Market leaders in temperature controlled
microscopy, Linkam Scientific Instruments, report on the work of Professor Yves
Henri Geerts from the Université Libre de Bruxelles where he uses a specially
designed temperature stage to study crystallisation processes in
opto-electronic thin films.
The Laboratory of Polymer Chemistry at the Université Libre de Bruxelles,
unlike the name of the laboratory might suggest, is focused on research into
"small molecules", namely, liquid crystalline semiconductors for
organic electronics application. Various organic semiconductors have been
receiving a great deal of attention in "plastic" electronic devices
such as organic photovoltaic cells, light-emitting diodes (OLED) and field
effect transistors (OFET). The important physical parameters of
microcrystalline films are strongly affected by dimensions of domains and
domain boundaries, while large defect-free single crystals are difficult to
fabricate and inappropriate for practical applications. At the same time,
liquid crystals have been recognized as a new type of organic semiconductors,
as they are capable to self-healing of structural defects and to
self-organization in large structurally homogeneous domains. Influence of
domain boundaries, if any, on carrier transport in liquid crystalline phases is
very small.
Professor Yves Henri Geerts and his colleagues in Bruxelles have undertaken a
study of single crystal thin films of terthiophene, the building block for the
organic semi-conductor polythiophene, by directional crystallization by means
of a thermal gradient using the Linkam GS350 stage. The background to this work
is to better understand how molecular structure and supramolecular organization
affects optoelectronic properties. These can also be affected by the method of
fabrication, therefore determining a method to control deposition and
crystallisation is important. As part of his research, Professor Geerts used
polarized optical microscopy (POM) and X-ray diffraction to characterise the
shape, size, and orientation (in and out of the plane of the substrate) of the
crystals produced by the thermal gradient technique. He found temperature
gradients could potentially be used to control crystal growth and these
conditions induce a preferential fast growth direction perpendicular to the
gradient direction. In addition it is found that nucleation and growth can be
decoupled for OSC crystallizing from the melt in a temperature gradient and
that these conditions lead to the generation of highly textured thin films with
uniaxial in-plane orientation of crystallites.
The Linkam GS350 was chosen for this work for its ability to accurately
programme temperature gradients across the sample. It has two heating elements
which are perfectly aligned to ensure uniform thermal contact between the
temperature-controlled surface and the sample media. The heating elements are
separated by a 2.5mm gap and can be controlled to 0.1°C from -196 to 350°C
allowing large, precise temperature gradients to be set up.
The accompanying T95-Linksys controller and Linksys 32 software enables the
precision stepper motor to control the position and the speed of sample
movement between the two elements and can be used to determine speed of crystal
growth and allow the crystallisation front to remain in the field of view. The
stepper motor control also enables extremely fast heating or cooling by quickly
transferring the sample from one element to the other.
Visit Linkam at
http://www.linkam.co.uk/
and learn about the broad range of applications in the field of temperature
controlled microscopy.
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