Chemistry Reference
In-Depth Information
Au-covered AFM tip
S
S
S
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
S
S
S
Au surface
Figure 1.6. Schematic representation of a CT complex formation of a SAM of
TMPD on an Au-coated AFM tip and a SAM of TCNQ on an Au substrate.
Adapted from Skulason & Frisbie, 2002.
2
D
slope of the elastic part is close to the Debye stiffness
k
D
, defined as
k
D
=
=
m
(
k
B
D
/
h
)
2
, where
m
stands for the mean atomic mass.
k
D
can be considered as
the stiffness of the crystal, because
m
ω
D
is a measure of the temperature above which
all vibrational modes begin to be excited and belowwhich modes begin to be frozen
out. An approximate estimate of the cohesive energy can thus be obtained. In the
case of TTF-TCNQ, force plots obtained with nanoindentation with ultrasharp tips
have the shape shown in Fig. 1.7.
Below
Y
stands for the deformation at the plastic yield
point, the material responds elastically and above
δ
12
.
7 nm, where
δ
Y
δ
Y
plasticity is evidenced by dis-
crete discontinuities inmultiples of
0.9 nm. Such discontinuities correspond to the
distance between two adjacent
ab
-planes. From the elastic region we can evaluate
the maximum accumulated energy that TTF-TCNQ can withstand without irre-
versible deformation by simply integrating the force plot for 0
≤
δ
≤
δ
Y
. The inte-
3000 nm
3
in this case). Thus, the estimated maximum mean energy per TTF-TCNQ pair is
2.3 eV. This value compares well to the experimentally derived enthalpy of sub-
limation of TTF-TCNQ, which amounts to 2.7 eV per TTF-TCNQ pair (de Kruif
10
−
15
J. The affected volume is larger than
R
2
gral gives
∼
2
.
0
×
π
δ
Y
(
∼
