Atomic Force Microscopy Phase Images Research Articles

Improved Performance for Inverted Organic Photovoltaics via Spacer between Benzodithiophene and Benzothiazole in Polymers

In this work, the effects of the spacer between benzo[1,2-b;3,4-b′]dithiophene (BDT) and dialkoxybenzothiadiazole (ROBT) in polymers were investigated for applications in organic solar cells. Polymer PBDT–2T–ROBT has a bithiophene (2T) spacer between the BDT and ROBT units, whereas PBDT–ROBT is a direct copolymer of BDT and ROBT. The polymer/PC70BM solar cells using both polymers were fabricated and optimized via polymer/fullerene ratio, solvent, and solvent additives. The spacer has significantly improved solar cell performance from 1.28% (Voc =0.77 V, Jsc = 3.13 mA/cm2, FF = 53.11%) to 5.11% (Voc = 0.66 V, Jsc = 13.33 mA/cm2, FF = 58.12%). The X-ray diffraction (XRD) spectra show the PBDT–2T–ROBT/PC70BM blended film is semicrystalline, whereas the PBDT–ROBT/PC70BM film is amorphous. This indicates that the spacer facilitates polymer organization for higher carrier mobility in the film. Atomic force microscopy (AFM) topography and phase images show that PBDT–2T–ROBT/PC70BM films form fibrillar networks, ...

MORPHOLOGY OF SILICA-REINFORCED NATURAL RUBBER: THE EFFECT OF SILANE COUPLING AGENT

ABSTRACT A good dispersion of silica in a rubber vulcanizate is important as it influences the filler-to-rubber interaction and consequently the final properties. This article presents an investigation into the morphology of silica-reinforced natural rubber (NR) in the presence and absence of a silane coupling agent, bis(triethoxysilylpropyl) tetrasulfide (TESPT). Micro- and nano-dispersion morphologies of silica in NR and deproteinized natural rubber (DPNR) are studied by using atomic force microscopy (AFM). Using a special network visualization technique based on transmission electron microscopy (TEM), insight into the silica and rubber interaction in the NR and DPNR is gained. In the absence of silane, vacuoles around the silica particles are formed as a result of a weak filler–polymer interaction, whereas the presence of silane leads to strong filler-to-rubber bonding, which prevents formation of vacuoles. Improvement of the micro-dispersion of silica in the NR and DPNR vulcanizates with the use of TESPT is observed from AFM phase imaging. The correlation between the filler-to-rubber interaction as analyzed by TEM and AFM and bound rubber contents as well as the Payne effect is discussed.

Large area graphene and graphene oxide patterning and nanographene fabrication by one-step lithography

A cheap and green technology based on electro-erosion, performed at room conditions and scalable to large area (cm2) is demonstrated for structuring graphene and graphene oxide films on any substrate. This one-step technique based on electrical discharges produced by a direct current voltage source competes favorably with laser patterning. The threshold voltage for complete graphene elimination as determined by Raman mapping is around 20V. At low relative humidity conditions (30%) the transformation to graphene oxide is also detected for operating voltages above 40V, the oxidation being probably mediated by the residual adsorbed water at the surface. We also show the close correlation of atomic force microscopy (AFM) phase images with the modified graphene characteristics while AFM topographic images are dominated by extrinsic aspects. The use of a spring probe relaxes the requirement of precision for the tip-sample distance and sample flatness to around 10μm which is very convenient for large scale applications. Furthermore, this technique allows the formation of high quality nanographene, with size around 18nm, and graphene micro-ribbon lattices in a very fast way and very well defined edges, with dimensions down to 1μm width and mm length, very promising for terahertz graphene plasmonic applications.

Antimicrobial and antioxidant surface modification of cellulose fibers using layer-by-layer deposition of chitosan and lignosulfonates

To confer cellulose fibers antimicrobial and antioxidant activities, chitosan (CS)/lignosulfonates (LS) multilayers were constructed on fibers surfaces through layer-by-layer deposition technique. The formation of CS/LS multilayers on cellulose fibers surfaces was verified by X-ray photoelectron spectroscopy (XPS) and zeta potential measurement. The surface morphologies of CS/LS multilayers on fibers surfaces were observed by atomic force microscopy (AFM). The results showed that characteristic element (i.e. N and S element) content increased with increasing bilayers number, the surface LS content increased linearly as a function of bilayers. Zeta potential of modified fibers was inversed after deposition of each layer. AFM phase images indicated that the cellulose microfibrils on fibers surfaces were gradually covered by granular LS aggregate. The antimicrobial testing results demonstrated that CS/LS multilayers modified fibers with CS in the outermost layer exhibited higher antimicrobial activity against Escherichia coli. The antioxidant testing results showed that antioxidant activity of CS/LS multilayers modified fibers was better than that of original fibers under the same oxidation conditions.

Quantitative characterization of phase separation in the photoactive layer of polymer solar cells by the phase image of atomic force microscopy

We have quantitatively characterized the phase separation of poly(3-hexylthiophene) (P3HT):C61-butyric acid methyl ester (PCBM) blend films and studied the effect of phase separation of photoactive layer on the performance of polymer solar cells. A mixed solvent of dichlorobenzene and chlorobenzene was adopted to prepare the P3HT:PCBM blend films, and a series of blend films with different morphologies were obtained by adjusting both the volume ratio of dichlorobenzene to chlorobenzene and the solvent annealing time. The surface morphology and phase distribution were measured by atomic force microscopy (AFM). The interface length between the domains of donor and acceptor, which is extracted from the AFM phase image of blend film, was used to quantitatively characterize the phase separation of photoactive layer. It was found that the short-circuit current density (JSC) of bulk heterojunction solar cells is proportional to the interfacial area of two phases, while it has a negligible effect on the open-circuit voltage. These results indicate that the larger interfacial area of donor and acceptor phases is beneficial to the exciton dissociation and carrier transport resulting in a significant increase of JSC and power conversion efficiency.

Morphological Differences in Short Side Chain and Long Side Chain Perfluorosulfonic Acid Proton Exchange Membranes at Low and High Water Contents

Ionomer membrane materials such as Dupont’s Nafion® (a sulfonated tetrafluoroethylene based fluoropolymer) have long been used as the proton conducting medium in proton exchange membrane (PEM) fuel cells. It has recently been shown that short side chain perfluorosulfonic acid membranes such as Hyflon Ion® developed by Solexis can outperform the benchmark long side chain material, DuPont’s Nafion®. Despite these promising results, a detailed understanding of the relationship between membrane morphology, hydration, and fuel cell performance has yet to be achieved. Using atomic force microscopy (AFM) phase imaging over a wide range of relative humidity and in solution, we compare a Nafion 212 membrane to Hyflon Ion® E87-05 and find evidence of the improved performance in the latter. Hyflon exhibits a higher hydrophilic surface area across all water contents, explaining its higher proton conductivity and fuel cell performance. At high water contents, the surface of Nafion® undergoes significant structural rearrangement and forms large rod-like features parallel to the surface, whereas the Hyflon Ion® membrane undergoes little if any such transition. [see the figure] We show using conductive AFM that these features are insulating, which indicates they most likely consist of a fluorocarbon layer surrounding a hydrophilic core and do not contribute to fuel cell performance. The ability of Hyflon to undergo changes in water content with minimal rearrangement of hydrophilic domains implies an ability to operate more effectively under variable relative humidity.

Encapsulation of 3-iodo-2-propynyl N-butylcarbamate (IPBC) in polystyrene-polycaprolactone (PS/PCL) blends

Polystyrene (PS, 1), polycaprolactone homopolymers (PCL, 2) and 3-Iodo-2-propynyl n-butylcarbamate (IPBC, 3) were physically mixed in dichloromethane (DCM) and processed into solid microspheres by using emulsion solvent evaporation method. Five different compositions with varying PS/PCL ratio were tested. The phase morphology of the microspheres was studied using Phase imaging atomic force microscopy (AFM) of polished cross-sections. Scanning electron microscopy was utilized to assess the distribution of IPBC in the polymer microspheres. The phase separation of the PS and PCL polymers in solvent cast films was assessed using polarized light optical microscopy of 11 polymer blends (0–100 wt-% PCL in PS). The PS/PCL-IPBC microspheres were incubated in water at RT and the release of IPBC was studied using high performance liquid chromatography (HPLC) at time points 1, 7 and 30 days. The microspheres dispersed in water borne outdoor paint matrix were tested for their antifouling activity against moulds in vitro.

In situ Determination and Imaging of Physical Properties of Soft Organic Materials by Analytical Transmission Electron Microscopy

Analytical transmission electron microscopy (ATEM) offers great flexibility in identification of the structural-chemical organization of soft materials at the level of individual macromolecules. However, the determination of mechanical characteristics such as hardness/elasticity of the amorphous and polycrystalline organic substances by ATEM has been problematic so far. Here, we show that energy filtered TEM (EFTEM) measurements enable direct identification and study of mechanical properties in complex (bio-)polymer systems of relevance for different industrial and (bio-)medical applications. We experimentally demonstrate strong correlations between hardness/elasticity of different polymers (polycaprolactone, polylactid, polyethelene, etc.) and their volume plasmon energy. Thickness and anisotropy effects, which substantially mask the material contrast in EFTEM bulk plasmon images, can be adequately removed by normalizing the latter by carbon elemental map. EFTEM data has been validated using atomic force microscopy phase images, where phase shift related to the hardness and elastic modulus of the materials.

A zoom into the nanoscale texture of secondary cell walls

BackgroundBesides classical utilization of wood and paper, lignocellulosic biomass has become increasingly important with regard to biorefinery, biofuel production and novel biomaterials. For these new applications the macromolecular assembly of cell walls is of utmost importance and therefore further insights into the arrangement of the molecules on the nanolevel have to be gained. Cell wall recalcitrance against enzymatic degradation is one of the key issues, since an efficient degradation of lignocellulosic plant material is probably the most crucial step in plant conversion to energy. A limiting factor for in-depth analysis is that high resolution characterization techniques provide structural but hardly chemical information (e.g. Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM)), while chemical characterization leads to a disassembly of the cell wall components or does not reach the required nanoscale resolution (Fourier Tranform Infrared Spectroscopy (FT-IR), Raman Spectroscopy).ResultsHere we use for the first time Scanning Near-Field Optical Microscopy (SNOM in reflection mode) on secondary plant cell walls and reveal a segmented circumferential nanostructure. This pattern in the 100 nm range was found in the secondary cell walls of a softwood (spruce), a hardwood (beech) and a grass (bamboo) and is thus concluded to be consistent among various plant species. As the nanostructural pattern is not visible in classical AFM height and phase images it is proven that the contrast is not due to changes in surfaces topography, but due to differences in the molecular structure.ConclusionsComparative analysis of model substances of casted cellulose nanocrystals and spin coated lignin indicate, that the SNOM signal is clearly influenced by changes in lignin distribution or composition. Therefore and based on the known interaction of lignin and visible light (e.g. fluorescence and resonance effects), we assume the elucidated nanoscale structure to reflect variations in lignification within the secondary cell wall.

Investigation of Thermal Instability of Additive-Based High-Efficiency Organic Photovoltaics

Most photovoltaics operate at high temperature under sunlight. In this work, the thermal instability of diiodooctane-based high-efficiency bulk heterojunction (BHJ) organic photovoltaics (OPVs) is studied. The BHJ layers were heated to various temperatures to investigate the changes in their physical properties using atomic force microscopy phase images. The mobilities of the carriers were characterized at various temperatures using the space-charge-limited current method, and the carrier lifetime was calculated by applying impedance spectroscopy to the simulated equivalent circuit of the OPV devices.

Solubilization of Genistein by Caseinate Micellar System

This study investigates the aggregation behavior of caseinate and the solubilization of genistein in aqueous caseinate solution. The critical aggregation concentration (CAC) of caseinate was obtained from the fluorescence intensity of 8-anilino-1-naphthalenesulfonic acid (ANS), which was enhanced by ANS-protein interactions and the hydrophobicity of caseinate. The increasing solubility of genistein in caseinate was confirmed by HPLC measurements; above and below the CAC, the genistein/caseinate molar ratio is 1:1 and 10:1, respectively. The latter ratio indicates that more caseinate molecules surround genistein below the CAC. However, the solubility of genistein in caseinate is unaffected by calcium ions. Atomic force microscopy (AFM) shows that casein sub-micelles are similarly structured in the presence and absence of genistein. In AFM phase images, the caseinate sub-micelle is brightened in the presence of genistein, implying that the particle becomes more rigid, probably because genistein attaches to the surface or to the narrow part of the sub-micelle. The diameter of sub-micelle aggregates is two times that of caseinate alone (24 nm versus 12 nm). These results were confirmed by cryo-TEM observations.

Enhancement of Compatibility between Ultrahigh-Molecular-Weight Polyethylene Particles and Butadiene-Nitrile Rubber Matrix with Nanoscale Ceramic Particles and Characterization of Evolving Layer

This article examines the modification of surface properties of ultrahigh-molecular-weight polyethylene (UHMWPE) with nanoscale ceramic particles to fabricate an improved composite with butadiene-nitrile rubber (BNR). Adhesion force data showed that ceramic zeolite particles on the surface of UHMWPE modulated the surface state of the polymer and increased its compatibility with BNR. Atomic force microscopy phase images showed that UHMWPE made up the microphase around the zeolite particles and formed the evolving layer with a complex interface. The complex interface resulted in improvements in the mechanical properties of the composite, especially its low-temperature resistance coefficients, thereby improving its performance in low-temperature applications.

Tapping Mode Atomic Force Microscopy (TMAFM) of Some Multi-Component Polymers

Atomic force microscopy (AFM) has been used frequently in polymer research in particular for imaging topography and phase morphology of multi-component polymers. In this work, we demonstrate the potential applications of the AFM in the study of morphology of multi-component polymers taking examples of some technically important semicrystalline polymers, blends and nanostructured block copolymers. The morphology of semicrystalline morphology could be determined ranging from molecular arrangement in the unit cells to the lamellar structure to the macroscopic morphology showing the spherulites of the polymers. Nanoscale morphology of block copolymers, nanocomposites and blends could be easily accessed by the aping mode AFM (TMAFM) phase imaging technique. It has been demonstrated that TMAFM phase imaging can be successfully utilized as a routine tool for the investigation of nanoscale morphology of the heterogeneous polymers.DOI: http://dx.doi.org/10.3126/jncs.v29i0.9258Journal of Nepal Chemical Society Vol. 29, 2012 Page: 96-103 Uploaded date: 12/5/2013

Fiber Modification of Unbleached Kraft Pulp with Laccase in the Presence of Ferulic Acid

Unbleached kraft pulp fibers were modified with laccase and ferulic acid (FRC) to improve their physical strength properties in paper products. The optimal conditions of laccase-FRC modification were examined in terms of the physical properties of pulps. The effects of laccase-FRC modification on the carboxyl group content and surface lignin content of pulps were investigated. The surface morphologies of laccase-FRC-modified pulp fibers were observed by atomic force microscopy (AFM). The carboxyl group and surface lignin contents for laccase-FRC-modified pulps increased compared to the control pulp. AFM phase images showed that the laccase-FRC-modified fiber surfaces were covered with large granular substances from the products of FRC grafting and lignin polymerization/condensation reactions. The observed strength improvements of laccase-FRC-modified pulp could be attributed to the grafting of FRC onto the fibers, the higher carboxyl group content of the modified fibers, and the formation of covalent bonding between the fibers via radical coupling.

SELF-ASSEMBLING BEHAVIOR AND INHIBITION EFFECT OF DODECANETHIOL SELF-ASSEMBLED MONOLAYERS ON COPPER SURFACE

The self-assembling behavior and inhibition effect of dodecanethiol self-assembled monolayers (SAMs) on copper surface were investigated by atomic force microscopy (AFM) and electrochemical methods. The assembling process was monitored by AFM phase images. The assembling time influences the corrosion protection efficiency of dodecanethiol SAMs. Surface friction significantly decreases when the copper surface is covered by SAMs.

Morphological characteristics of fulvic acid fractions observed by atomic force microscopy

Structural studies on fulvic acids (FAs) are significantly important since they are believed to be involved in many environmentally important processes, such as adsorption and transportation of organic and inorganic pollutants. In this research, morphology characteristics of FAs were studied by atomic force microscopy (AFM). FAs that were isolated from three soil layers (A1, B and C) of the same vertical profile in a Korean pine forest were divided into four fractions (FA-1, FA-2, FA-3 and FA-4) by a sequence of successive elution processes. Most of FAs appeared as a platy particle in the AFM topographic and phase images. Among these platy particles, some have a regular shape, such as round flake and oblong flake; others have irregular structures, such as sand heaps. Particle morphologies of different FA fractions, including hydrophilic and hydrophobic FAs fractions, were similar. However, particle sizes and distributions of FA fractions from different soil layers at the same vertical profile did differ. Particle sizes of hydrophobic FAs were relevant with respect to the soil depth. They were increased with the increasing of the soil depth. FAs from C layers were more heterogeneous with respect to the A1 and B. Our results may foster a better understanding for the relevance between the morphology of FA particles with the soil layers and the soil depth.

Wetting Behavior of Ionic Liquid on Mesoporous Titanium Dioxide Surface by Atomic Force Microscopy

Ionic liquids based on 1-butyl-3-methylimidazolium hexafluoro-phosphate (ILs [Bmim][PF6]) has been employed to wet the mesoporous and dense titanium dioxide (TiO2) films. It has been found from atomic force microscopy (AFM) analysis that ILs [Bmim][PF6] can form a wetting phase on mesoporous TiO2 films, but nonwetting and sphere shaped droplets on dense films. AFM topography, phase images, and adhesion measurements suggest a remarkable dependence of wetting ILs [Bmim][PF6] films on the TiO2 porous geometry. On mesoporous TiO2 films, the adhesive force of ILs [Bmim][PF6] reaches at 40 nN, but only 4 nN on dense TiO2 films. The weak interacting ILs [Bmim][PF6] on dense TiO2 films forms rounded liquid spheres (contact angle as 40°), which helps to reduce friction locally but not on the whole surface. The stronger adhesive force on mesoporous TiO2 films makes ILs [Bmim][PF6] adhere to the surface tightly (contact angle as 5°), and this feature remains after five months. The stable spreading ILs [Bmim][PF6] films provide low friction coefficient (0.0025), large wetting areas, and short CO2 diffusion distance on the whole mesoporous TiO2 surface, avoiding the significant decelerating effect through equilibrium limitations to enable CO2 capture rate up to 1.6 and 10 times faster than that on dense TiO2 and pure ILs, respectively. And importantly, ILs wetted on mesoporous TiO2 shorten the time reaching to the maximum adsorption rate (2.8 min), faster than that on mesoporous TiO2 (6.1 min), and dense TiO2 (11.2 min). This work provides an important guidance for the improvement of the efficiency of CO2 capture, gas separation, and the lubrication of micro/nanoelectromechanical systems (M/NEMs).

Surface Morphology of Nafion at Hydrated and Dehydrated Conditions

The relation between proton exchange membrane (PEM) hydration and fuel cell performance has been well documented at the macroscopic scale. Understanding how changes in membrane water content affect the organization of proton conducting domains at the micrometer and submicrometer scales is a sought-after goal in the rational design of higher performing PEMs. Using atomic force microscopy phase and current imaging, we have resolved proton conducting domains at the surface of Nafion membranes at dehydrated, ambient, and hydrated conditions, observing a unique morphology at each membrane water content. At ambient conditions, Nafion’s surface morphology resembles that proposed in the parallel-pore and bicontinuous network models, with the exception that hydrophilic domains are larger at the surface of Nafion compared to the bulk. At hydrated conditions, a network of wormlike, insulating domains extends several micrometers over Nafion’s surface with more conductive, water-rich regions found between these fibrillar features. Neither the surface morphology observed at ambient conditions nor at hydrated conditions persists in dehydrated membranes, which instead exhibit a low coverage of isolated hydrophilic surface domains that, remarkably, are similar in size to such domains at ambient conditions. These observations affirm properties distinct to Nafion’s surface and provide morphological evidence for the low conductivity observed in Nafion at dehydrated conditions and the high conductivity observed at hydrated conditions.

Compatibility of hydrazine and EPDM rubber containing polybutadiene coagent

Histogram analyses of atomic force microscopy (AFM) phase images demonstrated changes in the surface composition of ethylene propylene diene–modified (EPDM) elastomeric formulations containing polybutadiene after exposure to hydrazine. It was determined by Fourier-transform infrared (FT-IR) spectroscopic analysis that the change in the surface composition is caused by hydrogenation of the vinylic double bonds of the polybutadiene coagent, stemming from diimide generation from hydrazine. The diffusion and subsequent reaction of hydrazine with two cured EPDM elastomeric formulations were investigated by mapping FT-IR spectroscopy. The depth of diffusion and reaction was determined by monitoring changes in the intensity of the vinyl group signal from the polybutadiene in the FT-IR spectra. The FT-IR analysis of horizontal microtome specimens and vertical ‘cross section’ of the EPDM samples showed that hydrazine diffused less than 200 μm into the elastomeric materials over a period of 40 weeks. Sequential proton nuclear magnetic resonance spectroscopy of the polybutadiene coagent showed that the vinyl groups in polybutadiene were hydrogenated when mixed with hydrazine at room temperature. The reaction of hydrazine with the polybutadiene coagent of the EPDM elastomeric material was corroborated by matrix-assisted laser desorption/ionization time of flight mass spectrometry as well. A model compound, 4-vinyl cyclohexene, was used to assess the relative hydrogenation rate of primary and secondary double bonds using gas chromatography/mass spectrometry.

Morphological differences in short side chain and long side chain perfluorosulfonic acid proton exchange membranes at low and high water contents

Short side chain (SSC) perfluorosulfonic acid membranes such as Hyflon Ion® have shown improved proton conductivity and the ability to outperform the benchmark long side chain (LSC) material, Nafion®. Using atomic force microscopy phase imaging over a wide range of relative humidity, we compare the morphology of Nafion to Hyflon and find evidence for the improved performance in the latter. While the surface of Nafion undergoes significant structural rearrangement and forms large worm-like features at high water contents, Hyflon undergoes little, if any, transition. We also provide evidence of improved water retention in Hyflon at low water contents, explaining the improved conductivity under these conditions. Liquid phase imaging in water and methanol solutions was used to probe the extremes of swelling in both membranes and conductive AFM was used to characterize electrochemical activity at the membrane surface.