59th Annual Report on Research 2014

1. We have developed a simple and scalable colloidal lithography technology for fabricating periodic arrays of gold nanodonuts for sensitive SPR analysis. This new bottom-up approach leverages a unique polymer wetting layer between a non-close-packed monolayer silica colloidal crystal and a silicon substrate to template plasmonic nanodonuts with tunable geometries. Specular reflection measurements reveal that the efficient electromagnetic coupling of the incident light with the adjustable SPR modes of the templated nanodonut arrays enables good spectral tunability. Bulk refractive index sensing experiments show that a very high SPR sensitivity of ~758 nm per refractive index unit, which outperforms many plasmonic nanostructures created by both top-down and bottom-up approaches, can be achieved by the gold nanodonut arrays. Numerical simulations have also been performed to complement the optical characterization and the theoretical results match very well with the experimental measurements.

2. We have also developed a scalable colloidal templating approach for fabricating periodic arrays of metallic and silicon nanorings with complex nanostructures for plasmonic sensing. Non-close-packed monolayer silica colloidal crystal prepared by a simple spin-coating technology is first used as template in making periodic arrays of mushroom-like composite nanostructures consisting of silica spherical caps and polymer stems. Subsequent metal sputtering and reactive ion etching (RIE) lead to the formation of ordered asymmetric nickel nanorings which can be further utilized as etching masks for pattering periodic arrays of symmetric silicon nanorings. Moreover, periodic arrays of metallic and silicon concentric double nanorings can be fabricated by using the asymmetric nickel nanorings as templates. We have demonstrated that gold concentric double nanorings show strong surface-enhanced Raman scattering (SERS) with a SERS enhancement factor of ~9.5´10⁷ from adsorbed benzenethiol molecules.

3. In order to achieve simultaneous high SPR sensitivity and large SPR figure of merits which are critical for realizing ultrasensitive chemical sensing for investigating heterogeneous catalytic reactions, we have exploited a simple soft-lithography process for fabricating Au-covered titania and zirconia gratings by using digital versatile discs (DVDs) as structural templates. The resulting Au-covered titania gratings exhibit high SPR sensing sensitivity (859 nm/RIU) and figure of merit (up to ~62). We have obtained higher sensitivities (886 nm/RIU at 500°C and 896 nm/RIU at 1,100°C) by thermally annealing the titania gratings prior to Au sputter deposition. Meanwhile, smoother surface morphology, larger single-crystalline domain size of titania, and structural deformation of DVD track-pitch structure have been identified by X-ray diffraction (XRD) and high-magnification scanning electron microscope. Similar results are obtained with templated zirconia gratings which show higher degree of structural deformation than titania gratings. Theoretical simulations show that the SPR sensitivities of Au-covered oxide gratings are dominantly affected by the structural deformation.

4. To tune the surface plasmon responses of the templated plasmonic nanostructures, we have developed a bottom-up technique for layer-by-layer (LBL) assembling colloidal templates consisting of large (³ 1 μm) silica microspheres. Hexagonally close-packed monolayer colloidal crystal assembled at an air/water interface can be rapidly transferred onto the two surfaces of a glass substrate. High-quality multilayer colloidal photonic crystals with well-controlled number of colloidal layers, which could greatly expand the availability of the templated plasmonic nanostructures, are assembled by repeating this process in a LBL manner.

Educational Activities:

1. Four UF Ph.D. students and two self-funded Master students have worked on this multidisciplinary project since September 1, 2013.

2. Three 11^th-grade students worked on the project in summer 2014.

3. These students have co-authored in five peer-reviewed papers (2 published, 1 accepted, 2 under review) and one invited book chapter.

4. Eight oral presentations related to the project have been given by the students at various international, national, and local meetings.

Contributions within Discipline:

1. The templated plasmonic nanostructures with very high SPR sensitivities and figure of merits, as well as SERS enhancement factors are of great inportance in developing ultrasensitive chemical and biological sensors for real-time, label-free, specific, and quantitative detection of small molecules, biomarkers, viruses, and bacteria, as well as reaction intermediates in heterogeneous catalytic reactions. The fundamental experimental measurements and numerical simulations help us better understand the structure-property relationships of the templated plasmonic nanostructures.

2. The open-sourced optical simulation codes developed from this project facilitate basic understanding of optical diffraction, reflection, scattering, and transmission from a large variety of periodic dielectric gratings.

3. The new LBL colloidal self-assembly technology provides a simple and scalable approach for assembling large-area multilayer colloidal photonic crystals with NIR photonic band gaps, which are critical components for advanced optical systems (e.g., all-optical integrated circuits) but are difficult to be achieved by other bottom-up technologies.

Contributions to Other Disciplines:

1. The scalable spin-coating nanomanufacturing technology will advance many other areas not covered by this project ranging from highly efficient solar cells to bio-microanalysis, where the scalable and inexpensive production of large-area periodic nanostructures is important.

2. The open-sourced optical simulation source codes (freely downloadable from the PI's group website) can be easily implemented for many other optical applications, such as developing multilayer optical filters and broadband antireflection coatings.

3. In addition to sensitive chemical and biological sensing, the templated plasmonic nanostructures with high light transmission and electrical conductivity could be used as transparent conducting electrodes for flexible organic light emitting diodes (OLEDs) and organic photovoltaics.

Impact on Student Education and Training:

The students involved in this multidisciplinary project have gained experience in conducting innovative research in chemical synthesis (e.g., sol-gel chemistry and preparation of colloidal nanoparticles), standard microfabrication (like clean-room operation, metal deposition, dry etching), colloidal self-assembly, optical characterization and modeling, image analysis, heterogeneous catalysis, and chemical/biological sensing. They have also learned how to write technical papers and how to effectively communicate their research results to colleagues at international and national meetings. The Ph.D. students gained valuable mentoring experience in coaching the Master and high school students involved in this project.