Piezoresponse force microscopy and nanoferroic phenomena

REVIEW ARTICLE https://doi.org/10.1038/s41467-019-09650-8 OPEN Piezoresponse force microscopy and nanoferroic phenomena 1234567890():,; Alexei Gruverman1, Marin Alexe2 & Dennis Meier 3 Since its inception more than 25 years ago, Piezoresponse Force Microscopy (PFM) has become one of the mainstream techniques in the field of nanoferroic materials. This review describes the evolution of PFM from an imaging technique to a set of advanced methods, which have played a critical role in launching new areas of ferroic research, such as multiferroic devices and domain wall nanoelectronics. The paper reviews the impact of advanced PFM modes concerning the discovery and scientific understanding of novel nanoferroic phenomena and discusses challenges associated with the correct interpretation of PFM data. In conclusion, it offers an outlook for future trends and developments in PFM. T he invention of the atomic force microscope (AFM) in 1986 marked a dramatic shift in scientific research by providing a multifunctional toolbox to explore and manipulate functional properties of a wide range of materials at the nanometer scale. For ferroelectrics and other polar materials, the introduction of one of the voltage-modulated versions of AFM— piezoresponse force microscopy (PFM)—has produced a wealth of new opportunities. PFM enables non-destructive visualization and control of FE nanodomains, as well as direct measurements of the local physical characteristics of ferroelectrics, such as nucleation bias, piezoelectric coefficients, disorder potential, energy dissipation, and domain wall (DW) dynamics (see Box 1). With this, PFM has essentially driven the whole field into the realm of the nanoscale 1. Since the publication of the first review book 2, the experimental and physical principles of PFM operation have become common knowledge. A number of papers and books provided a comprehensive description of its technical details and gave abundant examples of PFM imaging and modification capabilities 3–5. As the field matured, new advanced PFM modes were developed (Box 2). However, the wide application of PFM revealed a growing number of challenges and concerns related to the imaging mechanism, data interpretation, and quantification. While nanoscale domain imaging has been crucial for the initial advent of nanoferroelectric research, it became also clear that a more careful analysis of the PFM image formation mechanism was necessary, along with comprehensive information on the structure, physics, and chemistry of the materials under investigation, to distinguish real effects from artifacts. This article, instead of describing the experimental issues of PFM and a variety of accumulated data, focuses on new science and discoveries enabled by PFM. After presenting a brief historical overview of the evolution of conventional PFM into a set of advanced modes, it describes the role of PFM in exploration of new emergent phenomena, including DW conductivity, magnetoelectric switching, voltage-free flexoelectric domain control, tunneling electroresistance, domain vertices, and polar vortices. Specific attention is paid to challenges in PFM application related to a variety of electromechanical coupling phenomena and the complex image formation 1 Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588, USA. 2 Department of Physics, University of Warwick, Coventry CV4 7AL, UK. 3 Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), N-7034 Trondheim, Norway. Correspondence and requests for materials should be addressed to A.G. (email: ) NATURE COMMUNICATIONS | (2019)10:1661 | https://doi.org/10.1038/s41467-019-09650-8 | www.nature.com/naturecommunications 1 REVIEW ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-09650-8 Box 1. | Main functions of PFM In conventional PFM, domain mapping is performed by scanning the sample surface with the probe in the contact regime while monitoring the local piezoelectric strain generated by a small a.c. (probing) electric bias. The role of the probe is two-fold: it is used both as: (i) an actuator, which allows electric field application through the nanoscale contact with the sample, and (ii) a sensor, which measures the electromechanical response of the sample by monitoring the cantilever mechanical motion (vertical displacement, torsion or bending). The response amplitude is a measure of the effective piezoelectric coefficient dzz, which, within certain conditions, can be related to the polarization magnitude, while the polarization direction can be determined from the PFM phase signal 1,2. The nanoscale lateral resolution of PFM is afforded by the use of a small integrated conducting tip with the apex curvature radius typically in the range of 20–30 nm. Domain imaging resolution, which also depends on the elastic and dielectric properties of the sample, surface conditions, indentation force, etc, can be in the sub-10-nm range as was demonstrated for a variety of ferroelectrics 32. d e c 3 PEM signal, a.u. b Probability a 2 f 1 0 –1 –2 –3 –10 –5 0 5 Bias, V 10 Box Fig. 1. a High-resolution imaging of static domain structures: 50-nm-thick PbTiO3 film with ac domains (the image size is 5 × 5 µm2). b Investigation of domain wall dynamics (adapted with permission from ref. 75); c high-density data storage: electrical writing of stable domains with the characteristic size <40 nm in a BaTiO3 thin film (the image size is 1.50 × 0.85 µm2); d investigation of fast domain switching kinetics: visualization of nanodomain nucleation during polarization reversal in an epitaxial Pb(Zr,Ti)O3 ferroelectric capacitor (the image size is 6 × 6 µm2) (reprinted from ref. 93, with the permission of AIP publishing); e Polarization control in µm-scale thin film Pb(Zr,Ti)O3 capacitors: upper row capacitors poled by negative voltage pulses, bottom row capacitors poled by positive pulses (the image size is 5 × 6 µm2) (reprinted from ref. 94, with the permission of AIP publishing). f Spectroscopic testing of local switching parameters: hysteresis loops acquired at different locations on the ferroelectric surface (reprinted with permission from ref. 3) The important role of PFM in the field on nanoferroics is determined by its ability to perform the following main functions: (1) high-resolution imaging, (2) nanoscale property manipulation, and (3) measurements of local parameters (Box Fig. 1)3,95. PFM has allowed mapping of nanoscale domain structures in FEs, primarily thin films, detection of the ultimate size limit for FE domains, delineation of unconventional domain configurations, such as closure domains, and their formation with respect to the electrical and mechanical boundary conditions. An electrically biased tip has provided unrivaled possibilities for manipulating polarization states at the nanoscale, DW engineering, and the acquisition of quantitative data related to domain switching dyna (...truncated)