Biomedical Engineering Reference
In-Depth Information
In this context it is relevant to underline, for instance, the results
[16] of molecular dynamic simulations on hydrogen molecules
liquefaction upon deformed external surfaces of single-walled carbon
nanotube (SWNT) bundles (at 80 K and 10 MPa). These studies are
relevant for the interpretation of the well-known experimental
finding for SWNT bundles [17].
It is also relevant to compare data from [5] with related results
from [9], regarding the graphite surface modifications induced by
interaction of hydrogen or deuterium atoms with perfectly crystalline
highly oriented pyrolytic graphite (HOPG) surfaces. Surface properties
have been probed [9] with high-resolution electron-energy-loss
spectroscopy (HREELS) revealing the formation of C-H units with
different vibrational energies. Comparison with calculations [18] by
density functional theory (DFT) led the authors [9] to establish the
models for hydrogen adsorption processes at the graphite surface. It
has been shown in Ref. [9] that the vibration at 295 meV is due to a
single H atom bonding to graphite (C-H), while the vibrations at 331
and 345 meV (and higher energy losses) are, respectively, related
to dimmer and quartet formation, or more generally, to a higher
number of clustering atoms (i.e., hydrogen clusters formation).
Subsequently, studies have been performed [9], using scanning
tunneling microscopy (STM). From the electronic point of view [9],
as hydrogen locally disturbs the electronic density near the Fermi
level, a charge density confinement has been observed between
the hydrogen clusters, particularly at low tip-sample bias voltage.
It has been also observed that after iteratively scans of the tip on
the same surface area, protuberances, which were attributed to the
hydrogen presence, eventually disappeared, evidencing a hydrogen
desorption. The authors in Ref. [9] assumed that the desorption
phenomenon derives from mechanical interaction between the tip
and the graphite surface.
It is also necessary to take into account the experimental data
[6-8] on hydrogen thermo-desorption (TD) from HOPG exposed
to atomic hydrogen at near-room temperatures (as in Ref. [5]),
and their thermodynamic analysis [3, 4]. Such treatment provoked
the protruding of surface nano-blisters with heights of 3-5 nm
and diameters of 40-75 nm, most of which disappeared after
hydrogen TD, that proceeded as a first-order reaction [7, 8]. The TD
measurements, carried out at a heating rate of
υ
= 25 K/s, revealed
two TD peaks: a peak
α
centered at
T
α
≈ 1123 K (half width height
∆
T
α
≈ 180 K; fraction of the total spectrum area
S
α
/
S
≈ 0.45; activation
Σ
