Nanomaterials are any type of material of nanosized thickness, i.e. less than 100 nm in thickness. There are various types, many of which exhibit different properties than bulk materials. One common factor of nanomaterials is that this thickness range is also known as the quantum regime, where quantum effects play a major role in defining the properties. Because of this, nanomaterials often fall into different dimensional categories, be it 2D, 1D or 0D. In this article, we look at these different subsets of nanomaterials and the common types found within each.
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Some people define the nanomaterial range as 1-1000 nm; however, the widely accepted range is between 1 and 100 nm. This range is also the quantum regime and is why many nanomaterials exhibit quantum effects that make them suitable for electronics, semiconductor technologies and quantum technologies. So, even though there are many different classes of nanomaterials, including thin films, coatings and nanoparticles, the focus of this article is to introduce the various nanomaterials by the dimension that they exist in.
Aside from the specific thickness range, 50% of the particles of the nanomaterial must be present in the 1-100 nm range and not be aggregated for materials to be classed as nanomaterials. For layered materials or single walled carbon nanotubes (SWCNT), one or more external dimensions need to be less than 1 nm thickness (i.e. Angstrom thickness).
Nanomaterials exist in different dimensions, not only because they can be one atomic layer thick, but by how their electrons can be confined to flow in a certain number of dimensions. For example 2D materials have their electrons confined in one direction, so the electrons then move in two directions, hence the name. The same principle applies for 1D and 0D materials which have their electrons confined in 2 and 3 dimensions respectively; and their electrons can move in 1 and 0 directions respectively. So, let’s look at some examples.
Uniatomic 2D Materials
There are many types of uniatomic 2D materials, such as germanene (made from germanium atoms), stanene (tin), silicene (silicon), phosphorene (phosphorous) and, of course, graphene (carbon).
Graphene is by far the most useful and the closest material to commercialization within this list, especially as some of these are still theoretical materials. However, graphene and the various other 2D atomic materials possess an excellent array of optical, physical and electrical properties that make them useful for a wide range of applications. Once graphene has been successfully used across many applications at a commercial level, it is expected that many of the other 2D materials will follow suit, although it could take a while.
Outside of graphene, the class of MXene 2D materials show some of the best electronic properties. The most common MXene is boron nitride, which exhibits a hexagonal array of alternating boron and nitrogen atoms. Many people think that the MXenes show better properties than graphene, but they are much harder to synthesize. As such, they have not been as widely studied as graphene. However interest in them is significantly growing.
Whilst boron nitride is the most common, and the most widely researched, MXenes come in many forms and are often made from a combination of early transition metals (M), such as Titanium, Vanadium, Chromium and Niobium, alongside carbon or nitrogen (X). Future applications involving the MXenes could include EMI shielding, water purification and in energy storage systems.
Transition metal dichalcogenides (TMDCs) are one of the oldest and longest studied class of 2D materials. They are widely used in semiconducting and electronic applications, but do not have as wide a range in properties in each material as other 2D materials. However, there are over 100 TMDCs to date, with the most common being tungsten diselenide (WSe2), tungsten disulphide (WS2), molybdenum disulphide (MoS2) and molybdenum diselenide (MoSe2), although many other transition metals and chalcogen atoms can be used.
One characteristic feature of TMDCs, is that one monolayer is composed of three atomic layers, where a layer of metal atoms is sandwiched between two layers of chalcogen atoms (note that these layers are physically bonded and are not held by van der Waals forces).
Nanotubes, be it a carbon nanotube or inorganic nanotube, are materials which are elongated in one dimension, with a length-to-diameter ratio of up to 132,000,000:1. Nanotubes direct electrons along the elongated axis and come in many forms, including single-walled nanotubes (SWNT), multi-walled nanotubes (MWNT), chiral nanotubes, armchair nanotubes and zigzag nanotubes.
There has been a lot of hype about how carbon nanotubes could be used for many applications, especially in structural applications. However, issues with dispersing and aligning carbon nanotubes led to them to go out of favour for a while. They have recently been making a resurgence as many of these issues have been negated.
Nanowires, otherwise known as quantum wires, are another well-known 1D material. Again, nanowires are elongated in one direction, albeit with a much lower width to length ratio of 1:1000. The most common nanowires are silver nanowires, which are also known to be highly electrically conductive. Nanowires are known for exhibiting many different quantum effects, which alongside their unidirectional electron movement, have made them ideal materials for various electronic applications.
Quantum dots are a very common and useful type of nanoparticle, where the electrons are confined in all 3-dimensions. Quantum dots are small semiconducting particles that have been greatly used in displays and solar cells. Quantum dots emit a certain wavelength of light when they encounter either light or electricity and many quantum dots can be easily tuned. Quantum dots composed of cadmium, such as cadmium selenide (CdSe), are the widest class of quantum dots that have been studied.
Fullerenes, also termed Buckminsterfullerenes (because of the geodesic shape they exhibit) come in two forms - pure carbon fullerenes and endohedral fullerenes, with the difference being that endohedral fullerenes contain additional atom(s) inside the carbon fullerene.
Fullerenes come in many shapes and sizes, but the most common is C60 as it is the most energetically and structurally stable form. Fullerenes composed of boron have also been predicted. Carbon fullerenes are composed of both single and double bonds, which are arranged into pentagons and hexagons. It is the pentagons which give the fullerenes their curvature. All fullerenes contain 12 pentagons, with a differing number of hexagons.
Overall, nanoparticles come in many forms. There are too many to individually discuss, but some of the most common are: single element nanoparticles, such as silver and gold nanoparticles which are used in medical imaging; metal oxide nanoparticles, including titanium dioxide nanoparticles used in white paint formulations; and amphiphilic nanoparticles such as Janus particles, which are used as stabilizers. Janus particles are an interesting class of nanoparticle, as one half of the surface is different to the other, and these two surfaces can differ in their external receptors, hydrophobicity or hydrophilicity, surface charge, and even in their magnetism.
Safe Nano: http://www.safenano.org/knowledgebase/resources/faqs/what-is-a-nanomaterial/
European Commision: http://ec.europa.eu/environment/chemicals/nanotech/faq/definition_en.htm
Drexel Univeristy: http://nano.materials.drexel.edu/research/synthesis-of-nanomaterials/mxenes/
“Chapter 2: Carbon Nanotube: Synthesis, Properties and Applications”- Kaushik B. A. and Majumder M. K., Carbon Nanotube Based VLSI Interconnects, SpringerBriefs in Applied Sciences and Technology, 2015, DOI: 10.1007/978-81-322-2047-3_2
“Synthesis, properties and applications of Janus nanoparticles”- Lattuada M., and Hatton T. A., Nanotoday, 2017, DOI: 10.1016/j.nantod.2011.04.008