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Surface potentials of patterned organosilane self-assembled monolayers acquired by Kelvin probe force microscopy and ab initio molecular calculation
30 November 2001
Chemical Physics Letters 349 (2001) 172±177 www.elsevier.com/locate/cplett
Surface potentials of patterned organosilane self-assembled monolayers acquired by Kelvin probe force microscopy and ab initio molecular calculation N. Saito a
a,*
, K. Hayashi a, H. Sugimura a, O. Takai a, N. Nakagiri
b
Department of Materials Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan b Core Technology Center, Nikon Co., Shinagawa, Tokyo 140-8601, Japan Received 26 April 2001; in ®nal form 12 September 2001
Abstract This study aimed experimentally and theoretically to reveal the surface potentials of organosilane self-assembled monolayers (SAMs) using Kelvin probe force microscopy (KPFM) and ab initio molecular orbital (MO) calculations, and to distinguish among the surface domains of the SAMs. We prepared the patterned SAMs of n-octadecyltrimethoxysilane [ODS: H3 C CH2 17 Si OCH3 3 ], heptadeca¯uoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [FAS: F3 C CF2 7 CH2 2 Si OCH3 3 ] and n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS: H2 N CH2 6 NH CH2 3 Si OCH3 3 ] by chemical vapor deposition (CVD). The surface potentials for FAS-SAM and AHAPS-SAM vs. ODS-SAM in the atmosphere were )170 and +50 mV, respectively. The experimental surface potentials agreed with the calculated ones at the surface area occupied by a molecule of 1.1±1:5 nm2 molecule 1 . Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Self-assembling organic molecular monolayers have been expected as a main basic technique in the ®eld of molecular devices since they may provide the desired control on monolayer structures at the molecular level and the expansions of the functional organic materials obtained through three-dimensional molecular assemblies [1]. In order to construct such three-dimensional molecular
*
Corresponding author. Fax: +81-52-789-2796. E-mail address: [email protected] (N. Saito).
assemblies, highly selective self-assembling reactions are required. Some minute and selected regions on substrates must recognize speci®c molecules among all the molecules in a chemical reaction system. Thus, evaluation of surface properties of molecular assemblies is of particular importance in order to design structures of molecular assemblies, including self-assembled monolayers (SAMs), that is, one of the simplest molecular assemblies. Many researchers have investigated various SAMs using scanning probe microscopy (SPM) since it is a promising method for measurement of molecular interactions which play key roles in molecular recognition reactions. Kelvin probe force microscopy (KPFM), which is
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 0 9 7 - 1
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
173
a kind of SPM, provides us with the surface potential of SAM [2]. The surface potential is strongly related to its molecular structure, the terminal functional group, orientation, packing density and etc. since the dipole of the SAM governs the change of surface potential. We think that result of the KPFM measurement play an important role in understanding molecular recognition mechanisms and in building molecular devices. KPFM measurements have been used for the molecular recognition of some thiol SAMs on gold substrates [2,3]. In the theoretical approaches of the surface potential, Taylor and Bayes [4±6] predicted the values of Langmuir±Blodgett (LB) ®lms and agreed with experimental ones in some extent. Miura et al. [7] calculated and measured the surface potentials of the thiols with helix peptides on gold substrates and showed that the longer the helix peptide, the larger was the negative surface potential obtained. However, few reports have been made on the surface potential for organosilane SAMs, especially those prepared by chemical vapor deposition (CVD). We investigated the patterned SAMs of n-octadecyltrimethoxysilane [ODS: H3 C CH2 17 Si OCH3 3 ], heptadeca¯uoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [FAS: F3 C CF2 7 CH2 2 Si OCH3 3 ] and n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS: H2 N CH2 6 NH CH2 3 Si OCH3 3 ] by CVD [8±10]. In these previous papers, we mainly focused on CVD and photopatterning conditions for respective SAMs. Moreover we optimized conditions of the KPFM measurements for organosilane SAMs using microstructure consisting of ODS- and FAS-SAMs as an example. In this study, we aim experimentally and theoretically to reveal the surface potentials of ODS, FAS and AHAPS-SAMs using KPFM and ab initio molecular orbital (MO) calculations, and to distinguish among the surface domains of the SAMs.
SPI-3800N) using a gold-coated silicon cantilever. The following details of the KPFM measurements were determined on the basis of our previous research [11,12]. Resonance frequency and Q-factor were 26.27 kHz and approximately 180, respectively. The cantilever was vibrated at a frequency of 27.98 kHz. An ac bias voltage of 2 V at a frequency of 5 kHz was applied between the probe and sample. KPFM images of the sample surface were acquired at a probe scan rate of 0.1 Hz.
2. Experiments
2.3. Calculation
2.1. KPFM measurement
All ab initio MO calculations were performed using GA U S S I A N 98 [13]. The structures and energies were obtained in the hybrid density functional theory with D95 basis set; B3LYP/D95. All
All samples in this study were observed in air with a KPFM (Seiko Instruments, SPA-300HV+
2.2. Preparation and Patterning of SAM The ODS-SAMs, FAS-SAMs and AHAPSSAMs were prepared on the substrates of n-type silicon by CVD. Fig. 1 shows the process from preparation to patterning. In step 1, a silicon surface covered with a thin oxide layer (ca. 1 nm) was hydroxylated and cleaned up at the same time by UV/ozone cleaning method. The source of light in this study was an excimer lamp with k 172 nm 2 and 10 mW=cm (Ushio Electric, UER20-172V). In step 2, a kind of SAM was deposited using the container in oven as a reactor. The reaction temperatures were from 373 to 423 K and the reaction time was 3 h for all samples. The precursor selectively reacts with the hydroxyls on the surface. Namely, we covered all over the surface with the SAM in this step. In step 3, the surface was irradiated under a reduced pressure of 10 Pa with vacuum ultra-violet (VUV) light through a photomask. This process decomposed the chains of SAMs and hydroxylated the siloxane surface by active oxygen species generated from atmospheric oxygen species. In step 4, another kind of SAM was deposited in a similar manner as the step 2. The pattern composing of two kinds of SAMs appeared on the surface covered by the photomask since the precursor for another kind of SAM reacted with the hydroxyls, not with the former SAM.
174
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
Fig. 1. Manufacturing process for patterned organosilane SAMs.
structural parameters were variable in the optimization. 3. Results and discussion Fig. 2 shows the KPFM images of (a) ODS/ FAS-SAM and (b) ODS/AHAPS-SAM. The dark and light domains correspond to the ones of low and high surface potentials in a sample, respectively. The surface potential for ODS-SAM was ca. 170 mV more positive than that of FAS-SAM and, while being, ca. 50 mV more negative than
that of AHAPS. Thus the surface potentials became more positive in the order of FAS-SAM, ODS-SAM and AHAPS-SAM. Here, we discuss how the surface potentials or dierences are obtained. Fig. 3 shows the simple principle of KPFM in this study. This system consists of ®ve parts; a gold-coated silicon probe, a galvanometer, a potentiometer, a gold electrode and a sample. The voltage vs. gold obtained from the potentiometer is equal to the surface potential vs. gold when the potentiometer is adjusted so that no current ¯ows through the galvanometer as a gold-coated silicon probe is vibrated. This potential is equal to the
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
175
Fig. 3. Schematic diagram showing principle of KPFM.
where / i and e are the work function of component i and proton charge, respectively. Eq. (1) is also equal to the following equation obtained from the Helmholtz equation [4±7] lSAM1 lSAM2 1 ; DVSAM1 vs: SAM2 eSAM1 ASAM1 eSAM2 ASAM2 e0 2
Fig. 2. KPFM images for ODS/FAS-SAM and ODS/AHAPSSAM.
sum of the contact potentials in respective layers. The dierence of surface potential between SAM1 and SAM2 is described as [2] DVSAM1
vs: SAM2
f/ SAM1
/ SAM2g=e; 1
where l is the actual value of the dipole moment normal to the substrate for a molecule, A the area occupied by each molecule, and e the dielectric constant, e0 the permittivity of free space. The dierence of surface potential is thus related to the dipole moment and work function of SAMs. The Helmholtz equation was used in the following discussion. We used the following molecules for the model of SAM: H3 C CH2 15 CH3 for ODS, F3 C CF2 7 CH2 CH3 for FAS and H2 N CH2 6 NH CH2 2 CH3 for AHAPS, since, except for these molecules, the other structures were the same. Fig. 4 shows the structures and dipole moments for those molecules. The structures of ODS-molecule, FASmolecule and AHAPS-molecule were anti, gauche and anti forms, respectively. The form of the FASmolecule stands to reason because of the large steric hindrance among ¯uorines. The ODS-molecule, FAS-molecule and AHAPS-molecule have net dipole moments of 0.03, 2.38 and 0.57 Debye, respectively. Assuming that these alkyls stand normal to the substrate, the dipole moments normal to the substrate for the ODS-molecule, FAS-molecule and AHAPS-molecule were 0.03 cos(90.0°),
176
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
Fig. 4. Molecular structures and dipole moments for ODSSAM, FAS-SAM and AHAPS-SAM.
2.38 cos(25.3°) and 0.57 cos(12.4°), respectively. Thus, the potential of ODS-SAM does not depend on its molecular packing density since the dipole
moment normal to the substrate for each of ODSmolecules is always equal to zero in this model. The dielectric constants for all SAMs used were approximately used the same value of 3.0 [6]. When these properties are substituted to Eq. (2), the relationship between the surface potential and the area occupied by each molecule are obtained. Fig. 5 shows the calculated surface potentials vs. ODS-SAM. The experimental surface potentials agreed with the calculated ones at the A of 1.1 to 1:5 nm2 molecule 1 . The organosilane LB ®lms generally had the A of 0.1 to 0:8 nm2 molecule 1 [14]. The packing densities of SAMs prepared by CVD were smaller than those of organosilane LB ®lms. Generally, LB ®lms are arti®cially prepared at the higher surface pressures than the equilibrium ones, which correspond to those of SAMs. The predicted packing densities for SAMs are reasonable from this viewpoint. This result also indicates that the surface potential for SAM aids the understanding of the reaction mechanism for SAM±CVD. In conclusion, we have reported the details of the surface potential of ODS-SAM, FAS-SAM and AHAPS-SAM using KPFM and ab initio MO calculation. We believe that the molecular recognition described in this study can be useful in creating molecular devices in the future. Acknowledgements This work has been supported by the Research Project `Biomimetic Materials Processing' (No. JSPS-RFTF 99R13101), Research for the Future (RFTF) Program, Japan Society for the Promotion of Science. References
Fig. 5. Calculated surface potential for FAS-SAM and AHAPS-SAM vs. ODS-SAM.
[1] A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, Boston, MA, 1991. [2] M. Fujihira, Annu. Rev. Mater. Sci. 29 (1999) 353. [3] J. Lu, E. Delamarche, L. Eng, R. Bennewitz, E. Meyer, H.J. Guntherodt, Langmuir 15 (1999) 8184. [4] D.M. Taylor, G.F. Bayes, Phys. Rev. E 49 (1994) 1439. [5] D.M. Taylor, G.F. Bayes, Mater. Sci. Eng. C8±C9 (1999) 65. [6] D.M. Taylor, Colloid Interface Sci. 87 (2000) 183.
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177 [7] Y. Miura, S. Kimura, S. Kobayashi, M. Iwamoto, Y. Imanishi, J. Umemura, Chem. Phys. Lett. 315 (1999) 1. [8] A. Hozumi, K. Ushiyama, H. Sugimura, O. Takai, Langmuir 15 (1999) 7600. [9] H. Sugimura, K. Ushiyama, A. Hozumi, O. Takai, Langmuir 16 (2000) 885. [10] A. Hozumi, Y. Yokogawa, T. Kameyama, H. Sugimura, et al., J. Vac. Sci. Technol. A 19 (2001) 1812.
177
[11] H. Sugimura, H. Hayashi, N. Saito, O. Takai, N. Nakagiri, Jpn. J. Appl. Phys. 40 (2001) L174. [12] H. Sugimura, H. Hayashi, N. Saito, O. Takai, N. Nakagiri, Jpn. J. Appl. Phys. 40 (2001) 4373. [13] M.J. Frisch et al., GA U S S I A N 98, Revision A.9, Gaussian Inc, Pittsburgh, PA, 1998. [14] K. Kojio, A. Takahara, T. Kajiyama, Colloids Surf. A 169 (2000) 295.
Chemical Physics Letters 349 (2001) 172±177 www.elsevier.com/locate/cplett
Surface potentials of patterned organosilane self-assembled monolayers acquired by Kelvin probe force microscopy and ab initio molecular calculation N. Saito a
a,*
, K. Hayashi a, H. Sugimura a, O. Takai a, N. Nakagiri
b
Department of Materials Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan b Core Technology Center, Nikon Co., Shinagawa, Tokyo 140-8601, Japan Received 26 April 2001; in ®nal form 12 September 2001
Abstract This study aimed experimentally and theoretically to reveal the surface potentials of organosilane self-assembled monolayers (SAMs) using Kelvin probe force microscopy (KPFM) and ab initio molecular orbital (MO) calculations, and to distinguish among the surface domains of the SAMs. We prepared the patterned SAMs of n-octadecyltrimethoxysilane [ODS: H3 C CH2 17 Si OCH3 3 ], heptadeca¯uoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [FAS: F3 C CF2 7 CH2 2 Si OCH3 3 ] and n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS: H2 N CH2 6 NH CH2 3 Si OCH3 3 ] by chemical vapor deposition (CVD). The surface potentials for FAS-SAM and AHAPS-SAM vs. ODS-SAM in the atmosphere were )170 and +50 mV, respectively. The experimental surface potentials agreed with the calculated ones at the surface area occupied by a molecule of 1.1±1:5 nm2 molecule 1 . Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Self-assembling organic molecular monolayers have been expected as a main basic technique in the ®eld of molecular devices since they may provide the desired control on monolayer structures at the molecular level and the expansions of the functional organic materials obtained through three-dimensional molecular assemblies [1]. In order to construct such three-dimensional molecular
*
Corresponding author. Fax: +81-52-789-2796. E-mail address: [email protected] (N. Saito).
assemblies, highly selective self-assembling reactions are required. Some minute and selected regions on substrates must recognize speci®c molecules among all the molecules in a chemical reaction system. Thus, evaluation of surface properties of molecular assemblies is of particular importance in order to design structures of molecular assemblies, including self-assembled monolayers (SAMs), that is, one of the simplest molecular assemblies. Many researchers have investigated various SAMs using scanning probe microscopy (SPM) since it is a promising method for measurement of molecular interactions which play key roles in molecular recognition reactions. Kelvin probe force microscopy (KPFM), which is
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 0 9 7 - 1
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
173
a kind of SPM, provides us with the surface potential of SAM [2]. The surface potential is strongly related to its molecular structure, the terminal functional group, orientation, packing density and etc. since the dipole of the SAM governs the change of surface potential. We think that result of the KPFM measurement play an important role in understanding molecular recognition mechanisms and in building molecular devices. KPFM measurements have been used for the molecular recognition of some thiol SAMs on gold substrates [2,3]. In the theoretical approaches of the surface potential, Taylor and Bayes [4±6] predicted the values of Langmuir±Blodgett (LB) ®lms and agreed with experimental ones in some extent. Miura et al. [7] calculated and measured the surface potentials of the thiols with helix peptides on gold substrates and showed that the longer the helix peptide, the larger was the negative surface potential obtained. However, few reports have been made on the surface potential for organosilane SAMs, especially those prepared by chemical vapor deposition (CVD). We investigated the patterned SAMs of n-octadecyltrimethoxysilane [ODS: H3 C CH2 17 Si OCH3 3 ], heptadeca¯uoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [FAS: F3 C CF2 7 CH2 2 Si OCH3 3 ] and n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS: H2 N CH2 6 NH CH2 3 Si OCH3 3 ] by CVD [8±10]. In these previous papers, we mainly focused on CVD and photopatterning conditions for respective SAMs. Moreover we optimized conditions of the KPFM measurements for organosilane SAMs using microstructure consisting of ODS- and FAS-SAMs as an example. In this study, we aim experimentally and theoretically to reveal the surface potentials of ODS, FAS and AHAPS-SAMs using KPFM and ab initio molecular orbital (MO) calculations, and to distinguish among the surface domains of the SAMs.
SPI-3800N) using a gold-coated silicon cantilever. The following details of the KPFM measurements were determined on the basis of our previous research [11,12]. Resonance frequency and Q-factor were 26.27 kHz and approximately 180, respectively. The cantilever was vibrated at a frequency of 27.98 kHz. An ac bias voltage of 2 V at a frequency of 5 kHz was applied between the probe and sample. KPFM images of the sample surface were acquired at a probe scan rate of 0.1 Hz.
2. Experiments
2.3. Calculation
2.1. KPFM measurement
All ab initio MO calculations were performed using GA U S S I A N 98 [13]. The structures and energies were obtained in the hybrid density functional theory with D95 basis set; B3LYP/D95. All
All samples in this study were observed in air with a KPFM (Seiko Instruments, SPA-300HV+
2.2. Preparation and Patterning of SAM The ODS-SAMs, FAS-SAMs and AHAPSSAMs were prepared on the substrates of n-type silicon by CVD. Fig. 1 shows the process from preparation to patterning. In step 1, a silicon surface covered with a thin oxide layer (ca. 1 nm) was hydroxylated and cleaned up at the same time by UV/ozone cleaning method. The source of light in this study was an excimer lamp with k 172 nm 2 and 10 mW=cm (Ushio Electric, UER20-172V). In step 2, a kind of SAM was deposited using the container in oven as a reactor. The reaction temperatures were from 373 to 423 K and the reaction time was 3 h for all samples. The precursor selectively reacts with the hydroxyls on the surface. Namely, we covered all over the surface with the SAM in this step. In step 3, the surface was irradiated under a reduced pressure of 10 Pa with vacuum ultra-violet (VUV) light through a photomask. This process decomposed the chains of SAMs and hydroxylated the siloxane surface by active oxygen species generated from atmospheric oxygen species. In step 4, another kind of SAM was deposited in a similar manner as the step 2. The pattern composing of two kinds of SAMs appeared on the surface covered by the photomask since the precursor for another kind of SAM reacted with the hydroxyls, not with the former SAM.
174
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
Fig. 1. Manufacturing process for patterned organosilane SAMs.
structural parameters were variable in the optimization. 3. Results and discussion Fig. 2 shows the KPFM images of (a) ODS/ FAS-SAM and (b) ODS/AHAPS-SAM. The dark and light domains correspond to the ones of low and high surface potentials in a sample, respectively. The surface potential for ODS-SAM was ca. 170 mV more positive than that of FAS-SAM and, while being, ca. 50 mV more negative than
that of AHAPS. Thus the surface potentials became more positive in the order of FAS-SAM, ODS-SAM and AHAPS-SAM. Here, we discuss how the surface potentials or dierences are obtained. Fig. 3 shows the simple principle of KPFM in this study. This system consists of ®ve parts; a gold-coated silicon probe, a galvanometer, a potentiometer, a gold electrode and a sample. The voltage vs. gold obtained from the potentiometer is equal to the surface potential vs. gold when the potentiometer is adjusted so that no current ¯ows through the galvanometer as a gold-coated silicon probe is vibrated. This potential is equal to the
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
175
Fig. 3. Schematic diagram showing principle of KPFM.
where / i and e are the work function of component i and proton charge, respectively. Eq. (1) is also equal to the following equation obtained from the Helmholtz equation [4±7] lSAM1 lSAM2 1 ; DVSAM1 vs: SAM2 eSAM1 ASAM1 eSAM2 ASAM2 e0 2
Fig. 2. KPFM images for ODS/FAS-SAM and ODS/AHAPSSAM.
sum of the contact potentials in respective layers. The dierence of surface potential between SAM1 and SAM2 is described as [2] DVSAM1
vs: SAM2
f/ SAM1
/ SAM2g=e; 1
where l is the actual value of the dipole moment normal to the substrate for a molecule, A the area occupied by each molecule, and e the dielectric constant, e0 the permittivity of free space. The dierence of surface potential is thus related to the dipole moment and work function of SAMs. The Helmholtz equation was used in the following discussion. We used the following molecules for the model of SAM: H3 C CH2 15 CH3 for ODS, F3 C CF2 7 CH2 CH3 for FAS and H2 N CH2 6 NH CH2 2 CH3 for AHAPS, since, except for these molecules, the other structures were the same. Fig. 4 shows the structures and dipole moments for those molecules. The structures of ODS-molecule, FASmolecule and AHAPS-molecule were anti, gauche and anti forms, respectively. The form of the FASmolecule stands to reason because of the large steric hindrance among ¯uorines. The ODS-molecule, FAS-molecule and AHAPS-molecule have net dipole moments of 0.03, 2.38 and 0.57 Debye, respectively. Assuming that these alkyls stand normal to the substrate, the dipole moments normal to the substrate for the ODS-molecule, FAS-molecule and AHAPS-molecule were 0.03 cos(90.0°),
176
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177
Fig. 4. Molecular structures and dipole moments for ODSSAM, FAS-SAM and AHAPS-SAM.
2.38 cos(25.3°) and 0.57 cos(12.4°), respectively. Thus, the potential of ODS-SAM does not depend on its molecular packing density since the dipole
moment normal to the substrate for each of ODSmolecules is always equal to zero in this model. The dielectric constants for all SAMs used were approximately used the same value of 3.0 [6]. When these properties are substituted to Eq. (2), the relationship between the surface potential and the area occupied by each molecule are obtained. Fig. 5 shows the calculated surface potentials vs. ODS-SAM. The experimental surface potentials agreed with the calculated ones at the A of 1.1 to 1:5 nm2 molecule 1 . The organosilane LB ®lms generally had the A of 0.1 to 0:8 nm2 molecule 1 [14]. The packing densities of SAMs prepared by CVD were smaller than those of organosilane LB ®lms. Generally, LB ®lms are arti®cially prepared at the higher surface pressures than the equilibrium ones, which correspond to those of SAMs. The predicted packing densities for SAMs are reasonable from this viewpoint. This result also indicates that the surface potential for SAM aids the understanding of the reaction mechanism for SAM±CVD. In conclusion, we have reported the details of the surface potential of ODS-SAM, FAS-SAM and AHAPS-SAM using KPFM and ab initio MO calculation. We believe that the molecular recognition described in this study can be useful in creating molecular devices in the future. Acknowledgements This work has been supported by the Research Project `Biomimetic Materials Processing' (No. JSPS-RFTF 99R13101), Research for the Future (RFTF) Program, Japan Society for the Promotion of Science. References
Fig. 5. Calculated surface potential for FAS-SAM and AHAPS-SAM vs. ODS-SAM.
[1] A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, Boston, MA, 1991. [2] M. Fujihira, Annu. Rev. Mater. Sci. 29 (1999) 353. [3] J. Lu, E. Delamarche, L. Eng, R. Bennewitz, E. Meyer, H.J. Guntherodt, Langmuir 15 (1999) 8184. [4] D.M. Taylor, G.F. Bayes, Phys. Rev. E 49 (1994) 1439. [5] D.M. Taylor, G.F. Bayes, Mater. Sci. Eng. C8±C9 (1999) 65. [6] D.M. Taylor, Colloid Interface Sci. 87 (2000) 183.
N. Saito et al. / Chemical Physics Letters 349 (2001) 172±177 [7] Y. Miura, S. Kimura, S. Kobayashi, M. Iwamoto, Y. Imanishi, J. Umemura, Chem. Phys. Lett. 315 (1999) 1. [8] A. Hozumi, K. Ushiyama, H. Sugimura, O. Takai, Langmuir 15 (1999) 7600. [9] H. Sugimura, K. Ushiyama, A. Hozumi, O. Takai, Langmuir 16 (2000) 885. [10] A. Hozumi, Y. Yokogawa, T. Kameyama, H. Sugimura, et al., J. Vac. Sci. Technol. A 19 (2001) 1812.
177
[11] H. Sugimura, H. Hayashi, N. Saito, O. Takai, N. Nakagiri, Jpn. J. Appl. Phys. 40 (2001) L174. [12] H. Sugimura, H. Hayashi, N. Saito, O. Takai, N. Nakagiri, Jpn. J. Appl. Phys. 40 (2001) 4373. [13] M.J. Frisch et al., GA U S S I A N 98, Revision A.9, Gaussian Inc, Pittsburgh, PA, 1998. [14] K. Kojio, A. Takahara, T. Kajiyama, Colloids Surf. A 169 (2000) 295.