Plasmons are quantized collective oscillations of electrons confined on the surface of a metal that interact strongly with light on the nanoscale. Such enhanced interaction could be exploited in a wide range of technologies, including metamaterials design, biosensing, therapeutics, solar energy harvesting and photocatalysis.

Bimetallic nanostructures such as nanowires, especially those made of silver and gold, are good plasmonic materials. Such elongated structures can be thought of as the plasmonic analogues of classical antennas. Plasmonic nanowires, or nano-antennas, collect and focus light at optical to near-infrared wavelengths (as opposed to conventional television or radio antennas that work at radio frequencies) and as such will be crucial for developing nanophotonics devices in the future.

Such devices possess "plasmonic modes" that can be tuned to resonate with the electronic transitions in molecules nearby. It is these plasmonic modes that increase the coupling between light emitted by neighbouring molecules and the antenna.

Controlling nanowire morphology and optical properties

To tailor the surface plasmon resonances of nanowires for specific device applications, researchers need to be able to accurately control the length of these structures and synthesise them so that they do not differ too much in weight and size.

Luis Liz-Marzán of the CIC biomaGUNE, San Sebastián, and the Basque Foundation for Science in Bilbao, Spain, and co-workers have developed a nanowire growth technique based on selectively depositing silver onto specific crystallographic facets of pre-fabricated gold cores (known as pentatwinned gold nanorods). To obtain highly elongated nanostructures that are microns in length, the researchers had to avoid secondary nucleation or other side reactions that would compromise the quality of the finished product.

"To this end, we exploited a millifluidic flow system and syringe pumps to precisely control the rate at which we added the silver precursor and the reductant employed in our reaction," explains Liz-Marzán. "In particular, we added the silver precursor at a slower rate than the reductant and in this way 'forced' the gold cores to grow linearly with the amount of silver added."

This technique avoids any side reactions, and at the same time allows the team to make nanowires with roughly the same molecular weight and size, he adds.

Analysing nanowire plasmons with EELS

The team, which includes researchers from the University of Bayreuth in Germany, the University Paris-Sud in France and the University of Antwerp in Belgium, together with ICFO and ICREA, both in Barcelona, Spain, analysed the nature of the plasmons in the nanowires using Electron Energy Loss Spectroscopy (EELS). They found that the plasmon modes were distributed over a very wide range of wavelengths (from the ultraviolet to the mid-infrared), which means that they could be used for a wider variety of applications than structures that possess plasmon modes in a more restricted wavelength range.

"The rich optical activity of these bimetallic nanowires provides new opportunities in an array of different fields," Liz-Marzán told "As well as the examples mentioned earlier, another good example is in surface-enhanced IR absorption spectroscopy (SEIRA), a technique that directly probes molecular vibrations through absorbed light and which requires precise tailoring of resonances in the near- and mid-infrared."

The researchers say that they are now busy looking into using these nanowires as building blocks for making metamaterials. These materials do not exist in nature and are sub-wavelength-patterned structures containing arrays of tiny elements such as rods and rings that respond to light and other electromagnetic waves in unusual ways. "We are also planning to try out our synthesis technique on different types of gold cores and metals," adds Liz-Marzán. "Replacing silver with palladium or platinum, for example, might allow us to make a bimetallic system that boasts both plasmonic and catalytic properties."

The present research is detailed in Nano Letters DOI: 10.1021/acs.nanolett.5b01833.

For more on nanofabrication techniques visit the topical review Low-cost fabrication technologies for nanostructures: state-of-the-art and potential 2015 Nanotechnology 26 042001