Feiyue Ma

Mechanisms of electromechanical coupling in strain based scanning probe microscopy

Electromechanical coupling is ubiquitous in nature and underpins the functionality of materials and systems as diverse as ferroelectric and multiferroic materials, electrochemical devices, and biological systems, and strain-based scanning probe microscopy (s-SPM) techniques have emerged as a powerful tool in characterizing and manipulating electromechanical coupling at the nanoscale. Uncovering underlying mechanisms of electromechanical coupling in these diverse materials and systems, however, is a difficult outstanding problem, and questions and confusions arise from recent experiment observations of electromechanical coupling and its apparent polarity switching in some unexpected materials. We propose a series of s-SPM experiments to identify different microscopic mechanisms underpinning electromechanical coupling and demonstrate their feasibility using three representative materials. By employing a combination of spectroscopic studies and different modes of s-SPM, we show that it is possible to distinguish electromechanical coupling arising from spontaneous polarization, induced dipole moment, and ionic Vegard strain, and this offers a clear guidance on using s-SPM to study a wide variety of functional materials and systems.

Mesoporous carbon nanofibers with a high surface area electrospun from thermoplastic polyvinylpyrrolidone

Carbon nanofibers (CNFs) have been synthesized from thermoplastic polyvinylpyrrolidone (PVP) using electrospinning in combination with a novel three-step heat treatment process, which successfully stabilizes the fibrous morphology before carbonization that was proven to be difficult for thermoplastic polymers other than polyacrylonitrile (PAN). These CNFs are both mesoporous and microporous with high surface areas without subsequent activation, and thus overcome the limitations of PAN based CNFs, and are processed in an environmentally friendly and more cost effective manner. The effects of heat treatment parameters and precursor concentration on the morphologies and porous properties of CNFs have been investigated, and their application as anodes for lithium ion batteries has also been demonstrated.

On the effective thermoelectric properties of layered heterogeneous medium

The effective thermoelectric behavior of layered heterogeneous medium is studied, with the distribution of temperature, electric potential, and heat flux solved rigorously from the governing equations, and the effective thermoelectric properties defined through an equivalency principle. It is discovered that the effective thermoelectric figure of merit of a composite medium can be higher than all of its constituents even in the absence of size and interface effects, in contrast to previous studies. This points toward a new route for high figure of merit thermoelectric materials.

Ferroelectric switching of elastin

Significance Ferroelectricity has long been speculated to have important biological functions, although its very existence in biology has never been firmly established. Here, we present, to our knowledge, the first macroscopic observation of ferroelectric switching in a biological system, and we elucidate the origin and mechanism underpinning ferroelectric switching of elastin. It is discovered that the polarization in elastin is intrinsic at the monomer level, analogous to the unit cell level polarization in classical perovskite ferroelectrics. Our findings settle a long-standing question on ferroelectric switching in biology and establish ferroelectricity as an important biophysical property of proteins. We believe this is a critical first step toward resolving its physiological significance and pathological implications. Ferroelectricity has long been speculated to have important biological functions, although its very existence in biology has never been firmly established. Here, we present compelling evidence that elastin, the key ECM protein found in connective tissues, is ferroelectric, and we elucidate the molecular mechanism of its switching. Nanoscale piezoresponse force microscopy and macroscopic pyroelectric measurements both show that elastin retains ferroelectricity at 473 K, with polarization on the order of 1 μC/cm2, whereas coarse-grained molecular dynamics simulations predict similar polarization with a Curie temperature of 580 K, which is higher than most synthetic molecular ferroelectrics. The polarization of elastin is found to be intrinsic in tropoelastin at the monomer level, analogous to the unit cell level polarization in classical perovskite ferroelectrics, and it switches via thermally activated cooperative rotation of dipoles. Our study sheds light onto a long-standing question on ferroelectric switching in biology and establishes ferroelectricity as an important biophysical property of proteins. This is a critical first step toward resolving its physiological significance and pathological implications.

Is thermoelectric conversion efficiency of a composite bounded by its constituents

We analyze the conversion efficiency of a bilayered thermoelectric composite, and conclude that thermoelectric conversion efficiency of a composite is not bounded by its constituents, and can be higher than all its constituents in the absence of size and interface effects. Conditions on constituent phases for enhanced conversion efficiency are also identified, and the upper bound on their conversion efficiency is established. This points to a new route for high efficiency thermoelectric materials.

High sensitivity piezomagnetic force microscopy for quantitative probing of magnetic materials at the nanoscale.

Accurate scanning probing of magnetic materials at the nanoscale is essential for developing and characterizing magnetic nanostructures, yet quantitative analysis is difficult using the state of the art magnetic force microscopy, and has limited spatial resolution and sensitivity. In this communication, we develop a novel piezomagnetic force microscopy (PmFM) technique, with the imaging principle based on the detection of magnetostrictive response excited by an external magnetic field. In combination with the dual AC resonance tracking (DART) technique, the contact stiffness and energy dissipation of the samples can be simultaneously mapped along with the PmFM phase and amplitude, enabling quantitative probing of magnetic materials and structures at the nanoscale with high sensitivity and spatial resolution. PmFM has been applied to probe magnetic soft discs and cobalt ferrite thin films, demonstrating it as a powerful tool for a wide range of magnetic materials.

Piezoelectric and piezomagnetic force microscopies of multiferroic BiFeO3-LiMn2O4 heterostructures

BiFeO3-LiMn2O4 (BFO-LMO) heterostructures were fabricated via pulsed laser deposition, and their ferroelectric and ferromagnetic properties were probed by magnetic force microscopy (MFM), piezoresponse force microscopy (PFM), and the newly developed piezomagnetic force microscopy (PmFM). MFM imaging shows no clear distinction between BFO and LMO phases, while PFM and PmFM mappings clearly distinguish LMO nanopillars from BFO matrix. Linear piezoelectric and piezomagnetic responses have been observed in both phases, with the effects more prominent in BFO. The strong piezomagnetic response in BFO is believed to arise from Mn doping, while piezoelectric-like response of LMO is attributed to ionic activities as well as vertical geometry of the heterostructure. The limitation of global excitation of PmFM is also discussed.

Probing multiferroic heterostructures of BiFeO3 -LiMn2O4 using magnetic, piezoelectric and piezomagnetic force microscopies

In this study magnetic force microscopy (MFM), piezoresponse force microscopy (PFM), and the newly developed piezomagnetic force microscopy (PmFM) techniques are used to probe the ferroelectric and ferromagnetic properties of BiFeO3-LiMn2O4 (BFO-LMO) heterostructures at nano-scale. The PmFM technique is also used to probe the ferromagnetic properties of CoFe2O4 (CFO) as a case study. The PFM and PmFM mappings of the BFO-LMO heterostructures clearly distinguish the BFO matrix and LMO nanopillars while the MFM mapping is ambiguous. The relatively high piezomagnetic response of BFO matrix is believed to be due to the Mn doping while the piezoelectric-like response of LMO nanopillars is due to the ionic activities and the vertical geometry of its heterostructure. Lastly, limitations of the PmFM technique are discussed.Copyright