Japanese Journal of Applied Physics Vol. 43, No. 11A, 2004, pp. 7722–7724 #2004 The Japan Society of Applied Physics
Growth of Cobalt-Phthalocyanine on KCl (001) Substrate and Copper-Phthalocyanine Whisker Shin-ichiro YANAGIYA, Hironori W AKAMATSU, Osamu NISHIKATA and Tetsuo I NOUE Department of Optical Science and Technology, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan (Received June 23, 2004; accepted July 22, 2004; published November 10, 2004)
Cobalt-phthalocyanine (CoPc) was grown on a KCl {001} substrate and on a copper-phthalocyanine (CuPc) whisker which was epitaxially grown on a KCl {001} substrate. Crystallization was carried out by a repeated vacuum deposition and thermal treatment method. CoPc whiskers were grown on a KCl {001}-cleaved substrate. A CoPc whisker with a smaller diameter was also grown on the CuPc whisker. The epitaxial growth of CoPc on CuPc was confirmed using high-resolution electron microscopy and transmission electron diffraction analysis. [DOI: 10.1143/JJAP.43.7722] KEYWORDS: thermal treatment, epitaxial growth, organic semiconductor, copper-phthalocyanine, cobalt-phthalocyanine, KCl
1.
Introduction
Metal-phthalocyanines (MPc’s) are promising organic semiconductor materials whose electronic and optical properties can be changed by substituting their central metals. In our previous study, we reported the epitaxial growth of copper-phthalocyanine (CuPc) on a KCl {001} substrate by a vapor deposition and thermal treatment (VDTT) method1) (Figure 1). A needle-like copper-phthalocyanine (CuPc) whisker, which was approximately 30 nm in diameter and 1 mm in length, was epitaxially grown on a KCl substrate. The CuPc whiskers were oriented so that their long axes were parallel to the h100i direction of the KCl substrate at an incline of about 56 with the substrate. The first question is whether these phenomena can be obtained when other phthalocyanines grow on the KCl substrate. Therefore, we first examined the growth of cobalt-phthalocyanine on a KCl {001} substrate. This direction-controlled crystallite has potential to be used in nano-sized organic devices. To realize the fabrication of such devices, a study of the heteroepitaxy of organic crystals is believed to be important. The correspondence of the lattice constant of the organic crystal with that of the substrate is one of the required conditions for heteroepitaxial growth. Hence, the epitaxial growth of organic molecules is generally difficult because of the different sizes and shapes of the molecules of the epitaxial crystals and substrates. However, CuPc and CoPc have quite similar crystal structures2) and should therefore satisfy this condition. The second purpose of this work is to study the heteroepitaxial growth of CoPc on CuPc by repeated VDTT. 2.
Experiments
A single crystal of KCl was grown by the Czochralski method and cleaved through its {001} plane. The CuPc and CoPc were purchased from Aldrich Co. and Tokyo Kasei Kogyo Co., Ltd., respectively, and were used without further purification. The crystallization of CoPc grown on the KCl {001} substrate was carried out by VDTT. 30 mg of CoPc powder was placed in a molybdenum boat, and the boat was mounted in the lower part of a vacuum chamber. A crystal substrate was placed in the upper part of the chamber so that its surface was facing downward. The distance between the
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(b) Fig. 1. TEM image of the surface of CuPc epitaxially grown on KCl {001} substrate. As shown in (a), the CuPc whiskers were oriented so that their long axes were parallel to the h100i direction of the KCl substrate. The shape of cross section was a quadrangle with a round corner (b).1)
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boat and the substrate was 10 cm. Under a base pressure of approximately 104 Torr, CoPc was evaporated at 573 K and deposition was carried out for 1 h. The thickness of the layer was approximately 30 nm. After the deposition, the sample was picked out from the chamber and annealed in air. The annealing temperature was maintained at 473 K, and the annealing duration was 24 h. The surface morphology of the sample was observed using a field-emission scanning electron microscope (FE-SEM, Hitachi Instruments Service Co., Ltd., S-4700) and a transmission electron microscope (TEM, HITACHI H-7100 and H-9000NAR). The crystallization of CoPc on the CuPc/KCl substrate was carried out by the following methods: First, CuPc was grown on the KCl substrate by VDTT. The conditions for CuPc crystallization were the same as those used for the growth of CoPc on KCl, as was mentioned above. After CuPc crystallization, CoPc was grown under the same protocol and conditions used for the growth of CoPc on the KCl substrate. 3.
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Results and Discussion
The effect of thermal treatment on the growth of CoPc was first investigated. Figure 2 shows a surface of CoPc grown by VDTT observed using the TEM. We observed a number of whiskers grown on the substrate. This whisker growth indicates that the thermal treatment affects not only CuPc/KCl but also CoPc/KCl. However, the orientation that can be seen in the case of CuPc/KCl can not be seen in Fig. 2. The cause of this difference is considered to be the difference in interfacial energy between CoPc/KCl and CuPc/KCl. CoPc was deposited and annealed on CuPc, which had been previously grown on KCl by VDTT. The surface of the sample is shown in Fig. 3. An FE-SEM image of an overall view of the sample is shown in Fig. 3(a) and an enlarged image of one crystallite is shown in Fig. 3(b). Most whiskers were oriented so that their long axis was parallel to the h100i
Fig. 2. The TEM image of the surface of the Cobalt phthalocyanine (CoPc) epitaxially grown on KCl {001} substrate. CoPc whiskers were grown on the substrate with random orientations. The line width of CoPc whiskers seemed to be slightly less than that of CuPc whiskers.
(b) Fig. 3. TEM image [Fig. 1(a)] and FE-SEM image [Fig. 1(b)] of the surface of CoPc and CuPc whiskers. The diameters of the whiskers were 30 nm at the thin part and 50 nm at the thick part.
direction of the KCl substrate. This epitaxial growth of CuPc on a KCl {001} substrate has already been reported by Yanagiya et al.1) In addition, it was also found that a number of thinner crystallites grew on the CuPc whiskers with a slight tilt. The diameters of the two parts of the crystallites were approximately 50 nm and 30 nm. These crystallites were not observed when CuPc was grown by VDTT [Fig. 1(b)]. In addition, the cross-sectional surface of the whisker is rectangular in contrast with the nearly square shape of the CuPc whisker [Fig. 1(b)]. Therefore, it is reasonable to conclude that these thinner crystallites are CoPc whiskers. To confirm this hypothesis, the chemical compositions of the samples was studied using an energy dispersive X-ray (EDX) analyzer. The composition was copper (approximately 88%) and cobalt (approximately 12%) in an area containing many thin whiskers. On the other hand, as little as 5% cobalt was observed in an area in which no thin whiskers could be seen. Hence, we concluded that the thin whiskers that grow on CuPc whiskers are mainly composed of CoPc. The reasons why most of the sample consists of CuPc are that CuPc covered the entire KCl substrate and that CoPc is difficult to grow on CuPc films. The coverage of CuPc on substrates has been reported in our previous study.1) This
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(a) Fig. 4. Diffraction pattern of the whiskers. The pattern had C4v symmetry.
idea is supported by the fact that few CoPc whiskers were seen to grow from films. Moreover, a slight tilt between the upper and lower parts of the whiskers was also observed. This is explained by the difference in face angle between CuPc and CoPc. A TED image is shown in Fig. 4. These diffraction spots correspond to the diffraction spots of CuPc reported by Yanagiya et al.,1) Hoshino et al.,2) and Ashida.3) This correspondence between the previous results and our results indicates that the CuPc and CoPc have quite similar crystal structures and orientations. We concluded that CoPc whiskers epitaxially grow on CuPc whiskers. Figure 5 shows the high-resolution electron microscope image of the boundary region of a CuPc and CoPc whisker. Grid lines of the CuPc and CoPc whisker can be seen in this figure. The lines are connected with each other and bend at the boundary of the whisker. This line bending corresponds to the bending of the whisker. The width of the lines is approximately 1.3 nm. Hoshino et al. reported that the unit cell parameters are as follows: for CuPc: a ¼ 12:91ð4Þ, b ¼ 3:81ð1Þ, c ¼ 12:00ð5Þ, ¼ 95:6ð3Þ, ¼ 90:1ð1Þ, ¼ 91:0ð2Þ; for CoPc: a ¼ 12:87ð3Þ, b ¼ 3:82ð1Þ, c ¼ 11:93ð6Þ, ¼ 95:2ð3Þ, ¼ 90:2ð2Þ, ¼ 92:2ð3Þ.2) Furthermore, they reported that CuPc molecules lie on the substrate and the intermolecular distance of the CuPc is estimated to be approximately 1.35 nm.4) Thus, this distance corresponds to the width of the lines of CuPc and CoPc. The lattice constant difference between CuPc and CoPc is quite small (0.3%). Therefore, we can not discuss the lattice constant change at the interface of each whisker. 4.
Conclusions
We have investigated the heteroepitaxial growth of CoPc on the CuPc which were epitaxially grown on a KCl {001} substrate by a vapor deposition and thermal treatment method. The whiskers were grown on a KCl {001} substrate. A number of the whiskers became thin in the center. From the analysis of the EDX, TED, and high-resolution TEM
(b) Fig. 5. High-resolution TEM image of (a) the head of a whisker and (b) boundary region framed by the dashed line in Fig. 3(a). The average width of the grid line was approximately 1.3 nm.
images, we concluded that thin whiskers of CoPc were epitaxially grown on CuPc whiskers. Acknowledgements The authors wish to thank Professor C. Kaito for the use of his transmission microscope and Dr. A. Hoshino for his helpful advice in this work. The authors are grateful for the partial support by Grant-in-Aid (No. 16710085) of Scientific Research of the Ministry of Education, Culture, Sports, Science, and Technology Japan. This work was performed under the inter-university cooperative research program of the Institute for Materials Research, Tohoku University. 1) S. Yanagiya, S. Nishikata, G. Sazaki, A. Hoshino, K. Nakajima and T. Inoue: J. Cryst. Growth 254 (2003) 244. 2) A. Hoshino, Y. Takenaka and H. Miyaji: Acta Cryst. B 59 (2003) 393. 3) M. Ashida: Bull. Chem. Soc. Jpn. 39 (1966) 2632. 4) A. Hoshino and H. Miyaji: Jpn. J. Appl. Phys. 43 (2004) 4344.
