Biomedical Engineering Reference
In-Depth Information
D[10
17
H°/cm
2
]
Figure 2.22
(a) Hydrogen storage efficiency of HOPG samples, desorbed
molecular hydrogen (
) of atomic hydrogen
exposure. (b) STM image for 600 × 600 nm area of the HOPG
sample subjected to atomic hydrogen dose of 1.8 × 1016 H
Q
) versus dose (
D
/
0
cm
2
, followed by hydrogen thermal desorption (TD) [5].
Desorption of hydrogen has been found on TD heating of the
HOPG samples under mass spectrometer control (Fig. 2.22a).
As is shown in Fig. 2.22a, with the increase of the total hydrogen
doses (D) to which HOPG samples have been exposed the desorbed
hydrogen amounts (Q) increase and the percentage of D retained
in samples (Q) decreases toward a saturation stage. After TD, no
bumps were visible on the HOPG surface, the graphite surface was
atomically flat, and covered with some etch-pits of nearly circular
shapes, one or two layers thick (Fig. 2.22b). This implies that after
release of the captured hydrogen gas, the bumps-blisters become
empty of hydrogen and the HOPG surface restores back a flat surface
morphology under action of van der Waals forces [5].
It was also found [5] that the bumps-blisters on HOPG surface,
containing hydrogen gas, have been removed after 12-14 successive
STM scannings (ambient conditions), leaving behind flat graphite
surfaces with irregular etch-pits one or two layers thick in them, and
smaller if compared with those formed in the TD case. It is supposed
[5] that during successive STM scanning observations, in some bumps
the holes created by STM tip becomes larger when the hydrogen
gas escapes through them, because some carbon atoms from holes
edges accompany the hydrogen release, contributing to enlarge the
holes' sizes. These results are consistent with the disappearance of
the “protuberances” under successive STM scanning of a graphite
