Magnetic Layer-by-Layer Assembly: From Linear Plasmonic Polymers to Oligomers.

One-dimensional (1D) nanostructures with controllable aspect ratios are essential for a wide range of applications. An approach for magnetic superparticle (SP) assembly over large areas (55 mm x 25 mm) is introduced via co-assistance of electrostatic and magnetic fields, so-called magnetic layer-by-layer (MLBL) assembly, on an arbitrary hydrophilic substrate within minutes. The SP structures (diameter (d.) = 120-350 nm) of Fe3O4 or Ag@Fe3O4 composites composed of hundreds of magnetite nanocrystals (d. 10-20 nm) are used as colloidal monomers to fabricate arrays of high-aspect-ratio (up to 102) linear nanochains, viz. colloidal polymers, where thermal effects induced by solvent evaporation were minimized. The arrays of colloidal polymers exhibit strong optical polarization effects owing to their geometrical anisotropy, which can be used as a simple optical filter. Furthermore, by using the binary colloidal mixture of different magnetic colloids including different-sized Fe3O4 and magnetoplasmonic Ag@Fe3O4, low-aspect-ratio (2~15) colloidal chains, viz. magnetic/plasmonic oligomers, with tunable lengths were fabricated, affording a facile but effective approach to modulate the optical properties of the chains. The scalable fabrication of well-aligned, linear colloidal polymers and oligomers opens up appealing opportunities for the development of sensors, sub-wavelength waveguides, optical tweezers, and enhanced solar harvesting devices.