential in the electronics/semiconductor industry. In addition, graphene’s strong carbon bonds give it an ultimate tensile strength of 130,000,000,000 Pa, compared to 400,000,000 Pa for structural steel, or 375,700,000 for Kevlar. Graphene is also extremely light (0.77 mg/m²) and elastic giving it enormous potential in the development of new materials [5][6][7].

PeakForce Tapping

PeakForce Tapping is similar to tapping mode AFM and was introduced by Bruker to increase the resolution of AFM into the sub-Ångstrom range using piconewton (pN) force control [8][9]. In PeakForce Tapping, the probe periodically taps the sample and the pN-level interaction force is measured directly and instantly by the deflection of the cantilever. The direct and linear force control at the 10pN level protects both tip and sample better than in tapping mode where peak forces are already typically higher on flat samples and transient forces easily exceed 1nN on steep slopes due to the nonlinear nature of the feedback. Furthermore, the instant force detection at a single point in space, as opposed to the cycle averaged ‘amplitude’ used in tapping, allows straightforward operation at the sweet spot for resolution. Finally, as a complete force curve is acquired for every image pixel, complete and unambiguous nanomechanical information is automatically available, separating adhesion stiffness information, and doing so with highest sensitivity and without any need to operate in complex multifrequency modes.

Figure 1. PeakForce QNM^® modulus images of graphene on hexagonal boron nitride, revealing a transition to a commensurate lattice upon alignment with highly localized strain relief. See also Nature Physics 10, 451–456 (2014)

Characterizing Graphene

Graphene supported upon other materials such as hexagonal boron nitride (Hbn) can undergo marked changes in optical and electronic properties [9][10]. When a ‘crystal’ such as single layer graphene is subjected to a periodic potential, which means adapting to the shapes and bond angles of an adjacent surface (hBN), two results are possible [10]. Either the graphene can stretch to follow the periodicity of hBN, resulting in a commensurate state or there may be little adjustment by the graphene bonds, giving an incommensurate state [10] and simple Moire pattern.

The topological defects between the two commensurate phases are called solitons and domain walls. These are of interest because as molecular structures they store the ‘strain’ imposed on the graphene layer of the hBN/ graphene composite structure and could be responsible for considerable changes that have been observed in the electronic and optical properties of graphene-boron nitride. In an investigation [10] using AFM, scanning tunnelling microscopy (STM) and Raman spectroscopy, the strain distribution in graphene on hBN for different misorientation (skewed) phase angles between the two crystalline structures was examined. The results showed a regular hexagonal ‘Moire Pattern’, which results from the effect of the hBN on graphene, and this confers a change on the charge carriers in the graphene and hence a difference in electronic properties [10].

PeakForce Tapping allowed the local elastic constants, including the Young’s modulus for the graphene/hBN structure to be determined. This is remarkable in that the required deformation of the graphene sheet is on the angstrom level, far below conventional indentation depths. Using PeakForce Tapping on a Dimension FastScan^® system revealed the classic Moire pattern in both Young’s modulus and conductivity measurements for misoriented phase angles. The high spatial resolution and sensitivity of the Young’s modulus measurement also revealed a transition to a commensurate state upon alignment of the crystalline structures, leading to localized stress strain relief in sharp domain walls (see Figure 1).

Conclusion

The Dimension FastScan AFM with PeakForce Tapping has extended the suitability of atomic force microscopy to a much larger range of sample types and allowed the measurement of nanoscale property data at a consummately high resolution [8]. PeakForce Tapping has not only revealed the traditional Moire pattern in Young’s modulus and conductivity measurements for misoriented phase angles, but it has also revealed the transition to a commensurate state in the Young’s modulus measurement. This led to localized stress strain relief in sharp domain walls. In addition, the PeakForce Tapping enables the use of ScanAsyst^® imaging optimization for such research, enabling the scanning parameters to be automatically adjusted in real-time to optimize the image and make high resolution measurements much more rapidly.

References

1. Bertolazzi S. and Brivio J., et al., Exploring flatland: AFM of mechanical and electrical properties of graphene, MoS₂ and other low-dimensional materials, 2013 (April), Microscopy and Analysis 27(3):21-24 (AM).
2. Giessibl, F. J. , Advances in atomic-force microscopy, 2003, Reviews of Modern Physics 75 (3): 949–983.
3. G. Binnig, C.F. Quate, C. Gerber, Atomic force microscope, 1986, Phys Rev Lett, 56, pp. 930–933.
4. Geim, A. K. (2012). ‘Graphene Prehistory’. Physica Scripta T146: 014003.
5. Geisse, N. A. (July–August 2009). AFM and Combined Optical Techniques. Materials Today 12 (7–8): 40–45.
6. Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G. (27 August 2009). The Chemical Structure of a Molecule Resolved by Atomic-Force Microscopy. Science 325 (5944): 1110–1114.
7. Zhang, X.,  Liu, C., Wang, Z., Force spectroscopy of polymers: Studying on intramolecular and intermolecular interactions in single molecular level, Polymer, Volume 49, Issue 16, 28 July 2008, Pages 3353–3361.
8. Pittenger, B., Erina, N., Su, C., Application Note #128, Quantitative Mechanical Property Mapping at the Nanoscale with PeakForce QNM (Bruker 2012), Bruker Nano Surfaces Division.
9. Xue, J., Sanchez-Yamagishi, J., et al., Scanning tunnelling microscopy and spectroscopy of ultra-flat graphene on hexagonal boron nitride, 2011, Nature Materials, 10, 282–285.
10. Woods, C.R., Britnell, L., Eckmann, A., et al., Commensurate–incommensurate transition in graphene on hexagonal boron nitride, Nature Physics, published online: 28 April 2014, DOI:10.1038/NPHYS2954.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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