Atomic Defect Measurement by Lateral Force Microscopy

Defects in two-dimensional (2D) materials impact important material properties, such as doping in semiconductor and localized quantum emission in semiconductors and insulators. A routine defect measurement technique is critical for understanding defect-property relationships and therefore essential for optimizing material performance. However, progress in understanding these critical relationships is hindered by the limitation of the existing defect characterization techniques. The existing techniques, such as conductive atomic force microscopy (CAFM) and scanning tunneling microscopy, cannot distinguish the out-of-plane defect location and require an electrically conductive sample for defect measurement in 2D materials without introducing additional defects. Here, for the first time, we demonstrate that lateral force microscopy (LFM) in ambient conditions can observe atomic defects in 2D materials. We improve the sensitivity of LFM through the consideration of cantilever mechanics and demonstrate the importance of minimizing tip-sample contact area for defect measurements. The defects observed with LFM are confirmed to be atomic defects based on a direct comparison of LFM with CAFM. Based on the Prandtl-Tomlinson model, LFM only interacts with the material surface. Thus, by comparing directly with CAFM defect measurement at the same region on MoSe₂, we found that the surface selenide site defects are conductive defects. We also show defect measurements on other transition metal dichalcogenides, WSe₂ and MoS₂. Due to the purely mechanical nature of LFM, LFM does not require a conductive pathway for defect measurement. Thus, LFM can also observe atomic defects on insulating 2D materials, such as hexagonal boron nitride (hBN). We show that LFM can differentiate high-quality hBN and low-quality hBN by measuring the intrinsic defect density in the hBN. To further establish the utility of LFM defect measurements, we introduced post-growth defects by annealing hBN in air and showed that increasing annealing temperature increases surface defect density in hBN exponentially, consistent with the Arrhenius equation. Our demonstration of a purely mechanical defect characterization technique not only provides new insight into the defects in electrically conductive materials but also enables routine defect-property determination for insulating materials. This new defect characterization technique will accelerate materials research in defect engineering.