X-ray study of structural reorganization in phthalocyanine containing Langmuir–Blodgett heterostructures
Introduction
Phthalocyanines are molecules with semiconducting properties and they are considered interesting for applications in molecular electronics, in particular, as optoelectronic devices [1], [2], [3], as diodes and transistors [4], [5], [6] and as sensitive elements of gas sensors [7], [8], [9], [10], [11].
One of the most important goals in order to realize new generation of molecular electronic devices based on quantum principles, is to obtain structures with regular molecular resolution, realizing the so called ‘molecular architecture’. Langmuir–Blodgett (LB) technique [12], [13] is one of the best candidates for the formation of such structures. It allows to deposit not only regular monocomponent layers with molecular control of their thickness, but also to create complicated structures with desirable alternation of layers of different materials [14], [15].
However, the structure of the realized molecular systems must be carefully studied, as it can be different from the one desired. It is well known that the film structure undergoes reorganization during passing the meniscus of the air/water interface. Up to three monolayers can be involved in flip-flop motion in order to come to the energy minimum of the realized structure [16]. Moreover, some structural variations can take place during ageing or after some treatment. The most obvious factor varying the sample structure is temperature.
X-ray measurements are among the most powerful methods for studying the structure of LB films in the direction perpendicular to the film plane [17], [18]. Precise analysis of X-ray curves allows to draw models of the electron density profiles across the monolayer thickness [19]. However, it requires some homogeneity of the film. Nevertheless, such parameter as repetition unit thickness (spacing) can be obtained directly from the angular position of Bragg reflections in the X-ray pattern. If the sample contains regions with different molecular packing it will be revealed by the presense of two or more different systems of reflections in the X-ray curve.
Therefore, the aim of the study is to form heterostructures containing phthalocyanines and fatty acid salt layers. Phthalocyanine layers can be considered as functional elements of molecular structures, while fatty acid salt layers are passive separating elements allowing to place active units on fixed distances one from another. The work presents the results of the X-ray scattering study of the structure of such superlattices and their heat-induced reorganization.
Section snippets
Materials and methods
Tetra-tert-butyl-substituted copper phthalocyanine was synthesized according to the procedure described in [20], [21]. The model of the molecule is shown in Fig. 1. Arachidic acid was purchased from Sigma.
LB films were deposited with a two-sectional LB trough (MDT, Russia) [22]. Milli-Q purified water with a resistivity of 18.2 MΩ cm was used as a subphase and 0.1 mM CdCl2 was added to the subphase. Cadmium arachidate layers were transferred at 29 mN/m and phthalocyanine layers were transferred at
Results and discussion
X-ray pattern of the sample, containing 40 periods of the cadmium arachidate monolayer and phthalocyanine monolayer, is presented in Fig. 2. The pattern contains two reflections corresponding to the spacing of 5.06 nm. The thickness of the cadmium arachidate monolayer is about 2.7 nm [23]; therefore, the residual thickness of the repetition unit (phthalocyanine monolayer) is about 2.3 nm. Such thickness corresponds to the diagonal arrangement of the phthalocyanine molecules in the film (Fig. 1).
Conclusions
These preliminary results allowed to determine the orientation of phthalocyanine molecules in LB superlattices. Interesting structural reorganisation of the film was observed after the sample heating. First reorganization of the film structure was observed after fatty acid salt melting and the second one took place at higher temperature but before the phthalocyanine melting.
Acknowledgements
The authors wish to thank Dr. Borovkov for the provision of the phthalocyanines. Thanks also to INFM (National Institute for Matter Physics) for the financial support.
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