Nanocube Assembly á La Carte

Arrays of nanopatterns are of high interest in the multidisciplinary field of nanoscale photonics. Conventionally there are two approaches to fabricating such nanopatterns: top-down nanofabrication via electron beam lithography and bottom-up self-assembly. The former can create complex structures but lacks control over crystallinity, introduces surface roughness due to etching¹, is expensive and time-consuming, making it unfavorable for large-scale fabrication. Conversely, the latter is typically straightforward, affordable, and has better control over atomic composition and crystallinity. However, it struggles to fabricate complex nanopatterns due to thermodynamic constraints². This has been partially circumvented with directed assembly strategies, which consist of colloidal nanoparticles in PDMS templates replicated from a nanofabricated master³. However, the nanopattern geometry that could be achieved effectively is still strongly limited to individual particles or low-density 1D arrays^4,5, whereas precise and deterministic arrangement of nanoparticles in truly complex and arbitrary nanopatterns (needed for nanophotonics and plasmonics applications) have so far remained elusive.<br/><br/>Here, we offer a new approach to obtain arbitrarily complex nanostructure arrays with a single nanoparticle resolution with no need for advanced equipment. Nanoparticles are first deposited at the air/water interface in a compact film and then transferred into a nanopatterned template. This highly versatile technique is independent from the specific nanopattern and can apply to a plethora of materials and shapes. We will show assembly of various materials (Ag, Au, Au-Cu₂O core-shell nanoparticles and nanocubes) into various complex nanopatterns, such as U-shapes, L-shapes, cross-shapes, S-shapes, T-shapes, Pancharatnam–Berry metasurfaces, and also split-ring resonators on PDMS, that cannot be obtained with conventional assembly techniques. Among other nanoparticle shapes, we focus on nanocubes because they are ideal building blocks for obtaining monocrystalline nanostructures^6,7. We will show that such assemblies, in which each nanocube unit is precisely assembled as a Lego brick in complex extended arbitrary motifs, can subsequently be transformed into continuous nanostructures via nanocube epitaxy and then transfer-printed to various substrates, including flat and microstructured substrates. Finally, we will show optical characterizations and simulations of the metasurfaces obtained. This strategy potentially offers an alternative to a bottom-up fabrication for metal nanostructuring, which can then be applied to optoelectronic applications.<br/><br/>References:<br/>1. Agrawal & Garnett. <i>ACS Nano</i> <b>2020</b>, 14 (9).<br/>2. Ostrovsky et al. <i>Langmuir</i> <b>2021</b>, 37 (30).<br/>3. Kraus et al. <i>Nat. Nanotechnol.</i> <b>2007</b>, 2 (9).<br/>4. Flauraud et al. <i>Nat. Nanotechnol.</i> <b>2017</b>, 12 (1).<br/>5. Capitaine & Sciacca. <i>Adv. Mater.</i> <b>2022</b>, 34 (24).<br/>6. Sciacca et al. <i>Adv. Mater.</i> <b>2017</b>, <i>29</i> (26).<br/>7. Capitaine et al. <i>ACS Nano</i> <b>2023</b>, 17 (10).