Stressing nano particles fine-tunes material properties

The newly patented method from Sandia researcher Hongyou Fan and colleagues uses pressure – described as a type of embossing - to produce finer and cleaner results in forming silver nanostructures than chemical methods, which are inflexible in their results and leave harmful by-products to dispose of. The research is detailed in Nature Communications.

Fan calls his approach ‘a simple stress-based fabrication method’ that, when applied to nanoparticle arrays, forms new nanostructures with tuneable properties.

‘There is a great potential market for this technology,’ he said in a statement. ‘It can be readily and directly integrated into current industrial manufacturing lines without creating new expensive and specialised equipment.’

‘This is a foundational method that should enable a variety of devices, including flexible electronics such as antennas, chemical sensors and strain detectors,’ said Sandia co-author Paul Clem. It also would produce transparent electrodes for solar cells and organic light-emitting diodes.

The method was inspired by industrial embossing processes in which a patterned mask is applied with high external pressure to create patterns in the substrate, Fan said.

‘In our technology, two diamond anvils were used to sandwich nanoparticulate thin films. This external stress manually induced transitions in the film that synthesised new materials,’ he said.

The pressure, delivered by two diamond plates tightened by four screws to any controlled setting, shepherds silver nanospheres into any desired volume. Propinquity creates conditions that produce nanorods, nanowires and nanosheets at chosen thicknesses and lengths rather than the one-size-fits-all output of a chemical process, with no environmentally harmful residues.

While experiments reported in the paper were performed with silver - the most desirable metal because it is the most conductive, stable and optically interesting and becomes transparent at certain pressures - the method also has been shown to work with gold, platinum and other metallic nanoparticles

Clem said the researchers are now starting to work with semiconductors.

Bill Hammetter, manager of Sandia’s Advanced Materials Laboratory, said, ‘Hongyou has discovered a way to build one structure into another structure - a capability we don’t have now at the nanolevel. Eight or nine gigapascal - the amount of pressure at which phase change and new materials occur - are not difficult to reach. Any industry that has embossing equipment could lay a film of silver on a piece of paper, build a conductive pattern, then remove the extraneous material and be left with the pattern. A coating of nanoparticles that can build into another structure has a certain functionality we don’t have right now. It’s a discovery that hasn’t been commercialised, but could be done today with the same equipment used by anyone who makes credit cards.’

The method can be used to configure new types of materials. For example, under pressure, the dimensions of ordered three-dimensional nanoparticle arrays shrink. By fabricating a structure in which the sandwiching walls permanently provide that pressure, the nanoparticle array will remain at a constant state, able to transmit light and electricity with specific characteristics. This pressure-regulated fine-tuning of particle separation enables controlled investigation of distance-dependent optical and electrical phenomena.

At even higher pressures, nanoparticles are forced to sinter, forming new classes of chemically and mechanically stable nanostructures that no longer need restraining surfaces. These cannot be manufactured using current chemical methods.