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Biomedical subjects

R Stanley Williams

Publications and source records attributed to R Stanley Williams.

11 recordsLinked to original sources
PubMed2006

Towards the silicon nanowire-based sensor for intracellular biochemical detection.

A microneedle sensor platform with integrated silicon nanowire tip was developed for intracellular biochemical detection. Because of the virtue of miniaturized size and high sensitivity, this sensor has a great potential for studying individual cell or localized bioenvironment by revealing the pH level and/or enzyme activities. The fabrication of the microneedle sensor was primarily based on conventional silicon processing, where a silicon-on-insulator (SOI) wafer with 50 nm thick (100) p-type Si device layer was used as the substrate. The silicon nanowires of 50 nm height and 50-100 nm width were created by electron beam (E-beam) lithography on the tip of microneedle with good electrical connection to the contact pads for convenient electrical measurement. A three layer structure with base, support cantilever, and needle tip was designed to ensure convenient handling of sensors and minimize the invasive penetration into biological cells. In this paper, we demonstrate a preliminary assessment of this novel intracellular sensor with electrical conductance measurement under different pH levels. It is expected that this sensor with proper chemical modification will enable localized biochemical sensing within biological cells, such as neurotransmitter activities during the synaptic communication between neuron cells.

Biosensing Techniques↗
PubMed2006

Self-assembled monolayers on Pt(111): molecular packing structure and strain effects observed by scanning tunneling microscopy.

Self-assembled monolayers (SAMs) of octanethiol and benzeneethanethiol were deposited on clean Pt(111) surfaces in ultrahigh vacuum (UHV). Highly resolved images of these SAMs produced by an in situ scanning tunneling microscope (STM) showed that both systems organize into a super-structure mosaic of domains of locally ordered, closely packed molecules. Analysis of the STM images indicated a (square root 3 x square root 3)R30 degrees unit cell for the octanethiol SAMs and a 4(square root 3 x square root 3)R30 degrees periodicity based on 2 x 2 basic molecular packing for the benzeneethanethiol SAMs under the coverage conditions investigated. SAMs on Pt(111) exhibited differences in molecular packing and a lower density of disordered regions than SAMs on Au(111). Electron transport measurements were performed using scanning tunneling spectroscopy. Benzeneethanethiol/Pt(111) junctions exhibited a higher conductance than octanethiol/Pt(111) junctions.

Journal Article↗
PubMed2006

Circuit fabrication at 17 nm half-pitch by nanoimprint lithography.

High density metal cross bars at 17 nm half-pitch were fabricated by nanoimprint lithography. Utilizing the superlattice nanowire pattern transfer technique, a 300-layer GaAs/AlGaAs superlattice was employed to produce an array of 150 Si nanowires (15 nm wide at 34 nm pitch) as an imprinting mold. A successful reproduction of the Si nanowire pattern was demonstrated. Furthermore, a cross-bar platinum nanowire array with a cell density of approximately 100 Gbit/cm(2) was fabricated by two consecutive imprinting processes.

Journal Article↗
PubMed2006

Regular arrays of monodisperse platinum/erbium disilicide core-shell nanowires and nanoparticles on Si(001) via a self-assembled template.

We have developed a process for fabricating monodisperse noble metal/rare earth disilicide core-shell nanoparticles and nanowires in regular arrays on Si(001) with a density of 5 x 10(10) / cm2, and over areas > 1 mm2. Pt deposited via physical vapor deposition on a self-assembled rare earth disilicide nanowire template combined with reactive ion etching produces arrays of nanostructures. SEM images demonstrate the ability to select nanowires or nanoparticles as a function of Pt coverage. Statistical analysis of images of Pt nanoparticle arrays yield a mean feature size of 8 nm with a size variation of +/- 0.9 nm and interparticle spacing of approximately 15 nm.

Crystallization↗
PubMed2005

Improved pattern transfer in nanoimprint lithography at 30 nm half-pitch by substrate-surface functionalization.

Resist detachment from the substrate during mold-substrate separation is one of the key challenges for nanoimprint lithography as the pitch of features decreases. We analyzed the problem by considering the surface and interfacial free energies of the initial state and the possible final states of the mold-polymer-substrate system and designed the chemistry of the system to provide the desired final state. We dramatically improved the resist adhesion to the substrate by assembling a monolayer of surface linker molecules on the substrate surface. A 37 nanowire pattern at 30 nm half-pitch was imprinted onto the surface-modified substrate.

Microscopy, Electron, Scanning↗
PubMed2005

Vapor-phase self-assembled monolayer for improved mold release in nanoimprint lithography.

Resist adhesion to the mold is one of the challenges for nanoimprint lithography. The main approach to overcoming it is to apply a self-assembled monolayer of an organosilane release agent to the mold surface, either in the solution phase or vapor phase. We compared the atomic force microscopy, ellipsometry, reflection-absorption infrared spectroscopy, and contact angle results collected from substrates treated by two different application processes and found that the vapor-phase process was superior. The vapor-treated substrates had fewer aggregates of the silane molecules on the surface, because the lower density of the agent in the vapor phase was not conducive to aggregation formation, and received a superior coating of the releasing agent, because the vapor was more effective than the solution in penetrating into the nanoscale gaps of the mold. A pattern transfer of 20 parallel nanowires with a line width of 40 nm at 100 nm pitch-size was performed faithfully with the vapor-treated mold without any resist adhesion.

Letter↗
PubMed2005

Cross-linked polymer replica of a nanoimprint mold at 30 nm half-pitch.

This letter reports the demonstration of a photocurable polymer process for replicating the master mold for nanoimprint lithography. The cross-linked polymer mold was fabricated directly with high fidelity from a master by imprinting and photocuring a low viscosity liquid prepolymer film spun onto a substrate. The surface of the cross-linked polymer mold can be treated using an O(2) plasma, and then vapor primed with a low surface energy mold release layer for repeatable imprinting. The imprinting results demonstrated that the cross-linked polymer mold could be faithfully used for both thermal and photocurable nanoimprint lithography.

Journal Article↗
PubMed2003

3D composition of epitaxial nanocrystals by anomalous X-ray diffraction: observation of a Si-rich core in Ge domes on Si(100).

Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 degrees C were obtained from grazing incidence anomalous x-ray scattering data at the Ge K edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.

Journal Article↗
PubMed2003

Equilibrium model of bimodal distributions of epitaxial island growth.

We present a nanostructure diagram for use in designing heteroepitaxial systems of quantum dots. The nanostructure diagram is computed using a new equilibrium statistical physics model and predicts the island size and shape distributions for a range of combinations of growth temperature and amount of deposited material. The model is applied to Ge on Si(001), the archetype for bimodal island growth, and the results compare well with data from atomic force microscopy of Ge/Si islands grown by chemical vapor deposition.

Journal Article↗