Large area Al2O3-Au raspberry-like nanoclusters from iterative block-copolymer self-assembly

Abstract

In the field of functional nanomaterials, core-satellite nanoclusters have recently elicited great interest due to their unique optoelectronic properties. However, core-satellite synthetic routes to date are hampered by delicate and multistep reaction conditions and no practical method has been reported for the ordering of these structures onto a surface monolayer. Herein we show a reproducible and simplified thin film process to fabricate bimetallic raspberry nanoclusters using block copolymer (BCP) lithography. The fabricated inorganic raspberry nanoclusters consisted of a ∼36 nm alumina core decorated with ∼15 nm Au satellites after infusing multilayer BCP nanopatterns. A series of cylindrical BCPs with different molecular weights allowed us to dial in specific nanodot periodicities (from 30 to 80 nm). Highly ordered BCP nanopatterns were then selectively infiltrated with alumina and Au species to develop multi-level bimetallic raspberry features. Microscopy and X-ray reflectivity analysis were used at each fabrication step to gain further mechanistic insights and understand the infiltration process. Furthermore, grazing-incidence small-angle X-ray scattering studies of infiltrated films confirmed the excellent order and vertical orientation over wafer scale areas of Al₂O₃/Au raspberry nanoclusters. We believe our work demonstrates a robust strategy towards designing hybrid nanoclusters since BCP blocks can be infiltrated with various low cost salt-based precursors. The highly controlled nanocluster strategy disclosed here could have wide ranging uses, in particular for metasurface and optical based sensor applications.

Fig. 1. Schematic representation of the raspberry nanoclusters fabrication: (A) formation of an out-of-plane cylindrical structure obtained from BCP1 self-assembly. (B) SIS of Al₂O₃ by ALD into the cylindrical PVP domains. (C) Controlled removal of 30 nm polymer film by UV/O₃ etching. (D) Formation of a second out-of-plane cylindrical structure obtained from BCP2 self-assembly on top of the first layer. (E) Au metal impregnation into the PVP domains followed by (F) Au metal reduction by RIE plasma.

Fig. 2. AFM topographical images of (A) as-cast BCP1 layer and (B) self-assembled BCP1 layer after 16 h SVA. GISAXS patterns of BCP1 films at different solvent vapour annealing (SVA) time: (C) as-cast and (D) 16 h. (E) Corresponding GISAXS line-cut along q_y integrated around the Yoneda band.

Fig. 3. AFM topographical images of the different steps of the process to obtain bimetallic raspberry-like nanoclusters: (A) alumina dots obtained after SIS and 30 min of treatment with UV/O₃. (B) Self-assembled BCP2 layer on top of the Al₂O₃ dots generated from the SIS of the self-assembled BCP1 film and (C) hexagonal ordered raspberry-like nanoclusters (Au@Al₂O₃) monolayers obtained by iterative self-assembly. Inset in (C) correspond to the FFT of the AFM image. Two different periodicities are clearly identified, corresponding to the Al₂O₃ dots and Au NPs respectively.

Fig. 4. GISAXS patterns of (A) the Al₂O₃ dots formed by the infiltration of BCP1, (D) the bimetallic raspberry-like nanoclusters obtained by the combination of the small Au⁰ NPs (∼15 nm diameter) on top of the Al₂O₃ dots (∼36 nm diameter) and (G) Au⁰ NPs obtained using a nanostructured BCP2 film as a template. (B), (E) and (H) correspond to the GISAXS line-cuts along q_y integrated around the Yoneda band (insets: AFM topographical images of the corresponding nanostructures (C), (F) and (I)).

Fig. 5. XRR reflectivity data (black line) obtained for (A) nanostructured BCP1 film, (B) Al₂O₃ dots array and (C) the final Au@Al₂O₃ bimetallic raspberry-like nanoclusters. Red lines correspond with fitting obtained by GenX.

Fig. 6. SLD profile (left) and imaginary SLD profiles (left: real part; right: imaginary part) obtained after fitting the experimental XRR data for: (A–C) neat BCP1 film, (D–F) BCP1 layer impregnated with Al₂O₃ and (G–I) bimetallic raspberry-like nanoclusters consisting in a Al₂O₃ core decorated with Au satellites. Graphical representations of the structures (B, E and H) are also presented to help the data interpretation.

Fig. 7. AFM topographical images of the different steps of the process to obtain decorated bimetallic nanowires (A) out-of-plane lamellar structure obtained from BCP3 self-assembly (B) Pt nanowires obtained by selectively impregnation out-of-plane BCP3 lamellar films and (C) bimetallic Au–Pt nanowires obtained by self-assembly of BCP2 on top of the Pt nanowires.