In-situ GISAXS study of gold sputtering onto a polymer template

E. Metwalli 1, S. Couet 2, K. Schlage 2, R. Röhlsberger 2, V. Körstgens 1, M. Ruderer1, W. Wang1,

G. Kaune1, S.V. Roth 2, and P. Müller-Buschbaum1

1 Technische Universität München, Physikdepartment LS E13, James-Franck-Str. 1, 85747 Garching, Germany 2 HASYLAB at DESY, Notkestr. 85, 22603 Hamburg, Germany

Recent advances in the patterning of polymers have enabled the fabrication of integrated micro- and nanosystems with a high degree of complexity and functionality [1-3]. For example, block copolymers have attracted immense interest for nanotechnology applications because of easy processability and low-cost fabrications. The chemically distinct and immiscible polymer blocks in block copolymers microphase-separate and self-assemble into ordered patterns on the scale of nanometers. This soft nanostructured polymer film can further be used as a template for patterning of hard inorganic materials such as metal nanoparticles. Metal nanoclusters in a matrix of insulating polymer have unique physical properties and have been proposed for optical, electrical, and magnetic applications. The techniques used for the metal patterning along with the various mechanisms (adsorption, surface diffusion, nucleation, and agglomeration) involved in the dispersion process of the nanoparticles within the polymer film make it difficult to understand the dynamics of nanoparticle formation in polymer matrix. In the present study we used the sputtering technique to investigate the deposition of gold nanoparticles onto commercially important types of copolymers—the thermoplastic elastomers. A mixture of polystyrene-block-polyisoprene-block-polystyrene P(S-b-I-b-S) triblock and polystyrene-block-polyisoprene P(S-b-I) diblock copolymers is known for its use in pressure sensitive adhesive (PSA) applications [4]. Here, we report on using a detachable DC magnetron sputtering deposition system mounted at a beamline allowing for grazing incidence small-angle X-ray scattering (GISAXS) to investigate the in-situ formation and growth of gold nanoparticles in the polymer film.

The GISAXS measurements were carried out at beamline BW4 of DORIS III storage ring [5, 6] at HASYLAB (DESY, Hamburg). The sample inside the sputtering chamber was placed horizontally (xy plane) and at an incidence angle αi =0.44° to the incidence X-ray beam by tilting the whole chamber using a goniometer. The incidence angle is well above the critical angle of both the polymer film and the substrate (αc(SIS/IS) = 0.133°, αc(Si) = 0.200°); therefore, the Yoneda peaks of both materials and specular peak are well separated on the 2D detector. At this angle of incidence both surface and bulk nanostructures of the polymer film are accessible. Structural information is obtained by horizontal (qy) and vertical (qz) cuts of the 2D intensity distribution. The resulting intensity profiles were fitted using a software IsGISAXS [7], which is based on the distorted-wave Born approximation and the DA (decoupling approximation) for particles encapsulated in a layer on a substrate. The interference function is fitted with a two parameter functions; the average distance, D and a disorder parameter, ω. The program was also used to model the full 2D intensity distribution. It allows us to determine the inter-particle distance, D, the particle shape, lateral size, and size distribution.

The GISAXS 2D images (Figure 1-a) of the thin blend copolymer film show two intensity maxima (marked with arrows), separated by the shadow of a rod-shaped beamstop, visible along qy direction, that originate from the interference of the PS-domains. This interference effect arises because the PS domains are separated by a preferential nearest neighbors (center-to-center) distance, D. The detection of only one interdomain interference and the absence of additional secondary bcc peaks (at q2/q1 = √2 and q3/q1 = √3) indicate that no regular spatial arrangements of the PS-rich domains and a more liquid-like ordered structure best describe the morphology of the polymer film.

The deposition of gold on this polymer template was performed for 6-seconds periods and the in-situ GISAXS measurements were collected for 30 second immediately after the deposition step. The deposition steps (at a rate of 4.3Å /min for six seconds) followed by the GISAXS measurements were both repeated for 10 times and the 2D intensity profiles are shown in Figure 1b-k. As the amount of deposited gold increases, the prominent peak at qy = 0.015 Å-1 on both sides

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of the beam stop gradually grows in intensity along the qz axis (Figure 1b-k). The fitting of the out-of-plane cuts (qy) of the 2D scattering intensity profile (at qz position of 0.025 Å-1) was performed for qy >0.005 Å-1 and indicates no strong influence of the accumulated gold particles on the overall lateral structures of the polymer film. In addition, no additional characteristic scattering peak is observed for the gold particles on the qy direction. On the other hand, the vertical cuts (qz) obtained at the prominent peak (at qy position 0.015 Å-1; shown in Figure 1 as two vertical growing intensity) shows a gradually increase of the intensity with increasing of the amount of gold. The polymer film model was simulated using the high x-ray refractive index gold particles instead of the PS domains. The results of this simulation compared with the experimental one indicates that the gold diffuse inward, and aggregate inside the PS domains. The polymer film sputtered with 4.3 Å gold was kept to relax for 10 h and then the GISAXS measurement was performed to investigate a possible structural reorganization of the gold nanoparticles inside the film. The GISAXS study indicates negligible changes in the film structure after 10 h relaxation time of the gold sputtered polymer film and reveals that the gold nanoparticles aggregate and form a stable morphology within the film.

Figure 1: Composite images showing 2D scattering patterns of phase separated thin blend copolymer film, (a) 10 repetitions of six-second corresponding to 0.43Å deposition of gold per repetitions onto the polymer film (b-k), and after 10 h relaxation of the 4.3 Å gold sputtered copolymer film (l). The intensity is shown on a logarithmic scale. The gray rectangle in the middle of the images indicates the rod beam stop and the horizontal black line with pointlike end is the specular beam stop. Two white arrows indicate the two scattering intensity maxima along the qy direction.

As a conclusion, the in-situ GISAXS investigation is critical for understanding how the arrangements of metal nanoparticles correlates with the structure of copolymer domains within the film and opens new possibilities for the investigation of metal-polymer interactions. Our results indicate that gold migrates to the central regions of the polymer film and decorates the spherical PS domains. The gold-gold interaction in PS regions dominates the gold-polymer interaction. Metal aggregation occurs at a rate faster than the diffusion of gold atoms in the polymer. Our study introduces the concept that, without thermal annealing of the gold attached polymer film, the gold assembly is not limited to the flat two dimensional but also included in three dimensional structures.

References [1] Y.N. Xia, E. Kim, X.M. Zhao et al., Science 273, 347 (1996) [2] S.R. Quake, and A. Scherer, Science 290, 1536 (2000) [3] J. Schmitt, G. Decher, W.J. Dressick et al., Advanced Materials 9, 61 (1997) [4] A. Roos, and C. Creton, Macromolecules 38, 7807 (2005) [5] S.V. Roth, R. Döhrmann, M. Dommach et al., Review of Scientific Instruments 77, 085106 (2006) [6] P. Müller-Buschbaum, Analytical and Bioanalytical Chemistry 376, 3 (2003) [7] R. Lazzari, Journal of Applied Crystallography 35, 406 (2002)

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