Cody M. Washburn

Lithographically defined three-dimensional graphene structures.

A simple and facile method to fabricate 3D graphene architectures is presented. Pyrolyzed photoresist films (PPF) can easily be patterned into a variety of 2D and 3D structures. We demonstrate how prestructured PPF can be chemically converted into hollow, interconnected 3D multilayered graphene structures having pore sizes around 500 nm. Electrodes formed from these structures exhibit excellent electrochemical properties including high surface area and steady-state mass transport profiles due to a unique combination of 3D pore structure and the intrinsic advantages of electron transport in graphene, which makes this material a promising candidate for microbattery and sensing applications.

Lithographically Defined Porous Carbon Electrodes

The special nature of the C C bond can lead to various polymorphic forms of carbon such as graphite, glassy-carbon, fullerenes (such as buckyballs), carbon nanotubes, and diamond. Electrodes made from carbon exhibit many useful properties including wide potential windows, low background capacitance, resistance to fouling, and catalytic activity for manyanalytes compared to solidmetal electrodes. In addition to the intrinsic material properties of carbon, functionalized films can be produced through chemical modification using a wide range of chemistries. Because of this flexibility and utility, fabrication of both macroand microporous carbon films, with their commensurate increase in surface area, continues to receive significant research interest. Some of the specific applications for porous carbon materials include fuel cells, electrochemical double layer capacitors, high surface area catalytic supports, water purification, and gas separation. Recently, it has been found that pyrolyzed photoresist films (PPFs) have the same unique properties of carbon electrodes with an advantage that they canbe lithographically defined. The goal of this work was to create lithographically defined porous pyrolyzedcarbonelectrodesandcharacterize thedepositionand electrochemical properties of metal nanoparticles on these electrodes. We report a robust fabrication method capable of producing large area ( 100s cm) submicrometer porous carbon films. In our approach, interferometric lithography (IL) is used topattern thickphotoresist films into 3Dperiodic lattices. These structures are then converted to carbon via pyrolysis under flowing forming gas. During pyrolysis, the non-carbon species in the resist polymer backbone are removed, while the bulk of the carbon remains. The patterned structures undergo significant shrinkage, but remarkably maintain their morphology and adhesion to the substrate. The degree of carbonization is a function of the pyrolysis temperature, which has a profound

Three dimensional nickel–graphene core–shell electrodes

The annealing of nickel-coated porous carbon structures results in a new three dimensional nanostructured graphene encapsulated nickel core–shell electrode. A highly interdependent and dynamic process is observed that results in the complete reversal of the spatial orientations of the two component system after the annealing process. We examine the mechanism of carbon diffusion and observe unexpected morphological changes of the nickel in response to carbon crystallization. The new nickel–graphene core–shell electrode demonstrates excellent electrochemical properties with promising applications in micro-batteries and biosensors.

VHF and UHF mechanically coupled aluminum nitride MEMS filters

This paper reports the development of narrow-bandwidth, post-CMOS compatible aluminum nitride (AlN) MEMS filters operating in the very (VHF) and ultra (UHF) high frequency bands. Percent bandwidths less than 0.1% are achieved utilizing a mechanically coupled filter architecture, where a quarter wavelength beam attached in low velocity coupling locations is used to connect two AlN ring resonators. The filter bandwidth has been successfully varied from 0.09% to 0.2% by moving the attachment of the coupling beam on the ring to locations with different velocity at resonance. Insertion losses of 11 dB are obtained for filters centered at 99.5 MHz with low termination impedances of 200 Omega. Utilizing a passive temperature compensation technique, the temperature coefficient of frequency (TCF) for these filters has been reduced from -21 ppm/C to 2.5 ppm/C. The reduced TCF is critical for narrow bandwidth filters, requiring only 13% of the filter bandwidth to account for military range (-55 to 125 C) temperature variations compared to 100% for uncompensated filters. Filters operating at 557 MHz are realized using overtone operation of the ring resonators and coupling beam where higher insertion losses of 32 dB into 50 Omega are seen due to the finite resonator quality factor and narrow bandwidth design. Overtone operation allows for the implementation of fully differential and balun type filters where the stop-band rejection is as high as 38 dB despite the increased insertion loss.

Vapor Sensing Using Conjugated Molecule-Linked Au Nanoparticles in a Silica Matrix

Cross-linked assemblies of nanoparticles are of great value as chemiresistor-type sensors. Herein, we report a simple method to fabricate a chemiresistor-type sensor that minimizes the swelling transduction mechanism while optimizing the change in dielectric response. Sensors prepared with this methodology showed enhanced chemoselectivity for phosphonates which are useful surrogates for chemical weapons. Chemoselective sensors were fabricated using an aqueous solution of gold nanoparticles that were then cross-linked in the presence of the silica precursor, tetraethyl orthosilicate with the -, -dithiolate (which is derived from the in situ deprotection of 1,4-di(Phenylethynyl-,-diacetylthio)-benzene (1) with wet triethylamine). The cross-linked nanoparticles and silica matrix were drop coated onto interdigitated electrodes having 8 m spacing. Samples were exposed to a series of analytes including dimethyl methylphosphonate (DMMP), octane, and toluene. A limit of detection was obtained for each analyte. Sensors assembled in this fashion were more sensitive to dimethyl methylphosphonate than to octane by a factor of 1000.

Highly ordered tailored three-dimensional hierarchical nano/microporous gold–carbon architectures

The preparation and characterization of three-dimensional hierarchical architectures, consisting of monolithic nanoporous gold or silver films formed on highly ordered 3D microporous carbon supports, are described. The formation of these nano/microporous structures involves the electrodeposition or sputtering of metal alloys onto the lithographically patterned multi-layered microporous carbon, followed by preferential chemical dealloying of the less noble component. The resulting hierarchical structure displays a highly developed 3D interconnected network of micropores with a nanoporous metal coating. Tailoring the nanoporosity of the metal films and the diameter of the large micropores has been accomplished by systematically changing the alloy compositions via control of the deposition potential, plating solution and coarsening time. SEM imaging illustrates the formation of unique biomimetic nanocoral- or nanocauliflower-like self-supporting structures, depending on the specific preparation conditions. The new 3D hierarchical nano/microporous architectures allow for enhanced mass transport and catalytic activity compared to common nanoporous films prepared on planar substrates. The functionality of this new carbon–gold hierarchical structure is illustrated for the greatly enhanced performance of enzymatic biofuel cells where a substantially higher power output is observed compared to the bare microporous carbon substrate.

Lithographically defined 3D nanoporous nonenzymatic glucose sensors.

Nonenzymatic glucose oxidation is demonstrated on highly faceted palladium nanowflower-modified porous carbon electrodes fabricated by interference lithography. Varying electrodeposition parameters were used to control the final shape and morphology of the deposited nanoparticles on the 3D porous carbon which showed a 12 times increase in the electrochemically active surface area over analogous planar electrodes. Extremely fast amperometric glucose responses (achieving 95% of the steady state limiting current in less than 5s) with a linear range from 1 to 10mM and a detection limit of 10 μM were demonstrated. The unusual surface properties of the pyrolyzed photoresist films produced strongly adhered palladium crystal structures that were stable for hundreds of cycles towards glucose oxidation without noticeable current decay.

Combined chemical and topographical guidance cues for directing cytoarchitectural polarization in primary neurons.

Chemical and topographical cues can be used to guide dissociated neurons into user-defined network geometries on artificial substrates, yet control of neuron polarity (differentiation into axons and dendrites) remains an elusive goal. We developed a dual guidance cue strategy for directing morphological maturity in neurons in vitro using combined chemical and topographical guidance cues on glass substrates. The surface chemistry provides chemical attraction and repulsion for controlling neuron placement and outgrowth, while the topography provides additional surface area for neuron attachment. Poly-l-lysine (PLL) was adsorbed into etched trenches in glass substrates, and an acetone liftoff process was used to produce bifunctional surfaces with a hydrophobic hexamethyldisilazane (HMDS) background and trench patterns of PLL. We examined the cytoarchitectural polarization of dissociated hippocampal pyramidal neurons on guidance cues designed to promote rapid outgrowth of neurites onto continuous line features and delayed neurite outgrowth onto interrupted line features. An optimum distance of approximately 5 μm between the cell body attachment node and the first interrupted line guidance cue led to specific cytoarchitectural polarization of ≥60% of neurons by 3 days of culture in vitro.

Lithographically-defined 3D porous networks as active substrates for surface enhanced Raman scattering

Interferometric lithographically fabricated porous carbon acts as active substrates for Surface Enhanced Raman Scattering (SERS) applications with enhancement factors ranging from 7 to 9 orders of magnitude.

Increased Mass Transport at Lithographically Defined 3-D Porous Carbon Electrodes

Increased mass transport due to hemispherical diffusion is observed to occur in 3D porous carbon electrodes defined by interferometric lithography. Enhanced catalytic methanol oxidation, after modifying the porous carbon with palladium nanoparticles, and uncharacteristically uniform conducting polymer deposition into the structures are demonstrated. Both examples result in two regions of hierarchical porosity that can be created to maximize surface area, via nanostructuring, within the extended porous network, while taking advantage of hemispherical diffusion through the open pores.