A study on the emergent piezoelectric and photonic properties of epitaxially grown bilayer MoS₂/WS₂

Abstract

This thesis is a comprehensive study of a chemical vapour deposition (CVD)-grown bilayer heterostructure of monolayer MoS2 on top of monolayer WS2 as a representative of the broader concept that layered heterostructures can manifest new optical and piezoelectric properties that are not found in the base materials. Using the principles of Group Theory it can be predicted that the as-grown bilayer MoS2/WS2 heterostructure classifies as a 3m point group material, even though both monolayer MoS2 and WS2 are 6m2 point group materials. While each mono-layer does not show out-of-plane (OOP) piezoelectricity it is shown here through piezoresponse force microscopy (PFM) mapping that, according to the symmetry properties of 3m crystals, the heterostructure has a non-zero d33 piezoelectric tensor component. A simple one-step atmospheric pressure CVD (APCVD) recipe is shown to reliably produce large amounts of this heterostructure on SiO2/Si, which has been analysed using Raman and photoluminescence (PL) spectroscopy, second harmonic generation (SHG) mapping, regular and cross-sectional scanning transmission electron microscopy (STEM), selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDS), as well as atomic force microscopy (AFM) and PFM. It is shown that MoS2/WS2 naturally grows as two distinct polytypes called 2H­like and 3R-like, named after the 2H and 3R polytypes of pure transition metal dichalcogenides (TMDCs). They are different from each other by the inter-layer rotation angle between the monolayers, 3R-like types have 0° and 2H-like types have 180° inter-layer rotation. These polytypes show very distinct characteristics in their photonic properties, 3R-like structures show very intense Raman and SHG emissions while those of 2H-like structures are relatively weak in intensity. This makes their differentiation very simple compared to the diffculty of identifying the different polytypes of pure TMDCs. Although very distinct in photonic characteristics, both polytypes classify as 3m point group materials which predicts OOP piezoelectricity, however also here the magnitudes are different. The d33 for 2H-like and 3R-like MoS2/WS2 has been measured to 2.25 pmV-1 and 3.67 pmV-1 , respectively. Finally, the strain dependency of the photonic properties of the heterostructure has been investigated via substrate bending. The in-plane (IP) Raman mode E' and acoustic mode 2LA(M) are strongly influenced by IP strain. The OOP A'1 Raman mode of both layers is far less impacted by IP strain, as one would expect. The direct bandgap A-peaks of both layers decrease in energy and but not in intensity through IP strain. All strain effects intensify up to a maximum strain of 2.1%, after which the heterostructure starts slipping and the changes revert slightly. In summary, a layered heterostructure has been produced that in many ways acts as a somewhat new material with unique structural, photonic and piezoelectric properties. This example serves to show off that through making layered heterostructures a wide variety of new materials can be discovered and hopefully used to drive modern material science forward.