Probing two-dimensional materials by advanced atomic force microscopy

Two-dimensional (2D) materials and their van der Waals heterostructures exhibit extremely rich mechanical, electrical, optical, magnetic and other novel physical properties, presenting abundant physics and potential applications. The intrinsic states of 2D materials, such as doping, twisting, and defect states, as well as the external environment, such as dielectric constant, stress, and electromagnetic fields, usually exhibit nanoscale heterogeneity, which can deeply affect their physical properties and corresponding nanodevice performance. Therefore, in-depth exploration of the novel properties of 2D materials and revealing the physical origins behind them often require nanoscale characterization techniques. Advanced atomic force microscopy (AFM) techniques, including nano-mechanical, nano-electrical, nano-optical, nano-magnetic and nano-etching modes, can measure the unique structures and novel properties of 2D materials at the nanoscale and investigate the structure-property relationships. In this paper, we first introduce the basic principles of AFM-based nano-mechanical, nano-electrical, nano-optical, nanomagnetic and nano-etching modes. We then summarize the research progress of these advanced AFM techniques in characterizing the novel properties of 2D materials. This nanoscale multi-dimensional characterization and manipulation technology has played a key role in promoting basic research and applied technologies based on 2D materials. Finally, we prospect the future innovative directions and development potential of advanced AFM techniques. Firstly, advanced AFM utilizing 2D quantum materials as the tip will continue to emerge in the future, opening up new windows for measuring the quantum properties of 2D materials. Secondly, combining ultrafast optical spectroscopy with near-field optics can realize ultrafast scanning near-field optical microscopy, providing new insights into the structure-property relationships and electron dynamics in 2D materials. Thirdly, emerging advanced AFM can simultaneously measure and image the mechanical, optical, and electrical properties of 2D materials at the nanoscale, improving the measurement efficiency and accuracy for the study of complex material systems. Lastly, machine learning will enable AFM platforms to have automated operational capabilities and precise data analysis abilities in the study of various 2D materials. These continually improving and evolving advanced AFM techniques will play a greater role in characterizing the novel properties of 2D materials and other interfacial systems.