Phenyl Functionalization of Atomically Precise Graphene Nanoribbons

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Phenyl Functionalization of Atomically Precise Graphene Nanoribbons for Engineering Inter-Ribbon Interactions and Graphene Nanopores Mikhail Shekhirev, Percy Zahl, and Alexander Sinitskii ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04489 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Phenyl Functionalization of Atomically Precise Graphene Nanoribbons for Engineering InterRibbon Interactions and Graphene Nanopores Mikhail Shekhirev,1 Percy Zahl,2 Alexander Sinitskii1,3* 1

2

Department of Chemistry, University of Nebraska – Lincoln, Lincoln, NE 68588, USA

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA

3

Nebraska Center for Materials and Nanoscience, University of Nebraska – Lincoln, Lincoln, NE 68588, USA *E-mail: [email protected]

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Abstract Graphene nanoribbons (GNRs) attract a great attention from researchers due to their tunable physical properties and potential for becoming nanoscale building blocks of electronic devices. GNRs can be synthesized with atomic precision by on-surface approaches from specially designed molecular precursors. While a considerable number of ribbons with very diverse structures and properties have been demonstrated in recent years, there have been only limited examples of on-surface synthesized GNRs modified with functional groups. In this study, we designed a nanoribbon, in which the chevron GNR backbone is decorated with phenyl functionalities, and demonstrate the on-surface synthesis of these GNRs on Au(111). We show that the phenyl modification affects the assembly of the GNR polymer precursors through π-π interactions. Scanning tunneling spectroscopy of the modified GNRs on Au(111) revealed that they have a band gap of 2.50±0.02 eV, which is comparable to that of the parent chevron GNR. The phenyl functionalization leads to a shift of the band edges to lower energies, suggesting that it could be a useful tool for the GNR band structure engineering. We also investigated lateral fusion of the phenyl-modified GNRs and demonstrate that it could be used to engineer different kinds of atomically precise graphene nanopores. A similar functionalization approach could be potentially applied to other GNRs to affect their on-surface assembly, modify their electronic properties, and realize graphene nanopores with a variety of structures.

Keywords: graphene nanoribbons, bottom-up synthesis, functionalization, electronic structure, self-assembly, graphene nanopore


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Because of their highly tunable electronic properties,1-3 graphene nanoribbons (GNRs) are often considered as potential building blocks of electronic devices and circuits. Following the pioneering publication by Cai et al.,4 a variety of different GNRs with atomically precise structures have been synthesized by the on-surface approach. The specially designed molecular precursors are first deposited in ultrahigh vacuum (UHV) on a metal substrate, such as Au(111), Ag(111) or Cu(111), on which they polymerize and eventually planarize upon annealing to form GNRs. Using the on-surface approach, GNRs with different structures and properties, including ribbons with armchair,4-14 zigzag,15 cove-type16 and chiral edges,17-20 heteroatom substitutions2125

and GNR heterojunctions,25-30 have been produced. With the development of synthetic approaches and growing portfolio of atomically

precise GNRs with very diverse properties, a question of how these ribbons could be assembled into ordered structures for integration into electronic devices becomes increasingly important. A step in this direction is to study how GNRs could be grown in an ordered fashion as opposed to randomly on a substrate. The GNR growth can be affected by the choice of a substrate as the ribbons can form along terraces of a stepped surface, such as Au(788),31,

32

or

crystallographic directions of Cu(111).33 Some alignment was observed for heteroatom-doped GNRs23, 25 and also the polymer stage of the on-surface synthesis of nanoribbons due to π-π interactions,9 which is illustrated by Figure 1a,b for the chevron GNR. This is one of the most studied atomically precise GNRs, which has been synthesized from 6,11-dibromo-1,2,3,4tetraphenyltriphenylene (molecule 1) both on metal substrates,4, 23, 25, 31, 33-35 and in solution.36 On Au(111), molecule 1 forms polymer 2 at about 250 °C, which then undergoes intramolecular cyclodehydrogenation at about 440 °C producing chevron GNRs (cGNRs, 3); see Figure 1a. As


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we discuss in a recent study,9 at the stage 2, the titled phenyls may engage in π-π interactions, resulting in the alignment of the adjacent polymer chains (Figure 1b).

Figure 1. On-surface synthesis of cGNRs and mGNRs. (a) Scheme of the on-surface synthesis of cGNRs on Au(111); see text for details. (b) Molecular model of neighbouring polymer 2


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chains. The region where the titled phenyls engage in π-π interactions is highlighted by red. (c) Scheme of the on-surface synthesis of mGNRs on Au(111); see text for details. (d) Molecular model of neighbouring polymer 5 chains. The region of anticipated π-π interactions is highlighted by red.

Intrigued by these results, in this study we investigated whether the π-π interactions could be purposefully engineered by modification of the cGNR structure. We propose that additional phenyls in the GNR precursor 4, which are highlighted by beige in Figure 1c, should extend the “tentacles” in the GNR polymer 5 that engage in π-π interactions, as shown in Figure 1d, which would result in stronger alignment of the polymer chains. The structure of the phenyl-modified chevron GNR 6, which we will refer to as mGNR, is shown in Figure 1c. While there have been a number of studies, in which GNRs were modified by incorporation of heteroatoms, such as nitrogen, boron and sulfur, within the graphitic core structure of a ribbon,21-25, 37, 38 there have been only limited examples of on-surface synthesized GNRs modified with functional groups.14, 28, 39 Therefore, the growth and characterization of mGNRs expands the repertoire of functionalization approaches in the on-surface synthesis of atomically precise nanoribbons. The laterally extended structure of mGNRs and their propensity for alignment is also interesting for the synthesis of graphene nanopores. Graphene nanopores have recently attracted attention due to their potential for electronic, sequencing and separation applications.39 It was demonstrated that they can be synthesized with atomic precision by lateral fusion of chevron GNRs9 or periodic GNRs comprising regions that are 7 and 13 carbon atoms wide.39 As we demonstrate in this study, the phenyl modification of cGNRs enables two scenarios of their


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lateral fusion, resulting in different kinds of atomically precise graphene nanopores. A similar approach could be potentially used to realize nanopores with other structures and properties.

Results and Discussion

Figure 2. Synthesis of the mGNR monomer (molecule 4); see text for details.

The general approach for the synthesis of the chevron GNR precursor 1, which was demonstrated by Saleh et al.,40 proved to be very flexible and convenient for the preparation of other molecular precursors for a variety of related nanoribbons. First synthesized is compound 11 (Figure 2),40, 41 which can then be used for Diels-Alder reactions with various 1,2-di-substitutedethynes to yield precursors for cGNRs,4, 36, 40 laterally extended chevron GNRs,29, 30, 42, 43 nitrogen-doped chevron GNRs,23, 25, 34, 38, 43 and functionalized chevron GNRs.28 In this work, we synthesized compound 11 using the published procedure,40 and refluxed it with 1,2-di([1,1'biphenyl]-4-yl)ethyne (compound 10) in diphenyl ether to produce the mGNR precursor (2,3di([1,1'-biphenyl]-4-yl)-6,11-dibromo-1,4-diphenyltriphenylene; compound 4). The detailed synthetic procedures for compound 10 and monomer 4 are described in Supporting Information.


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Thus, this work provides another example of versatility of this general chemical approach for producing molecular precursors for nanoribbons from the chevron GNR family. The molecules 4 were sublimed onto cold (4.5 K) Au(111) substrate in UHV conditions, which was then heated up to 250 °C to form polymer 5. All samples were imaged using scanning tunneling microscopy (STM) at 4.5 K.

Figure 3. STM characterization of polymer 5 on Au(111). (a, b) STM images of polymer 5 chains formed on Au(111) at low surface coverage. The inset shows fragment of the polymer chain indicated by the top arrow; some of the titled benzene rings are highlighted by green. Panel (b) shows two adjacent polymer chains that are indicated by the bottom arrow in (a). Scan parameters: 0.2 V, 20 pA. (c) Height profile along the line in (b). Blue part of the curve


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represents the area of a single polymer with extra phenyls lying flat on the surface. The red part of the curve represents the area in between two adjacent polymers, where the extra phenyl groups are rotated perpendicularly to the surface. (d) Molecular model of two polymer chains shown in (b,c). (e) STM image of polymer 5 chains formed on Au(111) at high surface coverage. Scan parameters: -2 V, 7 pA.

We first studied polymer 5 chains on Au(111) at low surface coverage. STM image in Figure 3a shows that the diffusing monomers primarily accumulate at the Au(111) step edges where they form polymer chains. However, the polymer structure could be better seen from an image of an isolated chain like the one shown by the top arrow; a close-up image of this polymer 5 chain is shown in the inset in Figure 3a. Previous studies demonstrated that cGNR polymers 2 on metal substrates have non-uniform heights in STM images: while triphenylene units lay flat on a surface, the phenyl groups are tilted relative to substrate due to intramolecular H-H repulsions (Figure 1b).4, 9, 33, 35 When some phenyls in a cGNR polymer 2 are replaced with biphenyls as in mGNR polymer 5, the benzene rings of the biphenyl groups that are immediately attached to the triphenylene units should still be tilted relative to substrate due to similar intramolecular H-H repulsions. However, the “outer” phenyls that are highlighted by beige in Figure 1c are sufficiently far from each other to lay flat on a gold surface. This structure can be seen in the inset in Figure 3a, in which the titled “inner” benzene rings appear as the bright features (some of these tilted rings are highlighted by green), while the flat “outer” phenyls and triphenylene units are visibly darker and have comparable brightness. In order to investigate the structure of the interacting polymer chains we focused our attention on areas where two adjacent polymers form a two-dimensional assembly. A close-up STM image of one of such areas indicated by the bottom arrow in Figure 3a is shown in Figure


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3b. Figure 3c demonstrates the height profile measured along the line in Figure 3b, which crosses “outer” phenyls of two adjacent polymer chains that are labelled as ‘1’ and ‘2’. The blue segment of the line crosses the “outer” phenyls of chain 1 only, while the red segment crosses such phenyls of both chains. The larger apparent height observed for the red segment indicates that in the region between chains 1 and 2 the phenyls are tilted relative to the surface due to π-π interactions. This arrangement is illustrated by the molecular model in Figure 3d; the regions highlighted by red (π-π interactions between tilted phenyls) and blue (phenyls lying flat on the substrate) correspond to the color-coded segments in Figure 3b,c. These results agree well with previous observations on cGNRs and N=7 armchair GNRs, for which π-π interactions play an important role in the assemblies of monomers and polymer precursors.4, 9 When the precursors are deposited at high coverage, the parallel assembly of the polymers becomes even more evident, as shown by the STM image in Figure 3e. The average length of the polymers was found to be 57 nm, and some of the chains were over 100 nm long. At 440 °C the polymers undergo intramolecular cyclodehydrogenation and form mGNRs as shown in Figure 1c. STM image of mGNRs on Au(111) is shown in Figure 4a. Unlike polymer 5 chains that exhibit brighter regions in STM images due to the tilted benzene rings (Figure 3a), the mGNRs look uniform, indicating their complete planarization. The structure of mGNRs was also visualized by non-contact atomic force microscopy (nc-AFM) using a CuO terminated tip.44 The nc-AFM image in Figure 4b reveals the perfect match of mGNRs with the molecular structure in Figure 1c. Noteworthy, in polymer 5 chains the “inner” phenyls in the biphenyl groups are tilted relative to the triphenylene units, which allows the “outer” phenyls to lay flat on Au(111) avoiding H-H repulsions, as shown in the inset in Figure 3a and the scheme in Figure 3e. Similarly, planarization of the “inner” phenyls during cyclodehydrogenation should


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force the “outer” phenyls to slightly twist to again avoid the H-H repulsions. The appearance of the side phenyls in the nc-AFM image in Figure 4b suggests that they are slightly tilted relative to the chevron GNR backbone.

Figure 4. Characterization of mGNRs. (a) STM image mGNRs on Au(111). Scan parameters: 0.1 V, 30 pA. (b) nc-AFM image of a fragment of mGNR. (c) STS point spectra of mGNR on


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Au(111) substrate. The inset shows STM image of mGNRs with red and black circles indicating the points at which the STS spectra were measured. Scan parameters: -0.3 V, 10 pA.

The mGNR can be considered as a laterally extended cGNR, since the side phenyl groups are in conjugation with the chevron GNR backbone. From this point of view, mGNR is another member of the family of laterally extended chevron GNRs, which have been discussed in recent studies.28, 30, 42, 43 We performed scanning tunneling spectroscopy (STS) study of mGNRs to determine their band gap and find how the phenyl functionalization/lateral extension changes the band gap of the parent cGNR. Chevron GNRs have been a subject of several STS investigations.28, 45, 46 When studying cGNRs on Au(111), Nguyen et al. determined the valence band (VB) edge at −0.83 eV, and the conduction band (CB) edge at 1.7 eV, producing a band gap of 2.53 eV,28 while Deniz et al. found the VB edge at –0.8 eV and the CB edge at 1.6 eV, yielding a band gap of 2.4 eV.46 Density functional theory (DFT) calculations have previously revealed that at the VB and CB energies the highest local densities of states (LDOS) are expected at the cGNR’s edges, which was shown experimentally for pristine and fluorenone-modified cGNRs.28, 29 Therefore, in order to measure band gaps of the ribbons from the cGNR family, STS spectra are typically measured at the nanoribbons’ edges.28, 46 Figure 4c shows two representative spectra measured at two different edge positions on a mGNR on Au(111); the colors of the spectra match the colors of the circles in the inset in Figure 4c, which indicate the points at which the STS was peformed. Both spectra show the VB edge located at -0.91 eV and the CB edge located 1.58 eV, resulting in the mGNR’s band gap of of 2.49 eV. We recorded mutiple spectra at the edges of several mGNRs, all of which exhibited similar VB and CB peaks. The average value of the mGNR’s band gap


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that was determined from multiple measurements is 2.50±0.02 eV, which is comparable to the band gap of cGNR on Au(111) found in prior STS studies.28, 46 However, these measurements also show that the phenyl functionalization leads to a shift of the GNR band edges to lower energies, which is an effect similar to those induced by the nitrogen doping25 or fluorenone modification of cGNRs.28 The latter approaches have been used to construct GNR heterojunctions,25, 28 so likewise the phenyl functionalization could be employed for engineering heterojunctions comprising, for example, mGNR and cGNR units.29, 30 The spectra in Figure 4c exhibit not only the band gap edges, but also several other prominent peaks. For example, the spectra recorded at all mGNR edge positions consistently showed a peak at about -1.05 V, which is likely associated with the VB-1 state. This assignment is consistent with the DFT-calculated VB-1 LDOS map of cGNR that, similarly to its VB LDOS map, predicts the highest electron density at the nanoribbon’s edges.45 On contrary, the DFTcalculated CB and CB+1 LDOS maps of cGNR look very different – while in the CB map the LDOS is high along all edges, in the CB+1 map it is concentrated in the concave edge positions.45 These DFT-calculated maps are also consistent with the experimental STS data, which show that while all probed edge positions of mGNRs show a prominent CB peak, the STS spectrum measured at the mGNR’s concave edge position (see the black point in the inset in Figure 4c) shows an additional peak at 1.8 V, which could be interpreted as the CB+1 state. While most of the mGNRs that we observed were well separated (Figure 4a), we also found a number of areas where ribbons were fused together. Fusion of nanoribbons is an important synthetic approach in the field of atomically precise GNRs as it allows fabrication of graphene structures that are difficult to directly produce from molecular precursors.5, 10, 13, 47, 48 The lateral fusion of nanoribbons can also be used for the synthesis of extended GNR structures


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and graphene nanopores.9, 39 As we demonstrate in this study, phenyl-modification of cGNRs enables two scenarios of their lateral fusion, resulting in different kinds of atomically precise graphene nanopores. Because of the planarization of polymer 5 chains upon annealing, the resulting mGNRs can no longer engage in π-π interactions. Furthermore, as the structures schematically shown in Figure 5a planarize, the distance between them should increase because of the intermolecular HH repulsions (Figure 5b). The resulting mGNRs have two options for lateral fusion, as the phenyl of one nanoribbon can engage in intermolecular cyclodehydrogenation with phenyls of another ribbon on one side (blue ovals) or the other (orange ovals). These fusion routes produce extended structures that have graphene nanopores with different sizes and shapes (Figure 5c,d). The first route results in graphene nanopores with the same size and shape (shown by the same blue color in Figure 5c), whereas the second route produces two kinds of alternating pores (shown by different colors in Figure 5d). Both of these arrangements were observed experimentally (Figure 5e,f), and the resulting nanopores differ in their sizes and shapes from those previously produced by the lateral fusion of cGNRs.9 These results demonstrate that different kinds of atomically precise graphene nanopores could be engineered through the phenyl modification of cGNRs, and a similar approach could be potentially applied to other GNRs to realize nanopores with a variety of structures and properties. The recent work by Moreno et al. provides another example of phenyl modification of a previously established GNR precursor, which produced fused graphene structures with precisely engineered nanopores.39


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Figure 5. Lateral fusion of mGNRs. (a) Scheme of the assembed polymer 5 chains. The region of π-π interactions is highlighted by red. (b) Scheme of the fully planarized mGNRs. Blue and orange ovals show possibilities for lateral inter-ribbon fusion. (c,d) Schemes of two variants of mGNR fusion. The resulting graphene nanopores that form between the ribbons are colored. (e)


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STM image of graphene nanopores formed according to the scheme (c). Scan parameters: 0.3 V, 20 pA. (f) STM image of graphene nanopores formed according to the scheme (d). Scan parameters: 0.1 V, 20 pA

In order to get further insights into the structure of graphene nanopores produced by the mGNR fusion, we studied them by nc-AFM. The left panel in Figure 6a shows nc-AFM image of the nanopore arrangement that is schematically illustrated by Figure 5c. In nc-AFM images of mGNRs and other ribbons from the chevron GNR family, including pristine cGNR,9 we occasionally observed bright dots in the concave regions of nanoribbons; one of these dots is shown by the black arrow in Figure 6a. Similar features were also observed in the N-doped GNRs synthesized from 9-methylcarbazole modified chevron GNR precursors, and were interpreted as methyl radicals.43 While methyl radicals were expected to form in that synthesis due to the homolytic cleavage of the C-N bonds in 9-methylcarbazole moieties,43 they are unlikely to be present in the on-surface synthesis of mGNRs. Therefore, these bright dots observed in nc-AFM images are likely other species, such as residual bromines. These results demonstrate that the concave areas of nanoribbons from the chevron GNR family could be attractive sites for certain on-surface species, which could be potentially utilized for designing complex GNR-based heterostructures.


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Figure 6. High-resolution nc-AFM imaging of graphene nanopores. (a) nc-AFM image of the nanopore arrangement that is schematically shown in Figure 5c. The left panel shows the original image, while in the right panel the graphene nanopores are overlaid by identical semitransparent blue shapes. (b) Molecular model of the nanopores observed in (a) showing two kinds of biphenyl bridges between the chevron GNR fragments. The benzene rings in the biphenyl bridges can be bonded either through meta (m) or para (p) positions.


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In order to better illustrate the shape of graphene nanopores in Figure 6a, we highlighted one of them (the second fully visible nanopore from the left edge of the image) by blue. Then, we overlaid exactly the same blue shape over all other nanopores in the nc-AFM image, see the right panel in Figure 6a. The resulting figure shows that the same semitransparent blue shapes accurately describes the geometry of all graphene nanopores, further demonstrating that they were prepared with atomic precision. Further analysis of the nc-AFM image in Figure 6a reveals slight differences in the appearances of biphenyl bridges that connect the chevron GNR fragments. In half of these bridges the benzene rings appear to be flat (see the middle bridge in the left panel in Figure 6a as an example), whereas in the other half of them the benzene rings look tilted. These two kinds of biphenyl bridges are present between the chevron GNR fragments in an alternating fashion. The corresponding molecular model of the biphenyl bridges in Figure 6a is shown in Figure 6b. The benzene rings in these biphenyl bridges can be bonded to the cGNR units either through meta (m) or para (p) positions. When the benzene rings are bonded through the para positions, the rings can rotate around the bond axes, which minimizes the H-H repulsions, as shown in Figure 6b. However, when the benzene rings are bonded through the meta positions, the rotations are not possible and despite the H-H repulsions the rings lay flat on the surface. These results demonstrate that high-resolution nc-AFM imaging can reveal subtle details of GNR and graphene nanopore structures. These results further show that it is possible to design on-surface GNR structures with out-of-plane components, as in the pristine (Figure 4b) and fused mGNRs (Figure 6), which is uncommon for most surface-synthesized GNRs demonstrated so far.


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Conclusions In summary, we demonstrated the on-surface synthesis of atomically precise chevron GNRs functionalized with phenyl groups. These ribbons have been grown on Au(111) in UHV conditions from a specially designed molecular precursor. We show that the phenyl modification affects the assembly of the mGNR polymers through π-π interactions. The mGNRs on Au(111) have a band gap of 2.50±0.02 eV, which is comparable to that of the parent chevron GNR. Also, the phenyl functionalization shifts of the band edges to lower energies, which means that it could be a useful tool for the GNR band structure engineering. The mGNRs could be considered for combining them with other GNRs from the chevron family into GNR heterojunctions. We also investigated the lateral fusion of mGNRs and demonstrate that it could be used to engineer different kinds of atomically precise graphene nanopores. A similar functionalization approach could be potentially applied to other GNRs to affect their on-surface assembly, modify their electronic properties, and realize graphene nanopores with a variety of structures.

Methods The detailed synthetic procedures for compound 10 and monomer 4 are provided in Supporting Information. Polymer 5 and mGNRs 6 were synthesized and investigated using an upgraded Createc based LT-STM system with custom GXSM control hardware and software.49 The substrates were prepared by three Ar+ sputtering/annealing cycles. The precursor 4 was sublimed onto cold (4.5 K) Au(111) substrate in UHV conditions using a home-built evaporator and the substrate was then heated up to 200-250 °C to form polymer 5 and subsequently to 400440 °C to form mGNRs 6. The samples were imaged using STM and nc-AFM at 4.5 K. The nc-


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AFM imaging was performed at a 50-150 mV STM bias using a typical tip oscillation amplitude of 50-80 pm and a Q-Plus [© F. Giessibl] sensor operated around 30 kHz.

ASSOCIATED CONTENT Supporting Information Detailed scheme of synthesis of the mGNR precursor and synthetic procedures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/XXXX.

Acknowledgements The work was supported by the Office of Naval Research (N00014-16-1-2899) and the National Science Foundation (CHE-1455330). This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. References 1.

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Son, Y.-W.; Cohen, M. L.; Louie, S. G., Half-Metallic Graphene Nanoribbons. Nature

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