# Introduction and Literature Review

## Motivation

Since the invention of the first germanium transistor by Bardeen, Brattain and Shockley in 1947 (Bardeen 1948), the semiconductor industry has striven for progress in on-chip circuit technologies that underpin our daily lives today. The modern age of electronics is rapidly approaching a paradigm shift, as Moore’s law begins to falter beyond the 10-nm node regime (Waldrop 2016). The International Roadmap for Semiconductor Technology (ITRS) has recently been replaced by the IDRS, reflecting the change in focus with regard to new integrated circuit (IC) design and implementations of nanotechnology in everyday devices. As the Internet of Things (IoT) and neural network computing come to the fore of scientific research worldwide, efforts have shifted to multidisciplinary approaches combining various areas of physics, chemistry, engineering and biology.
Spearheading this movement in the field of nanoscience is the area of two-dimensional (2D) materials, where recently-discovered allotropes of well-known chemical compounds have proven to exhibit extraordinary properties when studied on the nanoscale. Graphene, the prototypical 2D material, has attracted tremendous interest since its discovery in 2004 (Novoselov 2004). Owing to its superlative physical properties such as high carrier mobility (Morozov 2008), minimum electrical conductivity (Novoselov 2005), mechanical strength (Lee 2008) and thermal conductivity (Balandin 2008), it has garnered much institutional funding (Graphene Flagship - F...) and scrutiny from both experts and the public. In recent years, doubts have been cast as to the fruition of graphene-related research (Peplow 2016) when it comes to everyday applications. Long-heralded as the successor to silicon-based technology, planar carbon faces considerable challenges before it can become ubiquitous in high-frequency switching devices and flexible electronic displays (Schwierz 2010), (Schwierz 2013).
Researchers have thus moved beyond graphene in search of low-dimensional materials that could prove useful in new generation flexible electronics, photodetectors, LEDs and tunneling transistors. The family of transition metal dichalcogenides (TMDs) is populated by compounds whose unique functionalities and versatility of physical properties offers complementary properties to that of graphene and existing silicon-based fabrication processes. The ability to manipulate these properties on the nanoscale is a central theme of this thesis, with particular focus on electronic properties and devices which could be fabricated using TMDs.

## Two-dimensional van der Waals materials and devices

### Overview

Two-dimensional layered materials (2DLMs) can broadly be categorised into three groups: elemental, binary and complex compounds. Materials consisting of atoms of a single element of the periodic table, such as graphene (C) or phosphorene (P), are examples of elemental 2DLMs. TMDs such as MoS$${}_{2}$$, WSe$${}_{2}$$ (Jariwala 2014) and NbSe$${}_{2}$$ (Efetov 2015), belong to the class of binary compounds, along with their various alloys (e.g. WS$${}_{x}$$Se$${}_{2-2x}$$, MoS$${}_{x}$$Se$${}_{2-2x}$$) and related metal chalcogenides such as GeS and Bi$${}_{2}$$Te$${}_{3}$$. More complex variations of 2DLMs include organic framework thin films (Colson 2011), layered double hydroxides (LDHs) (Wang 2012) and complex metal oxides such as Bi$${}_{2}$$Sr$${}_{2}$$Co$${}_{2}$$O$${}_{8}$$ (Osada 2009).
The unusual nature of the covalently bonded, dangling-bond free lattice of many 2DLMs allows for new opportunities in design and implementation of nanoscale devices based on heterostructures of these materials. As the compounds are not constrained by lattice matching, they can be van der Waals (vdW)-stacked in various combinations on top of one another to study novel physics and properties of use to the electronic engineering industry. In addition, the layer-dependent properties of the individual materials themselves are of great interest for studying fundamental physics of low-dimensional systems, including charge transport and confinement, as well as photon, phonon and exciton manipulation (Choi 2016). The wide gamut of electronic properties in these 2DLMs allows for mixing and matching of atomically sharp interfaces to create novel nanodevices where each vdW-bonded layer can serve a unique and concrete function, as interdiffusion of atoms does not occur between each “Lego block” in the heterostructure stack.

Representative 2D materials visualised on the scale of associated bandgap energy (Liu 2016).