Nanoscale electrowetting effects observed by using friction force microscopy

We report the study of electrowetting (EW) effects under strong electric field on poly(methyl methacrylate) (PMMA) surface by using friction force microscopy (FFM). The friction force dependence on the electric field at nanometer scale can be closely related to electrowetting process based on the fact that at this scale frictional behavior is highly affected by capillary phenomena. By measuring the frictional signal between a conductive atomic force microscopy (AFM) tip and the PMMA surface, the ideal EW region (Young-Lippmann equation) and the EW saturation were identified. The change in the interfacial contact between the tip and the PMMA surface with the electric field strength is closely associated with the transition from the ideal EW region to the EW saturation. In addition, a reduction of the friction coefficient was observed when increasing the applied electric field in the ideal EW region.     

Electric-field oriented polymer blend film for proton conduction

A novel proton conductor has been realized by employing a ternary polymer blend system in combination with an electric-field orientation technique. A polymer film recast from a solution containing poly(acrylic acid), poly(vinyl butyral), and fluoroalkyl graft polymer under 2 kV.cm(-1) exhibited 10 times higher proton conductivity than that prepared without the external electric field. However, when the film was prepared under a field higher than 4 kV.cm(-1), proton conductivity decreased. The membranous character has been investigated by SEM and AFM observations. As a result, it has been proven that an electric-field treatment of 2 kV.cm(-1) forms the largest hydrophilic domains for proton conduction in the film. The alteration of the phase separation morphology induced by the electric field well explains the proton conductivity change.     

TiO2纳米粒子膜的制备、表面态性质和光催化活性

在酸性和碱性条件下,用TiCl4水解法制备了TiO2纳米粒子膜催化剂.采用原子力显微镜(AFM),X射线衍射(XRD),表面光电压谱(SPS)和场诱导表面光电压谱(EFISPS)测定了催化剂表面的微结构及能级结构.对催化剂进行了光催化降解苯酚实验,测定了其光催化活性.结果表明,酸性条件下制备的TiO2膜催化剂的光催化活性较高,其结果接近于P25.用能带理论解释了TiO2纳米粒子膜催化剂光催化活性的差异,分析了膜厚对光催化活性的影响.     

Chemical force titrations of amine- and sulfonic acid-modified poly(dimethylsiloxane)

Chemical force titrations-measurements of the adhesive interaction between a pair of suitably chemically modified atomic force microscopy (AFM) tip and sample surfaces as a function of pH-have been carried out for various combinations of silanol, amine, carboxylic acid, and sulfonic acid functional groups on both tip and sample. The primary surface material studied was poly(dimethylsiloxane) (PDMS). Surface modification was carried out using a plasma oxidation process to form silanol sites; further modification with amine or sulfonic acid sites was carried out by reaction of the silanol sites with the appropriate trialkoxysilane derivative. AFM tips were also modified using trialkoxysilane compounds. In the cases of tip/sample combinations with the same functional group on each, surface pK(1/2) values could be determined. In several "mixed" tip/sample combinations, a peak appeared in the titration curve midway between the surface pK(1/2) values of the tip and sample, consistent with an ionic H-bonding model for the interactions. The amine/sulfonic acid pair showed more complex behavior; the amine-terminated tip/sulfonic acid-terminated PDMS surface force titration curve consisted of two peaks centered at pH 4 and pH 8. Reversing the tip/sample pair resulted in the peak positions being shifted upward by 1.0 pH unit. The peak appearing at lower pH is assigned to electrostatic interactions between the two oppositely charged surfaces, whereas the higher pH peak is believed to arise due to ionic H-bonding interactions. AFM images show the effects on surface patterning of amine- and sulfonic acid-modified PDMS surfaces that have undergone two different oxidation methods (air plasma oxidation and Tesla coil oxidation). The surface morphologies of freshly prepared and 24 h aged air plasma oxidized PDMS are also discussed in this study.     

Latex film morphology and electrical potential pattern dependence on serum components: a scanning probe microscopy study

Dry films formed by surfactant-stabilized, peroxide-initiated styrene-butyl acrylate latex were examined by atomic and electric force microscopy (AFM and EFM). The effects of latex serum components on the films were observed by subjecting the latex to extensive dialysis prior to film formation, and comparing the results to as-prepared latex. The films formed with the dialyzed latex are smoother (as evidenced by roughness and fractal dimension measurements) than the films from the as-prepared latex, but they display large electric force gradients between neighboring domains. The films made with the as-prepared latex have the highest electric uniformity, with a maximum potential variation lower than 80 mV, while this reaches 200 mV in the dialyzed latex film.     

利用铁电薄膜研究体异质结型有机光伏器件的光电流极性

制备了铌镁酸铅-钛酸铅(PMN-PT)铁电薄膜,并通过紫外-可见(UV-Vis)透射光谱、X射线衍射(XRD)和原子力显微镜(AFM)对其进行了表征.为了研究体异质结型有机共混膜的光电流特性,制作了氧化铟锡(ITO)/PMN-PT/有机共混膜/铝(Al)的光伏器件,调制激光照射下外加偏压的极性和大小变化将直接改变瞬态光电流的极性和大小,从而可在实验上证明传统体异质结型有机光伏器件的光电流极性是由器件阴、阳电极的功函数差所导致的内建电场的方向决定.同时也提出了一种利用铁电薄膜来研究体异质结型有机光伏器件光电流特性的新方法.     

New SFA techniques for studying surface forces and thin film patterns induced by electric fields

We describe two ways to measure normal and/or lateral forces between two surfaces in a surface forces apparatus (SFA) while an electric field is applied between the surfaces. The first method involves depositing thin conductive layers on the exposed substrate (usually mica) sheets; the second involves using the optically reflecting silver layers on the back surfaces of the sheets as the electrodes. Two types of experiments were performed using these new techniques: (1) measuring the effects of an electric field on the rheology of an approximately 40-microm-thick film of zeolite particles suspended in silicone oil and (2) a dynamic study of electric field-induced pattern formation of a thin polymer film. In the first study, under an electric field of strength approximately 106 V/m the shear force or effective viscosity of the colloid suspension was found to be two orders of magnitude higher than in the absence of the field, when the expected bulk value was measured. In the dynamic study, the initially uniform film transformed into a 2-D honeycombed network of depressed cells bounded by elevated ridges that grew slowly with time in a way consistent with previously derived theories. The new techniques should be applicable to studies of other systems and interactions, such as double-layer forces, micro- and nanoelectrorheology, electric field-induced ordering of particles, and the effects of electric fields on adhesion, friction, and lubrication.     

Combined atomic force microscopy and fluorescence correlation spectroscopy measurements to study the dynamical structure of interfacial fluids

We have studied the dynamic structure of thin (approximately a few nanometers) liquid films of a nearly spherical, nonpolar molecule tetrakis(2-ethylhexoxy)silane (TEHOS) by using a combination of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS). Ultra-sensitive interferometer-based AFM was used to determine the stiffness (force gradient) and the damping coefficient of the liquid film. The experiments show oscillations in the damping coefficient with a period of approximately 1 nm, which is consistent with the molecular dimension of TEHOS as well as previous X-ray reflectivity measurements. Additionally, we performed FCS experiments for direct determination of the molecular dynamics within the liquid film. From the fluctuation autocorrelation curve, we measured the translational diffusion of the probe molecule embedded within the fluid film formed on a solid substrate. The autocorrelation function was best fitted with two components, which indicate that the dynamics are heterogeneous in nature. However, the heterogeneity is not as pronounced as had been previously observed for molecularly thin liquid films sandwiched between two solid substrates.     

Probing small unilamellar EggPC vesicles on mica surface by atomic force microscopy

Sonicated small unilamellar egg yolk phosphatidylcholine (EggPC) vesicles were investigated using atomic force microscopy (AFM) imaging and force measurements. Three different topographies (convex, planar, and concave shape) of the EggPC vesicles on the mica surface were observed by tapping mode in fluid, respectively. It was found that the topography change of the vesicles could be attributed to the interaction force between the AFM tip and vesicles. Force curves between an AFM tip and an unruptured vesicle were obtained in contact mode. During approach, two breaks corresponding to the abrupt penetration of upper and lower bilayer of vesicle were exhibited in the force curve. Both breaks spanned a distance of around 4 nm close to the EggPC bilayer thickness. Based on Hertz analysis of AFM approach force curves, the Young's modulus (E) and the bending modulus (kc) for pure EggPC vesicles were measured to be (1.97 +/- 0.75) x 10(6)Pa and (0.21 +/- 0.08) x 10(-19)J, respectively. The results show that the AFM can be used to obtain good images of intact and deformed vesicles by tapping mode, as well as to probe the integrity and bilayer structure of the vesicles. AFM force curve compare favorably with other methods to measure mechanical properties of soft samples with higher spatial resolution.     

聚苯胺-TCNQ复合薄膜的微观结构与电学特性

聚苯胺 (ＰＡＮＩ)作为高聚物材料 ,具有稳定性好 ,易于合成等优点．它在化学传感器、显示器、光化学电池等光电器件上有着许多潜在的应用前景．目前导电聚苯胺材料的合成、薄膜的制备与表征正在受到人们的重视．由于聚苯胺很难溶于一般的有机溶剂 ,用化学方法通过聚合物溶液用旋涂或自组装的方法成膜具有很大的局限性 ,特别是不易得到实用化的薄膜产物．而用真空蒸发沉积方法制备聚苯胺薄膜却有成膜质量高、易于控制 ,能很好与电子及微电子加工工艺相接轨等优点［１ -３］．国外已有一些关于真空蒸发沉积聚苯胺薄膜的研究报导［２ -５］,但…     

Direct Electrochemistry of Cytochrome c at Gold Electrode Modified with Fumed Silica

Direct electrochemistry of horse heart cytochrome c (cytc) has been obtained at a gold electrode constructed by self‐assembling fumed silica particles (FSPs) onto a silane monolayer. A pair of well‐defined and nearly symmetrical redox peaks of cytc is obtained at the FSPs film modified gold electrode. Cyclic voltammetry (CV) and tapping‐mode atomic force microscopy (AFM) are used to characterize the FSPs film modified electrode, showing that the FSPs can provide a favorable microenvironment for cytc and facilitate the direct electron transfer between the cytc and the gold electrode, which may propose an approach to realize the direct electrochemistry of other proteins.     

Controlled rearrangement of adsorbed undecanol films on mica surfaces induced by an atomic force microscopy tip

An undecanol film adsorbed on a mica surface was found to rearrange and spread in a position-controlled way induced by a tapping mode atomic force microscopy (AFM) probe. AFM images of varying scanning times showed that before forming an ordered monolayer the undecanol molecules were adsorbed on the mica surface in the disordered and disorganized status. With the proceeding of scanning, these undecanol molecules gradually formed an ordered and flat film. Such behavior was caused by the formation of a stable film and had never been reported for other alcohols.     

The use of nanotechnology to fabricate ultra-high density molecular memory in P(VDF/TRFE) copolymer: data storage

The atomic force microscope (AFM) has been used to investigate the morphological development and the local polarization under a bias voltage of the P(VDF/TrFE) copolymer. “Piezoresponse” and topography images were recorded using the contact mode. Ultra-thin films were used to successfully write memory bits via the local polarization induced by the electric field imposed by the Scanning Probe Microscopy tip between the gold-coated cantilever and the conductive substrate. This technqiue allows the simultaneous and direct observation of both the local ferroelectric polarity and the domain structure (surface topography). These results show the possibility of high data storage in such copolymer films.     

Bulk phase separation study by atomic force microscope friction imaging of natural rubber/poly(methyl methacrylate) film

The first observation of bulk phase separation in immiscible natural rubber (NR)/poly(methyl methacrylate) (PMMA) film using atomic force microscopy (AFM) is reported. Three different forms of AFM measurements: topographic, friction force imaging, and nanoindentation have been effectively used to investigate combined morphological and compositional mapping of the NR/PMMA system. The fracture temperature during sample microtoming and material physical properties could be responsible for the observed topographic contrast. The stronger contrast of friction imaging, relative to topographic imaging, is ascribed to local variations in mechanical properties of the phase-separated domains. Friction force imaging associated with nanoindentation response, performed under AFM force mode, highlights the AFM's ability for probing local friction, adhesion, and elastic properties, and for compositional mapping of heterogeneous polymer film. The resulting friction force imaging along with the response of the nanoindentation are in good agreement, indicating that PMMA exists mainly near the modified NR surface.     

Atomic force microscopy imaging of molybdenum oxide film electrodeposited on a carbon electrode

Non-stoichiometric mixed-valent molybdenum(VI, V) oxide film was grown on carbon substrates by the electrodeposition method. Responses of the prepared molybdenum oxide thin films to potential and to different solution acidities were studied by cyclic voltammetry, and the corresponding morphological changes of the film were monitored by atomic force microscopy (AFM). AFM images of the molybdenum oxide film show that the characteristic domed structure on the film surface increased during the transition from the oxidized state to the reduced state without signification change in the RMS surface roughness value. Furthermore, AFM studies show that the solution acidity has great effect on the morphology of the films, and the films undergo a homogenizing process with increasing pH of the solutions.     

Electrochemically nanopatterned conducting coronas of a conjugated polymer precursor: SPM parameters and polymer composition

Here we describe the formation of precisely controlled corona-type nanopatterns on electroactive polymer precursor films using scanning probe microscopy (SPM) methods. The binary composition of electroactive groups in the polymer triggers the formation of corona-type nanopatterns at particular voltages and tip writing speeds through the electrooxidation of the polymer precursor film. Various parameters such as tip speed and applied bias were explored in the nanopatterning process, and the formation of a conductive nanopattern was investigated using conducting atomic force microscopy (C-AFM). The formation of the nanopattern was attributed to the flow of electrons from the AFM tip to the polymer film in a controlled electric field distribution. We also report a new method to distinguish the polymer composition and distribution of a polymer blend film by characterizing biasing differences in the patterning of a polymer film.     

原子力显微镜研究APS化单晶硅衬底及单层MD膜表面

原子力显微镜研究ＡＰＳ化单晶硅衬底及单层ＭＤ膜表面张希，高芒来，王力彦，沈家骢（吉林大学化学系，长春，１３００２３）关键词原子力显微镜，ＡＰＳ修饰表面，分子沉积膜以静电相互作用为成膜推动力的各种功能体系分子沉积（ＭＤ）超薄膜已有报道［１￣４］．对ＭＤ...     

Electrochemical and atomic force microscopy study of carbon surface modification via diazonium reduction in aqueous and acetonitrile solutions

Electrochemical reduction of the diazonium salts of 4-nitrobenzene and 4-nitroazobenzene-4'- has been investigated in aqueous acid and acetonitrile media at carbon surfaces. Using pyrolyzed photoresist films as the substrate, we have examined the deposited films using electrochemistry and atomic force microscopy (AFM). Film thicknesses were measured by scratching through the film with an AFM tip. The procedure employed two AFM cantilevers with different lengths, located on the one device. When the shorter cantilever engages the surface in tapping mode, the longer cantilever (which is not resonating) imbeds into the surface with a constant force. For both modifiers and modification media, film thicknesses increase with deposition time to a limiting value. With equivalent modification conditions, films prepared in aqueous acid medium have lower limiting thicknesses than those prepared in acetonitrile. For nitrophenyl (NP) films, the same trends are found when calculating surface coverages from the charge associated with the reduction of surface -Ar-NO2 groups. Lower limiting film thicknesses and surface coverages for films prepared in aqueous conditions is attributed to growth of inherently more blocking films and is supported by examination of the response of the Fe(CN)6(3-/4-) couple at NP-modified surfaces. Combination of voltammetrically determined surface coverage and film thickness data yields a surface coverage of -Ar-NO2 groups of (2.5 +/- 0.5) x 10(-10) mol cm(-2) for a film thickness equivalent to a monolayer of NP groups.     

Pressure-induced restructuring of a monolayer film nanojunction produces threshold and power law conduction

The electrical conduction of metal-molecule-metal junctions formed between Au-supported self-assembled monolayers of structurally different 1-hexanethiol, 1-decanethiol, and ferrocenyl-1-undecanethiol and a Pt-coated atomic force microscope (AFM) tip has been measured under different compression forces using conducting-probe AFM. The observed junction resistance had two distinct power law scaling changes with the compression force. Different scaling regions were assigned to the change in the contact area, tunneling distance, number of conduction pathways, and structure of the film under compression.