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Microstructured π-Conjugated Organic Monolayer Covalently Attached to Silicon N. Saito,*,† K. Hayashi,† H. Sugimura,† and O. Takai‡ Department of Materials Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, and Center for Integrated Research in Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received February 3, 2003. In Final Form: June 30, 2003 A phenylacetylene (PA) monolayer was prepared on hydrogen-terminated silicon. Through vacuum ultraviolet (VUV) lithography, micropatterned PA-monolayer/SiO2 samples were subsequently fabricated. The surface potential and electrical conductivity of the microstructured PA monolayers were examined by Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), respectively. KPFM measurements showed that the PA-monolayer surface regions were negatively charged. C-AFM measurements demonstrated that, in the microdomains, the PA-monolayer regions had a higher conductivity than the SiO2 regions fabricated by VUV lithography. This is the first report of electronic property images of a monolayer directly attached to silicon being demonstrated in microregions.
Introduction Incorporating knowledge gained from silicon semiconductor devices, self-assembled monolayers (SAMs)1,2 have frequently been prepared on silicon wafers to fabricate molecular devices.3,4 These SAMs were prepared on native oxides from organosilane precursors in many cases. Recently, the π-conjugated organic monolayer directly attached to silicon has received attention because such an organic-molecule-terminated silicon has a potential for a functionalized sensor electrode. Over the past 10 years, many researchers have investigated organic monolayers directly attached to silicon.5-14 These monolayers have mainly been prepared from olefins, which do not have electrical conductivity. Cicero et al. prepared a phenylacetylene (PA) monolayer with π-orbital overlaps by making use of the photochemical reaction between unsaturated compounds and hydrogen-terminated silicon(111).10 Using attenuated total reflection spectroscopy, they reported the film thickness, water contact angle, and chemical bonding state * Author to whom correspondence should be addressed. † Department of Materials Engineering, Nagoya University. ‡ Center for Integrated Research in Science and Engineering, Nagoya University. (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, MA, 1991. (2) Bishop, A.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127-136. (3) Collet, J.; Vuillaume, D. Appl. Phys. Lett. 1998, 73, 2681-2683. (4) Amlani, I.; Rawlett, A. M.; Nagahara, L. A.; Tsui, R. K. Appl. Phys. Lett. 2002, 80, 2761-2763. (5) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631-12632. (6) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (7) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R. Y.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056-1058. (8) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R. Y.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213-221. (9) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305-307. (10) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695. (11) Quayum, E. E.; Kondo, T.; Nihonyanagi, S.; Miyamoto, D.; Uosaki, K. Chem. Lett. 2002, 2, 208-209. (12) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 1, 23-24. (13) Buriak, J. M. Chem. Rev. 2002, 102, 1271-1308. (14) Schmeltzer, J. M.; Porter, L. A.; Stewart, M. P.; Buriak, J. M. Langmuir 2002, 18, 2971-2974.
of their PA monolayer. On analogy with the results of refs 10 and 14, a PA monolayer may be formed onto a silicon substrate by the following reaction:
C6H5CtCH + HSit f C6H5CHdCHsSit
(1)
However, the electronic properties of π-conjugated monolayers directly attached to silicon have not yet been investigated. In the present study, we have investigated the electronic properties of micropatterned PA monolayers using conductive atomic force microscopy (C-AFM) with Kelvin probe force microscopy (KPFM), which provide us respectively with the electrical conductivity and surface potential in microregions. PA-monolayer/SiO2 microstructures were fabricated by vacuum ultraviolet (VUV) lithography, and the electronic properties of the PA monolayer were demonstrated with KPFM and C-AFM. This is the first report of electronic property images of organic monolayers directly attached to silicon being demonstrated in microregions. Experimental Section Materials. PA was obtained from Aldrich and used as was received. The other chemicals were reagent grade or else the highest available commercial grade and were used as were received. Deionized water (18 MΩ cm) was obtained using the AQUARIUS water system (Advantec Co., Ltd.). N-type silicon wafers with (111) orientation were obtained from Shin-Etsu Chemical Co., Ltd. Hydrogen-Terminated Silicon.15,16 The silicon substrates were cleaned ultrasonically in acetone, methanol, and deionized water in that order. The substrates were further etched in aqueous HF solution (5 vol %) at 25 °C for 3 min to remove surface oxide and to terminate with hydrogen. PA-Monolayer Preparation. The hydrogen-terminated Si(111) (Si-H) substrates were treated as follows. The Si-H substrates and PA were sealed together into an autoclave with a volume of 100 cm3, which had been modified for use under a reduced pressure. The volume of PA placed inside was confirmed to be 100 µL to avoid its disappearance in the autoclave. After evacuation down to 10 Pa, the autoclave was heated to 180 °C. (15) Saito, N.; Youda, S.; Hayashi, K.; Sugimura, H.; Takai, O. Surf. Sci., in press. (16) Saito, N.; Youda, S.; Hayashi, K.; Sugimura, H.; Takai, O. Chem. Lett., in press.
10.1021/la034187u CCC: $25.00 © 2003 American Chemical Society Published on Web 11/20/2003
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The PA liquid vaporized and reacted with the Si-H groups on the substrate, resulting in the formation of a PA monolayer (see formula 1). After deposition, the samples were ultrasonically cleaned in toluene, methanol, and deionized water in that order. VUV Photolithography.17 The monolayer was next micropatterned by VUV light. Our light source was an excimer lamp with λ ) 172 nm and 10 mW/cm2 (Ushio Electric, UER20-172V). The surface covered with the PA monolayer was irradiated with VUV light under a reduced pressure of 10 Pa through a photomask for 30 min. This condition is discussed in Results and Discussion in detail. This process decomposed the monolayer and oxidized the underlying silicon substrate with active oxygen species generated from atmospheric oxygen species by VUV excitation. Thus, the substrate surface became divided into two micropatterned regions, that is, undecomposed PA monolayer and silicon oxide. C-AFM and KPFM. All the samples in this study were observed in air by AFM and KPFM (Seiko Instruments, Inc., SPA-300HV+SPI-3800N) using a gold-coated silicon cantilever. The C-AFM measurements were carried out in the contact mode. Bias voltages (Vb) of 0-3.0 were applied. The following KPFM measurement details were determined on the basis of our previous research.17 The resonance frequency and Q factor were 23.41 kHz and approximately 180, respectively. The cantilever was vibrated at a frequency of 21.40 kHz. An alternating current bias voltage of 2 V at a frequency of 5 kHz was applied between the probe and the sample. KPFM images of the sample surface were acquired at a probe scan rate of 0.1 Hz. X-ray Photoelectron Spectroscopy (XPS) Measurement. XPS (ESCA-3300, Shimadzu Co., Ltd.) measurements were performed under the following conditions. The Mg KR X-ray source was operated at 10 mA and 30 kV. All the binding energies were referenced to metallic Si(2p) at 99.34 eV, and intensities were normalized to the total Si(2p) area. The take-off angle was 90°. The standard peaks used in this study were Si-C (284.1 eV),8 CdC in the aromatic ring (284.7 eV) in poly(phenylmethylsiloxane),18 and hydrocarbon (285.0 eV) in poly(tetramethyleneglycol).18
Figure 1. Relationship between the water contact angle and the preparation time.
Results and Discussion PA-Monolayer Preparation and Characterization. Figure 1 shows the relationship between the water contact angle and the reaction time. The water contact angle was saturated at about 90° after 9 h. This water contact angle approximately agreed with that of the monolayer prepared from phenyltrimethoxysilane () ca. 90°). Subsequently, we fixed the reaction time at 9 h. To confirm the synthesis of the PA monolayer, the organic compound attached to the substrates was analyzed by XPS. Figure 2 shows the XPS C(1s) spectrum of the sample. The spectrum corresponds to an as-deposited sample. The C(1s) spectrum of the PA monolayer seen in Figure 2 shows a strong peak attributed to the aromatic ring (284.6 eV). There was an inflection point (283.7 eV) in the spectrum. This indicated that a peak is located at this binding energy. The peak may correspond to a chemical bonding state originating from Si-C bonds. Furthermore, no peaks originating from oxidized carbon were observed. The film thickness of the PA monolayer, which was roughly estimated by the attenuation of Si(2p) spectra, was about 0.6 nm.19,20 A semiempirical molecular orbital calculation with the AM1 Hamiltonian shows that the film thickness is 0.67 nm when the tilt angle of the molecular chains is 0°. Thus, the film is a monolayer with a slight tilt angle. These results (17) Saito, N.; Hayashi, K.; Sugimura, H.; Takai, O.; Nakagiri, N. Chem. Phys. Lett. 2001, 349, 172-177. (18) Beanson, G.; Briggs, D. High-Resolution XPS of Organic Polymers-The Scenta ESCA300 Database; John Wiley & Sons: Chichester, U.K., 1992. (19) Li, Z.; Lieberman, M. Langmuir 2001, 17, 4887-4894. (20) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 21, 165-176.
Figure 2. XPS C(1s) spectra of as-deposited and argon-etched PA-monolayers.
Figure 3. Surface potential image of a microstructured PA monolayer (PA-monolayer/SiO2).
indicate that PA can be directly attached to silicon via the reaction described in formula 1. Microstructured PA Monolayer. To determine an appropriate VUV irradiation time, we examined its relationship with the water contact angle. The water
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Figure 4. Current images of a microstructured PA monolayer (PA-monolayer/SiO2).
contact angles of samples irradiated for 10, 20, and 30 min were about 50, 20, and 0°, respectively. The XPS C(1s) spectrum of the sample irradiated for approximately 30 min had an intensity nearly equal to that of UV/ozonecleaned silicon. These results indicate that VUV irradiation for 30 min completely removed the PA monolayer from the silicon substrate. The XPS Si(2p) spectrum of the sample irradiated for 30 min showed a SiO2 peak. This was due to photooxidation originating from atomic oxygen, produced during the VUV irradiation. We, thus, determined the optimum VUV-irradiation time to be 30 min. The microstructured PA monolayer (PA-monolayer/ SiO2) fabricated by irradiation through a photomask for 30 min was examined by KPFM. Figure 3 shows a surface potential image of the microstructured PA monolayer. The bright and dark regions correspond to PA-monolayer and SiO2 regions, respectively. The surface potential of the PA monolayer was +30 mV higher than that of the SiO2. The SiO2 region barely had a permanent dipole moment. On the other hand, the PA molecule had a permanent dipole moment along the vector from the substrate to the surface, as is shown in Figure 3. In previous research, we elucidated the relationship between the surface potentials acquired by KPFM and the dipole moments in a simplified molecular system and proposed a calculation model for the expression of the surface potential.17 The permanent dipole moment of the PA monolayer, which has the same direction as that of the PA molecule, results in its higher surface potential. Current Images of the Microstructured PA Monolayer. Figure 4 shows current images of the microstructured PA-monolayer/SiO2 samples under applied bias voltages (Vb) of 1.5-3.0 V at intervals of 0.5 V. Although the current images under Vb’s of 1.5 and 2.0 V were not clear, images could be observed under Vb’s of 2.5 and 3.0 V. Figure 4 also shows current-voltage characteristics of the PA monolayer and SiO2 regions. The current in the
SiO2 regions never flowed above 0.1 nA, which is below the detectable limit of this AFM. The current above 0.1 nA was measured in the PA-monolayer regions and became linearly higher with an increase in the bias voltage. The film thickness of the SiO2 and PA-monolayer regions was found to be approximately the same as that obtained by the topographic measurements in KPFM mode. Thus, the difference in current seen between the two types of regions did not depend on their film thicknesses but on their electrical conductivities. The current’s origin cannot be determined solely based on our approach in this study. Further investigation is necessary to elucidate differences in the electrical conduction between π-conjugated and alkyl monolayers in detail. Conclusions A PA monolayer directly attached to silicon was fabricated and characterized to understand π-conjugated monolayers on silicon without a SiO2 layer. We confirmed by XPS analysis that the PA monolayer was directly attached to the silicon. Microstructured PA-monolayer/ SiO2 samples were fabricated by VUV lithography. KPFM measurements showed that the PA-monolayer surface regions were positively charged. C-AFM measurements demonstrated that the PA-monolayer regions had a higher conductivity than the SiO2 regions fabricated by VUV lithography. These results indicate that the π-conjugated monolayers directly attached to silicon could find abilities not only to passivate the silicon surface but also to keep the electrical conduction. Acknowledgment. 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. LA034187U
