End‐to‐End Self‐Assembly of Semiconductor Nanorods in Water by Using an Amphiphilic Surface Design

Abstract

One-dimensional (1D) self-assemblies of nanocrystals are of interest because of their vectorial and polymer-like dynamic properties. Herein, we report a simple method to prepare elongated assemblies of semiconductor nanorods (NRs) through end-to-end self-assembly. Short-chained water-soluble thiols were employed as surface ligands for CdSe NRs having a wurtzite crystal structure. The site-specific capping of NRs with these ligands rendered the surface of the NRs amphiphilic. The amphiphilic CdSe NRs self-assembled to form elongated wires by end-to-end attachment driven by the hydrophobic effect operating between uncapped NR ends. The end-to-end assembly technique was further applied to CdS NRs and CdSe tetrapods (TPs) with a wurtzite structure.

Amphiphilic self-assembly of semiconductor nanocrystals with heterogeneous compositions

“…CdS nanorods with a wurtzite structure also self-assembled in an end-to-end manner [20]. CdSe and CdS nanorods capped with MESNA were co-assembled in water.…”

“…Amphiphilic modifications on the surface of nanoparticles successfully impart the nanoparticles a self-assembling capability, which were attained by employing amphiphilic ligands [14,15], the simultaneous use of hydrophilic and hydrophobic ligands [16,17], and reducing the density of water-soluble ligands to utilize the uncapped surface as a hydrophobic surface [18][19][20]. Recently, we have reported the surface modification of CdSe nanorods in an amphiphilic pattern, demonstrating a one-dimensional end-to-end self-assembly [20]. The tipsurface of a wurtzite nanorod with weak ligand affinity was selectively uncapped to be a hydrophobic part by using short-chained water-soluble thiolate ligands with weak binding capability.…”

“…-CdSe nanorods synthesized through a well-established protocol [20,25] were subjected to a ligand-exchange procedure with DMAET to give an aqueous solution. The static aging of the obtained aqueous solution overnight under dark afforded the endto-end self-assembly of CdSe nanorods [20]. The change in the chemical states of the components upon self-assembly was investigated by the XPS measurements ( fig.…”

“…The original peak could be assigned to Se 3d in a CdSe composition and the new one corresponds to the oxidized selenium such as SeO 2 (and/or SeO 3 ) [30]. The end-surfaces of nanorods exhibited weaker binding affinity to surface ligands compared to the side-wall surface because of the lower density of Cd atoms on the surface [20,31]. The ligand desorption process is therefore more prominent at the end-surface with the aid of electrostatic repulsion between neighboring ligands.…”

“…The ligand desorption process is therefore more prominent at the end-surface with the aid of electrostatic repulsion between neighboring ligands. The unbound Cd atoms after the ligand desorption were subjected to chemical etching by the co-existing Cl, giving a Solvophilic 100 75 25 Solvophobic 75 10 75 Solvent 25 75 25 bare Se-rich surface [20,32]. The selenium atoms on the end-surface exposed to the environment are most likely oxidized to form a SeO 2,3 layer on the end-surface.…”