Tunable and Directional Plasmonic Coupling within Semiconductor Nanodisk Assemblies

Abstract

Semiconductor nanocrystals are key materials for achieving localized surface plasmon resonance (LSPR) excitation in the extended spectral ranges beyond visible light, which are critical wavelengths for chemical sensing, infrared detection, and telecommunications. Unlike metal nanoparticles which are already widely exploited in plasmonics, little is known about the near-field behavior of semiconductor nanocrystals. Near-field interactions are expected to vary greatly with nanocrystal carrier density and mobility, in addition to properties such as nanocrystal size, shape, and composition. Here we demonstrate near-field coupling between anisotropic disk-shaped nanocrystals composed of Cu2-xS, a degenerately doped semiconductor whose electronic properties can be modulated by Cu content. Assembling colloidal nanocrystals into mono- and multilayer films generates dipole-dipole LSPR coupling between neighboring nanodisks. We investigate nanodisks of varying crystal phases (Cu1.96S, Cu7.2S4, and CuS) and find that nanodisk orientation produces a dramatic change in the magnitude and polarization direction of the localized field generated by LSPR excitation. This study demonstrates the potential of semiconductor nanocrystals for the realization of low-cost, active, and tunable building blocks for infrared plasmonics and for the investigation of light-matter interactions at the nanoscale.

Chemical Control of Plasmons in Metal Chalcogenide and Metal Oxide Nanostructures

“…When made hydrophobic and assembled into stacks, the out-of-plane LSPR appears as the dominant mode, while a side-by-side orientation red-shifts the in-plane LSPR. This strong particle orientation-dependence is also observed in Cu 1.96 S and Cu 7.2 S 4 [ 39 ] with peak shifts and relative intensities changing when assembled after surface treatment. The ability to tune the LSPR via self-assembly has the potential to create sophisticated metamaterials or to be directly integrated into optical devices.…”

“…[ 18 ] LSPR coupling occurs due to Coulomb interactions between nearest-neighbor nanodisks, and the coupling strength is highly dependent upon disk orientation and carrier density. [ 39 ] Though the particle shape does not change during self-assembly, orienting the particles so one face is more prevalent versus the other does infl uence the plasmon.…”

“…[ 39 ] Through dip-coating, the particles self-assemble and orient themselves either into stacks or in a side-by-side fashion depending upon ligand selection (Figure 2 C). Mentioned above, as-synthesized CuS hexagonal disks exhibit only one obvious absorbance peak corresponding to the in-plane LSPR mode.…”

Strategies for enhancing the sensitivity of plasmonic nanosensors

“…As the refractive indices of organic molecules are higher than those of typical buffer solutions, the binding of organic molecules to nanoparticles increases the local refractive index, leading to a spectral redshift of the peak wavelength in both the extinction and scattering spectra [17][18][19]. Unlike the propagating SPR derived from a thin noble metal film which exhibits a long sensing distance (in microns) [20], the effective sensing zone of LSPR nanosensors is highly localized (within tens of nanometres) because the LSPR decays exponentially with the distance from the nanoparticle surface [21][22][23][24][25]. Hence, the spectral response of LSPR is sensitive only to regions within the nanoscale environment surrounding metallic nanoparticles.…”

Synthesis and characterization of hexagonal faceted copper sulfide (Cu1.8S) nanodisks

“…Significant efforts have been devoted to the phase controlled synthesis of Cu 2 À x S NPs by varying the precursor concentration ratios [12], different precursor materials [13], varying the reaction temperatures [14] and using different combinations of stabilizing ligands [15] as well as by adopting more than one synthesis methods [16]. Lot of methods has been developed for the structure controlled synthesis of copper sulfide with desired morphologies.…”