Matching Items (43)
Description
Understanding charge transport in single molecules covalently bonded to electrodes is a fundamental goal in the field of molecular electronics. In the past decade, it has become possible to measure charge transport on the single-molecule level using the STM break junction method. Measurements on the single-molecule level shed light on charge transport phenomena which would otherwise be obfuscated by ensemble measurements of groups of molecules. This thesis will discuss three projects carried out using STM break junction. In the first project, the transition between two different charge transport mechanisms is reported in a set of molecular wires. The shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer wires show weakly length dependent and temperature dependent behavior. This trend is consistent with a model whereby conduction occurs by coherent tunneling in the shortest wires and by incoherent hopping in the longer wires. Measurements are supported with calculations and the evolution of the molecular junction during the pulling process is investigated. The second project reports controlling the formation of single-molecule junctions by means of electrochemically reducing two axial-diazonium terminal groups on a molecule, thereby producing direct Au-C covalent bonds in-situ between the molecule and gold electrodes. Step length analysis shows that the molecular junction is significantly more stable, and can be pulled over a longer distance than a comparable junction created with amine anchoring bonds. The stability of the junction is explained by the calculated lower binding energy associated with the direct Au-C bond compared with the Au-N bond. Finally, the third project investigates the role that molecular conformation plays in the conductance of oligothiophene single-molecule junctions. Ethyl substituted oligothiophenes were measured and found to exhibit temperature dependent conductance and transition voltage for molecules with between two and six repeat units. While the molecule with only one repeat unit shows temperature invariant behavior. Density functional theory calculations show that at higher temperatures the oligomers with multiple repeat units assume a more planar conformation, which increases the conjugation length and decreases the effective energy barrier of the junction.
ContributorsHines, Thomas (Author) / Tao, Nongjian (Thesis advisor) / Li, Jian (Thesis advisor) / Mujica, Vladimiro (Committee member) / Allee, David (Committee member) / Arizona State University (Publisher)
Created2013
Description
Circular Dichroism (CD) and electron paramagnetic resonance (EPR) were used to investigate the metal-binding sites of five different four-helix bundles, which have slight differences in the population of their side chains. Of the four-helix bundles, three have central dinuclear metal binding sites; two of these three also have outer dinuclear metal binding sites. The other two peptides have two identical, non-central, dinuclear metal binding sites. The CD spectra showed changes in the secondary structure of the peptides, and X-band EPR spectra of these peptides revealed the unique four peak signal of Cu(II). These findings improve our understanding of the metal binding environments of these peptides.
ContributorsCanarie, Elizabeth Rose (Author) / Allen, James (Thesis director) / Wolf, George (Committee member) / School of Mathematical and Statistical Sciences (Contributor) / School of Molecular Sciences (Contributor) / Barrett, The Honors College (Contributor)
Created2016-05
Description
Dielectrophoresis is a separations strategy that has the potential to separate small amounts of different proteins from each other. The forces at play in the channel used for dielectrophoresis are electroosmotic flow (EOF), electrophoresis (EP), and dielectrophoresis (DEP). EOF is the force exerted on liquid from an applied potential (1). EP is the force exerted on charged particles in a uniform electric field (2). DEP is the force exerted on particles (charged and uncharged) in a non-uniform electric field (3). This experiment was focused on the testing of a new microfluidic device to see if it could improve the focusing of proteins in dielectrophoresis. It was predicted that the addition of a salt bridge would improve focusing by preventing the ions created by the electrolysis of water around the electrodes from interacting with the proteins and causing aggregation, among other problems. Control trials using the old device showed that electrolysis was likely occurring and was the causal agent for poor outcomes. After applying the electric potential for some time a pH front traveled through the channel causing aggregation of proteins and the current in the channel decreased rapidly, even while the voltage was held constant. The resistance in the channels of the control trials also slightly decreased over time, until the pH shift occurred, at which time it increased rapidly. Experimental trials with a new device that included salt bridges eliminated this pH front and had a roughly linear increase of current in the channel with the voltage applied. This device can now be used in future research with protein dielectrophoresis, including in the potential differentiation of different proteins. References: 1) Electroosmosis. Oxford Dictionary of Biochemistry and Molecular Biology. 2. Oxford University Press: Oxford, England. 2006. 2) Electrophoresis. Oxford Dictionary of Biochemistry and Molecular Biology. 2. Oxford University Press: Oxford, England. 2006. 3) Dielectrophoresis. Oxford Dictionary of Biochemistry and Molecular Biology. 2. Oxford University Press: Oxford, England. 2006.
ContributorsHayes, Katelyn Donna (Author) / Hayes, Mark (Thesis director) / Borges, Chad (Committee member) / School of Life Sciences (Contributor) / Department of Psychology (Contributor) / Barrett, The Honors College (Contributor)
Created2016-05
Description
Hydrogen is considered one of the most potential fuels due to its highest gravimetric energy density with no pollutant emission during the energy cycle. Among several techniques for hydrogen generation, the promising photoelectrochemical water oxidation is considered a long-term solar pathway by splitting water. The system contains a photoanode and a cathode immersed in an aqueous electrolyte where charge separation takes place in the bulk of the semiconducting material on light absorption, leading to water oxidation/reduction at the surface of the photoelectrodes/cathode. It is imperative to develop materials that demonstrate high light absorption in the wide spectrum along with photoelectrochemical stability. N-type Monoclinic scheelite bismuth vanadate (BiVO4) is selected due to its incredible light absorption capabilities, direct bandgap (Eg ∼ 2.4-2.5 eV) and relatively better photoelectrochemical stability. However, BiVO4 encounters huge electron-hole recombination due to smaller diffusion lengths and positive conduction bands that cause slow charge dynamics and sluggish water oxidation kinetics. In order to improve the illustrated drawbacks, four strategies were discussed. Chapter 1 describe the fundamental understanding of photoelectrochemical cell and BiVO4. Chapter 2 illustrates details of the experimental procedure and state-of-the-art material characterization. Chapter 3 provide the impact of alkali metal placement in the crystal structure of BiVO4 systematically that exhibited ~20 times more performance than intrinsic BiVO4, almost complete bulk charge separation and enhancement in the diffusion length. Detailed characterization determined that the alkali metal getting placed in the interstitial void of BiVO4 lattice and multiple interbands formation enhanced the charge dynamics. Chapter 4 contains stoichiometric doping of Y3+ or Er3+ or Yb3+ at the Bi3+ site, leading to an extended absorption region, whereas non-stoichiometric W6+ doping at the V5+ site minimizes defects and increased charge carriers. To further enhance the performance, type-II heterojunction with WO3 along p-n junction with Fe:NiO enhance light absorption and charge dynamics close to the theoretical performance. Chapter 5 provides a comprehensive study of a uniquely developed sulfur modified Bi2O3 interface layer to facilitate charge dynamics and carrier lifetime improvement by effectively passivating the WO3/BiVO4 heterojunction interface. Finally, chapter 6 summarized the major findings, conclusion and outlook in developing BiVO4 as an efficient photoanode material.
ContributorsPrasad, Umesh (Author) / Kannan, Arunachala Mada (Thesis advisor) / Azeredo, Bruno (Committee member) / Chan, Candace (Committee member) / Segura, Sergio Garcia (Committee member) / Arizona State University (Publisher)
Created2021
Description
Transition metal oxides are used for numerous applications, includingsemiconductors, batteries, solar cells, catalysis, magnetic devices, and are commonly
observed in interstellar media. However, the atomic-scale properties which dictate the
overall bulk material activity is still lacking fundamental details. Most importantly, how
the electron shells of metals and O atoms mix is inherently significant to reactivity. This
thesis compares the binding and excited state properties of highly correlated first-row
transition metal oxides using four separate transition metal systems of Ti, Cr, Fe and Ni.
Laser ablation coupled with femtosecond pump-probe spectroscopy is utilized to resolve
the time-dependent excited state relaxation dynamics of atomically precise neutral
clusters following 400 nm (3.1 eV) photoexcitation. All transition metal oxides form
unique stable stoichiometries with excited state dynamics that evolve due to oxidation,
size, or geometry. Theoretical calculations assist in experimental analysis, showing
correlations between charge transfer characteristics, electron and hole localization, and
magnetic properties to the experimentally determined excited state lifetimes.
This thesis finds that neutral Ti and Cr form stable stoichiometries of MO2 (M =
Ti, Cr) which easily lose up to two O atoms, while neutral Fe and Ni primarily form MO
(M = Fe, Ni) geometries with suboxides also produced. TiO2 clusters possess excited
state lifetimes that increase with additional cluster units to ~600 fs, owing to a larger
delocalization of excited charge carriers with cluster size. CrO2 clusters show a unique
inversed metallic behavior with O content, where the fast (~30 fs) metallic relaxation
component associated with electron scattering increases with higher O content, connected
to the percent of ligand-to-metal charge transfer (LMCT) character and higher density of
states. FeO clusters show a decreased lifetime with size, reaching a plateau of ~150 fs at
the size of (FeO)5 related to the density of states as clusters form 3D geometries. Finally,
neutral (NiO)n clusters all have similar fast lifetimes (~110 fs), with suboxides possessing
unexpected electronic transitions involving s-orbitals, increasing excited state lifetimes
up to 80% and causing long-lived states lasting over 2.5 ps. Similarities are drawn
between each cluster system, providing valuable information about each metal oxide
species and the evolution of excited state dynamics as a result of the occupied d-shell.
The work presented within this thesis will lead to novel materials of increased reactivity
while facilitating a deeper fundamental understanding on the effect of electron
interactions on chemical properties.
ContributorsGarcia, Jacob M. (Author) / Sayres, Scott G (Thesis advisor) / Yarger, Jeffery (Committee member) / Steimle, Timothy (Committee member) / Arizona State University (Publisher)
Created2021
Description
Molecular structures and dynamics in amorphous materials present unique experimental challenges compared with the characterization of crystalline solids. Liquids and glassy solids have many applications in industry such as ionic liquids for fuel cells or biomolecule stabilizing agents, enhancing pharmaceuticals dissolution rates, and modified high performance biopolymers like silk for textile, biomedical, drug delivery, among many others. Amorphous materials are metastable, with kinetic profiles of phase transitions depending on relaxation dynamics, thermal history, plus factors such as temperature, pressure, and humidity. Understanding molecular structure and phase transitions of amorphous states of small molecules and biopolymers is broadly important for realizing their applications. The structure of liquid and glassy states of the drugs carbamazepine (CBZ) and indomethacin (IMC) were studied with solid-state nuclear magnetic resonance (ssNMR) spectroscopy, high energy X-ray diffraction, Fourier Infrared Transform Spectroscopy (FTIR), differential scanning calorimetry (DSC), and Empirical Potential Structure Refinement (EPSR). Both drugs have multiple crystalline polymorphs with slow dissolution kinetics, necessitating stable glassy or polymer dispersed formulations. More hydrogen bonds per CBZ molecule and a larger distribution of oligomeric states in the glass versus the liquid than expected. The chlorobenzyl ring of crystalline and glassy IMC measured with ssNMR were surprisingly found to have similar mobility. Crucially, humidity strongly affects glass structure, highlighting the importance of combining modeling techniques like EPSR with careful sample preparation for proper interpretation. Highly basic protic ionic liquids with low ∆pKa were synthesized with metathesis rather than proton transfer and characterized using NMR and dielectric spectroscopy. Finally, the protein secondary structure of spider egg sac silk was studied using ssNMR, FTIR, and scanning electron microscopy. Tubuliform silk found in spider egg sacs has extensive β-sheet domains which form nanocrystallites within an amorphous matrix. Structural predictions and spectroscopic measurements of tubuliform silk solution are mostly α-helical, with the mechanism of structural rearrangement to the β-sheet rich fiber unknown. The movement of spiders during egg silk spinning make in situ experiments difficult practically. This work is the first observation that tubuliform silk of Argiope aurantia after liquid crystalline spinning exits the spinneret as a predominantly (~70%) β-sheet fiber.
ContributorsEdwards, Angela Diane (Author) / Yarger, Jeffery L (Thesis advisor) / Liu, Yan (Committee member) / Mujica, Vladimiro (Committee member) / Arizona State University (Publisher)
Created2022
Description
As the simplest carboxylic acid, formic acid (FA) is ubiquitous to Earth’s atmosphere, helping seed cloud nucleation and leading to acid rain. By studying the interactions between FA and high intensity light under high vacuum, conditions similar to the upper atmosphere, on other planets (either in the solar system or beyond), and even in interstellar media are emulated. These results were produced from a home built vacuum chamber system, with a Wiley-McLaren time of flight mass spectrometer and using femtosecond (fs) laser pulses. The laser characteristics were as follows: a pulse width >35 fs, center wavelength of 400 nm (probe pulse was 800 nm for the pump-probe investigation), and laser intensities at ~1015 W/cm2.At high laser intensities, the first direct experimental evidence of CO3+ was recorded from the Coulomb explosion (CE) of the formic acid dimer (FAD) from a molecular beam. Theoretical calculations provided further evidence for the formation of CO3+ from the vertical ionization of FAD. When (FA)n(H2O)mH+ clusters (n = 1-7 and m = 0-1) were exposed to similar laser intensities, the larger clusters (n = 5-7) favored complete atomization from CE, indicating that the repulsive forces within the clusters at those sizes was too great to withstand to form CO3+. The protonated nature of the clusters and the peak shapes recorded in the mass spectra suggested that neutral (FA)n+ clusters undergo a dissociation mechanism within the extraction region. A novel technique was created to calculate these dissociation times on the order of 100s of nanoseconds (ns), increasing by ~10 ns for each additional FA molecule. Using pump-probe spectroscopy, it was observed similarly that neutral (FA)n clusters with n > 1, showed evidence of ion pair formation of the form [(FA)nH+·OOCH-] on the sub-picosecond timescale, increasing by 70 fs per FA molecule. Both trends indicate that the neutral clusters prefer to form compact 3d structures, but after photoexcitation the clusters have competing pathways to ionization, either through multiphoton ionization (ns dynamics) or ion pair formation (fs dynamics) that inevitably lead to the expansion and subsequent rearrangement into linear chains for the protonated cluster.
ContributorsSutton, Shaun (Author) / Sayres, Scott G. (Thesis advisor) / Richert, Ranko (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2023
Description
The movement of energy within a material is at the heart of numerous fundamental properties of chemistry and physics. Studying the process of photo-absorption in real time provides key insights into how energy is captured, stabilized, and dissipated within a material. The work presented in this thesis uses ultrafast time-of-flight mass spectrometry and computational modeling to observe and understand the properties of photo-excited states within molecules and clusters. Experimental results provide direct measurement of excited state lifetimes, while computational modeling provides a more thorough understanding of the movement of energy within an excited state. Excited state dynamics in covalent molecules such as n-butyl bromide (C4H9Br), presented in Chapter 4, demonstrate the significance of IVR of photo-excited states. Exciting to the high energy Rydberg manifold leads to predissociation into fragments of various lengths and degrees of saturation but the predissociation process is disrupted by energy redistribution into hot vibrational states. Experimental lifetimes show that IVR occurs over rapidly (~ 600 fs) leaving less energy for bond dissociation. Additionally, a long-lived feature in the dynamics of C4H9+ shows evidence of ion-pair formation – a known phenomenon which creates a stable A+/B- pair separated by several angstroms and occurring at energies lower than direct ionization. The results of this research show the dynamics of energy transfer into bond fragmentation, kinetic energy, and vibrational motion.
Metal-oxide clusters are unique materials which are representative of bulk materials but with quantized excited states instead of bands and as such can be used as atomically precise analogs to semiconducting materials. Excited state lifetimes and theoretical descriptors of electron-hole interactions of titanium oxide clusters, presented in Chapter 5, shows the significance of structure and oxidation of charge-transfer materials. Modeling the excited states of the photo-generated electrons and holes provides a window into the dynamics of charge-transfer and electron-hole separation and recombination in bulk materials. Furthermore, changes in the oxidation of the cluster have a dramatic impact on the nature of excited states and overall cluster properties. Such changes are analogous to oxygen defects in bulk materials and are critical for understanding reaction chemistry at defect sites.
ContributorsHeald, Lauren (Author) / Sayres, Scott G (Thesis advisor) / Seo, Dong-Kyun (Committee member) / Mujica, Vladimiro (Committee member) / Arizona State University (Publisher)
Created2022
Description
One strategic objective of the National Aeronautics and Space Administration (NASA) is to find life on distant worlds. Current and future missions either space telescopes or Earth-based observatories are frequently used to collect information through the detection of photons from exoplanet atmospheres. The primary challenge is to fully understand the nature of these exo-atmospheres. To this end, atmospheric modeling and sophisticated data analysis techniques are playing a key role in understanding the emission and transmission spectra of exoplanet atmospheres. Of critical importance to the interpretation of such data are the opacities (or absorption cross-sections) of key molecules and atoms. During my Doctor of Philosophy years, the central focus of my projects was assessing and leveraging these opacity data. I executed this task with three separate projects: 1) laboratory spectroscopic measurement of the infrared spectra of CH4 in H2 perturbing gas in order to extract pressure-broadening and pressure-shifts that are required to accurately model the chemical composition of exoplanet atmospheres; 2) computing the H2O opacity data using ab initio line list for pressure and temperature ranges of 10^-6–300 bar and 400–1500 K, and then utilizing these H2O data in radiative transfer models to generate transmission and emission exoplanetary spectra; and 3) assessing the impact of line positions in different H2O opacities on the interpretation of ground-based observational exoplanetary data through the cross-correlation technique.
ContributorsGharib-Nezhad, Ehsan (Author) / Line, Michael R. (Thesis advisor) / Lyons, James R. (Thesis advisor) / Sayres, Scott G. (Committee member) / Heyden, Bjorn Matthias (Committee member) / Arizona State University (Publisher)
Created2019
Description
Soft materials are matters that can easily deform from their original shapes and structures under thermal or mechanical stresses, and they range across various groups of materials including liquids, foams, gels, colloids, polymers, and biological substances. Although soft materials already have numerous applications with each of their unique characteristics, integrating materials to achieve complementary functionalities is still a growing need for designing advanced applications of complex requirements. This dissertation explores a unique approach of utilizing intermolecular interactions to accomplish not only the multifunctionality from combined materials but also their tailored properties designed for specific tasks. In this work, multifunctional soft materials are explored in two particular directions, ionic liquids (ILs)-based mixtures and interpenetrating polymer network (IPN).
First, ILs-based mixtures were studied to develop liquid electrolytes for molecular electronic transducers (MET) in planetary exploration. For space missions, it is challenging to operate any liquid electrolytes in an extremely low-temperature environment. By tuning intermolecular interactions, the results demonstrated a facile method that has successfully overcome the thermal and transport barriers of ILs-based mixtures at extremely low temperatures. Incorporation of both aqueous and organic solvents in ILs-based electrolyte systems with varying types of intermolecular interactions are investigated, respectively, to yield optimized material properties supporting not only MET sensors but also other electrochemical devices with iodide/triiodide redox couple targeting low temperatures.
Second, an environmentally responsive hydrogel was synthesized via interpenetrating two crosslinked polymer networks. The intermolecular interactions facilitated by such an IPN structure enables not only an upper critical solution temperature (UCST) transition but also a mechanical enhancement of the hydrogel. The incorporation of functional units validates a positive swelling response to visible light and also further improves the mechanical properties. This studied IPN system can serve as a promising route in developing “smart” hydrogels utilizing visible light as a simple, inexpensive, and remotely controllable stimulus.
Over two directions across from ILs to polymeric networks, this work demonstrates an effective strategy of utilizing intermolecular interactions to not only develop multifunctional soft materials for advanced applications but also discover new properties beyond their original boundaries.
First, ILs-based mixtures were studied to develop liquid electrolytes for molecular electronic transducers (MET) in planetary exploration. For space missions, it is challenging to operate any liquid electrolytes in an extremely low-temperature environment. By tuning intermolecular interactions, the results demonstrated a facile method that has successfully overcome the thermal and transport barriers of ILs-based mixtures at extremely low temperatures. Incorporation of both aqueous and organic solvents in ILs-based electrolyte systems with varying types of intermolecular interactions are investigated, respectively, to yield optimized material properties supporting not only MET sensors but also other electrochemical devices with iodide/triiodide redox couple targeting low temperatures.
Second, an environmentally responsive hydrogel was synthesized via interpenetrating two crosslinked polymer networks. The intermolecular interactions facilitated by such an IPN structure enables not only an upper critical solution temperature (UCST) transition but also a mechanical enhancement of the hydrogel. The incorporation of functional units validates a positive swelling response to visible light and also further improves the mechanical properties. This studied IPN system can serve as a promising route in developing “smart” hydrogels utilizing visible light as a simple, inexpensive, and remotely controllable stimulus.
Over two directions across from ILs to polymeric networks, this work demonstrates an effective strategy of utilizing intermolecular interactions to not only develop multifunctional soft materials for advanced applications but also discover new properties beyond their original boundaries.
ContributorsXu, Yifei (Author) / Dai, Lenore L. (Thesis advisor) / Forzani, Erica (Committee member) / Holloway, Julianne (Committee member) / Jiang, Hanqing (Committee member) / Zhuang, Houlong (Committee member) / Arizona State University (Publisher)
Created2020