Experimental and Theoretical Investigations of Surface-Assisted

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Experimental and Theoretical Investigations of Surface-Assisted Graphene Nanoribbon Synthesis Featuring Carbon−Fluorine Bond Cleavage Hironobu Hayashi,*,†,‡ Junichi Yamaguchi,§,‡ Hideyuki Jippo,§ Ryunosuke Hayashi,† Naoki Aratani,† Mari Ohfuchi,§ Shintaro Sato,*,§ and Hiroko Yamada*,† †

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan § Fujitsu Laboratories Ltd and Fujitsu Limited, 10-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0197, Japan S Supporting Information *

ABSTRACT: Edge-fluorinated graphene nanoribbons are predicted to exhibit attractive structural and electronic properties, which, however, still need to be demonstrated experimentally. Hence, to provide further experimental insights, an anthracene trimer comprising a partially fluorinated central unit is explored as a precursor molecule, with scanning tunneling microscopy and X-ray photoelectron spectroscopy analyses, indicating the formation of partially edgefluorinated polyanthrylenes via on-surface reactions after annealing at 350 °C on Au(111) under ultrahigh-vacuum conditions. Further annealing at 400 °C leads to the cyclodehydrogenation of partially edge-fluorinated polyanthrylenes to form graphene nanoribbons, resulting in carbon−fluorine bond cleavage despite its high dissociation energy. Extensive theoretical calculations reveal a defluorination-based reaction mechanism, showing that a critical intermediate structure, obtained as a result of H atom migration to the terminal carbon of a fluorinated anthracene unit in polyanthrylene, plays a crucial role in significantly lowering the activation energy of carbon−fluorine bond dissociation. These results suggest the importance of transient structures in intermediate states for synthesizing edge-fluorinated graphene nanoribbons. KEYWORDS: graphene nanoribbon, on-surface synthesis, dehalogenation, scanning tunneling microscopy, theoretical calculation

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substituted at proper positions are expected to form edgefluorinated GNRs via on-surface reactions, which are also influenced by molecular structure, halogenation position, and nature of the metal surface. In fact, these effects have demonstrated that even simple anthracene-based precursor molecules could exhibit variable reactivity, since on-surface synthesis features hierarchical reactions such as dehalogenation, polymerization, and cyclodehydrogenation (Figure S1).1−4,14−17 In addition, the high temperatures required for on-surface synthesis imply the possibility of concomitant C−F bond activation, since some transition-metal-based catalysts and γ-Al2O3 are known to efficiently cleave C−F bonds via the formation of a key intermediate state.18,19 Therefore, the mechanism of edge-fluorinated GNR formation from precursor

raphene nanoribbons (GNRs) have drawn much attention as promising materials for nanoelectronic devices, with their electronic properties determined by width and edge topology.1 From the viewpoint of structural modification, bottom-up synthesis based on surface-assisted reactions is a powerful approach to GNR fabrication, since precursor molecule design defines the ultimate GNR structure with atomic-level precision. Indeed, bottom-up approaches have allowed width,2−4 edge structure,5,6 and heteroatom incorporation7−9 to be modulated at precise positions on GNRs. Thus, the next big goal of this approach is to realize the synthesis of edge-functionalized GNRs.10,11 In particular, edgefluorinated GNRs have been predicted to show attractive electronic properties that have yet to be demonstrated experimentally.10−12 The carbon−fluorine (C−F) bond, which has never been previously introduced into GNR precursor molecules, is believed to be one of the strongest bonds in solution chemistry.13 Therefore, GNR precursor molecules fluorine © 2017 American Chemical Society

Received: April 3, 2017 Accepted: May 19, 2017 Published: May 19, 2017 6204

DOI: 10.1021/acsnano.7b02316 ACS Nano 2017, 11, 6204−6210

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HFHs and GNRs displayed lengths above 10 nm, indicating that the trimeric structure of HFH-DBTA did not affect the diffusion of molecules on Au(111) during polymerization. In fact, single-crystal X-ray crystallographic analysis (Figure S2) indicated that HFH-DBTA has a cross-shaped structure essential for efficient radical step-growth polymerization on Au(111).1 Further annealing at 400 °C induced the formation of planar structures with a width of ∼1.5−1.6 nm (Figure 1E), suggesting the formation of GNRs by intramolecular cyclodehydrogenation of poly-HFHs. In contrast to annealing at 350 °C, annealing at 400 °C led to the complete disappearance of the F 1s signal of C−F bonds (Figure 2B), suggesting that these bonds dissociate during cyclodehydrogenation to form defluorinated GNRs (H-7-GNRs) (Figure 1E), as confirmed by the absence of the signal of F atoms (682 eV) adsorbed on Au(111). The characteristic edge topology of H-7-GNRs (Figure 1E) was revealed by high-resolution STM imaging, with the 0.44 nm average periodicity of this topographic feature being well correlated with the estimated separation (∼0.43 nm) between H-7-GNR edge positions. Notably, density functional theory (DFT) simulations show that the presence/absence of F atoms is difficult to determine by STM imaging due to the insignificant difference between fluorinated and defluorinated GNRs (Figure S3). Thus, cyclodehydrogenation involving defluorination is particularly important for understanding the reaction mechanism of edge-functionalized GNR synthesis, as opposed to that of hydrogen-terminated GNRs.23 Importantly, partially reacted species (H-7-GNR/HFH-5+GNR)23 were clearly observed after annealing at 350 °C (Figure 1D), exhibiting protrusions spaced at a distance of 0.86−0.89 nm, which is in good agreement with the distance between the alternate ends of the upward-pointing anthryl moieties (see structure in Figure 1D). The observed partially reacted chain is reminiscent of the “one-side-domino” cyclodehydrogenation of polyanthrylene (poly-H),23 indicating a similar reaction mechanism for poly-HFHs despite the presence of edge F atoms. Notably, the F 1s XPS spectrum recorded after 10 min annealing at 350 °C still indicated the presence of C−F bonds attributed to unreacted poly-HFHs (Figure 2B), whereas the presence of these bonds in H-7GNR/HFH-5+-GNR was still not clear at this stage. Prolonged annealing (3 h) of poly-HFHs at 350 °C led to complete cyclodehydrogenation and F atom loss (Figure S4), as confirmed by XPS analysis (Figure 2B). Defluorination requires the dissociation of the C6H5−F bond (532 kJ/mol), which is much stronger than the C6H5−Br bond (351 kJ/mol).24 The unambiguous XPS signal of C−F bonds observed after annealing at 350 °C and the uniform H-7-GNR width in large-scale STM images (Figure 1E) clearly indicate differences in F/Br atom reactivity, which result in the selective formation of poly-HFHs. On the other hand, considering the C6H5−H bond dissociation energy (472 kJ/mol) required for dehydrogenation,24 edge C−F bonds in poly-HFHs can survive during cyclodehydrogenation. In fact, the chlorinated analog (DCBA) of DBBA (Figure S1) underwent cyclodehydrogenation before dechlorination,17 despite the C6H5−Cl bond dissociation energy (406 kJ/mol) being smaller than that of the C6H5−H. Therefore, an understanding of the specific F atom reactivity during on-surface synthesis would provide an important clue to creating tailored GNRs and other attractive carbon nanostructures in a controlled manner.25

molecules is expected to be more complex, with a deep mechanistic understanding of precursor molecule transformations based on experimental analyses and theoretical investigations expected to provide critical clues to the corresponding syntheses. Herein, we focus on understanding the effect of fluorine substitution on the on-surface reaction to realize edgefluorinated GNRs, using an anthracene trimer comprising a partially fluorinated central unit (HFH-DBTA) as a precursor molecule (Scheme 1). Initially, debromination-induced polymerization is expected, as seen for the well-studied 10,10′dibromo-9,9′-bianthracene (DBBA, see structure in Figure S1).1 Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were used to obtain detailed experimental evidence of the reactivity of HFH-DBTA on Au(111), which was combined with theoretical investigation results to understand the reaction mechanism. Scheme 1. Synthesis of HFH-DBTAa

a Reaction conditions: (a) (1) n-BuLi, TMEDA, THF, −50 °C to room temperature; (2) NaI, NaH2PO2·H2O, AcOH, reflux. (b) NBS, CHCl3/acetonitrile, reflux. TMEDA = Tetramethylethylenediamine. NBS = N-Bromosuccinimide.

RESULTS AND DISCUSSION Details of HFH-DBTA synthesis are provided in the Supporting Information.20 The experimental reactivity investigation began with the deposition of HFH-DBTA on Au(111) at room temperature under ultrahigh vacuum (UHV), with structural information obtained by STM and XPS analyses after 10 min annealing at several temperatures under UHV. Annealing at 350 °C for 10 min induced an on-surface reaction of HFH-DBTA (Figure 1A), leading to the initial formation of bright aligned periodic protrusions (Figure 1C). Line profile analysis determined the average height of these protrusions as ∼0.4 nm (Figure 1B), implying the formation of partially fluorinated polyanthrylenes (poly-HFHs), since this height could be attributed to the out-of-plane conformation of their anthryl moieties on Au(111). To shed light on molecular composition, XPS analysis was used for complementary characterization, with the Br 3d spectrum shown in Figure 2A. In this spectrum, no signal of Br atoms linked to anthracene units was detected at ∼70 eV.21 Moreover, after debromination of HFH-DBTA, the Br atoms appeared to be desorbed from Au(111), since no signal of adsorbed Br atoms at ∼68 eV21 was detected. In contrast, the signal at 688 eV in the F 1s spectrum clearly indicated the presence of C−F bonds (Figure 2B),22 confirming the formation of poly-HFHs (Figure 1C), which are important building blocks for edge-fluorinated GNRs. Moreover, the ribbon-like structure with an average height of ∼0.2 nm was assigned to GNRs (Figure 1A,B), indicating that they were already formed after annealing at 350 °C via the cyclodehydrogenation of poly-HFHs. The obtained poly6205

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Figure 1. (A) STM topographic images of HFH-DBTA deposited Au(111) surface after annealing at 350 °C for 10 min (sample voltage Vs = −1.2 V, tunneling current It = 100 pA). (B) Line profile analysis of HFH-DBTA deposited Au(111) surface after annealing at 350 °C for 10 min, performed along the blue arrow in (A). (C) Enlarged STM image of poly-HFHs in (A) (Vs = −1.2 V, It = 100 pA) overlaid with the corresponding molecular model. (D) High-magnification STM topographic image of H-7-GNR/HFH-5+-GNR in (A) (Vs = −1.6 V, It = 60 pA). (E) STM topographic image of Au(111) surface after annealing at 400 °C for 10 min (Vs = −1.7 V, It = 20 pA). Inset shows a highresolution image of H-7-GNR in (E) (Vs = −0.8 V, It = 80 pA). Proposed structures of poly-HFH, H-7-GNR/HFH-5+-GNR, and H-7-GNR are shown in (C−E), respectively.

Figure 2. (A) Br 3 d XPS spectrum of HFH-DBTA deposited Au(111) surface after annealing at 350 °C for 10 min. (B) F 1s XPS spectra of HFH-DBTA deposited Au(111) surface after annealing at several temperatures, with the XPS spectrum of Au(111) displayed as a reference (black). Intensities were normalized with respect to the height of the Au 4p peak. 6206

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Figure 3. Optimized geometries of (A) poly-HFH and (B) intermediate states on Au(111), with enlarged images shown below. Gray, pale pink, blue, and orange spheres represent C, H, F, and Au atoms, respectively. The H atom discussed in the text is represented by a yellow sphere. (C) and (D) Minimum energy paths for I0 → I1 and I1 → I2 transformations, respectively. Insets represent highest-energy structures of the transition state. Horizontal axes denote migration distances of (C) the H atom (yellow sphere) and (D) the F atom (blue dotted circle). Zero distances correspond to (B) I0 and (C) I1 states.

Waals interactions with Au(111) flatten poly-HFHs, creating an additional C−C bond between two neighboring anthracene units. Subsequently, the catalytic Au(111) surface cleaves the downward-pointing H atom to form the I0 state (Figure 3B).23 The following simulated reaction indicates that the outwardpointing H atom (yellow sphere in the I0 state) migrates to the neighboring C atom at the poly-HFH edge. During this step, the edge F atom of poly-HFH (blue dotted circle in the I0 state) is pushed downward (I0 → I1). Consequently, the edge C atom (red arrow in the I1 state) transiently becomes part of an H−C−F structure (I1 state). Finally, the downwardpointing F atom (blue dotted circle in the I1 state) is catalytically detached from poly-HFH and adsorbed at the fcc hollow site of Au(111) (I1 → I2).

For a deeper understanding of the reaction mechanism involving C−F bond dissociation, DFT calculations were performed using the generalized gradient approximationPerdew−Burke−Ernzerhof (GGA-PBE) functional implemented in the OpenMX code (see Methods).26 Figure 3A shows the most stable optimized geometries of poly-HFHs (P1 state) on Au(111), with the distance between the edge H atom and the topmost Au atom equaling 0.28 nm for poly-HFHs and poly-Hs (Figure S5), whereas the corresponding distance for F atoms in poly-HFHs equals 0.32 nm. Thus, poly-HFHs and poly-Hs are expected to exhibit the same geometry on Au(111). The one-side-domino cyclodehydrogenation of poly-HFHs is expected to proceed from their P1 state to the intermediate state (I0), similarly to the case of poly-Hs. Briefly, van der 6207

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ACS Nano The activation energies (Ea) of I0 → I1 and I1 → I2 reactions were estimated as ∼80 and ∼40 kJ/mol, respectively (Figure 3C,D), being surprisingly small for reactions involving C−F bond dissociation in view of the high C−F bond dissociation energy (532 kJ/mol). Further DFT calculations indicated the importance of the intermediate H−C−F structure (Figure S6), which lowers the Ea of C−F bond dissociation, decreasing it by ∼400 kJ/mol from the value of 538 kJ/mol observed for C−F bond dissociation from poly-HFH. Moreover, the catalytic effect of Au(111) enables a further decrease of ∼100 kJ/mol, leading to an overall small Ea of ∼40 kJ/mol. Notably, considering the Ea of I0 → I1 reaction (∼80 kJ/mol), H−C−F structure is likely to be formed from I0 state, with DFT calculations performed for C−F bond dissociation in fluorobenzene also supporting the importance of H−C−F structure formation for lowering the Ea of C−F bond dissociation (Figure S6). Arrhenius plots additionally indicate that the observed rate constants (k) of these reactions have reasonable values (Table S1). Several other on-surface reaction paths are possible (Figure S7), e.g., F atom detachment from poly-HFH (P1 → P2) or HFH-7-GNR (R1 → R2). However, the corresponding Ea and k values are not reasonable, implying that these reactions are much less favorable than the I0 → I2 transformation via I1 (Table S1). The performed simulations model only representative reactions, with the actual reaction mechanism being more complex due to several side reactions. Nevertheless, C−F bond dissociation is likely to proceed as proposed above (Scheme 2).

and the discovery of the critical intermediate state greatly contribute to the synthesis of a wide range of essential compounds and reaction yield improvement. Currently, the observed simple but interesting surface-assisted reactions26−28 indicate that on-surface reactions will become more important in the future as an indispensable way to create attractive nanostructures. Thus, an understanding of the C−F bond dissociation mechanism enables the proper design of precursor molecules for edge-functionalized GNRs.

METHODS Single-Crystal X-ray Structure. Pale yellow crystals of HFHDBTA suitable for X-ray diffraction were obtained by slow evaporation of 2-propanol vapor into a CH2Cl2 solution of HFH-DBTA (Figure S2). CCDC 1550562 contains the crystallographic data of HFHDBTA. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://summary.ccdc.cam.ac.uk/ structure summary form. Surface-Assisted Syntheses. Surface-assisted syntheses were carried out under UHV at a base pressure of 2 × 10−8 Pa. The surfaces Au(111)/mica substrates (Phasis, Switzerland) were cleaned by repeated cycles of 0.8 kV Ar+ sputtering and annealing at 450 °C. HFH-DBTA molecules were sublimed from a Ta crucible held at 350 °C onto the Au(111) surface held at room temperature, with the sample coverage controlled to be below one monolayer. After deposition, samples were postannealed between 350 and 400 °C for 10 min or 3 h. STM Characterization. STM (UNISOKU USM1400S-4P) characterization was carried out at a base pressure of 7 × 10−9 Pa at 77.6 K, with topographic images recorded in constant-current mode using electrochemically etched W tips. Typically, Vs between −1.8 and −0.8 V and It of 20−500 pA were used. STM data were processed using the SPIP software, with moderate filtering applied for noise reduction. XPS Characterization. XPS experiments (Kratos AXIS-HSi) were performed at 3 × 10−7 Pa at room temperature using monochromatized Al Kα line (hν = 1486.6 eV) radiation. Energy calibration was performed using the Au Fermi edge in each sample. The energy resolution was set to 550 meV for all spectra. Br 3d and F 1s spectra were normalized relative to Au 5p3/2 and Au 4p1/2 peak intensities, respectively. Computational Details. All calculations were performed using the OpenMX DFT code.29 The exchange−correlation potential was treated within the GGA using the PBE functional.30 Electron−ion interactions were described by norm-conserving pseudopotentials with partial core correction.31,32 Pseudoatomic orbitals (PAOs) centered on atomic sites were used as the basis function set,33 with the corresponding basis functions specified by C6.0-s2p2d1, F7.0s2p2d1, H5.0-s2p1, and Au7.0-s2p2d2f1. For example, C6.0-s2p2d1 indicates the PAO of the carbon atom with a cutoff radius of 6.0 Bohr and two s, two p, and one d components. van der Waals corrections were included by using a semiempirical DFT-D2 method.34 The STM image in Figure S3 was obtained using the Tersoff− Hamman scheme.35 Partial charge density was calculated in an energy window of −1 eV measured from the chemical potential and was visualized for an isovalue of 0.0001 Bohr−3 employing WSxM software.36 Figure 3A shows a structural model of poly-HFH on the Au(111) surface (P1 state), which was simulated using a repeated slab model comprising poly-HFH, three Au(111) layers, and a vacuum layer over 10 Å. The geometries of intermediate states on Au(111) were investigated using similar models, being optimized starting from the same lateral configuration of HFH-7-GNR on Au(111), where one carbon atom is situated above the topmost Au atom, and the other one is on a bridging site.37 The unit cell included two HFH moieties on a (9 × 4√3) Au(111) surface. Commensurability was imposed between the GNR and Au(111) lateral periodicities, resulting in a 3.3% compression of the Au(111) lateral lattice parameter from the

Scheme 2. Proposed Mechanism of C−F Bond Dissociation

CONCLUSIONS We investigated the mechanism of surface-assisted GNR synthesis involving C−F bond dissociation. Comprehensive STM and XPS analyses confirmed halogen atom reactivity, revealing that HFH-DBTA could be converted to GNRs, with the corresponding reaction mechanism being similar to that observed for DBBA. Theoretical calculations revealed the importance of the transient H−C−F structure for significantly lowering the activation energy of C−F bond dissociation. Finally, the synthetic aspect of this work must be emphasized. Although edge-fluorinated GNRs are predicted to exhibit attractive properties,10−12 the corresponding synthetic approach is poorly developed compared to that for standard Hterminated GNRs.1 Thus, our study strongly suggests the importance of understanding the reaction mechanism and transient structure role in the intermediate state. From a solution chemistry perspective, the deep investigation of the reaction mechanism by experimental and theoretical methods 6208

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equilibrium value to match that of GNRs (4.279 Å). Herein, only the Au(111) surface was deformed to focus on the poly-HFH to HFH-7GNR structural transformation. Spurious electrostatic interactions between periodic images in slab calculations were eliminated using an effective screening medium method.38,39 Geometries were optimized under a three-dimensional periodic boundary condition, allowing all atoms to relax in three dimensions. The minimum energy path (Figure 3C,D) was determined using the nudged elastic band (NEB) method.40 Convergence criteria for forces acting on atoms were set to 0.01 and 0.1 eV/Å for geometry optimization and NEB calculation, respectively.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02316. Additional experimental details and data (PDF) Crystallographic data (CIF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hironobu Hayashi: 0000-0002-7872-3052 Naoki Aratani: 0000-0002-3181-6526 Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was partly supported by CREST JST (no. JPMJCR15F1) and MEXT/JSPS KAKENHI grant nos. JP15K17843, JP26105004, JP26288038, JP15H00876 ‘AnApple’, and JP16H02286, the program for promoting the enhancement of research universities in NAIST supported by MEXT, Izumi Science and Technology Foundation, Kansai Research Foundation for Technology Promotion (KRF), The Kyoto Technoscience Center, and TEPCO Memorial Foundation. We thank Dr. A. Ando of the National Institute of Advanced Industrial Science and Technology for experimental support. We also thank Ms. Y. Nishikawa and Prof. L. McDowell, NAIST, for mass spectroscopy measurements and help with manuscript preparation, respectively. We would like to thank Editage for English language editing. REFERENCES (1) Talirz, L.; Ruffieux, P.; Fasel, R. On-Surface Synthesis of Atomically Precise Graphene Nanoribbons. Adv. Mater. 2016, 28, 6222−6231. (2) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.; Müllen, K.; Fasel, R. Atomically Precise Bottom-Up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470−473. (3) Zhang, H.; Lin, H.; Sun, K.; Chen, L.; Zagranyarski, Y.; Aghdassi, N.; Duhm, S.; Li, Q.; Zhong, D.; Li, Y.; Müllen, K.; Fuchs, H.; Chi, L. On-Surface Synthesis of Rylene-Type Graphene Nanoribbons. J. Am. Chem. Soc. 2015, 137, 4022−4025. (4) Chen, Y.-C.; de Oteyza, D. G.; Pedramrazi, Z.; Chen, C.; Fischer, F. R.; Crommie, M. F. Tuning the Band Gap of Graphene 6209

DOI: 10.1021/acsnano.7b02316 ACS Nano 2017, 11, 6204−6210

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DOI: 10.1021/acsnano.7b02316 ACS Nano 2017, 11, 6204−6210