In 1959, physicist Richard Feynman, in his talk “There’s plenty of room at the bottom”, envisioned a future where we could engineer materials and devices from bottom up, by directly manipulating individual atoms. This field is now known as Nanotechnology. It involves developing devices and materials on a nanoscale, which is a just a billionth of a metre, a concept that nature seems to have perfected. From the nano-sized hair on a lizard’s feet that helps it grip vertical walls, to the nanostructures on a butterfly’s wing which create its appealing colours and the flagella on bacteria which assist in their movement, are all examples of nature using nanotechnology. Although humanity is yet to achieve the level of complexity we see in nature, recent advances in the field, some by imitating nature, have had an enormous impact, especially in the fields of technology and medicine. The idea of building helical nanostructures and then using them as propellers on tiny robots swimming inside the human body or blowing electron bubbles from a two dimensional sheet of electrons may sound futuristic, but this is the kind of research happening in the Optics, Nanostructures and Quantum fluids lab at the Indian Institute of Science (IISc).
The group was established in 2009 by Prof. Ambarish Ghosh. Prof. Ghosh, an undergraduate from IIT, Kharagpur, received his PhD from Brown University in Rhode Island, USA and then spent 4 years as a Postdoctoral Fellow at Harvard University in Massachusetts, USA. On returning to India, he joined IISc as an Assistant Professor and founded the Optics, Nanostructures and Quantum fluids laboratory. The three terms represent the three main areas of research being carried out at the lab. The team conducts their research at the Centre for Nanoscience and Engineering and at the Department of Physics in IISc.
One of the research areas that the group explores is Nanophotonics, which is the investigation of the interaction between particles of light, called photons, and nanomaterials. When two metal nanoparticles are in close proximity, a highly enhanced electromagnetic field is created in the gap between the two particles due to a phenomenon called Plasmonic Coupling. The current research at the lab focuses on building three dimensional arrays of porous silver nanoparticles on a silicon dioxide substrate separated by atomic distances and studying the plasmonic interactions in what is called 3D Plasmonics. The group was among the first to use a technique called Glancing Angle Deposition (GLAD) to fabricate the a particular kind of nanostructures, called the metal-dielectric alternate arrays. The process involves depositing the required material, on to a suitable substrate, kept at an angle to the incoming vapour. The pores between the nanoparticles provide an ideal spacing for an enhanced electromagnetic field between the particles which can be used in various optical sensing applications, like measuring the refractive index of the medium where the substrate is immersed.
The GLAD technique was also used to fabricate chiral or helical nanostructures. The helical structure is coated with gold and silver nanoparticles in order to study their optical properties. These structures have shown to produce different optical responses when exposed to left and right circularly polarised light respectively. The group fabricated left and right handed helical nanostructures and then measured the optical activity of the nanostructures using a method called Circular Dichroism spectroscopy. Circular Dichroism (CD) is the difference in the absorption of left and right circularly polarised light and usually occurs when there is a chiral object present. The CD spectrum obtained from these chiral nanostructures, engineered using GLAD, was shown to be significantly higher than that of any of the other similar artificial chiral structures reported so far.
The chiral nanostructures also form the basis of the nano-propellers or nano-swimmers, wherein nanostructures submerged in fluids are manoeuvred using homogenous magnetic fields. Such devices could be used in a number of applications, but their most important use is in biomedical applications, like drug delivery and bio-rheological measurements, which is the measurement of fluid properties, like viscosity, inside a living organism. The helical nanostructures are, once again, fabricated using GLAD and then coated with a magnetic material, like cobalt. This makes them responsive to an external magnetic field. Manoeuvring using an external field removes the need for onboard batteries and wires. The use of an external magnetic field also means, once inserted into a living body, manoeuvring the nano-propellers will be a non-invasive procedure, i.e., moving the nano-propellers does not involve the introduction of any other instruments into the body. Once coated, the nano-propeller is about 3 to 4 micrometers in length, which is similar to the average size of bacteria, which varies from 2 to 8 micrometers.
Taking inspiration from nature, the motion of the nano-propellers mimics the corkscrew motion of bacterial flagella. Once immersed in the fluid, the permanent magnetic moments of the nanostructure are aligned along its axis, i.e., the magnetic moments of all the cobalt atoms, which act like tiny magnets, are aligned with the axis of the helical structure, using an external magnetic field. On rotating the external field, the structure rotates with it, propelling it forward in the liquid. Prof. Ghosh assures, “The magnetic field applied is very weak, much weaker than the ones used in MRI machines and hardly has any effect on the body”. The group was able to use the nano-propellers to measure rheological properties while travelling from one fluid into another. Inside the body, such devices could monitor the blood and other fluids in real time. “Currently we are working in collaboration with the biological sciences department. We get cells from them and insert these propellers into them to study their motion in a real world environment” says Prof. Ghosh. Talking about its motion, he goes on to say “We are also attempting to give it some intelligence, like how a bird flying in a swarm can read the disturbance in air flow, caused by another bird in front, and adjust itself. We want these (nano-propellers) to be autonomous in their motion”.
Apart from application-based research, the group also explores more fundamental aspects of nature. Their work on Multi Electron Bubbles (MEB), bubbles whose inside surface is decorated with electrons, provides insight into the behaviour of electrons on curved surfaces. The bubbles are formed by, first showering electrons from a tungsten tip on to the surface of liquid helium, where they form a two dimensional layer of electrons. Simultaneously, the sheet is given a pull using an electric pulse which leads to the formation of MEB’s sized between 1 and 1000 micrometres. The surface of the bubble is covered by electrons, with some helium vapours inside and provides an excellent platform to study the properties of electrons on curved surfaces.
The trick however, is in trapping the bubbles long enough to study them. The bubbles have a lifetime of about a hundredth of a second after which they evaporate. The group has been able to trap the bubble, with a self-devised electric trap, for about a second. This gives them just about enough time to study the properties of the electrons on the surface. “We are also interested in studying the properties of the bubble at the nanoscale. When it is formed, the bubble is micron-sized, but then it starts shrinking and becomes nano-sized at which point it starts showing some interesting properties, since at these scales it acts more like a quantum object than a classical one” says Prof. Ghosh while talking of the current research.
For a long time humans have used nanotechnology without realising it, like the renaissance craftsmen who used nanoparticles to make stained glass, without really knowing how it worked. Currently however, with our understanding of the behaviour of matter at quantum scales, we are in a much better position to engineer new materials and devices. Scientific advances in nanotechnology have started to catch up with, and in some cases even rival, its occurrence in nature. Considering the time it took for some of nature’s solutions, humanity has made rapid progress in this field in just over the last 30 years. With the pace at which the field is progressing, it may not be long before we realise Feynman’s dream of building devices atom-by-atom.
About the group:
The lab is headed by Prof. Ambarish Ghosh (Contact email: ambarish@ece.iisc.ernet.in) and currently has 9 members. They work out of two labs, one situated in the Centre for Nano Science and Engineering building and another in the Department of Physics building. More information on the group and their work can be found here.