Citation: Kim, S.; Moon, D.; Jeon, B.R.; Yeon, J.; Li, X.; Kim, S. Accurate Atomic-Scale Imaging of Two-Dimensional Lattices Using Atomic Force Microscopy in Ambient Conditions. Nanomaterials 2022, 12, 1542. https://doi.org/10.3390/ nano12091542 Academic Editor: Christophe Petit Received: 11 April 2022 Accepted: 29 April 2022 Published: 2 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Accurate Atomic-Scale Imaging of Two-Dimensional Lattices Using Atomic Force Microscopy in Ambient Conditions Sunghyun Kim 1,† , Donghyeon Moon 2,† , Bo Ram Jeon 1 , Jegyeong Yeon 1 , Xiaoqin Li 3,4 and Suenne Kim 2, * 1 Department of Applied Physics, Hanyang University, Ansan 15588, Korea; shdak6990@gmail.com (S.K.); brhu1018@gmail.com (B.R.J.); jaekyeong.yeon@gmail.com (J.Y.) 2 Department of Photonics and Nanoelectronics, Hanyang University, Ansan 15588, Korea; dhm227@hanyang.ac.kr 3 Center for Complex Quantum Systems, Department of Physics, The University of Texas at Austin, Austin, TX 78712, USA; elaineli@physics.utexas.edu 4 Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA * Correspondence: skim446@hanyang.ac.kr; Tel.: +82-31-400-5472 † These authors contributed equally to this work. Abstract: To facilitate the rapid development of van der Waals materials and heterostructures, scan- ning probe methods capable of nondestructively visualizing atomic lattices and moiré superlattices are highly desirable. Lateral force microscopy (LFM), which measures nanoscale friction based on the commonly available atomic force microscopy (AFM), can be used for imaging a wide range of two-dimensional (2D) materials, but imaging atomic lattices using this technique is difficult. Here, we examined a number of the common challenges encountered in LFM experiments and presented a universal protocol for obtaining reliable atomic-scale images of 2D materials under ambient en- vironment. By studying a series of LFM images of graphene and transition metal dichalcogenides (TMDs), we have found that the accuracy and the contrast of atomic-scale images critically depended on several scanning parameters including the scan size and the scan rate. We applied this protocol to investigate the atomic structure of the ripped and self-folded edges of graphene and have found that these edges were mostly in the armchair direction. This finding is consistent with the results of several simulations results. Our study will guide the extensive effort on assembly and characterization of new 2D materials and heterostructures. Keywords: AFM; TMD; LFM; graphene; transition metal dichalcogenides; atomic-scale imaging 1. Introduction In recent years, atomically thin layers and heterostructures of van der Waals (vdW) ma- terials prepared via chemical vapor deposition (CVD) or mechanical exfoliation/stacking have been intensively studied because of their unique properties and potential applications in quantum electronics and nanophotonics [1–4]. By changing the number of layers or controlling the twist angle between adjacent layers [5–7], remarkable quantum phases and properties have been discovered, including unconventional superconductivity [8,9], Hofstadter’s butterfly effect [10–12], Mott transition in graphene bilayers [7,13], and quan- tized exciton states in moiré crystals formed by twisting transition metal dichalcogenides (TMD) bilayers [14–17]. However, wrinkles and bubbles inevitably exist in stacked vdW bilayers [18–20], causing undesirable spatial variations in strains and disorder in moiré su- perlattices. Ultrafast laser nondestructive technology allows the study of strain, stress, and structural properties currently on the scale of tens to hundreds of nanometers [21–24]. The possibility of imaging atomic-scale strain-induced lattice distortion [25] or even controlling the twist angle using scanning probe methods [10] has been demonstrated. Nevertheless, such experiments remain very challenging in general. Nanomaterials 2022, 12, 1542. https://doi.org/10.3390/nano12091542 https://www.mdpi.com/journal/nanomaterials