B.S. in Biomedical Engineering at University of Connecticut (2008)
Periodic micro/nano architectures comprising all metallic components are interesting because they may exhibit large refractive index contrast, interesting optical phenomena and plasmonic physics, and, perhaps most importantly, thermal stability, thus broadening the scope of proposed applications. My research seeks to exploit the interesting characteristics of metallic micro architectures for applications including thermal photovoltaics, functional optical devices, and energy storage.
A. Thermal Photovoltaic Energy Conversion
Ultra high efficiency solar energy harvesting can be realized using an intermediate system that absorbs broadband radiation from the sun and subsequently emits that radiation toward a single junction solar cell in a narrowed bandwidth. Ideally, the emitted radiation energy would be concentrated just above the energy of the solar cell band gap. In this configuration, sub band gap photons are not wasted and excess energy due to thermalization of high-energy photons is minimized, thus resulting in higher overall efficiencies. My research focuses on engineering a practical intermediate structure for TPV systems using 3D metallic photonic crystals. Engineering thermally stable metallic photonic crystals, however, remains an unsolved challenge. I am exploring a variety of materials and fabrication methods to enhance the thermal stability of metallic photonic crystals, thus making them feasible candidates for thermal
photovoltaic energy conversion.
Figure 1 – Design of a thermal photovoltaic cell
B. Functional Optical Devices
In collaboration with Andre Radke and Prof. Harald Giessen, University of Stuttgart, Germany
We are interested in the advanced fabrication of complex metallic micro architectures with unique optical responses for applications as functional optical devices, metamaterials, etc. These complex structures can be patterned in a photoresist using direct laser writing and subsequently converted into metallic components using electrodeposition or electroless plating for positive or negative resists, respectively. Examples of components fabricated by silver electrodepositon directed by a direct laser written pattern in a positive photoresist are seen in Figure 2.
Figure 2 – Functional optical devices fabrication by direct laser writing in a positive photoresist, subsequent silver electroplating, and final photoresist removal
Multidimensional patterning with precision, speed, and reproducibility at the nanoscale is crucial for rapid evolution in computer logic, memory, metamaterials, photonic crystals, plasmonics, energy harvesting and storage, and nano-bio applications. At the micron scale, for example, multidimensional hydrogel structures can direct cell growth for tissue engineering applications, 3D photonic band gap materials containing designed defects demonstrate wave-guiding at telecom wavelengths, 3D arrays of metallic microstructures demonstrate metamaterial properties at optical frequencies, and enhanced light extraction in LED’s was realized using photonic band gap structures. Multidimensional patterning at nanometer scales (<100nm) has remained largely unexplored. Functional, multidimensional patterns at these scales (<100nm) will pioneer novel devices and properties that cannot be realized using unstructured materials. Moreover, patterning in the third dimension remains an unsolved challenge <100nm.
A. Directed Assembly of Functional Micelle Arrays
In collaboration with Honyou Fan, Sandia National Laboratories, Albuquerque, New Mexico and the University of New Mexico, Albuquerque, New Mexico
James H. Pikul and William P. King, Mechanical Science and Engineering, University of Illinois at Urbana-Champaign
Our approach to demonstrate mulit-scale and dimensional patterning combined top down lithographic nanopatterning with bottom up self-assembly. This study focused on the development of large scale, defect free, templates that could direct the assembly of our multi-functional nanoparticles. More specifically, we used nanoimprint lithography to fabricate patterned arrays that could then be modified to optimize the bottom up assembly. Our nanoparticles for bottom up assembly were fabricated using the block co polymer poly(styrene-b-4-vinyl pyridine), see Figure 3 below. These core (polystyrene) – shell (poly vinylpyridine) micelle structures are dispersed in aqueous solutions with pH ~ 2. In this environment, the poly vinylpyridine (PVP) chains are charged and thus repel each other and cause the shell to swell. However, when dry, or in higher pH solutions, these chains condense, changing the overall size of the micelle. The dynamic PVP shell can swell and condense over a range of ~200nm offering unique assembly opportunities not attainable with rigid nanoparticles. Assembly of these particles has, thus far, focused on 2D arrays (Figure 4). In the future, we will investigate the self-assembly of our dynamic micelles into three-dimensional architectures.
Figure 3 – Self-assembled PS (core) – PVP (shell) micelles
Figure 4 – Directed patterning of micelle arrays using multi-dimensional, lithographically defined templates
B. Photo-Directed Selective Area Functionalization inside 3D Templates
In collaboration with Agustin Mihi, Braun Group
Josh Ritchey and Jeff Moore, Chemistry, University of Illinois at Urbana-Champaign
Herein, we demonstrate the chemical functionalization of a three-dimensional template with a photoactive coumarin based molecular monolayer (Figure 5). Upon exposure with UV light, this molecule cleaves, inducing a change in surface charge. We exploit this change in surface charge to selectively decorate a 3D photonic crystal with functional nanoparticles. Figure 6 shows preliminary results of a silica opal decorated with silver nanoparticles.
Figure 5 – Photoactive coumarin molecule demonstrates charge inversion upon UV exposure
Figure 6 – Selective area decoration of a three-dimensional template using electroless silver plating