SBIR-STTR Award

This SBIR Phase I project will demonstrate that high sensitivity, radiation-resistance, and resolution can be elicited from nanostructured media composed of semiconducting nanoparticles derived from size-governed PbTe. In order to transform x-ray imaging instruments, nanocrystalline semiconductors provide an attractive material basis because they present a means of: 1) decreasing the underlying material cost by utilizing a solution-based fabrication methodology, 2) increasing the range of candidate materials by including the narrow-gap semiconductors, 3) increasing the exciton multiplicity upon the impingement of ionizing radiation by utilizing multi-exciton generation, and 4) increasing the radiation resistance because the introduction of a high density of nanoparticles can convey pronounced improvement in the radiation hardness of the material. In order to realize these properties, several experimental challenges must be overcome, the surmounting of which is one of the objects of the proposed research, during which we will: 1) utilize self-assembly to realize close-packed quantum-dot colloidal solids where the charge transport is optimized, and 2) extend the size of those domains to macroscopic size. The research is designed to not only deliver a high-performance x-ray sensor that can be commercialized but it will also advance basic physics by studying the interactions between energetic particles and strongly-confined charge carriers. By finding general material-design methods to suppress both radiation-induced damage and the stochastic thermal loss component in semiconductor materials, one can greatly increase the charge-conversion efficiency, which impacts the resolution of sensing devices, such as the extreme photon counting application targeted.

This SBIR project will exploit the nanoscale physics delivered by percolating networks of PbTe nanoparticles to develop an efficient, low-cost imager of hard x-rays (5 – 300 keV) that has high spatial (< 55 mm) and energy resolutions (< 2 %). We developed methods to assemble PbTe into scalable solids using either templated growth or self-assembly. PbTe gels self-assembled from colloidal nanoparticles (NPs) translate the size-dependent properties of nanostructures to materials with macroscale volumes. Large spanning networks of NP chains provide high interconnectivity within the material necessary for a wide range of properties from conductivity to viscoelasticity. In our Phase I work, we have: (1) optimized the aqueous hydrothermal synthesis recipe for PbTe NPs, (2) completed the development of low-cost fabrication methods that have form-factor flexibility and can be scaled to large areas (30.48 x 30.48 cm2 during Phase I) and thicknesses, (3) designed and fabricated pixelated rectifying metallic contacts to form a p-i-n diode, and (4) realized high-energy resolution (0.5 % at 81 keV) sensors across the hard x-ray energy range up to 383 keV. In contrast to single-crystalline, polycrystalline, or amorphous materials, nanostructured media allow one to increase the exciton multiplicity upon the impingement of ionizing quanta by utilizing multi-exciton generation. The surface-induced reduction in the relative participation of thermal loss processes results from a more-effective de-coupling of the phonon population from the information-carrying charge-carriers within the solid. Furthermore, one can exploit the accumulated effect of interfacial scattering events at the multitudinous boundaries with the nanostructured solid to enhance the stopping power of the solid relative to a homogeneous or single-crystalline equivalent. This quantum dot physics can be exploited to potentially transform x-ray imaging. In contrast to existing x-ray imaging implementations that measure x-ray generated currents, the EPIC-HXR project will deliver an x-ray panel that is spectroscopic on a photon-by-photon basis, expanding the capabilities of x-ray imagers to enable more precise atom-specific mapping of the interrogated targets, which is relevant for military applications, non-destructive evaluation, and medical imaging instrumentation. During Phase II, the PbTe-polymeric composite material and associated readout electronics will be first optimized for single-pixel performance in terms of detection efficiency and energy resolution. That pixel design will then be expanded into an x-ray imaging array with expandable low-noise readout electronics. That x-ray array demonstration will be followed by the material and readout integration into a 30 x 30 cm2 x-ray spectroscopic imager.