title: Two-Dimensional Bipyramid Plasmonic Nanoparticle Liquid Crystalline Superstructure with Four Distinct Orientational Packing Orders.
abstract: Anisotropic plasmonic nanoparticles have been successfully used as constituent elements for growing ordered nanoparticle arrays. However, orientational control over their spatial ordering remains challenging. Here, we report on a self-assembled two-dimensional (2D) nanoparticle liquid crystalline superstructure (NLCS) from bipyramid gold nanoparticles (BNPs), which showed four distinct orientational packing orders, corresponding to horizontal alignment (H-NLCS), circular arrangement (C-NLCS), slanted alignment (S-NLCS), and vertical alignment (V-NLCS) of constituent particle building elements. These packing orders are characteristic of the unique shape of BNPs because all four packing modes were observed for particles with various sizes. Nevertheless, only H-NLCS and V-NLCS packing orders were observed for the free-standing ordered array nanosheets formed from a drying-mediated self-assembly at the air/water interface of a sessile droplet. This is due to strong surface tension and the absence of particle-substrate interaction. In addition, we found the collective plasmonic coupling properties mainly depend on the packing type, and characteristic coupling peak locations depend on particle sizes. Interestingly, surface-enhanced Raman scattering (SERS) enhancements were heavily dependent on the orientational packing ordering. In particular, V-NLCS showed the highest Raman enhancement factor, which was about 77-fold greater than the H-NLCS and about 19-fold greater than C-NLCS. The results presented here reveal the nature and significance of orientational ordering in controlling plasmonic coupling and SERS enhancements of ordered plasmonic nanoparticle arrays.
authors.affiliation: The Melbourne Centre for Nanofabrication , 151 Wellington Road, Clayton 3168, Victoria, Australia.
authors.forename: Qianqian
authors.initials: Q
authors.lastname: Shi
authors.affiliation: The Melbourne Centre for Nanofabrication , 151 Wellington Road, Clayton 3168, Victoria, Australia.
authors.forename: Kae Jye
authors.initials: KJ
authors.lastname: Si
authors.forename: Debabrata
authors.initials: D
authors.lastname: Sikdar
authors.affiliation: The Melbourne Centre for Nanofabrication , 151 Wellington Road, Clayton 3168, Victoria, Australia.
authors.forename: Lim Wei
authors.initials: LW
authors.lastname: Yap
authors.forename: Malin
authors.initials: M
authors.lastname: Premaratne
authors.affiliation: The Melbourne Centre for Nanofabrication , 151 Wellington Road, Clayton 3168, Victoria, Australia.
authors.forename: Wenlong
authors.initials: W
authors.lastname: Cheng
keywords: SERS; bipyramid nanoparticles; orientational packing; plasmonic nanoparticle liquid crystalline superstructure; self-assembly