Curvature sculptured growth of plasmonic nanostructures by supramolecular recognition

Sui Yang, Yuan Wang, and Xiang Zhang

Phys. Rev. Materials 3, 116002 – Published 6 November 2019

Abstract

Nanoscale curvature is an important and powerful tool in understanding and tailoring chemical/surface functionalities of nanostructures that dictate a host of important applications from biochemical recognitions, catalysis to spectroscopy. However, it is a critical challenge in materials chemistry to rationally shape the local nanoscale curvatures of colloidal nanoparticles during the growth owing to the constraints of their flat facets. Here we demonstrate a synthetic mechanism that could cooperatively mediate local nanoparticle surface curvature patchiness and shape symmetries during one-step colloidal growth. The idea is to tailor host-guest supramolecular recognition using fluorocarbon and hydrocarbon molecules that regulate interfacial energy during the nanoparticle growth. Such delicate regulation enables a degree of freedom in control over the local nanoparticle curvatures during the growth, resulting in intriguing plasmonic properties. More interestingly, a morphological shape transformation was induced by such curvature changes from anisotropic nanorods to isotropic nanospheres. This unique approach of the spontaneous curvature/structural transformation of plasmonic nanoparticles exploits the mutual interplay between competing supramolecules and colloidal growth. It may ultimately allow for accurate controlling nanoscale objects with varied degree of complexity that could open the door to a myriad of surface chemical, optical, and biomedical applications.

Received 6 September 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.3.116002

Physics Subject Headings (PhySH)

Crystal growth Crystal structure Inorganic chemistry Nanocrystals Nanophotonics Plasmonics Solid-state chemistry Supramolecular phase separation

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & OpticalPolymers & Soft MatterInterdisciplinary Physics

Authors & Affiliations

Sui Yang¹, Yuan Wang¹, and Xiang Zhang^1,2,*

¹Nano-scale Science and Engineering Center (NSEC), 3112 Etcheverry Hall, University of California, Berkeley, California 94720, USA
²Faculties of Sciences and Engineering, University of Hong Kong, Hong Kong, China

^*xiang@berkeley.edu

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Issue

Vol. 3, Iss. 11 — November 2019

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Images

Figure 1
Schematic illustration of surface curvature sculpturing growth of colloidal plasmonic gold nanoparticles. The uniqueness of the approach is to introduce fluorocarbon molecule that possesses both hydrophobic and lipophobic properties. The compromised host (hydrocarbon $CT A^{+})$ -guest (fluorocarbon $PF O^{-})$ recognitions are through (1) electrostatic attraction, (2) hydrophobic insertion, and (3) lipophobic repulsion. Such synergistic supramolecular interactions thus generate microphased inhomogeneity of coupled micelle, structural directing template and modulates its interfacial energy. Due to the direct proportional relationship between surface energy and surface curvature, these unique compromised interactions simultaneously regulate surface curvature patchiness and growth of gold nanoparticles. More specifically, PFOA/CTAB micelle regulates the local packing parameter and charge density that binds to gold precursor ion, which governs the growth of curved gold nanoparticle and eventually leads to morphological transformation.
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Figure 2
Surface curvature sculptured growth of colloidal gold nanorods and related properties. (a)–(c) TEM images of gold nanorods at the equivalent aspect ratio of 2.9 but with distinct surface curvatures at synthetic parameters (a) $R = 0$ ; (b) $R = 0.11$ , and (c) $R = 0.125$ . Scale bar is 50 nm. (d) The quantitative analysis of local surface curvature $(k)$ of synthesized gold nanorods represented by different curvatures as values indicated in colormap as unit of $n m^{- 1}$ . (e) Plasmonic responses of curvature sculptured synthesized nanorods (extinction spectra normalized by the geometry cross section). Different colored curves indicate different $R$ values. (f) Comparison between experimental and simulated data at resonance maximum for both longitudinal and transverse plasmon. Appreciable blueshift of longitudinal resonances was shown due to the surface curvature change, while transverse plasmon peak almost remains as for the quasistatic limit. (g) Calculated plasmonic energy confinement with normalized electric-field enhancement $(E / E_{0})$ at resonant maximum for gold nanorods (distinct curvatures but same aspect ratio). Color bar indicates the relative electric-field enhancement values. The highly curved surface of nanorod leads to much stronger confinement of plasmon energy at designated high- $k$ location.
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Figure 3
Synergistic surface curvature and shape anisotropicity control. (a) TEM images of gold nanostructure transformation by systematically increasing $PF O^{-}$ ( $R$ ) in the synthesis. As increasing $R$ up to 0.055, the head tips of AuNRs become rounded and the shape elongated. However, the anisotropy of gold changes inversely with shortened aspect ratio and even more rounded edges as further increasing $R$ and finally leading to nanospheres $(R = 0.35)$ . (b) The mean curvature $κ_{m}$ (averaged along the particle's surface) of nanoparticles as a function of aspect ratios with respect to controlling parameter $R$ . The error bars are estimated by the standard deviation analysis from different batches. As increasing $κ_{m}$ , the nanoparticle experiences an anisotropic structural transformation. The dotted line indicates that surface curvatures of nanorods can be selectively tuned at a chosen aspect ratio. (c) The projected local surface curvature and AR profiles of each representative nanoparticle at different $R$ . The color bar indicates quantitative curvatures $(n m^{- 1})$ exhibiting both positive and negative values.
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Figure 4
FTIR reveals synergistic supramolecular coupling. Different colors indicate samples of various $R$ value. The dotted line is the spectrum measured from pure PFOA molecules. Vibration modes of $- C = O$ and $- C H_{2}$ are marked in the graph. Free-binding carbonyl group $(C = O)$ in pure PFOA experiences an appreciable redshift in wave numbers from 1765 to $1686 c m^{- 1}$ as raising $R$ , indicating strong recognitions and interactions between templating molecules of CTAB and PFOA.
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