Titolo della tesi: Bringing To Light Interesting Behaviours Of The Vanadium Dioxide and NV Centres in Diamond
Vanadium dioxide (VO2) shows a steep change in electrical and optical properties during the Semiconductor to Metal phase Transition (SMT), along to the temperature in which this change occurs ∼68°C, near the room one, are the reasons of the interest shown in this material in the scientific community. Some of the most captivating fields of study are the development of advanced fabrication methodologies to improve the performance of VO2 thin films for phase-change applications in optical devices, and the true physical reason for said transition to happen, lattice stress or electronic band distortion.
Usually the means to characterize the VO2 thin films are expensive and time consuming, i.e. x-ray diffractometers, electron microscopes and atomic force microscopes. This thesis work focus on finding a fast and effective technique to acquire the structural, morphological and thermal behaviour on polycrystalline VO2 thin films. To achieve this has been implemented a purely optical approach which combines Polarized Raman Mapping and Phase-Transition by Continuous Wave Optical Excitation (PTCWE), with these two techniques combined is possible to reconstruct a complete picture of the properties of the films, achieving the objective of being rapid and cheap. Furthermore an interesting stepped behaviour during the SMT, probably induced by the progressive stabilization of rutile metallic domains embedded in the semiconducting monoclinic matrix.
On the other hand a quantum magnetometry technique has been studied, particularly a Nitrogen-Vacancy (NV) embedded in diamond. Sensors exploiting the nature of quantum mechanics have attracted a lot of interest, this is due to their ability to have a strong sensitivity to external perturbation and to show a greater resolution. There exist a variety of these kind of sensors that in turn can measure distinct physical quantities, in the case of magnetic field there are different choices, such as superconducting quantum interference devices (SQUIDs) and atomic vapor cells other then the NV centres in diamond, respectively the limitation of SQUIDs are that they require cryogenic temperature to operate and vapor cells have a limiting factor given by their size.
NV centres on the other hand are able to overcome these limitations, having a room working temperature and can be incorporated in nanodiamonds as small as 5nm. Their implementation permits them to be used both as pixels in a magnetic field map and as tips for high resolution scanning probe microscopy, other then the magnetometry applications the NV centres may be used as detectors for temperature, pressure and electric field, making them multi-modal sensors.
The integration of quantum magnetometry techniques with optical methods could lead to a comprehensive framework for characterizing VO2 thin films. While the optical approach can give the data on the phase transition and crystal structure, NV centre magnetometry could provide complementary data on local magnetic fields, bringing to light the intrinsic nature of the transition. Together, these techniques could unveil complex interactions within the material, such as strain effects, domain formations, and electron correlations, which are essential for understanding and controlling the SMT.
The objectives of this research are thus twofold: first, to enhance the morphological and thermal characterization of VO2 thin films using advanced optical methods; and second, to explore the application of quantum sensors, particularly NV centres, and their possible application in studying the magnetic behaviour of thin films. By bridging material science and quantum technology, this work aims to contribute to the development of novel sensing methodologies and to deepen our understanding of phase transition phenomena in materials like VO2.