Chemistry Reference
In-Depth Information
In
the
homogeneous
case
γ
=
0
and
γ
⊥
=
2
γ
⊥
nv
since
γ
⊥
nv
+
γ
⊥
n
σ
−
γ
⊥
σ
v
=
0, thus:
γ
2
⊥
h
nv
µ
G
c
=
,
(5.4a)
=
γ
⊥
nv
µ
.
L
c
(5.4b)
Finally,
G
c
for the homogeneous case in the absence of steps is obtained under the
conditions
γ
=
0 and
γ
⊥
=
4
γ
⊥
nv
, which results in:
2
⊥
4
h
γ
nv
µ
G
c
=
,
(5.5a)
2
γ
⊥
nv
µ
=
.
L
c
(5.5b)
As mentioned before, low adhesion is expressed by
γ
>
0. In the case of
growth
L
c
>
0, which imples that
h
µ
−
γ
>
0, so that
µ >
0if
γ
≥
0.
Conditions
0 correspond to the Volmer-Weber mechanism
of growth (Chernov, 1984). Strong adhesion or complete wetting (Frank-van der
Merwe mode) implies that
γ
>
0 and
µ >
γ
<
µ <
0 and
0, while for the Stranski-Krastanov
mode
γ
<
0 and
µ <
0 for the first 2D layers and
γ
>
0 and
µ >
0 for
the 3D crystal growth.
The in-plane velocity
v
ip
can be approximated by:
∼
∂
∂
t
∝−
∂
L
G
∂
L
.
v
(5.6)
ip
L
∗
and that
It can be easily shown that
v
ip
∼
h
γ
⊥
at
L
=
v
ip
becomes independent
L
∗
, where
L
∗
is obtained under the condition
G
(
L
∗
)
of
γ
⊥
for
L
=
0 and is
given by
L
∗
=
2
L
c
. Hence,
v
ip
depends linearly on
γ
⊥
at the earlier stages of
L
∗
. In the case of 3D growth
we observe from Eq. (5.2a) that for a given positive
growth. In the absence of steps
v
ip
∼
4
h
γ
⊥
nv
at
L
=
µ
value,
G
c
decreases as
γ
→
γ
=
γ
nv
+
γ
ns
−
γ
sv
, in the low molecule-surface interaction
limit of physisorption (low
0. Since
γ
sv
), the
γ
→
0 condition will be fulfilled for low
γ
nv
and
γ
ns
values. Hence, nuclei with molecular planes of lower interface tension
parallel to the substrate have higher probabilities of formation at the beginning of the
nucleation process because of lower nucleation barriers and they grow faster since
γ
⊥
nv
γ
ns
values imply that the molecular planes possess strong
in-plane intermolecular interactions (the molecules are more strongly bound), which
results in weak interaction between adjacent molecular planes.
PTCDA films OMBD-grown on Ag(111) single crystals show a transition from
layer-by-layer growth with smooth and small-grained morphology towards the
>γ
nv
.Low
γ
nv
and
