Probe Force Research Articles

Kelvin Probe Force Microscopy and Electrochemical Atomic Force Microscopy Investigations of Lithium Nucleation and Growth: Influence of the Electrode Surface Potential.

Lithium metal is promising for high-capacity batteries because of its high theoretical specific capacity of 3860 mAh g-1 and low redox potential of -3.04 V versus the standard hydrogen electrode. However, it encounters challenges, such as dendrite formation, which poses risks of short circuits and safety hazards. This study examines Li deposition using electrochemical atomic force microscopy (EC-AFM) and Kelvin probe force microscopy (KPFM). KPFM provides insights into local surface potential, while EC-AFM captures the surface response evolution to electrochemical reactions. We selectively removed metallic coatings from current collectors to compare lithium deposition on coated and exposed copper surfaces. Observations from the Ag-coated Cu (Ag/Cu), Pt-coated Cu (Pt/Cu), and Au-coated Cu (Au/Cu) samples revealed variations in lithium deposition. Ag/Cu and Au/Cu exhibited two-dimensional growth, whereas Pt/Cu exhibited three-dimensional growth, highlighting the impact of electrode materials on morphology. These insights advance the development of safer lithium metal batteries.

Topography and localized charge of steps on CeO2(111) investigated by AFM/KPFM

The configuration of step edges on metal oxide surface is significant for understanding fundamental reactivity and predicting the performance in catalysis. In this work, the topography feature and charge of steps on CeO2(111) are investigated by atomic force microscopy /kelvin probe force microscopy combined with density functional theory. Hexagonal islands and pits containing step I edge and step II edge have been observed at annealing 1100 K. The irregular structures with the addition of step III edge have been found at 1200 K. The measurement of contact potential difference reveals that step II displays the strongest localized charge state, followed by step III, and step I with the weakest localized charge state. The calculations of density functional theory have confirmed that step II possesses a stronger localized charge state. In addition, the Ce atoms at the step II edge exhibit greater positive and negative charge accumulation. These results provide the surface structure and electrical potential of three step edges on the CeO2(111), and establish both experimental and theoretical foundations for subsequent investigations on adsorption and catalysis.

Electrostatic Surface Charging by Water Dewetting.

Water dewetting generates static electricity. We reviewed historical experiments of this phenomenon, and we studied the charging of polymer slides and metal electrode supported polymer films withdrawn vertically from a pool of aqueous solutions. For pure water, charging was negative and surface charge densities increased with the speed of dewetting, which we explain by the thermally activated entrainment of nanometer-sized water droplets or clusters charged by unbalanced adsorbed electric double-layer ions. Surface charge densities increased for reduced polymer film thickness following a power law, which we explain by reduced discharge of the entrained water volumes. At low salinity c ≲ 10 μM, charging was proportional to electrokinetic interfacial charge densities: the negative charging was increased for alkaline solutions and for most salts at μM concentrations and the charge polarity was inversed to positive for a cationic surfactant, a salt with a highly positively charged cation, and for a strong acid at approximately pH 4. Charging was reduced again for c ≳ 100 μM, especially at high dewetting speeds and for chaotropic ions, which we explain by the entrainment of larger and more discharged droplets. We determined adsorption energies of the charged water clusters on the dewetted surface from thermally stimulated discharge of the charged polymer slides and we show that the surface charge distribution, imaged by charged toner powders and measured microscopically by Kelvin probe force microscopy, is a record of the dewetting process that provides spatial and kinetic information about the three-phase contact line motion.

Electrostatic charge mitigation by graphene-based thin films for optical solar reflectors in spacecraft

In the field of spacecraft engineering, in particular for geostationary and geosynchronous applications, a significant need exists for a thermal control film to be electrically conductive, to effectively manage the dissipation of accumulated charges. In this study, transparent-electrically conductive reduced graphene oxide is strategically applied to heated rigid (quartz glass) and flexible (Fluorinated Ethylene propylene) optical solar reflector substrates. This innovation harnesses the unique properties of graphene to seamlessly integrate electrical conductivity into space-appropriate substrates with minimum deviation in their optical transparency and thermal properties. The tape peel-off test, conducted in accordance with space materials standards (ASTM D903) using 3 M Scotch tape, reveals the better adhesion of the developed coating. A noteworthy aspect of the present work involves visualizing charge dissipation using Field Emission Scanning Electron Microscope (FESEM) electron bombardment through obtained images and decreased surface potential measured by Kelvin Probe Force microscopy (KPFM). Furthermore, a basic OSR model is fabricated by depositing a reflective back with Aluminium and graphene thin film protecting from the top to investigate the alteration in reflection by varying incident angles. This research contributes to the advancement of spacecraft materials and coatings aimed at improving spacecraft durability and functionality.

Visualizing and Controlling of Photogenerated Electron-Hole Pair Separation in Monolayer WS2 Nanobubbles under Piezoelectric Field.

The piezoelectric properties of two-dimensional semiconductor nanobubbles present remarkable potential for application in flexible optoelectronic devices, and the piezoelectric field has emerged as an efficacious pathway for both the separation and migration of photogenerated electron-hole pairs, along with inhibition of recombination. However, the comprehension and control of photogenerated carrier dynamics within nanobubbles still remain inadequate. Hence, this study is dedicated to underscore the importance of in situ detection and detailed characterization of photogenerated electron-hole pairs in nanobubbles to enrich understanding and strategic manipulation in two-dimensional semiconductor materials. Utilizing frequency modulation kelvin probe force microscopy (FM-KPFM) and strain gradient distribution techniques, the existence of a piezoelectric field in monolayer WS2 nanobubbles was confirmed. Combining w/o and with illumination FM-KPFM, second-order capacitance gradient technique and in situ nanoscale tip-enhanced photoluminescence characterization techniques, the interrelationships among the piezoelectric effect, interlayer carrier transfer, and the funneling effect for photocarrier dynamics process across various nanobubble sizes were revealed. Notably, for a WS2/graphene bubble height of 15.45 nm, a 0 mV surface potential difference was recorded in the bubble region w/o and with illumination, indicating a mutual offset of piezoelectric effect, interlayer carrier transfer, and the funneling effect. This phenomenon is prevalent in transition metal dichalcogenides materials exhibiting inversion symmetry breaking. The implication of our study is profound for advancing the understanding of the dynamics of photogenerated electron-hole pair in nonuniform strain piezoelectric systems, and offers a reliable framework for the separation and modulation of photogenerated electron-hole pair in flexible optoelectronic devices and photocatalytic applications.

Pit initiation in quenching and partitioning processed martensitic stainless steels

The present article investigates the influence of chemical composition and phase fractions on the corrosion behaviour of industrially produced quenching and partitioning (Q&P) martensitic stainless steels. Localised corrosion was analysed by scanning Kelvin probe force microscopy (SKPFM) and scanning electrochemical microscopy (SECM) in 3.5 wt.% NaCl solution. SKPFM revealed a Volta-potential difference of around 40 mV between inclusions and the matrix, which is larger than the Volta potential variations within the matrix. This difference in surface potential is a driving force for selective dissolution (corrosion initiation) at inclusions and inclusion/matrix interfaces. SECM detected early pitting initiation, particularly in alloys containing MnS and TiN inclusions. Results suggest that pitting initiation and propagation occur at those specific regions. This study emphasised that irrespective of chemical composition and phase fraction, localised corrosion initiation in Q&P-processed martensitic stainless steels is predominantly governed by the presence of inclusions.

Hydrogen-induced failure analysis of bimetallic clad pipes

This study investigated the mechanical properties and fracture characteristics of an alloy 825/L360 bimetallic clad pipe after it was subjected to hydrogen charging. The results indicated that the elongation loss of the pipe increased from 12.3% to 50.0% with increasing hydrogen charging time. In addition, results obtained from hydrogen microprinting and scanning Kelvin probe force microscopy indicated that the hydrogen distribution characteristics of the aforementioned material influenced the hydrogen fracture failure mechanism of the clad pipe. Specifically, carbides and TiN initiated hydrogen cracking, and crack extension occurred through brittle cracking at grain boundaries, which was caused by the interaction of hydrogen with dislocation slip bands within the dendritic zone.

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

The presence of Urbach tails in Cu(In,Ga)Se2 (CIGSe) absorbers has been identified as a limiting factor for the performance of the CIGSe solar cells. The tail states contribute to both radiative and non-radiative recombination processes, ultimately leading to a reduction in the open-circuit voltage and, consequently, decreasing the overall efficiency of CIGSe devices. Urbach tails result from structural and thermal disorders. The Urbach tails can be characterized by the Urbach energy, which is associated with the magnitude of the tail states. Within polycrystalline CIGSe absorbers, grain boundaries can be considered as structural disorder and, therefore, can potentially contribute to the Urbach tails. In fact, it has been proposed that the band bending at grain boundaries contribute significantly to the tail states. This study focuses on examining the correlation between Urbach tails and the band bending at the grain boundaries. The Urbach energies of the CIGSe samples are extracted from photoluminescence (PL) measurements, which reveal that the introduction of Sodium (Na) into the material can lead to a reduction in the Urbach energy, and an even further decrease can be achieved through the RbF post-deposition treatment. The band bending at the grain boundaries is investigated by Kelvin probe force microscopy measurements. A thorough statistical analysis of more than 340 grain boundaries does not show any correlation between Urbach tails and grain boundaries. We measure small band bending values at the grain boundaries, in the range of the thermal energy (26 meV at room temperature). Furthermore, our intensity dependent PL measurements indicate that Urbach tails are, at least in part, a result of electrostatic potential fluctuations. This supports the model that the introduction of alkali elements mainly decreases the magnitude of electrostatic potential fluctuations, resulting in a subsequent reduction in the Urbach energy.

Invasion/Permeation Hydrogen in Cathodic Charged SUS316 Columnar Crystals Evaluated with a Scanning Kelvin Probe Force Microscope

Invasion/Permeation Hydrogen in Cathodic Charged SUS316 Columnar Crystals Evaluated with a Scanning Kelvin Probe Force Microscope

Degradation of MoOx Thin‐Films Properties in Excessive Oxygen Environments and Its Influence on Dopant‐Free Silicon Solar Cells

Transition metal oxides such as molybdenum oxide (MoOx) demonstrate significant potential as efficient hole‐selective passivating contacts in silicon heterojunction solar cells. Achieving efficient hole collection necessitates precise control over the optical and electrical properties of MoOx films. In this study, the effects of oxygen flow rate () on the growth, optical properties, and electrical properties of thermally evaporated MoOx films are investigated. In the Kelvin probe force microscopy results, it is indicated that MoOx thin‐film deposition followed an island‐to‐layer growth model. X‐ray photoelectron spectroscopy shows that MoOx films exhibit stoichiometric composition with fully oxidized Mo6+ ions, without additional oxygen. Notably, the O 1s orbital peak shifts toward higher binding energy with increased , indicating defect introduction. Consequently, the work function of MoOx films decreases from 5.93 to 5.51 eV as increases from 0 to 8 sccm. The maximum optical bandgap of the MoOx films exceeds 3.60 eV. As a proof of concept, 's impact on MoOx as a front buffer layer for dopant‐free silicon solar cells is analyzed. An efficiency of 20.8% was achieved for dopant‐free silicon solar cells after optimized MoOx film deposition, with an open‐circuit voltage of 713.7 mV, short‐circuit current density of 39.1 mA cm−2, and fill factor of 74.6%.

Predicting tablet properties using In-Line measurements and evolutionary equation Discovery: A high shear wet granulation study

High shear wet granulation (HSWG) is widely used in tablet manufacturing mainly because of its advantages in improving flowability, powder handling, process run time, size distribution, and preventing segregation. In line process analytical technology measurements are essential in capturing detailed particle dynamics and presenting real-time data to uncover the complexity of the HSWG process and ultimately for process control.This study presents an opportunity to predict the properties of the granules and tablets through torque measurement of the granulation bowl and the force exerted on a novel force probe within the powder bed. Inline force measurements are found to be more sensitive than torque measurements to the granulation process. The characteristic force profiles present the overall fingerprint of the high shear wet granulation, in which the evolution of the granule formation can improve our understanding of the granulation process. This provides rich information relating to the properties of the granules, identification of the even distribution of the binder liquid, and potential granulation end point. Data were obtained from an experimental high shear mixer across a range of key process parameters using a face-centred surface response design of experiment (DoE). A closed-form analytical model was developed from the DOE matrix using the discovery of evolutionary equations. The model is able to provide a strong predictive indication of the expected tablet tensile strength based only on the data in-line. The use of a closed form mathematical equation carries notable advantages over other AI methodologies such as artificial neural networks, notably improved interpretability/interrogability, and minimal inference costs, thus allowing the model to be used for real-time decision making and process control. The capability of accurately predicting, in real time, the required compaction force required to achieve the desired tablet tensile strength from upstream data carries the potential to ensure compression machine settings rapidly reach and are maintained at optimal values, thus maximising efficiency and minimising waste.

Controlling the Orientation‐Dependent Second Harmonic Generation in Hybrid Germanium Perovskites

AbstractManipulating the crystal orientation plays a crucial role in the conversion efficiency during second harmonic generation (SHG). Here, we provide a new strategy in controlling the surface‐dependent anisotropic SHG with the precise design of (101) and (2 0) MAGeI3 facets. Based on the SHG measurement, the (101) MAGeI3 single crystal exhibits larger SHG (1.3×(2 0) MAGeI3). Kelvin probe force microscopy imaging shows a smaller work function for the (101) MAGeI3 compared with the (2 0), which indirectly demonstrates the stronger intrinsic polarization on the (101) surface. X‐ray photoelectron spectroscopy confirms the band bending within the (101) facet. Temperature‐dependent steady‐state and time‐resolved photoluminescence spectroscopy show shorter lifetime and wider emission band in the (101) MAGeI3 single crystal, revealing the higher defect states. Additionally, powder X‐ray diffraction patterns show the (101) MAGeI3 possesses larger in‐plane polar units [GeI3]− density, which could directly enhance the spontaneous polarization in the (101) facet. Density functional theory (DFT) calculation further demonstrates the higher intrinsic polarization in the (101) facet compared with the (2 0) facet, and the larger built‐in electric field in the (101) facet facilitates surface vacancy defect accumulation. Our work provides a new angle in tuning and optimizing hybrid perovskite‐based nonlinear optical materials.

Comparative analysis of microstructure, electrical and optical performance in sidewall etching process for GaN-based green micro-LED

Micro-LEDs show the size-dependent external quantum efficiency (EQE) reduction problem, mainly owing to increased non-radiative recombination loss at the sidewall for smaller chip size. In this work, the evolution of microstructure, surface potential and optical performance of the green micro-LED sidewall was investigated comparatively after inductively coupled plasma (ICP) and tetramethylammonium hydroxide (TMAH) etching through transmission electron microscopy (TEM), Kelvin probe force microscope (KPFM), cathodoluminescence (CL) and time-resolved photoluminescence (TRPL). As confirmed by TEM and geometric phase analysis (GPA), ICP etching causes sidewalls to form atomically rough semi-polar surfaces and increases 25% compressive strain at the sidewall compared to the inside. TMAH solution introduces new sidewall defects due to excessive etching of three atomic layers of InGaN. Holes accumulate at the surface because of build-in electric field as showed by KPFM. The sidewall defects lead to a decrease in carrier lifetime resulting in uneven luminescence of micro-LED mesa. TMAH treatment removes the damaged layer and reduces the non-radiative recombination rate. ICP causes damage to the nanoscale structure, however the influence of sidewall defects on the carrier behavior is in the micron range due to unavoidable surface dangling bonds and surface lattice relaxation. A non-radiative recombination mechanism is proposed based on strain relaxation.

Controlling the Orientation-Dependent Second Harmonic Generation in Hybrid Germanium Perovskites.

Manipulating the crystal orientation plays a crucial role in the conversion efficiency during second harmonic generation (SHG). Here, we provide a new strategy in controlling the surface-dependent anisotropic SHG with the precise design of (101) and (2 0) MAGeI3 facets. Based on the SHG measurement, the (101) MAGeI3 single crystal exhibits larger SHG (1.3×(2 0) MAGeI3). Kelvin probe force microscopy imaging shows a smaller work function for the (101) MAGeI3 compared with the (2 0), which indirectly demonstrates the stronger intrinsic polarization on the (101) surface. X-ray photoelectron spectroscopy confirms the band bending within the (101) facet. Temperature-dependent steady-state and time-resolved photoluminescence spectroscopy show shorter lifetime and wider emission band in the (101) MAGeI3 single crystal, revealing the higher defect states. Additionally, powder X-ray diffraction patterns show the (101) MAGeI3 possesses larger in-plane polar units [GeI3]- density, which could directly enhance the spontaneous polarization in the (101) facet. Density functional theory (DFT) calculation further demonstrates the higher intrinsic polarization in the (101) facet compared with the (2 0) facet, and the larger built-in electric field in the (101) facet facilitates surface vacancy defect accumulation. Our work provides a new angle in tuning and optimizing hybrid perovskite-based nonlinear optical materials.

Chemically bonded CdS/Bi2MoO6 Z-scheme heterojunction synergises with strong internal electric field for photocatalytic CO2 reduction

Constructing strong interfacial electric fields to enhance the surface charge transport kinetics is an effective strategy for promoting CO2 conversion. Herein, we present the fabrication of CdS-Bi2MoO6 Z-scheme heterojunctions with a robust internal electric field (IEF) using an in situ growth technique, establishing chemical bonding between the components. The IEF at the interface can offer an impetus for the segregation and transportation of photogenerated carriers, while the Cd-O chemical bonding mode acts as a rapid conduit for these carriers, thereby reducing the charge transfer distance. As a result, the Z-scheme charge transfer is accelerated due to the synergistic influence of these two factors. Therefore, the optimized CdS/Bi2MoO6 Z-scheme heterojunction possesses significantly enhanced dynamic carrier mobility, thus promoting the conversion of CO2 to CO without the need for additional co-catalysts or sacrificial agents. This optimization yields a remarkable CO selectivity of up to 97%. Meanwhile, the expedited Z-scheme charge transfer mechanism is validated through X-ray photoelectron spectroscopy, Kelvin probe force microscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy.

Effect of repeating hydrothermal growth processes and rapid thermal annealing on CuO thin film properties.

This paper presents an investigation into the influence of repeating cycles of hydrothermal growth processes and rapid thermal annealing (HT+RTA) on the properties of CuO thin films. An innovative hydrothermal method ensures homogeneous single-phase films initially. However, their electrical instability and susceptibility to cracking under the influence of temperature have posed a challenge to their utilization in electronic devices. To address this limitation, the HT+RTA procedure has been developed, which effectively eliminated the issue. Comprehensive surface analysis confirmed the procedure's ability to yield continuous films in which the content of organic compounds responsible for the formation of cracks significantly decreases. Structural analysis underscored the achieved improvements in the crystalline quality of the films. The implementation of the HT+RTA procedure significantly enhances the potential of CuO films for electronic applications. Key findings from Kelvin probe force microscopy analysis demonstrate the possibility of modulating the work function of the material. In addition, scanning capacitance microscopy measurements provided information on the changes in the local carrier concentration with each repetition. These studies indicate the increased usefulness of CuO thin films obtained from the HT+RTA procedure, which expands the possibilities of their applications in electronic devices.

Enhancing the near-infrared upconversion photocatalytic activity of ZnO/Bi3Ti2O8F:Yb3+, Er3+ by modulating the internal electric field through Z-scheme heterojunction construction

Exploring strategies to improve the near-infrared response of photocatalysts is an urgent challenge that can be overcome by utilizing upconversion (UC) luminescence to enhance photocatalysis. This paper reports the fabrication of a ZnO/Bi3Ti2O8F:Yb3+, Er3+ (ZnO/BTOFYE) Z-scheme heterojunction based on a Bi3Ti2O8F:Yb3+, Er3+ (BTOFYE) UC photocatalyst via electrostatic self-assembly. Fermi energy difference at the interface of BTOFYE and ZnO generates a strong internal electric field (IEF) in the Z-scheme heterojunction, offering a novel charge transfer mode that promotes carrier transfer and separation while retaining the strong redox capability. These results are confirmed through in situ X-ray photoelectron spectroscopy, in situ Kelvin probe force microscopy, electron spin resonance, and density functional theory calculations. In addition, the effect of the IEF on the UC luminescence process of Er3+ enhances the luminescence intensity, considerably improving the UC utilization efficiency. The optimal ZnO/BTOFYE degrades 64 % of ciprofloxacin in 120 min, which is 2.3 times more than that degraded by BTOFYE. Overall, the results of this study offer a reference for the rational development of high efficiency UC photocatalysts by generating IEF in Z-scheme heterojunctions.

In situ investigation of hydrogen embrittlement induced by δ phase in selective laser-melted GH4169 superalloy

Direct evidence of hydrogen-assisted crack nucleation and propagation associated with the δ phase in the selective laser melted GH4169 superalloy was obtained. The analysis of hydrogen trapping sites using thermal desorption spectroscopy revealed that the δ phase exhibits strong hydrogen capture capability, with a hydrogen desorption activation energy of 35.45 ± 2.51 kJ/mol. In addition, spatially resolved hydrogen mapping conducted by scanning Kelvin probe force microscopy and hydrogen microprint technique provided further evidence for the δ phase as a deep hydrogen trapping site. The atomic-scale characterization sufficiently reveals the deformation mechanism of the δ phase induced by dislocation accumulation. Hydrogen-promoted dislocation slip localization facilitates the formation of microvoid defects in the δ phase, which is the main reason for the δ phase fracture, and induces intergranular and transgranular cracks.

Novel defect-transit dual Z-scheme heterojunction: Sulfur-doped carbon nitride nanotubes loaded with bismuth oxide and bismuth sulfide for efficient photocatalytic amine oxidation

The rational design of Z-scheme heterojunction hybrid photocatalysts is considered a promising way to achieve high photocatalytic activity. In this study, a dual Z-scheme heterojunction with bismuth sulfide (Bi2S3) nanorods and bismuth oxide (Bi2O3) nanoparticles anchored Sulfur-doped carbon nitride (S-CN) nanotubes (Bi2S3/S-CN/Bi2O3) is designed and fabricated through the ordinal metal ion adsorption, pyrolysis, and sulfidation processes using supramolecular rods as precursor. Compared with pristine Bi2S3, Bi2O3, and CN, the dual Z-scheme tube-shaped Bi2S3/S-CN/Bi2O3 catalyst exhibited a significantly improved photocatalytic activity in amine oxidation. The optimized Bi2S3/S-CN/Bi2O3 nanostructure exhibits a 97.6 % benzylamine conversion and 99.4 % imine selectivity within 4 h under simulated solar light irradiation. The excellent activity of Bi2S3/S-CN/Bi2O3 nanotubes can be attributed to the characteristic hollow defect band structure and efficient charge separation and transfer achieved by the dual Z-scheme charge transfer mechanism, which was systematically studied using electron spin resonance spectroscopy, Kelvin probe force microscope, and other techniques. The optimized dual Z-scheme heterojunction hybrid photocatalyst maintains the high oxidizing ability of Bi2S3 and Bi2O3 and the excellent reducing ability of CN, thereby significantly enhancing the photocatalytic activity. This research provides a facile and feasible synthesis strategy for designing dual Z-scheme heterojunctions with defect band structure to improve the photocatalytic activity.

Pulsed Force Kelvin Probe Force Microscopy-A New Type of Kelvin Probe Force Microscopy under Ambient Conditions.

Kelvin probe force microscopy (KPFM) is an increasingly popular scanning probe microscopy technique used for nanoscale imaging of surface potential for various materials, such as metals, semiconductors, biological samples, and photovoltaics, to reveal their surface work function and/or local accumulation of charges. This featured review outlines the operation principles and applications of KPFM, including several typical commercially available variants. We highlight the significance of surface potential measurements, present the details of the method operation, and discuss the causes of the limitation on spatial resolution. Then, we present the pulsed force Kelvin probe force microscopy (PF-KPFM) as an innovative improvement to KPFM, which provides an enhanced spatial resolution of <10 nm under ambient conditions. PF-KPFM is promising for the characterization of heterogeneous materials with spatial variations of electrical properties. It will be especially instrumental for investigating emerging perovskite photovoltaics, heterogeneous catalysts, 2D materials, and ferroelectric materials, among others.