OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 244

Moreover, owing to the excellent optical properties of semiconducting
nanotubes, both electronic and optoelectronic devices from the same material
seem feasible.

9.2.1.1 Electronic properes of CNTs

Although electrical properties of CNTs derive, to a large extent, from the
unusual electronic structure of graphene, the additional rolling process
forming the nanotube structure is further described by a pair of integers ( n ,
m ), deining the chiral vector C h = n a 1 + m a 2 , where a 1 and a 2 are the unit
vectors of the graphene honeycomb lattice. Depending on how the graphene
sheet is rolled up, nanotubes show either metallic or semiconducting
behaviour. The main electronic properties of CNTs can be summarised as
resistance, capacitance and inductance. 11
Electrical transport inside the CNTs is affected by scattering in
correspondence with defects and by lattice vibrations that lead to a new type
of quantised resistance ( R Q ) related to their contacts with three-dimensional
(3D) macroscopic objects, such as the metal electrodes. 12,13 When the only
resistance present is the quantum resistance, transport in the CNT is ballistic,
which means no carrier scattering or energy dissipation takes place in the
body of the CNTs, and this is generally achieved with tubes' length ≤ 100 nm.
Another electronic characteristic of CNTs is the capacitance that depends
on how their energy states are distributed. This quantum capacitance, C Q ,
is small (of the order of 10 -16 F/μm). 14 In addition, a CNT incorporated in a
structure has an electrostatic capacitance, C G , which arises from its coupling to
surrounding conductors; in a single-CNT FET, geometry leads to C G ≈ 1/ln( t ins ),
where t ins is the thickness of the gate insulator. Finally, CNTs have inductance,
which is a resistance to any changes in the current lowing through them. 15
The quantum inductance is usually called kinetic inductance, L K , and it is the
resistance to the change in the kinetic energy of the electrons of the CNT. It
leads to electron velocities in phase with the external driving ield.

9.2.1.2 CNT-FETs

Semiconducting CNTs present the interesting characteristic of an electronic
transport that can be switched “on” and “off ”, thus paving the way for their
use as novel FETs. In a typical FET, the basic features include a channel (made
up of a semiconductor, e.g., silicon) connected to two electrodes, namely a
source (S) and a drain (D). An insulating thin ilm, generally SiO 2 , separates
this part from a third electrode called the gate (G). By applying a voltage at
this gate electrode ( V g ), it is possible to modulate (switch) the conductance of
the semiconducting channel (Fig. 9.1) .