“[Our work] overcomes barriers to using nanocrystals routinely,” said Jeff Brinker of Sandia and the University of New Mexico. “The question in nanotechnology isn’t ‘where’s the beef?’, it’s ‘where’s the connectors?’ How does one make connections from the macroscale to the nanoscale?”

To make the structures, the researchers coated gold nanocrystals with thiols to stabilize them, and then encapsulated the crystals in a layer of surfactant molecules to create micelles. Adding tetraethyl orthosilicate to an aqueous solution of the micelles under basic conditions and in the presence of a catalyst led to the formation of silicic-acid moieties at the surface of the micelles. This caused the micelles to self-assemble into a structure consisting of a face-centred-cubic arrangement of gold nanocrystals within a silica matrix. The scientists say the silica matrix gave the structure increased chemical, mechanical and thermal robustness compared with other connected nanocrystal systems.

The researchers were able to tailor the size of the cubic lattice by altering factors such as the size of the nanocrystals, thickness of the stabilizing layer and thickness of the layer of surfactant molecules. Altering the reaction conditions also enabled them to produce the structure in thin-film form.

In addition, the scientists found they could use the first part of the technique to form water-soluble micelles from fluorescent semiconducting cadmium-selenide nanocrystals. The nanocrystals (or quantum dots) maintained their optical properties.

“The beauty of our approach is that it makes these quantum dots both water-soluble and biocompatible, two essential qualities if we want to use them for in vivo imaging,” said Sandia’s Hongyou Fan. “The functional organic groups on the quantum dots can link with a variety of peptides, proteins, DNA, antibodies etc. so that the dots can bind to and help locate targets like cancer cells, a critical issue in biomedicine.”

Brinker and colleagues believe the process could have applications in solid-state lasers, catalysts, cancer tracking, lighting devices and memory storage. The structures also proved useful for testing the electrical properties of nanodevices. “Before, there was no way to make precisely ordered 3D nanocrystalline solids, integrate them in devices and characterize their behaviour,” said Brinker. Initially, the team tested the properties of a planar metal-insulator-metal capacitor that used a gold nanocrystal/silica array as the insulator layer.

The researchers reported their work in Science.