California Team Directs Nano Self-Assembly for Device-Ready Thin Films
It could be the 21rst Century version of California Gold Rush. Researchers from the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) have cracked the code for device-ready nanoparticle self-assembly. In the bargain, the team is looking to change the game – and the economics – for large-scale nanofabrication.
Nanoreserachers Peter Bai, Joseph Kao and Ting Xu incorporated gold nanoparticles into solutions of block co-polymer supramolecules to form multiple-layers of self-assembled thin films.
The work is the latest step forward for using nanoscale materials efficiently for large-scale fabrication, according to team leader Ting Xu, a polymer scientist at both Berkeley Labs and UC. The thin films being produced have applications for computer memory storage, energy harvesting, energy storage, remote-sensing, catalysis, light management and other novel areas such as plasmonics, Xu added.
"We've demonstrated a simple yet versatile supramolecular approach to control the 3-D spatial organization of nanoparticles with single particle precision over macroscopic distances in thin films," Xu said. "While the thin gold films we made were wafer-sized, the technique can easily produce much larger films, and it can be used on nanoparticles of many other materials besides gold."
Specifically, the team produced multiple-layers of thin films from highly ordered one-, two- and three-dimensional arrays of gold nanoparticles.
"This is the first time that 2-D nanoparticle assembly, similar to those obtained using DNA linkers and controlled solvent evaporation, can be clearly achieved in multi-layers in supramolecule-based nanocomposite thin films," Xu said. "Our supramolecular approach does not require chemical modification to any of the components in the composite system and, in addition to providing a means of building nanoparticle-based devices, should also provide a powerful platform for studying nanoparticle structure-property correlations."
If nanoparticles can be coaxed into routinely assembling themselves into complex structures and hierarchical patterns, similar to what nature does with proteins, devices a thousand times smaller than those of today's microtechnologies could be mass-produced, she added.
The technique developed by Xu and her colleagues uses solutions of block co-polymer supramolecules (a group of molecules that act as a single molecule) to direct the self-assembly of nanoparticles.
Block copolymers are long sequences or "blocks" of one type of monomer bound to blocks of another type of monomer that have an innate ability to self-assemble into well-defined arrays of nano-sized structures over macroscopic distances.
"Block copolymer supramolecules self-assemble and form a wide range of morphologies that feature microdomains typically a few to tens of nanometers in size," Xu says. "As their size is comparable to that of nanoparticles, the microdomains of block copolymer supramolecules provide an ideal structural framework for the co-self-assembly of nanoparticles."
In this latest study, Xu and her colleagues incorporated gold nanoparticles into solutions of block co-polymer supramolecules to form films that ranged in thickness between 100 to 200 nanometers.
"Upon incorporation of nanoparticles, the block co-polymer supramolecules experience conformational changes, resulting in entropy that determines the placement and distribution of the nanoparticles, as well as the overall morphology of the nanocomposite thin films," Xu says. "Our results indicate that it should be possible to generate highly-ordered lattices of nanoparticles within block co-polymer microdomains and obtain 3-D hierarchical assemblies of nanoparticles with precise structural control."
This latest work follows a study Xu’s team published last year showing the ability to induce rod-shaped semiconductor nanocrystals to self-assemble into one-, two- and three-dimensional macroscopic structures.
The work appears in the journal Nano Letters as "Nanoparticle Assemblies in Thin Films of Supramolecular Nanocomposites." Aside from Xu, co-authors include Joseph Kao, Peter Bai, Vivian Chuang, Zhang Jiang and Peter Ercius.