Abstract
We demonstrate the large-scale organisation of anisotropic nanoparticles into linear assemblies displaying optical anisotropy on macroscopic areas. Monodisperse gold nanorods with a hydrophilic protein shell are arranged by dip-coating on wrinkled surfaces and subsequently transferred to indium tin oxide (ITO) substrates by capillary transfer printing. We elucidate how tuning the wrinkle amplitude enables us to precisely adjust the assembly morphology and fabricate single, double and triple nanorod lines. For the single lines, we quantify the order parameter of the assemblies as well as interparticle distances from scanning electron microscopy (SEM) images. We find an order parameter of 0.97 and a mean interparticle gap size of 7 nm. This combination of close to perfect uni-axial alignment and close-packing gives rise to pronounced macroscopic anisotropic optical properties due to strong plasmonic coupling. We characterise the optical response of the assemblies on ITO-coated glass via UV/vis/NIR spectroscopy and determine an optical order parameter of 0.91. The assemblies are thus plasmonic metamaterials, as their periodicity and building block sizes are well below the optical wavelength. The presented approach does not rely on lithographic patterning and provides access to functional materials, which could have applications in subwavelength waveguiding, photovoltaics, and for large-area metamaterial fabrication.
Highlights
Bottom-up assembly of noble metal nanoparticles into well-de ned structures has been a focus of scienti c interest for the past decade and is expected to foster innovation in the elds of nanoelectronics,[1,2,3] non-linear optics,[4,5,6] photovoltaics,[7,8] and metamaterials.[4,9,10,11,12] In particular, assemblies from gold and silver aPhysical Chemistry II, Universitatsstraße 30, 95440, Bayreuth, Germany
Monodisperse gold nanorods of high aspect ratio were synthesised using a method published by Vigderman and Zubarev in 2013.43 This protocol employs seeded growth of single-crystalline gold seeds in the presence of cetyltrimethylammonium bromide (CTAB) as a directing agent and hydroquinone as a rather mild reducing agent.[43]
Using a simple empiric linear equation (AR 1⁄4 (L-localised surface plasmon resonance (LSPR) À 420 nm)/95 nm), the aspect ratio (AR) was calculated to be 4.4 Æ 1.0.43,44 This result is in good agreement with the value determined from the determined dimensions (TEM) (3.9 Æ 0.6)
Summary
Bottom-up assembly of noble metal nanoparticles into well-de ned structures has been a focus of scienti c interest for the past decade and is expected to foster innovation in the elds of nanoelectronics,[1,2,3] non-linear optics,[4,5,6] photovoltaics,[7,8] and metamaterials.[4,9,10,11,12] In particular, assemblies from gold and silver aPhysical Chemistry II, Universitatsstraße 30, 95440, Bayreuth, Germany. Puri ed solutions of BSA-functionalised gold nanorods spread out completely on hydrophilised substrates, behaving similar to neat water, resulting in a contact angle close to zero.
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