Post-synthetic modification of epitaxially grown, highly oriented functionalized MOF thin films

Thin films of MOFs grown on solid substrates offer a huge potential with regard to tailoring the properties of a surface, in particular if used in connection with post-synthesis modification (PSM). Here, we report on the PSM of surface-supported crystalline MOFs, with target molecules using an amine-based coupling strategy.     

Substrate assisted electrochemical deposition of patterned cobalt thin films

The patterned Co layers deposited on the scratched Cu surfaces were investigated with the use of the scanning electron microscopy. Patterned cobalt thin films were electrochemically deposited from the cobalt sulfate bath at room temperature. Pattering of cobalt was carried out by simple means of substrate scratching. Gentle scratching induces a direct pattering of cobalt from vertical to horizontal. The prepared pattered films were characterized for their structural, surface morphological and compositional properties by means of X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. X-ray diffraction studies reveal that the films are of cobalt. From the SEM images fabrication of patterns of cobalt is apparent. This work demonstrates a novel approach for obtaining patterned cobalt for many technological applications.     

Substrate and surfactant effects on the glass-liquid transition of thin water films

Temperature-programmed time-of-flight secondary ion mass spectrometry (TP-TOF-SIMS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption, desorption, and glass-liquid transition of water on the graphite and Ni(111) surfaces in the temperature range 13-200 K. Water wets the graphite surface at 100-120 K, and the hydrogen-bonded network is formed preferentially in the first monolayer to reduce the number of nonbonding hydrogens. The strongly chemisorbed water molecules at the Ni(111) surface do not form such a network and play a role in stabilizing the film morphology up to 160 K, where dewetting occurs abruptly irrespective of the film thickness. The surface structure of the water film formed on graphite is fluctuated considerably, resulting in deweting at 150-160 K depending on the film thickness. The dewetted patches of graphite are molecularly clean, whereas the chemisorbed water remains on the Ni(111) surface even after evaporation of the film. The abrupt drop in the desorption rate of water molecules at 160 K, which has been attributed to crystallization in the previous TPD studies, is found to disappear completely when a monolayer of methanol is present on the surface. This is because the morphology of supercooled liquid water is changed by the surface tension, and it is quenched by termination of the free OH groups on the surface. The surfactant methanol desorbs above 160 K since the hydrogen bonds of the water molecules are reconstructed. The drastic change in the properties of supercooled liquid water at 160 K should be ascribed to the liquid-liquid phase transition.     

X-ray diffraction measurements and depth profiling by secondary neutral mass spectrometry on epitaxially grown high-Tc superconducting thin films

YBa₂Cu₃O_7– thin films were deposited by pulsed laser deposition (PLD) on various substrates with different misfit. X-ray diffraction (XRD) measurements were performed in order to characterize the growth quality of the films and to study the orientation between the films and the substrates. On the used substrates all investigated films are relaxed independently, and epitaxially grown who show a long range ordered two domain structure. The elemental concentration depth profiles of these films detected by secondary neutral mass spectrometry (SNMS) show homogeneous stoichiometric element distributions with slight deviation of the Y content.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Electrowetting of aligned carbon nanotube films

Electrowetting is one approach to reducing the interfacial tension between a solid and a liquid. In this method, an electrical potential is applied across the solid/liquid interface which modifies the wetting properties of the liquid on the solid without changing the composition of the solid and liquid phases. Electrowetting of aligned carbon nanotube (CNT) films is investigated by the sessile drop method by dispensing deionized (DI) water or 0.03 M NaCl droplets (contacted by Au wire) onto aligned CNT films assembled on a copper substrate. The results demonstrate that electrowetting can greatly reduce the hydrophobicity of the aligned CNTs; the contact angle saturation for DI water and 0.03 M NaCl droplets occurs at 98 and 50 degrees , respectively. The combined effects of the geometrical roughness and the electrical potential on the contact angle are briefly discussed and modeled. Such a strategy may be invoked to controllably reduce the interfacial tension between carbon nanotubes (CNTs) and polymer precursors when infiltrating the monomers into the prealigned nanotube films.     

Study of structure and polymerization of epitaxially grown SiPc(OH)2 thin films by high-resolution electron microscopy

Thin films of SiPc(OH)₂ (Pc = phthalocyanine) were formed epitaxially on the (001) surface of mica by vacuum deposition and were then polymerized by heat treatment. The molecular packing of the SiPc(OH)₂ was determined by electron diffraction and high-resolution electron microscopy as triclinic${\rm P\bar 1} $ having dimensions a = 0.727, b = 1.307, c = 0.688 nm, α = 102.5, β = 104.2, and γ = 97.4°. This monomer crystal grows with its c-axis parallel to the a-axis of the substrate mica and its bc-plane parallel to the (001) surface of mica. By heat treatment at 320°C, the SiPc(OH)₂ polymerized with the c-axis of the polymer parallel to the c-axis of the monomer. At 420°C, the c-axis of the polymer became parallel to the a*-axis of the monomer (i.e., perpendicular to the film surface). From high-resolution electron microscopy of partially polymerized specimens, the polymerization was shown to start at the edges of small monomer crystals. This may be considered to be due to the volume expansion during the polymerization. © 1993 John Wiley & Sons, Inc.     

Visualizing cell extracellular matrix (ECM) deposited by cells cultured on aligned bacteriophage M13 thin films

Topographical features ranging from micro- to nanometers can affect cell orientation and migratory pathways, which are important factors in tissue engineering and tumor migration. In our previous study, a convective assembly of bacteriophage M13 resulted in thin films which could be used to control the alignment of cells. However, several questions regarding its underlying reasons to dictate cell alignment remained unanswered. Here, we further study the nanometer topographical features generated by the bacteriophage M13 crystalline film, which results in the alignment of the cells and extracellular matrix (ECM) proteins. Sequential imaging analyses at micro- and nanoscale levels of aligned cells and fibrillar matrix proteins were documented using scanning electron microscopy and immunofluorescence microscopy. As a result, we observed baby hamster kidney cells with higher degree of alignment on the ordered M13 substrates than NIH-3T3 fibroblasts, a difference which could be attributed to the intrinsic nature of the cells' production of ECM proteins. The results from this study provide a crucial insight into the topographical features of a biological thin film, which can be utilized to control the orientation of cells and surrounding ECM proteins.     

Dendritic morphology in homeotropically aligned discotic films.

A hexa-alkylether-substituted hexa-peri-hexabenzocoronene, as a discotic liquid crystal, spontaneously homeotropically self-orients when cooled from the isotropic phase. During this process, a dendritic morphology is formed in which a significant optical difference is observed between the dendritic shapes and their periphery. However, local structural analysis using microfocus synchrotron radiation experiments demonstrate that there is an identical supramolecular order in both areas. Three-dimensional confocal surface measurements reveal that these dendritic structures result from a dewetting process, which occurs during solidification. Thereby, the contact area between the organic material and the surface is reduced. These results are important for the design of organic electronics, since the reduction of the contact area in an electronic device might inhibit the charge transport between the discotic semiconductor and the electrode.     

Gold nanoparticle mediated formation of aligned nanotube composite films

We report on the formation of highly anisotropic nanotube composite materials, made by the attachment of gold nanoparticles to the surface of the single-walled carbon nanotubes, followed by preparation of an aligned composite film by compression in a Langmuir-Blodgett trough. The gold is attached in a one-step sonication procedure. The gold-modified nanotube material forms a stable suspension in toluene and has been characterized by atomic force and scanning force microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The aligned films have highly anisotropic electrical properties, with a factor of approximately 3000 difference in the conductivity between the aligned and perpendicular directions.     

Formation of calcite thin films by cooperation of polyacrylic acid and self-generating electric field due to aligned dipoles of polarized substrates

We demonstrated the formation of calcite thin films on positively and negatively charged surfaces of a hydroxyapatite (HAp) electret coexisting with polyacrylic acid (PAA) and self-generating surface electric fields due to HAp electrets with electrically aligned dipoles. The cooperation of PAA and the self-generating surface electric field due to the electrets favored the formation of calcite thin films and acted remarkably on the negatively charged surface. Calcite thin films, 4–10 μm thick, with a shell-like microstructure were produced on the negatively charged surfaces with a small amount of PAA. In contrast, under other reaction conditions, calcite thin films with a fan-like structure in the cross section formed on the polarized substrates, and their thickness ranged from 2 to 7 μm. The films were composed of hemispheric- or flat island-shaped aggregates that were made of the calcite crystals that elongated along the c-axis. The morphology of the PAA–Ca²⁺ complex assembly, which adsorbed onto the polarized HAp substrates, was controlled by the balance of the spatial charge distribution in its structure and the properties of the self-generating surface electric field, which led to the different morphologies of the calcite thin films. We proposed that the formation mechanism of the films formed coexisting with PAA and the self-generating electric fields.     

Synthesis of surfactant-templated silica films with orthogonally aligned hexagonal mesophase

Thin silica films with orthogonally aligned hexagonal close-packed cylindrical structure are synthesized by dip coating silica precursors and poly(ethylene oxide)-polyproplyene oxide (PEO-PPO) triblock surfactants (P123) onto modified glass slides. All films cast from this sol display 2D hexagonal pore structures (a approximately 6.2 nm) under transmission electron microscopy (TEM). However, X-ray diffraction (XRD) shows that confining freshly deposited films between two chemically neutral modified slides completely aligns the pores toward the direction orthogonal to the substrate. Equally effective alignment is obtained by using slides modified with either a random PEO-PPO copolymer or P123 itself. The channels in films cast onto unmodified slides, onto modified slides which are exposed to air, or onto modified slides which are exposed to unmodified glass slides align at least partially parallel to the substrate. Parallel mesophase alignment is also observed in a control experiment with a sol containing the nonionic surfactant template decaethelyne glycol hexadecyl ether (Brij-56) sandwiched between copolymer-modified slides because the surfaces are not chemically neutral toward Brij-56. This study confirms that it is possible to use substrate surface chemistry to control the orientation of mesophases in mixtures of reactive silicates and low molecular weight nonionic surfactant templates.     

Equilibrium thin liquid films

The latest results are reviewed and a number of new concepts of the thermodynamics of thin films are formulated. Current definitions of disjoining pressure and their applications for introducing disjoining pressure into thermodynamics of phase equilibria, as well as the new thermodynamic definition of the thickness of thin film, are considered. New approaches to the rigorous definition of disjoining pressure in curved films and films with nonuniform thickness, including transition zones of wetting films, are analyzed. The modulus of Gibbs’ elasticity is derived for the case of a thin film. The role of the elasticity of this type in thin films and its correlation with traditional transverse (Derjaguin) elasticity related to the disjoining pressure are explained.     

Reversible super-hydrophobicity to super-hydrophilicity transition of aligned ZnO nanorod films

Remarkable surface wettability transition occurs with an inducement of ultraviolet (UV) for aligned ZnO nanorod films. The inorganic oxide films, which show super-hydrophobicity (left), become super-hydrophilic (right) when exposed to UV illumination. After the films are placed in the dark, the wettability evolves back to super-hydrophobicity. This reversible effect is ascribed to the cooperation of the surface photosensitivity and the aligned nanostructure. Such special property will greatly extend the applications of ZnO films.     

Birefringence of thin liquid films

Birefringences of oriented aggregates of Na-montmorillonite expanded in sodium chloride solutions were measured and compared with birefringences calculated from Wiener's equation. The difference between the calculated and the observed values is nearly constant in the region of swelling values 10–20 X. It is inferred that the difference comes from the birefringence of the polymolecular water layers and that it is evidence for a specific anisotropic structure. Anisotropy is also shown in some intercalated organic polymolecular films.     

Characterization of rhenium-silicon thin films

Rhenium-silicon thin films have been characterized as promising compounds of thermoelectric applications by means of analytical transmission electron microscopy and X-ray diffraction referring to composition, morphology, short-range order and phase formation. Beyond the discussion of the results some methodical aspects and problems are presented.     

Review of CdO thin films

Cadmium Oxide (CdO) thin film is one of the first transparent conducting oxide semiconductors. Its excellent optical and electronic properties have made CdO a promising material for flat panel displays. In this article, we provide a comprehensive review of the state-of-the-art research activities related to the ‘preparation-property-application’ triangle of CdO thin films.