Browsing by Author "Natelson, Douglas"
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Item Bicrystals and Bowties: Photothermoelectric and plasmonic effects of gold nanoscale(2020-01-08) Evans, Charlotte Irene; Natelson, DouglasThe interaction of light with gold dramatically changes when the size of the gold devices is reduced to the nanoscale. The free electron gas in the metal can collectively oscillate with the electric field of the incident laser in an excitation called plasmons which can result in dramatic enhancements of the local electric field. Electronic transport and open circuit voltage (photothermoelectric) measurements can be powerful tools to characterize a gold nanoscale device heated by an incident laser. This laser can be raster-scanned to probe the response of the device as a function of laser position. The thermoelectric effect has been well-studied since its discovery by Seebeck in the 1820s. A resurgence of thermoelectric studies have been published recently manipulating the thermoelectric properties of thin metal film devices via nanostructuring. Substantial variation of the local Seebeck coefficient has recently been observed in polycrystalline nanowires of uniform thickness and width, a regime where the Seebeck coefficient is generally regarded as a constant value. A study of the photothermoelectric voltage signal of single crystalline wires and bicrystal devices, two single crystals in contact at an individual grain boundary, provides evidence that grain structure does not substantially contribute to the local Seebeck coefficient. Instead, the photovoltages of these single and bicrystal devices are well-correlated with local variations in strain as detected by electron back-scatter diffraction measurements and with variations in platinum impurity concentration. Therefore, the photovoltage measurements are demonstrated to have sensitivity to intrinsic variations in nanoscale devices that are otherwise difficult to detect using traditional electronic transport and imaging techniques. In nanoscale molecular junctions, the large enhancements of the local electric field due to local surface plasmon resonances allows for single-molecule Raman measurements, which probe the inelastic energy exchanges between incident photons and the vibrational modes of the molecule. These optical measurements can be combined with electronic transport measurements to detect the energy exchanges between the vibrational and electronic energy states in the molecule. However, the plasmonic resonances that allow for the single-molecule sensitivity also result in high local heating making these energy exchanges difficult to detect. We will discuss one potential way to mitigate the local heating: remote excitation of the nanojunction using propagating surface plasmon polaritons. Electronic transport measurements estimate the local temperature rise of the device due to direct and remote excitation. Large enhancements of the open circuit photovoltages in nanogaps due to hot electron photocurrents from non-radiative plasmon decay are observed. The open circuit photovoltages are then used to probe the interactions of the propagating surface plasmon polaritons with the local modes in the nanogap, demonstrating that the open circuit photovoltages and electronic transport methods are powerful tools to not only characterize nanoscale devices but also to probe the plasmonic properties of the devices.Item Charge Transport and Transfer at the Nanoscale Between Metals and Novel Conjugated Materials(2012-09-05) Worne, Jeffrey; Natelson, Douglas; Kelly, Kevin F.; Mittleman, Daniel M.Abstract Organic semiconductors (OSCs) and graphene are two classes of conjugated materials that hold promise to create flexible electronic displays, high speed transistors, and low-cost solar cells. Crucial to understanding the behavior of these materials is understanding the effects metallic contacts have on the local charge environment. Additionally, characterizing the charge carrier transport behavior within these materials sheds light on the physical mechanisms behind transport. The first part of this thesis examines the origin of the low-temperature, high electric field transport behavior of OSCs. Two chemically distinct OSCs are used, poly-3(hexylthiophene) (P3HT) and 6,13- bis(triisopropyl-silylethynyl) (TIPS) pentacene. Several models explaining the low-temperature behavior are presented, with one using the Tomonaga-Luttinger liquid (TLL) insulator-to-metal transition model and one using a field-emission hopping model. While the TLL model is only valid for 1-dimensional systems, it is shown to work for both P3HT (1D) and TIPS-pentacene (2D), suggesting the TLL model is not an appropriate description of these systems. Instead, a cross-over from thermally-activated hopping to field-emission hopping is shown to explain the data well. The second part of this thesis focuses on the interaction between gold and platinum contacts and graphene using suspended graphene over sub-100 nanometer channels. Contacts to graphene can strongly dominate charge transport and mobility as well as significantly modify the charge environment local to the contacts. Platinum electrodes are discovered to be strong dopants to graphene at short length scales while gold electrodes do not have the same effect. By increasing the separation distance between the electrodes, this discrepancy is shown to disappear, suggesting an upper limit on charge diffusion from the contacts. Finally, this thesis will discuss a novel technique to observe the high-frequency behavior in OSCs using two microwave sources and an organic transistor as a mixer. A theoretical model motivating this technique is presented which suggests the possibility of retrieving gigahertz charge transport phenomena at kilohertz detection frequencies. The current state of the project is presented and discrepancies between devices made with gold and platinum electrodes measured in the GHz regime are discussed.Item Chemical tuning of electrical and magnetic properties in the transition metal dichalcogenides(2019-04-19) Choe, Jesse; Morosan, Emilia; Kelly, Kevin; Natelson, DouglasTransition metal dichalcogenides are a diverse class of layered materials. Due to their quasi two-dimensional nature, they are a sandbox for investigating low dimensional physics, but can be doped in a variety of ways. Not only can substitutional doping occur on either the transition metal or chalcogen site, but intercalation between the layers can tune the system as well. Here I report on the results of three transition metal dichalcogenide systems with three drastically different results. In the Copper-Platinum-Selenium system, initial results suggest two new superconductors in the ternary phase diagram. Doping platinum into TiSe$_2$ results in an increase in the resistivity of several order of magnitude. Angle-resolved photo-emission spectroscopy shows that Platinum doping induces a pseudo gap in the system. Scanning tunneling microscopy measurements show domain wall formation. Power law fits to the resistivity suggests that the electrical transport is dominated by Luttinger liquid behavior. Finally, the intercalation of Iron into TiS$_2$ gives rise to several magnetic features. Large bowtie magnetoresistance arises showing an increase of up to 40\%. Hysteresis in magnetization shows sharp switching behavior coinciding with the bowtie in magnetoresistance. Ferromagnetic order is seen in conjunction with glassy behavior. These results are compared and contrasted to the results in Fe$_x$TiS$_2$. My results in these systems show that the depth and breadth of physical phenomena in the transition metal dichalcogenide system make it a fascinating system for investigating strongly correlated systems.Item Creating Strontium Rydberg Atoms(2013-05-28) Zhang, Xinyue; Dunning, F. B.; Killian, Thomas C.; Natelson, DouglasDipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.Item Development of a Terahertz Leaky-Wave Antenna using the Parallel- Plate Waveguide(2014-07-23) McKinney, Robert Warren; Mittleman, Daniel M.; Kelly, Kevin F; Natelson, DouglasBecause of a growing bandwidth problem within wireless communications, the terahertz (THz) spectrum is being investigated as a possible technology for short-range, high-bandwidth communications. For this reason, it is worth implementing known communications technologies within the radio frequency (RF) and microwave bands, such as antennas, in the terahertz band. One such technology is the leaky-wave antenna. Leaky-wave antennas have been in use within the RF and microwave bands since the 1940’s. The leaky-wave antenna is a travelling wave antenna in which a fast wave with a phase velocity greater than the speed of light, c, propagates through a waveguide. This fast wave is allowed to leak out of the waveguide via an opening along the length of the waveguide. A THz leaky-wave antenna is implemented using the TE1 mode of a parallel-plate waveguide (PPWG). Various plate separations are used during this project in order to show the leaky-wave effect for different dispersion relations. Using a commercial THz time domain spectroscopy (THz-TDS) system, the input of the waveguide is a broadband THz signal. The expected output from such an input would be dispersed in the frequency domain. This is particularly interesting because it would allow the leaky-wave antenna to act as a THz demultiplexer by separating a broadband signal into individual frequency components that vary with angle. Our measured experimental results show that the waveguide indeed produces a dispersed output matching the analytical result. The propagation angle of lower frequencies is closer to perpendicular to the waveguide, with the cutoff frequency of the PPWG at the normal. Higher frequencies are transmitted closer to the axis of the waveguide. Since the phase-matching condition for a leaky-wave antenna can work in either direction, this THz leaky-wave antenna can also receive radiation. Our results show that when operating in this orientation, the receiving angle matches the angle of transmission from the transmitter setup for each frequency. This again shows agreement with the analytical result froe leaky-wave antennas. Using the leaky-wave antenna in this manner, we see the potential for THz frequency domain multiplexing. Varying the plate separation of a PPWG changes the dispersion relation. Since the angle of leaky-wave propagation depends on the dispersion, by varying the plate separation, one can vary the angle of the leaky-wave along the length of the waveguide. We implement such a waveguide in order to focus a chosen frequency to a point. Simulations of the field intensity show that this is possible. By mapping out the field intensity for each design frequency, our results validate this concept by showing that the field focuses within the plane of propagation. To the best of our knowledge, this work shows for the first time that these types of antennas can be implemented within the THz spectrum in order to transmit and receive THz signals.Item Electrical and Optical Characterization of Molecular Nanojunctions(2011) Ward, Daniel R.; Natelson, DouglasElectrical conduction at the single molecule scale has been studied extensively with molecular nanojunctions. Measurements have revealed a wealth of interesting physics. I3owever; our understanding is hindered by a lack of methods for simultaneous local imaging or spectroscopy to determine the conformation and local environment of the molecule of interest. Optical molecular spectroscopies have made significant progress in recent years, with single molecule sensitivity achieved through the use of surface-enhanced spectroscopies. In particular surface-enhanced Raman spectroscopy (SERS) has been demonstrated to have single molecule sensitivity for specific plasmonic structures. Many unanswered quest ions remain about the SERS process, particularly the role of chemical enhancements of the Raman signal. The primary goal of the research presented here is to combine both electrical and optical characterization techniques to obtain a more complete picture of electrical conduction at the single or few molecule level. We have successfully demonstrated that nanojunctions are excellent SERS substrates with the ability to achieve single molecule sensitivity. This is a major accomplishment with practical applications in optical sensor design. We present a method for mass producing nanojunctions with SERS sensitivity optimized through computer modeling. We have demonstrated simultaneous optical and electrical measurements of molecular junctions with single molecule electrical and SERS sensitivity. Measurements show strong correlations between electrical conductance and changes to the SERS response of nanojunctions. These results allow for one of the most conclusive demonstrations of single molecule SERS to date. This measurement technique provides the framework for three additional studies discussed here as well as opening up the possibilities for numerous other experiments. One measurement examines heating in nanowires rather than nanojunctions. We observe that, the electromigration process used to turn Pt nanowires into nanojunctions heats the wires to temperatures in excess of 1000 K, indicating that thermal decomposition of molecules on the nanowire is a major problem. Another measurement studies optically driven currents in nanojunctions. The photocurrent is a result of rectification of the enhanced optical electric field in the nanogap. From low frequency electrical measurements we are able to infer the magnitude of the enhanced electric field, with inferred enhancements exceeding 1000. This work is significant to the field of plasmonics and shows the need for more complete quantum treatments of plasmonic structures. Finally we investigate electrical and optical heating in molecular nanojunctions. Our measurements show that molecular vibrations and conduction electrons in nano-junctions under electrical bias or laser illumination can be driven from equilibrium to temperatures greater than 600 K. We observe that individual vibrations are also not in thermal equilibrium with one another. Significant heating in the conduction electrons in the metal electrodes was observed which is not expected in the ballistic tunneling model for electrons in nanojunctions this indicates a need for a more completely energy dissipation theory for nanojunctions.Item Embargo Electrically driven plasmonic processes: hot carriers and strong coupling(2023-11-29) Zhu, Yunxuan; Natelson, DouglasPlasmonic modes confined to metallic nanostructures at the atomic and molecular scale push the boundaries of light-matter interactions. Within these extreme plasmonic structures of ultrathin nanogaps and tunnelling junctions, new physical phenomena arise when plasmon resonances couple to electronic, exitonic, or vibrational excitations, as well as the generation of non-radiative hot carriers. This thesis will summarize experimental and theoretical advances in the regime of extreme nanoplasmonics, with an emphasis on plasmon-induced hot carriers, strong plexitonic effects, and electrically driven processes at the molecular scale. We examine above-threshold light emission in electromigrated tunnel junctions, which is consistent with a suggested theoretical model describing hot-carrier dynamics driven by nonradiative decay of electrically excited localized plasmons. By progressively altering the tunneling conductance of an aluminum junction, we tune the dominant light emission mechanism through different possibilities for the first time, finding quantitative agreement with theory in each regime. Using plasmonic tunnel junctions as a platform supporting both electrically and optically excited localized surface plasmons, we report a much greater (over 1000× ) plasmonic light emission at upconverted photon energies under combined electro-optical excitation, compared with electrical or optical excitation separately. We use electroluminescence to probe plasmon-exciton coupling in hybrid structures each consisting of a nanoscale plasmonic tunnel junction and few-layer two-dimensional transition-metal dichalcogenide transferred onto the junction. The resulting hybrid states act as a novel dielectric environment to affect the radiative recombination of hot carriers in the plasmonic nanostructure. Further potential of this work and possible future research directions will also be discussed.Item Electron phase coherence in mesoscopic normal metal wires(2007) Trionfi, Aaron James; Natelson, DouglasCorrections to the classically predicted electrical conductivity in normal metals arise due to the quantum mechanical properties of the conduction electrons. These corrections provide multiple experimental tests of the conduction electrons' quantum phase coherence. I consider if independent measurements of the phase coherence via different corrections are quantitatively consistent, particularly in systems with spin-orbit or magnetic impurity scattering. More precisely, do independent quantum corrections to the classically predicted conductivity depend identically on the ubiquitous dephasing mechanisms in normal metals? I have inferred the coherence lengths from the weak localization magnetoresistance, magnetic field-dependence of time-dependent universal conductance fluctuations, and magnetic field-dependent universal conductance fluctuations, three observable quantum corrections, in quasi one- and two-dimensional AuPd wires and quasi-1D Ag and Au wires between 2 and 20 K. While the coherence lengths inferred from weak localization and time-dependent universal conductance fluctuations are in excellent quantitative agreement in AuPd, the strong quantitative agreement is apparently lost below a critical temperature in both Ag and Au. Such a disagreement is inconsistent with current theory and must be explained. I developed a hypothesis attributing the coherence length discrepancy seen in Ag and Au to a crossover from the saturated to unsaturated time-dependent conductance fluctuation regime. Two experimental tests were then employed to test this hypothesis. One test examined the effects of a changing spin-flip scattering rate in Au while the second examined how passivation of the two level systems responsible for time-dependent conductance fluctuations at the surface of a Au nanowire affects the inferred coherence lengths. The results of the two tests strongly indicate that the observed disagreement in Au (and likely Ag) is indeed due to a crossover from saturated to unsaturated time-dependent conductance fluctuations.Item Electron transport in ferromagnetic nanostructures(2008) Lee, Sungbae; Natelson, DouglasAs the size of a physical system decreases toward the nanoscale, quantum mechanical effects such as the discretization of energy levels and the interactions of the electronic spins become readily observable. To understand what happens within submicrometer scale samples is one of the goals of modern condensed matter physics. Electron transport phenomena drew a lot of attention over the past two decades or so, not only because quantum corrections to the classical transport theory, but also they allow us to probe deeply into the microscopic nature of the system put to test. Although a significant amount of research was done in the past and thus extended our understanding in this field, most of these works were concentrated on simpler examples. Electron transport in strongly correlated systems is still a field that needs to be explored more thoroughly. In fact, experimental works that have been done so far to characterize coherence physics in correlated systems such as ferromagnetic metals are far from conclusive. One reason ferromagnetic samples draw such attention is that there exist correlations that lead to excitations (e.g. spin waves, domain wall motions) not present in normal metals, and these new environmental degrees of freedom can have profound effects on decoherence processes. In this thesis, three different types of magnetic samples were examined: a band ferromagnetism based metallic ferromagnet, permalloy, a III-V diluted ferromagnetic semiconductor with ferromagnetism from a hole-mediated exchange interaction, and magnetite nanocrystals and films. The first observation of time-dependent universal conductance fluctuations (TD-UCF) in permalloy is presented and our observations lead to three major conclusions. First, the cooperon contribution to the conductance is suppressed in this material. This is consistent with some theoretical expectations, and implies that weak localization will be suppressed as well. Second, we see evidence that domain wall motion leads to enhanced conductance fluctuations, demonstrating experimentally that domain walls can act as coherent scatterers of electrons. Third, the temperature dependence of the fluctuations is surprisingly strong, suggesting that the dominant decoherence mechanism in these wires is different than that in similar normal metal nanostructures. The first observation of TD-UCF in diluted magnetic semiconductors (DMS) is also presented. In contrast to analogous measurements on permalloy samples, we find a surprising suppression of TD-UCF noise in this material at low temperatures, independent of field orientation. We believe this implies that the suppression is not due to an orbital effect, and therefore some of the fluctuations originate with time-varying magnetic disorder. The temperature dependence of the TD-UCF implies either an unusual fluctuator spectrum or a nonstandard dephasing mechanism. Measurements of UCF as a function of magnetic field allow an order of magnitude estimate of the coherence length at 2 K of approximately 50 nm in this material. The last samples examined were magnetite nanocrystals and films. Magnetite has been used in technologies for millennia, from compasses to magnetoelectronic devices, although its electronic structure has remained controversial for seven decades, with a low temperature insulator and a high temperature "bad metal" separated by the Verwey transition at 120 K. A new electrically driven insulator-metal transition below the Verwey temperature in both magnetite films and nanocrystals was observed. The possibility that this was a thermal effect was tested through various methods, and we have shown that the transition is in fact truly electrically driven. This electrically driven transition also showed a great deal of rigidity against external magnetic field and high gate voltages.Item Electronic charge injection and transport in organic field-effect transistors(2007) Hamadani, Behrang Homayoun; Natelson, DouglasElectronic devices based on organic semiconductors, such as field-effect transistors (FETs) and light emitting diodes have attracted much interest as possible inexpensive and flexible alternatives to inorganic devices. Despite considerable improvement in device properties, a better understanding of the nature of charge transport in these devices and the physics of contacts is crucial to further development of optoelectronic organic devices. This work outlines our findings in understanding and characterizing the injection and transport mechanisms of charge carriers in solution processed poly (3-hexylthiophene) (P3HT) field-effect devices. We measured hole transport in P3HT FETs with Au electrodes at submicron channel lengths as a function of gate voltage and a wide range of temperatures. The strongly nonlinear and gate modulated transport is shown to be consistent with a model of Poole-Frenkel-like hopping mechanism in the space-charge limited current regime. Charge injection from different source/drain electrodes such as Au, Cu and Cr was examined over a broad temperature range, and the contact current-voltage characteristics were extracted from the dependence of conductance on channel length. The differences between linear vs. nonlinear charge injection were carefully studied and compared to recently developed models of charge injection. In addition, the effect of doping-dependent charge injection in devices with Au and Pt contacts was studied, revealing large contact resistances and marked non-Ohmic transport at low dopant concentrations. Ultraviolet photoemission spectroscopy (UPS) reveals that metal/P3HT band alignment is rearranged as samples are dedoped, leading to an increased injection barrier for holes, with a greater shift for Au/P3HT. We also performed a study using dipole-containing self-assembled monolayers on the Au source and drain electrodes to strongly manipulate the charge injection process across the metal/organic interface. We have shown that chemically increasing the injecting electrode work function significantly improves hole injection relative to untreated Au electrodes.Item Electronic devices containing switchably conductive silicon oxides as a switching element and methods for production and use thereof(2013-11-26) Tour, James M.; Yao, Jun; Natelson, Douglas; Zhong, Lin; He, Tao; Rice University; United States Patent and Trademark OfficeIn various embodiments, electronic devices containing switchably conductive silicon oxide as a switching element are described herein. The electronic devices are two-terminal devices containing a first electrical contact and a second electrical contact in which at least one of the first electrical contact or the second electrical contact is deposed on a substrate to define a gap region therebetween. A switching layer containing a switchably conductive silicon oxide resides in the gap region between the first electrical contact and the second electrical contact. The electronic devices exhibit hysteretic current versus voltage properties, enabling their use in switching and memory applications. Methods for configuring, operating and constructing the electronic devices are also presented herein.Item Electronic devices containing switchably conductive silicon oxides as a switching element and methods for production and use thereof(2015-09-08) Tour, James M.; Yao, Jun; Natelson, Douglas; Zhong, Lin; He, Tao; Rice University; United States Patent and Trademark OfficeIn various embodiments, electronic devices containing switchably conductive silicon oxide as a switching element are described herein. The electronic devices are two-terminal devices containing a first electrical contact and a second electrical contact in which at least one of the first electrical contact or the second electrical contact is deposed on a substrate to define a gap region therebetween. A switching layer containing a switchably conductive silicon oxide resides in the gap region between the first electrical contact and the second electrical contact. The electronic devices exhibit hysteretic current versus voltage properties, enabling their use in switching and memory applications. Methods for configuring, operating and constructing the electronic devices are also presented herein.Item Evanescent Wave Coupling in Terahertz Waveguide Arrays(2013-06-17) Reichel, Kimberly; Mittleman, Daniel M.; Xu, Qianfan; Natelson, DouglasAt optical frequencies, evanescent wave coupling in waveguides is an important concept underlying key technologies such as optical fiber splitters and combiners. At terahertz (THz) frequencies, there is a lack of such devices. In order to fill this gap, we investigate evanescent wave coupling at THz frequencies in an array of narrow-width parallel-plate waveguides (PPWGs). Although researchers have studied THz wave coupling between two adjacent wire waveguides, evanescent coupling in an array of PPWGs has not previously been considered. Metal PPWGs are ideal THz waveguide platforms since they offer low losses and negligible dispersion in the TEM waveguide mode. Additionally, PPWGs can exhibit energy leakage when the plates are narrow and the plate separation is large, indicating that an array of narrow-width PPWGs is a convenient platform for studying THz energy coupling between waveguides. By using the presented design of an array of identical narrow-width PPWGs in close proximity with their unconfined sides facing each other, we have demonstrated evidence of evanescent wave coupling in THz PPWG arrays. Thereby, we observed stronger coupling with larger waveguide plate separations and longer propagation paths. We confirmed these results through THz time-domain spectroscopy (THz-TDS) experiments and finite-element method (FEM) simulations. Based on evanescent wave coupling, this work establishes a platform to investigate new opportunities for THz waveguide devices and components such as splitters and power combiners.Item Exploration of Chemical Analysis Techniques for Nanoscale Systems(2013-09-16) Chang, Albert; Kelly, Kevin F.; Natelson, Douglas; Hafner, Jason H.As the critical dimensions of many devices, especially electronics, continue to become smaller, the ability to accurately analyze the properties at ever smaller scales becomes necessary. Optical techniques, such as confocal microscopy and various spectroscopies, have produced a wealth of information on larger length scales, above the diffraction limit. Scanning probe techniques, such as scanning tunneling microscopy and atomic force microscopy, provide information with an extremely fine resolution, often on the order of nanometers or angstroms. In this document, plasmon coupling is used to generate large signal increases, with clear future applications toward scanning probe optical spectroscopies. A variation on scanning tunneling microscopy is also used to study the surface structure of environmentally interesting nanoparticles. Traditional Raman spectroscopy is used to examine doped graphene, which is becoming a hot material for future electronic applications.Item Fast Electron Spectroscopy of Enhanced Plasmonic Nanoantenna Resonances(2014-07-31) Day, Jared K.; Halas, Naomi J.; Natelson, Douglas; Nordlander, Peter; Cox, Steven J.Surface plasmons are elementary excitations of the collective and coherent oscillations of conductive band electrons coupled with photons at the surface of metals. Surface plasmons of metallic nanostructures can efficiently couple to light making them a new class of optical antennas that can confine and control light at nanometer scale dimensions. Nanoscale optical antennas can be used to enhance the energy transfer between nanoscale systems and freely-propagating radiation. Plasmonic nanoantennas have already been used to enhance single molecule detection, diagnosis and treat cancer, harvest solar energy, to create metamaterials with new optical properties and to enhance photo-chemical reactions. The applications for plasmonic nanoantennas are only limited by the fundamental understanding of their unique optical properties and the rational design of new coupled antenna systems. It is therefore necessary to interrogate and image the local electromagnetic response of nanoantenna systems to establish intuition between near-field coupling dynamics and far-field optical properties. This thesis focuses on the characterization and enhancement of the longitudinal multipolar plasmonic resonances of Au nanorod nanoantennas. To better understand these resonances fast electron spectroscopy is used to both visualize and probe the near- and far-field properties of multipolar resonances of individual nanorods and more complex nanorod systems through cathodoluminescence (CL). CL intensity maps show that coupled nanorod systems enhance and alter nanorod resonances away from ideal resonant behavior creating hybridized longitudinal modes that expand and relax at controllable locations along the nanorod. These measurements show that complex geometries can strengthen and alter the local density of optical states for nanoantenna designs with more functionality and better control of localized electromagnetic fields. Finally, the electron excitations are compared to plane wave optical stimulation both experimentally and through Finite Difference Time Domain simulations to begin to develop a qualitative picture of how the local density of optical states affects the far-field optical scattering properties of plasmonic nanoantennas.Item Hydrogen Diffusion and Stabilization in Single-Crystal VO2 Micro/Nanobeams by Direct Atomic Hydrogenation(American Chemical Society, 2014) Lin, Jian; Ji, Heng; Swift, Michael W.; Hardy, Will J.; Peng, Zhiwei; Fan, Xiujun; Nevidomskyy, Andriy H.; Tour, James M.; Natelson, Douglas; Smalley Institute for Nanoscale Science and TechnologyWe report measurements of the diffusion of atomic hydrogen in single crystalline VO2 micro/nanobeams by direct exposure to atomic hydrogen, without catalyst. The atomic hydrogen is generated by a hot filament, and the doping process takes place at moderate temperature (373 K). Undoped VO2 has a metal-to-insulator phase transition at ∼340 K between a high-temperature, rutile, metallic phase and a low-temperature, monoclinic, insulating phase with a resistance exhibiting a semiconductor-like temperature dependence. Atomic hydrogenation results in stabilization of the metallic phase of VO2 micro/nanobeams down to 2 K, the lowest point we could reach in our measurement setup. Optical characterization shows that hydrogen atoms prefer to diffuse along the c axis of rutile (a axis of monoclinic) VO2, along the oxygen “channels”. Based on observing the movement of the hydrogen diffusion front in single crystalline VO2 beams, we estimate the diffusion constant for hydrogen along the c axis of the rutile phase to be 6.7 × 10–10 cm2/s at approximately 373 K, exceeding the value in isostructural TiO2 by ∼38×. Moreover, we find that the diffusion constant along the c axis of the rutile phase exceeds that along the equivalent a axis of the monoclinic phase by at least 3 orders of magnitude. This remarkable change in kinetics must originate from the distortion of the “channels” when the unit cell doubles along this direction upon cooling into the monoclinic structure. Ab initio calculation results are in good agreement with the experimental trends in the relative kinetics of the two phases. This raises the possibility of a switchable membrane for hydrogen transport.Item Hydrogen doping and the metal-insulator phase transition in vanadium dioxide(2015-04-22) Ji, Heng; Natelson, Douglas; Du, Rui-rui; Biswal, Sibani LStrongly correlated systems represent a major topic of study in condensed matter physics. Vanadium dioxide, a strongly correlated material, has a sharp metal-to-insulator phase transition at around 340 K (67 °C), a moderate temperature which can be easily achieved. Its potential as a functional material in optical switches and semiconductor applications has attracted a great deal of attention in recent years. In this thesis, after a detailed introduction of this material and the methods we used to grow VO2 in our lab (Chapter 1), I will discuss our attempts to modulate the electronic properties and phase transition of single-crystal VO2 samples. It started with a plan to use ionic liquid to apply an electrostatic gate to this material. Although modulation of the resistance was observed, we also discovered an unexpected electrochemical reaction, leading to a suspicion that hydrogen doping is the reason for the change of properties of VO2 (Chapter 2). Next, a series of experiments were performed to systematically study the mechanism of this hydrogen doping process and to characterize the hydrogenated VO2. Our collaborators also provided supporting simulation results to interpret these phenomena from a theoretical point of view, as well as results from synchrotron x-ray diffraction and neutron diffraction experiments. From all these studies, we confirmed the existence of the hydrogen intercalation in VO2 (Chapter 3), and further, plotted the phase diagram as a function of temperature and hydrogen concentration (Chapter 5). We also found that this diffusion process prefers the rutile crystal structure of VO2 (i.e. metallic phase) and specifically, its c-axis (Chapter 4). Finally, the low-temperature electric transport properties of the hydrogenated VO2 material have been studied for the first time, and interesting magneto-resistance responses will be discussed (chapter 6).Item In Situ Diffraction Study of Catalytic Hydrogenation of VO2: Stable Phases and Origins of Metallicity(American Chemical Society, 2014) Filinchuk, Yaroslav; Tumanov, Nikolay A.; Ban, Voraksmy; Ji, Heng; Wei, Jiang; Swift, Michael W.; Nevidomskyy, Andriy H.; Natelson, DouglasControlling electronic population through chemical doping is one way to tip the balance between competing phases in materials with strong electronic correlations. Vanadium dioxide exhibits a first-order phase transition at around 338 K between a high-temperature, tetragonal, metallic state (T) and a low-temperature, monoclinic, insulating state (M1), driven by electronヨelectron and electronヨlattice interactions. Intercalation of VO2 with atomic hydrogen has been demonstrated, with evidence that this doping suppresses the transition. However, the detailed effects of intercalated H on the crystal and electronic structure of the resulting hydride have not been previously reported. Here we present synchrotron and neutron diffraction studies of this material system, mapping out the structural phase diagram as a function of temperature and hydrogen content. In addition to the original T and M1 phases, we find two orthorhombic phases, O1 and O2, which are stabilized at higher hydrogen content. We present density functional calculations that confirm the metallicity of these states and discuss the physical basis by which hydrogen stabilizes conducting phases, in the context of the metalヨinsulator transition.Item In situ imaging of the conducting filament in a silicon oxide resistive switch(Nature Publishing Group, 2012) Yao, Jun; Zhong, Lin; Natelson, Douglas; Tour, James M.; Applied Physics ProgramThe nature of the conducting filaments in many resistive switching systems has been elusive. Throughᅠin situᅠtransmission electron microscopy, we image the real-time formation and evolution of the filament in a silicon oxide resistive switch. The electroforming process is revealed to involve the local enrichment of silicon from the silicon oxide matrix. Semi-metallic silicon nanocrystals with structural variations from the conventional diamond cubic form of silicon are observed, which likely accounts for the conduction in the filament. The growth and shrinkage of the silicon nanocrystals in response to different electrical stimuli show energetically viable transition processes in the silicon forms, offering evidence for the switching mechanism. The study here also provides insights into the electrical breakdown process in silicon oxide layers, which are ubiquitous in a host of electronic devices.Item Interfacial charge transfer in nanoscale polymer transistors(2009) Worne, Jeffrey Howard; Natelson, DouglasInterfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel (L < 100 nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero gate voltage conductance are consistent with such charge transfer. Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations. Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into P3HT, while much less charge transfer takes place at the Au/P3HT interface.