X-ray Diffraction Studies on the Metal-Insulator Transition in Vanadium Oxide Nanocrystals

Abstract

In this dissertation, we study the comprehensive structural changes accompanying metal insulator transition (MIT) in vandium dioxide (VO2) nanocrystals using in-situ synchrotron x-ray diffraction (XRD) measurements, which is an ideal and robust technique to precisely investigate changes in the lattice structure at the nanoscale level. Recently, transition-metal oxides (TMOs) such as VO, VO2, V2O3, Fe3O4 etc have attracted immense discussion due to their MIT transition property. Among them, VO2 is especially attractive since its MIT, where electrical conductivity and optical refctance change drastically, occurs near 68 °C, close to room temperature (RT). Due to the existence of this MIT, VO2 thin films and nanostructures have been investigated for practical applications such as sensor, energy harvesting systems, thermoelectric room temperature devices, ultrafast nonvolatile memories, and optical switches. This MIT transition is accompanied by a structural phase transition (SPT) where a low temperature (T) monoclinic (M) structure changes to a high temperature rutile (R) structure.

The origin of this MIT transition including the role of the structural modification has been extensively studied but still remains disputable. Recent research has been directed towards the possibility of controlling the MIT properties by modifying the morphology of nano structures, structural defects, stoichiometry, and strain. Since the electronic band structure of VO2 depends sensitively on the small lattice distortion leading to pairing of vanadium atoms, the MIT property is altered critically by the nature and density of structural defects. It is reported that grain boundaries affect the MIT characteristics as they might act as nucleation centers of the metallic phase, and the propagation of phase boundaries are controlled by them. In this dissertation we have also demonstrated the tuning of the MIT temperature using the morphology of the twinned VO2 nanocrystals. The details of studies are as follows.

In chapter 1, we write motivation for research related to vanadium oxide and its significance in the cutting-edge research. Principles, the theories and features of x-ray diffraction study are introduced in chapter 2. Chapter 3 contains literature survey and scientific research related to vanadium oxide such as synthesis study, MIT transitions and their control etc. Chapter 4 deals with the contents of experimental setups and methods used for the synthesis of VO2 nanocrystals and their characterizations.

In chapter 5 we discuss about the synthesis and structure of ’V’ shape twinned VO2 nanocrystals epitaxially grown on c-plane sapphire substrates using a vapor transport method. The (100)M twin plane played a key role in the determination of the morphology of VO2 nanocrystals. The growth of VO2 nanocrystals begins at the twin plane and proceeds toward two possible monoclinic [100] (aM -axis) direction resulting in ‘V’ shape twin crystals with the angle between the sides of around 115.4°. At relatively low growth temperature, 900 °C, the growth of the sides of ’V’ was limited producing ’coffee-bean’ shape crystals in which flat crystal facet regions are connected with rounded edges. The twin crystals were epitaxial to the c-plane sapphire substrate with the monoclinic [010] (bM -axis) normal to the substrate. In the in-plane direction, the (001)M planes of the VO2 twin crystals was aligned to the direction ±2.3° away from the sapphire (112 ̅0) plane. The sides of ’V’ exhibit a rectangular cross-section truncated by the substrate. A synchrotron x-ray diffraction measurement across the metal-insulator transition of the twin crystals implies that the phase transition of the coffee-bean shape nanocrystals occurs at a lower temperature with a smaller hysteresis gap than the fully grown V-shape nanocrystals.

In chapter 6 we investigated the structural transformation from the insulating monoclinic to the metallic rutile phase in VO2 using synchrotron x-ray diffraction in a grain orientation specific manner using a two-dimensional x-ray pixel detector. The XRD data averaged in all grain orientation corresponding to typical powder diffraction profile, the transition occurred over a broad temperature range of about 8.9K. However, the transition temperature and range vary greatly among the grains composing a bulk VO2 specific. We attribute the broadness of the transition in the averaged profile to this variance of the transition temperature among grains. Consistently, the transition was much sharper in the nanoparticle specimen, where were able to isolate signals from even fewer grains than the bulk power specimen. To understand the intrinsic physical characteristics, one has to carefully analyze data from a bulk system, and it is preferable to study VO2 single crystals.

And finally in the last, chapter 7 contains the conclusions of this dissertation.