Self-consistent modeling of electrochemical strain microscopy of solid electrolytes

Nanoscale Ferroelectric Characterization with Heterodyne Megasonic Piezoresponse Force Microscopy

Piezoresponse force microscopy (PFM), as a powerful nanoscale characterization technique, has been extensively utilized to elucidate diverse underlying physics of ferroelectricity. However, intensive studies of conventional PFM have revealed a growing number of concerns and limitations which are largely challenging its validity and applications. In this study, an advanced PFM technique is reported, namely heterodyne megasonic piezoresponse force microscopy (HM-PFM), which uses 106 to 108 Hz high-frequency excitation and heterodyne method to measure the piezoelectric strain at nanoscale. It is found that HM-PFM can unambiguously provide standard ferroelectric domain and hysteresis loop measurements, and an effective domain characterization with excitation frequency up to ≈110 MHz is demonstrated. Most importantly, owing to the high-frequency and heterodyne scheme, the contributions from both electrostatic force and electrochemical strain can be significantly minimized in HM-PFM. Furthermore, a special measurement of difference-frequency piezoresponse frequency spectrum (DFPFS) is developed on HM-PFM and a distinct DFPFS characteristic is observed on the materials with piezoelectricity. By performing DFPFS measurement, a truly existed but very weak electromechanical coupling in CH3NH3PbI3 perovskite is revealed. It is believed that HM-PFM can be an excellent candidate for the ferroelectric or piezoelectric studies where conventional PFM results are highly controversial.

Nonlinear electrochemomechanical modelling of electrochemical strain microscopy imaging

Electrochemical strain microscopy (ESM) is a powerful tool to resolve ionic transport and electrochemical processes with a nanoscale resolution. To ascertain the underlying mechanism that governs the signal generation of ESM imaging, a fully coupled nonlinear electrochemomechanical model based on the finite element method is developed and applied to LiMn₂O₄ particles. The frequency dependence of the ESM response, in particular the response at high frequencies used in the detection regime, is investigated in detail. The performed analysis demonstrates that the error induced by the decoupling approximation increases with decreasing bias frequency due to the relatively large variation in ion concentration. In the high frequency regime, the results reveal that the stress effect is negligible and local electroneutrality holds, providing the simplification of numerical simulation for ESM imaging. By applying an alternative current voltage, we suggest that the detectable signal observed in ESM imaging can be attributed to the Vegard effect, which was controversial in previous linear models. The local distribution of ion concentration shows that the ionic reorganization only takes place near the tip-surface junction, the spatial extent of which can be described by two relevant lengths, the contact radius and ion drift length, which determine the spatial lateral resolution and depth resolution, respectively, in ESM imaging. Through a parametric study, the electromigration is proved to be dominant at high frequencies and the relationship between ESM amplitude and some parameters may offer a strategy to measure local electrochemical reactivity. The impact of contact force is evaluated and the results indicate that the local compression reduces ion concentration and the resultant ESM signal in the detection regime. Thus attention must be paid to the contact force when a comparison between different measurements is conducted. The combination of the numerical model and experiment holds the promise of quantitative probing of local electrochemical parameters in solids.

Quantitative Electromechanical Atomic Force Microscopy

The ability to probe a material's electromechanical functionality on the nanoscale is critical to applications from energy storage and computing to biology and medicine. Voltage-modulated atomic force microscopy (VM-AFM) has become a mainstay characterization tool for investigating these materials due to its ability to locally probe electromechanically responsive materials with spatial resolution from micrometers to nanometers. However, with the wide popularity of VM-AFM techniques such as piezoresponse force microscopy and electrochemical strain microscopy there has been a rise in reports of nanoscale electromechanical functionality, including hysteresis, in materials that should be incapable of exhibiting piezo- or ferroelectricity. Explanations for the origins of unexpected nanoscale phenomena have included new material properties, surface-mediated polarization changes, and/or spatially resolved behavior that is not present in bulk measurements. At the same time, it is well known that VM-AFM measurements are susceptible to numerous forms of crosstalk, and, despite efforts within the AFM community, a global approach for eliminating this has remained elusive. In this work, we develop a method for easily demonstrating the presence of hysteretic (i.e., "false ferroelectric") long-range interactions between the sample and cantilever body. This method should be easy to implement in any VM-AFM measurement. We then go on to demonstrate fully quantitative and repeatable nanoelectromechanical characterization using an interferometer. These quantitative measurements are critical for a wide range of devices including MEMS actuators and sensors, memristor, energy storage, and memory.

Correlative Confocal Raman and Scanning Probe Microscopy in the Ionically Active Particles of LiMn2O4 Cathodes

In this contribution, a correlative confocal Raman and scanning probe microscopy approach was implemented to find a relation between the composition, lithiation state, and functional electrochemical response in individual micro-scale particles of a LiMn₂O₄ spinel in a commercial Li battery cathode. Electrochemical strain microscopy (ESM) was implemented both at a low-frequency (3.5 kHz) and in a high-frequency range of excitation (above 400 kHz). It was shown that the high-frequency ESM has a significant cross-talk with topography due to a tip-sample electrostatic interaction, while the low-frequency ESM yields a response correlated with distributions of Li ions and electrochemically inactive phases revealed by the confocal Raman microscopy. Parasitic contributions into the electromechanical response from the local Joule heating and flexoelectric effect were considered as well and found to be negligible. It was concluded that the low-frequency ESM response directly corresponds to the confocal Raman microscopy data. The analysis implemented in this work is an important step towards the quantitative measurement of diffusion coefficients and ion concentration via strain-based scanning probe microscopy methods in a wide range of ionically active materials.

Locally Controlled Cu-Ion Transport in Layered Ferroelectric CuInP2S6

Metal thiophosphates are attracting growing attention in the context of quasi-two-dimensional van der Waals functional materials. Alkali thiophosphates are investigated as ion conductors for solid electrolytes, and transition-metal thiophosphates are explored as a new class of ferroelectric materials. For the latter, a representative copper indium thiophosphate is ferrielectric at room temperature and, despite low polarization, exhibits giant negative electrostrictive coefficients. Here, we reveal that ionic conductivity in this material enables localized extraction of Cu ions from the lattice with a biased scanning probe microscopy tip, which is surprisingly reversible. The ionic conduction is tracked through local volume changes with a scanning probe microscopy tip providing a current-free probing technique, which can be explored for other thiophosphates of interest. Nearly 90 nm-tall crystallites can be formed and erased reversibly on the surface of this material as a result of ionic motion, the size of which can be sensitively controlled by both magnitude and frequency of the electric field, as well as the ambient temperature. These experimental results and density functional theory calculations point to a remarkable resilience of CuInP₂S₆ to large-scale ionic displacement and Cu vacancies, in part enabled by the metastability of Cu-deficient phases. Furthermore, we have found that the piezoelectric response of CuInP₂S₆ is enhanced by about 45% when a slight ionic modification is carried out with applied field. This new mode of modifying the lattice of CuInP₂S₆, and more generally ionically conducting thiophosphates, posits new prospects for their applications in van der Waals heterostructures, possibly in the context of catalytic or electronic functionalities.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Nanoscale Probing of Elastic-Electronic Response to Vacancy Motion in NiO Nanocrystals

Measuring the diffusion of ions and vacancies at nanometer length scales is crucial to understanding fundamental mechanisms driving technologies as diverse as batteries, fuel cells, and memristors; yet such measurements remain extremely challenging. Here, we employ a multimodal scanning probe microscopy (SPM) technique to explore the interplay between electronic, elastic, and ionic processes via first-order reversal curve I-V measurements in conjunction with electrochemical strain microscopy (ESM). The technique is employed to investigate the diffusion of oxygen vacancies in model epitaxial nickel oxide (NiO) nanocrystals with resistive switching characteristics. Results indicate that opening of the ESM hysteresis loop is strongly correlated with changes to the resonant frequency, hinting that elastic changes stem from the motion of oxygen (or cation) vacancies in the probed volume of the SPM tip. These changes are further correlated to the current measured on each nanostructure, which shows a hysteresis loop opening at larger (∼2.5 V) voltage windows, suggesting the threshold field for vacancy migration. This study highlights the utility of local multimodal SPM in determining functional and chemical changes in nanoscale volumes in nanostructured NiO, with potential use to explore a wide variety of materials including phase-change memories and memristive devices in combination with site-correlated chemical imaging tools.

Giant Electric-Field-Induced Strain in PVDF-Based Battery Separator Membranes Probed by Electrochemical Strain Microscopy

Efficiency of lithium-ion batteries largely relies on the performance of battery separator membrane as it controls the mobility and concentration of Li-ions between the anode and cathode electrodes. Recent advances in electrochemical strain microscopy (ESM) prompted the study of Li diffusion and transport at the nanoscale via electromechanical strain developed under an application of inhomogeneous electric field applied via the sharp ESM tip. In this work, we observed unexpectedly high electromechanical strain developed in polymer membranes based on porous poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) and, using it, could study a dynamics of electroosmotic flow of electrolyte inside the pores. We show that, independently of the separator membrane, electric field-induced deformation observed by ESM on wetted membrane surfaces can reach up to 10 nm under a moderate bias of 1 V (i.e., more than an order of magnitude higher than that in best piezoceramics). Such a high strain is explained by the electroosmotic flow in a porous media composed of PVDF. It is shown that the strain-based ESM method can be used to extract valuable information such as average pore size, porosity, elasticity of membrane in electrolyte solvent, and membrane-electrolyte affinity expressed in terms of zeta potential. Besides, such systems can, in principle, serve as actuators even in the absence of apparent piezoelectricity in amorphous PVDF.