PbSe Quantum Dots for X-ray and γ-Ray Sensors

Abstract

Nanocrystalline (NC) semiconductors exhibit exploitable properties, such as a tunable energy band gap and multiexciton generation, which arise due to strong quantum confinement. Lead selenide (PbSe), with a relatively large Bohr radius of 46 nm, possesses an extensive literature of organic synthesis routes with which one can explore the strong confinement regime in quantum dots, nanorods, and nanowires. The intrinsically high charge mobility combined with a high atomic number and density of the lead chalcogenides makes them attractive for sensing applications with highly penetrating quanta, such as X-rays and γ-rays. In sensing architectures, the exploitation of these properties for each individual nanocrystallite 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. Here, we prevent surface oxidation through the fabrication of PbSe NCs via tris(diethylamino)phosphine and a purified selenium precursor, a process that results in PbSe NCs that are chemically and optically stable for at least 1.6 years. If one endeavors to measure high-energy quanta, then the micrometer-scale stopping layers typical of optical photon sensors are insufficient and one must therefore find methods to interconnect the NCs through millimeter- to centimeter-scale thicknesses. Here, we report on the growth of millimeter-scale PbSe colloidal solids that are directly grown within the NC solution. Finally, in contrast to optical photon sensors that typically measure photocurrent, high-energy particle and photon sensors that measure the energy from each interacting quantum are typically hampered by the solid’s thermal noise and the counting statistics associated with discretizing a single quantum into a finite number of information carriers. We show that one can exploit the weaker phonon–electron coupling in nanocrystalline materials to produce room-temperature sensors of X-rays and γ-rays that have comparable resolution to state-of-the-art high-purity germanium detectors. The results thus suggest that multiexciton generation in nanocrystals can be profitably employed in sensing applications that target quanta that create hot-carrier populations that are well above the band gap.