Gemini‐Surfactant‐Directed Self‐Assembly of Monodisperse Gold Nanorods into Standing Superlattices

Abstract

Fabrication of ordered nanoparticle assemblies over extended areas and volumes is still a major challenge in nanomaterials research.[1] The current limitations in the production of such ordered assemblies dramatically hinder the application of nanoparticles in fields such as negative refractive index metamaterials or information technologies. In the particular case of metal nanocrystal assemblies, [2] nanoscale organization of readily accessible spherical gold nanoparticles [3,4] has been manipulated to produce a diverse range of topologies [5] with interesting optical and electrical properties. [6,7] However, the use of isotropic nanoparticles strongly limits potential applications that require the formation of lattices with vectorial properties. A recent report [8] demonstrated the formation of 3D gold nanorod (NR) superstructures from liquid-crystalline phases [9][10][11][12] with a limited degree of control over the dimensionality and directionality of the assembly. A major advance is demonstrated herein through the use of a gemini surfactant.[13] Replacement of cetyltrimethylammonium bromide (CTAB) by this unconventional surfactant during nanorod synthesis leads to production of monodisperse NRs that can undergo directional self-assembly into highly ordered 2D and 3D standing superlattices with anisotropic optical properties.The synthesis of highly monodisperse gold NRs is especially appealing because of their strong, polarizationdependent suface-plasmon-based optical properties, [14] which render their assemblies ideal candidates for the preparation of optically anisotropic lattices that allow manipulation of light in the nanoscale.[15] Nowadays, tuning of the longitudinal and transverse localized plasmon resonances of gold NRs by synthetic manipulation is a mature field of research. In particular, the seeded growth method in aqueous solution, [16] based on the reduction by a weak reducing agent of a gold salt on premade small seeds in the presence of CTAB and silver ions provides sufficient flexibility to synthesize nanorods (CTAB-NRs) with diverse sizes and shapes.[17] Besides being a shape-inducing agent, CTAB efficiently prevents aggregation through dynamic adsorption onto the gold NRs surface in a bilayer fashion.[18] This nanoparticle shielding of CTAB in water, together with the intense capillary forces generated at the solvent-air interfaces in aqueous solution and the typical Brownian motion of nanoparticles, brings colloidal stability face-to-face with controlled self-assembly of NRs in water and demands a rational search for new and simple strategies.To date, the construction of assemblies of standing gold NRs has mostly relied on postsynthesis surface functionalization with thiol and silane capping agents [19,20] and subsequent transfer into organic solvents. However, the degree of order in the self-assembly has still been limited to 2D sub-micrometer areas. Among the different capping agents that have been proposed for the preparation of metal nanoparticle arrays in organic solvents, thiol-func...

Directing semiconductor nanorod assembly into 1D or 2D supercrystals by altering the surface charge

“…[10] However, ζ of the pT rods in these solvents is found to exceed that of pT-CdSe rods in our study, which implies 1D assemblies should form. Similarly, in reports of orthogonal assemblies in solution of 15 CdSe/CdS and Au-CdSe, the preassembly of the NRs in the bulk solute is critical to vertically aligned order. [7,21] In both cases the equilibrium of dispersion and aggregation was deliberately modulated; 2D monolayers and not 3D supercrystals were formed from assembling NRs in the bulk.…”

“…[1] In the case of dispersions of mono-disperse, 15 spherical nanocrystals, the inter-particle interactions can be tuned to form highly ordered supercrystals in the solid state when the correct balance between nanoparticle diffusion rate, solvent volatility and seeding rate is struck. [2] Of recent interest are semiconductor nanorods (NRs) where assembly 20 allows individual properties such as single electron charging, linearly polarised emission and absorption to be collectively tuned and upscaled for application in electronics, photonics and solar energy conversion.…”

“…For rods with a zeta potential, ζ <~ 15 mV, the repulsion can be easily overcome by the thermal energy. 15 Therefore the dipole-dipole force would be sufficient to cause rods to align and assemble in solution. When ζ is higher, the Coulomb repulsion maintains the dispersion such that no aggregation occurs.…”

“…The dipole-dipole attraction is the driving force for alignment and is assisted by the affinity of surface ligands to interdigitate to reduce interfacial energy. [15][16] The higher ζ Py-CdSe rods will not assemble in solution as the Coulomb repulsion maintains a barrier to aggregation. 40 Through the combination of fluid flow and diffusion the higher ζ Py-CdSe rods will migrate to and get trapped at the liquid air interface.…”

Quantum dots on vertically aligned gold nanorod monolayer: plasmon enhanced fluorescence

“…A few previous studies have reported the formation of multilayer vertical arrays by decreasing the electrostatic repulsive force via modifying Au nanorods with weak polar ligands instead of cetyltrimethyl-ammonium bromide (CTAB). 20,21 Until recently, there have only been a few reports on the directional self-assembly of CTAB-stabilized Au nanorods into vertically aligned multilayer arrays. 22,23 Recently, our group has shown that it is possible to directly self-assemble CTAB-coated Au nanorods into highly organized vertical monolayer arrays by synergistically controlling the electrostatic repulsive force and the van der Waals attractive force.…”