STM Investigations of Self-assembled Janus Nanoparticles

M. Yu, J. Reguera, I.C. Pons-Sieperman, Q. Ong, S. Schrettl, S.C. Glotzer, F. Stellacci
Harbin Institute of Technology, CN

Keywords: Janus nanoparticle, self-assembly, scanning tunneling microscopy, phase separation, mixed ligands

Summary:

Janus particles, named by De Gennes after the double-faced Roman God, are defined as particles with two distinct sides. The asymmetry of Janus particles makes them unique in physical and chemical properties and great potential in a wide range of applications ranging from water-based paints, to heterogeneous nanocatalysis, electronic displays, nanomotors, drug or gene delivery, and so on. Various strategies have been used to produce Janus particles but with complex procedures and low yields. And small (<10 nm) nanoparticles (NPs) are scarcely reported. In the present work, monodisperse small Janus NPs synthesized by direct one-step based on the self-assembly of mixed ligands have been investigated systematically using scanning tunneling microscopy under both ultrahigh vacuum and ambient conditions. By tuning the length difference and composition ratio of binary mixtures of suitable ligand molecules of different lengths, the surface morphology of the resultant NP is varied in a controlled manner. Molecular dynamics simulations were employed to get better understanding of the phase-separation on the Janus NPs. Consistent with STM results, it is revealed that the enthalpic gains due to crystallization of the long ligands overcome the configurational and conformational entropy gains, which lead to the formation of Janus morphology.