Physics Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2800

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End date: 2009Subject: search.filters.subject.Physics, Condensed Matter

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    EXPLORATION OF NOVEL METHODS FOR THE FABRICATION AND CHARACTERIZATION OF ORGANIC FIELD-EFFECT TRANSISTORS AND EXAMINATION OF FACTORS INFLUENCING OFET PERFORMANCE
    (2009) Southard, Adrian Edward; Fuhrer, Michael S.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis explores novel methods for fabricating organic field effect transistors (OFETs) and characterizing OFET devices. Transfer printing is a promising process for fabricating organic thin-film devices. In this work, a transfer-printing process is developed for the polymer organic semiconductor P3HT. Pre-patterned P3HT is printed onto different dielectrics such as PMMA, polystyrene and polycarbonate. The P3HT layer is spun on a smooth silicon interface made hydrophobic by treatment with octyltrichlorosilane, which functions as a release layer. This method has distinct advantages over standard OFET fabrication methods in that 1) the active layer can be pre-patterned, 2) the solvent for the P3HT need not be compatible with the target substrate, and 3) the electrical contact formed mimics the properties of top contacts but with the spatial resolution of bottom contacts. Transparent, conducting films of carbon nanotubes (CNTs) are prepared by airbrushing, and characterized optically and electronically. OFETs with CNT films as source and drain electrodes are fabricated using various patterning techniques, and the organic/CNT contact resistance is characterized. CNT films make transparent, flexible electrodes with contact resistance comparable to that found for Au bottom-contacted P3HT transistors and comparable to CNT-film bottom-contacted pentacene transistors with CNTs deposited by other less flexible methods. A transparent OFET is demonstrated using transfer printing for the assembly of an organic semiconductor (pentacene), CNT film source, drain, and gate electrodes, and polymer gate dielectric and substrate. The dependence of the conductance and mobility in pentacene OFETs on temperature, gate voltage, and source-drain electric field is studied. The data are analyzed by extending a multiple trapping and release model to account for lowering of the energy required to excite carriers into the valence band (Poole-Frenkel effect). The temperature-dependent conductivity shows activated behavior, and the activation energy is lowered roughly linearly with the square-root of electric field, as expected for the Poole-Frenkel effect. The gate voltage dependence of the activation energy is used to extract the trap density of states, in good agreement with other measurements in the literature.
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    STEPS ON VICINAL SURFACES: DENSITY-FUNCTIONAL THEORY CALCULATIONS AND TRANSCENDING MINIMAL STATISTICAL-MECHANICAL MODELS
    (2009) Sathiyanarayanan, Rajesh; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Using both density-functional theory calculations and Monte Carlo simulations, we compute various key parameters that are used to model steps on vicinal surfaces. In the first part, we discuss the importance of multi-site interactions (trios and quartos) in the lattice-gas characterization of adatom interactions. Using density-functional theory calculations, we show that multi-site interactions with substantial contributions from direct interactions are sensitive to adatom relaxations. Such sensitivity to adatom relaxations complicates the lattice-gas approach to modeling overlayer systems. Our results show that a careful consideration of relaxation effects is required to make connections with experiments. In the second part, we use both density-functional theory calculations and kinetic Monte Carlo simulations to identify the impurity atom responsible for growth instabilities on Cu vicinals. In addition to that, we also show that a small quantity of codeposited impurities significantly alters the growth behavior. Our results indicate that growth morphologies could be controlled through the codeposition of an appropriate impurity. Hence, impurities could play a crucial role in nanostructuring of surfaces. Step configurations have fruitfully been related to the worldlines of spinless fermions in one dimension. However, in addition to the realistic no-crossing condition, the fermion picture imposes a more restrictive non-touching condition. in the third part of this thesis, we use Metropolis Monte Carlo method to study the effects of loosening this non-touching condition on the resulting TWDs. Our results show that allowing step touching leads to an effective attraction in the step-step interaction strength measurements. We show that this effective attraction can be incorporated into the fermion picture as a finite-size effect.
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    CARBON NANOTUBE THIN FILM AS AN ELECTRONIC MATERIAL
    (2009) Sangwan, Vinod Kumar; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Carbon nanotubes (CNT) are potential candidates for next-generation nanoelectronics devices. An individual CNT possesses excellent electrical properties, but it has been extremely challenging to integrate them on a large-scale. Alternatively, CNT thin films have shown great potential as electronic materials in low cost, large area transparent and flexible electronics. The primary focus of this dissertation is patterning, assembling, characterization and assessment of CNT thin films as electronic material. Since a CNT thin film contains both metallic and semiconducting CNTs, it can be used as an active layer as well as an electrode material by controlling the growth density and device geometry. The growth density is controlled by chemical vapor deposition and airbrushing methods. The device geometry is controlled by employing a transfer printing method to assemble CNT thin film transistors (TFT) on plastic substrates. Electrical transport properties of CNT TFTs are characterized by their conductance, transconductance and on/off ratio. Optimized device performance of CNT TFTs is realized by controlling percolation effects in a random network. Transport properties of CNTs are affected by the local environment. To study the intrinsic properties of CNTs, the environmental effects, such as those due to contact with the dielectric layer and processing chemicals, need to be eliminated. A facile fabrication method is used to mass produce as-grown suspended CNTs to study the transport properties of CNTs with minimal effects from the local environment. Transport and low-frequency noise measurements are conducted to probe the intrinsic properties of CNTs. Lastly, the unique contrast mechanism of the photoelectron emission microscopy (PEEM) is used to characterize the electric field effects in a CNT field effect transistor (FET). The voltage contrast mechanism in PEEM is first characterized by comparing measurements with simulations of a model system. Then the voltage contrast is used to probe the local field effects on a single CNT and a CNT thin film. This real-time imaging method is assessed for potential applications in testing of micron sized devices integrated in large scale.
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    Back-Action Evading Measurements of Nanomechanical Motion Approaching Quantum Limits
    (2009) Hertzberg, Jared Barney; Schwab, Keith C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The application of quantum mechanics to macroscopic motion suggests many counterintuitive phenomena. While the quantum nature of the motion of individual atoms and molecules has long been successfully studied, an equivalent demonstration of the motion of a near-macroscopic structure remains a challenge in experimental physics. A nanomechanical resonator is an excellent system for such a study. It typically contains > 1010 atoms, and it may be modeled in terms of macroscopic parameters such as bulk density and elasticity. Yet it behaves like a simple harmonic oscillator, with mass low enough and resonant frequency high enough for its quantum zero-point motion and single energy quanta to be experimentally accessible. In pursuit of quantum phenomena in a mechanical oscillator, two important goals are to prepare the oscillator in its quantum ground state, and to measure its position with a precision limited by the Heisenberg uncertainty principle. In this work we have demonstrated techniques that advance towards both of these goals. Our system comprises a 30 micron × 170 nm, 2.2 pg, 5.57 MHz nanomechanical resonator capacitively coupled to a 5 GHz superconducting microwave resonator. The microwave resonator and nanomechanical resonator are fabricated together onto a single silicon chip and measured in a dilution refrigerator at temperatures below 150 mK. At these temperatures the coupling of the motion to the thermal environment is very small, resulting in a very high mechanical Q, approaching ∼ 106. By driving with a microwave pump signal, we observed sidebands generated by the mechanical motion and used these to measure the thermal motion of the resonator. Applying a pump tone red-detuned from the microwave resonance, we used the microwave field to damp the mechanical resonator, extracting energy and "cooling" the motion in a manner similar to optical cooling of trapped atoms. Starting from a mode temperature of ∼ 150 mK, we reached ∼ 40 mK by this "backaction cooling" technique, corresponding to an occupation factor only ∼ 150 times above the ground state of motion. We also determined the precision of our device in measurement of position. Quantum mechanics dictates that, in a continuous position measurement, the precision may be no better than the zero-point motion of the resonator. Increasing the coupling of the resonator to detector will eventually result in back-action driving of the motion, adding imprecision and enforcing this limit. We demonstrated that our system is capable of precisions approaching this limit, and identified the primary experimental factors preventing us from reaching it: noise added to the measurement by our amplifier, and excess dissipation appearing in our microwave resonator at high pump powers. Furthermore, by applying both red- and blue-detuned phase-coherent microwave pump signals, we demonstrated back-action evading (BAE) measurement sensitive to only a single quadrature of the motion. By avoiding the back-action driving in the measured quadrature, such a technique has the potential for precisions surpassing the limit of the zero-point motion. With this method, we achieved a measurement precision of ∼ 100 fm, or 4 times the quantum zero-point motion of the mechanical resonator. We found that the measured quadrature is insensitive to back-action driving by at least a factor of 82 relative to the unmeasured quadrature. We also identified a mechanical parametric amplification effect which arises during the BAE measurement. This effect sets limits on the BAE performance but also mechanically preamplifies the motion, resulting in a position resolution 1.3 times the zero-point motion. We discuss how to overcome the experimental limits set by amplifier noise, pump power and parametric amplification. These results serve to define the path forward for demonstrating truly quantum-limited measurement and non-classical states of motion in a nearly-macroscopic object.
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    STRUCTURE TAILORED PROPERTIES AND FUNCTIONALITIES OF ZERO-DIMENSIONAL NANOSTRUCTURES
    (2009) Tang, Yun; Ouyang, Min; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The field of nanoscience and nanotechnology has achieved significant progress over last thirty years. Complex nanostructures with tunable properties for novel applications have been successfully fabricated and characterized. In this thesis, I will focus on our recent efforts on precise controlled synthesis of zero-dimensional nanostructures as well as fundamental understanding of the physical behavior of as-synthesized nanostructures. Particularly, three topics are presented: (1) Nanoscale crystallinity engineering: we have achieved nanoscale crystallinity control of noble metal nanoparticles with 100% yield by molecular engineering. We have used silver nanoparticles as example to demonstrate synthetic strategy and importance of such control in nanoscale chemical transformation, fundamental electron and phonon couplings and surface plasmon resonance based biological sensors. Such nanoscale crystallinity engineering provides a new pathway for design of complex nanostructures, tailoring nanoscale electronic and mechanical properties as well as controlling classical and quantum coupling interactions; (2) Precise control of core@shell nanostructures: we have developed a new universal strategy denoted as intermediated phase assisted phase exchange and reaction (iPAPER) to achieve layer-by-layer control of shell components in core@shell structures. Tunable plasmonic, optical and magnetic properties of core@shell structures enabled by our iPAPER strategy are further demonstrated. These characterizations are promising for understanding and manipulating nanoscale phenomena as well as assembling nanoscale devices with desirable functionality; and (3) Fundamental spin and structure manipulation of semiconductor quantum dots by hydrostatic pressure. Pressure provides a unique means of modifying materials properties. By measuring dependence of spin dynamics on pressure, we revealed that the spin states of semiconductor quantum dots are very robust. We further provided the first experimental evidence for the existence of a metastable intermediate state before the first-order phase transition of semiconductor quantum dots. Our results are crucial for the future development of quantum information processing based on spin qubits of quantum dots.
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    Diffusive Charge Transport in Graphene
    (2009) Chen, Jianhao; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The physical mechanisms limiting the mobility of graphene on SiO2 are studied and printed graphene devices on a flexible substrate are realized. Intentional addition of charged scattering impurities is used to study the effects of charged impurities. Atomic-scale defects are created by noble-gas ions irradiation to study the effect of unitary scatterers. The results show that charged impurities and atomic-scale defects both lead to conductivity linear in density in graphene, with a scattering magnitude that agrees quantitatively with theoretical estimates. While charged impurities cause intravalley scattering and induce a small change in the minimum conductivity, defects in graphene scatter electrons between the valleys and suppress the minimum conductivity below the metallic limit. Temperature-dependent measurements show that longitudinal acoustic phonons in graphene produce a small resistivity which is linear in temperature and independent of carrier density; at higher temperatures, polar optical phonons of the SiO2 substrate give rise to an activated, carrier density-dependent resistivity. Graphene is also made into high mobility transparent and flexible field effect device via the transfer-printing method. Together the results paint a complete picture of charge carrier transport in graphene on SiO2 in the diffusive regime, and show the promise of graphene as a novel electronic material that have potential applications not only on conventional inorganic substrates, but also on flexible substrates.
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    Low Temperature Scanning Tunneling Microscopy and Spectroscopy: A Study On Charge Density Waves and Vortex Dynamics
    (2009) Wang, Hui; Williams, Ellen D.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this thesis I describe the development of a low temperature scanning tunneling microscope system (LTSTM) and its application to the study of charge density waves and vortex dynamics. All the measurements are taken on different 2H-NbSe2 samples with or without impurities to examine the interesting coexistence of the charge density wave (CDW) phase and superconductive phase in the sample at 4.2 K. After creating a structural defect using a voltage pulse, we observed a new type of CDW in the vicinity of the defect. With a Sqrt(13) × Sqrt(13) reconstruction, the new CDW differs in many ways from the naturally occurring 3 × 3 CDW in 2H-NbSe2. This suggests a possible local phase transition induced by the tip-sample interaction. As a low-Tc type II superconductor, 2H-NbSe2 is also well-known for the formation of a vortex phase in magnetic fields. Although it was intensely studied for decades, many questions concerning the vortex system still remain unanswered. One of the most important and intriguing questions is the response of the system to a driving force well below the critical value fc¬. Due to an unexpected defect in our magnet, we are able to utilize a slowly decaying magnetic field with a rate at 5 nT/s to observe the dynamic creep motion of the vortex system which can be described as a Bragg glass. I will also present a study of the statics of this glass phase and demonstrate the use of LTSTM as a powerful imaging technique in the area of vortex physics.
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    INTERFACE EFFECTS ON NANOELECTRONICS
    (2009) Conrad, Brad Richard; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nanoelectronics consist of devices with active electronic components on the nanometer length scale. At such dimensions most, if not all, atoms or molecules composing the active device region must be on or near a surface. Also, materials effectively confined to two dimensions, or when subject to abrupt boundary conditions, generally do not behave the same as materials inside three dimensional, continuous structures. This thesis is a quantitative determination of how surfaces and interfaces in organic nanoelectronic devices affect properties such as charge transport, electronic structure, and material fluctuations. Si/SiO2 is a model gate/gate dielectric for organic thin film transistors, therefore proper characterization and measurement of the effects of the SiO2/organic interface on device structures is extremely important. I fabricated pentacene thin film transistors on Si/SiO2 and varied the conduction channel thickness from effectively bulk (~40nm) to 2 continuous conducting layers to examine the effect of substrate on noise generation. The electronic spectral noise was measured and the generator of the noise was determined to be due to the random spatial dependence of grain boundaries, independent of proximity to the gate oxide. This result led me to investigate the mechanisms of pentacene grain formation, including the role of small quantities of impurities, on silicon dioxide substrates. Through a series of nucleation, growth and morphology studies, I determined that impurities assist in nucleation on SiO2, decreasing the stable nucleus size by a third and increasing the overall number of grains. The pentacene growth and morphology studies prompted further exploration of pentacene crystal growth on SiO2. I developed a method of making atomically clean ultra-thin oxide films, with surface chemistry and growth properties similar to the standard thick oxides. These ultra-thin oxides were measured to be as smooth as cleaned silicon and then used as substrates for scanning tunneling microscopy of pentacene films. The increased spatial resolution of this technique allowed for the first molecular resolution characterization of the standing-up pentacene crystal structure near the gate dielectric, with molecules oriented perpendicular to the SiO2 surface. Further studies probed how growth of C60 films on SiO2 and pentacene surfaces affected C60 morphology and electronic structure to better understand solar cell heterojunctions.
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    CHARACTERIZATION OF METAL-OXIDE-SEMICONDUCTOR STRUCTURES AT LOW TEMPERATURES USING SELF-ALIGNED AND VERTICALLY COUPLED ALUMINUM AND SILICON SINGLE-ELECTRON TRANSISTORS
    (2008-11-21) Sun, Luyan; Kane, Bruce E; Drew, Howard D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    I incorporate an Al-AlOx-Al single-electron transistor (SET) as the gate of a narrow (~ 100 nm) metal-oxide-semiconductor field-effect transistor (MOSFET). Near the MOSFET channel conductance threshold, Coulomb blockade oscillations are observed at about 20 millikelvin, revealing the formation of a Si SET at the Si/SiO2 interface. Based on a simple electrostatic model, the two SET islands are demonstrated to be closely aligned, with an inter-island capacitance approximately equal to 1/3 of the total capacitance of the Si transistor island, indicating that the Si transistor is strongly coupled to the Al transistor. This vertically-aligned Al and Si SET system is used to characterize the background charges in a MOS structure at low temperature, which may also be sources of decoherence for Si quantum computation. A single charge defect, probably either a single charge trap at the Si/SiO2 interface or a single donor in the Si substrate, is detected and the properties of the defect are studied in this dissertation.
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    Studies of complex systems in condensed matter physics and economics
    (2008-11-21) Banerjee, Anand; Yakovenko, Victor M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation reports the study of complex systems from two very different fields. The dissertation is divided into two parts. The first part involves study of angular magnetoresistance in quasi-one-dimensional organic conductors and graphene bilayers (chapter 2 and 3). The second part is devoted to the modeling and empirical study of personal income distribution (chapter 4 and 5). First, we study the effect of crystal superstructures, produced by orientational ordering of the ReO4 and ClO4 anions in the quasi-one-dimensional organic conductors (TMTSF)2ReO4 and (TMTSF)2ClO4, on the angular magnetoresistance oscillations (AMRO) observed in these materials. Folding of the Brillouin zone due to anion ordering generates effective tunneling amplitudes between distant chains. These amplitudes cause multiple peaks in interlayer conductivity for the magnetic field orientations along the rational crystallographic directions (the Lebed magic angles). Different wave vectors of the anion ordering in (TMTSF)2ReO4 and (TMTSF)2ClO4 result in the odd and even Lebed angles, as observed experimentally. When a strong magnetic field is applied parallel to the layers and perpendicular the chains and exceeds a certain threshold, the interlayer tunneling between different branches of the folded electron spectrum becomes possible, and interlayer conductivity should increase sharply. This effect can be utilized to probe the anion ordering gaps in (TMTSF)2ClO4 and (TMTSF)4ReO4. An application of this effect to kappa-(ET)2Cu(NCS)2 is also briefly discussed. Next, we study AMRO in graphene bilayers. We calculate the interlayer conductivity and investigate the effects of a parallel magnetic field on the low energy bands of graphene bilayer. Next, we analyze the data on personal income distribution from the Australian Bureau of Statistics. We compare fits of the data to the exponential, log-normal, and gamma distributions. The exponential function gives a good (albeit not perfect) description of 98% of the population in the lower part of the distribution. The log-normal and gamma functions do not improve the fit significantly, despite having more parameters, and mimic the exponential function. We find that the probability density at zero income is not zero, which contradicts the log-normal and gamma distributions, but is consistent with the exponential one. The high-resolution histogram of the probability density shows a very sharp and narrow peak at low incomes, which we interpret as the result of a government policy on income redistribution. We also analyze data on individual income from Internal Revenue Service and University of Maryland. Finally, we discuss a model which captures the two-class structure of income distribution in the USA.