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J. Phys. Chem. B 1997, 101, 7808-7813
Chemical Vapor Deposition of Silver on Plasma-Modified Polyurethane Surfaces S. Serghini-Monim, P. R. Norton,* and R. J. Puddephatt Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, N6A 5B7, Canada ReceiVed: April 23, 1997; In Final Form: July 10, 1997X
The surface chemistry of [(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)(trimethylphosphine)silver(I)], [(fod)AgP(CH3)3], a silver precursor for chemical vapor deposition (CVD), has been investigated on plasma-modified polyurethane surfaces by means of reflection-absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). On the unmodified polyurethane surface, the RAIRS and XPS data show that [(fod)Ag(P(CH3)3)] reacts with polyurethane CdO, N-H, and C-O-C groups present at the surface by displacing the Lewis base P(CH3)3; these acid-base reactions yield the formation of surface Ag-O and Ag-N bonds. Air plasma treament of the polyurethane surface leads to the incorporation of CdO and C-O-O functional groups, as well as the conversion of C-N(H)- into CdN- functional groups by the abstraction of hydrogen. RAIRS data obtained following O2 and air plasma treatment showed that, in addition to grafting of functional groups, polyurethane chains change their conformation in the polyurethane film. The increase of silver concentration resulting from CVD is proportional to the increase of oxygen incorporated on the polymer surface. This study shows that CVD on soft polymers can be performed at room temperature.
Introduction The deposition of metal films on metal and semiconductor substrates using metal-organic chemical vapor deposition (MOCVD) is a well established field of research. By contrast, MOCVD on polymer surfaces is still an emerging field of research.1,2 The main obstacles to the use of MOCVD to grow metallic films on polymer substrates are (i) the low surface energy (reactivity) and (ii) the low melting points and low decomposition temperatures of many polymers. The modification of polymer surface reactivity has considerable importance in a variety of technological areas such as metallization, fabrication of composites, and enhancement of biomedical compatibility.3 Plasma surface modification is a commonly used technique to improve polymer surface wettability, adhesion, friction, and biocompatibility without altering the bulk properties of the polymer. This is accomplished by incorporating functional groups such as CdO, C-O, OdC-O, and NHx on polymer surfaces. We report in the present paper the results of adsorption of a silver complex [(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)(trimethylphosphine)silver(I)], [(fod)Ag(PMe3)], on a plasma-modified polyurethane surface. The purpose of the plasma modification treatment was to increase the concentration of adsorption sites for CVD reaction and to increase the surface biomedical compatibility through the incorporation of amine groups to mimic amino acids. The incorporation of silver atoms on catheter surfaces made from polyurethane is designed to provide antibacterial properties to the catheter. The use of a CVD process would permit comformal coating of convoluted shapes such as tubes. In a separate paper,2 we report RAIRS and XPS data on the adsorption of the [(fod)Ag(PMe3)] precursor on the untreated polyurethane surface. The RAIRS data show that for various substrate temperatures (90, 300, and 340 K) the silver complex reacts with CdO, NH, and C-O functional groups at the surface of the polyurethane film by displacement of the [PMe3] ligand. The reactivity of the polyurethane surface is higher at 340 K * To whom correspondence should be addressed. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, September 1, 1997.
S1089-5647(97)01382-5 CCC: $14.00
than at 90 and 300 K, due to a combination of factors including thermal activation of the complex, thermal enhancement of mobility, and diffusion of low molecular weight polyurethane chains from the bulk to the surface and reorientation of functional groups, making more active sites available to react with the incoming precursor. XPS results of the adsorption of the [(fod)Ag(PMe3)] on unmodified polyurethane surfaces at room temperature have shown the presence of Ag and the absence of P, thus providing direct evidence for [PMe3] ligand displacement by the functional groups at the polyurethane surface2 via an acid-base reaction between the polyurethane functional groups and the precursor. The O 1s and N 1s regions showed the appearance of new peaks with increasing [(fod)Ag(PMe3)] doses supporting the formation of [(fod)Ag-O-C] and [(fod)Ag-N-C] entities. The XPS and RAIRS data revealed that, at room temperature and above, adsorption of the Ag complex on the unmodified surface only occurs via interaction with an active site such as NH, C-O-C, and CdO 2 and that only chemisorbed Ag complex molecules remain on the surface. It thus is necessary to increase the number of active sites by a plasma surface treatment. This paper reports the results of such a study. Experimental Section RAIRS and XPS measurements were performed in two distinct UHV chambers. The XPS measurements were performed in a chamber described elsewhere4 using an SSL SSX100 X-ray photoelectron spectrometer with a monochromatized Al KR X-ray source and hemispherical analyzer at a takeoff angle of 53° and using an electron flood gun for charge compensation (1 V, 0.05 µA). Elemental analysis was performed using a spot size of 600 mm and a pass energy of 129 eV, whereas for the high-resolution measurements settings of 300 mm and 54 eV, respectively, were used. The RAIRS measurements were performed in the UHV chamber (base pressure < 5 × 10-9 Torr) described previously5,6 with the only difference being that the cold finger is terminated with a sample holder which can accommodate interchangeable polished aluminum disks of 14 mm diameter. A wobble stick was used to interchange samples which were held on a carrousel. © 1997 American Chemical Society
Chemical Vapor Deposition of Silver SCHEME 1: (A) Molecular Structure of Aromatic Polyurethane; (B) Molecular Structure of [(fod)Ag(PMe3)] Precursor
A heater cartridge (7.6 mm diameter and 10.2 mm long) was located at the back of the sample to allow sample warming, and a chromel-alumel thermocouple was set between the sample and the heater cartridge to monitor sample temperature from 90 to 600 K. This chamber is equipped with a mass spectrometer (Hiden-601), an ion gun (Varian-2046), and a quartz crystal monitor (Leybold Inficon). A Mattson Cygnus100 spectrometer was employed for RAIRS measurements. The RAIRS data were collected using a liquid nitrogen cooled, narrow band mercury-cadmium-telluride (MCT) detector. The spectra presented in this paper were averaged for 500 scans at a resolution of 4 cm-1. This chamber is also equipped with a UHV compatible plasma source developed by Winters et al.7 This source consists of a set of two nested, concentric quartz tubes. The plasma is created by a 2.45 GHz microwave generator and an Evenson cavity. Depending on the desired flux, the pressure inside the quartz tubes can be varied from 3 × 10-3 to 3 × 10-1 Torr (measured by a thermocouple gauge), while the pressure inside the UHV chamber can vary from 3 × 10-6 to 6 × 10-5 Torr. The distance between the substrate and the plasma source was 45 cm. The plasma source was moved to the XPS chamber for the relevant experiments to prevent exposure of the treated surface to air and humidity. All plasma parameters were kept the same in each chamber. The only difference was that it was not possible to carry out plasma treatments at low temperatures in the XPS chamber. The Ar purity was better than 99.99%, and the air was Matheson “Zero” grade with a purity with respect to organics of better than 99.98% and
