Langmuir 2003, 19, 5303-5310
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Self-Assembled Monolayers of ω-(3-Thienyl)alkanethiols on Gold Heejoon Ahn, Myunghwan Kim, Daniel J. Sandman, and James E. Whitten* The Department of Chemistry and Center for Advanced Materials, The University of Massachusetts Lowell, Lowell, Massachusetts 01854-5047 Received November 7, 2002. In Final Form: February 24, 2003 Thiophene-containing alkanethiols, Th-(CH2)n-SH (Th ) 3-thiophene) with n ) 2, 6, and 12, have been synthesized and self-assembled onto gold-coated Si(111) wafers. The properties of the monolayers have been compared with those of methyl-terminated self-assembled monolayers (SAMs) having the same number of methylene units. X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) demonstrate formation of surface thiolate bonds and assembly with the thiophene rings at the periphery of the monolayers. Dynamic contact angle measurements using water and hexadecane probe liquids are consistent with this conclusion and, particularly in the case of 12-(3-thienyl)dodecanethiol, indicate a densely packed, wellordered monolayer. Ellipsometry measurements yield thickness values of 6.7, 10.7, and 17.9 Å for n ) 2, 6, and 12, respectively. Assuming the alkyl chains are in a fully extended all-trans conformation, these data indicate tilt angles of 42, 41, and 35°, respectively. Quartz crystal microbalance measurements demonstrate that the thienyl-terminated alkanethiol monolayers have similar packing densities compared to methyl-terminated SAMs of similar lengths. Thermal stability measurements using XPS and UPS show that the thienyl-terminated SAMs are stable to at least 100 °C but desorb/decompose on heating to 150 °C.
Introduction Langmuir-Blodgett (LB) and self-assembly techniques are the most common methods for forming ultrathin organic films. Physisorbed films may be produced by the LB method by passing a solid substrate through the liquid/ air interface of an aqueous solution containing an organic monolayer film. When the substrate moves through the interface, organic molecules are transferred onto the solid substrate.1 Self-assembled monolayers (SAMs) are molecular assemblies that are based on monomolecular adsorption and arrangement between the liquid and solid phases. These are also referred to as “programmed assemblies”.2 The origin of SAMs can be traced to the work of Zisman et al.3 who formed well-oriented and nearly close-packed monolayer films by immersing glass into n-eicosyl alcohol, primary n-octadecylamine, and n-nonadecanoic acid solutions. These films exhibited wetting properties similar to those of LB films. Interest in SAMs increased tremendously after the discovery of the formation of alkanethiolates on gold by Nuzzo and Allara.4 Other types of SAMs on a variety of substrates have since been reported, but among these systems, those made by adsorbing alkanethiols on gold surfaces have been the most widely studied because of their ease of preparation, range of functionalities, and excellent stabilities.5,6 SAMs potentially have a wide range of applications, including * Corresponding author. Fax: (978)934-3013. Phone: (978)9343666. E-mail:
[email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991; pp 101-132. (2) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, 1995; pp 139-144. (3) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1, 513. (4) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (5) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991; pp 279-281 and references therein. (6) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
corrosion protection,7 nanolithography,8 and molecular electronics.9 Alkanethiol adsorbates with long alkyl chains (>C10) tend to form highly ordered, oriented monolayer films tilted 20-30° from the surface normal. However, for short alkyl chains ( 5. However, they noted a marked drop-off in the contact angles when n < 5, indicating either that the probe liquid sensed the underlying gold or an increased disorder in short-chain monolayers. In the case of the thienyl-terminated SAMs, we observe advancing water contact angles of 91°, 90°, and 90° for (52) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365.
methylene unit lengths of 2, 6, and 12, respectively. These lower values relative to those for the methyl-terminated monolayers may be attributed to the thienyl groups located at the periphery of the SAMs, consistent with similarly low advancing water contact angles for ω-thiophenefunctionalized n-alkyltrichlorosilane monolayers.22,25 Furthermore, Sullivan et al.22 measured a value of 88° for polythiophene films. Our results suggest that the periphery of the monolayers is comprised of a densely packed array of thiophene groups. The receding water contact angles for the methylterminated alkanethiol monolayers are 79°, 96°, and 100° for 2, 6, and 12 methylene units, respectively. The relatively large contact angle hysteresis values (10°-16°) may result from the surface roughness of the gold substrates on which the SAMs are assembled, consistent with previous reports of increased contact angle hysteresis for gold deposited on chromium/glass substrates relative to thermally annealed gold substrates.53 In general, gold surfaces prepared by deposition onto annealed substrates (i.e., silicon, mica, glass) show reduced surface roughness and flatter crystallites (∼0.2 µm in diameter) than gold substrates prepared by room-temperature deposition.54 In the present work, we have evaluated the surface roughness of our gold substrates using atomic force microscopy, and this technique reveals crystallites with diameters in the range of 40-60 nm and a valley-to-peak height of 1-3 nm. This roughness likely is the origin of the relatively large water contact angle hysteresis. Note that the thienyl-terminated alkanethiol hystereses are slightly larger than those corresponding to the methylterminated SAMs, indicating less homogeneity in the case of the thienyl-terminated monolayers. It is also interesting that the contact angle hysteresis for both the methyl- and thienyl-terminated alkanethiol SAMs decreases as the methylene unit length gets longer, indicating better ordering and higher packing density. Hexadecane has also been used as a probing liquid. Well-ordered n-alkanethiol monolayers have a hexadecane contact angle of 45°,6,11,44 and when hexadecane contacts only methylene groups, hexadecane completely wets the surfaces (0°).5,55 We have obtained hexadecane contact angles of 36°, 38°, and 43° for 1-propanethiol, 1-heptanethiol, and 1-tridecanethiol monolayers, respectively. The relatively low contact angles in the case of 1-propanethiol and 1-heptanethiol may arise from the hexadecane contacting the methylene groups, a result of relatively poor packing in the case of the short chains. We observe hexadecane contact angles of 26°, 25°, and 22° for (53) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G. Langmuir 2001, 17, 95. (54) Laibinis, P. E.; Palmer, B. J.; Lee, S.-W.; Jennings, G. K. SelfAssembled Monolayers of Thiols; Ulman, A., Ed.; Academic Press: New York, 1998; pp 4-7. (55) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Macromolecules 1993, 26, 5870.
SAMs of ω-(3-Thienyl)alkanethiols on Gold
Figure 8. Mass uptake change as a function of immersion time of a quartz crystal microbalance in 1 mM 2-(3-thienyl)ethanethiol (4), 6-(3-thienyl)hexanethiol (9), and 12-(3-thienyl)dodecanethiol (O) solutions. The lines through the data are included to guide the eye. Note that the measurements were performed ex situ, as discussed in the text.
Th-(CH2)n-SH, with n ) 2, 6, and 12, respectively. For hexadecane on polythiophene films and thiophene-capped monolayers, contact angles of approximately 16° and 20°, respectively, have been measured.22 The hexadecane contact angles we observe for thienyl-terminated alkanethiol SAMs are consistent with this value. QCM Measurements. Figure 8 shows mass uptake, converted from ex situ QCM measurements, as a function of immersion time in 1 mM solutions of thienyl-terminated alkanethiols in chloroform. Solution QCM measurements are known to be susceptible to complicating factors,56 and to verify the performance of the QCM, a test experiment was performed on 1-tridecanethiol. In this case, the equilibrium mass uptake was 190 ng/cm2, corresponding to a packing density of 5.2 × 1014 molecules/cm2, which is in good agreement with the reported value44 of 4.7 × 1014 molecules/cm2. The equilibrium mass uptake values obtained from the data in Figure 8 are 167, 220, and 310 ng/cm2 for 2-(3-thienyl)ethanethiol, 6-(3-thienyl)hexanethiol, and 12-(3-thienyl)dodecanethiol, respectively, corresponding to packing densities of 6.5, 6.6, and 7.0 × 1014 molecules/cm2. These values are larger than for 1-tridecanethiol, indicating greater packing densities in the case of the thiophene-terminated SAMs. This is inconsistent with the conclusions drawn from the ellipsometry data. It is possible that the QCM is overestimating the mass of self-assembled molecules due to physisorbed molecules on top of the SAM layers which are not completely removed by rinsing. This would be expected to be more severe in the case of the thiophene-substituted SAMs because of their longer lengths and increased intermolecular attractions. In any case, we conclude that the thiophene-substituted alkanethiols have comparable packing densities to n-alkanethiols. Thermal Stability Measurements. Figure 9 shows changes in the XPS C 1s/Au 4f and S 2p/Au 4f ratios as a function of heating temperature. Heating to 65 °C does not significantly change the sulfur intensity but causes a slight decrease in the carbon intensity. This may be due to thermal desorption of small molecules (e.g., CO or CO2) on the surface of the monolayers. Heating to 150 °C causes dramatic decreases in the C 1s/Au 4f and S 2p/Au 4f ratios. By 180 °C, all of the sulfur has desorbed, but the C 1s/Au 4f ratio is about 0.10, indicating that some carbon remains (56) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315.
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Figure 9. Variation in C 1s/Au 4f ratio (b) and S 2p/Au 4f ratio (O) for self-assembled 12-(3-thienyl)dodecanethiol monolayers on Au/Si(111) as a function of heating temperature. The left y-axis refers to the C 1s/Au 4f ratio (b); the right one refers to the S 2p/Au 4f ratio (O).
Figure 10. He I UPS of self-assembled 12-(3-thienyl)dodecanethiol monolayers on Au/Si(111) as a function of heating temperature. A spectrum of clean gold is included for reference.
on the surface. This residual carbon may have its origin in thermal decomposition of the SAM to carbon-containing species that remain on the surface. Figure 10 shows He I UPS spectra for the heating of self-assembled 12-(3-thienyl)dodecanethiol. A spectrum of clean gold (argon ion sputtered to remove any contaminants) is also included for reference. As discussed previously, the peak at 3.8 eV is mainly due to localized thiophene ring electronic states with some contributions from the thiol sulfur atoms. This peak persists to temperatures as high as 100 °C; by 150 °C it has essentially disappeared. This result is consistent with conclusions drawn from the XPS data. The UPS spectrum after heating to 180 °C shows features characteristic of bulk gold, indicating desorption of the monolayers. N-Alkanethiol SAMs are reportedly thermally stable in vacuum up to approximately 100 °C; heating to ca. 200 °C leads to monolayer desorption.57-59 Our results indicate that the thienyl-terminated SAMs behave similarly. (57) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (58) Li, J.; Liang, K. S.; Camillone, N.; Leung, T. Y. B.; Scoles, G. J. Chem. Phys. 1995, 102, 5012.
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Conclusions Thiophene-containing alkanethiols, Th-(CH2)n-SH (Th ) 3-thiophene) with n ) 2, 6, and 12, have been synthesized and self-assembled onto Au(111) surfaces. The properties of these monolayers have been compared with those of methyl-terminated SAMs having the same number of methylene units. Adsorption of the thiols via formation of a surface thiolate bond is demonstrated by angleresolved XPS measurements and UPS. Contact angle measurements also indicate that the thiophene functional group is at the periphery of the monolayer. Ellipsometry and QCM measurements show that 12-(3-thienyl)do(59) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775.
Ahn et al.
decanethiol forms well-ordered and closely packed monolayers on gold, exhibiting a tilt angle of 35°. 2-(3-Thienyl)ethanethiol and 6-(3-thienyl)hexanethiol form less packed monolayers and have greater tilt angles of 42° and 41°, apparently caused by disordering of the shorter alkyl chains. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. DMR-0089960. Insightful discussions with Professor Jayant Kumar and Dr. Dongwook Kim are acknowledged. M. Kim thanks the Council for Federated Centers and Institutes at the University of Massachusetts Lowell for a research assistantship. LA0208979
