Katyayani Seal

Switching spectroscopy piezoresponse force microscopy of polycrystalline capacitor structures

Polarization switching in polycrystalline PbZr0.52Ti0.48O3 films on Pt-coated Si substrates was studied by switching spectroscopy piezoresponse force microscopy (SSPFM). Acquisition of multiple hysteresis loops allows polarization switching parameters, including nucleation, coercive biases, and switchable response to be mapped in real space. In contrast to measurements made on the free surface, those on the metal-ferroelectric-metal capacitor structures show the evolution of correlated switching of 102–103 grain clusters with well-defined imprint and nucleation biases. The role of substrate bending on clustering and SSPFM detection mechanisms are discussed. These studies demonstrate real-space imaging of mesoscopic polarization reversal in real-world devices.

Local bias-induced phase transitions

Electrical bias-induced phase transitions underpin a wide range of applications from data storage to energy generation and conversion. The mechanisms behind these transitions are often quite complex and in many cases are extremely sensitive to local defects that act as centers for local transformations or pinning. Using ferroelectrics as an example, we review methods for probing bias-induced phase transitions and discuss the current limitations and challenges for extending the methods to field-induced phase transitions and electrochemical reactions in energy storage, biological and molecular systems.

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

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.

Local measurements of Preisach density in polycrystalline ferroelectric capacitors using piezoresponse force spectroscopy

Polarization switching in polycrystalline ferroelectric capacitors is explored using piezoresponse force microscopy (PFM) based first-order reversal curve (FORC) measurements. The band excitation method facilitates decoupling the electromechanical responses from variations in surface elastic properties. A simulated annealing method is developed to estimate the Preisach densities from PFM FORC data. Microscopic and macroscopic Preisach densities are compared, illustrating good agreement between the two.

Towards local electromechanical probing of cellular and biomolecular systems in a liquid environment

Electromechanical coupling is ubiquitous in biological systems, with examples ranging from simple piezoelectricity in calcified and connective tissues to voltage-gated ion channels, energy storage in mitochondria, and electromechanical activity in cardiac myocytes and outer hair cell stereocilia. Piezoresponse force microscopy (PFM) originally emerged as a technique to study electromechanical phenomena in ferroelectric materials, and in recent years has been employed to study a broad range of non-ferroelectric polar materials, including piezoelectric biomaterials. At the same time, the technique has been extended from ambient to liquid imaging on model ferroelectric systems. Here, we present results on local electromechanical probing of several model cellular and biomolecular systems, including insulin and lysozyme amyloid fibrils, breast adenocarcinoma cells, and bacteriorhodopsin in a liquid environment. The specific features of PFM operation in liquid are delineated and bottlenecks on the route towards nanometre-resolution electromechanical imaging of biological systems are identified.

Fabrication, dynamics, and electrical properties of insulated scanning probe microscopy probes for electrical and electromechanical imaging in liquids

Insulated cantilever probes with a high aspect ratio conducting apex have been fabricated and their dynamic and electrical properties analyzed. The cantilevers were coated with silicon dioxide and a via was fabricated through the oxide at the tip apex and backfilled with tungsten to create an insulated probe with a conducting tip. The stiffness and Q factor of the cantilevers increased after the modifications and their resonances shifted to higher frequencies. The coupling strength between the cantilever and the coating are determined. Electromechanical imaging of ferroelectric domains, current voltage probing of a gold surface, and a probe apex repair process are demonstrated.

High Tunability of the Surface-Enhanced Raman Scattering Response with a Metal-Multiferroic Composite

We demonstrate active control of the plasmonic response from Au nanostructures by the use of a novel multiferroic substrate-LuFe(2)O(4) (LFO)-to tune the surface-enhanced Raman scattering (SERS) response in real time. From both experiments and numerical simulations based on the finite-difference time-domain method, a threshold field is observed, above which the optical response of the metal nanostructure can be strongly altered through changes in the dielectric properties of LFO. This offers the potential of optimizing the SERS detection sensitivity in real time as well as the unique functionality of detecting multiple species of Raman active molecules with the same template.

Detection of percolating paths in polyhedral segregated network composites using electrostatic force microscopy and conductive atomic force microscopy

Composite specimens possessing polyhedral segregated network microstructures require a very small amount of nanosize filler, <1 vol %, to reach percolation because percolation occurs by accumulation of the fillers along the edges of the deformed polymer matrix particles. In this paper, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) were used to confirm the location of the nanosize fillers and the corresponding percolating paths in polymethyl methacrylate/carbon black composites. The EFM and C-AFM images revealed that the polyhedral polymer particles were coated with filler, primarily on the edges as predicted by the geometric models provided.

Piezoelectric response of nanoscale PbTiO3 in composite PbTiO3−CoFe2O4 epitaxial films

Piezoelectric properties of PbTiO3 in 1/3PbTiO3−2/3CoFe2O4 transverse epitaxial nanostructures on differently oriented SrTiO3 were analyzed using conventional and switching-spectroscopy piezoelectric force microscopy. The results confirmed that the individual PbTiO3 nanocolumns in the CoFe2O4 matrix exhibit a detectable piezoelectric response regardless of substrate orientation. For the {100} and {110} orientations, a bias of ±10 V produced ferroelectric domain switching; however, no switching was observed for the {111} films. Small values of piezoelectric constants dzz(100)≈11 pm/V, dzz(110)≈5 pm/V, and dzz(111)≈3 pm/V are attributed to the weak intrinsic response of the nano-PbTiO3 under strong mechanical and depolarizing-field constraints in the composite films.

High frequency piezoresponse force microscopy in the 1-10MHz regime

Imaging mechanisms in piezoresponse force microscopy (PFM) in the high frequency regime above the first contact resonance are analyzed. High frequency (HF) imaging enables the effective use of resonance enhancement to amplify weak signals, improves the signal to noise ratio, minimizes the electrostatic contribution to the signal, and improves electrical contact. The limiting factors in HF PFM include inertial stiffening, deteriorating signal transduction, laser spot effects, and the photodetector bandwidth. Analytical expressions for these limits are derived. High-quality PFM operation in the 1–10MHz frequency range is demonstrated and prospects for imaging in the 10–100MHz range are discussed.