Current-sensing Atomic Force Microscopy Research Articles

Quantitative Understanding of Ionic Channel Network Variation in Nafion with Hydration Using Current Sensing Atomic Force Microscopy.

Proton exchange membranes are an essential component of proton-exchange membrane fuel cells (PEMFC). Their performance is directly related to the development of ionic channel networks through hydration. Current sensing atomic force microscopy (CSAFM) can map the local conductance and morphology of a sample surface with sub-nano resolution simultaneously by applying a bias voltage between the conducting tip and sample holder. In this study, the ionic channel network variation of Nafion by hydration has been quantitatively characterized based on the basic principles of electrodynamics and CSAFM. A nano-sized PEMFC has been created using a Pt-coated tip of CSAFM and one side Pt-coated Nafion, and studied under different relative humidity (RH) conditions. The results have been systematically analyzed. First, the morphology of PEMFC under each RH has been studied using line profile and surface roughness. Second, the CSAFM image has been analyzed statistically through the peak value and full-width half-maximum of the histograms. Third, the number of protons moving through the ionic channel network (NPMI) has been derived and used to understand ionic channel network variation by hydration. This study develops a quantitative method to comprehend variations in the ionic channel network by calculating the movement of protons into the ionic channel network based on CSAFM images. To verify the method, a comparison is made between the NPMI and the changes in proton conductivity under different RH conditions and it reveals a good agreement. This developed method can offer a quantitative approach for characterizing the morphological structure of PEM. Also, it can provide a quantitative tool for interpretating CSAFM images.

Understanding of Ionic Structure Change Under Hydration Levels by Using CSAFM

Proton exchange membranes (PEMs) are a key component of proton exchange membrane fuel cells. They work as gas separator, insulator, and proton conductor. Thus, characterization of the membrane is a crucial process for developing PEM. However, this work is not easy because its complicated phase separated morphological structure and widely varying ionic structure under hydration. Current sensing atomic force microscopy (CSAFM) measures nano current flow between a tip and the sample surface and can map the nano current flow distribution of scanned area. In this study, the ionic structure of Nafion under different hydration condition is studied by using CSAFM. Specifically, the area variation of ionic domain on the membrane surface under different relative humidity is characterized by quantitative method based on CSAFM measurement. The study is done by several steps. First, the Nafion 212 is scanned in the environmental chamber by varying 15% to 65% relative humidity. In this process, the current distribution and the morphology are simultaneously mapped at each RH. Second, the local net charge value of protons, which is connected with area of ionic domain on the membrane surface, are calculated based on current distribution. In this process, the area of ionic domain on the membrane surface under different RH is estimated. The study shows several remarkable results. First of all, the non-uniform ionic domain increasing is observed from sequentially mapped current distribution on the surface under different RH. Second, the reliable estimation method of ionic domain variation is introduced by this study.

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.

Revealing corrosion behavior of B4C/pure Al composite at different interfaces between B4C particles and Al matrix in NaCl electrolyte

In the present work, the corrosion behavior of the B4C/pure Al composite in a Cl− containing environment is investigated. Thereby, the B4C/pure Al composites have been subjected to the different heat treatments to highlight the impact of the corrosion behavior at the different interfaces. The results of current sensing atomic force microscope (CSAFM) and scanning Kelvin probe microscopy (SKPFM) show that the dissolution of the Al matrix is relevant to the micro-galvanic corrosion between anodic Al matrix and cathodic B4C particles. The as quenched (AQ) composites with a higher volta potential difference (VPD) show inferior anticorrosion property than that of the as produced (AP) composites and the as annealed (AA) composites. By combining X-Ray diffraction (XRD) and geometric phase analysis (GPA), the higher residual stress and obvious interfacial strain between the B4C particles and the Al matrix of the AP samples is detected, which result in increasing the driving force of micro-galvanic corrosion.

Unraveling the Relevance of Electron and Hole Transfer in Lead Halide Perovskite Nanocrystals on Current Conduction.

Optimization of perovskite-based optoelectronic performance demands prudent engineering in the device architecture with facile transport of generated charge carriers. Herein, we explore the charge transfer (CT) kinetics in perovskite nanocrystals (PNCs), CsPbBr3, with two redox-active quinones, menadione (MD) and anthraquinone (AQ), and its alteration in halide exchanged CsPbCl3. With a series of spectroscopic and microscopic measurements, we infer that both electron and hole transfer (ET-HT) prevail in CsPbCl3 with quinones, resulting in a faster CT, while ET predominates for CsPbBr3. Furthermore, current-sensing atomic force microscopy measurements demonstrate that the conductance across a metal-PNC-metal nanojunction is improved in the presence of quinones. The contributions of ET and HT to current conduction across PNCs are well supported and validated by theoretical calculations of the density of states. These outcomes convey a new perspective on the relevance of ET and HT in the optimal current conduction and optoelectronic device engineering of perovskites.

Simultaneously Enhancing Exciton/Charge Transport in Organic Solar Cells by an Organoboron Additive.

Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).

Initiation Mechanism of Localized Corrosion Induced by Al2O3-MnS Composite Inclusion in Low-Alloy Structural Steel

The present work systematically investigated the initiation mechanism of localized corrosion induced by Al2O3-MnS composite inclusion in E690 steel under a simulated marine environment. The results showed that a micro-gap exists between the Al2O3-MnS inclusion and the matrix, and electron backscattered diffraction (EBSD) analysis revealed significant lattice dislocation zones around the Al2O3-MnS composite inclusion. The presence of the micro-gap and the lattice dislocation both promoted the localized corrosion initiation. The Volta potential of Al2O3 detected by scanning Kelvin probe force microscopy (SKPFM) was approximately 149.33 mV higher than that of the steel matrix, and the Volta potential of MnS was 10 mV lower than that of the steel matrix. The current-sensing atomic force microscopy (CSAFM) results showed that the Al2O3 was not conductive, while the MnS had good conductive properties. Therefore, it was not possible for a galvanic couple to be formed between Al2O3 and the adjacent steel matrix. A galvanic couple effect between the MnS and the adjacent steel matrix was directly demonstrated for the first time. The MnS acted as the anode phase for preferential dissolution in the corrosion process. The in situ immersion experiments and the Pourbaix diagram results confirmed that the dissolution of MnS was an electrochemical reaction process and the dissolution of Al2O3 was a chemical reaction.

Tunable Conductance of MoS2 and WS2 Quantum Dots by Electron Transfer with Redox-Active Quinone.

Due to their uniqueness in tunable photophysics, transition metal dichalcogenide (TMD) based quantum dots (QDs) have emerged as the next-generation quantum materials for technology-based semiconductor applications. This demands frontline research on the rational synthesis of the TMD QDs with controlled shape, size, nature of charge migration at the interface, and their easy integration in optoelectronic devices. In this article, with a controlled solution-processed synthesis of MoS2 and WS2 QDs, we demonstrate the disparity in their structural, optical, and electrical characteristics in bulk and confinement. With a series of steady-state and time-resolved spectroscopic measurements in different media, we explore the uncommon photophysics of MoS2 and WS2 QDs such as excitation-dependent photoluminescence and assess their excited state charge transfer kinetics with a redox-active biomolecule, menadione (MQ). In comparison to the homogeneous aqueous medium, photoinduced charge transfer between the QDs and MQ becomes more plausible in encapsulated cetyltrimethylammonium bromide (CTAB) micelles. Current sensing atomic force microscopy (CS-AFM) measurements at a single molecular level reveal that the facilitated charge transfer of QDs with MQ strongly correlates with an enhancement in their charge transport behavior. An increase in charge transport further depends on the density of states of the QDs directing a change in Schottky emission to Fowler-Nordheim (FN) type of tunneling across the metal-QD-metal junction. The selective response of the TMD QDs while in proximity to external molecules can be used to design advanced optoelectronic devices and applications involving rectifiers and tunnel diodes for future quantum technology.

In Situ Investigation of Migration of Cations during Electrolyte Spreading on Chromium Coated Steels Electrodeposited from Trivalent-Chromium Electrolyte

Resistance to corrosion and inhibition of cathodic delamination of polymer topcoat makes chromium coated steels suitable for application in the packaging industry. These coatings were traditionally produced from hexavalent chromium electrolytes. These coatings are now being phased out due to the carcinogenic nature of hexavalent chromium used in the production process (by the REACH regulation in the European Union). Electrodeposition from trivalent chromium containing electrolyte is being considered as an alternative to replace these traditional hexavalent-chromium coated steels. The cathodic delamination behavior of polymer topcoat on these trivalent-chromium coated steels were studied in detail previously [1-3]. However, the factors that result in inhibition of cathodic delamination are still not well understood.The two important factors influencing the rate of cathodic delamination are the electron transport and cation migration on the surface. The aim of this study is to understand the influence of complex composition of these trivalent-chromium coatings on the electrical conductivity and ion migration on the surface of the coating at the microscopic scale. The electronic conductivity of the coatings was studied using current-sensing atomic force microscopy to understand electron transfer occurring at the cathodic delamination front. Cation migration during oxygen-reduction induced spreading of electrolyte on these trivalent-chromium coated steel was studied using in situ scanning Kelvin probe force microscopy. The coatings are found to highly resist electron transport. Heterogeneities observed in the microscopic scale using the current-sensing atomic force microscope are correlated to the behavior observed during electrolyte spreading. The kinetics of spreading observed at microscopic scales are similar to that observed using a scanning Kelvin probe at larger scales. The results demonstrate in situ scanning Kelvin probe force microscopy as an effective tool to study the migration of cations during electrolyte spreading and the local heterogeneities at the microscopic scale. Warren, David & N. McMurray, H & de Vooys, Arnoud. (2013). Localised SKP Studies of Cathodic Disbondment on Chromium/Chromium Oxide Coated Steel. ECS Transactions. 50. 67-78.Wint, N & de Vooys, Arnoud & Mcmurray, Hamilton. (2016). The corrosion of chromium based coatings for packaging steel. Electrochimica Acta. 203. 326-336.James E. Edy, Hamilton N. McMurray, Koen R. Lammers, Arnoud C.A. deVooys (2019). Kinetics of corrosion-driven cathodic disbondment on organic coated trivalent chromium metal-oxide-carbide coatings on steel. Corrosion Science. 157. 51-61.

Towards a better understanding of localised corrosion induced by typical non-metallic inclusions in low-alloy steels

Detailed understanding of localised corrosion phenomena induced by inclusions in low-alloy steel is extremely critical as any corrosion-induced premature failure can potentially lead to a catastrophe. In particular, there is still ambiguity as to whether the localised corrosion initiation induced by non-metallic inclusions is an electrochemical process or a chemical process. Herein, the localised heterogeneous electrochemical mechanism of typical non-metallic inclusions–Al2O3, ZrO2-Ti2O3-Al2O3, and (RE)AlO3-(RE)2O2S-(RE)xSy – was investigated using scanning Kelvin probe force microscopy and current-sensing atomic force microscopy. The chemical stability of typical non-metallic inclusions was characterized by Pourbaix diagrams. The results indicate that the localised corrosion behaviour shows no correlation to the Volta potential difference between the non-metallic inclusions and the matrix. Moreover, the localised corrosion initiation process is mainly a chemical corrosion process rather than an electrochemical corrosion process. Thus, the applicability of Volta potential difference in predicating the preferential corrosion area in different system (conductive/nonconductive system) should be given full attention.

Properties and Morphologies of Anion Exchange Membranes for Fuel Cells with Different Lengths of Fluorinated Hydrophobic Chains

1. IntroductionPolymer electrolyte fuel cells are expected as next-generation energy devices because of their high energy conversion efficiency and no carbon dioxide emission. However, expensive noble metals such as platinum are used for the catalyst in the commercialized proton exchange membrane fuel cells. To solve this problem, anion exchange membrane fuel cells (AEMFCs) operable with non-noble metal catalysts have been attracting attentions. The low ionic conductivity and chemical instability of the anion exchange membranes (AEMs), however, are barriers to the practical use of AEMFCs. In our laboratory, we have been developing new AEMs with fluorinated hydrophobic groups, such as BAF-QAF1) and QPAF(C6)-42,3) (Fig. 1) to overcome the problems. QPAF(C6)-4 showed a high conductivity and a high mechanical strength but the thermal stability was insufficient. We have newly synthesized QPAF(C4)-4 with shorter perfluoroalkylene chains than those of QPAF(C6)-4 to tune the interpolymer interaction between the hydrophobic chains. The membrane structures and the properties of two membranes, QPAF(C6)-4 and QPAF(C4)-4, are compared.2. ExperimentalQPAF(C6)-4 ionomers were synthesized, and the membranes were formed as previously reported.2,3) QPAF(C4)-4 ionomers were synthesized in the same manner as QPAF(C6)-42) except PAF(C4) monomers were used instead of PAF(C6) monomers. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) were used to understand the arrangements of the hydrophobic/hydrophilic clusters in membranes. To observe the surface morphology and visualize the current images on the film surfaces, current-sensing atomic force microscopy (CS-AFM) was used.4,5) The water uptake and conductivity were measured in water, and the dynamic mechanical analysis (DMA) was conducted. The storage and loss elastic modules were also measured at different temperatures.3. Results and discussionBy TEM, hydrophilic clusters with a diameter of ca. 2 nm were confirmed in both membranes in vacuum (Fig. 2). In situ SAXS measurements under humidified nitrogen gas revealed the existence of periodic structures with the distance more than two times larger than those obtained between the hydrophilic clusters observed by TEM. The rate of the increase in periodicity was larger for QPAF(C4)-4 when the relative humidity was increased. The QPAF(C4)-4 membrane showed a higher water uptake than that of QPAF(C6)-4, but the conductivity was lower. This decrease in conductivity for QPAF(C4)-4, caused by an excessive film swelling (Fig. 3) confirmed by SAXS, may be related with the lower hydrophobicity with the shorter perfluoroalkylene chains.In DMA curves, QPAF(C4)-4 film retained high storage and loss modules up to ca. 80 ˚C, 15 ˚C higher than those of QPAF(C6)-4 (Fig. 4). By the stress-strain measurements, QPAF(C4)-4 showed a lower strain and a higher maximum stress than QPAF(C6)-4 (Fig. 5). The mechanical improvements of QPAF(C4)-4 may be explained by the improved packing between benzene rings due to the shorter perfluoroalkylene chains in the hydrophobic structure.Other properties are discussed based on the chemical structures (Fig. 1) and the three-dimensional conformations of the membranes.References Kimura, A. Matsumoto, J. Inukai, and K. Miyatake, ACS Appl. Energy Mater., 3, 469 (2020).Ono, J. Miyake, S. Shimada, M. Uchida, and K. Miyatake J. Mater. Chem. A, 43, 21779 (2015).Miyake and K. Miyatake, Sustain. Energy Fuels, 3, 1916 (2019). Hara, T. Kimura, T. Nakamura, M. Shimada, H. Ono, S. Shimada, K. Miyatake, M. Uchida, J. Inukai, and M. Watanabe, Langmuir, 32, 9557 (2016).Kimura, R. Akiyama, K. Miyatake, and J. Inukai, J. Power Sources, 375, 397 (2018). Figure 1

Insight into the intrinsic mechanism of improving electrochemical performance via constructing the preferred crystal orientation in lithium cobalt dioxide

Surface properties of cathode materials play important roles in the transport of lithium-ions/electrons and the formation of surface passivation layer. Optimizing the exposed crystal facets of cathode materials can promote the diffusion of lithium-ions and enhance cathode surface stability, which may ultimately dominate cathode’s performance and stability in lithium-ion batteries. Here, polycrystalline LiCoO2 (LCO) thin films with (0003) and {101-1} preferred orientations were prepared as the well-defined model electrodes. In situ Current-Sensing Atomic Force Microscopy (CSAFM) was employed to investigate the lithium de-intercalation and electronic conductivity evolution of the (0003) and {101-1} facts in organic electrolyte at the nanoscale. It was found that the lithium deintercalation following a “Li-rich core model” in the LCO grains, and the LCO grains with (0003) crystal face show less conductivity than those with {101-1} faces. Moreover, X-ray Photoelectron Spectroscopy characterization of the charged electrode surface indicates that a denser surface passivation layer is formed on {101-1} than that on (0003) crystal faces. This is caused by the lower adsorption energy of decomposition molecule on {101-1} crystal faces and higher work function (due to the surface atomic structure) for {101-1} crystal faces, as confirmed by Density Functional Theory (DFT) and Kelvin probe force microscopy (KPFM) results. In addition, electrochemical measurements confirm that the thin film electrodes with {101-1} preferred orientation not only show smaller electrode polarization, but also more readily form a stable surface passivation layer compared with the (0003) preferred orientation. This work highlights the importance of cathode conductivity, and suggests that the LCO {101-1} facet atomic structure may thermodynamically promote the physical/chemical adsorption and decomposition of electrolyte.

Current-sensing atomic force microscopy for analyzing conformations and properties of polymer membranes for fuel cells

Abstract The three-dimensional structures of polymer membranes are different at surfaces and inside bulks, and thus, in general, physical/chemical properties are also different. Morphologies and properties of membrane surfaces are now visualized by current-sensing atomic force microscopy. The increase in performances of a single cell is discussed based on the three-dimensional structures of the polymer membrane, anion-exchange membrane as an example, used for fuel cells. Other reports on Nafion®, proton-exchange membrane, are also introduced to show the importance of this microscopic method.

Tunnelling Current Measurements Using Current Sensing Atomic Force Microscope

To realise the miniaturised devices, the precise measurement of nanoscale tunnelling current in ultrathin films is of utmost importance. For the nanoscale current measurements, current sensing atomic force microscope (CSAFM) is one of the most powerful tool. CSAFM allows to map the current distribution on the film surface and it permits to perform current measurements as a function of applied bias voltage. It has turned out to be crucial for studies of organic films. In CSAFM, a physical contact is made on film with a precise control of the applied force in nanonewton (nN) range. For the preparation of ultrathin film, Langmuir-Blodgett (LB) technique is known to provide a uniform film with a good control over the thickness in the molecular level. In the last two decades, there have been many CSAFM studies for the tunnelling current measurements. This review is intended to cover the literature on the tunnelling current measurements using CSAFM.

Synergistic engineering of hole transport materials in perovskite solar cells

AbstractIn this work, methylammonium lead triiodide (CH3NH3PbI3) perovskite solar cells with efficiencies higher than 18% were achieved using a new nanocomposite hole transport layer (HTL) by doping poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with a mixed dopant of polyaniline (PANI) and graphene oxide (GO). A synergistic engineering between GO, PANI, and PEDOT:PSS was accomplished to introduce additional energy levels between perovskite and PEDOT:PSS and increase the conductivity of PEDOT:PSS. Kelvin probe force microscope results confirmed that adding GO to PEDOT:PSS/PANI composite significantly reduced the average surface potential. This increased the open circuit voltage (Voc) to 1.05 V for the GO/PEDOT:PSS/PANI nanocomposite perovskite solar cells from the pristine PEDOT:PSS (Voc = 0.95 V) and PEDOT:PSS/PANI (Voc = 0.99 V). In addition, adding PANI to the HTLs substantially enhanced short circuit current density (Jsc). This was supported by the current sensing‐atomic force microscopy (CS‐AFM) and conductivity measurements. The PANI doped films showed superior electrical conductivity compared with those without PANI as indicated by CS‐AFM results. PANI can fill the gaps between the microflakes of GO and give rise to more compact hole transport material (HTM) layer. This led to a higher Jsc after doping with PANI, which was consistent with the incident photon‐to‐current efficiency and electrochemical impedance spectroscopy results. The results of X‐ray diffraction (XRD) and AFM indicated the GO/PANI doped HTMs significantly improved the crystallinity, topography, and crystal size of the perovskite film grown on their surface. A higher efficiency of 18.12% for p‐i‐n perovskite solar cells has been obtained by adding the mixed dopant of GO, PANI, and PEDOT:PSS, demonstrating better stability than the pristine PEDOT:PSS cell.image

Localized Electrochemical Impedance Measurements on Nafion Membranes: Observation and Analysis of Spatially Diverse Proton Transport Using Atomic Force Microscopy

Nafion, the state-of-the-art-of proton exchange membrane in fuel cell, is a copolymer with a randomly tethered side-chain terminated by a pendant sulfonic acid groups adjacent to the polytetrafluoroethylene (PTFE) backbone. The PTFE backbone is hydrophobic and insulative, whereas the sulfonic acid groups are hydrophilic and proton-conductive. As a result of the intrinsic difference between the backbone and sulfonic acid groups, phase separation occurs during the formation of solid Nafion membranes from ionomer solution via a self-organizing process conducted at elevated temperature. The ionic clusters are formed in hydrophobic matrix by hydrophilic groups, interconnected by slender ionic channels which serve as the proton conductive pathways. Furthermore, the distribution of proton-conductive pathways not only inside but on the surface of the membrane plays a significant role in improving the cell performance. The latter determines the mass transport of H2O molecules and protons through the membrane, which is the basis of good performance in fuel cells. In addition, the three-phase boundary (TPB) plays a crucial role in the performance of fuel cells, which means that the electrocatalysts (such as Pt) in fuel cells must directly contact with the conductive hydrophilic regions on the surface of PEMs to minimize the contact resistance and invalid catalysts located in the hydrophobic regions will lead to lower catalyst utilization. However, an excess of hydrophilic groups would cause flooding, and the hydrophobic phase is necessary for supporting mechanical strength and separating reagents in fuel cells. Therefore, in order to further promote fuel cell performance, detailed knowledge of phase separation with Nafion is required, and a good balance between the distributions of hydrophilic and hydrophobic groups is extremely crucial for promising ion exchange membranes. Currently, current-sensing AFM is often used to obtain the imaging of local surface proton conductance. However, the direct current (DC) applied leads to an overall result that is affected by many factors, such as proton conductivity, mass transport and reaction kinetics. Compared to DC methods, alternate current (AC) methods such as electrochemical impedance spectroscopy (EIS) provides a powerful insight to the overall impedance response and individual processes including ionic transport can be distinguished at higher accuracy. In our study, we constructed a home-built AFM-EIS setup and measured the localized impedance by coupling AFM with an impedance tester in a Faraday cage. In contrast to traditional average and macroscopic bulk impedance measurements, we obtained a detailed and spatially diverse distribution of EIS spectra at selected points on Nafion with AFM-EIS. By equivalent electric circuit fitting, we derived the localized pure proton transfer resistance from the EIS spectra; such results cannot be obtained by current-sensing AFM. In addition, with a customized automatic procedure, we performed proton transfer resistance imaging on the Nafion surface in an area of 2 µm*2 µm at different humidities, the results of which confirmed the widely known fact that higher water activities significantly reduce the transport resistance. However, our experimental results show a wide variation in proton transport resistance at a well-equilibrated humidity level of the membrane. Our work has led to the conclusion that the large variation arises mainly from the surface concentration of water channels due to the fact that the transport and reaction kinetics are favoured by the three-phase interface. Furthermore, to elucidate the Nafion microstructure, a numerical simulation was constructed to incorporate morphological and physical variations of the membrane, such as the heterogeneity of the hydrophilic and hydrophobic groups in the membrane, which revealed that at higher water activities, SO3H groups cluster to create a matrix of interconnected nano-water channels through which the ions are transported. Figure 1

Localized Electrochemical Impedance Measurements on Nafion Membranes: Observation and Analysis of Spatially Diverse Proton Transport Using Atomic Force Microscopy.

The distribution of ion conductive channels on the Nafion membrane surface, which determines the formation of the three-phase boundary, plays a very important role in improving the performance of proton-exchange membrane fuel cells. Therefore, understanding the microstructures at the catalyst layer/membrane interfaces of proton-exchange membranes is essential. Although current-sensing atomic force microscopy (AFM) can present some surface conductance data, localized impedance measurement providing more accurate proton-transport information is desirable. To obtain this information, in our study, localized electrochemical impedance spectroscopy was measured automatically with a home-built AFM-electrochemical impedance spectroscopy setup in which AFM was coupled with an impedance tester by a customized procedure. By this method, the localized proton-transport resistance at different humidities was observed in spatially diverse locations, and the value decreased as the membrane became hydrated. Furthermore, the microstructure of the Nafion membrane was numerically reconstructed at different hydration levels to examine the relationship between the membrane microstructural morphology and proton-transport resistance. The results showed that the spatial diversity of proton-transport resistance arose from the variable concentration of hydrophilic groups at the contact location of the AFM tip and the membrane, and from the heterogeneity of dry sulfonic acid groups in the membrane that creates local variation in water content.

Advanced Characterization Methods for Electrical and Sensoric Components and Devices at the Micro and Nano Scales

The present study covers the nanoanalysis methods for four key material characteristics: electrical and electronic properties, optical, stress and strain, and chemical composition. With the downsizing of the geometrical dimensions of the electronic, optoelectronic, and electromechanical devices from the micro to the nanoscale and the simultaneous increase in the functionality density, the previous generation of microanalysis methods is no longer sufficient. Therefore, the metrology of materials' properties with nanoscale resolution is a prerequisite in materials' research and development. The article reviews the standard analysis methods and focuses on the advanced methods with a nanoscale spatial resolution based on atomic force microscopy (AFM): current‐sensing AFM (CS‐AFM), Kelvin probe force microscopy (KPFM), and hybrid optical techniques coupled with AFM including tip‐enhanced Raman spectroscopy (TERS), photothermal‐induced resonance (PTIR) characterization methods (nano‐Vis, nano‐IR), and photo‐induced force microscopy (PIFM). The simultaneous acquisition of multiple parameters (topography, charge and conductivity, stress and strain, and chemical composition) at the nanoscale is a key for exploring new research on structure–property relationships of nanostructured materials, such as carbon nanotubes (CNTs) and nano/microelectromechanical systems (N/MEMS). Advanced nanocharacterization techniques foster the design and development of new functional materials for flexible hybrid and smart applications.

Probing interlayer excitons in a vertical van der Waals p-n junction using a scanning probe microscopy technique

Two dimensional (2D) semiconductors feature exceptional optoelectronic properties controlled by strong confinement in one dimension. In this contribution, we studied interlayer excitons in a vertical p-n junction made of bilayer n-type MoS2 and few layers of p-type GaSe using current sensing atomic force microscopy (CSAFM). The p-n interface is prepared by mechanical exfoliation onto highly ordered pyrolytic graphite (HOPG). Thus the heterostructure creates an ideal layered system with HOPG serving as the bottom contact for the electrical characterization. Home-built Au tips are used as the top contact in CSAFM mode. During the basic diode characterization, the p-n interface shows strong rectification behavior with a rectification ratio of 104 at ±1 V. The I–V characteristics reveal pronounced photovoltaic effects with a fill factor of 0.55 by an excitation below the band gap. This phenomenon can be explained by the dissociation of interlayer excitons at the interface. The possibility of the interlayer exciton formation is indicated by density functional theory (DFT) calculations on this heterostructure: the valence band of GaSe and the conduction band of MoS2 contribute to an interface-specific state at an energy of about 1.5 eV. The proof of excitonic transitions to that state is provided by photoluminescence measurements at the p-n interface. Finally, photocurrent mapping at the interface under an excitation wavelength of 785 nm provides evidence of efficient extraction of such excitons. Our results demonstrate a pathway towards a 2D device for future optoelectronics and light harvesting assisted by interlayer excitons in a van der Waals (vdW) heterostructure.

Plasmon-mediated nonradiative energy transfer from a conjugated polymer to a plane of graphene-nanodot-supported silver nanoparticles: an insight into characteristic distance.

Hybrid nanostructures comprising conjugated polymers (CPs) and plasmonic metals show excellent performance in light harvesting. However, the energy transfer mechanism of the CP film to nearby metal nanoparticles, especially knowledge of the characteristic distance, is still unclear. Here, quenching of the emission of a CP film in proximity to a monolayer of graphene-nanodot-supported silver nanoparticles (GND-Ag NPs) is investigated. Uniform Ag NPs with D = 3.2 nm were grown on GNDs in situ under mild light irradiation, and a series of bilayer structures of GND-Ag NPs/CPs were constructed by spin-coating blue, green and red light-emitting poly(9,9-dioctylfluorene) (PFO), poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV),respectively, on top of the GND-Ag NP plane. The spacer distance was controlled by the layers of assembled polyelectrolytes. Both steady and transient photoluminescence (PL) spectra showed emission quenching of the bilayer structures, providing the maximum efficiency of 99% for the F8BT films. The surface density of GND-Ag NPs and the spacer distance-dependent PL quenching data were analyzed within the plasmonic resonant energy transfer model, and the extracted characteristic distances are 6 nm, 3 nm and 10 nm for the PFO, F8BT and MEH-PPV systems, respectively. Current-sensing atomic force microscopy shows that the GND-Ag NPs/F8BT film exhibits enhanced electrical conductivity. These results are believed to be important for the development of plasmonic enhanced polymer photovoltaics and photocatalysis.