Paul W. Bohn

Bio: Paul W. Bohn is an academic researcher from University of Notre Dame. The author has contributed to research in topics: Raman scattering & Raman spectroscopy. The author has an hindex of 51, co-authored 314 publications receiving 9601 citations. Previous affiliations of Paul W. Bohn include Bell Labs & Engineer Research and Development Center.

Metal-assisted chemical etching in HF/H2O2 produces porous silicon

Abstract: A simple and effective method is presented for producing light-emitting porous silicon (PSi) A thin (d<10 nm) layer of Au, Pt, or Au/Pd is deposited on the (100) Si surface prior to immersion in a solution of HF and H2O2 Depending on the type of metal deposited and Si doping type and doping level, PSi with different morphologies and light-emitting properties is produced PSi production occurs on the time scale of seconds, without electrical current, in the dark, on both p- and n-type Si Thin metal coatings facilitate the etching in HF and H2O2, and of the metals investigated, Pt yields the fastest etch rates and produces PSi with the most intense luminescence A reaction scheme involving local coupling of redox reactions with the metal is proposed to explain the metal-assisted etching process The observation that some metal remains on the PSi surface after etching raises the possibility of fabricating in situ PSi contacts

Gateable Nanofluidic Interconnects for Multilayered Microfluidic Separation Systems

Abstract: The extension of microfluidic devices to include three-dimensional fluidic networks allows complex fluidic and chemical manipulations but requires innovative methods to interface fluidic layers. Externally controllable interconnects, employing nuclear track-etched polycarbonate membranes containing nanometer-diameter capillaries, are described that produce hybrid three-dimensional fluidic architectures. Controllable nanofluidic transfer is achieved by controlling applied bias, polarity, and density of the immobile nanopore surface charge and the impedance of the nanocapillary array relative to the microfluidic channels. Analyte transport between vertically separated microchannels has three stable transfer levels, corresponding to zero, reverse, and forward bias. The transfer can even depend on the properties of the analyte being transferred such as the molecular size, illustrating the flexible character of the analyte transfer. In a specific analysis implementation, nanochannel array gating is applied to ...

Nanofluidics in chemical analysis

Abstract: Nanofluidic architectures and devices have already had a major impact on forefront problems in chemical analysis, especially those involving mass-limited samples. This critical review begins with a discussion of the fundamental flow physics that distinguishes nanoscale structures from their larger microscale analogs, especially the concentration polarization that develops at nanofluidic/microfluidic interfaces. Chemical manipulations in nanopores include nanopore-mediated separations, microsensors, especially resistive-pulse sensing of biomacromolecules, fluidic circuit analogs and single molecule measurements. Coupling nanofluidic structures to three-dimensional microfluidic networks is especially powerful and results in applications in sample preconcentration, nanofluidic injection/collection and fast diffusive mixing (160 references).

In-plane control of morphology and tunable photoluminescence in porous silicon produced by metal-assisted electroless chemical etching

Abstract: Photoluminescent porous silicon (PSi) was produced by Pt-assisted electroless etching of p−-Si (100) in a 1:2:1 solution of HF, H2O2, and methanol. The peak emission wavelength of the PSi could be tuned in the range 500 nm⩽λ⩽600 nm simply by changing the time of etching. The luminescence is sufficiently intense at all wavelengths to be visible by eye. Furthermore, by patterning the metal areas on the surface prior to etching, the luminescence can be controlled spatially. To investigate the relationship among processing variables — principally etch time and spatial proximity to Pt — and morphology, scanning electron microscopy (SEM), true color fluorescence microscopy, and spatially resolved phonon line shape studies were undertaken. SEM images show nanocrystalline features in the region where the luminescence originates, a region which shifts spatially as a function of etch time, as indicated by fluorescence microscopy. Raman scattering measurements of the shift and broadening of the longitudinal optical ...

Manipulating Molecular Transport through Nanoporous Membranes by Control of Electrokinetic Flow: Effect of Surface Charge Density and Debye Length

Abstract: Molecular transport through nanoporous nuclear-track-etched membranes was investigated with fluorescent probes by manipulating applied electric field polarity, pore size, membrane surface functionality, pH, and the ionic strength. Three forces contribute to analyte transport through membranes: ion migration, electroosmosis, and diffusion. Diffusion dominates under field-free conditions with surface hydrophobicity controlling solvent access to the nanochannels and hence the magnitude of transport by diffusion. In low ionic strength solutions (μ ∼ 10 mM), electroosmosis dominates transport when the membranes are biased, and the charge state of the surface determines the direction of flow. At high ionic strength (μ ∼ 1 M), ion migration dominates in hydrophobic membranes, and diffusion is controlling in hydrophilic membranes. The magnitude and polarity of the interior surface charge is controlled by surface functionality and displays the largest impact on molecular transport. The analyte can migrate in oppo...

Formation and Structure of Self-Assembled Monolayers.

Abstract: The field of self-assembled monolayers (SAMs) has witnessed tremendous growth in synthetic sophistication and depth of characterization over the past 15 years.1 However, it is interesting to comment on the modest beginning and on important milestones. The field really began much earlier than is now recognized. In 1946 Zisman published the preparation of a monomolecular layer by adsorption (self-assembly) of a surfactant onto a clean metal surface.2 At that time, the potential of self-assembly was not recognized, and this publication initiated only a limited level of interest. Early work initiated in Kuhn’s laboratory at Gottingen, applying many years of experience in using chlorosilane derivative to hydrophobize glass, was followed by the more recent discovery, when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions.3 Getting away from the moisture-sensitive alkyl trichlorosilanes, as well as working with crystalline gold surfaces, were two important reasons for the success of these SAMs. Many self-assembly systems have since been investigated, but monolayers of alkanethiolates on gold are probably the most studied SAMs to date. The formation of monolayers by self-assembly of surfactant molecules at surfaces is one example of the general phenomena of self-assembly. In nature, self-assembly results in supermolecular hierarchical organizations of interlocking components that provides very complex systems.4 SAMs offer unique opportunities to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. The ability to tailor both head and tail groups of the constituent molecules makes SAMs excellent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrates and molecule-solvent interactions like ordering and growth, wetting, adhesion, lubrication, and corrosion. That SAMs are well-defined and accessible makes them good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. SAMs provide the needed design flexibility, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the structure and stability of two-dimensional assemblies. These studies may eventually produce the design capabilities needed for assemblies of three-dimensional structures.5 However, this will require studies of more complex systems and the combination of what has been learned from SAMs with macromolecular science. The exponential growth in SAM research is a demonstration of the changes chemistry as a disciAbraham Ulman was born in Haifa, Israel, in 1946. He studied chemistry in the Bar-Ilan University in Ramat-Gan, Israel, and received his B.Sc. in 1969. He received his M.Sc. in phosphorus chemistry from Bar-Ilan University in 1971. After a brief period in industry, he moved to the Weizmann Institute in Rehovot, Israel, and received his Ph.D. in 1978 for work on heterosubstituted porphyrins. He then spent two years at Northwestern University in Evanston, IL, where his main interest was onedimensional organic conductors. In 1985 he joined the Corporate Research Laboratories of Eastman Kodak Company, in Rochester, NY, where his research interests were molecular design of materials for nonlinear optics and self-assembled monolayers. In 1994 he moved to Polytechnic University where he is the Alstadt-Lord-Mark Professor of Chemistry. His interests encompass self-assembled monolayers, surface engineering, polymers at interface, and surfaces phenomena. 1533 Chem. Rev. 1996, 96, 1533−1554

Science and technology for water purification in the coming decades

Abstract: One of the most pervasive problems afflicting people throughout the world is inadequate access to clean water and sanitation. Problems with water are expected to grow worse in the coming decades, with water scarcity occurring globally, even in regions currently considered water-rich. Addressing these problems calls out for a tremendous amount of research to be conducted to identify robust new methods of purifying water at lower cost and with less energy, while at the same time minimizing the use of chemicals and impact on the environment. Here we highlight some of the science and technology being developed to improve the disinfection and decontamination of water, as well as efforts to increase water supplies through the safe re-use of wastewater and efficient desalination of sea and brackish water.

Surface-Enhanced Raman Spectroscopy

Abstract: The ability to control the size, shape, and material of a surface has reinvigorated the field of surface-enhanced Raman spectroscopy (SERS). Because excitation of the localized surface plasmon resonance of a nanostructured surface or nanoparticle lies at the heart of SERS, the ability to reliably control the surface characteristics has taken SERS from an interesting surface phenomenon to a rapidly developing analytical tool. This article first explains many fundamental features of SERS and then describes the use of nanosphere lithography for the fabrication of highly reproducible and robust SERS substrates. In particular, we review metal film over nanosphere surfaces as excellent candidates for several experiments that were once impossible with more primitive SERS substrates (e.g., metal island films). The article also describes progress in applying SERS to the detection of chemical warfare agents and several biological molecules.