Multi-scale Functional Nanomaterials for the Spectroscopic Detection of Ionizing Radiation and Characterization of Complex Structural Networks

Abstract

Materials exhibiting unique properties in the nanoscale have potential to solve challenges that the macroscopic world faces. Translation of nanoscale effects into macroscopic solids is difficult because it requires the ordering of millions of precisely synthesized nanomaterials across multiple lengths scales without losing their spatial confinement. Multi-scale functional nanomaterials seek to combine the advantageous properties of nanoscale subunits with hierarchical organization to target specific applications. One such application that multi-scale functional nanomaterials can provide specific benefit is in the field of ionizing radiation detection. Nanoscale semiconductor materials can achieve high resolution gamma ray detection through efficient rates of multi-exciton generation and the suppression of optical phonons. Solid-state gamma ray detectors require macroscale thickness and pathways of organized semiconductors for charge percolation. This work advances our understanding and capability for nanoscale interactions with gamma rays through investigating new nanosemiconductor materials and approaches for assembling macroscale ordered solids from the nanoparticle subunits. Radiation detector performance was evaluated through the spectroscopic response of each material to a barium-133 gamma ray source, and measuring the peak resolution as the primary quantifiable metric. The first successful material approach utilized a robust aramid nanofiber matrix derived from Kevlar as a scaffold, and nanoparticles of cadmium telluride were grafted onto the polymer backbone to form efficient percolation networks. These aramid nanofiber composites measure tens-to-hundreds of micrometers in thickness, and successfully detect gamma rays with resolution < 1% at 81 keV, comparable to current commercial devices. The cadmium telluride/aramid nanofiber nanocomposites additionally demonstrated mechanical flexibility and resilience, with no degradation of performance up to 1000 bending cycles. Several formulations of aqueously colloidal lead telluride nanoparticles were investigated and developed to replace cadmium telluride in the nanocomposites in order to improve the materials’ stopping power and phonon suppression. The lead telluride/aramid nanofiber nanocomposites demonstrated gamma ray sensitivity, but the higher dielectric constant and limited device thickness constrained their performance with noise due to high capacitance. An alternative nanoparticle system was invented that showed lead telluride nanoparticles spontaneously self-assemble into macroscale transparent hydrogels consisting of a percolating nanoscale network. Graph theory was used as a tool to quantify network structure and develop correlations between electrolyte concentration/composition, the topological descriptor average nodal connectivity, and the rheological and electrical properties of the hydrogels. Functional detectors are prepared by reinforcing the spanning lead telluride networks with crosslinked polymers, which demonstrate scalability to several millimeters without exhibiting any apparent limitation to achieve thicknesses of several centimeters. The lead telluride polymer nanocomposites are shown to preserve the 3D nanoscale network in the macroscale devices and demonstrated resolved detection of the prominent 356 keV gamma ray from barium-133. The findings of this work prove the utility of nanosemiconductors for high-resolution gamma-ray detection, and provide a methodology for producing large-scale functional solids with conserved nanoscale features that retain desirable functionality. The goal of producing percolating networks of semiconducting nanoparticles that span macroscale volumes was demonstrated.