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Infrared spectroscopy of pentacene thin film on SiO2 surface
Applied Surface Science 244 (2005) 607–610 www.elsevier.com/locate/apsusc
Infrared spectroscopy of pentacene thin film on SiO2 surface Yoshinobu Hosoia,b, Koshi Okamuraa,b, Yasuo Kimuraa,b, Hisao Ishiia,b,*, Michio Niwanoa,b a
Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Received 31 May 2004; accepted 5 October 2004 Available online 6 January 2005
Abstract The thin film of pentacene on a SiO2 surface has been investigated by infrared spectroscopy in the multiple internal reflections (MIR) mode. It was found that the molecules in the monolayer are arranged with their molecular axes perpendicular to the surface, and that this arrangement is conserved during film growth up to a 70-nm thickness. In addition, the assignment of the infrared active vibrational modes is discussed. # 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 78.30.Jw Keywords: Pentacene; Infrared spectroscopy; Film structure; SiO2 surface
1. Introduction Pentacene is one of the promising materials for organic field effect transistors (OFETs) due to its large mobility. The electronic characteristics of a pentacene thin film are dependent on the film structure such as its molecular orientation and morphology. Several groups have investigated this film structure by X-ray diffraction (XRD) and atomic force microscopy (AFM), and shown that the well-ordered structure * Corresponding author. Tel.: +81 22 217 5504; fax: +81 22 217 5503. E-mail address: [email protected] (H. Ishii).
and the large grain size in the thin film contribute to the good FET performance [1,2]. However, few investigations have been performed with respect to the interface structure between the monolayer and the SiO2 surface as a typical dielectric layer, which plays a crucial role in the accumulation and transport of charges in OFETs [3,4]. Infrared spectroscopy in the multiple internal reflections (MIR) geometry is a powerful tool for the investigation of the structures of organic films based on molecular vibrations. Infrared light penetrates a silicon or GaAs substrate with a SiO2 layer, which works as a prism in the MIR setup, following multiple internal reflections at the interface between
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.131
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Y. Hosoi et al. / Applied Surface Science 244 (2005) 607–610
the substrate and SiO2 which generates the evanescent wave interacting with the molecular vibrations of the adsorbates on the surface. The reflections of several of tens of times allow us to detect weak bands at the submonolayer regime. In addition, the MIR measurement with polarized light can be applied to the analysis of the molecular orientation of pentacene in a thin film. In this study, the structure and growth process of the pentacene thin film on SiO2 from the submonolayer to multilayer have been investigated by in situ MIR measurement.
2. Experimental In our experiment, two substrates covered with SiO2 layers were used in order to discuss the entire region of the vibrational modes of pentacene. One was a non-doped GaAs(1 0 0) wafer with a SiO2 layer prepared by RF sputtering for the observation in the low-frequency region around 800–2000 cm 1. The other was an n-type phosphorus-doped Si(1 0 0) wafer (8–12 V cm) with an oxide layer prepared by the treatment of the mixture of H2SO4 and H2O2 (1:1) for the high frequency around 3000 cm 1. Preparation of the pentacene thin films and MIR measurements were carried out in a vacuum chamber with a base pressure of 1 10 6 Pa. The detailed geometry is described in Ref. [5]. Pentacene was deposited on the substrates at room temperature with the deposition rate of 0.1–0.4 nm/min.
3. Results and discussion At first, we have performed the assignment of the infrared-active vibrational modes. The spectrum in the low-frequency region of the 70-nm thick pentacene film is shown in Fig. 1 together with that by KBr method. In the KBr disk including the bulk crystal in random orientation, all of the vibrational modes with infrared activity are observable. On the other hand, the MIR measurement with s-polarized light enables us to observe only the modes which have transition dipole moments parallel to the surface. A pentacene molecule belongs to the D2h symmetry, and its vibrational modes are assigned to the three irreducible representations, b1u, b2u and b3u, corresponding to
Fig. 1. Infrared spectra of the bulk pentacene crystal measured by KBr method and the MIR spectrum of the thin film with the various thicknesses on the SiO2 surface from 800 to 2000 cm 1. The inset is a schematic view of the molecular structure of pentacene.
their transition moments parallel to the X, Yand Z axes, respectively. Our X-ray diffraction analysis revealed that the Y-axis of the pentacene molecules is nearly perpendicular to the surface in the 70-nm thick film. It is expected that the vibrational modes, which have the transition dipole moments parallel to the Y-axis (Y mode), cannot be observed in the MIR measurement. The bands at 1443, 1499, 1518 and 1537 cm 1 (gray area in Fig. 1) are actually missing in the MIR spectrum shown in Fig. 1(e), indicating that these bands are assigned to the Y modes. This assignment agrees with the previous observation using the matrix isolation technique and theoretical calculation [6]. The two bands at 904 and 1296 cm 1 assigned to the Z and X modes, respectively, are strongly observed in the MIR spectrum since these two modes are parallel to the surface. The frequencies and relative intensities of the principal normal modes in the experimental results are shown in Table 1. They are compared with those of the calculated infrared-active modes. In the high-frequency region around 3000 cm 1 where CH stretching vibrational modes are mainly observed, there are five main bands at 3015, 3028, 3043, 3053 and 3073 cm 1 in the bulk crystal measured by the KBr disk method shown in Fig. 2(a). We called these bands ‘‘nn(CH) (n = 1– 5)’’. Table 1 indicates that the experimental frequencies and intensities in most of the modes do not fit those of the calculation. The difference may be caused by the presence of overtones and combinations. Thus, the precise assignment requires a comparison between
Y. Hosoi et al. / Applied Surface Science 244 (2005) 607–610
609
Table 1 Frequencies (cm 1) and relative intensities for principal vibrational modes of pentacene in the experiment and theoretical calculations Theorya Frequency
CH out-of-plane (Z) Ring str. (X) Ring str. (Y) n1(CH) (Y) n2(CH) n3(CH) (X) n4(CH) (Y) n5(CH) (X) a b c
Experimental Relative intensity
879.1 1.00b 1269.9 0.30b 1428.4 0.00b 3047.2 0.04c Convolution of several modes 3057.0 0.06c 3070.4 1.00c 3082.2 1.46c
Bulk crystal in KBr
Thin film by MIR
Frequency
Relative intensity
Frequency
Relative intensity
907 1296 1443 3015 3028 3043 3053 3073
1.00b 0.23b 0.08b 0.38c 0.72c 1.49c 1.00c 0.79c
904 1296 1443 3012 3025 3043 3051 –
1.00b 0.14b 0.01b 0.55c 0.71c 0.40c 1.00c –
Gaussian’98 program at the B3LYP/6-31G(d,p) level [7] with a scaling factor of 0.9613 [8]. Intensities relative to the band around 900 cm 1. Intensities relative to the band at 3051–3053 cm 1 (3070.4 cm 1 in the theory).
the spectra of the KBr and MIR. In the MIR spectrum of the thin film at 50 nm in Fig. 2(c), the n1(CH) and n4(CH) modes have a strong intensity. The intensity of the n3(CH) mode, which is the strongest of the five modes in the bulk crystal, becomes weaker and the band of the n5(CH) mode is out of the detection limit. We conclude that the n3(CH) and n5(CH) modes are assigned to the Y modes and conversely the n1(CH) and n4(CH) modes are the X modes. The n2(CH) mode could not be assigned because of no significant change between the bulk crystal and thin film. It is expected that the broad band of n2(CH) includes several modes with their dipole moments parallel to the X or Y axis.
Fig. 2. Infrared spectra of the bulk pentacene crystal measured by KBr method and the MIR spectrum of the thin film with the various thicknesses on the SiO2 surface around 3000 cm 1.
Next, the growth process of the pentacene thin film has been examined from the MIR spectra, based on the above-mentioned assignment. The MIR spectra of the thin films for different thicknesses are demonstrated in Fig. 1(b)–(e) in the low-frequency region. The spectra of the submonolayer and monolayer regimes at 0.45 and 1.4 nm have features similar to those of the multilayer at 14 and 70 nm. The X and Z modes at 1296 and 904 cm 1 have bands with a strong intensity and the Y modes around 1400–1600 cm 1 disappear. The positions of the X, Y and Z modes are similar to those in the bulk crystal, and there is no additional band. This indicates that the molecules with their Yaxes perpendicular to the surface are arranged in the monolayer without any strong interactions between the molecules and the SiO2 surface. The evolution of the bands with respect to the film growth indicates that the molecular orientation is conserved from the submonolayer to multilayer. This finding is supported by the constant position, FWHM and relative intensity of each band with the increasing thickness. This result is consistent with the previous results [1–4]. A similar conclusion was obtained from the result of the high-frequency region, as shown in Fig. 2(b) and (c). The spectrum of the monolayer at 1.7 nm is similar to that of the multilayer at 50 nm although the spectrum includes the fluctuating background. The n1(CH) and n4(CH) modes assigned to the X modes have strong intensities. On the contrary, the intensity of the n3(CH) and n5(CH) modes assigned to the Y
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Y. Hosoi et al. / Applied Surface Science 244 (2005) 607–610
modes are very weak. From the submonolayer to multilayer, the position, FWHM and relative intensity of each band are constant, and no additional peaks were observed, suggesting a van der Waal’s interaction at the interfaces.
References [1] I.P.M. Bouchoms, W.A. Schoonveld, J. Vrijmoeth, T.M. Klapwijk, Synth. Met. 104 (1999) 175.
[2] C.C. Mattheus, A.B. Dros, J. Baas, G.T. Oostergetel, A. Meetsma, J.L. de Boer, T.T.M. Palstra, Synth. Met. 138 (2003) 475. [3] F.J. Meyer zu Heringdorf, M.C. Reuter, R.M. Tromp, Nature 412 (2001) 517. [4] R. Ruiz, B. Nickel, N. Koch, L.C. Feldman, R.F. Haglund, A. Kahn, G. Scoles, Phys. Rev. B 67 (2003) 125406. [5] M. Niwano, M. Terashi, M. Shinohara, D. Shoji, Surf. Sci. 401 (1998) 364. [6] D.M. Hudgins, S.A. Sandford, J. Phys. Chem. A 102 (1998) 344. [7] M.J. Frisch, et al. Gaussian 98, Revision A.7, Gaussian, Inc., Pittsburgh, PA, 1998. [8] M.W. Wong, Chem. Phys. Lett. 256 (1996) 391.
Infrared spectroscopy of pentacene thin film on SiO2 surface Yoshinobu Hosoia,b, Koshi Okamuraa,b, Yasuo Kimuraa,b, Hisao Ishiia,b,*, Michio Niwanoa,b a
Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Received 31 May 2004; accepted 5 October 2004 Available online 6 January 2005
Abstract The thin film of pentacene on a SiO2 surface has been investigated by infrared spectroscopy in the multiple internal reflections (MIR) mode. It was found that the molecules in the monolayer are arranged with their molecular axes perpendicular to the surface, and that this arrangement is conserved during film growth up to a 70-nm thickness. In addition, the assignment of the infrared active vibrational modes is discussed. # 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 78.30.Jw Keywords: Pentacene; Infrared spectroscopy; Film structure; SiO2 surface
1. Introduction Pentacene is one of the promising materials for organic field effect transistors (OFETs) due to its large mobility. The electronic characteristics of a pentacene thin film are dependent on the film structure such as its molecular orientation and morphology. Several groups have investigated this film structure by X-ray diffraction (XRD) and atomic force microscopy (AFM), and shown that the well-ordered structure * Corresponding author. Tel.: +81 22 217 5504; fax: +81 22 217 5503. E-mail address: [email protected] (H. Ishii).
and the large grain size in the thin film contribute to the good FET performance [1,2]. However, few investigations have been performed with respect to the interface structure between the monolayer and the SiO2 surface as a typical dielectric layer, which plays a crucial role in the accumulation and transport of charges in OFETs [3,4]. Infrared spectroscopy in the multiple internal reflections (MIR) geometry is a powerful tool for the investigation of the structures of organic films based on molecular vibrations. Infrared light penetrates a silicon or GaAs substrate with a SiO2 layer, which works as a prism in the MIR setup, following multiple internal reflections at the interface between
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.131
608
Y. Hosoi et al. / Applied Surface Science 244 (2005) 607–610
the substrate and SiO2 which generates the evanescent wave interacting with the molecular vibrations of the adsorbates on the surface. The reflections of several of tens of times allow us to detect weak bands at the submonolayer regime. In addition, the MIR measurement with polarized light can be applied to the analysis of the molecular orientation of pentacene in a thin film. In this study, the structure and growth process of the pentacene thin film on SiO2 from the submonolayer to multilayer have been investigated by in situ MIR measurement.
2. Experimental In our experiment, two substrates covered with SiO2 layers were used in order to discuss the entire region of the vibrational modes of pentacene. One was a non-doped GaAs(1 0 0) wafer with a SiO2 layer prepared by RF sputtering for the observation in the low-frequency region around 800–2000 cm 1. The other was an n-type phosphorus-doped Si(1 0 0) wafer (8–12 V cm) with an oxide layer prepared by the treatment of the mixture of H2SO4 and H2O2 (1:1) for the high frequency around 3000 cm 1. Preparation of the pentacene thin films and MIR measurements were carried out in a vacuum chamber with a base pressure of 1 10 6 Pa. The detailed geometry is described in Ref. [5]. Pentacene was deposited on the substrates at room temperature with the deposition rate of 0.1–0.4 nm/min.
3. Results and discussion At first, we have performed the assignment of the infrared-active vibrational modes. The spectrum in the low-frequency region of the 70-nm thick pentacene film is shown in Fig. 1 together with that by KBr method. In the KBr disk including the bulk crystal in random orientation, all of the vibrational modes with infrared activity are observable. On the other hand, the MIR measurement with s-polarized light enables us to observe only the modes which have transition dipole moments parallel to the surface. A pentacene molecule belongs to the D2h symmetry, and its vibrational modes are assigned to the three irreducible representations, b1u, b2u and b3u, corresponding to
Fig. 1. Infrared spectra of the bulk pentacene crystal measured by KBr method and the MIR spectrum of the thin film with the various thicknesses on the SiO2 surface from 800 to 2000 cm 1. The inset is a schematic view of the molecular structure of pentacene.
their transition moments parallel to the X, Yand Z axes, respectively. Our X-ray diffraction analysis revealed that the Y-axis of the pentacene molecules is nearly perpendicular to the surface in the 70-nm thick film. It is expected that the vibrational modes, which have the transition dipole moments parallel to the Y-axis (Y mode), cannot be observed in the MIR measurement. The bands at 1443, 1499, 1518 and 1537 cm 1 (gray area in Fig. 1) are actually missing in the MIR spectrum shown in Fig. 1(e), indicating that these bands are assigned to the Y modes. This assignment agrees with the previous observation using the matrix isolation technique and theoretical calculation [6]. The two bands at 904 and 1296 cm 1 assigned to the Z and X modes, respectively, are strongly observed in the MIR spectrum since these two modes are parallel to the surface. The frequencies and relative intensities of the principal normal modes in the experimental results are shown in Table 1. They are compared with those of the calculated infrared-active modes. In the high-frequency region around 3000 cm 1 where CH stretching vibrational modes are mainly observed, there are five main bands at 3015, 3028, 3043, 3053 and 3073 cm 1 in the bulk crystal measured by the KBr disk method shown in Fig. 2(a). We called these bands ‘‘nn(CH) (n = 1– 5)’’. Table 1 indicates that the experimental frequencies and intensities in most of the modes do not fit those of the calculation. The difference may be caused by the presence of overtones and combinations. Thus, the precise assignment requires a comparison between
Y. Hosoi et al. / Applied Surface Science 244 (2005) 607–610
609
Table 1 Frequencies (cm 1) and relative intensities for principal vibrational modes of pentacene in the experiment and theoretical calculations Theorya Frequency
CH out-of-plane (Z) Ring str. (X) Ring str. (Y) n1(CH) (Y) n2(CH) n3(CH) (X) n4(CH) (Y) n5(CH) (X) a b c
Experimental Relative intensity
879.1 1.00b 1269.9 0.30b 1428.4 0.00b 3047.2 0.04c Convolution of several modes 3057.0 0.06c 3070.4 1.00c 3082.2 1.46c
Bulk crystal in KBr
Thin film by MIR
Frequency
Relative intensity
Frequency
Relative intensity
907 1296 1443 3015 3028 3043 3053 3073
1.00b 0.23b 0.08b 0.38c 0.72c 1.49c 1.00c 0.79c
904 1296 1443 3012 3025 3043 3051 –
1.00b 0.14b 0.01b 0.55c 0.71c 0.40c 1.00c –
Gaussian’98 program at the B3LYP/6-31G(d,p) level [7] with a scaling factor of 0.9613 [8]. Intensities relative to the band around 900 cm 1. Intensities relative to the band at 3051–3053 cm 1 (3070.4 cm 1 in the theory).
the spectra of the KBr and MIR. In the MIR spectrum of the thin film at 50 nm in Fig. 2(c), the n1(CH) and n4(CH) modes have a strong intensity. The intensity of the n3(CH) mode, which is the strongest of the five modes in the bulk crystal, becomes weaker and the band of the n5(CH) mode is out of the detection limit. We conclude that the n3(CH) and n5(CH) modes are assigned to the Y modes and conversely the n1(CH) and n4(CH) modes are the X modes. The n2(CH) mode could not be assigned because of no significant change between the bulk crystal and thin film. It is expected that the broad band of n2(CH) includes several modes with their dipole moments parallel to the X or Y axis.
Fig. 2. Infrared spectra of the bulk pentacene crystal measured by KBr method and the MIR spectrum of the thin film with the various thicknesses on the SiO2 surface around 3000 cm 1.
Next, the growth process of the pentacene thin film has been examined from the MIR spectra, based on the above-mentioned assignment. The MIR spectra of the thin films for different thicknesses are demonstrated in Fig. 1(b)–(e) in the low-frequency region. The spectra of the submonolayer and monolayer regimes at 0.45 and 1.4 nm have features similar to those of the multilayer at 14 and 70 nm. The X and Z modes at 1296 and 904 cm 1 have bands with a strong intensity and the Y modes around 1400–1600 cm 1 disappear. The positions of the X, Y and Z modes are similar to those in the bulk crystal, and there is no additional band. This indicates that the molecules with their Yaxes perpendicular to the surface are arranged in the monolayer without any strong interactions between the molecules and the SiO2 surface. The evolution of the bands with respect to the film growth indicates that the molecular orientation is conserved from the submonolayer to multilayer. This finding is supported by the constant position, FWHM and relative intensity of each band with the increasing thickness. This result is consistent with the previous results [1–4]. A similar conclusion was obtained from the result of the high-frequency region, as shown in Fig. 2(b) and (c). The spectrum of the monolayer at 1.7 nm is similar to that of the multilayer at 50 nm although the spectrum includes the fluctuating background. The n1(CH) and n4(CH) modes assigned to the X modes have strong intensities. On the contrary, the intensity of the n3(CH) and n5(CH) modes assigned to the Y
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Y. Hosoi et al. / Applied Surface Science 244 (2005) 607–610
modes are very weak. From the submonolayer to multilayer, the position, FWHM and relative intensity of each band are constant, and no additional peaks were observed, suggesting a van der Waal’s interaction at the interfaces.
References [1] I.P.M. Bouchoms, W.A. Schoonveld, J. Vrijmoeth, T.M. Klapwijk, Synth. Met. 104 (1999) 175.
[2] C.C. Mattheus, A.B. Dros, J. Baas, G.T. Oostergetel, A. Meetsma, J.L. de Boer, T.T.M. Palstra, Synth. Met. 138 (2003) 475. [3] F.J. Meyer zu Heringdorf, M.C. Reuter, R.M. Tromp, Nature 412 (2001) 517. [4] R. Ruiz, B. Nickel, N. Koch, L.C. Feldman, R.F. Haglund, A. Kahn, G. Scoles, Phys. Rev. B 67 (2003) 125406. [5] M. Niwano, M. Terashi, M. Shinohara, D. Shoji, Surf. Sci. 401 (1998) 364. [6] D.M. Hudgins, S.A. Sandford, J. Phys. Chem. A 102 (1998) 344. [7] M.J. Frisch, et al. Gaussian 98, Revision A.7, Gaussian, Inc., Pittsburgh, PA, 1998. [8] M.W. Wong, Chem. Phys. Lett. 256 (1996) 391.