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.     

Site‐Specific Placement of Au Nanoparticles on Chemical Nanopatterns Prepared by Molecular Transfer Printing Using Block‐Copolymer Films

Inexpensive, large area patterning of ex‐situ synthesized metallic nanoparticles (NPs) at the nanoscale may enable many technologies including plasmonics, nanowire growth, and catalysis. Here, site‐specific localization of Au NPs onto nanoscale chemical patterns of polymer brushes is investigated. In this approach, patterns of hydroxyl‐terminated poly(styrene) brushes are transferred from poly(styrene‐block‐methyl methacrylate) (PS‐b‐PMMA) block copolymer films onto a replica substrate via molecular transfer printing, and the remaining areas are filled with hydroxyl‐terminated poly(2‐vinyl pyridine) (P2VP‐OH) brushes. Citrate‐stabilized Au NPs (13 nm) selectively bind to P2VP‐OH functionalized regions and the quality of the resulting assemblies depends on high chemical contrast in the patterned brushes. Minimization of the interpenetration of P2VP‐OH chains into PS brushes during processing is the key for achieving high chemical contrast. Large area hexagonal arrays of single Au NPs with a placement accuracy of 3.4 nm were obtained on patterns (～20 nm spots, ～40 nm pitch) derived from self‐assembled cylinder‐forming PS‐b‐PMMA films. Linear arrays of Au NPs were generated on patterns (40 nm lines, 80nm pitch) derived from lamellae‐forming PS‐b‐PMMA that had been directed to assemble on lithographically defined masters.     

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.     

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.     

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.     

The Effect of Nanoscale Confinement on the Collective Electron Transport Behavior in Au Nanoparticles Self‐Assembled in a Nanostructured Polystyrene‐block‐poly(4‐vinylpyridine) Diblock Copolymer Ultra‐thin Film

This study involves the collective electron transport behavior of sequestered Au nanoparticles in a nanostructured polystyrene‐block‐poly(4‐vinylpyridine). The monolayer thin films (ca. 30 nm) consisting of Au nanoparticles self‐assembled in the 30‐nm spherical poly(4‐vinylpyridine) domains of an polystyrene‐block‐poly(4‐vinylpyridine) diblock copolymer were prepared. From the current‐voltage characteristics of these thin films, the collective electron transport behavior of Au nanoparticles sequestered in the spherical poly(4‐vinylpyridine) nanodomains was found to be dictated by Coulomb blockade and was quasi one‐dimensional, as opposed to the three‐dimensional behavior displayed by Au nanoparticles that had been dispersed randomly in homo‐poly(4‐vinylpyridine). The threshold voltage of these composite increased linearly upon increasing the inter‐nanoparticle distance. The electron tunneling rate constant in the case of Au nanoparticles confined in poly(4‐vinylpyridine) nanodomains is eight times larger than that in the randomly distributed case and it increases upon increasing the amount of Au nanoparticles. This phenomenon indicates that manipulating the spatial arrangement of metal nanoparticles by diblock copolymer can potentially create electronic devices with higher performance.     

Tuning the Dimensions and Periodicities of Nanostructures Starting from the Same Polystyrene‐block‐poly(2‐vinylpyridine) Diblock Copolymer

The controlled tuning of the characteristic dimensions of two‐dimensional arrays of block‐copolymer reverse micelles deposited on silicon surfaces is demonstrated. The polymer used is polystyrene‐block‐poly(2‐vinylpyridine) (91 500‐b‐105 000 g mol^–1). Reverse micelles of this polymer with different aggregation numbers have been obtained from different solvents. The periodicity of the micellar array can be systematically varied by changing copolymer concentration, spin‐coating speeds, and by using solvent mixtures. The profound influence of humidity on the micellar film structure and the tuning of the film topography through control of humidity are presented. Light scattering, atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and X‐ray photoelectron spectroscopy were used for characterization. As possible applications, replication of micellar array topography with polydimethylsiloxane and post‐loading of the micelles to form iron oxide nanoparticle arrays are presented.     

High‐Performance Nonvolatile Transistor Memories of Pentacence Using the Green Electrets of Sugar‐based Block Copolymers and Their Supramolecules

Reported here are the nonvolatile electrical characteristics of pentacene‐based organic field‐effect transistor (OFET) memory devices created from the green electrets of sugar‐based block copolymer maltoheptaose‐block‐polystyrene (MH‐b‐PS), and their supramolecules with 1‐aminopyrene (APy). The very hydrophilic and abundant‐hydroxyl MH block is employed as a charge‐trapping site, while the hydrophobic PS block serves as a matrix as well as a tunneling layer. The orientation of the MH nanodomains could be well controlled in the PS matrix with random spheres, vertical cylinders, and ordered horizontal cylinders via increasing solvent annealing time, leading to different electrical switching characteristics. The electron‐trapping ability induced by the horizontal‐cylinder MH is stronger than those of the random‐sphere and vertical‐cylinder structures, attributed to the effective contact area. The electrical memory window of the device is further improved via the supramolecules of hydrogen‐bonding 1‐aminopyrene to the MH moieties of MH‐b‐PS for enhancing the hole‐trapping ability. The optimized device using the horizontal cylinders of the supramolecule electret exhibits the excellent memory characteristics of a wide memory window (52.7 V), retention time longer than 10⁴ s with a high ON/OFF ratio of >10⁵, and stable reversibility over 200 cycles. This study reveals a new approach to achieve a high‐performance flash memory through the morphology control of sugar‐based block copolymers and their supramolecules.     

Protein‐Enabled Synthesis of Monodisperse Titania Nanoparticles On and Within Polyelectrolyte Matrices

Here, the results of a study of the mechanism of bio‐enabled surface‐mediated titania nanoparticle synthesis with assistance of polyelectrolyte surfaces are reported. By applying atomic force microscopy, surface force spectroscopy, circular dichroism, and in situ attenuated total reflection Fourier‐transform infrared spectroscopy, structural changes of rSilC‐silaffin upon its adsorption to polyelectrolyte surfaces prior to and during titania nanoparticle growth are revealed. It is demonstrated that the adhesion of rSilC‐silaffin onto polyelectrolyte surfaces results in its reorganization from a random‐coil conformation in solution into a mixed secondary structure with both random coil and β‐sheet structures presented. Moreover, the protein forms a continuous molecularly thin layer with well‐defined monodisperse nanodomains of lateral dimensions below 20 nm. It is also shown that rSilC embedded inside the polylelectrolyte matrix preserves its titania formation activity. It is suggested that the surface‐mediated, bio‐enabled synthesis of nanostructured materials might be useful to develop general procedures for controlled growth of inorganic nanomaterials on reactive organic surfaces, which opens new perspectives in the design of tailored, in situ grown, hybrid inorganic–organic nanomaterials.     

Elaboration of P3HT/CNT/PCBM Composites for Organic Photovoltaic Cells

This Full Paper focuses on the preparation of single‐walled or multi‐walled carbon nanotube solutions with regioregular poly(3‐hexylthiophene) (P3HT) and a fullerene derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl[6,6]C₆₁ (PCBM) using a high dissolution and concentration method to exactly control the ratio of carbon nanotubes (CNTs) to the P3HT/PCBM mixture and disperse the CNTs homogeneously throughout the matrix. The CNT/P3HT/PCBM composites are deposed using a spin‐coating technique and characterized by absorption and fluorescence spectroscopy and by atomic force microscopy to underline the structure and the charge transfer between the CNTs and P3HT. The performance of photovoltaic devices obtained using these composites as a photoactive layer mainly show an increase of the short circuit current and a slight decrease of the open circuit voltage which generally leads to an improvement of the solar cell performances to an optimum CNT percentage. The best results are obtained with a P3HT/PCBM (1 : 1) mixture with 0.1 wt % multi‐walled carbon nanotubes with an open circuit voltage (V_oc) of 0.57 V, a current density at the short‐circuit (I_sc) of 9.3 mA cm^–2 and a fill factor of 38.4 %, which leads to a power conversion efficiency of 2.0 % (irradiance of 100 mW cm^–2 spectroscopically distributed following AM1.5).     

Hierarchical Directed Self‐Assembly of Diblock Copolymers for Modified Pattern Symmetry

Block copolymer lithography exploiting diblock copolymer thin films is promising for scalable manufacture of device‐oriented nanostructures. Nonetheless, its intrinsic limitation in the degree of freedom for pattern symmetry within hexagonal dot or parallel line array greatly diminishes the potential application fields. Here, we report multi‐level hierarchical self‐assembled nanopatterning of diblock copolymers for modified pattern symmetry. Sequential hierarchical integration of two layers of diblock copolymer films with judiciously chosen molecular weights and chemical composition creates nanopatterned morphology with modified pattern symmetry, including sparse linear cylinder or lamellar arrays. Internal structure of the hierarchically patterned morphology is characterized by grazing‐incidence small‐angle X‐ray scattering throughout the film thickness. Pattern transfer of the modified nanopattern generates linear metal nanodot array with uniform size and regular spacing as a typical example of functional nanopatterned structures.     

Fabrication of Nanoporous Alumina Ultrafiltration Membrane with Tunable Pore Size Using Block Copolymer Templates

Control over nanopore size and 3D structure is necessary to advance membrane performance in ubiquitous separation devices. Here, inorganic nanoporous membranes are fabricated by combining the assembly of cylinder‐forming poly(styrene‐block‐methyl methacrylate) (PS‐b‐PMMA) block copolymer and sequential infiltration synthesis (SIS). A key advance relates to the use of PMMA majority block copolymer films and the optimization of thermal annealing temperature and substrate chemistry to achieve through‐film vertical PS cylinders. The resulting morphology allows for direct fabrication of nanoporous AlO_x by selective growth of Al₂O₃ in the PMMA matrix during the SIS process, followed by polymer removal using oxygen plasma. Control over the pore diameter is achieved by varying the number of Al₂O₃ growth cycles, leading to pore size reduction from 21 to 16 nm. 3D characterization, using scanning transmission electron microscopy tomography, reveals that the AlO_x channels are continuous through the film and have a gradual increase in pore size with depth. Finally, the ultrafiltration performance of the fabricated AlO_x membrane for protein separation as a function of protein size and charge is demonstrated.     

Double Stimuli‐Responsive Isoporous Membranes via Post‐Modification of pH‐Sensitive Self‐Assembled Diblock Copolymer Membranes

Double stimuli‐responsive membranes are prepared by modification of pH‐sensitive integral asymmetric polystyrene‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer membranes with temperature‐responsive poly(N‐isopropylacrylamide) (pNIPAM) by a surface linking reaction. PS‐b‐P4VP membranes are first functionalized with a mild mussel‐inspired polydopamine coating and then reacted via Michael addition with an amine‐terminated pNIPAM‐NH₂ under slightly basic conditions. The membranes are thoroughly characterized by nuclear magnetic resonance (¹H‐NMR), Fourier transform infrared spectroscopy and X‐ray‐induced photoelectron spectroscopy. Additionally dynamic contact angle measurements are performed comparing the sinking rate of water droplets at different temperatures. The pH‐ and thermo‐double sensitivities of the modified membranes are proven by determining the water flux under different temperature and pH conditions.     

Hierarchically Ordered Arrays of Noncircular Silicon Nanowires Featured by Holographic Lithography Toward a High‐Fidelity Sensing Platform

A novel, highly uniform and tunable hybrid plasmonic array is created via ion‐milling, catalytic wet‐etching and electron‐beam evaporation, using a holographically featured structure as a milling mask. A simple and low‐cost prism holographic lithography (HL) technique is applied to create an unprecedentedly coordinated array of elliptic gold (Au) holes, which act as the silicon (Si) etching catalyst in the reaction solution used to fabricate an elliptic silicon nanowire (SiNW) array; here, the SiNWs are arrayed hierarchically in such a way that three SiNWs are triangularly coordinated, and the triangles are arranged hexagonally. After removing the polymeric mask and metal thin film, the highly anisotropic thick Au film is deposited on the SiNW arrays. This hybrid substrate shows tunable optical properties in the near‐infrared (NIR) region from 875 nm to 1030 nm and surface‐enhanced Raman scattering (SERS) activities; these characteristics depend on the catalytic wet etching time, which changes the size of the vertical gap between the Au thick films deposited separately on the SiNWs. In addition, lateral interparticle coupling induces highly intensified SERS signals with good homogeneity. Finally, the Au‐capped elliptical SiNW arrays can be hierarchically patterned by combining prism HL and conventional photolithography, and the highly enhanced fluorescence intensity associated with both the structural effects and the plasmon resonances is investigated.     

Synthesis of Amphiphilic Copolymers of Poly(ethylene oxide) and Poly(ϵ‐caprolactone) with Different Architectures,and Their Role in the Preparation of Stealthy Nanoparticles

Well‐defined copolymers of biocompatible poly(?‐caprolactone) (PCL) and poly(ethylene oxide) (PEO) are synthesized by two methods. Graft copolymers with a gradient structure are prepared by ring‐opening copolymerization of ?‐caprolactone (?CL) with a PEO macromonomer of the ?CL‐type. The ?CL polymerization is initiated by a PEO macroinitiator to prepare diblock copolymers. These amphiphilic copolymers are used as stabilizers for biodegradable poly(D,L ‐lactide) (PLA) nanoparticles prepared by a nanoprecipitation technique. The effect of the copolymer characteristic features (architecture, composition, and amount) on the nanoparticle formation and structure is investigated. The average size, size distribution, and stability of aqueous suspensions of the nanoparticles is measured by dynamic light scattering. For comparison, an amphiphilic random copolymer, poly(methyl methacrylate‐co‐methacrylic acid) (P(MMA‐co‐MA)), is synthesized. The stealthiness of the nanoparticles is analyzed in relation to the copolymer used as stabilizer. For this purpose, the activation of the complement system by nanoparticles is investigated in vitro using human serum. This activation is much less important whenever the nanoparticles are stabilized by a PEO‐containing copolymer rather than by the P(MMA‐co‐MA) amphiphile. The graft copolymers with a gradient structure and the diblock copolymers with similar macromolecular characteristics (molecular weight and hydrophilicity) are compared on the basis of their capacity to coat PLA nanoparticles and to make them stealthy.     

An Electrical Rectifier Based on Au Nanoparticle Array Fabricated Using Direct‐Write Electron Beam Lithography

Close‐packed arrays of Au nanoparticles are produced in patterned regions by electron beam (e‐beam) lithography using a highly sensitive direct–write resist, N⁺AuCl₄^?(C₈H₁₇)₄Br. While the e–beam causes dewetting of the resist to nucleate Au nanoparticles, the following step of thermolysis aids particle growth and removal of the organic part. Thus formed arrays contain Au nanoparticles. Such arrays are patterned into ≈10 μm wide stripes between Au contact pads on SiO₂/Si substrates to realize electrical rectification. Under forward bias, the device exhibits a threshold voltage of +4.3 V and a high current rectification ratio of 3 × 10⁵, which are stable over many repetitive measurements. The threshold voltage of the rectifier can be reduced by applying an electric stress or by varying the electron dosage used for array formation. The nanoparticle rectifier element could be transferred onto flexible substrates such as PDMS, where the nanoparticle coupling is influenced by swelling of the substrate. Obviously, the nanoparticle size, shape, and the spacing in array are all important for the rectifier device performance. Based on the electrical measurements the mechanism of rectification is found to be due to switching of electrical conduction with applied bias, from short–distance tunneling to F–N type tunneling followed by transient filament formation.     

Control of Optical Hysteresis in Block Copolymer Photonic Gels: A Step Towards Wet Photonic Memory Films

Polystyrene‐block‐poly(2‐vinyl pyridine) (PS‐b‐P2VP) block copolymer photonic gels are fabricated that exhibit controllable optical hysteresis in response to a cyclic pH sweep. The optical hysteresis is tuned by controlling the ion‐pairing affinity between various anions and the protonated pyridinium ions on the P2VP block, which is highly dependent on the hydration energy of the ions, the dielectric constant of the solvent, and the ionic strength of the medium. The pH coercivity defining the magnitude of hysteresis of the photonic gels could be varied from 0.26 to 7.4. Photonic gel films with strong optical hysteresis can serve as wet photonic memory films where information can be cyclically recorded and erased at least 15 times and maintained for at least 96 h. The memory colors can be further tuned by selection of the copolymer molecular weight.     

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.     

Highly Photoluminescent and Environmentally Stable Perovskite Nanocrystals Templated in Thin Self‐Assembled Block Copolymer Films

Ordered nanostructured crystals of thin organic–inorganic metal halide perovskites (OIHPs) are of great interest to researchers because of the dimensional‐dependence of their photoelectronic properties for developing OIHPs with novel properties. Top‐down routes such as nanoimprinting and electron beam lithography are extensively used for nanopatterning OIHPs, while bottom‐up approaches are seldom used. Herein, developed is a simple and robust route, involving the controlled crystallization of the OIHPs templated with a self‐assembled block copolymer (BCP), for fabricating nanopatterned OIHP films with various shapes and nanodomain sizes. When the precursor solution consisting of methylammonium lead halide (MAPbX₃, X = Br^?, I^?) perovskite and poly(styrene)‐block‐poly(2‐vinylpyridine) (PS‐b‐P2VP) is spin‐coated on the substrate, a nanostructured BCP is developed by microphase separation. Spontaneous crystallization of the precursor ions preferentially coordinated with the P2VP domains yields ordered nanocrystals with various nanostructures (cylinders, lamellae, and cylindrical mesh) with controlled domain size (≈40–72 nm). The nanopatterned OIHPs show significantly enhanced photoluminescence (PL) with high resistance to both humidity and heat due to geometrically confining OIHPs in and passivation with the P2VP chains. The self‐assembled OIHP films with high PL performance provide a facile control of color coordinates by color conversion layers in blue‐emitting devices for cool‐white emission.     

The Very‐Low Shear Modulus of Multi‐Walled Carbon Nanotubes Determined Simultaneously with the Axial Young's Modulus via in situ Experiments

The natural frequencies (f) as a function of the length (L) of single, multi‐walled carbon nanotubes (CNTs) are measured using the electric‐field‐induced resonance method together with the “nanoknife” technique for cutting nanotubes to the desired length. The experimental f‐L data for short CNTs are found to be adequately described by the Timoshenko beam model, but not by the widely‐used Euler‐Bernouilli beam model. The failure of the Euler‐Bernouilli beam model is due to its neglect of the significant effect of shear deformation caused by the extremely‐anisotropic mechanical properties of CNTs. The axial Young's modulus and radial shear modulus of CNTs are obtained simultaneously through fitting the experimental f‐L data with the Timoshenko beam model.