UV-NIR Eco-sustainable light sources based on halide perovskites for biomedical applications

Recent advancements in nanoscience, materials science, photonics, and electronics have sparked significant interest in the miniaturization and integration of light sources. Halide perovskites, including both hybrid organic-inorganic and fully inorganic variants, have emerged as a pivotal class of semiconductors with considerable importance in optoelectronics, energy harvesting, and, more recently, in medical diagnostics and therapy. These materials exhibit properties that are particularly well-suited for light-emitting devices. This PhD thesis will specifically focus on caesium lead halides, namely CsPbBr3 and CsPbCl3, with the objective of fabricating thin films on various substrates that demonstrate high uniformity in composition, thickness and optical properties. The primary goal is to assess their potential applicability in light-emitting devices and radiation detection and to investigate additional applications for these materials in the field of optoelectronics. The study will include a comprehensive analysis of the fabrication process for solvent-free, high-quality fully inorganic halide perovskite thin films utilizing RF-MS deposition. This technique was firstly demonstrated at the Department of Physics and Astronomy at the University of Florence for the deposition of fully inorganic lead halide thin films, thus facilitating their application for optoelectronic devices. This thesis presents pioneering analyses and results, including an in-depth examination of various parameters and a comprehensive characterization of the resulting thin films produced using this technique. It investigates and demonstrates Amplified Spontaneous Emission (ASE) and photoluminescence (PL) from polycrystalline CsPbBr3 thin films at room temperature: for the first time we utilize film thicknesses ranging from 100 to 300 nm having a gain length comparable to the film thickness without the use of variable length stripe method. Notably, this experimental configuration has not been previously reported in the literature concerning halide perovskite samples and it is of interest for the development of vertical emitters. The sample analysis covers structural, morphological, chemical and optical properties and it includes detailed results from the ASE experiments. Furthermore, this research explores an additional application of halide perovskite materials through the development of an innovative sensor architecture known as the CsPbCl3/Pd interdigitated electrode (IDE), specifically designed for proton therapy applications. The findings of this study underscore significant implications for medical treatment procedures and contribute to advancements in the biomedical field. Firstly, a comprehensive analysis is conducted on the fabrication process for solvent-free, high-quality fully inorganic halide perovskite thin films using RF-MS deposition, a technique demonstrated for the first time at the Department of Physics and Astronomy at the University of Florence for depositing fully inorganic lead halide thin films. This analysis encompasses the effects of varying parameters, such as different thicknesses and deposition rates, resulting in the production of halide perovskite monolayers with thicknesses of 3.5 Å, but also of 100 nm, 300 nm, 500 nm, and 1 μm. Furthermore, an extensive characterization of the samples is employed using techniques such as scanning electron microscopy (SEM), high-resolution scanning electron microscopy (HR-SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thin film uniformity and emission spectra. These characterizations were conducted to understand how variations in parameters affect the pursuit of achieving ASE at room temperature, thereby highlighting their potential application in light-emitting devices and exploring further applications in the biomedical field. The results obtained — encompassing chemical, morphological, structural, and optical emission characteristics — collectively support the conclusion that ASE at room temperature can be effectively achieved with samples deposited at the optimal rate of 0.3 Å/s, particularly for thicknesses of 100 nm, 300 nm, and 500 nm. These findings not only advance the fundamental understanding of metal halide perovskites but also improve their potential application in next-generation optoelectronic devices. Building upon the various aspects and preliminary findings from this research, two extensive studies are conducted to further investigate the potential applications of halide perovskite thin films. The first investigation detailed in this thesis centers on the study of ASE and PL from polycrystalline CsPbBr3 thin films at room temperature, which is a primary objective of this research. This study aims to elucidate the mechanisms driving ASE at room temperature, using film thicknesses ranging from 100 to 300 nm as the gain length. The goal is to demonstrate that this gain length can be minimized to just a few hundred nanometres through a transmission configuration. The experiments conducted reveal a robust ASE signal at room temperature, characterized by notable saturation behaviour and a significant reduction in the absorption coefficient, indicating the initiation of nonlinear optical processes. This pioneering finding shows that ASE can be effectively achieved even when the gain mechanism relies solely on the material's natural absorption length, rather than being confined within a traditional waveguide. This experimental demonstration marks a significant advancement in the understanding and application of halide perovskite thin films, particularly CsPbBr3, for light emission, which is a key aspect of this thesis. Further exploration using high spectral resolution techniques has unveiled a modal structure exhibiting remarkable stability in both spectral line shape and intensity across successive laser pulses. This stability suggests the emergence of lasing effects under transmission conditions, representing a notable discovery that indicates the potential for constructing laser devices without conventional guiding components. The second study focuses on the application of CsPbCl3 inorganic perovskite thin-film detectors for real-time monitoring in proton therapy. This investigation highlights another significant use of halide perovskite thin films: their potential as efficient radiation detectors. The specifical aim was to develop active devices that function as real-time dosimeters in proton therapy. By utilizing CsPbCl3 thin film devices integrated onto flexible substrates with interdigitated electrodes (IDE), their response to proton beams with energies ranging from 100 to 228 MeV was investigated. The experimental results demonstrate the exceptional effectiveness of the CsPbCl3/Pd IDE sensors, showcasing their ability to accurately monitor radiation in real time. The devices displayed a linear response both in terms of extraction currents and proton flux, making them highly effective tools for radiation detection. These attributes are particularly valuable in proton therapy applications, where precision and immediate response are critical for effective treatment outcomes. Future research endeavours will focus on leveraging CsPbBr3 for radiation detection applications. Specifically, we intend to utilize thin films of CsPbBr3, produced via RF-MS deposition, as ionizing detectors for proton beam irradiation, which is commonly employed in therapeutic settings such as cancer treatment. The choice of CsPbBr3 as a radiation detector is particularly promising due to its favourable optoelectronic properties, which include high photoluminescence efficiency and sensitivity to radiation. Proton therapy involves bombarding tumour tissue with protons, which require precise monitoring to ensure accurate dose delivery and minimize damage to surrounding healthy tissue. By integrating CsPbBr3 thin films into detection systems, we aim to exploit their capacity for real-time monitoring of radiation levels. The implementation of CsPbBr3 as an ionizing detector involves characterizing its response to proton beams and assessing its performance metrics, such as sensitivity and temporal resolution. These characteristics will be crucial in ensuring that the detector can efficiently convert incoming radiation into measurable electrical signals, ultimately leading to improved accuracy in dose measurement during proton therapy. This integration promises to enhance the safety and effectiveness of therapeutic applications, potentially paving the way for innovative advancements in radiation detection technology.