Marco Girolami

Some properties of electrochemical systems can be properly characterized by low- and extremely low-frequency impedance measurements. The frequency range down to 1 mHz requires peculiar attention in terms of accuracy, measurement speed, portability and costs, and commercial systems are often unable to guarantee good performances without expensive or bulky solutions. Here, we first review some of the applications of electrochemical impedance spectroscopy and their requirements and then report on recent works proposing circuits and systems focused on optimizing performances, reducing costs, and increasing versatility. Circuits for generating controllable current sources are described, optimizing output impedance and measurable impedance range. Then, integrated systems are taken into account, aiming at realizing portable and versatile instruments.

Two-photon sub-bandgap photocurrent (TPPC) is demonstrated in surface-nanotextured black diamond films fabricated by a two-step femtosecond laser treatment at an optimal total accumulated laser fluence of 5.0 kJ cm^-2, unequally split between the two steps with a split ratio of 2:1. A broad intermediate band of electrically active deep-level defects, located at an energy distance of 1.83 eV from the conduction band, is experimentally observed by sub-bandgap spectral response evaluation in the 190-1000 nm wavelength range, allowing for a significant enhancement of quantum efficiency under photovoltaic conditions. This work represents the first experimental demonstration of TPPC in bulk materials with deep-level impurities conceived for intermediate-band solar cells and provides a solid physical interpretation of the operating principle of defect-engineered black diamond-based devices for solar energy conversion.

FLASH radiotherapy, which delivers ultra-high dose rates (UHDR) in microsecond-scale pulses, demands real-time, high-precision dosimetry to investigate the so-called FLASH effect. However, conventional systems struggle to operate reliably under such extreme conditions. This work presents a novel dosimetric system combining a synthetic diamond detector with custom front-end and acquisition electronics, specifically designed for UHDR electron beam diagnostics. The system enables real-time measurements of pulse duration with ±8 ns resolution, supports charge-per-pulse values up to 40 nC with 10 bits of resolution, and provides instantaneous dose rate estimates in the MGy/s range. These features allow accurate quantification of pulse parameters and cumulative dose, making the system a robust tool for UHDR beam characterization and the commissioning of FLASH-capable linear accelerators, with direct relevance to preclinical FLASH research.

Accurate measurement of the dose delivered during FLASH radiotherapy remains a challenging task for assessing the so-called FLASH effect in preclinical studies. Electron beams generated by LINACs used in FLASH radiotherapy consist of microsecond pulses and are therefore characterized by instantaneous dose rates on the order of MGy/s. Diamond represents one of the most promising materials for fabricating reliable dosimeters for FLASH radiotherapy. So far, excellent results have been achieved using detectors based on high-quality single-crystal CVD diamond samples. Conversely, the literature lacks solid evidence demonstrating the effectiveness of lower-quality HPHT diamonds in this context. Here, we demonstrate for the first time that a suitable acquisition technique for the pulse signal generated by an HPHT type IIa diamond detector under ultra-high dose rate irradiation enables the development of an effective system for accurately measuring the dose delivered by a FLASH LINAC. The detection prototype exhibits a linear response to both the dose and dose rate of the incident radiation, demonstrating that the implemented measurement technique effectively mitigates spurious trap-related signals that typically compromise accuracy. Our research therefore opens the way for the development of a new generation of diamond dosimeters based on low-cost type IIa diamonds, potentially broadening the application of diamond-based dosimetry in both FLASH and conventional radiotherapy.

The demand for flexible and wearable sensors is more and more increasing in a wide range of applications. Specifically, in the field of medical diagnostics and radiation therapy, a flexible X-ray dosimeter is extremely desirable, due to the possibility of a conformal contact between the human body and the sensing device. Here we report on a flexible X-ray detector based on an ultrathin (500 nm) film of a wide-bandgap metal-halide-perovskite (FAPbBr3) as absorbing layer, showing sensitivity values up to 375.24 μC Gy-1 cm-2 to X-rays with energy up to 40 keV, excellent linearity with the radiation dose-rate (linearity coefficient 0.97), and very good operational stability under 12 h of continuous irradiation. The device, relying on a simple low-cost technology, exploits the built-in potential of a typical p-in architecture used in perovskite solar cells, hence it does not require a bias voltage to operate, allowing for continuous radiation monitoring with no risk of battery depletion.

Metal-halide perovskites are revolutionizing the world of X-ray detectors, due to the development of sensitive, fast, and cost-effective devices. Self-powered operation, ensuring portability and low power consumption, has also been recently demonstrated in both bulk materials and thin films. However, the signal stability and repeatability under continuous X-ray exposure has only been tested up to a few hours, often reporting degradation of the detection performance. Here it is shown that self-powered direct X-ray detectors, fabricated starting from a FAPbBr₃ submicrometer-thick film deposition onto a mesoporous TiO₂ scaffold, can withstand a 26-day uninterrupted X-ray exposure with negligible signal loss, demonstrating ultra-high operational stability and excellent repeatability. No structural modification is observed after irradiation with a total ionizing dose of almost 200 Gy, revealing an unexpectedly high radiation hardness for a metal-halide perovskite thin film. In addition, trap-assisted photoconductive gain enabled the device to achieve a record bulk sensitivity of 7.28 C Gy⁻¹ cm⁻³ at 0 V, an unprecedented value in the field of thin-film-based photoconductors and photodiodes for “hard” X-rays. Finally, prototypal validation under the X-ray beam produced by a medical linear accelerator for cancer treatment is also introduced.

Orthorhombic κ-Ga₂O₃ thin films were grown for the first time on polycrystalline diamond free-standing substrates by metal-organic vapor phase epitaxy at a temperature of 650 °C. Structural, morphological, electrical, and photoelectronic properties of the obtained heterostructures were evaluated by optical microscopy, X-ray diffraction, current-voltage measurements, and spectral photoconductivity, respectively. Results show that a very slow cooling, performed at low pressure (100 mbar) under a controlled He flow soon after the growth process, is mandatory to improve the quality of the κ-Ga₂O₃ epitaxial thin film, ensuring a good adhesion to the diamond substrate, an optimal morphology, and a lower density of electrically active defects. This paves the way for the future development of novel hybrid architectures for UV and ionizing radiation detection, exploiting the unique features of gallium oxide and diamond as wide-bandgap semiconductors.