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Formation of Highly Ordered Self-Assembled Monolayers of

8 Alkynes on Au(111) Substrate
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Tomasz Zaba, ‡ Agnieszka Noworolska, ‡ Carleen Bowers, § Benjamin Breiten, §

12 George M. Whitesides§ and Piotr Cyganik*‡
13

14 ‡Smoluchowski Institute of Physics, Jagiellonian University, ul. Reymonta 4, 30-059 Krakow, Poland

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§Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA

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18 19 Supporting Information Placeholder

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ABSTRACT: Self-assembled monolayers (SAMs), prepared

SAMs are sensitive to oxidation at an undefined point in their

22 23

by reaction of terminal n-DON\QHV+&Ł&&+2)nCH3, n = 5, 7, 9, and 11) with Au(111) at 60oC were characterized using scanning tunneling microscopy (STM), infra-red reflection absorption

formation; that is, oxidation occurs either during or after SAM formation (for example, by UHDFWLRQ RI WKH $X&Ł&5 ERQG ZLWK O2). Contact angle analyses of increasing lengths of alkynes

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spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS),

(HCŁC(CH2)nCH3, n = 5, 7, 9 and 11) also suggest4 that the quali-

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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

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spectroscopic experiments confirm formation of highly-ordered

adhesion,9,10 and charge transport3,11-13 (inter alia), most of this

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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

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properties—hydrophobicity, high surface order, and packing

the contribution of the anchoring groups of the SAM (as opposed

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density—also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The formation of high-quality SAMs from al-

to the terminal group, the thickness, or the electronic structure). In the current work, we characterize the SAMs formed by reactions

32 33

kynes requires careful preparation and manipulation of reactants in an oxygen-free environment: trace quantities of oxygen lead to

of n-alkynes (HCŁC(CH2)nCH3, n = 5, 7, 9 and 11) with Au(111). :H EHOLHYH WKDW WKH $X&Ł&5 JURXS LVSDUWLFXODrly 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)

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WKHí&Ł&íJURXSPRVWOLNHO\FDWalyzed by the gold substrate in

the orbital structure of the acetylene group, and the existence of a

37 the presence of O2).

variety of stable organometallic compounds containing the

38 Au&Ł&5 JURXS VXJJHVWV WKDW WKH LQWHUIDFH Eetween the metal,

39 Au, and the saturated organic component of the SAM (R =

40

Thin organic films based on self-assembled monolayers

(CH2)nCH3), might be especially informative in studies in which the interface connecting the SAM to the metallic substrate might

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(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 44 45

SAMs with composition Au/AgSR is the reaction most commonly used to prepare SAMs,1 although reactions that generate organosilanes on silicon2 (SiR) and organic carboxylates on silver3

order of n-alkyne-derived SAMs on gold through a combination of microscopic, spectroscopic and contact angle measurements. Our results show that the disorder and mixed organic functionality

46

(AgO2CR) have attractive properties, and a number of other pre-

implied by previous work4-6 are artefacts reflecting oxidation of

47 48 49

cursors have been surveyed. There have also been scattered descriptions of SAMs formed on gold from solutions of alkynes4 (HCŁC(CH2)nCH3, n = 3, 5, 7, 9, 11, and 13), ethynylbenzene5 (HCŁCC6H5) or n-alkylmercury(II) tosylates6 (CH3(CH2)nHgOTs,

the terminal acetylene by oxygen in solution during formation of the SAM (perhaps in a reaction catalyzed by gold),15 and that
using appropriate experimental conditions (e.g., rigorous exclu-
sion of oxygen; a slightly elevated temperature of 60ºC during

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n = 4 and 18) on Au(111). Although the potential interest of

formation of the SAM) results in well-organized SAMs of alkynes

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6$0VKDYLQJ0HWDOí&Ł&5ERQGVLVKigh, since they offer a new

that have qualities similar to that of alkanethiols on Au(111).

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type of metal-organic bond, most of these studies have used prep-

These studies thus establish that SAMs of the surface composition

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arations analogous to those employed with n-alkanethiols, and

$X&Ł&5 SURYLGH D QHZ W\SH RI 6$0 IRU XVH LQ SK\Vical- and

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have generated SAMs that have not seemed to be highly ordered,

physical-RUJDQLF VWXGLHV KDYLQJ DQ LQWHUIDFH WR JROG $X&Ł&5

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and are thus, perhaps, unsuitable for detailed studies of the physical chemistry of the surface. In particular, there are no procedures that describe the formation of SAMs that are highly ordered in

and are complementary to the well-understood SAMs of alkanethiols (AuSR).

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two dimensions—a key requirement for high-quality surface

SAMs of alkynes were prepared on gold by submerging freshly

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science. The most recent analyses of n-alkyl-based SAMs on

evaporated Au(111) substrates in a 1mM ethanolic solution of n-

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Au(111) indicate a “liquid-like” structure of the monolayer,6 and

alkyne (HCŁC(CH2)nCH3, n = 5, 7, 9, and 11) for 15 hours at

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XPS analyses of SAMs formed from alkynes4,5 suggest that these

60ºC. Importantly, the preparation of the SAMs was performed in

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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.

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29 molecules.17 The detection of similar features for alkynes suggests 30 the formation of densely packed chemisorbed structures on 31 Au(111).

32 This inference of a dense monolayer is confirmed by high33 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

38 schematically in Figure 1d) is consistent with the (—3u—3)R30o

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Figure 1. In (a-c) STM data for decyne/Au(111) SAMs. Panels assigned as A, B and C show cross-sections A, B and C indi-

lattice (which characterizes the structure formed by alkanethiols on Au(111)18 and leads to an area per molecule of 21.5 Å2. While
the quality of our data does not permit a more detailed description

41

cated in (a-c), respectively. The inset in (b) shows FFT spectrum

of the structure (as was possible for alkanethiols, which exhibit a

42

of image (b) with indicated six-fold symmetry pattern. Yellow

c(4u2) superlattice),18 it certainly shows that alkynes on Au(111)

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rectangle in (b) marks an area which corresponds to the image

form well-ordered structures with packing densities similar to

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shown in (c) obtained after FFT filtering of (b). In (d) scheme of

those of alkanethiols (area per molecule 21.5 Å2).

45 decyne adsorption in the (—3u—3)R30o 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-

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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

50 51 52

description of the experimental procedure, as well as additional details of measurements.
Figure 1 summarizes the results obtained by scanning tunneling

the C-H stretching range at 2965 cm-1 (QaCH3), 2938 cm-1 (QsCH3, FR), 2920 cm-1 (QaCH2), 2878 cm-1 (QsCH3, FR), and 2851 cm-1 (QsCH2) (Figure 2). For comparison, analogous IRRAS measure-

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microscopy (STM) for decyne (n = 7) chemisorbed on Au(111). Analysis on a larger scale (Figure 1a) shows formation of depressions on the substrate with depths compatible with substrate lat-

ments were performed on SAMs of dodecanethiol (DDT) on Au(111). This comparison shows that the frequencies, relative LQWHQVLWLHV DQG WKH EDQG ZLGWK RI WKH VSHFWUD IRU $X&Ł&5 DQG

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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).19 For

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respective cross-section, marked by A). Such depressions are also

the same orientation of n-alkynyl and alkanethiol on Au(111),

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characteristic of formation of both thiol-1 and selenol-16 based

however, we would expect the DDT (nCH2 = 11) spectrum to be

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SAMs on Au(111), and probably result mainly from lifting the

close to that of tetradecyne (nCH2 = 11), which has a comparable

59

Au(111) herringbone reconstruction upon chemisorption of these

aliphatic chain length ((CH2)nCH3). Instead, the DDT spectrum

60 intensity is similar to that of octyne (nCH2 = 5). As a result of

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12 Figure 5. Film thickness calculated in number of methylene 13 groups ((CH2)n) from the XPS data (see text) for SAMs of 14 HCŁC(CH2)nCH3, (n = 5, 7, 9, and 11) on Au(111). The solid 15 line indicates a linear fit.

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Figure 3. IRRAS data analysis for tetradecyne/Au(111) with

Au(111) is preserved across the entire series of alkynes that we

17 18

fitting individual bands. The inset shows linear increase in 2920 cm-1 peak intensity as a function of the CH2 group number.

investigated; Nuzzo et al. reported a similar observation in earlier 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

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orientation of the molecules within the SAM (with respect to the

alkyl-based SAMs—n-butylmercury tosylate (C4H9HgOTs) and

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metal substrate); the data in Figure 2 suggest slightly different

n-octadecylmercury tosylate (C18H37HgOTs) on gold—

23 24

twisting and/or tilting of the (CH2)nCH3 chain for the n-alkynyl and alkanethiolate on Au(111). These differences are not surpris-

demonstrated fundamental differences in the relative intensities and band broadening between these SAMs and analogous al-

25 26 27

ing considering the different bonding geometries for each SAM.
We also note that the observed differences in the IRRAS spectra
for n-alkynyl and alkanethiol on Au(111) are much smaller than those reported previously19 for alkanethiolates on Au(111) and

kanethiols: that is, the n-alkyls had a liquid-like structure (in contrast to the crystalline structure observed for n-alkanethiols). In contrast, the IRRAS data presented in Figures 2 and 3 demonstrates 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 3 shows a more detailed analysis of the IRRAS spectra

Figure 4 shows X-ray photoelectron spectroscopy (XPS) analy-

30 IRUWHWUDGHF\QHLQFOXGLQJDILWWLQJIRUWKH&í+VWUHWFKLQJPRGHV sis of the alkyne-based SAMs. In contrast to previously reported

31 32

The inset in Figure 3 shows that an increase in the length of the aliphatic chain (from n = 5 up to n = 11) correlates linearly with

XPS data for increasing lengths of alkynes (HCŁC(CH2)nCH3, n = 5, 7, 9, and 11),4 the C1s signal in Figure 4 shows a single sym-

33 34

the intensity of the CH2-related symmetric (2920 cm-1) mode. This observation indicates that the conformation of n-alkynes on

metric peak at 285 eV, with no additional higher energy components (e.g., peaks at 287 eV, 289 eV). The C1s spectra are also

consistent with the corresponding data obtained for DDT (Figure

35 4), as well as other literature data20 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 signal21 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

linear increase in the IR signal with an increasing number of

43 44

methylene units (CH2). Moreover, using the linear relation obtained 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&ŁCCH3 fragment. This 47 extrapolation is also consistent with the ca. 5.6 Å dimensions of

48 an upULJKW FRQILJXUDWLRQ RI WKH í&ŁCCH3 molecule on Au(111), 49 on the basis of the bonds lengths provided by previous DFT23

50 51

calculations. Thus, the estimation of thickness using XPS is conVLVWHQW ZLWK 6$0V RI +&Ł&&+2)nCH3 on Au(111) having the í&Ł&íJURXSSHUSHndicular to the surface.14

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Previously reported XPS data for SAMs derived from

53 54

ethynylbenzene (HCŁCC6H5) on gold5 suggested that the SAM 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-

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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 subcontract from Northwestern University from the United States

12 Department of Energy (DOE, DE-SC0000989).

13

14

Figure 6. XPS region scans (C1s and O1s) for SAMs of

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(2) Li, Y.; Calder, S.; Yaffe, O.; Cahen, D.; Haick, H.;

18 Kronik, L.; Zuilhof, H. Langmuir 2012, 28, 9920.

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tions) during preparation of the SAMs (see Figure 6). In the XPS

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23 24

cant asymmetry and broadening of the C1s peak, and are indicative of the R[LGDWLRQRIWKHí&Ł&íJURXSas reported in ref. 5. Our data demonstrate that the oxidation of alkynes does not involve

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25 26 27

WKH&í$XERQGformation (as had been suggested previously5), but instead involves oxidation of the alkynes in solution during formation of the SAM.

E.; Kohutova, A.; Mucha, M.; Stibor, I.; Michl, J.; Wrochem, F. J. Phys. Chem. Lett. 2013, 4í
(7) Drelich, J.; Wilbur, J. L.; Miller, J. D.; Whitesides,
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28 The interfacial free energy of n-alkyl SAMs on Au(111) was

(8) Hao, Y. J.; Soolaman, D. M.; Yu, H. Z. J. Phys.

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investigated using measurements of advancing contact angles of

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30 31 32

water (4aH2O). The data (see Table 1 in the SI file) are in a range (4aH2O = 111o-115o) that is consistent with the values measured here for DDT/Au(111), as well as literature data24 for SAMs of

(9) Vezenov, D. V.; Zhuk, A. V.; Whitesides, G. M.; Lieber, C. M. J. Am. Chem. Soc. 2002, 124, 10578.
(10) Khan, M. N.; Tjong, V.; Chilkoti, A.; Zharnikov, M. Angew. Chem. Int. Ed. 2012, 51, 10303.

33 alkanethiols on Au(111). The values of 4aH2O presented here are

(11) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.;

34 35

significantly higher than those reported previously (89o-98o) for alkynes on Au(111),4 and serve as additional evidence of a similar
degree of order for SAMs of alkynes (prepared properly) and

Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075.
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36 alkanethiols on Au(111). 37

Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904. (13) McCreery, R. L.; Bergren, A. J. Adv. Mater. 2009,

38

In conclusion, our STM and IRRAS data show that alkynes on Au(111) form SAMs that have organization and structure similar

21, 4303. (14)

Maity, P.; Takano, S.; Yamazoe, S.; Wakabayashi,

39

to alkanethiols on Au(111). High surface ordering and packing

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40 41

density of alkynes is also consistent with hLJKYDOXHVRIșaH20. The XPS data show that the presence of oxygen marks contamination

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42 43

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the C1s signal. Alkynes appear to be more sensitive to oxidation

(19) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.;

46

than alkanethiols during formation of the SAM, and require the

Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113,

47

formation to be carried out carefully in an oxygen-free atmos-

7152.

48 phere. 49 50 ASSOCIATED CONTENT

(20) Zharnikov, M.; Geyer, W.; Golzhauser, A.; Frey, S.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 1, 3163.
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51 Supporting Information 52

2037.

(22)

Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15,

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Experimental details are available free of charge via the Internet at http://pubs.acs.org

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54 (24) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.;

55 AUTHOR INFORMATION

Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

56 57 Corresponding Author

58 piotr.cyganik@uj.edu.pl,

59

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