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Temperature-Dependent Subsurface Growth during Atomic Layer Deposition on Polypropylene and Cellulose Fibers Jesse S. Jur,* Joseph C. Spagnola, Kyoungmi Lee, Bo Gong, Qing Peng, and Gregory N. Parsons Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695 Received December 6, 2009. Revised Manuscript Received February 5, 2010 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 °C) produced a more abrupt transition. Cross-sectional transmission electron microscopy images of polypropylene fibers coated at higher temperature (90 °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.
I. Introduction Inorganic integration with polymers is of interest for biological and chemical separations,1 tissue-engineering scaffolds,2 composite and hybrid organic-inorganic materials,3,4 and barrier layers for organic electronics.5-9 Surface modification of polymers, and fibers in particular, is also of interest for modulating wettability and controlling cell adhesion,10,11 and for high surface area electrodes for energy storage and conversion applications. Atomic layer deposition (ALD) of inorganic materials has previously been applied to a range of complex polymer materials and fibers, *Corresponding author. E-mail:
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including electrospun polymer nanofibers,12,13 nanospheres,14 and natural cotton,15 and it provides a unique ability to create conformal coatings at low temperatures.16,17 The detailed mechanisms associated with ALD on various polymers are of current interest for a wide range of materials integration applications,6-9,12,13,15,18-28 and several polymer systems of interest are summarized in Table 1. In addition, ALD growth of aluminum oxide has also been studied on self-assembled monolayers with various end-group terminations.29-31 Previous studies have established that when ALD is performed on nonreactive (16) Elam, J. W.; Routkevitch, D.; Mardilovich, P. P.; George, S. M. Chem. Mater. 2003, 15, 3507-3517. (17) Groner, M. D.; Fabreguette, F. H.; Elam, J. W.; George, S. M. Chem. Mater. 2004, 16, 639-645. (18) Cooper, R.; Upadhyaya, H. P.; Minton, T. K.; Berman, M. R.; Du, X. H.; George, S. M. Thin Solid Films 2008, 516, 4036-4039. (19) Fabreguette, F. H.; George, S. M. Thin Solid Films 2007, 515, 7177-7180. (20) Ferguson, J. D.; Weimer, A. W.; George, S. M. Chem. Mater. 2004, 16, 5602-5609. (21) Groner, M. D.; George, S. M.; McLean, R. S.; Carcia, P. F. Appl. Phys. Lett. 2006, 88 (5). (22) Hooper, A.; Fisher, G. L.; Konstadinidis, K.; Jung, D.; Nguyen, H.; Opila, R.; Collins, R. W.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 1999, 121, 8052-8064. (23) Kemell, M.; Farm, E.; Ritala, M.; Leskela, M. Eur. Polym. J. 2008, 44, 3564-3570. (24) Liang, X. H.; Hakim, L. F.; Zhan, G. D.; McCormick, J. A.; George, S. M.; Weimer, A. W.; Spencer, J. A.; Buechler, K. J.; Blackson, J.; Wood, C. J.; Dorgan, J. R. J. Am. Ceram. Soc. 2007, 90 (1), 57-63. (25) Wilson, C. A.; Grubbs, R. K.; George, S. M. Chem. Mater. 2005, 17, 5625-5634. (26) Wilson, C. A.; McCormick, J. A.; Cavanagh, A. S.; Goldstein, D. N.; Weimer, A. W.; George, S. M. Thin Solid Films 2008, 516, 6175-6185. (27) Yun, S. J.; Lim, J. W.; Lee, J. H. Electrochem. Solid State Lett. 2004, 7 (1), C13-C15. (28) Zhang, L. B.; Patil, A. J.; Li, L.; Schierhorn, A.; Mann, S.; Gosele, U.; Knez, M. Angew. Chem., Int. Ed. 2009, 48, 4982-4985. (29) Li, M.; Dai, M.; Chabal, Y. J. Langmuir 2009, 25, 1911-1914. (30) Kobayashi, N. P.; Williams, R. S. Chem. Mater. 2008, 20, 5356-5360. (31) Kobayashi, N. P.; Donley, C. L.; Wang, S. Y.; Williams, R. S. J. Cryst. Growth 2007, 299 (1), 218-222.
Published on Web 02/17/2010
DOI: 10.1021/la904604z
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Table 1. Literature Reports of the Aluminum Oxide Growth by ALD on Various Polymers polymer or molecule
reference
poly(3-hexylthiophene) (P3HT) poly(vinylphenol) (PVP) poly(methyl methacrylate) (PMMA) poly(ethylene naphthalate) (PEN) poly(ethylene terephthalate) (PET) poly(carbonate) (PC) polypropylene (PP) poly(vinyl alcohol) (PVA) poly(vinylpyrrolidone) (PVP) cellulose cotton polyimide (Kapton) polyethylene (PE) poly(ether ether ketone) (PEEK) polytetrafluoroethylene (PTFE) ethylenetetrafluoroethylene (ETFE) poly(vinyl chloride) polystyrene (PS) poly(ether sulfone) (PES) porphyrins
6 6 6, 23, 25, 26 7-9, 21, 19 9 9, 26 11, 25 12 13 15 18, 19, 21 20, 24, 25 23 23 23 25, 26 25, 26 27 28
polymers, growth can proceed through subsurface nucleation,25 and in some cases, ALD process reactants can penetrate through polymer films resulting in film coatings under the polymer layer.6 Reactant penetration and subsurface inorganic nucleation in a polymer film can negatively affect material properties. For example, oxide penetration in P3HT active layers in organic thin film transistors results in a decrease of device performance.6 Subsurface oxide growth can also impede the performance of ALD barrier layers formed on polymer films.5,9 While a polymer substrate generally constrains the temperature range allowed for deposition, previous studies have not specifically addressed the role of substrate temperature in ALD film nucleation and growth on polymers. This article demonstrates that within the temperature range of interest for inorganic coatings on polymers, film nucleation, coalescence, and growth depends strongly on the substrate temperature. The temperature and nucleation dependence is also shown to vary with the bonding units available within the polymer structure. Polymers without reactive groups in the polymer backbone structure, for example, show significant subsurface growth at elevated deposition temperatures, whereas materials such as cellulose cotton with reactive groups in the polymer chain, promote surface growth even at high deposition temperature. Subsurface growth in nonreactive polymers is ascribed to thermally driven polymer expansion and enhanced precursor penetration.
II. Materials and Methods Melt blown polypropylene ((C3H6)n) nonwoven fiber mats were prepared using a 0.56 m pilot-scale melt blowing line located in the Nonwoven Cooperative Research Center at NC State University. The polypropylene fibers in the resulting nonwoven mat have a radius between 500 nm and 5 μm, and the overall mat thickness is ∼0.3 mm. Polypropylene is a common semicrystalline thermoplastic with a highly nonreactive surface. The nonpolar structure results in fibers that have high chemical resistance to alcohols, organic acids, esters, ketones, inorganic acids, and alkalis, as well as low water absorption (95% naturally, 99% finished) consisting of linear chains of glucose (repeating unit of C6H10O5) with ss-glycosidic linkages. The hydrogen bonding between chains results in a stable material that is chemically resistant. The material used here was previously bleached and mercerized in sodium hydroxide solution to produce a transformation from R- to β-cellulose.15 Each glucose unit has three hydroxyl groups that make the material highly polar, resulting in significant water absorption (>8 wt %). The coefficient of linear expansion of cellulose cotton is between (7-8) 10-5 C-1 between 60 and 90 °C, but increases nonlinearly at temperatures above 100 °C. Cellulose fibers are stable at temperatures up to ∼150 °C. Atomic layer deposition was used to deposit thin films of aluminum oxide on the as-received polypropylene and cotton fiber substrates. Trimethyl aluminum (TMA) is used as the Al precursor and water is used as the source of oxygen. The ALD process consists of a vacuum-based two step reaction sequence where a gas phase precursor first reacts with available hydroxyl groups resulting in methyl-group termination on the substrate surface. One ALD cycle consists of a TMA dose, inert purge, H2O dose, and inert purge. Under ALD conditions on a dense solid receptive surface, increasing the number of cycles produces a welldefined incremental increase in oxide thin film thickness. As discussed below, initial deposition on nonreactive polymer surfaces generally produces less well-defined results. The ALD growth was performed in a custom built hot-wall reaction chamber consisting of a 24 in. length, 23/4 in. diameter stainless steel conflat-sealed nipple with independent 1/4 in. diameter gas inlets for the TMA (98% purity, STREM Chemicals) and H2O (UV-deionized). The system is pumped by a rotary mechanical pump, has a background pressure of
