Nanotech for next-gen colours

Nanostructures have, in recent years, seen increasing applicability in industrial processes that recreate natural colours through artificial methods. They are used to reflect light in ways that generate artificial structural colours which are inspired by colours from nature. The high-end nature of these processes, and the resultant carbon footprint, have made sections of the industry look for more robust and cost-effective alternatives.

Researchers at the Indian Institute of Science (IISc) are working on a technique that is in line with these efforts toward sustainability. A cost-effective, single-step fabrication technique for designing reflective displays and mechanochromic sensors is being developed at the Laboratory of Advanced Nanostructures for Photonics and Electronics (LANSPE), at the IISc’s Department of Instrumentation and Applied Physics (IAP).

Mechanochromism refers to a change of colour that a material undergoes when it is put under mechanical stress. The research team is looking at the new fabrication technique—it allows dynamic tunability of the medium of display—as a significant improvement on the existing options that involve complex, multi-phase fabrication methods. The technique comes with the promise of scalability and varied applications, including in soft robotics, pressure sensors, and smart windows.

Tapajyoti Das Gupta, assistant professor at the IAP, details how the process is different from the existing methods, like the use of lithographic techniques on nanoparticles, or the applications of 2D photonic crystals, where a new optical property is achieved through the variation in reflectivity among the multiple layers of the structures. These are complex processes that, still, do not offer the option of dynamic tunability.

The technique developed at the IAP, in contrast, causes a change in the optical property of the sample when it is bent, or stretched, demonstrates Das Gupta. The research team has applied for a patent on the technology.

A range of colours

At LANSPE, Das Gupta and his team work with samples that are made of polydimethylsiloxane (PDMS). PDMS is a unique and special substrate. It can be thought of as noodles with varying amounts of gravy where we have control over the amount of gravy. It is this gravy-like substance which interacts with the Liquid Gallium metal to form a layered nanodroplet structure. The research team has studied and modelled the interaction of this gravy with Liquid Gallium.

Das Gupta points out that the liquid content of the sample is key, based on which diverse colours—“more or less the whole spectrum”—could be generated. The research team plays with the properties of PDMS to develop dynamically tunable optical devices. “By controlling these PDMS properties, we can tune the particle sizes, which has not been done before,” he says. A comparison between the deposits made on the PDMS and glass/silicon substrates reveals how the particle sizes are substantially different, even under the same scale level.

“On the hard substrate, we can see a big and broad distribution of particles, resulting in a broadband optical spectrum and hence dull colours, whereas, on the PDMS sample, we have a well-defined distribution which is what gives its colour. Glass or silicon, when they are tested under the same conditions, gives out white or grey while PDMS, when its properties are tuned, gets us a variety of colours,” he says.

The researchers, by tuning the strain effect, have achieved a change in the hue, brightness, and CIE (a mapping system that quantifies colour and how humans perceive it). At the laboratory, the researchers show how the technology is tested for its robustness. The colour change achieved in the sample is analysed and repeated over “more than a thousand cycles” before its reliability is validated.

The technology has the potential to change the way high-colouration displays including advertisement installations are conceived and fabricated, says Das Gupta. The idea was to identify cheaper, simpler, and scalable fabrication techniques.

“It is a one-step, cost-effective fabrication that, with a deposition of just about a few hundred nanometres of film, could get you a variety of colours. The LED-based displays have been found to be extremely energy-intensive while the fabrication employed here ensures that the system could work even with a bit of surrounding light,” he says.

Robots and object detection

The researchers say the innovation can have applications across sectors, including robotics where the technology can enhance the performance in primary tasks like object detection. Picture a robotic arm that is holding an object, with fingers that have the technology attached to them.

By tracking the change in colour caused by the pressure which is applied on the object, through the bending of the fingers, a distinction could be made between what the arm is holding—an apple? a chunk of wood? jelly? In vision-guided systems, a small camera could be attached to detect and process these colours. Das Gupta says this visual effect, facilitated by the technique, comes as an important upgrade on some of the prevailing electrical identification methods. Detection of fingerprints is another area the innovation could prove effective in.

In the healthcare sector, the innovation carries the possibility of supplementing pressure sensors. In its basic applicability, the technology could be used in a wearable device to identify the optimal pressure required while tying a bandage. By calibrating colours in accordance with the effect that the bending/stretching has on the bandage, the optimum pressure for tying it could be identified.