Imaging nanometer scale optical heterogeneities in phospholipid monolayers deposited on metal island films

Near-field scanning optical microscopy was applied to study the distribution of fluorescently labeled phospholipid monolayers deposited on the surface of gold island films by the Langmuir-Blodgett technique. Nanometer scale (approximately 50 nm) optical heterogeneities were observed in near-field fluorescence images of the monolayer deposited at 10 mN/m surface pressure. At higher surface pressure (30 mN/m) the heterogeneities became less pronounced. Overlaying of the near-field transmission and fluorescence images from the same area of the sample shows local transmission of gold island film is at a minimum where the fluorescence of the lipid monolayer is at a maximum. It was concluded that coverage of the metal island film by the Langmuir-Blodgett phospholipid monolayer is incomplete, and lipid molecules are preferentially localized in crevices of the film.     

Stability of dimethyldioctadecylammonium bromide Langmuir-Blodgett films on mica in aqueous salt solutions—implications for surface force measurements

Langmuir-Blodgett (LB) monolayer films of dimethyldioctadecylammonium bromide (DDOA) on muscovite mica have been studied using Wilhelmy plate type wetting measurements, surface force measurements, atomic force microscopy (AFM), and Brewster angle microscopy (BAM) on insoluble monolayers of DDOA before deposition. In particular, the effect of exposure to aqueous KBr salt solutions was investigated. BAM shows a heterogeneous monolayer with small condensed domains of dendritic shape under conditions normally used for deposition. A stick-jump behaviour of the meniscus is seen during deposition, leading to a large-scale heterogeneity measurable in wetting studies. These also show breakdown and hydrophilization of the LB film at the three-phase contact line (meniscus) and when exposed to salt solutions of approximately 10⁻² M concentration. The advancing contact angle against water is approximately 105°. Surface force measurements show long-range attraction in water, but also a surface charge which depends on salt concentration, and breakdown when surfaces are brought into contact in high salt concentrations. AFM images of untreated films show small holes, and breakdown when exposed to salt solution, especially at the three-phase line. The LB film is judged to be less suitable as a model hydrophobic surface owing to its heterogeneity and instability in salt solution.     

Liquid-crystal micropolarizer array for polarization-difference imaging

Fabrication and applications are discussed for a visible-wavelength micropolarizer array consisting of a linear polarizer and a micropatterned liquid-crystal (LC) cell. LC alignment direction is controlled by means of depositing an optically transparent gold film at an oblique angle and coating the surface with an alkanethiol self-assembled monolayer. Microdomains of two perpendicular LC alignment directions are created by photolithography and etching of the gold layer, rotating the substrate 90 deg, and depositing a second oblique gold layer in the etched areas. The resulting array is used for polarization-difference imaging (PDI), a technique that enhances image contrast in the presence of scattering. Images obtained with the array require more processing than do conventional PDI images, but this method eliminates the need for an electronically activated LC filter and is especially suited to systems whose filters are closely integrated with optical sensor arrays.     

Adhesive properties of polypropylene (PP) and polyethylene terephthalate (PET) film surfaces treated by DC glow discharge plasma

In this study, the adhesive properties of the plasma modified polypropylene (PP) and polyethylene terephthalate (PET) film surfaces have been investigated. Hydrophilicity of these polymer film surfaces was studied by contact angle measurements. The surface energy of the polymer films was calculated from contact angle data using Fowkes method. The chemical composition of the polymer films was analyzed by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) was used to study the changes in surface feature of the polymer surfaces due to plasma treatment. The adhesion strength of the plasma modified film was studied by T-peel strength test. The results showed a considerable improvement in surface wettability even for short exposure times. The AFM and XPS analyses showed changes in surface topography and formation of polar groups on the plasma modified PP and PET surfaces. These changes enhanced the adhesive properties of polymer film surfaces.     

Controlled nucleation and growth of surface-confined gold nanoparticles on a (3-aminopropyl)trimethoxysilane-modified glass slide: a strategy for SPR substrates

The thickness of the gold film and its morphology, including the surface roughness, are very important for getting a good, reproducible response in the SPR technique. Here, we report a novel alternative approach for preparing SPR-active substrates that is completely solution-based. Our strategy is based on self-assembly of the gold colloid monolayer on a (3-aminopropyl)trimethoxysilane-modified glass slide, followed by electroless gold plating. Using this method, the thickness of films can be easily controlled at the nanometer scale by setting the plating time in the same conditions. Surface roughness and morphology of gold films can be modified by both tuning the size of gold nanoparticles and agitation during the plating. Surface evolution of the Au film was followed in real time by UV-vis spectroscopy and in situ SPRS. To assess the surface roughness and electrochemical stability of the Au films, atomic force microscopy and cyclic voltammetry were used. In addition, the stability of the gold adhesion is demonstrated by three methods. The as-prepared Au films on substrates are reproducible and stable, which allows them to be used as electrodes for electrochemical experiments and as platforms for studying SAMs.     

Structure and magnetic properties of mono- and bi-layer graphene films on ultraprecision figured 4H-SiC(0001) surfaces

Monolayer and bilayer graphene films with a few hundred nm domain size were grown on ultraprecision figured 4H-SiC(0001) on-axis and 8 degrees -off surfaces by annealing in ultra-high vacuum. Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction (LEED), Raman spectroscopy, and scanning tunneling microscopy, we investigated the structure, number of graphene layers, and chemical bonding of the graphene surfaces. Moreover, the magnetic property of the monolayer graphene was studied using in-situ surface magneto-optic Kerr effect at 40 K. LEED spots intensity distribution and XPS spectra for monolayer and bilayer graphene films could become an obvious and accurate fingerprint for the determination of graphene film thickness on SiC surface.     

Controlled formation and mixing of two-dimensional fluids

We introduce a novel technique for the controlled spreading and mixing of lipid monolayers from multilamellar precursors on surfaces covered by the hydrophobic epoxy resin SU-8. The lipid spreads as a monolayer as a result of the high surface tension between SU-8 and the aqueous environment. A micropatterned device with SU-8 lanes, injection pads, and mixing regions, surrounded by hydrophilic Au, was constructed to allow handling of lipid films and to achieve their mixing at controlled stoichiometry. Our findings offer a new approach to dynamic surface functionalization and decoration as well as surface-based catalysis and self-assembly.     

Mixed alkylsilane functionalized surfaces for simultaneous wetting and homeotropic anchoring of liquid crystals

Surfaces functionalized with a self-assembled monolayer (SAM) formed from a mixture of two alkylsilanes with different chain lengths have been designed to simultaneously improve the liquid crystal (LC) wettability and promote homeotropic anchoring of the LC. Most chemically functionalized surfaces (e.g., long alkyl chain SAMs) that promote homeotropic alignment of LC possess low surface energy and result in poor LC wettability, inhibiting LC infiltration into microstructured surfaces and sometimes resulting in LC dewetting from the surface. However, a surface modified with a mixed SAM of octadecyltriethoxysilane (C18) and ethyltriethoxysilane (C2) exhibited very low LC contact angle while providing homeotropic anchoring. Ellipsometry was used to correlate the bulk concentration of C18 in the deposition solution to the surface coverage of C18 in the mixed monolayer; these bulk and surface concentrations were found to be equal within experimental uncertainty. The LC contact angle was found to depend nonmonotically with the surface coverage density, with a minimum (14.4 ± 0.1°) at a C18 surface coverage of 0.26 ± 0.08. Homeotropic LC anchoring was achieved at a C18 surface coverage of ≥0.11 ± 0.04, in the regime where a minimum in the LC contact angle was observed. The practical application of this approach to surface modification was demonstrated using a micropillar array sensor substrate. When the array was functionalized with a conventional C18 SAM, the LC did not infiltrate the array and exhibited a contact angle of 47.4 ± 0.5°. However, the LC material successfully infiltrated and wetted the same microstructured substrate when functionalized with a C18/C2 mixed SAM, while still exhibiting the desired homeotropic anchoring.     

Pulsed ion beam characterization of CVD diamond surfaces under thin film deposition conditions

Diamond and diamond-like carbon have properties which in principle make them ideally suited to a wide variety of thin-film applications. The widespread use of diamond thin films, however, has been limited for a number of reasons related largely to the lack of understanding and control of the nucleation and growth processes. Real-time, in-situ studies of the surface of the growing diamond film are experimentally difficult because these films are normally grown under a relatively high pressure of hydrogen, and conventional surface analytical methods require an ultrahigh vacuum environment. Pulsed ion beam based analytical methods with differentially pumped ion sources and particle detectors are able to characterize the uppermost atomic layer of a film during growth at ambient pressures in the range 0.7–27 Pa (4–6 orders of magnitude higher than other surface-specific analytical methods). We describe here a system which has been developed for the purpose of determining the hydrogen concentration and bonding sites on diamond surfaces as a function of sample temperature and ambient hydrogen pressure under hot-filament chemical vapor deposition (CVD) growth conditions. It is demonstrated that as the hydrogen partial pressure increases the saturation hydrogen coverage of the surface of a CVD diamond film increases, but that the saturation level depends on the atomic hydrogen concentration and substrate temperature. At the highest temperatures studied (700 °C), it was found that the surface hydrogen concentration did not exceed 1/4 monolayer.     

Layer‐by‐Layer Fabrication of Covalently Crosslinked and Reactive Polymer Multilayers Using Azlactone‐Functionalized Copolymers: A Platform for the Design of Functional Biointerfaces

We report a method for modulating the physicochemical properties of surfaces that is based on the reactive layer‐by‐layer fabrication of covalently crosslinked thin films using azlactone‐functionalized copolymers. We demonstrate that copolymers containing different molar ratios of methyl methacrylate (MMA) and 2‐vinyl‐4,4‐dimethylazlactone (VDMA) can be alternately deposited with poly(ethyleneimine) to assemble covalently crosslinked thin films. Characterization using ellipsometry demonstrates that, in general, film growth and thickness decrease as the content of reactive, azlactone functionality in the copolymer used to assemble the film decreases. Reflective infrared spectroscopy experiments demonstrate that films fabricated from MMA:VDMA copolymers contain residual azlactone functionality and that these reactive groups can be exploited to modify film‐coated surfaces. Fabricating films from MMA:VDMA copolymers containing different compositions permitted modulation of the density of reactive groups within the films and, thus, the extent to which the films are functionalized by exposure to small molecule amines. For example, functionalization of MMA:VDMA copolymer films with the small molecule D ‐glucamine resulted in films with water contact angles that varied with the composition of the copolymer used to fabricate the film (e.g., as the azlactone content in the film increased, glucamine‐modified films became more hydrophilic). We demonstrate further that treatment of copolymer‐containing films with glucamine resulted in changes in the numbers of mammalian cells that grow on the surfaces of the films. Our results suggest the basis of methods that could be used to modulate or tune the density of chemical and biological functionality presented on surfaces of interest in a variety of fundamental and applied contexts.     

Electrochemical synthesis using a self-assembled Au nanoparticle template of dendritic films with unusual wetting properties

This report describes the electrochemical preparation of dendritic silver films with unusual wetting properties coming from the use of a self-assembled gold nanoparticle (Au NP) template. It shows that the Au NP self-assembled monolayer on the highly ordered pyrolytic graphite (HOPG) surface is responsible for the formation of the dendritic morphology, which is not observed for the same deposition conditions on a bare HOPG substrate. An interesting evolution of the wetting properties of these films during the electrodeposition process is observed. Field emission scanning electron microscopy (FEGSEM), energy-dispersive spectrometry (EDS), x-ray photoelectron spectroscopy (XPS) and contact angle measurements are used to reveal the dendritic structure of the deposited silver film at a later stage of the electrodeposition process. They also reveal surprising wetting properties in terms of hydrophobic surface.     

Surface characterization of the monolayer and Langmuir–Blodgett films of tetra-tert-butyl-copper phthalocyanine

The relaxation and hysteresis phenomena of the tetra-tert-butyl-copper phthalocyanine (ttb-CuPc) monolayer were investigated. The monolayer was then transferred to a hydrophilic glass surface to prepare one monolayer of Langmuir–Blodgett (LB) films. The uniformity, stability and molecular arrangement of the LB films were studied by measurements of dynamic contact angle (DCA), and were compared with the information obtained by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results show that the monolayer of ttb-CuPc is generally stable. The limiting area of a molecule is smaller than those reported in the literature, which is attributed to the aggregation of ttb-CuPc molecules into multilayer domains. The advancing and receding contact angles of water on these LB films are much smaller than those on a homogeneous film prepared by vacuum deposition. The wettability analysis on the LB films suggests that ttb-CuPc molecules are not arranged uniformly and continuously, and the LB film of ttb-CuPc contains a high ratio of exposed glass substrate. The surface morphology inspected by TEM and AFM shows the formation of separated domains of ttb-CuPc molecules, which is consistent with the surface condition evaluated from the surface wettability.     

Author index

Very thin organic or polymer films can be analyzed using several analytic techniques. The thickness of the films discussed ranges from monolayers to several thousand ångströms. The techniques described are X-ray photoelectron spectroscopy, IR spectroscopy, ellipsometry and use of the quartz crystal oscillator microbalance. The last technique was applied to a fluoropolymer deposited onto a silver surface. The parameters which control film thickness and unformity are discussed along with how these parameters are measured. The second example discussed is the application of nitrogen-containing organic molecules to copper and iron surfaces. The chemical bonding is discussed along with the orientation of the molecules in the resulting film and the analytical techniques used.     

Electrochemical thinning of thicker gold film with qualified thickness for surface plasmon resonance sensing

To meet the requirement of surface plasmon resonance (SPR) sensing, controlling the thickness of the gold film is very important. Here, we report an efficient and simple approach to prepare a SPR-active substrate when the thickness of the gold film is larger than the optimizing 50 nm and smaller than 100 nm. This method is based on anodic electrodissolution of gold in electrolyte containing chloride ions. Using this method, the thickness of gold films can be easily changed at a nanometer scale by controlling the number of potential scans and the concentrations of chloride ions in the electrolyte. At the same time, the influence of gold film thickness on the SPR signal is recorded by SPR in real time. To assess the change of the surface roughness and morphology of gold film through anodic electrodissolution, atomic force microscopy was used. The surface roughness of the same Au film before and after anodic electrodissolution is 1.179 and 2.767 nm, respectively. The change of the surface roughness of Au film brings out a slight angle shift of SPR. This indicates that surface electrodissolution of the gold does not affect the character of the original bulk film and this film can be used for SPR experiments. To confirm our expectation, a simple adsorption experiment of cytochrome c (Cyt c) on the gold film treated with anodic electrodissolution modified by 11-mercaptoundecanic acid was carried out. The angle shift of SPR confirmed the adsorption of Cyt c, and the cyclic voltammetry of Cyt c provided a complementary confirmation for the adsorption of Cyt c. These results show that this approach provides a good way to change the thicker gold film to an optimized thickness of SPR sensing. The great advantage brought by this approach is in that it can convert the waste gold films with greater thicknesses fabricated by the vacuum deposition method or other methods into useful materials as active SPR substrates.     

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.     

Tailoring of the morphology and chemical composition of thin organosilane microwave plasma polymer layers on metal substrates

The growth of thin microwave organosilicon plasma polymers on model zinc surfaces was investigated as a function of the film thickness and the oxygen partial pressure during film deposition. The evolution of the topology of the film was studied by atomic force microscopy (AFM). The nano- and micro-roughness was investigated at the inner and the outer surfaces of the plasma polymers. A special etching procedure was developed to reveal the underside of the plasma polymer and thereby its inner surface. Rough films contained voids at the interface, which reduced the polymer/metal contact area. The increase in oxygen partial pressure led to a smoother film growth with a perfect imitation of the substrate topography at the interface. The chemical structure of the films was determined by infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). ToF-SIMS at the outer and the inner surface of the plasma polymers showed that the density of methylsilyl groups increases in the outer surface layer of the plasma polymer and depends on the oxygen partial pressure. The chemical composition of the films could be altered to pure SiO₂ without changing the morphology by using oxygen-plasma post-treatment. This was proved by means of IRRAS and AFM. Chemistry and topology of the films were correlated with the apparent water contact angle. It was found that a linear relationship exists between the nanoscopic roughness of the plasma polymer and the static contact angle of water. Superposition of a nanoscopic roughness of the metal surface and the nanoscopic roughness of methylsilyl-rich films led to ultra-hydrophobic films with water contact angles up to 160°.     

Chemisorption and chemical reaction effects on the resistivity of ultrathin gold films at the liquid-solid interface

Ultrathin gold films, with thicknesses between the onset of conductivity (d ～ 5 nm) and the electron mean free path (d ～ 80 nm), display surface-sensitive resistivities, which have been exploited to follow the adsorption and desorption of molecular monolayers at the metal-solution interface with high precision. For nominal Au film thicknesses (d ～ 40 nm), strongly chemisorbed thiolate monolayers increase the resistivity of the thin Au films by ～4%, but weakly adsorbed species, such as pyridine or phenolate at open circuit, induce no observable change in the Au film resistance. Resistivity measurements implemented with a high-stability current source and high-precision digital voltmeter sampling at 1 Hz resulted in 3σ uncertainties in alkanethiolate coverage of 1.4 × 10(-)(4) monolayer. Surface plasmon resonance measurements, performed simultaneously with resistivity measurements, indicate that changes in resistivity vary monotonically with coverage with three distinct regions: a low-coverage region of heightened adsorbate mobility, an intermediate-coverage region with generally linear behavior, and a chain length-dependent saturation region at high coverages. Resistivity measurements were also capable of reproducibly following the chemical state of the Au surface through a complex set of redox manipulations, demonstrating the versatility of this simple measurement.     

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.     

Microstructured bioreactive surfaces: covalent immobilization of proteins on Au(1 1 1)/silicon via aminoreactive alkanethiolate self-assembled monolayers

Micrometer-scale patterns of a defined surface chemistry and structure were produced on both ultraflat Au(1 1 1) and on gold-coated monocrystalline silicon surfaces by a method combining microcontact printing, wet chemical etching and the replacement of etch-resist self-assembled monolayers (SAMs) by functionalized or reactive SAMs. Key steps in this methodology were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry and contact angle measurements. The covalent immobilization of (functional) biological systems on these surfaces was tested using an N-hydroxysuccinimide ester -functionalized disulphide (DSU), which covalently binds primary amines without the need for further activation steps. Atomic force microscope images of native collagen V single molecules immobilized on these patterned surfaces revealed both high spatial resolution and strong attachment to the monolayer/gold surface. Microcontact printing of DSU is shown to be feasible on specially prepared, ultraflat Au(1 1 1) surfaces providing a valuable tool for scanning probe experiments with biomolecules. The retention of enzymatic activity upon immobilization of protein was demonstrated for the case of horseradish peroxidase. The described approach can thus be used to confine biological activity to predetermined sites on microstructured gold/silicon devices – an important capability in biomedical and biomolecular research. © 1999 Kluwer Academic Publishers     

High-resolution chemical imaging through tissue with an X-ray scintillator sensor

We describe a novel method for high-resolution chemical imaging on a surface embedded in tissue. The sensor surface consists of an X-ray scintillator film coated in a thin film loaded with chemical indicator dye. A narrow scanning X-ray beam is used to excite luminescence from X-ray scintillators located within the beam. This luminescence passes through the indicator film, and the spectrum is analyzed to measure chemical concentrations at that location. A pH sensor is demonstrated with a dynamic range between pH 6-9 and noise level of 0.05 pH units using methyl-red dyed pH paper. The location of the interface between two types of scintillator films is obtained with 0.30 mm spatial resolution even though the images are highly blurred by 10 mm of chicken breast. This work has important applications for detecting pH changes on surfaces of implanted medical devices.