New Paradigms for Goniopolar Materials
Abstract
The goal of this program is to combine theory, synthesis, and property measurements to expand and understand the library of materials that simultaneously exhibit majority n-type and p-type behavior, respectively, along orthogonal crystallographic directions. We refer to this phenomenon as either goniopolarity or axis-dependent conduction polarity. First, we will develop the theoretical methodology to accurately predict the anisotropic conduction properties of semiconductors with large band gaps (Egap greater than 0.5 eV), and validate these predictions with measurements on our recently discovered intermediate gapped semiconductors. Second, using computational predictions, crystal growth, and experimental measurements, we seek to identify the canonical metallic and semiconducting material with axis-dependent conduction polarity; i.e. the material that that has all the properties needed for it to be as ubiquitous as silicon . The criteria for this includes ease of single crystalline growth with macroscopic dimensions, size of the band gap, scalability with respect to elemental abundance, air-stability, thermal stability of both the material and the anisotropic conduction polarity, robustness of anisotropy with respect to doping or impurity levels, and maximum thermopower and Hall anisotropy. Initial targets will include the refractory band gap tunable WSi2-Re4Si7 alloys, while we computationally build a database of the anisotropic axis-dependent conduction polarity of other semiconductors and metals to synthesize and test experimentally. Third, we will establish different design concepts to maximize the Seebeck and Hall anisotropy of goniopolar metals, including exploiting materials with multiple hyperboloidal Fermi surfaces stacked in a Russian Nesting Doll configuration as a result of supercell formulation (i.e. refractory NbAl0.6Si1.4). Finally, we will characterize the anisotropic transport properties of the photoexcited minority carriers in our largest gap semiconductor (NaSnAs) to evaluate whether goniopolar materials could produce a significant photovoltaic effect. Collectively, this will be accomplished via a computationally guided effort to identify and predict the conduction properties of semiconductors, grow single crystals, and measure the temperature-dependent anisotropic transport properties, as well as under photoexcitation, and compare these values to predictions from (time-dependent) DFT. Overall, this work extends the successes of the 2018-2021 cycle of our AFOSR program, in which we established band structure fingerprints for goniopolarity and developed models to accurately predict the magnitude of thermopower in metals and narrow gap semiconductors (Egapless than0.15), set a record for the largest thermopower anisotropy in Re4Si7 and expanded the library of experimentally confirmed materials with goniopolarity by greater than50 percent.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Mar 07, 2023
- Source ID
- FA95502110268
Entities
People
- Joshua E Goldberger
Organizations
- Air Force Office of Scientific Research
- Ohio State University
- United States Air Force