Surfaces and interfaces play pivotal roles in diverse fields such as catalysis, electronics, sensors, and photonics. Nature provides many examples of how micro- and nano-scale structures can lead to interesting macroscopic properties, such as adhesive setae in gecko feet and photonic crystals in butterfly wings. Motivated by these and other examples, we strive to create materials that have useful engineering applications.
Surface and Interfacial Science and Engineering of M/NEMS
Micro-/nanoelectromechanical systems (M/NEMS) technology is an emerging technology which uses the tools and techniques developed for the integrated circuit industry to build microscopic machines. Since these machines are created with the same tools used to create integrated circuits, they can be cofabricated with microelectronics devices. Fabricating the machines and the electronics side by side enables machines that can have intelligence. These tiny machines are becoming ubiquitous, and are quickly finding their way into a variety of commercial and defense applications. Examples include sensors for inertial navigation, Lab-on-a-Chip technology for chemical and biochemical analysis, large-area high-resolution displays, and nanomechanical computing. Due to the large surface-to-volume ratio of M/NEMS, surface forces dominate over body forces. While this leads to many of the key advantages of technology at this scale, some of the major issues that inhibit widespread application of MEMS/NEMS are strong adhesion, friction and wear that halt device operations or eventually destroy them. We are developing micro-instruments and methods for in situ measurements of these interfacial issues to provide fundamental understanding of them. We are also examining hard coatings (such as SiC), self-assembled monolayers (SAMs), and graphene to tailor surface properties such as wettability, adhesion, and biocompatibility.
Synthesis and Surface Science of Two-Dimensional Materials
Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) possess many unique electronic and optoelectronic properties that make them attractive choices for next-generation electronics and sensors. While scalable synthesis of these materials has been demonstrated, the fundamentals of the growth kinetics and chemical modifications are still not well understood. Our work explores bottom-up synthesis and defect engineering of few-layer TMDCs. In addition, we are demonstrating the use of various TMDC materials for microsensor applications.
Semiconducting nanowires have received much interest in recent years, due to their anticipated (and in some cases observed) novel optical, thermal, electrical, and mechanical behavior. To date, Si nanowires have been the most extensively studied owing both to the technological relevance of the material and to the relative simplicity of the synthesis. However, a number of critical applications, such as high-efficiency electrochemical capacitors and high brightness miniature neutron sources, highlight the limitations of Si nanowires and the need for more robust materials. While the exceptional physicochemical stability of SiC nanowire arrays is very desirable in such applications, much work remains to be done before the outstanding capabilities of this material can be unlocked and deployed. In particular, the ability to grow arrays with uniform size and/or accurate placement, uniform doping, and tailored electrical properties is of paramount importance in the applications mentioned above. The goal of our research is to gain a fundamental understanding of the growth mechanism of silicon carbide nanowires and of the parameters that control their morphological and electrical properties.
Noble-metal nanoparticles show strong resonances for light scattering and absorption, due to the excitation of localized surface plasmons (collective oscillation of the conduction electrons). At resonance, light resonantly excites the plasmon modes of the metal nanoparticle, which then acts as a radiating dipole. Its resonance frequency is strongly dependent on particle shape and dielectric environment, which enables tuning of its “color” throughout the visible and into the near-infrared regime of the spectrum, while keeping particle size well below 100 nm. We are using these phenomena to create complex composite nanoparticles (e.g., core-vest nanoparticles), yielding materials with enhanced catalytic and photocatalytic properties.
Recently Completed Projects
Geckos are known for their remarkable ability to vertically climb and stick to just about any surface. This is enabled by the hierarchical structures on their feet that range from stiff seta and spatula micro- and nano-structures to millimeter-scale lamellar arrays. Mimicking the multi-scale structure of the geckos has remained a challenge in the field. Utilizing the technique for parallel synthesis of silicon nanowires, master templates can be created with precisely controlled nanowire diameter, length, and density. Subsequent molding yield high-aspect-ratio polymer nanofiber arrays that exhibit high friction with low detachment force. The fibers can be molded with different types of polymers, e.g., polydimethylsiloxane, polypropylene, low- and high-density polyethylene. Furthermore, schemes are developed to mimic the additional levels of hierarchy into these nanostructures. Such adhesives have many potential applications, such as in biomedical and sports equipment, as well as for crawling micro-robots. Also, fabricating the fiber arrays with tailored geometry allows one to probe how the fundamental parameters of the nanofibers and the contacting substrate (e.g., fiber and substrate geometry, modulus, and surface energy) affect macroscopic properties like adhesion and friction. This effort provides broader insights for contact between non-ideal surfaces.