Chinese Chemical Letters  2018, Vol. 29 Issue (6): 967-969   PDF    
Sierpiński triangles formed by molecules with linear backbones on Au(111)
Xue Zhanga, Ruoning Lia, Na Lia, Gaochen Gua, Yajie Zhanga, Shimin Houa, Yongfeng Wanga,b    
a Key laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China;
b Peking University Information Technology Institute Tianjin Binhai, Tianjin 300450, China
Abstract: Fractals play an important role in mathematics, aesthetic, science, and engineering. The representative Sierpiński-triangle fractals have been successfully constructed by V-shape molecules in experiments. The molecular Sierpiński triangles formed by molecules with linear backbones have been theoretically predicted but not experimentally discovered. To achieve this goal in the experiment, we used[1, 1';4', 1'';4'', 1''']-quaterphenyl-3, 4"-dicarbonitrile molecules as building blocks and employed cobalt atoms as cements, then successfully obtained metal-organic Sierpiński triangles with an order up to 2 on the Au(111) surface. There are twenty-four types of three-fold coordination nodes formed between the metal atom and organic ligands via coordinate interactions. The coexistence of various nodes is responsible for that the highest order of Sierpiński triangles is limited to 2.
Key words: STM linear backbone     Coordination     Fractal     Sierpiński triangle    

Fractals, which have the similar patterns at different scales [1], are commonly found in nature [2], such as mountain ranges, craters, snowflakes, trees, and romanesco broccoli. Fractal patterns, being fantastic and intricate, play an important role in mathematics, aesthetic, science, and engineering. In the past two decades, various fractals have been constructed both on surfaces and in solutions through various sophisticated strategies [3-9]. Scanning tunneling microscopy (STM) was used to investigate the growth mechanism of fractals due to its resolution capability at the single molecule level.

The Sierpiński-triangle fractal, a typical representative of the fractal structure, could date from the 11th century, as decorative patterns on the floors of churches in Rome. Molecular Sierpiński triangles have been fabricated for the first time in experimental systems using self-assembly of DNA tiles [10, 11]. However, the triangles were full of defects. Recently, defect-free Sierpiński triangles were theoretically predicted through Monte Carlo simulations [12]. They were experimentally obtained on single crystalline surfaces in ultrahigh vacuum. Using 120° molecules, the molecular Sierpiński triangles were constructed via halogen bonding [13], metal organic coordinating [14-17], hydrogen bonding [18], and covalent bonding interactions [19, 20]. Monte Carlo simulations have shown that Sierpiński triangles could be prepared by molecules with linear backbones [21]. However, this has not been experimentally realized yet.

Herein, we used [1, 1';4', 1''; 4'', 1''']-quaterphenyl-3, 4''-dicarbonitrile (QPDCN, Fig. 1) as building blocks and employed cobalt atoms as cements, then successfully constructed a series of metalorganic Sierpiński triangles on Au(111) (Fig. 1). The structures were investigated by low-temperature STM. All twenty-four three-fold coordination nodes formed by QPDCN molecules and cobalt atoms are listed and only one third of them could form metal-organic Sierpiński triangles. This is responsible for that the highest order of Sierpiński triangles in our experiments is limited to 2.

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Fig. 1. The Sierpiński triangles formed by QPDCN molecules and their schematic diagrams with the marked order from 0 to 2. A 2-order Sierpiński triangle consists of three 1-order Sierpiński triangles.

The experiments were carried out with a low-temperature STM (Unisoku, USM1500) with a base pressure of 10-10 Torr. The single crystalline Au(111) surface was cleaned by repetitive cycles of Ar+ sputtering and annealing at 450 ℃ for 20 minutes. QPDCN molecules were thermally deposited onto Au(111) kept at room temperature. Then Co atoms were evaporated from a Ta boat onto the molecule-covered substrates. After annealing at 150 ℃ for 20 min, the samples were transferred to the STM scanner and imaged at liquid helium temperature. All STM images were acquired with a polycrystalline Pt/Ir tip and processed using the software of WSxM [22].

Fig. 2a shows the molecular self-assembled structure of QPDCN molecules deposited on Au(111). The molecular models are overlaid on the magnified STM image (Fig. 2b). The molecular structures are stabilized by the intermolecular C-H...N hydrogen bond between hydrogen atoms from phenyl groups andnitrogen atoms of cyano groups. The QPDCN molecule consists of linear quaterphenyl as the main chain and two cyano groups in the para and meta sites of outer phenyl rings. It is achiral in vacuum and becomes chiral on Au(111) due to the surface constraint. The two enantiomers, denoted as L and R, are show in Fig. 2a.

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Fig. 2. (a) Self-assembled structure of QPDCN on Au(111). The inset shows surfaceinduced L and R enantiomers. (b) Molecular models overlaid on the magnified STM images. The molecular superstructure is stabilized by C-H...N hydrogen bonds. The surface close-packed direction is marked by black arrows according to the herringbone reconstruction on Au(111). All arrows in other STM images mark the same direction. (c, d) High-resolution STM images of Sierpiński triangles composed of QPDCN molecules and Co atoms on Au(111) and their molecular models (e, f). Except for two molecules at the apex, all rest molecules in a Sierpiński triangle are in the same chirality. The two different types of three-fold coordination nodes are marked by 1 and 2. Tunneling parameters for (a) and (b): Vb = 0.1 V, It = 12 pA; (c) and (d): Vb = 0.5 V, It = 10 pA.

Interestingly, metal-organic Sierpiński triangles appeared on the substrate after depositing Co atoms on the molecule-covered surface and annealing at 150 ℃ for 20 minutes. It has been demonstrated that the Co atom and cyano group could form threefold coordinate nodes on the Au(111) substrate [17]. In Figs. 2c and d, the coordinated structures are also three-fold. The invisibility of Co atoms might be due to that they are much closer than QPDCN molecules to the substrate. The corresponding molecular models of Sierpiński triangles are shown in Figs. 2e and f. Except for two molecules at the apex, all rest molecules in a Sierpiński triangle are in the same chirality. There are two different types of elementary coordination nodes which can be observed from molecular models (Figs. 2e and f). We stipulate that the node 1 consists of two cyano groups at the para position and one cyano group at the meta position. The node 2 involves one cyano groups at the para position and two cyano group at the meta position.

In the previous research, the highest order of Sierpiński triangles using 120° molecules can reach to 4 both in experiments and simulations. In contrast, the maximum order of Sierpiński triangles constructed by molecules with linear backbones here is only 2. To get insight into the formation mechanism of low-order Sierpiński triangles, all 24 possible three-fold coordination nodes are listed in Fig. 3. Each node is formed by three QPDCN molecules and one cobalt atom. The light and dark blue sticks represent chiral molecules L and R, respectively. A QPDCN molecule has four different ways to coordinately bond to the Co atoms, through the end of para or meta sites in the L or R enantiomers. As shown at the top of Fig. 3, the four homotactic fan-blade nodes are the same as the structures which they rotate 120° or 240°. The remaining 20 heterotactic forms in Fig. 3 are different from the nodes that they rotated such degrees because of the three-fold symmetry of the Au (111). Hence, there are total 64 kinds of nodes on Au(111). The dashed line represents the mirror plane to separate nodes with different chirality. Only eight types of nodes can attach another QPDCN molecule to form closed triangles and get larger Sierpiński triangles. They are marked by the black checks at the lower right corners. The rest two third of nodes prevent Sierpiński triangles from growing larger and are responsible for the formation of erroneous structures. Same to our experiments, the highest order of Sierpiński triangles is also 2 in the previous simulation using similar molecules [21]. For a similar shorter linker on Ag(111), the second-order Sierpiński triangles were not experimentally observedbecause of the coexistence of three-tosix-fold coordination motifs [23].

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Fig. 3. All possible three-fold coordination nodes formed by QPDCN molecules and cobalt atoms. The dashed line represents the mirror plane to separate nodes with different chirality. The black check at the lower right corner indicates the nodes that could form Sierpinski triangles

The Sierpiński triangles in Fig. 2 consist of closed triangles and linked molecules between them, where all molecules except for three at the apex are in the same chirality. Three QPDCN molecules in one closed triangle must adopt same chirality. The linked molecule could be in the form with different chirality, as shown in the STM images and molecular models in Fig. 4. The mixed enantiomers make the whole triangles highly distorted. The coexistence of 64 types of coordinated nodes and various Sierpiński triangles of low orders makes it very challenging to construct high-order Sierpiński triangles because the essential error-correction processes are easy to be prohibited.

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Fig. 4. High-resolution STM images (a, c) showing the deformed Sierpiński triangles and their molecular models (b, d). In both structures, the chirality of the linked molecules is different from that of molecules in closed triangles. Tunneling parameters: (a) Vb =0.5V, It =10 pA; (c) Vb =1V, It =10 pA.

In summary, we have successfully constructed metal-organic Sierpiński triangles by using QPDCN molecules with linear backbones and Co atoms on Au(111) in ultrahigh vacuum. They arestabilized bythe three-fold Co-N coordination interactions. The growth of Sierpiński triangles was investigated by low-temperature STM at the single-molecule level. The coexistence of various nodes is responsible for that the highest order of Sierpiński triangles is limited to 2.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21522301, 21373020, 21403008, 61621061, 21433011, 61271050) and the Ministry of Science and Technology (Nos. 2014CB239302 and 2013CB933404).

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