Polymer surfaces with reversibly switchable ordered morphology

Honeycomb macroporous films fabricated by the "breath figures" method were composed of poly2-vinylpyridine (P2VP) distributed in the holes of polystyrene (PS). The porous films exhibited reversible behavior responding to water and different solvent vapors. When the porous film was treated with water, the honeycomb pattern would change to the hexagonal islandlike pattern. Once heated to remove the water, the honeycomb pattern emerged again. When the porous film was exposed to different solvent vapors, the same reversible process appeared. Carbon disulfide (CS(2)), toluene (TOL), and tetrahydrofuran (THF) solvent vapors induced the honeycomb pattern into the ordered islandlike pattern, and ethanol, chloroform, methyl ethyl ketone (MEK), and dimethylformamide (DMF) solvent vapors made the islandlike pattern come back to the honeycomb pattern. The hygroscopic property of P2VP and the polymer-solvent interaction are the driving force for the reversibly switchable morphology. The appropriate control of the hole depth is very crucial in determining the reversible changes.     

D-A structured high efficiency solid luminogens with tunable emissions: Molecular design and photophysical properties

The combination of an electron-accepting unit with aggregation-induced emission features and varying electron-donating arylamines yields high efficiency solid luminogens with tunable emissions from green to red.     

Bioinspired structured surfaces

Nature has evolved objects with desired functionality using commonly found materials. Nature capitalizes on hierarchical structures to achieve functionality. The understanding of the functions provided by objects and processes found in nature can guide us to produce nanomaterials, nanodevices, and processes with desirable functionality. Various natural objects which provide functionality of commercial interest have been characterized to understand how a natural object provides functionality. We have modeled and fabricated structures in the lab using nature's route and developed optimum structures. Once it is understood how nature does it, optimum structures have been fabricated using smart materials and fabrication techniques. This feature article provides an overview of four topics: Lotus effect, rose petal effect, gecko feet, and shark skin.     

Superwetting of structured surfaces

Superwetting of structured surfaces, sometimes referred to as hemi-wicking, was studied both experimentally and theoretically. Structured substrates with regular arrays of square pillars or frustra were machined from graphite blocks and then treated to render them lyophilic. Liquids spread over these surfaces to produce noncircular wetting areas. If the channels between the features were made shallower or narrower, liquids wicked more and spread over a larger area. The inherent wettability of the graphite was relatively unimportant; large differences in the contact angles had little influence on the spreading. Practically, this means that, to achieve extensive coverage, near-zero contact angles are not required. A combination of the appropriate surface structure and moderate inherent wettability can effectively flatten liquids, spreading them over very large areas.     

Photon control of liquid motion on reversibly photoresponsive surfaces

The movement of a liquid droplet on a flat surface functionalized with a photochromic azobenzene may be driven by the irradiation of spatially distinct areas of the drop with different UV and visible light fluxes to create a gradient in the surface tension. In order to better understand and control this phenomenon, we have measured the wetting characteristics of these surfaces for a variety of liquids after UV and visible light irradiation. The results are used to approximate the components of the azobenzene surface energy under UV and visible light using the van Oss-Chaudhury-Good equation. These components, in combination with liquid parameters, allow one to estimate the strength of the surface interaction as given by the advancing contact angle for various liquids. The azobenzene monolayers were formed on smooth air-oxidized Si surfaces through 3-aminopropylmethyldiethoxysilane linkages. The experimental advancing and receding contact angles were determined following azobenzene photoisomerization under visible and ultraviolet (UV) light. Reversible light-induced advancing contact-angle changes ranging from 8 to 16 degrees were observed. A large reversible change in contact angle by photoswitching of 12.4 degrees was achieved for water. The millimeter-scale transport of 5 microL droplets of certain liquids was achieved by creating a spatial gradient in visible/UV light across the droplets. A criterion for light-induced motion of droplets is shown to be consistent with the response of a variety of liquids. The type of light-driven fluid movement observed could have applications in microfluidic devices.     

Thiophene-based polyphosphazenes with tunable optoelectronic properties

The polyphosphazene backbone provides a versatile platform to explore numerous synthesis and structure–property relationships for many technological applications. In this study, a new class of polyphosphazene semiconducting materials was synthesized via macromolecular substitution, which integrates a  PN backbone with thiophene-based side groups. The synthesized thiophene-based polymers were subjected to chemical oxidation (oxidative coupling) to optimize their optoelectronic properties through side-chain chemistry. Both the spectroscopic and electronic analyses revealed that optical and electronic properties, as well as glass transition temperatures could be modulated by chemical oxidation of the polymers. The suitability of the polymers as potential semiconductors was further evaluated using their steady-state fluorescence quenching behavior in the presence of four different dopants (PC70BM, PC60BM, F4TCNQ, and TCNQ). It was found that the addition of dopant as a quencher to the polymer solutions does not form a complex in the ground state, and its excited state shows an efficient decrease in fluorescence intensity without altering the shape and peak position of the fluorescence band. The overall results demonstrate that the utilization of chemical oxidation via side-chain chemistry of polyphosphazenes offers an adaptable toolbox that can be used to make new and potentially useful polymeric semiconductors for applications in organic electronics.     

Equilibrium properties of reversibly flocculated dispersions

The structure of sonic flocculated dispersions can be changed reversibly by means of shearing. Often the changes are not instantaneous. The resulting shear-history effect gives rise to a complex but interesting rheological behaviour. Using non-aqueous suspensions of fumed silica, the rheological equilibrium properties of such systems are investigated. To change the floc structure, the water content of the particles is altered. As well as the steady-state shear viscosity, the equilibrium modulus and the yield stress are measured. Various techniques are compared. The effect of concentration on the equilibrium properties is used to test some structural models. The concentration dependence is best described by a power-law relation, the power being identical for modulus and yield stress. These results compare well with some theoretical predictions. Contrary to the assumptions used in the modelling, the yield stress is often dominated by kinetic phenomena. This shortcoming also shows up in the predictions for the critical strain.     

Droplet motion on designed microtextured superhydrophobic surfaces with tunable wettability

Superhydrophobic surfaces have shown promising applications in microfluidic systems as a result of their water-repellent and low-friction properties over the past decade. Recently, designed microstructures have been experimentally applied to construct wettability gradients and direct the droplet motion. However, thermodynamic mechanisms responsible for the droplet motion on such regular rough surfaces have not been well understood such that at present specific guidelines for the design of tunable superhydrophobic surfaces are not available. In this study, we propose a simple but robust thermodynamic methodology to gain thorough insight into the physical nature for the controllable motion of droplets. On the basis of the thermodynamic calculations of free energy (FE) and the free-energy barrier (FEB), the effects of surface geometry of a pillar microtexture are systematically investigated. It is found that decreasing the pillar width and spacing simultaneously is required to lower the advancing and receding FEBs to effectively direct droplets on the roughness gradient surface. Furthermore, the external energy plays a role in the actuation of spontaneous droplet motion with the cooperation of the roughness gradient. In addition, it is suggested that the so-called "virtual wall" used to confine the liquid flow along the undesired directions could be achieved by constructing highly advancing FEB areas around the microchannels, which is promising for the design of microfluidic systems.     

Wetting of regularly structured gold surfaces

In this study we report results for a systematic study of the wetting of structured gold surfaces formed by electrodeposition through monolayer templates of close-packed uniform submicrometer spheres. Removal of the template after deposition leaves a regular hexagonal array of sphere segment pores where the depth of the pores and, thus, the topography of the surface are controlled by the thickness of gold deposited through the template. We find that, as the thickness of the porous film increases up to the radius of the pores, the apparent contact angle for water on the surface increases from 70 degrees on the flat surface to more that 130 degrees , and then with increasing thickness above the radius of the pores the apparent contact angle decreases back toward 70 degrees . We show that these changes in the apparent contact angle agree with the model of Cassie and Baxter for nonwetted surfaces even though the gold itself is hydrophilic. We also show that the apparent contact angle is independent of the diameter of the pores over the range 400-800 nm. This is the first reported example showing the change of a hydrophilic surface (theta; < 90 degrees ) into a hydrophobic surface (theta; > 90 degrees ) purely by control of the surface topography. The role of the pore shape and size in stabilizing the nonwetting (Cassie-Baxter) droplet on the surface is discussed.     

Reproducible SERRS from structured gold surfaces

Metallic substrates with ordered spherical cavities have been shown to be very effective for surface-enhanced Raman scattering (SERS) and can be fabricated reproducibly using electrodeposition. The sensitivity of detection is increased by several orders of magnitude by using surface-enhanced resonance Raman scattering (SERRS). In this report we demonstrate SERRS for the first time on electrodeposited gold films templated with colloidal spheres and demonstrate the reproducibility of the response. We also obtain a direct comparison between SERRS and SERS by choosing two dyes, Cy5 and Cy3, which are similar in structure but differ in their excitation maxima, such that one is resonant and the other non-resonant with our laser excitation. As expected, the resonant enhancement is found to be of the order of 10(3) over and above that for SERS. The net SERRS enhancements are shown to be of the order of 10(9). We also find that the resonant enhancement profile of the different peaks for the chromophore follows the plasmonic resonance absorption spectrum obtained for the structured surface.     

Nanostructured block-random copolymers with tunable magnetic properties

It was recently shown that block copolymers (BCPs) produced room-temperature ferromagnetic materials (RTFMs) due to their nanoscopic ordering and the cylindrical phase yielded the highest coercivity. Here, a series of metal-containing block-random copolymers composed of an alkyl-functionalized homo block (C(16)) and a random block of cobalt complex- (Co) and ferrocene-functionalized (Fe) units was synthesized via ring-opening metathesis polymerization. Taking advantage of the block-random architecture, the influence of dipolar interactions on the magnetic properties of these nanostructured BCP materials was studied by varying the molar ratio of the Co units to the Fe units, while maintaining the cylindrical phase-separated morphology. DC magnetic measurements, including magnetization versus field, zero-field-cooled, and field-cooled, as well as AC susceptibility measurements showed that the magnetic properties of the nanostructured BCP materials could be easily tuned by diluting the cobalt density with Fe units in the cylindrical domains. Decreasing the cobalt density weakened the dipolar interactions of the cobalt nanoparticles, leading to the transition from a room temperature ferromagnetic (RTF) to a superparamagnetic material. These results confirmed that dipolar interactions of the cobalt nanoparticles within the phase-separated domains were responsible for the RTF properties of the nanostructured BCP materials.     

Bridged coordination polymer multilayers with tunable properties

Coordination multilayers consisting of Pd(II) pincer-type complexes and poly(vinyl pyridine) were synthesized and characterized. Film properties were found to be dependent on and could be tuned by varying bath deposition concentrations, polymer molecular weight, and solution additives that compete with binding. Generally, smoother, thinner films were obtained with lower poly(vinyl pyridine) deposition bath concentrations. Likewise, film thickness and roughness could be reduced by employing a higher-molecular-weight poly(vinyl pyridine). Film properties could also be influenced by using acetonitrile as a solution additive, effectively driving the binding equilibrium slightly toward the free species.     

Helical rosette nanotubes with tunable chiroptical properties

On the basis of transmission electron microscopy (TEM), dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and circular dichroism (CD) studies, compound 1 was shown to exist mainly in two states: (a) At high concentration (> or =1 mM, in methanol), 1 undergoes hierarchical self-assembly to generate rosette nanotubes with approximately 4 nm diameter and a concentration-dependent hydrodynamic radius in the range 10-100 nm. Under these conditions, addition of a chiral amino acid promoter (L-Ala), that binds to the crown ether moiety of 1 via electrostatic interactions, promotes a rapid transition (k(0) approximately equal 0.48 s(-1), for [1] = 0.046 mM, [L-Ala] = 2.8 mM) from racemic to chiral rosette nanotubes with predefined helicities as indicated by the resulting induced circular dichroism (ICD). (b) At low concentration (< or =0.04 mM, in methanol), 1 exists mainly in a nonassembled state as shown by TEM and DLS. Addition of L-Ala in this case triggers a relatively slow (k(0) approximately equal 0.07 s(-1) for [1] = 0.04 mM, [L-Ala] = 2.4 mM) sequence of supramolecular reactions leading to the hierarchical self-assembly of rosette nanotubes with predefined helicities. Under both conditions a and b, the kinetic data unveiled the intrinsic ability of the rosette nanotubes to promote their own formation (autocatalysis). The degree of chiral induction was found to depend dramatically upon the chemical structure of the promoter. This process appears also to follow an all-or-none response, as the vast majority of the crown ether sites must be occupied with a promoter for a complete transition to chiral nanotubes to take place. Finally, both supramolecular pathways a and b offer an efficient approach for the preparation of helical rosette nanotubes with tunable chiroptical properties and may also be viewed as a process by which a predefined set of physical and chemical properties that characterizes a molecular promoter is expressed at the macromolecular level.     

Nanoparticle-induced morphology and hydrophilicity of structured surfaces

Nanoparticles have been applied into the construction of micro- and nanoscaled surface structures with extreme wettability over the past few years. However, the details of processing and employing colloidal nanosuspensions for this purpose have not yet been fully investigated. In this work, we study the surface structures formed via nanosuspensions, in which nanoparticles of solid phase are presented, and the caused surface wettability. We disperse silica nanoparticles with different sizes into pure ethanol to prepare nanosuspensions with a series of concentrations. The suspensions are ultrasonically processed to prompt uniform distribution of nanoparticles before application. The deposited nanosuspensions are thermally treated to assist the regulation of surface patterns based on nanoparticles. Hence, the investigation explores a variety of experimental conditions that will lead to distinctive surface structures and wettabilities. Accordingly, the wettability of the induced surfaces is investigated using contact angle measurement, and the structures of those surfaces are mainly revealed by atomic force microscopy (AFM). Superhydrophilicity is observed on many of such formed surfaces, and the pattern of surface structures in micro- and nanoscale is closely related to the processing conditions and the size of nanoparticles. Thus, we report the characteristics of the surface patterns based on nanoparticles and the formed wettability.     

Phase-field modeling of wetting on structured surfaces

We study the dynamics and equilibrium profile shapes of contact lines for wetting in the case of a spatially inhomogeneous solid wall with stripe defects. Using a phase-field model with conserved dynamics, we first numerically determine the contact line behavior in the case of a stripe defect of varying widths. For narrow defects, we find that the maximum distortion of the contact line and the healing length is related to the defect width, while for wide defects, it saturates to constant values. This behavior is in quantitative agreement with the experimental data. In addition, we examine the shape of the contact line between two stripe defects as a function of their separation. Using the phase-field model, we also analytically estimate the contact line configuration and find good qualitative agreement with the numerical results.     

Morphological wetting transitions at chemically structured surfaces

Chemically structured surfaces are discussed which consist of patterns of lyophilic and lyophobic surface domains. Wetting layers on top of these surfaces attain a variety of morphologies and undergo morphological wetting transitions. One convenient way to explore these transitions experimentally is by changing the total volume of the wetting layer.     

SERS at structured palladium and platinum surfaces

Palladium and platinum are important catalytic metals, and it would be highly advantageous to be able to use surface enhanced Raman spectroscopy (SERS) to study reactive species and intermediates on their surfaces. In this paper we describe the use of templated electrodeposition through colloidal templates to produce thin (<1 microm) films of palladium and platinum containing close packed hexagonal arrays of uniform sphere segment voids. We show that, even though these films are not rough, when the appropriate film thickness and sphere diameter are employed these surfaces give stable, reproducible surface enhancements for Raman scattering from molecules adsorbed at the metal surface. We report SERS spectra for benzenethiol adsorbed on the structured palladium and platinum surfaces of different thicknesses and void diameters and show that, for 633 nm radiation, enhancements of 1800 and 550 can be obtained for palladium and platinum, respectively.     

Fluorinated Alq3 derivatives with tunable optical properties

This communication reports that not only the emission colour but also the photoluminescence quantum yield of Alq3 can be tuned by introducing fluorine atoms at different positions; with fluorination at C-5 the emission is red-shifted with a tremendously decreased intensity, fluorination at C-6 causes a blue-shift with a significantly increased intensity, and fluorination at C-7 has a minor effect on both the colour and intensity of Alq3's emission.     

Graphene-based multifunctional iron oxide nanosheets with tunable properties

We report the synthesis of graphenes with tunable properties due to the growth of needlelike iron oxide (IO) nanoparticles on their surfaces. The electrical conductivity, flexibility, and magnetic properties of graphene nanosheets (GNSs) could be tuned on demand by fine controlling both the surface coverage and the length of the IO nanoneedles. The degree of coverage of the IO nanoparticles on the surface of the GNSs made it possible to control the resulting properties of the IO/GNSs on demand. As examples of their utility, paperlike materials were generated by simple filtration, and the resulting IO/GNS nanocomposites showed extraordinary removal capacity and fast adsorption rates for As(V) and Cr(VI) ions in water. Another possible application is the preparation of multifunctional films equipped with conductivity, flexibility, and magnetic properties. The fabrication process is easy to scale up at a low cost. In addition, both the colloidal solution and film forms of the resulting IO/GNSs were effective for removal of heavy metal ions, meaning this material could be utilized for actual industrial applications.