Nonvolatile memory effects in an orthoconic smectic liquid crystal mixture doped with polymer-capped gold nanoparticles

Abstract

Promising applications of liquid crystal nanocomposites have driven extensive efforts to achieve non-volatile memory effects for the realization of electronic storage devices. In this context, non-volatile memory effects in an orthoconic smectic liquid crystal mixture, with and without polymer capped gold nanoparticles, were investigated. The dielectric spectroscopy technique was performed by applying a d.c. bias during the measurement or a d.c. potential before the start of the measurement in order to obtain pre-conditioning of the sample. Both techniques showed the presence of non-volatile memory effects in the pure orthoconic smectic liquid crystal mixture similar to the doped one. The results demonstrate that the addition of gold nanoparticles enhances the memory effect making it permanent. Our experimental evidence underlines the importance of the structure of the host liquid crystal and clearly suggests that the prolonged time memory effect, observed in the doped liquid crystal, is due to the electric field inducing charge transfer from the liquid crystal molecules to the gold nanoparticles, thanks to the polymer-capping which acts as an ionic charge trapper. Such an ionic trap effect is also responsible for strong reduction of total conductivity of the doped system.

Comparisons on Different Liquid Crystals Doped with Various Sized Gold Nanoparticles for Nonvolatile Memory Effects

“…In the past years the bistability or memory effect has been demonstrated in Deformed Helix Ferroelectric Liquid Crystals (DHFLCs) doped with Gold Nano-Particles (GNPs), in which the long nonvolatile memory effect has been observed probably as a result of the electric field induced charge transfer effect from liquid crystal molecules to the GNPs and the stabilization of the helix deformation process [3]. As reported in our previous work, furthermore, the capture of ions/charges inside the polymeric coatings on the GNP surfaces increased the resistivity of the GNPdoped liquid crystal composites [4]. Memory effects have been observed in both pure orthoconic smectic liquid crystal mixture and those doped with GNPs, indicative of the importance of the liquid crystal host.…”

“…For this reason they are historically used in display technology and other electrooptic devices [1]. More recently liquid crystals by their very nature are suitable for matrix-guided synthesis and self-assembly of nanoscale materials [2][3][4]. Since liquid crystals are anisotropic materials they provide excellent supports for selfassembly of nano-particles into larger ordered superstructures.…”

“…Hydrogen tetrachloroaurate(III) tetrahydrate (haucl 4 •4H 2 O 99.999% trace metals basis) product number 254169, Trisodium citrate dihydrate (Na 3 C 6 H 5 O 7 ≥ 99% FG product number W3 26 were purchased from Aldrich and silver nitrate (agno 3 99.999 %) was purchased from Alfa Aesar. All these chemicals were used as received.…”

“…All glassware was cleaned with aqua regia (3:1 v/v hcl (37%): HNO 3 (65%) solutions) and then rinsed thoroughly with Milli-Q water before use. The aqueous solution of haucl 4 (1 wt %) was prepared and stored at ca. 4°C before use.…”

“…The stabilization of the memory effect is probably due to the polymeric coatings of the GNPs acting as traps for ionic charges, from which a large reduction of complessive conductivity was derived [4].…”