Stephen Jesse

The band excitation method in scanning probe microscopy for rapid mapping of energy dissipation on the nanoscale

Mapping energy transformation pathways and dissipation on the nanoscale and understanding the role of local structure in dissipative behavior is a key challenge for imaging in areas ranging from electronics and information technologies to efficient energy production. Here we develop a family of novel scanning probe microscopy (SPM) techniques in which the cantilever is excited and the response is recorded over a band of frequencies simultaneously, rather than at a single frequency as in conventional SPMs. This band excitation (BE) SPM allows very rapid acquisition of the full frequency response at each point (i.e. transfer function) in an image and in particular enables the direct measurement of energy dissipation through the determination of the Q-factor of the cantilever–sample system. The BE method is demonstrated for force–distance and voltage spectroscopies and for magnetic dissipation imaging with sensitivity close to the thermomechanical limit. The applicability of BE for various SPMs is analyzed, and the method is expected to be universally applicable to ambient and liquid SPMs.

Polarization Control of Electron Tunneling into Ferroelectric Surfaces

Ferroelectric Patterning with High Fields Ferroelectric oxides have a net polarization that can switch direction upon application of a sufficiently high electric field. In principle, a ferroelectric thin film should be able to act as a polar switch—tunneling an electron through the film would effectively switch on or off depending on the direction of the polarization. In practice, the length scale needed for a sufficiently small tunneling barrier is nearly the same as the scale at which films no longer support ferroelectricity. Maksymovych et al. (p. 1421) now show that the tip of an atomic force microscope can be used to pattern polarization domains in a thin film of lead zirconate titanate in high electric fields similar to those for field emission tips. High electric fields delivered with an atomic force microscope tip pattern polarization domains in ferroelectric thin films. We demonstrate a highly reproducible control of local electron transport through a ferroelectric oxide via its spontaneous polarization. Electrons are injected from the tip of an atomic force microscope into a thin film of lead-zirconate titanate, Pb(Zr0.2Ti0.8)O3, in the regime of electron tunneling assisted by a high electric field (Fowler-Nordheim tunneling). The tunneling current exhibits a pronounced hysteresis with abrupt switching events that coincide, within experimental resolution, with the local switching of ferroelectric polarization. The large spontaneous polarization of the PZT film results in up to 500-fold amplification of the tunneling current upon ferroelectric switching. The magnitude of the effect is subject to electrostatic control via ferroelectric switching, suggesting possible applications in ultrahigh-density data storage and spintronics.

Switching spectroscopy piezoresponse force microscopy of ferroelectric materials

The application of ferroelectric materials for electronic devices necessitates the quantitative study of local switching behavior, including imprint, coercive bias, remanent and saturation responses, and work of switching. Here we introduce switching spectroscopy piezoresponse force microscopy as a tool for real-space imaging of switching properties on the nanoscale. The hysteresis curves, acquired at each point in the image, are analyzed in the thermodynamic and kinetic limits. We expect that this approach will further understanding of the relationships between material microstructure and polarization switching phenomena on the nanoscale, and provide a quantitative tool for ferroelectric-based device characterization.

Deterministic control of ferroelastic switching in multiferroic materials

Multiferroic materials showing coupled electric, magnetic and elastic orderings provide a platform to explore complexity and new paradigms for memory and logic devices. Until now, the deterministic control of non-ferroelectric order parameters in multiferroics has been elusive. Here, we demonstrate deterministic ferroelastic switching in rhombohedral BiFeO(3) by domain nucleation with a scanning probe. We are able to select among final states that have the same electrostatic energy, but differ dramatically in elastic or magnetic order, by applying voltage to the probe while it is in lateral motion. We also demonstrate the controlled creation of a ferrotoroidal order parameter. The ability to control local elastic, magnetic and torroidal order parameters with an electric field will make it possible to probe local strain and magnetic ordering, and engineer various magnetoelectric, domain-wall-based and strain-coupled devices.

Direct imaging of the spatial and energy distribution of nucleation centres in ferroelectric materials

Macroscopic ferroelectric polarization switching, similar to other first-order phase transitions, is controlled by nucleation centres. Despite 50 years of extensive theoretical and experimental effort, the microstructural origins of the Landauer paradox, that is, the experimentally observed low values of coercive fields in ferroelectrics corresponding to implausibly large nucleation activation energies, are still a mystery. Here, we develop an approach to visualize the nucleation centres controlling polarization switching processes with nanometre resolution, determine their spatial and energy distribution and correlate them to local microstructure. The random-bond and random-field components of the disorder potential are extracted from positive and negative nucleation biases. Observation of enhanced nucleation activity at the 90 composite function domain wall boundaries and intersections combined with phase-field modelling identifies them as a class of nucleation centres that control switching in structural-defect-free materials.

Real Space Mapping of Li-Ion Transport in Amorphous Si Anodes with Nanometer Resolution

The electrical bias driven Li-ion motion in silicon anode materials in thin film battery heterostructures is investigated using electrochemical strain microscopy (ESM), which is a newly developed scanning probe microscopy based characterization method. ESM utilizes the intrinsic link between bias-controlled Li-ion concentration and molar volume of electrode materials, providing the capability for studies on the sub-20 nm scale, and allows the relationship between Li-ion flow and microstructure to be established. The evolution of Li-ion transport during the battery charging is directly observed.

Quantitative mapping of switching behavior in piezoresponse force microscopy

The application of ferroelectric materials for nonvolatile memory and ferroelectric data storage necessitates quantitative studies of local switching characteristics and their relationship to material microstructure and defects. Switching spectroscopy piezoresponse force microscopy (SS-PFM) is developed as a quantitative tool for real-space imaging of imprint, coercive bias, remanent and saturation responses, and domain nucleation voltage on the nanoscale. Examples of SS-PFM implementation, data analysis, and data visualization are presented for epitaxial lead zirconate titanate (PZT) thin films and polycrystalline PZT ceramics. Several common artifacts related to the measurement method, environmental factors, and instrument settings are analyzed.

Vector Piezoresponse Force Microscopy

A novel approach for nanoscale imaging and characterization of the orientation dependence of electromechanical properties-vector piezoresponse force microscopy (Vector PFM)-is described. The relationship between local electromechanical response, polarization, piezoelectric constants, and crystallographic orientation is analyzed in detail. The image formation mechanism in vector PFM is discussed. Conditions for complete three-dimensional (3D) reconstruction of the electromechanical response vector and evaluation of the piezoelectric constants from PFM data are set forth. The developed approach can be applied to crystallographic orientation imaging in piezoelectric materials with a spatial resolution below 10 nm. Several approaches for data representation in 2D-PFM and 3D-PFM are presented. The potential of vector PFM for molecular orientation imaging in macroscopically disordered piezoelectric polymers and biological systems is discussed.

A decade of piezoresponse force microscopy: progress, challenges, and opportunities

Coupling between electrical and mechanical phenomena is a near-universal characteristic of inorganic and biological systems alike, with examples ranging from piezoelectricity in ferroelectric perovskites to complex, electromechanical couplings in electromotor proteins in cellular membranes. Understanding electromechanical functionality in materials such as ferroelectric nanocrystals and thin films, relaxor ferroelectrics, and biosystems requires probing these properties on the nanometer level of individual grain, domain, or protein fibril. In the last decade, piezoresponse force microscopy (PFM) was established as a powerful tool for nanoscale imaging, spectroscopy, and manipulation of ferroelectric materials. Here, we present principles and recent advances in PFM, including vector and frequency-dependent imaging of piezoelectric materials, briefly review applications for ferroelectric materials, discuss prospects for electromechanical imaging of local crystallographic and molecular orientations and disorder, and summarize future challenges and opportunities for PFM emerging in the second decade since its invention

Nanoscale Switching Characteristics of Nearly Tetragonal BiFeO3 Thin Films

We have investigated the nanoscale switching properties of strain-engineered BiFeO(3) thin films deposited on LaAlO(3) substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicates that the nearly tetragonal films have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements and piezoresponse force microscopy, we provide clear evidence of ferroelectric switching of the tetragonal phase, but the polarization direction, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large and reversible, electrically driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material, which is promising for a plethora of applications.