Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 27101
Chiral plasmonic nanocrescents: large-area fabrication and optical properties VLADIMIR E. BOCHENKOV1,* AND DUNCAN S. SUTHERLAND2 1
Lomonosov Moscow State University, Leninskie gory 1/3, Moscow 119991, Russia iNANO Center, Aarhus University, Gustav Wieds vej 14, Aarhus 8000, Denmark *
[email protected]
2
Abstract: Large-area arrays of substrate-supported plasmonic gold crescents are fabricated by using the new colloidal lithography technique, which is based on an in situ-deposited silica resistance layer. The method provides the means to control the particles’ asymmetry just by changing the mutual deposition angle of gold and silica. Asymmetric crescent structures exhibit a pronounced circular dichroism in near-infrared region, with the chiral asymmetry factor reaching 0.2. According to the simulation, the optical chirality enhancement reaches between one and two orders of magnitude and is localized near the crescents’ tips. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction Chiral metallic nanostructures supporting a Localized Surface Plasmon Resonance (LSPR) are a subject of current extensive research [1] due to the promising applications in photonics, including new optical devices [2], enantioselective sensing [3] and catalysis [4]. A good control over nanostructures’ size and shape is vital for these applications. To date, a number of accurate top-down fabrication techniques exists, which allow manufacturing of various 2Dand 3D chiral plasmonic structures with high precision, such as Electron Beam Lithography (EBL) or Focused Ion Beam etching. However, these approaches generally suffer from low throughput and high cost, both hampering their broad application. For an industrial scale production, a possible solution to this problem might be in using injection-molded polymer templates to create plasmonic metamaterial films [5]. In this case, EBL is only used to fabricate master, which can be then replicated multiple times to produce low-cost nanostructured samples [6]. An alternative fabrication route allowing efficient parallel production of plasmonic nanostructures without using EBL is based on self-assembly. For example, an interesting fabrication scheme has been proposed using micellar nanolithography [7]. This method uses a monolayer of gold-loaded micelles transferred to the substrate by spin-coating to create arrays of gold nanodots after plasma treatment. Then, various 3D nanostructures like chiral nanohelices, etc. can be grown on these posts via glancing-angle deposition. Another example of self-assembly-based methods is colloidal lithography (CL). In this approach, colloidal particles, adsorbed on a substrate, are used as a mask in further deposition and etching processes to produce arrays of nanostructures. A number of CL-based techniques have been developed so far for the fabrication of plasmonic structures of different shape [8– 10]. For instance, periodic arrays of triangular nanoprisms [11,12] as well as more complex periodic structures [9,13,14] can be produced by Nanosphere Lithography using a closepacked layer of monodisperse nanospheres. Another general approach, usually referred to as Sparse Colloidal Lithography (SCL), uses a mask composed of charged sulfate latex spheres distributed over the surface with short-range, but without long-range order [15]. Due to coulombic repulsion, the deposited particles are separated from each other by a certain distance, allowing each nanosphere to be used for producing an individual nanoparticle. When used directly, this approach can be used to fabricate nanohole arrays [16] and various patterns [17,18], and in combination with reactive ion etching it allows fabrication of #342694 Journal © 2018
https://doi.org/10.1364/OE.26.027101 Received 17 Aug 2018; revised 25 Sep 2018; accepted 26 Sep 2018; published 2 Oct 2018
Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 27102
symmetric naanorings [19], crescents [20 0] and arrays oof crescent-likke nanoholes [21]. Yet another modiffication of CL approach calleed Hole-Mask Lithography (H HCL) is basedd on using a thin sacrificcial polymer laayer formed on n a substrate pprior to particlee deposition [222]. After the formation of a hole mask k by deposition n of a thin mettal film and paarticles lift-off, followed by etching thee polymer thro ough the holes, various structtures can be obbtained by depoosition of materials thro ough such mask k. The method d allowed fabriccation of plasm monic nanostruuctures of various shapee, including disks [23], stack ked disks [24] , their dimers [25] and clussters [26], etc. HCL app proach combin ned with well-ccontrolled tilteed-angle rotatioon evaporationn allowed fabrication off split-rings [27 7] and 3D chiraal plasmonic paarticles [28]. ntial importancce of CL is itss relative simpllicity and One of thee main advantages and poten low fabricatio on costs, that facilitates f its broad b use in unniversities andd R&D labs woorldwide. For practical reasons, min nimizing the number n of exxperimental techniques usedd for the monstrate a sim mple CLfabrication off certain nanostructures is deesirable. In thiss work, we dem based route fo or the fabricatiion of 2D chiraal plasmonic sstructures, whicch is based onn Physical Vapor Deposiition only and unlike HCL does d not requiree the use of sppin-coating andd reactive ion etching. Our approach h, the In situ Resist Colloiidal Lithograpphy (IRCL) [229], is a ntly, it has succcessfully beenn applied for tthe fabricationn of large modification of SCL. Recen m2) arrays of gold rings and symmetric creescents [30,31 ]. Here we exxtend this area (few cm approach furtther to fabricaate 2D chiral asymmetric a creescent particlees. We show tthat these structures exh hibit strong CD D response at th he frequency off the fundamenntal LSPR modde. 2. Results and a discussio on 2.1. Fabricattion of chiral crescent c partiicles Gold nanocrescents are prod duced using thee procedure illuustrated by Figg. 1(A).
Fig. 1. 1 Fabrication of gold g asymmetric crescents. c A) Depposition scheme: i)) deposition of Tii adhesion layer at α = 40 0° with substrate rotation, r ii) deposiition of silica at α = 30° and β = 0°, a α = 40° and β = 90°, iv) lift-off oof the particles annd the top layer off iii) deeposition of gold at gold, v) resulting structu ures, vi) schematiccs demonstrating tthe convention forr measuring angless α and d β; B) Camera im mage of a 24 mm m glass slide covvered by gold asyymmetric crescentt structu ures and the corressponding SEM im mages.
Right after depositing 300 nm polystyren ne nanospherees on solvent aand UV-ozonee cleaned on layer of Ti is deposited aat an angle α = 40° to surfacce normal substrates, thee 2 nm adhesio along with su ubstrate rotatio on, that leads to formation oof Ti layer aroound and beneeath each sphere. In the next step, 25 nm n silica resistt layer is depossited at a lowerr constant anglle α = 30° without rotattion, covering all substrate’’s surface exccept the ellipttical areas unnder each particle. The subsequent deposition of 20 nm gold layerr, carried out aat α = 40° afteer turning mal, leads to fformation of aan asymmetricc crescent the substrate by angle β arround the norm de the silica well. w The polysstyrene spheress as well as thhe top layer off gold are structure insid then removed d by repeated tape t stripping.. Asymmetric gold crescent particles remaain at the bottom of the wells in silicaa layer, as demo onstrated by sccanning electroon microscopyy image in
Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 27103
Fig. 1(B). Particle P size distribution d iss defined by the polydisppersity of poolystyrene nanospheres. Silica resist deposition step is a distincctive part of ouur IRCL approaach, since it proovides an ntrol over the shape of fabriicated particless by surface m masking, while allowing additional con one the removal of the sacrrificial gold laayer due to its poor adhesionn to silica. Varriation of onditions, in particular, p the substrate rotaation angle β,, allows fabriccation of deposition co particles with h different assymmetry. Fig gure 2 demonnstrates the goold crescent sstructures obtained at β = 45, 90, 135, 180°.
Fig. 2. 2 Fabrication of various v crescent sttructures by changging deposition anngle β of gold: A)) 45°, B) B 90°, C) 135°, D) 180. Top: sch hematics, showingg gold deposition direction (arrow),, shado owed region (bluee) and in-hole deposited d gold (yyellow). Bottom: SEM images off fabriccated particles.
Asymmetrric crescent strructures obtain ned at β
