Showing papers by "Brian J. Rodriguez published in 2011"

Surface Domain Structures and Mesoscopic Phase Transition in Relaxor Ferroelectrics

Abstract: Relaxor ferroelectrics are a prototypical example of ferroic systems in which interplay between atomic disorder and order parameters gives rise to emergence of unusual properties, including non-exponential relaxations, memory effects, polarization rotations, and broad spectrum of bias- and temperatureinduced phase transitions. Despite more than 40 years of extensive research following the original discovery of ferroelectric relaxors by the Smolensky group, the most basic aspect of these materials – the existence and nature of order parameter – has not been understood thoroughly. Using extensive imaging and spectroscopic studies by variable-temperature and time resolved piezoresponse force microscopy, we fithat the observed mesoscopic behavior is consistent with the presence of two effective order parameters describing dynamic and static parts of polarization, respectively. The static component gives rise to rich spatially ordered systems on the ∼ 100 nm length scales, and are only weakly responsive to electric fi eld. The surface of relaxors undergoes a mesoscopic symmetry breaking leading to the freezing of polarization fl uctuations and shift of corresponding transition temperature.

Nano-textured self-assembled aligned collagen hydrogels promote directional neurite guidance and overcome inhibition by myelin associated glycoprotein

Abstract: The development of nerve guidance conduits is constantly evolving as the need arises for therapies for spinal cord injury In addition to providing a path for regrowing axons to reconnect with their appropriate targets, the structural and biochemical cues provided by these conduits should be permissive for directional neurite outgrowth and be protective against inhibition in the vicinity of the injury site Here, we adapted the use of iso-electric focusing to drive the alignment of supramolecular fibrils into self-assembled collagen hydrogels (∼300 µm diameter), and tested those hydrogels for the ability to direct and enhance the migration of neurites Structural characterization revealed anisotropic alignment of nanofibrillar aggregates (∼20 nm diameter), arranged in micron-scale bundles (∼1 to 2 µm diameter) similar to the hierarchical size scales observed in native tissues Neurite outgrowth extended bidirectionally along the axes of aligned hydrogels Furthermore, it was shown that, as opposed to poly-D-lysine, neurite outgrowth on aligned hydrogels is not inhibited in the presence of myelin-associated glycoprotein (p > 005) These results highlight for the first time a structural and biochemical role for iso-electrically aligned collagen hydrogels in controlling neuronal growth, and indicate that the short-term signaling associated with these hydrogels can be used in adjunct therapy following injury to the spinal cord

Compositional disorder, polar nanoregions and dipole dynamics in Pb(Mg1/3Nb2/3)O3-based relaxor ferroelectrics

Abstract: The complex structure of relaxor ferroelectrics comprises polar nanoregions (PNRs) which appear upon cooling below the Burns temperature and quenched com- positional (chemical) disorder. The relation between the polar nanostructure and compositionally ordered regions (CORs) often observed in relaxors has been the subject of extensive theoretical investigations; however, the experi- mental data, especially concerning Pb(B 0=3 B 00=3 )O3-type complex perovskite relaxors, are rather limited. In this pa- per, we analyse and discuss the results of our recent inves- tigations of the morphology of CORs and the dynamics of PNRs in Pb(Mg1/3Nb2/3)O3-based solid solutions in which the degree of compositional disorder was varied by means of changing the composition and/or by means of high-tem- perature annealing. The samples were characterised using X-ray diffraction, transmission electron microscopy, piezo- response force microscopy, Brillouin light scattering, di- electric spectroscopy, as well as by measuring pyroelectric effect and ferroelectric hysteresis loops. No influence of the size of CORs on the PNRs relaxation in the ergodic relaxor phase is found. Instead, the CORs size influences significantly the diffuseness of the transition from the field-induced ferroelectric phase to the ergodic relaxor state. The results are interpreted in the framework of a model suggesting the coexistence of static and dynamic PNRs in the ergodic relaxor phase.

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.

Bubble polarization domain patterns in periodically ordered epitaxial ferroelectric nanodot arrays

Abstract: In this work, bubble polarization domains in periodically ordered ferroelectric Pb(Zr0.4Ti0.6)O3 nanodot arrays and their formation mechanisms have been investigated by piezoresponse force microscopy (PFM) and Monte-Carlo simulations. The PFM observations reveal the coexistence of single domain and apparent bubble domain patterns within the same nanodot array, which also exhibit dissimilar polarization reversal processes. The formation of various polarization configurations can be accounted for by the interplay of various factors, such as polarization anisotropy and depolarization field. Using Monte-Carlo simulation, we are able to reproduce bubble and single domains and further predict that these patterns can be tailored by varying the nanodot parameters, including dot height, aspect ratio, etc.