Nanostructured materials can have unique properties that are not possible using macroscale materials. Some examples can be found in nature, where the surface texture and geometry of various organisms have evolved over time to optimize for certain operations. Our research approach is to first understand the physical origins, and then improve on nature’s design by using first-order scaling laws, rigorous analytical models, finite element methods, and numerical optimization routines.
Dynamic Color
Many creatures in nature have the amazing ability to change color, such as the chameleon and the fish neon tetra. These are based on active manipulation of periodic nanostuctures in their skin, which allows the reflectance spectra to change in real time. We have made an engineered material that mimics this behavior [1], as inspired by the neon tetra. Our approach is based on magnetic field-induced assembly of nanoparticles on a lithographically patterned template. The nanoparticle assembly aligned to the magnetic field, allowing the nanostructures to tilt when the field angle is changed. Initial prototype demonstrates reversible color change from green to yellow, a shift of roughly 30% (see Gallery page for demonstration). This research can find applications in low-power reflective display and dynamic camouflage.
Nanostructured Aerogel Films
Porous materials such as aerogel have many attractive properties, such as high strength and stiffness to density ratio, low thermal conductance, and has large surface area. We are investigating fabricating highly porous materials (>95% air) with periodic order using nanolithography techniques. This can potentially lead to porous materials with enhanced mechanical, optical, and thermal properties. We recently demonstrated an air-like porous film with refractive index of 1.025 (index of air is 1), the lowest reported value for a thin film [2]. However, it still has ~GPa stiffness, allowing the film to be integrated into devices and systems. This material can find application in thermal barrier coating and optical/thermal insulting layer in integrated photonics.
Stretchable Transparent Conductors
Naturally occurring electrical conductors such as metals are generally opaque and rigid. Our group has been examining using nano-accordion structures to create conductors that are also stretchable and transparent. Taking inspiration from springs, we use nanolithography to pattern nanoscale accordion structures made out of aluminum-doped ZnO, as shown in the figure above [3]. These structures can be stretched to 50%, which is 2 orders of magnitude better than bulk ZnO, and has ~70% transparency in the visible spectrum (see Gallery page for demonstration). Such structures can have many applications in wearable devices and display.
Multifunctional Nanostructured Glass
One example we have demonstrated is nanostructured glass with both improved optical and wetting properties. The nanostructured glass is anti-fogging, self-cleaning, anti-glare, and has enhanced transmission [4]. The surface of the glass consists of high aspect-ratio periodic nanocone array, shown above. We have also investigated using interfacial nanostructures between dissimilar materials to suppress thin-film interference [5] and enhanced optical transmission [6]. Such structures can have many applications in more efficient solar systems, high performance glass, and display.
References
[1] Z. Luo, B. A. Evans, and C.-H. Chang, “Magnetically Actuated Dynamic Iridescence Inspired by the Neon Tetra,” ACS Nano, 2019, 13, 4, 4657-4666 [link].
[2] X. A. Zhang, A. Bagal, E. C. Dandley, J. Zhao, C. J. Oldham, B.-I. Wu, G. N. Parsons, and C.-H. Chang, “Ordered 3D Thin-Shell Nanolattice Materials with Near-Unity Refractive Indices,” Advanced Functional Materials, 25, 6644-6649, 2015. [link] [3] A. Bagal, E. C. Dandley, J. Zhao, X. A. Zhang, C. J. Oldham, G. N. Parsons, and C.-H. Chang, “Multifunctional Nano-Accordion Structures for Stretchable Transparent Conductors,” Materials Horizons, 2, 486-494, 2015. [link]
[4] K.-C. Park, H. J. Choi, C.-H. Chang, R. E. Cohen, G. H. McKinley, and G. Barbastathis, “Nanotextured Silica Surfaces with Robust Superhydrophobicity and Omnidirectional Broadband Supertransmissivity,” ACS Nano, 6, no. 5, 3789–3799 (2012). [link]
[5] Q. Yang, X. A. Zhang, A. Bagal, W. Guo, and C.-H. Chang, “Antireflection effects at nanostructured material interfaces and the suppression of thin-film interference,” Nanotechnology, 24, 235202 (2013). [link] [6] Y.-A. Chen, S. V. Naidu, Z. Luo, and C.-H. Chang, “Enhancing optical transmission of multilayer composites using interfacial nanostructures,” Journal of Applied Physics 126, 063101, 2019. [link]
