Conference Agenda

Metal nanocatalysts typically consist of noble metal nanoparticles (NPs) anchored on a metal oxide support, where the NP surface exposes active sites to catalyse target chemical reactions. The catalyst’s economic viability demands high activity, high selectivity, and high stability. It is well established that the performance of catalytic NPs is closely related to their size, shape and interparticle distance. Synthesis methods that can tailor the structural properties of noble metal NPs are therefore attractive to elucidate performance-structure relationships. In this regard, there is an increasing interest in Atomic Layer Deposition (ALD), a vapour-phase deposition method which proved its efficiency in dispersing noble metal NPs on complex high surface area supports with atomic-scale control over the metal loading (atoms per cm²) and nanoparticle size [1]. However, an improved understanding of how the deposition parameters influence the formation and growth of the noble metal NPs is required to fully exploit the tuning potential of ALD.

We designed a high-vacuum setup for thermal and plasma-enhanced ALD that is compatible with synchrotron-based in situ X-ray fluorescence (XRF) and grazing incidence small-angle X-ray scattering (GISAXS) monitoring [2]. Using this setup, we resolved the dynamics of Pt and Pd NP formation and growth on planar SiO₂ and Al₂O₃ surfaces [3-5]. In situ XRF was used to quantify the evolution of metal loading with the number of ALD cycles, while analysis of the key scattering features allowed us to correlate the amount of deposited material with the evolution of structural parameters such as cluster shape, average size and areal density.

In a first study we focused on the growth of Pt deposits on SiO₂ with the thermal ALD process comprising sequential MeCpPtMe₃ and O₂ exposures at 300°C [3]. The results indicated a nucleation stage, followed by a diffusion-mediated particle growth regime during which the size and spacing of the Pt NPs is largely determined by adsorption of migrating Pt species on the surface and diffusion-driven particle coalescence. Interestingly, diffusion phenomena and ripening of the Pt NPs during ALD could be suppressed by using N₂ plasma as a reactant instead of O₂ in the ALD cycle. By combining O₂-based and N₂ plasma-based ALD processes, we developed a tuning strategy that offers independent control over the Pt NP size and areal density [4].

Secondly, we studied the initial nucleation of Pd NPs deposited at 150°C on oxide substrates (SiO₂ or Al₂O₃) by combining Pd(hfac)₂ and H₂ plasma in an ALD sequence [5]. The results confirmed a long nucleation process and revealed a relatively low NP areal density, in line with the occurrence of surface poisoning during the initial ALD cycles [6]. The reaction of the Pd precursor with the oxide surface leaves site blocking surface species behind, thereby inhibiting the nucleation. To enhance the nucleation, we explored two potential methods to ‘clean’ the surface: (1) introducing trimethylaluminum (TMA) exposures during the initial ALD cycles, and (2) introducing an O₂ plasma exposure, either before or after the H₂ plasma step, throughout the ALD process. Both these approaches had a significant impact on the evolution of NP size and spacing, and the insights obtained were used to develop a strategy that enables precise control of the Pd NP dimensions and coverage [5].

[1] Lu, J., Elam, J. W. & Stair, P. C. (2016). Surf. Sci. Rep. 71, 410.

[2] Dendooven, J., Solano, E., Minjauw, M. M., Van de Kerckhove, K., Coati, A., Fonda, E., Portale, G., Garreau, Y. & Detavernier, C. (2016). Rev. Sci. Instrum. 87, 113905.

[3] Dendooven, J., Van Daele, M., Solano, E., Ramachandran, R. K., Minjauw, M. M., Resta, A., Vlad, A., Garreau, Y. Coati, A., Portale, G. & Detavernier, C. (2020). Phys. Chem. Chem. Phys. 22, 24917.

[4] Dendooven, J., Ramachandran, R. K., Solano, E., Kurttepeli, M., Geerts, L., Heremans, G., Rongé, J., Minjauw, M. M., Dobbelaere, T., Devloo-Casier, K., Martens, J. A., Vantomme, A., Bals, S., Portale, G., Coati, A. & Detavernier, C. (2017). Nat. Commun. 8, 1074.

[5] Feng, J.-Y., Ramachandran, R. K., Solano, E., Minjauw, M. M., Van Daele, M., Vantomme, A., Hermida-Merino, D., Coatia, A., Poelman, H., Detavernier, C. & Dendooven, J. (2021). Appl. Surf. Sci. 539, 148238.

[6] Goldstein, D. N. & George, S. M. (2011). Thin Solid Films 519, 5339.

Soft-matter bicontinuous networks find a double gyroid structure from block copolymer (BCP) self-assembly. A gyroid structure composed of dissimilar blocks has proven its potential as a soft crystal, of which the lattice dimension is variable with molecular weight of the polymer. Using an asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA), recently we showed that the self-assembled gyroid films formed via a solvent vapor annealing (SVA) process undergo unique structural distortion due to directional deformation immediately upon deswelling [1]. During the SVA process with PS-b-PMMA films, initially transient cylinders developed from the as-cast morphology transform into a cubic gyroid structure in a swollen state. We then observed that upon solvent evaporation the gyroid lattice contracts along the film normal direction while retaining the swollen lateral dimension. The degree of contraction is turned out to be related to the evaporation speed. Rapid and spontaneous deswelling processes lead to triclinic gyroid structures with z-directional contraction ratios (C_z) of 2.5 and 2.0, respectively.

Our X-ray analysis reveals that symmetries of the resulting gyroid structures are partially broken due to the non-affine transformation, eliciting several forbidden reflections such as {110} and {200} reflections. For further characterization of the symmetry-breaking, we delineate the structural features of noncubic gyroid films by computing electron-density difference maps from grazing incidence small angle X-ray scattering (GISAXS) data. We employed iterative phase retrieval method to solve the phase problem. Level-set approach is accordingly developed to quantitate the structural characteristics of the maps in terms of inversion symmetry-breaking, suggesting its possible application to optical Weyl photonic crystals. This presentation will focus on X-ray data collection and analysis.

[1] Jo, S., Park, H., Jun, T., Kim, K., Jung, H., Park, S., Lee, B., Lee, S., Ryu, D. Y. (2021), Applied Materials Today 23, 101006.

GISAXS measurements were performed at Pohang Accelerator Laboratory (Korea) and Advanced Photon Source (APS) at Argonne National Laboratory (US). The APS is supported by the US department of Energy, Office of Basic Energy Sciences, under contract no. DE-AC0206CH11357. This research was supported by Samsung Research Funding & Incubation Center of Samsung Electronics under Project Number SRFC-MA1801-04.

Thin films of thickness 100 nm or less are typically deposited on a much thicker substrate, making it difficult to obtain the required high-quality total scattering data for analysis in real-space with pair distribution functions (PDF) [1]. In recent years, total scattering in reflection geometry at grazing incidence (GI), that is, below the critical angle of total external reflection, has been used with success to increase the surface sensitivity and scattering intensity [2, 3]. GI-PDF gives high-quality PDFs from films as thin as 3 nm with a 20 x 2.5 µm² focused X-ray beam with 100 keV photon energies at PETRA III [2]. Using this same setup we have developed a novel ultra-high vacuum compatible deposition chamber, that allows for the demanding sample alignment of the thin film sample under vacuum conditions as well as a 180 degree in-plane rotation [4]. Via a rotary feedthrough and bellows combination the surrounding vacuum chamber is not affected by translation of the sample, allowing for film deposition equipment such as a magnetron sputter source. This has been employed at the P07-EH2 beamline with a radio-frequency magnetron sputter source, as pictured in the figure below. We will show how this has been applied to observe the formation of thin films during the initial stages of deposition in real time with sub-second time resolution.

[Fig 1 with figure + picture]

Figure 1. PDFs of a Pt thin film at 10, 20, 30, 40, 50 and 60 seconds of sputter deposition with approximately 1 Å/s and a picture of the equipment installed on the surface diffractometer at beamline P07-EH2, PETRA III, Hamburg, Germany, as seen from the detector side.

[1] K. M. Ø. Jensen et al., IUCrJ 2 (2015), 481

[2] A.-C. Dippel et al., IUCrJ 6 (2019), 291

[3] K. Stone et al., APL Materials 4 (2016), 076103

[4] M. Roelsgaard et al., IUCrJ 6 (2019), 299