The two-dimensional (2D) materials have multiple applications in optoelectronics, energy storage, gas- and bio-sensors, and photocatalysis and solar energy conversion, to mention a few. In particular, the atomically layered transition metal dichalcogenides (TMDCs), such as WS2 and WSe2 are promising candidates due to their unique electronic and optical properties, ease of manufacturing,
mechanical robustness, low toxicity, as well as composed of relatively abundant elements on Earth . For this research, we study WSe2 and WS2 samples growth individually or simultaneously by the Chemical Vapour Deposition (CVD) on the Si or SiOx surfaces. These materials form complex trigonal and hexagonal faceted structures formed by the individual layers of material, typically with a screw dislocation in the centre of the crystallites defining the growth process .
To identify dislocations and faults between several stacked WSe2 and WS2 layers we have used nanomechanical mapping in SPM by combining the Atomic Force Microscopy (AFM) with the ultrasonic vibration – namely, the Ultrasonic Force (UFM) and the Heterodyne Force (HFM) Microscopies. In UFM/HFM amplitude modulated ultrasonic vibrations at frequency up to 10 MHz are applied to the sample resulting in its displacement of few nm normal to its surface. The AFM tip contacting the sample then produces dynamic force at the oscillation frequency that propagates to the inner layers of the 2D materials. The hidden subsurface features such as dislocations and stacking faults have a compressibility that differs from one of the perfect sample that, in turn, modifies the dynamic mechanical impedance sensed by the AFM tip. This is detected as cantilever deflection at the modulation frequency, thanks to the nonlinearity of the tip-surface interaction reflecting the hidden structure of the 2D material . If both tip and sample are vibrated at the adjacent frequencies without amplitude modulation (as in the HFM setup ), the amplitude of the nonlinear response reflects the subsurface nanomechanical elastic moduli modulation, whereas the phase detects the dynamic relaxation processes in nanometre volumes with a time-sensitivity of few nanoseconds.
The UFM and HFM study of WS2 and WSe2 materials revealed a clear contrast in areas, some of these linked with the topographical features, and some likely to reflect subsurface dislocations and stacking faults. Alternative reasons for the nanomechanical contrast of hidden features such as the misorientation of the crystallographic axis of layers and crystal-surface interaction and the demonstrated link between the screw and misfit dislocations are discussed.
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