A 2004 report on graphene transistors by Prof. Andre Geim and Prof. Konstantin Novoselov boosted Atomic Force Microscopy (AFM) studies of graphene.
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This article features the recent developments in AFM and its importance in graphene research.
Graphene: Wonder Material
Graphene is a zero bandgap semiconductor with high electrical conductivity, making it a potential candidate for advancing the semiconductor industry. It is a highly robust material, with a tensile strength of 130 GPa, the second most impressive property of graphene after its electronics characteristics.
Recent findings have shown that by adding a small amount of graphene, concrete can be strengthened by 30%. The thermal conductivity of single-layer graphene is in the range of 4800–5300 W/mK and is also highly light (0.77 mg/m2), giving it an enormous potential in developing novel textiles materials.
Need for Advanced Microscopy
The determination of topographic information and graphene layers is vital for the application of graphene-based materials. Some methods have been used to characterize graphene, such as transmission electron microscopy (TEM) and Raman spectroscopy. However, 2D Raman spectroscopy has a limitation of optical diffraction to a lateral resolution of only around 1 µm.
For instance, imagine if we have an all-in-one instrument that can characterize graphene film quality, such as morphology and roughness, mechanical, electrical properties, and magnetic fields. Atomic Force Microscopy is the name of that instrument.
Atomic Force Microscopy (AFM): Basic Overview
Atomic force microscopy is a powerful tool for imaging and characterizing various surfaces at the atomic level. The operating mechanism of conventional AFM depends on how the tip moves across the sample. There are two modes: contact and tapping.
In contact mode, the tip is 'drawn' across the sample in contact with the surface, and the deflection of the cantilever is directly recorded. However, there is a chance of damaging the surface in this mode, which can be avoided in tapping mode, in which the tip moves on and off the surface.
Advanced AFM to Study Properties of Graphene
Atomic Force Microscopy by Oxford Instrument has emerged as an advanced instrument that characterizes graphene's structural properties at the sub-nanometer scale. Cypher AFM by Oxford instrument can conduct repeated measurements of surface topography in three dimensions with high sensitivity. Additional information, such as magnetic, electrical, or nanomechanical information, can be captured concurrently, yielding a high amount of data from a single instrument.
Scanning Kelvin Probe Microscopy mode in Cypher AFM can measure the surface potential of materials to distinguish ABA and ABC graphene domain with nanometer scale lateral resolution. The conductive AFM mode available on Cypher AFM is sensitive to the material's local impedance, allowing metals and insulators to be probed.
Cypher AFM is also equipped with advanced AFM modes such as scanning capacitance microscopy (SCM), piezoresponse force microscopy (PFM), and Kelvin probe force microscopy (KPFM). These nano electrical characterization capabilities are more sensitive to factors like twist angle, strain, and the number of stacked layers than topographic imaging alone.
Prof. Guangyu Zhang and the team of researchers from the Chinese Academy of Sciences, Beijing, used Cypher AFM to quantify current fluctuations across a MoS2/graphene heterostructure as a function of relative twist angle. They utilized Cypher AFM's high resolution and sensitivity to confirm the expected 1.18 nm period of the moiré superlattice generated by the lattice mismatch.
Graphene offers an exciting possibility to control friction in cars and MEMS devices because of its ultra-low friction property. A team of researchers from Tsinghua University exhibited success in this area by stretching pressured graphene bubbles over holes in a substrate. The team conducted AFM friction and adhesion experiment using lateral force microscopy mode on Cypher AFM.
Cypher AFMs' exceptional force sensitivity and low noise level allowed precise measurements even in this ultralow friction domain.
Understanding stacking configuration in few-layer graphene is essential for modifying physical properties in few-layer 2D materials.
Prof. Zhiwen Shi and the researchers from Shanghai Jiao Tong University, China, demonstrated how Cypher AFMs could distinguish the ABA and ABC configurations in graphene domain using Scanning Kelvin Probe Microscopy (SKPM) and Scanning Capacitance Microscopy (SCM) modes. Their AFM image shows a clear contrast between ABA and ABC domains.
Moreover, the conductive AFM technique on Cypher AFMs offers significantly higher performance to measure current on twisted bilayer graphene. Prof. Qunyang li and the researchers from Tsinghua University have shown typical current images measured on graphene domains with different twist angles.
Further Development in the Instrument
Oxford Instrument has two significant AFM accessories other than Cypher AFM mode: Jupiter XR AFM and MFP-3D AFM.
Jupiter XR AFM is precisely designed for large-area graphene samples. Its AM-FM mode combines the advantages and benefits of normal tapping mode with nanomechanical property mapping. The Variable Field Module 4 (VFM4) is suited for magnetic force microscopy, where the sample is needed to study under a magnetic field.
Contact Resonance Viscoelastic Mapping Mode for MFP-3D AFMs offers a quantitative mapping of both viscoelastic and elasticity materials. The Conductive AFM probe holder in MFP-3D AFM mode provides conductive AFM imaging and I-V measurement.
Conclusion
Atomic Force Microscopy (AFM) by Oxford Instrument is the most powerful and first of its kind instrument designed to achieve high-resolution performance with a noise floor 50% lower than other commercial AFM. Moreover, CAFM, PFM, and KPFM modes available in AFM can characterize the domain structure of moiré patterns as well as the variation in electrical characteristics with twist angle. Undoubtedly, AFM plays a crucial role in continuing the discovery of novel properties of graphene.
Continue reading: Using AFM for Nanostructure Analysis.
References and Further Reading
Oxford Instruments (2021) AFM Systems and Accessories. [online] Available at: https://afm.oxinst.com/products/
M. Liao et al (2018) Twist angle-dependent conductivities across MoS2/graphene heterojunctions. Nat. Commun. 9, 4068. Available at: https://doi.org/10.1038/s41467-018-06555-w
S. Zhang et al (2019) Tuning friction to a superlubric state via in-plane straining. Proc. Natl. Acad. Sci. U.S.A. 116, 24452. Available at: https://doi.org/10.1073/pnas.1907947116
G. Chen et al (2019) Evidence of a gate-tunable Mott insulator in a trilayer graphene moiré superlattice. Nature Physics 15, 237–241. Available at: https://www.nature.com/articles/s41567-018-0387-2
Zhang, Shuai et al (2020) Abnormal conductivity in low-angle twisted bilayer graphene. Science advances 6, no. 47. Available at: https://www.science.org/doi/10.1126/sciadv.abc5555
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