Mechanistic formulation of inorganic membranes at the air-liquid interface.

Freestanding functional inorganic membranes, beyond the limits of their organic and polymeric counterparts1, may unlock the potentials of advanced separation2, catalysis3, sensors4,5, memories6, optical filtering7 and ionic conductors8,9. However, the brittle nature of most inorganic materials, and the lack of surface unsaturated linkages10, mean that it is difficult to form continuous membranes through conventional top-down mouldings and/or bottom-up syntheses11. Up to now, only a few specific inorganic membranes have been fabricated from predeposited films by selective removal of sacrificial substrates4-6,8,9. Here we demonstrate a strategy to switch nucleation preferences in aqueous systems of inorganic precursors, resulting in the formation of various ultrathin inorganic membranes at the air-liquid interface. Mechanistic study shows that membrane growth depends on the kinematic evolution of floating building blocks, which helps to derive the phase diagram based on geometrical connectivity. This insight provides general synthetic guidance towards any unexplored membranes, as well as the principle of tuning membrane thickness and through-hole parameters. Beyond understanding  a complex dynamic system, this study comprehensively expands the traditional notion of membranes in terms of composition, structure and functionality.

Giant polarization ripple in transverse pyroelectricity.

Pyroelectricity originates from spontaneous polarization variation, promising in omnipresent non-static thermodynamic energy harvesting. Particularly, changing spontaneous polarization via out-of-plane uniform heat perturbations has been shown in solar pyroelectrics. However, these approaches present unequivocal inefficiency due to spatially coupled low temperature change and duration along the longitudinal direction. Here we demonstrate unconventional giant polarization ripples in transverse pyroelectrics, without increasing the total energy input, into electricity with an efficiency of 5-fold of conventional longitudinal counterparts. The non-uniform graded temperature variation arises from decoupled heat localization and propagation, leading to anomalous in-plane heat perturbation (29-fold) and enhanced thermal disequilibrium effects. This in turn triggers an augmented polarization ripple, fundamentally enabling unprecedented electricity generation performance. Notably, the device generates a power density of 38 mW m-2 at 1 sun illumination, which is competitive with solar thermoelectrics and ferrophotovoltaics. Our findings provide a viable paradigm, not only for universal practical pyroelectric heat harvesting but for flexible manipulation of transverse heat transfer towards sustainable energy harvesting and management.

Macromolecule conformational shaping for extreme mechanical programming of polymorphic hydrogel fibers.

Mechanical properties of hydrogels are crucial to emerging devices and machines for wearables, robotics and energy harvesters. Various polymer network architectures and interactions have been explored for achieving specific mechanical characteristics, however, extreme mechanical property tuning of single-composition hydrogel material and deployment in integrated devices remain challenging. Here, we introduce a macromolecule conformational shaping strategy that enables mechanical programming of polymorphic hydrogel fiber based devices. Conformation of the single-composition polyelectrolyte macromolecule is controlled to evolve from coiling to extending states via a pH-dependent antisolvent phase separation process. The resulting structured hydrogel microfibers reveal extreme mechanical integrity, including modulus spanning four orders of magnitude, brittleness to ultrastretchability, and plasticity to anelasticity and elasticity. Our approach yields hydrogel microfibers of varied macromolecule conformations that can be built-in layered formats, enabling the translation of extraordinary, realistic hydrogel electronic applications, i.e., large strain (1000%) and ultrafast responsive (~30 ms) fiber sensors in a robotic bird, large deformations (6000%) and antifreezing helical electronic conductors, and large strain (700%) capable Janus springs energy harvesters in wearables.

Nanoscale Ferroelectric Characterization with Heterodyne Megasonic Piezoresponse Force Microscopy.

Piezoresponse force microscopy (PFM), as a powerful nanoscale characterization technique, has been extensively utilized to elucidate diverse underlying physics of ferroelectricity. However, intensive studies of conventional PFM have revealed a growing number of concerns and limitations which are largely challenging its validity and applications. In this study, an advanced PFM technique is reported, namely heterodyne megasonic piezoresponse force microscopy (HM-PFM), which uses 106 to 108 Hz high-frequency excitation and heterodyne method to measure the piezoelectric strain at nanoscale. It is found that HM-PFM can unambiguously provide standard ferroelectric domain and hysteresis loop measurements, and an effective domain characterization with excitation frequency up to ≈110 MHz is demonstrated. Most importantly, owing to the high-frequency and heterodyne scheme, the contributions from both electrostatic force and electrochemical strain can be significantly minimized in HM-PFM. Furthermore, a special measurement of difference-frequency piezoresponse frequency spectrum (DFPFS) is developed on HM-PFM and a distinct DFPFS characteristic is observed on the materials with piezoelectricity. By performing DFPFS measurement, a truly existed but very weak electromechanical coupling in CH3NH3PbI3 perovskite is revealed. It is believed that HM-PFM can be an excellent candidate for the ferroelectric or piezoelectric studies where conventional PFM results are highly controversial.

Dynamic thermal trapping enables cross-species smart nanoparticle swarms.

Bioinspired nano/microswarm enables fascinating collective controllability beyond the abilities of the constituent individuals, yet almost invariably, the composed units are of single species. Advancing such swarm technologies poses a grand challenge in synchronous mass manipulation of multimaterials that hold different physiochemical identities. Here, we present a dynamic thermal trapping strategy using thermoresponsive-based magnetic smart nanoparticles as host species to reversibly trap and couple given nonmagnetic entities in aqueous surroundings, enabling cross-species smart nanoparticle swarms (SMARS). Such trapping process endows unaddressable nonmagnetic species with efficient thermo-switchable magnetic response, which determines SMARS' cross-species synchronized maneuverability. Benefiting from collective merits of hybrid components, SMARS can be configured into specific smart modules spanning from chain, vesicle, droplet, to ionic module, which can implement localized or distributed functions that are single-species unachievable. Our methodology allows dynamic multimaterials integration despite the odds of their intrinsic identities to conceive distinctive structures and functions.

A 2D-SnSe film with ferroelectricity and its bio-realistic synapse application.

Catering to the general trend of artificial intelligence development, simulating humans' learning and thinking behavior has become the research focus. Second-order memristors, which are more analogous to biological synapses, are the most promising devices currently used in neuromorphic/brain-like computing. However, few second-order memristors based on two-dimensional (2D) materials have been reported, and the inherent bionic physics needs to be explored. In this work, a second-order memristor based on 2D SnSe films was fabricated by the pulsed laser deposition technique. The continuously adjustable conductance of Au/SnSe/NSTO structures was achieved by gradually switching the polarization of a ferroelectric SnSe layer. The experimental results show that the bio-synaptic functions, including spike-timing-dependent plasticity, short-term plasticity and long-term plasticity, can be simulated using this two-terminal devices. Moreover, stimulus pulses with nanosecond pulse duration were applied to the device to emulate rapid learning and long-term memory in the human brain. The observed memristive behavior is mainly attributed to the modulation of the width of the depletion layer and barrier height is affected, at the SnSe/NSTO interface, by the reversal of ferroelectric polarization of SnSe materials. The device energy consumption is as low as 66 fJ, being expected to be applied to miniaturized, high-density, low-power neuromorphic computing.

Probing the Coexistence of Ferroelectric and Relaxor States in Bi0.5Na0.5TiO3-Based Ceramics for Enhanced Piezoelectric Performance.

To tackle the global restriction on the use of lead-based materials, a feasible strategy of developing a piezoelectric ceramic with a ferroelectric- and relaxor-coexisted hybrid state is proposed in order to reduce the energy barrier as well as to assist polarization rotation. A significantly enhanced piezoelectric coefficient, d33, of 173 pC/N along with a broadened high-temperature stability above 300 °C has been obtained. Further probing via piezoresponse force microscopy unveils the grain boundary-governed domain structures with complicated configurations, suggesting close correlations with the coexistence of ferroelectric and relaxor states. This work demonstrates a recipe for establishing a novel grain-based ferroelectric-relaxor hybrid state with improved piezoelectric performance, which can further be beneficial for realistic applications.

Sustainable Fuel Production from Ambient Moisture via Ferroelectrically Driven MoS2 Nanosheets.

Unlike traditional water splitting in an aqueous medium, direct decomposition of atmospheric water is a promising way to simultaneously dehumidify the living space and generate power. Here, a tailored superhygroscopic hydrogel, a catalyst, and a solar cell are integrated into a humidity digester that can break down ambient moisture into hydrogen and oxygen, creating an efficient electrochemical cell. The function of the hydrogel is to harvest moisture from ambient humidity and transfer the collected water to the catalyst. Barium titanate and vertical 2D MoS2 nanosheets are integrated as the catalyst: the negatively polarized cathode can enhance the electron transport and attract H+ to the MoS2 surface for water reduction, while water oxidation takes place at the positively polarized anode. By employing this mechanism, it is possible to maintain the relative humidity in a medium-sized room at <60% without any additional energy input, and a stable current of 12.5 mA cm-2 is generated by the humidity digester when exposed to ambient light.

A Hybrid Artificial Photocatalysis System Splits Atmospheric Water for Simultaneous Dehumidification and Power Generation.

A new approach for artificial photocatalysis of electrical generation directly from atmospheric water is reported. A hybrid system comprising a hydrogel incorporated with Cu2 O and BaTiO3 nanoparticles is developed, wherein the Cu2 O is designed to expose two different crystal planes, namely (100) and (111). These planes exhibit different surface potentials and form a polarization electric field of 2.3 kV cm-1 that acts on a ferroelectric dipole. With the help of this electric field, the dipole is redirected for aiding in positive and negative polarizations with (100) and (111) planes, then boosting water reduction and oxidation kinetics separately at (100) and (111) planes. Additonally, zinc-/cobalt-based superhygroscopic hydrogels serve as a water-capturing "hand" to harness humidity from the ambient environment. The integrated hydrogel-Cu2 O@BaTiO3 hybrid is used to dehumidify air, which can split 36.5 mg of water by employing only 150 mg hydrogel and simultaneously generate a photocurrent of 224.3 µA cm-2 under 10 mW cm-2 illumination.

Probing the Ionic and Electrochemical Phenomena during Resistive Switching of NiO Thin Films.

Ionic transport and electrochemical reactions underpin the functionality of the memory devices. NiO, as a promising transition metal oxide for developing resistive switching random access memory, has been extensively explored in the terms of the resistive switching. However, there is limited experimental evidence to visualize the ionic processes of the NiO under the external electrical field. In addition, the correlation between the ionic processes and the resistive switching has not been established. To close this gap and also to determine the role of the ionic processes in resistive switching of the NiO, in this study, a series of scanning probe microscopy techniques, including electrochemical strain microscopy (ESM), conductive atomic force microscopy, Kelvin probe force microscopy, and a newly developed first-order reversal curve-IV, are employed to measure the ESM response, the resistive switching performance, the work function, and the ionic dynamics of NiO, respectively. The results in this work have clearly visualized the ionic transport and electrochemical reactions of NiO when subjected to the electrical field. It has been found that the ionic processes and the resistive switching accompanied each other. Furthermore, it is found that the electrochemical reactions play a determinative role in the resistive switching of the NiO, and this electrochemically induced resistive switching performance can be explained by an integrated mechanism that has combined the filamentary and the interfacial effects underlying resistive switching. In addition to providing a better understanding of the resistive switching of NiO, this work also provides effective methods to probe the ionic processes and to correlate these ionic processes to the performance of functional materials.

Understanding the Intrinsic Carrier Transport in Highly Oriented Poly(3-hexylthiophene): Effect of Side Chain Regioregularity.

The fundamental understanding of the influence of molecular structure on the carrier transport properties in the field of organic thermoelectrics (OTEs) is a big challenge since the carrier transport behavior in conducting polymers reveals average properties contributed from all carrier transport channels, including those through intra-chain, inter-chain, inter-grain, and hopping between disordered localized sites. Here, combining molecular dynamics simulations and experiments, we investigated the carrier transport properties of doped highly oriented poly(3-hexylthiophene) (P3HT) films with different side-chain regioregularity. It is demonstrated that the substitution of side chains can not only take effect on the carrier transport edge, but also on the dimensionality of the transport paths and as a result, on the carrier mobility. Conductive atomic force microscopy (C-AFM) study as well as temperature-dependent measurements of the electrical conductivity clearly showed ordered local current paths in the regular side chain P3HT films, while random paths prevailed in the irregular sample. Regular side chain substitution can be activated more easily and favors one-dimensional transport along the backbone chain direction, while the irregular sample presents the three-dimensional electron hopping behavior. As a consequence, the regular side chain P3HT samples demonstrated high carrier mobility of 2.9 ± 0.3 cm²/V·s, which is more than one order of magnitude higher than that in irregular side chain P3HT films, resulting in a maximum thermoelectric (TE) power factor of 39.1 ± 2.5 µW/mK² at room temperature. These findings would formulate design rules for organic semiconductors based on these complex systems, and especially assist in the design of high performance OTE polymers.

Probing electrochemically induced resistive switching of TiO2 using SPM techniques.

Resistive switching on the nanoscale is an emerging research field and Scanning Probe Microscopy (SPM) is a powerful tool for studies in this area. Under the SPM tip, the electrical field is very high due to the small tip radius on the order of tens of nanometers, and this can enable a range of ionic/electrochemical phenomena during the resistive switching of the materials under the SPM tip. Although the ionic/electrochemical phenomena have long been considered vital for the resistive switching of materials, a few pieces of experimental evidence, as well as the decoupling of the effects of the electrochemical processes at different stages, are still needed. In this work, we applied SPM based techniques to study resistive switching as well as the electrochemical phenomena during the resistive switching of the TiO2 thin films prepared using Pulse Laser Deposition (PLD). It was found that the reversible or irreversible electrochemical processes initiated at different voltages can promote or degrade the resistive switching behavior of TiO2. Combined with an electrical cell with environmental control, these electrochemical processes have been shown to require the involvement of moisture; the accumulation of oxygen vacancies, protons, and hydroxyls at the tip/TiO2 junction may contribute to the promoting effect of the reversible electrochemical process on resistive switching, while the oxygen vacancy ordering and the injection of protons and hydroxyls into the lattice may lead to the irreversible electrochemical process. This work provides a detailed insight into the characteristics, origins, and the effects of the electrochemical phenomena on resistive switching performance, and will provide a further understanding of the electrochemical phenomena in various functional materials.

Self-surface charge exfoliation and electrostatically coordinated 2D hetero-layered hybrids.

At present, the technological groundwork of atomically thin two-dimensional (2D) hetero-layered structures realized by successive thin film epitaxial growth is in principle constrained by lattice matching prerequisite as well as low yield and expensive production. Here, we artificially coordinate ultrathin 2D hetero-layered metal chalcogenides via a highly scalable self-surface charge exfoliation and electrostatic coupling approach. Specifically, bulk metal chalcogenides are spontaneously exfoliated into ultrathin layers in a surfactant/intercalator-free medium, followed by unconstrained electrostatic coupling with a dissimilar transition metal dichalcogenide, MoSe2, into scalable hetero-layered hybrids. Accordingly, surface and interfacial-dominated photocatalysis reactivity is used as an ideal testbed to verify the reliability of diverse 2D ultrathin hetero-layered materials that reveal high visible-light photoreactivity, efficient charge transfer and intimate contact interface for stable cycling and storage purposes. Such a synthetic approach renders independent thickness and composition control anticipated to advance the development of 'design-and-build' 2D layered heterojunctions for large-scale exploration and applications.