Author: Elizabeth Lee Radue
Publisher:
ISBN:
Category : Raman spectroscopy
Languages : en
Pages : 119
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Vanadium dioxide is an intensely studied material, since it goes through an insulator-metal transition at a critical temperature just above room temperature at 340~K. The dramatic change in conductivity and the easily accessible transition temperature makes it an attractive material for novel technologies. Thin films of VO2 have a reversible transition without any significant degradation in contrast, and depending on the microstructure of the films, the properties of the transition are tunable. In this work, I study the dynamics of the insulator-transition in thin films grown on different substrates using a pump-probe configuration. The energy needed to trigger the transition, as well as the time constants of the change in reflectivity are affected by the strain in the VO2 films. I also characterized the samples using Raman spectroscopy and XRD measurements in order to identify what underlies the differences in behavior. Finally, in collaboration with Dr. Yamaguchi's group at RPI, I show that it is possible to trigger the transition using a THz pulse that directly pumps energy into the lattice, and at lower energies than needed to pump films by photoinducing the electrons across the band gap.
Author: Vance Morrison
Publisher:
ISBN:
Category :
Languages : en
Pages :
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"In this thesis the design and implementation of an ultrafast electron diffractometer with radio frequency compression capabilities is presented. In addition, the results of ultrafast electron diffraction (UED) measurements on the semiconductor to metal phase transition in vanadium dioxide are shown. The ability to perform UED measurements on ultrafast time scales is first demonstrated by observing the expansion and coherent oscillation of the crystal lattice in thin film, single crystal gold. The evolution of the spatio-temporal charge density in ultrashort pulses was then studied using electron-laser cross correlation measurements mediated by the ponderomotive force. These measurements were compared with particle tracing simulations and theoretical models. Similar electron-laser cross correlation measurements were also performed in order to characterize the behaviour of a novel radio-frequency (RF) pulse compression technique. Using an RF cavity, an oscillating, 3 GHz electric field is synchronized to the electron pulse arrival time and allows for the compression of high bunch charge electron pulses (0.1 pC) to 334+/-10 fs. This represents a bunch charge increase of 10^2-10^3 over previous ultrafast electron sources that provide a sub 500 fs impulse response. Finally, the semiconductor to metal transition in vanadium dioxide was studied using RF compressed electron pulses. Here, distinct ultrafast structural and electronic phase transitions were observed providing insight into the long standing debate surrounding the roles of electron-electron interactions and electron-lattice interactions in this phase transition." --
Author: Mengkun Liu
Publisher:
ISBN:
Category :
Languages : en
Pages : 316
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Abstract: The metal insulator transition in vanadates has been studied for decades and yet new discoveries still spring up revealing new physics, especially among two of the most studied members: Vanadium sesquioxide (V2 0 3 ) and Vanadium dioxide (VO2 ). Although subtleties abound, both of the materials have first order insulator to metal phase transitions that are considered to be related to strong electron-electron (e-e) correlation. Further, ultrafast spectroscopy of strongly correlated materials has generated great interest in the field given the potential to dynamically distinguish the difference between electronic (spin) response versus lattice responses due to the associated characteristic energy and time scales.In this thesis, I mainly focus on utilizing ultrafast optical and THz spectroscopy to study phase transition dynamics in high quality V2 0 3 and VO2 thin films epitaxially grown on different substrates. The main findings of the thesis are:(1) Despite the fact that the insulator to metal transition (IMT) in V2 03 is electron-correlation driven, lattice distortion plays an important role. Coherent oscillations in the far-infrared conductivity are observed resulting from coherent acoustic phonon modulation of the bandwidth W. The same order of lattice distortion induces less of an effect on the electron transport in VO 2 in comparison to V2 03 . This is directly related to the difference in latent heat of the phase transitions in VO2 and V2 03 .(2) It is possible for the IMT to occur with very little structural change in epitaxial strained VO2 films, like in the case of Cr doped or strained V2 03 . However, in V02 , this necessitates a large strain which is only possible by clamping to a substrate with larger c axis parameter through epitaxial growth. This is demonstrated for VO 2 films on TiO2 substrates.(3) Initiating an ultrafast photo-induced insulator-to-metal transition (IMT) is not only possible with above bandgap excitation, but also possible with high-field far-infrared excitation. With the help of the field enhancement in metamaterial split ring resonator gaps, we obtain picosecond THz electric field transients of several MV/cm which is sufficient to drive the insulator to metal transition in V0 2 .
Author:
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ISBN:
Category :
Languages : en
Pages : 0
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During the summer of 2003, a picosecond Leopard YAG laser with an upgraded OPG system was purchased by the Laser Lab of the University of Puerto Rico, Mayaguez, after the contract award was announced. The laser arrived in August 2003, but due to problems with beam quality it was shipped back to the factory to be made right. The laser was corrected and was finally installed in the Laser Lab in mid-March 2004. The laser lab is now a well-established entity for the study of ultrafast photodynamic processes of optical materials. It functions in both an independent research role and a collaborative role in the development of optical materials. The primary focus of the project this year was on ultrafast spectroscopic studies of vanadium dioxide thin film as well as heavy metal nanoparticles embedded in melting glass optical materials. This effort included research on ultrafast passive optical switching derived by the enhancement of chi(exp 3), and ultrafast photo-induced insulator-metal transition (thermochromic effect). A large part of the initial focus was to develop heavy metal nanoparticle doped inorganic glass, which has been proven to show an enhanced chi(exp 3) effect. The other focal area was the study of the ultrafast phase-transition of VO2 thin film. This part of the work was begun right after the new laser was installed. With better laser output stability and beam profile, the authors were able to accurately characterize ultrafast time-resolved optical responses. (5 figures, 20 refs.).
Author: Yin Shi
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Vanadium dioxide (VO2) is a strongly correlated system which exhibits an intriguing metal-insulator transition (MIT) accompanied by a structural transition at a temperature slightly above the room temperature. It offers potential novel device applications such as sensors, Mott field-effect transistors, and memristors, which desire guidance from mesoscopic theoretical modeling. Based on symmetry consideration, we formulate a mesoscopic phase-field model of the MIT explicitly incorporating both structural and electronic instabilities as well as free electrons and holes. We employ this model to investigate the MIT in mesoscale VO2 subject to various stimuli such as heat, stress/strain, electric field, doping, electric current, and light. First, the temperature-stress/strain phase diagrams of VO2 nanobeams and thin films under different mechanical boundary conditions are calculated consistently, which show good agreement with existing experimental observations. We also calculate the temperature-radius phase diagrams of VO2 nanoparticles and nanofibers. Second, in a VO2 slab under an electric field in an open-circuit configuration, an abrupt universal resistive transition is shown to occur inside the supercooling region, in sharp contrast to the conventional Landau-Zener smooth electric breakdown. Third, the temperature-dopant-concentration phase diagrams of VO2 doped with various metal ions are calculated consistent with the experiments. Furthermore, hole doping in VO2 may induce a metastable metallic monoclinic phase, which could be stabilized through geometrical confinement and the size effect in VO2-VO_{2-delta} bilayers leading to the decoupling of the electronic and structural phase transitions. Fourth, we demonstrate that the electric current may drive the MIT isothermally via the current-induced electron correlation weakening, inducing a few-nanosecond ultrafast resistive switching consistent with experimental measurements. The isothermal temperature-current phase diagram is further calculated and the current is also found able to drive domain walls to move. Fifth, dynamic processes of the MIT in VO2 illuminated by femtosecond laser pulses are simulated, showing the emergence of the transient metallic monoclinic phase and the bias-induced shrinkage of the photoinduced metallic phase. We also prove that during a generic metal-insulator transition, a nonequilibrium homogeneous state may be unstable against charge density modulations with certain wavelengths, and thus evolves to the equilibrium phase through transient electronic phase separation. This transient electronic phase separation is shown to take place in VO2 upon photoexcitation.
Author: Joseph Woicik
Publisher: Springer
ISBN: 3319240439
Category : Science
Languages : en
Pages : 576
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This book provides the first complete and up-to-date summary of the state of the art in HAXPES and motivates readers to harness its powerful capabilities in their own research. The chapters are written by experts. They include historical work, modern instrumentation, theory and applications. This book spans from physics to chemistry and materials science and engineering. In consideration of the rapid development of the technique, several chapters include highlights illustrating future opportunities as well.
Author: Shriram Ramanathan
Publisher: Springer Science & Business Media
ISBN: 1441906649
Category : Technology & Engineering
Languages : en
Pages : 344
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Book Description
Thin Film Metal-Oxides provides a representative account of the fundamental structure-property relations in oxide thin films. Functional properties of thin film oxides are discussed in the context of applications in emerging electronics and renewable energy technologies. Readers will find a detailed description of deposition and characterization of metal oxide thin films, theoretical treatment of select properties and their functional performance in solid state devices, from leading researchers. Scientists and engineers involved with oxide semiconductors, electronic materials and alternative energy will find Thin Film Metal-Oxides a useful reference.
Author:
Publisher:
ISBN:
Category : Chemistry
Languages : en
Pages : 2540
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Author: Yasubumi Furuya
Publisher:
ISBN:
Category : Technology & Engineering
Languages : en
Pages : 408
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Book Description
Smart/intelligent systems is a primary technology for present and future applications in areas ranging from everyday life to aerospace missions, from civil to military environments, from robots to information technology. Smart materials are the critical foundation for high-performance smart devices, and smart devices are fundamental components for smart systems. The three cannot be separated. This book bridges the fields of smart materials, sensing and actuating devices, and intelligent systems, and provides an opportunity for researchers from all three arenas to channel information into a coherent, interdisciplinary community. Topics include: piezoelectric actuators; novel devices and systems; shape memory alloys and magnetostrictive devices; nanometer-scale processing and properties; piezoelectric materials; sensor materials and devices; and electroactive polymer actuators.
Author:
Publisher:
ISBN:
Category : Dielectric devices
Languages : en
Pages : 408
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