Electrical properties of ideal carbon nanotubes

Room temperature current-voltage characteristics of ideal single wall carbon nanotubes have been investigated using first principle quantum mechanical calculations. Three basic types of carbon nanotubes (metallic, small bandgap and large bandgap) have been simulated for comparison. Simulation model consists of an infinitely long nanotube with a central region through which current is driven in response to a voltage applied to left and right regions which are treated as electrodes. The central region has a width equal to an integer number of unit cells of the nanotube. (n, n) armchair nanotube is chosen as the test case, since its resistance is well known (∼6.45 kohm) from theoretical calculations. n is varied from 4 to 14 to cover a range of diameters up to ∼1 nm. Calculated resistance values agree well with the theoretical value to within a few per cent of error. Next I-V characteristics of two zigzag nanotubes (12, 0) and (13, 0) are calculated in the range 0-1 v using the same approach. I-V curves indicate that zigzag nanotubes exhibit nonlinear increase of current as a function of increasing voltage, as opposed to the armchair nanotube which exhibits ohmic behavior. Conductivity of (12, 0) nanotube decreases with increasing voltage. Its current values are greater than (14, 14) nanotube at the same voltage, suggesting that a small bandgap nanotube can be more conductive than a metallic one at room temperature. Large bandgap (13, 0) nanotube exhibits a threshold voltage of about 0.8 v beyond which a noticeable current starts to flow. Its current values are significantly smaller than both small bandgap and metallic nanotubes at the same voltage.