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Exceptional Dewetting of Organic Semiconductor Films: The Case of Dinaphthothienothiophene (DNTT) at Dielectric Interfaces Tobias Breuer,*,† Andrea Karthaü ser,† Hagen Klemm,‡ Francesca Genuzio,‡ Gina Peschel,‡ Alexander Fuhrich,‡ Thomas Schmidt,‡ and Gregor Witte† †
Fachbereich Physik, Philipps-Universität Marburg, 35032 Marburg, Germany Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, 14195 Berlin, Germany
‡
S Supporting Information *
ABSTRACT: The novel organic semiconductor dinaphthothienothiophene (DNTT) has gained considerable interest because its large charge carrier mobility and distinct chemical robustness enable the fabrication of organic field effect transistors with remarkable long-term stability under ambient conditions. Structural aspects of DNTT films and their control, however, remain so far largely unexplored. Interestingly, the crystalline structure of DNTT is rather similar to that of the prototypical pentacene, for which the molecular orientation in crystalline thin films can be controlled by means of interfacemediated growth. Combining atomic force microscopy, near-edge X-ray absorption fine structure, photoelectron emission microscopy, and X-ray diffraction, we compare substrate-mediated control of molecular orientation, morphology, and wetting behavior of DNTT films on the prototypical substrates SiO2 and graphene as well as technologically relevant dielectric surfaces (SiO2 and metal oxides that were pretreated with self-assembled monolayers (SAMs)). We found an immediate threedimensional growth on graphene substrates, while an interfacial wetting layer is formed on the other substrates. Rather surprisingly, we observe distinct temporal changes of DNTT thin films on SiO2 and the SAM-treated dielectric substrates, which exhibit a pronounced dewetting and island formation on time scales of minutes to hours, even under ambient conditions, leading to a breakup of the initially closed wetting layer. These findings are unexpected in view of the reported long-time stability of DNTT-based devices. Therefore, their future consideration is expected to enable the further improvement of such applications, especially since these structural modifications are equivalently observed also on the SAM-treated dielectric surfaces, which are commonly used in device processing. KEYWORDS: organic semiconductors, DNTT, NEXAFS, atomic force microscopy, interfaces, PEEM, thermal evolution, thin films
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INTRODUCTION On the route to the fabrication of novel electronic devices based on organic semiconductors (OSCs), large efforts were made to identify new materials for such applications and understand their physicochemical properties.1−4 While the electronic energy levels of molecular entities can be predicted prior to the synthesis process and various strategies have been developed to specifically modify characteristics of a parent OSC,5,6 the actual performance of devices is largely determined by morphological and solid state properties of the thin films.7−13 As the latter properties are rather hard to predict,14 adequate microstructural experimental analyses are required. Several structure−property relationships have been deduced for OSCs, underlining the strong implications of crystal polymorphism, crystallinity, and morphology on the final device characteristics.15,16 Of particular interest in this regard is the molecular orientation in thin films because the shape anisotropy of the molecular entities leads to anisotropic packing of molecular solids. As a consequence, also fundamental optoelectronic characteristics such as charge © 2017 American Chemical Society
carrier mobility or optical absorption which are decisive for organic field effect transistor (OFET)17−20 and organic photovoltaic (OPV) applications21,22 depend critically on the molecular alignment. Among the newly synthesized OSCs, dinaphthothienothiophene (DNTT, C22H12S2) has gained particular interest as it reveals a remarkably high charge carrier mobility of 8 cm2/(V s).23,24 Moreover, the fabricated OFETs exhibit an improved long-term stability25 as compared to previous OSC benchmark systems based on pentacene and rubrene.26−30 While previous studies on DNTT have focused mainly on realizations of OFETs24,31−35 and demonstrate its large potential for device applications, a comprehensive study of structural properties of the relevant thin films is also required to gain a detailed understanding of the observed device properties. Interestingly, the crystal packing motif of DNTT23 closely resembles that of Received: December 21, 2016 Accepted: February 20, 2017 Published: February 20, 2017 8384
DOI: 10.1021/acsami.6b15902 ACS Appl. Mater. Interfaces 2017, 9, 8384−8392
Research Article
ACS Applied Materials & Interfaces
diffraction was utilized to characterize their crystalline structure with a Bruker D8 Discovery diffractometer using Cu Kα radiation. C 1s NEXAFS and X-ray photoelectron spectroscopy (XPS) measurements were performed at the HE-SGM dipole beamline of the synchrotron storage ring BESSY II of the Helmholtz Center Berlin (Germany) providing linearly polarized light (polarization factor = 0.91) and an energy resolution at the carbon K-edge of about 300 meV. All NEXAFS spectra were recorded in partial electron-yield (PEY) mode. To determine the average molecular orientation relative to the sample surface, NEXAFS spectra were recorded at different angles of incidence (30°, 55°, and 90°). Details on the experimental setup and data evaluation are provided in the literature.48 PEEM measurements were carried out in the SMART microscope operating at the UE49-PGM beamline of BESSY II. This aberration corrected and energy filtered instrument combines microscopy (LEEM, PEEM), diffraction (LEED), and spectroscopy (XPS, NEXAFS) techniques for comprehensive characterization of surfaces.49,50 The DNTT films were grown in situ in an attached preparation chamber and transferred within the UHV apparatus into the specimen chamber of the microscope. Time-dependent lateral current measurements were performed using a MM3A-EM micromanipulator (Kleindiek) which was brought into contact with the DNTT films and a Keithley 2450 sourcemeter.
the widely studied OSC pentacene (PEN). In view of a variety of structural peculiarities found for pentacene thin films such as (i) the presence of an interface stabilized thin-film (TF) phase,36 (ii) the control of molecular orientation by choice of substrate,37,38 and (iii) a distinct postdeposition dewetting,39 the question arises whether these occur also for DNTT films. Here, we report on such a detailed investigation of DNTT thin films prepared on prototypical model substrates (SiO2 and graphene/graphite) as well as on dielectric substrates pretreated with self-assembled monolayers (SAMs) which are commonly used in OFET architectures (in particular, we used octyltrichlorosilane (OTS) on SiO2 and phosphonic acids on metal oxides). SiO2 and graphene substrates have been chosen since they are known to induce crystalline growth in different molecular orientations for several oligomers like acenes and coronenes as well as phthalocyanines without forming chemisorbed hybrids.37,38,40−44 For several compounds, even film crystallization in different molecular polymorphs on these substrates has been reported, hence underlining the impact of the substrates on the film growth.45,46 The SAM-treated dielectrics are used to survey whether our observations made for the model substrates are also found for these technologically relevant surfaces. Combining atomic force microscopy (AFM), near-edge X-ray absorption spectroscopy (NEXAFS), X-ray diffraction (XRD), and photoemission electron microscopy (PEEM), we present a precise morphological and microstructural analysis of the respective thin films. By this means, we show that in contrast to an upright molecular growth of DNTT on bare and functionalized SiO2 surfaces, indeed crystalline thin films in lying molecular configuration are formed on graphene and graphite. Surprisingly, we identify severe temporal changes of the film morphology, which puts the reported long-time stability of DNTT-based thin-film transistors into a new focus. This is in particular true since this dewetting process is equivalently found for films prepared on the SAM-treated surfaces.
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RESULTS AND DISCUSSION
A. Multilayer Growth of DNTT Films on SiO2 and Graphene. For the prototypical OSC pentacene and several of its derivatives thin films are found to exhibit upright molecular orientations on natively oxidized Si (denoted as SiO2) surfaces37,40,41 while lying configurations are observed on graphene.38,51 These different molecular orientations are typically accompanied by strikingly different film morphologies. For PEN, on SiO2 pyramidal, dendritic islands with characteristic molecular steps are formed;37,52 films grown on graphene yield tessellated mesa-like structures with rather smooth surfaces.38 On the basis of the close resemblance of the crystal structures of PEN and DNTT and the above-mentioned growth peculiarities of PEN films, we have studied at first the growth of DNTT films on SiO2 and graphene. Figure 1a shows the morphology of DNTT multilayer films (nominal thickness dnom = 40 nm) prepared on SiO2 as derived from AFM measurements. For these films, islands are formed which feature a dendritic, pyramidal shape, thereby closely resembling the shape of PEN islands formed on SiO2.37,52 On the surface of these islands, monomolecular steps of 1.6 nm height are observed which corresponds well to the lattice spacing in the molecular (001) planes. This suggests an upright molecular orientation in these islands (which is verified later by our NEXAFS and XRD analyses). Upon deposition of DNTT onto graphene surfaces, however, the situation is largely different: as presented in Figure 1b, extended, nonuniformly distributed fibers are formed which feature individual heights of up to 200 nm. This different morphology might be attributed to a different molecular orientation than in the DNTT films on SiO2. A clear answer in this regard can, however, not be provided based on the AFM measurements, especially because the large roughness hampers an analysis of molecular step heights. Therefore, additional information on the crystalline texture and orientation of such films was derived from specular X-ray diffraction measurements. For DNTT/SiO2, diffraction peaks are found at angles of 2Θ = 5.45°, 10.92°, and 16.40° (cf. Figure 1c). These signals can be attributed to the first-, second-, and third-order reflections of the (001)DNTT lattice planes (d(001) = 1.62 nm) and thus indeed confirm an upright
EXPERIMENTAL SECTION
The DNTT (Sigma-Aldrich, purity >99%) thin films have been prepared by organic molecular beam deposition under high-vacuum conditions onto oxidized (100)-oriented silicon wafers, graphenecoated quartz and silicon wafers (Graphenea, Spain), and highly ordered pyrolytic graphite (HOPG, SPI supplies, mosaicity
