Folding plasmene nanosheets

Self-folding plasmene nanosheets could be used for advanced identification applications Credit: Professor Wenlong Cheng

Professor Wenlong Cheng has been investigating how to make an unusual class of materials with exotic properties and unprecedented real-world applications.

Formally introducing plasmene nanosheets to the world, he and his team have created self-folding sheets of nanoparticles that can change the behaviour of light.

The nanosheets consist of a superlattice of nanoparticles, organised in a hexagonal pattern much like the arrangement of carbon atoms in the wonder-material, graphene.

Wenlong’s team creates the materials by sparsely distributing nanoparticles covered with protruding polystyrene hairs, or ligands, across the surface of a water droplet. By evaporating the water the surface area shrinks, bringing the particles closer together. Attractive van der Waals forces between the nanoparticles draw them together while repulsive forces between the ligands manoeuvre the nanoparticles into the desired hexagonal configuration.

The nanoparticle-laden water droplet is suspended over an open cavity, in this case a hole in a copper sheet, which gives the nanosheet a place to form. Once all the water has been removed, a continuous nanosheet forms neatly across this cavity like the membrane on a drum, allowing Wenlong to begin to play with it.

Using ANFF-Vic’s focused ion beam (FIB) instrument at the Melbourne Centre for Nanofabrication, the team “scores” specific patterns into the sheets, deliberately weakening them at key points. The weakened regions are akin to the paper folding lines introduced in origami, where the layout and severity of the folds define the final three-dimensional structures.

(Centre and Left) TEM image of a single free-standing soft plasmene nanosheet and zoomed-in view showing the ordered rhombic dodecahedral building blocks. (Right) SEM image of a complex 3D plasmene origami fabricated in conjunction with top down focused ion beam lithography.

This scoring process means the resultant shape’s properties can be tuned by manipulating the pattern cut into them and the order in which the folding is performed, enabling tailoring of the final product to meet specific end applications.

Researchers have investigated the properties of this self-folding before, but the designs of the well-defined origami structures of this kind have not been realised until now.

Light interacts and reflects off differing origami shapes in a way that is unique to each contour. By changing the structure, Wenlong can exploit his plasmene sheets to perform some useful tricks.
One useful application for these tiny sculptures is in nanophotonics as plasmonic waveguides for directing and switching light at the nanoscale by circumventing diffraction limits.

The plasmene nanosheets produced can also be used as dual-coded encryption security labels as it’s almost impossible to fraudulently create the shapes without the initial blueprint. This poses enormous potential benefits to the banknote industry, which is always searching for more secure methods of deterring counterfeiters.

The plasmene sheet’s semitransparency, mechanical softness and uniform response to light suggest that it could be an ideal material in the production of secure identifying labels for chemicals, currency and commodities, among others.