Shape-dependent dispersion and alignment of nonaggregating plasmonic gold nanoparticles in lyotropic and thermotropic liquid crystals

We use both lyotropic liquid crystals composed of prolate micelles and thermotropic liquid crystals made of rod-like molecules to uniformly disperse and unidirectionally align relatively large gold nanorods and other complex-shaped nanoparticles at high concentrations. We show that some of these ensuing self-assembled orientationally ordered soft matter systems exhibit polarization-dependent plasmonic properties with strongly pronounced molar extinction exceeding that previously achieved in self-assembled composites. The long-range unidirectional alignment of gold nanorods is mediated mainly by anisotropic surface anchoring interactions at the surfaces of gold nanoparticles. Polarization-sensitive absorption, scattering, and extinction are used to characterize orientations of nanorods and other nanoparticles. The experimentally measured unique optical properties of these composites, which stem from the collective plasmonic effect of the gold nanorods with long-range order in a liquid crystal matrix, are reproduced in computer simulations. A simple phenomenological model based on anisotropic surface interaction explains the alignment of gold nanorods dispersed in liquid crystals and the physical underpinnings behind our observations.

Figure 1

Schematic of (a) rod-like micelle of a ternary lyotropic LC and (b) chemical structure of the 5CB molecule with LC director N describing average orientation of their long axes. (c) Alignment of GNRs on average along the local director N describing average orientation of the rod-like micelles or 5CB molecules. (d) Oriented self-assembly of GNRs in a nematic LC, which exhibits long-range unidirectional ordering of rod-like nanoparticles along the far-field director ${\mathbf{N}}_{\mathbf{0}}$.

Figure 2

(a) SEM image of gold nanocubes. (b) Optical microscopy image of a dispersion of gold naoncubes in 5CB. The inset shows a vial with the gold nanocubes-5CB dispersion. (c) Polarization-independent extinction spectrum of gold nanocubes in 5CB. (d) Transmission electron microscopy (TEM) image of gold “nanostar.” (e) Optical microscopy image of dispersion of gold “nanostars” in 5CB. The inset shows a vial with the gold “nanostars”-5CB dispersion. (f) Polarization-independent extinction spectrum of gold “nanostars” in 5CB. (g) TEM image of GNRs. The inset shows a vial with the GNRs-5CB dispersion. Transmission-mode optical images of GNRs-5CB sample with (h) $\mathbf{P}\left|\right|{\mathbf{N}}_{\mathbf{0}}$ and (i) $\mathbf{P}\perp {\mathbf{N}}_{\mathbf{0}}$. (j) Polarization-dependent extinction spectra of GNRs-5CB sample.

Figure 3

(a) POM image of an unaligned concentrated GNR-5CB composite sandwiched between two parallel glass plates. (b, c) Transmission-mode optical images of a GNRs-5CB composite under polarized light. (d) POM image of GNR-5CB droplet suspended within a PDMS matrix. (e, f) Transmission-mode optical images of GNRs-5CB droplets obtained under polarized-light illumination showing polarization-dependent colors.

Figure 4

(a) SEM image of mPEG-functionalized GNRs. The inset of (a) shows the distribution of aspect ratios of used GNRs. (b, c) Transmission-mode optical microscopy micrographs of GNRs in aligned $N_C$ with the polarization of incident light (b) parallel and (c) perpendicular to the far-field director ${\mathbf{N}}_{\mathbf{0}}$. (d) POM image showing half-integer disclination defects in unaligned samples of the dispersions of GNRs in $N_C$. (e, f) Polarized dark-field images of the same area as shown in (d) obtained for two orthogonal polarizations denoted by white double arrows. (g) A schematic of the director field N(r) corresponding to (d–e).

Figure 5

(a) A POM micrograph of a “yin-yang”-shaped twist domain within a uniformly aligned cell with GNRs-5CB obtained by patterned illumination of the photoalignment layer. (b, c) The corresponding brightfield micrographs for polarizations of incident light (b) perpendicular and (c) parallel to N. (d) POM and (e, f) brightfield micrographs of a hexagonal pattern created by patterned illumination. (g) POM and (h, i) brightfield optical micrographs of a twisted “GNR” pattern created by spatially varying illumination. Inset in (a) depicts the π/2 twist of N and GNRs across the cell thickness within twist domains.

Figure 6

(a) A schematic of the setup used to measure the polarization-dependent transmitted spectra; the arrow indicates orientation of ${\mathbf{N}}_{\mathbf{0}}$ set by the shear force. (b) Experimental extinction spectra of GNRs in $N_C$ at 4.7 × 10${}^{-8}$ M. (c) Simulated extinction coefficients for the same concentration of GNRs in $N_C$ as (b) for two orthogonal polarizations of incident light and for $S_{\mathrm{GNR}}$ = 1. (d) Computer-simulated spectral dependencies of extinction, absorption, and scattering coefficients for two orthogonal polarizations of incident light for $S_{\mathrm{GNR}}$ = 0.71. (e) Calculated imaginary parts of refractive indices and its anisotropy. (f) Calculated spectral dependence of effective-medium refractive indices $n_{\parallel }$ and $n_{\perp }$.

Figure 7

(a) Order parameter $S_{\mathrm{GNR}}$ vs. $\delta =\pi LRW/\left(2k_{B}T\right)$. (b) Distribution of GNR orientations with respect to N at $S_{\mathrm{GNR}}$ = 0.71. (c) Order parameter $S_{\mathrm{GNR}}$ vs. effective radius $r_{\mathrm{eff}}$ of the nanoparticle. (d) Extinction coefficients at longitudinal SPR wavelength vs. $r_{\mathrm{eff}}$.