Showing papers by "Paul W. Bohn published in 1993"

Aspects of Structure and Energy Transport in Artificial Molecular Assemblies

Abstract: The last decade has seen an explosion of interest in the special properties that may be imparted to molecular assemblies prepared on planar sub­ strates by the self-assembly (SA) and Langmuir-Blodgett (LB) techniques. This interest is fueled by the concept of molecular engineering, or the purposeful design of structures that are supermolecular in scale and in which molecular units are arranged in specific spatial (and possibly tem­ poral) arrangements to realize a coordinated functional goal. This rather broad concept encompasses a wide variety of issues of keen current tech­ nical interest. Nonlinear optical devices play a central role in prototype photonic signal processing schemes, and researchers quickly appreciated that organic: materials, in which the nonlinear polarization is almost entirely electronic in origin, could provide large, fast nonlinearities for optical switching. However, organic molecules far too often crystallize in centro symmetric space groups, thus negating, on a macroscopic scale [X(2) = 0], the advantages associated with a large molecular hyper­ polarizability, /3. To circumvent difficulties with centrosymmetric arrange­ ments of molecules, noncentrosymmetric layered structures can be built by utilizing the LB (I, 2) and SA (3, 4) fabrication techniques to maintain the directionality of the molecular hyperpolarizability tensor. Surface

Evaluation of Polynomial Fitting Functions for Use with CCD Arrays in Raman Spectroscopy

Abstract: Calibration of wavelength is vital to the accurate assessment of Raman shifts using multichannel detectors, such as a charge-coupled device (CCD) detector in combination with multistage spectrometers. Although in zeroth order, the position in the image plane (i.e., CCD) may be approximated as linearly related to wavenumber, this approximation fails badly for accurate determinations of band position at even moderate spectral coverage (>50 cm−1). A Gaussian development of the imaging properties of a Czerny-Turner spectrometer leads to a quadratic relationship. However, the use of flat-field optics in modern triple monochromators results in position-wavenumber relationships that are not described well by a quadratic expression and are best fit by empirical relationships. Establishing the goodness of fit of several trial functions is the purpose of this current work. One way to achieve the relationship between position, or pixel number, and wavelength, as dispersed by a grating spectrometer, is to observe a standard sample that has several Raman lines of accurately known wavenumber shift. Indene, C9H8, has been used as such a standard for many years due to its well-calibrated and evenly spaced spectral features. This paper will explore the use of indene as a Raman shift calibration standard for CCD based spectrometers in the diagnostically important range of 700 to 1600 cm−1.

Influence of molecular orientation and proximity on spectroscopic line shape in organic monolayers

Abstract: Within a monomolecular layer, molecules may be present as isolated species that behave independently or in an aggregated form that acts collectively. Theoretical derivations of the energy shift expected for the aggregate transition relative to the isolated monomer are based on transition moment coupling and predict a significantly different orientational dependence for point- and extended-dipole approximations. In contrast to the commonly invoked transformation from H- to J-aggregate behavior as the molecular orientation increases from surface normal, purely H-aggregate behavior is predicted at small intermolecular separation distances using the extended dipole approximation, regardless of axial orientation

Characterization of an aggregate-sensitive single-component energy-transfer system

Abstract: Energy transfer is studied in a system consisting of Langmuir-Blodgett monolayers of the hemicyanine dye, N-(3-sulfopropyl)-4-(p-dioctylaminostyryl)pyridinium. Deposition from CHCl 3 solution onto the air-water interface results in self-assembly into two-dimensional aggregates with an absorption band significantly blue-shifted (9000 cm -1 ) from that of the monomer in solution. Calculation of structural parameters consistent with this extremely large blue-shift, in the extended dipole approximation, indicates the structure of the aggregate is considerably more compact than the average structure observed in the Π-A isotherm

Spectroscopic characterization of aggregation behavior in hemicyanine dye monolayer and multilayer systems

Abstract: The aggregation behavior of the hemicyanine dye N-(3-sulfopropyl)-4-(p-(dioctylamino)styryl)pyridinium, fabricated in Langmuir-Blodgett (LB) monolayer and multilayer systems and directly physisorbed onto quartz substrates has been studied by absorption and fluorescence spectroscopy. The LB monolayer films prepared on quartz substrates show entirely H-aggregate structures, characterized by a large blue-shifted (ΔE∼9000 cm -1 ) electronic transition at 340 nm, not only for the pure-dye system but also for the dye diluted in stearic acid at molar ratios down to 17%. The LB multilayer systems with pure dye show an identical absorption band (340 nm) to the LB monolayers

Optical determination of surface density in oriented metalloprotein nanostructures

Abstract: In the current work we circumvent a difficulty in estimating surface coverage by noting that iron-porphyrin proteins in solution have been assayed spectrophotometrically after conversion to pyridine hemochromes. By comparing the total adsorbance obtained from direct absorption measurements of oriented metalloprotein layers on SiO[sub 2] at the Soret resonance (410 nm in cytochrome b[sub 5]) to the total number density of surface protein, obtained from subsequent pyridine hemochrome assay (PHCA) analysis, the apparent surface molar absorptivity is obtained directly. In this correspondence we report the use of the PHCA to determine the surface molar absorptivity for oriented arrays of cytochrome b[sub 5] mutants. The heme is completely dissociated from the surface cytochrome b[sub 5] and converted to the reduced pyridine hemochrome in solution. Subsequently, the absorbance of reduced species in solution is determined colorimetrically. From the correlation of the absorbance of reduced hemochrome to the standard curve obtained from pyridine hemochrome assay of solution cytochrome b[sub 5], the surface concentration is estimated. 18 refs., 2 figs., 1 tab.

Structural modification in reactive‐ion‐etched i‐InP and n+‐InP studied by Raman scattering

Abstract: Structural and electrical property modifications in i‐InP and n+‐InP by reactive ion etching have been characterized by using Raman scattering to observe changes in the positions and intensities of intrinsic phonons and coupled phonon‐plasmon bands. Different etch gas compositions (Ar, He, CH4/H2, CH4/Ar, CH4/He, and CH4/H2/Ar), bias potentials (0–500 V), and etch times were examined. Electrical property changes were followed by correlating results with a one‐sided abrupt junction model. Ar and He, which interact with the material by physical sputtering processes, were found to induce the greatest structural and electrical modifications. Ar‐etched samples exhibited the greatest structural modification, with evidence of structural damage coming from far beyond the calculated penetration depth of low‐energy Ar+, while samples etched in methane‐based plasmas showed very little structural perturbation. Etching in all of the gas mixtures used produced some degree of electrical modification in n+‐InP, with He p...

Phonon‐electron interactions in the two‐dimensional electron gas in InGaAs‐InAlAs modulation‐doped field‐effect transistor structures studied by Raman scattering

Abstract: Raman scattering by coupled longitudinal optic phonons and two‐dimensional electron gas electrons in In0.53Ga0.47As‐In0.52Al0.48As δ‐doped heterostructures provides a powerful probe of electronic properties in these In‐based structures. The two highest frequency modes, of the three coupled electron‐phonon modes expected in this system, were observed, with the highest frequency mode being identified in InGaAs‐based systems. The large dispersion of this mode makes it a particularly sensitive probe for changes in such properties as carrier concentration and subband energy. For structures with higher carrier concentrations coupling of the longitudinal optic phonon to multiple electron intersubband transitions is resolved. These measurements are particularly useful for heavily doped structures for which room‐temperature Hall measurements cannot distinguish channel electrons from those in parallel conduction paths.

Mass transport phenomena in thin films of poly(2-vinylpyridine) studied via optical guided wave techniques

Abstract: The unique ability of optical waveguide techniques to determine simultaneously film thicknesses and mass transport behavior in swollen polymer films is utilized in these experiments to make accurate determinations of diffusion coefficients of fluorescein into H[sub 2]O-swollen poly(2-vinylpyridine) films from aqueous solution. The diffusion behavior is determined from fitting the fluorescence-time curves to an intensity expression derived from Fick's second law and the appropriate boundary conditions to obtain the diffusion coefficient of the fluorophore in the film. Two techniques for characterizing the fluorescence behavior spatially and temporally are critically compared. Fiber optic-based detection schemes suffer from inaccuracies relative to the use of an imaging camera based on a charge-coupled device (CCD) array. The diffusion coefficients obtained show no significant dependence on the bulk solution concentration in the range 1 nM [le] [fluorescein] [le] 10 [mu]M of the fluorophore. The value of the diffusion coefficient was found to be in the range 1 x 10[sup [minus]12] cm[sup 2]/s [le] D [le] 3 x 10[sup [minus]11] cm[sup 2]/s and was found to vary widely with small changes in sample preparation conditions. 25 refs., 5 figs., 1 tab.

Effects of Reactive Ion Etching on Phonon-Electron Interactions in Inalas-Ingaas Modulation-Doped Field-Effect Transistor Structures Studied by Raman Scattering

Abstract: Raman Scattering by coupled longitudinal optic phonons and two-dimensional electron gas electrons in In052Al048As-In053Ga047As δ-doped heterostructures provides a powerful probe of electronic properties in these In-based structures The two highest frequency modes, of the three coupled electron-phonon modes expected in this system, were observed, with the highest frequency mode being identified for the first time in InGaAs-based systems The large dispersion of this mode makes it a particularly sensitive probe for changes in such properties as carrier concentration and subband energy For structures with higher carrier concentrations coupling of the longitudinal optic phonon to multiple electron intersubband transitions is resolved These measurements are particularly useful for heavily-doped structures for which room-temperature Hall measurements cannot distinguish channel electrons from those in parallel conduction paths In addition HBr-based reactive ion etching has been performed on these structures, and the effects have been correlated with etch time and bias voltage for structures with different cap layer thicknesses