Dissertations / Theses on the topic 'Analytical chemistry|Physical chemistry|Engineering'

Over recent years, Ion Mobility–Mass Spectrometry (IMS–MS) measurements have become a widely used tool in a number of disciplines of scientific relevance, including, in particular, the structural characterization of mass-selected biomolecules such as proteins, peptides, or lipids, brought into the gas-phase using a variety of ionization methods. In these structural studies, the measured electrical mobilities are customarily interpreted in terms of a collision cross-section, based on the classic kinetic theory of ion mobility. For ideal ions interacting as smooth, rigid-elastic hard-spheres with also-spherical gas molecules, this collision cross-section (CCS) is identical to the true, geometric cross section. On the other hand, for real ions with non-perfectly spherical geometries and atomically-rough surfaces, subject to long-range interactions with the gas molecules, the expression for the CCS can become fairly intricate.

This complexity has frequently led to the use of helium as the drift gas of choice for structural studies, given its small size and mass, its low polarizability (minimizing long-range interactions), and its sphericity and lack of internal degrees of freedom, all of which contribute to reduce departures between measured and true cross-sections. Recently, however, a growing interest has arisen for using moderately-polarizable gases such as air, nitrogen, or carbon dioxide (among others) in these structural studies, due to a number of advantages they present over helium, including their higher breakdown voltages (allowing for higher instrument resolutions) and better pumping characteristics. This shift has, nevertheless, remained objectionable in the eye of those seeking to infer accurate structural information from ion mobility measurements and, accordingly, there is a critical need to study whether or not measurements carried out in such gases may be corrected for the finite size of the gas molecules and their long-range interactions with the ions, in order to provide cross-sections truly representative of ion geometry. A first step to address this matter is undertaken here for the special case of nearly-spherical, nanometer-sized ions.

In order to attain this goal, we have performed careful and accurate IMS–MS measurements of hundreds of electrospray-generated nanodrops of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ₄), in a variety of drift gases (air, CO₂, and argon), covering a wide range of temperatures (20-100 °C, for both air and CO₂), and considering nanodrops of both positive and negative polarity (the latter in room-temperature air only). Thanks to the combined measurement of the mass and mobility of these nanodrops, we are able to simultaneously determine a mobility-based collision cross-section and a mass-based diameter (taking into account the finite compressibility of the IL matter) for each of them, which then allows us to establish a comparison between the two.

Over the entire range of experimental conditions investigated, our measurements show that the electrical mobilities of these nearly-spherical, multiply-charged IL nanodrops are accurately described by an adapted version of the well-known Stokes—Millikan (SM) law for the mobility of spherical ions, with the nanodrop diameter augmented by an effective gas-molecule collision diameter, and including a correction factor to account for the effect of ion—induced dipole (polarization) interactions, which result in the mobility decreasing linearly with the ratio between the polarization and thermal energies of the ion–neutral system at contact. The availability of this empirically-validated relation enables us, in turn, to determine true, geometric cross-sections for globular ions from IMS—MS measurements performed in gases other than helium, including molecular or atomic gases with moderate polarizabilities. In addition, the observed dependence of the experimentally-determined values for the effective gas-molecule collision diameter and the parameters involved in the polarization correction on drift-gas nature, temperature, and nanodrop polarity, is further evaluated in the light of the results of numerical calculations of the electrical mobilities, in the free-molecule regime, of spherical ions subject to different types of scattering with the gas molecules and interacting with the latter under an ion–induced dipole potential. Among the number of findings derived from this analysis, a particularly notable one is that nanodrop–neutral scattering seems to be of a diffuse (cf. elastic and specular) character in all the scenarios investigated, including the case of the monatomic argon, which therefore suggests that the atomic-level surface roughness of our nanodrops and/or the proximity between their internal degrees of freedom, rather than the sphericity (or lack of it) and the absence (or presence) of internal degrees of freedom in the gas molecules, are what chiefly determine the nature of the scattering process.

The dissociative chemisorption of oxygen and water on Fe, Ti, and adlayers of each metal on the other have been explored via surface electron spectroscopies. Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) have been used to monitor the reactions of titanium and iron with low exposures (from one to several hundred langmuirs) of oxygen and/or water vapor. Comparisons and contrasts are drawn between bulk metals and adlayer alloys--alloys formed by vapor depositing a few angstroms of one metal on top of a relatively thick film of the other metal. Most notable is the ability of a very thin layer of titanium to lessen the reactivity of a thick iron film toward oxygen.

Increasing the efficiency and durability of organic light-emitting diodes (OLEDs) has attracted attention recently due to their prospective wide-spread use as flat-panel displays. The performance and efficiency of OLEDs is understood to be critically dependent on the quality of the device heterojunctions, and on matching the ionization potentials (IP) and the electron affinities (EA) of the luminescent material (LM) with those of the hole (HTA) and electron (ETA) transport agents, respectively. The color and bandwidth of OLED emission color is thought to reflect the packing of the molecules in the luminescent layer. Finally, materials stability under OLED operating conditions is a significant concern. LM, HTA, and ETA thin films were grown in ultra-high vacuum using the molecular beam epitaxy technique. Thin film structure was determined in situ using reflection high energy electron diffraction (RHEED) and ex situ using UV-Vis spectroscopy. LM, HTA, and ETA occupied frontier orbitals (IP) were characterized by ultraviolet photoelectron spectroscopy (UPS), and their unoccupied frontier orbitals (EA) estimated from UV-Vis and fluorescence spectroscopies in combination with the UPS results. The stability of the molecules toward vacuum deposition was verified by compositional analysis of thin film X-ray photoelectron spectra. The stability of these materials toward redox processes was evaluated by cyclic voltammetry in nonaqueous media. Electrochemical data provide a more accurate estimation of the EA since the energetics for addition of an electron to a neutral molecule can be probed directly. The energetic barriers to charge injection into each layer of the device has been correlated to OLED turn-on voltage, indicating that these measurements may be used to screen potential combinations of materials for OLEDs. The chemical reversibility of LM voltammetry appears to limit the performance and lifetimes of solid-state OLEDs due to degradation of the organic layers. The role of oxygen as an electron trap in OLEDs has also been verified electrochemically. Finally, a more accurate determination of the offset of the occupied energy levels at the interface between two organic layers has been achieved via in situ monitoring of the UPS spectrum during heterojunction formation.

The use of ion-selective field-effect transistors (ISFETs) as potentiometric microsensors was investigated. In the first stage, an instrument was designed and built to operate an array of ISFETs. A microcomputer was used for instrument control and acquisition of data.
The second phase of research focussed on the development of a pH sensitive radiotelemetric device that could eventually be used for the noninvasive monitoring of gastric pH. The first attempt used an ISFET as a variable resistor in a simple telemetry circuit. The drift in the pH dependent signal from this device was significant. The use of a differential sensor was studied as a possible way to minimize the effect of signal drift. This system measured the differential output of a pH ISFET and a pH insensitive ISFET. The pH insensitivity was due to an alkanethiol monolayer at the ISFET$ vert$solution interface.
It was shown that ISFETs are well suited for use as sensors in telemetry devices. The union of these previously independent research areas has been achieved.

The presented study investigated the Induced Density of Interface States (IDIS) model at different polymer interfaces by using photoemission spectroscopy in combination with electrospray deposition. In recent years, organic electronics have attracted considerable attention due to their advantages of low-cost and easy-fabrication. The performance of such devices crucially depends on the energy barrier that controls the interface charge transfer. A significant effort has been made to explore the mechanisms that determine the direction and magnitude of charge transfer barriers in these devices. As a result of this effort, the IDIS model was developed to predict the energy alignment at metal/organic and organic/organic interfaces. The validity of the IDIS model on molecular interfaces was confirmed by the results of a series of experiments with small molecular materials, which are in good agreement with the theoretical calculations from the IDIS model. The charge neutrality level (CNL) and screening factor for various organic materials can be determined from the linear correlation between the hole injection barrier at metal/organic interface and the work function of its corresponding metal substrate, which stands as one of the most important features of the IDIS model. The study presented here explores whether the IDIS model is also valid for polymer interfaces. Two prototypical polymer materials: poly(3-hexylthiophene) (P3HT) and poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) were selected for the investigation. In the first part of this study, a series of metal/polymer interfaces were prepared using electrospray and investigated with photoemission spectroscopy. The linear relationship between the hole barriers extracted from the metal/polymer interface and the work function of its respective metal substrate suggests that the IDIS model is also valid for metal/polymer interfaces. The CNLs and the screening factors of P3HT and MEH-PPV are determined respectively. The experiment results are also discussed with regard to the Integer Charge Transfer (ICT) model. The comparison between the two models suggests that the IDIS model should be applied to interfaces prepared in vacuum while the ICT model works on interfaces with an ambient contamination layer present. The second part of the dissertation discusses the photoemission results of the MEH-PPV/P3HT heterojunction from the perspectives of the two models. The results indicate that the IDIS model is valid for polymer/polymer heterojunctions. The IDIS model more accurately predicted the measured orbital line up by using its principles for organic/organic heterojunction than the ICT model.

We focused on a non-cooling room temperature microbolometer infrared imaging array device which includes a sensing layer of p-type a-Si:H component layers doped with boron. Boron incorporation and bonding configuration were investigated for a-Si:H films grown by plasma enhanced chemical deposition (PECVD) at varying substrate temperatures, hydrogen dilution of the silane precursor, and dopant to silane ratio using multiple internal reflection infrared spectroscopy (MIR-IR). This study was then confirmed from collaborators via Raman spectroscopy. MIR-IR analyses reveal an interesting counter-balance relationship between boron-doping and hydrogen-dilution growth parameters in PECVD-grown a-Si:H. Specifically, an increase in the hydrogen dilution ratio (H2/SiH4) or substrate temperature was found to increase organization of the silicon lattice in the amorphous films. It resulted in the decrease of the most stable SiH bonding configuration and thus decrease the organization of the film. The new chemical bonding information of a-Si:H thin film was correlated with the various boron doping mechanisms proposed by theoretical calculations. The study revealed the corrosion morphology progression on aluminum alloy (Al, 0.5% Cu) under acidic chloride solution. This is due to defects and a higher copper content at the grain boundary. Direct galvanic current measurement, linear sweep voltammetry (LSV), and Tafel plots are used to measure corrosion current and potential. Hydrogen gas evolution was also observed (for the first time) in Cu/Al bimetallic interface in areas of active corrosion. Mechanistic insight that leads to effective prevention of aluminum bond pad corrosion is explored and discussed. (Chapter 4) Aluminum bond pad corrosion activity and mechanistic insight at a Cu/Al bimetallic interface typically used in microelectronic packages for automotive applications were investigated by means of optical and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electrochemistry. Screening of corrosion variables (temperature, moisture, chloride ion concentration, pH) have been investigated to find their effect on corrosion rate and to better understand the Al/Cu bimetallic corrosion mechanism. The study revealed the corrosion morphology progression on aluminum alloy (Al, 0.5% Cu) under acidic chloride solution. The corrosion starts as surface roughening which evolves into a dendrite structure and later continues to grow into a mud-crack type corrosion. SEM showed the early stage of corrosion with dendritic formation usually occurs at the grain boundary. This is due to defects and a higher copper content at the grain boundary. The impact of copper bimetallic contact on aluminum corrosion was explored by sputtering copper microdots on aluminum substrate. Copper micropattern screening revealed that the corrosion is activated on the Al/Cu interface area and driven by the large potential difference; it was also seen to proceed at much higher rates than those observed with bare aluminum. Direct galvanic current measurement, linear sweep voltammetry (LSV), and Tafel plots are used to measure corrosion current and potential. Hydrogen gas evolution was also observed (for the first time) in Cu/Al bimetallic interface in areas of active corrosion. Mechanistic insight that leads to effective prevention of aluminum bond pad corrosion is explored and discussed. Micropattern corrosion screening identified hydrogen evolution and bimetallic interface as the root cause of Al pad corrosion that leads to Cu ball lift-off, a fatal defect, in Cu wire bonded device. Complete corrosion inhibition can be achieved by strategically disabling the mutually coupled cathodic and anodic reaction cycles.

The new third form of carbon, known collectively as fullerenes, is studied in several general aspects. Efficient production methods have been developed along with an increased understanding into the formation mechanisms. Several physical and chemical properties are investigated in attempts to characterize these unique molecules. An exciting feature of these all-carbon molecules is their ability to undergo chemical modification while maintaining their cage structure, unlike diamond or graphite. One of the fundamental alterations is the addition of oxygen atoms to the cage. Techniques to produce these derivatives and their characterization is also presented.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2006.
Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 3816. Adviser: Paul W. Bohn. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2006.
Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6426. Advisers: Paul J.A. Kenis; Andrzej Wieckowski. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Design, development, and application of novel all-solid-state spectroscopic gas sensors is reported. These compact instruments employ mid-infrared difference-frequency generation pumped by room-temperature semiconductor lasers, and are capable of real-time selective measurement of trace gases in ambient air with better than 1 ppb precision (part per billion, by mole fraction). Their features and performance in the laboratory and field environment are examined and compared to that of traditional spectroscopic sensors.