In a novel approach that mimics nature, Bengaluru-based researchers have designed crystalline materials that selectively scatter specific colours of light. Dyes and pigments produce colour predominantly through selective absorption of light. But scattering of light by particles which are arranged in an ordered, periodic pattern produces structural colour, which gives butterfly wings their colour and sheen.
The backlit colour display of a mobile or laptop monitor becomes difficult to read under intense light. But if the front panel were to be made of structural colour then the ambient light would become a source of colour. By producing crystals that scatter wavelengths corresponding to red, green and blue light, structural colours can be used in place of the conventional LED and LCD monitors, too.
In nature, nanosized particles and colloids are responsible for producing structural colours. Compared with atoms, colloidal particles are 10,000 times bigger, and, so, conventional lab techniques to move the particles over long distances to form an ordered, periodic pattern have been riddled with problems.
The novel approach adopted by researchers at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) and Indian Institute of Science (IISc) has overcome the challenge of transporting the particles to target sites; the size and symmetry of the growing crystallites are also controlled. The results were published in the journal Proceedings of the National Academy of Sciences.
structural colours can be used in place of the conventional LED and LCD monitors.“We need some mechanism which will drive the particle in a given direction to a long distance. We solved this by creating an energy gradient on the surface of a Moiré pattern. The energy gradient dictates where the colloidal particle should go and nucleate and grow in an ordered fashion,” says Chandan K. Mishra from the Chemistry and Physics of Materials Unit, JNCASR, and the first author of the paper.
To produce the Moiré pattern the researchers first imprinted an optical grating (which has linear trenches drilled on a glass surface) on a soft polymer. Rotating the optical grating at a small angle and repeating the imprinting on the soft polymer led to the creation of a geometrical pattern. The Moiré pattern with channels of non-uniform depth was the template on which the colloidal particles get deposited at specific sites in an orderly pattern.
“There is an energy gradient within the geometric pattern which drives the particles to the desired locations,” says Prof. Ajay Sood of the Department of Physics, IISc and a coauthor of the paper. The energy gradient comes from the variation in the depth of the channels and the presence of small particles driving the colloidal particles to the specific sites in the pattern.
In the presence of smaller particles (which are added along with the colloidal particles), the colloidal particles are attracted towards the channel wall. “Since the channel has a gradient in depth, the smaller particles drive the colloidal particles to the deeper portions of the channel where the particle is surrounded by tall ridges on either side. This is the final resting place of the colloidal particle. If all the particles come to this point they form a crystal,” says Prof. Rajesh Ganapathy from JNCASR and one of the corresponding authors of the paper.
The nucleation initially begins at the sites where the ridges have maximum height and then progressively spreads to sites where the channel height is less. The distance between two nucleation sites is predetermined to scatter a particular colour, for instance, red.
The process is repeated with colloidal particles of a different size which will grow into crystals with a different separation distance between them (periodicity). Due to a different separation distance the crystals will then scatter light of a different wavelength. “Our eventual goal is to make these patterns and drop particles of three different sizes at the same time and the Moiré pattern will decide where each particle size should go and form crystals that scatter red, green and blue wavelength,” says Prof. Ganapathy. “What we have done is the first step — controlling the colloidal self-assembly using the Moiré pattern.”