Lab talk
Jun 22, 2010
Anomalous n-type transport observed in Pd-contacted CNTFET
In a recent study, researchers from the University of Sheffield, UK, have observed for the first time an anomalous n-type (electron transport) behaviour in a palladium-contacted carbon nanotube field effect transistor (CNTFET). The peculiar behaviour is related to the diameter of the nanotube, measured by Atomic Force Microscopy to be about 0.57 nm (±10%). For such a small diameter, the curvature of the wall is so important that the π and σ orbitals that bond the carbon atoms together and that normally weakly interact in graphene, hybridize to form σ-π bonds, which strongly modify the electronic properties of the nanotube. In this region, the electronic bandgap is no longer inversely proportional to the diameter, neither is the workfunction constant at about 4.7&nbssp;eV as with large diameter nanotubes.
The stronger n-type branch observed in this transistor is explained by a (6, 1) carbon nanotube, which has a workfunction larger than palladium at low temperatures and a small bandgap. This enables palladium to inject electrons rather than holes within the channel at these temperatures.
The study offers new insight into electronics based on small-diameter carbon nanotubes, which have so far received little attention as they have been thought to be inappropriate for electronic devices. This study shows that some chiral nanotubes such as the (6,1) may have interesting properties that can facilitate n-type transport without any external doping by a suitable choice of metal and nanotube. The small bandgap facilitates operation at low supply voltages, a necessary requirement for future "green" electronics. Future challenges, however, exist in the growth, isolation and purification of such nanotubes.
More details can be found in the journal Nanotechnology.
About the author
The study was conducted in collaboration between the University of Sheffield (UK) and the University of Cambridge (UK) as part of the project "Device Electronics Based on Nanowires and Nanotubes" (DEWINT), under the programme "Fundamentals of Nanoelectronics" Fone, by the European Science Foundation (ESF). Dr S Jejurikar, research associate at the University of Sheffield, performed the electrical and physical characterization on the CNTFETs supplied and fabricated by Dr A Tahraoui from the University of Cambridge. Dr D Casterman, research associate at the University of Sheffield simulated the transport of the FET and the electronic properties of the CNTs via Density Functional Theory. Dr O Petrenko, research associate at the University of Sheffield, carried out the bandgap calculations using GW theory. P B Pillai, PhD student at the University of Sheffield, measured the workfunction of the nanotubes using Scanning Probe Microscopy in association with Dr C Durkan from the University of Cambridge. Prof. M M De Souza, chair of microelectronics at the University of Sheffield, and Prof. W I Milne, director of CAPE at the University of Cambridge, guided the work at the respective institutions.
