Weak Epitaxy Growth and Phase Behavior of Planar Phthalocyanines

May 8, 2008 - Fax: +86-431-85262266. ... i.e., metal-free phthalocyanine (H2Pc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), ... Large...
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J. Phys. Chem. B 2008, 112, 6786–6792

Weak Epitaxy Growth and Phase Behavior of Planar Phthalocyanines on p-Sexiphenyl Monolayer Film Tong Wang, Junliang Yang, Haibo Wang, Feng Zhu, and Donghang Yan* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Graduate School of Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: December 18, 2007; ReVised Manuscript ReceiVed: March 2, 2008

We systematically investigated the weak epitaxy growth (WEG) behavior of a series of planar phthalocyanine compounds (MPc), i.e., metal-free phthalocyanine (H2Pc), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), zinc phthalocyanine (ZnPc), iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), grown on a p-sexiphenyl (p-6P) monolayer film by selected area electron diffraction (SAED) and atomic force microscopy (AFM). Two types of epitaxial relations, named as incommensurate epitaxy and commensurate epitaxy, were identified between phthalocyanine compounds and the substrate of the p-6P film. The tiny variation of the lattice constant of phthalocyanine compounds can result in different crystal orientations. The change rule of incommensurate and commensurate epitaxy was extracted. The tendency of commensurate epitaxy becomes weaker as the lattice constant b increases, while it gets stronger as the substrate temperature is elevated. Large size and continuous H2Pc films can be obtained by controlling the growth conditions. The WEG method is generally applicable in the whole family of planar phthalocyanine compounds and may be used to fabricate other high-quality organic films. 1. Introduction Organic semiconductor films have attracted much interest as the key issue in organic optoelectronic devices, which possess many advantages in terms of low-cost, flexibility, large area processing, etc., in comparison with traditional semiconductors.1,2 The device performance has a strong relationship to the morphology and crystallinity of the thin film.3 The growth of high-quality organic thin films with a high order, low defect, and large size domain is required for improving the device performance. High quality organic film is fabricated generally by organic molecular beam epitaxy (OMBE) under ultrahigh vacuum (UHV)4,5 or oriented growth on highly oriented substrate.6,7 Recently, weak epitaxy growth (WEG) was developed and is well-suited to fabricate the highly oriented film of disk-like semiconductor molecules such as phthalocyanine compounds.8,9 “Weak” means to decrease the interaction between the molecules and the substrate by way of introducing a new substrate and elevating substrate temperature. Thus, phthalocyanine molecules are upright on the substrate, and the π-π conjugated direction is parallel to the film plane. It is different from conventional OMBE film in which molecules tends to lie flat on the substrate due to the strong interaction between molecules and substrate. Therefore, WEG is conducive to carrier transport in the film plane. The mobility of organic electronic devices with phthalocyanine on p-sexiphenyl (p-6P) as active layers by means of WEG was achieved at the same level as that of corresponding single crystals.8,9 The family of planar phthalocyanine (MPc) is considered as one of the most promising candidates for fabricating organic thin film transistors (OTFT) because of their chemical and thermal stability.3,8–11 The WEG mechanism of phthalocyanine compounds was revealed in the model system of H2Pc grown * Corresponding author. E-mail: [email protected]. Fax: +86-43185262266. Tel: +86-431-85262165.

on a p-6P monolayer and a double-layer film, in which the surface geometrical channels of p-6P substrate dominated the oriented nucleation and growth of H2Pc film.12 In the present paper, the WEG behavior of the family of planar phthalocyanine compounds, i.e., H2Pc, NiPc, CuPc, ZnPc, FePc, CoPc, on p-6P monolayer films was investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). Furthermore, the phase behavior of MPc crystals was also elucidated in detailed. The molecular structures of planar MPc and p-6P are given in Figure 1a and 1b. MPc molecules have upright growth on p-6P ultrathin films with large size domains. The family of planar phthalocyanine compounds exhibited similar WEG behavior on p-6P monolayer films. In each p-6P domain, MPc crystals showed two or three in-plane orientations which formed the angles of (13.5-16.5° or (13.5-16.5° and 0° between the c*-axis of MPc crystal and the a*-axis of the p-6P crystal. The tiny variation of lattice constants of various MPc resulted in the different crystal orientation. The tendency of commensurate epitaxy becomes weaker as the lattice constant b increases, while it gets stronger as the substrate temperature is elevated. 2. Experimental Section 2.1. Fabrication of Organic Films. The MPc were purchased from Aldrich Company (Milwaukee, WI), and the p-6P was synthesized according to ref 13. They were purified twice by thermal gradient sublimation prior to experiments. First, 2 nm p-6P films were deposited on SiO2 substrate at 180 °C (150 nm thermally oxidation SiO2 layer on silicon wafer), and the typical grain size is 80 µm2 ∼ 120 µm2. Then, MPc thin films were deposited on the p-6P thin film, and the substrate temperatures were varied from 180 to 210 °C. The thin films were deposited under pressure of 10-4 to 10-5 Pa at a rate of about 1 nm/min.

10.1021/jp7118795 CCC: $40.75  2008 American Chemical Society Published on Web 05/08/2008

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Figure 1. (a and b) Molecular structures of MPc (M ) H2, Ni, Cu, Zn, Fe, Co) and p-6P, respectively. AFM height images of (c) 2 nm H2Pc, (d) 2 nm NiPc, (e) 3 nm CuPc, (f) 2 nm ZnPc, (g) 2 nm FePc, and (h) 4 nm CoPc grown on 2 nm p-6P film (monolayer film) at the substrate temperature of 180 °C, respectively. The height data of MPc crystals shown under each figure are corresponding to the blue lines indicated in the figures.

TABLE 1: Summary of Lattice Constants, Mismatches, and Oriented Angles for MPc on p-6P Monolayer Film MPc grown on p-6P monolayer commensurate

p-6P H2Pc NiPc CuPc ZnPc FePc CoPc

incommensurate

d(100) (Å)

d(010) (Å)

d(001) (Å)

β (deg)

mismatch (%) bp-6P and bMPc

mismatch (%) ap-6P and cMPc

angle between ap-6P and cMPc (θ), deg

7.84 26.14 26.06 25.92 25.98 25.90 25.88

5.59 3.814 3.790 3.790 3.780 3.765 3.750

26.24 23.97 24.18 23.92 24.21 24.10 24.08

98.17 91.1 94.8 90.4 90.6 90.0 90.2

2.34 1.70 1.70 1.43 1.03 -

1.91 2.81 1.70 2.93 2.47 -

13.5 14.3 15.5 13.5 15.5 16.5

2.2. Characterization of Organic Films. AFM Measurements. Films were imaged by a SPI 3800N (Seiko Instruments Inc.) with tapping mode. AFM height images and phase images were obtained at the same time. A 150 µm scanner and a commercially available SiN4 cantilever with a spring constant of 2 N/m were used in all experiments. TEM Measurements. The organic films of MPc/p-6P were first deposited on a SiO2 substrate and then a carbon film was

deposited on MPc/p-6P, which was used as support layer. Gold was used for demarcation if necessary. The films were separated from the SiO2 surface by floatation in 10% HF solution. The organic film with the carbon coating was transferred to a copper grid for measurement. The selected area electron diffraction was imaged with a JEOL JEM-1011 transmission electron microscope operated at 100 kV. In order to provide a weaker intensity beam and higher contrast, dark field was used for the experi-

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Figure 2. (a) Electron micrograph morphology and (b) corresponding electron diffraction pattern of 3 nm NiPc grown on p-6P monolayer film.

ment. Simultaneously, MoO3 was used to calibrate the rotation angle of the image relative to the diffraction pattern. 3. Results and Discussion 3.1. WEG Behavior of the Family of Planar Phthalocyanine. The thin film morphologies of planar phthalocyanine compounds H2Pc, NiPc, CuPc, ZnPc, FePc, and CoPc grown on the p-6P monolayer film are shown in Figure 1. All phthalocyanine crystals have lamellar shape with excellent inplane orientation in each p-6P domain. Their sizes are all in the submicrometer range. The neighboring grains coalesce easily and unsynchronally along the growth direction. H2Pc grain is the largest, while FePc grain is the smallest. The height data of MPc crystals on p-6P monolayer films are shown under each figure corresponding to the blue lines. Each molecule layer of the MPc crystal is about 1.3 nm, just about half of crystal cell parameter a (2.588-2.614 nm) with the R-form as shown in Table 1. In Figure 1c, d, f, and g, the island height is about 2.5 nm corresponding to two molecule layers. Therefore, MPc molecules should grow with approximate standing-up mode and the (100) lattice plane is parallel to p-6P film–substrate. The third molecule layer emerges in 3 nm CuPc, where the coverage of the second layer is near to 90%, as shown in Figure 1e. The film is coalescing and exhibits a net-like feature. In the 4 nm CoPc image (Figure 1h), the fourth layer emerges, although the foregoing three layers have not coalesced completely. In order to understand the structural relation between phthalocyanine compounds and the p-6P substrate, the experiments of selected area electron diffraction (SAED) are performed. Figure 2 is an electron micrograph, and a corresponding SAED of 3 nm NiPc is grown on p-6P monolayer film. The indexed ED pattern consists of one [001] zone of p-6P and three [100] zones of NiPc with the R-form (monoclinic unit cell with a ) 26.15 Å, b ) 3.790 Å, c ) 24.26 Å, β ) 94.8°).14,15 Furthermore, there is an angle of about (14.3° between the c*-axis of NiPc and the a*-axis of the p-6P. It is similar to ZnPc grown on a p-6P monolayer film that the angle between the c*-axis of ZnPc and the a*-axis of p-6P is (13.5°.8 The only difference is the magnitude of the angle. Because the b direction is coincident with the directions of π-π conjugation and crystal growth, the discrepancy in angles may be related to the lattice constants b. The third in-plane orientation is that the c*-axis of NiPc crystal is parallel to the a*-axis of p-6P crystal. Figure 3 shows the SAED patterns of H2Pc, FePc, CuPc, and CoPc grown on p-6P monolayer film. The indexed ED patterns show one [001] zone of p-6P and three [100] zones of MPc in

the R-form except for CoPc which exhibits only two [100] zones. The angles of H2Pc, CuPc, FePc, and CoPc on the p-6P monolayer film are (13.5°, (15.5°, (15.5°, and (16.5°, respectively, between the c*-axis of MPc and the a*-axis of p-6P. Interestingly, the lattice constant b of corresponding MPc are 3.814 Å, 3.790 Å, 3.765 Å, 3.750 Å, respectively. The angle diminishes gradually with an increase in the lattice constants b of MPc, as shown in Figure 4. However, ZnPc grown on p-6P monolayer film is not compatible with the rule; the reason is not clear and requires further research. For comparison, their lattice constants of MPc and the angles are listed in Table 1. These two sets of diffraction only show the orientation of MPc crystals but not lattice matching with the p-6P substrate, which corresponds to incommensurate epitaxy.12 Furthermore, as to the third set of in-plane orientation corresponding to commensurate epitaxy, the c*-axis of MPc crystal is parallel to the a*-axis of the p-6P crystal. For H2Pc grown on p-6P monolayer film, the electron diffraction spots of this set can be observed at all times, and the intensity is stronger than the other two symmetrical sets of in-plane orientations. But for NiPc, CuPc, ZnPc, FePc, and CoPc, the electron diffraction spots of this set are somewhat weaker than the spots of their other two symmetrical sets and become weaker gradually. Finally, it is hard to observe the electron diffraction spots of this set for CoPc grown on p-6P monolayer film. In other words, the electron diffraction intensity of the commensurate epitaxy becomes weaker as the lattice constant b decreases, and eventually the commensurate epitaxy disappears. The two sets of in-plane orientations of incommensurate epitaxy result from the geometrical channels between the prominent H-atoms of (001) plane of p-6P film. The prime geometrical channels are along [110] and [1–10] directions with a channel distance of 3.507 Å.12 In the nucleation stage, the initial MPc molecules tend to diffuse into the channels where it is advantageous for MPc molecules to nucleate in an oriented manner. The distance of the parallel MPc molecules is related to π-π interaction of molecules and is equal to 3.4 Å of herringbone structure. Then, the schematic diagram of the change rule for different MPc grown on p-6P monolayer film is shown in Figure 5. The angle between [110] and [1–10] geometrical channels on the p-6P (001) plane is set as ω (111° for p-6P monolayer), and the angle of herringbone packing in R-MPc is set as λ (128° for H2Pc). In case of MPc molecules parallel to the direction of channel, the (λ-111°)/2 angle (set as θ1) is formed between the c*-axis of MPc and the a*-axis of p-6P. The schematic diagram is represented in Figure 5a.

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Figure 3. Electron diffraction patterns of 3 nm (a) H2Pc, (b) FePc, (c) CuPc, and (d) CoPc grown on p-6P monolayer film at a substrate temperature of 180 °C. The index diffraction patterns exhibit that H2Pc, FePc, and CuPc crystals have three sets of in-plane orientations, while CoPc crystals have only two sets of in-plane orientations. The angle between the c*-axis of MPc and the a*-axis of p-6P diminishes gradually as the lattice constant b of MPc increases.

Figure 4. Plot of the angle between c*-axis of MPc and the a*-axis of 6P vs the lattice constant b of MPc.

However, to ensure the lowest energy, MPc molecule clusters will deviate from the direction of channel by a small angle (set as θ2 in Figure 5b) to reach at a suitable location. The driving force of rotation comes from π-π electron interaction and van der Waals force between MPc and p-6P molecules. Because the channel is wide enough, after rotation for a certain degree, numerous MPc molecules still stay in channels. Apparently, MPc molecule clusters will also rotate to the opposite direction. In the event of θ2 equal to -θ1, i. e., the c*-axis of MPc crystal is parallel to the a*-axis of the p-6P crystal. This state corresponds to the third orientation, commensurate epitaxy. The λ degree of each kind of MPc can be approximately calculated on the basis of b values through formula sin (λ/2) ) b0/b; b0 denotes the distance of the parallel MPc molecules. For instance, fo H2Pc grown on p-6P monolayer, λ ) 128°, θ1 ) 8.5°, θ2 )

5° or -8.5°, so the angle between the c*-axis of H2Pc and the a*-axis of p-6P is 13.5° or 0°. Likewise, for NiPc, λ ) 129.2°, θ1 ) 9.1°, θ2 ) 5.2° or -9.1°, the angle between the c*-axis of NiPc and the a*-axis of p-6P is 14.3° or 0°. But for CoPc, λ ) 132.4°, θ1 ) 10.7°, θ2 ) 5.8° (can not reach at -10.7°), the angle between the c*-axis of CoPc and the a*-axis of p-6P is only 16.5°. There is not commensurate epitaxy because the b value and λ are so large that CoPc molecules cluster are difficult to rotate to the opposite direction of θ1 to make θ2 equal to -θ1. That means there is the unique lowest energy location in the channel. Consequently, the variation of lattice constants of various MPc resulted in the different orientation angle of crystal axes. Lattice mismatching is used to characterize the commensurate epitaxy. Here, lattice mismatching of each kind of MPc grown on p-6P monolayer film was calculated and shown in Table 1. (For [100]p-6P//[001]MPc, mismatching % ) (d(001)MPc - 3 × d(100)p-6P)/(3 × d(100)p-6P). For [010]p-6P//[010]MPc, mismatching % ) (3 × d(010)MPc - 2 × d(010)p-6P)/(2 × d(010)p-6P).) It is almost perfect mismatching in that the mismatching is much smaller than the upper limit (10–15%) for the occurrence of organic epitaxy.16 3.2. Phase Behavior of Planar Phthalocyanine. In order to acquire high-quality film, we focus on the growth habits of the initial ultrathin film of MPc in the system of H2Pc grown on p-6P monolayer film at different substrate temperatures. AFM micrographs of H2Pc films grown at 190 °C with film thicknesses from 0.8 to 2.0 nm are shown in Figure 6. At 0.8 nm film (Figure 6a), H2Pc crystals exhibit approximately three inplane orientations, just as analyzed by SEAD in Figure 3a. Their

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Figure 5. Schematic diagram of MPc grown on p-6P monolayer film. ω is the angle between [110] and [1–10] geometrical channels on the p-6P (001) plane (111° for p-6P monolayer), λ is the angle of herringbone packing in the R-form MPc, θ1 ) (λ-111°)/2, θ2 is the angle that MPc molecule cluster deviates from the direction of channel. The λ degree of each kind of MPc can be approximately calculated on the basis of b values through formula sin (λ/2) ) b0/b (b0 denote the distance of the parallel MPc molecules).The variation of oriented angles of MPc crystal axes resulted from the change of their lattice constant b.

Figure 6. AFM height images of (a) 0.8 nm, (b) 1.6 nm, (c) 2.0 nm H2Pc grown on p-6P monolayer film at the substrate temperature of 190 °C, respectively. H2Pc film exhibits excellent coalescence as thickness increases.

Figure 7. AFM micrographs of 2.5 nm H2Pc film grown at a substrate temperature of (a) 200 °C and (b) 210 °C on p-6P monolayer film, respectively. (b) Annealed at 210 °C for 30 min after growth.

height is about 2.5 nm corresponding to double molecule layers. Some molecule clusters scattered around the strip-like crystals are undergoing morphological transition or migrating to doublelayer crystals. As H2Pc film thickness is increased to 1.6 nm (Figure 6b), these strip-like crystals enlarge. Some adjacent striplike crystals begin to collide and coalesce gradually. One of

the in-plane orientations is dominant in quantity. With further increase of H2Pc thickness to 2.0 nm (Figure 6c), the strip-like crystals almost coalesce and exhibit single in-plane orientation. The cross-sectional profile data corresponding to the blue line indicate that the coalesced H2Pc crystals are a closed double molecule layer in coexistence with a few strip-like crystals of

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Figure 8. AFM micrographs of 10 nm H2Pc films grown at a substrate temperature of 210 °C on p-6P monolayer film. The image in b is collected over the region in the square in a.

Figure 9. (a) Electron micrograph morphology and (b) corresponding electron diffraction pattern of 2.5 nm H2Pc on p-6P monolayer film at a substrate temperature of 200 °C.

Figure 10. Phase diagram of H2Pc grown on p-6P monolayer film. Tc is the transition temperature between thin-film phase and crystalline phase, Tm1 is the melting temperature of crystalline phase, Tm2 is the melting temperature of thin-film phase, Tb is the boiling temperature. The gray rectangle is the temperature scope that can be observed in the experiment.

the third molecule layer. Furthermore, the coalesced H2Pc crystals exhibit liquid-like morphology and dewetting behavior. Similar experimental phenomena are also observed in other organic films, just as p-6P,12 pentacene,17 m-OSB,18,19 etc. These films are different from bulk phase and are named thin-film phase. Hence, this H2Pc film may be a new phase, i. e., thinfilm phase, that is different from the bulk phase of H2Pc crystals. Figure 7a and 7b are AFM micrographs of 2.5 nm H2Pc films grown at an elevated substrate temperature of 200 and 210 °C on p-6P monolayer film, respectively. At the time of doublelayer H2Pc film coalescence, some double-layer films also

transition to a multilayer film which also shows irregular morphology (Figure 7a). At 200 °C, the double-layer film is thermodynamically metastable and will transition to the thermodynamically stable three- or four-layer film. The velocity of transition is greater than that of double-layer growth. Hence, it is difficult to obtain a large size and continuous H2Pc film. Figure 7b shows the AFM micrographs of a 2.5 nm H2Pc film grown at a further elevated substrate temperature of 210 °C and annealed for 30 min. A double-layer film with large size and a multilayer film of regular morphology (cross-sectional feature indicated by white arrow) coexist. The double-layer film exhibits a dewetting-like shape, while the regular morphology film shows the characteristic of a crystalline phase. As similar to the m-OSB phase transition,19 besides absorbing vacuum-deposited molecules, the process of forming the crystalline phase includes three mechanisms: direct absorption of molecules from double molecule layer films, migration of small molecular clusters, and quasi-Ostwald ripening. At 210 °C, the double-layer film is also thermodynamically metastable and will transition to multilayer films. However, the transition velocity and the velocity of double-layer growth are about in equilibrium. Thus, it is feasible to obtain large size and high-quality H2Pc films by decreasing the velocity of transition and desorption. Therefore, by increasing the thickness of H2Pc film at a deposition temperature of 210 °C, a high-quality H2Pc film with only single orientation and long-range order on the p-6P monolayer film is obtained, as shown in Figure 8. In other words, the H2Pc single-crystal film is fabricated successfully. This film exhibits a very smooth

6792 J. Phys. Chem. B, Vol. 112, No. 22, 2008 morphology with smaller roughness, which the root meam square (rms) roughness determined from Figure 8b is as low as 0.58 nm. Figure 9 shows the electron micrograph morphology and corresponding electron diffraction pattern of 2.5 nm H2Pc grown on p-6P monolayer film at 200 °C. In comparison to an electron diffraction pattern of 3 nm H2Pc on p-6P monolayer film at 180 °C (Figure 3a), only the electron diffraction spots of the commensurate epitaxy orientation are clear, and the other two sets of incommensurate epitaxy orientation become rather weaker. Accordingly, we can conclude that high substrate temperature of above 190 °C is advantageous to form H2Pc film of a single in-plane orientation, even form single-crystal film. The consequence is coincident with the analysis above about the MPc molecule nucleation process in the geometrical channels of the p-6P (001) plane. Adequate activity energy is supplied for H2Pc molecules to rotate the appropriate angle in the channel to form commensurate epitaxy with the lowest potential energy. According to lots of experiment results and the growth habit of H2Pc films on p-6P monolayer film at different substrate temperatures, the phase diagram of H2Pc crystal can be deduced, as shown in Figure 10. Tc is the transition temperature from thin-film phase to crystalline phase, which is also named as critical temperature. Tm1 is the melting temperature of crystalline phase, Tm2 is the melting temperature of thin-film phase, Tb is the boilling temperature. The gray rectangle is the temperature scope that can be observed in experiment. For Figure 6–8, the substrate temperature of the film growth is close to critical temperature Tc. The transition temperature between the thinfilm phase and crystalline phase should be within 190-210 °C. At temperatures close to Tc, the formation of the thin-film phase and crystalline phase is determined by the growth velocity of the thin-film phase and the transition velocity from thin-film phase to crystalline phase. If the former is smaller than the latter, it is advantageous to form the crystalline phase (Figure 7a). Otherwise, the thin-film phase is formed easily. But the desorption effect is obvious at temperatures higher than Tc, and it is disadvantageous for the H2Pc film to grow. At temperatures lower than Tc, although the crystalline phase is more stable than the thin-film phase, film growth is dominated by dynamics and the thin-film phase will be obtained (Figure 6c). Then it will transition to the crystalline phase gradually. According to the phase diagram, on one hand, we can know the relative stability of crystalline phase and thin-film phase. On the other hand, we are able to choose an appropriate temperature to fabricate the film with a crystalline phase or thin-film phase. 4. Conclusions We have investigated a series of planar phthalocyanines grown on p-sexiphenyl (p-6P) monolayer film by selected area electron diffraction (SAED) and atomic force microscopy (AFM). Two types of epitaxial relations, named as incommensurate epitaxy and commensurate epitaxy, were identified between phthalocyanine compounds and the p-6P substrate. The two symmetric sets of MPc crystals on p-6P monolayer film are incommensurate epitaxy. The angle between the c*-axis of MPc and the a*-axis of p-6P diminishes gradually as the lattice

Wang et al. constant b of MPc increases. The tiny variation of lattice constant of MPc resulted in a different orientation of their crystal axes. The commensurate epitaxy becomes weaker as the lattice constant b increases and eventually disappears. However, the incommensurate epitaxy becomes weaker as the substrate temperature is elevated. Large size and continuous H2Pc films can be obtained by controlling the growth condition, and even the single-crystal film is fabricated successfully. The WEG method is generally applicable to the whole family of planar phthalocyanines and may be used to fabricate other high-quality organic films. Furthermore, the phase behavior of MPc film was studied in the system of H2Pc grown on p-6P monolayer film at different substrate temperatures. The double-layer film exhibits liquidlike morphology and the dewetting behavior, which correspond to a film phase. While the multilayer film with regular morphology shows the characteristic of a crystalline phase. Under experimental conditions, the film phase is thermodynamically metastable and will transition to the crystalline phase gradually. The transition temperature (Tc) between thin-film phase and crystalline phase is within 190 °C ∼ 210 °C. At a temperature close to Tc, the formation of a thin-film phase and crystalline phase is determined by the growth velocity of the thin-film phase and the transition velocity from the thin-film phase to crystalline phase. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (50773079, 20621401). References and Notes (1) Horowitz, G. AdV. Mater. 1998, 10, 365. (2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (3) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. AdV. Mater. 1997, 9, 42. (4) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (5) Hooks, D. E.; Fritz, T.; Ward, M. D. AdV. Mater. 2001, 13, 227. (6) Smith, P.; Wittmann, J. C. U.S. Patent Appl. No. 361,129,1989. (7) Wittmann, J. C.; Smith, P. Nature 1991, 352, 414. (8) Wang, H.; Zhu, F.; Yang, J.; Geng, Y.; Yan, D. AdV. Mater. 2007, 19, 2168. (9) Wang, H.; Song, D.; Yang, J.; Yu, B.; Geng, Y.; Yan, D. Appl. Phys. Lett. 2007, 90, 253510. (10) Zhang, J.; Wang, J.; Wang, H.; Yan, D. Appl. Phys. Lett. 2004, 84, 142. (11) Bao, Z.; Lovinger, A. J.; Brown, J. J. Am. Chem.Soc. 1998, 120, 207. (12) Yang, J.; Wang, T.; Wang, H.; Zhu, F.; Li, G.; Yan, D. J. Phys. Chem. B 2008, 112, 3132. (13) Garnier, F.; Horowitz, G.; Peng, X. Z. Synth. Met. 1991, 45, 16(14) (14) Ashida, M.; Uyeda, N.; Suito, E. Bull. Chem. Soc. Jpn. 1966, 39, 2616. (15) Iwatsu, F.; Kobayashi, T.; Uyeda, N. J. Phys. Chem. 1980, 84, 24– 3223. (16) Wittmann, J. C.; Lotz, B. Prog. Polym. Sci. 1990, 15, 909. (17) Ruiz, R.; Choudhary, D.; Nickel, B.; Toccoli, T.; Chang, K. C.; Mayer, A. C.; Clancy, P.; Blakely, J. M.; Headrick, R. L.; Iannotta, S.; Malliaras, G. G. Chem. Mater. 2004, 16, 4497. (18) Tang, Y.; Wang, Y.; Wang, G.; Wang, H.; Wang, L.; Yan, D. J. Phys. Chem. B 2004, 108, 12921. (19) Tang, Y.; Wang, Y.; Wang, X.; Xun, S.; Mei, C.; Wang, L.; Yan, D. J. Phys. Chem. B 2005, 109, 8813.

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