We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In semiconducting nanotubes, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent.¹ Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field assisted charge separation. We observe a sharp, well defined E₁₁ free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally. In addition, we investigate the competing photocurrent mechanisms in quasi-metallic nanotubes over a wide range of bandgaps (30–180 meV).² Here, we correlate homogenous pn-doping dependence ( V_g1 = – V_g2 ) of the photocurrent with the current–voltage ( I–V_g ) characteristics (V_g1 = V_g2 ) of each device. These I–V_g characteristics, in turn, are used to determine the bandgap of the nanotubes by fitting our data to a Landauer transport model. The mechanism underlying photocurrent generation is then established by correlating the photocurrent transport with the calculated thermoelectric power, the measured conductance, and the extracted bandgap for all devices.

1.            Chang, S.W., J. Theiss, J. Hazra, M. Aykol, R. Kapadia and S.B. Cronin, Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions. Applied Physics Letters, 107, 053107 (2015).

2.            Amer, M.R., S.W. Chang and S.B. Cronin, Competing Photocurrent Mechanisms in Quasi-Metallic Carbon Nanotube pn Devices. Small, 11, 3119-3123 (2015).