Pulsed-force-mode Atomic Force Microscopy Research Articles

Probing Ternary Solvent Effect in High Voc Polymer Solar Cells Using Advanced AFM Techniques

This work describes a simple method to develop a high V(oc) low band gap PSCs. In addition, two new atomic force microscopy (AFM)-based nanoscale characterization techniques to study the surface morphology and physical properties of the structured active layer are introduced. With the help of ternary solvent processing of the active layer and C60 buffer layer, a bulk heterojunction PSC with V(oc) more than 0.9 V and conversion efficiency 7.5% is developed. In order to understand the fundamental properties of the materials ruling the performance of the PSCs tested, AFM-based nanoscale characterization techniques including Pulsed-Force-Mode AFM (PFM-AFM) and Mode-Synthesizing AFM (MSAFM) are introduced. Interestingly, MSAFM exhibits high sensitivity for direct visualization of the donor-acceptor phases in the active layer of the PSCs. Finally, conductive-AFM (cAFM) studies reveal local variations in conductivity in the donor and acceptor phases as well as a significant increase in photocurrent in the PTB7:ICBA sample obtained with the ternary solvent processing.

Pulsed-Force-Mode AFM Studies of Polyphenylene Dendrimers on Self-Assembled Monolayers

Pulsed-force-mode atomic force microscopy (PFM-AFM) with a chemically modified tip was employed to measure the topography and adhesion force images of homoaggregates of fourth generation polyphenylene and carboxylic-acid-functionalized second generation polyphenylene dendrimers on hydrophilic self-assembled monolayers (SAMs). Although from the AFM topographic image the dendrimers could not be discriminated, from the adhesion image, the respective homoaggregates were easily discriminated. The determination is based on the different adhesive interactions between the dendrimers and the chemically modified tip, which are related to the chemical nature of the outer-surface functional groups, and the adsorbed water layer on hydrophilic surfaces under ambient conditions. It shows that PFM-AFM with chemically modified tips has nanoscale chemical spatial resolution.

Influence of the viscosity and the substrate on the surface hydrophobicity of polyurethane coatings

Tailor-made polyurethane (PU) dispersions were synthesized as coatings for paperboard for dry food packaging. For this purpose a low moisture-vapor transmission rate and a high surface hydrophobicity are desirable characteristics, which are both met by PU. However, it was found that the surface hydrophobicity of water-borne PU dispersions depends strongly on the viscosity of the dispersion. This dependency was studied by static contact angle measurements (SCA) as well as a novel technique using digital pulsed-force mode atomic force microscopy (DPFM-AFM). Comparison of the results validated that DPFM-AFM is a valuable tool to characterize the surface hydrophilicity. Both techniques confirmed that the surface hydrophobicity varies with the viscosity and that an optimum viscosity for the PU coating with a maximum surface hydrophobicity can consequently be determined. It was found that both lower as well as higher viscosities led to a less hydrophobic surface.

A review and comparative study of release coatings for optimised abhesion in resin transfer moulding applications

In this study, a number of abhesion-promoting coatings were considered in terms of their physicochemical and release properties. The techniques used to further this study include; field emission gun scanning electron microscopy, atomic force microscopy (AFM), profilometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy, static secondary ion mass spectrometry, Fourier transform infra-red analysis and contact angle analysis for coating physical and chemical characterisation along with pulsed force mode atomic force microscopy (PF-AFM) and other adhesion and mechanical tests to determine surface release properties. These coatings were applied to metal substrates and were based upon silicone, fluoropolymer or metal–PTFE composite chemistry, all being potentially useful as release films for resin transfer moulding applications. The semi-permanent Frekote B15/710 NC mould release coating system, which is based on PDMS, proved extremely effective in terms of release against a cured epoxide applied under pressure. Although fluoroalkylsilane coatings offer a number of technological advantages for release applications, they generally produce very thin coatings which conform to any existing surface topography and adhesion through mechanical interlocking. The commercial PTFE-based coatings were found to provide poor release properties due to the presence of surface microcracks which allowed epoxide penetration when cured under elevated pressure and temperature. Electroless Ni/PTFE composite coatings comprise a hard nickel–phosphorus matrix containing a very fine dispersion of PTFE particles. The matrix proved sufficiently robust for industrial applications and the low friction and surface energy provided by the embedded PTFE combined with macroscopic-scale surface roughness provided efficient mould release.

Polymer Nanowire Elastic Moduli Measured with Digital Pulsed Force Mode AFM

The mechanical bending behavior of polymer nanowires-polypyrrole and poly(3,4-ethylene dioxythiophene-co-styrene sulfonate)-produced by template molding were measured using a new innovation in atomic force microscopy (AFM). Digital pulsed force mode (DPFM) was used to image and simultaneously perform three-point bend tests along nanowires spanning microchannels in silicon. The bending profiles were analyzed for apparent elastic moduli variations along the suspended length of individually isolated nanowires and compared to classic beam deflection models for various geometric and boundary conditions. The elastic moduli calculated from these AFM data are 2-7 times that expected for bulk polymer values (approximately 1-3 GPa), demonstrating an apparent strengthening of nanostructured polymer even for diameters greater than 100 nm--the accepted boundary for nanoscience. Furthermore, detailed analysis of deflection data versus loading location demonstrates the experimental dependence on test geometry and inherent errors in relying solely on midpoint bending measurements or any single loading configuration for nanomechanical testing as well as the significant contribution of nanoindentation effects.

Effects of Chain Length on Adhesive Force between Gold Tip and Gold Substrate Covered with Alkanethiol Self-Assembled Monolayers

The effects of chain length on adhesive force between a gold-coated tip and a Au(111) substrate covered with alkanethiol self-assembled monolayers (SAMs) were studied. By microcontact printing (µ-CP), we prepared a patterned surface covered with two types of n-alkanethiol that have the same CH3-terminal functional group but different chain lengths. As a method for mapping adhesive force, pulsed-force-mode atomic force microscopy (PFM-AFM) was used. PFM-AFM enables the simultaneous imaging of surface topography and adhesive force. Regardless the use of the same CH3-terminal functional group to modify a surface, a difference in adhesive force corresponding to the printed pattern was observed. The adhesive force on a printed region covered with a longer alkanethiol SAM was slightly smaller than that of an unprinted region covered with a shorter alkanethiol SAM chemisorbed from solution after µ-CP. The possible molecular mechanisms for the difference in the observed adhesive force were discussed in detail.

Topographic effects on adhesive force mapping of stretched DNA molecules by pulsed-force-mode atomic force microscopy

Adhesive interaction between a tip and a sample surface was examined on a microscopic scale by pulsed-force-mode atomic force microscopy (PFM-AFM). The signal measured by monitoring pull-off force is influenced by various factors such as topography, elasticity, electrostatic charges, and adsorbed water on surfaces. Here, we focus on the topographic effects on the adhesive interaction. To clarify the topographic influence, the adhesive force measurement of a stretched DNA molecule with a smaller radius of curvature than that of a tip was carried out at low relative humidity (RH) with an alkanethiol-modified tip. The experimental conditions such as low RH and the use of the alkanethiol-modified tip were required to minimise the influence of water capillary force on hydrated DNA strands. The hydrophobic modification of a substrate surface was also important to minimise the adsorbed water effect. The DNA molecules were stretched on the substrate surfaces by an immobilisation process called a dynamic molecular combing method. The two-component vapour-phase surface modification with an alkylsilane mixed with a silane derivative containing an amino end group enhanced the DNA adsorption due to the electrostatic interaction. The experimental results for the topographic effects on the adhesive force mapping were reproducible.

Imaging stretched single DNA molecules by pulsed-force-mode atomic force microscopy

The effect of a surface water layer on DNA strands deposited on a substrate was studied by atomic force microscopy (AFM). DNA molecules were deposited and stretched on chemically modified glass coverslips by a molecular combing method. Lambda bacteriophage DNA molecules were aligned on the organosilane-modified substrate surfaces by chemical and physical adsorption during the molecular combing. The combed DNA molecules were observed in humidity-controlled air and in aqueous solutions by pulsed-force-mode AFM (PFM-AFM). Chemical modification of cantilevers with an Au-coated tip by organothiol compounds was also applied to DNA observation. Mapping adhesive forces in aqueous media was useful to discriminate chemically the DNA strands from the substrate surface. The results suggest that PFM-AFM can be used widely to image the stretched DNA molecules on the silane-modified substrates.

Chemically Selective Force Mapping of Electrochemically Generated Two-Component ω-Substituted Alkanethiol Monolayer Gradients by Pulsed-Force-Mode Atomic Force Microscopy

Pulsed-force-mode atomic force microscopy (PFM-AFM) was used to map the adhesion force between an AFM tip and samples prepared by microcontact printing (μCP) and electrochemically generated ω-substituted alkanethiol monolayer gradients. AFM tips chemically derivatized to terminate with either or -COOH end functional groups were used to measure the adhesion force difference between and COOH-terminated regions on a μCP sample in both air and deionized Images acquired in air with a -terminated tip yielded mean adhesion force differences between and COOH-terminated sample regions, of 7.5 ± 4.8 nN for tips prepared from alkanethiols on Au and 11.1 ± 3.0 nN for tips prepared from alkylsiloxanes on Si. The variability in was significantly reduced in deionized PFM-AFM was also able to distinguish and COOH-terminated regions in an electrochemically generated gradient in air and deionized The contrast of the image and adhesion force differences measured on electrochemically generated gradients in were comparable to those measured on μCP samples. PFM-AFM images of electrochemically generated gradients indicated a transition region ∼75 mV wide in rough correspondence to the 60 mV width of the Fermi-Dirac distribution at 300 K. © 2002 The Electrochemical Society. All rights reserved.

Chemical force microscopy of self-assembled monolayers on sputtered gold films patterned by phase separation

Patterned self-assembled monolayers (SAMs) were formed on gold films and observed by friction force microscopy (FFM) and adhesive force mapping with pulsed-force mode atomic force microscopy (PFM–AFM). The substrate gold films were prepared by sputtering gold on flat surfaces of osmium-coated cover glass with surface roughness, R a, of 0.3 nm. The patterned samples with the CH 3 and COOH terminated regions were prepared using the Langmuir–Blodgett (LB) method, partial removal of the LB film by ultrasonication, and SAM formation. The CH 3 and COOH terminated regions of the patterned SAMs in air and in water were observed by mapping friction and adhesive forces with FFM and PFM–AFM, respectively, using gold-coated AFM tips chemically modified with a thiol compound terminating in CH 3 or COOH. The adhesive forces measured in air increased in the order of CH 3/CH 3, CH 3/COOH (or COOH/CH 3), and COOH/COOH, while those in water increased in reverse order. The enormous high adhesive force observed in water for CH 3/CH 3 was attributed to hydrophobic interaction between the CH 3 tip and the CH 3 terminated sample surface. With CH 3 tip, the lower friction force was observed, however, in water on the CH 3 terminated region than on the COOH terminated region. This experimental finding raises a question as to what is the effective normal load in friction measurements in water.

Chemical force microscopy of CH 3 and COOH terminal groups in mixed self-assembled monolayers by pulsed-force-mode atomic force microscopy

Pulsed-force-mode atomic force microscopy (PFM-AFM), with functionalized probe tips, was applied to discrimination of chemical functionalities of a binary system of mixed self-assembled monolayers (SAMs) consisting of CH 3- and COOH-terminating alkane thiols. PFM-AFM enabled simultaneous imaging surface topography and distribution of adhesive forces between the tip and sample surfaces. Since the adhesive forces were directly related to interaction between the chemical functional groups on the tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with the chemically modified tips to accomplish imaging the sample surface with the chemical sensitivity. The adhesive force mapping by PFM-AFM with the CH 3-modified tips in pure water clearly discriminated the hydrophobic CH 3-terminating domains embedded in the COOH-terminating SAM matrix.

Chemical force microscopy of microcontact-printed self-assembled monolayers by pulsed-force-mode atomic force microscopy

A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH 3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH 3-terminated regions prepared by microcontact printing (μCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.

Mapping of electrical double-layer force between tip and sample surfaces in water with pulsed-force-mode atomic force microscopy

Pulsed-force-mode atomic force microscopy (PFM-AFM) using a cantilever with a Si3N4 tip was applied to map charge distribution on a sample surface in water. In order to confirm the applicability of the present PFM, we prepared a patterned sample by vapor deposition of Al on a quartz plate covered with silica beads, followed by oxidation of Al with O2 and removal of the beads with ultrasonication. The two different areas of Al2O3 and SiO2 had different isoelectric points and bore positive and negative charge, respectively, at pH 8.6. The lateral resolution of the present method was found to be ca. 30 nm.