© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 958 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Direct Probing of Charge Injection and Polarization- Controlled Ionic Mobility on Ferroelectric LiNbO 3 Surfaces Evgheni Strelcov,* Anton V. Ievlev, Stephen Jesse, Ivan I. Kravchenko, Vladimir Y. Shur, and Sergei V. Kalinin* For many decades, ferroelectric materials have remained an object of intense fascination in the condensed matter physics community and have been broadly incorporated into a number of applications, including piezoelectric transducers, sensors, and optoelectronic devices. Recent progress in the fabrication of ferroelectric films combined with advances in scanning probe microscopy (SPM) methods [1] has attracted significant attention to nanoscale ferroelectrics for applications such as ferroelectric random access memories, [2] data storage, [3] and tunneling devices (polar switchers). [4] The working principle of these applications relies on the ability of a ferroelectric mate- rial to switch the direction of spontaneous polarization in external electric fields and to retain the polarization direction in the zero field state. Correspondingly, ferroelectric switching mechanisms are of direct interest for these applications, and a broad range of studies using scanning probe microscopies, [5,6] focused X-ray, [7] and recently in situ scanning transmission electron microscopy [8] has been reported. It is by now well-realized that the bulk spontaneous polari- zation of ferroelectrics is necessarily screened by charge redis- tribution. Recent works illustrate the preponderant role the screening phenomena have in the stabilization of the ferroelec- tric domain structure [9] and highlight the universal presence of ionic charges as the primary mechanism for spontaneous polar- ization screening in ambient. [10–12] Correspondingly, screening charge dynamics can sensitively affect the spontaneous polari- zation switching dynamics in both local and macroscopic meas- urements. For example, thermodynamic analysis by Morozo- vska [13] demonstrated that switching in an SPM experiment is impossible unless polarization charge is almost completely screened; similarly, slow screening charge dynamics can be a significant factor controlling the domain wall velocity. [14] Experi- mentally, phenomena such as back-switching and formation of bubble domains in PFM, [6,15] formation of charge halos in the direction of wall motion, temperature-induced potential inversion, and humidity influence of domain dynamics [12,16] all illustrate the role of screening ionic and electronic charges on spontaneous polarization switching. These theoretical analyses and experimental observations suggest that spontaneous polarization switching on ferroelec- tric surfaces should properly be described as a coupled sponta- neous polarization switching electrochemical reaction process, with the nature of electrochemical component being deter- mined by the available chemical species. In an ambient envi- ronment these are likely to be the hydroxyl groups, OH - , and protons, H + , readily available as a result of dissociative water adsorption. [17,18] The combined process can be represented as (+P - OH - ) + H2O + 2e - ⇄ (-P - H + ) + 2OH - (1) Here, (+P – OH - ) is the positive spontaneous polarization charge electrostatically or chemically bound with the screening hydroxyl group (see Kalinin et al. [10] for discussion of the equi- librium degree of screening), and (-P – H + ) is the negative spontaneous polarization charge bound to a screening proton. The electrons (or holes) are provided directly by the conductive SPM tip. Correspondingly, the dynamics of the electrochemical process and mobility of ionic charges on the surface is key to understanding and modeling domain nucleation and wall dynamics. Notably, this problem is universal for other oxides as well, where the role of surface ionic charges in phenomena such as charge writing in lanthanum aluminate-strontium titanate (LAO-STO) heterostructures is by now well recognized. [18,19] However, experimental studies of the surface charge dynamics for local polarization reversal present a complex problem. Indeed, the amount of transferred charge is relatively small; for example, switching a domain of 300 nm radius in LiNbO 3 generates ca. 6×10 5 electrons, equivalent to a current of 0.1 nA over 1 millisecond. Hence, direct probing of the charge generation by normal current detection is impossible. Similarly, although the humidity-controlled surface leakage currents (i.e., surface ionic conductivity) are well known in the context of semiconductor device development, [20] no information on the spatial distribution of the surface charge in multidomain ferro- electrics and its interaction with the domain walls is available. Here, we use time-resolved Kelvin probe force microscopy (tr-KPFM) [21] for probing electrochemical phenomena and ionic transport on dielectric surface of LiNbO 3 crystal. This approach allows SPM analog of techniques, such as potentiostatic and galvanostatic intermittent titrations. We expect this approach to be universal for exploring surface ionic dynamics on low-con- ductive surfaces. In order to distinguish between the sponta- neous polarization of the LiNbO 3 crystal and the electrochem- ical polarization of its surface owing to charge motion, herein we call the former s-polarization and latter e-polarization. Dr. E. Strelcov, Dr. S. Jesse, Dr. I. I. Kravchenko, Dr. S. V. Kalinin Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge, Tennessee, 37831, United States E-mail: strelcove@ornl.gov; sergei2@ornl.gov Dr. A. V. Ievlev, Prof. V. Y. Shur Ferroelectric Laboratory Institute of Natural Sciences Ural Federal University Ekaterinburg, 620083, Russian Federation DOI: 10.1002/adma.201304002 Adv. Mater. 2014, 26, 958–963