Hydrophobic gold nanoparticles via silane conjugation: chemically and thermally robust nanoparticles as dopants for nematic liquid crystals

Abstract

We examine for the first time how chemically and thermally stable gold nanoparticles (NPs), prepared by a silane conjugation approach, affect both the thermal and the electro-optical properties of a nematic liquid crystal (LC), when doped at concentrations ranging from 0.25 to 7.5 wt%. We find that the octadecylsilane-conjugated gold NPs stabilize both the enantiotropic nematic and the monotropic smectic-A phases of the LC host with a maximum stabilization of 2 ° C for the nematic and 3.5 ° C for the smectic-A phases for the mixture containing 1 wt% of the silanized particles. The same mixture shows the lowest values for the Fréedericksz transition threshold voltage and the highest value for the dielectric anisotropy. Generally, all NP-containing mixtures, except mixtures with NP concentrations exceeding 5 wt%, reduce the threshold voltage, increase the dielectric anisotropy and reduce both rise and decay time; the latter particularly at temperatures at least 10 ° C below the isotropic–nematic phase transition on cooling.

Thermal optical non-linearity of nematic mesophase enhanced by gold nanoparticles – an experimental and numerical investigation

“…The temperature induced modulation of the effective refractive index [eqns. (10)(11)(12)] causes a phase modulation [eqn. (13)] that diffracts a probe beam with strength measured by the diffraction efficiency parameter η [eqn.…”

“…This nonlinear refractive index is related to the modulation of effective refractive index in the liquid crystal implicitly through eqns. (10)(11)(12)(13)(14)(15)(16). The temperature induced modulation of the effective refractive index [eqns.…”

“…8 Besides enhancing the optical nonlinearity of the host material, doping the LCs with GNPs improves the electrooptical properties of LCs by increasing the dielectric anisotropy, lowering the optical and electric Freedericksz threshold, improving the thermal stability of the LCs and reducing the switching times. 10 In order to gain insight into the mechanism of thermal optical Page 1 of 10 Physical Chemistry Chemical Physics nonlinearity and to identify the key variables of electro-optical systems comprising LC-GNPs composites, a mathematical model that estimates an effective optical nonlinearity coefficient by solving the thermal diffusion equation in the GNP doped LC layer of the system excluding the presence of bounding substrate was developed by Ouskova et al 8 This simple model, 8 however, estimated values of second order nonlinear refractive index that were an order of magnitude larger than those determined through experimental measurements. In an attempt to reduce this discrepancy, Acreman et al 13 have proposed an extended model that includes the attenuation of the incident light beams as it traverses the cell.…”

“…13,14,15 Enhancement of the optical nonlinearity of LCs is achieved through one of the following mechanisms: (I) localized orientational variations induced by (a) doping LCs with dye molecules that undergo photoinduced conformational changes leading to an increased orientational torque on the nematic host when illuminated; 4,5 (b) sandwiching the LCs between inorganic photorefractive crystals that generate surface charge field when illuminated; 6 (c) sandwiching the LCs between photoconducting polymer based substrates that can modulate an applied external field through localized electrical conductance variations when illuminated 7 or (II) localized temperature variations induced by doping the LCs with nanoparticles that can transform radiation at the surface plasmon frequency into heat. 8,9 Due to their surface plasmon resonance which occurs in the visible spectrum, large absorption cross-section, excellent thermal stability 10 and resistance to light induced degradation, gold nanoparticles (GNPs) are better absorbers than dye molecules.…”

“…From the spatial temperature and orientation distribution, the corresponding refractive index grating is obtained using the Extended Cauchy model detailed in eqns. (10)(11)(12). The total phase shift of the incident probe beam of wavenumber k resulting from this refractive index grating is determined using the expression…”