Strain Manipulation and Generalized Defect Characterization of 2D Materials with Atomic Force Microscopy

2D materials are exciting materials for exploring and leveraging quantum phenomena. The quantum electronic and optical properties of 2D materials depend strongly on defects and strain. Defects can be a source of unwanted heterogeneity or defects can introduce a desired effect. Random strain fluctuations within a 2D material can introduce inhomogeneity that obscures quantum phenomena while engineered strain profiles can lead to novel material behaviors. Clearly, understanding and controlling defects and strain in 2D materials is essential for controlling quantum phenomena in these materials. In the first part of this talk, I will describe our recent work on developing generalizable approaches to locating, quantifying, and differentiating defects in 2D materials with atomic force microscopy (AFM). In particular, I will show that conductive AFM locates the same defects in transition metal dichalcogenides as scanning tunneling microscopy. I will also present data demonstrating that lateral force microscopy (LFM), a purely mechanical technique, can image certain types of defects in both transition metal dichalcogenides and hexagonal boron nitride. We have demonstrated the ability of LFM to locate defects in monolayers on a variety of substrates (SiO₂, sapphire, and polymer) and in materials grown with different methods (flux-grown bulk crystals and chemical vapor deposition monolayers). Our results indicate that AFM-based techniques are viable for routine defect characterization of 2D materials, which is important for quantum material development. In the second part of this talk, I will discuss our work on manipulating the strain distribution within 2D materials. Our approach is to use AFM to indent 2D materials which are placed on top of polymer films. We previously showed that this technique can introduce single photon emitters in monolayer WSe₂. The focus in this talk will be on understanding the limits of nanoindentation for introducing strain. By changing the tip geometry (shape and size) and the indentation depth, we can introduce various levels of strain into the materials, which substantially alters the optical properties. We can cause localized photoluminescence shifts of up to 200 meV in monolayer WSe₂. Our work demonstrates a general technique for modifying the strain state of 2D materials in a controllable way, which may allow access to new quantum phenomena in 2D materials.

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