In this thesis, we present our findings on two major topics, both of which are studies of molecules on metal surfaces by scanning tunneling microscopy (STM). The first topic is on adsorption of a model amine compound, 1,4-benzenediamine (BDA), on the reconstructed Au(111) surface, chosen for its potential application as a molecular electronic device. The molecules were deposited in the gas phase onto the substrate in the vacuum chamber. Five different patterns of BDA molecules on the surface at different coverages, and the preferred adsorption sites of BDA molecules on reconstructed Au(111) surface, were observed. In addition, BDA molecules were susceptible to tip-induced movement, suggesting that BDA molecules on metal surfaces can be a potential candidate in STM molecular manipulations. We also studied graphene nanoislands on Co(0001) in the hope of understanding interaction of expitaxially grown graphene and metal substrates. This topic can shed a light on the potential application of graphene as an electronic device, especially in spintronics. The graphene nanoislands were formed by annealing contorted hexabenzocoronene (HBC) on the Co(0001) surface. In our experiments, we have determined atop registry of graphene atoms with respect to the underlying Co surface. We also investigated the low-energy electronic structures of graphene nanoislands by scanning tunneling spectroscopy. The result was compared with a first-principle calculation using density functional theory (DFT) which suggested strong coupling between graphene pi-bands and cobalt d-electrons. We also observed that the islands exhibit zigzag edges, which exhibits unique electronic structures compared with the center areas of the islands.

The physical properties of materials at the nanoscale are controlled to a large extent by their interfaces. While much knowledge has been acquired about the properties of material in the bulk, there are many new and interesting phenomena at the interfaces that remain to be better understood. This is especially true at the scale of their constituent building blocks - atoms and molecules. Studying materials at this intricate level is a necessity at this point in time because electronic devices are rapidly approaching the limits of what was once thought possible, both in terms of their miniaturization as well as our ability to design their behavior. In this thesis I present our explorations of the interplay between mechanical properties, electronic transport and binding energetics of single atomic contacts and single-molecule junctions. Experimentally, we use a customized conducting atomic force microscope (AFM) that simultaneously measures the current and force across atomic-scale junctions. We use this instrument to study single atomic contacts of gold and silver and single-molecule junctions formed in the gap between two gold metallic point contacts, with molecules with a variety of backbones and chemical linker groups. Combined with density functional theory based simulations and analytical modeling, these experiments provide insight into the correlations between mechanics and electronic structure at the atomic level. In carrying out these experimental studies, we repeatedly form and pull apart nanoscale junctions between a metallized AFM cantilever tip and a metal-coated substrate. The force and conductance of the contact are simultaneously measured as each junction evolves through a series of atomic-scale rearrangements and bond rupture events, frequently resulting in single atomic contacts before rupturing completely. The AFM is particularly optimized to achieve high force resolution with stiff probes that are necessary to create and measure forces across atomic-size junctions that are otherwise difficult to fabricate using conventional lithographic techniques. In addition to the instrumentation, we have developed new algorithmic routines to perform statistical analyses of force data, with varying degrees of reliance on the conductance signatures. The key results presented in this thesis include our measurements with gold metallic contacts, through which we are able to rigorously characterize the stiffness and maximum forces sustained by gold single atomic contacts and many different gold-molecule-gold single-molecule junctions. In our experiments with silver metallic contacts we use statistical correlations in conductance to distinguish between pristine and oxygen-contaminated silver single atomic contacts. This allows us to separately obtain mechanical information for each of these structural motifs. The independently measured force data also provides new insights about atomic-scale junctions that are not possible to obtain through conductance measurements alone. Using a systematically designed set of molecules, we are able to demonstrate that quantum interference is not quenched in single-molecule junctions even at room temperature and ambient conditions. We have also been successful in conducting one of the first quantitative measurements of van der Waals forces at the metal-molecule interface at the single-molecule level. Finally, towards the end of this thesis, we present a general analytical framework to quantitatively reconstruct the binding energy curves of atomic-scale junctions directly from experiments, thereby unifying all of our mechanical measurements. I conclude with a summary of the work presented in this thesis, and an outlook for potential future studies that could be guided by this work.

This thesis studies electronic properties of molecular devices in the limiting cases of strong and weak electrode-molecule coupling. In these two limits, we use the complementary techniques of X-Ray spectroscopy and Scanning Tunneling Microscopy (STM) to understand the mechanisms for electrode-molecule bond formation, the energy level realignment due to metal-molecule bonds, the effect of coupling strength on single-molecule conductance in low-bias measurements, and the effect of coupling on transport under high-bias. We also introduce molecular designs with inherent asymmetries, and develop an analytical method to determine the effect of these features on high-bias conductance. This understanding of the role of electrode-molecule coupling in high-bias regimes enables us to develop a series of functional electronic devices whose properties can be predictably tuned through chemical design. First, we explore the weak electrode-molecule coupling regime by studing the interaction of two types of paracyclophane derivates that are coupled `through-space' to underlying gold substrates. The two paracyclophane derivatives differ in the strength of their intramolecular through-space coupling. X-Ray photoemission spectroscopy (XPS) and Near-Edge X-ray Absorbance Fine Structure (NEXAFS) spectroscopy allows us to determine the orientation of both molecules; Resonant Photoemission Spectroscopy (RPES) then allows us to measure charge transfer time from molecule to metal for both molecules. This study provides a quantititative measure of charge transfer time as a function of through-space coupling strength. Next we use this understanding in STM based single-molecule current-voltage measurements of a series of molecules that couple through-space to one electrode, and through-bond to the other. We find that in the high-bias regime, these molecules respond differently depending on the direction of the applied field. This asymmetric response to electric field direction results in diode-like behavior. We vary the length of these asymmetrically coupled molecules, and find that we can increase the rectifying characteristics of these molecules by increasing length. Next, we explore the strong-coupling regime with an X-Ray spectroscopy study of the formation of covalent gold-carbon bonds using benzyltrimethyltin molecules on gold surfaces in ultra high vacuum conditions. Through X-ray Photoemission Spectroscopy (XPS) and X-ray absorption measurements, we find that the molecule fragments at the Sn-Benzyl bond when exposed to gold and the resulting benzyl species only forms covalent Au-C bonds on less coordinated Au surfaces like Au(110). We also find spectroscopic evidence for a gap state localized on the Au-C bond that results from the covalent nature of the bond. Finally, we use Density Functional Theory based Nudged Elastic Band methods to find reaction pathways and energy barriers for this reaction. We use our knowledge of the electronic structure of these bonds to create single-molecule junctions containing Au-C bonds in STM-based break junction experiments. In analogy with our approach for the weakly coupled `through-space' systems, we study the high-bias current-voltage characteristics of molecules with one strong Au-C bond, and one weaker donor-acceptor bond. These experiments reveal that the `gap state' created due to the covalent nature of the Au-C bond remains essentially pinned to the Fermi level of its corresponding electrode, and that most of the electric potential drop in the junction occurs on the donor-acceptor bond; as a result, these molecules behave like rectifiers. We use this principle to create a series of three molecular rectifiers, and show that the unique properties of the Au-C bond allow us to easily tune the rectification ratio by modifying a single electronic parameter. We then explore the process of molecular self-assembly to create organic electronic structures on metal surfaces. Specifically, we study the formation of graphene nanoribbons using a brominated precursor deposited on Au(111) surface in ultra high vacuum. We find that the halogen atoms cleave from the precursors at surprisingly low temperatures of <100C, and find that the resulting radicals bind to Au, forming Au-C and Au-Br bonds. We show that the Br desorbs at relatively low temperatures of <250C, and that polymerization of the precursor molecules to form nanoribbons proceeds only after the debrominization of the surface. Finally, with Angle-Resolved Photoemission and Density Functional Theory calculations, we quantify the interaction strength of the resulting nanoribbons with the underlying gold substrate. Taken together, the results presented in this thesis offer a mechanistic understanding of the formation of electrode-molecule bonds, and also an insight into the high-bias behavior of molecular junctions as a function of electrode-molecule coupling. In addition, our work in developing tunable, functional electronic devices serves as a framework for future technological advances towards molecule-based computation.

Metal-molecule-metal junctions have become a widely used test-bed for the study of nanoscale electronic phenomena. Single-molecule junctions in particular have provided a deeper understanding of charge transport across interfaces, and single-molecule electronic components have been proposed as a successor for silicon technology. This thesis presents an experimental approach for controlling the electronic properties of single-molecule junctions by manipulating the environment about the junction. With this tunable functionality, we are able to demonstrate single-molecule variants of transistors and diodes. We begin our work by probing charge transport through single-oligomers of commonly used molecules in organic electronic devices. We focus on these systems due to their narrow band gaps, giving them the potential for exhibiting high molecular conductances. Single-molecule junctions are formed using the Scanning Tunneling Microscope-based break junction (STM-BJ) technique. We first consider a family of oligothiophenes, ranging in length from 1 to 6 units. We find that this family of molecules exhibits an anomalous conductance decay with molecular length; this is mainly due to conformational effects. These conformational effects also result in very broad conductance distributions, further preventing oligothiophenes from being useful in molecular electronic devices. However, we find that thiophene dioxides are particularly well-suited for single-molecule devices, primarily due to exceptionally narrow band gaps. Oligothiophene dioxides also constitute a unique system where the dominant conductance orbital changes with molecular length. Specifically, we find that the shorter oligomers have transport dominated by the highest occupied molecular orbital (hole-type transport), but longer oligomers have transport dominated by the lowest unoccupied molecular orbital (electron-type transport). We next demonstrate a method for gating single-molecule junctions. In order to over- come the difficulty of lithographically defining a gate electrode in close enough proximity to the molecular junction so that the gate voltage impacts the electrostatics of the junction, we turn to measurements in electrolytic solutions. Ions in these solutions form compact layers of charge at metal surfaces, and these electric double layers can be controlled by the gate electrode; such electrolytic gating results in high gating efficiencies. Using this technique, we show that we are able to continuously modulate the conductance of non-redox active molecular junctions. Using ionic environments, we next develop a new technique for creating a single-molecule diode. Performing break junction measurements in electrolytic solutions without the presence of a gate electrode, we show that we still have control of the junction’s electrostatic environment. In particular, if the source and drain electrodes are of considerably different areas, we find that we asymmetrically control this environment. Using this technique, we demonstrate single-molecule diodes created from otherwise symmetric molecular junctions. Combining this with measurements on thiophene dioxide oligomers, we show single-molecule diodes with the highest reported rectification ratios to date. This technique has the potential for application in nano-scale systems beyond single-molecule junctions. These results constitute another step toward the development of single-molecule devices with commercial applications. Finally, the methods presented in this thesis offer further insights into the electronic structure of molecular junctions. We show that we can assess energy-level alignment at metal molecule interfaces– this alignment is a crucial parameter controlling the proper- ties of the interface. We also demonstrate that we can probe large regions ( 2eV) of the transmission function which governs charge transport through the junction. By being able to control level alignment, we are also able to offer preliminary studies on single-molecule junctions in the resonant transport regime. Combined, the results presented in this thesis grant new insights into electron transport at the nanoscale and provide new routes for the development of functional single-molecule devices.

<p>The prevalence of nanoparticles in the environment is expected to grow in the coming years due to their increasing pervasiveness in consumer and industrial applications. Once released into the environment, nanoparticles encounter conditions of pH, salinity, UV light, and other solution conditions that may alter their surface characteristics and lead to aggregation. The unique properties that make nanoparticles desirable are a direct consequence of their size and increased surface area. Therefore, it is critical to recognize how aggregation alters the reactive properties of nanomaterials, if we wish to understand how these properties are going to behave once released into the environment. The size and structure of nanoparticle aggregates depend on surrounding conditions, including hydrodynamic ones. Depending on these conditions, aggregates can be large or small, tightly packed or loosely bound. Characterizing and measuring these changes to aggregate morphology is important to understanding the impact of aggregation on nanoparticle reactive properties. Examples of decreased reactivity due to aggregation include the case where tightly packed aggregates have fewer available surface sites compared to loosely packed ones; also, photocatalytic particles embedded in the center of large aggregates will experience less light when compared to particles embedded in small aggregates. However, aggregation also results in an increase in solid-solid interfaces between nanoparticles. This can result in increased energy transfer between neighboring particles, surface passivation, and altered surface tension. These phenomena can lead to an increase in reactivity. The goal of this thesis is to examine the impacts of aggregation on the reactivity of a select group of nanomaterials. Additionally, we examined how aggregation impacts the removal efficiency of fullerene nanoparticles using membrane filtration.</p><p>The materials we selected to study include ZnS - a metal chalcogenide nanoparticle that photoluminesces after exposure to UV; TiO2 and ZnO nanoparticles - photocatalytic nanoparticles that generate reactive oxygen species upon UV irradition; and, fullerene nanoparticles used in the filtration experiments, selected for their potential use, small size, and surface chemistry. Our primary methods used to characterize particle and aggregate characteristics include dynamic light scattering used to describe particle size, static light scattering used to characterize aggregate structure (fractal dimension), transmission electron microscopy used to verify primary particle sizes, and electrophoretic mobility measurements to evaluate suspension stability. The reactive property of ZnS that was measured as a function of aggregation was photoluminescence, which was measured using a spectrofluorometer. The reactive property of TiO2 and ZnO that was studied was their ability to generate hydroxyl radicals; these were measured by employing a fluorescent probe that becomes luminescent upon interaction with the hydroxyl radical. To detect the presence of fullerene nanoparticles and calculate removal efficiencies, we used total organic carbon measurements. Additionally, we used UV-vis spectroscopy to approximate the impact of particle shadowing in TiO2 and ZnO aggregates, and Fourier transformed infrared spectroscopy to determine how different electrolytes interact with fullerene surface groups.</p><p>Our findings indicate that the impact of aggregation on nanoparticle reactivity is material specific. ZnS nanoparticles exhibit a 2-fold increase in band-edge photoluminescence alongside a significant decrease in defect-site photoluminescence. This is attributed to aggregate size-dependent surface tension. Additionally, we used photoluminescence measurements to develop a new method for calculating the critical coagulation concentration of a nanoparticle suspension. </p><p>The ability of both TiO2 and ZnO to generate hydroxyl radicals was significantly hampered by aggregation. The decline in hydroxyl radical generation could be attributed to two key parameters. First, increased aggregate size was associated with increased particle shadowing, as determined from the observed decrease in the rate of optically induced transitions. Secondly, aggregate structure was associated both with increased shadowing (denser aggregates exhibited more shadowing than similarly sized loose aggregates), and with an increase in radical quenching on neighboring particle surfaces in an aggregate. </p><p>Aggregation had a positive impact on hydroxylated fullerene membrane separation, increasing removal efficiency to around 80%, regardless of transmembrane pressure. However, the type of electrolyte used determined whether aggregation was successful at increasing removal. Divalent ions, capable of forming strong covalent bonds with surface oxygen groups, increased removal efficiency and made it pressure insensitive. In contrast, monovalent ions increased removal efficiency slightly, but maintained the pressure dependence of the removal efficiency. Evidence is presented to support the hypothesis that divalently aggregated hydroxylated fullerenes deform under increased pressure and partially penetrate the membrane.</p><p>Finally, nanoparticle reactive properties depend on the primary particle aggregation state. Both size and structure are key factors when evaluating nanomaterial reactivity under aggregation-inducing conditions. However, the impact of aggregation is not easily predicted. Some materials exhibit a decreased reactivity while others experience an increase. Therefore, the impact of aggregation on nanoparticle reactive properties must be evaluated on a material-by-material basis, while considering all of the particle and aggregate characteristics as well as environmental ones.</p> / Dissertation

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Thesis ( Ph.D.) -- University of Texas at Arlington, 2008.

<p>The overall purpose of this work is to address several of the roadblocks to use of thermoelectric materials for generation of electricity, namely inefficient processing of materials and low performance, commonly rated by the figure of merit, ZT=T?2?/?tot. The ZT includes ? as the Seebeck coefficient, ? as electrical resistivity, T as the average temperature, and ?tot as total thermal conductivity. ?tot is the sum of electronic charge carrier (?C) and lattice (?L) contributions to thermal conductivity. Attempts to increase ZT in the literature to values >1 have focused on decreasing the thermal conductivity via nanostructuring or optimizing the electrical conductivity and Seebeck coefficient by doping. In this work, two separate approaches are taken to tackle these issues: (1) Target higher ZT by assembling lead telluride (PbTe) nanoparticles from a multi-gram synthesis utilizing ligand stripping techniques or deliberately including discrete lead sulfide (PbS) NCs. (2) Develop a rapid, convenient synthesis of tetrahedrite (Cu12Sb4S13). Approach (1): Nanostructuring of PbTe and PbTe?PbS. Nanostructured PbTe and nanocomposites of PbTe?PbS are hypothesized to increase ZT by lowering thermal conductivity, while ligand stripping of PbTe NCs by sulfide or iodide is expected to increase ZT because it has been demonstrated to increase electrical conductivity in thin films of PbS. A new synthesis is in demand because mixing PbTe and PbS NCs requires that the PbTe be dispersible, and literature syntheses of such NCs suffer from small yields (<200 mg). Thus, applications of dispersible PbTe NCs are largely limited to thin films. The ZT values of these thin films are not reported due to difficulty in quantifying thermal conductivity. In the dissertation research, nanostructured PbTe pellets are prepared by hot-pressing PbTe NCs after either mixing with PbS NCs by incipient wetness, or ligand stripping with sulfide salt, iodide salt, or both. The PbTe NCs themselves are prepared in multi-gram quantities by hot-injection methods in solution. The NCs are characterized for crystallinity by powder X-ray Diffraction (XRD). The size and morphology of the NCs are probed via Transmission Electron Microscopy (TEM), and their composition is determined by Energy Dispersive Spectroscopy (EDS). The thermoelectric properties are studied on hot-pressed pellets of each sample. Approach (2): Developing a facile route to tetrahedrite and doped derivatives. Tetrahedrite is exciting the thermoelectric community due to its lack of rare or toxic elements, the tunability of its electronic properties by doping, the ability to dope by ball-milling with the plentiful natural mineral, and the ability to achieve a ZT of unity. However, the natural mineral is unsuitable on its own due to an excess of natural dopant, and reported tetrahedrite syntheses require heating at high temperature 650 ?C in a three day process followed by two weeks of heating at 450 ?C. This work establishes a new synthesis amenable to industrial production that reduces the heating time from over 2 weeks to 2 days for simultaneous batch production at moderate temperature (155 ?C for one day and 430 ?C for 30 min, cooling naturally). The tetrahedrite powder is prepared from chloride-free metal salts and thiourea by solvothermal methods and characterized by XRD for crystallinity. The composition is determined by Inductively Coupled Plasma analysis. Products from multiple batches are mixed by ball-milling alone or combined with the natural mineral as a means to dope with Zn2+ as a solid solution. The resulting powder is then hot-pressed to pellet form for thermoelectric characterization. The tetrahedrite is also doped in-situ by zinc over a range of 0.79 to 1.40 mol equivalents using chloride-free metal salts.

<p> I discuss the design, fabrication and measurement at millikelvin-temperatures of Al/AlO<i>_x</i>/Al Josephson junction-based transmon qubits coupled to superconducting thin-film lumped element microwave resonators made of aluminum on sapphire. The resonators had a center frequency of around 6GHz, and a total quality factor ranging from 15,000 to 70,000 for the various devices. The area of the transmon junctions was about 150 nm &times; 150 nm and with Josephson energy <i>E_J</i> such that 10GHz &le; <i>E_J</i> &le; 30 GHz. The charging energy of the transmons arising mostly from the large interdigital shunt capacitance, was <i>E_c/h</i> &ap; 300MHz. </p><p> I present microwave spectroscopy of the devices in the strongly dispersive regime of circuit quantum electrodynamics. In this limit the ac Stark shift due to a single photon in the resonator is greater than the linewidth of the qubit transition. When the resonator is driven coherently using a coupler tone, the transmon spectrum reveals individual "photon number'' peaks, each corresponding to a single additional photon in the resonator. Using a weighted average of the peak heights in the qubit spectrum, I calculated the average number of photons <i>n&macr;</i> in the resonator. I also observed a nonlinear variation of <i>n&macr;</i> with the applied power of the coupler tone <i>P_rf</i>. I studied this nonlinearity using numerical simulations and found good qualitative agreement with data. </p><p> In the absence of a coherent drive on the resonator, a thermal population of 5.474 GHz photons in the resonator, at an effective temperature of 120 mK resulted in a weak <i>n</i> = 1 thermal photon peak in the qubit spectrum. In the presence of independent coupler and probe tones, the <i> n</i> = 1 thermal photon peak revealed an Autler-Townes splitting. The observed effect was explained accurately using the four lowest levels of the dispersively dressed Jaynes-Cummings transmon-resonator system, and numerical simulations of the steady-state master equation for the coupled system. </p><p> I also present time-domain measurements on transmons coupled to lumped-element resonators. From <i>T</i>₁ and Rabi oscillation measurements, I found that my early transmon devices (called design LEv5) had lifetimes (<i>T</i>₁ &sim; 1 &mu;s) limited by strong coupling to the 50 &Omega; transmission line. This coupling was characterized by the the rate of change of the Rabi oscillation frequency with the change in the drive voltage (d<i>f_Rabi</i> /<i>dV</i>) &ndash; also termed the Rabi coupling to the drive. I studied the design of the transmon-resonator system using circuit analysis and microwave simulations with the aim being to reduce the Rabi coupling to the drive. By increasing the resonance frequency of the resonator &omega;<i>_r</i>/2&pi; from 5.4 GHz to 7.2 GHz, lowering the coupling of the resonator to the transmission line and thereby increasing the external quality factor <i>Q_e</i> from 20,000 to 70,000, and reducing the transmon-resonator coupling <i> g</i>/2&pi; from 70 MHz to 40 MHz, I reduced the Rabi coupling to the drive by an order of magnitude (&sim; factor of 20). The <i>T</i>₁ &sim; 4 &mu;s of devices in the new design (LEv6) was longer than that of the early devices, but still much shorter than the lifetimes predicted from Rabi coupling, suggesting the presence of alternative sources of noise causing qubit relaxation. Microwave simulations and circuit analysis in the presence of a dielectric loss tangent tan &delta; &sime; 5 &times; 10^-6 agree reasonably well with the measured <i>T</i>₁ values, suggesting that surface dielectric loss may be causing relaxation of transmons in the new designs.</p>

The vast majority of optical components and devices in use today can be grouped under the umbrella of ``bulk optics''; i.e. they generally have a non-negligible thickness compared to the wavelength of light. This is true of components from lenses to wave plates to Fabry-Perot etalons, all of which need sufficient thickness such that light waves can accumulate an appropriate amount of phase upon propagation through the structure. In this thesis, we develop and explore a variety of optical components that are thin compared to the wavelength of light and lie at the interface between two materials (i.e. a substrate and air). We explore approaches to filter, absorb, redirect, and re-shape light with flat, ultra-thin structures which are easy to fabricate with modern micro- and nanofabrication techniques. / Engineering and Applied Sciences