Kelvin Probe Force Microscopy Research Articles

Increased Deep Trap Density in Interfacial Engineered Nanocomposite Revealed by Sequential Kelvin Probe Force Microscopy for High Dielectric Energy Storage.

Nanocomposites combining inorganic nanoparticles with high dielectric constant and polymers with high breakdown strength are promising for the high energy density storage of electricity, and carrier traps can significantly affect the dielectric breakdown process. Nevertheless, there still lacks direct experimental evidence on how nanoparticles affect the trap characteristics of nanocomposites, especially in a spatially resolved manner. Here, a technique is developed to image the trap distribution based on sequential Kelvin probe force microscopy (KPFM) in combination with the isothermal surface potential decay (ISPD) technique, wherein both shallow and deep trap densities and the corresponding energy levels can be mapped with nanoscale resolution. The technique is first validated using the widely-used commercial biaxially oriented polypropylene, yielding consistent results with macroscopic ISPD. The technique is then applied to investigate polyvinylidene fluoride-based nanocomposites filled with barium titanate nanoparticles, revealing higher deep trap density around surface-modified nanoparticles, which correlates well with its increased breakdown strength. This technique thus provides a powerful spatially resolved tool for understanding the microscopic mechanism of dielectric breakdown of nanocomposites.

Visible light enhanced selective benzene response of hexagonal atomic structured CuS microcloves

We have reported CuS microcloves (MCs) for highly selective detection of benzene using scanning kelvin probe (SKP) technique. The electron microscopy investigations revealed the formation of clove shaped CuS with a size of about 1–2 µm at 180 °C and 6 h. The time of reaction significantly affected the crystallinity as well as the morphology of resulting CuS materials. The investigations on adsorption behavior of different types of common VOCs with varying charge and nature revealed an outstanding selectivity towards benzene when compared to other VOCs. The difference in work function between gold tip and sensor surface modified with CuS MCs was decreased under the exposure of light, most likely caused by transfer of electrons to conduction band of CuS under illumination of light. As a result, the CuS MCs showed enhanced photo response in visible light exposure in the presence of benzene. A chemiresistive type sensor device was fabricated using CuS MCs and exposed to benzene vapor in a concentration ranging from 2.8 to 11.2 ppm. The sensitivity was found to be 1.63 × 10-3 ppm−1. Notably, a rapid response time of ∼6 s and recovery time of ∼ 16 s was achieved for the sensor at a benzene concentration of 5.6 ppm at room temperature. Also, considerable and repeatable sensor response was observed for 560 ppb of benzene vapor at 70 °C. Hence, these results suggested that the CuS MCs have huge potential as novel material for selective benzene detection in gas phase towards breath analysis and other environmental monitoring.

Electrochemical corrosion behavior of 7075 aluminum alloy with different ultrasonic surface rolling microstructures

7075 Al alloy is strengthened by ultrasonic surface rolling process (USRP), and the process parameters are changed to regulate the microstructure. The thickness, grain size and distribution of different strengthening layer are characterized in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The influence of different strengthening structures on their electrochemical corrosion behavior are studied using scanning Kelvin probe (SKP) and electrochemical impedance spectroscopy (EIS). The results show that the average grain size of untreated Al alloy is 2.89 μm, the surface SKP potential is low and fluctuates greatly, and has high sensitivity to pitting corrosion. After USRP treatment, the surface roughness of 7075 Al alloy decreases, the grain size is significantly refined and shows a gradient distribution, SKP potential in the strengthening area increases and becomes more stable, the passive film on the surface thickens, the density of pits significantly decreases, and the corrosion resistance is improved. Besides, the samples with thicker plastic deformation layer and surface nanocrystallization (average size of 39.9 nm) exhibit better performance compared to other samples.

Fundamental Aspects of Conduction in Charged ErMnO3 Domain Walls

AbstractIt is now well‐established that ferroelectric domain walls, at which there are discontinuities in polarization, are usually electrically conducting. Yet, there is a dearth of rather basic information on the physics underpinning conductivity. Here, Kelvin Probe Force Microscopy (KPFM)‐based experiments are reported, which allow significant new insights regarding charge transport at domain walls in ErMnO3. In one set of experiments, KPFM is used to spatially map the Hall potential, developed at the surface of polished single crystals. These maps provide direct experimental evidence that n‐type head‐to‐head domain walls arise in otherwise p‐type material. In another set of experiments, the geometry for current flow is restricted, by cutting sub‐micron thick lamellar slices of ErMnO3 (using a Focused Ion Beam microscope). Separate contacts are made to n and p‐type walls and the potential profiles, when driving source‐drain currents, are measured (again using KPFM). Current‐electric field functions showed Ohmic behaviour for p‐type walls, with an intrinsic room temperature conductivity value of ≈0.4Sm−1. The n‐type walls showed non‐Ohmic behaviour and a significantly lower conductivity, supporting the prediction that electrons are in a polaronic state; an upper bound for the room temperature conductivity of the domains themselves is ≈6 × 10−6Sm−1 at 0.1 MVm−1.

Interface coupling induced built-in electric fields accelerate electro-assisted uranium extraction over Co3O4@FeOx nanosheet arrays

Revealing the nature of enhanced electro-assisted uranium extraction activity by hetero-interface and M-O-H coordination bonds is an important and challenging task. Hence, Co3O4@FeOx nanosheet arrays with abundant M-O-H coordination bonds is fabricated as a binder-free and self-supporting electrocatalyst for electro-assisted uranium extraction from fluorine-containing uranium wastewater. As revealed by self-consistent energy band calculations and in-situ Kelvin Probe Force Microscopy (KPFM) analysis, the p-n heterojunction formed by Co3O4@FeOx provides a well-designed built-in electric field (BIEF), triggering the interfacial accumulation of uranium and accelerating the electro reduction kinetics of uranium. Consequently, Co3O4@FeOx shows remarkable U(VI) extraction ability (>95 %) in fluorine-containing uranium wastewater. By virtue of the XAFS spectra, free uranyl ions are captured by M-O-H on Co3O4@FeOx to form a sturdy 2 Oax-1 U-3 Oeq configuration, confirming the electrically driven separation of uranium and fluorine. Furthermore, DFT calculation indicates that the existent of M-O-H enhances the adsorption energy of uranium species, thus promoting uranium extraction efficiency.

Influence of multilayered polymer coatings on the corrosion behaviour and microstructures of Al6061-T6 alloy

The influence of multilayered polymer coatings on the corrosion and microstructural properties of Al6061-T6 alloy was investigated. The electrochemical properties were determined in chloride solution via corrosion tests and electrochemical impedance spectroscopy (EIS) technique at room temperature. The microstructural properties and corrosion mechanism after polarization were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM)/Kelvin probe force microscopy (KPFM) techniques. From the corrosion tests and SEM-AFM/KPFM analysis, the multilayered polymer coated (using polyacrylic acid - PAA) Al6061-T6 alloy experienced a positive shift in the corrosion potential (-0.648 V), signifying a possible corrosion protection ability. With a charge transfer resistance of 2.506×10−6 Ω cm2, and lesser corrosion products featured on the coated sample, the present work demonstrates that the deposition of multilayered polymer PAA coating can enhance the corrosion resistance of Al6061-T6 alloy.

Structural, electrostatic force microscopy, work function, and optical characterization of pure and Al-doped ZnO nanoparticles

The electrical and optical characteristics of pure and Al-doped zinc oxide (ZnO) nanoparticles were analyzed. The work function of these nanoparticles was investigated using Kelvin Probe Force Microscopy (KPFM). The work function of the Al-doped ZnO nanoparticles was found to be lower than that of undoped ZnO nanoparticles. Electrostatic Force Microscopy (EFM) was employed to map the distribution of charges and conductivity in both ZnO and Al -doped ZnO, revealing enhanced charge trapping and increased conductivity in the Al-doped ZnO nanoparticles compared to the undoped ones. XRD analysis verified that both pure ZnO and Al-doped ZnO nanoparticles exhibited a hexagonal wurtzite crystal structure. Additionally, Raman spectroscopy revealed new vibrational modes at 572 cm−1, which were attributed to E1 (LO) in Al-doped ZnO. UV–visible spectroscopy indicated that the band gap of Al -doped ZnO nanoparticles is wider than that of pure ZnO nanoparticles, Additionally, photoluminescence spectroscopy demonstrated a blue shift in the emission spectrum of the Al-doped ZnO nanoparticles, accompanied by a reduction in green emission defects.

Multiparametric AFM insights into electron transport mechanisms in biomemristors

Biomemristors have attracted significant attention due to their applications in biodegradable and flexible electronic devices, and their prospective functions in information storage, neural synapses, and neuromorphic computation. Hence, understanding biomaterials' resistive switching (RS) mechanism is essential to design novel biomemristors with enhanced performance. In the last decade, various atomic force microscopy (AFM) modes, such as Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (cAFM), in situ/operando visualize the electron transport behaviors within biomemristors at the nanoscale. This review provides a comprehensive discussion of multiparametric AFM techniques to characterize biomemristors and inspire new ideas in developing next-generation consumer electronics. We also summarize the progress of revealing the internal mechanism of biomemristors with functional AFM to visualize and understand the switching behavior. Finally, the opportunities and challenges of further utilizing multiparametric AFM to explore the electron transport mechanisms in biomemristors are presented.

Interface interaction mediated surface plasmon resonance enhancement promoted visible-light-driven CO2 reduction with water

The conversion of carbon dioxide (CO2) into fuel using solar energy holds significant promise. However, the inefficient use of light and poor production activity have hindered its development. Here, we propose a simple in situ annealing oxidation method by coating a layer of TiO2 outside TiN, a material with a favorable price and localized surface plasmon resonance (LSPR) effect, to create an L-TiNO composite. The yield of CO over L-TiNO (50.8 μmol g−1 h−1) is 56.4 times that over TiO2 (0.9 μmol g−1 h−1) under visible light irradiation in pure aqueous environment, with a selectivity of 95.98%. In-situ Fourier transform infrared (FTIR) measurements reflect the CO2-COOH⁎-CO conversion route happening on L-TiNO. Characterizations like the Kelvin probe force microscopy (KPFM) technique confirm the generation of built-in electric field, which facilitates efficient carrier separation and migration. Density functional theory (DFT) calculations support that L-TiNO with LSPR effect alters the shape of absorbed CO2 to facilitate generation COOH⁎ via forming hydroxyl end group (Ti-OH) and promotes CO⁎ desorption to CO(g). This work provides valuable insights into the coupling of plasmonic materials with semiconductors to achieve efficient solar energy utilization.

Multi‐Dimensional Physically Unclonable Functions: Optoelectronic Variation‐Induced Multi‐Key Generation from Small Molecule PN Heterostructures

AbstractIn this era of demanding invulnerable security systems against the threat of hacking, physically unclonable functions (PUFs), especially in which can spawn multiple security keys within a single device, has gained attention. This investigation explores the multi‐key generable PUF devices employing organic small molecules, specifically C8‐BTBT and PTCDI‐C13. The variation stems from the formation of irregular PN junctions, haphazardly configured grain boundaries of C8‐BTBT. A comprehensive analysis including scanning electron microscopy (SEM), atomic force microscopy (AFM), kelvin probe force microscopy (KPFM), impedance spectroscopy (IS), and optical simulation, has been substantiated the underlying mechanisms. Exploiting the photo‐responsive characteristics within the light wavelength spectrum of 660 and 530 nm, alongside the electrical characteristics, the capability to generate a total of 30 distinct multi‐security keys in a single device is been successfully. These keys, distinguished by variable parameters such as voltage, light wavelength, and the calculated photo‐to‐dark current ratio (PDCR), manifest novel quantitative and qualitative dimensions in security protocol customization. Inter‐Hamming distance and entropy of these cryptographic keys exhibit commendable averages of 51.9–53.1%, and 0.81, respectively. Moreover, a noteworthy average bit‐aliasing mean value of 51.9%, derived from four distinct batches, underscores the pragmatic feasibility of the proposed conceptual framework for PUF devices.

COF‐Topological Quantum Material Nano‐heterostructure for CO2 to Syngas Production under Visible Light

AbstractEfficient solar‐driven syngas production (CO+H2 mixture) from CO2 and H2O with a suitable photocatalyst and fundamental understanding of the reaction mechanism are the desired approach towards the carbon recycling process. Herein, we report the design and development of an unique COF‐topological quantum material nano‐heterostructure, COF@TI with a newly synthesized donor‐acceptor based COF and two dimensional (2D) nanosheets of strong topological insulator (TI), PbBi2Te4. The intrinsic robust metallic surfaces of the TI act as electron reservoir, minimising the fast electron‐hole recombination process, and the presence of 6s2 lone pairs in Pb2+ and Bi3+ in the TI helps for efficient CO2 binding, which are responsible for boosting overall catalytic activity. In variable ratio of acetonitrile‐water (MeCN : H2O) solvent mixture COF@TI produces syngas with different ratios of CO and H2. COF@TI nano‐heterostructure enables to produce higher amount of syngas with more controllable ratios of CO and H2 compared to pristine COF. The electron transfer route from COF to TI was realized from Kelvin probe force microscopy (KPFM) analysis, charge density difference calculation, excited state lifetime and photoelectrochemical measurements. Finally, a probable mechanistic pathway has been established after identifying the catalytic sites and reaction intermediates by in situ DRIFTS study and DFT calculation.

Distribution of hydrogen atoms at a notch on X52 steel under various loads and displacements studied by scanning Kelvin probe and finite element modeling

Abstract In this work, the distribution of hydrogen (H) atoms at a notch on an X52 pipeline steel under various applied loads or displacements were investigated by Volta potential measurements using a scanning Kelvin probe (SKP) and finite element modeling. The stress concentration at the notch is elastic under the test condition. The H atom diffusion and distribution is driven by stress, rather than strain, in the steel. However, when displacements are applied on the steel specimen, the strain dominates the H atom distribution. As the local stress or strain increases, the H atom concentration increases, as indicated by negative shifts of the Volta potential. By establishing the quantitative relationships among Volta potential, von Mises stress, and H atom distribution, this work attempts to provide a novel method for H atom detection in steels through the Volta potential measurement.

Nanoscale Structure of Lipid-Gemini Surfactant Mixed Monolayers Resolved with AFM and KPFM Microscopy.

Drug delivery vehicles composed of lipids and gemini surfactants (GS) are promising in gene therapy. Tuning the composition and properties of the delivery vehicle is important for the efficient load and delivery of DNA fragments (genes). In this paper, we studied novel gene delivery systems composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-3-phosphocholine (DPPC), and GS of the type N,N-bis(dimethylalkyl)-α,ω-alkanediammonium dibromide at different ratios. The nanoscale properties of the mixed DOPC-DPPC-GS monolayers on the surface of the gene delivery system were studied using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We demonstrate that lipid-GS mixed monolayers result in the formation of nanoscale domains that vary in size, height, and electrical surface potential. We show that the presence of GS can impart significant changes to the domain topography and electrical surface potential compared to monolayers composed of lipids alone.

Engineering Domain Variants in 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 Single Crystals Using High-Frequency AC Poling.

Single crystals of (001)-oriented 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 (PMN-30PT) with a composition near the morphotropic phase boundary have attracted considerable attention due to their superior dielectric and electromechanical performance. Recently, a new alternating current (electric field) poling approach used for the enhancement of dielectric and piezoelectric properties. However, the microscopic domain variants that govern the performance, especially under high-frequency alternating current (AC) voltages, remain largely unexplored. In this work, the domain microstructure under AC poling reveals the presence of four monoclinic (MA) domain variants using a suite of scanning probe microscopy methods, and X-ray diffraction (XRD) reciprocal space mapping is tuned. It is reported on the emergence of hierarchical fine domains - needle-shaped, and 109° domain walls under applied high-frequency AC poling. Time-resolved Kelvin probe force microscopy (KPFM) reveals the charge dynamics and relaxation behavior of these needle domains and walls. The findings provide new insight and guidance to the domain engineering by high-frequency AC poling for the development of advanced transducer technology.

Atomic force microscopy correlates mechanical and electrical properties of HepG2 cells with curcumin concentration

Hepatocellular carcinoma (HCC) is a highly prevalent cancer with a significant impact on human health. Curcumin, a natural compound, induces cytoskeletal changes in liver cancer cells and modifies the distribution of lipids, proteins, and polysaccharides on plasma membranes, affecting their mechanical and electrical properties. In this study, we used nanomechanical indentation techniques and Kelvin probe force microscopy (KPFM) based on atomic force microscopy (AFM) to investigate the changes in surface nanomechanical and electrical properties of nuclear and cytoplasmic regions of HepG2 cells in response to increasing curcumin concentrations. CCK-8 assays and flow cytometry results demonstrated time- and concentration-dependent inhibition of HepG2 cell proliferation by curcumin. Increasing curcumin concentration led to an initial increase and then decrease in the mechanical properties of nuclear and cytoplasmic regions of HepG2 cells, represented by the Young's modulus (E), as observed through nanoindentation. KPFM measurements indicated decreasing trends in both cell surface potential and height. Fluorescence microscopy results indicated a positive correlation between curcumin concentration and phosphatidylserine translocation from the inner to the outer membrane, which influenced the electrical properties of HepG2 cells. This study provides valuable insights into curcumin's mechanisms against cancer cells and aids nanoscale evaluation of therapeutic efficacy and drug screening.

Polyelectrolyte modified black phosphorus/titania nanosheet heterojunction enhanced photocatalysis: Synergistic enhancement effect of interface affinity and electron transport channel

The instability and high electron-hole recombination have limited the application of black phosphorus (BP) as an excellent photocatalyst. To address these challenges, poly dimethyl diallyl ammonium chloride (PDDA), poly (allylamine hydrochloride) (PAH), and polyethyleneimine (PEI) are introduced to the functionalization of BP (F-BP), which can not only enhance its stability, but also boost the carrier transfer. Furthermore, a high-performance heterojunction photocatalyst is fabricated using F-BP and titania nanosheets (TNs) via a layer-by-layer self-assembly approach. The experimental outcomes unequivocally indicate that F-BP exhibits fast charge migration compared to BP. The density functional theory (DFT), in situ Kelvin-probe force microscopy (KPFM) and other advanced characterization techniques collectively unfold that PDDA modified BP can notably boost separation and propagation of charges, along with an enhanced carrier abundance. In summary, this novel strategy of using polyelectrolytes to enhance the electron transfer and the stability of BP permits immense potential in building next-generation BP-based high efficiency photocatalysts.

Regulation of electron delocalization between flower-like (Co, Ni)-MOF array and WO3/W photoanode for effective photoelectrochemical water splitting

A stable heterometallic (Co, Ni)-MOF array with a 3D hierarchical flower-like morphology was coated onto the coral-like WO3/W thin film. The optimal (Co, Ni)-MOF/WO3/W photoanode exhibits a high photocurrent density of 2.96 mA cm−2 at 1.23 V in the 0.5 M Na2SO4 solution and the photoelectrochemical (PEC) cell displays an excellent solar-to-hydrogen (STH) efficiency (2.17 %) under AM 1.5 G solar light. Moreover, the separation behavior and spatial distribution of photogenerated charges have been demonstrated by photo-assisted Kelvin probe force microscopy (KPFM). Furthermore, the shift of the d-band center and the change of Gibbs free energy for oxygen evolution reaction (OER) are realized by density functional theory (DFT) calculations. Overall, the coupling effect of these ions of Co2+, O2−, Ni2+, and W6+ could result in an electron delocalization in the (Co, Ni)-MOF/WO3/W photoanode, boosting the OER activity and finally improving the PEC water splitting performance.

D-A Conjugated Polymer/CdS S-Scheme Heterojunction with Enhanced Interfacial Charge Transfer for Efficient Photocatalytic Hydrogen Generation.

Owing to the improved charge separation and maximized redox capability of the system, Step-scheme (S-scheme) heterojunctions have garnered significant research attention for efficient photocatalysis of H2 evolution. In this work, an innovative linear donor-acceptor (D-A) conjugated polymer fluorene-alt-(benzo-thiophene-dione) (PFBTD) is coupled with the CdS nanosheets, forming the organic-inorganic S-scheme heterojunction. The CdS/PFBTD (CP) composite exhibits an impressed hydrogen production rate of 7.62mmolg-1 h-1 without any co-catalysts, which is ≈14 times higher than pristine CdS. It is revealed that the outstanding photocatalytic performance is attributed to the formation of rapid electron transfer channels through the interfacial Cd─O bonding as evidenced by the density functional theory (DFT) calculations and in situ X-ray photoelectron spectroscopy (XPS) analysis. The charge transfer mechanism involved in S-scheme heterojunctions is further investigated through the photo-irradiated Kelvin probe force microscopy (KPFM) analysis. This work provides a new point of view on the mechanism of interfacial charge transfer and points out the direction of designing superior organic-inorganic S-scheme heterojunction photocatalysts.

Charge-Localized Retention and Long-Term Memory Enabled by Cooperating Sterically Confined Molecular Crystallization with Spiro[fluorene-9,9'-xanthene]-Based Csp3-Hindrance.

Charge localization of memory materials plays a crucial role in the endurance and retention ability of organic nonvolatile memory, which is completely opposite from the charge delocalization of high-mobility materials. However, charge transfer of both though-space and through-bond based on molecular design principles still faces challenges. Herein, a nonplanar wide-bandgap semiconductor with Csp3-hindrance (DOCH3-DDPA-SFX) has been designed and synthesized. An effective crystallization effect of self-assembled two-dimensional nanosheets on charge trapping dynamics and kinetics is visualized by Kelvin probe force microscopy (KPFM). The trapped charges are localized completely on a single nanosheet, which has better charge trapping and retention properties than an amorphous film. Meanwhile, crystallization also greatly improves structure stability. Combining DFT theoretical calculations, the mechanisms of localization and long-term retention are discussed. The steric crystallization effects on the charge localization will guide the effective design of single-component semiconducting charge-memory materials by molecular assembly and aggregate control for high-performance organic memory.

Insight into the corrosion susceptibility and failure mechanism of Cu-Ni alloy pipeline welded joints in simulated marine environment

In this work, the corrosion behavior of Cu-Ni alloy pipeline welded joints is investigated in a simulated marine environment solution with coexisting chloride ions (Cl−) and sulfide ions (S2−). Microstructural observation, chemical component analysis, electrochemical measurements, scanning kelvin probe (SKP) and the finite element method (FEM) were adopted to probe the corrosion failure mechanism. The results showed that the welded joints at base metal (BM), heat affected zone (HAZ) and weld zone (WZ) exhibit various microstructures and reveal pitting and intergranular corrosion, moreover, the WZ appears inter-dendrite corrosion. WZ exhibits a higher corrosion sensitivity with the highest corrosion current density of about 1.23 × 10−5 A·cm−2. The defects in passive film and the thickness and density of corrosion product film differ for the three zones resulting in the protective effect of WZ being the poorest. BM with the thickest and densest corrosion product film effectively prevents the infiltration of corrosive ions (HS−, Cl−), meanwhile, the lowest defect concentration of the passive film of BM further protects the substrate. The transition of WZ in the WZ-HAZ galvanic couple from cathode to anode aggravates the corrosion of WZ.