Biomedical Engineering Reference
In-Depth Information
shows the profiles of Kr-release after ion implantation under the
same conditions (energy and dose of ions) in UDD, acetylene carbon
soot and diamond powder with grain size of ~1
μm
. The HT-peak
(near 1500°C) appears evident solely in the case of nanostructured
diamond, whereas it is characterized by a very low intensity in the
cases of micro-sized diamond and non-diamond carbon soot.
The HT-peak is somewhat shifted to higher temperatures with
increasing mass number of atoms in the raw Ar-Kr-Xe (Fig. 6.11).
A similar shift was observed increasing the ion energy, as shown in
Fig. 6.12. This shift can reflect the increase of ion penetration depth
into diamond crystal with increasing ion energy. One can suggest,
therefore, that the difference in the position of HT-peak for different
heavy noble gas atoms implanted at the same energies (Fig. 6.11)
could be caused by mass dependence of the penetration depth. The
calculations performed using the TRIM (transport of ions in matter)
mode predict the increase of implantation depth in diamond with
increasing ion mass of heavy noble gases (1.3, 1.4, and 1.6 nm for
700 eV ions of Ar, Kr, and Xe, respectively).
The following mechanism could be suggested for HT release of
noble gases from UDD. The graphitization of UDD under annealing
in vacuum occurs at temperatures of the HT-peak (above 1200°C)
[84]. Using the data obtained in Refs. [84, 85] we have calculated the
temperature dependence of transformed diamond fraction in UDD
sample; these data are plotted in Fig. 6.13 (lower part).
The similarity between this curve and the release profile of
noble gases suggests that HT-peak of noble gases from nanodiamonds
is caused by structural transformation of UDD. According to Refs.
[84, 85] this transformation proceeds from the surface toward
the particle bulk. In this case the temperature of HT-peak should
increase with ion penetration depth (the deep layer in the diamond
core will be destroyed at higher temperatures). This explanation was
confirmed by the mass-spectrometric detection of nitrogen, present
in UDD as bulk impurity, released during graphitization of UDD.
To our knowledge, this is the first direct observation of nitrogen
desorption from UDD.
These results indicate that the thermal release of noble gases
from nanodiamond grains is associated with atoms trapped inside
diamond crystal at sites belonging to two main types. The LT-peak
is formed by the atoms escaping the sites of low activation energies.
The HT release is caused by the atoms more tightly bounded in
