Sequential infiltration synthesis

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Sequential infiltration synthesis (SIS) is a technique derived from atomic layer deposition (ALD) in which a polymer is infused with inorganic material using sequential, self-limiting exposures to gaseous precursors, allowing precise control over the composition, structure, and properties of product materials.^{[1][2][3][4][5][6][7]} This synthesis involves metal-organic vapour-phase precursors and co-reactants dissolving and diffusing into polymers, interacting with the polymers functional groups via reversible complex formation and/or irreversible chemical reactions, and yielding desired composite materials, which may be nanostructured. The metal-organic precursor (A) and co-react vapour (B) are supplied in an alternating ABAB sequence. Following SIS, the organic phase can be removed thermally or chemically to leave only the inorganic components behind. The precise control over the infiltration and synthesis via SIS allows the creation of materials with tailored properties such as composition, mechanics, stoichiometry, porosity, conductivity, refractive index, and chemical functionality on the nanoscale.^[8]

SIS has been applied in various fields, including electronics, energy storage, and catalysis, due to its ability to precisely control material properties. SIS is sometimes referred to as "multiple pulsed vapour-phase infiltration" (MPI),^[9] "vapour phase infiltration" (VPI)^[10] or "sequential vapour infiltration" (SVI).^[11]

SIS involves the 3D distribution of functional groups in polymers, while its predecessor, ALD, is associated with the impermeable 2D distribution of reactive sites on solid surfaces. In typical ALD processes, the precursor pulses are much shorter in duration and have lower partial pressure compared to SIS, as they only need to provide sufficient exposure to saturate the surface chemical groups on the substrate surface.^[1] In SIS, the partial pressures and exposure times for the precursor pulse are typically larger compared to ALD to allow for sufficient precursor to infiltrate a 3D volume by dissolution and diffusion.^[1] SIS utilises the diffusive nature of precursor transport within polymers, making the distribution of precursors within the material sensitive to time, pressure, temperature, and polymer chemistry and microstructure.^[1]

The diffusion of precursors below the surfaces of polymers during ALD was observed in 2005 by the Steven M. George group when they observed that polymers could uptake trimethylaluminum (TMA) via absorption within their free volume.^[12] In this study, the interactions between the ALD precursors and the polymer functional groups were not recognised, and the diffusion of precursors into polymer films was considered a problem. Hence, the diffusion and reactions of ALD precursors into polymer films were considered challenges to address rather than opportunities. However, potential benefits of these phenomena were demonstrated by Knez and coworkers in a 2009 report describing an increased toughness of spider silk following vapour-phase infiltration.^[9]

Sequential infiltration synthesis was developed by Argonne National Laboratory scientists Jeffrey Elam and Seth Darling in 2010 as a means to synthesize nanoscopic materials starting from block copolymer templates.^[13] A patent application was filed in 2011 and the first patent was issued in 2016.^[14] SIS involves vapor diffusing into an existing polymer and chemically or physically binding to it. This results in the growth and formation of inorganic structures by selective nucleation throughout the bulk polymer.

With SIS, the shapes of various inorganic materials can be tailored by applying their precursor chemistries to patterned or nanostructured organic polymers, such as block copolymers.^[15] SIS was developed to intentionally enable the infusion of inorganic materials such as metal oxides and metals within polymers to yield hybrid materials with enhanced properties. Hybrid materials created via SIS can further be subjected to thermal annealing steps to remove the polymer constituents entirely to derive purely inorganic materials that maintain the structure of the original polymer morphology, including mesoporosity.^[16]

Although the early research in SIS focused on a small number of inorganic materials such as Al₂O₃, TiO₂, and ZnO, the technology diversified over the next decade and came to include a wide variety of both inorganic materials and organic polymers, as detailed in reviews.^{[1][2][3][4][5][6][7]}

SIS is based on the consecutive introduction of different precursors into a polymer, taking advantage of the material's porosity on the molecular scale. This allows the precursors to diffuse into the material and react with specific functional groups located along the polymer backbone or pendant group.^[4][5] Through the selection and combination of the precursors, a rich variety of materials can be synthesized, each of which can endow unique properties to the material.^[3][6][7]

The process of SIS involves various key steps, the first of which is materials selection. A suitable substrate material, such as a polymer film, and precursors, typically molecules that can react with the substrates functional groups, are selected for the infiltration synthesis. The pairing of polymer chemistry and precursor species is vital for acquiring the desired functionalization and modification.^[3]

The substrate is placed in a reactor with an inert atmosphere (typically an inert gas or vacuum). The first precursor vapour (e.g., trimethylaluminum, TMA) is introduced at a sufficiently high vapour pressure and duration such that the precursor molecules diffuse into the substrate.^[1] Thus the precursor infiltrates the material and then reacts with the interior functional groups.

After a suitable diffusion/reaction time, the reactor is purged with inert gas or evacuated to remove reaction byproducts and un-reacted precursor. A second vapour-phase species, often a co-reactant, such as H₂O, is introduced. Again, the precursor partial pressure and exposure time are selected to allow sufficient time and thermodynamic driving force for diffusion into the polymer and reaction with the functional groups left by the first precursor exposure.^[1] The second precursor is then purged or evacuated to complete the first SIS cycle.

The second precursor may also create new functional groups for reaction with the first precursor for subsequent SIS cycles. Sequential infiltration steps can then be repeated using the same or different precursor species until the desired modifications are achieved. When the desired infiltrations are achieved, the modified material can undergo further post-treatment steps to enhance the modified layers' properties, including the stability. Post-treatment may include heating, chemical treatment, or oxidation to remove the organic polymer.^[6][7]

SIS is especially natural to apply to block copolymer substrates.^[13][3] Block copolymers such as polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, can spontaneously undergo microphase separation to form a rich variety of periodic mesoscale patterns. If the SIS precursors are selected to react with just one of the BCP components but not with the second component, then the inorganic material will only nucleate and grow in that component. For instance, TMA will react with the PMMA side chains of PS-b-PMMA but not with the PS side chains. Consequently, SIS using TMA and H₂O as precursor vapors to infiltrate a PS-b-PMMA microphase-separated substrate will form Al₂O₃ specifically in the PMMA-enriched microphase subdomains. Subsequent removal of the PS-b-PMMA by using oxygen plasma or by annealing in air will convert the combined organic and inorganic mesoscale pattern into a purely inorganic Al₂O₃ pattern that shares the mesoscale structure of the block copolymer but is more chemically and thermally robust.

SIS has garnered attention in various fields due to its ability to accurately modify and functionalize polymeric materials.

One of the main applications of SIS is in the enhancement of etch resistance in lithographic photoresist, such as those used in photolithography, microfabrication, and nanolithography. This method involves the sequential deposition of inorganic materials within a patterned resist's micro/nanostructures. By carefully controlling the infiltration of these materials, SIS can precisely engineer the chemical composition and density of the resist, thus enhancing its resistance to common etching processes. This enables technologists to achieve finer feature patterns and greater durability in microfabrication, ultimately advancing the capabilities of semiconductor manufacturing and nanotechnology applications. Another recent application for SIS in lithography is to enhance the optical absorption of the photoresist in the extreme ultraviolet range^[17] to improve EUV lithography.

SIS has applications in the field of surface coatings, particularly in the development of coatings with specific functional properties. With the sequential infiltration of different precursors into the material, SIS allows for the creation of coatings with enhanced properties and performance such as durability, corrosion resistance, oleophilicity^[18][19]/lipophilicity, anti-reflection,^[20] and/or improved adhesion to substrates. Such an application of SIS can be used for protective coatings for metals, anti-fouling coatings for biomedical applications, and coatings for optical and electronic devices.^[21][22][23] In this application, the diffusion and reaction of the SIS precursors below the polymer surface facilitate a bulk-like transformation such that the effective thickness of the surface coating (e.g., several microns) is much larger than the film thickness that would result using the same number of atomic layer deposition (ALD) cycles on a conventional, dense substrate (e.g., a few nanometers).

SIS, with its precise control over material properties, is a powerful method to develop sensors and actuators.^[24][25] The functional layers created through the selective infiltration of specific precursors can enhance the sensitivity, selectivity, and response of sensors, which have applications in gas sensing, chemical sensing, biosensing, and environmental monitoring. SIS is also promising when used to engineer actuators with tunable properties, enabling the fabrication of devices on micro and nano scales for various applications.

SIS has also shown promise in energy devices, especially in improving the performance and stability of energy storage and conversion systems. Employing SIS and the correct precursors, the technique can modify the surfaces and interfaces of materials used in batteries, supercapacitors, and fuel cells, enhancing charge transport, electrochemical stability, and energy density. SIS is also being explored for its applications in photovoltaics,^[26] in which it can be used to engineer interfaces and increase light absorption.

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SIS is a suitable tool for surface modifications to improve biocompatibility, bioactivity, and controlled drug release, making it useful in some biomedical applications. Polymers and bioactive macro-molecules treated with SIS can obtain coatings with developed cell adhesion and reduced bacterial adhesion, as well as provide a medium for the controlled release of therapeutics. Such properties are applicable in biomedicine, such as implantable medical devices, tissue engineering, and drug delivery systems.

An early application of SIS was to improve the mechanical properties of protein structures in biomaterials. For instance, when spider dragline silk is infiltrated with Al₂O₃ using trimethyl aluminum (TMA) and H₂O, the toughness is improved by nearly an order of magnitude.^[9]

The advantages and disadvantages of SIS are outlined. It is important to note that the advantages and limitations of SIS are continually being explored, addressed, and improved upon via ongoing research and development efforts in the field.

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SIS allows for precise control over the composition, structure, and properties of materials. The sequential nature of the synthesis process enables the integration of multiple materials and the creation of complex and multi-functional nanostructures. SIS enables atomic-level precision in controlling the deposition of precursor materials. This high level of precision allows for creating nanostructures with uniform dimensions, well-defined interfaces, and tailored properties.

SIS is a versatile fabrication technique amenable to a diverse range of combinations of polymer chemistries and precursor species. By selecting specific precursor materials, researchers can tune the properties of the fabricated materials, which include but are not limited to electrical conductivity, optical properties, and catalytic activity. This empowers various applications in electronics, photonics, energy devices, separations, and more.

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One of the main challenges of SIS is the need to perform the process in an inert environment. Creation of a vacuum and/or introduction of inert gas carries costs that may be prohibitive for applications.

A second challenge is the complexity of the diffusion-reaction process. Specifics of the reactor configuration and process parameters can impact the final product material substantially, which can complicate process optimization, reproduction, and scalability. Even though SIS can be applied to a wide range of materials, some materials are not suitable for SIS. The relatively slow diffusion of SIS precursor vapors through polymers can make SIS over macroscopic distances time-consuming. For instance, the infiltration of millimeter distances into a polymer may require precursor exposure times of several hours. For comparison, ALD of thin films on dense surfaces that do not involve diffusion into the substrate would require exposure times of <1 s using the same precursors.^[1]