There has been rapid growth of interest in devices based on 2D materials. Defects in these 2D layers can affect their electronic and optoelectronic properties, especially in transitional metal dichalcogenides (TMDs). Recently, there has been a focus on characterizing atomic defects to control and understand device performance.
This tutorial series focuses on the use of atomic force microscopy (AFM) for routine detection and characterization of atomic-scale defects in 2D materials. Traditional approaches, such as scanning tunneling microscopy and scanning transmission electron microscopy, offer exceptional spatial resolution, but can pose challenges in accessibility and practicality for routine measurements, especially those required for process control. The series highlights a variety of AFM-based techniques for probing atomic defects in 2D materials. Presentations will combine representative results from a range of material systems with practical considerations for performing and interpreting these measurements, aiming to help researchers implement these techniques in their own laboratories.
Agenda
Thursday, November 12: CAFM of atomic defects in TMDs
Matthew Rosenberger, Notre Dame
Thursday, December 10: LFM of atomic defects in 2D materials
Matthew Rosenberger, Notre Dame
Wednesday, January 20: CAFM imaging and electronic spectroscopy of atomic defects in 2D materials
Mehmet Baykara, University of California, Merced
Thursday, February 11: Autonomous defects detection & back-contact free CAFM
Umberto Celano, Arizona State University
Thursday, February 25: Higher eigenmode imaging of defects
Nikolai Severin, Humboldt University of Berlin
Thursday, March 11: Title to be determined
Jairo Valasco, University of California, Santa Cruz
Thursday, March 25: Understanding the physical mechanism behind atomic resolution imaging in conductive atomic force microscopy in air
Mario Lanza, National University of Singapore
Thursday, April 8: CAFM of defects in hBN
Matthew Rosenberger, Notre Dame
Abstracts
Conductive atomic force microscopy for routine defect characterization of two-dimensional semiconductors
Routine defect characterization is a critical capability for understanding defect-property correlations, optimizing growth of two-dimensional (2D) materials, and understanding the impact of defects on devices. High-throughput optical methods for defect characterization, such as Raman spectroscopy, are useful for graphene, but are often insufficiently sensitive to defects in some other 2D materials, such as transition metal dichalcogenides (TMDs), particularly for defect densities of about 1012 cm-2 or less. Typical methods for directly detecting defects at the atomic scale, such as scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM), are effective, but they are relatively slow and often require arduous sample preparation. There is a need for 2D material defect characterization techniques that are routine, fast, and reliable. Here, we show that conductive AFM (CAFM) can locate and differentiate the defects in a way similar to STM by comparing conductive AFM and STM on the same region of a flux-grown TMD crystal1. We will also present measurements on TMD monolayers from different sources. We will share practical considerations for carrying out these measurements, such as sample preparation, tip selection, and choice of bias. We will also discuss a statistical framework for presenting defect density results and discuss situations in which these measurements are most effective. Overall, our work establishes conductive AFM as a higher-throughput alternative to STM for defect quantification that one can use as a routine characterization tool for 2D semiconductors.
(1) Xu, K.; Holbrook, M.; Holtzman, L. N.; Pasupathy, A. N.; Barmak, K.; Hone, J. C.; Rosenberger, M. R. Validating the Use of Conductive Atomic Force Microscopy for Defect Quantification in 2D Materials. ACS Nano 2023, 17 (24), 24743–24752. https://doi.org/10.1021/acsnan....
Lateral force microscopy of atomic defects in two-dimensional materials
Rapid and quantitative characterization of atomic defects in two-dimensional (2D) materials is crucial for understanding and improving 2D material performance. However, such defect metrology remains challenging due to limitations of existing characterization methods, which are generally destructive, slow, or insufficiently sensitive. Here, we describe the use of lateral force microscopy (LFM) to directly observe atomic defects in 2D materials1. Through a direct comparison with conductive atomic force microscopy, we show that LFM can observe intrinsic point defects in semiconducting 2D materials. As a purely mechanical technique, LFM has the advantage of not requiring a conductive pathway, which makes it useful on insulating 2D materials and semiconducting 2D materials that are on an insulating substrate without electrical contact. We demonstrate defect measurements on hexagonal boron nitride as well as 2D semiconductors on their growth substrates and within fabricated transistors. Finally, we discuss a few limitations to be aware of and guidance on how to practically implement this technique. This work demonstrates that LFM is a simple and quantitative tool for understanding defects in 2D materials.
(1) Yang, Y.; Xu, K.; Holtzman, L. N.; Yang, K.; Watanabe, K.; Taniguchi, T.; Hone, J.; Barmak, K.; Rosenberger, M. R. Atomic Defect Quantification by Lateral Force Microscopy. ACS Nano 2024, 18 (9), 6887–6895. https://doi.org/10.1021/acsnan...
(2) Yang, Y.; Xu, K.; Peña, T.; Neilson, K.; Zheng, X.; Hoang, A. T.; Yang, K.; Hennighausen, Z.; Zhang, T.; Holtzman, L. N.; Nattoo, C. A.; Hone, J. C.; Barmak, K.; Kong, J.; Mannix, A. J.; Pop, E.; Rosenberger, M. R. Nondestructive Atomic Defect Quantification of Two-Dimensional Materials and Devices. ACS Appl. Mater. Interfaces 2026, 18 (6), 10161–10170. https://doi.org/10.1021/acsami...
CAFM imaging and electronic spectroscopy of atomic defects in 2D materials
Thanks to their outstanding physical characteristics, two-dimensional (2D) materials have the potential to revolutionize next-generation electronics. 2D materials unavoidably contain atomic defects, the effects of which need to be evaluated to use them effectively in devices. Although the majority of applications utilizing 2D materials will take place under ambient conditions, conventional tools for characterizing materials with atomic resolution operate under strict environmental conditions imposed by ultrahigh vacuum (UHV). This critically limits the usefulness of the information provided by UHV-based methods such as scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM).
Our lab demonstrated the capability of true atomic-resolution imaging via conductive atomic force microscopy (CAFM) under ambient conditions [1]. Here, we will focus on atomic-resolution imaging of defects in various 2D electronic materials: (i) the prototypical transition metal dichalcogenide (TMD) molybdenum disulfide (MoS2) [2], (ii) a 2D dielectric (indium oxide, InO2) [3], and (iii) ultrathin crystals of tungsten carbide (WC), covered by monolayer graphene. We additionally demonstrate the capability of electronic spectroscopy on individual defects under ambient conditions, enabling chemical identification [4]. Our findings highlight CAFM as a powerful tool for atomic-resolution imaging and electronic spectroscopy of defects in 2D materials under ambient conditions.
[1] S.A. Sumaiya, J. Liu, M.Z. Baykara, ACS Nano 16, 20086 (2022)
[2] B. Kumral, et al. Nature Communications 16, 7105 (2025)
[3] F. Turker, et al. Advanced Materials 38, e16133 (2025)
[4] J.R. Evans, D.A. Garibay, A.N. Kuhls, M.Z. Baykara, ACS Omega, in press (2026)
Autonomous defects detection & back-contact free electrical atomic force microscopy in 2D materials
Electrical scanning probe microscopy is now a cornerstone of materials characterization with impact on the broader community of materials scientists, engineers and nanotechnologists. First, we report a rapid, generalizable pipeline that couples conductive atomic force microscopy (CAFM) imaging with a physics-aware machine-learning (ML)-based workflow to automatically detect, classify, and quantify defects from microscopic to mesoscopic scales in 2D materials. Second, we discuss how to overcome one of the most limiting design constraints that prevent electrical AFM adoption for in-line wafer and panel metrology, i.e., the requirement for electrical back-contact. This reliance on back-contact constitutes a significant roadblock, preventing the automated deployment of electrical AFM in complex manufacturing environments. To address the limitations imposed by physical back-contacts, we present a novel solution that replaces the mechanical connection with a low-energy electron beam focused on the sample surface near the AFM probe. We refer to this configuration as electron-beam excited AFM (EB-AFM). This approach yields results comparable to conventional techniques while unlocking applications previously inaccessible due to geometry or material constraints. Here, we demonstrate the efficacy of this strategy for contact-free electrical characterization on various systems including, as-grown, patterned and area-selectively grown 2D materials.
More information coming soon
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Understanding the physical mechanism behind atomic resolution imaging in conductive atomic force microscopy in air
Atomic scale characterization of nanomaterials is critical to understanding key properties such as atomic structure and density of native defects. Experiments of this type have traditionally been carried out using transmission electron microscopy and scanning tunneling microscopy, but both are complex and expensive: the former due to the sample preparation procedures, and the latter due to the use of ultrahigh vacuum. A few recent studies have demonstrated that conductive atomic force microscopy (CAFM) operated in air can resolve the atomic structure of gold, graphite, and a few two-dimensional layered materials, as well as detect the presence and density of atomic-sized defects. These groundbreaking observations significantly increase the throughput and reduce the cost of such experiments, and they may have a major positive impact not only in academic investigations but also in semiconductor-industry reliability tests. However, the physical mechanisms enabling such observations are still not well understood, and many scientists have been unable to reproduce such experiments; this has raised some skepticism. In this talk, I will analyze the status of this technique, and clarify under which measuring conditions atomic-resolution imaging becomes feasible using CAFM in air, highlighting the critical factors that affect reproducibility.
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