Rose Bengal Dye on Thiol-Terminated Bilayer for Molecular Devices

Components Laboratory, Electronics and Telecommunications. Research Institute (ETRI), 161 Gajeong-dong, Yuseong-gu, Daejeon 305-350 South Korea...
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Langmuir 2007, 23, 5195-5199

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Rose Bengal Dye on Thiol-Terminated Bilayer for Molecular Devices Gyeong Sook Bang, Jonghyurk Park, Junghyun Lee, Nak-Jin Choi, Hee Yoel Baek, and Hyoyoung Lee* National CreatiVe Research InitiatiVe, Center for Smart Molecular Memory, IT ConVergence Technology Research DiVision, IT ConVergence & Components Laboratory, Electronics and Telecommunications Research Institute (ETRI), 161 Gajeong-dong, Yuseong-gu, Daejeon 305-350 South Korea ReceiVed August 10, 2006. In Final Form: December 21, 2006 Recently, it has become increasingly important to control molecular layers, especially with regard to the formation of bilayers, in order to avoid electrical shorts in molecular electronics. In this paper, we report on the characterization of an in situ thiol-terminated bilayer that is formed by hydrogen bonding between the amine group of an aminoalkanethiol monolayer on a gold surface and the free amine group of aminoalkanethiolates in a bulk solution. We also report on the use of a rose bengal (RB) monolayer on a thiol-terminated bilayer for the purpose of application in a molecular memory device. Using surface-sensitive techniques such as grazing angle Fourier transform infrared (FT-IR) spectroscopy, quartz crystal microbalance (QCM) measurement, ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV), we characterized a thiol-terminated bilayer (TUA-AUT) and an RB functionalized monolayer on a bilayered surface (RB-TUA-AUT). For a control experiment, we prepared a single RB monolayer attached by an ethanethiol group to a gold surface. In order to assess the feasibility of the present approach with respect to application in molecular electronics, we tested the switching property of the self-assembled monolayers (SAMs) using conductingprobe atomic force microscopy (CP-AFM). The RB monolayer on the bilayered surface exhibited hysteresis, while a single RB monolayer gave an electrical short.

Introduction Recently, in the fields of nanoscience and nanotechnology, functional molecular materials have been applied to magnetic storage media, electronic and optical devices, and bioanalytical devices using self-assembled monolayers (SAMs).1,2 Selfassembly techniques for the fabrication of various molecular devices has attracted the attention of researchers.3 The fabrication of SAMs comprised of organic molecules, formed spontaneously on solid substrates by adsorption from solution, has been intensively studied as a powerful method of surface modification and functionalization.4 Recent reported results have demonstrated the importance of metal-molecular contact for the efficiency of * To whom correspondence should be addressed. Telephone: (82)-42860-1165. Fax: (82)-42-860-5404. E-mail: [email protected]. (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. Salomon, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes, V. B.; Frisbie, C. D. AdV. Mater. 2003, 15, 1881. Urbach, A. R.; Love, J. C.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12704. Lee, K. B.; Park, S. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 3048. Fullonier, S.; Miller, W. J. W.; Abbott, N. L.; Knoesen, A. Langmuir 2003, 19, 10501. (2) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885. Chaki, N. K.; Vijayamohanan, K. Biosens. Bioelectron. 2002, 17, 1. Bruckbauer, A.; Zhou, D. Kang, D.-J,; Korchev, Y. E.; Abell, C.; Klenerman, D. J. Am. Chem. Soc. 2004, 126, 6508. Park, S.-J.; Taton, T. A.; Mirkin, C. A. Science 2002, 295, 1503. Xiao, Y.; Piorek, B. D.; Plaxco, K. W.; Heeger, A. J. J. Am. Chem. Soc. 2005, 127, 17990. Polsky, R.; Gill, R.; Kaganovsky, L.; Willner, I. Anal. Chem. 2006, 78, 2268. Bang, G. S.; Cho, S.; Kim, B.-G. Biosens. Bioelectron. 2005, 21, 863. (3) Tans, S. J.; et al. Nature 1997, 386, 474. Salem, A. K.; Chao, J.; Leong, K. W.; Searson, P. C. AdV. Mater. 2004, 16, 268. Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. Service, R. F. Science 2003, 302, 556. Wassel, R. A.; Credo, G. M.; Fuierer, R. R; Feldheim, D. L.; Gorman, C. B. J. Am. Chem. Soc. 2004, 126, 295. Vilan, A.; Cahen, D. AdV. Funct. Mater. 2002, 12, 795. Chen, J.; Ma, D. Appl. Phys. Lett. 2005, 87, 023505/023501. Liu, Z.; Yasseri Amir, A.; Lindsey, S.; Bocian, D. F. Science 2003, 302, 1543. Shukla, A. D.; Das, A.; van der Boom, M. E. Angew. Chem., Int. Ed. 2005, 44, 3237. Sortino, S.; Di Bella, S.; Conoci, S.; Petralia, S.; Tomasulo, M.; Pacsial, E. J.; Raymo, F. M. AdV. Mater. 2005, 17, 1390. Yasutomi, S.; Morita, T.; Imanishi, Y.; Kimura, S. Science 2004, 304, 1944. (4) Ulman, A. An introduction to ultrathin organic films from LangmuirBlodgett to self-assembly; Academic Press: San Diego, 1991; p 237.

charge transfer, especially for alkanethiol-based junctions.5 SAMs allow the control of chemical (or physical) interfacial processes and offer uniformity of structure and potentially lower fabrication costs. For the realization of molecular electronics, however, it is necessary to overcome an electrical short problem. This problem can be addressed by increasing the film thickness with an in situ thiol-terminated bilayer, especially in the vertical structure of a metal-molecule-metal electrode. In the present study, we introduce an alkanethiol bifunctional molecule, 11-amino-1-undecanethiol (AUT), whose height is about 1.8 nm,21 in order to reduce the electrode surface roughness of an electrode, i.e., in the range of 0.5-1 nm. Accordingly, the film thickness of the bilayer was expected to be about 3-4 nm. Rose bengal (RB) is a xanthene molecular dye that is highly visible and easily absorbed.6 A physically adsorbed multilayered RB film formed by the Langmuir-Blodgett method exhibited interesting properties for molecular memory effect.7 In the present work, we develop a molecular memory device with a single monolayer using a self-assembly method. To this end, we synthesized RB-(CH2)2SH molecules and prepared a rose bengal monolayer on gold. The terminal group of RB-(CH2)2SH molecules having alkyl backbones and a thiol group is much bulkier than that of the simple alkanethiol SAMs.4,11 Even though the monolayer containing an RB derivative is directly formed through the thiol group on a gold surface, it leaves empty spaces on the gold surface because of its bulky terminal group.9 The empty spaces cause electrical shorts. Therefore, to minimize the (5) Cui, X. D.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Primak, A.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Nanotechnology 2002, 13, 5. Tivanski, A. V.; He, Y.; Borguet, E.; Liu, H.; Walker, G. C.; Waldeck, D. H. J. Phys. Chem. B 2005, 109, 5398. (6) Meei, S.; Neckers, D. C. J. Am. Chem. Soc. 1988, 110, 1257. (7) (a) Bandyopadhyay, A.; Pal, A. J. Appl. Phys. Lett. 2003, 82, 1215. (b) Bandyopadhyay, A.; Pal, A. J. J. Phys. Chem. B 2003, 107, 2531. (c) Majee, S. K.; Bandyopadhyay, A.; Pal, A. J. Chem. Phys. Lett. 2004, 399, 284. (8) Lee, H.; Do, H.; Kim, D.-H.; Zyoung, T. H.; Jeon, K. Korea Patent Appl. No. 04-91576, 2004. (9) Bang, G. S.; Jeon, I. C. Bull. Korean Chem. Soc. 2001, 22, 281.

10.1021/la062369t CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007

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Scheme 1. Direct Immobilization through Thiol Group of RB-(CH2)2SH on Gold Surface

empty spaces on the gold surface, we prepared an RB monolayer on a thiol-terminated bilayer (TUA-AUT) with simple alkanethiol by a stepwise method. As is well-known, 11-amino-undecane1-thiol (AUT) is a bifunctional building block, and we assume that it can form an in situ thiol-terminated bilayer structure by hydrogen bonding between the amine group of an aminoalkanethiol monolayer on a gold surface and the free amine group of aminoalkanethiolates in a bulk solution.16,20 Here, we report the formation, characterization, and switching effect of a thiol-terminated bilayer without RB (TUA-AUT) and an RB functionalized monolayer on a thiol-terminated bilayer (RB-TUA-AUT) using surface-sensitive techniques such as grazing angle Fourier transform infrared (FT-IR) spectroscopy, quartz crystal microbalance (QCM) measurement, spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and conducting-probe atomic force microscopy (CP-AFM). The surface coverage, film thickness, and chemical composition of the molecular layer were determined by QCM, spectroscopic ellipsometry, and XPS, respectively. For a control experiment, we prepared a single RB monolayer attached by an ethanethiol group to a gold surface. To assess the feasibility of the proposed approach with respect to application in molecular electronics, we tested the switching effect of a single RB monolayer, TUA-AUT, and RB-TUA-AUT using CP-AFM. We observed hysteresis only in RB-TUA-AUT. This is the first report on the molecular switching effect of an RB monolayer constructed on an in situ thiol-terminated bilayer using a selfassembly technique. We are currently working on the fabrication of a molecular memory device using an RB monolayer on a thiol-terminated bilayer. Experimental Section Materials. Potassium ferrocyanide(III) (K3[Fe(CN)6], 99+%, Aldrich), potassium chloride (99+%, Aldrich), ethanol (99.5%, HPLC grade, Aldrich), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC; >98%, Aldrich), rose bengal (RB; TCI), 11amino-1-undecanethiol hydrochloride (AUT; Dojindo Laboratories, Japan), sulfuric acid (H2SO4, electronic grade), hydrogen peroxide (H2O2, electronic grade), and dimethylformamide (DMF; 99.9+%, HPLC grade, Aldrich) were used without further purification. RB(CH2)2SH was synthesized as described in detail by Dr. H. Lee.8 Other chemicals were of an analytical reagent grade. Ionized water of 18 MΩ cm resistivity was used throughout the experiment. KCl (0.1 M) was used as a supporting electrolyte. Apparatus. Gold films of 800 Å thickness on a Ti/Si wafer were used for grazing angle FT-IR, ellipsometric, and electrochemical experiments. The gold film was evaporated by an E-beam evaporator. Before chemisorption, the gold film was cleaned with piranha solution (98% H2SO4:30% H2O2 ) 50:50, v/v). Caution: Piranha solution reacts Violently with most organic materials and should be handled with extreme care! The gold film was immersed in a freshly prepared piranha solution for 10 min, followed by washing several times with deionized water and ethanol, and finally drying under a stream of

nitrogen gas. FT-IR spectra were obtained in single reflection mode using a dry N2-purged Thermo Nicolet Nexus grazing angle FT-IR equipped with a SAGA (smart apertured grazing angle) accessory. In absorption mode, the background spectrum of bare gold was first obtained, followed by that of modified gold films. All spectra were obtained at 2 cm-1 resolution with 2000 scans. The thickness of the SAMs was measured using a spectroscopic ellipsometer (Model J. A. Woollam VASE) at an incident angle of 60°-70° in a wavelength range between 200 and 850 nm. An elemental analysis using the XPS results was carried out using a Scienta SES100 analyzer with an Mg KR source. The CP-AFM measurement was performed using a Digital Instrument Multimode with a Nanoscope IV. A QCM (SHIn EQCN 2000, Korea) with a Teflon-based cell9 was used to monitor changes in the frequency of the adsorption of the thiol compound at ambient conditions with no potential applied to the quartz crystal (see the Supporting Information). Cyclic voltammograms were obtained using a CHI 660A electrochemical analyzer. An Ag(s)|AgCl(s)|KCl(satd) reference electrode and a platinum wire auxiliary electrode were used. All electrochemical measurements were carried out in a completely deaerated solution. Preparation of Rose Bengal Monolayers. Monolayers were prepared by two different methods as shown in Schemes 1 and 2. One method was by direct formation through the thiol group containing RB derivative, RB-(CH2)2SH SAM, on the gold surface as outlined in Scheme 1. The other stepwise method is outlined in Scheme 2. First, bilayered TUA-AUT was prepared by soaking gold substrate in a solution containing 3 mM AUT in DMF for 24 h. The bilayered TUA-AUT was then introduced to a solution containing 0.5 mM RB and 10 mM EDC in ethanol and reacted by shaking for 12 h (RB-TUA-AUT SAM). The RB films were rinsed thoroughly with ethanol and dried under a stream of nitrogen gas. The modified films were immediately used in all measurements.

Results and Discussion First, the tendencies for the self-assembling process of RB(CH2)2SH were observed in pure DMF solution using QCM. Figure 1A shows the QCM results for 0.1 mM RB-(CH2)2SH solution and the fitting curve. When the frequency change was stabilized within (0.1 Hz with time for ca. 7 min, the RB(CH2)2SH solution was added and frequency data were obtained. The frequency rapidly decreased immediately after addition of the sample, followed by a gradual decrease. The results show that 80% of full coverage was reached within 10 min. The adsorption kinetics was observed by monitoring the frequency decrease, which is equivalent to the increase in mass on the gold surface (Supporting Information). From the frequency change (∆F), we estimated the surface density of RB-(CH2)2SH. The total frequency change was 33 Hz, which corresponds to 1.6 × 10-10 mol cm-2. If the roughness factor of the gold surface was 1.2,10 the coverage would be 1.3 × 10-10 mol cm-2. Assuming hexagonal close packing and a (10) Thomas, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1991, 7, 620.

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Scheme 2. Illustration of RB Functionalized Monolayer on the Thiol-Terminated TUA-AUT Bilayer on a Gold Surface

hard sphere of RB, this value indicates that a monolayer is formed. From the coverage value, the occupied area of an RB-(CH2)2SH molecule was estimated to be 132 Å2; thus, the diameter of the RB terminal group was estimated to be ca. 13 Å, which is in good agreement with the reported value.11 As estimated in this work, the RB terminal group (13 Å) is much bulkier than that (5 Å)4 of the simple alkanethiol. Even though RB-(CH2)2SH molecules have alkyl backbones and a thiol group, the diameter of RB is much larger than that of the distance between the hollow sites of gold, which leaves empty spaces around the molecules. The empty spaces make prediction and control of monolayercoated electrodes difficult. To minimize the empty spaces on the gold surface, we prepared an RB monolayer on a bilayered TUA-AUT by a stepwise method (Scheme 2). The self-assembling process of TUA-AUT was

Figure 1. Adsorption data for (A) 0.1 mM RB-(CH2)2SH in DMF and the fitting curve using a set of appropriate kinetic parameters: ka ) 88 M-1 s-1, kd ) 2.9 × 10-4 s-1, K ) 3.3 × 10-6 M, and ∆G° ) -7.34 kcal mol-1. (B) Adsorption data for 0.1 mM AUT in DMF.

observed using QCM in a pure DMF solution containing 0.1 mM AUT (Figure 1B). The total frequency change was -80 ( 3 Hz, which corresponds to 1.8 ((0.1) × 10-9 mol cm-2. Data were reproducibly obtained from four QCM experiments with the same conditions on a new gold surface. In consideration of the roughness of the gold surface, the coverage is 1.4 × 10-9 mol cm-2. This value is 2 times higher than that12 of a simple alkanethiol monolayer formed in the case of assuming hexagonal close packing. It is expected that a second layer would be formed during the SAM process. To confirm the second layer formation on the AUT SAM obtained from the QCM result, we measured the thickness of the SAMs using a spectroscopic ellipsometer. The thicknesses of TUA-AUT, RB-TUA-AUT, and RB-(CH2)2SH were 35, 45, and 20 Å, respectively. The thickness of TUAAUT is 2 times greater than the value (17 Å) theoretically calculated by Jaguar 5.5 software using Becke3 Lee-YangParr (B3LYP) three-parameter density functional theory (Supporting Information). Based on a comparison of the measured value (35 Å) and the calculated value (17 Å) of the film thickness, we can conclude that a second layer is formed, which is consistent with the QCM result. To further confirm the bilayered TUA-AUT, samples were examined via XPS and the S 2p spectra, as shown in Figure 2. Bound sulfur species (denoted by “A”) on the gold surface appeared around 162 eV, and unbound species (denoted by “B”) were detected around 164 eV, respectively. In addition, oxidized sulfur species (denoted by “C”), including sulfonate, were detected at 168 eV. These findings are in good agreement with previous results reported by several research groups.13 The oxidized sulfonate species were detected on all of the SAMs. The peak for unbound sulfur is observed only on TUA-AUT and RBTUA-AUT, not on RB-(CH2)2SH SAM. Furthermore, the bound sulfur peak for the RB-TUA-AUT SAM was enhanced in comparison with that of TUA-AUT. This is likely due to a relative decrease of the unbound sulfur species by coupling with RB. In addition, no peak of the unbound sulfur species was present at 164 eV for the RB-(CH2)2SH SAM. For a 1-dodecanethiol (DT) (11) Fini, P.; Castagnolo, M.; Catucci, L.; Cosma, P.; Agostiano, A. Thermochim. Acta 2004, 418, 33. (12) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (13) (a) Castner, D.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (b) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239. (c) Wallwork, M. L.; Smith, D. A.; Zhang, J.; Kirkham, J.; Robinson, C. Langmuir 2001, 17, 1126.

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Figure 2. XPS S 2p spectra for RB-(CH2)2SH, TUA-AUT, and RB-TUA-AUT films on gold surface. The S 2p peaks denoted by “A”, “B”, and “C” are bound sulfur, unbound sulfur, and oxidized sulfur species, respectively.

Figure 4. Cyclic voltammograms for 1.0 mM [Ru(NH3)6]2+ solution in 0.1 M KCl electrolyte: (A) bare gold, (B) RB-(CH2)2SH, (C) TUA-AUT, and (D) RB-TUA-AUT films on gold surface. Scan rate is 100 mV s-1. Figure 3. Grazing angle FT-IR spectra of TUA-AUT and RBTUA-AUT films on gold surface.

SAM (not shown here), we did not observe the peak at 164 eV. This means that RB-(CH2)2SH and DT SAMs without an amineterminated group and hydrogen bond do not show significant unbound sulfur species. Thus, from the evidence presented here and the reported data,14-16 we conclude that formation of hydrogen bonds take places between the terminal amine group of the AUT bounded on the gold surface and the free amine group of the AUT in the bulk solution, as illustrated in Scheme 2. To obtain RB functionalized film, RB groups were introduced to the TUA-AUT surface by a coupling reaction between the thiol terminal group of the second layer formed on the AUT SAM on gold and the activated carboxyl group of RB, as outlined (14) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (15) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639. (16) Wang, H.; Chem, S.; Li, L.; Jiang, S. Langmuir 2005, 21, 2633.

in Scheme 2. The resulting RB monolayer on TUA-AUT was characterized by grazing angle FT-IR spectroscopy (Figure 3). The IR spectra show two intense bands at 3000-2800 cm-1, which correspond to CH2 asymmetric and symmetric stretching of the hydrocarbon moiety. The values are 2928 and 2852 cm-1 for TUA-AUT and 2923 and 2850 cm-1 for RB-TUA-AUT. The stretching bands confirmed that the films are formed with a highly ordered structure.17,20 The stretching of primary NH shows a broad peak near 3200 cm-1 in TUA-AUT and RB-TUA-AUT. The peak around 1547 cm-1 results from the contribution of N-H bending with hydrogen bonding and C-N stretching (17) (a) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (b) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623. (18) Silverstein, R. M.; Francis, X. Spectrometric Identification of Organic Compounds, 6th ed.; John Wiley & Sons Inc.: New York, 1998; p 93. (19) Chen, A.; Hutchison, J. E.; Postlethwaite, T. A.; Richardson, J. N.; Muarry, R. W. Langmuir 1994, 10, 3332. (20) Wallwork, M. L.; Smith, D. A. Langmuir 2001, 17, 1126. (21) Sahoo, R. R.; Patnaik, A. Appl. Surf. Sci. 2005, 245, 26.

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Figure 5. I-V curves for (A) RB-(CH2)2SH, (B) TUA-AUT, and (C) RB-TUA-AUT films with 20, 35, and 45 Å thickness, respectively.

mode.13b,20 After the RB reaction on the TUA-AUT film, a CdO stretching band of the RB monolayer was shifted from carboxylic acid (1720 cm-1) to a lower frequency (1656 cm-1), which corresponds to thioester, as shown in Figure 3B (marked round circle).18 These results indicate that an RB monolayer is formed on the TUA-AUT surface. The thickness of the RB-TUA-AUT film, as measured by the spectroscopic ellipsometer, corresponds to the monolayer thickness that the activated carboxylic group of RB molecules (13 Å) covalently bounded to the thiol terminal group of the second layer of the TUA-AUT film (35 Å) formed due to hydrogen bonds. We next investigated the blocking properties of these films using CV in terms of their potential application in molecular devices. Figure 4 shows the cyclic voltammogram for 1.0 mM [Ru(NH3)6]2+ in solution on bare and modified gold electrodes. For the bare gold electrode, the redox peak of RuII/III shows typical cyclic voltammetric curves, as shown in Figure 4A. For the RB-(CH2)2SH monolayer, a reversible redox peak is clearly seen, and there is a blocking effect of 20% in comparison with the peak anodic current at the same potential.19 The shape of the redox peak suggests that open gold surfaces are present on the RB-(CH2)2SH monolayer. However, the voltammograms in Figure 4C,D show a dramatic difference from those in Figure 4A,B. The redox peak for RuII/III disappears from the TUA-AUT film and the RB-TUA-AUT film electrode, which shows a blocking effect of 98.7%. These results reflect that the redox peak decreases due to the more effective blocking layer, which is realized by decreasing the open spaces on the gold surfaces of TUA-AUT and RB-TUA-AUT. These results are consistent with the results predicted on the basis of the frequency change and grazing angle FTIR measurements. Finally, in order to assess the feasibility of the proposed approach with respect to possible application in molecular electronics, the I-V characteristics of the RB functionalized monolayer on bilayered TUA-AUT were examined. Figure 5 shows the I-V characteristics of RB-(CH2)2SH, TUA-AUT, and RB-TUA-AUT films, measured under a bias potential scan from -2 to 2 V at room temperature in air. While the RB-(CH2)2SH film showed ohmic behavior, the TUA-AUT film showed insulating behavior at the same bias region. This indicates that the TUA-AUT film serves as a blocking layer and does not entail any open gold surfaces. Under higher applied potentials, the RB-(CH2)2SH film was electrically shorted and the TUA-AUT film did not show any hysteresis. However, the RB monolayer on the bilayered AUT surface exhibited hysteresis. From these

results, we conclude that the hysteresis property is inherent in the RB monolayer, thus making it suitable for application in the organic semiconducting industry, especially in the area of molecular electronics.

Conclusion We have described the fabrication of a thiol-terminated bilayer and an RB functionalized monolayer on a thiol-terminated bilayer surface. The experimental thickness and frequency change of the thiol-terminated bilayer (TUA-AUT) were 2 times greater than the theoretical value of AUT monolayer according to spectroscopic ellipsometry and QCM measurements. These results are evidence of the possible formation of hydrogen bonds between the terminal amine groups of the AUT bonded on the gold surface and the free amine of the AUT in the bulk solution in DMF. In the RB monolayer on the bilayered TUA-AUT surface prepared by a stepwise method, RB molecules are covalently bonded to the thiol terminal group of the bilayered TUA-AUT through hydrogen bonds. The blocking effect of these films measured by cyclic voltammetry is 98.7%. The increased effectiveness of the blocking layer is attributed to a decrease in openings in the gold surfaces of the TUA-AUT and RB-TUA-AUT films. The RB functionalized film exhibits hysteresis, which may be useful for electronic applications. Therefore, the present study also suggests guidelines for the construction of various electrodes with functionalized organic materials on a bilayered TUA-AUT surface. We are currently working on enhancing the switching effect of the RB monolayer and fabricating a molecular device having a vertical structure of metal-molecule-metal. Acknowledgment. This work was supported by the Creative Research Initiative Program research fund (project title: Smart Molecular Memory) of the Korean Ministry of Science and Technology (MOST) in Korea. We would like to thank Professor Insung S. Choi and Mr. Young Shik Ghi of the Department of Chemistry at KAIST for grazing angle FT-IR measurements. We are also grateful to Dr. Yong-jai Cho and Dr. Yong-Seop Park of the Korea Research Institute of Standard and Science (KRISS) for spectroscopic ellipsometry and XPS measurements. Supporting Information Available: Information on quartz crystals, adsorption kinetics for RB-(CH2)2SH, and software for molecular length calculations. This material is available free of charge via the Internet at http://pubs.acs.org. LA062369T