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 309

still possible through ADF images, thus indicating a successful discrimination
with this technique.
Besides experimental microscopic detection, a theoretical simulation
could provide useful insights on the structural and electronic properties
of different material. For example, molecular dynamics (MD) calculations
have been performed on ice polygons inside CNTs. 217 The so-formed “ice-
nanotubes” (ice-NTs) consisted of polygonal (from pentagonal to heptagonal)
water rings placed along the SWCNT long axis. In the presence of an external
electric ield and in a temperature range between 100 and 350 K, these ice-
NTs represented the smallest ferroelectronic with spontaneous polarisation
of around 1 μC/cm 2 , thus opening the way to the encapsulation of other
dielectric materials in view of compact electronic devices.
In another study, the authors have incorporated water molecules inside
SWCNTs in spite of their hydrophobic nature, showing that encapsulated water
is interestingly transformed from a liquid-like state to a tube-like structure
(in agreement with the already mentioned ice-NTs). Having a shape similar to
hollow cylinders with diameters comparable to those of gas molecules, they
have stimulated a further evaluation of the competing behaviour between
water and gas molecules inside CNTs. 218 More precisely, SWCNTs bearing
an average diameter of 13.5 and 14.4 Å were puriied and subsequently
heated in air for 20-40 minutes between 350 and 450°C, in order to open
their extremities. After mixing methane (CH 4 ) and water molecules within
the same SWCNT, the results strongly indicated a so-called illing-ejecting
type transition of water. In other words, below a critical temperature ( T c ),
the water was ejected from the inside of the SWCNTs by the entering gas
(e.g., CH 4 ) molecules. The ejected water could be located around the open
ends of the SWCNTs, where it could reversibly re-enter the SWCNTs. Such
process should be possible when the gas-molecule-SWCNT interactions have
comparable strength to that of the water-SWCNT interactions, so that water
and gases are competing with each other to ill the SWCNTs.
In contrast, for a weaker gas (Neon, Ne) such replacement was not
observed. The observed on-off-like behaviour makes water-SWCNTs act as
“valves” for gas molecules, suggesting potential applications as gas selective
molecular nanovalves.
Throughout their investigations on molecules encapsulated inside CNTs,
the researchers have also carefully evaluated the behaviour of fullerene C 60
inside carbon nanopeapods (NPPs). 219 Surprisingly, by using inelastic neutron
scattering to probe the dynamics of nanopeapods (NPPs), they observed high
orientational mobility of the C 60 molecules when they are conined inside a
nanotube. In other words, these molecules undergo “free” rotations down to
very low temperatures, implying that the transition is continuously different