Probe Force Research Articles

Remote-Controllable Interfacial Electron Tunneling at Heterogeneous Molecular Junctions via Tip-Induced Optoelectrical Engineering.

Molecular electronics enables functional electronic behavior via single molecules or molecular self-assembled monolayers, providing versatile opportunities for hybrid molecular-scale electronic devices. Although various molecular junction structures are constructed to investigate charge transfer dynamics, significant challenges remain in terms of interfacial charging effects and far-field background signals, which dominantly block the optoelectrical observation of interfacial charge transfer dynamics. Here, tip-induced optoelectrical engineering is presented that synergistically correlates photo-induced force microscopy and Kelvin probe force microscopyto remotely control and probe the interfacial charge transfer dynamics with sub-10nm spatial resolution. Based on this approach, the optoelectrical origin of metal-molecule interfaces is clearly revealed by the nanoscale heterogeneity of the tip-sample interaction and optoelectrical reactivity, which theoretically aligned with density functional theory calculations. For a practical device-scale demonstration of tip-induced optoelectrical engineering, interfacial tunneling is remotely controlled at a 4-inch wafer-scale metal-insulator-metal capacitor, facilitating a 5.211-fold current amplification with the tip-induced electrical field. In conclusion, tip-induced optoelectrical engineering provides a novel strategy to comprehensively understand interfacial charge transfer dynamics and a non-destructive tunneling control platform that enables real-time and real-space investigation of ultrathin hybrid molecular systems.

Mesoporous TiO2 matrix embeded with Cs2CuBr4 perovskite quantum dots as a step-scheme-based photocatalyst for boosting charge separation and CO2 photoconversion

Halide perovskite quantum dots (PQDs) have shown great promise for the photochemical conversion of CO2. However, the fast carrier recombination rates and inadequate adsorption/activation for CO2 molecules have seriously restricted their practical application. Herein, an innovative step-scheme-based Cs2CuBr4/TiO2 (CCB/TiO2) photocatalyst was prepared by embedding Cs2CuBr4 PQDs in a mesoporous TiO2 matrix, which exhibits considerable potential as candidates for CO2 photoreduction under simulated sunlight. Remarkably, the optimised CCB/TiO2 photocatalyst delivered an impressive CO2 reduction activity, which was 3.1 and 16.0 times higher than those achieved by pure Cs2CuBr4 and TiO2, respectively. Furthermore, the S-scheme charge transfer mode was comprehensively corroborated by density functional theory (DFT) calculations, photo-assisted Kelvin probe force microscopy (KPFM), in situ irradiated X-ray photoelectron spectra (XPS), and electron spin resonance spectroscopy (ESR). On basis of theoretical calculations and in situ diffuse reflectance spectra (DRIFTS), the CCB/TiO2 exhibited a reduced energy barrier of the rate-determining step, which facilitated the CO2 photoreduction reaction.

Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization.

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

A Dynamic Memory for Reservoir Computing Utilizing Ion Migration in CuInP2S6

AbstractTime‐series analysis and forecasting play a vital role in the fields of economics and engineering. Neuromorphic computing, particularly recurrent neural networks (RNNs), has emerged as an effective approach to address these tasks. Reservoir computing (RC), a type of RNN, offers a powerful and efficient solution for handling nonlinear information in high‐dimensional spaces and addressing temporal tasks. CuInP2S6 (CIPS), a van der Waals material with ion conductivity, shows promise for sequential task processing. Here, a synapse device based on CIPS is demonstrated that exhibits temporal dynamics under electrical stimulation. By controlling Cu+ ion migration, this study successfully emulates synaptic performance, including potentiation and depression characteristics, and RC. Migration of Cu+ ions is confirmed using piezoresponse and Kelvin probe force microscopy. The device achieves low normalized root mean square errors (NRMSE) of 0.04762 and 0.01402 for the Hénon map and Mackey‐Glass series tasks, respectively. For real‐life time‐series prediction based on the Jena temperature database, an overall NRMSE of 0.03339 is achieved. These results highlight the potential of CIPS ion conductivity for real‐time signal processing in machine learning, expanding applications in neuromorphic computing.

Lattice distortion SnS2 piezoelectric self-Fenton system for efficient degradation and detoxification of pollutants

Both piezoelectricity and self-Fenton catalysis are effective ways to degrade water pollution, but little research has combined them to construct a more efficient water pollution treatment method. Here, a Fe-doped SnS2 (Sn1-xFexS2) piezoelectric self-Fenton system was constructed, which shows superior water treatment performance. The best piezoelectric properties of the Sn0.97Fe0.03S2 system were verified by degrading rhodamine B (RhB). The toxicity analysis of degradation intermediates and solutions confirmed that the toxicity of RhB decreased after degradation. In addition, Kelvin probe force microscopy and photoelectrochemical analysis confirmed the better piezoelectric properties of Sn0.97Fe0.03S2. It has demonstrated the enhancement of systematic piezoelectricity by Fe lattice defects and the formation of self-Fenton by Fe as an active center in the degradation of RhB. In this work, an efficient piezoelectric and self-Fenton technology is constructed to remove organic pollutants from water, which is significant for developing water treatment technology.

Fiber-Integrated Force Sensor using 3D Printed Spring-Composed Fabry-Perot Cavities with a High Precision Down to Tens of Piconewton.

Developing microscale sensors capable of force measurements down to the scale of piconewtons is of fundamental importance for a wide range of applications. To date, advanced instrumentations such as atomic force microscopes and other specifically developed micro/nano-electromechanical systems face challenges such as high cost, complex detection systems and poor electromagnetic compatibility. Here, it presents the unprecedented design and 3D printing of general fiber-integrated force sensors using spring-composed Fabry-Perot cavities. It calibrates these microscale devices employing varied-diameter m-scale silica particles as standard weights. The force sensitivity and resolution reach values of (0.436 ± 0.007) nmnN-1 and (40.0 ± 0.7) pN, respectively, which are the best resolutions among all fiber-based nanomechanical probes so far. It also measured the non-linear airflow force distributions produced from a nozzle with an orifice of 2 m, which matches well with the full-sized simulations. With further customization of their geometries and materials, it anticipates the easy-to-use force probe can well extend to many other applications such as air/fluidic turbulences sensing, micro-manipulations, and biological sensing.

Enhanced electrical conductivity and reduced work function of β-Ga2O3 thin films by hydrogen plasma treatment

As a common impurity, H plays a role in tuning the electrical properties of β-Ga2O3 and has attracted immense interest. Despite years of investigations, the influence of H-doping on electrical properties is not always clear due to the lack of comprehensive characterization on both micro- and macro scale. In this work, we investigate the effects of the H-plasma treatment on the electrical properties of β-Ga2O3 films by combining several techniques, from macroscale Hall and photoluminescence measurements to microscale conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM). The incorporation of H in β-Ga2O3 not only introduces shallow donor states such as Hi, but also passivates VGa defects by forming the VGa-4H complex. As a result, both the carrier concentration and mobility of H-doped β-Ga2O3 film are significantly increased, corresponding to an enhancement of conductivity by four orders of magnitude in comparison with the intrinsic one. These results correlate well with the local conductivity and surface potential mappings obtained from CAFM and KPFM. Moreover, we found that the work function of β-Ga2O3 thin films can also be tuned by the H-plasma treatment.

Probing and Modulation of the Electric Double Layer at the Insulating Oil-Paper Interface.

Charge accumulation in the insulating oil-paper system determines the operating safety of the converter transformers in high-voltage direct current (HVDC) transmissions. However, it has been a long-standing challenge to reveal the charge distribution of the electric double layer (EDL) at the insulating oil-paper interface and relate it to charge transport. In particular, the EDL and charging mechanisms at the oil-paper interface have not been fully understood. We herein demonstrate that the charge distribution of EDL at the oil-paper interface is probed through Kelvin probe force microscopy (KPFM). The origin charge distribution of EDL without any additives shows that the negative charge gathers on the insulating paper surface, while the positive charge diffuses in the insulating oil, which is derived from the electron affinity difference between insulating oil and insulating paper and acts as an additional obstacle to charge transportation at the oil-paper interface. Interestingly, the additive 3-amino-2,4-triazole (ATA) can tune the charge distribution of EDL by bringing extra hole traps, which significantly decreases the interface barrier and reduces the charge accumulation at the oil-paper interface. As well as increasing charge mobility in oil-paper insulation, ATA also ensures stabilization of operation under polarity inversion conditions by accelerating the dissipation rate of accumulated charge.

Effect of Si Content on Microstructures and Electrochemical Properties of Al-xSi-3.5Fe Coating Alloy

Hot-dip aluminum alloy is widely used in the engineering fields. However, during the aluminum plating process, Fe inevitably enters and reaches a saturation state, which has a significant impact on the corrosion resistance and microstructure of the coating. Currently, adding Si during the hot-dip aluminum process can effectively improve the quality of the coating and inhibit the Fe-Al reaction. To understand the effect of Si content on the microstructure and electrochemical performance of Al-xSi-3.5Fe coating alloys, the microstructure and post-corrosion morphology of the alloys were analyzed using SEM (Scanning Electron Microscope) and XRD (X-ray Diffraction). Through electrochemical tests and complete immersion corrosion experiments, the corrosion resistance of the coating alloys in 3.5 wt.% NaCl was tested and analyzed. The results show that the Al-3.5Fe coating alloy mainly comprises α-Al, Al3Fe, and Al6Fe. With the increase in Si addition, the iron-rich phase changes from Al3Fe and Al6Fe to Al8Fe2Si. When the Si content reaches 4 wt.%, the iron-rich phase is Al9Fe2Si2, and the excess Si forms the eutectic Si phase with the aluminum matrix. Through SKPFM (Scanning Kelvin Probe Force Microscopy) testing, it was determined that the electrode potentials of the alloy phases Al3Fe, Al6Fe, Al8Fe2Si, Al9Fe2Si2, and eutectic Si phase were higher than that of α-Al, acting as cathode phases to the micro-galvanic cell with the aluminum matrix, and the corrosion form of alloys was mainly galvanic corrosion. With the addition of silicon, the electrode potential of the alloy increased first and then decreased, and the corrosion resistance results were synchronous with it. When the Si content is 10 wt.%, the alloy has the lowest electrode potential and the highest electrochemical activity.

Reaction-diffusion effects and spatiotemporal oscillations under SEM, STM and AFM-assisted charging in fiber-like and wire-like systems: From molecular and quantum wires to cooperative ferroelectric nanofibers and microfibers

This review addresses the problem of reaction-diffusion effects and spatiotemporal oscillations in fiber-like and wire-like systems under the electron beam in SEM and in the presence of electric field in some special AFM techniques, such as current sensing atomic force microscopy (CS-AFM)/conductive atomic force microscopy (C-AFM), electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) also known as surface potential microscopy. Some similar reaction-diffusion effects also can be observed in scanning capacitance microscopy (SCM), scanning gate microscopy (SGM), scanning voltage microscopy (SVM) and piezoresponse force microscopy (PFM). At the end of this paper the authors provide analysis of their own results and approaches. In particular, the possibility of achieving the ion transfer controlled growth of cells along the ion concentration gradients in reaction-diffusion fibers and actuators is indicated. This fundamental idea is discussed within the framework of the implantable fiber “bioiontronics” and “neuroiontronics” controlled by acoustic and electrical signals that regulate the reaction-diffusion or chemical oscillation activity of such fiber structures as reaction-diffusion actuators and sensors. The literature review includes more than 130 references.

Quantitative characterization of built-in potential profile across GaAs p–n junctions using Kelvin probe force microscopy with qPlus sensor AFM

The electrostatic potential distribution in materials and devices plays an important role in controlling the behaviors of charge carriers. Kelvin probe force microscopy (KPFM) is a powerful technique for measuring the surface potential at a high spatial resolution. However, the measured surface potential often deviates from the potential deep in the bulk owing to certain factors. Here, we performed KPFM measurements across the p–n junction, in which such factors were eliminated as much as possible by selecting the sample, force sensor, and measurement mode. The measured surface potential distribution agrees well with the line shape of the simulated bulk potential. Our results demonstrate that KPFM is capable of quantitatively characterizing potential distributions whose changes occur on the order of 10 nm.

Characterizing and controlling infrared phonon anomaly of bilayer graphene in optical-electrical force nanoscopy

We, for the first time, report the nanoscopic imaging study of anomalous infrared (IR) phonon enhancement of bilayer graphene, originated from the charge imbalance between the top and bottom layers, resulting in the enhancement of E1u mode of bilayer graphene near 0.2 eV. We modified the multifrequency atomic force microscope platform to combine photo-induced force microscope with electrostatic/Kelvin probe force microscope constituting a novel hybrid nanoscale optical-electrical force imaging system. This enables to observe a correlation between the IR response, doping level, and topographic information of the graphene layers. Through the nanoscale spectroscopic image measurements, we demonstrate that the charge imbalance at the graphene interface can be controlled by chemical (doping effect via Redox mechanism) and mechanical (triboelectric effect by the doped cantilever) approaches. Moreover, we can also diagnosis the subsurface cracks on the stacked few-layer graphene at nanoscale, by monitoring the strain-induced IR phonon shift. Our approach provides new insights into the development of graphene-based electronic and photonic devices and their potential applications.

Microregion Characterization of Grain Boundary Defects and Electron Capture of CsPbI2Br Perovskite

All‐inorganic CsPbI2Br perovskite solar cell shows superior thermal stability, but the power conversion efficiency falls behind organic devices, due to the existence of abundant defects. Halogen and metal ions at the grain boundaries induce the formation of defect states, which increases the probability of carrier recombination. Grain boundaries are also the channels of ion migration. Herein, as relatively simple interface engineering strategy is used at the molecular level, using a multifunctional 2,5‐thiophene dicarboxylic acid to passivate the interface between the perovskite surface and the carbon electrode. The 2,5‐thiophene dicarboxylic acid can effectively coordinate the undercoordinated Pb2+, reduce the defect concentration of perovskite, and suppress the nonradiative recombination. The device with carbon electrode exhibits a power conversion efficiency of 13.4%. The microscopic passivation mechanism is investigated by Kelvin probe force microscopy. It is observed that the surface treatment effectively passivates the defects at the grain boundary and reduces the capture rate of photogenerated carriers of the grain boundary.

Young's Modulus Characterization of Nano‐Level Silicon Nanomembrane

Silicon nanomembrane (SiNM) has drawn great attention for the application in nanoelectrical devices as it shows excellent flexibility and is compatible with the integrated circuit process. The mechanical property measurement of the SiNM with nanoscale thickness is critical. A suspended SiNM (40 nm thick) for mechanical measurements is fabricated by transferring a chemically etched ultrathin monocrystalline silicon film from silicon on insulator wafer to a substrate with a multi‐hole array. And then, the atomic force probe is utilized to load force on the free‐standing SiNM to obtain a force deflection curve, and then the Young's modulus of such floating SiNM can be directly calculated based on the large deflection plane model. It shows that the Young's modulus of such SiNM is basically consistent with that of the bulk silicon. However, the SiNMs’ floating area significantly affects the results, i.e., the Young's modulus varies with the ratio of the suspended area diameter (i.e., hole diameter) to the film thickness. The Young's modulus is independent of hole diameter when the ratio is greater than 425. According to this relationship, the variation of Young's modulus can be predicted for arbitrary thick SiNMs and any transferable nanofilms.

Insight of the high switching window and data retention in lead-free 2D layered double perovskite resistive memory device

Lead-free robust halides double perovskites (DPs) are evolving as the key materials for the multi-functional resistive memory application. Herein, we aimed to enhance the switching window, i.e., current On/Off ratio by the cutting edge dimensional reduction of the three dimensional Cs2AgBiBr6 DP into two dimensional (2D) BA4AgBiBr8 (BA = butylammonium) DP and studied the insight of their resistive switching anomaly. We affirmed that the improved On/Off ratio (∼103 to ∼106) is attributed to the restricted charge transport in the high resistance state (HRS) of the BA4AgBiBr8 based switching device. The low HRS current can be ascribed to the synergies of higher Schottky barrier at the Au/BA4AgBiBr8 junction and higher thermal activation energy in the layered DP. Owing to the higher switching window in the 2D DP device, reproducible endurance (tested up to 500 cycles) and retentivity (tested up to 104 s) of the resistance states establish that the high On/Off ratio could be retained without significant deviation. The resistive memory behavior could be hypothesized by the charge trapping phenomenon in the BA4AgBiBr8 DP, as the charge retention was persisted over 60 min as evident from surface potential images of Kelvin probe force microscopy.

Direct Visualization of Charge Migration in Bilayer Tantalum Oxide Films by Multimodal Imaging

AbstractInspired by biological neuromorphic computing, artificial neural networks based on crossbar arrays of bilayer tantalum oxide memristors have shown to be promising alternatives to conventional complementary metal‐oxide‐semiconductor (CMOS) architectures. In order to understand the driving mechanism in these oxide systems, tantalum oxide films are resistively switched by conductive atomic force microscopy (C‐AFM), and subsequently imaged by kelvin probe force microscopy (KPFM) and spatially resolved time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). These workflows enable induction and analysis of the resistive switching mechanism as well as control over the resistively switched region of the film. In this work it is shown that the resistive switching mechanism is driven by both current and electric field effects. Reversible oxygen motion is enabled by applying low (&lt;1 V) electric fields, while high electric fields generate irreversible breakdown of the material (&gt;1 V). Fully understanding oxygen motion and electrical effects in bilayer oxide memristor systems is a fundamental step toward the adoption of memristors as a neuromorphic computing technology.

Ferroelectric Domain Wall p-n Junctions.

We have used high-voltage Kelvin probe force microscopy to map the spatial distribution of electrical potential, dropped along curved current-carrying conducting domain walls, in x-cut single-crystal ferroelectric lithium niobate thin films. We find that in-operando potential profiles and extracted electric fields, associated with p-n junctions contained within the walls, can be fully rationalized through expected variations in wall resistivity alone. There is no need to invoke additional physics (carrier depletion zones and space-charge fields) normally associated with extrinsically doped semiconductor p-n junctions. Indeed, we argue that this should not even be expected, as inherent Fermi level differences between p and n regions, at the core of conventional p-n junction behavior, cannot occur in domain walls that are surrounded by a common matrix. This is important for domain-wall nanoelectronics, as such in-wall junctions will neither act as diodes nor facilitate transistors in the same way as extrinsic semiconducting systems do.

3D Imaging and Quantitative Subsurface Dielectric Constant Measurement Using Peak Force Kelvin Probe Force Microscopy

AbstractNoninvasive and depth‐sensitive measurements of dielectric properties are becoming of great interest in advanced and complex nanostructured architectures. Here, a straightforward parallel approach applicable in peak force Kelvin probe force microscopy for a 3D measurement of dielectric constants at the nanoscale is demonstrated. The proposed approach features a simultaneous measurement of the mechanical, electrical, and depth‐dependent dielectric properties applied to embedded nanostructures. The findings provide initial elements for further development of experimental dielectric nano‐tomography methods for characterizing buried and embedded systems and dielectric interfaces.

High-speed mapping of surface charge dynamics using sparse scanning Kelvin probe force microscopy

Unraveling local dynamic charge processes is vital for progress in diverse fields, from microelectronics to energy storage. This relies on the ability to map charge carrier motion across multiple length- and timescales and understanding how these processes interact with the inherent material heterogeneities. Towards addressing this challenge, we introduce high-speed sparse scanning Kelvin probe force microscopy, which combines sparse scanning and image reconstruction. This approach is shown to enable sub-second imaging (>3 frames per second) of nanoscale charge dynamics, representing several orders of magnitude improvement over traditional Kelvin probe force microscopy imaging rates. Bridging this improved spatiotemporal resolution with macroscale device measurements, we successfully visualize electrochemically mediated diffusion of mobile surface ions on a LaAlO3/SrTiO3 planar device. Such processes are known to impact band-alignment and charge-transfer dynamics at these heterointerfaces. Furthermore, we monitor the diffusion of oxygen vacancies at the single grain level in polycrystalline TiO2. Through temperature-dependent measurements, we identify a charge diffusion activation energy of 0.18 eV, in good agreement with previously reported values and confirmed by DFT calculations. Together, these findings highlight the effectiveness and versatility of our method in understanding ionic charge carrier motion in microelectronics or nanoscale material systems.

Nitrogen-defect-modified g-C3N4/BaFe12O19 S-scheme heterojunction photocatalyst with enhanced advanced oxidation technology synergistic photothermal degradation ability of antibiotic: Insights into performance, electron transfer pathways and toxicity

In this research, nitrogen-defect-modified g-C3N4/BaFe12O19 S-scheme heterojunction composites were prepared by in-situ thermal polymerization approach and employed as a productive photo-Fenton catalyst for degrading antibiotic. The micromorphology, crystal structure, chemical composition and optical characteristics of the photocatalysts were evaluated by various testing approaches. The formation of the internal electric field between g-C3N4 and BaFe12O19 and the construction of the S-scheme heterojunction were confirmed by density functional theory and Kelvin probe force microscopy. Meanwhile, the nitrogen defect and photothermal effect of the g-C3N4/BaFe12O19 composite further accelerate the electron migration rate. The optimized g-C3N4/BaFe12O19-30 photocatalyst achieved 100% enrofloxacin removal within 10 min compared to monomer g-C3N4. The bactericidal activity experiment showed that the photocatalytic degradation products were low or non-toxic. Based on the above characterization experiments and density functional theory, the possible degradation mechanism of enrofloxacin was proposed. This research offers new insights for the synthesis of photocatalyst for wastewater treatment.