The oxygen stoichiometry has a large influence on the physical and chemical properties of complex oxides. Most of the functionality in e.g. catalysis and electrochemistry depends in particular on control of the oxygen stoichiometry. In order to understand the fundamental properties of intrinsic surfaces of oxygen-deficient complex oxides, we report on in situ temperature dependent scanning tunneling spectroscopy experiments on pristine oxygen deficient, epitaxial manganite films.

Although these films are insulating in subsequent ex situ in-plane electronic transport experiments at all temperatures, in situ scanning tunneling spectroscopic data reveal that the surface of these films exhibits a metal-insulator transition (MIT) at 120 K, coincident with the onset of ferromagnetic ordering of small clusters in the bulk of the oxygen-deficient film.

The surprising proximity of the surface MIT transition temperature of nonstoichiometric films with that of the fully oxygenated bulk suggests that the electronic properties in the surface region are not significantly affected by oxygen deficiency in the bulk. This carries important implications for the understanding and functional design of complex oxides and their interfaces with specific electronic properties for catalysis, oxide electronics and electrochemistry.

—Snijders et al.

While the properties of the manganite material below the surface change as expected with the removal of oxygen, becoming an insulator rather than a metal, or conductor, researchers found that the sample showed remarkably stable electronic properties at the surface.

Zheng Gai, a member of DOE’s Center for Nanoscale Materials Sciences at ORNL, emphasized that the robustness of a surface matters because it is precisely the surface properties that determine, influence and affect the functionality of complex oxides in catalysis and batteries.

While this work provides a fundamental understanding of a material used and researched for catalysts, oxide electronics and batteries, Gai and lead author Paul Snijders noted that it’s difficult to speculate about possible impacts.

I always say that in basic science we are discovering the alphabet. How these letters will be designed into a useful technological book is hard to predict.

—Paul Snijders

The authors did their experiment using scanning probe microscopy in a vacuum system with no exposure of the samples to the atmosphere. This contrasts with the conventional approach of growing a sample and then installing it in analysis equipment. During such a transfer, scientists expose the material to the water, nitrogen and carbon dioxide in the air.

By studying pristine samples, the ORNL team gained a surprising new understanding of the physics of the material surfaces—an understanding that is necessary to design new functional applications, Snijders said.

CNMS is one of the five DOE Nanoscale Science Research Centers supported by the DOE Office of Science, premier national user facilities for interdisciplinary research at the nanoscale. Together, the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative.

The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos national laboratories.

Resources

• Paul C Snijders, Min Gao, Hangwen Guo, Guixin Cao, Wolter Siemons, Hong-Jun Gao, T.Z. Ward, Jian Shen and Zheng Gai (2013) Persistent metal-insulator transition at the surface of an oxygen-deficient, epitaxial manganite film. Nanoscale. doi: 10.1039/C3NR02343E