Initial states of epitaxial growth of vanadyl- and titanyl-phthalocyanines on MoS2 cleaved surfaces

Thin Solid Films 331 (1998) 148±151

Initial states of epitaxial growth of vanadyl- and titanyl-phthalocyanines on MoS2 cleaved surfaces Yasunori Fujikawa*, Shinro Mashiko Kansai Advanced Research Center, Communications Research Laboratory, 588-2, Iwaoka, Iwaoka-cho, Nishi-ku, Kobe 651-24, Japan

Abstract The initial states of epitaxial growth of vanadyl-phthalocyanine (VOPc) and titanyl-phthalocyanine (TiOPc) on MoS2 cleaved surfaces have been investigated, using atomic force microscopy (AFM) and electron energy loss spectroscopy (EELS). In the case of VOPc, formation of the ®rst layer at room temperature which consists of one molecular layer on a MoS2 surface was observed in the AFM image. In the case of TiOPc, in contrast, the ®rst layer consisted of two molecular layers formed at room temperature, and the formation of one molecular layer occurred at the annealing at 420 K. These results indicate that the interaction between the VOPc molecule and the MoS2 surface is stronger than that between TiOPc and MoS2. This suggests that the overlap of orbitals of the transition metals at the center of the phthalocyanines with the orbitals of MoS2 substrate creates a large stabilization energy. The results of EELS measurement indicate that the molecular plane of VOPc is parallel to the MoS2 surface, while the molecular plane of TiOPc is inclined. q 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Phthalocyanine; Epitaxial growth; Atomic force microscopy; Electron energy loss spectroscopy

1. Introduction Epitaxial growth of thin organic ®lms has attracted much attention toward applying unique properties of organic molecules to optical and electronic devices. Among these, epitaxial growth and structural study by electron diffraction of metal-phthalocyanine (MPc) ®lms have been extensively pursued because of their unique properties and stability against heat and electron beam [1±7]. MPc ®lms, whose properties such as non-linear optical susceptibility are easily modi®ed by the crystal structure [8], are particularly interesting from the viewpoint of structure-controlled epitaxial growth. Control of the crystal structure of epitaxially grown organic ®lms requires investigation of the ®lm structure at monolayer-order thickness as the initial state of the ®lm growth. Growth of vanadyl-phthalocyanine (VOPc) ®lm on a MoS2 cleaved surface is known to proceed in layerby-layer mode [9], and it is suitable for this purpose because monolayer ®lm can be easily obtained. In this work, we report on the structure of VOPc ®lm on MoS2 cleaved surface at the initial stage of epitaxial growth revealed by atomic force microscopy (AFM) and electron

* Corresponding author. Present address: Venture Business Laboratory, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan; e-mail: [email protected].

energy loss spectroscopy (EELS) measurements, in comparison with that of titanyl-phthalocyanine (TiOPc) ®lm. AFM is one of the most powerful methods to observe directly an actual image of the ®lm, and information about the orientation and electronic state of molecules can be obtained by EELS measurements. The effect of the central transition metals of Pc molecules, V and Ti, which are neighborhoods in the periodic table, on growth manner and molecular arrangement of MPc ®lms is discussed. 2. Experimental Epitaxial growth was performed in an ultrahigh vacuum chamber with a base pressure of 2 £ 1027 Pa. Commercial VOPc and TiOPc powder was puri®ed by vacuum sublimation and charged into a Knudsen cell. MoS2 substrates were prepared by cleaving natural molybdenite and annnealing the specimens at 620 K for 1 h. After annealing, the substrate was cooled to room temperature and epitaxial growth was carried out. The thickness of the ®lms was estimated from the reading of a quartz oscillator thickness monitor placed close to the sample holder. The growth rate was set at 0.3 molecular layers/min when the source temperature was about 570 K. The samples were transferred in vacuo into the ultrahigh vacuum chamber for EELS measurements with a base pressure of 5 £ 1028 Pa. The EELS system is composed of a Vacuum Generators

0040-6090/98/$ - see front matter q 1998 Published by Elsevier Science Ltd. All rights reserved. PII S0 040-6090(98)009 11-0

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Fig. 1. AFM images (10 £ 10 mm 2) of VOPc ®lms on MoS2 cleaved surfaces with 0.8 MLE (a) and 1.3 MLE thickness (b). The step height of the ®rst unit layer observed in (a) and (b) is 0.4 nm and that of the second unit layer observed in (b) is 0.7 nm.

ARUPS10 angle-resolving analyzer and EMU50 electron monochromator. The energy resolution was set at about 0.05 eV. EELS measurements were performed with an incident electron energy (Ep) of 10 eV. The incident angle was ®xed at 608 and the scattering angles were set at 60 and 508 for specular and off-specular re¯ection conditions, respectively. AFM images were taken in air just after taking the samples out of the vacuum chamber, using the tapping mode of Digital Instruments Nanoscope III system. 3. Results and discussion 3.1. AFM studies Fig. 1a,b shows AFM images of VOPc ®lms on MoS2 cleaved surfaces with 0.8 MLE and 1.3 MLE thickness, respectively. Here, MLE stands for the amount of Pc mole-

cules necessary to cover a MoS2 surface with the molecular arrangement shown in Fig. 1c in Ref. [9]. In Fig. 1a, the ®rst unit layer of the VOPc ®lm with 0.4 nm thickness can clearly be seen. Unit layer stands for the layer observed as a single step in the AFM image. This indicates that the ®rst unit layer consists of one monolayer. In Fig. 1b, the second unit layer with 0.7 nm thickness, which is equivalent to two monolayers, can be seen on the ®rst unit layer with 0.4 nm thickness. Film growth in which the unit layer consists of two monolayers was observed by Tada et al. in an AFM image of a VOPc multilayer ®lm on a MoS2 surface [9], and also observed by Aoki et al. [10,11] and Ueno et al. [12] in the growth of ClAlPc ®lms analyzed by Penning ionization electron spectroscopy. It is proved by the present results that the second unit layer of the VOPc ®lm also consists of two monolayers as in the case of multilayer ®lm. Fig. 2a±c shows AFM images of TiOPc ®lms on MoS2

Fig. 2. AFM images of TiOPc ®lms on MoS2 cleaved surfaces. (a) (2 £ 2 mm 2) and (b) (5 £ 5 mm 2) are those of 1.0 MLE ®lm before and after annealing at 420 K for 1 h, respectively. (c) (5 £ 5 mm 2) is that of 2.0 MLE ®lm after annealing at 420 K. The step height of the ®rst unit layer observed in (a) is 0.7 nm, while that in (b) is 0.4 nm. The step heights of the second and third unit layers in (c) are 0.7 nm.

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Fig. 3. EEL spectra of a VOPc ®lm with the thickness of 1.0 MLE on a MoS2 surface, taken under specular (solid line) and off-specular (dotted line) re¯ection conditions.

second layer are also composed of two monolayers in the case of TiOPc growth. The difference of the metal atom at the center of phthalocyanine (Pc) ring is supposed to be ineffective to electrostatic and van der Waals energies between the molecules and MoS2 surface because electronegativity and electronic polarizability are similar between Ti atom and V atom, which are neighbors in the periodic table. Therefore, it is necessary to take account of the effect of the electronic state of the central metal to explain the difference of the growth manner between VOPc and TiOPc. In the case of TiOPc, all of the valence electrons of the Ti atom are used by the TivO and TiZN bonds. In contrast, the VOPc molecule has an electron remaining at the 3d orbital of the V atom. The overlap of 3d orbital of central metal atom and the valence and conduction band of MoS2 may stabilize the remaining electron in the case of VOPc. 3.2. EELS measurements

cleaved surfaces. Fig. 2a,b shows 1.0 MLE ®lm before and after annealing at 420 K for 1 h, respectively. Fig. 2c is that of 2.0 MLE ®lm after annealing at 420 K. In the case of the ®lm as-grown at room temperature (Fig. 2a), the ®rst unit layer is seen to have the thickness of 0.7 nm, which is equivalent to two monolayers. This result indicates that TiOPc molecules on a MoS2 surface are less stable than VOPc molecules. Small grains remain on the ®rst unit layer, which reveals the suppression of migration of the TiOPc molecule on the surface of the TiOPc itself. The ®rst unit layer, consisting of one monolayer with the thickness of 0.4 nm, is observed after annealing (Fig. 2b). In Fig. 2c, the second and third unit layers with the thickness of 0.7 nm are observed. This indicates that the unit layer after the

Fig. 4. The ®lm thickness dependence of the intensity enhancement of Q band peak which is normalized by the intensity enhancement of primary peak at 0 eV. (ILs 2 ILos ) stands for the difference between the intensity of loss peak at 1.8 eV under specular condition and that under off-specular condition. (IPs 2 IPos ) stands for the difference between the intensity of primary peak at 0 eV under the specular condition and that under the offspecular condition.

Fig. 3 shows electron energy loss (EEL) spectra of a VOPc ®lm with the thickness of 1.0 MLE on a MoS2 surface, taken under specular and off-specular re¯ection conditions. The peak observed at 0.38 eV corresponds to the n (C-H) vibration, and the peaks at 1.80 and 3.66 eV correspond to the electronic transitions called Q band and B band, respectively. The selection rule of EELS [13,14] requires that the transitions with dipole moments normal to the sample surface are emphasized in the spectrum under the specular condition, compared with the off-specular condition. Because the Q band is attributed to 2a1u±6eg (highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) of Pc ring) transition in the case of MPcs with D4h symmetry [15], Q band should be attributed to a2±e transition in the case of MPcs with C4v symmetry. Because the direct product representation of A2 and E is reduced to E representation, both the Q band and the n (C-H) mode have transition dipole moments parallel to the VOPc molecular plane. However, only the Q band peak at 1.8 eV is enhanced in the spectrum under the specular condition. The behavior of the n (C-H) peak indicates that the orientation of the VOPc molecular plane is parallel to the MoS2 surface. Therefore, the enhancement of the Q band peak cannot be attributed to the inclination of VOPc molecules. Fig. 4 shows the ®lm thickness dependence of the intensity enhancement of the Q band peak, which is normalized by the intensity enhancement of the primary peak at 0 eV. The intensity enhancement vanishes with increasing ®lm thickness. This result suggests that the dipole moment normal to the surface exists only at the interface between VOPc and MoS2. The change in the dipole moment of the Q band indicates that the symmetry of VOPc molecules (C4v) is reduced to Cs in respect to HOMO and LUMO of Pc ring, which are concerned with Q band transition. This change in symmetry can be induced by the interaction between the orbitals (HOMO and LUMO of Pc ring) of VOPc molecules

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Fig. 5. EEL spectra of a TiOPc ®lm with 1.0 MLE thickness after annealing at 420 K taken under specular (solid line) and off-specular (dotted line) re¯ection conditions.

Fig. 6. Intensity enhancement (IS 2 IOS ) of the spectra of a TiOPc ®lm with 1.0 ML thickness before (broken line) and after annealing (solid line). They are normalized by the value of intensity enhancement at 0 eV.

which have 4-fold symmetry and the orbitals of MoS2 substrate which have 3-fold symmetry. Fig. 5 shows the EEL spectra of a TiOPc ®lm with 1.0 ML thickness after annealing at 420 K and the formation of the monolayer ®lm. The intensity enhancement of the n (C-H) peak under the specular condition suggests inclination of the molecular plane of TiOPc, and the enhancement of the Q band peak is also observed. The inclination of the TiOPc molecules on MoS2 surface suggests that the interaction between TiOPc and MoS2 is weaker than that between VOPc and MoS2. Fig. 6 shows intensity enhancements of the spectra of a TiOPc ®lm with 1.0 ML thickness before and after annealing. The decrease in the n (C±H) enhancement by the annealing indicates that the molecular plane is less inclined by the formation of monolayer ®lm, and the increase in the Q band enhancement indicates the increase of the molecules in contacting with the MoS2 surface.

that the enhancement of Q band was observed in EELS measurements of monolayer ®lms suggests that the HOMO and LUMO of the Pc ring are affected by the orbitals of the MoS2 substrate.

4. Conclusions The initial states of epitaxial growth of VOPc and TiOPc ®lms on a MoS2 surface were investigated using AFM and EELS. AFM studies revealed that VOPc molecules form monolayer ®lm with room temperature while TiOPc molecules need annealing to form a monolayer. The EELS measurements revealed that the molecular plane of VOPc is parallel to the MoS2 surface while that of TiOPc is inclined. These differences indicate that the interaction between the VOPc molecule and the MoS2 surface is stronger than that between TiOPc and MoS2. This difference in the interaction was ascribed to the strong interaction of the V atom at the center of Pc ring with MoS2 surface. The fact

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