Surface Structure of Organic Semiconductor [n]Phenacene Single

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Surface structure of organic semiconductor [n]phenacene single crystals Yusuke Wakabayashi, Masanari Nakamura, Kaori Sasaki, Takahiro Maeda, Yuutaro Kishi, Hiroyuki Ishii, Nobuhiko Kobayashi, Susumu Yanagisawa, Yuma Shimo, and Yoshihiro Kubozono J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08811 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Journal of the American Chemical Society

Surface structure of organic semiconductor [n]phenacene single crystals Yusuke Wakabayashi, Hiroyuki Ishii,

†Division

‡

∗,†

Masanari Nakamura,

Nobuhiko Kobayashi,

‡

†

Kaori Sasaki,

†

Takahiro Maeda,

¶

Susumu Yanagisawa, Kubozono

§

Yuma Shimo,

§

‡

Yuutaro Kishi,

‡

and Yoshihiro

of Materials Physics, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan

‡Department

of Applied Physics, Faculty of Pure and Applied Science, University of Tsukuba, Ibaraki

¶Department

of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, Okinawa

§Research

305-8573, Japan 903-0213, Japan

Institute for Interdisciplinary Science, Okayama University, Okayama, 700-8530, Japan Received October 10, 2018; E-mail: [email protected] u.ac.jp

Surface structure relaxation of organic semiconductors aects the properties of organic devices, although such relaxation has not been well explored. Only two examples have been experimentally reported; tetracene shows a large surface relaxation while rubrene exhibits no relaxation. Therefore, a systematic investigation of the surface relaxation is conducted on [n]phenacenes (n = 5, 7, and 9). Electron density analyses are performed based on the synchrotron surface X-ray scattering with the aid of rst-principles calculations. The results show little surface relaxation in [n]phenacenes. Abstract:

The recent development of organic semiconductor techniques allows us to design and fabricate various molecular electronic devices. Light emitting diodes and solar cells are commercially available, and laser devices have also been studied. 1 The physical properties of organic semiconductors are aected by the molecular arrangement. Such a structural dependence of the transport, optical, or mechanical properties has been studied from the point of view of polymorphism. 2,3 The distance and angle between neighboring molecules dominate the transfer integral. 46 Molecular vibrations in the lattice structure also aect the carrier conductivity owing to the electron-vibration couplings. 711 When we think about organic eld-eect transistors (OFETs), the carriers are accumulated in a few molecular layers at the surface. Therefore, the surface structure, rather than the crystal structure, is relevant to the OFET properties. It is well known that the surface structure is sometimes dierent from the bulk crystals; such structural modulation is called surface relaxation or surface reconstruction, and is one of the central issues of surface science. This can be regarded as surface-induced polymorphism. Theoretical calculations predict little surface relaxation in many cases, 1215 while the techniques for energy calculation of van der Waals crystals are in the development phase. 16,17 Experimentally, surface structures are often studied by scanning probe microscopy and grazing incidence X-ray diraction. The former, which observes the in-plane structure, reports no superstructure in many organic semiconductors. 1820 The latter is usually used on lms, and derives the lattice parameters and preferred orientation. 21,22 The molecular arrangement in the lm can also be studied by this technique, although it requires much eort and is less common. 2325 Although the best device performance is al-

ways found in single crystals, structural studies of the surface relaxation in organic semiconductor crystals are sparse. It has been reported 26,27 that a large surface relaxation is observed in a tetracene single crystal, and little relaxation is observed in a rubrene single crystal. Hence, it is important to clarify if surface relaxation is common in organic semiconductors, and determining the conditions that produce a large surface relaxation will be even more interesting. To clarify the general tendency of the surface relaxation, a systematic study of the surface structure is required. In this study, we performed a surface electron density analysis of [n]phenacenes (n = 5, 7, and 9) having herringbone structure, 28 which is similar to tetracene. [n]phenacenes exhibit a carrier mobility of 5 to 18 cm2 V−1 s−1 , 29,30 which is comparable with a rubrene OFET. The larger the value of n, the larger the mobility. The depth dependence of the crystal structure can be studied by the X-ray scattering from surfaces, 31,32 and the technique was applied also on organic crystals. 33 While there are several analyzing methods, 3436 only coherent Bragg rod analysis (COBRA) 37,38 has been successfully used to derive the depth prole of the electron density of organic semiconductor surfaces. 26,27 This method uses the scattering amplitude from the bulk structure part as the reference wave of the hologram to obtain the unknown scattering amplitude from the relaxed surfaces, which is treated as the object wave in holography. Therefore, we need the bulk crystal structure for all the samples. Unfortunately, [n]phenacenes with n ≥ 6 crystallize in the shape of a lm, which prevents us from performing single crystal X-ray structure analysis. Instead, we used the calculated structure. Surface electron density analysis based on the calculated bulk crystal structure has never been done, and is, therefore, a challenge in terms of both theoretical and analysis techniques. The rst-principles calculations were performed using the van der Waals density functional (vdW-DF) 39 with a planewave basis set. The internal degrees of freedom were fully relaxed using the Quantum Espresso 40 codes with the lattice parameters xed to the experimentally reported values. 30,41 The cuto energy for the plane wave was 120 Ry, and the Brillouin zone integration was performed with a 4 × 4 × 2 k-point set. For the vdW-DF family, rev-vdW-DF2 42 was employed. The atomic congurations of [n]phenacenes for n = 7 and 9 were calculated starting from the initial guess constructed from a similar molecular orientation as that in a n=5 crys-

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(a) Crystal structure of [5]phenacene obtained by single crystal structure analysis. 43 (b), (c) Crystal structures of [n]phenacenes (n=7 and 9) obtained by DFT calculation (see text). Figure 1.

Surface X-ray scattering proles for [n]phenecanes along the (00L) lines are presented in Fig. 2 by the open symbols. Note that the horizontal axis is expressed in reciprocal lattice unit (r.l.u.) for each crystal. As can be seen, the intensity proles were similar to each other. Sharp peaks at integer L positions are the Bragg peaks. A relatively strong signal at the anti-Bragg position was found at L = n + 1.5 for all [n]phenecanes. 1018 10

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datasets. The data treatment was the same as the rubrene and tetracene cases. 26,27 The eect of the interpolation was minor (see supporting information). The thick blue curves in Fig. 2 show the interpolated experimental results, which were used for electron density analysis. Electron density proles derived by COBRA for [n]phenacenes are presented in Fig. 3 by the black curves. The red curves represent the electron density proles calculated for the ideal surface. The similarity between them was clear. For n = 5, the topmost layer had smaller contrast in the electron density prole than the ideal surface, showing that the topmost molecules have larger positional uctuation than those in bulk. All n = 5, 7 and 9 proles show negligible amount of deviation from the ideal surface structure except for the uctuation. They showed a clear contrast with the electron density prole of tetracene presented in panel (d), which exhibited a large surface relaxation. Our experiment shows that the [n]phenacenes have little surface relaxation. Another nding is that no structural change below the second molecular layer has been observed so far, whereas the topmost layer often show larger positional uctuation or relaxation (old rubrene, 26 [5]phenacene and tetracene). The surface eect appears to be limited to the topmost layer, which may be a consequence of the weak inter-layer interaction. 0.8 -3 Electron density (Å )

tal structure. We used the experimental values of lattice parameters, 30,41 and assumed no additional symmetry for n = 7 and 9 samples. The resulting DFT-derived structures are presented in Fig. 1 together with the results of the single crystal structure analysis of [5]phenacene. 43

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L(r.l.u) Figure 2. Surface X-ray scattering intensity distribution of [n]phenacenes along the 00L-rod. The horizontal axis is expressed in reciprocal lattice unit for each crystal. The signal from the Si sample holder for n = 9 data is also ploted with gray symbols. The blue curves show the interpolated experimental data for COBRA, and the red curves show the calculated intensity proles based on the crystal structure (n = 5) or DFT-derived structures (n = 7 and 9). The red dashed curve shows the calculated intensity distribution from modied bulk crystal structure (see text.)

The thin red curves show the calculated intensity proles from ideal surfaces, that is, the sudden termination of the bulk structure without any distortion. The observed proles were well reproduced by this calculation for n = 5 and n = 7. For n = 9, slight modication of the bulk crystal structure, 3.5◦ rotation of the molecules from the DFT-derived structure, improved the similarity between the measured and calculated intensity (red dashed curve, see supporting information for detail). Those features showed that the degree of surface relaxation was small. Coherent Bragg rod analysis was performed using a homemade code, which has already been successfully used for rubrene and tetracene, 26,27 for interpolated experimental

Electron density (Å-3 )

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Electron density prole as a function of depth for [n]phenacenes together with that of tetracene. 27 The origin of the horizontal axis is chosen as the surface. The oscillation of the electron density above the surface (negative-z region) is caused by the termination error of the Fourier transformation. Figure 3.

The surface X-ray scattering is caused by not only the surface but also the interface between the phenacene and SiO2 sample holder. Previous studies have shown that the interface is either very rough or as at as the surface. 26,27,44 The large FET mobility of phenacenes 29,30 suggests that the interface is at. The surface scattering intensity prole for each phenacene was well explained by one specic surface structure presented in Fig. 3, showing that the interface structure was the same as the surface structure. Let us examine what makes the surface relaxed. Morisaki 27 et al. pointed out that pentacene, which has a very similar structure as tetracene, has various kinds of polymorph structures, which suggests that the surface relaxation in tetracene is caused by the various structures having similar energy in the bulk form. The phenacenes reported in this study crystallized in a specic structure very stably. At this stage, we propose that the number of possible polymorph structures for the bulk crystal structure reects the possible degree of


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surface relaxation. Recently, the FET mobility, which is what we can measure experimentally, of phenacenes was calculated 45 based on the bulk crystal structure by using Marcus theory, and the values were close to the experimentally reported ones. 29,30 Although the Marcus theory has been used for ages, modern understanding of the transport in organic semiconductors does not support using it. 46,47 Theoretical examination based on modern transport theory for organic semiconductor is desired. On the other hand, using the bulk crystal structure to estimate the surface transport properties for phenacenes is supported by our present work. Surface structure relaxation of [n]phenacenes is examined by the combination of rst-principles calculations and surface X-ray scattering measurements. The phase retrieval analysis of the surface X-ray scattering using the calculated bulk structure was successfully demonstrated. As a result, [n]phenacenes exhibit little surface relaxation, which is very dierent from the tetracene case.

Acknowledgement The authors thank Mr. Y. Araya and Ms. S. Hamao for the experimental support. This work was supported by Grants-In-Aid for Scientic Research (JSPS KAKENHI, Grant Nos JP26287080, JP26105004, JP26105008, and JP26105011). The synchrotron radiation experiments at the Photon Factory were performed with the approval of the Photon Factory Program Advisory Committee (Proposal No. 2015S2-009). The computations were performed at the Supercomputer Center, ISSP, University of Tokyo, and Center for Computational Sciences, University of Tsukuba.

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(21) (22)

Supporting Information Available Experimental procedures and detail of the electron density analysis are provided in the supporting information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

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