Tuning Internal Strain in Metal-Organic Frameworks via Vapor Phase Infiltration for CO2 Reduction

A gas-phase approach to form Zn coordination sites on metal-organic frameworks (MOFs) by vapor-phase infiltration (VPI) was developed. Compared to Zn sites synthesized by the solution-phase method, VPI samples revealed approximately 2.8 % internal strain. Faradaic efficiency towards conversion of CO₂ to CO was enhanced by up to a factor of four, and the initial potential was positively shifted by 200-300 mV. Using element-specific X-ray absorption spectroscopy, the local coordination environment of the Zn center was determined to have square-pyramidal geometry with four Zn-N bonds in the equatorial plane and one Zn-OH₂ bond in the axial plane. The fine-tuned internal strain was further supported by monitoring changes in XRD and UV/Visible absorption spectra across a range of infiltration cycles. The ability to use internal strain to increase catalytic activity of MOFs suggests that applying this strategy will enhance intrinsic catalytic capabilities of a variety of porous materials.

SCIP: a new simultaneous vapor phase coating and infiltration process for tougher and UV-resistant polymer fibers

Simultaneous coating and infiltration of Kevlar fibers with two different inorganic materials significantly improves the modulus of toughness of the polymer and provides protection against UV-induced degradation.

Atomic-Layer-Deposition Growth of an Ultrathin HfO2 Film on Graphene

Direct growth of an ultrathin gate dielectric layer with high uniformity and high quality on graphene remains a challenge for developing graphene-based transistors due to the chemically inert surface properties of graphene. Here, we develop a method to realize atomic-layer-deposition (ALD) growth of an ultrathin high-κ dielectric layer on graphene through premodifying the graphene surface using electron beam irradiation. An amorphous carbon layer induced by electron beam scanning is formed on graphene and then acts as seeds for ALD growth of high-κ dielectrics. A uniform HfO₂ layer with an equivalent oxide thickness of 1.3 nm was grown as a gate dielectric for top-gate graphene field-effect transistors (FETs). The achieved gate capacitance is up to 2.63 μF/cm², which is the highest gate capacitance on a graphene solid-state device to date. In addition, the fabricated top-gate graphene FETs present a high carrier mobility of up to 2500 cm²/(V·s) and a negligible gate leakage current of down to 0.1 mA/cm², showing that the ALD-grown HfO₂ dielectric layer is highly uniform and of very high quality.

Porphyrins as Templates for Site-Selective Atomic Layer Deposition: Vapor Metalation and in Situ Monitoring of Island Growth

Examinations of enzymatic catalysts suggest one key to efficient catalytic activity is discrete size metallo clusters. Mimicking enzymatic cluster systems is synthetically challenging because conventional solution methods are prone to aggregation or require capping of the cluster, thereby limiting its catalytic activity. We introduce site-selective atomic layer deposition (ALD) on porphyrins as an alternative approach to grow isolated metal oxide islands that are spatially separated. Surface-bound tetra-acid free base porphyrins (H2TCPP) may be metalated with Mn using conventional ALD precursor exposure to induce homogeneous hydroxide synthetic handles which acts as a nucleation point for subsequent ALD MnO island growth. Analytical fitting of in situ QCM mass uptake reveals island growth to be hemispherical with a convergence radius of 1.74 nm. This growth mode is confirmed with synchrotron grazing-incidence small-angle X-ray scattering (GISAXS) measurements. Finally, we extend this approach to other ALD chemistries to demonstrate the generality of this route to discrete metallo island materials.

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.

Graphene oxide monolayers as atomically thin seeding layers for atomic layer deposition of metal oxides

Graphene oxide (GO) was explored as an atomically-thin transferable seed layer for the atomic layer deposition (ALD) of dielectric materials on any substrate of choice. This approach does not require specific chemical groups on the target surface to initiate ALD. This establishes GO as a unique interface which enables the growth of dielectric materials on a wide range of substrate materials and opens up numerous prospects for applications. In this work, a mild oxygen plasma treatment was used to oxidize graphene monolayers with well-controlled and tunable density of epoxide functional groups. This was confirmed by synchrotron-radiation photoelectron spectroscopy. In addition, density functional theory calculations were carried out on representative epoxidized graphene monolayer models to correlate the capacitive properties of GO with its electronic structure. Capacitance-voltage measurements showed that the capacitive behavior of Al2O3/GO depends on the oxidation level of GO. Finally, GO was successfully used as an ALD seed layer for the deposition of Al2O3 on chemically inert single layer graphene, resulting in high performance top-gated field-effect transistors.

Temperature-dependent infiltration of polymers during sequential exposures to trimethylaluminum

Atomic layer deposition provides the opportunity to introduce nanoscale inorganic coatings to organic polymers creating coatings of varied compositions of finish with distinctive interfaces. Prior research has shown that ALD materials nucleation on polymers varies in composition and structure based on how the precursor interacts with the polymer chemistry and the process conditions. To study this in more detail, in situ quartz crystal microgravimetry is employed to understand the infiltration and saturation behavior of trimethylaluminum in polyamide-6, poly(acrylic acid), poly(ethylene terephthalate), and poly(methyl methacrylate). Emphasis is placed on understanding reactive vapor diffusion into these polymers as the exposure temperature is varied. Finally, we propose potential growth mechanisms based on the temperature-dependent observations in this work that enables the ability to produce a customized interface for ALD materials growth on polymer substrates.

Integrative self-assembly of functional hybrid nanoconstructs by inorganic wrapping of single biomolecules, biomolecule arrays and organic supramolecular assemblies

Synthesis of functional hybrid nanoscale objects has been a core focus of the rapidly progressing field of nanomaterials science. In particular, there has been significant interest in the integration of evolutionally optimized biological systems such as proteins, DNA, virus particles and cells with functional inorganic building blocks to construct mesoscopic architectures and nanostructured materials. However, in many cases the fragile nature of the biomolecules seriously constrains their potential applications. As a consequence, there is an on-going quest for the development of novel strategies to modulate the thermal and chemical stabilities, and performance of biomolecules under adverse conditions. This feature article highlights new methods of "inorganic molecular wrapping" of single or multiple protein molecules, individual double-stranded DNA helices, lipid bilayer vesicles and self-assembled organic dye superstructures using inorganic building blocks to produce bio-inorganic nanoconstructs with core-shell type structures. We show that spatial isolation of the functional biological nanostructures as "armour-plated" enzyme molecules or polynucleotide strands not only maintains their intact structure and biochemical properties, but also enables the fabrication of novel hybrid nanomaterials for potential applications in diverse areas of bionanotechnology.

ALD/MLD of novel layer-engineered Zn-based inorganic-organic hybrid thin films using heterobifunctional 4-aminophenol as an organic precursor

Here we present novel hybrid inorganic-organic thin films of the (-Zn-O-C(6)H(4)-NH-)(n) type deposited in an atomic/molecular layer-by-layer manner through sequential gas-surface reactions of separately introduced inorganic (diethyl zinc) and organic (4-aminophenol) precursor pulses. The organic precursor employed is heterobifunctional (containing both hydroxyl and amino groups) and possesses a rigid benzene backbone; these precursor characteristics are believed to suppress the unwanted double surface reactions and promote the ideal growth mechanism such that the film thickness is linearly controlled by the number of deposition cycles. The appreciably high growth rate of ~1.1 Å per cycle is found to remain constant in the deposition temperature range of 140-200 °C, but in practice our atomic/molecular layer deposition (ALD/MLD) process yields high-quality, uniform, smooth and relatively air-stable films even in a much wider temperature range from 140 up to 330 °C. The refractive index of the films is ~1.94 ± 0.01 independent of the deposition temperature and the density ranges within 1.5-1.7 g cm(-3).

A route to nanoscopic materials via sequential infiltration synthesis on block copolymer templates

Sequential infiltration synthesis (SIS), combining stepwise molecular assembly reactions with self-assembled block copolymer (BCP) substrates, provides a new strategy to pattern nanoscopic materials in a controllable way. The selective reaction of a metal precursor with one of the pristine BCP domains is the key step in the SIS process. Here we present a straightforward strategy to selectively modify self-assembled polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) BCP thin films to enable the SIS of a variety of materials including SiO(2), ZnO, and W. The selective and controlled interaction of trimethyl aluminum with carbonyl groups in the PMMA polymer domains generates Al-CH(3)/Al-OH sites inside the BCP scaffold which can seed the subsequent growth of a diverse range of materials without requiring complex block copolymer design and synthesis.

Improved mechanical stability of dried collagen membrane after metal infiltration

A few percent of transition metals impregnated inside some biological organisms in nature remarkably improve such organisms' mechanical stability. Although the lure to emulate them for development of new biomimetic structural materials has been great, the practical advances have been rare because of the lack of proper synthetic approaches. Multiple pulsed vapor phase infiltration proved successful for the preparation of such transition metal impregnated materials with highly improved mechanical stability. The artificially infiltrated metals (Al, Ti, or Zn) from gas phase lead to around 3 times increase of toughness (in terms of breaking energy) of natural collagen in a dried state. In addition, the infiltrated metals apparently induce considerable crystallographic changes in the natural collagen structures. This infiltration approach can be used as guide for the synthesis of bioinspired structural materials related to metal infiltration.

Temperature-dependent subsurface growth during atomic layer deposition on polypropylene and cellulose fibers

Nucleation and subsequent growth of aluminum oxide by atomic layer deposition (ALD) on polypropylene fiber substrates is strongly dependent on processing temperature and polymer backbone structure. Deposition on cellulose cotton, which contains ample hydroxyl sites for ALD nucleation and growth on the polymer backbone, readily produces a uniform and conformal coating. However, similar ALD processing on polypropylene, which contains no readily available active sites for growth initiation, results in a graded and intermixed polymer/inorganic interface layer. The structure of the polymer/inorganic layer depends strongly on the process temperature, where lower temperature (60 degrees C) produced a more abrupt transition. Cross-sectional transmission electron microscopy images of polypropylene fibers coated at higher temperature (90 degrees C) show that non-coalesced particles form in the near-surface region of the polymer, and the particles grow in size and coalesce into a film as the number of ALD cycles increases. Quartz crystal microbalance analysis on polypropylene films confirms enhanced mass uptake at higher processing temperatures, and X-ray photoelectron spectroscopy data also confirm heterogeneous mixing between the aluminum oxide and the polypropylene during deposition at higher temperatures. The strong temperature dependence of film nucleation and subsurface growth is ascribed to a relatively large increase in bulk species diffusivity that occurs upon the temperature-driven free volume expansion of the polypropylene. These results provide helpful insight into mechanisms for controlled organic/inorganic thin film and fiber materials integration.