An optical rectification device – that is, a device that converts free-propagating electromagnetic waves at optical frequencies to d.c. electricity – was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectification device requires that an antenna be coupled to a diode that operates on the order of 1 petahertz (switching speed on the order of a femtosecond). Ultralow capacitance, on the order of a few attofarads, could allow a diode to operate at these frequencies; and the development of metal-insulator-metal tunnel junctions with nanoscale dimensions has emerged as a potential path to diodes with ultralow capacitance, but these structures remain extremely difficult to fabricate and couple to a nanoscale antenna reliably. Here we demonstrate an optical rectification device by engineering metal-insulator-metal tunnel diodes, with ultralow junction capacitance of approximately 2 attofarads, at the tips of multiwall carbon nanotubes, which act as the antenna and metallic electron emitter in the diode. This demonstration is achieved using very small diode areas based on the diameter of a single carbon nanotube (about 10 nanometers), geometric field enhancement at the carbon nanotube tips, and a low work function semi-transparent top metal contact. Using vertically-aligned arrays of the diodes, we measure d.c. open-circuit voltage and short-circuit current at visible and infrared electromagnetic frequencies that is due to a rectification process, and quantify minor contributions from thermal effects. Our devices show evidence of photon-assisted tunneling that reduces diode resistance by two orders of magnitude under monochromatic illumination. Additionally, power rectification is observed under simulated solar illumination. Numerous current-voltage scans on different devices, and between 5-77 degrees Celsius, show no detectable change in diode performance, indicating a potential for robust operation.
Baratunde “Bara” Cola is an assistant professor in the George W. Woodruff School of Mechanical Engineering and the School of Materials Science and Engineering at the Georgia Institute of Technology. He received his B.E (2002) and M.S. (2004) from Vanderbilt University, while a member of the Vanderbilt Football Team, and his Ph.D. (2008) from Purdue University, all in mechanical engineering. Bara has received prestigious early career research awards from DARPA (2009), NSF (2011), and the Army (2013), and received the Presidential Early Career Award for Scientist and Engineers (PECASE) in 2012 from President Obama for his work in nanotechnology, energy, and outreach to high school art and science teachers and students. In 2013, Bara was awarded the AAAS Early Career Award for Public Engagement with Science. In addition to building knowledge and training students, his journal articles, book chapters, and conference proceedings have helped to produce 3 issued patents and 5 pending patents, which lead to Bara founding Carbice Nanotechnologies, Inc. in 2012 to commercialize carbon nanotube thermal interface materials.
Bara’s work is currently focused on characterization and design of thermal transport and energy conversion in nanostructures and devices. He is also interested in the scalable fabrication of organic and organic-inorganic hybrid nanostructures for novel use in technologies such as thermal interface materials, thermoelectrics and thermo-electrochemical cells, infrared and optical rectenna, and materials that can be tuned to regulate the flow of heat.
Host: Professor Megan Valentine