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

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Formation of Highly Ordered Self-Assembled Monolayers of Alkynes on Au(111) Substrate Formation of Highly Ordered Self-Assembled Monolayers of Alkynes on Au(111) Substrate The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters 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 Chemistry and Chemical Biology, 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 $X&Ł&5 ERQG ZLWK 23 and 11) with Au(111) at 60 C were characterized using scanning tunneling microscopy (STM), infra-red reflection absorption O2). Contact angle analyses of increasing lengths of alkynes 24 4 spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), (HC ŁC(CH2)nCH 3, n = 5, 7, 9 and 11) also suggest that the quali- 25 and contact angles of water. In contrast to previous spectroscopic ty of these SAMs is lower than those based on n-alkanethiols. 26 studies of this type of SAMs, these combined microscopic and Although SAMs have enabled studies of wetting, 7,8 27 spectroscopic experiments confirm formation of highly-ordered adhesion, 9,10 and charge transport 3,11-13 (inter alia), most of this 28 SAMs having packing densities and molecular chain orientations work has focused on the terminal part of the SAM that is exposed 29 very similar to those of alkanethiolates on Au(111). Physical to air, and there is relatively little work devoted to understanding 30 properties—hydrophobicity, high surface order, and packing the contribution of the anchoring groups of the SAM (as opposed 31 density—also suggest that SAMs of alkynes are similar to SAMs to the terminal group, the thickness, or the electronic structure). In of alkanethiols. The formation of high-quality SAMs from al- 32 the current work, we characterize the SAMs formed by reactions kynes requires careful preparation and manipulation of reactants of n-alkynes (HC C(CH ) CH , n = 5, 7, 9 and 11) with Au(111). 33 Ł 2 n 3 in an oxygen-free environment: trace quantities of oxygen lead to :HEHOLHYHWKDWWKH$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 acetylene 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 surface science. The reaction of organ- contribute to its properties. 42 ic thiols (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 silver 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-alkyl-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 molecules. 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 ( 3u3)R30 lattice (which characterizes the structure formed by alkanethiols 39 18 2 40 Figure 1. In (a-c) STM data for decyne/Au(111) SAMs. Pan- on Au(111) and leads to an area per molecule 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 ( 3u3)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 octyne (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.
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