Non-contact AFM imaging in water using electrically driven cantilever vibration.

An atomic force microscopy (AFM) imaging mode is presented that can simultaneously record surface topography and local electrical properties in aqueous solutions without mechanical contact between the AFM tip and the sample. The interaction between the electrically biased tip and the grounded sample in aqueous medium causes the AFM cantilever to vibrate. This operation mode is based on the previously developed SPFM technique, though using water as the medium instead of air introduces some important practical and theoretical differences, and also greatly extends the applicability of this technique. There are two vibration modes, one at the frequency of the applied voltage (ω) and one at twice this frequency (2ω). The surface topography can be imaged using feedback control of the 2ω vibration amplitude, which is very sensitive to the tip-sample separation distance in the range of 1-10 nm. The amplitude and phase of the 1ω vibration can be recorded simultaneously during imaging to obtain information on local surface charge or potential differences. Similar techniques exist for imaging in air or vacuum, but the addition of a polarizable medium such as water adds significant theoretical and practical complexities. This paper addresses those complexities and demonstrates the effectiveness of the technique for surface imaging and analysis in aqueous environments.

Hydrophobic but hygroscopic polymer films--identifying interfacial species and understanding water ingress behavior.

The hydrophobic but hygroscopic nature of polydimethylsiloxane (PDMS) with quaternary ammonium cationic side chains adsorbed on a SiO(2) surface was investigated with sum frequency generation vibration spectroscopy (SFG) and attenuated total reflectance infrared spectroscopy (ATR-IR). PDMS with cationic side chains, named cationic polymer lubricant (CPL), forms a self-healing boundary lubrication film on SiO(2). It is interesting that CPL films are externally hydrophobic but internally hydrophilic. The comparison of SFG and ATR-IR data revealed that the methyl groups of the PDMS backbone are exposed at the film/air interface and the cationic side groups and counterions are embedded within the film. The hydrophobicity must originate from the surface CH(3) groups, while the ionic groups inside the film must be responsible for water uptake. The surface hydrophobicity can alleviate the capillary adhesion while the hygroscopic property enhances the mobility and self-healing capability of the CPL boundary lubrication film.

Understanding run-in behavior of diamond-like carbon friction and preventing diamond-like carbon wear in humid air.

The friction behavior of diamond-like carbon (DLC) is very sensitive to the test environment. For hydrogen-rich DLC tested in dry argon and hydrogen, there was always an induction period, so-called "run-in" period, during which the friction coefficient was high and gradually decreased before DLC showed an ultralow friction coefficient (less than 0.01) behavior. Regardless of friction coefficients and hydrogen contents, small amounts of wear were observed in dry argon, hydrogen, oxygen, and humid argon environments. Surprisingly, there were no wear or rubbing scar on DLC surfaces tested in n-pentanol vapor conditions, although the friction coefficient was relatively high among the five test environments. Ex situ X-ray photoelectron and near-edge X-ray absorption fine-structure spectroscopy analyses failed to reveal any differences in chemical composition attributable to the environment dependence of DLC friction and wear. The failure of getting chemical information of oxygenated surface species from the ex situ analysis was found to be due to facile oxidation of the DLC surface upon exposure to air. The removal or wear of this surface oxide layer is responsible for the run-in behavior of DLC. It was discovered that the alcohol vapor can also prevent the oxidized DLC surface from wear in humid air conditions.

Identification of mobile species in cationic polymer lubricant layer on silicon oxide from AFM and XPS analyses.

The nanoscale spreading of a cationic polymer lubricant (CPL) film consisting of polydimethylsiloxane with quaternary ammonium salt side chains on a SiO(2) surface was studied with the disjoining pressure measurements using atomic force microscopy. CPL shows a monotonic decrease in disjoining pressure as the film thickness increases from 1.3 to 4.5 nm, which suggests stable spreading in this thickness range. Comparing the spreading rates calculated from disjoining pressure and the viscosity of CLP to the self-healing time after tribo-contacts revealed that the ionic form may not be the main mobile species. The X-ray photoelectron spectroscopy analysis found that the CPL film on SiO(2) has about 30% of the quaternary ammonium salts (cationic groups) reduced to tertiary amines (neutral groups). The reduced CPL polymer has much lower viscosity than the original CPL polymer and yields a spreading rate consistent with that measured at the macroscale. Thus, the mobile component in the CPL/SiO(2) film responsible for self-healing is concluded to be the reduced tertiary amine components of CPL.

One-step production of superhydrophobic coatings on flat substrates via atmospheric Rf plasma process using non-fluorinated hydrocarbons.

This paper describes the direct deposition of hydrocarbon coatings with a static water contact angle higher than 150 using simple C6 hydrocarbons as a reactive gas in helium plasma generated in ambient air without any preroughening of the silicon (100) substrate. The film morphology and hydrophobicity are found to strongly depend on the structure of the reagent hydrocarbon. The films deposited with n-hexane and cyclohexane exhibited relatively smooth morphology and the water contact angle was only ∼95°, similar to polypropylene. When benzene was used as a main reactive gas, the deposited film surface showed nanoscale textured morphology and superhydrophobicity with a water contact angle as high as 167°. Because the plasma is generated in air, all films show some degree of oxygen incorporation. These results imply that the incorporation of a small amount of oxygenated species in hydrocarbon films due to excitation of ambient air is not detrimental for superhydrophobicity, which allows the atmospheric rf plasma with the benzene precursor to produce rough surface topography needed for superhydrophobicity.

Maskless fabrication of nanowells using chemically reactive colloids.

This letter describes the maskless fabrication of nanowells on a silicon substrate using chemically reactive nanoparticles. The amidine-functionalized polystyrene latex (APSL) colloids are adhered onto a silicon wafer, and hydrolysis of the particles' amidine groups generates the ammonium hydroxide etchant locally. The localized release of reactive species and its fast diffusion into the bulk liquid ensure that the silicon etching takes place only under the APSL colloids. Thus, the basal length of the nanowells is precisely controlled by the diameter of the APSL particles. The shape of the nanowells depends on the structure of the substrate: inverted pyramids on silicon (100) and hexagonal pits on silicon (111). The method described here provides an easy, inexpensive, safe, and high-throughput approach for generating nanowells on silicon surfaces. This maskless and simple nanofabrication method will open doors for new applications with locally generated or locally delivered chemistry from nanoparticles.

Highly stable enzyme precipitate coatings and their electrochemical applications.

This paper describes highly stable enzyme precipitate coatings (EPCs) on electrospun polymer nanofibers and carbon nanotubes (CNTs), and their potential applications in the development of highly sensitive biosensors and high-powered biofuel cells. EPCs of glucose oxidase (GOx) were prepared by precipitating GOx molecules in the presence of ammonium sulfate, then cross-linking the precipitated GOx aggregates on covalently attached enzyme molecules on the surface of nanomaterials. EPCs-GOx not only improved enzyme loading, but also retained high enzyme stability. For example, EPC-GOx on CNTs showed a 50 times higher activity per unit weight of CNTs than the conventional approach of covalent attachment, and its initial activity was maintained with negligible loss for 200 days. EPC-GOx on CNTs was entrapped by Nafion to prepare enzyme electrodes for glucose sensors and biofuel cells. The EPC-GOx electrode showed a higher sensitivity and a lower detection limit than an electrode prepared with covalently attached GOx (CA-GOx). The CA-GOx electrode showed an 80% drop in sensitivity after thermal treatment at 50°C for 4 h, while the EPC-GOx electrode maintained its high sensitivity with negligible decrease under the same conditions. The use of EPC-GOx as the anode of a biofuel cell improved the power density, which was also stable even after thermal treatment of the enzyme anode at 50°C. The excellent stability of the EPC-GOx electrode together with its high current output create new potential for the practical applications of enzyme-based glucose sensors and biofuel cells.

Effects of gas adsorption isotherm and liquid contact angle on capillary force for sphere-on-flat and cone-on-flat geometries.

This paper explains the origin of the vapor pressure dependence of the asperity capillary force in vapor environments. A molecular adsorbate layer is readily formed on solid surface in ambient conditions unless the surface energy of the solid is low enough and unfavorable for vapor adsorption. Then, the capillary meniscus formed around the solid asperity contact should be in equilibrium with the adsorbate layer, not with the bare solid surface. A theoretical model incorporating the vapor adsorption isotherm into the solution of the Young-Laplace equation is developed. Two contact geometries--sphere-on-flat and cone-on-flat--are modeled. The calculation results show that the experimentally-observed strong vapor pressure dependence can be explained only when the adsorption isotherm of the vapor on the solid surface is taken into account. The large relative partial pressure dependence mainly comes from the change in the meniscus size due to the presence of the adsorbate layer.

Effects of ionic side groups attached to polydimethylsiloxanes on lubrication of silicon oxide surfaces.

The boundary film formation and lubrication effects of low molecular weight silicone lubricant molecules with cationic side groups were studied. Poly(N,N,N-trimethylamine-3-propylmethylsiloxane-co-dimethylsiloxane) iodide was synthesized and deposited on silicon oxide surfaces to form a bound-and-mobile lubricant film. The bound nature was investigated with ellipsometry, water contact angle, and X-ray photoelectron spectroscopy for the polymers with cationic mole percent of 6, 15, and 30 mol % (monomer based). The bound layer thickness decreased as the cationic content increased. The quaternary ammonium cations in this layer were electrostatically bound to the substrate surface. The mobile nature of the multilayers was explored with scanning polarization force microscopy. The multilayer films exhibited characteristic topographic features due to ionic interactions within the polymer film. Contact scratching of these films altered the multilayer topography within the contact scanned area. Even after high load contact scanning, the bound layer was not removed from the scanned region. These results implied that the molecules in the first layer are strongly bound and the molecules in the multilayers are mobile. Both nanoscale and macroscale tribological tests of these films revealed that the polymer with 15 mol % cationic groups gives lower friction and adhesion than the 6 and 30 mol % cationic polymers as well as the polydimethylsiloxane control sample. This seems to be due to a synergistic effect between the bound and the mobile layers.

Corrected direct force balance method for atomic force microscopy lateral force calibration.

This paper reports corrections and improvements of the previously reported direct force balance method (DFBM) developed for lateral calibration of atomic force microscopy. The DFBM method employs the lateral force signal obtained during a force-distance measurement on a sloped surface and relates this signal to the applied load and the slope of the surface to determine the lateral calibration factor. In the original publication [Rev. Sci. Instrum. 77, 043903 (2006)], the tip-substrate contact was assumed to be pinned at the point of contact, i.e., no slip along the slope. In control experiments, the tip was found to slide along the slope during force-distance curve measurement. This paper presents the correct force balance for lateral force calibration.