The implementation of two-dimensional (2D) materials has potential to revolutionise optoelectronics, energy storage, gas- and bio-sensors, photocatalysis and solar energy conversion. In particular, the atomically layered transition metal dichalcogenides (TMDCs), such as WS2 and WSe2 show unique electronic and optical properties, ease of manufacturing, mechanical robustness, low toxicity, as well as being composed of relatively abundant elements on Earth [1]. The growth of these materials, individually or via co-deposition by the Chemical Vapour Deposition (CVD) on the Si or SiOx surfaces, can create complex trigonal and hexagonal structures formed by the individual layers of material, typically staked around a single screw dislocation which defines the crystallite growth process [2].
For this research, we have used the combination of Atomic Force Microscopy (AFM) with the ultrasonic vibration – namely, the Ultrasonic Force (UFM) and the Heterodyne Force (HFM) Microscopies, for the mapping of nanomechanical properties and subsequent identification of dislocations and faults in multiple stacked WSe2 and WS2 layers. In UFM/HFM, the 2-8 MHz high frequency ultrasonic vibration of 1-2nm amplitude is applied to the sample, resulting in a displacement of few nm normal to its surface, causing AFM tip indentation. 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 kHz modulation frequency, thanks to the nonlinearity of the tip-surface interaction reflecting the hidden structure of the 2D material [3]. If both tip and sample are vibrated at the adjacent frequencies, the amplitude of the response at the difference frequency maps the subsurface nanomechanical elastic moduli, whereas the phase reflects the local 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, not always linked to the topographical features, and likely reflected subsurface dislocations and stacking faults. Alternative reasons for the nanomechanical contrast - the misorientation of crystallographic axis of the layers and crystal-surface interaction are discussed. We complemented our SPM study with Raman spectroscopy to identify the distribution of the different materials in the co-grown WSe2 and WS2 samples. The results show that the bottom layer and the edges of the upper layer are predominantly of WS2 and the central layers correspond to the WSe2.
[1] A. Eftekhari, J. Mater. Chem. A 5, 18299 (2017).
[2] M. J. Shearer et al., Journal of the American Chemical Society 139, 3496 (2017).
[3] J. L. Bosse et al., Journal of Applied Physics 115, 144304, 144304 (2014).