Chemistry Reference
In-Depth Information
experiments. Along the course of an EC experiment in solution, where neutral or
charged species are delivered in a steady-state mode to the working electrode, the
limiting kinetic step is essentially the electron transfer rate. In a confined envi-
ronment, the molecular species will progress very much more slowly to the active
electrode vicinity. For the growth of RCSs it is therefore indicated to deposit a
thin film of neutral
π
-donor molecules on the substrate surface, e.g., by sublima-
tion under vacuum, prior to immersion in the electrolyte solution, resulting in a
combined dry-wet process (Miura
et al.
, 1996; Deluzet
et al.
, 2002b). Hence, the
effect of slow movement of the electroactive species between the two substrates
is counterbalanced by a sizeable concentration of neutral donor molecules at the
anode vicinity.
In CEC, the set value of the constant current density has no effect on the nucle-
ation of the RCSs because of the high current densities achieved. Indeed, with a few
µ
m of gold deposits acting as the anode, for a given constant current intensity, the
actual current density is typically three orders of magnitude higher than when using
a standard platinum wire of 1 mm in diameter. Thus, in order to achieve identical
current density, a set current of 1
µ
A in the confined experiment configuration ought
to be increased to 1 mA in the classical EC cell. For example, EC of EDT-TTF in
the presence of the polyoxometallate anion [PW
12
O
40
]
−
3
has been achieved in a
CEC experiment with a set constant current of 1
µ
A. A similar experiment con-
ducted in standard EC conditions in solution also yielded single crystals, this time
using a constant current set at 100
µ
A in order to ensure that the current density be
kept as constant as possible between the two experiments. In both cases, the same
phase, formulated (EDT-TTF)
3
[PW
12
O
40
](CH
3
CN)(CH
2
Cl-CHCl
2
) was obtained
(Deluzet
et al.
, 2002b).
In order to increase the thin crystal dimensions, a CEC experiment should be
run at temperatures typically higher than those for the parent EC in solution. This,
however, has its limit when one observes that the superconducting
-
(BEDT-TTF)
2
Cu(NCS)
2
is selectively grown at 298 K, while the insulating phase,
(BEDT-TTF)Cu
2
(NCS)
3
, is obtained instead at 333 K, all other parameters being
identical.
Elevated temperatures favour fully oxidized over mixed valence formula-
tions
. Figure 3.5(b) shows a scanning electron microscopy (SEM) image of a thin
(BEDT-TTF)Cu
2
(NCS)
3
single crystal.
Thin crystals of
κ
-phase,
κ
6K
at ambient pressure, which forms bulk crystals with a parallelepiped morphology in
standard EC experiments, have also been obtained with CEC displaying the same
transition (Deluzet
et al.
, 2002b).
In addition, in Section 6.4 we shall see how CEC has allowed the preparation of
polymorphs of BFS, a significant example because only one crystallographic phase
was known, after countless EC syntheses in many laboratories around the world
κ
-(BEDT-TTF)
2
Cu[(N(CN)
2
Br], a 2D metal with
T
c
=
11
.
