Abstract
Although challenging, assembling and orienting non-spherical nanomaterials into two- and three-dimensional (2D and 3D) ordered arrays can facilitate versatile collective properties by virtue of their shape-dependent properties that cannot be realized with their spherical counterparts. Here, we report on the self-assembly of gold nanorods (AuNRs) into 2D films at the vapor/liquid interface facilitated by grafting them with poly(ethylene glycol) (PEG). Using surface sensitive synchrotron grazing incidence small angle X-ray scattering (GISAXS) and specular X-ray reflectivity (XRR), we show that PEG-AuNRs in aqueous suspensions migrate to the vapor/liquid interface in the presence of salt, forming a uniform monolayer with planar-to-surface orientation. Furthermore, the 2D assembled PEG functionalized AuNRs exhibit short range order into rectangular symmetry with side-by-side and tail-to-tail nearest-neighbor packing. The effect of PEG chain length and salt concentration on the 2D assembly are also reported.
Highlights
Assembling nanomaterials into two-or three- dimensional (2D or 3D) ordered structures can lead to novel collective physical and chemical properties that cannot be realized in the original bulk nanomaterials[1,2,3]
By grafting AuNRs with poly(ethylene glycol) (PEG), we have demonstrated that PEG-AuNRs can be assembled into 2D films at the vapor/liquid interface induced by salt
Using X-ray reflectivity (XRR), grazing incidence small angle X-ray scattering (GISAXS), and Scanning transmission electron microscopy (S/TEM) we determine the nature of the films that are formed
Summary
Assembling nanomaterials into two-or three- dimensional (2D or 3D) ordered structures can lead to novel collective physical and chemical properties that cannot be realized in the original bulk nanomaterials[1,2,3]. Gold nanorods (AuNRs) in particular possess essential anisotropic plasmonic characteristics, such that, when assembled, exhibit tunable surface plasmon resonances with local electromagnetic fields that are two to five times more intense than those achieved with spherical AuNPs23,24 These emerging properties can be further controlled by geometric parameters such as the aspect ratio (radius-to-length), size, interparticle distances, and orientation[23,25]. AuNRs grafted with poly(ethylene glycol) (PEG) have been assembled into highly packed two dimensional (2D) arrays at the water/vapor interface by spreading and manipulating them as Langmuir monolayers and subsequently transferring them to quartz or silicon wafers by the Langmuir-Blodgett (LB) technique[30] Another approach took advantage of grafting NRs and nanowires with short chain hydrophobic alkyl chains, and spread and manipulate them at the air-water interface achieving a high degree of orientational order[31,32,33,34]. We propose that the method presented here can be potentially further extended to assemble a variety of complex nanostructures such as nanotriangles, nanocubes and other nanostructure shapes[41,42,43]
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