Jiangyu Li

Diisopropylammonium Bromide Is a High-Temperature Molecular Ferroelectric Crystal

Environmentally Friendly Ferroelectrics Ferroelectrics—which are widely used as piezo elements, sensors, and actuators—maintain charge polarization even in the absence of an external electric field. The best ferroelectric properties are found in perovskites such as barium titanate (BTO) and lead zirconate titanate; however, environmentally friendly, lead-free alternatives are highly desirable. Fu et al. (p. 425; see the Perspective by Bonnell) find that the organic molecular crystal diisopropylammonium bromide has ferroelectric properties comparable to those of BTO and may represent a viable alternative to perovskites. An organic molecular crystal is found to have ferroelectric properties comparable to those of barium titanate. [Also see Perspective by Bonnell] Molecular ferroelectrics are highly desirable for their easy and environmentally friendly processing, light weight, and mechanical flexibility. We found that diisopropylammonium bromide (DIPAB), a molecular crystal processed from aqueous solution, is a ferroelectric with a spontaneous polarization of 23 microcoulombs per square centimeter [close to that of barium titanate (BTO)], high Curie temperature of 426 kelvin (above that of BTO), large dielectric constant, and low dielectric loss. DIPAB exhibits good piezoelectric response and well-defined ferroelectric domains. These attributes make it a molecular alternative to perovskite ferroelectrics and ferroelectric polymers in sensing, actuation, data storage, electro-optics, and molecular or flexible electronics.

Domain Dynamics During Ferroelectric Switching

The role of defects and interfaces on switching in ferroelectric materials is observed with high-resolution microscopy. The utility of ferroelectric materials stems from the ability to nucleate and move polarized domains using an electric field. To understand the mechanisms of polarization switching, structural characterization at the nanoscale is required. We used aberration-corrected transmission electron microscopy to follow the kinetics and dynamics of ferroelectric switching at millisecond temporal and subangstrom spatial resolution in an epitaxial bilayer of an antiferromagnetic ferroelectric (BiFeO3) on a ferromagnetic electrode (La0.7Sr0.3MnO3). We observed localized nucleation events at the electrode interface, domain wall pinning on point defects, and the formation of ferroelectric domains localized to the ferroelectric and ferromagnetic interface. These results show how defects and interfaces impede full ferroelectric switching of a thin film.

An organic-inorganic perovskite ferroelectric with large piezoelectric response

Finding a more flexible mechanical sensor Piezoelectric materials allow conversion between electricity and mechanical stresses. The most efficient piezoelectric materials are ceramics such as BaTiO3 or PbZrO3, which are also extremely stiff. You et al. identified an organic perovskite structured piezoelectric material that is far more pliable yet has a piezoelectric response similar to that of traditional ceramics. This material may be a better option to use as a mechanical sensor for flexible devices, soft robotics, biomedical devices, and other micromechanical applications that benefit from a less stiff piezoelectric material. Science, this issue p. 306 Trimethylchloromethyl ammonium trichloromanganese(II) may be a flexible material competitive for piezoelectric applications. Molecular piezoelectrics are highly desirable for their easy and environment-friendly processing, light weight, low processing temperature, and mechanical flexibility. However, although 136 years have passed since the discovery in 1880 of the piezoelectric effect, molecular piezoelectrics with a piezoelectric coefficient d33 comparable with piezoceramics such as barium titanate (BTO; ~190 picocoulombs per newton) have not been found. We show that trimethylchloromethyl ammonium trichloromanganese(II), an organic-inorganic perovskite ferroelectric crystal processed from aqueous solution, has a large d33 of 185 picocoulombs per newton and a high phase-transition temperature of 406 kelvin (K) (16 K above that of BTO). This makes it a competitive candidate for medical, micromechanical, and biomechanical applications.

CoO-carbon nanofiber networks prepared by electrospinning as binder-free anode materials for lithium-ion batteries with enhanced properties.

CoOx-carbon nanofiber networks were prepared from cobalt(ii) acetate and polyacrylonitrile by an electrospinning method followed by thermal treatment. The XPS results demonstrated that the cobalt compound in CoOx-carbon obtained at 650 °C was CoO rather than Co or Co3O4. The CoO nanoparticles with diameters of about 8 nm were homogeneously distributed in the matrix of the nanofibers with diameters of 200 nm. As binder-free anodes for lithium-ion batteries, the discharge capacities of such CoO-carbon (CoO-C) composite nanofiber networks increased with the pyrolysis and annealing temperature, and the highest value was 633 mA h g(-1) after 52 cycles at a current density of 0.1 A g(-1) when the CoO-C was obtained at 650 °C. In addition, the rate capacities of the CoO-C obtained at 650 °C were found to be higher than that of the sample annealed at a lower temperature and pure carbon nanofiber networks annealed at 650 °C. The improved properties of CoO-C nanofiber networks were ascribed to nanofibers as the framework to keep the structural stability, and favorable mass and charge transport. The present study may provide a new strategy for the synthesis of binder-free anodes for lithium-ion batteries with excellent properties.

Anomalous piezoelectricity in two-dimensional graphene nitride nanosheets

Piezoelectricity is a unique property of materials that permits the conversion of mechanical stimuli into electrical and vice versa. On the basis of crystal symmetry considerations, pristine carbon nitride (C3N4) in its various forms is non-piezoelectric. Here we find clear evidence via piezoresponse force microscopy and quantum mechanical calculations that both atomically thin and layered graphitic carbon nitride, or graphene nitride, nanosheets exhibit anomalous piezoelectricity. Insights from ab inito calculations indicate that the emergence of piezoelectricity in this material is due to the fact that a stable phase of graphene nitride nanosheet is riddled with regularly spaced triangular holes. These non-centrosymmetric pores, and the universal presence of flexoelectricity in all dielectrics, lead to the manifestation of the apparent and experimentally verified piezoelectric response. Quantitatively, an e11 piezoelectric coefficient of 0.758 C m(-2) is predicted for C3N4 superlattice, significantly larger than that of the commonly compared α-quartz.

Molecular ferroelectrics: where electronics meet biology

In the last several years, we have witnessed significant advances in molecular ferroelectrics, with the ferroelectric properties of molecular crystals approaching those of barium titanate. In addition, ferroelectricity has been observed in biological systems, filling an important missing link in bioelectric phenomena. In this perspective, we will present short historical notes on ferroelectrics, followed by an overview of the fundamentals of ferroelectricity. The latest developments in molecular ferroelectrics and biological ferroelectricity will then be highlighted, and their implications and potential applications will be discussed. We close by noting molecular ferroelectric as an exciting frontier between electronics and biology, and a number of challenges ahead are also described.

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.

Three dimensional architecture of carbon wrapped multilayer Na3V2O2(PO4)2F nanocubes embedded in graphene for improved sodium ion batteries

A novel Na3V2O2(PO4)2F@carbon/graphene three dimensional (3D)architecture (NVPF@C/G) is developed through a simple approach for the first time. It exhibits greatly improved rate capability and delivers a reversible capacity of 113.2 mA h g−1 at 1C.

Direct Observations of Retention Failure in Ferroelectric Memories

Nonvolatile ferroelectric random-access memory uses ferroelectric thin films to save a polar state written by an electric field that is retained when the field is removed. After switching, the high energy of the domain walls separating regions of unlike polarization can drive backswitching resulting in a loss of switched domain volume, or in the case of very small domains, complete retention loss.

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.