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On-surface fabrication of small-sized nanoporous graphene Hui Lu, Haochen Wang, Wenlong E, Dongxu Dai, Hongjun Fan, Zhibo Ma, and Xueming Yang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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The Journal of Physical Chemistry

Figure 1. Chemical structure of hexakis(4-bromopheny)benzene [HEX-6Br, C42H24Br6] molecule. Hydrogen atoms are white, carbon atoms gray, and bromine atoms red, respectively.

Figure 2. Typical STM images of HEX-6Br molecules self-assembled on a cold Au(111) surface with a densely packed hexagonal structure. (a) Large-area STM image of the monolayer structure of HEX-6Br deposited on a cold Au(111) surface (100x100 nm2, U=-1.31 V, I=17.1 pA). (b) Close-up STM image shows a densely

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packed hexagonal phase overlaid with a molecular structural model and the unit cell: a=1.430.02 nm, b=1.430.02 nm (6.25x6.25 nm2, U=−1.28 V, I=11.4 pA). (c) The calculated atomic structure of the self-assembled structure. The intermolecular interactions are drawn with dotted blue lines. (d) Simulated STM image at bias voltage of -1.5 V with respect to the calculated Fermi level.

Figure 3. STM images show the formation of patches of covalently bonded structures after deposition of HEX-6Br molecules at 363K and post-annealing at 438 K on Au(111): (a)100x100 nm2, U=-1.31 V, I=12.4 pA; (b) 30x30 nm2, U=-1.31 V, I=20.9 pA. (c) Line profile along white line shown in (b). (d) The relaxed model of a covalently bonds dimer structure.


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Figure 4. (a) STM image of 2D nanoporous graphene after further annealing the sample at 523 K, U=-1.31 V, I=18.3 pA. (b) Proposed structural model superimposed on the STM image.



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On-surface fabrication of small-sized nanoporous graphene Hui Lu,†,‡,▽Haochen Wang,†,‡,▽ Wenlong E,†,‡ Dongxu Dai,† Hongjun Fan,† Zhibo Ma*,† and Xueming Yang*,†,§ †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics,

Chinese Academy of Science, 457, Zhongshan Road, Dalian 116023, Liaoning, P. R. China ‡

University of Chinese Academy of Sciences, 19 A Yuquan Road, Shijingshan District, Beijing

100049, P. R. China §

Department of Chemistry, Southern University of Science and Technology, 1088 Xueyuan

Road, Guangdong, Shenzhen 518055, P. R.China *E-mail: [email protected] *E-mail: [email protected]

ABSTRACT: From the interplay of high-resolution STM and DFT calculations, we have successfully obtained nanoporous graphene via a hierarchical reaction pathway involving two different kinds of reactions (Ullmann coupling and cyclodehydrogenation) of hexakis(4bromopheny)benzene molecules (HEX-6Br) on Au(111).


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INTRODUCTION In the past few years, the surface-assisted synthesis of molecular or polymeric covalent nanostructures based on aromatic building blocks under both ultrahigh vacuum (UHV) conditions and at liquid/solid interfaces, has aroused significant interests as an efficient approach for precisely synthesizing 1D and 2D covalently bonded nanostructures.

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A variety of on-

surface reactions have been achieved, including dehalogenative and dehydrogenative homocoupling

reactions

from

sp1,

sp2,

sp3

carbons,8-14

Bergman

cyclization,15

cyclodehydrogenation,16-19 etc. One of the most popular and widely used reactions is Ullmann coupling of aromatic halides to form covalent C–C bonds. This reaction has been employed to build up diverse atomically precise nanostructures, including different shape and size graphene nanoribbons,20-24 polymeric chains,25,26porous molecular networks,27-29 and so on.30 In particular, nanoporous graphene can be grown using diphenyl-10,10’-dibromo-9,9’-bianthracene as the precursor according to three thermally activated reaction steps.31 It was recently demonstrated that nanosized pores can turn semimetallic graphene into a semiconductor with a bandgap, thus, nanoporous graphene structures are considered to be promising nanomaterials in the field of molecular electronic devices.32 It is therefore of general interest to make graphene structures with controllable pore sizes by virtue of atomically precise bottom-up fabrication. Herein, we choose the hexakis(4-bromopheny)benzene(C42H24Br6) molecule [shortened as HEX-6Br as shown in Figure 1] as a precursor and Au(111) as the substrate for the surfaceassisted synthesis. This molecule is of 6-fold symmetry and consists of a central benzene ring


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surrounded by six peripheral 4’-bromophenyl groups. It has a nonplanar configuration due to the steric hindrance between H atoms. From the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we demonstrate the formation of porous graphene nanostructures. The nanostructures are obtained through temperature-induced Ullmann coupling and cyclodehydrogenation reactions after depositing HEX-6Br molecules on the Au(111) surface under UHV conditions. It is shown that the Ullmann coupling occurs at 423 K, and subsequently, cyclodehydrogenation reaction takes place at 523 K to form the nanoporous graphene. Such a study shows the feasibility on the fabrication of nano porous graphene structures by on-surface synthesis strategy of HEX-6Br molecules, which may shed light on the bandgap engineering of graphene from controllable pore sizes point of view.

Figure 1. Chemical structure of hexakis(4-bromopheny)benzene [HEX-6Br, C42H24Br6] molecule. Hydrogen atoms are white, carbon atoms gray, and bromine atoms red, respectively. METHODS STM measurements were performed on a Unisoku low-temperature scanning tunneling microscope (LT SPM1200) operated in ultrahigh vacuum (UHV) chamber (base pressure of 10-10


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torr) at 80K. Single crystalline Au(111) surface was cleaned by several cycles of 1 keV Ar+ ions sputtering and then annealed at 723 K until the surface is clean. Commercially available HEX6Br molecules were evaporated onto Au(111) substrate from a Ta crucible at temperature of 588K. The molecular sources were degassed overnight prior to deposition. A cold Au(111) sample was taken from the STM chamber(80 K) to the preparation chamber(RT), at which molecules were deposited for 30 min. Afterwards, the sample were transferred to STM chamber then cooled down to LN2 temperature for scanning. Electrochemically etched W tips were used to obtain constant current STM images and the voltages refer to the bias on samples with respect to the tip. The sample temperatures were measured using an infrared thermometer. The DFT calculations are carried out using Vienna Ab Initio Simulation Package (VASP) code within the Projector augmented-wave (PAW) scheme.33 Perdew-Burke-Ernzerhof (PBE) functional is used to describe the exchange-correlation energy between electrons.34 Van der Waals (VDW) interactions are included using DFT-D3 method.35 The structure geometry is optimized until all forces are ≤0.03eV/Å, and the convergence criteria of each electronic step is ≤1.0E-4 eV.36 RESULTS AND DISCUSSION Figure 2a shows a large-scale STM image of the self-assembled structure of HEX-6Br molecules on a cold Au(111) surface (in which the Au(111) sample was taken from the STM chamber(at 80K) to the preparation chamber, the HEX-6Br molecules were deposited immediately for 30 min and then the sample was cooled to LN2 temperature again). It is observed that the molecules self-assemble into a highly ordered close-packed monolayer structure. Notably, the Au(111) herringbone reconstruction is still visible indicating the interaction between molecules and the


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substrate is relatively weak.37,38 Figure 2b shows a close-up STM image of the densely packed hexagonal structure in which individual HEX-6Br molecules are resolved. A molecular model and the unit cell are overlaid on the high-resolution STM image. The HEX-6Br molecule exhibits a relatively dark center surrounded by six bright protrusions and six dark ones, which are attributed to the peripheral benzene rings and the Br atoms, respectively. Normally, the bromine atoms were imaged as bright protrusions in the STM topography.39-41 However, in this present case, the bromine atoms are darker than the peripheral benzene rings owing to the steric hindrance between H atoms making the benzene rings titled on the surface. From the STM images, we observe that each HEX-6Br molecule is surrounded by six neighboring ones. The hexagonal molecular arrangement appears to be stabilized by intermolecular halogen–halogen interactions. Three Br atoms from the adjacent molecules form 3-synthons (the green triangle), similar halogen bonded 3-synthons have been reported previously.42,43 The intermolecular interactions are drawn with dotted blue lines (Figure 2c). The simulated STM image agrees well with the experimental one as shown in Figure 2d. Figure 3a, b show the formation of patches of covalently bonded structures after deposition of HEX-6Br molecules on the surface held at 373 K and post-annealing at 423 K. We observe that the molecules form branched network structures as shown in a large-scale STM image of Figure 3a, which is dramatically different from what we observed for the self-assembled hexagonal structure. The close-up STM image in Figure 3b and a closer inspection of branched network structures in the blue rectangle show that most of the molecules are covalently linked to the neighboring ones forming molecular patches. A line-profile analysis (Figure 3c) on adjacent species based on the high-resolution STM of Figure 3b(see white rectangle) reveals molecular center to center distance of 1.32nm, which is in good agreement with the DFT relaxed molecular


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model of a covalently bonded dimer structure, based on a gas-phase model (see Figure 3d). The formation of disordered molecular patches may be attributed to steric hindrance between molecules or other reasons. In fact, well-order covalent structures are typically difficult to be obtained.44,45 Although six bromine atoms are equivalent in molecular structure, the C-Br cleavage always occur step-by-step. In fact, if the first C-Br cleavage occurs, the molecule will be tilted because of the radical-surface interaction, thus other C-Br bonds are relatively difficult to be broken.

Figure 2. Typical STM images of HEX-6Br molecules self-assembled on a cold Au(111) surface with a densely packed hexagonal structure. (a) Large-area STM image of the monolayer structure of HEX-6Br deposited on a cold Au(111) surface (100x100 nm2,U=1.31 V, I=17.1 pA). (b) Close-up STM image shows a densely packed hexagonal phase overlaid with a molecular structural model and the unit cell: a=1.430.02 nm, b=1.430.02


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nm (6.25x6.25 nm2, U=−1.28 V, I=11.4 pA). (c) The calculated atomic structure of the selfassembled structure. The intermolecular interactions are drawn with dotted blue lines. (d) Simulated STM image at bias voltage of -1.5 V with respect to the calculated Fermi level.

Figure 3. STM images show the formation of patches of covalently bonded structures after deposition of HEX-6Br molecules at 363 K and post-annealing at 438 K on Au(111): (a)100x100 nm2, U=-1.31 V, I=12.4 pA; (b) 30x30 nm2, U=-1.31 V, I=20.9 pA. (c) Line profile along white line shown in (b). (d) The relaxed model of a covalently bonds dimer structure. An increase of the annealing temperature to 523 K gives rise to cyclodehydrogenation within HEX-6Br molecules and as a result the formation of nanoporous graphene structures as shown in Figure 4. Each unit can be considered as a hexabenzocoronene, referred to in the following as HBC,46 which is marked by a blue circle in Figure 4a. To determine the C-C bond formed between HBC molecules we measured the center-to-center distance of adjacent molecules，


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which was 1.32nm. This indicates intermolecular covalent bonding and thus reveals that the coupling between HBC molecules results in the formation of nanoporous graphene.46 A proposed model is superimposed on the STM image as shown in Figure 4b, in which a small patch of porous nanographene structure can be identified. The hindrance on the formation of larger nanoporous graphene islands is most probably because the following reason: the results for the formation of disordered structures are complicated. At this temperature not only the cyclodehydrogenation process can occur but also some other C-H bonds are dissociated, leading to the disordered connections. For example, the connection between the molecular unit at bottom right corner in the molecular model with its neighboring one.

Figure 4. (a) STM image of 2D nanoporous graphene after further annealing the sample at 523 K, U=-1.31 V, I=18.3 pA. (b) Proposed structural model superimposed on the STM image. CONCLUSIONS In summary, we have demonstrated the feasibility on the fabrication of small-sized nanoporous graphene structures by Ullmann coupling and cyclodehydrogenation reaction of HEX-6Br molecules on the Au(111) surface. These processes were investigated using UHV-STM imaging combined with DFT calculations. This strategy may shed light on the fabrication of graphene with controllable pore sizes for future electronics device applications.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions ▽

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge financial support from the National Natural Science Foundation of China grant 21688102, the Strategic Priority Research Program of Chinese Academy of Science grant XDB17000000, the National Key Research and Development Program of the MOST of China, grant 2016YFA0200603, National Natural Science Foundation of China, grant 21673236. We thank Shaoshan Wang (Harbin Institute of Technology) for the useful discussions and Prof. Wei Xu (Tongji University) for carefully revising our manuscript.

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