Electrical Transport in CNTs and Membranes
Electrical Transport in Carbon Nanotubes
The AFM-based technique developed at NPL has enabled the intrinsic electrical conductance of N-doped NTs to be measured for the first time. These measurements are important for the development of N-doped NT biosensor devices. These measurements were made in collaboration with Prof John Ryan's group, and have been published in Applied Physics Letters: APL, 89, 143110 (2006).
Carbon nanotubes (CNTs) are composed of sheets of carbon atoms rolled up into cylinders; they can be metallic or semiconducting depending on their chirality (the 'rolling-up angle' of the carbon sheet). CNTs are one-dimensional quantised electrical conductors, where the conductance quantum is G0 = 2e2/h = 12.9 k. A metallic CNT has conductance 2 G0 due to the presence of two conducting subbands at the Fermi Energy, EF.
Doping CNTs with nitrogen introduces an n-type electron donor state near to EF, which has been predicted to lead to metallic properties and high conductances (~2 G0) independent of chirality. Previous electrical measurements by Xiao et al. were dominated by the electrical properties of the CNT-electrode contacts and resulted in surprisingly low conductance values (~0.01 G0). Our aim was to measure the intrinsic electrical conductance of N-doped CNTs.
The N-doped CNTs used in this work were synthesized by Nicole Grobert. A bundle of N-doped CNTs was mounted on an AFM probe, which was then installed in an NPL-modified AFM to enable combined electrical and thermal conductance measurements. The CNT bundle was lowered until mechanical contact was made between an individual N-doped CNT and the graphite surface and then electrical conductance measurements were performed.
Figure 1 shows electrical conductance measurements made during the approach of an N-doped CNT towards a graphite surface, with a bias voltage of 0.1 V. Mechanical contact was observed at time, t = 40 s, and the measured conductance was initially 1.0 G0. The voltage was then swept from 0 to +3 V to –3 V whilst recording the current and conductance (Figure 1, inset). The voltage was then returned to the original value of 0.1 V and it was noticed that the conductance had increased to 1.2 G0. This is due to an 'annealing' of the electrical contacts as poorly conducting material is burnt away.
Figure 2 shows a histogram of the electrical conductance values measured for over 50 individual N-doped CNTs at low bias (≤ 0.1 V) after annealing. The histogram shows a peak at a conductance value of 1.0 G0. This is half of the expected conductance of 2 G0 and may be attributed to a reduced transmission at the CNT/bundle interface, or scattering at defects or N-doping sites.
Our measurements are in good agreement with the metallic behaviour and high conductance values predicted by theory and scanning tunnelling spectroscopy measurements.
Electrical Transport in Membranes
Electrical transport measurements of ion channel proteins embedded in membrane have been made;these demonstrates a new capability at NPL for studying a class of drug targets (ion channels) that are of great importance in the pharmaceutical industry.
Figure 3 shows the signal current measured due to the transport of potassium ions (K+) through a single gramicidin protein, which was inserted into an Egg-PC membrane. An extremely low signal noise of just 0.11 pA rms was achieved, such that the spontaneous opening and closing of a single ion channel was clearly resolved. For a bias voltage of +50 mV, the measured current was ~0.03 pA when the ion channel was closed and ~0.8 pA when the ion channel was open.