Research

Investigating Nanoscale Materials Functionality

The Balke Research Lab investigates the functionality of nanoscale materials, with a focus on electromechanical coupling, ionic transport, and solid–liquid interface phenomena. By combining advanced atomic force microscopy (AFM) with multi-modal characterization, our team connects fundamental nanoscale processes to macroscopic material performance in energy and information technologies.

Coupling and Control Over Polar Properties

The Balke Lab studies how polarization in materials can be controlled and transformed to enable new functionality in energy and information technologies. By applying electric fields, researchers explore complex dielectric behaviors in materials such as ferroelectrics, relaxors, and antiferroelectrics, including transitions between states that typically do not coexist. Of particular interest are out-of-equilibrium polar phases at interfaces, the role of ionic defects in shaping extrinsic properties, and the coupling of intrinsic and extrinsic polarization. Using advanced AFM techniques, the lab quantifies nanoscale polar properties, such as piezoelectric constants, and connects these insights to theory to discover and design new material behaviors.

AFM tip scanning a quantum materials moire graphene heterostructure.

Polarization of materials is at the heart of many applications in energy and information technology, often based on the possibility of controlling polarization through externally applied forces. The action of an electric field, in particular, enacts both linear and non-linear dielectric responses, creating a rich phenomenology of dipolar glasses, paraelectrics, relaxors, antiferroelectrics, and ferroelectrics. New and unexpected properties can emerge through the coupling of or transition between distinct polar behaviors, including the possibility of changing polar material properties on demand between states of materials that typically do not coexist.

One way to realize this is the stabilization of out-of-equilibrium polar phases, often confined to interfaces or surfaces due to broken symmetry, but which are topologically protected. Of fundamental interest are not only intrinsic but also extrinsic polar phenomena, which are associated with polar properties introduced by ionic defects, which can be controlled via chemical modification routes. The project goal can only be achieved through the fundamental understanding of polar properties and how to transform them on multiple length scales, from the atomic to micrometer scale properties, which require the development of new probing methodologies and integrated microscopic approaches.

Specifically, this includes (a) polar transitions based on order-disorder phenomena in ferroelectric ion conductors such as CuInP2S6, (b) the coupling of intrinsic and extrinsic polar properties, for example, the coupling of intrinsic polarization with oxygen vacancies, and (c) the creation of new functionality at boundaries of dissimilar polar properties. The discovery of new phenomena is enabled through the quantification of AFM measurables and the extraction of polar material properties, for example, the piezoelectric constant, to allow for integration with theory.

Highlights

Piezoresponse force microscopy detects the co-existence of multiple polarization states in van-der-Waals layered CuInP2S6 based on quantification of local piezoelectric responses and comparison with theory.

Brehm, J. A. et al. Nature Materials 19, 43–48 (2019).

AFM can be used to measure the normalized inverse tunability of (Ba,Sr)TiO3 through quantified cantilever responses based on electrostriction and highlight the spatial variations of dielectric tunability (red areas show less tunability of the dielectric constant).

Tselev, A. et al. Advanced Materials Interfaces 2, (2015).

The thorough investigation of cantilever dynamics allows us to quantify electromechanical material responses, enabling the extraction of functional properties at the nanoscale, compatible with complementary characterization methods and theoretical predictions.

Balke, N. et al. Nanotechnology 27, 425707 (2016).

Local Insights Into Electrochemical Processes

Electrochemically induced ion insertion underpins energy storage devices such as batteries, capacitors and pseudocapacitors, as well as electrochemical actuators, neuromorphic computing and desalination, where electrodes experience significant volume changes due to electro-chemo-mechanical coupling. Understanding how electrode mechanics correlate with electrochemical performance and charge storage is critical for advancing these technologies. Atomic force microscopy (AFM) enables in-situ tracking of local volume and stiffness changes at the grain level, offering insights beyond current-based methods and contributing to a library of strain-charge coupling phenomena across materials. This research focuses on electrode responses to electric double layer formation, surface and bulk redox processes, with particular emphasis on layered materials that combine fast ionic transport, tunable interlayer spacing and mechanical resilience, allowing current-strain coupling to be mapped and compared with complementary in-situ X-ray studies.

Electrochemically induced ion insertion is the key phenomenon involved not only in different electrochemical energy storage devices, including batteries, electrochemical double-layer capacitors, and pseudocapacitors, but also in electrochemical actuators, neuromorphic computing, and electrochemical water desalination. The ion-hosting electrode most often undergoes significant volume changes driven by Coulomb interactions, bond length changes, or phase changes, which is described as electro-chemo-mechanical coupling. Understanding the mechanical response of an electrode during electrochemical cycling and its correlation to the device’s electrochemical performance and stored charge is crucial to improving the performance of insertion-type energy storage devices, electrochemical actuators, water purification, ion separation, and neuromorphic computing applications.

AFMs can be used to track local volume changes in situ and can, therefore, be used to study local electrochemical processes on the level of single grains or electrode components, which is not possible with current-based characterization techniques. One of the goals of this research direction is to build a library of strain-charge coupling phenomena for different electrochemical energy storage devices and explore the consequences of local heterogeneity of electro-chemo-mechanical on electrochemical performances and local ionic transport.

Specifically, electrode volume changes and stiffness changes as a result of (a) electric double layer formation, as well as (b) surface and (c) bulk redox processes, are investigated. Of high interest are layered electrode materials since they allow for fast ionic transport, have an interlayer space that can be modified, and typically accommodate strain without mechanical degradation, such as crack formation. Using an AFM, local current-strain coupling coefficients can be identified and correlated with information about individual grain orientation and can be directly compared to in-situ X-ray studies, which track the volume changes on the unit cell level.

Highlights

Spatially Resolved Activation — Spatially resolved activation energy map overlaid with sample topography for Li-ion transport in LiCoO2 thin film cathode material. The average activation energy can be extracted to 0.26 eV, which fits well with macroscopic measurements and theoretical calculations (blue areas indicate not enough signal to be measured).

Balke, N. et al. Nano Letters 2012, 12 (7), 3399–3403.

Electro-chemo-mechanical coupling — The electro-chemo-mechanical coupling behaviors of proton insertion into WO3-based electrodes and the charging heterogeneity were revealed by the mechanical CV (mCV) approach via operando AFM.

Tsai, W.-Y. et al. Nano Energy 81, 105592 (2020).

Tunable elastic properties — Large and tunable elastic properties of two-dimensional MXene (Ti3C2) during charging/discharging have been quantified at different Li-ion contents in situ in a liquid environment, allowing for identification of ion insertion pathways.

J. Come et al. Advanced Energy Materials 2016, 6.

Solid-Liquid Interface-Driven Functionality

The structure of electrical double layers (EDLs) at electrified interfaces plays a critical role in electrochemical energy storage and emerging devices such as ion-gated transistors, with ionic liquids (ILs) offering wide electrochemical windows and high charge density for tuning metal oxide properties through electrostatic doping or vacancy modification. However, links between molecular-scale EDL structure and device-level behavior remain limited, particularly regarding ion layering defects that influence charge storage and gating. Atomic force microscopy (AFM), sensitive to density variations near interfaces, enables three-dimensional mapping of EDLs under operating conditions, connecting nanoscale processes to macroscopic performance. This approach also extends to modifying polar materials like ferroelectrics via solid-liquid interfaces, leveraging liquid environments for electrochemical control of surface chemistry, screening mechanisms, and order parameter tuning, while AFM in liquid further allows direct study of piezoelectric and ferroelectric properties.

Energy field in rainy weather with small lightnings.

The structure of electrical double layers (EDL) at electrified interfaces is of utmost importance for electrochemical energy storage as well as printable, flexible, and bio-electronic devices, such as ion-gated transistors. Ionic liquids (IL) have attracted considerable attention not only due to their large electrochemical window but also due to the high energy charge density they can provide.

This is, for example, used to modify the current-voltage characteristics of metal oxides due to electrostatic doping or modification of oxygen vacancy concentrations. While this is explored on the device level, connections to the molecular-level structure of the EDL are largely missing. An important consideration is the existence of defects in the ion layering observed in the EDL of IL and its consequences for charge storage or gating properties. Since an AFM probe is sensitive to the mass density changes when approaching the solid interface inside the liquid, this approach can be used to image the EDL structure and map it in three dimensions under in situ operation of electrochemical capacitors or gated transistors.

This approach shines a light on the correlation of molecular length scale processes and the macroscopically measured device performance. This concept can be expanded to use solid-liquid interfaces to modify polar materials and their functionality. The use of a liquid provides a well-defined chemical potential at the solid-liquid interface, which can provide additional screening mechanisms for polar materials, such as ferroelectrics. The use of a liquid has the advantage of electrochemical control of surface chemistry and thus order parameter tuning.

Since AFM can be used to study piezoelectric and ferroelectric properties in liquid, this platform offers a unique opportunity to link the interfacial structure of the liquid and underlying polar properties..