Self-Assembled Phases of Block Copolymer Blend Thin Films

The patterns formed by self-assembled thin films of blended cylindrical and lamellar polystyrene-<i>b</i>-poly(methyl methacrylate) block copolymers can be either a spatially uniform, single type of nanostructure or separate, coexisting regions of cylinders and lamellae, depending on fractional composition and molecular weight ratio of the blend constituents. In blends of block copolymers with different molecular weights, the morphology of the smaller molecular weight component more strongly dictates the resulting pattern. Although molecular scale chain mixing distorts microdomain characteristic length scales from those of the pure components, even coexisting morphologies exhibit the same domain spacing. We quantitatively account for the phase behavior of thin-film blends of cylinders and lamellae using a physical, thermodynamic model balancing the energy of chain distortions with the entropy of mixing

Gas Transport Selectivity of Ultrathin, Nanoporous, Inorganic Membranes Made from Block Copolymer Templates

We report the fabrication of ultrathin, nanoporous silicon nitride membranes made from templates of regular, nanoscale features in self-assembled block copolymer thin films. The inorganic membranes feature thicknesses less than 50 nm and volume porosities over 30%, with straight-through pores that offer high throughout for gas transport and separation applications. As fabricated, the pores are uniformly around 20 nm in diameter, but they can be controllably and continuously tuned to single-digit nanometer dimensions by atomic layer deposition of conformal coatings. A deviation from expected Knudsen diffusion is revealed for transport characteristics of saturated vapors of organic solvents across the membrane, which becomes more significant for membranes of smaller pores. We attribute this to capillary condensation of saturated vapors within membrane pores, which reduces membrane throughput by over 1 order of magnitude but significantly improves the membrane’s selectivity. Between vapors of acetone and ethyl acetate, we measure selectivities as high as 7:1 at ambient pressure and temperature, 4 times more than the Knudsen selectivity

Quantifying Bulk and Surface Recombination Processes in Nanostructured Water Splitting Photocatalysts via In Situ Ultrafast Spectroscopy

A quantitative description of recombination processes in nanostructured semiconductor photocatalystsone that distinguishes between bulk (charge transport) and surface (chemical reaction) lossesis critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface. This is achieved by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode in a working photoelectrochemical cell. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications that are not perceivable by conventional X-ray diffraction can drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast time scales can limit the overall efficiency even in nanostructures with dimensions smaller than the minority carrier diffusion length. This is particularly true for materials with high carrier concentration, where losses as high as 37% are observed. Our methodology provides a means of evaluating the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity

Enhancing Water Splitting Activity and Chemical Stability of Zinc Oxide Nanowire Photoanodes with Ultrathin Titania Shells

Zinc oxide nanowire photoanodes are chemically stabilized by conformal growth of an ultrathin shell of titania through atomic layer deposition, permitting their stable operation for water splitting in a strongly alkaline solution. Because of the passivation of zinc oxide surface charge traps by titania coating, core/shell nanowire arrays supply a photocurrent density of 0.5 mA/cm² under simulated AM1.5G sunlight at the thermodynamic oxygen evolving potential, demonstrating 25% higher photoelectrochemical water splitting activity compared to as-grown zinc oxide wires. By thermally annealing the zinc oxide wire arrays prior to surface passivation, we further increase the photocurrent density to 0.7 mA/cm²the highest reported value for doped or undoped zinc oxide photoanodes studied under similar simulated sunlight. Photoexcitations at energies above the zinc oxide band gap are converted with efficiency greater than 80%. Photoluminescence measurements of the best-performing nanowire arrays are consistent with improved water splitting activity from removal of deep trap states

Chemically Enhancing Block Copolymers for Block-Selective Synthesis of Self-Assembled Metal Oxide Nanostructures

We report chemical modification of self-assembled block copolymer thin films by ultraviolet light that enhances the block-selective affinity of organometallic precursors otherwise lacking preference for either copolymer block. Sequential precursor loading and reaction facilitate formation of zinc oxide, titanium dioxide, and aluminum oxide nanostructures within the polystyrene domains of both lamellar- and cylindrical-phase modified polystyrene-<i>block</i>-poly(methyl methacrylate) thin film templates. Near-edge X-ray absorption fine structure measurements and Fourier transform infrared spectroscopy show that photo-oxidation by ultraviolet light creates Lewis basic groups within polystyrene, resulting in an increased Lewis base–acid interaction with the organometallic precursors. The approach provides a method for generating both aluminum oxide patterns and their corresponding inverses using the same block copolymer template

Nanostructured Surfaces Frustrate Polymer Semiconductor Molecular Orientation

Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C₆₁-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction

Aberration-Corrected Electron Beam Lithography at the One Nanometer Length Scale

Patterning materials efficiently at the smallest length scales is a longstanding challenge in nanotechnology. Electron-beam lithography (EBL) is the primary method for patterning arbitrary features, but EBL has not reliably provided sub-4 nm patterns. The few competing techniques that have achieved this resolution are orders of magnitude slower than EBL. In this work, we employed an aberration-corrected scanning transmission electron microscope for lithography to achieve unprecedented resolution. Here we show aberration-corrected EBL at the one nanometer length scale using poly­(methyl methacrylate) (PMMA) and have produced both the smallest isolated feature in any conventional resist (1.7 ± 0.5 nm) and the highest density patterns in PMMA (10.7 nm pitch for negative-tone and 17.5 nm pitch for positive-tone PMMA). We also demonstrate pattern transfer from the resist to semiconductor and metallic materials at the sub-5 nm scale. These results indicate that polymer-based nanofabrication can achieve feature sizes comparable to the Kuhn length of PMMA and ten times smaller than its radius of gyration. Use of aberration-corrected EBL will increase the resolution, speed, and complexity in nanomaterial fabrication

Photo-Cross-Linkable Azide-Functionalized Polythiophene for Thermally Stable Bulk Heterojunction Solar Cells

We have synthesized photo-cross-linkable azide-functionalized poly­(3-hexylthiophene) to explore improvements in the thermal stability of bulk heterojunction solar cells. Exposing blends of photo-cross-linkable polythiophene and [6,6]-phenyl-C₆₁-butyric acid methyl ester to ultraviolet light preferentially cross-linked the polythiophene without degrading its optical or electrical properties. X-ray scattering measurements showed that cross-linking slightly compacted the polythiophene chain lamellar stacking while increasing the polymer crystal coherence length by 20%. Optimized solar cells having cross-linked active blend layers retained 65% of their initial photovoltaic power conversion efficiency after 40 h of thermal annealing at 110 °C, while devices using un-cross-linked commercial polythiophene underwent significant phase separation and retained less than 30% of their initial efficiency after annealing

Air–Liquid Interfacial Self-Assembly of Conjugated Block Copolymers into Ordered Nanowire Arrays

The ability to control the molecular packing and nanoscale morphology of conjugated polymers is important for many of their applications. Here, we report the fabrication of well-ordered nanoarrays of conjugated polymers, based on the self-assembly of conjugated block copolymers at the air–liquid interface. We demonstrate that the self-assembly of poly(3-hexylthiophene)-<i>block</i>-poly(ethylene glycol) (P3HT-<i>b</i>-PEG) at the air–water interface leads to large-area free-standing films of well-aligned P3HT nanowires. Block copolymers with high P3HT contents (82–91%) formed well-ordered nanoarrays at the interface. The fluidic nature of the interface, block copolymer architecture, and rigid nature of P3HT were necessary for the formation of well-ordered nanostructures. The free-standing films formed at the interface can be readily transferred to arbitrary solid substrates. The P3HT-<i>b</i>-PEG films are integrated in field-effect transistors and show orders of magnitude higher charge carrier mobility than spin-cast films, demonstrating that the air–liquid interfacial self-assembly is an effective thin film fabrication tool for conjugated block copolymers