Breaking the Fundamental Limitations of Nanoscale Ferroelectric Characterization: Non-Contact Heterodyne Electrostrain Force Microscopy.

Perceiving nanoscale ferroelectric phenomena from real space is of great importance for elucidating underlying ferroelectric physics. During the past decades, nanoscale ferroelectric characterization has mainly relied on the Piezoresponse Force Microscopy (PFM) invented in 1992, however, the fundamental limitations of PFM have made the nanoscale ferroelectric studies encounter significant bottlenecks. In this study, a high-resolution non-contact ferroelectric measurement, named Non-Contact Heterodyne Electrostrain Force Microscopy (NC-HEsFM), is introduced. It is demonstrated that NC-HEsFM can operate on multiple eigenmodes to perform ideal high-resolution ferroelectric domain mapping, standard ferroelectric hysteresis loop measurement, and controllable domain manipulation. By using a quartz tuning fork (QTF) sensor, multi-frequency operation, and heterodyne detection schemes, NC-HEsFM achieves a real non-contact yet non-destructive ferroelectric characterization with negligible electrostatic force effect and hence breaks the fundamental limitations of the conventional PFM. It is believed that NC-HEsFM can be extensively used in various ferroelectric or piezoelectric studies with providing substantially improved characterization performance. Meanwhile, the QTF-based force detection makes NC-HEsFM highly compatible for high-vacuum and low-temperature environments, providing ideal conditions for investigating the intrinsic ferroelectric phenomena with the possibility of achieving an atomically resolved ferroelectric characterization.

Intrinsic low sodium/NASICON interfacial resistance paving the way for room temperature sodium-metal battery.

Sodium-metal batteries have strong potential to be utilized as stationary high energy density storage devices. Owing to its high ionic conductivity, low electronic conductivity and relatively easy fabrication, NASICON-structure electrolyte (Na3Zr2Si2PO12) is one of the potential candidates to be considered in the solid-state sodium-metal batteries at room temperature. However, the large interfacial resistance between the solid-state electrolyte and the metallic sodium is known to limit the critical current density (CCD) of the cell. In this study, a simple and cost-effective annealing process is introduced to the electrolyte preparation to improves its interface with metallic sodium. X-ray photoelectron spectroscopy and scanning probe microscopy show that Si forms bonds with the surface functional groups when exposed to the ambient condition. With the removal of surface contamination as well as a partially reduced electrolyte surface, the annealed electrolyte shows an extremely small interfacial resistance of 11 Ω cm2 and a high CCD of 0.9 mA cm-2. This study provides an insight on the electrolyte surface preparation and its significant in a sodium-metal solid-state battery.

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.

Preparation of Eucommia ulmoides lignin-based high-performance biochar containing sulfonic group: Synergistic pyrolysis mechanism and tetracycline hydrochloride adsorption.

In this study, the sulfonic group was introduced to prepare high-performance Eucommia ulmoides lignin-based biochar, which was used to remove tetracycline hydrochloride. The BET area (2008 m2 g-1) of sulfonated biochar was twice that of unmodified biochar. Through XRD and Raman analysis, the synergetic pyrolysis mechanism of the sulfonic group in the formation of the porous structure was discussed. Sulfonated biochar had excellent adsorption performance for tetracycline hydrochloride (Qm: 1163 mg g-1), while the adsorption performance of unmodified biochar was about only one-fourth (Qm: 277.7 mg g-1) of that. The adsorption of tetracycline hydrochloride by the sulfonated biochar was spontaneously endothermic and conformed to the Langmuir isotherm model. The adsorption process was confirmed by pseudo-second-order kinetic model. Moreover, the sulfonic group on the sulfonated biochar significantly promoted the formation of the hydrogen bond and greatly improved the adsorption performance.

Probing nanoscale oxygen ion motion in memristive systems.

Ion transport is an essential process for various applications including energy storage, sensing, display, memory and so on, however direct visualization of oxygen ion motion has been a challenging task, which lies in the fact that the normally used electron microscopy imaging mainly focuses on the mass attribute of ions. The lack of appropriate understandings and analytic approaches on oxygen ion motion has caused significant difficulties in disclosing the mechanism of oxides-based memristors. Here we show evidence of oxygen ion migration and accumulation in HfO2 by in situ measurements of electrostatic force gradient between the probe and the sample, as systematically verified by the charge duration, oxygen gas eruption and controlled studies utilizing different electrolytes, field directions and environments. At higher voltages, oxygen-deficient nano-filaments are formed, as directly identified employing a CS-corrected transmission electron microscope. This study could provide a generalized approach for probing ion motions at the nanoscale.

Local Dielectric Property Detection of the Interface between Nanoparticle and Polymer in Nanocomposite Dielectrics.

The interface between nanoparticles and polymer matrix is considered to have an important effect on the properties of nanocomposites. In this experimental study, electrostatic force microscopy (EFM) is used to study the local dielectric property of the interface of low density polyethylene (LDPE)/TiO2 nanocomposites at nanometer scale. The results show that the addition of TiO2 nanoparticles leads to a decrease in local permittivity. We then carry out the finite element simulation and confirm that the decrease of local permittivity is related to the effect of interface. According to the results, we propose several models and validate the dielectric effect and range effect of interface. Through the analysis of DSC and solid-state NMR results, we find TiO2 nanoparticles can suppress the mobility of local chain segments in the interface, which influences the dipolar polarization of chain segments in the interface and eventually results in a decrease in local permittivity. It is believed the results would provide important hint to the research of the interface in future research.

Electrically configurable graphene field-effect transistors with a graded-potential gate.

A device architecture for electrically configurable graphene field-effect transistor (GFET) using a graded-potential gate is present. The gating scheme enables a linearly varying electric field that modulates the electronic structure of graphene and causes a continuous shift of the Dirac points along the channel of GFET. This spatially varying electrostatic modulation produces a pseudobandgap observed as a suppressed conductance of graphene within a controllable energy range. By tuning the electrical gradient of the gate, a GFET device is reversibly transformed between ambipolar and n- and p-type unipolar characteristics. We further demonstrate an electrically programmable complementary inverter, showing the extensibility of the proposed architecture in constructing logic devices based on graphene and other Dirac materials. The electrical configurable GFET might be explored for novel functionalities in smart electronics.