October 18, 2012
RedOrbit Exclusive Interview: Professor Shu Yang, University of Pennsylvania
Jedidiah Becker for redOrbit.com — Your Universe Online
Color is all around us - from painted buildings, to stop signs, to butterfly wings. Traditionally, colorful compounds, called pigments, are added to material to give them the desired color. However, color can often fade and is susceptible to damage from environmental conditions, like rain. A more novel way to create color is to synthesize polymers to create surface types and colors for specific applications. However, creating such compounds is a challenging process and is still being explored in laboratories around the world.
RO: From your personal webpage, we can see that you´ve been involved in several research projects that involve synthesizing polymers and other inorganic materials. Has any of your previous work focused on materials that exhibit superhydrophobicity or structural color, and what led you to want to combine these two properties?
Yang: My lab has been working on fabrication of photonic crystals using holographic lithography technique over the last 10 years. Three-dimensional (3D) photonic crystals are crystalline materials where the refractive index is periodically modulated on a length scale comparable to the light wavelength of interest. Interference of the light waves scattered from the crystal leads to complete stop bands or photonic band gaps (PBG), where the light of a particular wavelength is totally reflected in a photonic crystal, thus, illustrating a specific color.
The colorful display exhibited in butterfly wings, beetle scales and opals in nature — so-called “structural color” — is based on similar mechanisms; that is, the coloration is caused by the interference, diffraction, or scattering of light by periodic arrays of transparent materials. In the meantime, my lab has always been interested in surface properties, such as wetting and adhesion. Dated back to my graduate study at Cornell, I worked on the synthesis of low surface energy coatings and their potential application to create superhydrophobic surfaces that mimic the lotus leaves. At Bell Laboratories, Lucent Technologies, I worked on the fabrication of superhydrophobic surfaces using the top-down lithography process, which I continue at Penn with more emphasis on self-assembly approaches.
It is interesting to note that in nature, bioorganisms often possess hierarchical architecture with multiple functions. In the case of the butterfly wing, it has both structural color and superhydrophobicity, while a gecko´s foot hairs are both sticky and superhydrophobic. So in my lab, we have always been thinking whether we can combine different traits displayed in nature in synthetic materials.
RO: What kind of technical challenges did your team face in trying to combine these two unique properties?
Yang: To combine the two unique properties in a single material, one needs to be careful about the feature size that is responsible for each property but not interfering with each other. The two qualities – structural color and superhydrophobicity – are related by structures: structural color is the result of periodic patterns in the bulk material with a periodicity comparable with the wavelength of light, while superhydrophobicity is the result of surface roughness on micro- and/or nanoscales.
For color in the visible light, the size of the periodic structure should be in the range of submicron to a few microns. If the roughness responsible for superhydrophobicity is also in this range, it will compete with the structural color, even making the film opaque due to random scattering. Therefore, we carefully introduced nanoroughness (~ 100 nm or less) onto the 3D microstructure, where the 3D microstructure is responsible to the structural color, and the nanoroughness prevents the wetting of water on the surface.
RO: We understand that your lab team took a somewhat outside-the-box approach to combining these two characteristics. In your own words, could you briefly describe the problems with previously used techniques as well as where you got the idea to try a different approach?
Yang: We are not the first group that combines structural color and superhydrophobicity. To do so, it typically requires sophisticated chemical reactions and multiple nano- and microfabrication processes to introduce nano-roughness onto the microstructures. For example, researchers have used sequential assembly of different sized colloidal particles to create an inverse opal with nanoroughness. It is an art to control the assembly of the two different particles to ensure the smaller nanoparticles will fill into the voids between the larger submicron particles, rather than aggregating around the large particles.
Later, the larger particles were removed to create a microporous structure (or inverse opal) for the structural color, while the nanoparticles were solidified to maintain the 3D network and contribute to the non-wettability. Other approaches include multiple lithography steps, or the combination of either lithography or self-assembly with plasma etching or assembly of nanoparticles. Each step needs fine-tuning the processing parameters.
So we have been interested in methods that can simplify these steps. In my lab, we have investigated the solvent effect in self-assembly of polymeric nanoparticles and surface nanotextures. However, the discovery of the solvent effect in the 3D photonic crystals was rather accidental. In the past, we always tried to create 3D photonic crystals with smooth surface for the high optical quality.
The co-lead author, Dr. Guanquan Liang is an optical physicist. He was playing with different solvents in the holographic lithography process, hoping to improve the quality of the 3D photonic crystals he fabricated. Instead, after dipping the fabricated film in some solvents that were commonly used in holographic lithography, he often got spongy, nanospheres on the 3D microstructures. Frustrated that he could not get smooth surface, he came to my office. I was actually very excited that this could be a simple way to create superhydrophobic surface on a photonic film.
RO: Can you give us an idea of what this material would actually look like when applied to a large surface like, say, an office building or a house? Would it really have that same intense, shimmering quality that we associate with peacock feathers and butterfly wings?
Yang: Yes. Since the structural color is a reflective color that is dependent on the structure, it does not suffer photobleaching like pigmentation. As long as the structure maintains its integrity, we will always see the intense shiny color from these materials. However, to fabricate the 3D photonic structures reported in our paper, we used a state-of-art non-conventional 3D lithography technique. So it is not intended for low-cost, large area fabrication. We believe that the concept we demonstrated here is applicable to other fabrication methods.
RO: Aside from its potential use in beautifying the outsides of buildings, have you imagined any other potential uses for such this material, or is that something you plan on leaving to the marketing experts?
Yang: It could be used as a traffic sign, which needs to be shiny and clean in the rainy or snowy days. It could be used as a bulletin board on the highway or on the building. It could be used as a fancy, protective cover of the iPhone or iPad. It could also be used as camouflage or something that could be worn by the soldiers, for example, as blast injury dosimeters.
We are currently looking into new methods that will allow us to mass-produce these materials for potential commercialization. Of course, we welcome any suggestion from experts about market needs.
RO: Looking at your resume, you´ve been a very busy materials engineer in recent years. Do you already have your eyes on a new research project? Care to give us a sneak peek?
Yang: I am interested in applying the fundamental research to practical applications. As mentioned above, our current interest is to further simply the fabrication method for mass-production. We are on track with that goal. One drawback of structural color is that the color is angle dependent due to Bragg diffraction. However, Morpho Blue butterfly wings have angle-independent color. So we plan to mimic that.
We are also working on dynamically tuning colors for display, specifically from colored films to transparent windows, as well as creating transparent, superhydrophobic surfaces. We are interested in integrating the materials in optical sensors for energy efficient buildings, while the transparency is very important to solar cells, windows and eyewear.
Shu Yang is an Associate Professor in the Department of Materials Science and Engineering at University of Pennsylvania. She received her B.S. degree in Materials Chemistry from Fudan University, China in 1992, and Ph.D. degree from Chemistry and Chemical Biology at Cornell University in 1999. She then joined Bell Laboratories, Lucent Technologies as a Member of Technical Staff before moving to Penn in 2004.
She has coauthored approximately 100 peer reviewed technical papers, holds over 20 patents that are issued or pending, and has edited 2 books. She is a recipient of ICI (1999) and Unilever student awards (2001) for outstanding research in polymer science and engineering from American Chemical Society (ACS). She was selected to present at the “Japan-America Frontiers of Engineering symposium” (2011) by the National Academy of Engineering, and MIT's TR35 as one of the world´s top 100 young innovators under age of 35 (2004). She received the prestigious Faculty Early Career Development (CAREER) award from NSF in 2006.
Her group is interested in developing innovative protocols (i.e. materials synthesis and fabrication techniques) at the convergence of top-down and bottom-up approaches for directed assembly of complex, multi-functional structures from polymers, gels, biomaterials, and organic-inorganic hybrids. By coupling of chemistry, fabrication and external stimuli, they address the fundamental questions at surface-interface in a precisely controlled environment, and study the structure-property relationship. Specifically, her research focuses on synthesis and engineering polymers and hybrids with controlled size, shape, and morphology over multiple length scales, study of their unique surface (wetting, adhesion and biocompatibility), optical, and mechanical properties and instability, as well as their dynamic tuning.