Features of the Growth of p-Quaterphenyl Crystalline Films from Solution Drops on Substrates Keywords: p-quaterphenyl, 2D crystal growth, solutions, organic semiconductor crystalline films, sessile drop method, surface tension, wetting angle INTRODUCTION Linear conjugated semiconducting oligomers are of great interest for organic electronics due to their ability to form large single-crystal plates or films during growth from solutions [1–3]. Thin-film crystal structures have a great potential for the development of planar optoelectronic technologies, since they are not significantly inferior to bulk crystals in terms of their electron transport characteristics [4]. Ultrathin crystalline films of organic semiconductors with a thickness of one or several layers can be deposited on a substrate using the Langmuir–Blodgett method. The crystal structures formed in this way make it possible to combine efficient charge transfer and luminescence [5, 6]. Inkjet technologies for the formation of thin-film semiconductor coatings based on organic conjugated oligomers [7, 8] are of great interest in the manufacture of optoelectronic devices. Recent studies have shown that linear conjugated oligomers can crystallize at the interface upon the drying of solution drops on the substrate surface (sessile drop method) in the form of extensive single-crystal films with a thickness of one to several single layers [9–11]. Ultrathin single-crystal films formed by this method are characterized by a higher mobility of electric charges [10] in contrast to polydisperse films obtained using the Langmuir–Blodgett method [5]. However, the surface- mediated 2D crystallization of conjugated oligomers at interphase boundaries is generally a rather poorly studied phenomenon. Therefore, the disadvantages of this approach are practically uncontrollable nucleation, poorly controlled crystal growth in length and thickness, and, as a result, a deposited crystal film without clear localization in a given place on the substrate. It is known that the nucleation of 2D crystals can occur both on the contact line of a drop followed by the formation of a single-crystal film on the solution– air interface and its further deposition on the substrate during movement of the contact line as a result of solvent evaporation (the technique described in [10]) and on the substrate–solution interface far from the contact line [11]. There is the following information on the influence of the medium and external conditions on the formation of large thin-film crystals. The high surface tension of the solution and high viscosity promote the nucleation and growth of crystal films at the solution–air interface, and small bulk crystals are mainly formed at low surface tension of the solution and low viscosity [9, 10]. The authors of [11] report that the length of thin-film crystals increases and the thickness of thin-film crystals decreases with increasing temperature. However, there are practically no systematic data on the effect of the temperature factor on the growth features of crystalline films based on linear conjugated oligomers. This work is devoted to studying the growth of single-crystal films of p-quaterphenyl (4P) upon the slow evaporation of a sessile drop of solution on a glass substrate depending on the ambient temperature and the type of solvent. Studies on crystallization from solutions and the crystal structure of bulk single-crystal films and 4P plates were recently presented in [3]. In this study, p-quaterphenyl serves as a model object as it is a linear conjugated oligomer with a relatively low solubility under normal conditions (~0.2 g L–1 in toluene at 20°C). In the future, we plan to investigate p-quaterphenyl derivatives with various terminal substituents that improve the solubility and adsorption activity near the surface layer of the liquid phase. EXPERIMENTAL p-Quaterphenyl synthesized according to the technique described in [3] was used for the studies. Solutions of the substance in toluene (high purity grade) and 1,2,4-trichlorobenzene (chemical
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