Formation and Domain Structure of Self-Assembled Monolayers by

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J. Phys. Chem. C 2007, 111, 2691-2695

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Formation and Domain Structure of Self-Assembled Monolayers by Adsorption of Tetrahydrothiophene on Au(111) Jaegeun Noh,*,† Youngdo Jeong,† Eisuke Ito,‡ and Masahiko Hara‡,§ AdVanced Nanomaterials Laboratory, Department of Chemistry, Hanyang UniVersity, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea, Local Spatio-Temporal Functions Laboratory, Frontier Research System, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, and Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8502, Japan ReceiVed: October 29, 2006; In Final Form: December 16, 2006

The formation and structure of tetrahydrothiophene (THT) self-assembled monolayers (SAMs) on Au(111) were examined using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). XPS measurements revealed that THT molecules, containing endo-sulfur aliphatic rings, can form chemisorbed SAMs, in contrast with the formation of physisorbed SAMs by dialkyl monosulfide, suggesting that the adsorption ability of monosulfide compounds on gold strongly depends on the structure of tail groups, such as an aliphatic ring or two alkyl groups attached to sulfur head groups. In addition, high-resolution STM imaging revealed, for the first time, that the adsorption of THT molecules on Au(111) results in long-range, two-dimensional, ordered SAMs, having a (3 × 2x3) superlattice with many unique structural defects and a few vacancy islands. It is suggested that the unique surface structures of THT SAMs on Au(111) are mainly due to the weak van der Waals interactions between THT rings, as well as a dynamic structural variation of the THT ring in the SAMs. Our results from this study will be very useful in understanding SAM formation and the structures of organosulfur molecules containing an endo-sulfur ring.

Introduction Self-assembled monolayers (SAMs), formed by the spontaneous adsorption of organosulfur compounds on gold, offer an excellent means to control the physical and chemical properties of surfaces for various technological applications in biosensing, nanopatterning, corrosion inhibition, and molecular electronic devices.1-11 It has been noted that understanding fundamental issues of organic SAMs, such as packing structure, structural stability, interfacial properties, binding condition, and selfassembly phenomena, is necessary for the fabrication of functional molecular layers and their potential applications.12-21 As a result of many studies, it has been realized that the formation of two-dimensional (2D) SAM structures is driven by both interactions between the sulfur head group of a molecule and the Au substrate and molecule-molecule interactions.1,2,11,12 The van der Waals interaction is one of the key factors determining the final SAM structure, and the interactions would be easily tuned by changing the alkyl tail groups or aromatic tail groups. A high-resolution scanning tunneling microscopy (STM) study revealed that the closely packed alkanethiol SAMs on Au(111) have a (x3×x3)R30° structure1,2,12,13 or c(4×2) superstructure1,2,12,13,15,21 with a different molecular brightness in a unit cell. On the other hand, it was found that SAMs of aromatic thiols, containing a biphenyl or terphenyl backbone, on Au(111) at saturation coverage have several different structures, such as a commensurate (x3×x3)R30° structure,22 a commensurate (x3 × 2x3) structure,19,23,24 and a more complex incommensurate structure.19 Recently, different types of * Corresponding author: Telephone: +82-2-2220-0938. Fax: +82-22299-0762. E-mail: [email protected]. † Hanyang University. ‡ RIKEN. § Tokyo Institute of Technology.

SAMs, formed by thiols containing spherical hydrocarbon cage backbones, were fabricated and characterized, and an STM study revealed that the bulky molecular backbone of the molecules leads to the formation of unique molecular structures.25-27 SAMs formed by cyclohexanethiol (CHT), containing a flexible sixmembered aliphatic ring, on Au(111) have a (5×2x10)R48° superlattice, resulting from the existence of two geometric isomers, that is, the equatorial and axial chair isomers, in CHT SAMs.28 On the other hand, to understand the adsorption behavior of sulfur head groups in the thiophene ring on gold, π-conjugated thiophene or its derivative SAMs were fabricated and investigated by various surface characterization tools.29-33 Theoretical studies were also performed to investigate the interactions between thiophene and the gold surface.34,35 From these results, it was suggested that the sulfur atom in thiophene chemically interacts with the gold surface. Molecular-scale STM imaging reveled that the adsorption of thiophene on Au(111) led to the formation of SAMs consisting of various ordered phases depending on the preparation conditions.29-31 It has been reported that visualization of nanometer-scale SAM structure can provide useful information for understanding the nature of self-assembly phenomena of organic molecules on gold surfaces and tailoring the chemical and physical properties of solid surfaces. Although the reaction and adsorption of tetrahydrothiophene (C4H8S, THT; a heterocyclic compound without π-conjugated system) on Pt(111) surfaces36,37 from the gas phase have been investigated from the viewpoint of catalytic desufurization, the self-assembly phenomena and structure of THT SAMs have not yet been discovered. Interestingly, investigating the structure of THT SAMs can provide meaningful information to understand the effects of a π-conjugation system for SAM formation by adsorption of heterocyclic molecules containing

10.1021/jp067093c CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007

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Figure 1. XPS spectra in the S 2p region of THT SAMs on Au(111), showing the existence of strongly bound sulfur at 162 eV (S 2p3/2).

sulfur atoms on gold. In addition, the sulfur atom in the THT molecule directly binds to the aliphatic ring, and the sulfur atom in dialkyl monosulfide is attached to two alkyl chains. Both molecules can be classified as monosulfide systems. It has been realized that the structure and adsorption behavior of dialkyl monosulfide SAMs on gold are quite different from those of dialkyl disulfide SAMs or alkanethiol SAMs.38-42 Therefore, it would be interesting to compare the SAM formation of cyclic monosulfide and noncyclic dialkyl monosulfide on Au(111). In this study, the binding conditions and structure of THT SAMs on Au(111) were examined by X-ray photoelectron spectroscopy (XPS) and STM, and we report that the first molecular-scale features of THT SAMs on Au(111) demonstrate unique surface structures, as compared to SAMs of thiophene with a π-conjugated system or alkanethiol SAMs. Experimental Section THT was purchased from Tokyo Chemical Industry (Tokyo, Japan) and used without further purification. The Au(111) substrates were prepared by the vacuum deposition of gold onto freshly cleaved mica, as described in a previous paper.28 THT SAMs were formed by the immersion of the Au(111) substrates in a 1 mM ethanolic solution of THT at room temperature for 1 day. After the SAM samples were removed from the solutions, they were thoroughly rinsed with a pure ethanol solution to remove weakly adsorbed molecules from the SAM surface. XPS measurements were performed using an ESCALAB 250 system (Thermo VG Scientific, U.K.). A monochromatized Al KR line (10 kV, 150 W) was used as the excitation source (1486.6 eV). STM measurements were carried out using a NanoScope E (Veeco, Santa Barbara, CA) with a Pt/Ir tip (80:20), and all STM images were taken under ambient conditions at room temperature, using the constant current mode. Results and Discussion To reveal the interactions between the sulfur head groups in THT and the gold surface, we examined XPS spectra in the S 2p region of THT SAMs on Au(111). It is generally believed that the S 2p peak appears as a doublet, composed of 2p3/2 and 2p1/2 peaks, with an intensity ratio of 2:1 originating from the spin-orbital splitting.43-45 Three S 2p3/2 peaks for THT SAMs in Figure 1 were observed at 161.1, 162, and 163.1 eV. The main chemisorbed sulfur peak (A peak) was observed at 162 eV, which is often observed from well-ordered alkanethiol

Noh et al. SAMs on gold. One additional chemisorbed peak (B peak) with a weak intensity was also observed at 161.1 eV. Although the origin of this sulfur peak is still unclear, it has been suggested that the peak may be due to the bound atomic sulfur45,46 or differently bound sulfur resulting from a change in the adsorption configuration of the molecules44,45,47,48 or from a change in the adsorption site of the sulfur atoms on the Au(111) surface.45,49 The third peak, with a weak intensity, was observed at 163.1 eV, which is assigned to the unbound sulfurs that were usually found in SAM samples prepared from organosulfur compounds consisting of complicated chemical structures45 or dialkyl monosulfides.50,51 We found that the XPS spectra for THT SAMs are nearly the same as those of thiophene SAMs, which implies that, when the sulfur atoms in thiophene adsorb to the Au(111) surface,30 the π-conjugated system of thiophene does not significantly affect the adsorption behavior of sulfur atoms during the self-assembly of thiophenes. On the other hand, whereas the XPS spectra for THT SAMs on Au(111) have a strong bound peak at 162 eV, the XPS spectra for dialkyl sulfide SAMs have a very strong unbound sulfur peak at around 163.2 eV.39,51 This result implies that the sulfur atoms in the THT ring exhibit stronger adsorption activity than those in dialkyl monosulfides. This may be related to an increase in the s-bonding character of two nonbonding orbitals attached to the sulfur atom in THT, which results from the presence of a fivemembered ring.52 However, two nonbonding orbitals attached to the sulfur atom in dialkyl sulfide have larger p-bonding characters, compared to THT molecules. Therefore, we assumed that s-bonding character plays an important role in the formation and bonding nature of organosulfur SAMs. STM imaging reveals that the spontaneous adsorption of THT molecules on Au(111) results in the formation of closely packed, ordered SAMs with many structural defects, as shown in Figure 2. In comparison to alkanethiol SAMs or aromatic thiol SAMs on Au(111), the STM image (100 nm × 100 nm) in Figure 2a shows that THT SAMs have markedly different surface structures in the formation of ordered domains and the formation and distribution of vacancy islands. Although many small domains, ranging from 5 to 30 nm and separated by domain boundaries, were usually observed from well-defined alkanethiol SAMs or aromatic thiol SAMs, THT SAMs have a single, large, ordered domain with a size of more than 100 nm. It was also found that the domain sizes of THT SAMs were mainly limited by the size of atomically flat Au(111) terraces. On the other hand, a few vacancy islands were observed in THT SAM samples, and the ratio of the area of vacancy islands to the total surface area was measured to be approximately 2.5-3.5%, which is markedly less than the observed value of 5-9% in alkanethiol SAMs.53,54 Compared with other SAM systems, these noticeable differences in the domain structure and vacancy islands can be attributed to weak lateral interactions between the five-membered rings of THT molecules. It is easily assumed that the weak van der Waals interactions result from the small size of the ring and a slightly distorted adsorption geometry of the ring with conformation dynamics, resulting in the lack of a clear domain boundary. The formation of low-density vacancy islands seems also to be related to weak van der Waals interactions because similar structural behaviors were observed from alkanethiol SAMs with shorter alkyl chains.55,56 Due to weak lateral interactions, THT molecules can diffuse more rapidly on the surface than alkanethiols with longer alkyl chains or aromatic thiols, resulting in the formation of a large, single domain, as seen in Figure 2a. It is also suggested that Au adatoms and vacancy islands diffuse rapidly on the surface and

Formation and Domain Structure of THT SAMs

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Figure 2. STM images of THT SAMs on Au(111). (a) Surface structures showing single ordered domains and a few vacancy islands. (b) STM image showing structural defects in the ordered domains. (c) Molecularly resolved STM image showing conformational defects in 2D ordered SAMs and the molecules with low STM imaging contrast in the dark regions, as indicated by the arrows.

merge near step edges because of the reduction in total step edge energy, which results in the formation of low-density vacancy islands.2,6 Interestingly, we observed many structural defects in wellordered domains (see the many small, dark areas in Figure 2a) that have never been observed in alkanethiol SAMs and aromatic SAMs. The high-resolution STM image (50 nm × 50 nm) in Figure 2b clearly shows many structural defects that contain disordered phases in the ordered domains. In general, the disordered phases have often appeared as an intermediate phase at the SAM growth stage from the striped phases with a low surface coverage to the close-packed phases at saturation coverage.57 The disordered phases are mainly separated by the ordered phases with phase separation. However, for the THT SAMs on Au(111), the two phases are relatively well-mixed without any clear phase separation. We assumed that this result may be due to the structural dynamics of the five-membered ring. When the sulfur atoms in the THT ring chemisorbed on the Au(111) surface, the conformational change of molecules would be largely confined to the SAMs. Therefore, the ring structure of THT on Au(111) can only be slightly distorted with a small structural variation. However, these kinds of conformational dynamics can affect the two-dimensional (2D) structure of THT SAMs, resulting in the formation of partially disordered phases in the ordered domains. Figure 2c shows a molecularly resolved STM image (11 nm × 11 nm) of THT SAMs, showing conformational defects in well-ordered domains with a high degree of structural order. The structural details will be discussed later. We observed a single molecule in the dark regions with darker molecular contrast (as indicated by the arrows), compared with the surrounding adsorbed molecules. A difference in STM imaging contrast of molecules in a 2D SAM packing arrangement is believed to be mainly due to a difference in molecular conformation21,58,59 or adsorption sites of sulfur head groups.21,60 Although the apparently low contrast of molecules in the dark region is not fully understood, it may be attributed to a metastable adsorption configuration of the THT ring and/or a metastable adsorption state of sulfur atoms on a gold surface, which can provide a different STM imaging mechanism. On the other hand, the surface structures of THT SAMs on Au(111) are totally different from those of thiophene SAMs.35 From this result, we demonstrated that the π-conjugated system of the thiophene ring strongly affects 2D SAM growth and the final SAM structure. The high-resolution STM image (6 nm × 6 nm) in Figure 3a shows well-ordered 2D packing arrangements of THT SAMs on Au(111). The inset is a 2D, fast Fourier transform, indicating

Figure 3. (a) High-resolution STM image of THT SAMs on Au(111). (b, c) Height profiles along lines a′ and b′ on the image showing the superperiodicities of the adsorbed molecules.

hexagonal packing arrangements of THT SAMs. The crosssectional profile in Figure 3b, taken along line a′, shows uniform periodicity with the molecular distances of 8.4 ( 0.2 Å for the adsorbed THT molecules, and the cross-sectional profile in Figure 3c, taken along line b′, shows alternating periodicity with the distances of 10.1 ( 0.2 Å between two bright molecular spots. We extracted the lattice constants of a rectangular unit cell containing four molecules: a ) 8.4 ( 0.2 Å ) 3ah and b

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Noh et al. completely different from those of the thiophene SAMs with a π-conjugated system. From these results, we clearly revealed that the π-conjugation system of molecules strongly affects 2D SAM growth of endo-sulfur heterocyclic molecules, resulting in the formation of a unique SAM structure. References and Notes

Figure 4. Proposed structural model for THT SAMs on Au(111). The lattice constants extracted from the high-resolution image are a ) 8.4 ( 0.2 Å ) 3ah and b ) 10.1 ( 0.2 Å ) 2x3, where ah ) 2.89 Å denotes the interatomic spacing of the Au(111) lattice. The structure of THT SAMs on Au(111) can be assigned as a (3 × 2x3) superlattice. We note that the orange molecules correspond to the bright molecular features and the green molecules correspond to the dark molecular features in the STM image of Figure 3a. The inset shows a side view of the THT molecule.

) 10.1 ( 0.2 Å ) 2x3, where ah ) 2.89 Å denotes the interatomic spacing of the Au(111) lattice. Based on the STM image of Figure 3a, we propose a schematic structural model for THT SAMs on Au(111) in Figure 4, which can be described as a (3 × 2x3) superlattice. In this model, we assumed that all sulfur atoms in the THT SAMs occupy threefold hollow sites of the Au(111) lattice. The average areal density for the adsorbed molecule was calculated to be 21.6 Å2/molecule, which is the same as that observed from the closely packed alkanethiol SAMs with a (x3×x3)R30° structure or c(4×2) superlattice. As we mentioned before, the unit cell of c(4×2) superlattice for alkanethiol SAMs on Au(111) contains two or three different molecules with different STM imaging contrasts, which may be due to the differences in twist about the alkyl chain axis21,58,59 or the differences in adsorption site of sulfur head groups on Au(111).21,60 Similar to alkanethiol SAMs, THT SAMs have a (3 × 2x3) superlattice, and two different molecular spots, with a height difference ranging from 0.2 to 0.35 Å, were observed from the unit cell, as shown in Figure 3a. We suggest that the two different molecular features in THT SAMs originate from a small variation in adsorption configuration of THT molecules, driven by the optimization of van der Waals interactions between THT rings because all sulfur atoms occupy identical threefold hollow sites of an Au(111) surface. Conclusion We have demonstrated that the adsorption of THT molecules, containing endo-sulfur rings on Au(111), leads to chemisorbed, 2D, ordered SAMs with many unique structural defects and few vacancy islands, with a low vacancy island area to total surface area ratio of 2.5-3.5%. We expect that the unique surface structure is mainly due to the weak van der Waals interactions between THT rings, as well as dynamic structural variations of the THT rings in the SAMs. A high-resolution STM image reveals that the THT SAMs on Au(111) have a (3 × 2x3) superlattice, containing two different molecular spots in the unit cell, which might be originating from a small variation in the adsorption configuration of THT molecules, driven by the optimization of van der Waals interactions between the THT rings. The surface structures of the THT SAMs on Au(111) were

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