Shane Juhl

Amplitude-modulated electrostatic nanolithography in polymers based on atomic force microscopy

Amplitude modulated electrostatic lithography using atomic force microscopy (AFM) on 20–50 nm thin polymer films is discussed. Electric bias of AFM tip increases the distance over which the surface influences the oscillation amplitude of an AFM cantilever, providing a process window to control tip-film separation. Arrays of nanodots, as small as 10–50 nm wide by 1–10 nm high are created via a localized Joule heating of a small fraction of polymer above the glass transition temperature, followed by electrostatic attraction of the polarized viscoelastic polymer melt toward the AFM tip in the strong (108–109 V/m) nonuniform electric field.

Precise formation of nanoscopic dots on polystyrene film using z-lift electrostatic lithography

Z-lift electrostatic lithography on thin (10–50nm) polystyrene (PS) films is discussed. The height of nanostructures can be controlled via mechanically drawing or depressing the cantilever height (z-lift) during the application of a voltage. Since polymer is not removed or crosslinked during structure formation, the features are erasable. Various aspects such as voltage doses, film thickness, z-lift height, and rate are explored. Structure height formation relies mainly on, and is proportional, to the z-lift magnitude; however, only a narrow range of voltages yields structures for any given film thickness. Structures ranging from 0–10nm are produced on a 40nm thick PS film using −36V by varying the z-lift on a 0.1–0.9N∕m cantilever from −20nm to +400nm.

Electric charging and nanostructure formation in polymeric films using combined amplitude-modulated atomic force microscopy-assisted electrostatic nanolithography and electric force microscopy

A hybrid technique, combining lithography which exploits atomic force microscope tip manipulation with modified electric force microscopy was used to study surface electric charging (deposition and evolution) of polymethyl methacryalate and polystyrene films. Upon charging the films past a threshold voltage, two distinct regimes were observed: (1) stable feature formation related to electric breakdown and mass transport resulting in stable film deformation due to the negative surface charging (negative tip bias) and (2) no stable feature formation regime attributed to viscoelastic deformation of polymer surface followed by the surface relaxation in the case of positive surface charging (positive tip bias).

Femtosecond laser damage threshold and nonlinear characterization in bulk transparent SiC materials

Semi-insulating and conducting SiC crystalline transparent substrates were studied after being processed by femtosecond (fs) laser radiation (780 nm at 160 fs). Z-scan and damage threshold experiments were performed on both SiC bulk materials to determine each samples nonlinear and threshold parameters.

Femtosecond micromachining in transparent bulk materials using an anamorphic lens

A unique anamorphic lens design was applied to a circular 780nm femtosecond laser pulse to transform it into an elliptically shaped beam at focus. This lens was developed to give an alternative method of micromachining bulk transparent materials. The challenge for femtosecond laser processing is to control the nonlinear affect of self-focusing, which can occur when using a fast f-number lens. Once the focused spot is dominated by self-focusing the predicted focused beam becomes a filament inside the bulk, which is an undesirable effect. The anamorphic lens resolves this self-focusing by increasing the numerical aperture (NA) and employing an elliptical beam shape. The anamorphic lens was designed to furnish a 2.5mum by 190mum line at focus. Provided the pulse energy is high enough, transparent bulk material will be damaged with a single femtosecond laser pulse. Damage in this text refers to visual change in the index of refraction as observed under an optical microscope. Using this elliptical shape (or line), grating structures were micro-machined on the surface of SiC bulk transparent substrate. SiC was chosen because it is known for its micromachining difficulty and its crystalline structure. From the lack of self-focusing and using energy that is just above the damage threshold the focused line beam generated from the anamorphic lens grating structures produced a line shape nearly identical to the geometrical approximation. In this paper we discuss a new method of writing gratings (or other types of structures) in bulk transparent materials using a single femtosecond laser pulse. We will investigate the grating structures visually (inspected under an optical microscope) and also by use of an atomic force microscopy (AFM). In addition, we test the grating diffraction efficiency (DE) as a function of grating spacing, d.

Bio-scaffolds for ordered nanostructures and metallodielectric nanoparticles

The use of virus nanoparticles, specifically Chilo and Wiseana Invertebrate Iridovirus, as building blocks for iridescent nanoparticle assemblies and core substrates in the fabrication of metallodielectric nanostructures is discussed. Virus particles are assembled in vitro, yielding films and monoliths with optical iridescence arising from multiple Bragg scattering from close packed crystalline structures of the iridovirus. Bulk viral assemblies are prepared by centrifugation followed by the addition of glutaraldehyde, a cross-linking agent. Long range assemblies were prepared by employing a cell design that forced virus assembly within a confined geometry followed by cross-linking. In addition to these assemblies core-shell particles were created from the same virus. A gold shell is assembled around the viral core by attaching small gold nanoparticles to the virus surface by means of the inherent chemical functionality found within the protein cage structure of the viral capsid. These gold nanoparticles act as nucleation sites for electroless deposition of gold ions from solution. UV/Vis spectroscopy and electron microscopy, were used to verify the creation of the virus assemblages. The optical extinction spectra of the metallo-viral complex were compared to Mie scattering theory and found to be in quantitative agreement. These investigations demonstrate that direct harvesting of biological structures, rather than biochemical modification of protein sequences, is a viable route to create unique, optically active materials.

Nanocomposite materials based on sulfonated polyarylenethioethersulfone and sulfonated polybenzimidazole for proton exchange membrane fuel cell applications

Fabrication of novel nanocomposite membranes comprising a fully sulfonated polyarylenethioether sulfone (SPTES) and sulfonated poly(p-phenylene benzobisimidazole)(SPBI) and the evaluation of the membrane properties are described. The nanocomposite membrane was obtained via a solvent cast process in a mixture of DMAc and methanol as solvents. The nanocomposite membrane proton conductivity, as measured by four probe impedance spectroscopy, was found to increase with increase in the SPTES content in the nanocomposite. The highest proton conductivity obtained was ~80mS/cm at 65ºC and 85 % relative humidity for the SPTES/SPBI 70/30 nanocomposite membrane which was considerably less than the 300 mS/cm for the pure SPTES membrane, but it was found that the swelling of the nanocomposite membranes was reduced due to the reduced water uptake of the nanocomposite membrane relative to SPTES. The morphology of the SPTES/SPBI nanocomposite membranes was examined by a combination of techniques, such as X-ray diffraction and scanning electron microscopy, to confirm the dispersion of SPBI in the nanocomposite. The membrane electrode assembly performance of the nanocomposite membranes was preliminary studied for H2/O2 fuel cells applications.