well-established discipline, the idea of crafting materials and components on the nanoscale and designing and creating tailored materials for smart applications is now developing as the relatively new disciplines of nanoscience and nanotechnology.
Figure 2.1. Schematic of length scale with respect to a nanometre, showing common examples. Source: Abhijit Tembhekar/LadyofHats/Alan Cann/Acharya KR, Fry E E, Logan D T, Stuart D I/; visualisation: Astrojan/Белых Владислав Дмитриевич.
In 1959, Professor Richard Feynman delivered his seminal lecture entitled ‘There’s plenty of room at the bottom’, in which he imagined what could be achieved if we shrunk technology down to the nanoscale. This lecture inspired material scientists and technologists to challenge themselves to venture to the smallest length scales possible for materials. The lecture has thus been defined as the birth of nanoscience and nanotechnology. As a physicist, Professor Feynman was fully aware that this shrinking would not be a simple linear scaling issue, because smaller components will not display the same properties as the bulk material. Rather, materials at the nanoscale display rich and complex properties specifically dependent on their dimensions, proportion of surface atoms, and morphology (see the examples in the sections below).
Nanomaterial can be defined as having at least one dimension in the nanoscale (between 1–100 nm). This ranges from thin nanometre films (one dimension at the nanoscale) to nanowires (two dimensions at the nanoscale) to nanoparticles (all three dimensions at the nanoscale). Dimensions smaller than 1 nm are in the territory of clusters, made up of a few atoms, which are outside the realm of nanomaterials. We should also consider that the nanoscale can apply to voids as well as matter, and thus nanoporous materials can also be considered as nanomaterials, particularly as they have large surface areas. While 100 nm is conventionally the upper limit, this can be blurred. Materials termed nanoscale are reported over this size, if the properties displayed are still within the realms of nanoproperties (discussed in section 2.2) or, (more frequently as time goes on) it suits to classify them as a nanomaterial for political (non-scientific) reasons!
The nanoscale is a most interesting length scale for solid state materials for several reasons. First, from a philosophical viewpoint, it can be asked: how small can a particle be before it is no longer a material? Indeed, there is a valid scientific debate about whether or not a nanoparticle can or cannot dissolve, or if it is already a solute, when in a solution.
Second, nanomaterials have a large percentage of surface compared to bulk material, and surfaces have very different chemical and physical properties to the bulk. To illustrate this, let us consider two spherical particles with radii of 1 μm and 10 nm. The ratio of the external surface area to volume (A:V) of each particle is inversely proportional to its radius (figure 2.2):
AV=4πr243πr3=3r.(2.1)
For these two particles, A:V will be respectively 0.003 nm−1 and 0.3 nm−1. This means that the 10 nm particle will have 100 times more surface for the same volume. As a result, the 10 nm particle will have 100 times more atoms on the surface available to interact with the external environment. This phenomenon leads to significantly increased catalytic activities, for example. This effect also manifests into other special effects and properties such as optical and magnetic properties, which are discussed further below.
Figure 2.2. Graphical representation of how surface area increases with decreasing radius of a spherical particle (on a log 10 scale).
Third, the nanoscale lies at the boundary of bulk material properties and atomic chemical properties (figure 2.3). The former is generally described by continuous transitions, while the latter must be considered as quantum mechanical. Quantum mechanics is both difficult and abstract to conceptualise but can be linked to the macroscale world by the theory of wave particle duality, where any wave can also be described as a particle with the same energy, and vice versa.
E=hf=ρc.(2.2)
These are linked by the work of both Einstein and Broglie, that shows that a particle and wave with the same energy are interchangeable (equation (2.2), where E = energy, h = Planck’s constant, f = frequency of the wave, p = particle momentum = mass × velocity, (momentum of a particle) and c = the speed of light). At this stage however, it is simple to see that properties at the atomic scale are quantised because electrons (the principle particle (or wave) behind all chemistry) reside in discrete quantised electronic orbitals (defined by having an integer wavelength). This is simply described as if one considered the energy a particle can have in a defined narrow box, which is the energy that equates to any integer wavelength that will fit in the box as a standing wave (figure 2.3). These energy levels are equal distance apart as the box width remains constant (figure 2.3), however, the energy well of an atom follows a curve and as such the energy levels become closer together the higher the energy (figure 2.3). This is also directed by the shape of the energy well; the deeper the well, the further apart the energy levels, while the value of each energy level is also directed by the element (the pull of the nucleus etc). Thus, there is only one series of discrete energies that relates to that electron and its energy levels, of that particular atom. As the number of atoms in a material increases, so too does the number of interactions between atoms and electrons. The more interactions, the harder it is to define discrete energy levels as more and more energy levels occur, forming a continuum which is what we can describe in a bulk material (figure 2.3). One can similarly conceptualise the particle in a box on a macroscale. The width of the box is now much greater and the wavelength associated with a macroscale (large mass) particle is now so small, the particle can now be considered to occupy any energy, so it becomes a continuous system. In Newtonian physics a particle can have any energy, velocity and thus momentum within its box (figure 2.3). The interface where quantum states transition to continuous phases is at the nanoscale and this leads to a range of exotic behaviours, some of which are briefly described below in section 2.3.
Figure 2.3. Depiction of how nanoscale materials lie at the crossover between the atomic quantum laws of physics and the macroscale Newtonian laws of physics. In quantum physics: the left shows how a particle in a very narrow box can only have the energy quantised by the size of the wavelength. The change in energy of these is the same as the box sides are parallel. For an atomic system, the change in energy between levels decreases at increasing number of n, due to the energy potential. Conversely, a particle in a macroscale box can move in any direction at any given energy, so there is a continuum of energy a particle can possess.
2.2 Tangible and historical examples of nanomaterials
It is no coincidence that nanoscience has grown in parallel with the growth and development of instrumentation, imaging and spectroscopic techniques that have the resolution to allow probing on the nano- to the atomic scale (see characterisation in chapter 3). This has been essential to analyse and understand these materials as well as aid design and synthesis. However, while we may not have been able to visualise or understand them, we have been using the properties of nanomaterials for millennia, with examples stretching through history (figure 2.4). One of the earliest examples is the 4th century Roman Lycurgus cup. This beautiful glass cup, mounted in a carved bronze exterior, is now housed in the British Museum. It is an excellent example of nano-optics (specifically surface plasmon resonance), where, because it contains different metallic nanoparticles it looks green if illuminated from the front, because silver nanoparticles scatter the reflective light, and red if illuminated from behind, due to surface plasmon
