Browsing by Subject "Strongly correlated materials"

Now showing 1 - 4 of 4

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
Effects of thickness on the metal-insulator transition in free-standing vanadium dioxide nanocrystals
(American Chemical Society, 2017) Fadlelmula, M. M.; Sürmeli, E. C.; Ramezani, M.; Kasırga, T. S.
Controlling solid state phase transitions via external stimuli offers rich physics along with possibilities of unparalleled applications in electronics and optics. The well-known metal-insulator transition (MIT) in vanadium dioxide (VO2) is one instance of such phase transitions emerging from strong electronic correlations. Inducing the MIT using electric field has been investigated extensively for the applications in electrical and ultrafast optical switching. However, as the Thomas-Fermi screening length is very short, for considerable alteration in the material’s properties with electric field induced MIT, crystals below 10 nm are needed. So far, the only way to achieve thin crystals of VO2 has been via epitaxial growth techniques. Yet, stress due to lattice mismatch as well as interdiffusion with the substrate complicate the studies. Here, we show that free-standing vapor-phase grown crystals of VO2 can be milled down to the desired thickness using argon ion-beam milling without compromising their electronic and structural properties. Among our results, we show that even below 4 nm thickness the MIT persists and the transition temperature is lowered in two-terminal devices as the crystal gets thinner. The findings in this Letter can be applied to similar strongly correlated materials to study quantum confinement effects.
Open Access
Experiments on strongly correlated materials: magneto-transport properties of VO2 AND V2O3
(2019-09) Sürmeli, Engin Can
Vanadium oxides provide unusual electrical and magnetic phenomena emerging from strong electronic correlations, which include, among other things, a thermally induced metal-insulator transition (MIT). Investigation of the changes in carrier concentration and mobility across the MIT in vanadium oxides, such as vanadium dioxide (VO2) and vanadium sesquioxide (V2O3), carries great importance for understanding the micromechanisms behind suchfirst-order phase transitions. A well-known approach to measuring such parameters in semiconductor materials is Hall effect measurement. So far, magnetotransport studies have only been conducted on polycrystalline thinfilms of VO2/V2O3. As a result, reports on the Hall mobility of these materials often contradict with each other due to the non-uniform stress building on the crystal by adhesion to the substrate. Thus, a thorough investigation of Hall effect measurements on single-crystalline, stress-free VO2 nanobeams and V2O3 nanoplates is required. However, achieving this task is not a straightforward process. First of all, the relatively small size of nanobeams compared to the epitaxialfilms creates the necessity to utilize a bridge-type Hall-bar shaping of the crystal. Additionally, in order to produce a stress-free environment, the crystals must be detached from the substrate and transferred to an atomically at surface, such as hexagonal boron nitride (h-BN). Therefore, the device fabrication method demands many steps despite that VO2 is a very fragile material. In this work, we provide a new fabrication method for shaping VO2 and V2O3 into Hall-bar structure via Gallium and Argon-ion milling while inducing minimal damage on the crystal. We also investigate the strain level of shaped crystals and provide methods to prevent cracking in the devices upon structural phase As a second objective, we investigate the resistivity behavior and magnetic response of VO2 nanobeams at low temperature ranges. We show that the high magnetoresistance of VO2 creates demand for very high magneticfields in the Hall effect measurements. Finally, we demonstrate a Hall effect measurement on an as-grown V2O3 nanoplatelet across its phase transition.
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.