Liquid-liquid Assembled 2D Materials for Large-Area Electronics

Abstract

This thesis addresses the key challenge of deposition of films of 2D nanoflakes from liquid dispersions and presents a study of the self-assembled tiling at the molecularly flat interface of immiscible liquids. The optoelectronic properties of a 2D material film is highly dependent on its morphology, which for 2D conducting films should ideally be a densely packed and neatly tiled monolayer with a high proportion of edge-to-edge nanoflake contact. The water-hexane interfacial assembly of a range of conducting, semiconducting, and insulating 2D nanoflakes has enabled the deposition of large-area films that display these ideal morphological features. A statistical analysis technique found that the liquid-liquid assembled monolayers (LAMs) are of near perfect flatness, with over 70 % covered by a single nanoflake thickness and a total covered area of approximately 90 %. In contrast, commonly used deposition methods of spin or spray coating result in stacked, non-uniform film morphology as predicted by the Poisson distribution. Further, the liquid-liquid assembly technique is applicable to a continuous production strategy, enabled by Marangoni flows at the interface and demonstrating its potential towards roll-to-roll processing. In the deposition of devices with vertically stacked geometries, the separation of certain features with pinhole-free and uniform dielectric layers is necessary. To afford greater performance, these dielectric layers should additionally be thin, flat and non-porous. Here we report a method for the repeated stacking of LAMs of the insulating 2D material hexagonal boron nitride. These films possess ideal morphologies for these applications, as confirmed by statistical analysis, building upon the findings of single-layer LAMs. This assembly method provides excellent control over the placement of 2D materials monolayers, and could provide new opportunities for the fabrication of 2D materials-based large-area electronics. To determine the suitability of these films for applications in large-area electronics, several demonstrator devices were presented. Field-effect transistors fabricated from molybdenum disulphide monolayers demonstrate maximum mobility and on/off current ratio of 0.73 cm^2.V^-1.s^-1 and 10^5 respectively, comparable to equivalent devices fabricated from single-crystalline channel material. Transparent conductive electrodes were produced from monolayers of reduced graphene oxide using the continuous production strategy. Their transparency and sheet resistance of 88 % and 850 ohms.sq^-1 respectively meet the requirements of current technologies such as touch panels. By deposition onto elastomeric substrates, these monolayers also demonstrate tuneable strain-sensing capabilities. Here, strain gauges are presented with tuneable sensitivity (gauge factor) from 10^4 to 10^2 and tuneable maximum working strain from 0.6 % to 20 %, to accommodate applications in either structural health monitoring or human-motion sensing respectively.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Brian Derby (Supervisor) & Ian Kinloch (Supervisor)