What if one day, your computer, TV or smart phone could process data with light waves instead of an electrical current, making those devices faster, cheaper and more sustainable through less heat and power consumption? That’s just one possibility that could one day result from an international research collaboration that’s exploring how to improve the performance of plasmonic devices. The research led by Masoud Kaveh-Baghbadorani, a doctoral student in the University of Cincinnati’s Department of Physics, will be presented on March 5, at the American Physical Society Meeting in San Antonio, Texas. 
Masoud Kaveh-Baghbadorani, a doctoral student in UC's Department of Physics, conducts research to improve the performance of plasmonic devices. The researchers are investigating the manipulation of light in plasmonic nanostructures using the dephasing and population dynamics of electron-hole-pairs in metal coated, core-shell semiconductor nanowires. The technique would minimize energy loss and heat production. The research focuses on guiding light through nanometer-thick metal films – about a thousand times thinner than a human hair – to propagate light with plasmon waves, a cumulative electron oscillation. Plasmonics is an emerging research field, but it has limitations due to high resistivity losses in the metal films. Kaveh-Baghbadorani has been exploring the development of hybrid metal/organic semiconductor nanowires that work as an energy pump to compensate for energy losses in the metal coating. “We have tried this with an alloy of silver, now we’re trying it with gold. The purpose is to better understand and try to model how energy is getting transferred from the semiconductor nanowire into the metal. There are many different variables here to better understand this energy transfer or energy coupling,” explains Kaveh-Baghbadorani. “We are working on improving the coupling between the semiconductor nanowires and the metal coating.” Masoud Kaveh-Baghbadorani conducts research in a UC lab in partnership with the Australian Research Council to improve the performance of plasmonic devices. In addition to using a different metal, the researchers are also using a vertical alignment of nanowire structures. They also developed a method to completely surround the nanowires with layers of 10-nanometer-thick gold films. An inserted organic material works as a spacer layer to control the energy transfer from the nanowire into the metal. “The metal results in high resistivity losses,” explains co-researcher Hans Peter-Wagner, a UC professor of physics and Kaveh-Baghbadorani’s advisor. “We want to overcome these losses by pumping energy from nanowire excitons, or electronic excitations, into the metal. This is the reason why we do this research.” The research is also exploring the effect of using different organic spacer layer thicknesses on the energy coupling. “When we use different organic materials in the plasmonic structure, we can extend the lifetime of excited charge carriers, therefore they can travel longer within the structure before they get captured by the metal,” says Kaveh-Baghbadorani. “By changing the organic spacer thickness, we can control the energy transfer process.” Future applications could include faster and enhanced performance of computers and other smart electronic devices, solar cells or even lead to a super-lens that results in a vast improvement of the current generation of microscopes. “We’re far from being at the end of potential applications for this research and constantly thinking about new uses. The research field is extremely rich, there’s no end in sight,” says Wagner.
Masoud Kaveh-Baghbadorani, a doctoral student in UC's Department of Physics, conducts research to improve the performance of plasmonic devices. The researchers are investigating the manipulation of light in plasmonic nanostructures using the dephasing and population dynamics of electron-hole-pairs in metal coated, core-shell semiconductor nanowires. The technique would minimize energy loss and heat production. The research focuses on guiding light through nanometer-thick metal films – about a thousand times thinner than a human hair – to propagate light with plasmon waves, a cumulative electron oscillation. Plasmonics is an emerging research field, but it has limitations due to high resistivity losses in the metal films. Kaveh-Baghbadorani has been exploring the development of hybrid metal/organic semiconductor nanowires that work as an energy pump to compensate for energy losses in the metal coating. “We have tried this with an alloy of silver, now we’re trying it with gold. The purpose is to better understand and try to model how energy is getting transferred from the semiconductor nanowire into the metal. There are many different variables here to better understand this energy transfer or energy coupling,” explains Kaveh-Baghbadorani. “We are working on improving the coupling between the semiconductor nanowires and the metal coating.” Masoud Kaveh-Baghbadorani conducts research in a UC lab in partnership with the Australian Research Council to improve the performance of plasmonic devices. In addition to using a different metal, the researchers are also using a vertical alignment of nanowire structures. They also developed a method to completely surround the nanowires with layers of 10-nanometer-thick gold films. An inserted organic material works as a spacer layer to control the energy transfer from the nanowire into the metal. “The metal results in high resistivity losses,” explains co-researcher Hans Peter-Wagner, a UC professor of physics and Kaveh-Baghbadorani’s advisor. “We want to overcome these losses by pumping energy from nanowire excitons, or electronic excitations, into the metal. This is the reason why we do this research.” The research is also exploring the effect of using different organic spacer layer thicknesses on the energy coupling. “When we use different organic materials in the plasmonic structure, we can extend the lifetime of excited charge carriers, therefore they can travel longer within the structure before they get captured by the metal,” says Kaveh-Baghbadorani. “By changing the organic spacer thickness, we can control the energy transfer process.” Future applications could include faster and enhanced performance of computers and other smart electronic devices, solar cells or even lead to a super-lens that results in a vast improvement of the current generation of microscopes. “We’re far from being at the end of potential applications for this research and constantly thinking about new uses. The research field is extremely rich, there’s no end in sight,” says Wagner.
UC doctoral student Yan Jin. Jin will present on how a blend of conjugated polymers resulted in structural and electronic changes that increased efficiency three-fold, by incorporating pristine graphene into the active layer of the carbon-based materials. The technique resulted in better charge transport, short-circuit current and a more than 200-percent improvement in the efficiency of the devices. “We investigated the morphological changes underlying this effect by using small-angle neutron scattering (SANS) studies of the deuterated-P3HT/F8BT with and without graphene,” says Jin. The partnership with the Oak Ridge National Laboratory, U.S. Department of Energy, is exploring how to improve the performance of carbon-based synthetic polymers, with the ultimate goal of making them commercially competitive. Unlike the silicon-or germanium-powered solar cells on the market, polymer substances are less expensive and more malleable. “It would be the sort of cell that you could roll up like a sheet, put it in your backpack and take it with you,” explains Vikram Kuppa, Jin’s advisor and a UC assistant professor of chemical engineering and materials science. One of the main challenges involving polymer-semiconductors is that they have significantly lower charge transport coefficients than traditional, inorganic semiconductors, which are used in the current solar technology. Although polymer cells are thinner and lighter than inorganic devices, these films also capture a smaller portion of the incoming light wavelengths and are much less efficient in converting light energy to electricity. UC doctoral student researcher Yan Jin is photographed with her advisor, Assistant Professor Vikram Kuppa. UC Assistant Professor Vikram Kuppa and Yan Jin. “Our approach is significant because we have now shown peak improvement of over 200 percent on a few different systems, essentially a three-fold increase in the efficiency of the cell by addressing the fundamental problem of poor charge transport,” says Kuppa. Jin led the research conducted at Oak Ridge National Laboratory and at UC’s Organic and Hybrid Photovoltaics Laboratory in the UC College of Engineering and Applied Science (CEAS). “We’re finding that these enhancements resulted from improvements in both charge mobility and morphology,” says Jin. “The morphology is related to the physical structure of the blend in the polymer films and has a strong impact on the performance and the efficiency of the organic photovoltaic (OPV) cells.” Yan’s future research is continuing on the examination of morphology and its connection to solar cell performance. Part of that research will be conducted on state-of-the-art, Ultra Small Angle X-ray Scattering (USAXS) equipment coming to the College of Engineering and Applied Science at UC, the result of a Major Instrumentation Award to Kuppa from the National Science Foundation. Kuppa says the $400,000 piece of equipment is only the second of its kind in a university in the U.S. and the first such instrumentation with multiple-sources and broad-measurement range.
Combining oblique angle deposition with standard graphene transfer protocols, two planar arrays of metal nanoparticles are fabricated that are vertically separated by atomic dimensions, corresponding precisely to the thickness of a single layer of graphene, i.e., 0.34 nm. Upon illumination of light at an appropriate wavelength, the local electromagnetic field at the junction of the dimers can be increased dramatically, thereby resulting in the most sensitive graphene–plasmonic hybrid photodetector reported to date. (© Wiley) Decorating a material with metal nanoparticles is a known way of increasing the interaction with light. When light falls on such tiny structures it forces all electrons to dance back and forth in a collective way, generating a very large electric field nearby. This large electric field extracts a response out of the material on which these metal nanoparticles rest. If there are two such nanoparticles close by, they interact and the electric field between them further increases. Researchers have tried to exploit this idea but with limited success due to the stringent requirement of separating the nanoparticles by just a few nanometers. In the present work, graphene acts as a perfect spacer and this produces an unprecedented field enhancement of nearly a million times in between the nanoparticles, boosting the interaction with light. As a result, the Raman signal in this device was found to be 100 times more intense. (Raman Effect, whereby light is reflected by a material in a signature pattern, is routinely used to study graphene.) This is of significance because although graphene responds to a large range of light frequencies (colours) and has a very fast response, it does not absorb light very well by itself. The authors also show that graphene may be irreplaceable, providing the optimum separation distance between nanoparticles, reducing the distance further may reduce the field enhancement. This design can be extended to other materials as well, Prof. Ambarish Ghosh who led the research says "The technique will allow a variety of plasmonic (metal nanoparticles) and 2D materials, which can be used to make devices of well designed functionality." This interdisciplinary research included researchers from the Centre for Nano Science and Engineering, Department of Electrical Engineering and the Department of Physics. They are already looking for industrial partners to take this technique out of the lab and into applications. With more work the device may become well suited for applications like sensitive colour-based detection, efficient medical diagnosis and single molecule detection.