Showing papers by "Arthur B. Ellis published in 1999"
TL;DR: In this article, a simple synthesis of an aqueous-based ferrofluid that may be used in an introductory science or engineering laboratory is described, where magnetite (Fe3O4) particles are precipitated by combining FeCl3 and FeCl2 in a 2:1 stoichiometric ratio.
Abstract: Ferrofluids are colloidal suspensions of surfactant-coated magnetic particles in a liquid medium. This paper describes a simple synthesis of an aqueous-based ferrofluid that may be used in an introductory science or engineering laboratory. Magnetite (Fe3O4) particles are precipitated by combining FeCl3 and FeCl2 in a 2:1 stoichiometric ratio in aqueous ammonia solution. The resulting particles, ~10-20 nm in diameter based on powder X-ray diffraction, are then treated with aqueous tetramethylammonium hydroxide, which acts as a surfactant. When the resulting ferrofluid is placed near a magnet, it forms conical spikes. This paper also describes a method for repelling both oil- and water-based ferrofluid from solid surfaces that would otherwise be stained by the fluid. Finally, a demonstration of the interaction between ferrofluid and magnetic fields, in which ferrofluid is induced to leap upward by a stack of magnets, is described.
365 citations
TL;DR: In this article, simple classroom demonstrations and laboratory experiments based on properties of polydimethylsiloxane (PDMS) are described and compared to micro-contact printing processes, where the raised surface features of the resulting cured PDMS block can transfer ink images to paper in a fashion similar to microcontact printing.
Abstract: This paper describes simple classroom demonstrations and laboratory experiments based on properties of polydimethylsiloxane (PDMS). PDMS is a colorless, transparent elastomer. Spheres cast from PDMS can be cross-linked to varying extents to affect their rigidity, as seen in their ability to bounce when dropped. PDMS recently has been used by Whitesides et al. to replicate submicron-scale patterns by casting a negative relief image from a master template, and to progressively reduce the dimensions of these patterns through cycles based on mechanical compression. Curing PDMS in contact with features pressed into aluminum foil transfers the foil features to the elastomer. The raised surface features of the resulting cured PDMS block can transfer ink images to paper in a fashion that is similar to microcontact printing processes. Stretching or compressing the block will alter the dimensions of the transferred image. Curing PDMS in contact with the microscopic features on optical transform slides can transfer ...
127 citations
TL;DR: In this article, the shift of the surface Fermi level within the band gap was monitored during controlled thermal annealing, allowing for the identification of specific chemical entities responsible for the reduction in surface band bending.
Abstract: SeS2-passivated n-type GaAs (100) surfaces, formed by treatment of GaAs in SeS2:CS2 solution at room temperature, were studied with high-resolution core-level photoemission spectroscopy excited with synchrotron radiation source. The SeS2-treated surface consists of a chemically stratified structure of several atomic layers thickness. Arsenic-based sulfides and selenides reside in the outermost surface layer while gallium-based selenides are adjacent to the bulk GaAs substrate. The shift of the surface Fermi level within the band gap was monitored during controlled thermal annealing, allowing for the identification of the specific chemical entities responsible for the reduction in surface band bending. Arsenic-based species are removed at low annealing temperature with little shift of the Fermi level. Gallium-based selenides are shown to be associated with the unpinning of the surface Fermi level.
28 citations
TL;DR: Flexible sheet refrigerator magnets (RMs) have interesting magnetic structures that allow them to be used to demonstrate a number of chemical principles, such as scanning probe microscopy and models of mechanical properties of metals and salts.
Abstract: Flexible sheet refrigerator magnets (RMs) have interesting magnetic structures that allow them to be used to demonstrate a number of chemical principles. RMs are a composite of strontium ferrite dispersed in the elastomer Hypalon. The magnetic alignment of the ferrite particles gives rise to a striped array of alternating north and south poles on the back side of the RM that may be visualized by a number of techniques and altered with a strong magnet. When the back sides of two RMs are rubbed against each other, the magnets may slide or alternately attract and repel one another ("chatter"), depending on their relative orientation. These effects lend themselves to demonstrations of scanning probe microscopy and models of mechanical properties of metals and salts. Concepts of composite materials and surface chemistry may be illustrated with student laboratory fabrications of refrigerator magnets utilizing strontium ferrite dispersed in polydimethylsiloxane elastomer.
19 citations
TL;DR: In this paper, the authors show that the band-gap photoluminescence (PL) intensity of the semiconductor is reversibly quenched by exposure to CO2, while uncoated CdSe samples display no response relative to a nitrogen reference ambient.
Abstract: Films of ethanolamine, 3-aminopropanol, and 4-aminobutanol, 1−3, serve as transducers for the detection of CO2 when deposited onto emissive CdSe substrates: the band-gap photoluminescence (PL) intensity of the semiconductor is reversibly quenched by exposure to CO2, while uncoated CdSe samples display no response relative to a nitrogen reference ambient. Infrared (IR) spectra obtained with films of these alkanolamines and related alcohols and amines, 4−13, are consistent with selective reaction of the amine functionality in these films to form carbamates. The direction of the PL response corresponds to an enhancement of Lewis acidity accompanying the reaction with CO2, and use of a dead-layer model permits an estimation of the adduct-induced expansion of the depletion width as being roughly a few hundred Angstroms. Both IR and PL data reveal that the film response to CO2 begins at ∼0.1 atm and saturates by ∼0.3 atm, and their similarity indicates that the partition coefficient for binding CO2 to the bulk...
18 citations
TL;DR: The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is modulated through adsorption of 17 porphyrins and metalloporphyrin from methylene chloride solution as mentioned in this paper.
Abstract: The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is modulated through adsorption of 17 porphyrins and metalloporphyrins from methylene chloride solution. In nitrogen-saturated solution, the PL intensity is reversibly enhanced through adduct formation with octaethylporphyrin (OEP), tetraphenylporphyrin (TPP), their divalent Co, Cu, and Cd derivatives, and with tetra(pentafluorophenyl)porphyrin (TFP), tetra-(4-tolyl)porphyrin (TTP), and tetra-(4-pyridyl)porphyrin (TPyP); in contrast, the PL intensity is reversibly quenched by exposure to divalent Mg, Ni, and Zn complexes of OEP and TPP. The concentration dependence of the PL changes can be fit to the Langmuir adsorption isotherm model to yield binding constants that range from ∼103 to 105 M-1. Saturating methylene chloride with oxygen rather than nitrogen does not affect the CdSe PL nor does oxygen affect the (metallo)porphyrin absorption spectra in methylene chloride solution. However, many of the metalloporphyrin-induced PL ...
15 citations
8 citations
TL;DR: This activity, which allows students to extract DNA from plant and animal cells, serves as a spectacular example of the complexity of biochemical structure and function and fits well with a discussion of nucleic acids, hydrogen bonding, genetic coding, and heredity as discussed by the authors.
Abstract: This activity, which allows students to extract DNA from plant and animal cells, serves as a spectacular example of the complexity of biochemical structure and function and fits well with a discussion of nucleic acids, hydrogen bonding, genetic coding, and heredity. DNA extraction can also be used in conjunction with a discussion of polymers and their properties. This activity can be used to complement a diffraction experiment illustrating how the double helix structure of DNA was determined.
6 citations
TL;DR: In this article, a novel Nearfield Scanning Optical Microscopy (NSOM) technique was used to obtain simultaneous topology, photoluminescence and photoreflectance (PR) spectra.
Abstract: A novel Near-field Scanning Optical Microscopy (NSOM) technique is used to obtain simultaneous topology, photoluminescence and photoreflectance (PR) spectra. PR spectra from GaAs surfaces were obtained and the local electric fields were calculated. Sub-wavelength resolution is expected for this technique and achieved for PL and topology measurements. Photovoltages, resulting from the high intensity of light at the NSOM tip, can limit the spatial resolution of the electric field determination.
1 citations
TL;DR: In this article, an ammonia-based, in situ passivation of GaAs surfaces conducted within a metalorganic vapor phase epitaxy reactor is presented, and the shift of the GaAs surface Fermi level, and hence the surface charge density, resulting from this in situ passingivation, has been studied using photoreflectance (PR) spectroscopy.
Abstract: An ammonia-based, in situ passivation of GaAs surfaces conducted within a metalorganic vapor phase epitaxy reactor is present. The shift of the GaAs surface Fermi level, and hence the surface charge density, resulting from this in situ passivation, has been studied using photoreflectance (PR) spectroscopy. Samples consisting of an undoped GaAs layer on highly doped n-GaAs (UN+) and p-GaAs (UP+) structures allow for the exact determination of the surface Fermi level position using PR These structures were grown by MOVPE and in situ thermal nitridation was performed after growth within the MOVPE system without exposure to the air. After nitridation, the surface Fermi level can be shifted by ~ 0.23 eV towards the conduction band edge for UN+ structures and by ~ 0.11 eV towards the valence band edge for UP+ structures from the normally mid-gap ‘pinned’ positions.