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Formation of Highly Ordered Self-Assembled Monolayers of Alkynes on Au(111) Substrate

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Citation Zaba, Tomasz, Agnieszka Noworolska, Carleen Morris Bowers, Benjamin Breiten, George M. Whitesides, and Piotr Cyganik. 2014. Formation of Highly Ordered Self-Assembled Monolayers of Alkynes on Au(111) Substrate. Journal of the American Chemical Society 136, no. 34: 11918–11921. doi:10.1021/ja506647p.

Published Version doi:10.1021/ja506647p

Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:33490483

Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP Page 1 of 5 Journal of the American Chemical Society

1 2 3 4 5 6 7 Formation of Highly Ordered Self-Assembled Monolayers of 8 9 Alkynes on Au(111) Substrate 10 ‡ ‡ § § 11 Tomasz Zaba, Agnieszka Noworolska, Carleen Bowers, Benjamin Breiten, 12 § ‡ 13 George M. Whitesides and Piotr Cyganik* 14 ‡Smoluchowski Institute of Physics, Jagiellonian University, ul. Reymonta 4, 30-059 Krakow, Poland 15 § 16 Department of and , Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA 17 18 19 Supporting Information Placeholder 20 21 ABSTRACT: Self-assembled monolayers (SAMs), prepared SAMs are sensitive to oxidation at an undefined point in their formation; that is, oxidation occurs either during or after SAM 22 by reaction of terminal n-DON\QHV +&Ł& &+ 2)nCH 3, n = 5, 7, 9, o formation (for example, by UHDFWLRQ RI WKH $X&Ł&5 ERQG ZLWK 23 and 11) with Au(111) at 60 C were characterized using scanning tunneling microscopy (STM), infra-red reflection absorption O2). Contact angle analyses of increasing lengths of alkynes 24 4 (IRRAS), X-ray photoelectron spectroscopy (XPS), (HC ŁC(CH2)nCH 3, n = 5, 7, 9 and 11) also suggest that the quali- 25 and contact angles of water. In contrast to previous spectroscopic ty of these SAMs is lower than those based on n-alkanethiols. 26 studies of this type of SAMs, these combined microscopic and Although SAMs have enabled studies of wetting, 7,8 27 spectroscopic experiments confirm formation of highly-ordered adhesion, 9,10 and charge transport 3,11-13 (inter alia), most of this 28 SAMs having packing densities and molecular chain orientations work has focused on the terminal part of the SAM that is exposed 29 very similar to those of alkanethiolates on Au(111). Physical to air, and there is relatively little work devoted to understanding 30 properties—hydrophobicity, high surface order, and packing the contribution of the anchoring groups of the SAM (as opposed 31 density—also suggest that SAMs of alkynes are similar to SAMs to the terminal group, the thickness, or the electronic structure). In of alkanethiols. The formation of high-quality SAMs from al- 32 the current work, we characterize the SAMs formed by reactions kynes requires careful preparation and manipulation of reactants of n-alkynes (HC C(CH ) CH , n = 5, 7, 9 and 11) with Au(111). 33 Ł 2 n 3 in an oxygen-free environment: trace quantities of oxygen lead to :HEHOLHYHWKDWWKH$X&Ł&5JURXSLVSDUWLFXOD rly interesting as 34 oxidized contaminants and disordered surface films. The oxida- the basis for SAMs on Au for two reasons: i) the group 35 tion process occurs during formation of the SAM; via oxidation of FRQQHFWVZLWKJROGDWRPVRQWKHVXUIDFHE\DVWURQJı-bond; 14 ii) 36 WKHí&Ł&íJURXS PRVWOLNHO\FDW alyzed by the gold substrate in the orbital structure of the acetylene group, and the existence of a 37 the presence of O 2). variety of stable organometallic compounds containing the 38 Au &Ł&5 JURXS VXJJHVWV WKDW WKH LQWHUIDFH E etween the metal, 39 Au, and the saturated organic component of the SAM (R = 40 (CH2)nCH 3), might be especially informative in studies in which Thin organic films based on self-assembled monolayers the interface connecting the SAM to the metallic substrate might 41 (SAMs) 1 are ubiquitous in . The reaction of organ- contribute to its properties. 42 ic (RSH) with group Ib metals (Au and Ag) to generate The objective of this work was to address the question of the 43 SAMs with composition Au/AgSR is the reaction most commonly order of n-alkyne-derived SAMs on gold through a combination 44 used to prepare SAMs, 1 although reactions that generate orga- 2 3 of microscopic, spectroscopic and contact angle measurements. 45 nosilanes on silicon (SiR) and organic carboxylates on Our results show that the disorder and mixed organic functionality 46 (AgO 2CR) have attractive properties, and a number of other pre- implied by previous work 4-6 are artefacts reflecting oxidation of cursors have been surveyed. There have also been scattered de- 47 4 the terminal acetylene by oxygen in solution during formation of scriptions of SAMs formed on gold from solutions of alkynes 15 48 5 the SAM (perhaps in a reaction catalyzed by gold), and that (HC ŁC(CH2)nCH 3, n = 3, 5, 7, 9, 11, and 13), ethynylbenzene using appropriate experimental conditions (e.g., rigorous exclu- 49 6 (HC ŁCC 6H5) or n-alkylmercury(II) tosylates (CH3(CH2)nHgOTs, 50 sion of oxygen; a slightly elevated temperature of 60ºC during n = 4 and 18) on Au(111). Although the potential interest of formation of the SAM) results in well-organized SAMs of alkynes 51 6$0VKDYLQJ0HWDOí&Ł&5ERQGVLVK igh, since they offer a new that have qualities similar to that of alkanethiols on Au(111). 52 type of metal-organic bond, most of these studies have used prep- These studies thus establish that SAMs of the surface composition 53 arations analogous to those employed with n-alkanethiols, and $X&Ł&5 SURYLGH D QHZ W\SH RI 6$0 IRU XVH LQ SK\V ical- and 54 have generated SAMs that have not seemed to be highly ordered, physical-RUJDQLF VWXGLHV KDYLQJ DQ LQWHUIDFH WR JROG $X&Ł&5  55 and are thus, perhaps, unsuitable for detailed studies of the physi- and are complementary to the well-understood SAMs of al- cal chemistry of the surface. In particular, there are no procedures kanethiols (AuSR). 56 that describe the formation of SAMs that are highly ordered in 57 two dimensions—a key requirement for high-quality surface SAMs of alkynes were prepared on gold by submerging freshly 58 science. The most recent analyses of n--based SAMs on evaporated Au(111) substrates in a 1mM ethanolic solution of n- 59 Au(111) indicate a “liquid-like” structure of the monolayer, 6 and alkyne (HC ŁC(CH2)nCH 3, n = 5, 7, 9, and 11) for 15 hours at 60 XPS analyses of SAMs formed from alkynes 4,5 suggest that these 60ºC. Importantly, the preparation of the SAMs was performed in ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 5

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 Figure 2. Overview of IRRAS data for n = 5, 7, 9 and 11 26 alkynes on Au(111) together with the corresponding 27 DDT/Au(111) in the characteristic C-H stretching range. 28

29 17 30 . The detection of similar features for alkynes suggests the formation of densely packed chemisorbed structures on 31 Au(111). 32 This inference of a dense monolayer is confirmed by high- 33 resolution STM, accompanied by Fourier analysis (using Fast 34 Fourier Transform, FFT), which shows a hexagonal lattice. The 35 data obtained after FFT filtering (Figure 1c) suggest a hexagonal 36 structure with a period of ca. 5 Å, as indicated by the line scans 37 along the axes. The structure inferred from this analysis (shown o 38 schematically in Figure 1d) is consistent with the ( —3u—3)R30 lattice (which characterizes the structure formed by alkanethiols 39 18 2 40 Figure 1. In (a-c) STM data for /Au(111) SAMs. Pan- on Au(111) and leads to an area per of 21.5 Å . While the quality of our data does not permit a more detailed description 41 els assigned as A, B and C show cross-sections A, B and C indi- cated in (a-c), respectively. The inset in (b) shows FFT spectrum of the structure (as was possible for alkanethiols, which exhibit a 42 18 of image (b) with indicated six-fold symmetry pattern. Yellow c(4 u2) superlattice), it certainly shows that alkynes on Au(111) 43 rectangle in (b) marks an area which corresponds to the image form well-ordered structures with packing densities similar to 44 shown in (c) obtained after FFT filtering of (b). In (d) scheme of those of alkanethiols (area per molecule 21.5 Å 2). o 45 decyne adsorption in the ( —3u—3)R30 structure with arbitrary To characterize the orientational order of alkyne-based SAMs 46 taken adsorption seats. further, we used infra-red reflection absorption spectroscopy 47 (IRRAS). IRRAS measurements of the SAMs—formed by reac- 48 an oxygen-free environment to avoid oxidation of the acetylene tion of (n = 5), decyne (n = 7), dodecyne (n = 9) and 49 group. The supporting information (SI) provides a more detailed tetradecyne (n = 11) with Au(111)—showed vibrational bands in -1 -1 50 description of the experimental procedure, as well as additional the C-H stretching range at 2965 cm (QaCH 3), 2938 cm (QsCH 3, -1 -1 -1 51 details of measurements. FR ), 2920 cm (QaCH 2), 2878 cm (QsCH 3, FR ), and 2851 cm 52 Figure 1 summarizes the results obtained by scanning tunneling (QsCH 2) (Figure 2). For comparison, analogous IRRAS measure- ments were performed on SAMs of dodecanethiol (DDT) on 53 microscopy (STM) for decyne (n = 7) chemisorbed on Au(111). Au(111). This comparison shows that the frequencies, relative 54 Analysis on a larger scale (Figure 1a) shows formation of depres- sions on the substrate with depths compatible with substrate lat- LQWHQVLWLHV DQG WKH EDQG ZLGWK RI WKH VSHFWUD IRU $X&Ł&5 DQG 55 19 tice steps due to a single atomic layer of gold (as indicated by the AuSR are similar (for more details see Figure 1S in the SI). For 56 respective cross-section, marked by A). Such depressions are also the same orientation of n-alkynyl and alkanethiol on Au(111), 57 characteristic of formation of both -1 and -16 based however, we would expect the DDT (n CH2 = 11) spectrum to be 58 SAMs on Au(111), and probably result mainly from lifting the close to that of tetradecyne (n CH2 = 11), which has a comparable 59 Au(111) herringbone reconstruction upon chemisorption of these aliphatic chain length ((CH 2)nCH 3). Instead, the DDT spectrum 60 intensity is similar to that of octyne (n CH2 = 5). As a result of ACS Paragon Plus Environment Page 3 of 5 Journal of the American Chemical Society

1 2 3 4 5 6 7 8 9 10 11 12 Figure 5. Film thickness calculated in number of 13 groups ((CH 2)n) from the XPS data (see text) for SAMs of 14 HC ŁC(CH2)nCH 3, (n = 5, 7, 9, and 11) on Au(111). The solid 15 line indicates a linear fit. 16 Figure 3. IRRAS data analysis for tetradecyne/Au(111) with Au(111) is preserved across the entire series of alkynes that we 17 fitting individual bands. The inset shows linear increase in 2920 -1 investigated; Nuzzo et al . reported a similar observation in earlier 18 cm peak intensity as a function of the CH 2 group number. spectroscopic studies of alkanethiols on Au. 19 In contrast to our 19 observations, recent PM-IRRAS experiments by Scholz et al .6 20 surface selection rules, IRRAS intensities are sensitive to the EDVHG RQ DQDO\VLV RI WKH &í+ VWUHWFKLQJ UDQJH  SHUIRUPHG IRU 21 orientation of the molecules within the SAM (with respect to the alkyl-based SAMs—n-butylmercury tosylate (C 4H9HgOTs) and 22 metal substrate); the data in Figure 2 suggest slightly different n-octadecylmercury tosylate (C 18 H37 HgOTs) on gold— demonstrated fundamental differences in the relative intensities 23 twisting and/or tilting of the (CH 2)nCH 3 chain for the n-alkynyl 24 and alkanethiolate on Au(111). These differences are not surpris- and band broadening between these SAMs and analogous al- 25 ing considering the different bonding geometries for each SAM. kanethiols: that is, the n- had a liquid-like structure (in We also note that the observed differences in the IRRAS spectra contrast to the crystalline structure observed for n-alkanethiols). 26 In contrast, the IRRAS data presented in Figures 2 and 3 demon- 27 for n-alkynyl and alkanethiol on Au(111) are much smaller than those reported previously 19 for alkanethiolates on Au(111) and strates that it is possible to form alkynes that have order similar to 28 Ag(111), which have similar surfaces structures. that of alkanethiols on Au(111). 29 Figure 4 shows X-ray photoelectron spectroscopy (XPS) analy- 30 Figure 3 shows a more detailed analysis of the IRRAS spectra IRUWHWUDGHF\QHLQFOXGLQJDILWWLQJIRUWKH&í+VWUHWFKLQJPRGHV sis of the alkyne-based SAMs. In contrast to previously reported 31 XPS data for increasing lengths of alkynes (HC ŁC(CH2)nCH 3, n = The inset in Figure 3 shows that an increase in the length of the 4 32 aliphatic chain (from n = 5 up to n = 11) correlates linearly with 5, 7, 9, and 11), the C1s signal in Figure 4 shows a single sym- -1 metric peak at 285 eV, with no additional higher energy compo- 33 the intensity of the CH 2-related symmetric (2920 cm ) mode. 34 This observation indicates that the conformation of n-alkynes on nents (e.g., peaks at 287 eV, 289 eV). The C1s spectra are also 35 consistent with the corresponding data obtained for DDT (Figure 4), as well as other literature data 20 for un-oxidized alkanethiols 36 on Au(111). We calculated the film thickness using the C1s/Au4f 37 intensity ratios (assuming an exponential attenuation of the photo- 38 electron signal 21 and using attenuation lengths reported earlier). 22 39 The calculated values (summarized in Figure 5) show a linear 40 increase in thickness as a result of an increase in the length of the 41 chain (from n = 5 to n = 11). Importantly, the XPS data are con- 42 sistent with the IRRAS data shown in Figure 3; they indicate a 43 linear increase in the IR signal with an increasing number of methylene units (CH 2). Moreover, using the linear relation ob- 44 tained from our XPS measurements data, we can extrapolate the 45 thickness of the film for n = 0 to a value of 5.9 Å; this value cor- 46 responds to the length exSHFWHG IRU D í&ŁCCH 3 fragment. This 47 extrapolation is also consistent with the ca. 5.6 Å dimensions of 48 an upULJKWFRQILJXUDWLRQRIWKHí&ŁCCH3 molecule on Au(111), 23 49 on the basis of the bonds lengths provided by previous DFT 50 calculations. Thus, the estimation of thickness using XPS is con- ) CH on Au(111) having the 51 VLVWHQW ZLWK 6$0V RI +&Ł& &+ 2 n 3 í&Ł&íJURXSSHUSH ndicular to the surface. 14 52 Previously reported XPS data for SAMs derived from 53 5 ethynylbenzene (HC ŁCC 6H5) on gold suggested that the SAM 54 contained oxidized components, as is inferred from both a signifi- 55 cant O1s signal and higher-energy contributors to the C1s peak 56 (e.g., peaks at 287 eV, 289 eV). In contrast, data shown in Figure 57 Figure 4. XPS C1s and O1s data for n = 5, 7, 9 and 11 4 show virtually no O1s signal for all SAMs analyzed, which 58 alkynes on Au(111) together with the corresponding were prepared carefully in an oxygen-free environment (the SI 59 DDT/Au(111). details the procedure). The oxidation of alkynes is indeed clearly 60 visible for samples that were exposed to oxygen (ambient condi- ACS Paragon Plus Environment Journal of the American Chemical Society Page 4 of 5

Notes 1 The authors declare no competing financial interests. 2 3 ACKNOWLEDGMENT 4 The authors would like to thank Prof. Marek Szymonski for 5 providing access to the STM microscope at the Department of 6 Physics of Nanostructures and Nanotechnology at the Jagiellonian 7 University. This work was supported financially by the National 8 Science Centre Poland (grant DEC-2013/10/E/ST5/00060). The 9 XPS equipment was purchased with the financial support of the 10 European Regional Development Fund (grant POIG.02.02.00-12- 11 023/08). The work at Harvard University was supported by a 12 subcontract from Northwestern University from the United States Department of Energy (DOE, DE-SC0000989). 13 14 Figure 6. XPS region scans (C1s and O1s) for SAMs of REFERENCES 15 HC {C(CH2)11 CH 3 on Au(111) prepared in an oxygen-free (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. 16 atmosphere (upper spectra) and an oxygen-containing atmos- G.; Whitesides, G. M. Chem. 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Soc. 1989 , 111 , 321. 56 57 Corresponding Author 58 [email protected] , 59 60 ACS Paragon Plus Environment Page 5 of 5 Journal of the American Chemical Society

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