Modulating the Charge Transfer Plasmon in Bridged Au Core‐Satellite Homometallic Nanostructures

Abstract

Among plasmonic metals, Au is the best candidate for practical applications due to its intrinsic visible LSPR absorption, excellent stability and facile shape control and surface functionalization. [21][22][23][24][25] Generally, the LSPR of AuNPs is basically dependent on their shape, size, and structural aspect ratio (AR). [2,21,26,27] Much effort has been devoted to the controlled synthesis of AuNPs with various shapes. [28][29][30][31][32][33] To date, the synthesis of AuNPs in spheres, [34] cubes, [35] spikes, [36] rods, [37] plates, [38] bipyramids, [39] octahedra, [35] stars, [40] etc., has been extensively studied and is well controlled. As a result of this structural diversity, the LSPR absorptions of the different AuNPs are distributed widely within the visible and near-infrared (NIR) spectral range.Beyond shape control, structural hybridization is another effective way to tune the LSPR absorptions of Au nanostructures, which can lead to different plasmonic modes due to the intraparticle couplings of the elementary structural units. In general, plasmonic coupling can be divided into two types: capacitive and conductive couplings. Capacitive coupling, which is inversely proportional to the interparticle distance, usually occurs between separated AuNP units. When the AuNPs are linked by a conductive bridge, the charge transfer across the bridge induces the occurrence of the charge transfer plasmon (CTP) mode. [41][42][43][44] The resonance energy of the CTP is highly dependent on the conductance of the bridge, which determines the position of the CTP peaks.Recently, Duan, [45] Shi [46] , and Halas [47] reported Au matryoshka structures with tunable broad spectral absorptions in the NIR spectral range by engineering capacitive couplings among multiple Au shells. In Duan's and our recent works, the strong intraparticle capacitive couplings among the Au branches led to full spectrum absorption (black body) properties. [30,45] Effective conductive couplings were first achieved by introducing a conductive Au bridge between two Au nanodisks on a substrate by the lithography method, leading to CTP modes. By varying the conductance of the bridge, i.e., the different widths and lengths of the bridge, the CTP absorptions were readily tuned to within the middle IR spectral region.For the CTP structures synthesized by the lithography method, their applications are limited on substrate. To expand the study and application of CTP to colloidal systems, the solution phase synthesis of uniform nanostructures with tunable CTP is thus highly desired.The localized surface plasmon resonance (LSPR) is one of the important properties for noble metal nanoparticles. Tuning the LSPR on demand thus has attracted tremendous interest. Beyond the size and shape control, manipulating intraparticle coupling is an effective way to tailor their LSPR. The charge transfer plasmon (CTP) is the most important mode of conductive coupling between subunits linked by conductive bridges that are well studied for structures prepared on substrates b...

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