Chemistry Reference
In-Depth Information
sensitive because the interaction of a helium atom with a surface is dominated by
a long range van der Waals-like attraction and a shorter range repulsive interaction
due to electronic overlap between the helium and the surface species.
An example of
in situ
real-time characterization of the growth of thin films of
the radical
p
-NPNN in high vaccum will be discussed in detail in Chapter 5.
Liquid environment
The growth of ordered MLs in liquids can be
in situ
characterized with SPMs. One
of the great advantages of SPMs is that they give real-space images with very high
resolution in a variety of environments. In addition they can provide information
on physical properties such as electrical conductivity, friction, mechanical resis-
tance, etc. Operating under fluid allows imaging of samples in chemically active or
electrochemical environments. For such studies, AFMs have become widely used,
although many scanning tunnelling microscopy studies have been undertaken. The
main reason lies in their working principle, force versus tunnelling, implying the
absence or action of an electric field for the AFM or STM, respectively. In the case
of the STM, the presence of such a field may strongly perturb the growth conditions.
With an AFM, capillary forces, so important in air, are reduced when operating in
a fluid.
In Section 4.2 we discussed the examples of the
in situ
characterization of MLs of
cyclotrimeric terthiophenediacetylene (Mena-Osteritz & Bauerle, 2001) and of the
isomers of bis-cyanoethylthio-bis-octadecylthio-TTF (Gomar-Nadal
et al.
, 2003) at
the liquid/HOPG interface with scanning tunnelling microscopy. We end this chap-
ter with an example of
in situ
studies performed with an AFMalso at a liquid/HOPG
interface. The experiments were performed in CH
3
CN with a single compartment
fluid cell made of quartz with ports for fluid entry and exit. Freshly cleaved HOPG
substrates were used as electrodes and platinum counter and silver reference elec-
trodes were inserted through the outlet port of the fluid cell. Under these conditions
layers of (BEDT-TTF)
2
I
3
have been prepared and studied (Hillier
et al.
, 1994). In
this case the choice of experimental conditions influences the selectivity toward
the
polymorphs of (BEDT-TTF)
2
I
3
. High (low) overpotentials favour the
formation of the
α
and
β
α
-(
β
-)phase and electrochemically etched HOPG favours the for-
mation of the
-phase forms on pristine HOPG. AFM images
allow a topographical view of the layer as well as details of the structure when
operating in high resolution. In this case the unit cell of the layer can be obtained,
within the experimental limitations of the technique, allowing in some favourable
cases to differentiate between known crystallographic phases.
α
-phase while the
β
