Pulsed Force Mode Research Articles

Pulsed Force Kelvin Probe Force Microscopy.

Measurement of the contact potential difference (CPD) and work functions of materials are important in analyzing their electronic structures and surface residual charges. Kelvin probe force microscopy (KPFM), an imaging technique of atomic force microscopy, has been widely used for surface potential and work function mapping at the nanoscale. However, the conventional KPFM variants are often limited in their spatial resolution to 30-100 nm under ambient conditions. The continuingly decreasing size and increasing complexity of photoactive materials and semiconductor devices will present future challenges in uncovering their nanometer-scale electrical properties through KPFM. Here, we introduce a KPFM technique based on the pulsed force mode of atomic force microscopy. Our technique, named pulsed force Kelvin Probe Force Microscopy (PF-KPFM), is a single-pass technique that utilizes the intrinsic Fermi level alignment between the AFM tip and the conductive sample without the need for an external oscillating voltage. Induced cantilever oscillations generated by a spontaneous redistribution of electrons between tip and sample are extracted and used to obtain the cantilever oscillation amplitude and to derive the surface potential. The spatial resolution of PF-KPFM is shown to be <10 nm under ambient conditions. The high spatial resolution surface potential mapping enables in situ determination of ohmic and nonohmic contacts between metals and semiconductors, mapping boundaries of ferroelectric domains of BaTiO3, as well as characterization of protein aggregates. High spatial resolution measurements with PF-KPFM will facilitate further studies directed at uncovering electrical properties for emerging photoactive materials, biological samples, and semiconductor devices.

In Situ Preparation of Thin and Rigid COF Film on Li Anode as Artificial Solid Electrolyte Interphase Layer Resisting Li Dendrite Puncture

AbstractMetallic Li is considered the most promising anode material for high‐energy density batteries due to its high theoretical capacity and low electrochemical potential. However, commercialization of the Li anode has been hampered by the safety issue associated with Li‐dendrite growth resulting from uneven Li‐ion deposition and an unstable solid electrolyte interphase (SEI). Herein, an in situ prepared 10 nm thin film of covalent organic framework (COF) uniformly covered on the Li anode (COF‐Li) is used as an artificial SEI layer for Li plating/striping stabilization and Li dendrite inhibition. Abundant microcellular structures in the COF can redistribute the Li‐ion flux and lead to the homogeneous plating/stripping process. Meanwhile, the superhard mechanical properties and mechanical behavior during needling of the ultrathin COF film is studied via the digital pulsed force mode equipped in atomic force microscopy, illustrating a high Young's modulus of 6.8 GPa that is strong enough to resist dendrite growth. As a result, stable cycling for 400 h is achieved in the COF‐Li symmetrical cell at a current density of 1 mA cm−2, and the internal short circuit is effectively blocked by COF‐Li in Li–S batteries.

Methodology for estimating the modulus of elasticity of bitumen under different aging conditions by AFM

This paper presents a micro and nano mechanical analysis method to measure the elastic modulus and properties of bitumen. Tests were performed using the AFM in pulsed force mode to determine the adhesion and stiffness of bitumen. In AFM contact mode, a methodology was developed to determine the modulus of elasticity of the bitumen using the nanoindentation technique for small-volume specimens using a Berkovich tip. Two different sources of bitumen were studied with a performance grade of PG 64 S (22) and PG 64 H (25), in its neat, RTFO, and RTFO + PAV aged conditions. The mechanical properties of the bitumen were analysed from the responses of the measured materials. The AFM images and mechanical results reveal that the properties of the bitumen are considerably affected by aging, associated with oxidation and variations in its morphology. Modulus measured by AFM was also compared to traditional G* measurements from a rheometer, and their relationship analysed.

Effect of ageing on micromechanical properties of bitumen by means of atomic force microscopy

This study focuses on morphological changes and micromechanical properties of bitumen and its components (maltenes and asphaltenes) under neat, laboratory short- and long-term, and long-term field ageing. The material was analysed by means of atomic force microscopy in pulsed force mode, allowing for material nano-mechanical measurements. It was concluded that the colloidal equilibrium of asphalt fractions is affected by a significant loss of low molecular weight components, and air oxidising the bitumen. In addition, stiffness and adhesion were measured for each condition. It was observed that the greatest contribution in stiffness is associated with polar components, while the adhesiveness is associated mainly with non-polar components. Finally, to relate the ageing process to chemical changes, Fourier transform infrared spectroscopy measurements were performed to quantify the functional groups associated with oxidation. Furthermore, thermodynamic analysis by means of differential scanning calorimetry was performed to explain some of the observed changes in micromechanical behaviour.

Probing Chemical and Physical Properties of Poplar Tension Wood Using Confocal Raman Microscopy and Pulsed Force Mode Atomic Force Microscopy

ABSTRACTLignocellulosic biofuels have been identified as a possible solution to contribute to the world’s demands in energy and environmental sustainability. However, the fundamental understanding of the physical and chemical traits hindering key reactions during biomass to biofuel conversion processes has been limited by the lack of suitable tools and by the large natural variability in such systems. Reaction wood constitutes a good model system to study variations of cellulose content, given the increase in cellulose content in the cell walls of the region under tension in the plant during growth. In this work, we use confocal Raman mapping and Pulsed Force Mode Atomic Force Microscopy (PFM) to explore the effect of variation in cellulose content on the structure and composition of the plant cell wall at the nanoscale. Using statistical analysis on Raman datasets, the characteristic peaks for cellulose and lignin are examined to reveal changes in peak positions across the different scanned regions of the cross section. PFM is used to study local mechanical properties of the different layers of the cell wall. Our approach facilitates the correlation of structure-composition traits of the plant cell wall for a more fundamental understanding of processes involved in biofuel research.

Tip in-light on: Advantages, challenges, and applications of combining AFM and Raman microscopy on biological samples.

Scanning probe microscopies and spectroscopies, especially AFM and Confocal Raman microscopy are powerful tools to characterize biological materials. They are both non‐destructive methods and reveal mechanical and chemical properties on the micro and nano‐scale. In the last years the interest for increasing the lateral resolution of optical and spectral images has driven the development of new technologies that overcome the diffraction limit of light. The combination of AFM and Raman reaches resolutions of about 50–150 nm in near‐field Raman and 1.7–50 nm in tip enhanced Raman spectroscopy (TERS) and both give a molecular information of the sample and the topography of the scanned surface. In this review, the mentioned approaches are introduced, the main advantages and problems for application on biological samples discussed and some examples for successful experiments given. Finally the potential of colocated AFM and Raman measurements is shown on a case study of cellulose‐lignin films: the topography structures revealed by AFM can be related to a certain chemistry by the colocated Raman scan and additionally the mechanical properties be revealed by using the digital pulsed force mode. Microsc. Res. Tech. 80:30–40, 2017. © 2016 Wiley Periodicals, Inc.

The Influence of High Pressure Treatment and Thermal Pasteurization on the Surface of Polymeric Packaging Films

Several common single layer films (PE-HD, PE-LD, PP-BO, PA6-BO and PET-BO) and multilayer (PS/PE, PP-BO/PEpeel and PET-BO/PE) films were treated by either high pressure (600 MPa) or temperature (80 °C/90 °C) to simulate a high pressure or thermal pasteurization process. The samples were tested by atomic force microscopy (AFM), profile method and surface energy measurements to obtain information about the influence of the treatments on the surface topography and surface energy of the samples and by differential scanning calorimetry and by tensile testing concerning material properties. As key figures arithmetic surface roughness (by AFM at Pulsed Force Mode and profile method), surface energy by surface energy measurement and adhesion between tip and surface by AFM were extracted. Results indicate an influence of both high-pressure processing and thermal-processing on the surface roughness of biaxial oriented polymer films as single layer films. Laminated biaxially oriented polymer films showed no changes regardless of which processing was performed. The surface energy was hardly affected by both of the treatments for any stretched, non-stretched, single or laminated films.

Distribution and localization of hydrophobic and ionic chemical groups at the surface of bleached human hair fibers.

A chemical mapping with high lateral resolution using an atomic force microscope in the pulsed force mode with chemically modified tips, introduced as "dynamic chemical force microscopy" (dCFM), was carried out to investigate the chemical properties of the cuticle of human hair and its changes following an oxidative treatment. Chemically modified atomic force microscopy (AFM) tips, CH3- and NH2-terminated, were applied to achieve a defined chemical contrast (hydrophobic and ionic) in aqueous medium. A comparative Fourier transform infrared spectroscopy-attenuated total reflection identified the dominant chemical groups of the surface vicinity of the hair fiber resulting from the bleaching process. The combined experimental results lead to the conclusion that the hydrophobic top layer is partially removed after bleaching, resulting mostly in hydrophilic SO3(-) end groups at the top of the surface of the hair, with a mean surface density "δ(mean)" of negatively charged groups of approximately 2.2 molecules/nm(2), corresponding to ∼600 μg/m(2) cysteic acid. This indicates that thioester bonds are disrupted and fatty acids are removed as a result of cysteine oxidation. At the molecular level, our results indicate a clustered "self-assembled monolayer" alignment of cysteic acid with a crystal-like structuring, reminiscent of the "fluid mosaic model of cell membranes", with a surface energy of approximately 0.04 N/m. Despite previous extensive works of AFM on human hair, this is, to our knowledge, the first time that the hydrophobic and ionic sites at the top surface of hair have been imaged at the nanoscale with dCFM.

Atomic force microscopy on phase-control pulsed force mode in water: Imaging and force analysis on a rhodium-octaethylporphyrin layer on highly oriented pyrolytic graphite

Abstract We developed phase-control pulsed force mode (p-PFM) as the operation mode for atomic force microscopy (AFM). The p-PFM allowed us to observe soft or weakly adsorbed materials in a liquid environment using a conventional AFM apparatus, and allowed for force curve mapping (FCM) after offline data processing. We applied the p-PFM to a rhodium-octaethylporphyrin (RhOEP) layer on highly oriented pyrolytic graphite (HOPG), which is applicable to anode catalysts of fuel cells. The RhOEP/HOPG system was stably observed in water by this mode. In the p-PFM image, we found both large and small protrusions, which were not observed in the dynamic force mode, in air. The detailed force analysis suggested that these protrusions are nanobubbles located on the HOPG substrate exposed in holes or pits of the RhOEP layer.

A fast and quantitative evaluation of the Aspergillus fumigatus biofilm adhesion properties by means of digital pulsed force mode

The opportunistic pathogenic mould Aspergillus fumigatus (A. fumigatus) is an increasing cause of morbidity and mortality in immunocompromised and in part immunocompetent patients. A. fumigatus can grow in multicellular communities by the formation of a hyphal network embedded in an extracellular matrix (ECM) meanly composed by polysaccharides, melanin, proteins. Because adhesion properties is one primary factor affecting the balance between growth, detachment and biofilm formation, its quantification is essential in understanding, predicting, and modelling biofilm development.Atomic force microscopy (AFM) imaging and force spectroscopy have recently opened a range of novel applications in microbiology including the imaging and manipulation of membrane proteins at the subnanometer level, the observation of the surface of living cells at high resolution, the mapping of local properties such as surface charges, the measurement of elastic properties of cell-surface constituents and the probing of cellular interactions using functionalized probes. Nevertheless, the principal disadvantage of this approach is the relatively slow acquisition rate that makes AFM is not able to detect fast dynamics.In this study we demonstrated that digital pulsed force mode (DPFM) atomic force microscopy can be used to obtain high-resolution topographical images and to quantify the adhesion properties of the A. fumigatus biofilm with an high acquisition rate.Here we show by means of DPFM-AFM that Alginate Lyase (AlgL), an enzyme known to reduce negatively charged alginate levels in microbial biofilm, is able to reduce the biofilm adhesion forces forming several nano-fractures in the ECM. These results suggest that the AlgL could used to enhance the antifungal drugs transit through the ECM.

Molybdenum disulfide dc contact MEMS shunt switch

Atomic force microscopy pulsed force mode verifies that molybdenum disulfide (MoS2) has a smaller surface adhesion energy than graphene. MEMS switches based on MoS2 may have less stiction problems. Suspended MoS2 two-end fixed beams were fabricated, and their mechanical properties including Young's modulus were characterized by atomic force microscope (AFM) indentation. MoS2 dc contact MEMS (micro-electro-mechanical systems) switches were demonstrated with a pull-in voltage of less than 10 V.

Pulsed contact resonance for atomic force microscopy nanomechanical measurements

We demonstrate an improved technique for nanomechanical imaging in atomic force microscopy. By merging the sensitivity to contact stiffness inherent to contact resonance (CR) spectroscopy with the delicate nature and potential for adhesion data of pulsed-force mode, we address major shortcomings of both techniques. Fast CR data are recorded during each pulsed cycle by driving the sample at two frequencies near the CR frequency and modeling the contact as a harmonic oscillator. The technique provides nanomechanical parameters including frequency, quality factor, and adhesion force. Compared to continuous contact, the technique should reduce damage and support more complex analysis models.

Ultrasensitive Force Spectroscopy with Tuning Fork Based Frequency Modulation

The standard method that is used with atomic force microscopy to monitor mechanical properties of materials such as elasticity and adhesion is based on beam bounce technology. With such an approach there are two major problems: one is jump to contact and the other is adhesion ringing. Numerous methods have evolved for trying to resolve these problems from methods call pulsed force mode to peak force. However, what is desired is a smooth approach and retract from a surface or molecule where force measurements need to be implemented. Over the last few years, it has been realized that the best method of force feedback in atomic force microscopy is based on tuning fork force modulation but there have been few studies implementing these advances into the realm of force spectroscopy. In this paper, force spectroscopic analysis is implemented based on tuning forks and it is demonstrated that there is close to single pN force sensitivity. These efforts use the pioneering theory of Sader and Jarvis that has shown theoretically that is possible to derive accurate formulas for the force versus frequency in such Frequency Modulations methods [J.E. Sader and S. P. Jarvis, “Accurate formulas for interaction force and energy in frequency modulation force spectroscopy” Appl. Phys. Lett.84, 1801 (2004)]. It will be further shown that such normal force turning fork based force spectroscopy can readily be integrated with other chemical and structural tools such as Raman microSpectroscopy and Scanning Electron Microscopy.

Interaction between water-soluble polysaccharides and native nanofibrillar cellulose thin films

The objective of this work was to compare the adsorption of different polysaccharides and cellulose derivatives on cellulose nanofibril films. Cellulose films having the native cellulose I structure were prepared from hardwood kraft pulp by extensive mechanical disintegration. Further fractionation enabled the preparation of reproducible, nanometer-scale thickness films. Systematic comparison by Quartz Crystal Microbalance with Dissipation (QCM-D) showed that various industrially available galactomannans have almost as good affinity to cellulose surface as xyloglucan and that most of the polysaccharides attach irreversibly to cellulose nanofibrils at low pH (4.5) and intermediate ionic strength (10 mM). SPR results support the QCM-D findings. Atomic Force Microscopy (AFM) imaging and Digital Pulsed Force Mode (DPFM) measurements further confirmed that a uniform non-aggregated layer of polysaccharides was formed that changed the properties of the NFC film.

Compositional Analysis of Low Quantities of Phase Separation in Hot-Melt-Extruded Solid Dispersions: A Combined Atomic Force Microscopy, Photothermal Fourier-Transform Infrared Microspectroscopy, and Localised Thermal Analysis Approach

To characterise phase separations in aged hot-melt-extruded solid dispersions at a micron to submicron scale. Hot-melt-extruded felodipine and Eudragit® E PO systems at a range of compositions were studied after a standard period of aging to allow phase separation to occur. The samples were characterised using combined nano-thermal analysis, photothermal FTIR microspectroscopy coupled with pulsed force mode AFM as a novel characterisation approach. Crystalline felodipine presents in all formulations with drug loadings from 10-70% (w/w). In formulations with high drug loadings (50 and 70%), amorphous felodipine co-exists with crystalline forms, and higher drug concentration is observed in the centre compared to the outer surface of the extrudates. Drug crystal dimensions in extrudates with low drug loadings (10-30%) are small, in the micron to submicron range. We propose that uneven drug distribution is principally caused by processing-associated factors such as expansion of extrudates during extrusion. We have demonstrated that the novel combined approach allows site-specific characterisation of the extruded systems and that drug distribution may be uneven across the extrudates, with concomitant implications for understanding stability and drug release behaviour.

Digital Pulsed Force Mode AFM and Confocal Raman Microscopy in Drug-Eluting Coatings Research

ABSTRACTControlled release of amorphous drug from a polymer matrix depends intimately upon the degree of mixing of drug and polymer, the susceptibility of the drug to crystallization, and the ability of the drug to dissolve and diffuse through water-swollen polymer. Characterization methods ideally would follow these processes on the molecular level in situ and in real time. We move closer to this ideal state of characterization through application of two imaging methods: digital pulsed force mode atomic force microscopy (D-PFM AFM) and confocal Raman microscopy (CRM). We examine model spin-coated films ~1 μm thick containing the drug dexamethasone dispersed in poly(n-alkyl methacrylate) homopolymer and blend coatings. We report aqueous-immersion studies of surface and subsurface structural changes due to drug elution over time frames ranging from very fast (a few minutes) to slow (tens of hours).

Plasma Polymer Tailoring of the Topography and Chemistry of Surfaces at the Nanoscale

We demonstrate in this paper that plasma polymer can be advantageously used to provide surfaces with topography and chemical control at the nanoscale. Moreover, this technique was also proved to be of high interest to functionalize atomic force microscopy tips that were used to probe the patterned surfaces in pulsed force mode. This approach allowed demonstration by a direct observation of the possibility of generating alternating hydrophilic/hydrophobic surfaces at the nanoscale prepared by DUV laser irradiation. Such a versatile and simple route opens new possibilities in the field of smart surfaces engineering.

Nanoscale Investigation of Nafion Membranes after Artificial Degradation

In this contribution we report on the nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes investigated by contact atomic force microscopy (AFM), conductive AFM, and pulsed force-mode (PFM)-AFM. The different techniques can provide complementary information on structure and conductivity. Conductive AFM is used to investigate the conductivity of Nafion membrane surfaces. The images show a quite inhomogeneous distribution of current at the surface. The percentage of conductive surface increases with humi¬dity, but regions without any current are still present up to 80% relative humidity [1,2]. Comparison with PFM-AFM images, where differences in adhesion forces are measu¬red, indicates that hydrophobic regions, which are attribu¬ted to PTFE-like polymer back bone without conductivity, are present at the surface with comparable dimensions. Conductive AFM gives information about the internal structure of ionic clusters during current flow. High resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature (Fig. 3). The influence of H2O2 treatment as a method for artificial degradation is investigated. The analysis of adhesion forces demonstrates a significant change of the surface properties with different membrane treatment. Topography and adhesion measurements clearly show materials changes with high resolution and correlate with changes in conductivity distributions. In particular, small protrusions in the topography correlating with strongly diminished conductivity in the current images are observed. These areas are interpreted as crystalline polymer backbone which higher stability against radical decomposition.

Surface Mechanical Properties of Thin Polymer Films Investigated by AFM in Pulsed Force Mode

Atomic force microscopy in the pulsed force mode (PFM) is applied in this work to the study of thin dewetting patterns formed by drying an aqueous solution of poly(N-isopropylacrylamide) (PNIPAM) and sodium dodecyl sulfate (SDS) on mica. This technique allows the automated acquisition of typically 4 x 10(6) force-distance curves on the sample surface together with maps showing nanodomains differentiated by their stiffness and adhesion to the tip. Topography images of dry films revealed a morphology formed by droplets distributed on the substrate. Adhesion and stiffness images with good lateral resolution show droplets containing polymer and surfactant contrasting with the substrate and also nanosized heterogeneities inside these droplets. They also revealed very small dewetted structures which could not be observed in the topography map by noncontact AFM. Adhesion interactions between the AFM tip and the polymer or the dewetted mica substrate were measured in terms of adhesion force and detachment energy, and can be used as new information to understand dewetting patterns containing silica particles, PNIPAM, and SDS. Other surface mechanical parameters such as stiffness, maximum indentation, hardness, compliance, hysteresis, and Young's modulus were obtained by sampling many points and used to characterize the PNIPAM/SDS films formed in the dewetting process.

Characterizing interfacial structure of TPO/CPO/TPO adhesive joints by PFM–AFM and SEM

The influence of annealing and EBR component in injection-molded thermoplastic polyolefin (TPO) plaques on adhesion strength of CPO to TPO was investigated by a lap shear test. The TPO was fabricated as a blend of highly crystalline isotactic polypropylene (iPP) and low crystalline poly(ethylene-butene) impact modifier (EBR28). The CPO was a maleated chlorinated polypropylene containing 21.8wt% Cl. High resolution pulsed force mode–atomic force microscopy (PFM–AFM) combined with the image analysis was used to characterize the interfacial properties of the lap shear joints. Based on PFM–AFM stiffness images, a “transition zone” with a width on the order of 600–1500nm was observed between CPO and the TPO substrate that may play an important role in affecting the adhesion strength. This zone exhibits enhanced stiffness after annealing at 120°C. The PFM–AFM images further show that the interface between iPP and CPO without annealing is very sharp and the interface between TPO and CPO without annealing is wider than the interface between iPP and CPO. Annealing (120°C/20min) leads to broadening of the interface between TPO and CPO. The thickness of the interface in lap shear joints was obtained from the z-directional line profiles of the stiffness maps. The fracture surface morphology was revealed by scanning electronic microscope (SEM), which showed that the fracture structure varied with both the addition of EBR28 in TPO plaques and the annealing condition. Finally, a correlation of interfacial properties to adhesion was obtained: higher stiffness in the transition zone coupled with a thicker interface resulted in stronger adhesion and cohesive failure within the CPO and TPO. In the case of CPO/iPP, the narrow interface and absence of a clearly defined transition zone correlated with interfacial failure between these components.