One-Step Preparation of Large-Scale Self-Assembled Monolayers of

In this paper, a kind of robust self-assembled monolayer (SAM) directed by lateral triple hydrogen bonding interactions was prepared by a one-step rea...
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J. Phys. Chem. C 2008, 112, 4209-4218

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One-Step Preparation of Large-Scale Self-Assembled Monolayers of Cyanuric Acid and Melamine Supramolecular Species on Au(111) Surfaces Hai-Ming Zhang, Zhao-Xiong Xie,* La-Sheng Long, Hui-Ping Zhong, Wei Zhao, Bing-Wei Mao, Xin Xu,* and Lan-Sun Zheng State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen UniVersity, Xiamen 361005, China ReceiVed: August 29, 2007; In Final Form: December 14, 2007

In this paper, a kind of robust self-assembled monolayer (SAM) directed by lateral triple hydrogen bonding interactions was prepared by a one-step reaction of cyanuric acid (CA) and melamine (M) molecules on reconstructed Au(111) surfaces in ambient conditions. The structures of the CA‚M SAMs were found to be similar to those of bulk crystals. These physisorbed SAMs, directed by triple hydrogen bonds, were found to be stable at the air/substrate interface and possessed a high thermal stability (up to 400 K). According to the high-resolution STM images, a commensurate structure with respect to the reconstructed Au(111) surfaces was established. Combined with density functional theory (DFT) calculations, the roles of the adsorbateadsorbate and the adsorbate-substrate interactions, as well as the origin of the high stability of the CA‚M SAMs, were discussed. In addition to the preparation of the CA‚M SAMs, SAMs of melamine and cyanuric acid molecules alone were also studied on the Au(111) surfaces. Chiral structures were found in both of these SAMs. Structural models of melamine SAMs and cyanuric acid SAMs were established.

1. Introduction Supramolecular chemistry, defined as chemistry beyond chemical bonds, concerns how to exploit noncovalent bonds to make supramolecular species.1,2 Guided by directional and selective noncovalent bonds, modern supramolecular chemistry has made great success in the preparation of stable heteromolecular species in solutions and as bulk crystals.3-6 In recent years, the concepts of supramolecular chemistry have been applied to building the two-dimensional ordered supramolecular structures on surfaces.7-9 These surfaces, fabricated by supramolecular structures, are able to exhibit features of molecular recognition, becoming a powerful tool in the applications of chemical sensors, nanoelectronics, and so forth.10-12 Although several heteromolecular species have been prepared at the liquid/ solid interface13-16 or in ultrahigh vacuum (UHV) conditions,7,17,18 only one successful preparation of long-range monolayers of heteromolecular species has been reported very recently in the UHV chamber by carefully choosing appropriate template surfaces, such as the Au(11,12,12) surface, to guide the long-range propagation.19 However, the Au(11,12,12) surfaces are not common and the UHV conditions demand a complicated and expensive apparatus. Exploration of feasible and inexpensive methods for the preparation of large-scale ordered monolayers of heteromolecular species is therefore important and challenging. The cyanuric acid and melamine complex (CA‚M) is one of the typical supramolecular species in which cyanuric acid (CA) and melamine (M) are held together by N-H‚‚‚O and N-H‚‚‚N hydrogen bonds, yielding a rosette form as shown in Scheme 1.20,21 In 1999, Rao et al. first reported the structure of the CA‚M adduct by using the single-crystal X-ray crystallography.22 Very recently and in the UHV conditions, Beton’s * Corresponding authors. Tel: +86-592-2180627. Fax: +86-5922183047. E-mail: [email protected]; [email protected].

group and Besenbachers’ group reported the fabrication of the CA‚M bimolecular arrays on the Au(111) surfaces. However, preparation of large ordered domains of the CA‚M SAMs with uniform structures is difficult in the UHV conditions by a sequential or simultaneous deposition of M and CA on the surfaces.23-25 In this paper, we report a systematic study of the self-assembly of CA, M, and the CA‚M complex on the Au(111) surfaces. Especially, large-scale ordered monolayers of the CA‚M supramolecular complex on the Au(111) surfaces are prepared in ambient conditions by a simple method in which the aqueous solutions and the Au(111) substrate are preheated to a certain temperature. Combining the structure details of CA, M, and the CA‚M SAMs from the STM technique and density functional theory (DFT) calculations, the roles of the adsorbateadsorbate and the adsorbate-substrate interactions, as well as the origin of the high stability of the CA‚M SAMs are discussed. 2. Experimental Section The reconstructed Au(111) surfaces of a single-crystalline bead fixed on a gold sheet were prepared by Clavilier’s method followed by careful annealing in a hydrogen flame. The solutions of CA and M molecules (0.1 mM) were prepared with Milli-Q water at room temperature. A droplet of hot solution (about 350 K) was put onto an Au(111) surface that was preheated to about 380-390 K. After the evaporation of the solution, the surface was rinsed carefully with water. For the preparation of SAMs of the CA‚M complex, the solutions of CA and M molecules (0.1 mM) with equal volume were mixed first at room temperature. The followed preparation procedure was similar to that of the M or CA SAMs by using the blended solutions. A mechanically sharpened Pt/Ir tip was used for the STM observation (Nanoscope IIIa, Digital Instrument) at room temperature. Typical imaging conditions were 200 mV in bias voltage and 300 pA in tunneling current.

10.1021/jp076916a CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008

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Figure 1. Self-assembly of the M molecules on the Au(111) surfaces. (a) Large-scale STM image of the M SAMs. A number of black dots, highlighted by the white arrows, are found with a random distribution. (b) High-resolution STM image showing detailed information of the M SAMs. The black and white circles represent the empty unit and the filled unit, respectively. (c) A proposed structural model of the host-guest structure. Hydrogen bondings are signified by using dashed lines.

SCHEME 1: Bonding Structure of the CA‚M Complex

3. Results and Discussion 3.1. Self-Assembly of the Melamine (M) Molecules on Au(111) Surfaces. Figure 1a shows a typical large-scale STM image of the M SAMs, in which well-ordered SAMs are found. The M SAMs consist of close-packed bright dots. Several black dots, as indicated by white arrows in Figure 1a, distribute randomly in the SAMs. Detailed structures can be recognized easily in the high-resolution STM image as shown in Figure 1b. The black circle marks the composition of the black dots (hereafter, empty unit). The white circle marks the contrast unit,

the filled unit, which prevails in the M SAMs. Each filled unit is composed of six bright petals and a central bright dot. The size of each petal is about 0.5 nm, which corresponds well to the size of a single M molecule. Therefore, the bright petal in each unit can be considered as an individual M molecule. Furthermore, the triangular shape of each petal suggests that the M molecules adsorb flatly on the Au(111) surfaces. A unit cell marked in Figure 1b includes two M molecules, if we ignore the central dots of the filled units. The mean value of the cell parameters measured from tens of the high-resolution STM

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Figure 2. Chiral structures in the M SAMs. (a) Two adjacent homochiral domains formed by pure enantiomers. Each white triangle represents a single M molecule. The angle between unit cells of the adjacent homochiral domains is measured to be 28°. High-resolution STM images of homochiral domains formed by pure enantiomers: (b) the “R” structure and (c) the “S” structure. (d) A structural model of the M SAMs on the Au(111) surfaces, showing the formation of the angle between unit cells of two adjacent homochiral domains.

images are a ) b ) 1.02 ( 0.02 nm. The angle γ is measured around 120 ( 2°. The filled unit, as marked by the white circle of Figure 1b, has packing features similar to those of the empty unit, except for the central bright dot. We consider that the bright dot in the center of each filled unit is attributed to a guest M molecule. This phenomenon is similar to the observation in SAMs of trimesic acid on the HOPG surfaces, in which the central dot, found in the chicken wire and flower structures, was attributed to a guest trimesic molecule.26 In the M SAMs, the diameter of each hole formed by six M molecules is measured to be about 0.5 nm, which is not large enough to locate a single M molecule with a flat-lying geometry on the Au(111) surfaces. Therefore, the bright dot in the center of each filled unit is assigned as a single M molecule adsorbed perpendicularly by the NH2 group facing the Au(111) surfaces, as shown in Figure 1c. More evidence to support such a proposed model may be found in the Supporting Information. Although chiral structures in the M SAMs were proposed by Beton’s group,23,24 there have been no direct STM results exhibiting structural information of the chiral packing modes. On the basis of tens of high-resolution STM images, the chiral packing structures can be identified, and a reasonable structural model is established as in Figure 2. Such a model is different from the previous one, as proposed by Beton et al.23,24 Figure 2a shows two adjacent domains in which different packing structures of the M molecules are found, and are denoted as “R” and “S”, respectively. Each white triangle marked in the figure represents a single M molecule. The unit cell of each domain is superimposed in the left and right side of Figure 2a. The angle between the long axes of the unit cells of two domains is measured to be about 28 ( 2°. In the empty unit of the “R” and “S” structures, as marked by the six white triangles in Figure 2b and c, each M molecule does not point straight toward the center of the unit. They are found to rotate

by 14 ( 2° clockwise or counterclockwise with respect to the principal axes of the unit. Extension of each structure forms homochiral arrays of the “R” and “S” domains. According to the cell parameters and the angle of the unit cell between the “R” and “S” domains, we establish the model of the M SAMs on the Au(111) surfaces, as depicted in Figure 2d. In this model, every M molecule is adsorbed with its center on the hollow site of the Au(111) surfaces with a flat-lying geometry. Each N atom in a melamine molecule is facing to an underlying gold atom, which maximizes the interactions between the adsorbed melamine molecule and the underlying substrate. This adsorption geometry is supported by our DFT calculations, which will be given in the following section. A (x13 × x13) R ( 14° structure of the M SAMs is thus established. The cell parameters are a ) b ) 1.04 nm and angle γ ) 120°. In such a packing model, the angle of the long axes of the unit cell between two adjacent chiral domains is 28°, which corresponds very well to the experimental observations. The M molecules contact each other with perfect arrangement of the hydrogen bonds. The hydrogen bond of N-H‚‚‚N is calculated to be around 2.9 Å for N‚‚‚N, which is well within the range of N-H‚‚‚N bonds 2.7-3.0 Å in the literature.22 3.2. Self-Assembly of the Cyanuric Acid (CA) Molecules on Au(111) Surfaces. A typical STM image of SAMs of CA on Au(111) surfaces is shown in Figure 3a. Two kinds of structures are found, corresponding to two-dimensional closepacked structures, as indicated by the white circle, and onedimensional linear structures, as indicated by the black arrow. Although the reconstructed Au(111) surfaces are used for the preparation of the CA SAMs, no typical reconstructed ridges are found in the STM image. Detailed structural information of the unit cell can be collected from the high-resolution STM images. In Figure 3b, a primitive unit cell, as marked by a white circle in Figure 3a, is composed of six bright dots that form a sixfold ring. The lattice belongs

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Figure 3. Self-assembly of the CA molecules on the Au(111) surfaces. (a) Large-scale STM image of the CA SAMs. The white circle shows the primary unit cell of two-dimensional close-packed structures, which is composed of six bright dots, forming a sixfold ring. The black arrow indicates one-dimensional linear structures. (b) High-resolution STM image of the CA SAMs. The cell parameters are a ) b ) 1.32 ( 0.02 nm and angle γ ) 120° ( 2°. (c) Optimized geometric structures of one primary unit of the CA SAMs. Two hydrogen bonds were found between adjacent CA molecules as marked with dashed lines.

to the space group P6 with lattice constants being equal to a ) b ) 1.32 ( 0.02 nm and angle γ ) 120 ( 2°. The distance of two nearest-neighbor bright dots is measured to be 0.50 ( 0.02 nm. Each bright dot is considered as a single CA molecule because the distance between two neighboring dots is too large to fit the distance of groups within a single molecule. However, the average area of each molecule (0.19 nm2) is smaller than the maximum area of a CA (about 0.44 nm2). This suggests that the CA molecules do not lie flat on the gold substrate. We consider that the CA molecules adsorb perpendicularly on the substrate with O or the N-H group facing the Au(111) surfaces. This packing structure and the cell parameters are similar to the self-assembly of 2-pyrrolidone molecules on Au(111) surfaces in which the 2-pyrrolidone molecule is found to adsorb perpendicularly on the Au(111) surfaces.27 Directed by lateral hydrogen bonds, six 2-pyrrolidone molecules form a sixfold ring to compose the primary unit.27 Although the CA molecule shares the same imido group with the 2-pyrrolidone molecule, the hydrogen-bonding pattern between adjacent CA molecules may not be similar to that of 2-pyrrolidone molecules. In order to clarify the hydrogenbonding pattern between CA molecules, we performed a DFT calculation at the level of B3LYP with basis sets of 6-31++G(d,p) to optimize the structures of the primary CA unit. Figure 3c shows the optimized structures of the primary unit, in which six CA molecules form a threefold ring with every molecular plane perpendicular to the plane of the ring. Two hydrogen bonds were found between adjacent CA molecules as marked by dashed lines in Figure 3c. The average length and strength of the hydrogen bonds of N-H‚‚‚O is calculated to be 1.90 Å and 19.5 kJ/mol, respectively. This calculation result corresponds well to the proposed model from STM images that CA

molecules are adsorbed perpendicularly to the substrate and six CA molecules form a primary unit cell under the direction of lateral hydrogen bonds. Possibly because of the omission of the substrate effect and the inter-ring interactions, the calculated ring diameter (1.15 nm) is 0.15 nm larger than the observed value from STM. An estimation based on the single point calculation at the experimental ring diameter (1.0 nm) will lead to a maximum decrease of 4.4 kJ/mol on the average strength per hydrogen bond. Such a gentle sacrifice of the strength in the intra-ring hydrogen-bonding shall be compensated by the optimized ring-substrate interactions and the inter-ring interactions. More detailed DFT calculation results are presented in Section 3.4, where we will discuss the substrate effect. Chiral packing structures are also found in the SAMs of the CA molecules. Figure 4a shows two adjacent homochiral domains formed by pure enantiomers, denoted as “R” and “S”, respectively. The unit cell of each domain is marked in Figure 4a. The angle between the long axis of the unit cell of each domain is measured to be 22 ( 2°. Detailed structural information is shown in the high-resolution STM images of the “R” structure (Figure 4b) and the “S” structure (Figure 4c) in which a single CA molecule is marked by a white circle. On the basis of the high-resolution STM images and the cell parameters, a (x21 × x21) R ( 11° structure for the CA SAMs is established as in Figure 4d. In this model, the CA molecules adsorb perpendicularly to the substrate and form a commensurate structure with respect to the Au(111) surfaces. Six molecules form a sixfold ring, which is identified as the primitive unit cell. The cell parameters are a ) b ) 1.32 nm and angle γ ) 120°. This packing structure differs from the observation of the CA SAMs in the UHV conditions in which the CA molecules were discovered to adsorb flatly on the

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Figure 4. Chiral packing structures in the SAMs of the CA molecules. (a) Two adjacent homochiral domains formed by pure enantiomers. The angle between the long axis of the unit cells of each homochiral domain is measured to be 22°. High-resolution STM images exhibit the detailed structural information: (b) the “R” structure and (c) the “S” structure. (d) A structural model of the CA SAMs on the Au(111) surfaces in which a(x21 × x21) R ( 11° structure is established.

reconstructed Au(111) surfaces forming a hydrogen-bonddirected network.23 We consider that the difference of the packing ways may originate from the effects of water molecules. In a CA solution, hydrogen bonds exist not only between the CA molecules but also between CA and the water molecules. These water molecules interfere with the formation of the networks directed by hydrogen bonds between the CA molecules when the CA molecules are separated out of the solutions to form SAMs on the Au(111) surfaces. 3.3. Self-Assembly of the CA‚M Complex on Au(111) Surfaces. Figure 5a shows a high-quality STM image observed from a mixed solution of CA and M molecules (0.1 mM) with equal volume, where a well-ordered domain can be recognized to extend over the terrace of the reconstructed Au(111) surfaces with an area larger than 50 × 50 nm2. SAMs prepared in this way do not lift the reconstruction of gold because the typical gold reconstructed ridges can be recognized easily in Figure 5a. Detailed structures are shown by a closer view of the wellordered domains. Figure 5b exhibits a high-resolution STM image in which each bright dot can be considered as a single molecule. An open honeycomb structure can be observed with unit parameters of a ) b ) 0.98 ( 0.05 nm and angle γ ) 120 ( 4°. A unit cell, marked in Figure 5b, is composed of two molecules. Structures of the CA‚M SAMs are not similar to either the structures of the M SAMs or those of the CA SAMs. Moreover, the unit parameters measured from the STM images are closer to that of the 1:1 CA and M complex observed from X-ray crystallography (0.96-0.97 nm). Further attention should be paid to the detailed molecular information of the unit cell, as shown in Figure 5c. The unit cell is composed of two molecules that exhibit features of a big and a small triangular shape, respectively. These features correspond well to the electronic properties of the M and CA molecules, respectively. Thus, molecules with features of big triangular shapes in the highresolution STM image can be assigned as the M molecules;

the others with features of small triangular shapes can be associated with the CA molecules. Accordingly, we believe that the SAMs of CA‚M complexes are prepared successfully on the reconstructed Au(111) surfaces. Directions of the molecular rows, defined as the direction of the same molecules, are found along the direction of the reconstructed Au(111) surfaces, as shown in Figure 6b. This observation implicates the relationship between the reconstructed Au(111) surfaces and the CA‚M SAMs. Figure 6a shows a schematic model of the reconstructed Au(111) surface. The periodic length of gold atoms along the [112h] direction is 0.50 nm, whereas those along the [2h11] and [12h1] directions are reduced to 0.48 nm. By measuring the periodic length of the CA‚M SAMs in Figure 6b, we found that the mean values are 0.95 ( 0.02 nm along the [2h11] direction and 0.99 ( 0.02 nm along the [112h] direction. These correspond to the two times periodicity of the gold substrate along the [2h11] and [112h] directions. Furthermore, a slight deviation in the direction of the long axis of the unit cell is found along the [011h] direction, as indicated by the solid and dashed line in Figure 6b. The angle is measured around 3°. This deviation reflects the intrinsic structural property of the reconstructed Au(111) surfaces because the gold atom troughs along the [011h] direction have a slight deviation, which is about 2.6° in theory. The deviation angle we found in the CA‚M SAMs along the [011h] direction indicates the relationship between the CA‚M SAMs and the gold substrate. The same phenomena have been found in the SAMs of n-alkanes on reconstructed Au(111) surfaces in which alkane molecules were found to form a commensurate SAM with respect to the gold substrate.28 Thus, we conclude that the M and CA molecules are adsorbed along the direction forming a commensurate CA‚M SAM with respect to the reconstructed Au(111) surfaces (see Figure 6c). The structural model of the SAMs reveals that CA and M are held together by N-H‚‚‚O and N-H‚‚‚N hydrogen bonds yielding an extended network. Directions of the hydrogen bonds are along

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Figure 5. SAMs of the CA‚M complexes on the reconstructed Au(111) surfaces. (a) Low-resolution STM image showing that a single domain of the well-ordered CA‚M SAMs extends over the terrace of the reconstructed Au(111) surfaces larger than 50 × 50 nm2. (b) High-resolution STM image showing the packing structures of the CA‚M SAMs. (c) Higher magnification of image b, exhibiting detailed molecular information of the unit cell.

the direction. The average length of N-H‚‚‚O and N-H‚‚‚N hydrogen bonds is about 2.9 Å. These packing structures of the CA‚M SAMs are similar to the 3D crystal structures observed by X-ray crystallography.22 3.4. Density Functional Theory (DFT) Calculations on the Role of the Substrate. Structures of the physisorbed SAMs are determined by the subtle balance between the adsorbateadsorbate and adsorbate-substrate interactions.8,18,29-31 When the adsorbate-adsorbate interactions are relatively weak, such as van der Waals and single hydrogen bonds, the adsorbatesubstrate interactions play an important role on the structures of the physisorbed SAMs. For these SAMs, a commensurate structure is often observed because the adsorbate molecules can always adjust their intermolecular orientation to obtain an optimal adsorbate-substrate interaction.28,32 With the increasing of the adsorbate-adsorbate interactions, it becomes increasingly hard for the adsorbate molecules to rotate and translate on the surfaces. Thus, incommensurate structures appear. Recent studies on the SAMs of MOCPs (metal-organic coordination polymers) revealed that the structure of the MOCP SAMs was independent of the symmetry of the substrate and was dominated mainly by the adsorbate-adsorbate interactions.17 For example, incommensurate structures observed in the SAMs of the CuDHBQ (2,5-dihydroxybenzo-quinone) MOCPs showed that the adsorbate-adsorbate interactions dominated the final structures on the surfaces.33 The typical bonding energy of the metalorganic coordination bonds amounts to 10-30 kcal/mol per interaction, ranging from weak bonds, normally found in biologic systems, to strong covalent bonds.34 Considering the bonding strength, the lateral interactions of multiple hydrogen bonds can contribute a bonding of medium strength, such that

the strength of multiple hydrogen bonds can be comparable to that of the metal-organic coordination bonds. In SAMs of CA‚M, the lateral interactions of triple hydrogen bonds can direct the self-assembly of CA‚M on the surfaces. Up to date, the role of the substrate, such as Au(111), is not fully understood in the formation of SAMs directed by medium-strength molecular interactions. Besenbacher’s group has reported the self-assembly of guanine on reconstructed Au(111) surfaces in UHV conditions.35 The adsorbate-adsorbate interactions in the G-quartet network were calculated to be 80.9 kJ/mol (0.84 eV), corresponding to the strength of two hydrogen bonds. Compared with the molecule-substrate interaction (about 24.1 kJ/mol), they considered that the packing features of the guanine networks were determined completely by the H-bonding interactions among the guanine molecules. The Au(111) surfaces only served as a template to locate the molecules in a planar geometry. However, recent studies on the bicomponent self-assembly of BDATB(1,4-bis-(2,4-diamino-1,3,5,-triazine)-benzene) and PTCDI(3,4,9,10-peryleneteracarboxylic diimide) on the Au(11,12,12) and Au(111) surfaces revealed that the structures of the substrate determined the final structures of the bicomponent SAMs, despite of the triple hydrogen bonds between BDATB and PTCDI.19 Accordingly, quantitative understanding of the role of the substrate will be helpful for the reasonable design of the physisorbed SAMs. Figure 7a shows an optimized geometry of a CA‚M pair, calculated by the DFT method at the level of B3LYP with basis sets of 6-31+G(d). The calculated hydrogen bond length of 2.875 Å for N-H‚‚‚N and 2.993 Å for N-H‚‚‚O are in good agreement with the experimental data (2.85-2.88 Å for N‚‚‚N

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Figure 6. Structural relationship between the CA‚M SAMs and the reconstructed Au(111) surfaces. (a) A schematic model of a reconstructed Au(111) surface. (b) Large-scale STM image of the CA‚M SAMs showing a slight deviation of molecular rows along the [011h] direction. (c) A structural model of the CA‚M SAMs on the reconstructed Au(111) surfaces.

Figure 7. Optimized structures by using the DFT method at the level of B3LYP with basis sets of 6-31+G(d). (a) A CA‚M pair. (b) A CA1‚M3 pair.

and 2.94-2.98 Å for N‚‚‚O) observed by the X-ray crystallography.22 In such an arrangement, the unit cell parameters will be a ) b ) 0.96 nm, corresponding well to the experimental observations. This agreement between theory and experiment further indicates that the packing structures of the CA‚M motif with such hydrogen bond lengths for N-H‚‚‚N and N-H‚‚‚O are the optimal structures directed by the lateral hydrogen bonds. The formation energy of a CA‚M pair is found to be 62.7 kJ/mol, consisting of triple hydrogen bonds (see Figure 7a). When a CA molecule adsorbs flatly to form an extended network with three M molecules, nine hydrogen bonds will be found around one CA molecule (see Figure 7b). The total strength of the lateral interactions around one CA molecule is calculated

to be 182.8 kJ/mol. The strong lateral interactions between M and the CA molecules ensure the formation of the CA‚M SAMs rather than the independent domains of the M SAMs and the CA SAMs. We first calculate the adsorption of a melamine molecule on the Au(111) surfaces. A cluster of Au22 is chosen to mimic the Au(111) surface (Figure 8a). The initial geometry is established according to the experimental results as shown in Figure 2d. All of the gold atoms were fixed at their original positions during the process of optimization. The top view of the optimized structure (Figure 8b) shows that every N atom in the melamine molecule locates on the top site of the underlying gold atoms. The side view of the optimized structure (Figure 8c) shows that the molecule is adsorbed flatly on the substrate with an

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Figure 8. DFT calculations for the adsorption of a melamine molecule on the Au(111) surfaces. (a) A cluster model of Au22 to mimic the Au(111) surface. (b) Top view of the optimized structure. (c) Side view of the optimized structure. The average distance between the molecule and the substrate is 3.71 Å.

Figure 9. DFT calculations for the adsorption of M and CA in the geometries similar to those in the CA‚M SAMs on the Au(111) surfaces. The initial geometries are established according to the experimental results (cf. Figure 6c). (a) A Au20 cluster used to simulate the Au(111) surfaces. (b) The optimized geometries for the M adsorption. (c) The optimized geometries for the CA adsorption.

equivalent distance of 3.71 Å. The adsorption energy Ea is calculated according to

Ea ) Emol-Au22 - (Emol + EAu22) where Emol-Au22 is the total energy of the adsorbed system and Emol and EAu22 represent the energies of the free molecule and the gold cluster, respectively. The calculated adsorption energy is 20.4 kJ/mol. We define Eamol as the energy of the isolated molecule with the geometric structure of adsorption. The value of Eamol is very similar to that of Emol with a difference of 0.42 kJ/mol. This suggests that the adsorption does not involve the variation of the geometric structure of the M molecule. Other geometries have also been checked by locating the melamine molecule on different sites of the substrate. Energy optimizations conclude that the geometry of Figure 8b is the most stable structure. This result is consistent with our proposed model of the M SAMs and supports our discussions for the formation of the chiral packing structures in the M SAMs. The molecule-molecule interactions are found along the direction when a melamine molecule adopts the adsorption geometry as depicted in Figure 8b. This is consistent with the findings in the SAMs of melamine molecules. However,

directions of the molecular interactions are found along the direction in the CA‚M SAMs (see Figure 6c), which suggests a different adsorption geometry in the formation of the CA‚M SAMs. To calculate the adsorption of M and CA molecules in the geometries similar to those in the CA‚M SAMs on Au(111) surfaces, a Au20 cluster is chosen to simulate the Au(111) surfaces (Figure 9a). The initial orientation of the molecules is based on the experimental observation (Figure 6c). The optimized structures for the adsorption of an M molecule and a CA molecule in the geometries of the CA‚M SAMs are shown in Figure 9b and c, respectively. Flat geometric structures are found for the adsorption of both M and CA. The adsorption heights of M and CA are 3.82 and 3.98 Å, respectively. The adsorption energies are summarized in Table 1. The calculated adsorption energies of 19.6 kJ/mol for M and 20.8 kJ/mol for CA reveal that the molecule-molecule interaction in the supramolecular species of the CA‚M complex is four times larger than the molecule-substrate interaction. Hence, the structures of the CA‚M SAMs on the Au(111) surfaces are determined mainly by the lateral hydrogen bonds. This result is consistent with the findings in other SAMs directed by the hydrogen bonds on the gold surfaces.19,35 Studies on the selfassembly of guanine on reconstructed Au(111) surfaces con-

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TABLE 1: Adsorption Energy of M and CA Molecules on the Gold Cluster of Au20 a molecuels

Eamol - Emol (kJ/mol)

Ea (kJ/mol)

melamine cyanuric acid

0.21 0.25

19.6 20.8

Eamol and Emol are defined as the energies of the isolated molecule with the adsorption geometry, and the free molecule geometry, respectively. Ea is the adsorption energy, calculated according to Ea ) Emol-Aun - (Emol + EAun). a

cluded that the packing features of the guanine networks were determined completely by the H-boning interactions among guanine molecules, and the surface served primarily as a template to accommodate the molecules in a planar geometry.35 However, as will be shown latter, our work demonstrate that the Au(111) surfaces are able to adjust the final structures of the CA‚M SAMs. Hence, even though the adsorbate-substrate interactions are indeed relatively weak, the role of the substrate cannot be ignored. As can be calculated from Figure 6c, the length of the hydrogen bonds N-H‚‚‚N among M and CA molecules is 3.05 Å along the [011h] and [1h01] directions and 2.80 Å along the [11h0] direction. Our DFT calculations show that lengthening the N-H‚‚‚N hydrogen bond length from the optimized value (2.875 Å) in the free CA‚M pair to 3.05 Å decreases the hydrogen bond strength by 4.6 kJ/mol, while shortening the N-H‚‚‚N hydrogen bond length to 2.80 Å decreases the hydrogen bonding by only 1.1 kJ/mol. These data reveal that the potential energy surface of the molecule-molecule interaction is very floppy, such that the adsorbate-substrate interaction can adjust the final structures of the CA‚M SAMs on the Au(111) surfaces in order to achieve an optimal adsorbatesubstrate interaction. The sacrifice of the hydrogen bonds is less than the gain for the optimal adsorbate-substrate interactions. Such kinds of packing structures make the heteromolecular species of CA‚M easily form large, well-ordered structures and make the CA‚M SAMs stable in air. Description of nonbonded weak interactions with DFT methods represents a great challenge. Even though the numbers presented here may be semiquantitative, we emphasize that weak interactions between adsorbates and substrates can play an important role in adjusting the final SAM structures. Conclusions We have successfully prepared CA‚M SAMs on reconstructed Au(111) surfaces, which exhibit features of hexagonal hydrogenbonded networks on surfaces with high thermal stabilities. Combined with DFT calculations, the role of the adsorbateadsorbate and adsorbate-substrate interactions has been discussed. The main conclusions are the following: (1) M molecules form well-ordered monolayers on the Au(111) surfaces with a flat-lying geometry in air. Every molecule contacts others with a perfect arrangement of hydrogen bonds to form a hexagonal extended network. A commensurate (x13 × x13) R ( 14° structure of M SAMs is established. (2) Chiral structures are found in the M SAMs. In a hexagonal unit, the M molecules are found to rotate by 14 ( 2° clockwise or counterclockwise with respect to the principal axes of the unit. The extension of each structure forms homochiral arrays of the “R” and “S” domains. (3) The CA molecules form well-ordered structures on the Au(111) surfaces by chemisorption with O and the N-H group facing the Au(111) surfaces. Chiral structures are also found in

the CA SAMs. A (x21 × x21) R ( 11° structure of the CA SAMs is established on the Au(111) surfaces. (4) SAMs of the CA‚M complex are prepared by reaction of M and CA molecules on the reconstructed Au(111) surfaces by the evaporation of hot aqueous M and CA mixtures. The M and CA molecules are adsorbed along the direction with a flat-lying geometry to form a commensurate structure with respect to the reconstructed Au(111) surfaces. A (2 x3 × 2 x3) R30° structure is established on the reconstructed Au(111) surfaces. (5) DFT calculations reveal that the strength of the moleculemolecule interaction is three times larger than that of the molecule-substrate interaction. Structures of the CA‚M SAMs are determined mainly by the molecule-molecule interaction. The reconstructed Au(111) surfaces also play an important role in determining the final structure of the SAMs, despite the relatively weak molecule-substrate interaction. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20533030, 20525311, 20673085, 20725310, and 20721001), the National Basic Research Program of China (Grant Nos. 2007CB815303 and 2007CB815206), Key Scientific Project of Fujian Province of China (Grant No. 2005HZ01-3), and NCET from the Ministry of Education of China. Supporting Information Available: STM images sequentially captured in tunneling conditions of I ) 800 pA and V ) 150 mV, exhibiting more detailed information of the central dots in the M SAMs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lehn, J. M. Supramolecular Chemistry, Concepts and PerspectiVes; VCH: Weinheim, 1995. (2) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley: Chichester, 2000. (3) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W. S.; Withersby, M. A.; Schroder, M. Coord. Chem. ReV. 1999, 183, 117. (4) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022. (5) Swiegers, G. F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483. (6) Zeng, F. W.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (7) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (8) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290. (9) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (10) Gooding, J. J.; Mearns, F.; Yang, W. R.; Liu, J. Q. Electroanalysis 2003, 15, 81. (11) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020. (12) Furukawa, S.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2007, 46, 2831. (13) Surin, M.; Samori, P. Small 2007, 3, 190. (14) Surin, M.; Samori, P.; Jouaiti, A.; Kyritsakas, N.; Hosseini, M. W. Angew. Chem., Int. Ed. 2007, 46, 245. (15) Tao, F.; Bernasek, S. L. J. Am. Chem. Soc. 2005, 127, 12750. (16) Xu, S. L.; Dong, M. D.; Rauls, E.; Otero, R.; Linderoth, T. R.; Besenbacher, F. Nano Lett. 2006, 6, 1434. (17) Stepanow, S.; Lin, N.; Payer, D.; Schlickum, U.; Klappenberger, F.; Zoppellaro, G.; Ruben, M.; Brune, H.; Barth, J. V.; Kern, K. Angew. Chem., Int. Ed. 2007, 46, 710. (18) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (19) Canas-Ventura, M. E.; Xiao, W.; Wasserfallen, D.; Mullen, K.; Brune, H.; Barth, J. V.; Fasel, R. Angew. Chem., Int. Ed. 2007, 46, 1814. (20) Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc. 1990, 112, 6409. (21) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D. N.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37. (22) Ranganathan, A.; Pedireddi, V. R.; Rao, C. N. R. J. Am. Chem. Soc. 1999, 121, 1752. (23) Staniec, P. A.; Perdigao, L. M. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. C 2007, 111, 886.

4218 J. Phys. Chem. C, Vol. 112, No. 11, 2008 (24) Perdigao, L. M. A.; Perkins, E. W.; Ma, J.; Staniec, P. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 12539. (25) Xu, W.; Dong, M. D.; Gersen, H.; Rauls, E.; Vazquez-Campos, S.; Crego-Calama, M.; Reinhoudt, D. N.; Stensgaard, I.; Laegsgaard, E.; Linderoth, T. R.; Besenbacher, F. Small 2007, 3, 854. (26) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25. (27) Xie, Z. X.; Charlier, J.; Cousty, J. Surf. Sci. 2000, 448, 201. (28) Zhang, H. M.; Xie, Z. X.; Mao, B. W.; Xu, X. Chem.sEur. J. 2004, 10, 1415. (29) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297.

Zhang et al. (30) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619. (31) De Feyter, S.; Miura, A.; Yao, S.; Chen, Z.; Wurthner, F.; Jonkheijm, P.; Schenning, A.; Meijer, E. W.; De Schryver, F. C. Nano Lett. 2005, 5, 77. (32) Zhang, H. M.; Yan, J. W.; Xie, Z. X.; Mao, B. W.; Xu, X. Chem.s Eur. J. 2006, 12, 4006. (33) Zhang, H. M.; Zhao, W.; Xie, Z. X.; long, L. S.; Mao, B. W.; Xu, X.; Zheng, L. S. J. Phys. Chem. C 2007, 111, 7570. (34) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853. (35) Otero, R.; Schock, M.; Molina, L. M.; Laegsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Angew. Chem., Int. Ed. 2005, 44, 2270.