Nanomaterials makes the foundation of nanoscience and nanotechnology. Nanomaterial research has attracted great number of scientists and the field has seen explosive growth over the years. So why so many scientists are after these materials? And what are the promises they hold for the future? This is exactly what we are going to find out in this section.
Nanomaterials are extremely small, at least in one dimension. To be classified as a nanomaterial at least one dimension (width, length or height) need to be less than 100 nanometers. Once again, a nanomaterial is one billionth of a meter or 1000 million times smaller than a meter. It’s so small that its 100,000 times smaller than a diameter of a human hair.
Nanomaterials are a subject of great interest due to unique properties these materials show. Scientists have observed that nanoparticles show unique mechanical, optical, magnetic and electrical properties that are not seen in their bulk counterparts. These materials promise potential for great achievements in fields such as electronics, medicine, transportation, textiles, etc.
Although the subject of nanomaterials may seem futuristic, number of nanoparticles occur naturally in high abundance than most of us think. There are many nanosized particles in fine sand and dust. Volcanic ash contains large amount of nanoparticles and even viruses that are quite plentiful in our surrounding can be classified as nanoparticles. There are also man made nanoparticles, which are made incidentally from certain situations like in engine smoke, burning, factory smoke, etc. However, engineered nanoparticles, deliberately synthesized by humans usually with a specific objective in mind, holds a particular interest.
Classification of Nanomaterials
Nanomaterials are classified according to the length scale of each of its dimension. The definition states that at least one dimension of a nanomaterial should be in between 1 to 100 nm. Accordingly, nanomaterials can be nanoscale in all three dimensions (nano-particles), in two dimensions (thin films and sheet like nanomaterial), in one dimension (nano wires or nanofibers) and all three dimensions in macroscale (combination of other nanomaterials, like bundle of nanofibers, bundle of nanosheets,etc). Fairly frequent identification method for nanomaterial is presented below.
0 D : all three dimensions in the nanoscale (nanoparticles)
1 D : one dimension in nanoscale and other two in macroscale ( nanofibers, nanowires)
2 D : two dimensions in nanoscale and the other in the macroscale ( nano sheets, thin films)
3 D : no dimensions at the nanoscale, all are in the macroscale (nanostructures with nanomaterials
The reason for strange properties of nanomaterials
Nanomaterials show unique properties at the nanoscale compared to its bulk counterparts. There are two fundamental reasons for this behavior of nanomaterials: surface effects and quantum effects. Let’s discuss both in bit details.
Effect of surface area
If you are given a cube of 1 cm x 1 cm x 1 cm and asked it to cut in to million equal pieces, you will end up with a 0.1 mm x 0.1 mm x 0.1 mm size cubes. If you were able to cut these small cubes without any material loss, we can see that volume of the all million small cubes and the cube we started with has the same volume. But what can we say about the surface area? If you do the mathematics you can see that the surface area of all the small cubes is 100 times more than that of the cube we started with. If we can continue the process, more and more atoms that were previously in the inner structure of the cube has now come to the surface. If it’s possible to cut this big cube in to a small cube that’s only 1 nm x 1 nm x 1 nm, the surface area would be 10 million more compared to the original cube.
This presents us the first interesting phenomenon occur at this scale. Nanomaterials have extremely high surface to volume ratios. This enables the nanomaterials to interact with the environment much intensely compared to bulk materials. On the other hand, Interior atoms of a material are more coordinated compared to surface atoms due to more bonds. This results in stable atoms. However, the atoms in the corners and the edges are less saturated due to less coordination and much less stable compared to interior atoms. If we take our cube cutting experiment, in a cube having 1 nm x 1 nm x 1 nm dimensions, almost all the atoms are in the surface and they are quite reactive. The fact that, some nanomaterials show extraordinary catalytic and absorbance activity is a good example for this effect.
Quantum effects (quantum confinement)
In quantum mechanics, a characteristic radius of electron mobility can be defined and called as Bohr exciton radius. Usual bulk semiconductor materials are much bigger than the Bohr exciton radius thus electron mobility is not disturbed or in other words “not confined”. Quantum confinement occurs when the particle size of the material is too small to be comparable to the Bohr excition radius, thus confining the electron mobility. This results, that the electron holes being squeezed in to small particle resulting “quantum confinement” of the electron hole pairs. This confinement means to confine the motion of otherwise randomly moving electrons to restrict its motion to specific energy levels. If the particle size decreases till the nanoscale dimensions the confining dimensions makes energy levels way specific that the material band gap increases. In simple words, if the electron and holes are constrained by reducing a particle size in to the nanoscale, then significant change of semiconductor properties can be seen. This is a key aspect that has great potential in many emerging electronic applications.