Optoelectronics in Carbon Nanotube Photodiodes and Graphene Hetero-Interface Devices

The excellent thermal, electronic and optical properties of carbon nanotubes (NTs) and graphene strongly motivate the use of these materials in optoelectronic devices. Here, we review our recent investigations of NT and graphene optoelectronic devices. By studying individual NT and graphene devices, we aim to uncover novel physical phenomena and establish a foundation for future applications in carbon nanoelectronics. We present photocurrent measurements of NT gated p-n junctions and graphene hetero-interface field effect transistors. In NTs, we observe dramatic photocurrent gain at high photon energies that we attribute to multiple electronhole pair generation. In graphene, we observe pronounced photocurrent generation at the interface between single and multi-layer graphene as well as at the interface between suspended and unsuspended graphene sheets. Summary of Research: The NT p-n junction devices (Figure 1) consist of individual NTs in a split-gate field effect geometry. Three independent gates, V1, V2, and global back gate VG, allow selective electrostatic doping along the length of the NT. By applying voltages of opposite polarities on V1 and V2, a p-n junction is realized, which yields a built-in electric field E along the length of the nanotube. Photo-generated electron hole-pairs created in the junction are separated by the built-in potential and accelerated to the device contacts, leading to photocurrent at zero bias. Current-voltage characteristics are measured while the p-n junction is illuminated at low temperatures. Figure 2 shows I-VSD data at 60 K under illumination at several photon energies (labeled). As the photon energy is increased, we observe very striking behavior. The reverse-bias photocurrent increases with increasing photon energy and evolves into a series of steps with increasing reverse bias. By combining spatially and spectrallyresolved photocurrent measurements, we attribute these photocurrent steps to highly efficient particle-antiparticle creation by high-energy charge carriers in the Figure 1: Schematic of carbon nanotube p-n junction in multiple gate geometry. A p-n junction (with built-in electric field E) is formed by electrostatic gating of the nanotube. Figure 2: I-VSD photocurrent characteristics of the nanotube p-n junction at various photon energies. Inset: potential energy diagram showing electron-hole pair production.