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Steric effects on adsorption and desorption behaviors of alkanethiol self-assembled monolayers on Au(111)
Chemical Physics Letters 462 (2008) 209–212
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Steric effects on adsorption and desorption behaviors of alkanethiol self-assembled monolayers on Au(111) Eisuke Ito a,b,*, Jaegeun Noh b,c, Masahiko Hara a,d a
Flucto-Order Functions Asian Collaboration Team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Institute of Nano Science and Technology, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea c Department of Chemistry, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea d Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan b
a r t i c l e
i n f o
Article history: Received 18 April 2008 In final form 22 July 2008 Available online 29 July 2008
a b s t r a c t Adsorption conditions and desorption processes of self-assembled monolayers (SAMs) formed on Au(111) by propanethiol (PT) and PT-derivatives containing methyl groups on the a-carbon atom were examined by X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). XP spectra showed the formation of S–Au bonds, while adsorption condition depends on the molecular backbone structure. TDS indicated that PT SAM has two desorption fragments, dimer and monomer, at 450 K and 370 K, respectively. By increasing the number of methyl group, the dimerization was significantly suppressed. The monomer desorption peak shifted to a lower temperature, suggesting that the desorption process of alkanethiol SAMs is influenced by side-groups. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled monolayers (SAMs) of organosulfur molecules on Au(111) have been widely studied because of their well-ordered structures and their potential application to organic or biomolecular devices [1–3]. Growth of the uniform monolayer results from (1) covalent bond formation between the head group of a molecule and the substrate surface and (2) a self-assembly process due to intermolecular interactions. In the case of alkanethiol SAMs, the S–Au bond formation and van der Waals interactions of the alkyl chains play important roles in the formation of well-ordered structures on Au(111) substrates. Thermal desorption spectroscopy (TDS), also called as temperature programmed desorption (TPD), is useful for estimating the adsorption energy of adsorbates on metal surfaces. So far, there have been many reports on the TDS of alkanethiols (ATs) in the SAMs on metal surfaces to estimate the adsorption energy of the molecules [4–14]. It has been reported that AT SAMs on Au(111) substrates exhibit a dimerization reaction by sample heating in the TDS measurement. Desorption of the dimer fragments has been observed from closely-packed ordered phases of AT SAMs with ‘standing-up’ molecular orientation, suggesting that dimerization of sulfur head groups on gold is related to the molecular packing and orientation [6,9]. In contrast, most TDS studies of AT SAMs * Corresponding author. Address: Institute of Nano Science and Technology, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea. Fax: +82 2 2220 1935. E-mail address: [email protected] (E. Ito). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.07.075
have focused on the dependence of alkyl chain length or molecular coverage to understand the desorption behavior of AT SAMs. To date, there have been only a few reports concerning the desorption behavior of AT SAMs with a bulky molecular backbone so far. Lavrich et al. reported that the desorption energies of these SAMs prepared under the ultrahigh vacuum are weak relative to those of AT SAMs [7]. However, the desorption of the dimers was not experimentally analyzed in this Letter. In this study, we investigated the steric effects of ATs on the adsorption and desorption behaviors of AT SAMs by S 2p X-ray photoelectron spectroscopy (XPS) and TDS. SAMs were prepared on Au(111) by using propanethiol (PT) and PT-derivatives with two or three methyl groups on the a-carbon atom. Fig. 1 shows the structures of the organic compounds in this study. PT, 2-propanethiol (2PT), and 2-methyl-2-propanethiol (MPT) have an ethyl group, two methyl groups, and three methyl groups attached to the a-carbon atom, respectively, expecting the different occupied area of a molecule on the Au(111) substrates. A schematic illustration of the adsorption conditions for these SAMs is shown in Fig. 2.
2. Experiments Target organic compounds were purchased from TCI and used without any further purification. Au(111) substrates were fabricated by vacuum deposition on a mica plate as described previously [15–17]. The SAMs were fabricated by immersing the Au(111) substrates for one day into freshly prepared 1 mM ethanol solutions of the corresponding compounds. After the SAM samples
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S2p
(a) PT
SH (b) 2PT
SH (c) MPT
Fig. 1. Chemical structures of the compounds in this study: (a) PT, (b) 2PT, and (c) MPT.
Au(111) (a) PT SAM
Intensity / Normalized intensity
SH
(a) PT
(b) 2PT
(c) MPT
168
Au(111) (b) 2PT SAM
Au(111) (c) MPT SAM Fig. 2. Schematic view of the adsorption conditions for (a) PT, (b) 2PT, and (c) MPT SAMs on Au(111) substrate, assuming a standing orientation.
were removed from the solutions, they were carefully rinsed with an excess of pure ethanol to remove weakly adsorbed molecules. XP spectra of the SAMs were obtained using an Escalab250 (Thermo Fisher Scientific). The pass energy was set to 20 eV. Monochromatic Al Ka line was used as the excitation source. The obtained spectra were calibrated with the Au 4f7/2 peak at 84.0 eV and were analyzed by a peak fitting procedure [17]. TDS measurements were performed using a WA-1000S (ESCO Ltd.) with a quadrupole mass spectrometer (QMG-421, Balzers). The base pressure, before heating of the sample, was less than 1 10 7 Pa, and the heating rate was about 1 K/s. 3. Results and discussion Fig. 3 shows the S 2p XP spectra of the SAMs. The intensities of the spectra are normalized with the area of the Au 4f peak. The S 2p XP spectrum of the PT SAM (Fig. 3a) shows two dominant peaks at 162.0 eV and 163.2 eV that are assigned to bound S atoms (S 2p3/2 and S 2p1/2) on the Au surface [18–23]. The shape of the 2PT SAM spectrum is very similar to that of the PT SAM, while the bound S peak of the 2PT SAM is observed at a slightly lower binding energy of 161.9 eV (S 2p3/2). Although the other two S states at 161.2 eV and 162.9 eV (S 2p3/2) are confirmed by the curve-fitting analysis, their intensities are negligibly small. This indicates that the molecules are mostly chemisorbed on the Au(111) surface through the
166
164
162
160
158
Binding Energy / eV Fig. 3. S2p XP spectra of the SAMs. (a) PT, (b) 2PT, and (c) MPT. The intensity of the spectra was normalized with the area of the Au 4f peak. The fitting results are shown below each spectrum.
formation of S–Au chemical bonds. The area ratio of the S 2p and Au 4f peaks is almost the same in the two spectra (0.0062 for PT SAM and 0.0065 for 2PT SAM), indicating the same amount of adsorption on the Au(111) surfaces. STM studies have revealed that PT molecules on gold can form well-ordered SAMs [24,25], as in the case of AT SAMs with longer alkyl chains [26–30]. Our recent STM observation showed that 2PT molecules can form closelypacked ordered SAMs with a paired row structure (data not shown here). STM results showing ordered phases for PT and 2PT SAMs are consistent with the XPS results showing similar peaks in the S 2p regions, as shown in Fig. 3a and b. In contrast, the spectral feature for the MPT SAM (Fig. 3c) is broad, compared to that for the PT and 2PT SAMs. This means that the MPT molecules do not form a uniform monolayer film on the Au(111) surface. According to the curve-fitting analysis, three different S states are necessary to explain the spectrum. The state at 161.9 eV corresponds to the bound S atoms on the Au surface. The peak area of the bound S state (S 2p3/2 at 161.9 eV) relative to the Au 4f peak area (0.0019) is about half of the peak areas for the PT and 2PT spectra (0.0041 and 0.0036), while the total peak area for the S 2p is similar among the three molecules (0.0063 for MPT SAM). The degree of adsorption of the MPT molecules on the Au(111) substrate is almost the same as in the PT and 2PT SAMs. The feature located at 162.9 eV is assigned to the unbound S state [19–21,23]. Its intensity is very small, similar to the PT and 2PT SAMs. The strong peak observed at 161.2 eV corresponds to the different state of S bound on the Au(111) surface [19,20,22,23]. Thus, in the MPT SAM, other binding state (at 161.2 eV) intermingles considerably with the usual adsorption state of S atoms (at 161.9 eV). It is reasonable to expect that the occupied area of a MPT molecule containing three methyl groups bonded to the a-carbon atom is larger than that of PT and 2PT molecules, as shown in Fig. 2. This suggests that some molecules cannot be adsorbed on the stable bound site on the Au surface. In addition, MPT molecules do not form well-ordered SAMs due to weak van der Waals
E. Ito et al. / Chemical Physics Letters 462 (2008) 209–212
interactions between bulky alkyl groups. STM imaging revealed that MPT molecules hardly form ordered SAMs. Thus, the large contribution of the peak at 161.2 eV in XPS spectra for MPT SAMs can be ascribed to diminished structural order due to steric hindrance by the alkyl groups at the a-carbon, as revealed in our STM study. The STM results of the SAMs, together with the results for other related molecules, will be published elsewhere separately. Fig. 4a shows the TD spectra of a PT SAM. The solid line is the spectrum of the monomer (parent mass), and the circles represent the signal of the dimer fragment (disulfide). Two desorption processes can be seen at different temperatures, which is very similar to other reports for AT SAMs [6,9]. Firstly, dimer desorption was observed at around 375 K, and then, the parent mass was detected at a higher temperature of 450 K. This shows that the dimerization reaction of two thiol molecules occurs on the Au surface at a temperature lower than that of the molecular desorption. Kondoh et al. discussed the desorption process of AT SAMs [9]. Dimer desorption was observed only for the high coverage of the molecules. AT molecules in the closely-packed phase are in a standing orientation on the Au(111), and neighboring S atoms are adsorbed with a molecular spacing distance of about 0.5 nm. By heating the sample, the two neighboring sulfides form a disulfide, and then the disulfide molecules are desorbed. The molecular density on the Au surface decreases after dimers are desorbed, and the averaged intermolecular space becomes larger, leading to suppression of the dimerization reaction. Then, at the higher temperature, molecules start to desorb as the monomers through breaking of the S–Au bonds. In the previous TDS studies of AT SAMs with the chain lengths longer than six, a sulfide fragment, which is smaller than the parent mass by 1 (one hydrogen atom), was apparently observed [6,9,10], indicating that the molecules are adsorbed as the sulfide, while desorption of the sulfides was not detected in our experiments.
2 (a) PT 1
Intensity / arb. units
0 2 (b) 2PT 1
0 2 (c) MPT 1
0 300
350
400
450
500
550
Temperature / K Fig. 4. TD spectra of SAMs of PT and its derivatives. (a) PT, (b) 2PT, and (c) MPT. The solid lines are desorption spectra for monomer fragments, and the circles represent the spectra for dimer fragments.
211
Short propylsulfide species on the Au surface might react with the hydrogen atoms during desorption to form the monomer. Fig. 4b shows the TD spectra for a 2PT SAM. Although dimer desorption was also detected in the 2PT SAM spectrum, the intensity was weak compared with that of the PT SAM. Two methyl groups are attached to the a-carbon atom, leading to a wider intermolecular distance even in the final phase. In contrast, in the case of the MPT SAM shown in Fig. 4c, only the parent mass was detected and the dimer desorption was not observed. Three methyl groups are attached to the a-carbon atom bonded to the S atom, leading to a larger occupied area per molecule as mentioned above. This means that the distance between the neighboring S atoms increases. This reduces the reactivity of disulfide formation on the Au(111) surface, since the molecules cannot approach each other due to steric hindrance. Therefore, the larger intermolecular spacing prevents from the dimerization. Thus, two standing thiolates should be closely packed for formation of a disulfide on the Au surface upon heating. Desorption of the monomer fragment (parent mass of a thiol molecule) is clearly observed for all of the SAMs. The peak temperatures for the monomers in the spectra of the PT, 2PT, and MPT SAMs are about 450 K, 430 K, and 390 K, respectively. The monomer desorption temperature decreases with an increasing number of methyl groups on the a-carbon atom. The MPT SAM shows the lowest monomer desorption temperature among the measured SAMs, suggesting the weakest adsorption energy on the Au(111). This shows that attaching a side group at the a-carbon atom affects the adsorption energy on the Au surface, since this desorption occurs by scission of the S–Au bonds. According to Redhead’s equation [3,4,6–9,31], the adsorption activation energy can be estimated from the peak-top temperature. Assuming the first-order process for the monomer desorption and a pre-exponential factor m0 of 1015 s 1 [9], the adsorption energy is calculated to be 37, 35, and 32 kcal/mol for the PT, 2PT, and MPT SAMs, respectively. The value for the PT SAM is comparable to the reported value of about 40 kcal/mol [3,9] for sulfide desorption and is slightly smaller than the S–Au bonding energy (45 kcal/mol [3,6,9]). The difference in desorption energy between the PT and MPT SAMs is estimated to be about 5 kcal/mol. Intermolecular interactions between the neighboring alkyl chains do not affect the desorption temperature of the monomer. In our case, only 3 or 4 carbon atoms are in a molecule, indicating small intermolecular interactions. Also, chain length dependence of the monomer desorption was not observed between the PT and hexanethiol SAMs. The monomer desorption peaks for both SAMs are positioned at around 450 K. Thus, the lowering of the desorption temperature of the 2PT and MPT SAMs does not result from a decrease in the intermolecular interactions. Fig. 5 summarizes the desorption processes of the three SAMs. After deposition at the room temperature, the molecular density decreases in the order of PT, 2PT, and MPT, which relates to the dimerization efficiency. Some of the molecules in the standing phase of the PT and 2PT SAMs are desorbed as the dimers, while dimerization cannot occur in the MPT SAM due to large intermolecular distance. The difference in desorption energy of the monomer fragments could arise from the strength of the S–Au bond. Several possibilities are considered for the change in the bond strength. Lavrich et al. reported a weak S–Au bond energy due to the steric hindrance of the methyl group [7]. Attachment of the methyl side group makes S–Au distance long, corresponding to a weakening of the bonding energy. The second possibility is that the S–Au bond strength depends on the density of the molecules. Hayashi et al. [32] found, based on theoretical calculations, that the adsorption energy of methylthiolate adsorbed on Au(111) increases with decreasing molecular density. From the peak intensities of the
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dimer
monomer HS
S
PT
HS
S
S S S S S
S
S
Au(111)
Au(111)
Au(111) HS
HS
S
2PT
S
S
S
S
S
S
HS
S
S
S
Au(111)
Au(111)
Au(111) HS
HS
tion of S–Au chemical bonds, while a non-uniform monolayer is suggested for the MPT SAM because of the bulky molecular structure. By attaching a methyl group, desorption of dimer fragments in the TD spectrum was significantly suppressed. The dimerization reaction of AT SAMs by heating occurs only with the short distances between the S atoms in a closely-packed structure. Different monomer desorption temperatures suggest that the adsorption energy of the AT molecules changes upon introduction of the methyl groups at the a-carbon atom. Today, many varieties of thiol derivatives are applied to device fabrication. Our results show that the intermolecular interaction or molecular spacing in AT SAMs on Au electrodes significantly affects thermal stability of the films as well as adsorption condition.
HS
MPT
S
S
S
S
Au(111)
density
Acknowledgements
HS
S
PT>2PT>MPT
Au(111)
PT<2PT
Fig. 5. Schematic view of the desorption processes for PT, 2PT, and MPT SAMs on Au(111) substrate.
dimer fragments, the dimer desorption is less for 2PT than for PT SAM. Thus, the ordering of the molecular density before monomer desorption is reversed (PT < 2PT < MPT), as shown in Fig. 5. The trend of the desorption temperature relates with the expected molecular density, consistent with the theoretical prediction. Third is that the chemical modification of the a-carbon atom in an AT molecule leads to a change in the adsorption energy on the Au(111) surface. A theoretical calculation by Higai et al. [33] shows the dependence of the bond strength of S–Au on the chemical environment of a-carbon atom. The binding energy of the S–Au bond may decrease by insertion of the methyl group at the a-carbon atom. As another possibility, we cannot exclude the different adsorption sites among these SAMs at present. Bond lengths and adsorption sites should be experimentally determined by other methods in order to understand the adsorption energy on the Au(111) surface. Relationship between the desorption processes and the adsorption states (S 2p XP spectra and STM images) cannot be fully explained at present. In the S 2p spectrum of the MPT SAM (Fig. 3c), two bound states are observed equally, while only the S–Au bound state similar to PT SAM is dominantly observed for the 2PT SAM. The peak intensity of the lower energy state (at 161.2 eV for S 2p3/2) is not related linearly with the intensity of the dimer desorption and the monomer desorption temperature for these SAMs. The bound state in the S2p XP spectra appears at the same binding energy (162.0 ± 0.1 eV for S 2p3/2) for all SAMs, indicating the same electron density of the bound S atom, while desorption of the monomer depends on the compounds. The difference in the desorption energy (5 kcal/mol) is small compared to the energy of the S–Au chemical bond. Thus, we suggest that the modification of the electronic structure in the S atom is small. The peak energy of the S–Au chemical bond in S2p spectra may be insensitive to the strength of the chemical bond or desorption temperature. In summary, we investigated the adsorption conditions and desorption processes of SAMs of ATs with a methyl group on the a-carbon atom by XPS and TDS. The XPS results showed the forma-
We would like to thank Dr. T. Hayashi of the Tokyo Institute of Technology for fruitful discussion on the desorption processes of alkanethiol SAMs. We acknowledge Dr. A. Nakao of RIKEN for her help with the XPS measurements. This work was supported by the Korea Foundation for International Cooperation of Science & Technology (KICOS) through a grant provided by the Korean Ministry of Science & Technology (MOST) (No. K20501000002-07E0100-00210). This work was also partially supported by the Seoul R&BD Program (10919). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
A. Ulman, Chem. Rev. 96 (1996) 1533. G.E. Poirier, Chem. Rev. 97 (1997) 1117. S. Krämer, R.R. Fuierer, C.B. Gorman, Chem. Rev. 103 (2003) 4367. R.G. Nuzzo, B.R. Zegarski, L.H. Dubois, J. Am. Chem. Soc. 109 (1987) 733. R.G. Nuzzo, L.H. Dubois, D.L. Allara, J. Am. Chem. Soc. 112 (1990) 588. N. Nishida, M. Hara, H. Sasabe, W. Knoll, Jpn. J. Appl. Phys. 35 (1996) 5866. D.J. Lavrich, S.M. Wetterer, S.L. Bernasek, G. Scoles, J. Phys. Chem. B 102 (1998) 3456. V. Bondzie, St. J. Dixon-Warren, Y. Yu, L. Zhang, Surf. Sci. 431 (1999) 174. H. Kondoh, C. Kodama, H. Sumida, H. Nozoe, J. Chem. Phys. 111 (1999) 1175. C. Kodama, T. Hayashi, H. Nozoe, Appl. Surf. Sci. 169–170 (2001) 264. G. Liu, J.A. Rodriguez, J. Dvorak, J. Hrbek, T. Jirsak, Surf. Sci. (2002) 295. T.H. Lin, T.P. Huang, Y.L. Liu, C.C. Yeh, Y.H. Lai, W.H. Hung, J. Phys. Chem. B 109 (2005) 14079. J.G. Lee, J. Lee, J.T. Yates Jr, J. Am. Chem. Soc. 126 (2004) 440. I.I. Rzeznicka, J. Lee, P. Maksymovych, T. Yates Jr, J. Phys. Chem. B 109 (2005) 15992. J. Noh, M. Hara, Langmuir 18 (2002) 9111. J. Noh, M. Hara, Langmuir 16 (2000) 2045. E. Ito, J. Noh, M. Hara, Jpn. J. Appl. Phys. 42 (2003) L852. Ch. Zubrägel, C. Deuper, F. Schneider, M. Neumann, M. Grunze, A. Schrtel, Ch. Wöll, Chem. Phys. Lett. 238 (1995) 308. T. Ishida, M. Hara, I. Kojima, S. Tsuneda, N. Nishida, H. Sasabe, W. Knoll, Langmuir 14 (1998) 2092. T. Ishida et al., Langmuir 15 (1999) 6799. M. Wirde, U. Gelius, L. Nyholm, Langmuir 15 (1999) 6370. D.L. Pugmire, M.J. Tarlox, R.D. van Zee, Langmuir 19 (2003) 3720. Y. Jeong, C. Lee, E. Ito, M. Hara, J. Noh, Jpn. J. Appl. Phys. 45 (2006) 5905. S.Y. Lee, J. Noh, E. Ito, H. Lee, M. Hara, Jpn. J. Appl. Phys. 42 (2003) 236. J. Zhang, Q. Chi, J. Ulstrup, Langmuir 22 (2006) 6203. S. Xu, P.E. Laibinis, G. Liu, J. Am. Chem. Soc. 120 (1998) 9356. G.E. Poirier, W.P. Fitts, J.M. White, Langmuir 17 (2001) 1176. J. Noh, M. Hara, Langmuir 18 (2002) 1953. T. Fukuma, T. Ichii, K. Kobayashi, H. Yamada, K. Matsushige, J. Appl. Phys. 95 (2004) 1222. B. Lüssem, L. Müller-Meskamp, S. Karthäuser, R. Waser, Langmuir 21 (2005) 5256. P.A. Readhead, Vacuum 12 (1962) 203. T. Hayashi, Y. Morikawa, H. Nozoe, J. Chem. Phys. 114 (2001) 7615. H. Higai, J. Nara, T. Ohno, J. Chem. Phys. 121 (2004) 970.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Steric effects on adsorption and desorption behaviors of alkanethiol self-assembled monolayers on Au(111) Eisuke Ito a,b,*, Jaegeun Noh b,c, Masahiko Hara a,d a
Flucto-Order Functions Asian Collaboration Team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Institute of Nano Science and Technology, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea c Department of Chemistry, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea d Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan b
a r t i c l e
i n f o
Article history: Received 18 April 2008 In final form 22 July 2008 Available online 29 July 2008
a b s t r a c t Adsorption conditions and desorption processes of self-assembled monolayers (SAMs) formed on Au(111) by propanethiol (PT) and PT-derivatives containing methyl groups on the a-carbon atom were examined by X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). XP spectra showed the formation of S–Au bonds, while adsorption condition depends on the molecular backbone structure. TDS indicated that PT SAM has two desorption fragments, dimer and monomer, at 450 K and 370 K, respectively. By increasing the number of methyl group, the dimerization was significantly suppressed. The monomer desorption peak shifted to a lower temperature, suggesting that the desorption process of alkanethiol SAMs is influenced by side-groups. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled monolayers (SAMs) of organosulfur molecules on Au(111) have been widely studied because of their well-ordered structures and their potential application to organic or biomolecular devices [1–3]. Growth of the uniform monolayer results from (1) covalent bond formation between the head group of a molecule and the substrate surface and (2) a self-assembly process due to intermolecular interactions. In the case of alkanethiol SAMs, the S–Au bond formation and van der Waals interactions of the alkyl chains play important roles in the formation of well-ordered structures on Au(111) substrates. Thermal desorption spectroscopy (TDS), also called as temperature programmed desorption (TPD), is useful for estimating the adsorption energy of adsorbates on metal surfaces. So far, there have been many reports on the TDS of alkanethiols (ATs) in the SAMs on metal surfaces to estimate the adsorption energy of the molecules [4–14]. It has been reported that AT SAMs on Au(111) substrates exhibit a dimerization reaction by sample heating in the TDS measurement. Desorption of the dimer fragments has been observed from closely-packed ordered phases of AT SAMs with ‘standing-up’ molecular orientation, suggesting that dimerization of sulfur head groups on gold is related to the molecular packing and orientation [6,9]. In contrast, most TDS studies of AT SAMs * Corresponding author. Address: Institute of Nano Science and Technology, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea. Fax: +82 2 2220 1935. E-mail address: [email protected] (E. Ito). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.07.075
have focused on the dependence of alkyl chain length or molecular coverage to understand the desorption behavior of AT SAMs. To date, there have been only a few reports concerning the desorption behavior of AT SAMs with a bulky molecular backbone so far. Lavrich et al. reported that the desorption energies of these SAMs prepared under the ultrahigh vacuum are weak relative to those of AT SAMs [7]. However, the desorption of the dimers was not experimentally analyzed in this Letter. In this study, we investigated the steric effects of ATs on the adsorption and desorption behaviors of AT SAMs by S 2p X-ray photoelectron spectroscopy (XPS) and TDS. SAMs were prepared on Au(111) by using propanethiol (PT) and PT-derivatives with two or three methyl groups on the a-carbon atom. Fig. 1 shows the structures of the organic compounds in this study. PT, 2-propanethiol (2PT), and 2-methyl-2-propanethiol (MPT) have an ethyl group, two methyl groups, and three methyl groups attached to the a-carbon atom, respectively, expecting the different occupied area of a molecule on the Au(111) substrates. A schematic illustration of the adsorption conditions for these SAMs is shown in Fig. 2.
2. Experiments Target organic compounds were purchased from TCI and used without any further purification. Au(111) substrates were fabricated by vacuum deposition on a mica plate as described previously [15–17]. The SAMs were fabricated by immersing the Au(111) substrates for one day into freshly prepared 1 mM ethanol solutions of the corresponding compounds. After the SAM samples
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E. Ito et al. / Chemical Physics Letters 462 (2008) 209–212
S2p
(a) PT
SH (b) 2PT
SH (c) MPT
Fig. 1. Chemical structures of the compounds in this study: (a) PT, (b) 2PT, and (c) MPT.
Au(111) (a) PT SAM
Intensity / Normalized intensity
SH
(a) PT
(b) 2PT
(c) MPT
168
Au(111) (b) 2PT SAM
Au(111) (c) MPT SAM Fig. 2. Schematic view of the adsorption conditions for (a) PT, (b) 2PT, and (c) MPT SAMs on Au(111) substrate, assuming a standing orientation.
were removed from the solutions, they were carefully rinsed with an excess of pure ethanol to remove weakly adsorbed molecules. XP spectra of the SAMs were obtained using an Escalab250 (Thermo Fisher Scientific). The pass energy was set to 20 eV. Monochromatic Al Ka line was used as the excitation source. The obtained spectra were calibrated with the Au 4f7/2 peak at 84.0 eV and were analyzed by a peak fitting procedure [17]. TDS measurements were performed using a WA-1000S (ESCO Ltd.) with a quadrupole mass spectrometer (QMG-421, Balzers). The base pressure, before heating of the sample, was less than 1 10 7 Pa, and the heating rate was about 1 K/s. 3. Results and discussion Fig. 3 shows the S 2p XP spectra of the SAMs. The intensities of the spectra are normalized with the area of the Au 4f peak. The S 2p XP spectrum of the PT SAM (Fig. 3a) shows two dominant peaks at 162.0 eV and 163.2 eV that are assigned to bound S atoms (S 2p3/2 and S 2p1/2) on the Au surface [18–23]. The shape of the 2PT SAM spectrum is very similar to that of the PT SAM, while the bound S peak of the 2PT SAM is observed at a slightly lower binding energy of 161.9 eV (S 2p3/2). Although the other two S states at 161.2 eV and 162.9 eV (S 2p3/2) are confirmed by the curve-fitting analysis, their intensities are negligibly small. This indicates that the molecules are mostly chemisorbed on the Au(111) surface through the
166
164
162
160
158
Binding Energy / eV Fig. 3. S2p XP spectra of the SAMs. (a) PT, (b) 2PT, and (c) MPT. The intensity of the spectra was normalized with the area of the Au 4f peak. The fitting results are shown below each spectrum.
formation of S–Au chemical bonds. The area ratio of the S 2p and Au 4f peaks is almost the same in the two spectra (0.0062 for PT SAM and 0.0065 for 2PT SAM), indicating the same amount of adsorption on the Au(111) surfaces. STM studies have revealed that PT molecules on gold can form well-ordered SAMs [24,25], as in the case of AT SAMs with longer alkyl chains [26–30]. Our recent STM observation showed that 2PT molecules can form closelypacked ordered SAMs with a paired row structure (data not shown here). STM results showing ordered phases for PT and 2PT SAMs are consistent with the XPS results showing similar peaks in the S 2p regions, as shown in Fig. 3a and b. In contrast, the spectral feature for the MPT SAM (Fig. 3c) is broad, compared to that for the PT and 2PT SAMs. This means that the MPT molecules do not form a uniform monolayer film on the Au(111) surface. According to the curve-fitting analysis, three different S states are necessary to explain the spectrum. The state at 161.9 eV corresponds to the bound S atoms on the Au surface. The peak area of the bound S state (S 2p3/2 at 161.9 eV) relative to the Au 4f peak area (0.0019) is about half of the peak areas for the PT and 2PT spectra (0.0041 and 0.0036), while the total peak area for the S 2p is similar among the three molecules (0.0063 for MPT SAM). The degree of adsorption of the MPT molecules on the Au(111) substrate is almost the same as in the PT and 2PT SAMs. The feature located at 162.9 eV is assigned to the unbound S state [19–21,23]. Its intensity is very small, similar to the PT and 2PT SAMs. The strong peak observed at 161.2 eV corresponds to the different state of S bound on the Au(111) surface [19,20,22,23]. Thus, in the MPT SAM, other binding state (at 161.2 eV) intermingles considerably with the usual adsorption state of S atoms (at 161.9 eV). It is reasonable to expect that the occupied area of a MPT molecule containing three methyl groups bonded to the a-carbon atom is larger than that of PT and 2PT molecules, as shown in Fig. 2. This suggests that some molecules cannot be adsorbed on the stable bound site on the Au surface. In addition, MPT molecules do not form well-ordered SAMs due to weak van der Waals
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interactions between bulky alkyl groups. STM imaging revealed that MPT molecules hardly form ordered SAMs. Thus, the large contribution of the peak at 161.2 eV in XPS spectra for MPT SAMs can be ascribed to diminished structural order due to steric hindrance by the alkyl groups at the a-carbon, as revealed in our STM study. The STM results of the SAMs, together with the results for other related molecules, will be published elsewhere separately. Fig. 4a shows the TD spectra of a PT SAM. The solid line is the spectrum of the monomer (parent mass), and the circles represent the signal of the dimer fragment (disulfide). Two desorption processes can be seen at different temperatures, which is very similar to other reports for AT SAMs [6,9]. Firstly, dimer desorption was observed at around 375 K, and then, the parent mass was detected at a higher temperature of 450 K. This shows that the dimerization reaction of two thiol molecules occurs on the Au surface at a temperature lower than that of the molecular desorption. Kondoh et al. discussed the desorption process of AT SAMs [9]. Dimer desorption was observed only for the high coverage of the molecules. AT molecules in the closely-packed phase are in a standing orientation on the Au(111), and neighboring S atoms are adsorbed with a molecular spacing distance of about 0.5 nm. By heating the sample, the two neighboring sulfides form a disulfide, and then the disulfide molecules are desorbed. The molecular density on the Au surface decreases after dimers are desorbed, and the averaged intermolecular space becomes larger, leading to suppression of the dimerization reaction. Then, at the higher temperature, molecules start to desorb as the monomers through breaking of the S–Au bonds. In the previous TDS studies of AT SAMs with the chain lengths longer than six, a sulfide fragment, which is smaller than the parent mass by 1 (one hydrogen atom), was apparently observed [6,9,10], indicating that the molecules are adsorbed as the sulfide, while desorption of the sulfides was not detected in our experiments.
2 (a) PT 1
Intensity / arb. units
0 2 (b) 2PT 1
0 2 (c) MPT 1
0 300
350
400
450
500
550
Temperature / K Fig. 4. TD spectra of SAMs of PT and its derivatives. (a) PT, (b) 2PT, and (c) MPT. The solid lines are desorption spectra for monomer fragments, and the circles represent the spectra for dimer fragments.
211
Short propylsulfide species on the Au surface might react with the hydrogen atoms during desorption to form the monomer. Fig. 4b shows the TD spectra for a 2PT SAM. Although dimer desorption was also detected in the 2PT SAM spectrum, the intensity was weak compared with that of the PT SAM. Two methyl groups are attached to the a-carbon atom, leading to a wider intermolecular distance even in the final phase. In contrast, in the case of the MPT SAM shown in Fig. 4c, only the parent mass was detected and the dimer desorption was not observed. Three methyl groups are attached to the a-carbon atom bonded to the S atom, leading to a larger occupied area per molecule as mentioned above. This means that the distance between the neighboring S atoms increases. This reduces the reactivity of disulfide formation on the Au(111) surface, since the molecules cannot approach each other due to steric hindrance. Therefore, the larger intermolecular spacing prevents from the dimerization. Thus, two standing thiolates should be closely packed for formation of a disulfide on the Au surface upon heating. Desorption of the monomer fragment (parent mass of a thiol molecule) is clearly observed for all of the SAMs. The peak temperatures for the monomers in the spectra of the PT, 2PT, and MPT SAMs are about 450 K, 430 K, and 390 K, respectively. The monomer desorption temperature decreases with an increasing number of methyl groups on the a-carbon atom. The MPT SAM shows the lowest monomer desorption temperature among the measured SAMs, suggesting the weakest adsorption energy on the Au(111). This shows that attaching a side group at the a-carbon atom affects the adsorption energy on the Au surface, since this desorption occurs by scission of the S–Au bonds. According to Redhead’s equation [3,4,6–9,31], the adsorption activation energy can be estimated from the peak-top temperature. Assuming the first-order process for the monomer desorption and a pre-exponential factor m0 of 1015 s 1 [9], the adsorption energy is calculated to be 37, 35, and 32 kcal/mol for the PT, 2PT, and MPT SAMs, respectively. The value for the PT SAM is comparable to the reported value of about 40 kcal/mol [3,9] for sulfide desorption and is slightly smaller than the S–Au bonding energy (45 kcal/mol [3,6,9]). The difference in desorption energy between the PT and MPT SAMs is estimated to be about 5 kcal/mol. Intermolecular interactions between the neighboring alkyl chains do not affect the desorption temperature of the monomer. In our case, only 3 or 4 carbon atoms are in a molecule, indicating small intermolecular interactions. Also, chain length dependence of the monomer desorption was not observed between the PT and hexanethiol SAMs. The monomer desorption peaks for both SAMs are positioned at around 450 K. Thus, the lowering of the desorption temperature of the 2PT and MPT SAMs does not result from a decrease in the intermolecular interactions. Fig. 5 summarizes the desorption processes of the three SAMs. After deposition at the room temperature, the molecular density decreases in the order of PT, 2PT, and MPT, which relates to the dimerization efficiency. Some of the molecules in the standing phase of the PT and 2PT SAMs are desorbed as the dimers, while dimerization cannot occur in the MPT SAM due to large intermolecular distance. The difference in desorption energy of the monomer fragments could arise from the strength of the S–Au bond. Several possibilities are considered for the change in the bond strength. Lavrich et al. reported a weak S–Au bond energy due to the steric hindrance of the methyl group [7]. Attachment of the methyl side group makes S–Au distance long, corresponding to a weakening of the bonding energy. The second possibility is that the S–Au bond strength depends on the density of the molecules. Hayashi et al. [32] found, based on theoretical calculations, that the adsorption energy of methylthiolate adsorbed on Au(111) increases with decreasing molecular density. From the peak intensities of the
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dimer
monomer HS
S
PT
HS
S
S S S S S
S
S
Au(111)
Au(111)
Au(111) HS
HS
S
2PT
S
S
S
S
S
S
HS
S
S
S
Au(111)
Au(111)
Au(111) HS
HS
tion of S–Au chemical bonds, while a non-uniform monolayer is suggested for the MPT SAM because of the bulky molecular structure. By attaching a methyl group, desorption of dimer fragments in the TD spectrum was significantly suppressed. The dimerization reaction of AT SAMs by heating occurs only with the short distances between the S atoms in a closely-packed structure. Different monomer desorption temperatures suggest that the adsorption energy of the AT molecules changes upon introduction of the methyl groups at the a-carbon atom. Today, many varieties of thiol derivatives are applied to device fabrication. Our results show that the intermolecular interaction or molecular spacing in AT SAMs on Au electrodes significantly affects thermal stability of the films as well as adsorption condition.
HS
MPT
S
S
S
S
Au(111)
density
Acknowledgements
HS
S
PT>2PT>MPT
Au(111)
PT<2PT
Fig. 5. Schematic view of the desorption processes for PT, 2PT, and MPT SAMs on Au(111) substrate.
dimer fragments, the dimer desorption is less for 2PT than for PT SAM. Thus, the ordering of the molecular density before monomer desorption is reversed (PT < 2PT < MPT), as shown in Fig. 5. The trend of the desorption temperature relates with the expected molecular density, consistent with the theoretical prediction. Third is that the chemical modification of the a-carbon atom in an AT molecule leads to a change in the adsorption energy on the Au(111) surface. A theoretical calculation by Higai et al. [33] shows the dependence of the bond strength of S–Au on the chemical environment of a-carbon atom. The binding energy of the S–Au bond may decrease by insertion of the methyl group at the a-carbon atom. As another possibility, we cannot exclude the different adsorption sites among these SAMs at present. Bond lengths and adsorption sites should be experimentally determined by other methods in order to understand the adsorption energy on the Au(111) surface. Relationship between the desorption processes and the adsorption states (S 2p XP spectra and STM images) cannot be fully explained at present. In the S 2p spectrum of the MPT SAM (Fig. 3c), two bound states are observed equally, while only the S–Au bound state similar to PT SAM is dominantly observed for the 2PT SAM. The peak intensity of the lower energy state (at 161.2 eV for S 2p3/2) is not related linearly with the intensity of the dimer desorption and the monomer desorption temperature for these SAMs. The bound state in the S2p XP spectra appears at the same binding energy (162.0 ± 0.1 eV for S 2p3/2) for all SAMs, indicating the same electron density of the bound S atom, while desorption of the monomer depends on the compounds. The difference in the desorption energy (5 kcal/mol) is small compared to the energy of the S–Au chemical bond. Thus, we suggest that the modification of the electronic structure in the S atom is small. The peak energy of the S–Au chemical bond in S2p spectra may be insensitive to the strength of the chemical bond or desorption temperature. In summary, we investigated the adsorption conditions and desorption processes of SAMs of ATs with a methyl group on the a-carbon atom by XPS and TDS. The XPS results showed the forma-
We would like to thank Dr. T. Hayashi of the Tokyo Institute of Technology for fruitful discussion on the desorption processes of alkanethiol SAMs. We acknowledge Dr. A. Nakao of RIKEN for her help with the XPS measurements. This work was supported by the Korea Foundation for International Cooperation of Science & Technology (KICOS) through a grant provided by the Korean Ministry of Science & Technology (MOST) (No. K20501000002-07E0100-00210). This work was also partially supported by the Seoul R&BD Program (10919). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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