Browsing by Subject "Metal-insulator transition"

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Open Access
Crystal growth and investigations on the effects of hydrogen doping of VO2
(2019-03) Yavuz, Koray
Vanadium Dioxide(VO2) has been studied extensively for its interesting electronic structure that allows it to go through Metal-Insulator Transition(MIT) at 65 C. The nature of this phenomena is not entirely clear and more research is needed to firmly establish the science behind it and to realize possible applications; such as ultra-fast electrical and optical switching, sensor devices and Mott-Field Effect Transistors. One of the important experiments to understand the electronic structure of a material is Hall-effect measurements but due to acicular (needle like) nature of VO2 crystals, this subject is only studied either on millimeter sized samples which are not suitable for many device applications or on poly crystalline thin films that are under non-uniform stress due to the substrate effects which gives unsatisfactory results when performing experiments. This thesis suggest a new method of chemical vapour deposition(CVD) growth for low aspect ratio VO2 crystals that have lengths between 50-100 m and thicknesses between 40- 170 nm. These crystals can be mechanically removed from the substrate and transferred to use in different applications such as Hall-effect measurements or Transmission Electron Microscope(TEM) studies. Additionaly this work shows some aspects of the surface chemistry of the widely used Silica, Si, quartz and Sapphire substrates; relating with the control of oxygen saturation on the surface. Another VO2 growth method for c-plane sapphire that leads to considerably more crystal yield is shown. Hydrogenation of the VO2 crystals suppresses the MIT so understanding this phenomena might help us better understand the effects lying behind the transition. To study this phenomena a crystal is doped only from half by blocking the passage of hydrogen to other half so the interplay between the insulating phase and hydrogenated conductive phase can be observed. As the analysis tool, TEM is used on this sample. Using a two-terminal device of a VO2 crystal, the effects of hydrogenation on the electronic properties have also been studied. Overall this thesis introduces a new method for CVD growth of VO2 which is used in various applications such as Hall-effect experiments, two terminal devices and TEM studies. To control the growth process the interplay between oxygen and surface chemistry of sapphire, silica, Si and quartz substrates have been investigated. With these studies a better understanding of the mechanics of growth is intended.
Open Access
Investigation of the effects of thickness on the metal-insulator transition in vanadium dioxide nanocrystals, and development of a novel vanadium dioxide mott field-effect transistor
(2017-07) Fadlelseed, Mustafa Mohieldin Fadlelmula
Vanadium dioxide (VO2) is a material that has attracted a lot of attention for its prospective potential to be utilized in the eld of electrical and ultrafast optical switching in one hand, and for the fundamental physics that can be revealed through studying this strongly correlated material on the other hand. One of the most attractive qualities of VO2 is the metal-insulator transition (MIT) which takes place slightly above room temperature in this material. Controlling such phase transition through external stimuli would open unprecedented avenues of electrical and optical applications. However, thin VO2 nanocrystal are required to overcome the limitation imposed thought the Thomas-Fermi screening length which limits the changes and the control that external electrical stimuli would have on any crystal that exceeds this length. The screening length in VO2 is known to be no more than 6 nm. Here, we avoided the use of epitaxial and sputtered lms for the complications in such materials that arise from the stress due to lattice mismatch and the interdi usion with substrates in epitaxial lms, and the polycrystalline nature of sputtered lms. In this work, vapor-phase grown VO2 nanocrystals are used instead. One reason behind this is that unlike epitaxial lms vapor-phase grown VO2 nanocrystals can be released out of the growth substrate and transferred in order to eliminate the stress induced on the crystals due to adhesion to the substrate. The main shortcoming of this type of crystals, which is addressed thoroughly in this study, is that vapor-phase grown VO2 nanocrystals are produced with dimensions no less than 30 nm due to the lack of thickness control in physical vapor deposition technique. Mainly in this study, a systematic method to mill down vapor-phase grown VO2 nanocrystals to sub-5 nm thicknesses is developed. Ar-ion milling is utilized to achieve this goal. Photoresist protection and shadowing methods are introduced and used to reveal the etch rate of VO2 nanocrystals which is found to be equal to 3.3 0:3 nm/min using ion-gun energy of 1 KeV with medium monatomic ux. Our results show some surface damage caused by the Ar-ions bombardment that is limited maximum to the top 5.6 nm of the surface of the etched crystals. This damage and related changes in the electrical properties in the milled crystals are completely eliminated by short duration treatment in a 37% hydrochloric acid (HCl(aq)) solution of these crystals. The results presented here in this regards show complete recovery of the relative order of changing in resistance that accompanies the MIT of treated etched crystals when compared to their pristine form. The last part of this study is dedicated to the investigation of implementing mill down vapor-phase grown VO2 nanocrystals in possible prospective applications. Mainly, the use of these crystals in constructing Mott-Field E ect Transistors (Mott-FETs) is investigated. Further investigation are yet to be done in this regards in order to draw a nal conclusion in the possibility of using VO2 nanocrystals in reliable Mott-FETs. However, the results presented here along with the suggestions related to the fabrication of vapor-phase grown VO2 nanocrystals based three-terminal devices are of a vital importance in setting directions for future works.
Open Access
Synthesis of V2 O3 nanoplates for the exploration of the correlated supercritical state
(American Physical Society, 2019) Rasouli, Hamid Reza; Mehmood, Naveed; Çakıroğlu, Onur; Sürmeli, Engin Can; Kasırga, T. Serkan
Peculiar features exist in the stress-temperature phase stability diagram of V2O3, such as a first-order phase transition between the paramagnetic insulating and metallic phases that ends with a critical point, quantum phase transition, and a triple point. These features remain largely unexplored, and the exact nature of the phase transitions is not clear due to very limited control over the stress in bulk or film samples. Here, we show the synthesis of single-crystal V2O3 nanoplates using chemical vapor deposition via van der Waals epitaxy. Thickness of the V2O3 nanoplates range from a few to hundreds of nanometers, and they can be mechanically exfoliated from the growth substrate. Using Raman spectroscopy on the nanoplates, we reveal that, upon heating, V2O3 enters a supercritical state for both tensile strained and relaxed crystals with a similar out-of-plane response. Transmission electron microscopy on V2O3 nanoplates hints at the existence of a structural change when the crystals are heated. Our results show that V2O3nanoplates should be useful for studying the physics of the supercritical state and the phase stability of V2O3 to enable new horizons in applications.