Sequential Infiltration Synthesis Process Research Articles

Sequential Infiltration Synthesis of Al2O3 in PMMA Thin Films: Temperature Investigation by Operando Spectroscopic Ellipsometry.

Sequential infiltration synthesis (SIS) is a scalable and valuable technique for the synthesis of organic-inorganic materials with several potential applications at the industrial level. Despite the increasing interest for this technique, a clear picture of the fundamental physicochemical phenomena governing the SIS process is still missing. In this work, infiltration of Al2O3 into thin poly(methyl methacrylate) (PMMA) films using trimethyl aluminum (TMA) and H2O as precursors is investigated by operando dynamic spectroscopic ellipsometry (SE) analysis. The TMA diffusion coefficient values at temperatures ranging from 70 to 100 °C are determined, and the activation energy for the TMA diffusion process in PMMA is found to be Ea = 2.51 ± 0.03 eV. Additionally, systematic data about reactivity of TMA molecules with the PMMA matrix as a function of temperature are obtained. These results provide important information, paving the way to the development of a comprehensive theory for the modeling of the SIS process.

Sequential Infiltration Synthesis of Al2O3 in PMMA Thin Films: Temperature Investigation by Operando Spectroscopic Ellipsometry.

Sequential infiltration synthesis (SIS) is a scalable and valuable technique for the synthesis of organic-inorganic materials with several potential applications at the industrial level. Despite the increasing interest for this technique, a clear picture of the fundamental physicochemical phenomena governing the SIS process is still missing. In this work, infiltration of Al2O3 into thin poly(methyl methacrylate) (PMMA) films using trimethyl aluminum (TMA) and H2O as precursors is investigated by operando dynamic spectroscopic ellipsometry (SE) analysis. The TMA diffusion coefficient values at temperatures ranging from 70 to 100 °C are determined, and the activation energy for the TMA diffusion process in PMMA is found to be Ea = 2.51 ± 0.03 eV. Additionally, systematic data about reactivity of TMA molecules with the PMMA matrix as a function of temperature are obtained. These results provide important information, paving the way to the development of a comprehensive theory for the modeling of the SIS process.

Atomic Layer Depostion within Polymers Templates for Spatially Controlled Growth and Doped Materials

Today’s nanofabrication techniques require multistep and costly processes in order to fabricate complex, multi-materials nanostructures. Performing atomic layer deposition (ALD) within polymeric templates can offer a simple solution for nanostructure fabrication. In this process, named sequential infiltration synthesis (SIS), high partial pressures and long exposures times lead to inorganic materials growth within polymers. Sequential polymer removal results in polymer-templated inorganic nanostructure. While SIS shows great potential in fabricating large variety of structures, it is currently limited to a single material growth process.In our research, we utilized SIS properties to achieve two different objectives: selective spatial growth and homogenous growth for doped materials. We demonstrated, for the first time, multi-material SIS process with control over the spatial location of each material and fabricate heterostructure nanorods and nanowires. Doping materials were fabricated using simultaneously growth of two precursors.We studied SIS within self-assembled block copolymer (BCP) films and electrospun polymer fibers and developed multi-material SIS, where two metal oxides are grown together in a single process, with precise control over their location within the polymer template. We used cylinder forming poly (styrene-block-methyl methacrylate) (PS-b-PMMA) films and electrospun PMMA as the polymeric template and DEZ (diethyl zinc), TMA (trimethyl aluminum) as the organometallic precursors. We achieved control over the growth location of each metal oxide by tuning the organometallic precursors diffusion time, forming heterostructures after polymer removal. A short exposure of the first precursor resulted in a limited growth only at the outer part of the polymer, while a long exposure of the second precursor enabled it to reach the full depth of the polymer besides the section which was already occupied by the first precursor. An exposure to water completed the cycle. We demonstrated this process on BCP films to achieve AlOx-ZnO nanorods arrays (Figure 1A-B) and on polymer fibers to achieve AlOx-ZnO fibers (Figure 1C-D). We performed structural characterization using scanning and transmission electron microscopy (SEM and TEM, respectively) to characterize the nanowires and nanorods as well as three-dimensional characterization scanning TEM (STEM) tomography and energy-dispersive X-ray spectroscopy (EDS) STEM tomography in order to probe the structure and the chemical composition in 3D. To fabricate doped materials, we applied SIS process with TMA and DEZ in the same exposure within homopolymer film. We preformed SEM-EDS to quantify each element and evaluate the doping percentage. This research opens new pathways for multi-materials nano scale structure fabrication through ALD-based growth within polymers.Figure 1 caption: AlOx-ZnO nanorods array: cross-sectional views using A) EsB detector and B) secondary electron in-lens detector. AlOx-ZnO fibers C) EDS map D) HAADF image [1] R. Azoulay et al., Small, 15, 1904657 (2019).[2] R. Azoulay et al., ACS Appl. Nano Mater., 5, 7228 (2022). Figure 1

Developing Quantitative Nondestructive Characterization of Nanomaterials: A Case Study on Sequential Infiltration Synthesis of Block Copolymers.

The sequential infiltration synthesis (SIS) of inorganic materials in nanostructured block copolymer templates has rapidly progressed in the last few years to develop functional nanomaterials with controllable properties. To assist this rapid evolution, expanding the capabilities of nondestructive methods for quantitative characterization of the materials properties is required. In this paper, we characterize the SIS process on three model polymers with different infiltration profiles through ex situ quantification by reference-free grazing incidence X-ray fluorescence. More qualitative depth distribution results were validated by means of X-ray photoelectron spectroscopy and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy.

Spatially Templated Nanolines of Ru and RuO2 by Sequential Infiltration Synthesis

Nanoscale patterning of inorganics is crucial for the fabrication of advanced electronic, photonic, and energy devices. The emerging sequential infiltration synthesis (SIS) method fabricates nanofeatures by block-selective vapor-phase growth in block copolymer templates with tunable patterns. Yet, SIS has been demonstrated mainly for Al2O3 and a few other metal oxides, while deriving metal nanostructures from a single SIS process is a challenge. Here, we present SIS of the Ru metal in polystyrene-block-polymethyl methacrylate (PS-b-PMMA) templates without any pretreatment, using alternating infiltration of RuO4 and H2. RuO4 interacts selectively and strongly with the aromatic C═C and C–H groups in PS, leaving the PMMA domains inert. Density functional theory calculations corroborate that the PS–RuO4 interaction is energetically favorable, with a calculated interaction energy of −1.65 eV, whereas for PMMA–RuO4, the calculated energy of −0.05 eV indicates an unfavorable interaction. Morphological analysis on the di-BCP after the RuO4-H2 process indicates an increase in contrast as a function of SIS cycles and templated Ru incorporation. The crystalline nature of the Ru deposits is confirmed using grazing incidence wide-angle X-ray scattering. Plasma-aided removal of the organic components yields Ru nanolines with lateral dimensions of ca 20 nm. We further highlight the broad potential of RuO4 as a reactant for SIS by generating RuO2 nanopatterns via alternating RuO4 and methanol infiltration.

Micro and Nanopatterned Ion Exchange Membranes for Applications in Electrochemical Devices

Topographical patterns on ion exchange membranes’ (IEMs’) surfaces have been show to improve the performance and energy efficiency in electrochemical energy conversion and water treatment processes. Substituting flat IEMs with patterned/profiled IEMs in electrodialysis reduces spacer channel mass transfer limitations by increasing the interfacial area between the liquid solution and membrane materials. Additionally, the extended surface of the profiled cation-exchange and anion-exchange membranes augment the spacer channel conductivity without obfuscating bulk liquid flow. Topographically patterned ion exchange membranes are fabricated using several methodologies that include photo-/soft-lithography, hot pressing, solution casting and 3D printing. The minimum lateral feature size that can be obtained by these methods is practically on the order of a few microns and potentially could be as low as few hundred nanometers. Block copolymer (BCP) lithography is a nanopatterning platform that generates lateral feature sizes of 5 to 50 nm. Thin films of diblock copolymers can self-assemble on a neutral layer resulting in lamella, cylinder, or sphere morphologies when the film is thermal or solvent vapor annealed. The morphology of the film primarily depends on volume fraction (fA, fB) of each block and interaction parameter between the blocks (χ). Poly(styrene-block-methylmethacrylate) (PS-b-PMMA) is one of the most studied block copolymer for BCP lithography because it form perpendicular aligned nanostructures via thermal annealing without topcoat layers to control the free surface interfacial energy. The etch contrast between the styrene and MMA blocks also makes it easier to transfer the pattern onto substrates – especially if sequential infiltration synthesis (SIS) is used.In this work, for the first time, a method to prepare nanopatterned IEMs using BCP lithography is reported. The SIS process post PS-b-PMMA self-assembly converts MMA into alumina creating organic-inorganic hard mask. The styrene block is removed via oxygen plasma in a reactive ion-etcher leaving behind a nanoporous alumina template on a silicon wafer. Sulfonated poly(arylene ether ether ketone) (SPEEK) is dropcasted onto the nanoporous alumina and the solvent is evaporated. Once releasing the SPEEK from the substrate, a nanopatterned cation-exchange membrane is attained. The electrochemical properties of the nanopatterned membrane such as conductivity, permselectivity and transference number are measured and compared to a flat, non-patterned membrane. The efficiency of these nanopatterned membranes for simple electrochemical devices, like a hydrogen pump, will also be presented.This work is supported from NSF Award # 1703307. Figure 1

Metal Oxide Heterostructure Array via Spatially Controlled-Growth within Block Copolymer Templates.

Nanofabrication is continuously searching for new methodologies to fabricate 3D nanostructures with 3D control over their chemical composition. A new approach for heterostructure nanorod array fabrication through spatially controlled-growth of multiple metal oxides within block copolymer (BCP) templates is presented. Selective growth of metal oxides within the cylindrical polymer domains of polystyrene-block-poly methyl methacrylate is performed using sequential infiltration synthesis (SIS). Tuning the diffusion of trimethyl aluminum and diethyl zinc organometallic precursors in the BCP film directs the growth of AlOx and ZnO to different locations within the cylindrical BCP domains, in a single SIS process. BCP removal yields an AlOx -ZnO heterostructure nanorods array, as corroborated by 3D characterization with scanning transmission electron microscopy (STEM) tomography and a combination of STEM and energy-dispersive X-ray spectroscopy tomography. The strategy presented here will open up new routes for complex 3D nanostructure fabrication.

Tin oxide nanostructure fabrication via sequential infiltration synthesis in block copolymer thin films

Tin oxide (SnO2) nanostructures are attractive for sensing, catalysis, and optoelectronic applications. Here we investigate the fabrication of SnOx nanostructures through sequential infiltration synthesis (SIS) in block copolymer (BCP) film templates. While the growth of metal and metal oxides within polymers and BCP films via SIS has been demonstrated until now using small precursors such as trimethyl aluminum and diethyl zinc, we hypothesize that SIS can be performed using larger precursors and demonstrate SnOx SIS with tetrakis(dimethylamino)tin (TDMASn) and hydrogen peroxide. Tuning the SIS reaction and BCP chemistry resulted in highly ordered, polystyrene-block-poly(2-vinyl pyridine) (P2VP)-templated porous SnOx - AlOx and SnOx nanostructures. Detailed investigation using in-situ microbalance, high resolution electron microscopy, elemental analysis and infra-red spectroscopy shows that SnOx can directly grow within P2VP homopolymer and BCP films. Simultaneously with the growth, SnOx SIS process also contributes to the polymer etch. Performing SnOx SIS with pretreatment of a single AlOx SIS cycle increases the SnOx growth and protects the BCP template from etching. This is the first report of SnOx SIS opening a pathway for additional tetrakis-based organometallic precursors to be utilized in growth processes within polymers.

Effect of Nanostructured Domains in Self-Assembled Block Copolymer Films on Sequential Infiltration Synthesis.

There are broad interests in selective and localized synthesis in nanodomains of self-assembled block copolymers (BCPs) for a variety of applications. Sequential infiltration synthesis (SIS) shows promise to selectively grow a controllable amount of materials in one type of nanodomain of a self-assembled BCP film. However, the effects of nanostructured domains in a BCP film and SIS cycles on the material growth behavior of SIS are rarely studied. In this work, we investigated thegrowth behavior of TiO2 SIS within self-assembled polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films and the two corresponding pure homopolymer films (PS and PMMA) by using in situ quartz crystal microbalance (QCM). According to the experimental results, reactant purge steps are essential to enable a high selectivity of SIS in PMMA nanodomains in the BCP films by eliminating the undesired homogeneous reactions. The continuous PS nanodomain acts as the main channel in transporting reactants to PMMA nanodomains in theself-assembled PS-b-PMMA BCP films. The segregated nanoscale PMMA nanodomains in the BCP films show dramatically different TiCl4 diffusion/reaction behavior than a continuous PMMA film. The mass gain per SIS cycle within PMMA nanodomains decreases quickly with increasing cycle number. After 7 TiO2 SIS cycles, TiO2 SIS can only take place at the interface between PS and PMMA nanodomains in the BCP film. The TiO2 SIS process can uniformly modify PMMA nanodomains throughout a self-assembled PS-b-PMMA film up to the diffusion depth owing to the unique nanostructure-enabled diffusion. SIS cycle number and chemistry of a BCP will strongly affect the material growth behavior of a SIS chemistry on the BCP film and, therefore, the final morphology of the resulting nanomaterial. Detailed studies are warranted for a SIS process on a self-assembled BCP film of different chemistry.

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 AlOx 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 AlOx channels are continuous through the film and have a gradual increase in pore size with depth. Finally, the ultrafiltration performance of the fabricated AlOx membrane for protein separation as a function of protein size and charge is demonstrated.

Sequential Infiltration Synthesis for the Design of Low Refractive Index Surface Coatings with Controllable Thickness.

Control over refractive index and thickness of surface coatings is central to the design of low refraction films used in applications ranging from optical computing to antireflective coatings. Here, we introduce gas-phase sequential infiltration synthesis (SIS) as a robust, powerful, and efficient approach to deposit conformal coatings with very low refractive indices. We demonstrate that the refractive indices of inorganic coatings can be efficiently tuned by the number of cycles used in the SIS process, composition, and selective swelling of the of the polymer template. We show that the refractive index of Al2O3 can be lowered from 1.76 down to 1.1 using this method. The thickness of the Al2O3 coating can be efficiently controlled by the swelling of the block copolymer template in ethanol at elevated temperature, thereby enabling deposition of both single-layer and graded-index broadband antireflective coatings. Using this technique, Fresnel reflections of glass can be reduced to as low as 0.1% under normal illumination over a broad spectral range.

Sequential Infiltration of Self-Assembled Block Copolymers: A Study by Atomic Force Microscopy

Sequential infiltration synthesis (SIS), when combined with novel polymeric materials capable of self-assembly, such as block copolymers (BCPs), has been shown to effectively improve the pattern transfer of nanoscale templates. Herein, we present a study of the SIS process aimed at elucidating some critical aspects such as the evolution of the BCP morphology and mechanical properties after infiltration. Atomic force microscopy nanomechanical mapping was able to measure a consistent stiffness change within the SIS-infiltrated poly(methyl methacrylate) (PMMA) blocks. Interestingly, the increase in Young’s modulus of the infiltrated blocks is small compared to the final stiffening of the same infiltrated features after a treatment with oxygen plasma.

Ozone-Based Sequential Infiltration Synthesis of Al2O3 Nanostructures in Symmetric Block Copolymer.

Sequential infiltration synthesis (SIS) provides an original strategy to grow inorganic materials by infiltrating gaseous precursors in polymeric films. Combined with microphase-separated nanostructures resulting from block copolymer (BCP) self-assembly, SIS selectively binds the precursors to only one domain, mimicking the morphology of the original BCP template. This methodology represents a smart solution for the fabrication of inorganic nanostructures starting from self-assembled BCP thin films, in view of advanced lithographic application and of functional nanostructure synthesis. The SIS process using trimethylaluminum (TMA) and H2O precursors in self-assembled PS-b-PMMA BCP thin films was established as a model system, where the PMMA phase is selectively infiltrated. However, the temperature range allowed by polymeric material restricts the available precursors to highly reactive reagents, such as TMA. In order to extend the SIS methodology and access a wide library of materials, a crucial step is the implementation of processes using reactive reagents that are fully compatible with the initial polymeric template. This work reports a comprehensive morphological (SEM, SE, AFM) and physicochemical (XPS) investigation of alumina nanostructures synthesized by means of a SIS process using O3 as oxygen precursor in self-assembled PS-b-PMMA thin films with lamellar morphology. The comparison with the H2O-based SIS process validates the possibility to use O3 as oxygen precursor, expanding the possible range of precursors for the fabrication of inorganic nanostructures.

New Insights into Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is a process derived from ALD in which a polymer is infused with inorganic material using sequential, self-limiting exposures to gaseous precursors. SIS can be used in lithography to harden polymer resists rendering them more robust towards subsequent etching, and this permits deeper and higher-resolution patterning of substrates such as silicon. Herein we describe recent investigations of a model system: Al2O3 SIS using trimethyl aluminum (TMA) and H2O within the diblock copolymer, poly(styrene-block-methyl methacrylate) (PS-b-PMMA). Combining in-situ Fourier transform infrared absorption spectroscopy, quartz-crystal microbalance, and synchrotron grazing incidence small angle X-ray scattering with high resolution scanning transmission electron microscope tomography, we elucidate important details of the SIS process: 1) TMA adsorption in PMMA occurs through a weakly-bound intermediate; 2) the SIS kinetics are diffusion-limited, with desorption 10x slower than adsorption; 3) dynamic structural changes occur during the individual precursor exposures. These findings have important implications for applications such as SIS lithography.

Direct fabrication of high aspect-ratio metal oxide nanopatterns via sequential infiltration synthesis in lithographically defined SU-8 templates

Nanopatterning high aspect-ratio metal oxide structures remains challenging for conventional nanofabrication methods based on reactive ion etching due to marginal etch selectivity between target oxides and typical mask materials. Here, the authors report the application of sequential infiltration synthesis (SIS) on lithographically defined SU-8 polymer templates for patterning arbitrarily designed, high aspect-ratio metal oxide nanostructures with sub-50 nm linewidths, smooth vertical profiles, and three-dimensional (3D) morphologies difficult to achieve by the conventional fabrication methods. As examples, various AlOx nanostructures with ∼40 nm linewidths and up to 16 aspect ratios were demonstrated, along with TiOx in-plane nanowire arrays of controlled positional registrations. Detailed scanning and transmission electron microscopy studies revealed nanocrystalline and amorphous internal structures of respective AlOx and TiOx, as well as the swelling and contraction behaviors of polymer templates during the SIS process, which allowed the facile fabrication of high aspect-ratio, sub-50 nm-featured oxide nanopatterns with 3D morphologies. These results confirm the potential of vapor-phase material infiltration in directly nanopatterning complexly structured metal oxides.

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.