Clays and Shales

Abstract

An optical rectenna—a device that directly converts free-pro-pagating electromagnetic waves at optical frequencies to direct current—was first proposed over 40 years ago1,yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale2,3. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 pHz (switching speed on the order of 1 fs). Diodes operating at these frequencies are feasible if their capacitance is on the order of a few atto-farads3,4, but they remain extremely difficult to fabricate and to reliably couple to a nanoscale antenna2. Here we demon-strate an optical rectenna by engineering metal–insulator–metal tunnel diodes, with a junction capacitance of ∼2aF, at the tip of vertically aligned multiwalled carbon nanotubes (∼10 nm in diameter), which act as the antenna5,6. Upon irradiation with of vertically aligned multiwalled carbon nanotubes (∼10 nm in diameter), which act as the antenna5,6. Upon irradiation with visible and infrared light, we measure a d.c. open-circuit voltage and a short-circuit current that appear to be due to a rectification process (we account for a very small but quantifiable contribution from thermal effects). In contrast to recent reports of photodetection based on hot electron decay in a plasmonic nanoscale antenna7,8, a coherent optical antenna field appears to be rectified directly in our devices, consistent with rectenna theory4,9,10.Finally,power rectification is observed under simulated solar illumination, and there is no detectable change in diode performance after numerous current–voltage scans between 5 and 77 °C, indicating a potential for robust operation. A single-frequency energy conversion efficiency greater than