Self-assembled gold nanoparticle chains for nanoplasmonics

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Making new materials that have small scale structures and multiple components is expected to be of great importance in a wide range of applications such as sensing, data storage, electronics and catalysis. One new area where small-scale structures could make a significant breakthrough is in the channelling of light around optical circuits . Guiding light along a glass fibre is a fast and efficient means of sending information but is limited by the size of the cables used because light has a wavelength of around half a micrometer, so once this scale is reached in the fibre then diffraction occurs. To get light to travel through smaller wires one needs to induce electronic effects in the material, and the current way of doing this is to use small dielectric or metallic patterns that interact with the light such that they are able to confine and guide the light along particular directions. These optical circuits work by light-induced localized excitation in the surface electrons of the metal (so-called plasmons), and can be produced by patterning processes (lithography). Although these plasmonic devices have been shown to efficiently confine and channel optical information, they remain restricted in size because of the difficulty of preparing very small ( nanoscale ) and complex patterns by lithographic techniques. Moreover, they tend to dissipate a lot of the light during propagation along the structures. The proposed research programme intends to explore a new approach to nanoplasmonics . Instead of preparing the small metallic structures by top-down lithography, we will use a bottom-up self-assembly approach involving the spontaneous alignment of very small gold nanoparticles into linear chains. In this way, we get round the problem of the small size required because we can make the gold nanoparticles beforehand using chemical synthesis that allows us to fine-tune the size. For example, it is relatively easy to prepare gold nanoparticles with sizes of 5, 10, 20 nm etc. What is particularly novel about our work is that we have discovered a method for inducing the nanoparticles to line up like beads on a string, and when they do this, their optical properties change dramatically because light can propagate down the nanoparticle chains. We now want to explore how we can control this assembly to produce chains that are straight or branched, have different lengths, or include other types of nanoparticles in them such as fluorescent or magnetic nanoparticles. We also want to embed the chains in a liquid crystalline array so that we can change the direction of light propagation along the chains by electrical switching of the liquid crystal matrix. Finally, we have some very advanced techniques (photon scanning tunnelling microscopy and near-field scanning optical microscopy), and mathematical methods that will allow us to understand the underlying physics of our new approach. Together, all these experiments should allow us to pioneer new ideas and methods in optical guidance in very small scale structures. And once we understand how these systems work, then it should be possible to use the results to develop new types of nanoscale devices that could be used as low cost, ultra-compact and sensitive analytical devices for use in health care, food control and drug tracking, as well as in super-fast computers driven by optical processing.