Chemistry Reference
In-Depth Information
(Debe &Kam, 1990). It remains to be seen whether gravity is ultimately responsible
for such behaviour or if subtle differences in the experimental conditions are the
cause.
3.3 Patterning
Micrometre and nanometre-sized artificial structures based on MOMs can be pro-
duced by patterning with the goal of devising technological applications. This is a
necessary step to adapting such materials into real devices. Because of the molec-
ular character of these materials the conventional lithographic procedures used for
inorganic materials and polymers (electron beam, UV irradiation, chemical etch-
ing, etc.) can hardly be applied, hence several soft (non-aggresive) strategies are
being developed. Here we briefly review some of these strategies and illustrate
some selected examples.
Micropatterning
Micrometre-scale patterned thin films of small molecular weight organic semicon-
ductors can be achieved by organic vapour phase deposition (Shtein
et al.
, 2003).
This growth technique is essentially type II CVD, or OCVD, described before, but
using only one type of molecule and cooling the substrate. In the literature it is
quite common to find many different names to describe very similar processes. For
researchers not familiar with such techniques it can be puzzling at the beginning.
Patterned structures are achieved by means of masks placed close to the substrates.
It is clear that the optimal experimental conditions (pressure of the carrier gas,
distance mask-substrate, temperature, etc.) have to be met in order to produce effi-
cient structures. For instance, the background pressure of the carrier gas has to be
of the order of the mask dimensions (0.1-10 Torr approximately corresponds to a
mean free path of 100-1
ยต
m). This technique has been successfully applied e.g., to
Alq
3
.
Adifferent way of preparing patterned films is by using patterned substrates. Such
substrates can be prepared by well-controlled lithographic techniques or simply by
mechanical preparation. The most common example is the case of PTFE films
obtained by friction transfer onto smooth substrates such as silicon, glass, ITO, etc.
PTFE films are easily prepared by sliding a PTFE rod at a constant pressure against
a clean glass slide held at 573 K. The PTFE surfaces consist of a succession of
atomically flat areas and oriented few nm high steps separated by typically 25 to
500 nm. PTFE films are chemically inert, thermally stable up to
c
. 600 K, optically
transparent in the visible range and, very important, of low cost. The oriented PTFE
films induce the preferential orientation of organic films grown on top of them,
