Growth, Characterization and Assembly of Lead Chalcogenide Nanosemiconductors for Ionizing Radiation Sensors

Abstract

The demand for precise and efficient nuclear radiation detection has increased dramatically, particularly in numerous scientific disciplines, as well as homeland security and medical imaging applications. Modern methods by which ionizing radiation was sensed has now heavily considered using nano-scale materials for the sake of effective counting. Nanocrystalline (NC), or quantum dot (QD), semiconductors themselves exhibit exploitable properties—such as tunable energy band gap and charge carrier multiplication (multi-exciton generation) which arise due to strong quantum confinement. With this, fabricating a quantum-dot-based semiconductor to operate as a high-performance detector, via a low-cost solution-based manufacturing method, can truly alter the capabilities of radiation detectors. Using this NC approach which primarily focuses on high atomic number and density materials, was investigated as a means to maximize charge creation while minimizing the uncertainty in that conversion, as the approach is based on favorable features of NC materials for their application to the detection of ionizing radiation. The intrinsically high charge mobility combined with high atomic number and density of the lead chalcogenides makes them attractive for sensing applications with highly penetrating quanta, such as x-rays and gamma-rays, as the lead chalcogenide materials possesses an extensive literature of synthetic routes with which one can explore the strong confinement regime in quantum dots. By varying the reaction conditions, NCs of various sizes and shapes were synthesized, and their physical and opto-electric properties were investigated. Drop-, float-, or dip-coating NC dispersions on various metal contacts resulted in close-packed NC assemblies of lead chalcogenides. However, in sensing architectures, the exploitation of various properties for each individual nanocrystallite (NC) is hampered by the need to transport the charge carriers throughout the active volume, a motion that can be retarded by energetic surface barriers typically in the form of insulating oxides. Various synthetic routes are investigated to fabricate lead chalcogenide QDs while the feasibility of utilizing NC materials as a basis for detecting ionizing radiation is also explored. QDs and their assembled structures were carefully investigated through characterization to determine their overall quality. Methods to improve NC interconnectivity were studied and mentioned. The prevention of surface oxidation through the fabrication of NCs was also explored, resulting in chemically and optically stable NCs for at least 1.6 years. Overall, this study focuses on using solution-based methods to fabricate nano-semiconductor nuclear radiation detectors. Various recipes will be presented, as well as the electrical results of developed NC assembly samples, with the focus on improving the charge carrier transport properties of these NC assemblies.