Biomedical Engineering Reference
In-Depth Information
(a)
150
100
50
0%
5
7.6%
9.5%
10.5%
0
0
200
400
600
Anneal temperature (°C)
(b)
0.25
1.00
0.15
0.05
0.50
-0.05
-0.15
HMEC
CPD
-0.25
0.00
0
2
4
6
8
10
% Si
FIGURE 2.26
(a) Attachment of HMEC (after 6 h) as a function of anneal temperatures and silicon content; (b) contact poten-
tial difference (CPD) variation with silicon content compared with HMEC count (normalized). (Reprinted with
permission from Okpalugo et al.,
Diamond Rel. Mater.
, 13, 1088, 2004.)
a reduction in the a-C:H(Si) work function, due to an increased sp
3
fraction or a change in
the density of surface charge [89].
HMEC attachment on a-C:H is lower than a-C:H(Si) and peaks at 300°C, beyond the
onset of graphitization (as reviewed by an increasing
I
D
/
I
G
ratio of Raman spectroscopy),
whereas attachment seems constant up to 600°C on a-C:H(Si). Without silicon, it appears
that an optimal film structure exists at approximately 200-300°C, which promotes HMEC.
It may be that loss of hydrogen is required for optimal attachment surfaces, but that con-
tinued hydrogen loss and the consequent conversion to sp
2
bonding has a negative effect.
The polar nature of the surface is clearly modified with the introduction of silicon. Contact
angle measurements show a change in surface structure from polar to dispersive and an
increase in hydrophobicity. It may be that in a-C:H(Si), the nature of the surface remains
sufficiently hydrophobic even as the sp
2
/sp
3
ratio increases at high anneal temperatures.
In hydrogenated amorphous carbon films subjected to post-deposition nitrogen doping,
Okpalugo et al. [90] found that the introduction of nitrogen atoms/molecules to the a-C:H
structures leads to an atomic structural change favorable to the attachment of human
microvascular endothelial cells. In this study a “sweep plate” was employed to attract
charged ions, so these are excluded from the doping species (samples named SN). Nitrogen
doping therefore involved either atomic species (named SN) or a mixture of nitrogen atoms
and ions (named N), via post deposition exposure to the flux for up to 2.5 h.
