Chemistry Reference
In-Depth Information
Figure 5.13. TMAFM image of a thin film (thickness
∼
1
m) of
α
-
p
-NPNN
µ
on NaCl(100): 500 nm
500 nm. Reprinted from
Surface Science
, Vol. 415,
J. Fraxedas, J. Caro, A. Figueras, P. Gorostiza and F. Sanz,
Dislocation hollow
cores observed on surfaces of molecular organic thin films: p-nitrophenyl nitronyl
nitroxide radical
, 241-250, Copyright (1998), with permission from Elsevier.
×
stress field accumulated in the hollow cores at distances above
R
hc
, which is in turn
a consequence of the weak intermolecular interactions and of the low rotational
barrier of the radical around the carbon-carbon bond that bridges the nitrophenyl
with the nitronyl nitroxide groups.
The decay of the stress field can be approximated by the following expression:
u
(
nR
hc
)
u
(0)
1
<
R
c
)
,
(5.20)
1
+
n
2
/
(1
+
2
R
hc
/
where
u
(0) and
u
(
nR
hc
) represent the strain energy density in the centre and at a
distance
nR
hc
from the hollow core, respectively, and
n
∈
N
.
2
In the case of
α
-
p
-
NPNN
R
hc
∼
7-10 nm and
R
c
<
25 nm, so that if we assume
R
hc
/
R
c
∼
1, at a
distance
nR
hc
∼
40-50 nm (
n
∼
5) from the centre of the hollow core, we obtain
u
(
nR
hc
)
9, thus a drastic reduction at a short distance. Compared to an
inorganic material such as YBa
2
Cu
3
O
7
−
δ
,
R
hc
∼
<
u
(0)
/
23 nm for thin
films (Klemenz, 1998) so that at distances as large as e.g., 2300 nm the stress field
is only reduced by a factor 1/3, which is manifested by regular shapes of the steps
at such large distances.
330 nm and
R
c
∼
2
This expression can be derived from Eqs. (5.17a), (5.17b) and (5.18) assuming that
R
F
>
0,
R
c
>
0 and
R
hc
>
0
(
R
H
>
0) and replacing
R
by
nR
hc
.
