Self-assembled Block Copolymer Templates Research Articles

(Invited) Extended Thin Film Platinum Group Metal Electrocatalysts with Different Mesoscale Morphologies

The lack of sufficiently active oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts is a bottleneck to the proliferation of PEM fuel cells and water electrolyzers. Commercially available PGM electrocatalysts fail to attain DOE technical targets due to instabilities and activity losses incurred at low loadings. Morphology control of PGM nanostructures is an enticing route towards activity and stability enhancements of electrocatalysts, but not much is known about the structure-property relationships between PGM mesoscale geometry (5 to 100 nm) and their activity and stability.This talk presents our work on model Pt extended surface electrocatalysts of tunable feature sizes and morphologies prepared from self-assembled block copolymer templates. The morphology, either perpendicular lamellae, cylinders, or perforated cylinders, of the extended surface electrocatalyst was controlled by the block copolymer volume fraction. The periodic lateral feature sizes (10 to 40 nm in this work) were controlled by the block copolymer degree of polymerization. The self-assembled block copolymers were transformed into extended surfaces using sequential infiltration synthesis to prepare aluminum oxide followed by reactive ion etching to remove the block copolymer template. Pt was sputtered onto the aluminum oxide supports. ORR data in a RDE with the new extended nanostructured thin films of Pt showed mass activity values of 0.35 A mgPt-1 to 0.46 A mgPt -1 at 0.9 V vs. RHE. The measured electro-chemically active surface area (ECSA) values obtained from hydrogen underpotential deposition were 64 to 70 m2 gPt -1. The lower values hailed from lamellae morphology while the larger values corresponded to a cylindrical morphology. The measured mass activity values, which have no cobalt or nickel additives and are unoptimized and display competitive mass activity values for ORR. We envision that the block copolymer templated extended surfaces could be an alternative to 3M’s state-of-the-art nanostructured thin film technology.

Structural Changes during the Conversion Reaction of Tungsten Oxide Electrodes with Tailored, Mesoscale Porosity.

In-plane tungsten oxide nanostructures, including hexagonally patterned cylinders and holes in a matrix, were fabricated via sequential infiltration synthesis (SIS) on self-assembled block copolymer templates. Using the tailored morphology and porosity of these model electrodes with in situ grazing incidence small-angle X-ray scattering, the intrinsic structural change of nanoscale active materials during the conversion reaction of WO3 + 6Li ↔ W + 3Li2O was investigated at controlled electrochemical conditions. Reversible electrode volume expansion and contraction was observed during lithiation and delithiation cycles, respectively. The potential where the electrode's thickness expansion started was ∼1.6 V, which is close to the thermodynamically expected one for the conversion reaction of WO3 with lithium (1.65 V). The temporal evolution of the electrode volume at constant electrode potentials revealed high overpotential for bulk lithiation and slow conversion reaction kinetics, despite the tailored porosity of the SIS electrodes. Oxide cylinders showed a smaller overall electrode thickness change, likely due to unconstrained lateral volume change, as compared to a matrix with holes. On the other hand, better connectivity and guided volume change of the latter electrode morphology provided improved cycling stability. In addition, heterogeneity in an electrode, from internal pores and density gradients, was found to aggravate the fragmentation of the electrode during the conversion reaction. Insights into oxide conversion reaction kinetics and the relationship between electrode mesostructure and cycling behavior obtained from this study can help guide the more rational design of conversion electrodes for high-performing batteries.

(Invited) Nanopatterning Functional Metal Oxide Nanostructures By Vapor-Phase Infiltration in Polymer Templates

Infiltration synthesis is a process technique derived from atomic layer deposition (ALD), where polymeric templates, such as those lithographically patterned and self-assembled, are infiltrated by vapor-phase inorganic precursors to form novel organic-inorganic hybrids with enhanced materials properties. These hybrids can be further transformed by selectively removing the organic matrix to finally form metal oxide nanostructures with morphology and positional registry as dictated and designed by the starting polymer templates. In this talk, I will highlight a few examples of our recent efforts to generate functional inorganic nanostructures by using infiltration synthesis including, patterning arbitrary metal oxide nanostructures with sub-40 nm linewidth and over 15 aspect ratio, structural characteristics extremely difficult to achieve by conventional microfabrication techniques; fully CMOS compatible, wafer-scale synthesis and integration of in-plane aligned ZnO nanowire array phototransistors; and three-dimensional (3D) ZnO nanomesh structures derived from self-assembled block copolymer (BCP) templates with layer-number-dependent percolative electrical conductance.

Understanding the Influence of Porosity on Product Selectivity for Copper CO2 Reduction Electrocatalysts

The carbon dioxide reduction reaction (CO2RR) provides a process for producing valuable chemicals and energy-dense fuels from CO2; however, selectively producing a specific high-value product remains challenging. In this contribution, we present our study on how to harness nanoporous morphologies to address this challenge. Our work is driven by several results in literature which have now established that nanoporous morphologies can affect the CO2RR product distribution on Cu catalysts (1-3). To gain insight into this effect, we use model nanoporous Cu catalysts prepared using a self-assembled block copolymer (BCP) template. The BCP-template approach allows precise control over pore size and geometry as well as thickness. Using these structures and the dependence of the product distribution on pore sizes, pH, and mass transport, we will discuss the influence of nanoporous morphologies on the CO2RR efficacy and the implications for future CO2RR catalyst design, particularly within the context of reaction intermediate trapping for selectivity towards high-value products. References (1) A. Dutta, M. Rahaman, N. C. Luedi, M. Mohos, and P. Broekmann, ACS Catal., 6(6), 3804 (2016). (2) K. D. Yang, W. R. Ko, J. H. Lee, S. J. Kim, H. Lee, M. H. Lee, and K. T. Nam, Angew. Chem. Int. Ed., 56(3), 796 (2017). (3) T. Kim, A. Kargar, Y. Luo, R. Mohammed, E. Martinez-Loran, A. Ganapathi, P. Shah, and D. P. Fenning, ACS Appl. Energy Mater., 1(5), 1965 (2018).

Relationship between Nd content and magnetic properties of Nd2Fe14B/Nd nanocomposites chemically synthesized using self-assembled block copolymer templates

To synthesize nanocomposite magnets with controlled high magnetic performance, Nd2Fe14B/Nd nanocomposites were chemically synthesized using self-assembled block copolymer templates. The Nd, Fe, and B ratios in the raw materials used to synthesize the nanocomposites were adjusted to control the nanocomposites’ magnetic properties. Nd2Fe14B/Nd nanocomposites with large coercivity were obtained from a Nd-rich mixture of raw materials. In the nanocomposites, Nd2Fe14B was surrounded by a nonmagnetic Nd phase that weakened the magnetic coupling of Nd2Fe14B grains and prevented magnetic field inversion. With increasing Nd content, the coercivity increased and the magnetization decreased. The simple process described herein, which involved chemical synthesis using self-assembled block copolymer templates, enabled the synthesis of nanocomposite magnets with controlled high magnetic performance through adjustment of the ratios of the raw materials.

LiCoO2/Li7La3Zr2O12 nanocomposite cathodes synthesized via self-assembled block copolymer templates and used in all-solid-state lithium batteries

Nanocomposite cathodes comprising an active cathode material (LiCoO2) and a solid lithium electrolyte (Li7La3Zr2O12) were synthesized by using a self-assembled polystyrene‑b‑poly(4‑vinylpyridine) block copolymer template containing precursors of the active cathode material and solid electrolyte. Calcination at 1023 K was used to remove the block copolymer template and crystallize the active cathode material and solid electrolyte. The molecular weight of the block copolymer template was controlled to optimize the structure of the nanocomposite cathode and maximize capacity. When the nanocomposite cathode that was obtained by using the block copolymer template with the highest molecular weight was used in an all-solid-state lithium battery, the obtained discharge capacity per electrode (98.2 mAh/g) was substantially higher than literature values for other cathodes used in all-solid-state lithium batteries. We conclude that increasing the molecular weight of the block copolymer template allowed for greater dispersion of the precursors and binding of the well-dispersed nanoparticles of active cathode material and solid lithium electrolyte, which promoted the formation of fine conducting paths and an electrochemically effective interface between the active cathode material and solid lithium electrolyte.

Tunable Fabrication of SERS-Active Noble Metal Nanoarrays

This work demonstrates the fabrication of uniform gold nanoarrays by utilizing self-assembled block copolymer (BCP) template, which enables us to finely tune the gold nanostructures by simply adjusting reaction time. We fabricated a series of evolutional gold nanostructures with different reaction time, and investigated their corresponding SERS effects. Results show that the nanostructures have strong size-dependent Raman signal enhancement. This work provides a fast, simple and reproducible way to fabrication of SERS-active substrate with tunable nanostructures.

Ordered Pt Nanopattern Catalysts through Self-Assembled Block Copolymer Template

Patterning of metallic nanoparticles (NPs) via BC micelles plays an important role in the manufacture of conductive integrated devices. BCs containing a metal-binding block of poly(vinylpyridine) (P2VP or P4VP) can be used as a direct chemical reagent to coordinate to or to be protonated by metal ions and compounds such as chloroplatinic acid (H2PtCl6) or auric acid (HAuCl4). In this work, Platinum nanoparticles (Pt NPs) were synthesized by dip-coated technique on titanium wafers. The dip-coating withdrawal rate and concentration of H2PtCl6 precursor had been investigated as two of most important factors on the morphology of Pt-NP arrays. The result shows that diameter of nanodots decreases with the increase of withdrawal rate. The initial ECSA and durability of the Pt/Ti catalysts, with an initial ECSA of 106.5 m2.g-1 and a nearly zero particle-aggregation during 3000-cycle CV, can be controlled through particle size and spacing.

Synthesis of metal oxide nanocomposites from self-assembled block copolymer templates as cathode materials for all-solid-state lithium batteries

Self-assembled structures are useful for fabrication of nanostructures as a sustainable process with minimum energy consumption. Alternately stacked layers on a nanometre scale are produced by adsorption process denoted as layer by layer process, however, shape is limited in layers and chemical composition is also limited. Superlattice structures are produced by sputtering or self-assembly process using nanoparticles. But the construction of three-dimensional ordered inorganic nanocomposite structures remains a great challenge. All-solid-state lithium batteries (LiBs) are attracting much attention as innovative devices for upsized safety secondary batteries with high energy density. However, bulk-type all-solid-state LiBs face a number of hurdles, particularly the construction of an electrochemically favourable interface between the solid Li electrolytes and the active materials. In this study, we synthesised inorganic nanocomposite cathode materials by using self-assembled block copolymers (BCPs) as templates; this nanocomposite was composed of Li0.5La0.5TiO3 (LLT) as the electrolyte and LiCoO2 (LCO) as the cathode active material. Several LCO/LLT ratios were examined with the goal of producing a nanocomposite for use in all-solid-state LiBs. This nanostructured material, with the electrochemically favourable interface between electrode and electrolyte, resulted in a high capacity even at a high LCO content. The process we used to prepare the nanocomposite is expected to be versatile for producing other highly functional devices, such as quantum dot solar cells, thermoelectrics, and solid oxide fuel cells.

The effect of the LiCoO2/Li7La3Zr2O12 ratio on the structure and electrochemical properties of nanocomposite cathodes for all-solid-state lithium batteries

A LiCoO2/Li7La3Zr2O12 nanocomposite synthesized using a self-assembled block copolymer template exhibited a high capacity per active material, 135 mA h g−1.

Synthesis and magnetic properties of FePt nanocomposite magnets via self-assembled block copolymer templates

Nanocomposite magnets consisting of hard and soft magnetic phases with controlled structures have attracted much attention for permanent magnetic applications owing to their large energy products. However, construction of structurally controlled nanocomposite magnets with the high magnetic performance that is theoretically predicted remains a challenge. We recently developed a novel route for synthesis of inorganic nanocomposites with three-dimensional, periodically ordered spheres, cylinders, or layers in a matrix using self-assembled block copolymer templates. Metal complexes were selectively introduced into a specific copolymer block of the template, and subsequent removal of the copolymers produced nanocomposites that replicated the template structure. Here, we describe a novel method for fabricating FePt nanocomposite magnets consisting of exchange-coupled hard and soft magnetic phases via self-assembled block copolymer templates. The magnetization curves of nanocomposite samples showed single-phase-like hysteresis loops that confirmed efficient exchange coupling between the hard and soft phases. The measured maximum energy product (BH)max exceeded the theoretical value for non-exchange-coupled isotropic FePt. Our method of compositing an inexpensive and abundant element (iron) to yield nanocomposites with properties superior to those of homogeneous materials demonstrates that it is possible to create novel, highly functional materials in a way that is much more cost-effective than conventional approaches.

Palladium nanoparticle array-mediated semiconductor bonding that enables high-efficiency multi-junction solar cells

A detailed study on the application of Pd nanoparticle arrays, produced by self-assembled block copolymer templates, in bonding of III–V-based solar cell materials was carried out. The Pd nanoparticle array-mediated bonding (mechanical stacking) of GaAs-based thin-films (cells) was readily performed on the surface of GaAs or InP-based substrates (cells) to form multi-junction device architectures. Using the optimized Pd NP array, a 30.4%-efficiency four-junction two-terminal cell, consisting of an InGaP/GaAs top cell and an InGaAsP/InGaAs bottom cell, was achieved owing to the excellent electrical and optical bonding properties (bonding resistance, 1.81 Ω cm2; optical loss, 2.9%). Together with the verification of the long-term reliability of the Pd nanoparticle array-mediated bonding, our approach would become practically attractive for producing high-efficiency multi-junction solar cells.

Directing Hybrid Structures by Combining Self-Assembly of Functional Block Copolymers and Atomic Layer Deposition: A Demonstration on Hybrid Photovoltaics.

The simplicity and versatility of block copolymer self-assembly offers their use as templates for nano- and meso-structured materials. However, in most cases, the material processing requires multiple steps, and the block copolymer is a sacrificial building block. Here, we combine a self-assembled block copolymer template and atomic layer deposition (ALD) of a metal oxide to generate functional hybrid films in a simple process with no etching or burning steps. This approach is demonstrated by using the crystallization-induced self-assembly of a rod-coil block copolymer, P3HT-b-PEO, and the ALD of ZnO. The block copolymer self-assembles into fibrils, ∼ 20 nm in diameter and microns long, with crystalline P3HT cores and amorphous PEO corona. The affinity of the ALD precursors to the PEO corona directs the exclusive deposition of crystalline ZnO within the PEO domains. The obtained hybrid structure possesses the properties desired for photovoltaic films: donor-acceptor continuous nanoscale interpenetrated networks. Therefore, we integrated the films into single-layer hybrid photovoltaics devices, thus demonstrating that combining self-assembly of functional block copolymers and ALD is a simple approach to direct desired complex hybrid morphologies.

Graded porous inorganic materials derived from self-assembled block copolymer templates.

Graded porous inorganic materials directed by macromolecular self-assembly are expected to offer unique structural platforms relative to conventional porous inorganic materials. Their preparation to date remains a challenge, however, based on the sparsity of viable synthetic self-assembly pathways to control structural asymmetry. Here we demonstrate the fabrication of graded porous carbon, metal, and metal oxide film structures from self-assembled block copolymer templates by using various backfilling techniques in combination with thermal treatments for template removal and chemical transformations. The asymmetric inorganic structures display mesopores in the film top layers and a gradual pore size increase along the film normal in the macroporous sponge-like support structure. Substructure walls between macropores are themselves mesoporous, constituting a structural hierarchy in addition to the pore gradation. Final graded structures can be tailored by tuning casting conditions of self-assembled templates as well as the backfilling processes. We expect that these graded porous inorganic materials may find use in applications including separation, catalysis, biomedical implants, and energy conversion and storage.

Manipulating the Morphology and Surface Plasmon Resonance of Silver Nanoparticle Arrays with Self-Assembled Block Copolymer Templates

Two-dimensional hexagonal ordered perforated triblock copolymer poly(styrene-b-butadiene-b-styrene) (SBS) films were fabricated via solvent annealing process and used as templates to modulate the morphologies of the silver nanoparticle arrays deposited on them by gas phase cluster beam deposition. The evolution of the extinction spectra of the silver nanoparticle arrays with the deposition time was in situ investigated. Their surface plasmon resonance properties can be significantly manipulated with the formation of the regular nanoparticle arrays that decorating the ordered SBS templates through near-field plasmon coupling.

Morphology‐Driven Modulation of Charge Transport in Radical/Ion‐Containing, Self‐Assembled Block Copolymer Platform

A TEMPO-substituted ionic liquid was selectively incorporated into well-defined, self-assembled block copolymer templates, which served as an active layer for organic nonvolatile memory. Phase structures (sphere, cylinder, and lamellae) and their orientation modulated the resistive switching behavior, which demonstrated the unprecedented, morphology-driven charge transport in the organic electronic devices.

Non-lift-off Block Copolymer Lithography of 25 nm Magnetic Nanodot Arrays

Although nanolithographic techniques based on self-assembled block copolymer templates offer tremendous potential for fabrication of large-area nanostructure arrays, significant difficulties arise with both the lift-off and etch processes typically used for pattern transfer. These become progressively more important in the limit of extreme feature sizes. The few techniques that have been developed to avoid these issues are quite complex. Here, we demonstrate successful execution of a nanolithographic process based on solvent annealed, cylinder-forming, easily degradable, polystyrene-b-polylactide block copolymer films that completely avoids lift-off in addition to the most challenging aspects of etching. We report a "Damascene-type" process that overfills the polystyrene template with magnetic metal, employs ion beam milling to planarize the metal surface down to the underlying polystyrene template, then exploits the large etch rate contrast between polystyrene and typical metals to generate pattern reversal of the original template into the magnetic metal. The process is demonstrated via formation of a large-area array of 25 nm diameter ferromagnetic Ni(80)Fe(20) nanodots with hexagonally close-packed order. Extensive microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation. We argue that the approach is generalizable to a wide variety of materials, is scalable to smaller feature sizes, and critically, minimizes etch damage, thus preserving the essential functionality of the patterned material.

Enhanced thermal stability of monodispersed silver cluster arrays assembled on blockcopolymer scaffolds

Triblock copolymer poly(styrene-b-butadiene-b-styrene) (SBS) films with long-rangeordered self-assembled nanopatterns are used as templates to selectively adsorb soft-landingsilver clusters. Closely spaced cluster arrays with high monodispersity are formed throughthe confinement of the block copolymer scaffolds, and show a much enhancedthermal stability as compared with the cluster assemblies on the surfaces of covalentamorphous solids, or even on the disordered SBS films. Their morphologies arebarely influenced by long time thermal annealing at a temperature as high as180 °C, while in the latter case intense aggregations and coalescences of silver clusters arecommonly observed upon annealing. The different thermal stabilities of the clusterassemblies also induce different evolutions of their optical extinction spectra underannealing. This promises a simple way to control the monodispersity and thermalstability of metal cluster assembly via self-assembled block copolymer template.

Fabrication of Vertical Carbon Nanotube Arrays from Self-Assembled Block Copolymer Films

Highly uniform vertical carbon nanotube arrays were prepared by a novel hybrid approach combining a plasma-enhanced chemical vapor deposition (PECVD) process with self-assembled block copolymer nanotemplate. Monodisperse catalyst nanoparticles were prepared on a substrate using self-assembled nanoporous block copolymer templates. The subsequent PECVD growth of vertical CNTs from the nanopatterned catalyst particles provided highly oriented vertical CNT arrays, which are potentially useful for device integration of CNTs.

Electron spectroscopy and microscopy on chromium oxide nanowires on templated block copolymers

Electronic and geometric properties of chromium oxide nanowires deposited on self-assembled block copolymer templates are evaluated by means of synchrotron radiation based spectroscopic methods like X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The geometric distribution of the nanowires is monitored with X-ray photoelectron emission microscopy (XPEEM) and atomic force microscopy (AFM). XAS and XPS of Cr 2p emissions for chromium oxide nanostructures determine an oxidation state different from the most common chromium oxides and additional satellite peaks are derived from localization effects induced by the nanostructuring.