Size-Dependent Assemblies of Nanoparticle Mixtures in Thin Films

Abstract

Hybrid nanoparticle (NP) arrays based on particles of different sizes and chemistries are highly desirable to obtain tunable properties for nanodevices. A simple approach to control the spatial organization of NP mixtures within supramolecular frameworks based on NP size has been developed. By varying the ratio of the NP size to the periodicity of the block-copolymer-based supramolecule, a range of hybrid NP assemblies in thin films, ranging from 1D chains to 2D lattices and 3D arrays and networks of NPs, can be readily generated.

End-to-End Alignment of Nanorods in Thin Films

“…For thin films, similar to what has been shown previously in nanocomposites containing spherical nanoparticles, 40,41 the low surface tension of the NR and the entropy associated with polymer chain deformation upon particle incorporation must be balanced to prevent surface aggregation of NRs. For cylindrical supramolecular nanocomposites, nanoparticles larger than the interstitial sites between BCP cylinders are expelled to the film surface.…”

“…In thin films, the change in free energy upon incorporation of NRs can be expressed as ΔG = (ΔH surface + ΔH ligand-polymer ) − T(ΔS con + ΔS trans + ΔS geometric ) + (ΔG p−p − TΔS orient ), where the first two groups of terms are similar to the free energy contributions considered for nanocomposites containing spherical nanoparticles. 40 Briefly, the first group of terms represents contributions from the surface tensions, and the nanoparticle ligand−polymer interaction. The second group accounts for the entropic changes upon nanoparticle incorporation originating from the conformational change of the polymer chain, the translational entropy of the nanoparticles, and the geometric chain packing of polymer chain, respectively.…”

“…For cylindrical supramolecular nanocomposites, nanoparticles larger than the interstitial sites between BCP cylinders are expelled to the film surface. 40 The long axis of NRs are generally several times the critical particle diameter and could be entropically excluded from the film interior to form aggregates on the surface. This also emphasizes the importance of the orientation dependence of ΔS con , that is, NRs must assemble end-to-end in a cylindrical nanocomposite for it to be energetically favorable to sequester NRs in the interior of film.…”

“…40,48,49 In thin films, it forms a cylindrical morphology where parallel PS cylinders are embedded in the P4VP(PDP) 1.7 matrix. The PS cylinders are packed in a distorted hexagonal lattice and the NPs, passivated with alkyl ligands, are selectively sequestered in the interstitial region between PS cylinders.…”

“…40 CdS NRs (60 nm in length) or CdSe/CdS seeded NRs (20 or 40 nm in length) capped with alkylphosphonic acids were used. The NR lengths were selected to tailor the degree of confinement imposed by the supramolecular structure on the NR arrangement.…”