Size does matter: Nano vs. Macroscopic world

We live in an era of nanomaterials, nanotechnology, and nanoscience. What is so special about this nano world? How different is it from the macroscopic world of conventional bulk materials? How size influences the difference in properties in these two distinct worlds, although the basic material is same? For example, the properties of gold nanoparticles are distinctly different from the bulk gold. One simple answer is nanoparticles consist of fewer atoms to few thousand atoms while the bulk materials generally Fig 1 gold macro vs nano composed of billions of atoms. Look at the image below. At nanoscale, gold does not look even yellow! All of us know that gold (in bulk) is an inert metal. However, the same metal at nanosize of about 5 nm works as a catalyst in oxidizing carbon monoxide (CO). Therefore, size does influence the property. But, how? What happens when a material breaks down to nanoscale? Part of the answer lies in the number of surface atoms. Let’s elaborate it. We know that at bulk state gold forms face centered cubic (fcc) lattice where each gold atom remains surrounded by 12 gold atoms, even the gold atoms at surface is surrounded by six adjacent atoms. In a gold nanoparticle, a higher number of atoms sit at the surface, and surface atoms are always more reactive. These large numbers of exposed atoms in gold nanoparticles compared to the bulk material enable gold nanoparticles to function as a catalyst.

Now what happens to the color? At nanoscale, gold loses its vibrant yellow color. While light gets reflected from the surface of the gold at bulk state, the electron clouds resonates with certain wavelength of light at nanoscale. Depending on the size of the nanoparticle, it absorbs light of certain wavelength and emits light at different wavelength. For example, nanoparticles of sizes about 90 nm absorb red to yellow light and emit blue-green, whereas particles around 30 nm in size absorb blue and green light and appear red in color.

The physical properties such as melting point, boiling point, conductivity, etc. also change in nanoscale. For example, Fig 2a when gold melts in its bulk state regardless whether it’s a small ring or big gold bar, all melts at the same temperature. But this is not true for nanoparticles; with decrease in size, the melting point lowers and it varies by hundreds of degrees (Check the inset picture). This is because when a matter reaches nano-regime, it no longer follows Newtonian or classical physics, rather it obeys the rules of quantum mechanics. The nanoeffects which are relevant for nanomaterials are as follows: (i) Gravitational force no longer controls the behavior due to the very small mass of the nanoparticles, rather electromagnetic field determines the behavior of the atoms and molecules; (ii) Wave-particle duality applicable for such small masses, where wave nature shows pronounced effect; (iii) As a result of wave-particle duality, a particle (electron) can penetrate through an energy region or barrier (i.e. energy potential) which is classically forbidden and this is known as quantum tunneling. In classical physics, a particle can jump a barrier only when it has energy more than the barrier; Fig 2_tunneling therefore, the probability of finding the particle on the other side the barrier is nil if the particle possesses less energy than the barrier. On the other hand, in quantum physics, the probability of finding a particle, with less energy required to jump the barrier, on the other side is finite. However, to have a tunneling effect, the thickness of the barrier should be comparable with the wavelength of the particle and this is only possible in nanoscale level. Based on quantum tunneling, scanning tunneling microscope (STM) is created to characterize the nanosurfaces.

(iv) Quantum confinement i.e. electrons are not freely movable in bulk material rather these are confined in space. Size tunable electronic properties of nanoparticles arise due to quantum confinement.

(v) Energy quantization i.e. energy is quantized. An electron can exist only at discreet energy levels. Quantum dots, a special class of nanoparticles of size 1-30 nm, show the effect of energy quantization.

(vi) Random molecular motion: At absolute zero molecules are always moving owing to their kinetic energy, although this motion is not comparable to the object at macroscale. However, at nanoscale, this motion becomes comparable to the size of the particle; hence, influence the behavior of the particle.

(vii) Increased surface-to-volume ratio: The changes in the bulk properties (mp, bp, hardness, etc.) can be attributed to the enhanced surface-to-volume ratio of nanoparticles.

Therefore, in a nut shell, because of the above mentioned changes, the properties of a material in nanoregime differ from macroscale.

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