Polymeric Materials: Science & Engineering 2001, 85, xxx X-ray standing wave measurements of gold nanoparticles in polymeric thin films Rodney S. Guico 1 , Andrew G. Richter 1 , Jin Wang 1 , and Kenneth R. Shull 2 1 User Program Division, Argonne National Laboratory, Argonne, IL 60439, guico@aps.anl.gov 2 Department of Materials Science and Engineering, Northwestern University INTRODUCTION Polymer/metal nanocomposites have emerged in recent years as an important research area due to their practical and fundamental significance. While much of the current work in this field has focused on the diffusive properties of nanoparticles in polymeric matrices, specifically diblock copolymer templates, fundamental questions can be addressed by studying individual particle interactions with homopolymers. To observe the physics of the situation, we probed polymer/metal interactions using a model system. The diffusion of gold nanoparticles was measured in real time as the polymer thin film is annealed above the glass transition temperature (T g ). Total external reflection x-ray standing waves (TER-XSWs) were used to determine the profile of the metal particles perpendicular to the polymer surfaces over short anneal times before the particles have coalesced. EXPERIMENTAL SETUP Polished silicon wafers were first placed in a thermal evaporation system and coated with a layer of silver (using chromium as an underlayer). Poly(tert-butyl acrylate) (PtBA, M W =352,000 g/mol) thin films (250 Å) were then spun cast from a butanol solution. The samples were then placed back in the evaporation system, where a thin layer of gold was deposited on the polymer films. A quartz crystal monitor residing in the vacuum chamber measured the thickness of the evaporated layers. The gold coverage was equivalent to a continuous layer 4 Å thick. The gold layer was then sandwiched by another thin polymer film (250 Å) floated from a water bath. XSW experiments were performed at the 1-BM beamline at the Advanced Photon Source. The x-rays were monochromated to an energy of 12.1 keV, suitable for promoting Au L fluorescence. During XSW measurements, the samples were kept under a helium environment both at room temperature and throughout the annealing process. RESULTS & DISCUSSION XSWs are generated during total external reflection when the incident angle of the x-rays is less than the critical angle of the silver mirror. Previously, it has been demonstrated that the XSW method can be used to locate a heavy atom marker layer in thin films with Angstrom spatial resolution. 1,2 The measurement was performed in real time thanks to the intense synchrotron x-ray beams and the strong Au L fluorescence. The spatial distribution of the gold marker can be obtained by fitting the fluorescence profile as a function of incident angle with a realistic model. The reflectivity (a) and fluorescence (b) experimental data at room temperature along with the fits are shown in Figure 1. The reflectivity and fluorescence data were fitted up to the critical angle of the silver mirror, which is 5.08 mrad, as seen by the sharp decrease in the reflectivity. The data above this angle also contained information for the silver, chromium, and silicon layers. Since the critical period of the XSWs was about 100 Å for a silver mirror, the electric field oscillations passed through the heavy atom layer 250 Å above the mirror surface three times, as indicated by the three fluorescence peaks in the profile. The fluorescence yield is a convolution of the electric field intensity and an assumed spatial distribution. In the current work, a Gaussian distribution was assumed with two fitting parameters: the half-width-half-max (HWHM) of the distribution and its mean position above the mirror surface. The fit in 0 1 Reflectivity (a) 0 4 Au L Fluorescence (a.u.) (b) 0 4 0 5 10 Au L Fluorescence (a.u.) (c) Incident Angle (mrad) Figure 1. Experimental data (circle) and fits (line) at room temperature for (a) reflectivity and (b) fluorescence. The fluorescence profile after the sample has been annealed at 80°C for 206 minutes is shown in (c). Figure 1b showed that the gold layer has a HWHM of 17 Å and is situated 250 Å above the mirror surface. The samples were then annealed above T g (=49°C) to allow the gold nanoparticles to diffuse. The fluorescence profile taken after 206 minutes at 80°C is shown in Figure 1c. The fit to the fluorescence profile showed significant broadening (HWHM=68 Å) of the gold distribution as well as movement of the mean position towards the air/film interface. This is partially attributed to the asymmetric nature of the polymer films, since motion of the bottom PtBA layer is hindered by the substrate. The HWHM values collected in real time during the anneal were used to determine effective diffusion coefficients and indicated that, within the polymer relaxation times, diffusion distances were comparable to the interparticle spacing. ACKNOWLEDGEMENTS This work was supported by the U.S. DOE under Contract No. W- 31-109-ENG-38. REFERENCES 1. Wang, J.; Bedzyk, M.J.; Penner, T.L.; Caffrey, M. Nature 1991, 354, 377. 2. Bedzyk, M.J.; Bommarito, G.M.; Schildkraut, J.S. Phys. Rev. Lett. 1989, 62, 1376. View publication stats View publication stats