AVS 68 Session TF1+SE-WeM: Vapor Deposition and Vapor Infiltration of Organic, Polymeric, and/or Hybrid Materials

Vapor phase infiltration (VPI) is a process for synthesizing organic-inorganic hybrid materials by infiltrating polymers with inorganic vapor molecules. Recently, our research team has used VPI to synthesize hybrid membranes with consistent nanofiltration performance in a variety of organic solvents that would otherwise swell or dissolve the parent polymer membrane. The properties of these hybrid materials can vary with the amount of VPI generated inorganic loading. However, the relationship between VPI processing conditions and inorganic loading is still not fully understood. This talk will discuss the effects of VPI dose pressure and exposure time on inorganic loading using the technologically relevant membrane material polymer of intrinsic microporosity 1 (PIM-1). At sufficiently low dose pressures and infiltration times (i.e., before saturation), inorganic loading can be controlled with both vapor pressure and exposure time. However, inorganic loading appears to saturate for this system at around 13 wt% inorganic. This apparent saturation point can be reached by infiltrating for 5 hours at TMA dose pressures above 0.4 Torr or after 40 hours at lower vapor pressures of 0.1 Torr. Inorganic loading appears to saturate for this system when the polymer’s functional groups become fully populated with bound VPI precursors. These experimental results can be understood with the use of a recently developed reaction-diffusion model for VPI. Critical to applying this model to these post-deposition measurements is re-normalizing the mass loading to the total number of functional groups in the polymer. These findings suggest that the vapor pressure during infiltration affects the kinetics of this infiltration process but does not appear to alter the thermodynamically determined saturation point for inorganic loading.

Sequential infiltration synthesis (SIS) enables the creation of inorganic or polymer-inorganic hybrid nanocomposite materials through the diffusion of metal-organic vapor reagents into polymers and subsequent reactions with the polymer functional groups. Here we report a detailed study of aluminum oxide (Al₂O₃) SIS using trimethyl aluminum (TMA) and H₂O in polycarbonate (PC) thin films and membranes. We employed in situ Fourier transform infrared spectroscopy (FTIR) to elucidate the chemical interactions and in situ spectroscopic ellipsometry (SE) to measure the physical and optical changes that occur during Al₂O₃ SIS in PC. During the first SIS cycle, FTIR measurements revealed a rapid and irreversible reaction between TMA and the PC carbonyl groups to form Al-CH₃ species which then converted to Al₂O₃ upon H₂O exposure during.Virtually no spectral changes were observed by FTIR in subsequent SIS cycles.The SE measurements showed that the PC film thickness increased by ~75% and the refractive index (RI) decreased by ~2% during the initial TMA exposure.These effects were partially reversed during the following H₂O exposure such that the net thickness increase was ~36% and the RI returned to its initial value.During subsequent SIS cycles, the thickness and RI remained essentially constant.Ex situ X-ray photoelectron spectroscopy (XPS) depth profiling measurements showed ~30 at% Al uniformly distributed through the depth of the PC film following a single Al₂O₃ SIS cycle. SIS-derived nano/microstructure materials with tunable structural and optoelectronic properties could be developed for a variety of applications.

Keywords: atomic layer deposition; sequential infiltration synthesis; aluminum oxide, polymer-inorganic nanocomposite.

Vapor phase infiltration (VPI) is a post-polymerization modification technique that imbues inorganic materials into polymers to create organic-inorganic hybrid materials with new properties distinct from the parent polymer. While several VPI precursor-polymer chemistries have been explored, a lack of chemical intuition remains for fully understanding the thermodynamics and kinetics that govern the VPI process. This study seeks to continue to build this knowledge by examining the VPI process kinetics for TiO₂ infiltration into PMMA via the use of TiCl₄ and H₂O precursors. In this research, polymethylmethacrylate / TiO₂ hybrid materials are prepared using VPI. The depth of infiltration of the TiO₂species into ~200 nm PMMA thin films is studied using x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy and spectroscopic ellipsometry. The kinetics for TiCl4 infiltration increases with both VPI process temperature and TiCl₄ exposure time. However, the rates of infiltration are considerably slower than those observed in the more commonly studied trimethylaluminum (TMA) / PMMA system. Even at 150 °C, process times of at least 12 hours are required to fully infiltrate a 200 nm PMMA film whereas using TMA similar films are fully infiltrated within 1 hour at the same process temperature. Films that we believe to be fully infiltrated at 150 °C and 24 hours of TiCl₄ exposure have a 6 at% Ti in the innermost bulk, as determined by XPS. Interestingly, unlike AlO_x-PMMA hybrids, these TiO_x – PMMA hybrids exhibit significant changes in their optical properties. Increased titanium loading leads to a 4% increase in refractive index and increased UV absorbance in the UV range of 270-350nm. Furthermore, films infiltrated at 150 °C and 24 hours of TiCl₄ exposure had a 50 % and 70 % reduction in coefficients of thermal expansion (CTE) below and above T_g respectively. Reduced CTE trends with increased titanium loading. We will discuss possible opportunities to use these new properties for various applications.

This project is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Conjugated polymers such as polyethylenedioxythiophene (pEDOT), polypyrrole (pPy), and polyaniline (pAni) exhibit high electrochemical capacities, making them appealing as electrode materials for energy storage, electrochemical desalination, and chemical sensing. Recent work has established the growth of thin-films of pEDOT using alternating gas-phase exposures of EDOT monomers and a metal-chloride (e.g. MoCl₅) oxidant in a process termed oxidative molecular layer deposition (oMLD). Here, we report on recent work establishing oMLD of amine-containing conjugated polymers. We find that pyrrole (Py) and MoCl₅ undergo self-limiting surface reactions during oMLD exposures to form conformal pPy thin films, but oMLD using aniline (Ani) and p-phenylenediamine (PDA) monomers yield unexpected azo functionality. The formation of azo groups is attributed to an MoCl₅-amine surface adduct that spatially constrains polymerization reactions near the amine group and produces azo groups when coupling two primary amines. pPy grown by oMLD exhibits a record-breaking 282 mAh/g capacity in aqueous electrolyte, and PDA/MoCl₅ oMLD yields azo-polymers of interest as anode materials for alkali-ion batteries. Alternating between Py and PDA monomers during oMLD produces molecularly mixed copolymers with qualitatively different electrochemical responses from the isolated monomer structures. This work lays the foundation for the growth of conformal thin films of conjugated amine polymers with molecular-level control of composition and thickness.

The deposition rate and properties of MLD films are for a large part determined by what happens during the precursor exposure step. In some cases, however, the purge step is of equal importance, for example in the MLD of alucone films using trimethylaluminum (TMA) and ethylene glycol (EG). Due to the porosity of alucone films, the reactants during their exposure step not only react at the film surface but also tend to infiltrate into the film. The subsequent outgassing of the infiltrated reactant can take relatively very long thereby becoming the deposition rate-limiting step. If enough purge time is not provided for the reactant to outgas, it will lead to an additional CVD component alongside MLD in the overall growth. To employ/avoid the CVD component in the deposition process, we have developed a kinetic model to correlate parameters like exposure times, partial pressures, purge times and deposition temperature to the amount of CVD component in the growth. We have also investigated the impact of this additional CVD component on the step coverage of the alucone films and evaluated its usage in superconformally filling dielectric gaps.

Additionally, we also looked into solutions to increase the deposition rate of the alucone films and improve their conformality and amongst others found that using a bulkier precursor like DMAI instead of TMA can overcome the problem of precursor infiltration, increase the deposition rate of alucone processes by at least an order of magnitude and provide quite conformal films. We also performed a detailed investigation of MLD of alucone using DMAI as the aluminum precursor wherein the effect of deposition temperature and reactant purge times on deposition kinetics has been investigated and the DMAI alucone films have been compared with those prepared using TMA for their chemical environment and degradation showing striking similarities between both. We believe that the above work could be extended to other MLD systems and can serve as a guide in designing efficient MLD reactors and processes.