Recently, molecular materials have emerged as challenger to inorganic materials for the active layers in opto-electronic devices. However, if they require comparatively low operating voltages, it is still difficult and a major challenge to grow organic macroscopic single crystals. Therefore, these molecular materials are incorporated in the devices as ordered thin films rather than as single crystals. Different procedures, among which epitaxial growth, are used to elaborate ‘‘single-crystal like’’ thin films as perfect as possible. The macroscopic performances of these films rely strongly on their quality. Therefore, both their morphology and their structure have to be carefully characterised and understood. Among the various ex-situ techniques commonly available to control thin films, electron microscopy (EM) that can examine both the real and the reciprocal space is most appropriate. Combining the various EM modes one can visualise the crystal habits and morphology down to a few nanometres, the sizes, number and shapes of micro-defects or grain boundaries at the heart of the crystallites. The potentiality of EM as a versatile tool to characterise the fine structure of thin films is illustrated via the description of some distinctive cases of epitaxially grown thin films. We first recall the geometrical matching rules for usual epitaxy and then describe two cases of specific selection rules: a topographic selection for some polyolefins, a molecular selection for some diacetylenes. Finally, the morphology, structure and quality of ordered thin films for opto-electronic will be reported. Though very diverse at first sight, the four chosen examples illustrate the process of understanding step by step the epitaxial growth of organic thin films on orienting substrates. These examples demonstrate that the origin of the orientation of a crystal onto another crystal can be very diverse. The major and ultimate goal is to yield large well-oriented thin films based on polymer or molecular crystals. This can only be achieved by a careful selection of the depositsubstrate pair. Such films will optimise the opto-electronic response of organic materials at the macroscopic scale, a prerequisite for future development.
Thierry, A., Wittmann, J., Lotz, B., da Costa, V., Le Moigne, J., Campione, M., et al. (2004). Organisation, structure and morphology of organic thin films via electron microscopy. ORGANIC ELECTRONICS, 5(1-3), 7-22 [10.1016/j.orgel.2004.04.003].
Organisation, structure and morphology of organic thin films via electron microscopy
CAMPIONE, MARCELLO;BORGHESI, ALESSANDRO;SASSELLA, ADELE;
2004
Abstract
Recently, molecular materials have emerged as challenger to inorganic materials for the active layers in opto-electronic devices. However, if they require comparatively low operating voltages, it is still difficult and a major challenge to grow organic macroscopic single crystals. Therefore, these molecular materials are incorporated in the devices as ordered thin films rather than as single crystals. Different procedures, among which epitaxial growth, are used to elaborate ‘‘single-crystal like’’ thin films as perfect as possible. The macroscopic performances of these films rely strongly on their quality. Therefore, both their morphology and their structure have to be carefully characterised and understood. Among the various ex-situ techniques commonly available to control thin films, electron microscopy (EM) that can examine both the real and the reciprocal space is most appropriate. Combining the various EM modes one can visualise the crystal habits and morphology down to a few nanometres, the sizes, number and shapes of micro-defects or grain boundaries at the heart of the crystallites. The potentiality of EM as a versatile tool to characterise the fine structure of thin films is illustrated via the description of some distinctive cases of epitaxially grown thin films. We first recall the geometrical matching rules for usual epitaxy and then describe two cases of specific selection rules: a topographic selection for some polyolefins, a molecular selection for some diacetylenes. Finally, the morphology, structure and quality of ordered thin films for opto-electronic will be reported. Though very diverse at first sight, the four chosen examples illustrate the process of understanding step by step the epitaxial growth of organic thin films on orienting substrates. These examples demonstrate that the origin of the orientation of a crystal onto another crystal can be very diverse. The major and ultimate goal is to yield large well-oriented thin films based on polymer or molecular crystals. This can only be achieved by a careful selection of the depositsubstrate pair. Such films will optimise the opto-electronic response of organic materials at the macroscopic scale, a prerequisite for future development.
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