Breakdown of the Small‐Polaron Hopping Model in Higher‐Order Spinels

Abstract

charge transfer at surfaces [2] or by potentially increasing catalytic selectivity by the localization of minority carriers, [3] and have been predicted to form better transparent conducting oxides (TCOs) than materials that conduct through band transport. [4] But polaron transport is sluggish, and the polaron-induced limitations to charge carrier mobility are well known to the applied science community. [3,5,6] These limitations are especially prevalent in transition metal oxides, which are prone to form smallpolarons (tightly bound) due to the strong electron correlation within the valence d-orbitals. [7] For instance, next-generation p-type TCOs like CuAlO 2 and ZnRh 2 O 4 , which are essential to realizing low-power transparent electronics, have limited performance due to the low mobilities of smallpolarons. [8] The transport of small-polarons in energy-conversion materials have necessitated decreasing diffusion path dimensions to the nanometer length scale [3] and smallpolarons can establish a fundamental limit for the Fermi level in materials like Fe 2 O 3. [9] Similarly, in small-polaron oxides used as active materials in Li-ion batteries, the low mobility of polarons has been observed to slow down Li diffusion, leading to accumulation of Li-ions at the electrode interface and low battery capacities. [2] The small-polaron hopping model has been used for six decades to rationalize electronic charge transport in oxides. The model was developed for binary oxides, and, despite its significance, its accuracy has not been rigorously tested for higher-order oxides. Here, the small-polaron transport model is tested by using a spinel system with mixed cation oxidation states (Mn x Fe 3−x O 4). Using molecular-beam epitaxy (MBE), a series of single crystal Mn x Fe 3−x O 4 thin films with controlled stoichiometry, 0 ≤ x ≤ 2.3, and lattice strain are grown, and the cation site-occupation is determined through X-ray emission spectroscopy (XES). Density functional theory + U analysis shows that charge transport occurs only between like-cations (Fe/Fe or Mn/Mn). The site-occupation data and percolation models show that there are limited stoichiometric ranges for transport along Fe and Mn pathways. Furthermore, due to asymmetric hopping barriers and formation energies, the Oh Mn + + 2 2 polaron is energetically preferred to the Oh Fe + + 2 2 polaron, resulting in an asymmetric contribution of Mn/Mn pathways. All of these findings are not contained in the conventional small-polaron hopping model, highlighting its inadequacy. To correct the model, new parameters in the nearestneighbor hopping equation are introduced to account for percolation, cross-hopping, and polaron-distribution, and it is found that a near-perfect correlation can be made between experiment and theory for the electronic conductivity.

High-sensitivity CdTe phototransistors with the response spectrum extended to 1.65 μm

“…Epitaxial growth with atomic and/or sublattice layers can be realized by the molecular beam epitaxy (MBE) technique. 29 Therefore, high-quality single crystalline thin films, quantum wells and superlattices have been grown by MBE. 29 More crucially, the stoichiometry and doping content of the epitaxial systems can be precisely controlled.…”

“…29 Therefore, high-quality single crystalline thin films, quantum wells and superlattices have been grown by MBE. 29 More crucially, the stoichiometry and doping content of the epitaxial systems can be precisely controlled. 29 Nowadays, we, the MBE team at ECNU, have mastered the MBE technique to grow Te-rich and Cd-rich CdTe thin films, 19 where different defect levels are demonstrated.…”

“…29 More crucially, the stoichiometry and doping content of the epitaxial systems can be precisely controlled. 29 Nowadays, we, the MBE team at ECNU, have mastered the MBE technique to grow Te-rich and Cd-rich CdTe thin films, 19 where different defect levels are demonstrated. Here, Cd-rich CdTe thin films are grown on mica substrates by MBE with deep defect states located at 0.8 eV and 1.1 eV including the emission peaks of 1.4 eV, 1.55 eV, 1.59 eV and E g (Fig.…”

Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe2O4 and Fe3O4

“…Fe 3 O 4 was grown at the same conditions on (001)oriented Nb:STO, except that the O 2 gas pressure was maintained at ∼4.5 × 10 −6 Torr. 31 Characterization of MnFe 2 O 4 and Fe 3 O 4 Films. Reflection high-energy electron diffraction (RHEED), a technique sensitive to the first few atomic layers of a film surface, was used to monitor the growth process.…”

“…The island growth results are typical for spinel ferrites grown on Nb:STO, where there is a significant lattice mismatch (∼7%) between the MnFe 2 O 4 film lattice parameter (a/2 ≈ 4.24 Å) and that of the substrate (a = 3.905 Å). 31 The {111}-type surfaces are also the minimum energy surface for spinel films, 34 which leads to faceting into pyramid-type islands when grown on a (001) substrate. This Volmer−Weber growth mode has been observed previously for CoFe 2 O 4 films grown on STO.…”