Electrically conductive, solventless nanoparticle liquids, consisting of gold nanoparticles chemically functionalized with large organic molecular groups, have been investigated for potential utility in electronic and electrical applications. These and other solventless nanoparticle liquids, including electrically nonconductive ones, have been topics of recent research directed toward understanding and exploiting their unusual properties. The most obvious unusual property is that a collection of nanoparticles of this type can flow in a liquid-like fashion, notwithstanding the absence of free solvent molecules. By modifying the attractive and repulsive forces between the nanoparticles through modifications of the surface chemistry of the organic ligands, the properties of the resulting nanoparticle liquids can be tailored for specific applications.

A Nanoparticle-Liquid Specimen was made by exchanging 30-nm citrate-capped gold nanoparticles MPS, then treating the MPS-functionalized nanoparticles with methyltrialkyl(C8-C10)ammonium chloride. In this transmission electron micrograph, the gold nanoparticles appear dark and a corona of methyltrialkyl(C8-C10)ammonium chloride linking the nanoparticles is shown at low contrast against the background. The scale bars represent a length of 50 nm.
In the investigation, citrate-capped and dodecanethiol-capped gold nanoparticles were synthesized, then solutions containing the capped nanoparticles were exposed to a variety of alkythiol solutions and the resulting chemical reactions allowed to proceed to effect tailored functionalization of the nanoparticles with 3-mercapto-1-propanesulfonic acid (MPS). Next, nanoparticle liquids were prepared by treating the variously-MPS-functionalized gold nanoparticles with a series of cationic surfactants. In each case, the cationic species formed an ionic liquid corona around the anionic MPS-functionalized nanoparticles (see figure).

The nanoparticle liquids were chemically characterized by x-ray photoelectron spectroscopy, transmission electron microscopy, zeta-potential measurements, inductively-coupled-plasma atomic-emission spectroscopy, and nuclear magnetic resonance. These characterizations were performed to contribute to understanding of the effects of the chemical synthesis and treatment processes and thereby to enable improvement of these processes.

Mechanical tests were performed to study the potential utility of nanoparticle liquids as electrically conductive lubricants in relay switches in micro-electromechanical systems. For these tests, the nanoparticle liquids were spin-coated to an average thickness of 2 to 3 nm (amounting to sub-monolayer coverage) on gold substrates. In the test of each such coated substrate, a ball was pressed onto the substrate with a controllable, reproducible force, and the resulting contact resistance was measured as the ball was slid across the surface in a cyclic motion at a frequency of 5 Hz. For comparison, the same tests were performed on uncoated gold substrates. The results of these tests were interpreted as demonstrating that the gold-nanoparticle coatings increased the durabilities (quantified in terms of numbers of cycles until failure) by factors ranging from about 10 to about 103.

It is speculated that a nanoparticle-liquid coat of the type tested can reconfigure itself through fluid-like motion and thereby can heal itself when it is damaged by rubbing in use as an electrically conductive lubricant. More specifically, it is speculated that as such a lubricant in a relay-switch contact becomes worn out through mechanical deformation of the nanoparticles and ashing of the carbonaceous coats on the nanoparticles, the undamaged portions of the lubricant can flow into the damaged areas and thereby heal the coat.

This work was done by Robert I. MacCuspie, Andrea M. Elsen, Steve Patton, J. David Jacobs, Steve Diamanti, Michael Arlen, Andrey A. Voevodin, and Richard A. Vaia of the Air Force Research Laboratory.


This Brief includes a Technical Support Package (TSP).
Gold-Based Nanoparticle Liquids for Electronic Applications

(reference AFRL-0054) is currently available for download from the TSP library.

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