| AVS 58th Annual International Symposium and Exhibition | |
| Late Breaking Session | Friday Sessions |
| Session SS2-FrM |
| Session: | Surface Science Late Breaking Session |
| Presenter: | Amit Kumar, Oak Ridge National Laboratory |
| Authors: | A. Kumar, Oak Ridge National Laboratory Y. Kim, Oak Ridge National Laboratory A. Tselev, Oak Ridge National Laboratory S.V. Kalinin, Oak Ridge National Laboratory A. Baddorf, Oak Ridge National Laboratory |
| Correspondent: | Click to Email |
Multiple reports of unexpected ferroelectricity in nanoscale systems have been made based on electromechanical hysteresis detected by piezoresponse force microscopy (PFM). These include thin films of ferroelectric materials below theoretical stability limits and materials without bulk ferroelectricity, such as strontium titanate and manganites. However, recent studies of ionic systems, such as Li-ion conductors, suggest that local electrochemistry, usually ignored in these studies, may have an important role to play in the origin of the hysteresis loops. We report the role of local electrochemistry, including surface and bulk oxygen vacancy generation and dynamics on ionic conductors such as Yttria stabilized zirconia (YSZ) and lantanum strontium cobaltites (LaSrCoO) and compare results with less understood systems including strained and unstrained SrTiO3 and the lantanum aluminate - strontium titanate (LAO-STO) interface.
The role of oxygen vacancies in oxides, of obvious relevance in fuel cells, memristors, superconductors, etc., have traditionally been studied in macroscopic volumes and has been limited by high activation temperatures. We have developed an approach for spatially resolved local assessment of the thermodynamics and kinetics involved in the generation and diffusion of oxygen vacancies by utilizing volume variations of these oxides upon application of a concentrated electric field. In band excitation electrochemical strain microscopy (ESM), a strongly confined electric field at a tip is used to drive the oxygen vacancies in these oxide materials and the tip used to detect the resulting local electrochemical strain due to migration of oxygen vacancies. Vacancy diffusion is relatively slow, so that local strain hysteresis loops are open. Mapping the loop opening as a function of the final bias establishes the onset and kinetics of the diffusion process. Signal relaxation experiments locally characterize the diffusion dynamics of the vacancies. In mixed ionic-electronic oxide systems, current-voltage measurements provide complementary information on electronic transport.
Mapping of local oxygen vacancy diffusivity with 30 nm resolution has been achieved on a number of oxides ranging from purely ionic conductors (YSZ) to mixed ionic electronic conductors (LaSrCoO), including ferroelectrics and resistive switching in TiO2 and SrTiO3 films. This approach was further applied to the LAO-STO interface, now a prototypical 2D electron gas with ferroelectric behavior proposed due to local hysteresis loops. While the role of static oxygen vacancies has been well discussed, we explore the local dynamic processes in the system. We observe two parallel dynamic processes that we tentatively ascribe to surface charge dynamics and bulk vacancy injection. This behavior is unique for LAO-STO and is not observed in YSZ and LaSrCoO. These observations both explain hysteresis without ferroelectricity and provide insight into the behavior of this system.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy.