Formation of n-Alkyl Monolayers by Organomercury Deposition on Gold

Letter pubs.acs.org/JPCL

Formation of n‑Alkyl Monolayers by Organomercury Deposition on Gold Frank Scholz,† Eva Kaletová,‡ Elizabeth S. Stensrud,‡ William E. Ford,† Anna Kohutová,‡ Malgorzata Mucha,‡ Ivan Stibor,‡ Josef Michl,‡,§,* and Florian von Wrochem*,† †

Sony Deutschland GmbH, Materials Science Laboratory, Hedelfinger Str. 61, 70327 Stuttgart, Germany Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague, Czech Republic § Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States ‡

S Supporting Information *

ABSTRACT: n-Alkyl self-assembled monolayers can be directly attached to gold through C−Au bonds by the deposition of organomercury salts on gold substrates, as shown here using nbutylmercury and n-octadecylmercury tosylate derivatives. The Hg atoms, which are codeposited during this process, are removed by thermal annealing at 95 °C, resulting in alkyl monolayers having a significantly enhanced thermal stability compared with alkanethiol monolayers, however, a lower degree of conformational order. The monolayer properties are elucidated by X-ray photoemission and IR spectroscopy, STM, ellipsometry, and contact-angle goniometry. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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disclosure of monolayers formed by soaking Au in a solution of trialkylstannyl salts of tosic, trifluoroacetic, and triflic acids.20 Recently, the first evidence of alkyl−Au21 and fullerene−Au22 contacts in mechanically controlled break junction studies appeared. In this Letter, we provide evidence that organomercurial compounds, n-alkylHgOTs, form well-defined n-alkyl monolayers on gold surfaces, and we present a simple procedure for the realization of dense n-butyl and n-octadecyl monolayers via the solution deposition of n-butylmercury tosylate (C4H9HgOTs) and n-octadecylmercury tosylate (C18H37HgOTs)23 on Au, followed by a thermal annealing cycle for Hg removal. As a result, the n-alkyl chains are directly attached to Au, even though the precise alkyl-Au binding mode is still under investigation. The SAM is chemically and thermally highly stable, showing molecular densities close to those of alkanethiol monolayers. An alternative approach for removing Hg in n-octadecyl/Hg SAMs has been developed in parallel, involving anodic oxidation of the initially formed monolayers.23 The n-alkyl SAMs are grown by soaking clean Au substrates in a 1 mM THF solution of either C 4H9HgOTs or C18H37HgOTs for 3 h. For XPS and STM analysis, atomically flat Au(111) surfaces are used as a substrate, as obtained by

he stable attachment of organic molecules to metal substrates has numerous applications in several fields such as optoelectronic devices, sensing, corrosion inhibition, and biomedical applications. To date, hydrocarbons have been commonly attached to noble metals by using thiol anchor groups, which are prototypical for the formation of selfassembled monolayers (SAMs). Thiols form covalent bonds between the sulfur atom and the metal substrate1 and are known to yield highly uniform, often crystalline self-assembled monolayers.2−5 However, despite their dense packing, high molecular order, and ease of preparation, thiol-based monolayers have certain disadvantages, such as fluctuations in the electrical coupling to the metal, low thermal stability, and facile oxidation of the sulfur moiety.6 For this reason, the need for alternative contacts7−9 has spurred research in recent years. Among others, the direct attachment of molecules to the metal substrate by carbon−metal bonds has been considered, for example, by reacting diazonium salts with a variety of metals.10,11 However, diazonium salts do not allow a selflimiting growth of well-defined SAMs. Alternatively, thermally stable alkylidene layers on molybdenum carbide12 have been realized, including the versatile modification via olefin metathesis reactions,13 and terminal alkynes have been adopted for direct metal−carbon bonding.14 Also, the attachment of alkynyls to Au has been recently demonstrated.15 The serendipitous discovery of the attachment of organomercury salts to gold16−18 and investigations of the adsorption of organoplatinum complexes on platinum19 led to the © 2013 American Chemical Society

Received: June 9, 2013 Accepted: July 22, 2013 Published: July 22, 2013 2624

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Figure 1. XP spectra of n-butyl/Hg (a,b) and n-octadecyl/Hg (c,d) monolayers in the Hg 4f (a,c) and C 1s (b,d) regions: as-prepared (T = 24 °C) and upon thermal annealing (T = 95 °C). The oxidation state of Hg is extracted from the position of the Hg 4f7/2 and Hg 4f5/2 core levels, showing a binding energy of 99.7 and 103.7 eV, respectively. The inset in panel a shows the removal of Hg upon annealing at 95 °C. XP spectra in the C 1s region (b,d) demonstrate the stability of the monolayers toward thermal annealing.

flame annealing of Au on mica.24 For PM-IRRAS and contactangle goniometry measurements, template-stripped gold (TSG) substrates with a RMS roughness of 0.3 nm are prepared using reported procedures.25 Upon molecular assembly, the samples are rinsed in THF and dried under a stream of nitrogen. n-Alkyl monolayers are formed in this stage, showing clear spectroscopic evidence of the presence of elemental Hg (vide infra). The self-assembly process takes place at a slower rate than reported for alkanethiol monolayers (see Figure S5 in the Supporting Information) and is followed by thermal annealing of the samples either in vacuum or under an inert atmosphere (argon or nitrogen) at 95 °C for ∼30 min. The annealing process triggers either the diffusion of the Hg atoms into the bulk Au26 or its evaporation,27 leaving a metalfree n-alkyl monolayer. Further details of synthesis and sample preparation are provided in the Supporting Information. The nature of the as-prepared and thermally treated SAMs is investigated by X-ray photoemission spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), contact-angle goniometry, scanning tunnelling microscopy (STM), and ellipsometry. As shown in Figure 1a,c, XP spectra of the Hg 4f and Au 4f region, acquired upon self-assembly, demonstrate the presence of Hg0 at the surface. The oxidation state of Hg is extracted from the chemical shift of the Hg 4f7/2 core level, whose binding energy of 99.7 eV is in good agreement with previous reports from elemental Hg layers on Au (99.826 and 99.6 eV28) as well as from solid Hg (99.9 eV).29 Elemental quantification using XPS emission intensities reveals that the surface density of Hg on Au is about two times as high as that of sulfur in densely packed n-alkanethiol monolayers. (See Table S1 in the Supporting Information.)

The high density of Hg atoms at the surface is attributed to the breaking of C−Hg bonds during the solution-based adsorption process, which causes part of the n-alkyl chains to leave the substrate. As a result, an accumulation of Hg atoms at the surface is induced (two Hg atoms/n-alkyl chain). On the basis of the sulfonate S 2p signal at 167.4 eV, most tosylate groups (OTs) leave the n-butyl/Hg monolayer upon self-assembly. The presence of this signal depends on the thoroughness with which the sample was rinsed during preparation. Thermal annealing of the monolayers to temperatures above 95 °C leads to a complete removal of elemental Hg from the surface, as shown in the Hg 4f spectrum of n-butyl/Hg (Figure 1a, inset) and n-octadecyl/Hg SAMs (Figure 1c). Figure 1b,d demonstrates that in both cases the XPS C 1s core level signal is not modified by thermal annealing, thus indicating that the nature of the SAM remains unaffected by the annealing process. This is confirmed by PM-IRRAS measurements (vide infra), showing the invariance of the asymmetric and symmetric inplane C−H stretching modes upon annealing. Notably, no carbon oxidation or alkene formation30 occurs, as indicated by the stability of the higher binding energy components in the C 1s spectrum (Figure 1b,d) and by the absence of related peaks in the IR spectra. From the C 1s emission in n-butyl SAMs (Figures 1b and 2), the presence of aliphatic carbon (284.45 eV for n-butyl/Hg) is shown, exhibiting peak components that are identical to those found in n-butanethiol monolayers (Figure 2). In the case of longer alkyl chains, a trend toward higher binding energies is observed, expressed in a shift to 284.62 and 285.0 eV for noctadecyl/Hg and n-dodecanethiol SAMs, respectively. (See Figure S1b in the Supporting Information.) This upward shift in the C 1s peak position has been assigned to a reduced 2625

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Figure 2. XP spectra of as-prepared n-butyl/Hg and n-butanethiol monolayers in the C 1s region. The peak position for C 1s is found at 284.45 eV for both monolayers, whereas the fwhm of the main C 1s component is 1.24 eV for both SAMs.

screening of the photoemission hole by the metal substrate when increasing the physical separation between hole and substrate.31 A chemical shift of C 1s components upon carbon−gold bond formation is not detectable within the resolution limit of the spectrometer. This is clear when C 1s signals from short and long chain n-alkyl monolayers are compared in terms of line shapes and binding energies, as shown in Figure 1b,d. A chemical shift should be in principle more apparent for short chain n-butyl SAMs, which is however not the case. Also, when comparing the C 1s emission from n-butyl and n-butanethiol SAMs (Figure 2), the peak components are in perfect coincidence, both with respect to binding energy and fwhm. This analogy of the two C 1s spectra represents an important piece of evidence for the Au surface being really covered by a uniform layer of n-alkyl chains that have structural properties close to those of n-butanethiol SAMs. The higher energy component at ∼285.8 eV, which is common for both n-alkyl/ Hg and n-butanethiol SAMs, might be caused by intrinsic energy losses in the photoemission process due to vibronic excitations.31 Note that in Figure 1b a shift in the C 1s binding energy upon annealing is observed, which is not seen for noctadecyl/Hg SAMs (Figure 1d). The C 1s binding energy progressively decreases from 284.5 (at 24 °C) to 284.0 eV (at 120 °C), reaching a plateau at higher temperatures. (See Figure S3 in the Supporting Information.) At this time, we have no convincing explanation for this effect, and possible interpretations range from a shift in the Au work function caused by the removal of Hg atoms (interface dipole change) to a monolayer phase transition occurring from the solid to the liquid phase.32 On the basis of the computed C 1s/Au 4f ratios, the surface density of alkyl chains in n-butyl/Hg and n-octadecyl/Hg SAMs is found to be close to the molecular density observed for nbutanethiol and n-dodecanethiol SAMs, but it appears to be lower for n-octadecyl/Hg SAMs than for n-butyl/Hg SAMs (Table S1 in the Supporting Information). PM-IRRAS spectra of the n-butyl/Hg SAM (Figure 3) show characteristic vibrational modes in the C−H stretching region that are well-known from alkanethiol monolayers,33−35 although the relative peak intensities differ significantly from those in a n-butanethiol reference SAM (Figure 3b). For asprepared SAMs, the resonances are located at 2963 cm−1 (υa(CH3)), 2880 cm−1 (υs(CH3)), 2938 cm−1 (υs(CH3, FR)),36 2926 cm−1 (υa(CH2)), 2857 cm−1 (υs(CH2)), and 2907 cm−1 (υ (CH3, FR or 2δ)).37 The bands at 2938 and 2907

Figure 3. (a) PM-IRRAS-spectrum for an n-butyl/Hg monolayer on Au in the 2800−3000 cm−1 region: as-prepared (T = 24 °C) and upon thermal annealing (T = 95 °C). (b) Analogous spectrum for a nbutanethiol monolayer. The peaks, assigned to their respective IR bands, are significantly broader in panel a than in panel b. Detailed fits using Lorentz−Gaussian components are shown in Figure S2 in the Supporting Information.

cm−1 are apparent as shoulders but can be assigned based on a Lorentz−Gaussian fitting that well represents the whole spectrum. (See Figure S2 in the Supporting Information for detailed peak fitting.) Upon thermal annealing, negligible changes in peak positions and intensities are found, except for a slightly lower intensity of the υs(CH3) resonance. The IR spectra from as-prepared n-octadecyl/Hg SAMs (Figure S2d in the Supporting Information) closely resemble those of n-butyl/ Hg SAMs (Figure S2a in the Supporting Information), only the relative intensity ratio of CH3 and CH2 lines (υs(CH3)/ υs(CH2) and υa(CH3)/υa(CH2)) expectedly decreases due to the higher fraction of methylene groups present in long octadecyl chains. This IR intensity/chain length dependence is, however, much less pronounced than in alkanethiol monolayers.34 The intensity ratios υs(CH3)/υs(CH2) and υa(CH3)/ υa(CH2) of the (symmetric and asymmetric) methyl bands over their methylene counterparts are higher for n-butanethiol than for n-butyl/Hg, which is attributed to differences in the orientation of the transition dipoles of υ(CH3) and υ(CH2) relative to the surface plane (e.g., for n-butyl/Hg SAMs, the transition dipoles of the CH2 stretch vibration are on average pointing in a direction more perpendicular to the surface than for n-butanethiol SAMs). Notably, for n-butyl/Hg SAMs, the IR lines are significantly broader than those for n-butanethiol 2626

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SAMs, which reflect a greater structural disorder present in nbutyl/Hg SAMs. Furthermore, a shift toward higher frequencies in the υ(CH2) peak position is observed for n-butyl/Hg SAMs (3 cm−1 for υs(CH2) and 4 cm−1 for υa(CH2)), indicative of more liquid-like packing than in n-butanethiol SAMs. Even though molecular resolution is difficult to achieve by STM, scans of n-octadecyl monolayers consistently show the presence of dense and disordered monolayers. This is deduced from (i) the reconstruction of Au(111) step edges and (ii) the presence of speckled and disordered structures on the Au(111) terraces (Figure S6 in the Supporting Information). The thermal stability of the monolayer surface upon annealing is verified by contact angle goniometry. A static water contact angle of 86.8° ± 1.5° and 86.8° ± 1.2° for nbutyl/Hg SAMs before and after annealing, respectively, compared to a static contact angle of 85.3° ± 2.4° for nbutanethiol SAMs, indicate that (i) the surface structure of the SAM is not affected by the annealing process and (ii) the interaction of water with the methylene terminal groups is comparable for n-butanethiol and n-butyl SAMs. The contact angle for n-butanethiol SAMs is in reasonable agreement with previously reported data,38 considering that static contact angles are about 10° lower than advancing contact angles. Further details on XPS, contact angle goniometry and PMIRRAS are provided in the Supporting Information. Insight into monolayer stability and into the dynamics of the Hg removal process is obtained from thermodesorption experiments. For this purpose, a series of XPS scans is acquired in the C 1s, O 1s, and Hg 4f regions while the sample temperature is ramped up from 20 to 240 °C at 1.6 K/min under ultrahigh vacuum using a PID-controlled sample heater.7 The steep decrease in the Hg 4f signal intensity marks the threshold for Hg removal, centered at a temperature between 90 and 100 °C (Figure 4a). At higher temperatures, that is, in the range of 100−190 °C, the carbon atomic density gradually drops (Figure 4b). Above 190 °C, it stabilizes at about 1/2 of the initial carbon concentration. An analogous behavior is observed with n-octadecyl/Hg monolayers (Figure S4 in the Supporting Information), showing that the process is essentially independent of chain length. On the basis of the overall body of data, we conclude that the Hg atoms, initially facilitating the adsorption of n-alkyl/Hg molecules onto Au surfaces, are removed from the surface at temperatures above ∼95 °C. The whole process leaves either nbutyl or n-octadecyl chains directly attached to the substrate through one or more C−Au bonds. The stable anchoring of nalkyl chains to Au is substantiated by STM data (Figure S6 in the Supporting Information) and by thermodesorption data (Figures 1b and 4b) that demonstrate the higher thermal stability of n-alkyl monolayers compared with n-alkanethiol monolayers. (See Figure 4b.) A progressive loss of alkyl density is observed for n-alkyl SAMs only at temperatures far above 100 °C. Importantly, the selective removal of Hg at temperatures ranging from 90 to 100 °C does not affect the structural characteristics of n-alkyl monolayers. In summary, we have shown the feasibility of the direct alkylation of Au for the formation of dense, but disordered, nalkyl monolayers. The result constitutes a proof of concept toward a new class of functional monolayers, whose building blocks could potentially include a wide range of aromatic or aliphatic substituents. Whereas they are more disordered than alkanethiols, the higher permeability of these monolayers could

Figure 4. Evolution of the XPS Hg 4f signal intensity (a) and of the normalized C 1s (b) signal intensity as a function of temperature for nbutyl/Hg SAMs (filled symbols). Open symbols show the change in the normalized XPS C 1s signal intensity for n-butanethiol monolayers. The analogy in the behavior of long chain n-alkyl/Hg monolayers, investigated using n-octadecyl/Hg SAMs, is demonstrated in Figure S4 in the Supporting Information. Thermodesorption was performed at a heating rate of 1.6 K/min and under ultrahigh vacuum conditions.

make them suitable for catalysis or electrochemical action by the Au surface as well as for sensing applications.

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ASSOCIATED CONTENT

S Supporting Information *

Additional details of sample preparation, XPS data, PM-IRRAS analysis, contact-angle goniometry, ellipsometry and STM data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +49-711-5858-838, Fax: +49-711-5858-99838, E-mail: [email protected] (F.v.W.). Tel: (001) 303 492 6519, Fax: 303 492 0799, E-mail: [email protected]. edu (J.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 FUNMOL 213382 and ERC Grant Agreement 227756). Initial funding was provided by the Grant Agency of the Czech Republic (203/07/1619 and 203/ 09/0705) and the Institute of Organic Chemistry and 2627

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