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. SECTION: Plasmonics, Optical Materials, and Hard Matter C over the chemical interactions between nanoparticles has recently led to the formation of new heterostructures and even quasi-crystals. Photonic interactions have lead to the creation of cold atom lattices. Furthermore, optical forces have become a powerful noncontact approach for studying individual nanoparticles and biomolecules. Yet, using light to simultaneously control and assemble multiple nanoparticles into ordered arrays is still challenging. Strongly localized (or focused) optical fields are usually required for optical manipulation. The regimes for this manipulation can be classified into far-field (d ≫ λ), intermediate-field (d ≈ λ), and near-field (d ≪ λ). A widely used far-field technique is the traditional optical tweezers with a tightly focused Gaussian beam. Integration of spatial light modulators with optical tweezers has enabled the production of structured light fields for advanced manipulation. Still, farfield light shaping is limited by diffraction. Near-field techniques based on user-imposed light shaping overcome the diffraction limit yet usually require ex situ fabrication of designed trapping devices. Subwavelength manipulation by evanescent light fields has recently been demonstrated, and plasmonic tweezers, a near-field technique, has attracted much attention. In the latter, the plasmon resonance of noble-metal structures with nanometer-scale features can strongly enhance the EM fields near the metal surface and thus create localized EM fields. These far-field and near-field techniques are being widely applied in nanoscience and biology. Exciting opportunities will emerge from shaping EM fields in the intermediate-scale region, including strong coupling of photon sources to nanophotonic systems and assembling hybrid photonic architectures. One approach involves optical trapping by light shaping and focusing in fabricated photonic lattices. The light−matter interaction termed “optical binding” is another approach to shape light in the intermediate-scale regime and has the potential to create dynamically reconfigurable structures. Optical binding of micrometer or submicrometer dielectric (e.g., polymer) beads occurs when the scattered light from these beads interferes with the incident field, resulting in field gradients and thus spatial arrangement of the beads. The process of array formation by the optical binding interaction in the presence of a continuous applied EM field is a type of “dynamic selfassembly”. Metal particles are stronger light scatterers (per volume) than polymer particles, allowing strong optical binding of Au nanoparticle pairs, and optical binding interaction of Ag nanoparticles as small as 40 nm in diameter has recently been demonstrated. Here we report that chains of large Ag nanoparticles (∼140 nm diameter) formed by optical binding interactions can serve as an in situ beam shaper to create localized optical fields and that these nanoparticles and the structured fields they produce can serve as backbones for hierarchical assembly of hybrid nanostructures. The chains assemble in a simple (initially unstructured) optical field and behave as rigid bodies. However, Received: May 14, 2013 Accepted: July 22, 2013 Letter