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Papers by Paul Bohn
Faraday Discussions, 2022
Potential-induced wetting and dewetting behaviors in pH-responsive block copolymer membranes are ... more Potential-induced wetting and dewetting behaviors in pH-responsive block copolymer membranes are investigated to establish a promising mass transport control system for ultrasensitive sensing applications.
A three-dimensional microfluidic circuit with nanofluidic interconnecting gates has been develope... more A three-dimensional microfluidic circuit with nanofluidic interconnecting gates has been developed for the injection and extremely rapid mixing of analytes for chip-based biochemical assays. Polycarbonate nanoporous membranes containing 15-to 200-nm diameter cylindrical pores are employed as electrically-controlled molecular gates to inject bands of ultrasmall analyte samples (-10 attomoles) from one microchannel channel to another, and to completely mix and react analytes within microns of the injection point. The molecular gating, injection, and rapid mixing functionalities depends on the electrokinetic flow through the nanopores, which separates the microchannels.
The Journal of Chemical Physics
Pseudomonas aeruginosa is an opportunistic human pathogen implicated in both acute and chronic di... more Pseudomonas aeruginosa is an opportunistic human pathogen implicated in both acute and chronic diseases, which resists antibiotic treatment, in part by forming physical and chemical barriers such as biofilms. Here, we explore the use of confocal Raman imaging to characterize the three-dimensional (3D) spatial distribution of alkyl quinolones (AQs) in P. aeruginosa biofilms by reconstructing depth profiles from hyperspectral Raman data. AQs are important to quorum sensing (QS), virulence, and other actions of P. aeruginosa. Three-dimensional distributions of three different AQs (PQS, HQNO, and HHQ) were observed to have a significant depth, suggesting 3D anisotropic shapes—sheet-like rectangular solids for HQNO and extended cylinders for PQS. Similar to observations from 2D imaging studies, spectral features characteristic of AQs (HQNO or PQS) and the amide I vibration from peptide-containing species were found to correlate with the PQS cylinders typically located at the tips of the ...
2018 IEEE 18th International Conference on Nanotechnology (IEEE-NANO), 2018
Because electron transfer reactions are fundamental to life processes, such as respiration, visio... more Because electron transfer reactions are fundamental to life processes, such as respiration, vision, and energy catabolism, it is critically important to understand the relationship between functional states of individual redox enzymes and the macroscopically observed phenotype, which results from averaging over all copies of the same enzyme. To address this problem, we have developed a new technology, based on a bifunctional nanoelectrochemical-nanophotonic architecture - the electrochemical zero mode waveguide (E-ZMW) - that can couple biological electron transfer reactions to luminescence, making it possible to observe single electron transfer events in redox enzymes. Here we describe E-ZMW architectures capable of supporting potential-controlled redox reactions with single copies of the oxidoreductase enzyme, glutathione reductase, GR, and extend these capabilities to electron transfer events where reactive oxygen species are synthesized within the ~ 100 zL volume of the nanopore.
This document constitutes the final technical report for DE-SC0006642, In Situ Correlated Molecul... more This document constitutes the final technical report for DE-SC0006642, In Situ Correlated Molecular Imaging of Chemically Communicating Microbial Communities, a project carried out collaboratively by investigators at Notre Dame and UIUC. The work carried out under DOE support in this project produced advances in two areas: development of new highly sophisticated correlated imaging approaches and the application of these new tools to the growth and differentiation of microbial communities under a variety of environmental conditions. A significant effort involved the creation of technical enhancements and sampling approaches to allow us to advance heterocorrelated mass spectrometry imaging (MSI) and correlated Raman microscopy (CRM) from bacterial cultures and biofilms. We then exploited these measurement advances in heterocorrelated MS/CRM imaging to determine relationship of signaling molecules and excreted signaling molecules produced by P. aeruginosa to conditions relevant to the rhizosphere. In particular, we: (1) developed a laboratory testbed mimic for the rhizosphere to enable microbial growth on slides under controlled conditions; (2) integrated specific measurements of (a) rhamnolipids, (b) quinolone/quinolones, and (c) phenazines specific to P. aeruginosa; and (3) utilized the imaging tools to probe how messenger secretion, quorum sensing and swarming behavior are correlated with behavior.
Physical Review B, 1994
Room-temperature Raman spectroscopy was used to characterize n-type-doped InQ53GaQ47As and Ino, z... more Room-temperature Raman spectroscopy was used to characterize n-type-doped InQ53GaQ47As and Ino, zA)0 4, As layers by observing coupled longitudinal-optical (LO)-phonon-plasmon mode spectra as a function of carrier density. Carrier densities determined from the high-frequency I. + coupled mode shift in the Raman spectra were compared to carrier densities determined from electrochemical capacitance-voltage profiling measurements. In InQ 53GaQ47As with carrier densities n & SX10"cm accurate determination of carrier densities from the Raman spectra was hindered by Landau damping and small I. + mode dispersion. Better agreement between the two techniques was observed in InQ 53GaQ 47As and InQ»AlQ 48As for carrier densities of about 8 X 10' cm ' and higher, doping levels at which Landau damping is less pronounced. Surface band bending was evaluated from changes in Raman intensity as a function of carrier density of dipole-allowed LO-phonon modes originating in the surface space-charge region. Values for the surface built-in potential determined from the Raman spectra were found to be smaller than those obtained from electrical measurements. The discrepancy is attributed to screening of the surface dipole by photogenerated carriers.
Langmuir, 1991
The kinetics and structural chemistry associated with the displacement of acetic acid by lon er c... more The kinetics and structural chemistry associated with the displacement of acetic acid by lon er chain alkanoic acids, CHs(CH2),C02H, n = 12, 14, 16, and 18, from cyclohexane solution were stutied by a combination of surface plasmon resonance measurements and surface enhanced Raman scattering. The early time kinetics show linear behavior for all chain lengths with the initial rate of adsorption increasing with longer chain lengths. The early kinetics for stearic acid (n = 16) also show a monotonic dependence on solution concentration up to the critical reverse micelle concentration. These observations are interpreted in terms the solvent-solute interactions becoming less favorable as the chain length becomes longer. Kinetics over all time scales are fit to an Elovich equation, and the role of surface heterogeneity is interpreted in terms of alkyl tails blocking some sites but not others. Surface Raman spectra are also obtained in real time as a function of surface coverage, r. The lack of a carbonyl stretch shows that the molecules adsorb as the carboxylates. The ratio of asymmetric to symmetric carboxylate stretching intensities shows a complex behavior as a function of r, which is interpreted in terms of relative surface populations and reorientation of tail and head groups at high r.
Applied Physics Letters, 1999
GaN grown by selective area epitaxy and subsequent lateral overgrowth exhibits sharply peaked ani... more GaN grown by selective area epitaxy and subsequent lateral overgrowth exhibits sharply peaked anisotropic structures in the form of hexagonal pyramids and ridges. Spatially resolved optical emission from these structures, using both cathodoluminescence and collection-mode near-field scanning optical microscopy, of radiation centered near 550 nm, the so-called yellow band, indicates that the emission arises predominantly from the apex regions of the pyramids and ridges. In contrast, transmission electron microscopy shows that the apex region is nearly dislocation free and that dislocations cluster at the vertical growth core region. The spatial separation of the dislocations and optical emission indicates that the yellow-band emission has no direct relationship to dislocations. The observation of yellow-band emission strongly localized in the apical regions of both types of structures and the tendency of impurity species to concentrate in these areas argues that it arises instead fro...
Biomicrofluidics, Jul 1, 2014
Electrochemistry is a promising tool for microfluidic systems because it is relatively inexpensiv... more Electrochemistry is a promising tool for microfluidic systems because it is relatively inexpensive, structures are simple to fabricate, and it is straightforward to interface electronically. While most widely used in microfluidics for chemical detection or as the transduction mechanism for molecular probes, electrochemical methods can also be used to efficiently alter the chemical composition of small (typically <100 nl) microfluidic volumes in a manner that improves or enables subsequent measurements and sample processing steps. Here, solvent (H 2 O) electrolysis is performed quantitatively at a microchannel Pt band electrode to increase microchannel pH. The change in microchannel pH is simultaneously tracked at a downstream electrode by monitoring changes in the i-V characteristics of the proton-coupled electro-oxidation of hydroquinone, thus providing real-time measurement of the protonated forms of hydroquinone from which the pH can be determined in a straightforward manner. Relative peak heights for protonated and deprotonated hydroquinone forms are in good agreement with expected pH changes by measured electrolysis rates, demonstrating that solvent electrolysis can be used to provide tunable, quantitative pH control within a microchannel.
Solar Energy Materials and Solar Cells, Aug 1, 2012
ABSTRACT Ag particle-assisted chemical etching of silicon wafers in HF/H2O2 is of interest for it... more ABSTRACT Ag particle-assisted chemical etching of silicon wafers in HF/H2O2 is of interest for its potential to produce antireflective layers for solar cells. In this work, Ag films containing both nanoscale (d<100 nm) and microscale (d<1 μm) particles were deposited through the silver-mirror reaction on planar p-Si(111), planar p-Si(100) and p-Si(100) pre-etched in KOH/isopropanol to produce pyramidal textures. Subsequently, these wafers were subjected to metal-assisted chemical etching (MacEtch) in 1:1:1 (v:v:v) HF(49%):H2O2(30%):EtOH solutions, to produce porous silicon (PSi) containing both micro- and nanoscale roughness features. The resulting surfaces exhibit morphologies that evolve with processing conditions, especially the absence/presence of pyramidal textures and the time the structure is subjected to MacEtch. Under optimal conditions excellent anti-reflection behavior is observed with surface reflectivities being reduced below 10% for either p-Si(100) or p-Si(111) surfaces. For p-Si(100) better results (R∼5%) were obtained for 30 min KOH/isopropanol pre-etch than for either no pre-etch or longer (60 min) pre-etch. The influence of the reductant on Ag particle deposition on p-Si(111) was studied, and MacEtch catalyzed by Ag produced from acetaldehyde reductant produced surfaces with lower reflectivities than those with glucose reductant.
APS March Meeting Abstracts, Mar 1, 1997
ABSTRACT
The Journal of Physical Chemistry, 1996
A novel multisulfur heterocyclic compound with alkyl chains, C 3 S 5 (C 18 H 37) 2 (4,5-bis(octad... more A novel multisulfur heterocyclic compound with alkyl chains, C 3 S 5 (C 18 H 37) 2 (4,5-bis(octadecylthio)-1,3-dithiole-2-thione), has been synthesized and fabricated into Langmuir-Blodgett (LB) films. The structure of this film on SiO 2 slides has been studied using X-ray photoelectron spectroscopy (XPS) and IR and UV transmission linear dichroism (LD) measurements. From the isotherm, it is known that the Langmuir film at the airwater interface is a bilayer. Angle-resolved XPS of films transferred to SiO 2 show that the alkyl chains of the molecule form the topmost layer of the film, with the C 3 S 5 ring underneath the alkyl layer. Electronic structure calculations indicate that the transition moment associated with the observed UV band lies coincident with the C 2 axis of the C 3 S 5 ring group. In order to properly interpret the polarized absorption data, a simple model addressing the local field effect in LD measurements was developed. Incorporating measured information on the polarizability of the chromophore, UV and IR LD measurements showed the ring plane and the chains to be stacked at 69°and 20°, respectively, relative to the surface normal. These data coupled with the XPS results permitted a structural model to be constructed which accounts for the multilayer structure of the transferred film.
Czechoslovak Journal of Physics, Jun 1, 1996
Temperature-dependent Raman spectroscopy is used to investigate the effect of superconductivity o... more Temperature-dependent Raman spectroscopy is used to investigate the effect of superconductivity on the near-surface electronic structure of a semiconductor in good electrical contact with a superconductor. The light scattering is performed through a high-quality thin (60-100A) film of Nb, grown directly onto in-situ Ar+-etched (100)-n+InAs. Below To, the LO mode, associated with the surface charge accumulation layer in the InAs, is enhanced by-40% in comparison with the nearby L_ bulk phonon mode. This change, reversible upon temperature cycling, is observed only when the Nb is in good electrical contact with the InAs. Preliminary results show a similar effect on NbN/InAs. Our results constitute the first optical detection of the superconducting proximity effect. 1.
Thin Solid Films, Dec 1, 2001
Exposure of thin (20 nmFdF80 nm) Au films to solutions of strongly interacting Lewis bases, e.g. ... more Exposure of thin (20 nmFdF80 nm) Au films to solutions of strongly interacting Lewis bases, e.g. organothiols, I , results y in adsorption which raises the resistivity of the metal film ;2%. The relative ease of measuring resistivity via a four-point probe and the large signal-to-background ratio which characterizes the measurement makes electrical conduction measurements an attractive candidate for chemical sensors and studies of the liquid-solid interface. It is widely accepted that surface contributions to resistivity are determined both by electronic structure in the near-surface region and by morphology. In these experiments Au film sensitivity to molecular adsorption was varied by controlling surface roughness. Resistivity changes were measured on smooth annealed Au films, on the same films after etching with CN , and then after subsequent treatment with I. After each y y treatment the film morphology was characterized by atomic force microscopy (AFM), and the resulting power spectral densities were correlated with resistivity changes resulting from standard exposures to C H SH. These measurements were used to 16 33 understand the role of morphology in determining the sensitivity of resistivity measurements to molecular adsorption. As expected CN significantly roughened the annealed Au films, a change accompanied by an increase in sensitivity to adsorption. Subsequent y I exposure smoothed the films at all length scales, but surprisingly the sensitivity increased again. A two-site model is proposed y which explains the results in terms of the competing effects of increased surface area and changes in surface site densities caused by chemical etching.
Environmental Science & Technology, Apr 15, 2005
A miniaturized lead sensor has been developed by combining a lead-specific DNAzyme with a microfa... more A miniaturized lead sensor has been developed by combining a lead-specific DNAzyme with a microfabricated device containing a network of microfluidic channels that are fluidically coupled via a nanocapillary array interconnect. A DNAzyme construct, selective for cleavage in the presence of Pb 2+ and derivatized with fluorophore (quencher) at the 5′ (3′) end of the substrate and enzyme strands, respectively, forms a molecular beacon that is used as the recognition element. The nanocapillary array membrane interconnect is used to manipulate fluid flows and deliver the small-volume sample to the beacon in a spatially confined detection window where the DNAzyme is interrogated using laser-induced fluorescence detection. A transformed log plot of the fluorescent signal exhibits a linear response (r 2) 0.982) over a Pb 2+ concentration range of 0.1-100 µM, and a detection limit of 11 nM. The sensor has been applied to the determination of Pb 2+ in an electroplating sludge reference material, the result agreeing with the certified value within 4.9%. Quantitative measurement of Pb 2+ in this complex sample demonstrates the selectivity of this sensor scheme and points favorably to the application of such technologies to analysis of environmental samples. The unique combination of a DNAzyme with a microfluidic-nanofluidic hybrid device makes it possible to change the DNAzyme to select for other compounds of interest, and to incorporate multiple sensing systems within a single device for greater flexibility. This work represents the initial steps toward creation of a robust field sensor for lead in groundwater or drinking water.
Mrs Bulletin, 2008
Trace contaminant detection in water represents both a grand challenge and great opportunity for ... more Trace contaminant detection in water represents both a grand challenge and great opportunity for materials scientists and engineers. The recent discovery that functional DNA can be obtained to bind selectively to a wide range of contaminants makes it possible to interface these molecules with nanoscale materials, such as gold nanoparticles and quantum dots, to transform the molecular reorganization between functional DNA and contaminants into physically detectable colorimetric and fluorescent signals. Micro-and nanofluidic devices have also played a critical role in lowering the detection limits of functional-DNA sensors, promoting sensor regeneration and thus improving sensor performance and allowing long-term unattended monitoring of water quality.
Proceedings of SPIE, Jun 10, 1996
ABSTRACT
Physical review, Oct 31, 2002
High-quality thin Nb and NbN films ͑60-100 Å͒ are grown on ͑100͒ n ϩ-InAs (nϭ10 19 cm Ϫ3) substra... more High-quality thin Nb and NbN films ͑60-100 Å͒ are grown on ͑100͒ n ϩ-InAs (nϭ10 19 cm Ϫ3) substrates by dc-magnetron sputter deposition. Studies of the electronic properties of interfaces between the superconductor and the semiconductor are done by Raman scattering measurements. The superconducting proximity effect at superconductor-semiconductor interfaces is observed through its impact on inelastic light scattering intensities originating from the near-interface region of InAs. The InAs longitudinal optical phonon LO mode (237 cm Ϫ1) and the plasmon-phonon coupled modes L Ϫ (221 cm Ϫ1) and L ϩ ͑1100 to 1350 cm Ϫ1), for n ϩ ϭ1ϫ10 19 Ϫ2ϫ10 19 cm Ϫ3 are measured. The intensity ratio of the LO mode ͑associated with the near-surface charge accumulation region, in InAs͒ to that of the L Ϫ mode ͑associated with bulk InAs͒, is observed to increase by up to 40% below the superconducting transition temperature. This temperature-dependent change in light scattering properties is only observed with high quality superconducting films and when the superconductor and the semiconductor are in good electrical contact. A few possible mechanisms of the observed effect are proposed.
Journal of Nanoparticle Research, Jul 4, 2012
Species transport in nanocapillary membrane systems has engaged considerable research interest, p... more Species transport in nanocapillary membrane systems has engaged considerable research interest, presenting technological challenges and opportunities, while exhibiting significant deviations from conventionally well understood bulk behavior in microfluidics. Nonlinear electrokinetic effects and surface charge of materials, along with geometric considerations, dominate the phenomena in structures with characteristic lengths below 100 nm. Consequently, these methods have enabled 3D micro-and nanofluidic hybrid systems with high-chemical selectivity for precise manipulation of mass-limited quantities of analytes. In this review, we present an overview of both fundamental developments and applications of these unique nanocapillary systems, identifying forces that govern ion and particle transport, and surveying applications in separation, sensing, mixing, and chemical reactions. All of these developments are oriented toward adding important functionality in micro-total analysis systems. Keywords Membranes Á Nanostructures Á Nanofluidics Á Microfluidics Á Ion transport Á Electrokinetics Á l-TAS Á Nanopore Á Nanocapillary Á Water filtration Á Sustainable development
Faraday Discussions, 2022
Potential-induced wetting and dewetting behaviors in pH-responsive block copolymer membranes are ... more Potential-induced wetting and dewetting behaviors in pH-responsive block copolymer membranes are investigated to establish a promising mass transport control system for ultrasensitive sensing applications.
A three-dimensional microfluidic circuit with nanofluidic interconnecting gates has been develope... more A three-dimensional microfluidic circuit with nanofluidic interconnecting gates has been developed for the injection and extremely rapid mixing of analytes for chip-based biochemical assays. Polycarbonate nanoporous membranes containing 15-to 200-nm diameter cylindrical pores are employed as electrically-controlled molecular gates to inject bands of ultrasmall analyte samples (-10 attomoles) from one microchannel channel to another, and to completely mix and react analytes within microns of the injection point. The molecular gating, injection, and rapid mixing functionalities depends on the electrokinetic flow through the nanopores, which separates the microchannels.
The Journal of Chemical Physics
Pseudomonas aeruginosa is an opportunistic human pathogen implicated in both acute and chronic di... more Pseudomonas aeruginosa is an opportunistic human pathogen implicated in both acute and chronic diseases, which resists antibiotic treatment, in part by forming physical and chemical barriers such as biofilms. Here, we explore the use of confocal Raman imaging to characterize the three-dimensional (3D) spatial distribution of alkyl quinolones (AQs) in P. aeruginosa biofilms by reconstructing depth profiles from hyperspectral Raman data. AQs are important to quorum sensing (QS), virulence, and other actions of P. aeruginosa. Three-dimensional distributions of three different AQs (PQS, HQNO, and HHQ) were observed to have a significant depth, suggesting 3D anisotropic shapes—sheet-like rectangular solids for HQNO and extended cylinders for PQS. Similar to observations from 2D imaging studies, spectral features characteristic of AQs (HQNO or PQS) and the amide I vibration from peptide-containing species were found to correlate with the PQS cylinders typically located at the tips of the ...
2018 IEEE 18th International Conference on Nanotechnology (IEEE-NANO), 2018
Because electron transfer reactions are fundamental to life processes, such as respiration, visio... more Because electron transfer reactions are fundamental to life processes, such as respiration, vision, and energy catabolism, it is critically important to understand the relationship between functional states of individual redox enzymes and the macroscopically observed phenotype, which results from averaging over all copies of the same enzyme. To address this problem, we have developed a new technology, based on a bifunctional nanoelectrochemical-nanophotonic architecture - the electrochemical zero mode waveguide (E-ZMW) - that can couple biological electron transfer reactions to luminescence, making it possible to observe single electron transfer events in redox enzymes. Here we describe E-ZMW architectures capable of supporting potential-controlled redox reactions with single copies of the oxidoreductase enzyme, glutathione reductase, GR, and extend these capabilities to electron transfer events where reactive oxygen species are synthesized within the ~ 100 zL volume of the nanopore.
This document constitutes the final technical report for DE-SC0006642, In Situ Correlated Molecul... more This document constitutes the final technical report for DE-SC0006642, In Situ Correlated Molecular Imaging of Chemically Communicating Microbial Communities, a project carried out collaboratively by investigators at Notre Dame and UIUC. The work carried out under DOE support in this project produced advances in two areas: development of new highly sophisticated correlated imaging approaches and the application of these new tools to the growth and differentiation of microbial communities under a variety of environmental conditions. A significant effort involved the creation of technical enhancements and sampling approaches to allow us to advance heterocorrelated mass spectrometry imaging (MSI) and correlated Raman microscopy (CRM) from bacterial cultures and biofilms. We then exploited these measurement advances in heterocorrelated MS/CRM imaging to determine relationship of signaling molecules and excreted signaling molecules produced by P. aeruginosa to conditions relevant to the rhizosphere. In particular, we: (1) developed a laboratory testbed mimic for the rhizosphere to enable microbial growth on slides under controlled conditions; (2) integrated specific measurements of (a) rhamnolipids, (b) quinolone/quinolones, and (c) phenazines specific to P. aeruginosa; and (3) utilized the imaging tools to probe how messenger secretion, quorum sensing and swarming behavior are correlated with behavior.
Physical Review B, 1994
Room-temperature Raman spectroscopy was used to characterize n-type-doped InQ53GaQ47As and Ino, z... more Room-temperature Raman spectroscopy was used to characterize n-type-doped InQ53GaQ47As and Ino, zA)0 4, As layers by observing coupled longitudinal-optical (LO)-phonon-plasmon mode spectra as a function of carrier density. Carrier densities determined from the high-frequency I. + coupled mode shift in the Raman spectra were compared to carrier densities determined from electrochemical capacitance-voltage profiling measurements. In InQ 53GaQ47As with carrier densities n & SX10"cm accurate determination of carrier densities from the Raman spectra was hindered by Landau damping and small I. + mode dispersion. Better agreement between the two techniques was observed in InQ 53GaQ 47As and InQ»AlQ 48As for carrier densities of about 8 X 10' cm ' and higher, doping levels at which Landau damping is less pronounced. Surface band bending was evaluated from changes in Raman intensity as a function of carrier density of dipole-allowed LO-phonon modes originating in the surface space-charge region. Values for the surface built-in potential determined from the Raman spectra were found to be smaller than those obtained from electrical measurements. The discrepancy is attributed to screening of the surface dipole by photogenerated carriers.
Langmuir, 1991
The kinetics and structural chemistry associated with the displacement of acetic acid by lon er c... more The kinetics and structural chemistry associated with the displacement of acetic acid by lon er chain alkanoic acids, CHs(CH2),C02H, n = 12, 14, 16, and 18, from cyclohexane solution were stutied by a combination of surface plasmon resonance measurements and surface enhanced Raman scattering. The early time kinetics show linear behavior for all chain lengths with the initial rate of adsorption increasing with longer chain lengths. The early kinetics for stearic acid (n = 16) also show a monotonic dependence on solution concentration up to the critical reverse micelle concentration. These observations are interpreted in terms the solvent-solute interactions becoming less favorable as the chain length becomes longer. Kinetics over all time scales are fit to an Elovich equation, and the role of surface heterogeneity is interpreted in terms of alkyl tails blocking some sites but not others. Surface Raman spectra are also obtained in real time as a function of surface coverage, r. The lack of a carbonyl stretch shows that the molecules adsorb as the carboxylates. The ratio of asymmetric to symmetric carboxylate stretching intensities shows a complex behavior as a function of r, which is interpreted in terms of relative surface populations and reorientation of tail and head groups at high r.
Applied Physics Letters, 1999
GaN grown by selective area epitaxy and subsequent lateral overgrowth exhibits sharply peaked ani... more GaN grown by selective area epitaxy and subsequent lateral overgrowth exhibits sharply peaked anisotropic structures in the form of hexagonal pyramids and ridges. Spatially resolved optical emission from these structures, using both cathodoluminescence and collection-mode near-field scanning optical microscopy, of radiation centered near 550 nm, the so-called yellow band, indicates that the emission arises predominantly from the apex regions of the pyramids and ridges. In contrast, transmission electron microscopy shows that the apex region is nearly dislocation free and that dislocations cluster at the vertical growth core region. The spatial separation of the dislocations and optical emission indicates that the yellow-band emission has no direct relationship to dislocations. The observation of yellow-band emission strongly localized in the apical regions of both types of structures and the tendency of impurity species to concentrate in these areas argues that it arises instead fro...
Biomicrofluidics, Jul 1, 2014
Electrochemistry is a promising tool for microfluidic systems because it is relatively inexpensiv... more Electrochemistry is a promising tool for microfluidic systems because it is relatively inexpensive, structures are simple to fabricate, and it is straightforward to interface electronically. While most widely used in microfluidics for chemical detection or as the transduction mechanism for molecular probes, electrochemical methods can also be used to efficiently alter the chemical composition of small (typically <100 nl) microfluidic volumes in a manner that improves or enables subsequent measurements and sample processing steps. Here, solvent (H 2 O) electrolysis is performed quantitatively at a microchannel Pt band electrode to increase microchannel pH. The change in microchannel pH is simultaneously tracked at a downstream electrode by monitoring changes in the i-V characteristics of the proton-coupled electro-oxidation of hydroquinone, thus providing real-time measurement of the protonated forms of hydroquinone from which the pH can be determined in a straightforward manner. Relative peak heights for protonated and deprotonated hydroquinone forms are in good agreement with expected pH changes by measured electrolysis rates, demonstrating that solvent electrolysis can be used to provide tunable, quantitative pH control within a microchannel.
Solar Energy Materials and Solar Cells, Aug 1, 2012
ABSTRACT Ag particle-assisted chemical etching of silicon wafers in HF/H2O2 is of interest for it... more ABSTRACT Ag particle-assisted chemical etching of silicon wafers in HF/H2O2 is of interest for its potential to produce antireflective layers for solar cells. In this work, Ag films containing both nanoscale (d<100 nm) and microscale (d<1 μm) particles were deposited through the silver-mirror reaction on planar p-Si(111), planar p-Si(100) and p-Si(100) pre-etched in KOH/isopropanol to produce pyramidal textures. Subsequently, these wafers were subjected to metal-assisted chemical etching (MacEtch) in 1:1:1 (v:v:v) HF(49%):H2O2(30%):EtOH solutions, to produce porous silicon (PSi) containing both micro- and nanoscale roughness features. The resulting surfaces exhibit morphologies that evolve with processing conditions, especially the absence/presence of pyramidal textures and the time the structure is subjected to MacEtch. Under optimal conditions excellent anti-reflection behavior is observed with surface reflectivities being reduced below 10% for either p-Si(100) or p-Si(111) surfaces. For p-Si(100) better results (R∼5%) were obtained for 30 min KOH/isopropanol pre-etch than for either no pre-etch or longer (60 min) pre-etch. The influence of the reductant on Ag particle deposition on p-Si(111) was studied, and MacEtch catalyzed by Ag produced from acetaldehyde reductant produced surfaces with lower reflectivities than those with glucose reductant.
APS March Meeting Abstracts, Mar 1, 1997
ABSTRACT
The Journal of Physical Chemistry, 1996
A novel multisulfur heterocyclic compound with alkyl chains, C 3 S 5 (C 18 H 37) 2 (4,5-bis(octad... more A novel multisulfur heterocyclic compound with alkyl chains, C 3 S 5 (C 18 H 37) 2 (4,5-bis(octadecylthio)-1,3-dithiole-2-thione), has been synthesized and fabricated into Langmuir-Blodgett (LB) films. The structure of this film on SiO 2 slides has been studied using X-ray photoelectron spectroscopy (XPS) and IR and UV transmission linear dichroism (LD) measurements. From the isotherm, it is known that the Langmuir film at the airwater interface is a bilayer. Angle-resolved XPS of films transferred to SiO 2 show that the alkyl chains of the molecule form the topmost layer of the film, with the C 3 S 5 ring underneath the alkyl layer. Electronic structure calculations indicate that the transition moment associated with the observed UV band lies coincident with the C 2 axis of the C 3 S 5 ring group. In order to properly interpret the polarized absorption data, a simple model addressing the local field effect in LD measurements was developed. Incorporating measured information on the polarizability of the chromophore, UV and IR LD measurements showed the ring plane and the chains to be stacked at 69°and 20°, respectively, relative to the surface normal. These data coupled with the XPS results permitted a structural model to be constructed which accounts for the multilayer structure of the transferred film.
Czechoslovak Journal of Physics, Jun 1, 1996
Temperature-dependent Raman spectroscopy is used to investigate the effect of superconductivity o... more Temperature-dependent Raman spectroscopy is used to investigate the effect of superconductivity on the near-surface electronic structure of a semiconductor in good electrical contact with a superconductor. The light scattering is performed through a high-quality thin (60-100A) film of Nb, grown directly onto in-situ Ar+-etched (100)-n+InAs. Below To, the LO mode, associated with the surface charge accumulation layer in the InAs, is enhanced by-40% in comparison with the nearby L_ bulk phonon mode. This change, reversible upon temperature cycling, is observed only when the Nb is in good electrical contact with the InAs. Preliminary results show a similar effect on NbN/InAs. Our results constitute the first optical detection of the superconducting proximity effect. 1.
Thin Solid Films, Dec 1, 2001
Exposure of thin (20 nmFdF80 nm) Au films to solutions of strongly interacting Lewis bases, e.g. ... more Exposure of thin (20 nmFdF80 nm) Au films to solutions of strongly interacting Lewis bases, e.g. organothiols, I , results y in adsorption which raises the resistivity of the metal film ;2%. The relative ease of measuring resistivity via a four-point probe and the large signal-to-background ratio which characterizes the measurement makes electrical conduction measurements an attractive candidate for chemical sensors and studies of the liquid-solid interface. It is widely accepted that surface contributions to resistivity are determined both by electronic structure in the near-surface region and by morphology. In these experiments Au film sensitivity to molecular adsorption was varied by controlling surface roughness. Resistivity changes were measured on smooth annealed Au films, on the same films after etching with CN , and then after subsequent treatment with I. After each y y treatment the film morphology was characterized by atomic force microscopy (AFM), and the resulting power spectral densities were correlated with resistivity changes resulting from standard exposures to C H SH. These measurements were used to 16 33 understand the role of morphology in determining the sensitivity of resistivity measurements to molecular adsorption. As expected CN significantly roughened the annealed Au films, a change accompanied by an increase in sensitivity to adsorption. Subsequent y I exposure smoothed the films at all length scales, but surprisingly the sensitivity increased again. A two-site model is proposed y which explains the results in terms of the competing effects of increased surface area and changes in surface site densities caused by chemical etching.
Environmental Science & Technology, Apr 15, 2005
A miniaturized lead sensor has been developed by combining a lead-specific DNAzyme with a microfa... more A miniaturized lead sensor has been developed by combining a lead-specific DNAzyme with a microfabricated device containing a network of microfluidic channels that are fluidically coupled via a nanocapillary array interconnect. A DNAzyme construct, selective for cleavage in the presence of Pb 2+ and derivatized with fluorophore (quencher) at the 5′ (3′) end of the substrate and enzyme strands, respectively, forms a molecular beacon that is used as the recognition element. The nanocapillary array membrane interconnect is used to manipulate fluid flows and deliver the small-volume sample to the beacon in a spatially confined detection window where the DNAzyme is interrogated using laser-induced fluorescence detection. A transformed log plot of the fluorescent signal exhibits a linear response (r 2) 0.982) over a Pb 2+ concentration range of 0.1-100 µM, and a detection limit of 11 nM. The sensor has been applied to the determination of Pb 2+ in an electroplating sludge reference material, the result agreeing with the certified value within 4.9%. Quantitative measurement of Pb 2+ in this complex sample demonstrates the selectivity of this sensor scheme and points favorably to the application of such technologies to analysis of environmental samples. The unique combination of a DNAzyme with a microfluidic-nanofluidic hybrid device makes it possible to change the DNAzyme to select for other compounds of interest, and to incorporate multiple sensing systems within a single device for greater flexibility. This work represents the initial steps toward creation of a robust field sensor for lead in groundwater or drinking water.
Mrs Bulletin, 2008
Trace contaminant detection in water represents both a grand challenge and great opportunity for ... more Trace contaminant detection in water represents both a grand challenge and great opportunity for materials scientists and engineers. The recent discovery that functional DNA can be obtained to bind selectively to a wide range of contaminants makes it possible to interface these molecules with nanoscale materials, such as gold nanoparticles and quantum dots, to transform the molecular reorganization between functional DNA and contaminants into physically detectable colorimetric and fluorescent signals. Micro-and nanofluidic devices have also played a critical role in lowering the detection limits of functional-DNA sensors, promoting sensor regeneration and thus improving sensor performance and allowing long-term unattended monitoring of water quality.
Proceedings of SPIE, Jun 10, 1996
ABSTRACT
Physical review, Oct 31, 2002
High-quality thin Nb and NbN films ͑60-100 Å͒ are grown on ͑100͒ n ϩ-InAs (nϭ10 19 cm Ϫ3) substra... more High-quality thin Nb and NbN films ͑60-100 Å͒ are grown on ͑100͒ n ϩ-InAs (nϭ10 19 cm Ϫ3) substrates by dc-magnetron sputter deposition. Studies of the electronic properties of interfaces between the superconductor and the semiconductor are done by Raman scattering measurements. The superconducting proximity effect at superconductor-semiconductor interfaces is observed through its impact on inelastic light scattering intensities originating from the near-interface region of InAs. The InAs longitudinal optical phonon LO mode (237 cm Ϫ1) and the plasmon-phonon coupled modes L Ϫ (221 cm Ϫ1) and L ϩ ͑1100 to 1350 cm Ϫ1), for n ϩ ϭ1ϫ10 19 Ϫ2ϫ10 19 cm Ϫ3 are measured. The intensity ratio of the LO mode ͑associated with the near-surface charge accumulation region, in InAs͒ to that of the L Ϫ mode ͑associated with bulk InAs͒, is observed to increase by up to 40% below the superconducting transition temperature. This temperature-dependent change in light scattering properties is only observed with high quality superconducting films and when the superconductor and the semiconductor are in good electrical contact. A few possible mechanisms of the observed effect are proposed.
Journal of Nanoparticle Research, Jul 4, 2012
Species transport in nanocapillary membrane systems has engaged considerable research interest, p... more Species transport in nanocapillary membrane systems has engaged considerable research interest, presenting technological challenges and opportunities, while exhibiting significant deviations from conventionally well understood bulk behavior in microfluidics. Nonlinear electrokinetic effects and surface charge of materials, along with geometric considerations, dominate the phenomena in structures with characteristic lengths below 100 nm. Consequently, these methods have enabled 3D micro-and nanofluidic hybrid systems with high-chemical selectivity for precise manipulation of mass-limited quantities of analytes. In this review, we present an overview of both fundamental developments and applications of these unique nanocapillary systems, identifying forces that govern ion and particle transport, and surveying applications in separation, sensing, mixing, and chemical reactions. All of these developments are oriented toward adding important functionality in micro-total analysis systems. Keywords Membranes Á Nanostructures Á Nanofluidics Á Microfluidics Á Ion transport Á Electrokinetics Á l-TAS Á Nanopore Á Nanocapillary Á Water filtration Á Sustainable development