Direct visualization of dewetting of molecularly thin liquid films on solid surfaces

The effect of the surface energy gamma, disjoining pressure, Pi, and roughness on the dewetting of molecularly thin liquid lubricant films on magnetic disks, which have sub-nanometer surface topography, has been investigated by visualizing the dewetting process directly using ellipsometric microscopy. The dewetting process of thin liquids on the rough surface is determined not only by the well-known instability of films, which is determined by the sign of dPi/dh, but also by the sign of Pi and the surface topography of the substrate even if its roughness is of the sub-nanometer order. The dewetting film formed small droplets, which were not along the surface topography of the substrate, when Pi < 0. On the other hand, it formed grooves along the surface topography with a sub-nanometer roughness when Pi > 0. Moreover, the sub-nanometer roughness initiated the dewetting of the metastable liquid thin films.     

Pentacene films on Cu(119)

The molecular structure of thin pentacene film grown on a Cu(119) surface has been studied by near-edge X-ray absorption fine structure spectroscopy and scanning tunneling microscopy. The interaction between the π-molecular orbitals delocalized on the aromatic rings and the underlying copper substrate was deduced from XAS spectra. Pentacene molecules arrange with the main axis almost parallel with the Cu terraces according to the measured polarization dependence of the C 1s absorption spectra. For thickness exceeding 4 nm an upright arrangement of the molecules was observed with a dense herringbone-like ordering. The present study thus demonstrates that highly ordered pentacene films can be obtained on a Cu(119) vicinal surface both in a flat orientation for low coverages and in a bulk-like herringbone orientation for higher coverages.     

Dendrimer pattern formation in evaporating drops

The redistribution of organic solutes during drop evaporation is a nanoscale self-assembly process with relevance to technologies ranging from inkjet printing of organic displays to synthesis of biosmart interfaces for sensing and screening. We have used solutions of dendrimer molecules with incrementally varying terminal site chemistry to explore whether the condensed dendrimer patterns resulting from microdroplet evaporation sensitively depend on, and are characteristic of, the surface chemistry of the solute molecules. This hypothesis has been experimentally confirmed by comparing the behavior of microdroplets of G4, G4-25%C12, and G4-50%C12 dendrimers dissolved in pentanol and deposited on mica substrates. For the dilute concentration studied here, the presence of periodically 'scalloped' dendrimer rings is ubiquitous. The instability wavelength of the scalloped rings is found to be proportional to the width of the ring, similar to observations of the rim instability in dewetting holes. The effect of dendrimer surface chemistry is obvious in the detailed structure of the self-assembled rings. G4 rings are diffuse and disordered with no evidence for layered growth. G4-25%C12 exhibits highly ordered ring structures and the onset of monomolecular terracing. G4-50%C12 exhibits highly periodic scallops and very distinct monomolecular height terraced growth of the rings with flat terraces and sharply defined steps. On the basis of these results, it is likely that the morphology of condensed molecule-based ring patterns formed by evaporation of microdroplets on surfaces can be used as a 'fingerprint' to identify, for example, solute molecule surface chemistry and concentration and function as a sensor for a variety of biochemical events.     

Substrate surface energy dependent morphology and dewetting in an ABC triblock copolymer film

A gradient combinatorial approach was used to examine the effect of substrate surface energy on the morphology and stability of films of a poly(isoprene-b-styrene-b-ethylene oxide) triblock copolymer that exhibits an alternating gyroid morphology in the bulk. Atomic force microscopy data across our surface energy (water contact angle) library suggest a transformation to predominantly surface parallel lamellae with an antisymmetric ordering. For substrate water contact angles below 70 degrees the film exhibited autophobic dewetting from an adsorbed half-period triblock copolymer monolayer at longer annealing times. X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure analysis along gradient specimens indicated that the substrate surface energy governed the composition profile of the monolayer, and this variation in chemical expression was key to whether the film was stable or autophobically dewet. These observations demonstrate that enthalpic interactions, in addition to entropic considerations, can play a major role in autophobic dewetting of block copolymer films.     

Photoluminescence properties of a perfluorinated supramolecular columnar liquid crystal with a pyrene core: effects of the ordering and orientation of the columns

We investigated the effects of the alignment and ordering of pi-conjugated perfluorinated dendrimers containing pyrene moieties in their cores on their photoluminescence (PL) properties. The pyrene molecules are stacked in columns surrounded by aromatic and semifluorinated tails, which can be conjugated and act as chromophores. Polarized light microscopy (PLM), cross-sectional scanning electron microscopy (SEM), and atomic force microscopy (AFM) results show that variation of the cooling rate of the dendrimers produces variation in their orientation and ordering: Slow cooling (approximately <0.5 degrees C/min) of the isotropic melt in a sandwich glass cell results in a high degree of ordering and the vertical alignment of the columns on the substrate, in which the stacked pyrene molecules are oriented parallel to the surface over large areas. In contrast, rapid cooling (approximately >10 degrees C/min) leads to the planar alignment of the columns with significant disorder on the same substrate. UV-vis, PL, SEM, and AFM results show that the quenched columns with a planar orientation produce a broad emission band and a second weak shoulder, which indicates the presence of isolated molecules. However, the high degree of ordering of the columns with a vertical alignment produces a red-shift in the PL spectrum, with very few isolated molecules. By comparing two films with different alignments but similar ordering, we show that the ordering of this material has a greater influence on the PL spectrum than the alignment. This effect of the ordering of the columns was further verified by comparing the optical properties of the isolated dendrimers with those of small pi-conjugated molecules in solution and solid films.     

Roughness and ordering at the interface of oxidized polystyrene and water

For the first time, atomistically detailed molecular dynamics calculations revealed molecular ordering of the water-oxidized atactic polystyrene (aPS) interface. Both ordering of the water molecules and the phenyl rings occur. In addition, the natural roughness of the surface has been simulated and compared to experimental values. The composition of the simulated aPS films is based on spin-coated aPS films that have been oxidized and characterized experimentally. The aPS surfaces are oxidized with ultraviolet-ozone radiation and have been characterized by XPS, AFM, and water contact angle measurements. XPS measurements show that the oxygen content in the sample increases rapidly with exposure and reaches saturation near 24 at. % of oxygen. The molecular dynamics simulations show smoothening of an hydrophobic aPS surface upon transition from vacuum to water. The smoothening decreases with increasing hydrophilicity. The calculations reveal ordering of oxidized phenyl rings for aPS surfaces in water. The order increases with increasing hydrophilicity. Additionally, we investigated the water structure near the aPS-water interface as a function of the surface hydrophilicity. With increasing hydrophilicity, the density of water at the aPS-water interface increases. The water density profile is steeper in the presence of hydrophobic aPS. The water shows an ordered layer near both the hydrophobic and hydrophilic surfaces; the position of this layer shifts toward the interface with increasing hydrophilicity.     

Achieving structural control with thin polystyrene-b-polydimethylsiloxane block copolymer films: The complex relationship of interface chemistry,annealing methodology and process conditions

The structure of thin microphase-separated polystyrene-block-polydimethylsiloxane (PS–PDMS) films has been studied using state-of-the-art top-down and cross-sectional electron microscopy. This is the first time that the profile of PS–PDMS films has been measured in situ and these measurements allowed us to image the shape of the PDMS domains within the film as well as examine the wetting behavior of the block copolymer film on a variety of substrates. It was found that for each polymer, substrate chemistry and annealing method combination examined, there was a small range of film thicknesses whereby the films exhibited the optimal characteristics of high levels of ordering without dewetting or multilayering. Specifically, the optimum thickness for films treated by thermal annealing was greater than that for the equivalent solvent annealed film; a change that was correlated with morphology variations related to solvent swelling of the solvent annealed films. The surface chemistry also induced changes in the optimum film thickness. Selective surfaces were shown to control whether a PDMS wetting layer was formed or not, leading to either thicker or thinner wetting optimum film thicknesses; while undulating morphologies were observed for less selective surfaces. Concomitant changes in the periodicity were then hypothesized to occur as a result of confinement effects and the selectivity of the surface.     

Scanning tunnelling microscopic images of the surface ordering of the liquid crystal 4-n-octyl-4'-cyanobiphenyl

Images of the surface ordering of 4-n-octyl-4'-cyanobiphenyl, a room temperature smectic liquid crystal deposited upon graphite, have been obtained by scanning tunnelling microscopy. The microscope was operated in air using the constant-current mode. Under certain tunnelling conditions it has been possible to resolve both the aliphatic and aromatic parts of the molecule, and to observe individual benzene rings. Two previously unreported conformations have been observed: an overlapping bilayer structure with spacing 3.7 nm, and a monolayer structure with spacing 2.4 nm. The latter structure may represent the first visual evidence for a surface polar ordered structure.     

Synthesis and photoinitiated polymerization of diacrylate derivates of halobenzoates

The synthesis, characterization, mesomorphism and photoinitiated polymerization of new thermotropic derivatives of bis{4-[6-(acryloyloxy)hexyloxy]halobenzoate}-1,4-phenylene were studied. Halogen incorporation in the mesogenic unit decreases the stability of the mesophases of monomers containing three aromatic rings. The monomer MIII exhibits a nematic phase which has been macroscopically oriented by surface pre-treatment and fixed by photopolymerization in situ, yielding a uniaxially crosslinked network. The molecular ordering of liquid crystalline diacrylate films was visualized by examining the fracture surface by scanning electron microscopy. These films exhibit a fine texture, illustrating the presence of an oriented layered structure.     

Patterned magnetic rings fabricated by dewetting of polymer-coated magnetite nanoparticles solution

Here we present a simple and controlled method for direct fabrication of ordered 2D arrays of magnetic rings. This method utilizes polystyrene-coated magnetite nanoparticles as a solution, and the magnetic rings are fabricated on patterned self-assembled monolayers by dewetting of the solution. Polystyrene-coated magnetite nanoparticles were synthesized by atom-transfer radical polymerization, which promoted the dispersibility and stability of magnetite nanoparticles in chloroform. Magnetic rings were studied using optical photograph, SEM, and magnetic force microscopy. This approach offers a new way for patterning nanoparticulate rings with deliberate control over feature composition, size, as well as interfeature distance.     

Chiral ordering and conformational dynamics for a class of oligo-phenylene-ethynylenes on Au(111)

Adsorption structures formed from a class of planar organic molecules on the Au(111) surface under ultrahigh vacuum conditions have been characterized using scanning tunneling microscopy (STM). The molecules have different geometries, linear, bent, or three-spoke, but all consist of a conjugated aromatic backbone formed from three or four benzene rings connected by ethynylene spokes and functionalized at all ends with an aldehyde, a hydroxyl, and a bulky tert-butyl group. Upon adsorption, the molecules adopt different surface conformations some of which are chiral. For the majority of the observed adsorption structures, chirality is expressed also in the molecular tiling pattern, and the two levels of chirality display a high degree of correlation. The formation and chiral ordering of the self-assembled structures are shown to result from dynamic interchanges between a diffusing lattice gas and the nucleated islands, as well as from a chiral switching process in which molecules alter their conformation by an intramolecular rotation around a molecular spoke, enabling them to accommodate to the tiling pattern of the surrounding molecular structures. The kinetics of the conformational switching is investigated from time-resolved, variable temperature STM, showing the process to involve an activation energy of approximately 0.3 eV depending on the local molecular environment. The molecule-molecule interactions appear primarily to be of van der Waals character, despite the investigated compounds having functional moieties capable of forming intermolecular hydrogen bonds.     

Compound formation and solvent (dis)ordering in syndiotactic polystyrene/benzylmethacrylate systems

In-situ small- and wide-angle X-ray scattering (SAXS and WAXS) experiments combined with Raman spectroscopy have been performed to study the phase behaviour of syndiotactic polystyrene (sPS) and benzylmethacrylate (BzMA). In the quenched gels, sPS adopts a helical conformation which is stabilised by the solvent molecules, in fact compound formation occurs. From the combined experimental data it was concluded that two different structural modifications exist within the solvent-included helical δ-phase, respectively the δ' phase in which the solvent molecules are intercalated and ordered between the phenyl rings of sPS and a δ” phase where this solvent ordering is lost.     

On sub-T(g) dewetting of nanoconfined liquids and autophobic dewetting of crystallites

The glass transition temperature (T(g)) of thin films is reduced by nanoconfinement, but it is also influenced by the free surface and substrate interface. To gain more insights into their contributions, dewetting behaviors of n-pentane, 3-methylpentane, and toluene films are investigated on various substrates as functions of temperature and film thickness. It is found that monolayers of these molecules exhibit sub-T(g) dewetting on a perfluoro-alkyl modified Ni substrate, which is attributable to the evolution of a 2D liquid. The onset temperature of dewetting increases with film thickness because fluidity evolves via cooperative motion of many molecules; sub-T(g) dewetting is observed for films thinner than 5 monolayers. In contrast, monolayers wet substrates of graphite, silicon, and amorphous solid water until crystallization occurs. The crystallites exhibit autophobic dewetting on the substrate covered with a wetting monolayer. The presence of premelting layers is inferred from the fact that n-pentane crystallites disappear on amorphous solid water via intermixing. Thus, the properties of quasiliquid formed on the crystallite surface differ significantly from those of the 2D liquid formed before crystallization.     

Surface orientation of 3,4,5-tris-substituted benzoic acid amphiphiles

Regarding the molecular orientation on flat substrates, thin films have been studied of a series of wedge-shaped molecules (3,4,5-tris-substituted benzoate-benzo crown ether compounds) consisting of a hydrophobic outer rim and a polar group at the thin end which form columnar mesomorphic and crystalline structures. For most substrates studied here, autophobic dewetting is demonstrated to be caused by the formation of a monomolecular adlayer in which the molecules are oriented normal to the substrate surface with the hydrophobic tails directed away from the substrate. For thick films, this adlayer is shown to cause an "in-plane" orientation of the axis of the columnar state. An ordered in-plane oriented adlayer is observed only for highly ordered pyrolytic graphite as the substrate. In this case, specific interactions with the substrate cause formation of a well-ordered 2D pattern that might favor homeotropic orientation of the columnar structures but has to be optimized by further structural variation. The structure of the adsorbed monolayer is elucidated by combining contact angle measurements, plasmon resonance spectroscopy, and optical and scanning tunneling microscopy.     

Dewetting of liquids on ceramic surfaces at high temperatures.

The influence of surface structure and chemistry on high-temperature dewetting of silicate liquids on ceramic surfaces has been investigated. Model systems based on well-defined crystallography and known chemistry have been used to illustrate the effect of surface roughness and chemistry on the dewetting process. Reconstructed ceramic surfaces provide ideal substrates to study effects of surface roughness. It has been shown that the morphology of dewet droplets depend on the length scale and the crystallography of the facets on the surface. Complex pattern formation due to solute redistribution during dewetting is illustrated in the case of SiO2 dewetting on (001) rutile substrates. The role of kinetics on the dewetting process has also been clarified.     

Dewetting Behavior of Random Copolymer Films Induced by Solvent Vapor Annealing

In recent years, the dewetting behavior of block copolymer films has been studied a lot, but that of random copolymer films was rarely studied. In this study, effects of film thickness and solvent vapor annealing duration (0 s–24 h) on the dewetting behavior of the spin-coated poly(styrene-co-acrylonitrile) (SAN) random copolymer films were mainly investigated by atomic force microscopy and contact angle method for the first time. The film thicknesses of the SAN films prepared at different concentrations were characterized by X-ray reflectometry to be 6–34 nm. With the annealing of acetone vapor, the SAN films first appear holes and then rupture into droplets which fuse and break periodically. The periodic evolutions of the droplets are due to the preferred affinity of acetone molecules with the AN segments and the change of surface energy. This phenomenon is different from the single evolutions in the spin-coated polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymer films. This illustrates the interactions between AN segments and the substrate are stronger than those between PMMA segments and the substrate in the spin-coated films.     

Toward mechanical switching of surface-adsorbed [2]catenane by in situ copper complexation

Using scanning tunneling microscopy (STM), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS), we demonstrate that a free [2]catenane consisting of two interlocking 30-membered rings (cat-30) can be deposited on a Ag(111) surface by vacuum sublimation without decomposition. The deposited cat-30 molecules self-organize as ordered dimer chain structures at the surface, presumably via intermolecular pi-pi stacking. An in situ addition of Cu atoms to the surface-adsorbed catenanes induces a drastic change in the molecular organization, i.e., from the dimer chain structure to isolated species. The nitrogen core level spectra suggest that the cat-30 phenanthroline units coordinate with Cu, indicating that the free catenane has been transformed into a Cu-complexed [2]catenane. Since it is known that the two interlocked macrocyclic rings of the free ligand cat-30 completely rearrange, i.e., circumrotate, upon complexation to copper, our results reveal that when adsorbed on the silver surface, the two macrocyclic rings of the free [2]catenane can glide within one another so as to generate the corresponding copper complex by in situ Cu complexation.     

Controlled nanoparticle assembly by dewetting of charged polymer solutions

In this paper, we present an alternative approach for controlled nanoparticle organization on a solid substrate by applying dewetting patterns of charged polymer solutions as a templating system. Thin films of charged polymer solutions dewet a solid substrate to form complex dewetting patterns that depend on the polymer charge density. These patterns, ranging from polygonal networks to elongated structures that are stabilized by viscous forces during dewetting, serve as potential templates for two-dimensional nanoparticle organization on a solid substrate. Thus, while nanoparticles dried in pure water undergo self-assembly to form close-packed arrays, addition of charged polymer in the dispersion leads to the formation of open structures that are directed by the dewetting patterns of the polymer solution. In this study, we focus on the application of elongated structures resulting from dewetting of high-charge-density polymer solutions to align nanoparticles of silica and gold into long chains that are several micrometers in length. The particle ordering process is a two-step mechanism: an initial confinement of the nanoparticles in the dewetting structures and self-assembly of the particles within these structures upon further drying by lateral capillary attractions.     

Self-assembled organic radicals on Au(111) surfaces: a combined ToF-SIMS, STM, and ESR study

Electron spin resonance (ESR), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and scanning tunneling microscopy (STM) have been used in parallel to characterize the deposition on gold surface of a series of nitronyl nitroxide radicals. These compounds have been specifically synthesized with methyl-thio linking groups suitable to interact with the gold surface to form self-assembled monolayers (SAMs), which can be considered relevant in the research for molecular-based spintronics devices, as suggested in recent papers. The degree of the expected ordering on the surface of these SAMs has been tuned by varying the chemical structure of synthesized radicals. ToF-SIMS has been used to support the evidence of the occurrence of the deposition process. STM has shown the different qualities of the obtained SAMs, with the degree of local order increasing as the degree of freedom of the molecules on the surface is decreased. Finally, ESR has confirmed that the deposition process does not affect the paramagnetic characteristics of radicals and that it affords a complete single-layered coverage of the surface. Further, the absence of angular dependence in the spectra indicates that the small regions of local ordering do not give rise to a long-range order and suggests a quite large mobility of the radical on the surface, probably due to the weak interaction with gold provided by the methyl-thio linking group.     

Dewetting of thin polystyrene films absorbed on epoxy coated substrates

Various characteristics of dewetting of thin polystyrene (PS) films absorbed on highly cross-linked epoxy-coated and silicon oxide covered substrates are studied as a function of PS film thickness (20h(c1) whereas the spinodal dewetting (SD) occurs through the growth of surface undulations for hh(c2) while the SD mechanism is observed for h相似文献