Gold-Coated Nanowires for Sensor Technology

The future of the engineering world has found a place in the development and refinement of nanowire technology that has been conducted over the last several years. While the diameter within a nanowire typically falls between 1-100 nanometers (nm), the length of these wires can extend for hundreds of micrometers, which can be useful for a wide variety of technological applications.

Similarly, their extremely high ratio of surface area to volume allows for these wires to be very good candidates as detectors, as this extended surface area can be treated to bind with specific biological molecules or chemicals¹. Depending upon the type of material in which nanowires are grown upon will determine the way in which this structure can be used. Nanowires also exhibit an impressive slenderness in their design, which allows for the electrons and photons present within these structures to experience what is known as “quantum confinement effects,” which can be applied for use in a wide range of macroscopic tools.

Recently, researchers around the world have successfully applied nanowires for sensor and light-emitting diode technologies. For example, on January 19, 2015, a team of researchers based in Japan found that a silica coating placed onto nanowires showed a high reflectivity in the complete visible wavelength of the distributed light, which can be used further as a white light coating for a wide range of optoelectronic products².

As a result of the outstanding properties that nanowires can offer an extensive amount of applications, physicists at the University of Cincinnati have recently found a way to enhance this luminescence potential of nanowires by the addition of a gold coating. Primary graduate student researcher Fatemesadat Mohammadi utilized a technique known as high-vacuum organic molecular beam deposition in order to spread organic and metal layers onto the gallium-nitride nanorods³. In a technique that is uniquely specific to the experiment conducted by the Cincinnati research team, this created nanofilm function as a spacer that can control the flow of energy between the exciton present in the nanowire, as well as the flow of energy occurring between the plasmons, or metal electrons. These organic spacers also allow for an extended emission lifetime of the gold-coated nanorods, which prevents quenched photoluminescence, a process that is typical of gold-coated nanorods that shortens the emission lifetime, from occurring³.

As pulses of laser light are fired at a complicated array of mirrors and beam spitters, all of which are arranged at precise angles in order to accurately measure the emission produced by these nanofilms. Luminescence of the nanowire was measured by observing how the emitted light is able to be coupled onto the metal film through a process known as plasmonic waveguiding. Mohammadi found that the nanowire reaction took just 10 picoseconds to occur, which can better be understood as one trillionth of a second⁴.

The project conducted by Mohammadi, under the direction of Associate Physics Professor Hans-Peter Wagner, found an easily replicable experiment that demonstrated the potential of these gold-coated nanowires to serve as an energy pump for future semiconductor purposes³. Not only does this gold nanofilm exhibit an exceptionally fast rate of production, but it also avoids a troublesome physical property that can occur with materials known as resistivity. As the temperature of the metal increases, the resistance of the metal to corporate properly also increases, which can be a direct result of the electron-photon interactions witnessed in this experiment. While the use of gold-coated nanowires showed a great deal of promise in this experiment, more traditional coatings such as silver and silica can be even more beneficial to enhancing the metallic properties of the applied nanowires for any given application of its use.

References:

1. Chandler, David L. "Explained: Nanowires and Nanotubes." MIT News. N.p., 11 Apr. 2013. Web. http://news.mit.edu/2013/explained-nanowires-and-nanotubes-0411.
2. Xi, Shuang, Tielin Shi, Xiaoping Li, and Zirong Tang. "Silica Nanowires as New Coating Material for High-efficiency Light Reflection in LED System." IEEE CPMT Symposium Japan 2014 (2014). Web.
3. "The Magic of Nanotechnology." University of Cincinnati. Web. http://magazine.uc.edu/editors_picks/recent_features/nanotech.html.
4. "Using Gold Coating to Control Luminescence of Nanowires." ScienceDaily. ScienceDaily, 16 Mar. 2017. Web. https://www.sciencedaily.com/releases/2017/03/170316141136.htm.
5. Image Credit: shutterstock.com/KaterynaKon