Inspirations from nature for nanotechnology: Optical effects

Inspirations from nature for nanotechnology: Optical effects

by Chamali Malaarachchi

Nanotechnology is a highly diversified and widely spoken topic in science. The modern concepts of nanoscience and nanotechnology started with a talk entitled “There’s Plenty of Room at the Bottom” by physicist Richard Feynman in 1959.  Extraordinary properties at nanoscale that are unachievable at macroscopic level can be achieved by nanomanipulation of materials. Nanotechnology is the design, synthesis, characterization, and application of particles by the controlled manipulation of their shapes and sizes in the nano range of 1-100 nm with at least one dimension of the particle below 100 nm. Going into deep, nanometer is one-billionth of a meter. Making it more imaginable, a human hair is approximately 100,000 nanometers in width and an inch contains 25,400,000 nanometers.

Nanomaterials often possessing different, unusual properties when compared with their macroscale materials which can be explained mainly by four reasons where the first would be their greater surface area to volume ratio. It causes the material to get more exposed to the surrounding and therefore, be more reactive. Secondly, electromagnetic forces become more prominent over the gravitational forces given the fact that, nanomaterials are very light in weight. Quantum effects like quantum entanglement and surface plasmonic vibrations become more prominent with the reduction of particle size from macroscale to nanoscale. At nanoscale, properties in nanoparticles will strongly depend on its size and the movements of electrons. The fourth possible reason would be the random motion of nanoparticles. Attributed to the above reasons, along with many of the other properties including chemical, electrical, and magnetic properties, etc., optical properties dramatically vary at the nanoscale. Controlling the size, shape and surface functionality of the nanoparticles causes different optical effects.

The optical effects of nanomaterials and their wide variety of applications are not a novel concept even though it is widely spoken at present. There are shreds of evidence for the utilization of optical effects of nanoparticles since ancient times, possibly unintentionally without knowing the concepts behind them. Stained glass made in 4th BC contained gold nanoparticles dispersed in the silicon matrix to absorb all frequencies except red in the visible light resulting in a bright red color reporting to be the first attempt of a nanotechnology application. Another well-known example, the Lycurgus cup made with dichroic glass appears green in reflection and red in transmission due to the gold and silver nanoparticles dispersed throughout the glass.

The Lycurgus cup. The glass appears green in reflected light (A) and red-purple in transmitted light (B). (Courtesy: British Museum)

There are many fascinating applications of the optical effect of nanoparticles in nature. For instance, the colors in the morpho butterfly wings independent of a pigment, solely originate with the basis of structural colors. The ordered hexagonal packed surface patterns usually in nanoscale made with chitin, comparable with the visible range wavelengths of the electromagnetic radiations provide different colors by manipulating the incident and reflected wavelengths.

                The colors in the morpho butterfly wings (Courtesy:

Moths’ eye having hexagonal-shaped small bumps falling into few nanometers range, on the surface which are smaller than the wavelength range of visible light results in the absorption of more light attributing for a better vision even at dark and dim light conditions and giving out a low reflectance helping it to get protected from predators. Mimic of moth-eye creates the antireflective films giving sunlight readability to smartphones and similar devices and enhances the efficiency in thermo-voltaic cells by improved IR light absorption.

Chameleon cool coating (C3) has been developed by scientists for simultaneous achievement of two properties black color and coolness said to be contradicting as black objects absorb all incident radiation making the temperature in its vicinity to be increased. The chameleon skin is composed of two layers: the upper layer with superficial iridophores that provides camouflage by changing the skin color in response to emotions and environmental factors and the lower layer with deep iridophores that provides thermal protection. The layer of superficial iridophores contains different shaped and sized nanocrystals to manipulate the visible light which make cells to reflect shorter wavelengths like blue and green by coming closer to each other in the relaxed state and to reflect shorter wavelengths like red and orange in the excited state by nanocrystals getting distanced apart. The deep iridophores reflect a large amount of near-infra-red (NIR) radiation making the chameleon to stay cool. The C3 has been developed by imitating this concept in a single layer composite having spherical CuO nanoparticles to control visible light to obtain the black color while spherical TiO2 nanoparticles are developed to reflect NIR radiation achieving thermal barrier properties.

Concept of the C3 (Courtesy: Nature research journal)

Such fascinating technologies have been developed as a direct inspiration of natural phenomena in nanotechnology and it is a privilege that nanotechnology has been developed to a level that could imitate certain natural concepts for the betterment of the future of mankind.