Mixed Triboelectric and Flexoelectric Charge Transfer at the Nanoscale.

The triboelectric effect is a ubiquitous phenomenon in which the surfaces of two materials are easily charged during the contact-separation process. Despite the widespread consequences and applications, the charging mechanisms are not sufficiently understood. Here, the authors report that, in the presence of a strain gradient, the charge transfer is a result of competition between flexoelectricity and triboelectricity, which could enhance charge transfer during triboelectric measurements when the charge transfers of both effects are in the same direction. When they are in the opposite directions, the direction and amount of charge transfer could be modulated by the competition between flexoelectric and triboelectric effects, which leads to a distinctive phenomenon, that is, the charge transfer is reversed with varying forces. The subsequent results on the electrical power output signals from the triboelectrification support the proposed mechanism. Therefore, the present study emphasizes the key role of the flexoelectric effect through experimental approaches, and suggests that both the amount and direction of charge transfer can be modulated by manipulating the mixed triboelectric and flexoelectric effects. This finding may provide important information on the triboelectric effect and can be further extended to serve as a guideline for material selection during a nanopatterned device design.

Recent Progress in the Nanoscale Evaluation of Piezoelectric and Ferroelectric Properties via Scanning Probe Microscopy.

Piezoelectric and ferroelectric materials have garnered significant interest owing to their excellent physical properties and multiple potential applications. Accordingly, the need for evaluating piezoelectric and ferroelectric properties has also increased. The piezoelectric and ferroelectric properties are evaluated macroscopically using laser interferometers and polarization-electric field loop measurements. However, as the research focus is shifted from bulk to nanosized materials, scanning probe microscopy (SPM) techniques have been suggested as an alternative approach for evaluating piezoelectric and ferroelectric properties. In this Progress Report, the recent progress on the nanoscale evaluation of piezoelectric and ferroelectric properties of diverse materials using SPM-based methods is summarized. Among the SPM techniques, the focus is on recent studies that are related to piezoresponse force microscopy and conductive atomic force microscopy; further, the utilization of these two modes to understand piezoelectric and ferroelectric properties at the nanoscale level is discussed. This work can provide guidelines for evaluating the piezoelectric and ferroelectric properties of materials based on SPM techniques.

Data Mining of Heterogeneous Electrical Conduction in the Electrode Components of Fuel Cells.

The electrodes of a polymer electrolyte membrane fuel cell (PEMFC) primarily contain a Pt/C catalyst and Nafion binder. Since these components play crucial roles in the redox reaction and proton transport, respectively, their distributions can directly affect the electrochemical reactivity and thus the device performance. Although analyzing the component distribution is important to understand its electrochemical reactivity and improve the device performance, determining it for the PEMFC electrode remains a challenging task. Herein, we propose a strategy for visualizing the spatial distribution of the electrode components and their heterogeneous electrical properties using multidimensional current-voltage (I-V) spectroscopy combined with data mining. The electrical properties of the electrode components, i.e., the Pt/C catalyst and Nafion binder, were explored by I-V spectroscopy, and their electrical heterogeneity was spatially classified based on the shapes of the measured I-V curves by cluster analysis. The results show that the components and their interfacial structure can be spatially visualized from the surface electrical heterogeneity. The proposed method is expected to be applicable for investigating in detail not only the spatial properties of PEMFC electrodes but also the properties of various material systems.

Mechanically programmed 2D and 3D liquid crystal elastomers at macro- and microscale via two-step photocrosslinking.

Liquid crystal elastomers (LCEs) are a unique class of active materials with the largest known reversible shape transformation in the solid state. The shape change of LCEs is directed by programming their molecular orientation, and therefore, several strategies to control LC alignment have been developed. Although mechanical alignment coupled with a two-step crosslinking is commonly adopted for uniaxially-aligned monodomain LCE synthesis, the fabrication of 3D-shaped LCEs at the macro- and microscale has been rarely accomplished. Here, we report a facile processing method for fabricating 2D and 3D-shaped LCEs at the macro- and microscales at room temperature by mechanically programming (i.e., stretching, pressing, embossing and UV-imprinting) the polydomain LCE, and subsequent photocrosslinking. The programmed LCEs exhibited a reversible shape change when exposed to thermal and chemical stimuli. Besides the programmed shape changes, the actuation strain can also be preprogrammed by adjusting the extent of elongation of a polydomain LCE. Furthermore, the LCE micropillar arrays prepared by UV-imprinting displayed a substantial change in pillar height in a reversible manner during thermal actuation. Our convenient method for fabricating reversible 2D and 3D-shaped LCEs from commercially available materials may expedite the potential applications of LCEs in actuators, soft robots, smart coatings, tunable optics and medicine.

The device level modulation of carrier transport in a 2D WSe2 field effect transistor via a plasma treatment.

Tungsten diselenide (WSe2) has received significant attention because it shows the pristine ambipolar property arising from the Fermi level located near the midgap and can be converted to uni-polar form. In this study, we observe the formation of tungsten oxide (WOx) on the WSe2 surface after oxygen plasma treatment and show that the p-type WOx dopes WSe2. In our devices that underwent plasma treatment, it was interesting to find a strong correlation between the changes in the work function of WSe2 and a gold electrode, and the channel and contact resistances. The channel resistance changes very sensitively at a rate of 64 meV per dec with the increase in the WSe2 channel work function, which is close to the thermal limit; this indicates the defect-free oxidized WSe2 channel. The carrier transport in the oxidized WSe2 FET is shown to change to a high performance p-type device with greatly reduced channel and contact resistances with the increase in the plasma oxidation time.

Significance of electrostatic interactions due to surface potential in piezoresponse force microscopy.

Piezoresponse force microscopy (PFM) has gradually becomes indispensable tool to investigate local piezoelectric and ferroelectric properties in diverse material systems. However, numerous reports have shown that the PFM signal can originate from several non-piezoelectric origins. Among them, because the electrostatic interaction between the AFM tip/cantilever and sample surface can be readily involved, it can be the most important factor during PFM measurement. In particular, in materials with relatively low piezoelectricity, the situation can be more significant because the PFM signals from weak piezoelectricity can be hidden or buried by the electrostatic interactions. Herein, we examined the significance of the electrostatic interactions induced by the surface potential in PFM. Using piezoelectric and non-piezoelectric materials, we examined how the surface potential-dependent electrostatic interactions can significantly affect the PFM signal. We observed that the electrostatically induced PFM amplitude have a linear relationship with the magnitude of surface potential when the instrumental noise floor is properly considered. Our results demonstrate that electrostatic interactions can be significant and their recognition and minimization are essential during PFM and other AFM-based measurements.

A Room-Temperature Ferroelectric Ferromagnet in a 1D Tetrahedral Chain Network.

Ferroelectricity occurs in crystals with broken spatial inversion symmetry. In conventional perovskite oxides, concerted ionic displacements within a 3D network of transition-metal-oxygen polyhedra (MOx ) manifest spontaneous polarization. Meanwhile, some 2D networks of MOx foster geometric ferroelectricity with magnetism, owing to the distortion of the polyhedra. Because of the fundamentally different mechanism of ferroelectricity in a 2D network, one can further challenge an uncharted mechanism of ferroelectricity in a 1D channel of MOx and estimate its feasibility. Here, ferroelectricity and coupled ferromagnetism in a 1D FeO4 tetrahedral chain network of a brownmillerite SrFeO2.5 epitaxial thin film are presented. The result provides a new paradigm for designing low-dimensional MOx networks, which is expected to benefit the realization of macroscopic ferro-ordering materials including ferroelectric ferromagnets.

Ferroelectric Polarization Rotation in Order-Disorder-Type LiNbO3 Thin Films.

The direction of ferroelectric polarization is prescribed by the symmetry of the crystal structure. Therefore, rotation of the polarization direction is largely limited, despite the opportunity it offers in understanding important dielectric phenomena such as piezoelectric response near the morphotropic phase boundaries and practical applications such as ferroelectric memory. In this study, we report the observation of continuous rotation of ferroelectric polarization in order-disorder-type LiNbO3 thin films. The spontaneous polarization could be tilted from an out-of-plane to an in-plane direction in the thin film by controlling the Li vacancy concentration within the hexagonal lattice framework. Partial inclusion of monoclinic-like phase is attributed to the breaking of macroscopic inversion symmetry along different directions and the emergence of ferroelectric polarization along the in-plane direction.

Tunable Out-of-Plane Piezoelectricity in Thin-Layered MoTe2 by Surface Corrugation-Mediated Flexoelectricity.

Piezoelectricity crystallographically exists only in the in-plane direction in two-dimensional transition metal dichalcogenides. Here, we demonstrated flexoelectricity-tunable out-of-plane piezoelectricity in semiconducting 2H-MoTe2 flakes by creating surface corrugation. In particular, the strong out-of-plane piezoelectricity and its spatial variation depending on local flexoelectricity was observed even though crystallographically there exists only in-plane piezoelectricity. Surface corrugation-mediated flexoelectricity tuning can be applied to other two-dimensional or thin-layered materials and, furthermore, the results could provide useful information on the interweaving nature between mechanical stimulus and electric dipole in low-dimensional materials.

Dynamic mechanical control of local vacancies in NiO thin films.

The manipulation of local ionic behavior via external stimuli in oxide systems is of great interest because it can help in directly tuning material properties. Among external stimuli, mechanical force has attracted intriguing attention as novel stimulus for ionic modulation. Even though effectiveness of mechanical force on local ionic modulation has been validated in terms of static effect, its real-time i.e., dynamic, behavior under an application of the force is barely investigated in spite of its crucial impact on device performance such as force or pressure sensors. In this study, we explore dynamic ionic behavior modulated by mechanical force in NiO thin films using electrochemical strain microscopy (ESM). Ionically mediated ESM hysteresis loops were significantly varied under an application of mechanical force. Based on these results, we were able to investigate relative relationship between the force and voltage effects on ionic motion and, further, control effectively ionic behavior through combination of mechanical and electrical stimuli. Our results can provide comprehensive information on the effect of mechanical forces on ionic dynamics in ionic systems.

Direct Probing of Polarization Charge at Nanoscale Level.

Ferroelectric materials possess spontaneous polarization that can be used for multiple applications. Owing to a long-term development of reducing the sizes of devices, the preparation of ferroelectric materials and devices is entering the nanometer-scale regime. Accordingly, to evaluate the ferroelectricity, there is a need to investigate the polarization charge at the nanoscale. Nonetheless, it is generally accepted that the detection of polarization charges using a conventional conductive atomic force microscopy (CAFM) without a top electrode is not feasible because the nanometer-scale radius of an atomic force microscopy (AFM) tip yields a very low signal-to-noise ratio. However, the detection is unrelated to the radius of an AFM tip and, in fact, a matter of the switched area. In this work, the direct probing of the polarization charge at the nanoscale is demonstrated using the positive-up-negative-down method based on the conventional CAFM approach without additional corrections or circuits to reduce the parasitic capacitance. The polarization charge densities of 73.7 and 119.0 µC cm-2 are successfully probed in ferroelectric nanocapacitors and thin films, respectively. The obtained results show the feasibility of the evaluation of polarization charge at the nanoscale and provide a new guideline for evaluating the ferroelectricity at the nanoscale.

Evenly transferred single-layered graphene membrane assisted by strong substrate adhesion.

We explored the transfer of a single-layered graphene membrane assisted by substrate adhesion. A relatively larger adhesion force was measured on the SiO2 substrate compared with its van der Waals contribution, which is expected to result from the additional contribution of the chemical bonding force. Density functional theory calculations verified that the strong adhesion force was indeed accompanied by chemical bonding. The transfer of single-layered graphene and subsequent deposition of the dielectric layer were best performed on the SiO2 substrate exhibiting a larger adhesion force. This study suggests the selection and/or modification of the underlying substrate for proper transfer of graphene as well as other 2D materials similar to graphene.

Electrostatic-free piezoresponse force microscopy.

Contact and non-contact based atomic force microscopy (AFM) approaches have been extensively utilized to explore various nanoscale surface properties. In most AFM-based measurements, a concurrent electrostatic effect between the AFM tip/cantilever and sample surface can occur. This electrostatic effect often hinders accurate measurements. Thus, it is very important to quantify as well as remove the impact of the electrostatic effect on AFM-based measurements. In this study, we examine the impact of the electrostatic effect on the electromechanical (EM) response in piezoresponse force microscopy as a model AFM mode. We quantitatively studied the effects of increasing the external electric field and reducing the spring constant of a cantilever. Further, we explored ways to minimize the electrostatic effect. The results provide broad guidelines for quantitatively analyzing the EM response as well as, eventually, for obtaining the electrostatic-free EM response. The conclusions can be applied to other AFM-based measurements that are subject to a strong electrostatic effect between the AFM tip/cantilever and sample surface, regardless of contact and non-contact modes.

Nanosculpting of complex oxides by massive ionic transfer.

Scanning probe microscopy (SPM)-based approaches have been extensively studied as methods to control the structure and properties of materials on the nanoscale. In many cases, the SPM probe is physically utilized to control structure and properties. In addition to physical modulation, it has been reported that voltage can be effectively used to modulate electrochemical phenomena on the sample surface. These studies suggest that electrochemical modulation of the structure and properties is possible by applying a voltage. Herein, in order to demonstrate voltage induced modulation of surface structure, we explored surface nanosculpting by creating electrochemically induced pits on the surface of TiO2 thin films through the application of voltage using the atomic force microscope tip. Using a unipolar negative voltage sweep, pits were successfully generated. Further, the electric potential distribution was simulated to unravel the relationship between the pit volume and the magnitude of the applied voltage. Finally, surface protrusion induced by positive voltage sweep was also observed to elucidate the complete process of electrochemically induced surface modulation. These results can offer fundamental information for understanding how surface structure can be modulated by electrochemical phenomena.

Determination of ferroelectric contributions to electromechanical response by frequency dependent piezoresponse force microscopy.

Hysteresis loop analysis via piezoresponse force microscopy (PFM) is typically performed to probe the existence of ferroelectricity at the nanoscale. However, such an approach is rather complex in accurately determining the pure contribution of ferroelectricity to the PFM. Here, we suggest a facile method to discriminate the ferroelectric effect from the electromechanical (EM) response through the use of frequency dependent ac amplitude sweep with combination of hysteresis loops in PFM. Our combined study through experimental and theoretical approaches verifies that this method can be used as a new tool to differentiate the ferroelectric effect from the other factors that contribute to the EM response.

Probing of multiple magnetic responses in magnetic inductors using atomic force microscopy.

Even though nanoscale analysis of magnetic properties is of significant interest, probing methods are relatively less developed compared to the significance of the technique, which has multiple potential applications. Here, we demonstrate an approach for probing various magnetic properties associated with eddy current, coil current and magnetic domains in magnetic inductors using multidimensional magnetic force microscopy (MMFM). The MMFM images provide combined magnetic responses from the three different origins, however, each contribution to the MMFM response can be differentiated through analysis based on the bias dependence of the response. In particular, the bias dependent MMFM images show locally different eddy current behavior with values dependent on the type of materials that comprise the MI. This approach for probing magnetic responses can be further extended to the analysis of local physical features.

Multi-floor cascading ferroelectric nanostructures: multiple data writing-based multi-level non-volatile memory devices.

Multiple data writing-based multi-level non-volatile memory has gained strong attention for next-generation memory devices to quickly accommodate an extremely large number of data bits because it is capable of storing multiple data bits in a single memory cell at once. However, all previously reported devices have failed to store a large number of data bits due to the macroscale cell size and have not allowed fast access to the stored data due to slow single data writing. Here, we introduce a novel three-dimensional multi-floor cascading polymeric ferroelectric nanostructure, successfully operating as an individual cell. In one cell, each floor has its own piezoresponse and the piezoresponse of one floor can be modulated by the bias voltage applied to the other floor, which means simultaneously written data bits in both floors can be identified. This could achieve multi-level memory through a multiple data writing process.

Strong anisotropy of ferroelectricity in lead-free bismuth silicate.

Bismuth silicate (Bi2SiO5) was recently suggested as a potential silicate based lead-free ferroelectric material. Here, we show the existence of ferroelectricity and explore the strong anisotropy of local ferroelectricity using piezoresponse force microscopy (PFM). Domain structures are reconstructed using angle-resolved PFM. Furthermore, piezoresponse hysteresis loops and piezoelectric coefficients are spatially investigated at the nanoscale. The obtained results confirm the existence of ferroelectricity with strong c-axis polarization. These results could provide basic information on the anisotropic ferroelectricity in Bi2SiO5 and furthermore suggest its considerable potential for lead-free ferroelectric applications with silicon technologies.

Humidity effect of domain wall roughening behavior in ferroelectric copolymer thin films.

We have demonstrated that domain switching in ferroelectric copolymer films can be significantly affected by humidity. With increasing relative humidity (RH), we observed larger domains with highly irregular boundaries as a result of lateral spreading of the tip-induced electric field that originates from water adsorption. Fractal dimension study of irregular domains reveals that the fractal dimension is higher in cases where the RH is higher. The results show that the RH is one of the major switching parameters in ferroelectric copolymers, and therefore could allow clear understanding with regard to domain switching behavior in the ferroelectric copolymer films under ambient conditions.