Nanoscience

Volume 4

P John Thomas, Neerish Revaprasadu, P John Thomas, Neerish Revaprasadu

• Publisher: Royal Society of Chemistry
• Year: 2017
• ISBN:
  9781788012492
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Nanotechnology & MEMS
• Index: Technology & Engineering

The field of nanoscience continues to grow at an impressive rate and, with such a vast landscape of material, careful distillation of the most important discoveries will help researchers find the key information they require. Nanoscience Volume 4 provides a critical and comprehensive assessment of the most recent research and opinion from across the globe. Coverage includes diverse topics such as 2D nanomaterials, quantum dot solar cells and core nanoparticles for drug delivery applications. Anyone practising in any nano-allied field, or wishing to enter the nano-world will benefit from this resource, presenting the current thought and applications of nanoscience.

A review of two-dimensional nanomaterials beyond graphene

Aleksander A. Tedstone^*a, Jack R. Brent,^*b and David J. Lewis,^*c

DOI: 10.1039/9781782620358-00108

The principal interest in two-dimensional (2D) nanomaterials has been in their electronic properties, but as understanding of them has broadened, a wide range of applications have been explored. This chapter outlines recent literature concerning the synthesis, modification, and applications of two-dimensional nanomaterials beyond graphene. Recent theoretical propositions of these novel two-dimensional nanomaterials and their applications are also discussed.

1Introduction

Nanomaterials with one or more dimension of nanometre length scale (1–100 nm) have been of growing interest to the scientific community due to their intrinsic properties which arise from physical confinement of materials at a miniscule length scale and the socio-economic driver of the prospect of ever-more miniaturised devices. The principal interest in two-dimensional (2D) nanomaterials has been in their electronic properties, but as understanding of them has broadened, a wide range of applications have been explored. For instance, their extremely high surface areas are beneficial for catalysis, an application that has been demonstrated with a number of 2D materials,^1–4 and is likely to continue evolving as the library of materials expands and their synthesis becomes more refined. Biomedical applications of many types of nanomaterials have been the subject of intensive research, and 2D materials are no exception, finding use in imaging, drug delivery and targeted therapeutic methods.^5–9 Separation membranes consisting of composites with 2D materials as a major component are also an emerging application with the potential to outperform the state of the art technology.¹⁰ 2D nanomaterials can be created and used as colloidal dispersions or supported on substrates,¹¹ and this versatility makes them very exciting to the wider scientific community, as well as reinvigorating interest in the bulk materials from which they are derived from such as graphitic carbon, black phosphorus and molybdenite (MoS₂).

2Two-dimensional (2D) nanomaterials

Two-dimensional (2D) nanomaterials are ultra-thin materials derived from bulk crystalline solids with intrinsically layered atomic structures. These bulk materials are also known as van der Waals solids by virtue of having no molecular bonds between layers and are held together by non-covalent interactions. The archetypal 2D nanomaterial is graphene, the atomically thin material derived from bulk graphite, the properties of which were elucidated in 2004, and sparked interest in the exploration of analogous materials with different compositions. Since this discovery, various allotropes of other main group elements have been identified as 2D material candidates, as have materials comprised of more than one element that exhibit layered structures. The extremely thin nature of these materials can give rise to unique properties and facilitates the creation of layered heterostructures of different 2D materials, opening up a massive field of potential study that has already had significant achievements and may become the basis of emerging technologies that rely on these unique material properties. Not all 2D materials have known bulk counterparts, although in principal any sufficiently stable monolayer compound should be capable of forming an extended multilayer crystal.

2D materials can be prepared by a number of routes, principally falling into the categories of top-down or bottom-up preparation.¹² Top-down routes typically involve the naturally occurring parent material being processed into 2D form via exfoliation of layers, either mechanically, in solution or via intercalation–expansion. Bottom-up routes utilise the direct synthesis of the material in controllable conditions such as chemical vapour deposition (CVD), solution syntheses or gas-phase methods such as physical vapour transport (PVT). Variants of each of these techniques are legion and judicious selection is required for any application as each technique carries its own benefits and disadvantages.

Where a 2D material has a naturally occurring bulk parent material, it is important to note that some properties only arise at the few- and mono-layer length scales due to confinement of electronic and magnetic states in the dimension of thickness reduction. Not all research or application requires monolayer sheets, and few-layer forms of these materials can be considered 2D depending upon the property of interest, but the distinction is important and context dependant.

2.1Homoatomic main group 2D materials

There are a number of homoatomic main group analogues of graphene, many of which have been isolated or synthesised in monolayer form in recent years, and some of which remain theoretical possibilities not yet empirically proven to be stable. Silicene, germanene, stanene, and phosphorene have been the subject of reviews and comparative studies of graphene analogues in 2014 and 2015,^13–15 and more recent developments will be discussed herein. The topological insulator and photo-induced topological superconductor properties of silicene, germanene and stanene are collectively discussed in the work of Ezawa,^16,17 – and it is notable that the heavier group IV 2D materials have more in common with each other than with graphene in term of structure and properties. No 2D allotropes of lead or bismuth have been proposed as stable compounds at the time of writing.

2.1.1 Graphene

Graphene has been the subject of great interest and investment,¹⁸ and is the archetypal 2D nanomaterial that has spurred on interest in related 2D structures. Potential technological applications are numerous due to its exceptionally high conductivity and tensile strength. The material is chemically stable in atmospheric conditions yet readily functionalized, and has many derivatives, for example graphene oxide, graphane and fluorographene.^19–22 A principle challenge in bringing graphene into real-world applications is the size and homogeneity of the sheets that can be produced – although efforts and improvements have been made in this area and wafer-scale CVD graphene synthesis has been demonstrated as a viable route for electronics-grade material.^23,24 It is also possible to create superstructures of non-covalently linked graphene crystals for certain applications that do not require an extended homogenous sheet.²⁵ CVD growth on a liquid Cu substrate can allow isotropic growth of the graphene and this minimizes the density of grain boundaries in the material; crystal defects that have a detrimental effect on graphenes desired properties.²⁶ The review literature on graphene is extensive so will not be covered in depth in the current work, which will instead focus on more recently proposed and isolated 2D materials.

2.1.2 Silicene

A layered allotrope of elemental silicon analogous to graphite is not found in nature, but this has not prevented theoretical and experimental studies into a monolayer silicon material analogous to graphene.^27,28 Silicene was first proposed in 2009²⁹ and first isolated in ...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Preface
• Author biographies
• Role of ligands in the synthesis of bi- and multi-metallic nanocrystals
• Recent highlights in advanced transmission electron microscopy techniques: applications to nanomaterials
• Gold fabricated core–shell nanoparticles as innovative cancer therapeutic strategies to improve drug delivery
• Supramolecular chemistry of AIE-active tetraphenylethylene luminophores
• A review of two-dimensional nanomaterials beyond graphene
• Colloidal quantum dots solar cells
• The role of ligands in mediating charge transport in nanocrystalline medium