Development of Colloidal Quantum Dots-based Ionising Radiation Sensors

The development of novel materials and detectors for ionising radiation has gathered consistent interest in the scientific community in the last years. Efficient detection of ionising radiation is crucial in several fields, that span from radiation protection to high energy physics. Airports, nuclear plants, research institutes, hospitals and radiotherapy centers all employ ionising radiation on a daily basis. Radiation detection therefore plays a key role in measuring the dose and the characteristic of the particles that the personnel and the general public could be exposed to in these facilities. Today, current commercial X-ray and gamma solid state detectors rely on inorganic semiconductors such as Cadmium Zinc Telluride crystals, amorphous Selenium, High Purity Germanium or Silicon based technologies. These materials show excellent performance. However, these inorganic semiconductors are rigid, brittle, some are very expensive and have severe limitations for large-area applications. Thus, the research community is actively studying promising alternatives to inorganic solid crystals that can overcome these limitations. In this thesis, the promising use of Colloidal Quantum Dots as novel sensing material in X-ray detection is demonstrated. As solution-processable semiconductors, these nanocrystals meet the requirements for ease of fabrication, low cost, large-area application, but their detection capabilities have been investigated in only but a few studies. However, Lead Sulfide quantum dots have extensively been employed in infrared and visible light photodetectors, with ever improving results in terms of performance and reliability. Being a heavy element, Lead also has a high photoabsorption cross-section for X-ray photons. These reasons motivated me to investigate and develop Lead Sulfide Colloidal Quantum Dots based photodetectors as direct detectors for X-ray photons. The detectors described in this work have been developed, fabricated and tested during the PhD research project. A complete laboratory production procedure was designed, starting from the very synthesis of the nanomaterial to the final testing of the detectors. The research line included the synthesis and purification of the quantum dots, their deposition and functionalisation onto the sample substrates, the surface characterisation of the material, the electrical bonding and housing into supports, the benchmarking of their electrical characteristics with optical light and, finally, their testing under X-ray radiation. For all these steps, I adapted to ionising radiation detection methods well known in other fields to design optimised recipies and procedures for the objectives of the project. I developed and demonstrated Hard X-ray detection by a simple drop-cast photodetector with Lead Sulfide QD sensing layer. The thin (3 um) detecting layer showed record sensitivities for Quantum Dots-based detectors with a low, microelectronic-compatible, bias voltage of just 1V. Motivated by the excellent proof-of-concept results, I designed a similar direct detector, with a different ligand molecule for the QD and a different deposition technique. These devices were fabricated with Electrohydrodynamic jet printing, which allowed to precisely deposit the sensing material onto the electrical contacts in a drop-on-demand approach. These devices showed a good sensitivity to hard X-rays and an improved limit of detection. The samples showed a significant improvement in response speed, compared to the drop-cast devices described earlier.