Edge State Engineering of Graphene Nanoribbons - Nano Letters

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Edge state engineering of graphene nanoribbons Xuelei Su, Zhijie Xue, Gang Li, and Ping Yu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02356 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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Nano Letters

Edge State Engineering of Graphene Nanoribbons Xuelei Su, Zhijie Xue, Gang Li,∗ and Ping Yu∗ School of Physical Science and Technology, ShanghaiTech University, 201210 Shanghai, China E-mail: [email protected]; [email protected] Abstract Zigzag edges of graphene nanoribbons, which are predicted to host spin-polarized electronic states, hold great promise for future spintronic device applications. The ability to precisely engineer the zigzag edge state is of crucial importance for realizing its full potential functionalities in nanotechnology. By combining scanning tunneling microscopy and atomic force microscopy, we demonstrate the zigzag edge states get energy splitting upon fusing manganese phthalocyanine molecule with the short armchair graphene nanoribbon termini. Moreover, the edge state splitting can be reversibly switched by adsorption and desorption of atom hydrogen on the magnetic core of manganese phthalocyanine. These observations can be explained by tuning the zigzag edge local doping through the charge transfer process, which provides a new route to functionalize graphene-based molecular devices.

Keywords Two-dimensional material, graphene nanoribbon, noncontact atomic force microscopy, scanning tunneling microscopy, edge state Graphene nanoribbons (GNRs), a novel quasi one-dimensional graphene nanostructure, are promising building blocks for nanoelectronics and spintronics. 1,2 Among all strategies 1

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developed for preparing GNRs, recent advances in the bottom-up on surface synthesis of GNRs from precursor molecules, have the advantage to precisely control the width and edge topology. 3,4 Thus, it gives access to various GNR structures with tunable electronic properties, such as bandgap engineering 5–7 and controllable chemical doping. 8–10 More intriguingly, the zigzag edge states (ZES) of GNRs are predicted to be spin-polarized and have potential applications in spintronics. 11,12 According to different theoretical predictions, the ZES of GNRs should show ferromagnetic order along the edge and couple antiferromagnetically between the edges giving rise to energy splitting of edge states by taking electron-electron interaction into account. 13–15 By using top-down fabrications, the edge state splitting of GNRs has been observed in unzipping carbon nanotubes 16 and in etched graphene on silicon carbide, 17 whereas in these studies an atomically precise modification of the edges of GNRs could hardly be realized. In strong contrast, armchair graphene nanoribbons (AGNRs) and zigzag topology graphene nanoribbons (ZGNRs) have been synthesized on surface by bottom-up approach with atomic controls, 3,4 where a large energy splitting of the ZES have been observed on both the zigzag termini of AGNRs and the extended zigzag edge of ZGNRs by transferring the GNRs from Au(111) growth substrate onto insulation islands of NaCl. 4,18 Generally, previous experimental studies mainly concentrate on investigating the intrinsic electronic properties of ZES by improving the edge precision or reducing the substrate interaction. Nevertheless, the response of ZES to neighboring molecules remains still largely unexplored, and a more detailed understanding of how to engineer the ZES over atomic-scale control is required to realize the full potential of GNR-based electronics. Here, we focus on engineering the ZES electronic properties through surface-assisted fusing single manganese phthalocyanine (MnPc) molecule to the zigzag termini of AGNRs, where the edge states are isolated from the delocalized GNRs bulk states. 18 The mostappealing feature of integrating transition-metal (TM) phthalocyanine molecules with GNRs is their versatility especially the tunable electronic and magnetic properties, providing extensive opportunities to be implemented as switches to engineer the ZES electronic structure

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through creating TM-phthalocyanine-GNR hybrid nanostructure. Indeed, graphene edge has been modified by phorphine fusing, although no electronic structure of graphene at the Fermi level is affected by phorphine functionalization. 19 Recently, iron porphyrin bonded to chiral GNRs with atomically precise contacts has been reported, but the work focused on investigating the magnetic properties of porphyrin molecule instead of the electronic structure of GNRs. 20 By using combined atomically resolved atomic force microscopy (AFM) and scanning tunneling microscopy (STM), the ZES of AGNRs is found to show energy splitting upon fusing MnPc to the zigzag termini. Moreover, the edge state splitting can be switched on and off reversibly with atomic precision by detaching and attaching of atom hydrogen on MnPc metal core. 0

Atomically well-defined AGNRs were grown on a Au(111) surface using 10,10 -dibromo0

9,9 -bianthryl (DBBA) molecules as precursors followed a recently established bottom-up method. 3 Figure 1A schematically illustrates the stepwise annealing process for fusing single MnPc molecule to the zigzag termini of AGNRs. After evaporating DBBA molecules on to the Au(111) substrate, the sample was annealed at 433 K as a first step, so that the precursor molecules are linearly polymerized. Subsequently, MnPc molecules were deposited on the sample and annealed together with linear polymer chains for the second step at 593 K. At an elevated temperature annealing, the surface-assisted cyclodehydrogenation results in the formation of aromatic AGNRs. Employing this reaction, some of MnPc molecules were also made covalent coupling to the zigzag edge of AGNRs. Apart from the MnPc connection site, the edges of AGNRs are all hydrogen satruated. A typical overview STM image is shown in Fig.1B. Besides some ribbons attached to other ribbons or without any MnPc coupling, we can also find individual, free ribbon, of which the short zigzag edge is fused with a single MnPc molecule (MnPc-AGNR). Figure 1C,D show respectively the STM images of the complete structure and the zoomed view of junction for MnPc-AGNR, revealing that one isoindole ring of MnPc connects straight with the zigzag termini of AGNR. But these ordinary STM images do not provide the direct bond configuration of the formed junction.

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In contrast, noncontact atomic force microscopy (nc-AFM) with functionalized tip has the ability to resolve the inner structure of single molecules, 21–23 which is complementary to the STM electronic orbital imaging. 24,25 To determine the detailed bond configuration, we performed nc-AFM measurements with CO-functionalized tungsten tip. Atomically resolved constant-height AFM images of the complete structure and the junction of MnPc-AGNR are shown in Fig. 1E,F. The nc-AFM images clearly reveal that one of the outmost benzene ring in MnPc is covalent coupling with the terminal zigzag edge of AGNR by forming two carbon-carbon (C-C) bonds in between. Thus, a pentagon ring is formed in the MnPc-AGNR junction as presented in schematic model of Fig. 1A. To investigate the influence of the MnPc fusing on the electronic structure of AGNRs zigzag edge state, the differential conductance (dI /dV ) spectroscopy measurements were carried out over several different positions along the AGNR, as marked in Fig. 2A. The corresponding dI /dV spectra and one spectrum on Au(111) for reference are shown in Fig. 2B. When tip locates above the free zigzag end, the dI /dV spectra exhibit only one peak with the energy close to the Fermi level around 30 mV, which decay within 1.2 nm along the AGNR and vanish in the centra part of the ribbon. This state has been observed before and is attributed to the edge state localized at the zigzag edge of AGNRs. 2,26 Exactly as the results shown here, the free ZES of AGNRs termini on Au(111) substrate is observed only at positive bias, indicating one possibly degenerate state near the Fermi level, which has been explained by quenching of the energy splitting through the substrate doping. 27,28 In striking contrast to the free end, the dI /dV spectra measured on top of the zigzag edge fused with MnPc show a double-peak structure at both sides of the Fermi level with energy splitting of 50 mV (or can be viewed as a dip feature at the Fermi level at current stage). To probe and compare the spatial distribution of the edge states of both ends, dI /dV maps were measured at specific edge state resonance energies as well as the Fermi level in the constant height mode. As shown in Fig. 2C, the dI /dV maps on the zigzag edge of AGNR fusing with MnPc display pronounced local density of state contribution at both -25 and 25

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Figure 1: Bottom-up synthesis of MnPc fusing to the zigzag edge of AGNRs. (A) Reaction scheme for fusing MnPc to the zigzag edge of AGNRs through stepwise annealing at 433 K and 593 K respectively. (B) STM overview image showing a free MnPc-AGNR structure (V = 300 mV, I = 0.1 nA). (C,D,E,F) STM images (C,D) and constant-height nc-AFM images (E,F) of the complete structure and zoomed view of junction for MnPc-AGNR with CO tip. In (B) scale bar, 3 nm, other scale bars, 1 nm

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mV, whereas the dI /dV maps on the free end of AGNR present the high density of state only at positive bias voltage. Fusing MnPc to the zigzag termini of AGNRs induces a double-peak/dip feature of ZES, which can be a pair of occupied and unoccupied edge states from coulomb repulsion, or a kondo effect from the localized spin of MnPc thus showing a dip feature at the Fermi level. To determine whether the electronic structure of the zigzag edge fusing with MnPc is a Kondo effect or not, dI /dV spectroscopy measurements of MnPc-AGNR junction have been performed at 77 K, which are shown in Fig. S1. Comparing the dI /dV spectra measured on top of MnPc center at 5 K and 77 K shown in Fig. S1A, we find the spectra change from a sharp step to featureless at Fermi level, which clearly suggests the quenching of MnPc Kondo resonance by increased temperature. This result is consistent with previous experimental reports that the Kondo temperature of MnPc on Au(111) is lower than 77 K. 29 In contrast, double-peak structure still survives at 77 K as shown in the Fig. S1B. Considering that the double-peak structure of ZES shows no Kondo temperature dependence except the spectra width getting broadened only from thermal effect, we attribute the electronic structure of zigzag edge fusing with MnPc to a pair of occupied and unoccupied edge states instead of Kondo effect. As well known, the magnetic properties of MnPc is highly tunable, which quantum spin number can be switched from 3/2 to 1 through attaching and detaching of hydrogen atom on the manganese core. 29 It is of great interest to investigate whether the energy splitting of ZES can be manipulated through remote control by attaching and detaching of hydrogen atom on the manganese core, which will equip us with a new strategy to engineer the ZES electronic structure. In our experiments, actually after cross-dehydrogenative reaction at 593 K, there are two types of MnPc molecule appearances have been observed on the sample surface, which can be distinguished from their STM morphology images. One kind has already been shown in Fig. 1,2, in which the bright protrusion were observed in the STM images of MnPc center. The other type is shown in Fig. 3A, where the MnPc molecule is

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Figure 2: Electronic structure of MnPc fused zigzag edge of AGNRs. (A,B) Constantcurrent STM image and corresponding dI /dV spectra measured at different positions (1-14) as well as on Au(111) for reference (C) Constant-height dI /dV mappings measured with a CO tip at the bias of -25 mV, 1 mV, 25 mV and 30 mV. Scale bars, 1 nm. found to show a depression at the cross center. According to previous experiment results, 29 the change of MnPc molecule center morphology from a bright protrusion to a depression feature is induced by single hydrogen atom attachment on top of MnPc, moreover, the morphology can be switched back to the bright protrusion by applying a positive pulse for detaching the hydrogen atom from MnPc. Here, we attributed the depression morphology of MnPc center in Fig. 3A to the chemical adsorption of a single hydrogen atom onto the MnPc molecule (labeled as H-MnPc), since dehydrogenative reaction occurs with a loss of hydrogen gas as byproduct providing the possibility for hydrogen atom adsorption on MnPc molecule. In addition, we can detach the hydrogen atom from MnPc by applying a positive voltage pulse of 2 V. For comparison, the overview STM image of the same area after applying the voltage pulse on the H-MnPc molecule center is shown in Fig. 3B. It is found that the depression feature at the MnPc center can be switched back to a bright protrusion. Fig. 3C and D demonstrate the high resolution of STM images on the whole H-MnPc/MnPc-AGNR structure before and after pulsing. The atomically resolved constant 7


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height AFM images before and after voltage pulse are shown in Fig. 3E and F respectively. By hydrogen atom attachment and detachment, the spin state of MnPc can be tailored from low spin to high spin characterized by dI /dV measurements on the MnPc center showing featureless or a pronounced step feature at zero bias (See the dI /dV spectra measured on MnPc center in Supporting Information Fig. S2). Most strikingly, we found the electronic structure of the ZES can be manipulated by attaching hydrogen atom on top of MnPc. As shown in Fig. 3G, dI /dV spectrum taken on the zigzag edge fusing with H-MnPc shows no peaks near the Fermi level instead of a broad resonance around -150 mV. After pulsing for detachment of hydrogen atom from MnPc, namely recovering the high spin state of MnPc, the dI /dV spectrum measured on the same position of zigzag edge exhibits the double peak structure at both sides of the Fermi level. Two representative dI /dV mappings at bias of 30 mV before and after pulsing are shown in Fig. 3H (More dI /dV mappings at different energies can be found in Supporting Information Fig. S3 ). It clearly demonstrates that the ZES splitting vanishes once fused by H-MnPc, while the the edge state splitting recovers as soon as the hydrogen atom is pulsed away. Note that the edge state at the free zigzag end of GNR is not influenced by the manipulation process. We have also checked other free ribbons fused by H-MnPc on different adsorption sites, which all show the same engineering process suggesting this manipulation method is robust and reliable. (See another example in Supporting Information Fig. S3). Moreover, we have tried dosing the sample with 60 Langmuir of hydrogen at room temperature and then cooled down to 5 K, more than 90 percent of the MnPc molecules can be turned into H-MnPc with a depression at the center. The H-MnPc-AGNRs all exhibit the electronic states of -150 mV at the the zigzag edge fusing with H-MnPc as discussed above. Comparing the dI /dV spectra measured at different positions across the H-MnPc-AGNR junction as shown in Fig. S4, the electronic state of -150 mV measured at the zigzag edge shows the most pronounced intensity among all the measured positions, which suggests that this -150 mV electronic state should be the zigzag edge state of H-MnPc-AGNR instead of

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the H-MnPc molecular state. A

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Figure 3: Engineering the ZES electronic structure. (A,B) STM overview images of HMnPc/MnPc fusing to the zigzag edge of AGNR before (A) and after (B) applying a voltage pulse. (C,D) High resolution of constant-current STM images before (C) and after (D) voltage pulse (V = 300 mV, I = 0.1 nA). (E,F) Constant-height nc-AFM images before (E) and after (F) the voltage pulse. (G) dI /dV spectra measured at the same position on the zigzag edge of AGNR fused with H-MnPc/MnPc, before (black) and after (red) voltage pulse. (H) Constant-height dI /dV mappings at bias of 30 mV before and after voltage pulse. In (A,B) scale bars, 5 nm, other scale bars, 1 nm. To further understand the engineering mechanism, we performed ab-inito calculations on a free-standing graphene nanoribbon fusing with a MnPc molecule. Although a possible charge transfer between AGNRs and a metallic substrate can dramatically modify the electronic structure of the edge states of AGNRs, 27,28 it is a too colossal task for including the substrate in the calculation. For simplicity, no substrate was considered in our calculations, such that possible charge transfer between substrate and the MnPc-AGNR hybrid system cannot be addressed by our calculation. The electronic correlations between MnPc 9


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d-electrons was partially taken into account by using DFT + U functional with U = 3.0 eV in both self-consistent and partial density of states calculations. The free-standing AGNR is known to be spin polarized with two edges being antiferromagnetic aligned, 13–15 as shown in our calculation result of Fig. 4 A, where the spin density is overlaid on the structure model with red and blue colors denoting the two different spin polarization directions. First of all, in agreement with our experimental observations we found that assembling MnPc with AGNR caused qualitative structure change in neither MnPc molecule nor AGNR, i.e. only the interface is slightly reconstructed within a full DFT relaxation (see the relaxed structure model in Fig. 4). The Mn atom remains in the same plane of phthalocyanine and the local D4h symmetry is kept unchanged. The connection of the MnPc molecule to the zigzag edge of AGNRs causes notable changes to the electronic structure of the AGNR edge states. As clearly demonstrated in a comparison of Fig. 4 A and B, the spin polarization of the edge states calculated at the carbon atom indicated by arrows is clearly reduced after fusing with MnPc, i.e. the separation of the two main spin-polarized states below and above the Fermi level shrinks. In Fig. 4 the spin-polarized local density of states of chosen carbon atom is shown in red and blue colors corresponding to the minority and majority components. To highlight the role played by the adsorption of hydrogen atom, we have considered both one and two hydrogen atoms attached to the central Mn site as shown in Fig. 4C and D, respectively. In addition to the obvious reduction of the local magnetic moment of Mn, absorbing hydrogen atoms to Mn center also reduces the spin polarization of the edge states, resulting in a reduction in the energy separation of the two main spin-polarized states below and above the Fermi level, as clearly shown in the comparison of Fig. 4B, C and D. However, further increasing the amount of absorbed hydrogen atoms does not lead to a complete suppression of the edge state spin polarization. The free-standing MnPc-AGNR system remains spin polarized throughout our calculations. As a result, we conclude that, without the metallic substrate, the edge sates of the AGNRs with MnPc connection site can be partially tuned by MnPc fusing, while the edge states of the opposite far zigzag edges are not influenced

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at all ( The calculation results of the electronic states of far zigzag edges can be found in the supporting information). The persisting spin-polarization suggests that the edge state engineering observed in our experiment is not solely determined by the magnetic properties of MnPc. Other mechanism such as charge transfer between substrate and MnPc-AGNR

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Figure 4: The density of states of carbon atom at the near edges of the MnPc-AGNRs indicated by the arrow in the inset structure model. (A-D) correspond to the AGNR, MnPcAGNR, H-MnPc-AGNR and H2-MnPc-AGNR, correspondingly. The spin-polarized local density of states of chosen carbon atom is shown in red and blue colors corresponding to the minority and majority components. The inset shows the fully relaxed structure model and the overlaying isovalued spin density surface with red and blue colors denoting the minority and majority of spin directions. As we known, the edge state of AGNR is very sensitive to the charging effect, which report that the zigzag edge states of the neutral free-standing graphene nanoribbons (GNRs) should

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be energy splitting around the Fermi level and the energy splitting will be quenched due to the charging of GNRs. 27,28 Once the zigzag edge is positively charged, only the unoccupied edge state can be detected above the Fermi level, while when the zigzag edge is negatively charged, only the occupied edge state can be detected below the Fermi level. Since the edge state of H-MnPc-AGNR shifts below the Fermi level of about 150 mV, an intuitive interpretation is that there could be additional charge transfer to the zigzag edge of HMnPc-AGNR due to the hydrogen adsorption, which makes the zigzag edge with H-MnPc site negatively charged thus leading the edge state below the Fermi level. For proving this picture, we have performed the Kelvin probe force spectroscopy (KPFS) measurements along H-MnPc-AGNR and MnPc-AGNR respectively as marked in Fig. 5A. For each KPFS, the frequency shift ∆f (V ) was recorded as a function of sample voltage V . From fitting ∆f (V ) as a function of sample voltage V to a parabola, the voltage at maximum value of ∆f (V ) is extracted, which is equal to the local contact potential difference (LCPD) between tip and sample. 30–32 The representative KPFS are shown in Fig. S5. Since surface charges and dipoles affect the local work function, thus, LCPD should qualitatively reflects the charge state of individual atom, 33 the electron affinity, 34 and the charge distribution at surfaces. 32,35 The corresponding results of LCPD along lines are summarized in Fig. 5B. In principle, the LCPD values crucially depend on the exact tip shape. 36 Thus, it is necessary to compare the LCPD values measured with the same tip. Fortunately, a short AGNR fusing with HMnPc at both zigzag edges can be found occasionally. The hydrogen atom can be detached from one H-MnPc by applying a voltage pulse thus forming a AGNR structure with zigzag edges fusing with H-MnPc and MnPc respectively (H-MnPc-AGNR-MnPc). The LCPD results show that the LCPD measured on the AGNR lower than that of Au(111), which is agree with the well known fact that GNRs have lower work function than that of Au(111). Moreover, increased LCPD were observed on the center of H-MnPc and the zigzag edge of HMnPc-AGNR respectively, while increasing of LCPD was also detected on the pentagon ring position of the MnPc-AGNR junction as shown another peak feature in Fig. 5B. Generally,

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a positive/negative charge gives rise to a decreased/increased local work function and thus a decreased/increased LCPD. We can conclude that the zigzag edge of H-MnPc should be more negatively charged than that of MnPc-AGNR. This could be the reason why the edge state of H-MnPc-AGNR shifts below the Fermi level. In two different junctions of H-MnPc/MnPc-AGNR, the increases of LCPD are both observed on the pentagon rings. According to previous results, pentagon rings can functionalize the zigzag edge and modify the ZES electronic structure due to the sub lattice symmetry broken. 37 In their calculation, they also find the pentagon rings can modify the interaction of the GNRs with Au(111) substrate by 0.05 nm difference in adsorption height. Therefore, it could be due to the pentagon ring formation that the charge transfer from the zigzag termini with MnPc site is weakened thus leading it to nearly neutral. As a consequence, the edge state is turned into a pair of occupied and unoccupied edge states below and above the Fermi level. So generally speaking, upon fusing MnPc with the zigzag edge of AGNRs and adsorbing hydrogen on top of MnPc afterwards, the hole doped zigzag edge on Au(111) is tuned to the nearly neutral state and electron doped zigzag edge respectively. Therefore, the electronic structure of the edge state is engineered from above the Fermi level to the energy splitting and below the Fermi level correspondingly. In conclusion, we report a first step towards engineering the zigzag edge state of AGNRs by fusing with single MnPc molecule. In the combined STM and AFM experiments, we fabricate the MnPc-AGNR nanostructures and tune the energy splitting of ZES by reversibly switching the local charge doping level. A corresponding charge transfer mechanism is proposed and supported by local contact potential measurements as well as density-functional calculations. Our method would be further extended to longer zigzag edges of GNRs, where different bonding configurations to nearby molecules will allow to explore the full potential of edge state engineering. Moreover, our proof-of-principle experiments may be generalized to systems where charge transfer processes can be manipulated, such as between GNRs and conducting molecules or other semiconducting/insulating materials. We anticipate that in-

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Figure 5: LCPD measurements along the H-MnPc-AGNR-MnPc and MnPc-AGNR structures. (A) STM images as well as the measured lines for KPFS measurement marked in the figure. (B) LCPD values shown along lines. Scale bars, 1nm. corporating functional molecular units with AGNRs will open bright prospects not only for controlling modification of AGNR electronic properties but also expanding its applications in diverse fields such as gas sensing or optoelectronics.

Author Information Corresponding Authors G.L. (email: [email protected]). P.Y. (email: [email protected]) Author contributions X.S. and P.Y. conceived the experiments. X.S. and Z.X. carried them out. G.L. performed the DFT calculations. X.S., P.Y. and G.L. wrote the manuscript. All authors discuss the results and commented on the manuscript.

Supporting Information Available Sample preparation, details on the STM/AFM measurements and DFT calculations. Figures: The electronic structure of MnPc-AGNRs at 77 K, spectra measured on the center of H-

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Nano Letters

MnPc/MnPc, another example for engineering the ZES electronic structure of MnPc-AGNR, dI /dV spectra measured across the H-MnPc-AGNR junction, KPFS measurements, band gap of the AGNRs, dI /dV spectra measured on the MnPc-AGNRs with different lengths, density of states of carbon atom at the far edge of MnPc-AGNRs.

Acknowledgement P.Y. and G.L. are supported by ShanghaiTech start-up funding. P.Y. gratefully acknowledges the financial support from National Natural Science Foundation of China (11704250), Shanghai Natural Science Foundation (17ZR1443200). G.L. acknowledges the Program for Professor of Special Appointment (Shanghai Eastern Scholar). Calculations were carried out at the HPC Platform of ShanghaiTech University Library and Information Services, as well as School of Physical Science and Technology. The authors declare no conflict of interest.

References (1) Li, X., Wang, X., Zhang, L., Lee, S. & Dai, H. Chemically derived, ultrasmooth graphene nanoribbon semiconductors. Science 319, 1229–1232 (2008). (2) Koch, M., Ample, F., Joachim, C. & Grill, L. Voltage-dependent conductance of a single graphene nanoribbon. Nature Nanotechnology 7, 713–717 (2012). (3) Cai, J., Ruffieux, P., Jaafar, R., Bieri, M., Braun, T., Blankenburg, S., Muoth, M., Seitsonen, A., Saleh, M., Feng, X., M¨ ullen, K. & Fasel, R. Atomically precise bottomup fabrication of graphene nanoribbons. Nature 466, 470–4734 (2010). (4) Ruffieux, P., Wang, S., Yang, B., Sánchez-Sánchez, C., Liu, J., Dienel, T., Talirz, L., Shinde, P., Pignedoli, C. A., Passerone, D., Dumslaff, T., Feng, X., M¨ ullen, K. & Fasel,

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R. On-surface synthesis of graphene nanoribbons with zigzag edge topology. Nature 531, 489–492 (2016). (5) Ruffieux, P., Cai, J., Plumb, N. C., Patthey, L., Prezzi, D., Ferretti, A., Molinari, E., Feng, X., M¨ ullen, K., Pignedoli, C. A., & Fasel, R. Electronic structure of atomically precise graphene nanoribbons. ACS Nano 6, 6930–6935 (2012). (6) Chen, Y.-C., de Oteyza, D. G., Pedramrazi, Z., Chen, C., Fischer, F. R., & Crommie, M. F. Tuning the band gap of graphene nanoribbons synthesized from molecular precursors. ACS Nano 7, 6123–6128 (2013). (7) Zhang, H., Lin, H., Sun, K., Chen, L., Zagranyarski, Y., Aghdassi, N., Duhm, S., Lin, Q., Zhong, D., Li, Y., M¨ ullen, K., Fuchs, H., & Chi, L. On-surface synthesis of rylenetype graphene nanoribbons. Journal of the American Chemical Society 137, 4022–4025 (2015). (8) Bronner, C., Stremlau, S., Gille, M., Brauße, F., Haase, A., Hecht, S., & Tegeder, P. Aligning the band gap of graphene nanoribbons by monomer doping. Angewandte Chemie International Edition 52, 4422–4425 (2013). (9) Cloke, R. R., Marangoni, T., Nguyen, G. D., Joshi, T., Rizzo, D. J, Bronner, C., Cao, T., Louie, S. G., Crommie, M. F., & Fischer, F. R., Site-specific substitutional boron doping of semiconducting armchair graphene nanoribbons. Journal of the American Chemical Society 137, 8872–8875 (2015). (10) Kawai, S., Saito, S., Osumi, S., Yamaguchi, S., Foster, A. S., Spijker, P., & Meyer, E., Atomically controlled substitutional boron-doping of graphene nanoribbons. Nature Communications 6, 8098 (2015). (11) Son, Y.-W., Cohen, M. L. & Louie, S. G. Half-metallic graphene nanoribbons. Nature 444, 347–349 (2006).

16


Page 16 of 20

Page 17 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(12) Han, W., Kawakami, R. K., Gmitra, M. & Fabian, J. Graphene spintronics. Nature Nanotechnology 9, 794–807 (2014). (13) Nakada, K., Fujita, M., Dresselhaus, G & Dresselhaus, M. S. Edge state in graphene ribbons: Nanometer size effect and edge shape dependence. Physical Review B 54, 17954–17961 (1996). (14) Yang, L., Park, C.-H., Son, Y.-W, Cohen, M. L. & Louie, S. G. Quasiparticle energies and band gaps in graphene nanoribbons. Physical Review Letters 99, 186801 (2007). (15) Yazyev, O. V. Emergence of magnetism in graphene materials and nanostructures. Reports on Progress in Physics 73, 056501 (2010). (16) Tao, C., Jiao, L., Yazyev, O. V., Chen, Y.-C, Feng, J., Zhang, X., Capaz, R. B., Tour, J. M., Zettl, A., Louie, S. G., Dai, H. & Crommie, M. F. Spatially resolving edge states of chiral graphene nanoribbons. Nature Physics 7, 616–620 (2011). (17) Li, Y. Y., Chen, M. X., Weinert, M. & Li, L. Direct experimental determination of onset of electron-electron interactions in gap opening of zigzag graphene nanoribbons. Nature Communications 5, 4311 (2014). (18) Wang, S., Talirz, L., Pignedoli, C. A., Feng, X., M¨ ullen, K., Fasel, R. & Ruffieux, P. Giant edge state splitting at atomically precise graphene zigzag edges. Nature Communications 7, 11507 (2016). (19) He, Y., Garnica, M., Bischoff, F., Ducke, J., Bocquet, M.-L., Batzill, M., Auwärter, W. & Barth, J. V. Fusing tetrapyrroles to graphene edges by surface-assisted covalent coupling. Nature Chemistry 9, 33–38 (2017). (20) Li, J., Merino-D´ıez, N., Carbonell-Sanromà, E., Vilas-Varela, M., de Oteyza, D. G., Pe˜ na, D., Corso, M., & Pascual, J. I.

Survival of spin state in

magnetic porphyrins contacted by graphene nanoribbons. 17


Science Advances 4

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(2018).

URL http://advances.sciencemag.org/content/4/2/eaaq0582.

Page 18 of 20

http:

//advances.sciencemag.org/content/4/2/eaaq0582.full.pdf. (21) Gross, L., Mohn, F., Moll, N., Liljeroth, P. & Meyer, G. The chemical structure of a molecule resolved by atomic force microscopy. Science 325, 1110–1114 (2009). (22) Gross, L., Mohn, F., Moll, N., Meyer, G., Ebel, R., Abdel-Mageed, W. M. & Jaspars, M. Organic structure determination using atomic-resolution scanning probe microscopy. Nature Chemistry 2, 821–825 (2010). (23) de Oteyza, D. G., Gorman, P., Chen, Y.-C, Wickenburg, S., Riss, A., Mowbray, D. J., Etkin, G., Pedramrazi, Z., Tsai, H.-Z., Rubio, A., Crommie, M. F., & Fischer, F. R Direct imaging of covalent bond structure in single-molecule chemical reactions. Science 1238187 (2013). (24) Repp, J., Meyer, G., Stojković, S. M., Gourdon, A. & Joachim, C. Molecules on insulating films: scanning-tunneling microscopy imaging of individual molecular orbitals. Physical Review Letters 94, 026803 (2005). (25) Soe, W.-H., Manzano, C., De Sarkar, A., Chandrasekhar, N. & Joachim, C. Direct observation of molecular orbitals of pentacene physisorbed on au (111) by scanning tunneling microscope. Physical Review Letters 102, 176102 (2009). (26) Talirz, L., Söde, H., Cai, J., Ruffieux, P., Blankenburg, S., Jafaar, R., Berger, R., Feng, X., M¨ ullen, K., Passerone, D., Fasel, R. & Pignedoli, C. A. Termini of bottomup fabricated graphene nanoribbons. Journal of the American Chemical Society 135, 2060–2063 (2013). (27) Ijäs, M., Ervasti, M., Uppstu, A., Liljeroth, P., van der Lit, J., Swart, I., & Harju, A. Electronic states in finite graphene nanoribbons: Effect of charging and defects. Physical Review B 88, 075429 (2013).

18


Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(28) van der Lit, J., Boneschanscher, M. P., Vanmaekelbergh, Daniël., Ijäs, M., Uppstu, A., Ervasti, M., Harju, A., Liljeroth, P., & Swart, I. Suppression of electron-vibron coupling in graphene nanoribbons contacted via a single atom. Nature Communications 4, 2023 (2013). (29) Liu, L., Yang, K., Jiang, Y., Song, B., Xiao, W., Li, L., Zhou, H., Wang, Y., Du, S., Ouyang, M., Hofer, W. A., Castro Neto, A. H., & Gao, H.-J. Reversible single spin control of individual magnetic molecule by hydrogen atom adsorption. Scientific Reports 3, 1210 (2013). (30) Nonnenmacher, M., o’Boyle, M. & Wickramasinghe, H. K. Kelvin probe force microscopy. Applied Physics Letters 58, 2921–2923 (1991). (31) Sadewasser, S., Jelinék, P., Fang, C.-K., Custance, O., Yamada, Y., Sugimoto, Y., Masayuki, A., & Morita, S. New insights on atomic-resolution frequency-modulation kelvin-probe force-microscopy imaging of semiconductors. Physical Review Letters 103, 266103 (2009). (32) Mohn, F., Gross, L., Moll, N. & Meyer, G. Imaging the charge distribution within a single molecule. Nature Nanotechnology 7, 227–231 (2012). (33) Gross, L., Mohn, F., Liljeroth, P., Repp, J., Giessible, F. J. & Meyer, G. Measuring the charge state of an adatom with noncontact atomic force microscopy. Science 324, 1428–1431 (2009). (34) Schuler, B., Liu, S.-X., Geng, Y., Decurtins, S., Meyer, G. & Gross, L. Contrast formation in kelvin probe force microscopy of single π-conjugated molecules. Nano Letters 14, 3342–3346 (2014). (35) Albrecht, F., Repp, J., Fleischmann, M., Scheer, M., Ondrá˘cek, S., & Jel´ınek, P. Probing charges on the atomic scale by means of atomic force microscopy. Physical Review Letters 115, 076101 (2015). 19


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(36) Enevoldsen, G. H., Glatzel, T., Christensen, M. C., Lauritsen, J. V. & Besenbacher, F. Atomic scale kelvin probe force microscopy studies of the surface potential variations on the tio 2 (110) surface. Physical Review Letters 100, 236104 (2008). (37) Shinde, P. P. bibinfoauthorGröning, O., Wang, S., Ruffieux, P., Pignedoli, C. A., Fasel, R. & Passerone, D. Stability of edge magnetism in functionalized zigzag graphene nanoribbons. Carbon 124, 123–132 (2017).

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Edge State Engineering of Graphene Nanoribbons - Nano Letters

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