Materials Science & Engineering
Stephen H. Foulger
Stephen H. Foulger

Professor
Greg-Granitevillle Endowed Chair & Professor

Materials Science and Engineering

Phone: (864) 656-1045
Fax: (864) 656-1049
Office: 91 Technology Drive
E-mail: foulger@clemson.edu

Education

Ph.D., Materials Science, Massachusetts Institute of Technology, 1996
B. S. , Mechanical Engineering, University of California, Santa Barbara, 1990

Dr. Foulger graduate work was performed under the guidance of Prof. Gregory Rutledge and explored the structural characteristics of the disordered state of liquid crystalline polymers through molecular simulation. Prior to joining the faculty of Clemson University, he served as a polymer scientist in the Division of Research, Development & Engineering of the Italian company Pirelli Tyres / Cables & Systems.


Research

Materials science researchers have been striving over the last decade to develop a full repertoire of dielectric, semiconductor, and metallic nanomaterials and characterize their size dependent electronic and optical properties. These particles are of interest not only because they can be used to make very small structures, but also because the commonly understood properties of ordinary bulk materials are dramatically different at the nano-length scale. Transitioning this knowledge from the realm of basic science into a technology that can be exploited in the development of novel optical & electronic devices will require a multidisciplinary approach. To this end, our group is composed of people with backgrounds spanning the scientific and engineering disciplines.

Self-assembly.

Since the original proposal that three dimensional periodic dielectric structures could exhibit a photonic bandgap (PBG), considerable attention has been focused on developing these materials into a form which is suitable for use in photonic applications. Unfortunately, the general exploitation of visible photonic crystals as devices has been hindered by the difficulties in creating 3D periodic dielectric structures with a feature size comparable to the wavelength of visible light, as well as achieving dielectric contrasts that result in a forbidden gap that overlaps in all directions within the Brillouin zone. Though a number of groups have made progress applying conventional microlithographic techniques to this end, this objective remains a challenge employing these techniques. Focus has now turned to systems which undergo self organization at a nanometer length scale, such as colloidal crystals.

Two self-assembly approaches to generating photonic crystals have emerged. One approach involves the assembly of nanoparticles into close-packed arrays through sedimentation and typically relies on non-specific particle-particle repulsion to induce order. Particle assembly via this method is attractive in terms of both simplicity and versatility. Because close packed order typically involves concentrating particles into a confined space, these studies often involve hard spheres (e.g., ceramic, metal, dense polymer) that undergo little deformation upon packing. The second approach involves the longer range electrostatic repulsive interactions of suspended colloidal spheres bearing a high surface charge and arranged in ordered arrays.

Our group has focused on crystalline colloidal arrays. A crystalline colloidal array is a three dimensionally ordered lattice of self-assembled monodisperse colloidal particles, most commonly amorphous silica or a polymer latex, dispersed in aqueous or non-aqueous media. At high particle concentrations, long-range electrostatic interactions between particles result in a significant inter-particle repulsion which yields the adoption of a minimum energy colloidal crystal structure with either BCC or FCC symmetry. The ordering of the particles in the media results in spatial periodicities that range from ca. 100-1000 nm, often resulting in optical bandgap effects.

Unfortunately, the low elastic modulus exhibited by a liquid dispersion results in weak shear, gravitational, electric field, or thermal forces having the propensity to disturb the crystalline order and is a severe drawback to the practical application of these arrays in photonic devices. Our group has devised ways to encapsulate the ordered arrays in water-free, robust matrixes, resulting in PBG composites. The devised route to encapsulation offers the material designer the ability to tune the glass transition of the composite through a judicious choice in the encapsulation monomer(s) and allows for a wide range of coupled optical and mechanical properties, such as the variation of the stop band with stress (i.e., mechanochromic response).

Nanoparticle Synthesis

Photonic crystals are usually fabricated from dielectric materials with relatively frequency independent refractive indexes, though a number of experiments and theoretical calculations have demonstrated that materials with significant index contrast and frequency dependent refractive indexes can be used to construct PBG composite materials with unique features, and possibly with a complete photonic bandgap. To increase the refractive index contrast between spheres and matrix, as well introduce optical nonlinearities into the crystals, a number of approaches are being pursued in the group, such as the synthesis of metal & synthetic metal particles, as well combining different materials in a core-shell topology.

The exploitation of a core-shell topology results in the appearance of particles that exhibit new properties. Our group is active in developing ways to combine dissimilar materials in one particle to enhance the physical or optical performance of a self-assembled structure composed of the core-shell particles. Examples (highlighted in the previous figure) include the synthesis of monodisperse polystyrene particles coated with the intrinsically conducting polymer (ICP) poly(3,4-ethylenedioxythiophene) (PEDOT) .

An Eye towards the device

A main point of concentration in our group is on formulating approaches to fabricating device-ready PBG composites for direct integration with opto-electronic components. Our emphasis on the integration of nanoscale materials with established device fabrication routes is an effort to establish the scientific principles that govern the design, performance, and integration of these materials into optical and sensory components. One technique exploited within the group is soft lithography, a technique developed by George Whitesides of Harvard University to allow for the rapid prototyping of microfluidic devices. The previous figure presents the optical characteristics at various viewing angles of a polymerized crystalline colloidal array templated via soft lithography into a Clemson University tiger paw ensignia: a. blue photonic crystal at ca. 20° viewing angle; b. green photonic crystal at ca. 15° viewing angle; c. green-red photonic crystal at ca. 10° viewing angle; and d. red photonic crystal at ca. 0° viewing angle.


Recent Publications and Awards

H. Bao, G. Chumanov, R. Czerw, D. L. Carroll, and S. H. Foulger, “Synthesis of Core-Shell Silver Colloidal Particles by Surface Immobilization of an Azo-Initiator”, Colloid & Polymer Science, in press (2005).

P. Jiang, J. Ballato, D. W. Smith, and S. H. Foulger, “Multicolor Pattern Generation in Photonic Bandgap Composites”, Advanced Materials, 17, 179 (2005). (Note: image featured on the cover)

J. Ballato, S. H. Foulger, and D. W. Smith, “Optical properties of perfluorocyclobutyl polymers. II. Theoretical and Experimental Attenuation”, Journal of the Optical Society of America B-Optical Physics, 21, 958 (2004).

J. Ballato, S. H. Foulger, and D. W. Smith, “Optical properties of perfluorocyclobutyl polymers. II. Theoretical and Experimental Attenuation”, Journal of the Optical Society of America B-Optical Physics, 21, 958 (2004).

M. Han and S. H. Foulger, “Crystalline Colloidal Arrays Composed of Poly(3,4-ethylenedioxythiophene) Coated Polystyrene Particles with a Stop Band in the Visible Regime”, Advanced Materials, 16, 231 (2004).

S. Chen, Z. L. Wang, J. Ballato, S. H. Foulger, and D. L. Carroll, “Monopod, Bipod, Tripod, Tetrapod Gold Nanocrystals”, Journal of the American Chemical Society, 10, 1021 (2003).

J. Ballato, S. H. Foulger, and D. W. Smith, “Optical Properties of Perfluorocyclobutyl Polymers”, Journal of the Optical Society of America B-Optical Physics, 20, 1838 (2003).

M. W. Perpall K. P. U. Perera J. DiMaio, J. Ballato, S. H. Foulger, and D. W. Smith, “Novel Network Polymer for Templated Carbon Photonic Crystal Structures”, Langmuir, 19, 7153 (2003).

Z. Xu, Z. Wang, M. E. Sullivan, D. J. Brady, S. H. Foulger, A. Adibi, “Multimodal Multiplex Spectroscopy Using Photonic Crystals” Optics Express, 11 2126 (2003).

S. H. Foulger, P. Jiang, A. Lattam, J. Ballato, D. W. Smith, Jr. D. Dausch, S. Grego, and B. Stoner, “Photonic Bandgap Composites with Reversible High-Frequency Stop Band Shifts,” Advanced Materials, 15, 685 (2003).

D. W. Smith, Jr., S. Chen, S. Kumar, J. Ballato, C. Topping, and S. H. Foulger, “Pefluorocyclobutyl (PFCB) Copolymers for Microphotonics,” Advanced Materials, 14, 1585 (2002).

H. Zengin, W. S. Zhou, J. Y. Jin, R. Czerw, D. W. Smith, L. Echegoyen, D. L. Carroll, S. H. Foulger, and J. Ballato, “Carbon Nanotube Doped Polyaniline”, Advanced Materials, 14, 1480 (2002).

K. Perera, H. V. Shah, S. H. Foulger, and D. W. Smith, Jr., “Substituent Effects on Bergman Cyclopolymerization Kinetics of Bis-Ortho-Diynylarene (BODA) Monomers by Dynamic Scanning Calorimetry”, Thermochima Acta 1-2, 371-375 (2002).

S. H. Foulger, P. Jiang, Y. Ying, A. Lattam, J. Ballato, and D. W. Smith, Jr., “Photonic Bandgap Composites,” Advanced Materials, 13, 1898-1901 (2001).

S. H. Foulger, A. Lattam, P. Jiang, D. W. Smith, Jr., and J. Ballato, “Optical and Mechanical Properties of Poly(ethylene glycol) Methacrylate Hydrogel Encapsulated Crystalline Colloidal Arrays,” Langmuir 17, 6023-6026 (2001).

National Science Foundation CAREER Award - MPS/DMR, Clemson University Award for Faculty Achievement in the Sciences, Clemson University Board of Trustees Faculty Excellence Award, National Textile Center Scientific Excellence Award, 3M Corporation Pre-tenured Faculty Award.