AVS2010 Session TF-MoA: 42

Inorganic thin film growth by atomic layer deposition (ALD) on polymer fibers is shown to be significantly affected by the process conditions and materials. Transmission electron microscopy in combination with in situ quartz crystal microbalance analysis shows a strong influence on processing temperature and polymer backbone structure that can lead to varying types of interface modifications between the inorganic and organic polymer. Deposition on cellulose cotton shows conformal surface growth due to the presence of hydroxyl sites, resulting in conformal thin film growth. Polypropylene, which has no reactive sites, does allow for vapor phase diffusion of the ALD precursors, resulting in subsurface growth. The subsurface growth on polypropylene is significantly impeded at lower processing temperatures. The strong temperature dependence of interface modification is ascribed to a relatively large increase in bulk species diffusivity that occurs upon the temperature-driven free volume expansion of the polymer. The interface structure can significantly modify the physical properties of resulting inorganic-coated polymer fibers. For example, we measured the electrical properties of ALD ZnO coatings on polypropylene fiber mesh, and we find that modifying the polymer inorganic interface using low temperature ALD Al₂O₃ before ZnO ALD produces fibers with conductance that is increased by a factor >100x as compared to ZnO coated polypropylene without the interface pretreatment. For a range of polymer materials studied, we find distinct trends between polymer structure, ALD process conditions and resulting interface structure, and these results will be presented and discussed.

Nonaqueous sol-gel routes are elegant approaches for the synthesis of high quality metal oxide nanomaterials such as pure inorganic nanocrystals [1], ordered hybrid organic-inorganic materials [2] or thin films by atomic layer deposition (ALD) [3]. In this communication, after a short introduction to nonaqueous sol-gel chemistry we will focus on the ALD using nonaqueous sol-gel. In particular, the similarities and differences of the chemical processes taking place in solution and in ALD will be discussed through two examples of recent approaches of metal oxide film deposition

Our approach makes use of metal alkoxides and carboxylic acids as metal and oxygen source, respectively [4]. In the first step, it is expected that the reaction of surface alkoxide species with carboxylic acids leads to surface carboxylate species (eq. 1). In the second step an aprotic condensation reaction between surface carboxylate species and metal alkoxides leads to metal-oxide bonds formation (eq. 2).

≡M-OR’ + RCOOH → ≡M-OOCR + R’OH (1)

≡M-OOCR + M-OR’ → ≡M-O-M≡ + RCOOR’ (2)

This process enables the growth of metal oxides at temperatures as low as 50 °C on various supports.

A comparison of this nonaqueous sol-gel route to ALD approaches using ozone as oxygen source will be made. Although, from a first look it seems that the chemistry responsible for metal oxide formation is very different, this is certainly not the case. As a matter of fact it was recently reported that during the ALD of metal oxides, the exposure to ozone leads to formate, carbonate and hydroxyl groups on the surface [5-7]. According to Goldstein et al., a large amount of formate surface species were formed by the reaction of trimethylaluminium (TMA) and O₃ [5]. Therefore, the formation of surface carboxylic species upon reaction with ozone leads to similar surface states as in the case of reaction with carboxylic acids [4]. These findings will be used to discuss the chemical reactions responsible for the deposition of PbTiO_x using lead bis(3-N,N-dimethylamino-2-methyl-2-propoxide), titanium(IV) isopropoxide and a combination of water and ozone as oxygen sources as recently reported by Hyun Ju Lee et al. [8]

References:

[1] Pinna, N. & Niederberger, M. Angew. Chem. Int. Ed., 2008, 47, 5292

[2] Pinna, N. J. Mater. Chem., 2007, 17, 2769

[3] Clavel, G. et al. J. Mater. Chem, 2009, 19, 454

[4] Rauwel, E. et al. Angew. Chem., Int. Ed., 2008, 47, 3592

[5] Goldstein, D. N. et al. J. Phys. Chem. C, 2008, 112, 19530

[6] Rai, V. R.; Agarwal, J. Phys. Chem. C, 2008, 112, 9552

[7] Kwon, J. et al. Chem. Mater., 2008, 20, 3248

[8] Lee, H. J. et al. ECS Transactions, 2009, 19, 829

Electrospun nanofibers are of interest for air filtration, thermal insulation, biochemical sensing, and a range of other applications. Nanofibers can be readily electrospun from polyamide-6 (PA-6) nylon, but nylon is highly susceptible to water uptake and rapid degradation when exposed to chemical reactants, including those used in ALD. The high surface area of nanofibers makes these issues more acute and difficult to manage. We have recently investigated the deposition of Al₂O₃ and ZnO on nylon nanofibers and found distinctly different results for these two material processes. Aluminum oxide ALD from trimethylaluminum and water resulted in significant fiber degradation, whereas fibers remained intact upon ALD using diethyl zinc and water. The ZnO coating was not sufficient, however, to impede attack and degradation during subsequent exposure to Al₂O₃ ALD using TMA. We have recently explored a new MLD process in our lab using TMA and glycidol, where the resulting film is a hybrid organic-metal oxide polymer. This MLD coating onto a ZnO-coated nylon nanofiber results in only minimal reaction with the polymer under the ZnO. After 75 MLD cycles, exposing the nylon nanofibers to 25 cycles of trimethylaluminum/water results in no visible degradation, indicating successful encapsulation by the ZnO/MLD bilayer. We will present the chemistry associated with the reaction between the TMA and PA-6. Additionally, the surface chemistry of the PA-6 nanofibers after encapsulation, and the air stability of the hybrid organic-metal oxide coatings will be discussed.

Initiated chemical vapor deposition (iCVD) is a low temperature variant of hot-wire chemical vapor deposition (HWCVD) which has been used to produce a wide variety of free radical polymers like fluoropolymers, acrylic polymers and silicone polymers without the use of solvents. An initiator is activated in the gas phase by a resistively heated filament array. The radicals thus generated undergo surface polymerization on a cooled substrate by attaching to multiple monomer units. In this work, we produced thin films of poly(2-hydroxyethyl methacrylate) (PHEMA), an important hydrogel, for potential biomaterials applications such as skin substitutes. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies showed that the chemistry of PHEMA obtained by iCVD synthesis matched closely to PHEMA obtained from solution synthesis. Deposition kinetics were tuned to achieve high deposition rates (~1.5 μm/min), which yielded mechanically robust ultra-high molecular weight polymer films. As a result, the films displayed a high degree of physical chain entanglement giving rise to high tensile modulus and storage modulus. This eliminates the need for chemical crosslinking, thus preserving maximum hydrophilicity for cell growth. The films showed good water transport and swellability. In vitro cytotoxicity studies of adult human dermal fibroblasts on PHEMA showed these films were non-cytotoxic and displayed viable cell adhesion. Non-specific protein adsorption was shown to be reduced compared to controls.

Molecular layer deposition (MLD) can be used to grow hybrid organic-inorganic films using organic MLD and inorganic ALD precursors. The alucones are the most studied of these hybrid organic-inorganic films. Alucone MLD depends on the sequential, self-limiting reactions between trimethylaluminum (TMA) and various organic alcohols. In this work, alucone MLD was performed using glycerol, a sugar alcohol with three hydroxyl groups or glycidol, an alcohol molecule that contains an epoxide group. Both of these alcohols should promote cross-linking in the alucone film for enhanced film stability and film toughness.

Studies of alucone MLD using TMA and glycerol (GL) or glycidol (GLY) were performed using in situ quartz crystal microbalance (QCM), Fourier transform infrared (FTIR) and quadrupole mass spectrometry (QMS) techniques. The FTIR studies showed that the TMA and GL or GLY reactions were both self-limiting. For the TMA + GL reaction, the FTIR spectra observed the growth/loss of AlCH₃ and C-OH vibrations versus TMA/GL exposures. The mass gain measured by the QCM was 43 ng/cm²/cycle at 150ºC. QMS confirmed the presence of TMA and GL parents and observed the CH₄ reaction products. Ex situ XRR measurements yielded a growth rate of 2.40 Å/cycle after 50 cycles at 150ºC.

Alucone films grown using TMA and GL or GLY were more stable than alucone films grown using TMA and ethylene glycol. The instability of alucone films has been attributed to unreacted AlCH₃ species. To explore this issue, alucone films were grown using TMA, GL and H₂O exposures in an ABC sequence. QCM measurements showed that the mass gain increased to 56 ng/cm²/cycle for this ABC sequence at 150°C. In addition, the QMS observed CH₄ intensities during H₂O exposures that were ~1/4 the intensity observed during GL exposures. These results indicate that H₂O exposures can remove unreacted AlCH₃ species and increase the mass gain per cycle.