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Polymorph-Selective Preparation and Structural Characterization of Perylene Single-Crystals Andre Pick, Michael Klues, Andre Rinn, Klaus Harms, Sangam Chatterjee, and Gregor Witte Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 7, 2015
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Crystal Growth & Design
Polymorph-Selective Preparation and Structural Characterization of Perylene Single-Crystals André Pick,† Michael Klues,† Andre Rinn,† Klaus Harms,‡ Sangam Chatterjee†,§ and Gregor Witte†,§* †
Faculty of Physics, Philipps-Universität Marburg, D-35032 Marburg, Germany Faculty of Chemistry, Philipps-Universität Marburg, D-35032 Marburg, Germany § Materials Sciences Center, Philipps-Universität Marburg, D-35032 Marburg, Germany ‡
ABSTRACT: Organic semiconductors occurring in polymorphic structures represent excellent model systems for fundamental studies of optoelectronic excitations in different crystalline configurations. Perylene is an archetypal polycyclic aromatic hydrocarbon appearing in two polymorphs known as α- and β-phases which adopt different molecular packing motifs. However, the growth of high quality single-crystals with appropriate sizes and polymorph selectivity remains challenging. In this study, we compare various approaches towards a polymorph-selective perylene single-crystal growth. Though crystals of both polymorphs are obtained from toluene solution (either by cooling of saturated solution or by evaporation of solvents) they exhibit numerous defects and their size cannot be precisely controlled. Vapor deposition and re-sublimation favors the formation of α-crystals which can be rationalized by a newly identified thin-film phase forms initially. Further, we demonstrate that organic molecular beam deposition onto silicone-oil covered substrates enables the fabrication of high quality crystals of both phases. The relative occurrence of the individual polymorphs is controlled by the actual deposition parameters. Combining the results of X-ray diffraction, atomic-force microscopy, and fluorescence analysis enables an unambiguous polymorph identification solely based on the characteristic crystal shape. The morphological characterization reveals characteristic screw-dislocations at crystals grown from solution or by re-sublimation while the liquid mediated crystals exhibit exceptionally flat surfaces and enable detailed fluorescence studies without defect-related emission signals.
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INTRODUCTION Organic semiconductors and dye materials are presently receiving significant research interest because of their promising potential for the fabrication of future optoelectronic devices such as, e.g., organic light emitting diodes or organic photo-voltaic cells [1,2]. Besides their advantages of low cost, high quantum yields, flexibility, and low temperature processing, organic materials conceptually allow a tuning of optoelectronic properties by chemical design [3,4], while this task requires complex band structure engineering in the case of their inorganic counterparts. However, it should be noted that the optical and electronic properties as well as the dynamics of excited electronic states of molecular solids are distinctly different from that of single molecules (or solutions) due to intermolecular coupling [5]. Striking examples for such solid-state effects are the formation of exciton-states or efficient charge-carrier transport. Well defined model studies are mandatory to develop a microscopic understanding of the elementary excitations since intrinsic properties of molecular solids are frequently masked by impurities or structural defects. Studies on single crystalline samples are of particular interest along this direction of research as they enable the direct correlation between the molecular packing and the resulting electronic properties of solids [6-9]. Apart from the difficulty of growing suitably sized single crystals allowing for optical or electronic measurements, an additional complexity arises from the fact that molecular materials can exist in different crystalline forms (so-called polymorphisms) [10] which affects the resulting optoelectronic and charge transport properties of the molecular solids [11-15]. Various strategies to control polymorphism in organic crystals have been reported including inter alia variation of thermodynamic growth conditions, seeding technique, use of surfactants or crystallization under nanoscale confinement [16-19]. Despite significant research efforts, a rational approach of phase selective crystallization, however, still remains challenging and many attempts result in crystalline mixtures of various polymorphs.
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Crystal Growth & Design
In the present study we focus on the phase-selective crystallization of perylene (C20H12), a widely used dye molecule and also an organic semiconductor with notable charge carrier mobility [20,21]. Perylene is of particular interest for optoelectronic case studies of organic solids as it crystallizes in two monoclinic phases, which both reveal P21/c-symmetry: the α-polymorph (a=10.24Å, b=10.79 Å, c=11.13 Å, α=90°, β=100.92°, γ=90°) contains four molecules per unit cell and is packed in a sandwich-herringbone motif (also considered as dimeric structure), while the β-polymorph (a=9.76Å, b=5.84 Å, c=10.61 Å, α=90°, β=96.77°, γ=90°) has two molecules within the unit cell arranged in the so-called γ-type herringbone packing (monomeric structure) [22]. Both are schematically depicted in Fig. 1.
Figure 1: (a) Molecular structure of perylene and molecular arrangements adopted in the two polymorphs: (b) α-phase and (c) β-phase [22]. The viewing direction is perpendicular to the (bc)-plane (i.e. along a*). The complete crystallographic structure data are listed in the Supporting Information.
As a consequence of the various molecular packing motifs and intermolecular couplings both polymorphs exhibit different luminescence and absorption properties [23-25]. In addition, significant differences in the charge-carrier mobility are proposed for both phases [26]. Unfortunately, a direct experimental verification of this proposal as well as detailed optical measurements are, however, hampered by the lack of appropriate crystals of the β-polymorph. This problem is partly related to the fact that perylene crystals reveal an irreversible phase transition from the β-phase to the α-phase while the latter is stable up to the melting point [22]. So far, perylene crystal growth are reported either from solution [22,23,27] or by sublimation method [24,25,28,29], both resulting in a preferred crystallization of the α-phase. By pipetting saturated droplets of a perylene solution onto glass, followed by rapid cooling, Yago et al. [30,31] observe a preferential crystallization of the β3 ACS Paragon Plus Environment
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phase, hence indicating this process to be kinetically driven, rather than thermodynamically. Another route to control the phase of perylene is reported by Lei et al. [32], who utilized cetyltrimethylammonium bromide (CTAB) as a surfactant upon crystallization. Though this yields individual crystals of both phases depending on the actual CTAB-concentration, only micron-sized crystallites are obtained which still hamper detailed optical studies. More recently, Urbelis and Swift [33] reported a polymorph-selective crystallization of perylene from solution onto supports coated before by self-assembled monolayers of different chemical termination. So far, most optical studies of perylene samples were performed without any microscopic characterization of the crystals surface and morphology, thus neglecting the influence of defects. In addition, we note that the crystalline shapes (so called tracht) of perylene crystals often have not been correctly described, so that belonging crystallographic directions were wrongly assigned [24,28,32]. Here, we compare different crystallization procedures (including re-sublimation and solution growth) and demonstrate a reliable method allowing for a controlled preparation of well-defined perylene-crystals of both polymorphs. By employing liquid mediated growth [34] under high vacuum conditions and optimizing growth parameters, polymorph-selective crystallization is achieved yielding highly ordered, platelet-like perylene single crystals with lateral extensions of more than 100 µm. Combining X-ray diffraction (XRD) and atomic force microscopy (AFM) allowed an unambiguous correlation between crystallographic axes and the tracht of the respective single crystals. The exceptional quality of selected platelet-like crystals is also affirmed by AFM data showing virtually molecularly flat surfaces with very few monomolecular steps, while pyramidal crystals obtained from vapor deposition exhibit characteristic screw dislocations. Finally, the well-ordered crystallites were used for exemplary photoluminescence measurements to quantify the prevailing characteristic color impressions.
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Crystal Growth & Design
EXPERIMENTAL SECTION Epi-ready silicon-wafers coated with a native oxide (Siegert Wafer GmbH) or object glass slides are used as supporting substrates. Initially, all supports are cleaned in an ultrasonic bath by a mixture of ethanol and acetone and subsequently blown dry in a nitrogen stream. Films and crystallites of perylene (Sigma-Aldrich, purity 99.9%) are grown under HV conditions (
