Self-assembly of trithia-9-crown-3 and octathia-24-crown-8 on Au(111) surfaces

Abstract

We have investigated the formation of trithia-9-crown-3 (9S3) and octathia-24-crown-8 (24S8) self-assembled monolayers (SAMs) at liquid n-tetradecane/solid Au(111) interface using scanning tunneling microscopy (STM) at room temperature. STM images with a molecular resolution reveal completely different shape- and size-dependent SAM structures for the two kinds of thiacrown molecules. The experimental results demonstrate that 9S3 molecules lie flat, while 24S8 stand up on the Au(111) surfaces. Both of them form long-range ordered SAM structures. The unexpected standing conformations of 24S8 molecules on Au(111) provide a new opportunity for designing and forming shape- and size-controlled nanosized ordered SAM structures via strong chemical interactions between TCEs and metallic gold substrate as well as between TCEs and bio-macromolecules simultaneously for advanced sensors.

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Introduction

Self-assembled monolayers (SAMs) formed on solid substrate surfaces provide ideal mode systems for wide applications including surface patterning, chemical and bio-sensors, lubrication, corrosion resistance, and molecular electronics.¹ Organo-sulfur derivatives are the most typical systems for creating highly organized SAMs on various metal surfaces. Compared with extensive investigations on self-assembly of n-alkanethiols and aromatic thiols,^2–5 very few thioether SAMs on metal surfaces have been reported.⁶ Noh and co-workers⁷ studied adsorption structures of dioctadecylsulfide (DOS) self-assembled monolayers (SAMs) on Au(111) by scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy. C–S–C bond of DOS has been found to remain intact in the ordered SAMs without cleavage of S–C bond when forming Au–S coordinative bond between DOS and gold.

Thiacrown ether (TCE), which is defined by partial or total replacement of oxygen atoms on macro-cyclic crown ring by sulfur atoms, is a kind of multi-sulfur cyclic organic compound.^8,9 It is well known that the TCE can act as a soft Lewis base to interact selectively with metal ions, such as Ag⁺, Hg²⁺, Cu⁺ and Pd²⁺ to form inclusion complexes.^10,11 The basic processes involved in the coordination of the TCE with metal ions have been proven in 1980s.^12–14 However, TCE interaction with Au single crystal substrate has been less investigated. Because of the ring structures of TCEs, size-dependent shape-controlled ordered TCE nanopatterns can be expected to realize on single crystalline metallic surface, which can serve further as active sites for advanced studies on electron-transfer processes between metallic ions or bio-macromolecules, such as enzymes and metals. In previous investigations, Kunitake et al.¹⁵ reported ordered self-organized structures of dibenzo-18-crown-6 and its inclusion complex with potassium ion on Au(111) surfaces. Itaya et al.¹⁶ studied the formation of crown ether substituted phthalocyanine arrays on Au substrate. Although shape-controlled ordered nanostructures can be realized by the self-organization of the compounds on the solid metallic substrate, their interactions with the substrate may be too weak to guarantee further application for tethering active bio-macromolecules. TCE offers a possible opportunity to construct chemically bonded size-dependent ordered nanostructures on Au substrate due to strong Au–S chemical coordinative interaction between the substrate and the TCE molecules.

In this work, we investigate the formation of ordered SAMs of two kinds of TCEs, i.e., trithia-9-crown-3 (9S3) and octathia-24-crown-8 (24S8), by STM at liquid n-tetradecane/solid Au(111) interface. For the first time, we observed using STM that 24S8 molecules stand up, while 9S3 lie flat on the Au(111) surfaces and both of them form long-range ordered SAM structures. The unexpected standing ordered arrangement of 24S8 molecules on Au(111) opens a new door for designing and forming size-dependent shape-controlled nanosized ordered structures via strong chemical interactions between TCEs and metallic gold substrate as well as between TCEs and bio-macromolecules at the same time for advanced sensors.

Results and discussion

To see self-assembly behaviour of various organic molecules on clean annealed Au(111) under ambient conditions effectively, inert solvent is necessary to be used to cover the substrate surface being imaged by STM. The liquid not only acts as a solvent, but also as a tunneling medium protecting the imaging surface from contamination. Here, we selected nonpolar high-purity n-tetradecane as the inert liquid due to its low vapor pressure against rapid evaporation. Fig. 1a presents a typical STM image of a fresh bare Au(111) substrate covered by the n-tetradecane solvent. Monatomic step edge lines intersected with 60° can be found in the large-sized STM image (see Fig. 1a). When STM imaging was located in a small region on a single terrace, a clear 22 × √3 reconstruction structure can be observed on the Au(111) (see Fig. 1b). As well known, Au(111) reconstruction happens when the surface is extremely clean, as shown by STM in ultrahigh vacuum¹⁷ and under potential control in water.¹⁸ The direct seeing of the surface herringbone verifies that the clean Au(111) surface can also be obtained by covering the n-tetradecane on it under ambient condition. In addition, the n-alkane SAMs on the reconstructed Au(111) and Au(100) can even be imaged as stripes with a lamellar structure by STM at room temperature.^19–27 This further means that the n-tetradecane molecules are physisorbed on the Au(111) and do not damage the original herringbone structure. However, once the organic molecules with a higher affinity to the Au(111) are solvated in the n-tetradecane liquid film on the substrate, adsorption of these molecules occurs preferentially.

Fig. 1 STM images of a typical n-C₁₄H₃₀-covered Au(111) substrate at different resolutions. (a) 500 × 500 nm²; imaging condition: U_t = 0.5 V, I_t = 30 pA; several monatomic steps with the height of 0.24 nm can be clearly found. (b) 80 × 80 nm²; imaging condition: U_t = 0.3 V, I_t = 1 nA, presenting 22 × √3 reconstruction structure on one of the n-C₁₄H₃₀-covered Au(111) terraces. n-C₁₄H₃₀ monolayer can be observed as ordered stripes adsorbed physically on the Au(111) terrace.

Fig. 2 and 3 show typical STM images with different molecular resolutions when depositing ∼0.02 mg ml⁻¹ n-tetradecane solution of the 9S3 and 24S8 on the clean annealed Au(111) substrate, respectively. The 22 × √3 Au(111) herringbone structure disappeared immediately after depositing the two molecular n-tetradecane solutions. Instead, ordered SAM domain structures were found on the Au(111) substrates after several hours, showing the strong interaction between the molecules and the Au(111) substrate. In both cases, the ordered strip-like domains with an average lateral size of several tens of nanometers are arranged along three preferential directions intercrossing with 60° or 120° angle (see Fig. 2a and 3a), reflecting three-fold symmetry of Au(111). In addition, these domains are usually separated by apparently disordered regions. Some depressions, randomly distributed, appear at the borders of the ordered domains as well as in the ordered phases. The pits existing at the domain edges exhibit an average depth of ∼2.4 Å, which corresponds to the thickness of a layer of Au(111), showing individual Au atoms have been removed in the depressions. The depressions in the ordered domains have different heights, reflecting the features of the molecular defects or some adsorbates. The existence of the etched-Au pits implies that strong Au–S chemical bonds can be formed between the thiacrown ethers and Au substrate.²⁸

Fig. 2 STM images of highly organized domains of trithia-9-crown-3 self-assembled monolayer on Au(111). Lower resolution images of (a) 60 × 60 nm², (b) 50 × 50 nm² and higher resolution images of (c) 10 × 10 nm², (d) 5 × 5 nm².

Fig. 3 STM images of highly organized domains of octathia-24-crown-8 self-assembled monolayer on Au(111). Lower resolution images of (a) 50 × 50 nm², an inset showing directions of different crystalline planes, (b) 25 × 25 nm² and higher resolution images of (c) 10 × 10 nm² α ordered phase, (d) 10 × 10 nm² β ordered phase.

For the 9S3 SAMs, the highly ordered domains consist of many bright dot-like protrusions, which adopt a close-packed hexagonal structure (see Fig. 2c). Judging by the average size (∼4 Å) of the bright dots in the STM images, each bright dot can be attributed to an individual 9S3 molecule. The direction of the molecule arrangement can be deduced by triangular terrace edge line which is parallel to 〈110〉 direction of the Au(111) substrate (see Fig. 2b). Cross-section profiles along the 〈110〉 and 〈112〉 directions give a rectangle unit cell with a = 0.6 ± 0.05 nm, b = 1.2 ± 0.05 nm, α = 90 ± 1° for the 9S3 ordered molecule structure, which contains two molecules with an average area of 36 Ų per molecule.

In comparison, the ordered 24S8 domains have a uniaxial lamellae structure aligned along three preferential 〈112〉 directions, as shown in Fig. 3a inset. Higher resolution STM images (see Fig. 3b) reveal that each line consists of well-arranged strip-shaped protrusions. Each of the stripes has the average length of ∼1.2 nm and can be regarded as one 24S8 molecule. These findings imply that the individual 24S8 molecules in the SAMs seem to stand up on Au(111) surface and all protrusions are parallelly arranged to each other. Some higher or lower morphologies on the protrusions or between two protrusions were also observed, which may be induced by flexibility of the molecules due to the larger ring size and different conformations. Interestingly, two kinds of ordered phases (α and β) coexist in the 24S8 SAMs. The α phase (see Fig. 3c) has a rectangle unit cell with a = 0.6 ± 0.05 nm, b = 2.4 ± 0.05 nm, α = 90 ± 1°, containing one molecule with an average area of 144 Ų per molecule. The β phase (see Fig. 3d) can be described as a rectangle unit cell with a = 0.6 ± 0.05 nm, b = 2.9 ± 0.05 nm, α = 90 ± 1°, but containing two molecules with an average area of 87 Ų per molecule. Apparently, 24S8 molecules take more close-packed arrangement in β phase than in α phase.

Based on the sizes of the protrusions and the distances between two molecules for the two kinds of TCEs, theoretical calculations were performed using density functional theory (DFT) provided by the DMol³ code (see ESI† for details). The calculated lattice parameters for the self-assembled monolayers on Au(111) are summarized in Table S1, ESI.† The calculated parameters agree well with the experimental data. Molecular adsorption models for the 9S3 and the 24S8 are suggested in Fig. 4. In the simulation process, one of the sulfur atoms for both TCE molecules is assumed to take a three-fold face-centered cubic (fcc) hollow site of the gold lattice. The 9S3 molecule is assumed to arrange in cis-conformation, while the 24S8 molecule takes a standing-up conformation. Three of six sulfur atoms in the standing 24S8 molecule take place in direct strong Au–S coordinative interactions with the Au substrate and alkane groups connected to the three sulfur atoms nearly lie flat on the Au substrate. DFT results show that the interaction between each 9S3 molecule and Au(111) in the 9S3 assembly structure is about −1.75 eV per molecule and in 24S8-α and 24S8-β phases, the interactions between 24S8 and Au(111) are −2.12 eV per molecule and −2.35 eV per molecule, respectively. The three interaction energies theoretically calculated are all far higher than room temperature thermal fluctuation energy (∼26 meV), showing that the three ordered SAM structures can exist at room temperature steadily. The model also presents that the 24S8 molecules with a large macrocycle do not form flat lying structures once adsorbed on Au(111) substrate due to high flexibility. This is in agreement with the strip-like lamellae structures observed in STM images. It is worth noting that the standing conformation for TCE molecules has never been reported on Au(111). It is not clear as to why the 24S8 molecules take the standing conformation on Au(111). A possible explanation may originate from the synergy of three different factors including changeable flexible conformations for 24S8 molecules, S–S interaction among 24S8TCE molecules, and interaction between 24S8TCE molecules and Au substrate. Therefore, the lying flat (for trithia-9-crown-3) or standing up (for octathia-24-crown-8) conformations of the molecules might ultimately result from a competition between the largest number of S–Au bonds that they can form and optimization of the van der Waals interactions between the molecules.

Fig. 4 Theoretical conformations and molecular self-assembly packing models for 9S3 and 24S8 on Au(111). Top-view models for (a) 9S3 and (b) 24S8, side-view models for (c) 9S3 and (d) 24S8.

To further understand surface chemical states of the anchor groups on Au(111) in both trithia-9-crown-3 (9S3) and octathia-24-crown-8 (24S8) SAMs, X-ray photoelectron spectroscopy (XPS) experiment has been performed to detect S 2p spectra of the two SAMs on Au(111) substrates. The S 2p spectra obtained for the two SAMs exhibit three S 2p double-peak structures at 161.2, 162.2, and 163.4 eV (S 2p_3/2). The existence of the S 2p_3/2 and S 2p_1/2 peaks are typical features for thioether SAMs on Au substrate.²⁹ The spectra could be fit using a 2 : 1 peak area ratio and a 1.2 eV splitting, as shown in Fig. 5a and b. A doublet at 163.4 eV can be generally attributed to unbound sulfur or a disulfide moiety. The peak might originate from unbound disulfide moieties of the part-bounded thiacrown ether molecules in disordered SAM regions for 9S3/Au system and standing SAM for 24S8/Au system. A doublet peak at ∼162.0 eV is the dominating emission signal that is usually assigned to a thiolate species, i.e., sulfur atoms strongly bound to the Au substrate. This suggests that thiolate-type thiacrown ether molecules predominate. The doublet peak at 161.0 eV was frequently observed for various thiol-derived SAMs on both Au(111) and Ag(111) substrates, which is quite remarkable in the XPS spectra of 9S3 and 24S8 SAMs, might be associated either with atomic sulfur or with a specially adsorbed SAM constituent (other than by a typical thiolate bond). The former assignment originates from the findings in the study of n-alkanethiolate SAMs on Au, annealed at 415–490 K. It demonstrated that the thiolate-related doublet could shift to ∼161.0 eV once the C–S bonds are broken in the SAM constituents.³⁰ The latter assignment results from kinetic observation on the thiol–SAM formation on Au substrate. The peak at 161.0 eV was found at the early stage of molecular assembly.^31–33 In our case, it is impossible that the TCE molecules decompose into chemisorbed atomic sulfur, on the Au surface at room temperature, unless X-ray radiation leads to the cleavage of C–S bonds of the TCE molecules, adsorbed on Au(111) in XPS detection. Therefore, we are inclined to assign the doublet at 161.0 eV to non-standard binding thiolate types. This probably stems from adsorption site differences, i.e., defect sites, such as the step edges and corners.³⁴ However, XPS study provides evidences for the formation of Au–S chemical coordination bonds between 9S3 (or 24S8) molecules and Au.

Fig. 5 XPS S 2p for trithia-9-crown-3 (a) and octathia-24-crown-8 (b) SAMs on Au(111). The respective spectrum decomposition for trithia-9-crown-3 and octathia-24-crown-8 and background lines are also shown. S3, S2, and S1 peaks correspond to free thiacrown, thiolates, and thiolates on other adsorption sites on Au substrate.

Conclusions

To summarize, we have investigated trithia-9-crown-3 (9S3) and octathia-24-crown-8 (24S8) self-assembled monolayers on Au(111) by using scanning tunneling microscopy in n-tetradecane environment. High resolution STM images reveal different SAM structures for the two kinds of thiacrown ether molecules on Au(111) surface. The occurrence of the pits formed by etching surface Au atoms, implies a strong Au–S coordinative chemical interaction between gold substrates and the TCE molecules. Furthermore, XPS study also shows the formation of Au–S chemical coordination bonds between 9S3 (or 24S8) molecules and Au. The unexpected standing conformation for the 24S8 macrocycle molecules on Au(111) provides fire-new information in understanding the effect of molecule conformation on SAM structures. The realization of the chemically bonded TCE SAMs with various structures opens a new opportunity for designing size-dependent and shape-controlled SAMs for advanced bio- and chem-sensors.

Experimental

n-Tetradecane (99%), trithia-9-crown-3 (9S3) (99%), octathia-24-crown-8 (24S8) (99%) were purchased from Aldrich. All the chemical reagents were used as received without further purification.

Sample preparation

Firstly, a Au(111) substrate was formed by depositing 200 nm thick gold film on the freshly cleaved mica surface preheated to ∼600 K under ultra high vacuum (∼4 × 10⁻⁸ Pa) conditions. Secondly, the Au(111) surface was further annealed to produce large terraces in a propane–air flame under ambient condition. Thirdly, thus-obtained Au(111) substrate covered with pure n-tetradecane was detected using STM before depositing molecules. Finally, a droplet of n-tetradecane solution containing a small amount (∼0.02 mg ml⁻¹) of trithia-9-crown-3 (9S3) (99%) or octathia-24-crown-8 (24S8) (99%) was deposited on the Au(111) substrate after 6–12 h for STM observation.

STM characterization

STM imaging of bare, trithia-9-crown-3 (9S3) and octathia-24-crown-8 (24S8)-modified Au(111) surfaces was performed in the n-tetradecane environment using a Pico-SPM (Molecular Imaging/Agilent Technology) in a constant-current mode at room temperature. The STM tip was prepared by mechanically cutting a Pt/Ir (80 : 20) wire. Typical tunneling parameters for STM imaging were in the range of 0.5–1 V for bias voltage and 10–30 pA for tunneling current. The measurement error of STM images in lateral dimensions was within 5%. These STM images were recorded with various sizes and different samples to avoid artifacts from tips and samples.

X-ray photoelectron spectroscopy characterization

Trithia-9-crown-3 (9S3) and octathia-24-crown-8 (24S8)-modified Au(111) surfaces were prepared by depositing the bare Au(111) substrates into n-tetradecane solution containing a small amount (∼0.02 mg ml⁻¹) of trithia-9-crown-3 (9S3) (99%) or octathia-24-crown-8 (24S8) (99%) for 12 h. After that, these substrates were washed with ethanol solvent and subsequently dried with nitrogen gas for XPS detection. XPS was performed on a VG ESCALA BMKP spectrometer with a Mg Kα excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV.

Conflict of interest

The authors declare no competing financial interests.

Acknowledgements

The work was supported by grants from National Basic Research Program of China (2012CB933402, 2012CB933001), and National Key Scientific Instrument and Equipment Development Projects of China (No. 2014YQ090709).

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Footnotes

† Electronic supplementary information (ESI) available: Theoretical computation details. See DOI: 10.1039/c6ra10063e

‡ The two authors contributed equally to this work.

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