Download or read book Vanadium Oxide Phase Transitions written by Ilya Valmianski and published by . This book was released on 2017 with total page 92 pages. Available in PDF, EPUB and Kindle. Book excerpt: Vanadium oxides are a prototypical family of highly correlated oxides. In his dissertation, I present the study of two vanadium oxides in particular, V2O3 and VO2, which undergo simultaneously both a structural phase transition and a metal to insulator transition. While traditionally these phase transitions were studied in equilibrium, bulk, or in meso/macro-scale devices, in my work I focused on different modalities: fast, small, and strained. In my work on fast time scales during photoexcitation of V2O3 we found a novel meta-stable intermediate state that appears due to symmetry change in the monoclinic phase. This change occurs in the proximity of high temperature rhombohedral domains on length scales similar to those of electronic correlation. Our finding shows that the electronic and structural transitions in V2O3 have similar length scales but very different time scales. In VO2 and V2O3 nanoscale devices, we found a length-scale competition between Joule heating and electric field driven current induced metal to insulator transition. We proposed a novel thermoelectric model and performed simulations using finite element methods. Our modeling showed that the transition is highly inhomogeneous and the resulting filaments are surface bound with thermal gradients generating Seebeck electric fields on the order of 1000 V/cm. Finally, we studied pressurized and strained thin films in V2O3 and discovered strong strain relaxation for pressures of up to 500 MPa, which cause a deviation of thin film Pressure-Temperature phase diagram from bulk behavior. This strain relaxation relies on the difference between the structural and morphological length scales, which allows the formation of strain relaxing creases. Once those creases are fully strained, the thin films respond similarly to bulk samples.

Download or read book Inhomogeneous Phase Transition of Vanadium Oxide on Mesoscopic Scale written by Siming Wang and published by . This book was released on 2014 with total page 126 pages. Available in PDF, EPUB and Kindle. Book excerpt: Vanadium oxides are a prototypical family of materials that exhibit first order metal insulator transitions (MIT). In the past 15 years, the research has been focused on the role of different driving forces and the inhomogeneity in the phase transitions of vanadium oxides. Multiple stimuli, such as voltage, current and laser pulses, have been used to induce a MIT in vanadium oxides. Inhomogeneity can give rise to phase coexistence and multiple avalanches in mesoscopic scale vanadium oxides. In this thesis, I will focus on understanding the MIT of mesoscopic vanadium oxides. I will address the phase transition mechanism through resistance - temperature (R-T) and current - voltage (I-V) characteristics. I will present the R-T characteristic of nano-sized vanadium oxide devices, which exhibits multiple avalanches over two orders of magnitude. Statistics on the avalanches indicate different MIT mechanisms for different vanadium oxides. The I-V characteristic of micro-sized vanadium oxide devices has been previously interpreted as evidence for a voltage induced transition, a non-thermal pure electronic transition in vanadium oxides. I will present a comprehensive study of the I-V characteristic supported by various techniques, including fluorescent local temperature measurement, low temperature scanning electron microscopy and numerical simulation. The results prove that Joule heating plays a significant role in the voltage induced transition of vanadium oxides. I will also discuss the other important aspect of the phase transition, the structural phase transition (SPT) in vanadium oxides. The SPT can be used to manipulate the magnetic properties of ferromagnetic materials, e.g. coercivity and magnetization. In a vanadium oxide/ferromagnet bilayer, the coercivity increases as the SPT occurs, due to the stress anisotropy induced by the SPT. In the special case of a V2O3/Ni bilayer with a smooth interface, a large coercivity enhancement appears at the middle of the V2O3 SPT. This effect is attributed to the phase coexistence in V2O3 at the nanoscale and supported by micromagnetic simulations.

Download or read book Phase Transitions for Better Computers written by Suhas Kumar and published by . This book was released on 2016-08-14 with total page 92 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Semiconductor to Metallic Phase Transitions from Vanadium and Titanium Oxides Induced by Visible Light written by David Michael Lamb and published by . This book was released on 2009 with total page 128 pages. Available in PDF, EPUB and Kindle. Book excerpt: Transition metal oxides have been the subject of intense study by material scientists and chemists for many years. They represent a unique solid state material which can undergo reversible phase transitions from insulators (or semiconductors) to metals when they undergo changes in ambient conditions. It has long been established that vanadium oxides undergo these reversible phase transitions. When irradiating infrared light on thin films of VO2, a phase transition from a semiconductor to a metal causes the IR light to go from transmitting to reflecting. This study investigates phase transitions of metal oxide thin films in which visible light can be changed from transmitting to reflecting. Such a device would broaden the field of optoelectronics, optical neural networks, and advanced detector systems. An optoelectronic device from a thin film heterostructure of vanadium and titanium oxides is fabricated in this study. When a voltage was applied to the device, HeNe light responded to a reversible phase transition from a semiconductor to a more metallic material. Light on the transmission side experienced a significant drop while simultaneously experiencing a corresponding increase in reflectivity.

Download or read book The Photoinduced Phase Transitions of Vanadium Dioxide written by Kunal Tiwari and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "The insulator-to-metal transition of vanadium dioxide has attracted the interest of condensed matter physicists for over half a century. In its high-temperature phase, VO2 is metallic with tetragonal rutile crystallography. In its low-temperature phase, it has correlated semiconducting electronic character and a charge-density-wave- like paired monoclinic lattice structure. Determining the relative roles of electron-electron and electron-phonon interactions in the electronic structure of the low temperature phase has been the source of the physics community's interest in VO2.Over the past two decades, it has been shown that the insulator to metal transition may be photoinduced with ultrafast laser pulses. In this thesis we present ultrafast electron diffraction and ultrafast time resolved terahertz spectroscopy measurements of this photoinduced phase transition. Our ultrafast electron diffraction results reveal, at low fluences, a novel metastable phase. This phase has the crystallography of the insulating state, but a dramatically collapsed band gap. A reorganization of valence charge density accompanies this modulated spectroscopic activity.These results have twofold significance. They show that the insulating behavior of the low temperature phase is affected primarily by electron-electron correlations, not by lattice structure. Importantly, they also show that ultrafast electron diffraction may be used to probe both electronic and lattice structure dynamics--it is sensitive to valence charge density reorganizations.Our time resolved terahertz spectroscopy results complement these ultrafast electron diffraction data. We show that, in the novel metastable monoclinic phase, the band gap does not collapse below 50 meV. We also show that dynamics in the time resolved terahertz conductivity through the full photoinduced phase transition occur on two timescales--one fast (240 femtosecond) timescale, characteristic of the coherent athermal photoinduced phase transition; and one slow (picosecond) timescale, characteristic of the astructural transition to the metastable monoclinic phase. In conjunction with our ultrafast electron diffraction measurements, these results suggest that the slow dynamics of the astructural phase transition, and the structural phase transition may be affected by the same underlying mechanism." --

Download or read book Synthesis Characterization and Phase Transitions of Single crystalline Vanadium IV Oxide Nanostructures written by Luisa Whittaker and published by . This book was released on 2011 with total page 210 pages. Available in PDF, EPUB and Kindle. Book excerpt: The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid-solid phase transitions such as those involved in metal$ndashinsulator switching, ferroelectricity, and superconductivity. The peculiarities in the electronic structure of the seemingly simple binary vanadium oxide VO2, as manifested in a pronounced metal & mdashinsulator phase transition in proximity to room temperature, have made it the subject of extensive theoretical and experimental investigations over the last several decades. VO2 exhibits a first-order metal-insulator phase transition near room temperature at 68 & degC in the bulk.^Associated with the phase transition are dramatic changes in the electrical conductivity, optical properties of VO2 at all wavelengths, and a structural transition from an insulating, low-temperature monoclinic phase to a metallic, high-temperature tetragonal phase. Such properties make VO2 a suitable material for Mott field & ndasheffect transistors, optical switching devices, thermochromic coatings, and electronic devices exhibiting sharp thresholdlike variation of electrical and optical properties in response to external stimuli such as temperature and voltage. Scaling VO2 to nanoscale dimensions has recently been possible and has allowed well-defined VO2 nanostructures to serve as model systems for measurements of intrinsic properties without obscuration from grain boundary connectivities and domain dynamics.^Geometric confinement, substrate interactions, and varying defect densities of VO2 nanostructures gives rise to an electronic and structural phase diagram that is substantially altered from the bulk. In my talk, I will outline two distinct hydrothermal approaches for the synthesis of 1D single-crystalline VO2 nanostructures exhibiting a substantial diminution in the metal-insulator phase transition temperature based on a) the hydrothermal hydration, exfoliation, and recrystallization of bulk V2O4 and b) the hydrothermal reduction and doping of V2O5 using small-molecule reducing agents and tungsten precursors. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution WxV1 & ndashxO2.^Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal & rarr insulator and insulator & rarr metal phase transformations. Notably, the in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W6+ doping in the WxV1 & ndashxO2 nanobelts. We postulate that design principles extracted from fundamental understanding of phase transitions in nanostructures will allow the predictive and rational design of systems with tunable charge and spin ordering.

Download or read book Vanadium Dioxide Phase Transition Modeling and Bias Control for Photodetection written by Zhongnan Qu and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Vanadium dioxide (VO_2) is a transition material that demonstrates phase transitions between the insulator and metallic states when under thermal, electrical or optical stimuli. It is a promising material for novel devices, for instance, switches, volatile memory, oscillatory neural network units, and photodetectors. Despite being a long-standing interest in the field of condensed matter physics, the numerous observed properties of VO_2 remain insufficiently explained, which impedes the progress of its utilization for electronic devices. This thesis develops theoretical explanations and corresponding analytical and numerical models that fit our fabricated VO_2 devices. This thesis also investigates the feasibility of utilizing VO_2 as photodetectors and develops a bias control circuit with low-cost off-the-shelf circuit components, enabling repeated operations despite the hysteresis effect, demonstrated the potential of VO_2 to be integrated with existing electronics.

Download or read book Phase Stability and Transformations in Vanadium Oxide Nanocrystals written by Amy Jo Bergerud and published by . This book was released on 2016 with total page 127 pages. Available in PDF, EPUB and Kindle. Book excerpt: Vanadium oxides are both fascinating and complex, due in part to the many compounds and phases that can be stabilized as well as the phase transformations which occur between them. The metal to insulator transitions (MITs) that take place in vanadium oxides are particularly interesting for both fundamental and applied study as they can be induced by a variety of stimuli (i.e., temperature, pressure, doping) and utilized in many applications (i.e., smart windows, sensors, phase change memory). Nanocrystals also tend to demonstrate interesting phase behavior, due in part to the enhanced influence of surface energy on material thermodynamics. Vanadium oxide nanocrystals are thus expected to demonstrate very interesting properties in regard to phase stability and phase transformations, although synthesizing vanadium oxides in nanocrystal form remains a challenge. Vanadium sesquioxide (V2O3) is an example of a material that undergoes a MIT. For decades, the low temperature monoclinic phase and high temperature corundum phase were the only known crystal structures of V2O3. However, in 2011, a new metastable polymorph of V2O3 was reported with a cubic, bixbyite crystal structure. In Chapter 2, a colloidal route to bixbyite V2O3 nanocrystals is presented. In addition to being one of the first reported observations of the bixbyite phase in V2O3, it is also one of the first successful colloidal syntheses of any of the vanadium oxides. The nanocrystals possess a flower-like morphology, the size and shape of which are dependent on synthesis time and temperature, respectively. An aminolysis reaction mechanism is determined from Fourier transform infrared spectroscopy data and the bixbyite crystal structure is confirmed by Rietveld refinement of X-ray diffraction (XRD) data. Phase stability is assessed in both air and inert environments, confirming the metastable nature of the material. Upon heating in an inert atmosphere above 700 C, the nanocrystals irreversibly transform to the bulk stable corundum phase of V2O3 with concurrent particle coarsening. This, in combination with the enhanced stability of the nanocrystals over bulk, suggests that the bixbyite phase may be stabilized due to surface energy effects, a well-known phenomenon in nanocrystal research. In Chapter 3, the reversible incorporation of oxygen in bixbyite V2O3 is reported, which can be controlled by varying temperature and oxygen partial pressure. Based on XRD and thermogravimetric analysis, it is found that oxygen occupies interstitial sites in the bixbyite lattice. Two oxygen atoms per unit cell can be incorporated rapidly and with minimal changes to the structure while the addition of three or more oxygen atoms destabilizes the structure, resulting in a phase change that can be reversed upon oxygen removal. Density functional theory (DFT) supports the reversible occupation of interstitial sites in bixbyite by oxygen and the 1.1 eV barrier to oxygen diffusion predicted by DFT matches the activation energy of the oxidation process derived from observations by in situ XRD. The observed rapid oxidation kinetics are thus facilitated by short diffusion paths through the bixbyite nanocrystals. Due to the exceptionally low temperatures of oxidation and reduction, this material, made from earth-abundant atoms, is proposed for use in oxygen storage applications, where oxygen is reversibly stored and released. Further oxidation of bixbyite V2O3 under controlled oxygen partial pressure can lead to the formation of nanocrystalline vanadium dioxide (VO2), a material that is studied for its MIT that occurs at 68 C in the bulk. This transformation is accompanied by a change in crystal structure, from monoclinic to rutile phase, and a change in optical properties, from infrared transparent to infrared blocking. Because of this, VO2 is promising for thermochromic smart window applications, where optical properties vary with temperature. Recently, alternative stimuli have been utilized to trigger MITs in VO2, including electrochemical gating. Rather than inducing the expected monoclinic to rutile phase transition as originally proposed, electrochemical gating of the insulating phase was recently shown to induce oxygen vacancy formation in VO2, thereby inducing metallization, while the characteristic V-V dimerization of the monoclinic phase was retained. In Chapter 4, the preparation and electrochemical reduction of VO2 nanocrystal films is presented. The nanocrystalline morphology allows for the study of transformations under conditions that enhance the gating effect by creating a large VO2-electrolyte interfacial area and by reducing the path length for diffusion. The resulting transitions are observed optically, from insulator to metal to insulator and back, with in situ visible-near infrared spectroelectrochemistry and correlated with structural changes monitored by Raman and X-ray absorption spectroscopies. The never-before-seen transition to an insulating phase under progressive electrochemical reduction is attributed to an oxygen defect induced phase transition to a new phase. This is likely enabled by the nanocrystalline nature of the sample, which may enhance the kinetics of oxygen diffusion, support a higher degree of lattice expansion-induced strain, or simply alter the thermodynamics of the system.

Download or read book Vanadium Dioxide written by Jaime Monzon Reyes and published by . This book was released on 1974 with total page 456 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Solid solid Phase Transition in Vanadium Dioxide Thin Films written by Joyeeta Nag and published by . This book was released on 2011 with total page 192 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Study of Phase Transition Mechanisms in Vanadium Dioxide Thin Films for Flat Panel Display Appliciations written by Scott H. Beasor and published by . This book was released on 2000 with total page 190 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Shining Light on the Phase Transitions of Vanadium Dioxide written by Tyler J. Huffman and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The salient feature of the familiar structural transition accompanying the thermally-driven metal-insulator transition in bulk vanadium dioxide (VO2) is a pairing of all the vanadium ions in the monoclinic M¬1 insulating phase. Whether this pairing (unit cell doubling) alone is sufficient to open the energy gap has been the central question of a classic debate which has continued for almost sixty years. Interestingly, there are two less familiar insulating states, monoclinic M2 and triclinic, which are accessible via strain or chemical doping. These phases are noteworthy in that they exhibit distinctly different V-V pairing. With infrared and optical photon spectroscopy, we investigate how the changes in crystal structure affect the electronic structure. We find that the energy gap and optical inter-band transitions are insensitive to changes in the vanadium-vanadium pairing. This result is confirmed by DFT+U and HSE calculations. Hence, our work conclusively establishes that intra-atomic Coulomb repulsion between electrons provides the dominant contribution to the energy gap in all insulating phases of VO2. VO2 is a candidate material for novel technologies, including ultrafast data storage, memristors, photonic switches, smart windows, and transistors which move beyond the limitations of silicon. The attractiveness of correlated materials for technological application is due to their novel properties that can be tuned by external factors such as strain, chemical doping, and applied fields. For advances in fundamental physics and applications, it is imperative that these properties be measured over a wide range of regimes. Towards this end, we study a single domain VO2 crystal with polarized light to characterize the anisotropy of the optical properties. In addition, we study the effects of compressive strain in a VO2 thin film in which we observe remarkable changes in electronic structure and transition temperature. Furthermore, we find evidence that electronic correlations are active in the metallic rutile phase as well. VO2 films exhibit phase coexistence in the vicinity of the metal-insulator transition. Using scanning near-field infrared microscopy, we have studied the patterns of phase coexistence in the same area on repeated heating and cooling cycles. We find that the pattern formation is reproducible each time. This is an unexpected result from the viewpoint of classical nucleation theory that anticipates some degree of randomness. The completely deterministic nature of nucleation and growth of domains in a VO2 film with imperfections is a fundamental finding. This result also holds promise for producing reliable nanoscale VO2 devices.

Download or read book X Ray Metrology in Semiconductor Manufacturing written by D. Keith Bowen and published by CRC Press. This book was released on 2018-10-03 with total page 296 pages. Available in PDF, EPUB and Kindle. Book excerpt: The scales involved in modern semiconductor manufacturing and microelectronics continue to plunge downward. Effective and accurate characterization of materials with thicknesses below a few nanometers can be achieved using x-rays. While many books are available on the theory behind x-ray metrology (XRM), X-Ray Metrology in Semiconductor Manufacturing is the first book to focus on the practical aspects of the technology and its application in device fabrication and solving new materials problems. Following a general overview of the field, the first section of the book is organized by application and outlines the techniques that are best suited to each. The next section delves into the techniques and theory behind the applications, such as specular x-ray reflectivity, diffraction imaging, and defect mapping. Finally, the third section provides technological details of each technique, answering questions commonly encountered in practice. The authors supply real examples from the semiconductor and magnetic recording industries as well as more than 150 clearly drawn figures to illustrate the discussion. They also summarize the principles and key information about each method with inset boxes found throughout the text. Written by world leaders in the field, X-Ray Metrology in Semiconductor Manufacturing provides real solutions with a focus on accuracy, repeatability, and throughput.

Download or read book Principles of Nano Optics written by Lukas Novotny and published by Cambridge University Press. This book was released on 2012-09-06 with total page 583 pages. Available in PDF, EPUB and Kindle. Book excerpt: Fully revised and in its second edition, this standard reference on nano-optics is ideal for graduate students and researchers alike.

Download or read book High Energy Spectroscopy on Vanadium Oxides written by Hermen Folken Pen and published by . This book was released on 1997 with total page 168 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Induction of Semiconductor metal Phase Transition in Vanadium Dioxide Through Photon phonon Conversion written by Douglas Craig Barnard and published by . This book was released on 1979 with total page 66 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Theoretical Exploration of the Metal Insulator Transition in Vanadium Dioxide with an Eye Towards Applications A First Principles Approach written by and published by . This book was released on 2009 with total page 22 pages. Available in PDF, EPUB and Kindle. Book excerpt: Vanadium oxides are very interesting compounds which exhibit exotic transport phenomena. In particular vanadium dioxide (VO2) undergoes a first-order transition from a high-temperature metallic phase to a low-temperature insulating phase at almost the room temperature (T = 340 K). The resistivity jumps by several orders of magnitude through this transition, and the crystal structure changes from rutile (R-phase) at high-temperature to monoclinic (so-called M1-phase) at low-temperature. The latter is characterized by a dimerization of the vanadium atoms into pairs, as well as a tilting of these pairs with respect to the c-axis. VO2 has also attracted a great deal of attention for its ultrafast optical response, switching between the R and the M1 phase. Despite the large number of experimental studies focusing on this material the physics driving this phase transition and the resulting optical properties is still mysterious. There are intensive reports around the world to make devices such as switches, transistors, detectors, varistors, phase change memory, exploiting the unique properties of VO2. Two physical effects, Peierls, i.e. dimerization, and the Mott mechanism due to strong Coulomb repulsion are important in the metal-insulator transition (MIT) of VO2. Understanding the detailed interplay and the relative importance of both Peierls and Mott mechanism is important for controlling this material with an eye towards applications. For example, whether the driving force of this transition is electronic (i.e. occurring on femtosecond timescales) or structural (occurring on the picosecond timescale) is important to understand the speed of the switching from the M1 to the rutile phase. The insights obtained in this study together with the computational machinery developed, will serve as a basis for rational material design of VO2 based applications.