D4.1– Characterization of nanosheets, networks and heterostacks built from initially available 2D materials

The report outlines the progress made regarding the characterisation of initially available 2D materials, as well as their printed networks and heterostacks. While an abundance of layered materials have been theoretically predicted to exist, only a fraction of them has been successfully exfoliated and characterised. Upon exfoliation, it is crucial to assess if the predicted physicochemical properties of the produced nanosheets have been preserved and are retained in solution for further processing. Furthermore, the development of characterisation techniques to quantify the morphological properties of solution-processed networks made from 2D materials will be crucial for device applications. This is an area of intensive research and forms the basis of the work described in this report. We first will present the characterisation results on liquid phase exfoliated (LPE) and electrochemically exfoliated (EE) nanosheets, such as MoS2, AsSbS3, As2S3, SnGe, InSe, Mo0.5W0.5Se2, and HfSe3. Using atomic force microscopy (AFM), it was determined that the aspect ratio (AR) of EE nanosheets can reach values > 103, which is nearly 30 times larger than those obtained by the LPE method (AR ~ 10 - 30). Further characterisation by UV-Vis and Raman spectroscopy was used to investigate the intrinsic properties of the produced nanosheets. For EE MoS2, we began to explore whether information on dimensions or doping is potentially accessible from extinction spectroscopy. The Langmuir-Schaefer (LS) method is believed to be an ideal method to fabricate ultra-thin and uniform nanosheet networks. However, the morphology of such networks is difficult to measure quantitatively due to the length-scales involved, and information on technique optimisation has been limited. To address this, we have used AFM to characterise these networks and have developed a layer-by-layer fabrication protocol with precisely controlled layer thicknesses. Work to tailor the thin-film morphology by tuning the deposition parameters has also been performed. We show that spatial variations in the network morphology can be assessed using optical transmission scanning measurements. Finally, we have applied the focused ion beam – scanning electron microscopy nanotomography (FIB-SEM-NT) technique to reconstruct both 2D networks and heterostacks in 3D with unprecedented resolution. This allowed the internal structure of nanosheet films deposited under different conditions and using different nanosheet types to be characterised.