1,721,019 research outputs found
Surface-enhanced Raman and surface plasmon resonance measurements of Case II diffusion events on the nanometer length scale
No full textA study of spatially-localized phenomena associated with Case II transport is reported. Steady-state aspects of viscoelastic diffusion were investigated with the aid of SERS-active planar films implanted in m-scale polymer films. The use of Au rather than Ag metal-island films is shown to eliminate spectral interference resulting from photolytic degradation reported in typical SERS studies of glassy polymers. A sample probe consisting of an Au metal film sandwiched between polymer thin films was shown to provide a spatial resolution of less than 30 nm in the direction of penetrant motion i.e. normal to the plane of the superstrate-polymer interface. This high resolution probe may be used to generate SERS spectra of the polymer/penetrant system during permeation experiments. Penetrant motion and polymer relaxation processes may be simultaneously monitored in situ by interpretation of vibrational mode evolution. Polymer relaxation and deformation in response to osmotic stress caused by the presence of permeant molecules appears to lag behind solvent arrival and continue long after the diffusion front has passed.Surface plasmon resonance position measurements were used to study permeant-induced interfacial swelling processes in ultra-thin (350 A) poly(methylmethacrylate) films. A one-dimensional swelling model was developed and used in conjunction with Fresnel reflectivity relationships for multilayer systems to relate observed shifts in surface plasmon resonance position to changes in sample thickness and dielectric profile. Precision control of sample thermal history was accomplished with a feedback temperature regulation system based on a digital velocity PID control algorithm. Changes in sample thickness of less than 1A were easily observed. The swelling behavior of films in the 10 to 30 nm thickness range immediately subsequent to permeant introduction was investigated. Evidence that penetrant molecules are incorporated directly from the vapor stream, rather than from an adsorbed surface layer is reported. Sample thickness modulations at 0.3 Hz and of amplitude ca. 1A were observed at penetrant volume fractions between 0.08 and 0.12. The initial evolution of permeant concentration profiles on this length scale appears to be generally well described by the Thomas and Windle model, however, a strong temperature dependence is observed.Made available in DSpace on 2011-05-07T14:06:30Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9543573.pdf: 4278380 bytes, checksum: 3636a174297579faa69311cf2eb7baa1 (MD5)
Previous issue date: 1995Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T15:03:00Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:30:01-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Optical properties of III-V semiconductors
No full textA GaAs-Al\sb{0.22}Ga\sb{0.78}As heterostructure was prepared and used as a multimode optical waveguide in order to measure the sub bandgap energy- and temperature-dependent refractive index of GaAs. Propagation constants for the individual modes were measured by exciting one mode at a time using a real-space diffraction grating coupler. The resulting eigenmode distributions were used to obtain the refractive index of GaAs at a matrix of sub bandgap photon energies (1.40 eV h 1.50 eV) and temperatures (40 K T 300 K). Values for the temperature dispersion (dn/dT) at each photon energy, and the extrapolated refractive index at T = 0 K are presented. For the energy dispersion results, Sellmeier parameters including the long wavelength dielectric constant at each temperature are presented. The dominant error source in these measurements was uncertainty in the angular placement of the sample.In order to exploit the inherent sensitivity of modulation spectroscopy to measure the extrinsic contribution to the Urbach tail, the electro-optic tuning of a Ti:sapphire laser was investigated using a potassium dideuterophosphate (KD*P) crystal. The limited success in tuning and producing wavelength modulated electromagnetic radiation is discussed, and a model is presented for the observed laser output. Suggestions for the improvement of the studied modulation system are also given.Another phase of this project was to quantitate disorder in III-V semiconductor crystal lattices using Raman scattering in the backscattering geometry. The source of disorder in these materials was from a reactive ion etch plasma. The goal of this work is to use Raman scattering on a variety of etched epitaxial layers of GaAs and InP to diagnose etch induced lattice disorder. The desire was to minimize the disorder in the sample, while maintaining a reasonable amount of etch anisotropy in the desired structures. The Raman depolarization ratio of the longitudinal phonon can be used to diagnose the amount of disorder in the epitaxial layer. Using phonon scattering, reactive ion etch conditions which led to a compromise between effective etch profiles and minimal etch damage were identified for GaAs and InP.Made available in DSpace on 2011-05-07T13:23:39Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9210871.pdf: 9544748 bytes, checksum: e94f0fae12d8bdc2574e5f562f240f8f (MD5)
Previous issue date: 1991Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:54:11Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:25:09-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Enhanced Raman spectroscopy at dielectric interfaces
No full textThe examination of the merits of a new type of sampling structure for performing enhanced Raman scattering is presented here. These structures typically consist of 50 A of SiO\sb2 sputtered on Ag or Au island films, all coated on a lithographically defined opening in a masked SiO\sb2 substrate and use internal reflection in the SiO\sb2 substrate to excite plasma resonances in the island films with higher incident field strengths than possible with external excitation. Enhanced scattering from adsorbate molecules adsorbed on top of the SiO\sb2 overlayer is driven by the long ranged electromagnetic enhancement provided by the resonantly driven island film particles. Evidence for the long range enhancement comes from chemical passivation, scanning Auger, and distance dependence experiments, as well as comparing enhanced Raman scattering signals obtained from coated and uncoated Au island films. The dependence of the scattering on the excitation angle on incidence can be interpreted in terms of the strengths of the evanescent field components at the substrate-metal interface. Evidence suggesting that -nitrobenzoic acid (PNBA) photodegrades to an azodibenzoate species when adsorbed on top of the SiO\sb2 overlayers as a result of laser illumination is obtained.The ability to obtain polarized Raman scattering for monolayer adsorbates on these structures has been examined. The relationship of the relative intensities of the in-plane enhanced electric field components to the totally symmetric vibrations of p-nitrobenzoic acid and phthalazine was found to indicate a constant depolarization on the in-plane electric field components induced by the island film particles themselves. With this information and polarized Raman scattering data from non-totally symmetric phthalazine vibrations, the ability to determine the average surface molecular orientation of phthalazine monolayers at sputtered SiO\sb2 surfaces is reported.The ability to utilize these structures for sensitive vibrational spectroscopic analyses of different types of monolayer adsorbate systems is addressed. Systems which have been examined include molecular self assembled films, Langmuir-Blodgett assemblies, and adsorbed protein layers. Implications for extending the scope of these experiments will also be discussed.Made available in DSpace on 2011-05-07T12:04:46Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9011068.pdf: 10641749 bytes, checksum: cb96bd80ff3e851c200b144b55376d03 (MD5)
Previous issue date: 1989Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:36:21Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:14:39-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Plasma-enhanced chemical vapor deposition of hydrogenated silicon carbide films from novel precursors
No full textMade available in DSpace on 2011-05-07T12:36:59Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9210972.pdf: 12369824 bytes, checksum: fd525d929ff7e6afc7ee9c5de5cd2d57 (MD5)
Previous issue date: 1991Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:43:22Z
Item is restricted indefinitely.The influences of precursor molecular structure and electronic properties on the molecular structure, stoichiometry, and optical properties of a-SiC:H alloy films prepared through plasma enhanced chemical vapor deposition were investigated using infrared spectroscopy, ultraviolet-visible absorption spectroscopy, and sputtered neutral atom mass spectrometry (SNMS). Members of the homologous series tetramethylsilane (TeMS), trimethylsilane (TrMS), and dimethylsilane (DMS) as well as methane-silane (MS) were characterized as a-SiC:H precursors. Film structure, optical properties, and stoichiometry were studied as a function of precursor structure and deposition conditions, with deposition pressure serving as the manipulated variable.The infrared spectra of films prepared from the alkylsilane precursors revealed a strong dependence of the film structure on the deposition pressure, with high pressures (0.1 torr) producing linear, polymeric films, and low pressures (0.1 torr) producing amorphous, crosslinked films. The structure of the polymeric films was dominated by carbon atoms incorporated as chain terminating methyl groups. The crosslinked films contained fewer methyl groups and proportionately more bridging methylene and methylidyne groups. Similar film structures were obtained from all of the alkylsilane precursors.SNMS measurements revealed that the carbon-to-silicon ratio was inversely dependent on deposition pressure, with carbon content increasing as deposition pressures decrease Carbon content was also dependent on the precursor stoichiometry, with films prepared from TeMS exhibiting the highest carbon content (55-68 atomic percent) and films prepared from DMS exhibiting the lowest carbon content (35-52 atomic percent).UV-visible absorption measurements indicated that the optical properties of the films were strongly dependent on the deposition pressure, particularly at deposition pressures below 0.1 torr, and were only weakly dependent on the precursor structure. Films deposited from alkylsilane monomers exhibited larger optical gaps at higher deposition pressures. Observed gaps ranged from 2.1 eV to 3.6 eV, with TrMS producing materials with consistently larger optical gap energies.From these results, a phenomenological film structure evolution model is postulated, stressing the decomposition pathways of the methyl substituents as the primary determinant of the film structure.Restriction data tranferred 2014-07-01T11:18:58-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Orientation of adsorbate-adsorbate complexes using a novel waveguide linear dichroism technique
No full textMade available in DSpace on 2011-05-07T12:30:00Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9210774.pdf: 9143067 bytes, checksum: 78808011bb0bbcb2817b90e5a0e28b5a (MD5)
Previous issue date: 1991Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:41:48Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:18:08-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionA novel approach to linear dichroism (LD) measurements is utilized to investigate orientations of adsorbed monolayer assemblies. Polarized radiation in integrated optical waveguide structures is employed to maximize interactions between incident radiation and adsorbed molecules at the waveguide surface. In this manner, the experimental applications and detection limits of orientation measurements have been greatly expanded.Orientations of monophenyl silyl ether monolayers on glass substrates were initially investigated for three primary reasons: (1) the absorption cross section is small which provides a strenuous test for the applicability of this LD approach on weakly absorbing molecules, (2) the structure of the silane permitted an unambiguous relationship between the absorption transition moment and the molecular orientation, and (3) these silanes provide a surface which closely mimics a reversed-phase liquid chromatographic (RPLC) phenyl stationary phase commonly used for the separation of aromatic compounds. Successful experiments revealed an average orientation angle of the phenyl rings from the O - Si - C\sb{\rm aromatic} bond plane of close to 60\sp\circ in both air and combinations of a water/methanol solvent.These monophenyl silyl ether structures were then used in an RPLC capacity to observe the orientation of polycyclic aromatic hydrocarbon (PAH) solute molecules as complexation occurs with the aromatic bonded phase. A consistent orientation behavior is seen for the five different PAH solutes studied as the concentration of methanol is increased in the methanol/water solvent bath. At small methanol concentrations, the solute molecules orient at an angle commensurate with the orientation angle of the phenyl moieties in the bonded monolayer. As the methanol concentration increases, the solute molecules move toward an alternate orientation until a methanol concentration is reached where no further absorption by the solute molecule is observed. The data appear to support a model where a face-to-face orientation between the PAH solute and the phenyl ring is favored in solvents with a large aqueous character while the solutes prefer an edge-to-phenyl ring face orientation as the solvent becomes increasingly organic in nature. As a result of these experiments, it appears that the retention mechanism in RPLC can be described with a combination of an orientation factor with the solvophobic theory which is commonly accepted.This technique has proved to be a powerful new probe of the orientation of adsorbate-adsorbate complexes at solid-liquid interfaces. It will also prove useful for the study of other weakly absorbing monolayer structures which have eluded easy investigation because of small absorption cross sections. In addition, the simple mathematical treatment and the straightforward relationship between the obtained data and molecular orientation relieves the interpretational difficulties of other orientation measurement techniques.ETDs are only available to UIUC Users without author permissionU of I Onl
Spectroscopic characterization of dry etch-induced modification in III-V semiconductors and semiconductor heterostructures
No full textThe effects of dry etching on the structural and electrical properties of III-V semiconductors and semiconductor heterostructures were studied using vibrational and electronic Raman spectroscopy and low and room temperature photoluminescence spectroscopy. The frequency, intensity, and lineshape of the dipole-allowed and dipole-forbidden longitudinal optic (LO) phonon as well as the dipole-forbidden transverse optic phonon were used to evaluate structural modification in doped and undoped material. The intensities and frequencies of the coupled LO phonon-plasmon modes relative to those of the unscreened LO phonon were used to probe changes in the surface space charge region and free carrier concentration. Photoluminescence peak energies, intensity, and lineshape were used to evaluate crystal quality and defect identity.Structural modification was investigated in GaAs etched in SiCl\sb4, SiCl\sb4/SiF\sb4, and CH\sb4/H\sb2 plasmas and sputtered in an Ar plasma. SiCl\sb4 and SiCl\sb4/SiF\sb4 etches both produced significant structural modification. CH\sb4/H\sb2 plasmas produced less structural modification, however, a highly disordered surface layer was still created. Electrical modification by the methane-based plasmas was found to depend weakly on the self-bias voltage and was confined to within ca. 125 A of the surface.The effects of reactive ion etching (RIE) in methane-based plasmas as well as sputtering in Ar and He plasmas on InP were investigated. In general, methane-based plasmas showed far less structural damage than sputtering. Samples sputtered in Ar plasmas exhibited the most structural modification, however, at larger bias voltages some structural modification was observed for methane-based etches (except CH\sb4/H\sb2 plasmas). Of the methane-based etches CH\sb4/H\sb2 and CH\sb4/H\sb2/Ar plasmas exhibited the least structural modification. Electrical modification was also seen to be more extensive with sputter etches than methane-based etches. Samples sputtered in He plasmas showed more electrical perturbations than samples sputtered in Ar plasmas. Methane-based etches did affect electrical properties of InP. No clear trends were observed, although, in general, CH\sb4/H\sb2 etches resulted in the largest depletion widths.The effects of HBr RIE and Ar sputtering on InGaAs and InAlAs were investigated. Ar sputtering of InGaAs resulted in significant structural modification. HBr-etching lead to significantly less structural modification, although some disorder was created by these etches. Ar sputtering of InAlAs also resulted in structural modification, although to a lesser degree than observed for InGaAs. Ar sputtering results in the formation of a high-donor density surface region, which leads to the creation of an electron accumulation layer in the near-surface region. A decrease in carrier concentration was observed to ca. 230 A in HBr-etched n\sp+-InAlAs.The presence and shifts of the coupled phonon-two-dimensional electron gas (2DEG) plasmon modes in InGaAs/InAlAs heterostructures were used to investigate the effects of HBr RIE on the 2DEG properties. Etch-induced 2DEG modification was greatly reduced when the InAlAs barrier thickness was increased from 75 A to 350 A. For the structures with the smallest barrier thicknesses the decrease in 2DEG concentration was more strongly dependent on increased etch times than increased self-bias voltage.Made available in DSpace on 2011-05-07T13:02:51Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9416404.pdf: 14913016 bytes, checksum: 4a4f88e3e59aae84939b196c4b47c434 (MD5)
Previous issue date: 1994Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:49:28Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:22:30-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Spectroscopic characterization of the near-surface electronic structure of surface-modified III-V semiconductors
No full textInvestigations of the near-surface electronic structure of n\sp+-GaAs and n\sp+-InAs are presented. Recent studies of the GaAs surface have been carried out which demonstrate that the surface states which arise from termination of the crystal lattice, which are known to cause pinning of the Fermi energy level and an increase in the Schottky barrier in the semiconductor, can be passivated using inorganic sulfides. Although treatment with the inorganic sulfides is effective in reducing the surface recombination velocity, the loss of the surface As atoms during the treatment causes an electronic reorganization in the semiconductor which actually pins the Fermi level closer to the valence band and subsequently increases the band-bending at the surface. In this research, passivation with a long-chain alkane thiol demonstrates the ability to passivate the surface while unpinning the Fermi level and reducing the surface band-bending. This change is measured quantitatively by applying Raman spectroscopy is a probe of the near-surface region. The distinct electronic regions of the semiconductor surface can be differentiated by monitoring the predominant crystal vibrations from each region which give rise to Raman scattering.In a second system, Raman spectroscopy is again used to probe the surface electronic characteristics of a semiconductor. A semiconductor/superconductor interface is investigated to explore Cooper pair transport across the interface while the system is in a superconducting state. Previous investigations of a Nb/InGaAs system show an anomalous current at zero bias which arises when the system is superconducting. However, this phenomenon is not explained by current superconducting models. Typical contact probes which are often used to investigate small superconducting currents are difficult to employ in this system, so the Raman optical probe was used to carry out the research. An InAs/Nb structure was fabricated such that an Ohmic contact was made to provide a barrier-free interface for electron flow. The temperature of the system was lowered below the superconducting transition temperature and the resulting optical measurements were compared to those of the system taken in a normal state. These initial investigations show a change in the electronic character of the InAs supporting a case for Cooper pair transport in the system.Made available in DSpace on 2011-05-07T12:35:15Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
9702503.pdf: 5080970 bytes, checksum: 4ffde5711dd1972836cde1b422cef1f2 (MD5)
Previous issue date: 1996Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:42:57Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:18:44-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
