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Notes Low Energy Electron Beam Irradiation Promoted Selective Cleavage of Surface Furoxan Chang Ok Kim,† Jie Won Jung,† Minju Kim,† Tai-Hee Kang,‡ Kyuwook Ihm,‡ Ki-Jeong Kim,‡ Bongsoo Kim,‡ Joon Won Park,*,† Hyun-Woo Nam,§ and Kwang-Jin Hwang§ Center for Integrated Molecular Systems, Department of Chemistry, Division of Molecular and Life Sciences, Pohang University of Science and Technology, San 31 Hyoja-dong, Pohang, 790-784, Korea, Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang, 790-784, Korea, and Department of Chemical System Engineering, Hongik University, Jochiwon, Chungnam, 339-701, Korea Received November 7, 2002. In Final Form: March 1, 2003
Introduction Lithography is at the foundation of the great interest given to the semiconductor and biochip fields. Three patterning methods have been employed to obtain submicrometer resolution in the lithographic process.1 The first of these is photolithography, in which a masked surface is irradiated by using UV light.2-5 A second technique, referred to as soft lithography, uses an elastomer stamp to transfer desired materials with spatial resolution.6 The final method, e-beam lithography, relies on the use of either a focused electron beam7,8 or a projection of low energy electron beam through a mask.9-14 The former type of e-beam lithography provides the highest * To whom correspondence should be addressed. Phone: +8254-279-2119. Fax: +82-54-279-8365. E-mail:
[email protected] † Center for Integrated Molecular Systems, Department of Chemistry, Division of Molecular and Life Sciences, Pohang University of Science and Technology. ‡ Pohang Accelerator Laboratory, Pohang University of Science and Technology. § Department of Chemical System Engineering, Hongik University. (1) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (2) Moreau, W. M. Semiconductor Lithography: Principles and materials; Plenum: New York, 1988. (3) Huang, J. Y.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (4) Woollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (5) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (6) (a) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (b) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (7) Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Appl. Phys. Lett. 2000, 76, 2466. (8) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. (9) Harnett, C. K.; Satyalakshmi, K. M.; Criaghead, H. G. Langmuir 2001, 17, 178. (10) Geyer, W.; Stadler, V.; Eck, W.; Go¨lzha¨user, A.; Grunze, M.; Sauer, M.; Weimann, T.; Hinze, P. J. Vac. Sci. Technol., B 2001, 19, 2732. (11) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1995, 13, 1139.
resolution, while e-beam projection lithography has excellent throughput. The availability of high-resolution masks and an expanded pool of selective and sensitive set of electron beam promoted chemical reactions will enhance the number of general applications of the last approach. Processes which take full advantage of electron beam irradiation techniques must employ resist materials that are thin and uniform owing to the very short low energy electron beam penetration depths. Therefore, self-assembled monolayers (SAMs) are frequently used as resist materials for the low energy e-beam lithography.9-14 In most cases, SAMs are damaged by e-beam irradiation15-19 thus limiting the applicability when further modification of the SAMs is desired. Recently, Grunze and co-workers showed that nitro groups on aromatic ring containing SAMs are selectively converted to amino groups by low energy electron beam irradiation.20 In addition, these workers examined the site-selective immobilization of organic anhydrides by reaction with the generated amine group. This work exemplifies the importance of exploratory studies which are aimed at developing selective chemical reactions induced by electron beam irradiation. Below, we report the results of a study of a low energy e-beam irradiation induced cleavage reaction of surface localized furoxan, by using X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The furoxan system was selected for this effort because it is known that this substances undergoes selective and efficient fragmentation to form two NO molecules with concomitant production of a triple bond when it is irradiated in the gas phase (Scheme 1).21 If this same reaction pathway is followed upon irradiation of surface immobilized furoxan, the process might be used not only for generating patterns with sub-micrometer resolution but also for forming a carbon triple bond on irradiated surfaces and for releasing biologically relevant NO.22 (12) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1994, 12, 3663. (13) Lercel, M. J.; Whelan, C. S.; Craighead, H. G.; Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol., B 1996, 14, 4085. (14) Dressick, W. J.; Chen, M.-S.; Brandow, S. L.; Rhee, K. W.; Shirey. L. M.; Perkins, F. K. Appl. Phys. Lett. 2001, 78, 676. (15) Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900. (16) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750. (17) Zharnikov, M.; Geyer, W.; Go¨lzha¨user, A.; Frey, S.; Grunze, M. Phys. Chem. Chem. Phys. 1999, 1, 3163. (18) Go¨lzha¨user, A.; Geyer, W.; Stadler, V.; Eck, W.; Grunze, M.; Edinger, K.; Weimann, T.; Hinze, P. J. Vac. Sci. Technol., B 2000, 18, 3414. (19) Carr, D. W.; Lercel, M. J.; Whelan, C. S.; Craighead, H. G.; Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol., A 1997, 15, 1446. (20) (a) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (b) Go¨lzha¨user, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806. (c) Geyer, W.; Stadler, V.; Eck, W.; Go¨lzha¨user, A.; Grunze, M.; Sauer, M.; Weimann, T.; Hinze, P. J. Vac. Sci. Technol., B 2001, 19, 2732. (21) Hwang, K.-J.; Jo, I.; Shin, Y. A.; Yoo, S.; Lee, J. H. Tetrahedron Lett. 1995, 36, 3337. (22) (a) Pfeiffer, S.; Mayer, B.; Hemmens, B. Angew. Chem., Int. Ed. 1999, 38, 1714. (b) Murad, F. Angew. Chem., Int. Ed. 1999, 38, 1856. (c) Furchgott, R. F. Angew. Chem., Int. Ed. 1999, 38, 1870. (d) Ignarro, L. J. Angew. Chem., Int. Ed. 1999, 38, 1882.
10.1021/la026816q CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003
Notes
Langmuir, Vol. 19, No. 10, 2003 4505 Scheme 1. The NO Release Pathway from Furoxan in the Gas Phase (Ref 21)
Experimental Section General Procedures. The silane coupling agent, (3-aminopropyl)diethoxymethylsilane, was purchased from Gelest, Inc., purified through fractional distillation and stored under nitrogen. The 11-amino-1-undecanthiol hydrochloride was purchased from Dojindo Laboratories. Aziridine and 3-methylfuroxan-4-carbaldehyde (or furoxancarbadehyde) were synthesized as by using procedures described in the literature.23,24 UV-grade fused silica plates were purchased from CVI Laser Co. The polished prime Si(100) wafers (dopant, phosphorus; resistivity, 1.5-2.1 Ω‚cm) were purchased from MEMC Electronic Materials, Inc. The goldcoated substrates, prepared from the sequential deposition of Ti (10 nm) and Au (200 nm) onto a Si(100) wafer, were purchased from Lance Goddard Associates (Foster City, CA). Deionized water (18 MΩ‚cm) was obtained by passing distilled water through a Barnstead E-pure three-module system. UV-visible spectra were recorded by using a Hewlett-Packard diode-array 8453 spectrophotometer. The thickness of self-assembled thin films was measured by using a spectroscopic ellipsometer system (J. A. Woollam Co., model M-44). Contact angle was determined by using a Kru¨ss DSA 10 contact angle goniometer. Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) spectra were recorded by using a Bruker IFS 66v FT-IR spectrometer equipped with a MCT detector and an A513 variable angle reflection kit. p-Polarized light at an incidence angle of 80 ° was used, and spectra were collected from 1024 scans at 4 cm-1 resolution. The synchrotron radiation source at the Pohang Accelerator Laboratory (photoemission electron microscopy beamline, 4B1) was utilized for X-ray photoelectron spectroscopic (XPS) analysis of the furoxan imine layers. The photon energy was selected to give the optimized photoemission intensity and/or resolution. Binding energies were calibrated against the Si(2p) emission at Eb ) 99.3 eV. Electron exposures were performed by using a LFG63 electron gun system (VG Microtech) in the ultrahigh vacuum (UHV) chamber of the synchrotron beam line used for the photoemission experiment. The monolayers were irradiated with electrons of 400 eV with a beam current density of 0.087 µA/cm2 (0.07 µA beam current, 10 × 8 mm2 beam size). Total doses of the electron irradiations were estimated by multiplication of the current densities and the exposure times (from 0 to 190 s). The NEXAFS experiment was carried out at a photoemission electron microscopy (4B1 PEEM) beam line, and NEXAFS spectra were obtained at the carbon K-edge, nitrogen K-edge, and oxygen K-edge in a total electron yield mode, while measuring the sample current. The X-ray incidence angle is 0° to the surface normal in order to detect the expected triple bond. To minimize the damage by probe soft X-ray, carbon K-edge, nitrogen K-edge, and oxygen K-edge spectra of the furoxan-immobilized surface were recorded at new positions of the sample. The normalized NEXAFS spectra were obtained by dividing the sample current (I) by gold mesh current (I0) measured simultaneously to remove the beam fluctuation. Subsequently, a spectrum of bare silicon substrate taken under the same measurement conditions was subtracted to eliminate the influence of substrate. In our notation, the bare wafer means the substrate without treatment with the silane reagent. The photon energy was calibrated by recording the XPS spectrum of Au (4f) at the initial and final energies of the scan. For example, for a carbon K-edge spectrum, the XPS spectra were recorded at 270 and 345 eV. The energy deviation (23) (a) Furniss, B. S.; Hannaford, A. J.; Rogers, V.; Smith, P. W. G.; Tatchell. A. R. Textbook of Practical Organic Chemistry Including Qualitative Organic Analysis, 4th ed.; Longman: London, 1978; Chapter 6, p 873. (b) Allen, C. F. H.; Pangler, F. W.; Webster, E. R. In Organic Synthesis; Rabjohn, N., Ed.; Wiley: New York, 1963; Vol. IV, p 433. (24) (a) Hwang, K.-J.; Park, Y. C.; Kim, H. J.; Lee, J. H. Biosci. Biotechnol. Biochem. 1998, 62, 1693. (b) Fruttero, R.; Ferrarotti, B.; Serafino, A.; Stilo, A. D.; Gasco, A. J. Heterocycl. Chem. 1989, 26, 1345.
at both points was calculated, and the deviation was used for the interpolation. The interpolated deviation was used for the calibration of the entire energy region. The energy resolution was 0.2-0.3 eV. During the experiments, the base pressure of the experimental chamber was kept at ca. 1 × 10-10 Torr. Preparation of the Aziridine-Modified Gold Substrate. The gold substrates were immersed in piranha solution (70% H2SO4:30% H2O2) for 30 s, washed with copious amounts of deionized water, and dried in a vacuum. (Caution: Piranha solution can react violently with organic materials and should be handled with extreme caution. Piranha solution should not be stored in a sealed bottle.) Then, the substrates were annealed with a hydrogen flame for 5 min. Clean gold substrates were immersed in a 1.0 mM ethanolic solution of 11-amino-1undecanthiol hydrochloride under ambient condition for 6 h. After thiolation, the substrates were washed with ethanol and dried under vacuum. The aminothiolated substrates were immersed in dichloromethane (15 mL) containing aziridine (0.15 mL) and catalytic amounts of acetic acid. (Caution: Aziridine is toxic, carcinogenic, and teratogenic. Use only in a well-ventilated hood.) The heavy wall glass tube having a large Teflon screw cap was used as a reaction vessel and heated at 75 °C for 20 h. The resulting substrates were thoroughly washed with copious amounts of methanol and sonicated in methanol for 15 s. Finally, the washed substrates were dried under vacuum. Preparation of Furoxan Layers. The aminosilylated or aziridine-modified substrates were immersed in anhydrous ethanolic solutions (20 mL) containing the furoxancarbaldehyde (20 mg) under nitrogen atmosphere at 50 °C for 1 day. The substrates were then sonicated for 1 min in ethanol, dichloromethane, and methanol sequentially. Finally, the substrates were dried under vacuum.
Results and Discussion The furoxan substrate was prepared by reacting (3aminopropyl)diethoxymethylsilane (APDES) treated silicon wafers with 4-furoxancarbaldehyde (Scheme 2). The formation of furoxan imine layers was confirmed by the observation of an UV-absorption maximum at 269 nm associated with the furoxan chromophore. In addition, the imine-forming reaction resulted in an increase in the thickness of the organic layer on the silicon surface from 9 to 13 Å, and the contact angle value changed from 68° to 75° upon imine formation. Even though grafting favored formation of furoxan monolayer, the possibility that a submonolayer was also produced cannot be ruled out. Finally, unreacted amine groups must remain on the surface on the silicon wafer after the imine-forming process (Schemes 2 and 3). XPS surface analysis of the furoxan-modified wafer was performed by using synchrotron radiation. The survey spectrum of the furoxan-modified layer obtained from irradiation at 650 eV showed bands associated with O(1s), C(1s), N(1s), Si(2s), and Si(2p). The positions of these bands are in accordance with the literature values.25 The bands for C(1s) and N(1s) arise from the surface organic layer, while the O(1s) band is due to both the furoxan and SiO2 layer. The Si(2p) band consists of two peaks, one from the SiO2 layer at 103.0 eV and the other from the bulk Si layer at 99.3 eV. The N(1s) binding energy region was carefully scrutinized by using 530 eV irradiation. Two N(1s) bands were observed (Figure 1a). The band at 405.2 (25) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Co.: Eden Praire, MN, 1992.
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Notes
Scheme 2. The Formation of Furoxan Imine Layer on (a) a Silicon Substrate and (b) a Gold Substrate
Scheme 3. Chemical Transformation Triggered by e-Beam Irradiation on Furoxan-Immobilized Surface
eV is attributed to the quaternary nitrogen atom bonded to two oxygens in the furoxan ring and the band at 400.9 eV is likely associated with the other nitrogen of the furoxan ring, the imine nitrogen, and the remaining nitrogen of unreacted the APDES layer. The presence of two types of nitrogen atoms of furoxan ring was confirmed by independent XPS measurements on a polymer composed of furoxan and benzene rings. The XPS spectrum of the polymer shows peaks at 404.8 and at 401.0 eV (Supporting Information). Because the quaternary nitrogen atom bonded to two oxygens in the furoxan ring is positively charged, its binding energy is higher. Therefore, it is quite reasonable to assign two peaks (405.2 and 401.8 eV) for the nitrogen atoms of the immobilized furoxan. N(1s) bonding energies of imines and the amimes are very close, so peaks for these nitrogen atoms are overlapped at 400.3 eV. Upon 190 s electron beam (400 eV) irradiation of the surface of the modified wafer, the N(1s) peak at higher binding energy (405.2 eV) almost completely disappears (Figure 1b). The measured total electron dose during this time span was 16.6 µC/cm2. The result indicates that the quaternary amine of the ring is selectively eliminated in
the process promoted by beam irradiation. If the reaction of the surface furoxan is the same as that in the gas-phase reaction of this substance, the loss of the higher binding energy XPS band is a result of the extrusion of two NO molecules and concomitant generation of a triple bond (Scheme 3). It is expected that the peak at lower binding energy should be diminished by about 29%, but in actuality it decreased 12% (Figure 1). This discrepancy could be a result of incomplete conversion, baseline drift, or the combination of these factors. Careful examination of C(1s) and O(1s) peaks before and after irradiation revealed that the intensities of both do not change during the course of irradiation. The constancy in the intensities of these peaks is understandable because a carbon atom is not eliminated during the NO release process. Two sp2 carbon atoms of the substrate are converted to two sp carbon atoms in the product of this process. Therefore, in principle, the shape of the peak should change, but the large number of saturated carbon atoms must be the reason for the lack of resolution needed to observe this change. Also, the dominant nature of SiO2 layer dwarfs the reduction of O(1s) peak intensity occurring upon elimination of two oxygen atoms during the process. The FT-IRRAS technique was used in order to gain positive evidence for the generation of carbon triple bond in the product of this surface reaction. An 11-amino-1undecanethiol hydrochloride treated gold substrate was employed for immobilization of furoxancarbaldehyde and to enhance the surface signal intensity. A significant level of imine formation in this process requires 1 day minimally, but desorption of the thiol from the gold surface during this time period is problematic. Therefore, the thiolmodified gold substrates were first reacted with aziridine (Scheme 2) for the surface polymerization. As a result, the stability of the thiol layer was increased. Desorption of thiol from the resulting surface does not occur even
Notes
Langmuir, Vol. 19, No. 10, 2003 4507
Figure 1. XPS spectra showing the N(1s) region of the furoxanimmobilized APDES monolayer: (a) three deconvoluted bands of the pristine furoxan molecular layer; (b) bands before (0) and after (b) the e-beam irradiation.
Figure 3. (A) Carbon K-edge NEXAFS spectra of (a) bare wafer, (b) APDES-modified surface, and (c) furoxan-immobilized surface. The measured current was divided by mesh current (I0) measured simultaneously. The spectra obtained after (d) furoxan immobilization and (e) 400 eV e-beam irradiation. The normalized current (I/I0) was subtracted by bare substrate spectrum taken under the same measurement conditions. The before and after e-beam irradiation difference spectra (f). (B) The magnified spectrum (f) for the clear comparison.
over a 7 day period at 50 °C. The amine density on the surface produced by this polymerization process is very high.26 This leads to a significantly increased number of immobilized furoxan molecules on the surface. FT-IR spectroscopic analysis (Figure 2) enables observation of the imine bond and the furoxan ring stretching region at each reaction step. Following the imine-forming
reaction, furoxan ring stretching as well as newly generated imine at 1612 cm-1 and another furoxan ring stretching at 1468 cm-1 is observed (Figure 2c).27 Upon 400 eV e-beam irradiation, the band at 1612 and 1468 cm-1 are reduced owing to the furoxan ring cleavage (Figure 2d). We were unable to assign the absorption at 1658 cm-1 (Figure 2c) and 1666 cm-1 (Figure 2d). In addition, irradiation results in the disappearance of the absorption at 1468 cm-1 and leaves a minor absorption at 1448 cm-1. Although it is difficult to assign unambiguously the band at 1448 cm-1, the significant red shift (20 cm-1) indicates that it is not associated with the furoxan moiety. Absorption of the C-C triple bond is expected to appear at ca. 2200 cm-1. Since IR absorption of the triple bond is weak, the absorption at 2203 cm-1 is extremely weak (Figure 2f). To gain additional evidence for the presence of a carboncarbon triple bond in the surface product generated by irradiation, NEXAFS analysis was utilized. NEXAFS is
(26) Kim, H. J.; Moon, J. H.; Park, J. W. J. Colloid Interface Sci. 2000, 227, 247.
(27) Socrates, G. Infrared Characteristic Group Frequencies Tables and Charts; John Wiley & Sons Inc.: New York, 1994.
Figure 2. FT-IR spectra of furoxan-immobilized thiol layer on a gold substrate. Each spectrum was obtained after (a) 11amino-1-undecanthiol treatment, (b) aziridine treatment, (c and e) furoxan imine formation, and (d and f) 400 eV e-beam irradiation. The range for the C-C triple bond is highlighted in the inset. Spectra were recorded using p-polarized light at an incidence angle of 80°.
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a powerful tool for detecting specific bonds in molecules and is able to determine the lengths of these intramolecular bonds.28 Figure 3A shows the carbon K-edge NEXAFS spectra at each reaction step. Figure 3A(a-c) are NEXAFS spectra (I/I0) of a bare wafer, substrate after APDES reaction, and substrate after furoxan imine immobilization, respectively. Two peaks at 288.0 and 292.9 eV in the spectra of the sample produced by silanization are due to the σ*(C-H) and σ*(C-C) peaks, and these values are in harmony with the literature value.28,29 When the furoxan is immobilized on the APDES-modified surface, new peaks at 286.1 and 288.7 eV are observed. These peaks are assigned to π1*(CdN) and π2*(CdN), in accord with literature values.30-32 Because the two peaks partially overlap with each other and the latter with the σ*(C-H) peak, coupled with the fact that all of the sp2 carbon atoms are in different chemical environments, the exact assignment of the positions of these peaks is problematic. For the analysis, the signal associated with the bare wafer was subtracted from the spectra obtained during the irradiation reaction. Spectra d and e in Figure 3 are spectra obtained after furoxan immobilization and after the 400 eV e-beam irradiation, respectively. Subtraction from spectrum e of spectrum d generates the spectrum f, which shows the changes which occur upon irradiation. For the purpose of clarity, the relevant band is magnified in Figure 3B. A positive band at 286.5 eV and two negative bands at 285.7 and 288.7 eV can be seen in the difference spectrum. Because many of compounds containing the acetylene group show π* transition at ∼286 eV,28,33-39 the positive band positioning at slightly higher energy than the π1*(CdN) peak is assigned to the generated triple bond. The transition of acetylene gas adsorbed on a silicon wafer at 286.4 eV confirms this (28) Sto¨hr, J. NEXAFS Spectroscopy; Ertl, G., Gomer, R., Mills, D. L., Lotsch, H. K. V., Eds.; Springer-Verlag: Berlin, 1992. (29) Dhez, O.; Ade, H.; Urquhart, S. G. J. Electron Spectrosc. Relat. Phenom. 2003, 128, 85. (30) Huang, S. X.; Fischer, D. A.; Gland, J. L. J. Phys. Chem. 1996, 100, 10223. (31) Horsley, J. A.; Sto¨hr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. J. Chem. Phys. 1985, 83, 6099. (32) Okajima, T.; Fujimoto, H.; Sumitomo, M.; Araki. T.; Ito, E.; Ishii, H.; Ouchi, Y.; Seki, K. Surf. Rev. Lett. 2002, 9, 441. (33) (a) Matsui, F.; Yeom, H. W.; Matsuda, I.; Ohta, T. Phys. Rev. B 2000, 62, 5036. (b) Matsui, F.; Yeom, H. W.; Imanishi, A.; Isawa, K.; Matsuda, I.; Ohta, T. Surf. Sci. 1998, 401, L413. (34) Rochet, F.; Dufour, G.; Stedille, F. C.; Sirotti, F.; Prieto, P.; De Crescenzi, M. J. Vac. Sci. Technol., B 1998, 16, 1692. (35) Polzonetti, G.; Iucci, G.; Altamura, P.; Ferri, A.; Paolucci, G.; Goldoni, A.; Parent, Ph.; Laffon, C.; Rosso, M. V. Surf. Interface Anal. 2002, 34, 588. (36) (a) Polzonetti, G.; Carravetta, V.; Russo, M. V.; Contini, G.; Parent, P.; Laffon, C. J. Electron Spectrosc. Relat. Phenom. 1999, 9899, 175. (b) Carravetta, V.; Polzonetti, G.; Iucci, G.; Russo, M. V.; Paolucci, G.; Barnaba, M. Chem. Phys. Lett. 1998, 288, 37.
Notes
assignment. The negative, lower energy band is assigned to the π*(CdN) peak despite its slight red shift from the previously assigned energy. The factors discussed previously could explain the shift. Overall, the two negative bands for π1*(CdN) and π2*(CdN) reflect the disappearance of the CdN bonds through the release of NO from the ring. During each reaction step, nitrogen K-edge and oxygen K-edge NEXAFS spectra were also recorded. New peaks generated upon furoxan imine formation disappeared completely upon e-beam irradiation. This confirms nitrogen and oxygen atoms are eliminated in the 400 eV e-beam irradiation reaction (for the spectral data, see Supporting Information). Conclusion The XPS, FT-IR, and NEXAFS studies, discussed above, show that a major reaction pathway promoted by electron irradiation of immobilized furoxans involves release of nitrogen oxide and the concomitant generation of a carbon triple bond containing product on the surface. However, the yield of carbon triple bond generation was not determined in this effort. The results suggest that the selective chemical reaction that is triggered by electron impact on the surface is the same as that occurring in the gas phase. Reactions of this type are expected to play a useful role in the nanopattern formation by e-beam irradiation. Acknowledgment. Student fellowships of the Brain Korea 21 are greatly acknowledged. These studies were supported by the Korea Foundation of Science and Engineering through the Center for Integrated Molecular Systems. Supporting Information Available: UV-vis spectra of the substrates at each reaction step, XPS survey spectrum of furoxan-modified substrate, XPS spectra of C(1s) and O(1s) regions for furoxan-immobilized APDES monolayer before and after the irradiation, the structure of polymer used for N(1s) peak assignment, XPS spectrum of N(1s) region for the polymer, and nitrogen K-edge and oxygen K-edge NEXAFS spectra at each reaction step. This material is available free of charge via the Internet at http://pubs.acs.org. LA026816Q (37) (a) Sto¨hr, J.; Sette, F.; Jhonson, A. L. Phys. Rev. Lett. 1984, 53, 1684. (b) Arvanitis, D.; Baberschke, K.; Wenzel, L.; Do¨bler, U. Phys. Rev. Lett. 1986, 57, 3175. (c) Arvanitis, D.; Do¨bler, U.; Wenzel, L.; Baberschke, K.; Sto¨hr, J. Surf. Sci. 1986, 178, 686. (38) Rabus, H.; Arvanitis, D.; Domke, M.; Baberschke, K. J. Chem. Phys. 1992, 96, 1560. (39) Carr, R. G.; Sham, T. K.; Eberhardt, W. Chem. Phys. Lett. 1985, 113, 63.
