The Interplay of Shape and Crystalline Anisotropies in Plasmonic Semiconductor Nanocrystals

Abstract

Doped semiconductor nanocrystals are an emerging class of materials hosting localized surface plasmon resonance (LSPR) over a wide optical range. Studies so far have focused on tuning LSPR frequency by controlling the dopant and carrier concentrations in diverse semiconductor materials. However, the influence of anisotropic nanocrystal shape and of intrinsic crystal structure on LSPR remain poorly explored. Here, we illustrate how these two factors collaborate to determine LSPR characteristics in hexagonal cesium-doped tungsten oxide nanocrystals. The effect of shape anisotropy is systematically analyzed via synthetic control of nanocrystal aspect ratio (AR), from disks to nanorods. We demonstrate the dominant influence of crystalline anisotropy, which uniquely causes strong LSPR band-splitting into two distinct peaks with comparable intensities. Modeling typically used to rationalize particle shape effects is refined by taking into account the anisotropic dielectric function due to crystalline anisotropy, thus fully accounting for the AR-dependent evolution of multiband LSPR spectra. This new insight into LSPR of semiconductor nanocrystals provides a novel strategy for an exquisite tuning of LSPR line shape.

Impacts of surface depletion on the plasmonic properties of doped semiconductor nanocrystals

“…Related phenomena have been reported in ligand capped or core-shell metal-dielectric systems 20,47,48 . The sensitivity (S) of NC LSPR wavelength (λ LSPR ) to the changes in the surrounding refractive index unit (RIU) is defined as (λ LSPR /RIU) 2 (see section S9 of the SI for details) 49 . To evaluate NC plasmon sensitivity, NCs with various sizes and doping concentrations were dispersed in solvents with refractive indices raging from 1.3-1.55.…”

Plasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives

“…The poor plasmonic coupling of stacks of nanodisks suggests that the LSPR decay length for Cu sulfide nanodisks is anisotropic, due to the inherent anisotropy of the electrical conductivity, which is much greater along one specific crystallographic direction, [237] and of the hole mobility as well. Such peculiarities in some doped semiconductors need to be taken into account in strongly anisotropic nanostructures in order to correctly understand their optical properties, as discussed previously for in CsWO 3 [36] (details in the section 5 Optical modelling of degenerately doped semiconductor nanostructures.). On the other hand, they can also represent an extra handle in designing electromagnetic hot spots or coupled plasmonic architectures.…”

“…[9,34,35] Understanding the structure-properties relationship of plasmonic nanostructures requires a combined study of the local distribution of the dopant atom within the crystal, the role of vacancy formation, the presence of additional scattering centers or crystal anisotropies, and the extension of existing models (such as the Drude model) to simulate the plasmonic response of the nanostructure. [34][35][36] The possibility to tailor the plasmonic response of the degenerate nanostructure in the near to mid IR has been exploited for IR enhancing spectroscopies. [37][38][39][40] Photothermal heat release benefits from the strong light absorption in the NIR at wavelengths within the biological window.…”

“…The Drude parameters effective mass m*, the high frequency dielectric constant ε ∞ and the damping Γ describing the dielectric response of the free carriers in the NIR are known to be strongly dependent on the crystallographic axis. And indeed, only by taking into account the anisotropy of the dielectric function in the different crystallographic directions were the authors able to reproduce the experimental spectra (compareFigure 24aand b) [36]. This finding demonstrates that the conventional description of the LSPR that assumes isotropic dielectric constants is insufficient to describe the plasmonic response of doped metal oxide NCs with anisotropic crystal structures.…”

“…Solid lines represent the overall LSPR response by summing the longitudinal mode (dashed lines) and two transverse modes (dotted lines). The insets give the simulation of the dipolar surface charge distributions calculated at longitudinal and transverse peak wavelengths.Adapted with permission from ref[36] Copyright 2016 American Chemical Society.In another work by Kim et al, the influence of anisotropic NC shape was further correlated to the intrinsic crystal structure and their collaborative effects on the LSPR explored [36]. By synthesizing CsWO 3 nanorods of various aspect ratios, it was concluded that the anisotropy of the underlying crystal strongly influences the location of the LSPR modes.…”