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UTS: Institute for Nanoscale Technology

'Smart' colour-change coatings - coatings that vary the colour of the light reflected or transmitted by a surface on cue - are of great interest. Such materials have countless applications: as window coatings; in consumer devices of many kinds; in architecture; in optoelectronics. Thermochromic coatings for windows are already known in principle (and in the form of VO2 were explored in the 90s at UTS). But the idea of the MPD - the Mesoscale Plasmonic Device - is rather different: to work towards building a physical nanoscale device with a tiny gold arm that can be swung between vertical and horizontal positions by means of a conformation-changing organic molecule tether. With the arm in the vertical position, such a device would have a different plasmon resonance from when the arm was in the horizontal position. The organic tether would be a molecule that changes its conformation in response to a chemical, optical, thermal or other stimulus. The arm would be made from a single gold nanorod. Of course a single device would not good for much, but if the little nano-machine could be self-assembled by the billions onto a surface, then the entire surface could be activated to change colour. In this case theoretical considerations show that the surface coating of gold MPDs would be pinky-red when in the vertical orientation and blue to transparent in the horizontal orientation. The exact colour would depend upon the length of the nanorods, and their material. The longer the rod, the more red-shifted its absorption spectrum. A blue coating can be obtained with rods that have a 1:3 aspect ratio, but even longer rods can become transparent in the visible while absorbing strongly in the near infra-red. The intriguing possibilities for this device have led UTS to apply for a patent to protect the idea, while we attempt to demonstrate it together with our collaborator, Prof. Paul Mulvaney at University of Melbourne.

Self-assembly of billions of such devices is quite possible in principle, and a simpler analogue - the 'nano-capacitor' - has already been assembled and characterised at UTS in two different versions. Also, methods to manufacture gold nanorods have been demonstrated both at University of Melbourne in Mulvaney's group, and at UTS. The current challenge is therefore to identify suitable organic tether molecules that will show a reversible conformation change in response to a suitable external stimulus, and to demonstrate the attachment of the nano-rods to such tether molecules. The schematic illustration (above) shows how a reversible cis- to trans- transformation induced by light or temperature can change the shape of a molecule and, in principle, move a metallic rod attached to it.