Hierarchical Porosity in Self‐Assembled Polymers: Post‐Modification of Block Copolymer–Phenolic Resin Complexes by Pyrolysis Allows the Control of Micro‐ and Mesoporosity

It is shown that self‐assembled hierarchical porosity in organic polymers can be obtained in a facile manner based on pyrolyzed block‐copolymer–phenolic resin nanocomposites and that a given starting composition can be post‐modified in a wide range from monomodal mesoporous materials to hierarchical micro‐mesoporous materials with a high density of pores and large surface area per volume unit (up to 500–600 m² g^–1). For that purpose, self‐assembled cured composites are used where phenolic resin is templated by a diblock copolymer poly(4‐vinylpyridine)‐block‐polystyrene (P4VP‐b‐PS). Mild pyrolysis conditions lead only to monomodal mesoscale porosity, as essentially only the PS block is removed (length scale of tens of nanometers), whereas during more severe conditions under prolonged isothermal pyrolysis at 420 °C the P4VP chains within the phenolic matrix are also removed, leading to additional microporosity (sub‐nanometer length scale). The porosity is analyzed using transmission electron microscopy (TEM), small‐angle X‐ray scattering, electron microscopy tomography (3D‐TEM), positron annihilation lifetime spectroscopy (PALS), and surface‐area Brunauer–Emmett–Teller (BET) measurements. Furthermore, the relative amount of micro‐ and mesopores can be tuned in situ by post modification. As controlled pyrolysis leaves phenolic hydroxyl groups at the pore walls and the thermoset resin‐based materials can be easily molded into a desired shape, it is expected that such materials could be useful for sensors, separation materials, filters, and templates for catalysis.     

Organic–Inorganic Nanohybridization by Block Copolymer Thin Films

A simple route for fabricating highly ordered organic–inorganic hybrid nanostructures, using polystyrene‐block‐poly(ethylene oxide) diblock copolymer (PS‐b‐PEO) thin films coupled with sol–gel chemistry, is presented. Hexagonally packed arrays of titania nanodomains were generated by one‐step spin‐coating from solutions containing a titania precursor and PS‐b‐PEO, where the precursor was selectively incorporated into the PEO domain. The PS‐b‐PEO template was subsequently removed by UV treatment, leaving behind a highly dense array of hexagonally packed titania dots. The size of the dots, as well as the lattice spacing of the array, could be fine‐tuned by simply controlling the relative amount of sol–gel precursor to PS‐b‐PEO.     

A General Strategy to Disperse and Functionalize Carbon Nanotubes Using Conjugated Block Copolymers

A general strategy to disperse and functionalize pristine carbon nanotubes in a single‐step process is developed using conjugated block copolymers. The conjugated block copolymer contains two blocks: a conjugated polymer block of poly(3‐hexylthiophene), and a functional non‐conjugated block with tunable composition. When the pristine carbon nanotubes are sonicated with the conjugated block copolymers, the poly(3‐hexylthiophene) blocks bind to the surface of de‐bundled carbon nanotubes through non‐covalent π–π interactions, stabilizing the carbon nanotube dispersion, while the functional blocks locate at the outer surface of carbon nanotubes, rendering the carbon nanotubes with desired functionality. In this paper, conjugated block copolymers of poly(3‐hexylthiophene)‐b‐poly(methyl methacrylate), poly(3‐hexylthiophene)‐b‐poly(acrylic acid), and poly(3‐hexylthiophene)‐b‐poly(poly(ethylene glycol) acrylate) are used to demonstrate this general strategy.     

Arrays of Inorganic Nanodots and Nanowires Using Nanotemplates Based on Switchable Block Copolymer Supramolecular Assemblies

Here, a novel and simple route to fabricate highly dense arrays of palladium nanodots and nanowires with sub‐30 nm periodicity using nanoporous templates fabricated from supramolecular assemblies of a block copolymer, polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) and a low molecular weight additive, 2‐(4′‐hydroxybenzeneazo) benzoic acid (HABA) is demonstrated. The palladium nanoparticles, which are directly deposited in the nanoporous templates from an aqueous solution, selectively migrate in the pores mainly due to their preferential attraction to the P4VP block covering the pore wall. The polymer template is then removed by oxygen plasma etching or pyrolysis in air resulting in palladium nanostructures whose large scale morphology mirrors that of the original template. The method adopted in this work is general and versatile so that it could easily be extended for patterning a variety of metallic materials into dot and wire arrays.     

Sub‐Nanometer Level Size Tuning of a Monodisperse Nanoparticle Array Via Block Copolymer Lithography

The fabrication and catalytic application of a size‐tunable monodisperse nanoparticle array enabled by block copolymer lithography is demonstrated. Highly uniform vertical cylinder nanodomains are achieved in poly(styrene‐block‐4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer thin‐films by solvent annealing. The prominent diffusion of the anionic metal complexes into the protonated P4VP cylinder nanodomains occurs through specific electrostatic interactions in a weakly acidic aqueous solution. This well‐defined diffusion with nanoscale confinement enables preparation of the laterally ordered monodisperse nanoparticle array with sub‐nanometer level precise size tuning. The controlled growth of monodisperse nanoparticle arrays is proven by their catalytic use for vertical carbon nanotube (CNT) growth via plasma enhanced chemical vapor deposition (PECVD). Since the size of the catalyst particles is the decisive parameter for the diameters and wall‐numbers of CNTs, the highly selective growth of double‐walled or triple‐walled CNTs could be accomplished using monodisperse nanoparticle arrays.     

Soft Graphoepitaxy for Large Area Directed Self‐Assembly of Polystyrene‐block‐Poly(dimethylsiloxane) Block Copolymer on Nanopatterned POSS Substrates Fabricated by Nanoimprint Lithography

Polyhedral oligomeric silsequioxane (POSS) derivatives have been successfully employed as substrates for graphoepitaxial directed self‐assembly (DSA) of block copolymers (BCPs). Tailored POSS materials of tuned surface chemistry are subject to nanoimprint lithography (NIL) resulting in topographically patterned substrates with dimensions commensurate with the BCP block length. A cylinder forming polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) BCP is synthesized by sequential living anionic polymerization of styrene and hexamethylcyclotrisiloxane. The patterned POSS materials provide a surface chemistry and topography for DSA of this BCP and after solvent annealing the BCP shows well‐ordered microphase segregation. The orientation of the PDMS cylinders to the substrate plane could be controlled within the trench walls by the choice of the POSS materials. The BCP patterns are successfully used as on‐chip etch mask to transfer the pattern to underlying silicon substrate. This soft graphoepitaxy method shows highly promising results as a means to generate lithographic quality patterns by nonconventional methods and could be applied to both hard and soft substrates. The methodology might have application in several fields including device and interconnect fabrication, nanoimprint lithography stamp production, nanofluidic devices, lab‐on‐chip, or in other technologies requiring simple nanodimensional patterns.     

Nanoparticle Arrays: Sub‐Nanometer Level Size Tuning of a Monodisperse Nanoparticle Array Via Block Copolymer Lithography (Adv. Funct. Mater. 2/2011)

The fabrication and catalytic application of a size‐tunable monodisperse nanoparticle array enabled by block copolymer lithography is demonstrated. Highly uniform vertical cylinder nanodomains are achieved in poly(styrene‐block‐4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer thin‐films by solvent annealing. The prominent diffusion of the anionic metal complexes into the protonated P4VP cylinder nanodomains occurs through specific electrostatic interactions in a weakly acidic aqueous solution. This well‐defined diffusion with nanoscale confinement enables preparation of the laterally ordered monodisperse nanoparticle array with sub‐nanometer level precise size tuning. The controlled growth of monodisperse nanoparticle arrays is proven by their catalytic use for vertical carbon nanotube (CNT) growth via plasma enhanced chemical vapor deposition (PECVD). Since the size of the catalyst particles is the decisive parameter for the diameters and wall‐numbers of CNTs, the highly selective growth of double‐walled or triple‐walled CNTs could be accomplished using monodisperse nanoparticle arrays.     

Composites of Polyacrylonitrile and Multiwalled Carbon Nanotubes Prepared by Gelation/Crystallization from Solution

Composite films of polyacrylonitrile (PAN) and multiwalled carbon nanotubes (MWNTs) have been prepared by gelation/crystallization from solution. The contents of MWNTs were 5–10 wt.‐%, measured against PAN. The electrical and mechanical properties have been studied in comparison with those of the homopolymer PAN films prepared from the same method. Furthermore, stabilization and the carbonization have been carried out by using the drawn PAN–MWNTs as a new precursor to prepare carbon films with a cross‐sectional area much larger than that of a commercial carbon fiber (> 3000 times). The MWNTs within the PAN matrix promote the formation of a condensed aromatic ladder structure during the stabilization process and play an important role in preparing PAN‐based carbon material with high carbon quality and high mechanical properties. When the stabilized composites with 10 wt.‐% MWNTs are carbonized at 1000 °C, the Young's modulus reaches 37.5 GPa, and the electrical conductivity reaches 10² S cm^–1. The carbonized PAN homopolymer does not form an adequately robust bulk film for the mechanical properties to be measured.     

Tuning Electrical Conductance of Serpentine Single‐Walled Carbon Nanotubes

Changes in resistivity of serpentine single‐walled carbon nanotubes are presented as a function of bending radius, r_b, in the range of 100–2000 nm. Resistivity (ρ) is observed to increase with curvature (1/r_b), which is consistent with theoretical speculation on strain‐induced bandgap increment. Furthermore, a sharp bend (r_b < 50nm) in the nanotubes results in a drastic change in the field‐effect behavior, i.e., from ambipolar to p type across the bend. Local Raman spectra show that the G‐band Raman frequencies shift along the curvature, which may be attributed to local deformation and broken cylindrical symmetry in the nanotubes. The results suggest the possibility to tune the electrical properties by bending nanotubes and to build an all‐nanotube device by modulating the structure of the same tube.     

Hierarchical Structuring in Block Copolymer Nanocomposites through Two Phase‐Separation Processes Operating on Different Time Scales

Tailoring the size and surface chemistry of nanoparticles allows one to control their position in a block copolymer, but this is usually limited to one‐dimensional distribution across domains. Here, the hierarchical assembly of poly(ethylene oxide)‐stabilized gold nanoparticles (Au‐PEO) into hexagonally packed clusters inside mesostructured ultrathin films of polystyrene‐block‐poly(methyl methacrylate) (PS‐b‐PMMA) is described. A close examination of the structural evolution at different nanoparticle filling fractions and PEO ligand molecular weights suggests that the mechanism leading to this structure‐within‐structure is the existence of two phase separation processes operating on different time scales. The length of the PEO ligand is shown to influence not only the interparticle distances but also the phase separation processes. These conclusions are supported by novel mesoscopic simulations, which provide additional insight into the kinetic and thermodynamic factors that are responsible for this behavior.     

In Situ Tracking of Composition and Morphology of a Diblock Copolymer Film with GISAXS during Exchange of Solvent Vapors at Elevated Temperatures

Solvothermal vapor annealing at elevated temperature is applied to a thin film from a cylinder‐forming polystyrene‐block‐poly(dimethyl siloxane) (PS‐b‐PDMS) diblock copolymer. At this, the film is swollen in the vapor of n‐heptane (highly selective for PDMS). This vapor is stepwise replaced by the vapor of toluene (weakly selective for PS). The morphologies are investigated using in situ, real‐time grazing‐incidence small‐angle X‐ray scattering (GISAXS). The initial cylindrical morphology is transformed into, among others, the lamellar one. This novel type of experiments allows probing a trajectory in the state diagram of the PS‐b‐PDMS/n‐heptane/toluene mixture. To corroborate the morphologies, they are generated by molecular simulations, and the 2D GISAXS maps are calculated using the distorted‐wave Born approximation. To relate the morphologies to the solvent distribution in the two types of nanodomains, the latter is estimated from the intensities of the Bragg reflections in the 2D GISAXS maps along with the swelling ratio of the film. Comparison with the results from a similar experiment carried out at room temperature results in the same sequence of morphologies; however, at elevated temperature, more well‐ordered structures are obtained. This new approach proves to be efficient to achieve a block copolymer thin film having a desired morphology and orientation.     

Vertical Orientation of Nanodomains on Versatile Substrates through Self‐Neutralization Induced by Star‐Shaped Block Copolymers

Vertical orientation of lamellar and cylindrical nanodomains of block copolymers on substrates is one of the most promising means for developing nanopatterns of next‐generation microelectronics and storage media. However, parallel orientation of lamellar and cylindrical nanodomains is generally preferred due to different affinity between two block segments in a block copolymer toward the substrate and/or air. Thus, vertical orientation of the nanodomains is only obtained under various pre‐ or post‐treatments such as surface neutralization by random copolymers, solvent annealing, and electric or magnetic field. Here, a novel self‐neutralization concept is introduced by designing molecular architecture of a block copolymer. Star‐shaped 18 arm poly(methyl methacrylate)‐block‐polystyrene copolymers ((PMMA‐b‐PS)₁₈) exhibiting lamellar and PMMA cylindrical nanodomains are synthesized. When a thin film of (PMMA‐b‐PS)₁₈ is spin‐coated on a substrate, vertically aligned lamellar and cylindrical nanodomains are obtained without any pre‐ or post‐treatment, although thermal annealing for a short time (less than 30 min) is required to improve the spatial array of vertically aligned nanodomains. This result is attributed to the star‐shaped molecular architecture that overcomes the difference in the surface affinity between PS and PMMA chains. Moreover, vertical orientations are observed on versatile substrates, for instance, semiconductor (Si, SiO_x), metal (Au), PS or PMMA‐brushed substrate, and a flexible polymer sheet of polyethylene naphthalate.     

Siloxane Copolymers for Nanoimprint Lithography

Presented here is the novel use of thermoplastic siloxane copolymers as nanoimprint lithography (NIL) resists for 60 nm features. Two of the most critical steps of NIL are mold release and pattern transfer through dry etching. These require that the NIL resist have low surface energy and excellent dry‐etching resistance. Homopolymers traditionally used in NIL, such as polystyrene (PS) or poly(methyl methacrylate) (PMMA), generally cannot satisfy all these requirements as they exhibit polymer fracture and delamination during mold release and have poor etch resistance. A number of siloxane copolymers have been investigated for use as NIL resists, including poly(dimethylsiloxane)‐block‐polystyrene (PDMS‐b‐PS), poly(dimethylsiloxane)‐graft‐poly(methyl acrylate)‐co‐poly(isobornyl acrylate) (PDMS‐g‐PMA‐co‐PIA), and PDMS‐g‐PMMA. The presence of PDMS imparts the materials with many properties that are favorable for NIL, including low surface energy for easy mold release and high silicon content for chemical‐etch resistance—in particular, extremely low etch rates (comparable to PDMS) in oxygen plasma, to which organic polymers are quite susceptible. These properties give improved NIL results.     

Electrochemically Triggered Selective Adsorption of Biotemplated Nanoparticles on Self‐Assembled Organometallic Diblock Copolymer Thin Films

The controlled adsorption of the iron‐containing cage protein ferritin at the nanoscale using stimuli‐responsive self‐assembled diblock copolymer thin‐film templates is reported. The diblock copolymer used study consists of a cylinder‐forming polystyrene‐block‐polyferrocenylsilane (PS‐b‐PFS), with PFS as the minor block, and shows reversible redox properties. To prevent any spontaneous protein adsorption on either block, the electrolyte pH is selected to leave the ferritin negatively charged, and the protein concentration and solution ionic strength are carefully tuned. Selective adsorption of ferritin on the PFS domains of the self‐assembled thin films is then triggered in situ by applying a positive potential, simultaneously oxidizing the PFS and attracting the ferritin electrostatically.     

Facile Preparation of PbTiO3 Nanodot Arrays: Combining Nanohybridization with Vapor Phase Reaction Sputtering

A facile route is presented for the fabrication of spherical PbTiO₃ (PTO) nanodot arrays on platinized silicon substrates using PbO vapor phase reaction sputtering on micellar monolayer films of polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) loaded with TiO₂ sol–gel precursor. Short exposure to PbO transforms the amorphous TiO₂ into polycrystalline PTO, while keeping the inherent size and periodicity of TiO₂ nanodots. HRTEM images show that the spherical PTO nanodots, with an average size and height of 63 nm and 40 nm, respectively, are fixed on the Pt supported by residual carbon. XPS narrow scan spectra of Ti 2p and O 1s strongly verify the evolution of chemical identity and the reduction of the Ti‐O binding energy from TiO₂ to PTO. The amplitude and phase images of piezoelectric force microscopy (PFM) confirm a multidomain structure attributed by the crystalline orientation of the PTO nanodots. Furthermore, the discrete PTO nanodots show remarkable switching properties due to the low strain field induced by the small lateral size, and the absence of domain pinning effects by grain boundary.     

A Step‐Wise Approach for Dual Nanoparticle Patterning via Block Copolymer Self‐Assembly

A novel step‐wise approach for fabrication of periodic arrays of two different types of nanoparticles (NPs), selectively localized at different block copolymer phases is demonstrated. In the first step, pre‐synthesized ≈12 nm silver nanoparticles (AgNPs), stabilized with thiol‐terminated polystyrene, are mixed with poly(styrene‐block‐vinylpyridine) (PS‐b‐PVP) block copolymer in a common solvent. After film casting and consequent solvent vapor annealing the AgNPs are selectively localized within the PS phase of the block copolymer matrix due to the interaction with PS shell of the nanoparticles. In the second step, ≈2–5 nm gold, platinum, or palladium nanoparticles are directly deposited from their aqueous dispersion on the PVP domains of the self‐assembled block copolymer thin films. In such a way, thin films of nanostructured block copolymer with two types of nanoparticles, separated by the two distinct block copolymer phases, are prepared in a step‐wise manner. The presented method is very simple and can be applied for various combinations of pre‐synthesized nanoparticles where the characteristics of either type of nanoparticles are tuned accordingly in advance, which is more difficult to achieve for in situ synthesized nanoparticles.     

Block‐Copolymer‐Assisted One‐Pot Synthesis of Ordered Mesoporous WO3−x/Carbon Nanocomposites as High‐Rate‐Performance Electrodes for Pseudocapacitors

An ordered mesoporous tungsten‐oxide/carbon (denoted as m‐WO_3?x‐C‐s) nanocomposite is synthesized using a simple one‐pot method using polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) as a structure‐directing agent. The hydrophilic PEO block interacts with the carbon and tungsten precursors (resol polymer and WCl₆), and the PS block is converted to pores after heating at 700 °C under a nitrogen flow. The m‐WO_3?x‐C‐s nanocomposite has a high Brunauer–Emmett–Teller (BET) surface area and hexagonally ordered pores. Because of its mesoporous structure and high intrinsic density of tungsten oxide, this material exhibits a high average volumetric capacitance and gravimetric capacitance as a pseudocapacitor electrode. In comparison with reduced mesoporous tungsten oxide (denoted as m‐WO_3?x‐h), which is synthesized by a tedious hard template approach and further reduction in a H₂/N₂ atmosphere, m‐WO_3?x‐C‐s shows a high capacitance and enhanced rate performance, as confirmed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The good performance of m‐WO_3?x‐C‐s is attributed to the high surface area arising from the mesoporous structure, the large interconnected mesopores, and the low internal resistance from the well‐dispersed reduced tungsten oxide and amorphous carbon composite structure. Here, the amorphous carbon acts as an electrical pathway for effective pseudocapacitor behavior of WO_3‐x.     

Hydrophobic Chromophores in Aqueous Micellar Solution Showing Large Two‐Photon Absorption Cross Sections

A series of new hydrophobic two‐photon absorbing (2PA) chromophores with varied electron‐donating groups in quasi‐linear and multibranched structures are synthesized to correlate their structure/photophysical property relationships. The feasibility of using these large two‐photon absorption cross‐sectional (δ, expressed in GM = 1 × 10^–50 cm⁴ s photon^–1 molecule^–1) materials in aqueous solution is also explored. All four hydrophobic 2PA materials can be encapsulated into micelles generated by dispersing an amphiphilic block copolymer, poly(methacrylic acid)‐block‐polystyrene (PMAA‐b‐PS), into water. The micellar nanostructures are characterized using dynamic light scattering, atomic force microscopy, and transmission electron microscopy. After these dyes are incorporated into micelles, they exhibit strong fluorescence in water. It is found that the quantum yield and δ values of these chromophores are strongly dependent on the diameters of the micelles, concentrations of the PMAA‐b‐PS, and molecular structures of the 2PA chromophores. One of the compounds that has a strong triarylamino donor and a multibranched structure exhibits a large δ value of 2790 GM and high quantum yield (0.56) in micelle‐containing water. Although this value is smaller than the original value of 5300 GM in toluene, it is still substantially larger than the values of most water‐soluble 2PA materials, which have δ values of less than 100 GM.     

Ultrafast Responsive Non‐Volatile Flash Photomemory via Spatially Addressable Perovskite/Block Copolymer Composite Film

The exotic photophysical properties of organic–inorganic hybrid perovskite with long exciton lifetimes and small binding energy have appeared as promising front‐runners for next‐generation non‐volatile flash photomemory. However, the long photo‐programming time of photomemory limits its application on light‐fidelity (Li‐Fi), which requires high storage capacity and short programming times. Herein, the spatially addressable perovskite in polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO)/perovskite composite film as an photoactive floating gate is demonstrated to elucidate the effect of morphology on the photo‐responsive characteristics of photomemory. The chelation between lead ion and PEO segment promotes the anti‐solvent functionalities of the perovskite/PS‐b‐PEO composite film, thus allowing the solution‐processable poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) to act as the active channel. Through manipulating the interfacial area between perovskite and P3HT, fast photo‐induced charge transfer rate of 0.056 ns^?1, high charge transfer efficiency of 89%, ON/OFF current ratio of 10⁴, and extremely low programming time of 5 ms can be achieved. This solution‐processable and fast photo‐programmable non‐volatile flash photomemory can trigger the practical application on Li‐Fi.     

Protein‐Enabled Layer‐by‐Layer Syntheses of Aligned,Porous‐Wall,High‐Aspect‐Ratio TiO2 Nanotube Arrays

An aqueous, protein‐enabled (biomimetic), layer‐by‐layer titania deposition process is developed, for the first time, to convert aligned‐nanochannel templates into high‐aspect‐ratio, aligned nanotube arrays with thin (34 nm) walls composed of co‐continuous networks of pores and titania nanocrystals (15 nm ave. size). Alumina templates with aligned open nanochannels are exposed in an alternating fashion to aqueous protamine‐bearing and titania precursor‐bearing (Ti(IV) bis‐ammonium‐lactato‐dihydroxide, TiBALDH) solutions. The ability of protamine to bind to alumina and titania, and to induce the formation of a Ti–O‐bearing coating upon exposure to the TiBALDH precursor, enables the layer‐by‐layer deposition of a conformal protamine/Ti–O‐bearing coating on the nanochannel surfaces within the porous alumina template. Subsequent protamine pyrolysis yields coatings composed of co‐continuous networks of pores and titania nanoparticles. Selective dissolution of the underlying alumina template through the porous coating then yields freestanding, aligned, porous‐wall titania nanotube arrays. The interconnected pores within the nanotube walls allow enhanced loading of functional molecules (such as a Ru‐based N719 dye), whereas the interconnected titania nanoparticles enable the high‐aspect‐ratio, aligned nanotube arrays to be used as electrodes (as demonstrated for dye‐sensitized solar cells with power conversion efficiencies of 5.2 ± 0.4%).