Graphdiyne and graphyne as well as their derivatives are 2D semiconducting allotropes of carbon with non-zero band gaps , which make a distinct advantage over graphene for transistors with high on-off ratios . The 1D limit of graphyne and graphdiyne consists of repeating units of phenylene and ethynylene or butadiynylene, respectively [3, 4]. The ultimate small width (0.5 nm) and moderate band gap (~1.5 eV) of graphdiyne wire make it a promising candidate for miniature nanoelectronics and optoelectronic devices [5-7]. More interestingly, the band structure of the 1D graphdiyne wire can be further tuned by extending the numbers of phenylene unit , which can be regarded as an extended graphdiyne molecular wires. Recently, graphdiyne wires have been successfully synthesized on surfaces of noble metals by dehydrogenative homocoupling of terminal alkynes  and dehalogenative homocoupling of terminal alkynyl bromides . Both methods are based on the mechanism of terminal acetylenic coupling [11, 12]. However, due to the high reactivity of the terminal alkynyl groups in presence of noble metal atoms, unwanted side reactions of the alkyne homocoupling are always there and lead to branched graphdiyne wires with very short length or cross-linked 2D polymeric networks [13, 14]. So far, guided growth of long graphdiyne wires with regioselective reaction pathway can only be obtained by surface templating at the vicinity of surface step edges [7, 15]. A facile approach to synthesizing high-quality graphdiyne wires with long length and high yield is desired.
In this work, we present a new approach to the synthesis of isolated graphdiyne nanowires [-C
All sample preparation procedures were performed in ultra-high vacuum with a base pressure of 5
The periodic density functional theory (DFT) calculations were performed by using VASP software  with the projected augmented wave method  and the Perdew-Burke-Ernzerhof exchange-correlation functional . The plane wave basis set was expanded to a kinetic energy cut-off of 520 eV. Structural optimizations were performed until the forces acting on all atoms were smaller than 0.01 eV/Å. Iterative method of energy calculation is Davidson blocked iteration scheme. We generated 5
FIG. 1(a) is a schematic view of the CVD method we employed in this study. DBYP molecules were evaporated from a ceramic crucible and deposited on the Au(111) substrate held at 483 K. The CVD method with a hot Au surface generates considerable numbers of diffusive Au atoms that will provide sufficient Au adatoms during the dehalogenative homocoupling reactions of the DBYP molecules once they reached the surface. A typical STM image of the wires synthesized by CVD method is shown in FIG. 1(b). The ratios of precursor molecules polymerized into PYP wires is shown in FIG. 1(c). We found that more than 60% of the precursors were polymerized into PYP wires longer than 50 nm and the yield of aryl-aryl Ullmann reaction is counted to be higher than 95%. For a comparison, we made a controlled sample by depositing DBYP molecules on Au(111) held at room temperature, followed by an annealing procedure to the same temperature of 483 K. FIG. 1(d) shows the STM image of the self-assembled domains of DBYP along [11
To reveal the origin of the high yield of terminal homocoupling reactions and inhibit branched coupling in the on-surface synthesis of PYP wires in the CVD method, we performed a set of experiments at different substrate temperatures. STM images in FIG. 2 show the oligomers and the wires on-surface synthesized by CVD method with different substrate temperatures. For the sample prepared at 393 K, most DBYP molecules were polymerized into dimers by dehalogenative homocoupling (FIG. 2(a)). For the sample prepared at 423 K, tetramers were observed along with dimers (FIG. 2(c)). For the sample prepared at 483 K, most molecules were polymerized into long PYP wires and only very few oligomers can be found (FIG. 2(d)). The interesting thing is that for all three samples branched coupling was scarcely observed. If we take a close look at the assembled structure in FIG. 2(a), we found that the polymerized dimers are not closely packed but separated with adatoms (FIG. 2(b)). The line profile in FIG. 2(b) shows that the distance between the two Br atoms from two neighboring dimers is ~0.94 nm and an atomic protrusion shows up in the middle. These adatoms look dimmer than the DBYP dimers in STM images and cannot be assigned to dissociated Br atoms since Br adatoms typically have a much higher apparent height . We therefore attribute these interstitial adatoms to single Au atoms trapped in the molecular assembly during the on-surface reaction.
Although FIG. 2(a, b) indicates the presence of Au adatoms on the heated surface during the dimer formation [25, 26], it is hard to identify the role of the Au adatoms in the formation of long PYP wires simply from STM images like FIG. 2(d). To solve the problem, non-contact AFM experiment with single-bond resolution was performed on the CVD sample prepared at 483 K (FIG. 2(d)). The single-bond resolution was achieved by using a tuning fork AFM (qPlus) tip decorated with a single carbon monoxide molecule . FIG. 3 (a) and (c) show respectively the STM image and the AFM image of the same area of the sample in FIG. 2(d). The corresponding structural model is illustrated in FIG. 3(b). In the AFM image, the hexagons in the PYP chains can be identified as benzene rings and the two bright knots correspond to the alkyne pairs [8, 9, 27]. The dissociated Br atoms shown in the STM image are not prominent in AFM image due to the reduced electron density upon bonding to the underneath Au substrate. Although it is hard to tell the presence and position of Au adatoms only from STM image, the Au adatoms manifest as bright spots near every butadiynylene unit in the AFM image, as indicated by the yellow arrows in FIG. 3(c). The line profile along the orange line in FIG. 3(a) shows a shoulder structure at the same site of the bright spot in AFM image, further supporting the presence of the Au adatoms. The high-resolution AFM image in FIG. 3(e) shows clear frequency shift lines connecting the Au adatom and the butadiynylene units. This is a strong indication that there are covalent-like bonds between them, in accordance to the Dewar-Chatt-Duncanson model  which explains the type of chemical bonding between unsaturated ligands and a metal forming a
The above analysis demonstrates the protected synthesis of long isolated graphdiyne nanowires in presence of Au adatoms. The as-grown long nanowires are perfect examples of 1D infinite semiconductors. First-principles calculations show that the graphdiyne nanowire is a semiconductor with a band gap of ~1.5 eV (FIG. 4(a)). We performed STS measurements on our synthesized PYP wires, the results are shown in FIG. 4(b). The conductance (dI/dV) spectra obtained at the butadiynylene units (green line) and the biphenylene units (purple line) are nearly identical, indicating a delocalized electronic property owing to the fully
In summary, we report a new approach to the on-surface synthesis of long and branchless graphdiyne nanowires by CVD method. The main finding of this work is that we identified that single Au adatoms act as protecting groups in this on-surface polymerization with the aid of high-resolution AFM characterization. The butadiynylene units in the graphdiyne wires are stabilized by Au adatoms by forming Au-
This work was supported by the National Key R & D Program of China (No.2016YFA0200603 & No.2017YFA0205004), the Anhui Initiative in Quantum Information Technologies (AHY090300), the National Natural Science Foundation of China (No.21473174), and the Fundamental Research Funds for the Central Universities (No.WK2060190084 and No.WK2340000082). Ai-di Zhao acknowledges a fellowship from the Youth Innovation Promotion Association of Chinese Academy of Science (2011322).
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