Biomedical Engineering Reference
In-Depth Information
a pure carbon-onion or nanodiamond is not the final product. More
often than not an intermediary is formed with a diamond-like core
and an onion-like outer shell of varying thickness. These particles are
known as
[9, 15], and they form spontaneously in
the absence of surface passivation. Both the nanodiamond-to-onion
and onion-to-nanodiamond transformations have been modeled
computationally [16-22], and the structure and thermodynamic
stability of the pure and mixed carbon nanoparticles have been
examined theoretically [9, 23]. It is now well established that this
transformation is initiated at the (111) diamond surface, which has
a structure very similar to the hexagonal lattice of graphene, but
does not occur on the (100) and (110) surfaces [24].
Complementary investigations have also been undertaken on the
structure and stability of quasi-one dimensional nanocarbons [25].
Once again it was found that the (100) and (110) surfaces remain
stable in the diamond configuration [26], but the (111) surfaces
delaminate to form nanotubular cages parallel to the principal axis
of the nanowire. These structures have been termed
bucky-diamonds
bucky-wires
[27, 28], due to their similarities with the quasi-zero dimensional
bucky-diamonds mentioned above. The delamination in both
cases is related to the thermodynamic preference for fullenenic/
graphenic structures at small diameters [29], or in structures with
a small number of atoms [9, 23], but also indicates that an sp
2
-
bonded structure is often exposed to the surrounding environment
(irrespective of the structure beneath).
However, the situation can be very different when these carbon
nanostructures are removed from ultra-high vacuum and exposed to
gases. For example, it has been found that complete passivation of
the unstable surfaces of diamond-like nanocarbons with hydrogen
eliminates this surface instability, and promotes the formation of
entirely sp
3
-bonded materials [30]. A similar change can also occur
2
in sp
-bonded materials, where it has been found that exposure of
CNTs to atomic H during plasma treatment produces in defects on
the outer walls of nanotubes due to chemisorption of individual
H atoms. As one would expect, these changes can have a dramatic
effect on the nanostructure and its properties, as demonstrated
by a number of studies conducted in the field [11, 31, 32]. In an
experimental study of the interaction of atomic hydrogen with
various forms of sp
2
-bonded carbon (fullerenes, CNTs, and planar
graphene) Ruffieux
et al.
[33] found that the energy barrier for
