Conductive Atomic Force Microscopy as a Routine, Fast and Reliable Method for 2D Material Atomic Defect Characterization

Defects are very influential on material behavior in terms of mechanical, electrical, optical and chemical properties. In two-dimensional materials, defects are inevitable because of impurities[1], thermodynamic equilibrium, or non-optimized growth conditions. Previous research has revealed that defects could act as carrier donors, recombination centers, or electrochemical reaction sites, thus inducing n-type doping, trap states, or increasing the material chemical activity[2]. However, the quantitative effect of defects on material behavior remains insufficiently studied, largely because we lack a simple and routine method to characterize the defect densities. The common defect detection methods, such as Transmission Electron Microscopy (TEM), Scanning Tunneling Microscopy (STM), and Raman Spectroscopy, have practical disadvantages rendering them not ideal for routine defect quantification. Specifically, TEM requires complicated sample preparation, and frequently induces additional defects during the detection process itself, causing complex interpretation of experimental results. STM relies on strict environmental conditions and time-consuming experiments, making it difficult to operate quickly on different samples. Raman spectroscopy does not locate atomic defects accurately at the nanoscale, and is not sensitive to defects at low density levels.<br/><br/>Here, we show that conductive atomic force microscopy (CAFM) is a swift, simple, reliable and routine technique to characterize atomic defects. CAFM operates with ease under ambient environment, and does not require complicated sample preparation. We conduct CAFM measurement on MoSe₂ and WSe₂ bulk samples, finding that the sample electrical bias significantly influences defect appearance. Optimizing the sample bias enables CAFM to differentiate multiple types of atomic defects by their distinct appearance. We also show that CAFM can locate defects with true atomic resolution. To show the reliability of CAFM, we conduct both STM and CAFM defect measurement on MoSe₂ and WSe₂ crystals from the same growth batch, i.e., crystals with the same growth conditions, and compare the results to each other. We show that both CAFM and STM consistently reveal similar atomic defects. Also, both techniques reveal an additional kind of 10-nm-scale “large” defect. Such consistency between CAFM and STM indicates that the two techniques reveal defects in a similar fashion. Quantitatively, to determine the precision of our density results, we use a statistic model applying Poisson distribution to defect locations because the defect locations are random, which is confirmed by Moran’s Index based on the locations of more than 2000 defects measured by CAFM in a 250nm×240nm area. As a result, the density results of atomic defects and 10-nm-scale “large” defects obtained by CAFM and STM match with each other closely. Therefore, the qualitative (defect type differentiation) and quantitative (density results) consistency between CAFM and STM on TMD crystals from the same growth batch shows that CAFM is a reliable, accurate and precise technique for detecting defects in two-dimensional materials. Realization of a routine defect detection method opens many opportunities for exploring the influence of defects. The related fields can range from material science to electronics, chemical engineering and beyond.<br/><br/>References<br/>1. Jiang J, Xu T, Lu J, Sun L, Ni Z. Defect Engineering in 2D Materials: Precise Manipulation and Improved Functionalities. Research. 2019;2019:2019/4641739. doi:10.34133/2019/4641739<br/>2. Qiu H, Xu T, Wang Z, et al. Hopping transport through defect-induced localized states in molybdenum disulphide. Nat Commun. 2013;4(1):2642. doi:10.1038/ncomms3642