Hierarchical Photonic Synthesis of Hybrid Nanoparticle Assemblies

Optical “nano-manipulation” to control small objects with nanoscale precision requires strongly localized optical fields that are usually based on user-imposed shaping of the incident optical beam. Here we report an in situ approach to reshape and enhance electromagnetic (EM) fields using scattering and interference that is concomitant with “dynamic self-assembly” of nanoparticle arrays using simple (unstructured) applied EM fields. We show that Ag nanoparticles (∼140 nm diameter) illuminated by coherent light can form linear chains with nanometer precision via strong optical binding interactions. The chains, in turn, create highly shaped EM fields via coherent scattering from the particles, allowing less polarizable particles to be “co-trapped” in both intermediate-scale and near-field regimes. These less polarizable particles include quantum dots (CdSe/ZnS or CdSe/CdZnS core/shell nanocrystals; both are smaller than 10 nm, while the latter are further coated by ∼30 nm thick silica shells) and small Ag nanoparticles (60 nm diameter). This hierarchical optical-field-induced assembly is a starting point for photonically building artificial nanomaterials.