3D Ordered gold strings by coating nanoparticles with mesogens

Metallic nanoparticles have attracted great interest for their potential electronic [1], optical [2], photonic [3] and medical [4] applications. These applications normally require the fabrication of bulk 3-D arrays of metal nanoparticles, or “meta-atoms”, with prescribed geometrical layouts. So far most of these arrays could only be produced with exceptionally uniform nanoparticles, and the range of array types was limited by the particle shape; e.g. cubic arrays from spherical particles. Recently Mehl and Cseh of Hull University attempted to change this situation by grafting liquid crystal molecules side-on to the nanoparticles [5]. In a recent collaboration between research groups from Sheffield, Hull and Cardiff Universities, self-assembled 3-D and 2-D arrays of these liquid crystal-coated gold nanoparticles have now been created.

Two different mesogen-covered gold nanoparticle systems were studied, as shown in Figure 1. Small nanoparticles (ca. 2 nm in diameter) were used. Such materials preserve the ability of the mesogens to form a liquid crystalline phase. The alignment of the mesogens breaks the cubic symmetry that the nanoparticles would normally adopt, and introduce anisotropy to the periodic lattices that form.

Figure 1. Schematic structure of the gold nanoparticles coated with mesogens and alkylthiols.

Grazing incidence small-angle X-ray scattering (GISAXS) experiments, carried out at beamline BM28 (XMaS), were extremely useful in determining the structures of these anisotropic arrays. GISAXS patterns from well-oriented films of systems 1 and 2, prepared on silicon substrate, are shown in Figure 2. On the basis of these patterns, 1 was found to form a 3-D rhombohedral lattice with space group R3m, while 2 was shown to organize into a 2-D hexagonal columnar lattice. The schematic models of the two phases are shown in Figure 3.

GISAXS experiments on oriented films of 2 provided additional information about the columnar phases. Figure 2b shows that the nanoparticle columns stand perpendicular to the substrate surface. A weak diffuse meridional scattering peak indicates that the average distance between neighbouring gold nanoparticles within a column is just 2.0 nm.

Figure 2. GISAXS diffraction patterns of a) system 1 and b) system 2.

The mesophase structure of both 1 and 2 can be described as consisting of strings of nanoparticles surrounded by a sheath of axially aligned mesogens. A significant feature illustrated by the present results is the remarkable scope for varying the interparticle distance in the column direction simply by varying the length of the co-ligand thioalkyl chains. Thus, while in 1 the interparticle gap is 1.7 nm, in 2 the gap virtually disappears and the nanoparticles touch in the direction of the string.

This work demonstrates that highly ordered superlattices of metal nanoparticles other than those expected from mere packing of spheres can be created by coating the particles with liquid crystals. The results establish the first rules on which to base future design of more complex lattices with a view to build self-assembled metamaterials.

Figure 3. Schematic models of a) the rhombohedral phase in system 1; b) the hexagonal columnar phase in system 2; yellow: gold nanoparticles, green: mesogens.

References
[1] S.W. Boettcher, N.C. Strandwitz, M. Schierhorn, N. Lock, M.C. Lonergan, G.D. Stucky, Nat. Mater. 6, 592-596 (2007).
[2] S. Eustis, M.A. El-Sayed, Chem. Soc. Rev. 35, 209-217 (2006).
[3] P.N. Prasad, Nanophotonics, John Wiley & Sons, Inc. Hoboken, New Jersey (2004).
[4] J. Kim, Y. Piao, T. Hyeon, Chem. Soc. Rev. 38, 372-390 (2009).
[5] L. Cseh, G.H. Mehl, J. Mater. Chem. 17, 311-315 (2007).

Principal publication and authors
X. Zeng (a), F. Liu (a), A.G. Fowler (a), G. Ungar (a), L. Cseh (b,d), G.H. Mehl (b), J.E. Macdonald (c), 3D Ordered Gold Strings by Coating Nanoparticles with Mesogens, Advanced Materials 21, 1746-1750 (2009).
(a) Department of Engineering Materials, University of Sheffield (UK)
(b) Department of Chemistry, University of Hull (UK)
(c) School of Physics and Astronomy, Cardiff University (UK)
(d) Present address: Romanian Academy, Institute for Chemistry, Timisoara (Romania)

Top image: Gold string structures consisting of gold nanoparticles, mesogens and alkylthiol co-ligands