Aug 17, 2012
Dewetting synthesis delivers tunable plasmonic bimetals
The field of plasmonics extends across a number of practical applications, such as disease detection via localized surface plasmon resonance (LSPR) or surface enhanced Raman (SERS) sensing, energy harvesting and novel optical devices. One of the continuing goals of research in this field is to find materials that can perform as well or better than the pure noble metals gold (Au) and silver (Ag), which have naturally strong plasmonic responses. In recent work, researchers have reported a new synthesis route inspired by the natural phenomenon of dewetting, which can be used to make mixtures of Co-Ag that perform as well as pure Ag, and might even be better suited in certain cases due to some unique properties resulting from the multi-functional nature of such bimetallic systems.
When bilayer films of Co and Ag were melted by a nanosecond pulsed laser, the dewetting self-organization spontaneously produced arrays of nanoparticles with a narrow size distribution. By varying the individual film thickness, the scientists were able to vary the particle size by one order of magnitude, while independently controlling the composition of the sample over a large range.
Since Co and Ag are immiscible, the nanoparticles consisted of segregated regions of the metals. As a consequence, the team was able to tune the localized surface plasmon resonance (LSPR) to change by one order of magnitude in wavelength compared with pure Ag, by varying either the size and/or composition of the particles. The group also discovered that the sensitivity of the bimetallic nanoparticles to detecting local dielectric change, which is central to LSPR sensing, was as good as that of pure Ag.
Another interesting discovery was that the Co-Ag material appears to have much higher environmental stability compared with the pure Ag, an effect that is currently being investigated. In the near future, the researchers also plan to study the magneto-optical properties of the arrays, which have applications in optical communication and sensing.
This inter-disciplinary research was supported by the US National Science Foundation (NSF) and the Department of Energy (DOE) as well as the US NSF-supported TN-SCORE programme. The study was led by researchers at the University of Tennessee-Knoxville, US, and was a collaboration between Oak Ridge National Laboratory and other academic institutions (Southern Illinois University in Edwardsville and Washington University in St. Louis).
A full description of study can be found in the journal Nanotechnology.
About the author
This study was primarily conducted by the Group of Nano and Thin film Sciences (GNATS) at the University of Tennessee-Knoxville (UTK), led by Dr Ramki Kalyanaraman. The first author, Ritesh Sachan, is a PhD candidate co-supervised by Dr Ramki Kalyanaraman and Dr Gerd Duscher at UTK. Ritesh Sachan conducted the synthesis, TEM characterization and optical analysis for this work. The other collaborators include Dr Hernando Garcia (Southern Illinois University in Edwardsville), Dr Anup Gangopadhyay (Washington University in St. Louis), Dr Stephen Pennycook (Oak Ridge National Laboratory), graduate students Vanessa Ramos and Sagar Yadavali (all from UTK), as well as Dr Nozomi Shirato (from UTK) and Dr Hare Krishna (who graduated from Washington University and is now at Intel). This collaborative and inter-disciplinary team is involved in the design, synthesis and characterization of nano-related materials and phenomena with the goal of making new materials for applications in biosensing, catalysis, energy harvesting, optical processing and information storage.