Year: 2021
Type: article
Abstract: We simulated electron transport across a binary nanocrystal solid (BNS) of PbSe NCs with diameters of 6.5 nm and 5.1 nm. We used our hierarchical nanoparticle transport simulator (HINTS) to model the transport in these BNSs. The mobility exhibits a minimum at a large-NC-fraction ${f}_{\mathrm{LNC}}=0.25$. The mobility minimum is deep at $T=80\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and partially smoothed at $T=300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We explain this minimum as follows. As the LNC fraction ${f}_{\mathrm{LNC}}$ starts growing from zero, the few LNCs act as traps for the electrons traversing the BNS because their relevant energy level is lower. Therefore, increasing the ${f}_{\mathrm{LNC}}$ concentration of these traps decreases the mobility. As increasing ${f}_{\mathrm{LNC}}$ reaches the percolation threshold ${f}_{\mathrm{LNC}}$=${f}_{P}$, the LNCs form sample-spanning networks that enable electrons to traverse the entire BNS via these percolating LNC networks. Transport through the growing percolating LNC networks drives the rapid growth of the mobility as ${f}_{\mathrm{LNC}}$ grows past ${f}_{P}$. Therefore, the electron mobility exhibits a pronounced minimum as a function of ${f}_{\mathrm{LNC}}$, centered at ${f}_{\mathrm{LNC}}={f}_{P}$. The position of the mobility minimum shifts to larger LNC fractions as the electron density increases. We have studied the trends of this mobility minimum with temperature, electron density, charging energy, ligand length, and disorder. We account for the trends by a renormalized-trap model, in which capturing an electron renormalizes a deep LNC trap into a shallow trap or a kinetic obstacle, depending on the charging energy. We verified this physical picture by constructing and analyzing heat maps of the mobile electrons in the BNS.
Authors: Luman Qu, Davis Unruh, Gergely T. Zimanyi
Institution: University of California, Davis
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
Funder: National Science Foundation
OpenAlex ID: W3107946097