Mechanistic insights of enhanced spin polaron conduction in CuO through atomic doping

• Published in: npj computational materials Vol. 4; no. 1; pp. 1 - 8
• Main Authors: Smart, Tyler J., Cardiel, Allison C., Wu, Feng, Choi, Kyoung-Shin, Ping, Yuan
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 19.11.2018; Nature Publishing Group
• Subjects: 639/301/1034/1038; 639/301/299/890; Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Computational Intelligence; Conduction; Copper oxides; Coupling (molecular); Couplings; Doping; Electrode polarization; Electrodes; First principles; Hopping conduction; Impurities; Ionization; Localization; Materials Science; Mathematical and Computational Engineering; Mathematical and Computational Physics; Mathematical Modeling and Industrial Mathematics; Mobility; Oxides; Photocathodes; Photoelectric effect; Photoelectric emission; Polarons; Theoretical
• ISSN: 2057-3960; 2057-3960
• DOI: 10.1038/s41524-018-0118-3

The formation of a “spin polaron” stems from strong spin-charge-lattice interactions in magnetic oxides, which leads to a localization of carriers accompanied by local magnetic polarization and lattice distortion. For example, cupric oxide (CuO), which is a promising photocathode material and shares important similarities with high T c superconductors, conducts holes through spin polaron hopping with flipped spins at Cu atoms where a spin polaron has formed. The formation of these spin polarons results in an activated hopping conduction process where the carriers must not only overcome strong electron−phonon coupling but also strong magnetic coupling. Collectively, these effects cause low carrier conduction in CuO and hinder its applications. To overcome this fundamental limitation, we demonstrate from first-principles calculations how doping can improve hopping conduction through simultaneous improvement of hole concentration and hopping mobility in magnetic oxides such as CuO. Specifically, using Li doping as an example, we show that Li has a low ionization energy that improves hole concentration, and lowers the hopping barrier through both the electron−phonon and magnetic couplings' reduction that improves hopping mobility. Finally, this improved conduction predicted by theory is validated through the synthesis of Li-doped CuO electrodes which show enhanced photocurrent compared to pristine CuO electrodes. We conclude that doping with nonmagnetic shallow impurities is an effective strategy to improve hopping conductivities in magnetic oxides. Cathode materials: Cupric oxide invigorated Calculations reveal the mechanism through which doping increases the electronic conductivity of cupric oxide (CuO). Spin polarons—quasiparticles where lattice distortions couple to spin excitations—have an adverse effect on the electronic transport in CuO, which is a problem for applying this material as the cathode in photoelectrochemical cells or gas sensors. Researchers from the University of California, Santa Cruz and the University of Wisconsin, Madison led by Yuan Ping have analyzed the process through which doping CuO with lithium alleviates this problem. Their calculations reveal that the Li dopants lower the hopping barrier for polaron-mediated transport through reducing both the electron-phonon and magnetic couplings, and the low ionization energy of the Li dopants improves hole concentrations. They also fabricate Li−CuO electrodes and demonstrate that these allow enhanced photocurrent compared to devices made from pristine CuO. This suggests a general strategy for enhancing the conductivity of magnetic oxides.