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Orientation and Electronic Structures of Multilayered Graphene Nanoribbons Produced by Two-Zone Chemical Vapor Deposition Takahiro Kojima,*,† Yang Bao,‡ Chun Zhang,‡,§ Shuanglong Liu,‡,⊥ Hai Xu,‡ Takahiro Nakae,† Kian Ping Loh,*,‡,∥ and Hiroshi Sakaguchi*,† †

Institute of Advanced Energy Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 § Department of Physics and Graphene Research Centre, National University of Singapore, 2 Science Drive 3, Singapore 117542 ∥ Centre for Advanced 2D Materials (CA2DM) and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546 ‡

S Supporting Information *

ABSTRACT: The orientation and electronic structure of multilayered graphene nanoribbons with an armchair-edge (AGNRs) were determined by low-temperature scanning tunneling microscopy in this study. The orientation of AGNRs was found to be an edge-on structure when positioned as a top layer, while previous reports showed a face-on structure for monolayered AGNRs on Au(111). According to density functional theory calculations, AGNRs in a top layer preferentially form as edge-on structures rather than face-on structures due to the balance of CH−π and π−π interactions between AGNRs. Scanning tunneling spectroscopy and density functional theory calculations revealed that the electronic structures of multilayered AGNRs are similar to those in a gas-phase due to the lack of interaction between AGNRs and the Au(111) substrate. The observation of AGNRs in mutilayers might suggest the conformation-assisted mechanism of dehydrogenation when there is no contact with the Au(111) substrate.

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INTRODUCTION Although graphene has excellent electronic properties, the lack of an electronic band gap limits its application for electronics and optoelectronics.1,2 In contrast, graphene nanoribbons (GNRs), which are one-dimensional carbon materials, have attracted much attention because of opened band gaps that exhibit semiconducting properties.3,4 The properties of GNRs strongly depend on their widths and edge-structures. A large enough band gap for semiconducting applications requires a width less than 10 nm.5 Top-down approaches of GNR synthesis, such as lithographical patterning of graphene,6−8 cannot produce GNRs having widths less than 10 nm because of the limitation in electron beam focusing. Therefore, a bottom-up synthetic method is required for widths less than 10 nm because it enables precise control of the width and edge structure by regulating the molecular precursors. On-surface synthesis techniques such as ultrahigh vacuum (UHV) deposition 9−18 and chemical vapor deposition (CVD)19−21 have attracted much attention as bottom-up methods. A 7-AGNR (the number before “AGNR” indicates its width, that is, the number of carbon atoms in the direction perpendicular to the long axis) was synthesized using 10,10′dibromo-9,9′-bianthryl (DBBA) as a precursor by a UHV deposition method.9 Since then, various types of bottom-up © XXXX American Chemical Society

synthesized AGNRs with different widths have been reported. In the UHV deposition method, biradicals of precursors were generated by dehalogenation reactions on the heated metal surface, resulting in the formation of prepolymers. Subsequently, dehydrogenation reactions of prepolymers yielded GNRs by thermal annealing. Previous reports showed the orientation of the AGNR monolayer fabricated on Au(111) to be a face-on structure.9−14,17 Experiments on the band gaps of monolayered GNRs reported so far indicate wider band gaps than predicted by density functional theory (DTF) calculations for a gas-phase due to the orbital hybridization between GNRs and the Au substrate. 12,13,17,22 The mechanism of a dehydrogenation reaction of the monolayered 7-AGNR on Au(111) has been believed to be catalytic, in which the hydrogens of prepolymers were eliminated by the Au surface to form GNRs.23,24 However, it is still unclear whether a catalytic metal surface is essential to the dehydrogenation. Recently, we have reported a new method of bottom-up GNR synthesis, called two-zone chemical vapor deposition (2Z-CVD), which successfully produced a series of multilayered Received: June 3, 2017 Revised: August 15, 2017

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DOI: 10.1021/acs.langmuir.7b01862 Langmuir XXXX, XXX, XXX−XXX

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Langmuir AGNR,19 and cove-edged (acene) GNRs.20 The schematic presentation of the 2Z-CVD system is depicted in Figure 1a.

mantle heater for evaporating the precursor monomer. The quartz tube was cleaned by annealing at 1000 °C for 20 min for removing impurities, which deactivates radicals. The precursor (1 mg or 10 μg, prepared by a casting solution) on a quartz boat and the Au(111)deposited mica substrates were placed in the quartz tube. Ar gas was fed into the quartz tube at a flow rate of 500 sccm, resulting in a vacuum of 1 Torr. GNRs were synthesized in two stages. In the first stage, the temperatures of zone 1 and zone 2 in the quartz tube were set to 350 and 250 °C, respectively, with the subsequent evaporation of the precursors by heating to a proper temperature to sublimation using the mantle heater. In the second stage, the temperature of zone 2 was increased to 400 °C for converting the prepolymers into GNRs by dehydrogenation. After the CVD growth, the samples were cooled down from 400 °C to rt for 10 min in an Ar atmosphere. Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) Measurements. STM measurements were performed using commercial instruments (LT-STM, Omicron Nano Technology). After the CVD growth, all the samples were taken out of the CVD system in air, and then transferred to the LT-STM chamber. STM images and STS data were taken in the constant current mode under ultrahigh vacuum conditions (∼7.5 × 10−8 Torr) at 77 K. Electrochemically etched W chain was used as the tip after removal of oxide by electrical heating.

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RESULTS AND DISCUSSION Synthesis of Multilayered 5-AGNRs and 7-AGNRs by 2Z-CVD. By using the 2Z-CVD method, 5-AGNRs were prepared on a Au(111)-deposited substrate from 1 mg of 3,9and 3,10-dibromoperylene mixture (DBP), while 7-AGNRs were prepared from 1 mg of 10,10′-dibromo-9,9′-bianthryl (DBBA). Obtained samples were measured by LT-STM at 77 K. The STM images of these samples showed the multilayered chains evident in Figure 1b and c. The chains with nodes shown in Figure 1b were epitaxially arranged along the threefold axis of the Au substrate, and the chains with nodes shown in Figure 1c were partially arranged in parallel. The histograms of chainlength distribution at the top layer are shown in Figure S1. The maximum lengths of 5-AGNR and 7-AGNR were found to be 11.5 and 12.7 nm, respectively. Structural Analysis of Multilayered 5-AGNR. The upper layer of multilayered 5-AGNR is evident as bright chains in the STM image of Figure 1b, while the underlayer appears as darker chains. The high resolution image of chains in the underlayer is shown in Figure 2a. The periodicity and width of chains in the underlayer were analyzed as 4.2 and 8.6 Å, respectively (Figure 2b and c), which were in a good agreement with those of the structural model of 5-AGNR (Figure 2d). Thus, the chains in the underlayer could be assigned to 5AGNRs with a face-on structure, similar to the previous reports.13,14 The high resolution image of chains in the top layer is shown in Figure 2e, where each chain in the top layer is numbered as chain 1, 2, and 3. The chains 1, 2, and 3 have nodes with a periodicity of 4.2 Å (Figure 2f), corresponding to that of dot structures at the edge of 5-AGNR in the underlayer (Figure 2b). The height of chain 1 at 4.5 Å (Figure 2g) was remarkably longer than 1.8 Å, which corresponds to the previous experimental thickness of GNRs measured by STM.9 However, this is shorter than the width of 8.6 Å for 5-AGNR in the model (Figure 2d). In general, the topographic height is inconsistent with that of the object as viewed in the STM, because the topographic height measured by constant-current STM is influenced by the barrier height originating from the electronic structure of substances25,26 From these facts, the chains

Figure 1. (a) Experimental setup of 2Z-CVD with an illustration of the GNR growth mechanism through polymerization from DBP to form multilayered prepolymers and 5-AGNRs. (b) STM image of 5-AGNR obtained from 1 mg of DBP (It = 10 pA, Vs = −2.0 V). (c) STM image of 7-AGNR obtained from 1 mg of DBBA (It = 20 pA, Vs = −2.0 V).

The attractive feature of this method originates from an independent temperature control of the radical generation process and the GNR growth process. Concentrated biradicals generated by the collision of the precursors with a hot wall in zone 1 are supplied to a Au(111) substrate placed in zone 2, resulting in a highly efficient formation of prepolymers, followed by a high yield of AGNRs despite the low vacuum condition of 1 Torr. However, the orientation and electronic structures of multilayered AGNRs are still unknown. Therefore, we report here that the orientation and electronic structures of multilayered AGNRs produced by 2Z-CVD were revealed by lowtemperature scanning tunneling microscopy (LT-STM). Additionally, we discuss the mechanism of dehydrogenation in multilayered AGNRs, comparing with that of monolayered GNRs on Au(111).

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EXPERIMENT AND METHODS

Two-Zone Chemical Vapor Deposition (2Z-CVD). The 2ZCVD system consisted of a quartz tube (ϕ 26 mm diameter, 86 cm length) as the reactor; a rotary pump which can evacuate the system to less than 7 × 10−4 Torr; a two-zone electric furnace with a temperature controller; an Ar gas flow system with a mass-flow controller; and a B


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Figure 2. (a) STM images of 5-AGNR in the underlayer in Figure 1b (It = 10 pA, Vs = −2.0 V). (b) Cross-sectional analysis of the blue dotted line in (a). (c) Cross-sectional analysis of the red dotted line in (a). (d) Periodicity and width of the 5-AGNR model. (e) STM image containing top layer and underlayer (It = 10 pA, Vs = −2.0 V). (f) Cross-sectional analysis of the blue dotted line in (e). (g) Cross-sectional analysis of the red dotted line in (e). (h) Structural model of orientation of 5-AGNRs.

alignment (Figure S2d). These results are summarized in Table S1. Electronic Structure of 5-AGNR. To investigate the electronic structure of multilayered 5-AGNR, scanning tunneling spectroscopy (STS) was performed in the top layer and the underlayer of 5-AGNR. The obtained differential conductance curves are shown in Figure 3. The band gaps of 5-

observed in the STM images could be assigned to the 5AGNRs with an edge-on structure. Additionally, the interchain distance in the STM image (Figure 2g) was measured at 3.5 Å, which is similar to twice the van der Waals radius of carbon at 1.7 Å. Thus, the STM image (Figure 2e) is explained by the structural model, in which a pair of aggregated 5-AGNR chains with an edge-on structure lies on top of those with a face-on structure (Figure 2h). DFT calculations were carried out to clarify the edge-on orientation of 5-AGNR in a top layer. For modeling purposes, these calculations contain four configurations in order to compare energy states. The first model is one in which 5AGNRs with edge-on structures in a top layer lie on GNRs with face-on structures in the underlayer (Figure S2a). The second model represents 5-AGNRs with face-on structures in a top layer lying on the underlayer with face-on structures (Figure S2b). The third model represents 5-AGNRs with face-on structures on Au(111) (Figure S2c). The fourth model describes 5-AGNRs with edge-on structures on Au(111) (Figure S 2d). The result of the calculations shows that the edge-on alignment of GNRs on top layers over face-on structures in the underlayer (Figure S2a) is remarkably less than that of face-on alignment (Figure S2b) by 7.9 kcal mol−1. This result suggests that an edge-on structure is favored for the top layer of GNRs. The reason is considered to be due to the CH-π interaction between the top layer and the underlayer, aided by the lateral π−π interaction between aggregated 5AGNRs with an edge-on structure. In contrast, the calculation indicates that the monolayered GNRs on Au(111) prefer the face-on alignment (Figure S2c) rather than the edge-on

Figure 3. dI/dV curves of 5-AGNR on a top layer (a) and underlayer (b). Insets show the positions of STS measurements marked by a light blue dot, showing the experimental and calculated band gaps.

AGNRs located in a top layer and an underlayer were measured as 0.36 and 0.39 eV, respectively. These values were in a good agreement with 0.36 eV obtained from theoretical calculations. However, the band gap of monolayered 5-AGNR on Au(111) synthesized by an UHV deposition method was reported to be 2.8 ± 0.1 eV,13 larger than the theoretical prediction of 0.4 eV.4 In general, the contact of organic molecules with a metal surface is known to influence the electronic structure of the organic molecules due to the orbital hybridization between the molecules and the metal surface.27,28 The agreement of band C


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Figure 4. (a) STM image of prepolymer on a top layer obtained using 1 mg of DBBA (It = 20 pA, Vs = −2.0 V). Inset shows the space-filling model of prepolymers. (b) Cross-sectional analysis of the dotted line in (a). (c) dI/dV curve of prepolymer on a top layer. Inset shows the positions of STS measurements marked by a light blue dot, showing the experimental band gap. (d) Raman spectrum of the sample obtained using 1 mg of DBBA.

Figure 5. (a) STM image of 7-AGNR obtained from 10 μg of DBBA (It = 20 pA, Vs = −2.9 V). Inset shows the model of 7-AGNR with an edge-on structure. (b) Cross-sectional analysis of the line in (a). (c) Raman spectrum of the sample obtained using 10 μg of DBBA. (d) dI/dV curve of 7AGNR on a top layer. Inset shows the positions of STS measurements marked by a light blue dot, showing the experimental and calculated band gap.

the prepolymer was seen as an alternate dot structure that was measured at the positive bias (+1.0 V), and as the comb structure that was measured at the negative bias (−2.0 V). Our STM image was acquired at the negative bias at −2.0 V. These facts indicate that the chains observed in the STM image are attributed to the prepolymers. Furthermore, a STS measurement was performed to investigate the electronic structure of the prepolymer in a top layer. The band gap of the prepolymer obtained from the differential conductance curve, as shown in Figure 4c, was analyzed as 3.24 eV. According to a previous report, the band gap of monolayered prepolymers on Au(111) obtained from STS was measured to be 3.7 eV.32 The reason for the smaller band gap of the prepolymer in a top layer is ascribed to its lack of contact with Au(111). The obtained STM image indicates that the top layer prepared from 1 mg of DBBA

gaps between the experimental and the theoretical prediction in a gas-phase might originate from the decoupled interaction between GNRs and Au(111). Our assumption is supported by the previous report on STS measurements of bilayer pentacene on Cu(111).29 In addition, the DFT calculations using LDA of AGNRs reported by Louie et al.4 are in good agreement with the experimental results reported by Magda et al.30 and Wang et al.31 These results support our observation on the agreement between the calculations and experimental results on AGNRs. Chemical and Electronic Structure of Top Layer of Multilayered 7-AGNR. The high resolution STM image of the sample prepared with 1 mg of DBBA is shown in Figure 4a. The chain in the top layer was observed as a comb structure having a periodicity of 8.4 Å (Figure 4b). According to a previous report18 on second layered GNRs, the STM image of D


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Au(111) substrate. Thus, the dehydrogenation mechanism of a multilayered system attracts interest. The mechanism of dehydrogenation in multilayered prepolymers is supposed to be a conformation-assisted mechanism, involving the following steps: (1) the prepolymers in multilayers are flattened without contact with the Au substrate, (2) the C−C bond formation of neighboring anthracene units takes place, (3) hydrogens of anthracene units are removed as a hydrogen molecule (Figure S4). Prepolymers in the top layer of the sample that was prepared using 10 μg of DBBA are assumed to be densely packed, as can be seen in the STM image of 7-GNRs with an edge-on structure (Figure 5a). These conformations might be restricted to a shape that can bring the hydrogens in neighboring anthracene units to proximity, resulting in the dehydrogenation reaction. This might be the reason why the dehydrogenation reaction could take place without contacting Au(111). In contrast, the prepolymers in the top layer of the sample that was prepared using 1 mg of DBBA might allow a free conformation, as can be seen in the STM image of Figure 1c. The distance between the hydrogen atoms of neighboring anthracene units in a 1 mg system might be large, resulting in the low efficiency of the dehydrogenation reaction.

was covered by prepolymers (Figure 1c). However, the Raman spectrum of this sample corresponded to that of 7-AGNR, which has peaks at 393 cm−1 attributed to a radial breathing mode (RBM), 1216, 1256, and 1338 cm−1 characteristic of a Dband, and 1596 cm−1 a trait of a G-band (Figure 4d).9 In addition, Raman spectroscopy is a bulk measurement because the size of laser irradiation is on a micron scale. Furthermore, the STM image showed the 7-AGNR in the underlayer (Figure S3a), because the structure was similar to the previous report of monolayers with face-on structures on Au(111).9,11 The periodicity of the chain was analyzed as 4.3 Å (Figure S3b). However, the width of 7-AGNR was analyzed as 10 Å (Figure S3c), which was narrower than that of the previous report of 15 Å,9 suggesting a tilted 7-AGNR with a face-on structure. Our result is supported by the STM simulation that is able to reproduce the STM image (Figure S3d). Orientation and Electronic Structure of Multilayered 7-AGNR. To investigate the underlayer of 7-AGNR, a thin layer on Au(111) was prepared by 2Z-CVD by reducing the amount of DBBA (10 μg). The obtained STM image of the sample, as shown in Figure 5a, represents the narrow chains in a top layer, in remarkable contrast to that prepared from 1 mg of DBBA, showing the prepolymer in the top layer. The width of chains was analyzed to be 3.5 Å (Figure 5b), which is much narrower than that of 12.6 Å for 7-AGNR in the model (Figure S3d). The interchain distance in the STM image was analyzed as 3.7 Å (Figure 5b), which is similar to twice the van der Waals radius of carbon at 1.7 Å. The Raman spectrum of the same sample (Figure 5c) showed the peaks at 393 cm−1 attributed to a radial breathing mode (RBM), 1216, 1256, and 1338 cm−1 characteristic of a D-band, and 1596 cm−1 a trait of a G-band, which is in good agreement with that of 7-AGNR. From these facts, the narrow chains prepared from a 10 μg of DBBA correspond to 7-AGNR with an edge-on structure. The aggregation of chains with edge-on structures was seen in the STM image (Figure 5a). Up to four aggregated chains with edge-on structures lie on the underlayer. The interchain distance was analyzed to be 3.7 Å (Figure 5b), similar to that of 5-AGNR at 3.5 Å (Figure 2g). The aggregated 7-AGNRs are supposed to prefer the edge-on structure because of its stabilization by CH-π interaction from the underlayer, in addition to the π−π interaction between the aggregated 7AGNRs with edge-on structures. The STM image is consistent with the model of orientation of 7-AGNRs, in which aggregated 7-AGNRs with edge-on structures lie on top of those with faceon structures (Figure 5a, inset). To investigate the electronic structure of multilayered 7AGNR, STS studies were performed. The band gap of multilayered 7-AGNR is analyzed as 1.42 eV, which is in a good agreement with that of 1.58 eV calculated by DFT in a gas-phase (Figure 5d). From this fact, the electronic structure of multilayered 7-AGNRs is considered similar to that in a gasphase, which ascribes to the decoupled interaction with Au(111) as seen in 5-AGNR. The mechanism of a dehydrogenation reaction of the monolayered 7-AGNR on Au(111) has been believed to be catalytic, in which the hydrogens of prepolymers were eliminated by the Au surface to form GNRs.9,23 In contrast, no dehydrogenation reaction has taken place in the gas-phase up to temperatures of 1500 K.33 Moreover, prepolymers from DBBA on TiO2 were unable to be converted to 7-AGNRs by thermal annealing at 450 °C.34 Compared with these reports, our system is based on the multilayer not contacting the

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CONCLUSION

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ASSOCIATED CONTENT

In conclusion, we examined the orientation and electronic structure of multilayered 5-AGNR and 7-AGNR synthesized by 2Z-CVD using LT-STM. Previous reports showed the orientation of AGNR monolayers on Au(111) to be a face-on structure. In remarkable contrast, our work finds that the orientation of AGNRs to be edge-on structures in the top layer, whereas those in the underlayer are face-on structures. The DFT calculations of the orientation of AGNRs in a top layer indicates a preference for an edge-on structure rather than a face-on structure due to the balance of CH−π and π−π interactions between AGNRs. The band gaps of multilayered GNRs were in good agreement with the DFT calculations in a gas-phase due to a lack of orbital hybridization between GNRs and Au(111) substrates. Additionally, the observation of AGNRs in mutilayers might suggest the presence of a conformation-assisted mechanism of dehydrogenation when there is no contact with the Au(111) substrate. This result implies that GNRs can be produced not only on metal but also on other substrates such as insulators. Our investigations of the dehydrogenation reaction of prepolymers in a multilayer system show that it is possible to carry out efficient on-surface synthesis without a metal substrate. We believe our findings can be applied to future electronic and optoelectronic devices made of GNRs.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01862. Additional experimental details; histograms; illustrations of models of DFT calculations; calculation of orientation 1 energies of 5-AGNR; Structural analysis of 7-AGNR; illustrations of proposed and reported dehydrogenation mechanisms (PDF) E


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AUTHOR INFORMATION

Corresponding Authors

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

Yang Bao: 0000-0001-9868-4946 Takahiro Nakae: 0000-0002-7653-432X Kian Ping Loh: 0000-0002-1491-743X Present Address ⊥

S.L.: Department of Physics, University of Florida, 2001 Museum Rd., Gainesville, FL 32603, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by JSPS KAKENHI Grant Numbers JP17K14471, JP16K13608, JP16H00967, JP15H00995, and JP16K17893; “Zero-Emission Energy Research” of IAE, Kyoto University, John Mung Program of Kyoto University, Super Computer System, Institute for Chemical Research, Kyoto University was used for the calculation. K. P. L. acknowledges funding support from National Research Foundation CRP funded project "Two dimensional covalent organic framework: synthesis and applications, award number NRF-CRP16-201502".

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