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Epitaxial growth of C on rubrene single crystals for a highly ordered organic donor/acceptor interface Hiroki Mitsuta, Tetsuhiko Miyadera, Noboru Ohashi, Ying Zhou, Tetsuya Taima, Tomoyuki Koganezawa, Yuji Yoshida, and Masafumi Tamura Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00467 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on August 5, 2017

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Crystal Growth & Design

Epitaxial growth of C60 on rubrene single crystals for a highly ordered organic donor/acceptor interface Hiroki Mitsuta a,b, Tetsuhiko Miyadera * b, Noboru Ohashi c, Ying Zhou d, Tetsuya Taima e, Tomoyuki Koganezawa f, Yuji Yoshida b, Masafumi Tamura a a

Graduate School of Science and Technology, Tokyo University of Science, 2641 Yamazaki,

Noda, Chiba 278-8510, Japan. b

Research Centre for Photovoltaics, National Institute of Advanced Industrial Science and

Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan. c

Faculty of Engineering, Tokyo University of Science, Suwa, 5000-1, Toyodaira, Chino-City,

Nagano, 391-0292, Japan. d

Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science

and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan. e

Institute for Frontier Science Initiative (InFiniti), Kanazawa University, Kakuma, Kanazawa

920-1192 Japan. f

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo

679-5198, Japan

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KEYWORDS: Organic semiconductor, crystal growth, heteroepitaxy, RHEED, GIWAXS

ABSTRACT The highly ordered epitaxial growth of C60 films on rubrene single crystals was demonstrated. The C60 crystals growth commensurate with rubrene (001) surface lattice was confirmed by reflection high energy electron diffraction and X-ray diffraction and grazing incident wide angle X-ray scattering. Depending on growth conditions, several surface morphologies (rounded, hexagonal and triangular grains) of C60 grains were observed at the initial growth process. Large grains with layer-by-layer step and terrace structure of C60 were observed at the terrace region of rubrene (001) surface. The growth of high-crystallinity C60 films was achieved at high substrate temperature and slow deposition rate. Long migration length on the substrate which was given by enough substrate temperature enabled the formation of large single crystalline domains.


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1. INTRODUCTION

Hetero interface between donor molecule and acceptor molecule is important subject of research from both fundamental and practical aspects of organic semiconductors. Construction of well-ordered organic donor and acceptor interface is important for the understanding the physical properties of hetero interfaces. Fundamental analysis such as exciton and carrier transport dynamics

[1, 2]

and energy alignment

[3, 4]

for organic hetero interfaces is beneficial for the

practical applications e.g. organic photovoltaics. For the practical application, the random distribution of donor and acceptor materials prevents efficient carrier transport because of the formation of carrier trapping sites.[5] The crystallinity of the active layer of OPVs needs to be improved for the efficient transport of charge carriers and excitons. Two types of film structure can be assumed for OPV one is simple planar structure and another is mixed structure, which is so-called bulk heterojunction structure.[6] Improvement of crystallinity in the BHJ film can be one of the challenging research topics.[7, 8] Another research direction, which is the research focus of this paper, can be constructing well-ordered donor/acceptor planer interfaces. The use of single crystals organic semiconductor can be promising work with this regard and we had demonstrated the OPV using rubrene single crystal.[9] In our previous work, only donor compound is single crystal. The method for constructing single-crystalline donor/acceptor interface can be important work both for fundamental analysis and practical application. Based on this perspective, the method of heteroepitaxy is important where the lattice matched interface of different materials can be constructed. The heteroepitaxy of organic thin films has been studied for a long time on the basis of the concept of van der Waals epitaxy,[10] where the


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substrate and deposited films have a weak interaction. However, the studies of heteroepitaxy using different type of organic materials are still underway.[11-13] Materials with similar molecular structures tend to form well-ordered heterointerfaces because of the small mismatch in their lattice constants.[14 - 18] In the field of organic transistor, organic heteroepitaxy has been studied for fabricating ordered thin films on wetting layers. The growth of C60 on pentacene monolayer films[19], the growth of C8BTBT on nano-patterned Graphene[20], and the growth of pentacene on BN[21] were reported and field-effect-transistor characteristics were demonstrated. In the field of OPV, the construction of highly ordered donor/acceptor interface is important and a challenging research from fundamental and practical aspect. The growth of phthalocyanines on wetting layers such as 2,5-bis(4-biphenylyl)-bithiophene was reported for the fabrication of OPVs.[8, 22, 23]. We have been focusing on donor and acceptor interface and published the results of heteroepitaxial growth of C60 on the tetracene single crystal.[24] Recently, Nakayama et al. reported the heteroepitaxial growth of C60 on pentacene single crystal.[25, 26] Moreover, Wu et al., reported the epitaxial growth constructing well ordered C8BTBT and PTCDA on graphene and analyzed the photovoltaic performance[27] In our present study, we analyzed the detail of the crystal growth of C60 (acceptor) on rubrene (donor) single crystal substrates, where rubrene single crystals are among the most interesting organic semiconductors because of their large LD (~ 5 µm)[28] and high carrier mobility (~ 40 cm2/Vs)[29]. The C60 films (lattice constant 1.4 nm) grown on a rubrene (001) substrate (a = 0.718 nm, b = 1.44 nm) were investigated by varying a substrate temperature (Ts) and deposition rate (Rd) in order to clarify the lattice matching relationship and the film morphology.

2. EXPERIMENTAL SECTION


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Rubrene (Aldrich) single crystals were fabricated by physical vapor deposition. The (001) orientation of the rubrene single crystal was verified by X-Ray diffraction (XRD, RIGAKU Smart Lab., CuKα1 for X-ray source). The rubrene single crystal thin plates were mounted on a glass substrate and transferred into a vacuum chamber with a base pressure of 10-6 Pa. C60 (Frontier carbon) were evaporated onto the rubrene single crystal with the Rd monitored by using a quartz crystal microbalance. Growth of the C60 film on the rubrene single crystal was investigated under several conditions by varying the Ts and Rd (Ts: 25, 70 and 150°C; Rd: 0.001, 0.01 and 0.03 nm/s). The orientation and crystallinity of the film were observed in situ by reflection high energy electron diffraction (RHEED) with a micro-channel plate screen. Precise crystal orientation was analyzed by grazing incident wide angle X-ray scattering (GIWAXS) using synchrotron radiation facility (SPring-8 BL46XU). The surface morphologies of C60 thin films were examined by atomic force microscopy (AFM). The crystallinity and orientation of C60 films on the rubrene single crystal were determined by XRD.

3. RESULTS AND DISCUSSION

3.1 Initial growth stage The rubrene single crystal, which was fabricated by physical vapor deposition, exhibited step and terrace structure, as shown in Fig. 1(a). The initial growth process of the C60 films on the rubrene single crystal was investigated by RHEED and AFM. In situ RHEED analysis of a 1-nm-


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thick C60 film grown at Ts of 25°C and Rd of 0.01 nm/s (Fig.1(c)) yielded a streak and ring pattern. This indicated that the C60 film included single-crystal-like and poly-crystal-like regions. The streak pattern was observed from the initial stage of crystal growth (0.1 nm thick). The fact that the streaks were oriented in a particular direction suggested the epitaxial growth of C60, being commensurate with the rubrene single crystal. AFM analysis of 1-nm-thick C60 films revealed island growth of C60 (Fig. 2). The surface morphologies of the C60 films were different at the step and on the terrace of the rubrene single crystal: C60 tended to grow into small rounded grains along the step edge, whereas it formed rather large polygonal grains on the terrace. In addition, several types of C60 grain morphologies were observed on the rubrene terrace depending on Ts and Rd: at low Ts and high Rd, the C60 film tends to be made up of rounded grains, whereas high Ts and low Rd values tend to yield polygonal (e.g., hexagonal and triangular) grains. The symmetry of C60 grains suggests that rounded grains grown along the step edge and on the terrace exhibit an amorphous structure, while polygonal grains exhibit a form of the C60 fcc (111) face.[30] C60 grains with a polygonal hole were observed in the film grown at Ts = 70°C, which can be the fused grains of polygonal crystals (Figs. 2(d)–(f)). Large C60 grains were observed under high Ts and low Rd conditions. This is because lateral grain growth was enhanced by the long-range migration of C60 molecules at high Ts and the low density formation of crystal nuclei at low Rd. The C60 grain contains layer-by-layer step and terrace structure (detail image is represented in supporting information), which can be an evidence for the achievement of molecular-scale crystal growth control. The crystal orientation can be discussed by analyzing the orientation of triangular or hexagonal crystal habit (supporting info.). The frequency peak was observed every 30° in the histogram (fig. S3.). In addition to the RHEED streaks, this can be another evidence for the achievement of epitaxial growth of C60 on rubrene single crystal.


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3.2 Thick film (30 nm) region Next, C60 film growth in the thick (30 nm) film region was investigated by AFM (Fig. 3) and out-of-plane XRD (Fig. 4). Different types of C60 film morphology and crystallinity were observed depending on the growth conditions. At Ts = 25°C, there was little Rd dependence: a very small XRD peak, indicative of C60(111), and an AFM image of rounded grains with a size of around 100 nm were obtained (Figs. 3(a) and (b)). At low Ts, the crystal orientation of C60 was presumably disturbed with an increase in film thickness, resulting in poor crystallinity. At Ts = 150°C, the XRD peak intensities of C60(111) were increased and higher-order diffraction peaks of C60(222) were also observed. The crystallinity of the C60 film was improved by heating the substrate during growth. The C60 films grown at 150°C were observed to be a mixture of large pyramidal (~ 400 nm in width) and rounded grains (~ 100 nm in width) (Figs. 3(c) and (d)). The improvement in C60 crystallinity at Ts = 150°C can be attributed to the fact that the orientation of C60 grains was maintained during the initial growth stage up to the thick film region. Moreover, at Rd = 0.001 nm/s and Ts = 150°C, the ratio of C60 pyramidal grains on the terrace is higher than that at Rd = 0.01 nm/s and Ts = 150°C. Thus, high Ts and low Rd enable the growth of C60 films with high crystallinity and large grains on rubrene single crystals. 3.3 Crystal orientation Detail crystal orientation was analyzed by GIWAXS, where the two dimensional diffraction pattern was detected with PILATUS 300K and the sample was rotated to obtain three dimensional reciprocal space mapping (detail of the data conversion is described in supporting info.). The reciprocal space is horizontally clipped at the kz ~ 0.3 nm-1 to show the in-plane diffraction pattern for rubrene and C60 (Fig. 5 (a)). Diffraction peak of C60{220} was observed at


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every 30° that indicates 12-fold symmetry. Since the C60(111) plane has 6-fold symmetry, it can be concluded that there are two types of growth orientation for C60 films grown on rubrene single crystals. The coincidence of C60 [2 0 2] and rubrene [0 1 2] can be confirmed in Fig. 5 (a). Outof-plane reciprocal mapping along with ∥ is shown in fig. 5 (b) where ∥ is defined in fig. 5 (a). Based on the reciprocal mapping, the crystal orientation of C60 on rubrene single crystal can be illustrated as shown in Fig. 6. The lattice vector is rotated in ±15° from the

, where the lattice vectors are defined in fig. 6. The relationship between the lattice vectors of C60 ( , ) and rubrene (

,

) for both orientation can be expressed as: 1 = ⁄3 4

− 1⁄2

+ , for orientation I, 1⁄6

(1)

− 1⁄6

+ , for orientation II. 1⁄2

(2)

and 4⁄3 = 1

Here δ1 and δ2 denotes the lattice mismatch ( | |/| | = 0.025 , | |/| | = 0.016). The lattice mismatch is within the range of several percent, which can be beneficial for the heteroepitaxy. Especially, the C60 molecules marked as black in fig. 6 is well matched with rubrene molecules underneath. The discussion of lattice coincidence (point-to-point, point-online, and line-on-line coincidence) is important to understand the epitaxial relationship

[31 - 32]

.

The elements of transform matrix in eq. (1) and (2) consist of rational number and there are no column where all elements consist of integer. Considering from the matrix element, it can be expressed as weak coincident relationship. Contrary, all the C60 molecules along with the line of orientation I or line of orientation II are on the broken lines described in fig. 6, where the lines are linearly drawn along with rubrene molecules. Considering from this figure,


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the growth mode can be line-on-line coincidence. The line-on-line coincidence can be confirmed by analyzing the relationship of reciprocal lattice vector between both lattices [32]. When the two dimensional reciprocal lattices ( ∗ , ∗ , ∗

, ∗

, details were defined in the supporting info.) were taken into account, the relationship can be described as: −3∗ + ∗ = 6∗

.

(3)

Since the reciprocal lattice vector can be described by the linear relationship with integer factors, the growth mode of C60 on rubrene single crystal is determined as line-on-line coincidence.

4. CONCLUSIONS

We have succeeded in growing well-ordered C60 crystals on rubrene single crystals. The epitaxial growth of C60(111) films commensurate with the rubrene (001) surface lattice was verified by RHEED and GIWAXS. The C60 growth mode is categorized as line-on-line coincidence. Several types of morphologies were obtained by controlling the substrate temperature and deposition rate, where the hexagonal and triangular grains are oriented into certain direction. Higher-crystallinity C60 films with large pyramidal grains and layer-by-layer grown terrace structure were obtained at a substrate temperature of 150°C and deposition rate of 0.001 nm/s. This method enables the fabrication of highly controlled organic donor/acceptor hetero interface, which would open the way for the fundamental analysis of organic semiconductor interfaces.


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FIGURE CAPTIONS Figure 1. (a) Schematic drawing of the deposition of C60 on rubrene (001) surface. AFM image of rubrene (001) surface is also shown. (b) RHEED pattern of rubrene single crystal. (c) RHEED pattern of C60 film (1 nm thick) on the rubrene single crystal. The substrate temperature and deposition rate were 25ºC and 0.01 nm/s, respectively. Figure 2. AFM images (2×2 µm) of C60 films (1 nm) grown on rubrene single crystals. The substrate temperature and deposition rate are: (a) 25ºC, 0.03 nm/s; (b) 25ºC, 0.01 nm/s; (c) 25ºC, 0.001 nm/s; (d) 70ºC, 0.03 nm/s; (e) 70ºC, 0.01 nm/s; (f) 70ºC, 0.001 nm/s; (g) 150ºC, 0.03 nm/s; (h) 150ºC, 0.01 nm/s; (i) 150ºC, 0.001 nm/s. Figure 3. AFM images (2×2 µm) of C60 films (30 nm) on rubrene single crystals. The substrate temperature and deposition rate are: (a) 25ºC, 0.01 nm/s; (b) 25ºC, 0.001 nm/s; (c) 150ºC, 0.01 nm/s; (d) 150ºC, 0.001 nm/s. Figure 4. Out-of-plane XRD patterns of C60 films (30 nm) with various growth conditions. Figure 5.

Reciprocal mapping which were calculated from the data set of GIWAXS

measurement with sample rotation. The sample preparation condition was Ts = 25ºC, Rd = 0.01 nm/s. (a) in-plane diffraction mapping (kz ~ 0.3 nm-1 ). (b) out-of-plane diffraction mapping along with ∥ . The definition of ∥ is represented in (a). Diffraction indices were represented with green (rubrene), gray (C60 orientation I), and white (C60 orientation II) labels. Figure 6. Growth model for C60 films on rubrene(001) surface.


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AUTHOR INFORMATION Corresponding Author *Tetsuhiko Miyadera E-mail: [email protected] Tel: +81-29-861-3054. Fax: +81-29-861-6232.

ACKNOWLEDGMENT This work was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan. The GIWAXS measurement was performed at SPring-8 BL46XU with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, proposal nos. 2016A1514 and 2016B1861).

SUPPORTING INFORMATION Detail discussion of the morphology, crystal orientation, raw data of GIWAXS and two dimensional reciprocal lattices are described. Supporting information is available free of charge via the Internet at ttp://pubs.acs.org/.

REFERENCES


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(29) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Elastomeric Transistor Stamps: Reversible Probing of Charge Transport in Organic Crystals. Science 2004, 303, 1644. (30) Takashima, H.; Nakaya, M.; Yamamoto, A.; Hashimoto, A. Two-step growth of C60 films on H-terminated Si(111) substrate. J. Cryst. Growth 2001, 227/228, 825-828. (31) Hooks, D. E.; Fritz, T.; Ward, M. D. Epitaxy and Molecular Organization on Solid Substrates Adv. Mater. 2001, 13, 227-241. (32) Mannsfeld, S. C. B.; Leo, K.; Fritz, T. Line-on-Line Coincidence: A New Type of Epitaxy Found in Organic-Organic Heterolayers Phys. Rev. Lett. 2005, 94, 056104.


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Page 18 of 24

For Table of Contents Use Only

“Epitaxial

growth of C60 on rubrene single crystals for a highly ordered organic

donor/acceptor interface”

Hiroki Mitsuta, Tetsuhiko Miyadera *, Noboru Ohashi, Ying Zhou, Tetsuya Taima, Tomoyuki Koganezawa, Yuji Yoshida, Masafumi Tamura

C60 thin films epitaxially grown on the rubrene single crystal. Highly ordered C60 crystals with step and lattice structure were obtained.


18

Page 19 of 24


Fig. 1, Mitsuta et al. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

(a)

(b) C60 Rubrene

(c) Rubrene (001)

Rubrene


C60

Fig. 2, Mitsuta et al. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

(a)


(c)

(b)

Page 20 of 24

step terrace

(d)

(e)

(f)

(g)

(h)

(i)


0

[nm]

15

Page 21 of 24


Fig. 3, Mitsuta et al. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

(a)

(b)

(c)

terrace

(d)

step

step

terrace

0


[nm]

30


Page 22 of 24

Rubrene [008]

Rubrene [006] C60 [222]

C60 [022]

Rubrene [004]

C60 [111]

Rubrene [002]

Fig. 4, Mitsuta et al. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

0.01 Å/s 150 ºC 0.1 Å/s 150 ºC 0.01 Å/s 25 ºC 0.1 Å/s 25 ºC


Page 23 of 24


Fig. 5, Mitsuta et al.

(a)

(b)

-2 -2 4 -4 2 2

-4 2 2

-2 0 2

2 -2 0

2 -4 2 200 2 -2 0 2 -1 0 4 -2 -2 1 -1 0 2 0 -2 2 -2 0 0 -2 0 1 -2 0 2 -3 0 2 0 -2 4 -2 -2 0 -4 0 1 -4 0 -2 4 -2

-2 4 -2 0 2 -2 0 2 -2 -2 4 -2

-20 -10

0

10

20

008

20 042

15

-1 3 3

511 402

006 311

2 -1 6

022

10

004

-4 2 2

0

-20

-10

2 -1 2 2 -2 0

-2 2 0

0

10

k// [nm-1]

kx [nm-1] ACS Paragon Plus Environment

5 -1 -1

3 -1 -1

002

-1 1 1

5

3 -1 1

111

-3 3 3 -1 3 1 -2 2 2

2 2 -4

-20

2 -1 10

422

0 -2 2

-2 2 0

-10

2 -1 12

25

0 -2 2

-2 2 0

0

30

2 -4 2 -2 0 2

10

-2 -2 4

kz [nm-1]

20

ky [nm-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

2 -1 0 4 -2 -2

20


Page 24 of 24

Fig. 6, Mitsuta et al.

Orientation I

Orientation II

𝒃𝒃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

𝒂𝒂𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟

Rubrene

C60


Epitaxial Growth of C60 on Rubrene Single ... - ACS Publications

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