Development and Investigation of a Novel Liquid-Liquid Interface Deposition Technique for Thin Films of Two-Dimensional Materials

Two-dimensional (2D) nanosheet films and the van der Waals heterostructures (vdWHs) produced from them offer enormous potential for a wide range of applications, due to the large library of 2D materials. However, many of the approaches used to make these films and vdWHs are time consuming, labour-intensive, offer little control over the thickness of films, and aren't environmentally sustainable. In this thesis, a novel method for the production of 2D nanosheet films and heterostructures is presented, which aims to overcome all of these issues. The method, Liquid Interface Deposition (LID), enables the production of 2D nanosheet films at an organic-water interface, with a degree of control over the film thickness. The LID method is rapid, taking a maximum of 7 hours from precursor material to deposited film, with each deposition cycle taking less than an hour, meaning approximately 12 samples can be produced per day, or one sample of 12 subsequent depositions. Due to the nature of the LID method, surfactant-stabilised aqueous suspensions of 2D nanosheets can be used directly, not typically possible for other film fabrication methods, as the surfactant would remain in the film, contaminating the films and limiting its properties. The LID method is uniquely able to remove the surfactant from the suspension during film formation, thus resulting in a contaminant-free 2D nanosheet film.

Prior to film formation, a program has been developed to analyse the aqueous surfactant-stabilised suspensions. This program looked at scanning electron microscopy (SEM) image of the filtered suspension, and from these was able to determine the area and information about the shape of the nanosheets. The SEM images were acquired and the image analysis was undertaken automatically, enabling a statistical analysis of suspensions of nanosheets. This should facilitate comparison between the area and shapes of nanosheets, and the properties of films formed from them.

The LID method was shown to be applicable to multiple 2D materials and multiple substrates, without needing modifications to the method. A graphene-molybdenum disulphide-tungsten disulphide vdWH was produced using the LID method, which is the first demonstration of a liquid-liquid interface produced vertical vdWH. An investigation into how multiple layers of LID produced films interact with each other was undertaken, and it was shown that multiple layers remain decoupled from each other, such that a thick film can have the properties of a few-layer nanosheet. A study into the possibility of scaling up the LID method to form large-area films was performed, which showed that while this was possible, it produced films of lower homogeneity, and therefore requires further work.

The electrical properties of LID produced films were evaluated as transparent conductors and photodetectors. Graphene films were found to have conductivities ranging between for transmittances of 55-75. Additionally, MoS films were found to perform as photodetectors, with all films showing an increase in current magnitude on illumination with light.