From the size of rockets to the height of skyscrapers and magnitude of gigafactories, technological and engineering feats seem to go bigger and bigger. However, bigger is not always better!
Science has taught us that everything exists as a combination of smaller and smaller matter, a remarkable fact that stimulated engineers to move in the direction of smaller things. As a result, our modern world has become essentially based on technology and engineering done at scales 100,000 times smaller than the thickness of a sheet of paper.
Such technologies are usually categorised under the term nanotechnology.
Like with any new groundbreaking technology, the hype around it is substantial, and strongly polarised opinions begin to arise. For some people, nanomaterials are the solution for everything and the cure for every illness. To others, they are very scary and a threat for all of humanity.
So what is it about those nanomaterials? What are they exactly and how do they affect our world?
One common definition is that a nanomaterial is any man-made material with one of its spatial dimensions below 100 nm. Typically, there follows a comparison to human hair, which has a diameter ranging between 50 and 100 µm – so nanomaterials are hundreds and thousands of times smaller than a single strand of hair!
This is certainly impressive, but it can’t be everything there is to it, can it?
What‘s really special about them: when you shrink your materials to the nanoscale, an impressive change in their physical properties occurs compared to their macro counterparts; their properties become highly dependent on size and shape.
And this is quite unusual! From your everyday experience you know that gold always looks like gold – no matter its size or shape. A giant, irregular-shaped nugget, a medium-sized well-shaped bar, or a tiny crumb, they all have the same yellow-ish glow (if they are pure and clean on the surface).
Well, it turns out that when you make gold particles with dimensions below 100 nm, this is no longer true; in fact, they become red in colour. This is actually one of the first known applications of nanomaterials that such gold particles were used to make stained glass used for church windows.
Nowadays, as we have the tools to make well-controlled nanoparticles in different sizes, we can observe that it’s not just red, but different shades of red could be obtained depending on the particle size. For the smallest particles, colours as different as blue can be achieved.
Not only is it colour, but also all kinds of material properties get affected at the nanoscale, some of which are electrical conductivity, magnetic behaviour and melting temperature (bulk gold melts at around 1000°C, while 10 nm gold nanoparticles melt already at 500°C).
This means by going nano, you can change the properties of elements that have been known for hundreds and thousands of years. So for engineers and material scientists, nanomaterials are, first and foremost, a huge playground!
Size vs surface: a battle for dominance
Nanomaterials can be classified by how many of their dimensions are in the nano regime: one dimension in layers and sheets like graphene, two dimensions in tubes, wires and rods, and three dimensions in nanoparticles.
A special subset of the nanoparticles are quantum dots (QDs): nanocrystals out of semiconducting materials that extend only 10nm or less in all three dimensions and can conduct electrons. They are so small that sometimes they are described as artificial atoms and they show the strongest nano effects.
But, where do their unfamiliar size-dependant properties come from? There are two major factors at play: quantum confinement and surface effects. Quantum confinement only kicks in when particles are so small that their spatial boundaries confine the movement (called the wavefunction) of the electrons they contain. This changes the energy the electrons can carry and accordingly, it changes which part of the light they absorb, reflect and emit.
The second factor is QD’s very high surface-to-volume-ratio. While atoms inside quantum dots are completely immersed in a crystal lattice, surface atoms have some sides exposed to the environment and thus, unsatisfied bonds.
As a result, surface atoms behave differently from bulk atoms. Knowing that the surface area of a body scales with the square of its radius, while the volume changes with the radius cubed, increasing particle size would result in the number of bulk atoms vastly outnumbering the number of surface atoms very quickly.
This means that in the macro world, surface atoms have a negligible contribution to the behaviour of the whole material due to their relatively low amount. However, on the nanoscale, the number of surface atoms is much closer to the number of bulk atoms, resulting in a much bigger impact on the particle’s properties.
Like quantum confinement, this effect is size-dependent. But apart from size we now gain another way to manipulate the material properties, and that is by controlling its surface. Often, it’s not entirely clear if it’s actually quantum confinement or surface effects that dominate the particle’s properties.
Many studies have shown QDs having the same surface change their colour with size, and other studies described particles of the same size change their colour with different surface treatments. This debate has coined the phrase size vs surface.
Surface engineering: the power of functionalisation
The relatively large surface area of quantum dots also results in distinct challenges. Surface atoms tend to saturate their unsaturated bonds via two ways:
- Agglomeration, where atoms would stick to a neighbouring QD, which gives rise to the phenomenon of stickiness at the nanoscale. Here, QDs would form large agglomerated clusters, which renders them difficult to handle.
- Oxidation, where atoms would react with their environment; typically, the reactive oxygen in the surrounding air. In this case, an oxide shell would cover the surface QDs. Such oxides are typically electrically isolating. This is an undesired property, especially in applications such as electronics.
To address these issues, the particle’s surface bonds have to be saturated with something else, something useful. For this, we use short organic chains. The process of grafting such chains on the QD’s surface is called surface functionalisation.
With the proper functionalisation, the QDs are protected from oxidation and agglomeration. Since we are using organic materials, we need to make sure we don’t add too much and end up with another insulating shell. Then we can go crazy and experiment with how the functionalisation changes the QDs.
Printed electronics: rolling like newspapers
Surface functionalisation can also render QDs soluble in liquids. Such freestanding QD-solutions are called colloidal QDs. This is to differentiate them from QDs grown inside or on top of a solid matrix.
Being soluble is a big deal when it comes to processing the particles, especially for application in electronic devices. Conventionally, semiconductors are solid at room temperature and grown into crystals by evaporation in a furnace (physical or chemical vapour deposition). This requires high temperatures (>1000°C) and vacuum chambers, and it can only be done on a batch-by-batch basis. In other words: it’s slow and expensive.
Dealing with liquids, however, is much easier. Cheap and fast fabrication methods like spray coating or other printing techniques enable large-area processing at low temperatures; welcome to the field of printed electronics! QDs can also form thin, soft films that are somewhat flexible. This allows us to print them on foils and move to a high throughput roll-to-roll printing press; in other words: printing electronics like newspapers!
Applications of nanomaterials
Nanomaterials’ applicability spans over a wide variety of industries and application areas. Maybe the most established is the use of zinc and titanium oxides in sunscreen. Making TiO2 nano-sized retains its ability to absorb UV light, but diminishes its reflectivity of visible light. This makes the sunscreen transparent instead of white.
A more exotic use is quantum processors that take advantage of the QDs ability to trap single electrons so their spin can be used as a quantum bit.
In the medical field, QDs can be used in diagnostics and therapy by functionalising them with suitable receptors so they stick to specific targets. Then, they can serve as labels for biological imaging or as drug carriers for cancer treatment.
However, the possibility to fine-tune the optical properties of QDs makes them especially interesting for optoelectronic applications: LEDs for displays and lighting; light-absorbing devices in solar cells and photodetectors.
Such devices consist of very thin QD films sandwiched between conductive electrodes. The QD films have a thickness of a few 10-100 nm and therefore, can be soft, light, transparent, flexible and even stretchable. This enables new applications not accessible for conventional electronics. A popular topic in this regard is wearable electronics that can be integrated into clothes or even directly on the skin like a tattoo .
Materials: element combinations
Today, most QDs are made from compound semiconductors that combine elements from two different periodic table groups. The best results so far have been achieved with combinations based on either cadmium (CdSe, CdS, CdTe) or lead (PbSe, PbS, PbTe) . However, cadmium and lead are toxic heavy metals.
This makes them unusable for biological applications, and their use is heavily restricted for consumer applications (in the EU, lead is limited to 0.1 wt.-% and cadmium to 0.01 wt.-% in any material employed for a product).
Also, compounds based on the non-toxic indium (InP, InAs) have been shown to make good devices; however, indium is quite scarce and increasingly expensive as it is a key material for transparent conductive layers in displays and touchscreens.
An exciting recent alternative is QDs based on silicon. Silicon is made from sand and therefore, is highly abundant, non-toxic and biocompatible. While the majority of all electronics are already made of silicon (it’s also the go-to material for solar cells), it usually does not emit light and therefore, could not be used for LEDs.
A long-lasting dream of the industry came true when it was found that silicon QDs with a diameter of 5 nm and smaller show strong luminescence . LEDs made of such silicon QDs have been shown in different colours and while they are currently not yet very bright, they are very actively researched .
So we have seen that by going nano, we can open up a whole new world of materials and long known elements that suddenly change their properties depending on size, shape and surface.
The first commercial applications are already existing, as in cosmetics, repellent surface coatings or as phosphors in QLED TVs. However, the most promising applications are still in varying stages of research and have some way to go. But this is ok.
While nanomaterials became an actual research topic around the middle of the 20th century, only recently have the tools to reliably manufacture and characterise such small matter become sufficiently good, affordable and widely available. So we can expect a lot more to happen in the coming years – so, stay tuned and enjoy the ride!
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