H NMR study of the Assembly of Gold Nanoparticles Dispersed in Nematic Liquid Crystal

JONATHAN MILETTE; LINDA REVEN; R. BRUCE LENNOX; ALEJANDRO D. REY; EZEQUIEL R. SOULÉ

Conferencia; APS March Meeting; 2012

The assembly of 5 nm gold nanoparticles (AuNPs) dispersed in the liquid crystal (LC) 4'-pentyl-4-cyanobiphenyl (5CB) was followed by wideline 2H NMR. Upon cooling to the isotropic-nematic phase transition, the AuNPs form a reversible, long range network with the topology controlled by the particle concentration, cooling rate and film thickness. AuNPs with only mesogenic ligands, 4’-(12-mercaptododecyloxy)biphenyl-4- carbonitrile, CBO(CH2)12SH, and highly miscible AuNPs with mixed ligand shells of hexanethiol and CBO(CH2)12SH were studied. Both 5CB and CBO(CH2)12SH were selectively deuterated to separately follow the orientational order of the host LC and the NP ligands by variable temperature 2H NMR as a function of particle concentration. The 2H NMR spectrum of completely miscible AuNPs-d2 in isotopic 5CB consists of a sharp singlet. Below TNI, the AuNP-d2 spectrum transforms into a doublet superimposed on a broadened singlet, assigned to field aligned and conformationally disordered ligands respectively. The intensities of the two AuNP signals are strongly correlated with those of the host 5CB-d2 that displays a nematic-isotropic biphasic range that increases with particle concentration.  The NMR results are inputted into a mean field thermodynamic model to construct the phase diagrams of the AuNP-LC mixtures.