Browsing by Subject "Surface chemistry"

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Adhesion of particles on indoor flooring materials
(2007-12) Lohaus, James Harold, 1968-; Siegel, Jeffrey A.
This dissertation involved a theoretical and experimental investigation of the adhesive forces between spherical particles of four different diameters and two selected flooring materials under different air velocities. Previous theoretical work and experiments described in the literature tended to be conducted with idealized surfaces, and therefore have limited applicability to indoor environments. Controlled experiments were designed, constructed and executed to measure the air velocity required to overcome adhesion forces. The diameters of the particles investigated were 0.5, 3.0, 5.0 and 9.9 [mu]m, and the flooring materials were linoleum and wooden flooring. The critical velocity, the flow at which 50% of the particles detached, is presented as a function of particle diameter for each surface. The measured values were then compared to empirical and theoretical models as well as to a scaling analysis that considers component forces that act on a particle-surface system. The results suggest that critical velocity decreases with increasing particle diameter and that existing models have limited applicability to resuspension from flooring materials.
Adsorbate interactions at organic/metal interfaces
(2005) Scharff, Robert Jason; Campion, Alan
The vibrational and electronic structure of 2-12˚A thick films of pyromellitic dianhydride (PMDA) adsorbed on clean and H2O pre-covered Cu(111) surfaces have been measured at 110K and 295K using high resolution electron energy loss spectroscopy (HREELS) and surface Raman spectroscopy. On both surfaces at low temperature, the adsorption geometry favors a flatlying orientation. Adsorption at 295K results in an upright orientation that is consistent with previous accounts in the literature. The adsorption at 110K leads to the now well documented ring-opening reaction of one of the anhydride groups producing a carboxylate and carbonyl species, whereas, at room temperature CO is lost to the gas phase. The presence of pre-adsorbed H2O at 110K results in a hydrogen bonded complex with the carboxylate group, dramatically altering the electronic structure of the PMDA film. Surface Raman scattering experiments of the H2O-PMDA co-adsorbate complex exhibit resonant scattering behavior that is attributed to the so-called “first layer” or “chemical” SERS effect due to dynamical charge transfer excitations on “atomically smooth” Cu(111) at 110K
Adsorption, reaction and interfacial electronic structures of aromatic molecules on single crystal surfaces
(2005) Wei, Wei; White, John M.
Electron transfer at organic/metal interfaces is fundamental to a large number of problems in surface science. Electronic interactions at such an interface are responsible for charge injection from an electrode to the molecular film. The efficiency or rate of charge injection is determined by the energetic alignment of molecular orbitals to the metal Fermi level and the electronic coupling strength (wavefunction mixing) between molecular orbitals and metal bands. Two experimental investigations were performed with two-photon photoemission spectroscopy (2PPE). First, the energetic alignments of naphthalene/Cu(111) were probed. Three transitions involving unoccupied orbitals were found and identified as having π* molecular orbital character—the first lying 0.4 eV above the vacuum level vii (π* b1u), the second 0.3 eV below the vacuum level (π* b3g), and the third 1.1 eV below the vacuum level (π* b2g). In the second experiment, the interfacial electronic structures of chemisorbed styrene on Cu(111) were successfully investigated. We observed unoccupied states 3.5 eV above the Fermi level and occupied states 2.0 eV below the Fermi level. Polarization results reveal that the occupied and unoccupied states arise from bonding and antibonding orbitals formed by hybridization of copper (surface state and d-band orbitals) and styrene (π1* and π2* orbitals). For the first time, two-photon photoemission spectroscopy was employed to explore a surface chemical reaction: epoxidation of styrene on Cu(111). With 100 L oxygen on a Cu(111) surface, the atomic oxygen occupies three-fold hollow HCP sites rather than FCC sites. Its 2p states hybridize strongly with the dz2 states of the Cu atoms in the second layer. After styrene is adsorbed on Cu metal sites of this oxygen-covered surface, it undergoes efficient epoxidation to styrene oxide. The 2PPE results show that the change in the electronic structures of the adsorbed reactant is consistent with the surface reaction: the oxygen-induced feature from the Cu-O bonding disappears and a new state appears. However, 1000 L oxygen-covered Cu(111) is catalytically inert for styrene epoxidation: as styrene is added, no new features appear in 2PPE, and there is no evidence for chemical reaction in thermal desorption. This study could open up a new area of solid state and surface catalytic chemistry.
Catalytic activity of 2-D bimetallic surfaces and 3-D metal organic frameworks
(2020-04-10) Han, Sungmin (Ph. D. in chemistry); Mullins, C. B.; Keitz, Benjamin K; Hwang, Gyeong S; Sitz, Greg O; Henkelman, Graeme A
The first part of this dissertation involves understanding the fundamentals of 2-D palladium (Pd) - gold (Au) bimetallic surfaces. The special catalytic capabilities of Pd-Au bimetallic surfaces are mainly attributed to ensemble effects related to the compositions of inert Au atoms and active Pd atoms. In particular, we focus on the activation of O₂ and various oxidative reactions on the Pd–Au surfaces. Using molecular beam techniques under ultrahigh vacuum (UHV), we prove that H₂O and O₂ admolecules form hydrogen bonded clusters, which enhance O₂ activation. The direct dissociative adsorption of O₂ is also possible on the Pd–Au surfaces. We experimentally estimate the activation barriers for O₂ dissociation and the reactivity of oxygen adatoms as a function of the Pd coverage on the Au(111) surface, where a relatively low O₂ dissociation barrier (high Pd coverages) corresponds to a higher reaction barrier for oxygen adatoms (due to the higher Pd coverages). Based on these results, we also discover that acetaldehyde molecules can be selectively oxidized to acetic acid on less Pd deposited surfaces, since the small Pd ensembles on this surface can prevent the decomposition of acetate, which is an intermediate state in the formation of acetic acid. The second part of this dissertation involves growing and analyzing HKUST-1 metal-organic framework (MOF) thin films. MOFs are a new class of ultra-porous material based on inorganic-organic hybrid structures. We explain a new growth method for HKUST-1 thin films by sequential deposition of Cu and H₃BTC under vacuum. This procedure resulted in first MOF thin film to be controllably grown under vacuum, and the strategy can be applied in various applications. Since this growth method allows delicate quantity control of Cu, we successfully grow 4-6 nm or 8-12 nm Cu nanoparticles (NP) incorporated in HKUST-1 thin films. Applying temperature programmed desorption under vacuum, Cu NP incorporated HKUST-1 thin films show different catalytic activity for the methanol oxidation depending on the Cu NP sizes. The film with smaller Cu NP’s has improved selectivity for formaldehyde, and the film with larger Cu NP’s generates formaldehyde along with other products, CO₂ and H₂.
Current progress on the synthesis and characterization of a biosensing cascade on a patterned gold-silicon substrate
(2023-04-20) Wolff, Erich Paul; Webb, Lauren J.
The specificity and functionality of enzymes make them ideal for use in a range of devices from biosensors to catalytic reactors. These applications require the immobilization of enzymes onto substrates to preserve activity and reusability of these devices. In order to expand the functionality of these devices, it is increasingly more important to find ways to generate biomimetic devices with multiple enzymes in a controlled manner. These multiple enzymes-based device work together through enzymatic cascades to perform more advanced functions than possible with a single enzyme. Herein, I document my research on generating a substrate composed of Au and Si that will be used to immobilize two enzymes, Choline Oxidase (ChOx) and Horseradish peroxidase (HRP), to detect choline in solution, as a test for controlled enzyme immobilization on biosensing devices. Au and Si substrates were functionalized with alkyl thiol and alkyl trichlorosilane self-assembled monolayers for enzyme immobilization. ChOx will be immobilized on the Si portion of the substrate with electrostatic attachment, and HRP immobilized on the Au portion with (N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride) /(N-hydroxysuccinimide) coupling. Experiments in this study were performed in the manner of least to most complex mediums. Therefore, the enzymes were first studied in solution, then studied on one element substrates composed of either Au or Si, and finally studied on a substrate patterned with Au and Si domains. The properties, including native folding and activity (HRP V [subscript max] = 265.2 ± 20.1 U/mg, ChOx V [subscript max] = 2.4 ± 0.1 U/mg), of both enzymes were first collected in their native solutions; so that they could be compared to the enzymes once immobilized on their corresponding substrates with ultraviolet – visible (UV-Vis) and circular-dichroism spectroscopy. The successful immobilization, surface concentration (1.97 ± 0.27 pmol/cm²), and activity of HRP on uniform Au substrate were determined with infrared spectroscopy, ellipsometry, and UV-Vis. Future experiments in this project will focus on the properties of ChOx on Si, and both enzymes on a patterned Au/Si substrate. Long term, the goal is to expand the complexity of these multi-enzyme biodevices by adding more domains for attachment of enzymes with other metal domains and metal nanoparticles.
Effect of chemical treatment and trivalent doping on the surface structure and surface chemistry of Li1-xNi0.5-yMn1.5+yO4 spinel
(2018-02-05) Amos, Charles Dallas; Goodenough, John B.; Ferreira, Paulo J. (Paulo Jorge); Mullins, Charles B; Yu, Guihua; Varela, Maria
The surface structure and surface chemistry of Li₁[subscript -x]Ni₀̣.₅[subscript -y]Mn₁.₅[subscript +y]O₄ was examined by first analyzing as-prepared Li[Mn₂]O₄, the basis cubic spinel structure without Ni or trivalent dopants. It was found that Li[Mn₂]O₄ undergoes a surface reconstruction, which results in the production of a thin, stable surface layer of Mn₃O₄, a subsurface region of Li₁[subscript -x][Mn₂]O₄ with retention of bulk Li[Mn₂]O₄. This observation is compatible with the surface disproportionation of Mn coupled with oxygen deficiency and a displacement of surface Li⁺ from the Mn₃O₄ surface phase to a subsurface layer. Li₁[subscript -x][Mn₂]O₄ was then subjected to chemical treatments to further understand and isolate the role of Li and oxygen in the surface reconstruction. An aqueous acid treatment, a non-aqueous chemical delithiation, and an oxygen plasma treatment were applied to Li[Mn₂]O₄. It was found that Mn₃O₄ is a robust surface phase in the Li₁[subscript -x][Mn₂]O₄ system regardless of the chemical treatment and level of lithiation. The surface Mn₃O₄ phase is cubic whereas bulk Mn₃O₄ undergoes a cooperative Jahn-Teller distortion to tetragonal symmetry. Thicker Mn₃O₄ surface layers are tetragonal. Analysis of as-prepared LiNi₀.₅Mn₁.₅O₄ revealed a surface composed of mixed Mn₃O₄ and a well-known Ni-rich rock-salt phase that occurs in the LiNi₀.₅Mn₁.₅O₄ system. Trivalent doping of LiNi₀.₅Mn₁.₅O₄ with Cr creates an ordered spinel/rock-salt phase that has not been previously observed in either the LiNi₀.₅Mn₁.₅O₄ or LiMn₂O₄ systems.
Fundamental surface science investigations of systems designed to address technological issues
(2003) Yan, Xiaoming; White, John M.
Organometallic chemical vapor deposition of (MeCp)Ir(COD) onto Rh is simulated experimentally with and without co-reactant oxygen via isothermal reaction mass spectrometry. Auger electron spectroscopy (AES) is used to analyze the resulting film purity. Without oxygen, continuous film deposition occurs above 750 K. A large amount of carbon incorporates, and a final composition of C4Ir is inferred. At the steady state of film growth, acetylene is the only volatile product. Before reaching steady state, various hydrocarbon species are observed. With enough oxygen, the precursor combusts and pure Ir is deposited above 600 K. At steady state, the main by-products are CO and H2O. The thermal decomposition of tert-butoxy (TBO) with co-adsorbed O and NO is studied using temperature programmed desorption and AES on Rh foil and Cu(111). On Cu(111) with NO(a), some TBO decomposes below 240 K to form H2O, CO, CO2, C2H2, proposed imide and acetate, and others disproportionate to tert-butyl alcohol, isobutene and adsorbed oxygen at 610 K. On Rh with NO(a), two oxygen-containing fragments—TBO and a stabilized oxametallacycle—coexist. The proposed oxametallacycle decomposes at 350 K to acetone, while TBO, relatively stable in the presence of N and NO, decomposes to isobutene at 500 K. On Rh with O(a), TBO is stable only up to 380 K where, assisted by O, it decomposes to acetone and butene via a transient form of the oxametallacycle. Thermally evaporated Ag is deposited onto a thin solid water layer on clean hafnia, titania and functionized titiania surfaces. After thermal desorption of water, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) reveals Ag particles on these surfaces. On HfO2, particles have lateral dimensions between 5 and 20 nm and, in many cases, with heights exceeding the thickness of the original water layer. More interesting, particles form 1D bead-like strings spontaneously on 18 L ice. However, on trimethyl acetic acid (TMAAH) pre-saturated TiO2(110), only a few huge particles form. The difference is attributed to the different surface hydrophilicities, which affect both the initial ice layer growth and the competition between dewetting and desorption of adsorbed water.
Growth, structure, and chemistry of 1B metal nanoclusters supported on TiO₂(110)
(2006) Pillay, Devina; Hwang, Gyeong S.
Cu, Ag, and Au nanoclusters dispersed on TiO2(110) surfaces are utilized in a wide variety of applications ranging from microelectronics to heterogeneous catalysis. The unique chemical reactivity of these clusters is largely dependent on their size, shape, spatial distribution, and interfacial interaction with the oxide support. This implies that atomic level control of these properties can offer great opportunities in the development of novel devices based on supported metal nanoclusters. It is therefore necessary to understand how formation and restructuring of these clusters alter their geometric and electronic characteristics. This thesis involves the development of a theoretical foundation for studying the growth, structure, and chemistry of Cu, Ag, and Au on TiO2(110) surfaces. Using density functional theory calculations, we have identified factors that control the chemical reactivity of these supported metal nanoclusters. First we investigated the electronic and geometric structures of the stoichiometric and reduced rutile TiO2(110) surfaces. Then we examined the surface chemistry of TiO2 towards gaseous CO and O2, as well as the structure and growth of 1B metal nanoclusters on TiO2(110). We also examined how the electronic and geometric properties of mixed metal nanoclusters, CuAun(n≤ 3), differ versus their single metal counterparts, Cum and Aum (m ≤ 4). Finally, we considered CO oxidation reactions on TiO2(110)-supported small Au clusters. While current experimental techniques are limited to providing complementary atomic-level real space information, first principles-based atomic level simulations greatly contribute to elucidating the fundamental behavior and properties of Cu, Ag, and Au nanoclusters on TiO2(110). First principles modeling has paved the way for new catalyst development by investigating how the geometric, electronic, and chemical properties of TiO2-supported 1B metal nanoclusters vary with surface defects, adsorbates, and metal dopants before valuable time and manpower is invested in experimental synthesis and characterization.
Investigating thermodynamics and kinetics of hydrate phase change phenomena using experimental and machine learning tools
(2021-12-07) Acharya, Palash Vadiraj; Bahadur, Vaibhav; Ezekoye, Ofodike; Shi, Li; Bonnecaze, Roger
Hydrates are ice-like crystalline solids which form under high pressure and low-temperature conditions from water (forming a cage of host molecules) and another liquid or gas (guest molecule). Hydrates can enable numerous industrial applications in the fields of carbon capture and sequestration (CCS), flow assurance, natural gas transportation/storage and desalination. A significant technological barrier to many hydrates-related applications is the slow rate of formation of hydrates, which is a result of thermodynamic and kinetics-related limitations. This dissertation investigates the role of electric field and surface chemistry in accelerating the nucleation kinetics of clathrate hydrates (CO₂, tetrahydrofuran). It also investigates the role of amino acids in inhibiting the nucleation kinetics and thermodynamics of CO₂ hydrate formation. It also evaluates the utility of machine learning models in predicting the thermodynamic formation conditions for gas hydrates. In addition to the focus on fundamental investigations, this dissertation also evaluates the utility of hydrates as a carbon capture tool when coupled with a steam reforming system to generate blue hydrogen from landfill gas. The content of the dissertation work is motivated by three objectives, as described ahead. Objective 1 investigates the influence of electric fields and surface chemistry on nucleation kinetics of hydrate formation for two kinds of hydrate forming systems (considered as two separate subtasks): miscible liquid-liquid systems (Tetrahydrofuran-water) and gas-liquid (CO₂-water) systems. As background, it is noted that the role of electric field has been widely studied for accelerating freezing of water. Subtask 1-1 investigates the role of electric field when used in conjunction with open-cell aluminum metal foam-based electrodes in accelerating the formation of THF hydrates. It is demonstrated that aluminum foam electrodes trigger near-instantaneous nucleation (in only tens of seconds) of THF hydrates at very low voltages (~20V). The promotion effect can be ascribed to two distinct interfacial mechanisms at play: namely, electrolytic bubble generation and the formation of metal ion complex-based coordination compounds. While THF hydrates form under atmospheric pressure, CO₂ gas hydrates form at much higher pressures and are therefore studied using a custom-built high-pressure cell. Subtask 1-2 highlights the role of aluminum in accelerating nucleation kinetics of CO₂ gas hydrates. Statistically meaningful measurements of induction times for CO₂ hydrate nucleation are undertaken using water droplets as individual microsystems for hydrate formation. The influence of various metal surfaces, droplet size, CO₂ dissolution time, and the presence of salts in water on nucleation kinetics have been characterized. It is observed that Al metal significantly accelerates the nucleation kinetics of CO₂ hydrates (the effect of which cannot be replicated by salts of Al) with nucleation initiating from the Al-water interface. Prediction of thermodynamic conditions of hydrate formation is critical to their synthesis and Objective 2 is centered around developing modeling and experimental tools for effective prediction of thermodynamic phase equilibria for hydrates. Subtask 2-1 demonstrates the utility of machine learning models to predict hydrate dissociation temperature (HDT) as a function of constituent hydrate precursors and salt inhibitors. Importantly, and in contrast to most previous studies, thermodynamic variables such as the activity-based contribution due to electrolytes, partial pressure of individual gases, and specific gravity of the overall mixture have been used as input features in the prediction algorithms. Using such features results in more physics-aware ML algorithms, which can capture the individual contributions of gases and electrolytes in a more fundamental manner. Three ML algorithms: Random Forest (RF), Extra Trees (ET), and Extreme Gradient Boosting (XGBoost) are trained and their performance is evaluated on an extensive experimental dataset comprising of more than 1800 experimental data points. The overall coefficient of determination (R²) is greater than 97% for all the three ML models with XGBoost exhibiting the best prediction performance with an R² metric of 99.56%. Subtask 2-2 investigates the role of amino acids on the kinetics and thermodynamics of CO₂ hydrate formation using droplet-based microsystems. Amino acids are environmentally friendly and inexpensive hydrate formation inhibitors. Nucleation kinetics as well as the depression in thermodynamic hydrate formation temperature for CO₂ hydrates in the presence of five amino acids containing non-polar side chains have been evaluated. All the amino acids inhibit nucleation with tryptophan exhibiting the slowest nucleation rate. Isoleucine exhibits the highest thermodynamic inhibition effect with the highest depression in freezing point temperature corresponding to 0.2 K for the concentrations studied in the present analysis. Landfills produce significant amounts of methane, which is a potent greenhouse gas. Steam reforming of the landfill gas generates CO₂ + H₂ as byproducts. The generated hydrogen can be used in refineries, to produce fertilizers or to produce electricity in a fuel cell. Objective 3 investigates the techno-economic factors associated with a facility coupling a sorption-enhanced steam methane reforming system with a hydrates-based capture system for landfills across Texas. The electrical energy requirements, water use, operating and capital costs required to set up and keep such a facility running have been evaluated in detail. The cost of producing hydrogen for all counties is about $0.5/kg of H₂ (excluding the cost for natural gas). The total carbon capture cost lies in the range of $96-$145/metric ton of CO2 with the lowest/highest cost corresponding to Harris/Brazoria county producing the highest/lowest amount of CO₂. A minimum cost of $0.9(2.4)/kg of H₂ would be required for Harris (Brazoria) county for a positive 30-year net present value; a 5-year payback period would require a minimum cost of $1.35(4.95)/kg of H₂. In summary, this dissertation significantly advances the current understanding of hydrate formation by introducing novel techniques (consuming ultra-low energy as well as passive tools) for enhancing hydrate formation kinetics. It also develops novel ML and experimental tools for predicting thermodynamic formation conditions of hydrates in the presence of various inhibitors. Finally, this work assesses the technical and economic viability of a hydrates-centered future for the natural gas industry.
Ion-surface interactions relevant to semiconductor plasma processing
(1995-05) Tesauro, Mark Richard; Not available
Network thermodynamic modeling of a multicellular tissue during cryopreservation : a bond graph model of the islets of Langerhans
(1995-12) Freitas, Robson Cintra de; Not available
Optical second-harmonic electro- and thermoreflectance spectroscopy of Si(001)/SiO₂, H-Si(001), and Si(001)-(2x1) interfaces by femtosecond pulses
(1995-08) Dadap, Jerry Icban, 1962-; Not available
Phage display technology for surface functionalization of a synthetic biomaterial
(2005) Sanghvi, Archit Bharat; Schmidt, Christine E.
The rapid growth in the use of synthetic polymers in medicine and biotechnology has prompted the development of advanced biomaterials that present unique surface properties to control cellular activity. To control the surface properties of biomaterials numerous methods have been developed for immobilization of biomolecules. The goal of this work was to develop a new method for surface functionalization of synthetic biopolymers using phage display technology. This approach has traditionally been utilized for both biological and non-biological materials to select peptides expressed on the bacteriophage using a combinatorial approach. As presented in this thesis chloride doped polypyrrole (PPyCl) was used as a model biopolymer to screen for a peptide insert 4 selected from a combinatorial library with diversity of 109. A PPyCl-binding peptide (T59) was successfully identified using this phage display approach. As a biomaterial, polypyrrole presents many unique opportunities in the field of biomedicine, specifically in tissue engineering, drug delivery and biosensor development. A peptide-expressing phage (T59) that binds to PPyCl, when compared to other selected materials, was identified. Furthermore, the T59 peptide, independent of the phage, was synthesized and its binding ability and characteristics were analyzed using both qualitative and pseudo-quantitative analysis. Furthermore, the stability of the peptides in the presence of serum proteins was explored using indirect methods to compare to a control condition. Finally, we explored a potential application of the selected T59 peptides by attaching a cell-adhesion promoting sequence that permitted cell attachment on PPyCl surface without the presence of serum proteins. Although not directly shown here, this approach, which is highly versatile, simple and imparts not changes to the material’s bulk properties, can potentially be applied to various biopolymers.
Pre-atomization processes occurring on graphite surfaces used in electrothermal atomization
(1995-05) Jackson, Jason Garnett; Not available
Quantum state-resolved studies of sticking and elastic scattering of H₂ from Cu(100)
(2006) Kim, Jonghyuk; Sitz, Greg Orman
The sticking and elastic scattering of vibrationally excited H2 from Cu(100) were studied using Laser Induced Thermal Desorption (LITD), laser spectroscopy, and molecular beam techniques. The scattering of H2 from Cu(100) has been the subject of numerous experimental and theoretical stud ies. There is general agreement between theory and experiment for elastic scattering results but not in sticking or inelastic scattering measurements. LITD is developed to measure the H/Cu(100) coverage and to deter mine the sticking coeﬃcients. For the ground vibrational state, the H2 sticking coeﬃcient is 1.2±0.4×10−4 at Ei = 74 meV and ≤ 2.5±0.6×10−4 at Ei = 189 meV. For the ﬁrst excited vibrational state, the upper limit of the H2 sticking coeﬃcient is 0.065 at Ei = 74 meV and 0.17 at Ei = 189 meV. The kinetic energy transfer during the elastic scattering of H2(v = 1,j = 1) from Cu(100) increased rapidly from ∆E = 3.1±.16 meV at Ei = 74.7 meV to ∆E = 29.5±.04 meV at Ei = 122 meV and ∆E = 54.3±.25 meV at Ei = 189 meV. This trend is hard to explain using classical mechanics and may show an electronic friction eﬀect. The survival probabilities in the elastic scattering branch were dropped from 0.97 to 0.56 with increasing incident kinetic energies.
Removal of formaldehyde from indoor air : enhancing surface-mediated reactions on activated carbon
(2013-08) Carter, Ellison Milne; Katz, Lynn E.; Speitel, Gerald E.
Formaldehyde is a ubiquitous and hazardous indoor air pollutant and reducing concentrations in indoor environments is a public health priority. The goals of this doctoral work were to advance analytical methods for continuous monitoring of formaldehyde at very low concentrations (sub-20 ppb[subscript v]) and to improve fundamental, mechanistic understanding of how structural and chemical properties of activated carbon influence removal of formaldehyde from indoor environments. To achieve these goals, emerging sensor-based technology was evaluated for its ability to detect and quantify ppb[subscript v]-level formaldehyde concentrations on a continuous basis at relative humidity levels characteristic of residential indoor environments. Also, a combination of spectroscopic and selective titration techniques was employed to characterize molecular-level structural and chemical properties of traditional and chemically treated granular activated carbon (GAC). In addition to selecting two different commercially available GACs for study, design and preparation of a laboratory-prepared, chemically treated GAC was pursued to create nitrogen-doped GAC with desirable surface chemical properties. Performance of all GACs was evaluated with respect to formaldehyde removal through a series of packed bed column studies. With respect to continuous formaldehyde monitoring, a method detection limit for emerging sensor technology was determined to be approximately 2 ppb[subscript v], and for relative humidity levels characteristic of indoor environments (> 40%), quantitative, continuous formaldehyde measurements less than 10 ppb[subscript v] were robust. The two commercially available GACs tested were both capable of removing formaldehyde; however, the GAC with greater density of basic surface functional groups and greater electron-donating potential (Centaur) removed twice as much formaldehyde (on a GAC mass basis) as the less basic GAC (BPL). A laboratory-prepared GAC (BPL-N) was successfully created to contain pyridinic and pyrrolic nitrogen, which was associated with increased surface density of basic functional groups, as well as with increased electron-donating potential. BPL-N exhibited better removal capacity for formaldehyde than BPL and Centaur. Furthermore, packed bed column studies of BPL-N and BPL formaldehyde removal performance yielded evidence to support the hypothesis that electron-donating potential, especially nitrogen functional groups at the BPL-N surface, promote catalytic removal of gas-phase formaldehyde via oxidation.
Semiconductor nanowires : from a nanoscale system to a macroscopic material
(2011-12) Holmberg, Vincent Carl; Korgel, Brian Allan, 1969-
Semiconductor nanowires are one-dimensional nanoscale systems that exhibit many unique properties. Their nanoscale size can lead to defect densities and impurity populations different than bulk materials, resulting in altered diffusion behavior and mechanical properties. Synthetic methods now support the large-scale production of semiconductor nanowires, enabling a new class of materials and devices that use macroscopic quantities of nanowires. These advances have created an opportunity to fabricate bulk structures which exhibit the unique physical properties of semiconductor nanowires, bridging the properties of a nanoscale system with macroscopic materials. High aspect ratio germanium nanowires were synthesized in supercritical organic solvents using colloidal gold nanocrystal seeds. The nanowires were chemically passivated inside the reactor system using in situ thermal hydrogermylation and thiolation. The chemical stability of the passivated nanowires was studied by exposure to highly corrosive and oxidative environments. Chemical surface functionalization of germanium nanowires was investigated by covalently tethering carboxylic acid groups to the surface, as a general platform for the further functionalization of nanowire surfaces with molecules such as polyethylene glycol. Surface functionalization with dopant-containing molecules was also explored as a potential route for doping nanowires. In addition, static charging was exploited in the development of an electrostatic deposition method for semiconductor nanowires. In situ transmission electron microscopy experiments were conducted on gold-seeded germanium nanowires encapsulated within a volume-restricting carbon shell. A depressed eutectic melting temperature was observed, along with strong capillary effects, and the solid-state diffusion of gold into the crystalline stem of the germanium nanowire, occurring at rates orders of magnitude slower than in the bulk. Copper, nickel, and gold diffusion in silicon nanowires were also investigated. The rate of gold diffusion was found to be a strong function of the amount of gold available to the system. Finally, germanium nanowires were found to exhibit exceptional mechanical properties, with bending strengths approaching that of an ideal, defect-free, perfect crystal, and strength-to-weight ratios greater than either Kevlar or carbon fiber. Macroscopic quantities of nanowires were used to fabricate large sheets of free-standing semiconductor nanowire fabric, and the physical, morphological, and optical properties of the material were investigated.
Surface and interfacial chemistry of high-k dielectric and interconnect materials on silicon
(2001-08) Kirsch, Paul Daniel; Ekerdt, John G.
Surfaces and interfaces play a critical role in the manufacture and function of silicon based integrated circuits. It is therefore reasonable to study the chemistries at these surfaces and interfaces to improve existing processes and to develop new ones. Model barium strontium titanate high-k dielectric systems have been deposited on ultrathin silicon oxynitride in ultrahigh vacuum. The resulting nanostructures are characterized with secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). An interfacial reaction between Ba and Sr atoms and SiOxNy was found to create silicates, BaSixOy or SrSixOy. Inclusion of N in the interfacial oxide decreased silicate formation in both Ba and Sr systems. Furthermore, inclusion of N in the interfacial oxide decreased the penetration of Ba and Sr containing species, such as silicides and silicates. Sputter deposited HfO2 was studied on nitrided and unnitrided Si(100) surfaces. XPS and SIMS were used to verify the presence of interfacial HfSixOy and estimate its relative amount on both nitrided and unnitrided samples. More HfSixOy formed without the SiNx interfacial layer. These interfacial chemistry results are then used to explain the electrical measurements obtained from metal oxide semiconductor (MOS) capacitors. MOS capacitors with interfacial SiNx exhibit reduced leakage current and increased capacitance. Lastly, surface science techniques were used to develop a processing technique for reducing thin films of copper (II) and copper (I) oxide to copper. Deuterium atoms (D*) and methyl radicals (CH3*) were shown to reduce Cu2+ and/or Cu1+ to Cu0 within 30 min at a surface temperature of 400 K under a flux of 1×1015 atoms/cm2 ·s. Temperature programmed desorption experiments suggest that oxygen leaves the surface as D2O and CO2 for the D* and CH3* treated surfaces, respectively.
Surface chemistry and directed assembly of nanostructures on dielectric surfaces
(2006) Stanley, Scott Kendyl; Ekerdt, John G.
Surface chemistry and nanoparticle growth relevant for flash memory applications has been investigated with a number of surface science techniques and imaging methods. Germanium chemistry on SiO2 surfaces is investigated and a series of temperature dependant sequential reactions are identified explaining how Ge reactively etches SiO2 at low temperatures. These reactions hinder the accumulation of Ge adatoms on SiO2 surfaces during chemical vapor deposition (CVD). Germanium is seen to form an unusually stable contacting oxide on HfO2 surfaces and nanoparticles may be grown on HfO2 during CVD. The surface chemistry of Si is also examined on both SiO2 and HfO2 surfaces and Si is seen to be relatively stable on both surfaces, with only slight difference in desorption kinetics. A kinetically-driven patterning scheme was developed to direct the self assembly of nanoparticles within top-down defined regions on the surface by exploiting the reaction kinetics of Si and Ge. Using this method, adatoms are corralled into top-down defined regions where they bottom-up self assemble to form nanoparticles and no nanoparticles form elsewhere. The effect of feature size on the self assembly of nanoparticles is studied and reactive pathways for adatoms in confined spaces are examined.
Surface chemistry of FeHx with dielectric surfaces : towards directed nanocrystal growth
(2008-08) Winkenwerder, Wyatt August, 1981-; Ekerdt, John G.
The surface chemistry of GeH[subscript x] with dielectric surfaces is relevant to the application of germanium (Ge) nanocrystals for nanocrystal flash memory devices. GeH[subscript x] surface chemistry was first explored for thermally-grown SiO₂ revealing that GeH[subscript x] undergoes two temperature dependent reactions that remove Ge from the SiO₂ surface as GeH₄ and Ge, respectively. Ge only accumulates due to reactions between GeH[subscript x] species that form stable Ge clusters on the SiO₂ surface. Next, a Si-etched SiO₂ surface is probed by GeH[subscript x] revealing that the Si-etching defect activates the surface toward Ge deposition. The activation involves two separate reactions involving, first, the capture of GeH[subscript x] by the defect and second, a reaction between the captured Ge and remaining GeH[subscript x] species leading to the formation of Ge clusters. Reacting the defect with diborane, deactivates it toward GeH[subscript x] and also deactivates intrinsic hydroxyl groups toward GeH[subscript x] adsorption. A structure is proposed for the Si-etching defect. The surface chemistry of GeHx with HfO₂ is studied showing that the hafnium germinate that forms beneath the Ge nanocrystals exists as islands and not a continuous film. Annealing the hafnium germinate under a silane atmosphere will reduce it to Ge while leading to the deposition of hafnium silicate (HfSiO[subscript x]) and silicon (Si). Treating the HfO₂ with silane prior to Ge nanocrystal growth yields a surface with hafnium silicate islands on which Si also deposits. Ge deposition on this surface leads to the suppression of hafnium germinate formation. Electrical testing of capacitors made from Ge nanocrystals and HfO₂ shows that Ge nanocrystals encapsulated in Si/HfSiO[subscript x] layers have greatly improved retention characteristics.