Biomedical Engineering Reference
In-Depth Information
It must be emphasized that outgassing a material of high surface energy is not suitable to
prevent adventitious contamination of the surface. Materials cleaned with UV-ozone
treatment and showing a water contact angle below 5° reached an appreciable contact angle
(20° for silica, 40° for stainless steel and gold) after a stay of 5 minutes in the vacuum of the
spectrometer chamber. The high rate of contamination may be due to evacuation itself,
owing to the increased proportion of organic compounds in the residual gas and to their
increased rate of transfer (Caillou et al., 2008).
4.4. Thickness of the organic adlayer
No variation of the relative concentrations of organic compounds was revealed by angle-
resolved XPS analysis (Figure 4) but an effect of stratification may be masked by the surface
roughness. The evaluation of the adlayer thickness may clarify whether the increase of the
silane or protein concentration resulting from the respective treatments reflects an addition
of or a substitution by new compounds in the adlayer.
If the surface is considered as atomically smooth and the organic layer is continuous with a
constant thickness, the apparent concentration ratio [
C
]/[
Cr
] may be computed using the
following equation:
⎡
⎤
⎛
⎞
t
λ C1 p
λ cosθ
−
Org Org
CC
−
⎢
⎜
⎟
⎥
[]
[]
Org
C
C
i σ
⎢
⎥
⎣
⎝
⎠
⎦
=
Cr
C
(8)
Cr
i σ
t
λ Cexp
λ cosθ
⎛
−
⎞
C
r
Ox
Ox
⎜
⎟
Cr
Cr
Org
Cr
⎝
⎠
i
C
and i
Cr
are the relative sensitivity factors of C and Cr, respectively, provided by the
spectrometer manufacturer. The photoionization cross sections σ are 1 for C 1s and 11.7 for
Cr 2p (Scofield, 1976). The superscripts Org and Ox designate the organic adlayer and the
passive oxide layer, respectively. The concentration of Cr in the oxide layer (
Org
C
C ; between
0.015 and 0.020 mol.cm
-3
, depending on the sample), was determined on the basis of the
above discussion indicating that the oxide layer is constituted with FeOOH, CrOOH,
Ni(OH)
2
and MoO
3
(
section 4.1
). Note that this is in agreement with a concentration of
inorganic oxygen close to (
O
tot
−
O
org
), owing to the low concentration of silane. The
concentration of carbon in the organic adlayer was determined on the basis of the surface
composition modeled as detailed above, using the densities given in Table 3. The density of
silane, was taken as the average between the densities of 3-aminopropyl(trimethylsilane)
(0.8 g.cm
-3
) and 3-aminopropyl(trimethoxysilane) (1.0 g.cm
-3
). The electron inelastic mean
free paths (IMFP) were calculated using the Quases program (http://www.quases.com
)
and the TPP2M formula (Tanuma et al., 1997), considering the matrix composition deduced
above. For
C
λ
, values of 2.04 and 1.99 nm were computed for FeOOH and CrOOH,
respectively (energy gap 2.3 and 1.6 eV, respectively). Considering an energy gap of 6 eV for
the organic compounds, respective values of
Ox
Org
C
λ
were 3.79 and 3.04 nm in
hydrocarbon-like compounds [(CH
2
)
n
], 3.59 nm and 2.89 nm in protein, 3.67 and 2.00 nm in
polysaccharides [(C
6
(H
2
O)
5
)
n
], and 3.68 and 2.95 nm in silane [H
2
N(CH
2
)
3
SiO
1.5
].
The thickness of the organic adlayer deduced for photoelectron collection angles
θ
= 0° and
60° is given in Table 4. The difference between the values computed for the two
C
λ
and
Org
