Showing papers by "Brian J. Rodriguez published in 2010"

Local polarization dynamics in ferroelectric materials

Abstract: Ferroelectrics and multiferroics have recently emerged as perspective materials for information technology and data storage applications. The combination of extremely narrow domain wall width and the capability to manipulate polarization by electric field opens the pathway toward ultrahigh (>10 TBit inch−2) storage densities and small (sub-10 nm) feature sizes. The coupling between polarization and chemical and transport properties enables applications in ferroelectric lithography and electroresistive devices. The progress in these applications, as well as fundamental studies of polarization dynamics and the role of defects and disorder on domain nucleation and wall motion, requires the capability to probe these effects on the nanometer scale. In this review, we summarize the recent progress in applications of piezoresponse force microscopy (PFM) for imaging, manipulation and spectroscopy of ferroelectric switching processes. We briefly introduce the principles and relevant instrumental aspects of PFM, with special emphasis on resolution and information limits. The local imaging studies of domain dynamics, including local switching and relaxation accessed through imaging experiments and spectroscopic studies of polarization switching, are discussed in detail. Finally, we review the recent progress on understanding and exploiting photochemical processes on ferroelectric surfaces, the role of surface adsorbates, and imaging and switching in liquids. Beyond classical applications, probing local bias-induced transition dynamics by PFM opens the pathway to studies of the influence of a single defect on electrochemical and solid state processes, thus providing model systems for batteries, fuel cells and supercapacitor applications.

Microstructure and properties of well-ordered multiferroic Pb(Zr,Ti)O(3)/CoFe(2)O(4) nanocomposites.

Abstract: A nanofabrication technique combining pulsed laser deposition and a nanoporous anodic aluminum oxide membrane mask is being proposed to prepare various types of multiferroic nanocomposites, viz. periodically ordered CoFe2O4 dots covered by a continuous Pb(Zr,Ti)O3 layer, Pb(Zr,Ti)O3 dots covered with CoFe2O4, and Pb(Zr,Ti)O3/CoFe2O4 bilayer heterostructure dots. By properly tuning the processing parameters, epitaxial nanodot-matrix composites can be obtained. For the composite consisting of CoFe2O4 nanostructures covered by a Pb(Zr,Ti)O3 film, an unexpected out-of-plane magnetic easy axis induced by the top Pb(Zr,Ti)O3 layer and a uniform microdomain structure can be observed. The nanocomposites tested by piezoresponse force microscopy (PFM) exhibit strong piezoelectric signals, and they also display magnetoelectric coupling revealed by magnetic-field dependent capacitance measurement.

Direct evidence of mesoscopic dynamic heterogeneities at the surfaces of ergodic ferroelectric relaxors

Abstract: Spatial variability of polarization relaxation kinetics in the relaxor ferroelectric $0.9\text{Pb}({\text{Mg}}_{1/3}{\text{Nb}}_{2/3}){\text{O}}_{3}\text{\ensuremath{-}}0.1{\text{PbTiO}}_{3}$ is studied using time-resolved piezoresponse force microscopy at room temperature. Both the statistical principal component and correlation function analysis and the stretched exponent fits of relaxation curves illustrate the presence of mesoscopic ``fast'' and ``slow'' 100--200 nm regions. The spatial distribution of activation energies is reconstructed using a neural-network-based inversion of the relaxation data. The results directly prove the presence of mesoscopic heterogeneities associated with static and dynamic components of the order parameter on the surfaces of ferroelectric relaxors in the ergodic phase.

Resolution theory, and static and frequency-dependent cross-talk in piezoresponse force microscopy.

Abstract: Probing the functionality of materials locally by means of scanning probe microscopy (SPM) requires a reliable framework for identifying the target signal and separating it from the effects of surface morphology and instrument non-idealities, e.g. instrumental and topographical cross-talk. Here we develop a linear resolution theory framework in order to describe the cross-talk effects, and apply it for elucidation of frequency-dependent cross-talk mechanisms in piezoresponse force microscopy. The use of a band excitation method allows electromechanical/electrical and mechanical/topographic signals to be unambiguously separated. The applicability of a functional fit approach and multivariate statistical analysis methods for identification of data in band excitation SPM is explored.

Defect-mediated polarization switching in ferroelectrics and related materials: from mesoscopic mechanisms to atomistic control.

Abstract: The plethora of lattice and electronic behaviors in ferroelectric and multiferroic materials and heterostructures opens vistas into novel physical phenomena including magnetoelectric coupling and ferroelectric tunneling. The development of new classes of electronic, energy-storage, and information-technology devices depends critically on understanding and controlling field-induced polarization switching. Polarization reversal is controlled by defects that determine activation energy, critical switching bias, and the selection between thermodynamically equivalent polarization states in multiaxial ferroelectrics. Understanding and controlling defect functionality in ferroelectric materials is as critical to the future of oxide electronics and solid-state electrochemistry as defects in semiconductors are for semiconductor electronics. Here, recent advances in understanding the defect-mediated switching mechanisms, enabled by recent advances in electron and scanning probe microscopy, are discussed. The synergy between local probes and structural methods offers a pathway to decipher deterministic polarization switching mechanisms on the level of a single atomically defined defect.

Real space mapping of polarization dynamics and hysteresis loop formation in relaxor-ferroelectric PbMg1/3Nb2/3O3–PbTiO3 solid solutions

Abstract: Polarization switching in ergodic relaxor and ferroelectric phases in the PbMg1/3Nb2/3O3–PbTiO3 (PMN-PT) system is studied using piezoresponse force microscopy, single point electromechanical relaxation measurements, and voltage spectroscopy mapping. The dependence of relaxation behavior on voltage pulse amplitude and time is found to follow a universal logarithmic behavior with a nearly constant slope. This behavior is indicative of the progressive population of slow relaxation states, as opposed to a linear relaxation in the presence of a broad relaxation time distribution. The role of relaxation behavior, ferroelectric nonlinearity, and the spatial inhomogeneity of the tip field on hysteresis loop behavior is analyzed in detail. The hysteresis loops for ergodic PMN-10%PT are shown to be kinetically limited, while in PMN with larger PT content, true ferroelectric hysteresis loops with low nucleation biases are observed.

Double-Layer Mediated Electromechanical Response of Amyloid Fibrils in Liquid Environment

Abstract: Harnessing electrical bias-induced mechanical motion on the nanometer and molecular scale is a critical step toward understanding the fundamental mechanisms of redox processes and implementation of molecular electromechanical machines. Probing these phenomena in biomolecular systems requires electromechanical measurements be performed in liquid environments. Here we demonstrate the use of band excitation piezoresponse force microscopy for probing electromechanical coupling in amyloid fibrils. The approaches for separating the elastic and electromechanical contributions based on functional fits and multivariate statistical analysis are presented. We demonstrate that in the bulk of the fibril the electromechanical response is dominated by double-layer effects (consistent with shear piezoelectricity of biomolecules), while a number of electromechanically active hot spots possibly related to structural defects are observed.

Individual switching of film-based nanoscale epitaxial ferroelectric capacitors

Abstract: We have investigated the individual switching of nanoscale capacitors by piezoresponse force microscopy. Nanoscale epitaxial ferroelectric capacitors with terabyte per inch square equivalent density were fabricated by the deposition of top electrodes onto a pulsed laser deposited lead zirconate titanate thin film by electron beam evaporation through ultrathin anodic aluminum oxide membrane stencil masks. Using bias pulses, the nanoscale capacitors were uniformly switched and proved to be individually addressable. These film-based nanoscale capacitors might be a feasible alternative for high-density mass storage memory applications with near terabyte per inch square density due to the absence of any cross-talk effects.

Web-like domain structure formation in barium titanate single crystals

Abstract: The evolution of ferroelectric domain structure in Remeika-grown BaTiO3 crystals under an inhomogeneous electric field has been investigated using piezoresponse force microscopy. The ac imaging bias was found to affect the metastable polarization state and lead to the formation of a web-like domain structure. It is suggested that this behavior is due to the existence of subsurface domains arising in single crystals with a layered structure.

Dynamic and Spectroscopic Modes and Multivariate Data Analysis in Piezoresponse Force Microscopy

Abstract: Nonvolatile bias-controlled polarization states in ferroelectric materials offer unique opportunities for information technology and data storage applications. The ability to probe ferroelectric properties at the nanoscale by piezoresponse force microscopy (PFM) has enabled fundamental studies of polarization dynamics and the role of defects and disorder on domain nucleation and wall motion and has led to advances in the design and implementation of such applications. This has resulted in the development of fast spectroscopic modes to collect polarization switching data from every point in an image. The emergence of fast, configurable data processing electronics has prompted the development of dynamic and nonsinusoidal data acquisition methods for PFM. Further, the recent synergy of spectroscopic and dynamic modes has necessitated the development of multivariate data analysis and processing in PFM. These recent advances in the applications of PFM for imaging and spectroscopy of the ferroelectric switching processes will be discussed.

Vortex polarization states in nanoferroelectrics

Abstract: Two-dimensional arrays of ferroelectric lead zirconate titanate (PZT) nanodots fabricated using pulsed laser deposition (PLD) through ultrathin anodic aluminum oxide (AAO) membrane stencil masks have been investigated using piezoresponse force microscopy (PFM) Core-polarization states, which may indicate the presence of quasi-toroidal polarization ordering, have been observed Existing theoretical data have been used to determine the signature of a vortex polarization state in a PFM experiment and compared to the measured data