Colour has captivated the human imagination for centuries, inspiring artists, poets, and scientists alike. While most of the colours we encounter in everyday life are produced by pigments, there exists a fascinating alternative known as structural coloration. In this blog, we embark on a journey through the captivating realm of structural coloration within the field of nanoscience, uncovering the intricate mechanisms behind nature’s vibrant hues.
Understanding Structural Coloration:
Unlike pigments, which absorb certain wavelengths of light and reflect others to produce colour, structural coloration arises from the interaction of light with nanostructured materials. At the heart of this phenomenon lies the manipulation of light waves through precise control of nanoscale structures. These structures can be found in diverse organisms. Most famously seen in the Morpho butterfly but these organic nanostructures also occur in birds to beetles and even some plants. Scanning Electron Microscopy (SEM) images of the wing of a morpho butterfly are shown [below] [1]. Where colour is typically seen from a material absorbing certain regions of light within the visible spectrum, thus reflecting the rest and this reflection being what we perceive as colour, this is not present in these wings.
As seen in the figure above, the striking blue hue of the Morpho butterfly is due the arrangement of nanostructures on its wings as opposed to a blue pigment. Thus structural colour must be a result of the interference of light being scattered and reflected off these structures. Comparing the SEM images of the butterfly wings above to a diffraction grating, a tool commonly used to demonstrate how white light can be separated into a spectrum via these same principles of diffraction and interference shows this. [2]
In butterfly wings, these ‘organic diffraction gratings’ are formed from tree shaped arrays of the polymer ‘chitin’, an amide derivative of glucose – a sugar found in most living organisms. These structures are so small in fact that when the first scientists investigating the iridescent nature of butterfly wings and peacock feathers, two rather famous blokes – Newton and Hooke – could not resolve them with simple light microscopes. This property of these structures is not only the reason we need SEM to gain a good understanding of their physical and optical properties but will also be one of the main challenges faced by scientists in mimicking these architectures.
So how do we currently make these patterned nanostructures??
For years different lithography methods have been used, with photolithography being the most common by far due to its scalability. This method works by laying a thin layer of a polymeric photoresist on top of a wafer (typically silicon). A mask is then placed atop this photoresist in the shape of the desired structure, or the negative of this structure depending on the photoresist’s nature. Exposing this to light results in the unmasked photoresist undergoing a reaction which either results in the polymers comprising the photoresist becoming stronger or weaker depending on its nature. This allows for selective removal of either the photoresist protected by the mask or that which was exposed to light. Naturally this method is very scalable as the only limiting factor is the size of the wafer on which you can print your photoresist.
Due to the limited resolution which photolithography can provide, other methods for forming nanostructures have needed to be developed. Minimum feature size is proportional to wavelength given by the Rayleigh criterion: 1.22(λ/D)
Where D is the diameter of the aperture of the lens used to focus the light on the sample. Visible light, having a large wavelength (yes in this case a few hundred nanometres is large) in the best-case scenario can only be used to form structures approximately 100 nm in size. To combat this electron (E) beam lithography was developed, taking advantage of the wave-particle duality of electrons discovered by Louis de Broglie. Electrons, having a miniscule wavelength in comparison to visible light can be used to make features on length scales unfathomably small with reference to what was possible with photolithography. These wavelengths operate within a range depending on the voltage at which the electrons are accelerated with but for example at 100 keV the electron will have a wavelength of 3.88 pm. [3] Each picometre is 1 thousandth of a nanometre. To show how small this is, the average diameter of a human hair is 50 micrometres, this is equivalent to approximately 500 million picometres.
Perfect right? We are now able to make things as small as, and even smaller than the nanostructures we are trying to mimic, all problems sorted. Unfortunately, no. Where advances in E beam lithography have allowed us to create nanoscale architectures with sub 10 nm resolution, this method also has its flaws. Despite being very slow as you need to direct each and every electron individually, like writing with a pen, and hence very expensive we also cannot make structures as complex and intricate as billions of years of evolution have allowed the Morpho butterfly. Another huge problem is the fact that these methods are only capable of working on flat surfaces as incident light/electron beams need to be orthogonal to the photoresist they intend to activate.
This inability to make these nanostructures on curved surfaces was a large problem faced by lithographic techniques. Without extending our ability to form these nanoscale architectures into three dimensions we could never hope to achieve the same control over light which flora and fauna have so gracefully managed through millennia of evolution.
An entirely new, collaborative approach was needed. Enter the humble physical chemist. As mentioned, the Morpho butterfly forms its nanostructures with tree like architectures from the polymer chitin and in the immortal words of Aristotle – “If you can’t beat them, join them”. By gaining control of Block Copolymers, [4] a method in which two polymers with wildly different chemical properties are linked via a covalent bond. Polymer chemistry allows the formation of chains with almost any different structure desired. Choosing the right one may allow structures as seen for the Morpho butterfly to be made, but why stop there? The limitation is simply your imagination. Once the polymer is chosen, the two polymers can be easily phase separated into different domains but essentially remain linked via their covalent bond. This is known as microphase separation. This bond stops the two polymers from fully avoiding each other once in solvent and thus forms region after region of phases with opposite properties. These phases can be as big or as little as one likes (within reason i.e. 5-100 nm). Now with the phases separated, one can take either a chemical approach to remove the undesired polymer and leave the desired one or return to lithography if the undesired polymer has been designed to separate in a reaction with light. This process is shown below, where the first step has the chains of polymers tangled as much as your headphones are when you take them out of your pocket.
This method has allowed for many different styles of nanostructure be formed, mimicking many different architectures found in nature such as the nanowires shown above which are akin to the structures found in peacock feathers or the nanorods found in the eyeballs of moths which are used for light absorption to avoid predators/prey seeing them at night. These structures have many prospective uses in tech and optics industries. Think about how hard it is to see your phone screen on a bright day. An antireflective coating with nanostructures like those found in moths would render this problem null and void.
Further applications include the superhydrophic nature of the nanostructures found in cicada wings. [5] These structures have patterns so small that they go beyond the Rayleigh criterion for distinguishable features with natural light. This essentially means that they are transparent to the human eye, a wonderful feature for laying a coating on top of screens and anything else we wish to see behind. As mentioned, this was an area where E-beam lithography excelled. These tiny structures repel water and thus all the germs and bacteria present in water. These structures are shown in the Atomic Force Microscopy image below.
For this reason, when a cicada dies its body decays as any typical organism would, but its wings do not. It’s wings last unaffected by decay for hundreds of years due to these nanostructures. While I said above that E-beam lithography is expensive and difficult to scale, in the future if it is possible to scale this technology up, or if further advances are made in the block copolymer process, this technology could be used in the healthcare sector. Coating floors, walls and screens in hospitals, areas which are constantly being touched by both sanitized and unsanitized hands and feet may aid in the quelling of disease spread.
Links and references
- https://www.smartmaterialsolutions.com/blog/tag/Blog
- https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.plymouthgrating.com%2Fproduct%2Ftransmission-diffraction-gratings%2F&psig=AOvVaw1GrGzVqljOxIWKV357pwS5&ust=1713006595193000&source=images&cd=vfe&opi=89978449&ved=0CBIQjRxqFwoTCIDbhZDFvIUDFQAAAAAdAAAAABAO
- https://advanced-microscopy.utah.edu/education/electron-micro/index.html
- Mokarian, Parvenah. “Chemistry of Polymers and Macromolecules”, JS NPCAM course, Trinity College Dublin.
