Probing Optical Transitions in Individual Carbon Nanotubes Using Polarized Photocurrent Spectroscopy

Abstract

Carbon nanotubes show vast potential to be used as building blocks for photodetection applications. However, measurements of fundamental optical properties, such as the absorption coefficient and the dielectric constant, have not been accurately performed on a single pristine carbon nanotube. Here we show polarization-dependent photocurrent spectroscopy, performed on a p-n junction in a single suspended semiconducting carbon nanotube. We observe an enhanced absorption in the carbon nanotube optical resonances, and an external quantum efficiency of 12.3% and 8.7% was deduced for the E11 and E22 transitions, respectively. By studying the polarization dependence of the photocurrent, a dielectric constant of 3.6 ± 0.2 was experimentally determined for this semiconducting carbon nanotube.

Single carbon nanotube photovoltaic device

“…The external quantum efficiency has been previously measured for this device geometry 14 and was found for approximately 6 lW excitation power to be 12% on the E11 and approximately 3% for off resonance excitation. These values agree with other reported quantum efficiencies for carbon nanotube devices.…”

“…After locating the position of the nanotube diode, we performed photocurrent spectroscopy 4,14,17 to identify the optical transitions in the carbon nanotube. Fig.…”

“…The photocurrent spectrum was measured with the polarization aligned along the nanotube axis. 14,18 A plot of the photocurrent at E11 as a function of the polarization angle of the excitation laser can be found Thereafter we investigated the diode's response to illumination. The I-V characteristic of the carbon nanotube was measured for on resonance excitation on the E11 (0.84 eV), as well as for off resonance excitation at 0.98 eV, indicated by arrows in Fig.…”

“…It is also worth mentioning that although the nanotubes have an enhanced energy conversion efficiency at the resonances their efficiency at higher energies is not zero. The absorption coefficient for light off resonance is approximately 3%, 14 and the efficiency of converting this energy into electrical current is 5%-10% (see blue triangles in Figs. 3 and 4).…”

“…Details about the device fabrication has been reported elsewhere. 13,14 Two local gates at the bottom of the trench are used for electrostatic doping. A schematic representation of the device geometry can be found in the inset in Fig.…”