Subsurface imaging of stacking faults and dislocations in WS2 CVD grown flakes via Ultrasonic and Heterodyne Force Microscopy

The two-dimensional (2D) materials have multiple applications including optoelectronics [1] and fabrication of micro and nanoelectromechanical systems (MEMS and NEMS respectively), in particular, the layered transition metal dichalcogenide tungsten disulphide (WS2) already applied in the aerospace, automotive, and defence industries due to its high robustness. One of the WS2 synthetic methods is the Chemical Vapour Deposition (CVD) growth. By this method, the material is deposited creating complex structures formed by the orientation change of the individual layers of material making screw dislocations [1]. Therefore, analysing the structure under the surface is possible to understand how the spiral structures are stacked. We used SPM nanomechanical techniques combined with ultrasound - the Ultrasonic Force Microscopy (UFM) and the Heterodyne Force Microscopy (HFM) to identify the dislocations and faults between several stacked WS2 layers. The UFM images allowed to identify different areas with different stiffness which in the topographic AFM images do not show any particular features. The HFM images have better contrast when the difference frequency is equal to the contact resonance of the cantilever (54.4kHz). References [1] M.J. Shearer, L. Samad, Y. Zhang, Y. Zhao, A. Puretzky, K.W. Eliceiri, J.C. Wright, R.J. Hamers, S. Jin, Journal of the American Chemical Society, 139 (2017) 3496-3504. [2] F. Dinelli, M.R. Castell, D.A. Ritchie, N.J. Mason, G.A.D. Briggs, O.V. Kolosov, Philosophical Magazine A, Physics of Condensed Matter Structure Defects and Mechanical Properties, 80 (2000) 2299-2323. [3] M.T. Cuberes, H.E. Assender, G.A.D. Briggs, O.V. Kolosov, Journal of Physics D-Applied Physics, 33 (2000) 2347-2355.