Department of Materials Science and Engineering
Massachusetts Institute of Technology
Adding a new Dimension in Nanoscale Materials: Metal Nanoparticles with Phase Separated Ligand Shells Ligand coated metal nanoparticles are promising nanosize materials for novel electronic and optical devices. Their main strength is in the fact that they combine the advantages of small metals, such as single electron transistor (SET) behavior, together with most of the advantages of the organic ligands that are used to coat them, such as solubility in organic solvents and processability for example via self-assembly methods. Moreover, even complex nanoparticles coated with multiple types of ligands can be synthesized in just one step. Here we present a new family of mixed ligand nanoparticles that shows sub-nanometer patterns (e.g. ridges) on their ligand shell. This unique sub-nano-structuring of their ligand shell provides new properties to the particles. In particular, we focus on silver and gold particles that have ridges composed of hydrophilic valleys and hydrophobic peaks. For the first time we show the ability to control the supramolecular ordering of the ligands on the nanoparticle surfaces. Indeed, by systematically varying the mixture of ligands introduced during nanoparticle synthesis, one can control the resulting surface properties of the nanoparticles. Scanning tunneling microscopy images show ridges 3 Ã… deep and 6 Ã… wide on the ligand shell of nanoparticles. Control of both these parameters is provided by the choice of the ligands and of their molar ratio respectively. We also demonstrate that the nanoparticle ligands interact so as to align the stripes of neighboring nanoparticles over large length scales. The synthetic mechanism that leads to the formation of this supramolecular ordering will be discussed. These particles show unique and unexpected solubility, self-assembly and surface chemistry properties, in fact they show a remarkable resistance to protein nonspecific adsorption.