Small polaron hopping in epitaxial hematite films
EMSL Project ID
48533
Abstract
Iron oxide (hematite) has emerged as a promising material for photoelectrochemical (PEC) water splitting, where charge transport is often cited as a bottleneck to high efficiencies. It is also ubiquitous in nature and hence, electron transport in hematite is vitally important in many biogeochemical processes. Despite its importance, several fundamental transport parameters (drift mobility, activation barriers) are missing from the literature. Electrons in hematite move via thermally assisted hops between iron sites, a mechanism termed “small polaron hopping”. Many metal oxides are small polaronic conductors, characterized by a carrier mobility that is low and thermally activated. Also in contrast with conventional semiconductors, the magnitude and sign of the Hall effect may be inconsistent with the carrier’s true drift mobility, making resistivity, Seebeck and Hall effect an essential (but hard to acquire) suite of transport measurements. However, the difficulties associated with measurements on low mobility, high resistivity samples mean that only a few small polaron conductors (e.g., LiNbO3, NbO2, some manganites) have been fully characterized. Full analyses on hematite are rare as natural samples are insulating and typically contain hard-to-characterize impurities and defects. Near perfect samples are required so our data and conclusions are of the highest quality. The oxygen assisted molecular beam epitaxy apparatus at EMSL represents the state-of-the-art in the synthesis of epitaxial iron oxide thin films. Once fabricated, resistivity, Seebeck and Hall effect as functions of temperature will be measured at UT Austin. We recently applied this procedure to single crystalline bismuth vanadate – another promising material for PEC water splitting that is a small polaron conductor – finding a massive discrepancy between Hall and drift mobilities both in their magnitude and temperature dependence. We expect our results to be a significant contribution to our understanding of small polaron transport and be valuable in future iron oxide device and geochemical research.
Project Details
Project type
Limited Scope
Start Date
2014-07-01
End Date
2014-08-31
Status
Closed
Released Data Link
Team
Principal Investigator