Nanostructures; a transformative technology Sub-wavelength nanostructures can affect a variety of a material’s surface properties, including its reflectivity, hydrophobicity and anti-viral properties. We spoke to Dr David Nugent and Professor Parvaneh Mokarian about the work of the SUN-PILOT project in developing a process for creating such nanostructures, and the potential wide-ranging applications of the technology. A variety of functions can be introduced to a material by nanotexturing its surface, including hydrophobicity, self-cleaning, and anti-reflectivity, and researchers are drawing inspiration from the natural world as they seek to develop such nanostructures. Moths for example have anti-reflective eyes, with subwavelength, periodic structures that suppress reflections, while certain other animals and insects have anti-microbial surfaces. “Those functions are the result of nanotexturing,” explains Dr David Nugent, the founder of technology development company Elucidare, a partner in the EU-funded SUN-PILOT project. Injection moulded plastic with nanoengineered surfaces.
SUN-PILOT project The ability to create similar sub-wavelength nanostructures and apply them on different materials could bring significant benefits across many sectors, from healthcare to the automotive industry, a topic at the heart of SUN-PILOT. An initiative bringing together academic and commercial partners from across Europe, the aim in SUN-PILOT is to develop a cost-effective means of producing periodic nanostructures, utilising a technology called block co-polymers. “This is a class of polymers made of two chemically incompatible polymers joined by a very strong covalent bond,” says Professor Parvaneh Mokarian, founder of the technology and SUN-PILOT coordinator from Trinity College Dublin. This covalent bond means that two blocks in a polymer chain can only separate to a limited degree when mobility is introduced into the system, and periodic self-assembled nanostructures then form on the substrate. Changing the molecular weight of each block enables researchers to modify the periodic structure, an issue Professor Mokarian is exploring. “We are working on high molecular weight block co-polymers. The higher the molecular weight, the larger the domain size and periodicity you get,” she outlines. “We basically need to have these high molecular weight block co-polymers to go towards higher wavelength anti-reflective surfaces.” There are other means of creating subwavelength structures that lead to the creation
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of random structures, which are used to some extent in nature to create anti-reflective surfaces. However, the aspiration from the outset in SUN-PILOT has been to replicate the periodic structures found in the moth eye. “If you look at a moth eye under a microscope, you can see, categorically, that a moth eye has a periodic structure. That is not by accident,” stresses Dr Nugent. In recent research, done through the SUN-PILOT project, the benefits of periodic structures over random ones have been demonstrated. “You get better optical performance and better mechanical stability, or robustness,” he continues. A key aim now in the project is to increase
There is not yet a means of manufacturing these devices at large scale and low cost, an issue that partners in SUN-PILOT are working to address, with the project covering each step of the innovation cycle. Alongside developing the periodic structures, researchers in the project are also investigating how they can be transferred onto a substrate. “In SUN-PILOT we’re using a solution-based process. We dip the optic into a bath of the block co-polymers, and then they naturally form these microdomains via an annealing process,” outlines Dr Nugent. “As a result we’re theoretically able to create these nanostructures over any 3-d structure.”
How often do you have to hunch over your phone to block out ambient sunlight to see what’s on the screen? the periodicity of these nanostructures, which would then open up new possibilities in terms of applications, for example in the optics industry. The periodicity of the structure should be somewhere between 0.5-0.7 times the smallest wavelength of interest, which in the case of the UV spectrum would be about 300 nanometres, although there are different definitions. “That would imply that you want to have periodicity of about 150 nanometres. Wavelengths above that will see these structures as sub-wavelength, and therefore we will be able to achieve these anti-reflective properties,” says Dr Nugent.
Applications The process could be applied to any 3-d object, yet this is not an immediate objective in the project, with researchers at this stage targeting applications in the optics and automotive industries. The workhorse of the optics industry is fused silica, which is commonly used in lenses on tablets, mobile phones and other devices; Dr Nugent says anti-reflective surfaces would make it much easier to use these devices outside. “How often do you have to hunch over your phone to block out ambient sunlight to see what’s on the screen?” he points out. Anti-reflective
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surfaces would also significantly improve device efficiency. “We would get a better battery lifetime, as you wouldn’t have to compensate for ambient light by cranking up the power of the display,” explains Dr Nugent. A second major area of application is in the automotive industry, which involves etching into stainless steel. This is a more challenging material to etch onto, yet valuable insights have been gained during the project. “We’ve learned a lot of important lessons from the automotive industry,” says Professor Mokarian. Looking beyond the scope of the project, Professor Mokarian believes there are a wide range of potential applications of these nanostructures. “I would envisage that the process will be adopted by different companies and different industries, and modified to suit their particular needs,” she says. “The use of block co-polymers – engineered to achieve domains and periodicities that are suitable for the particular application – will be the common theme.” The development of a pilot line for producing tailored polymeric fluorinated additives is another useful outcome of the automotive application. Engineered by SUNPILOT partner micro resist technology GmbH, these mould-release chemicals will serve an important role in the advancement of injection moulded parts featuring functional nano-patterned surfaces. One exciting possibility is in using nanostructures to give a surface anti-microbial properties, which again is inspired to a large degree by observations of the natural world. For a long time it was thought that the antimicrobial properties of nanostructured surfaces in nature arose because they are inherently hydrophobic, and while that is indeed true, Dr Nugent says it doesn’t tell the whole story. “Another phenomenon is going on, which is that microbes become impaled on the sharp ends of these nanostructures,” he explains. “While it has been known for over ten years that something similar happens to bacteria, it’s only within the last two years that it’s become known that the same thing happens to viruses as well.” This results in a surface that is not just self-
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Ambient light reflections dramatically reduced by surface nanotexturing.
SUN-PILOT SUbwavelength Nanostructure PILOT Project Objectives
The objective of SUN-PILOT is to develop a novel and cost-effective platform for upscaling the fabrication of sub-wavelength nanostructures across large and non-planar surfaces. This will be achieved using stateof-the-art block copolymer chemistry and highly scaleable etching and injection moulding methods. Specific objectives include the demonstration of a clean and sustainable nano-patterning technology capable of reducing the maintenance and capital investment costs for optical component users whilst enhancing the lifetime of anti-reflection parts.
Project Funding
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 760915.
cleaning, but is also inherently anti-microbial and anti-viral. Similar nanostructures could be applied to door handles in hospitals, to prevent the transmission of viruses, or to make surgical devices self-cleaning, while there are many further possibilities. “Sub-wavelength structuring – nanotexturing – can change many aspects of an object’s properties. For example utility companies could use nanotechnology to prevent pipes getting clogged and dirty, as nothing could stick to the walls, so reducing maintenance costs,” outlines Dr Nugent. Nanostructures could also be used to improve the overall absorption efficiency of photovoltaic panels, underlining their versatility. “Sub-wavelength nanostructures can be used in many different ways,” stresses Dr Nugent.
Project Partners
www.sunpilot.eu/partners
Contact Details
Professor Parvaneh Mokarian Research Associate Professor AMBER and School of Chemistry Trinity College Dublin Ireland T: +353 1 896 3852 E: parvaneh.mokarian@tcd.ie W: www.sunpilot.eu
Parvaneh Mokarian
Parvaneh Mokarian is a Research Associate Professor in School of Chemistry and a PI in AMBER centre, Trinity College Dublin. Her research interest is on polymer thin films, polymers at surfaces and interfaces, lightnanostructure interaction, cell-nanosurface interaction and soft nanotechnology. Her research team is currently focused on creating templates for sub-wavelength nanostructures for industrial use for applications in optics, antireflective surfaces, metamaterials, self-cleaning and functional/smart surfaces
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