Quantum Dots in Graphene Nanoribbons - Nano Letters (ACS

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Quantum Dots in Graphene Nanoribbons Shiyong Wang, Neerav Kharche, Eduardo Costa Girão, Xinliang Feng, Klaus Müllen, Vincent Meunier, Roman Fasel, and Pascal Ruffieux Nano Lett., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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Quantum Dots in Graphene Nanoribbons Shiyong Wang1*, Neerav Kharche2*, Eduardo Costa Girão3, Xinliang Feng4, Klaus Müllen5, Vincent Meunier2, Roman Fasel1,6 and Pascal Ruffieux1† 1

Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.

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Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, 12180 New York, USA

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Departamento de Física, Universidade Federal do Piauí, CEP 64049-550, Teresina, Piauí, Brazil 4

Department of Chemistry and Food Chemistry, Technische Universität Dresden, Mommsenstrasse 4, 01062 Dresden, Germany

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Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

ACS Paragon Plus Environment

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ABSTRACT: Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics and bioelectronics, but the fabrication of widely tunable GQDs has remained elusive. Here, we report the fabrication of atomically precise GQDs consisting of lowbandgap N = 14 armchair graphene nanoribbon (AGNR) segments that are achieved through edge fusion of N = 7 AGNRs. The so-formed intraribbon GQDs reveal deterministically defined, atomically sharp interfaces between wide and narrow AGNR segments and host a pair of lowlying

interface

states.

Scanning

tunneling

microscopy/spectroscopy

measurements

complemented by extensive simulations reveal that their energy splitting depends exponentially on the length of the central narrow bandgap segment. This allows tuning of the fundamental gap of the GQDs over one order of magnitude within a few nanometers length range. These results are expected to pave the way for the development of widely tunable intraribbon GQD-based devices. KEYWORDS: Graphene quantum dot, Graphene nanoribbon, scanning tunneling spectroscopy, density functional theory, screening

TEXT: Graphene quantum dots (GQDs) host quantized states with discrete energies offering opportunities in a wide field of electronic, optoelectronic, and spintronic applications. Methods such as lithographic patterning1–3, chemical synthesis4–7, doping8–10, and recently established electrostatic confinement11–13 have been used to realize GQDs, but the fabrication of widely tunable GQDs with atomically defined dot and interface structure has remained elusive. Realization of such GQDs not only requires graphene nanostructures with an ultra-narrow energy gap, but also needs their controlled embedding in a wide bandgap environment. Armchair graphene nanoribbons (AGNRs) with specific widths exhibit ultra-small band gaps, which can be


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used to design highly tunable GQDs. AGNRs can be grouped into three families according to ribbon width N = 3p, 3p+1, and 3p+2, with the 3p+2 AGNRs having much smaller band gaps14– 16

(also called quasi-metallic AGNRs17) , where p is an integer. In addition, atomically controlled

interfaces between AGNR segments exhibiting different band gaps give rise to well-defined lowlying interface states18. These unique electronic properties of AGNR heterostructures allow us to design GQDs with energy gaps over a wide range. Realization of such widely tunable GQDs requires the ability to fabricate atomically precise graphene nanostructures, which is difficult to achieve via common-used top-down methods. The bottom-up growth of graphene nanoribbons (GNRs) opened the opportunity of synthesizing ribbons with predefined electronic band gap by precise control over the width and edge topology19,20. Recent advances have consolidated this approach by identifying suitable precursor monomers for the fabrication of AGNRs of different width17,19,21,22, and hence band gaps and, most recently, zigzag GNRs for which spin-polarized edge states and related applications in spintronics have been predicted20. Here, we report the realization of highly tunable GQDs by bottom-up synthesis of atomically precise 7-14-7 AGNR heterostructures where the ribbon width N changes from 7 to 14 and back to 7 carbon atoms along the ribbon axis. This induces sharp energy gap transitions from an AGNR segment belonging to the wide band gap family (N = 3p+1) to the quasi-metallic family (N = 3p+2) and back to the wide band gap family (see details in Figure S6). Figure 1a schematically shows that the lateral edge fusion of two 7-AGNRs via crossdehydrogenative coupling leads to the formation of a 14-AGNR QD sandwiched between two 7AGNR barriers with sharp, atomically defined interfaces23. The STM image in Figure 1b reveals several such QDs, with the shortest QD being ~2 nm long and the longest QD exceeding 20 nm.


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Figure 1c details the chemical structure of a typical QD as resolved by non-contact atomic force microscopy imaging using a CO functionalized tip24, revealing a defect free 14-AGNR structure formed by cross-dehydrogenative coupling of two 7-AGNRs at the respective edge segments. An example of a short 14-AGNR QD sandwiched between two 7-AGNR barriers (i.e. a 7-14-7 AGNR heterostructure) is shown in Figures 1d, where the atomically defined end structure of the original 7-AGNRs is maintained and forms atomically sharp interfaces between the GNR segments. Figure 1e sketches the energy levels of the GQD, with two red lines indicating a pair of low-lying interface states (that will be discussed below) and the black lines the higher energy QD states arising from longitudinal quantum confinement between the two side 7-AGNR barriers. The electronic properties of a short GQD on Au(111) are explored experimentally using differential conductance (dI/dV) spectroscopy, which accesses the local density of states (LDOS) of nanostructures in a rather direct way. Figure 2a shows an STM image of the GQD, and differential conductance dI/dV spectra recorded at different positions above this GQD are given in Figure 2b. The spectrum taken in the middle of a 7-AGNR barrier segment (red) exhibits two pronounced peaks at -0.8 V and 1.7 V, which are similar to the previously reported valence and conduction band features of isolated 7-AGNRs on Au(111)21,25,26. Dramatically different electronic properties are observed in the sandwiched 14-AGNR QD, which exhibits a manifold of discrete states at low energies. The spectrum recorded in the middle of the 14-AGNR QD (green) shows three peaks at -0.2 V, 0.3 V, and 0.8 eV inside the energy gap of the 7-AGNR barrier segments, while the spectrum recorded near the interface between 7-AGNR barrier and 14-AGNR QD (blue) shows three additional peaks at -0.4 eV, 0.1 eV and 0.6 eV (henceforth labeled as states 1-6). A dI/dV curve (black) taken nearby on the bare Au(111) is shown as a


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reference spectrum. We notice that dI/dV spectra taken on bare Au(111) and on the edge of 7AGNR also show two minor contributions at -0.4 V and -0.2 V, which are most probably given by specific sensitivity of the tip. The energetic overlap of ribbons states with Au(111) surface states makes an accurate assignment of these peaks difficult. However, spatial dI/dV mapping reveals that states 1-6 are localized within the 14-AGNR QD (Figure 2e), and hence originate from quantum confinement within the QD. At larger sample bias, we observe an increased LDOS intensity in dI/dV spectra taken above the QD (cf. Figure 2b) near -1.0 V, 1.3 V and 2.0 V, which can be ascribed to the high density of states at higher energies (see details in Supporting Figure S6). Density functional theory (DFT) calculations on free-standing QDs were carried out to elucidate these experimental observations. We first examined the dependence of the computed LDOS on the tip-ribbon distance (Figures S1 and S2). In agreement with previous studies26,27, the wavefunctions in the interior of the AGNRs decay much faster with increasing tip-ribbon distance than those at the ribbon edges, resulting in enhanced LDOS intensity at armchair edges. Interestingly, we observe a strong contrast inversion for some of the unoccupied states (cf. Figure S2), such as states 5 and 6, which can be explained by the cancellation of positive and negative regions of wavefunctions at finite tip-ribbon distance26. More importantly, the calculated LDOS maps at a realistic tip-height of 3.5 Å above the carbon plane (Figure 2d) agree well with the experimental measurements (Figure 2c). To characterize the band alignment between 7- and 14-AGNR segments (i.e. the wide band gap barrier segments and the central QD segment), we further took dI/dV spectra along two armchair edges of a 7-14-7 heterostructure (Figure 3b and 3c). These reveal sharp transitions from an energy gap of 2.5 eV in the left 7AGNR barrier segment to 0.15 eV in the 14-AGNR QD and back to 2.5 eV in the right 7-AGNR


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barrier segment. The DFT simulated LDOS map sampled at a height of 3.5 Å (Figure 3e) convincingly reproduces these experimental observations. By comparing Figures 3b-c with Figure 3e, we clearly identify the states 1-6 in experiment as the 6 states from HOMO-2 to LUMO+2 of the free standing 7-14-7 heterostructure, respectively. This assignment implies that the 14-AGNR QD supported on the Au(111) substrate is positively charged (+2|e|), which can be understood from the band alignment between the substrate and the 14-AGNR as discussed in the Supporting Information (Figure S7). The good agreement between calculations on free-standing QDs and experimental measurements on Au(111) supported QDs also indicates that AGNRs are weakly physisorbed on Au(111), in agreement with previous studies17,25,27. This physisorption picture has further been verified by STM manipulation experiments, where isolated QDs can easily be manipulated laterally and vertically. While AGNRs are physisorbed on Au(111), the Au(111) surface states energetically overlap with ribbon states which makes an accurate assignment of all the ribbon states difficult. To access their intrinsic properties, we thus transferred individual QDs onto insulating NaCl films by means of STM manipulation28. Thin NaCl films on metals, in contrast to bulk insulating substrates, allow STM/STS investigation while electronically decoupling GNRs from the metal substrate29. Figure 4a shows a successful transfer of a 7-14-7 AGNR heterostructure from Au(111) onto a monolayer NaCl island by STM manipulation. While one 7-AGNR terminus is still in contact with Au(111), the other parts of the heterostructure have successfully been immobilized on the NaCl island (cf. upper panel of Figure 4c). As shown in Figure 4b, dI/dV spectra clearly resolve all the six low-lying ribbon states with negligible contributions from the gold surface state, confirming that the monolayer NaCl film effectively decouples the GNRs from the metal substrate. Figure 4c shows dI/dV maps of the six states, which agree with the


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results on Au(111) (cf. Figure 2c). Figure 4d shows dI/dV spectra taken along the upper edge of the heterostructure, which reveal an energy gap transition from 2.9 eV to 0.19 eV when crossing the interface between the decoupled 7-AGNR barrier and the central 14-AGNR QD. The larger value for the 7-AGNR energy gap compared to the situation on Au(111) (2.9 eV vs 2.5 eV, respectively) is ascribed to the reduced screening from the metal substrate due to the presence of the NaCl30. Figure 4e shows the computed DFT-LDOS map of the free-standing GQD obtained at 3.5 Å above the carbon plane, which agrees very well with the experimental data (Figure 4d), further corroborating the atomic precision achieved in the QD fabrication. We note that, as a consequence of their fabrication protocol, two different atomic configurations of the GQD heterostructures were observed: i) the inline configuration (a short GNR is entirely fused to a longer one, such as the one in Figure 3a), and ii) the staggered configuration (two GNRs are partially fused as shown in Figure 4a). Interestingly, our results indicate that both configurations exhibit very similar electronic structure (cf. Figures 3e and 4c) despite their different atomic configurations, suggesting weak hybridization of the states contributed by the two 7-AGNR barrier segments with the states of the 14-AGNR QD. Please also note that, as a consequence of the fabrication protocol, it is impossible to achieve finite 14AGNRs with perfect zigzag termini, that is, without two 7-14 AGNR interfaces. Theoretically, we performed DFT calculations on finite 14-AGNR QDs with and without two side 7-AGNR barriers to understand how side barriers affect the electronic properties of the sandwiched 14AGNR QD (cf. Supporting Figure S3). We find that finite 14-AGNR QDs without side barriers host four low-lying end states, with two of them being highly localized at the zigzag termini and the other two showing similar behavior as the states 3 and 4 of the 7-14-7 heterostructure. In


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short, the presence of two side 7-AGNR barriers suppresses the two highly localized end states, and slightly weakens the longitudinal quantum confinement. To gain further insight into the tunability of the GQDs, we have studied the length-dependent electronic properties of 14-AGNR QDs using a parameterized semi-empirical tight-binding (TB) model31. This TB model is very effective for calculating large systems with comparable accuracy as DFT calculations (see comparison of TB calculations with DFT calculations in Figure S4). Simulated TB-LDOS maps of three different GQD heterostructures with 14-AGNR segment lengths of 5 nm, 15 nm and 30 nm are shown in Figure 5a. Please note that the DFT-LDOS maps in Figures 3e and 4e look different than the TB-LDOS maps in Figure 5 because in contrast to the DFT-LDOS maps in Figures 3e and 4e, which are taken at a height of 3.5 Å above the carbon plane, the TB-LDOS maps are integrated along the direction perpendicular to the carbon plane. If the integration perpendicular to the carbon plane is performed for both methods, the TB- and DFT-LDOS maps agree very well with each other, as is demonstrated in Figure S4. Combing back to the TB-LDOS maps shown in Figure 5, these clearly indicate that the HOMO and LUMO states of the GQD, corresponding to states 3 and 4 marked in Figure 3c, have dominant contribution from the 7-14 AGNR interfaces with a characteristic decay length of 5 nm, while the other four states are delocalized within the 14-AGNR QD. We thus assign the states 3 and 4 to interface states, which are absent in infinitely long 14-AGNRs, and assign the remaining four states to bulk states. In Figure 5b, we plot the energy of all 6 states as a function of the length of the central 14-AGNR QD. The interface states (HOMO and LUMO) converge rapidly and are located inside the band gap of the periodic 14-AGNR for 14-AGNR QDs longer than 4 nm. The pronounced sensitivity of the energy splitting of these interface states with respect to the 14AGNR segment length allows the realization of highly tunable GQDs. Figure 5c and 5d show the


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energy levels of two GQDs with respective 14-AGNR segment lengths of 2 nm and 10 nm, where the shorter GQD shows an energy splitting of 400 meV while for the longer GQD it is lowered to 80 meV. We thus observe an impressive change in fundamental energy gap of the QD by a factor of 5 by a mere change in QD length from 2 to 10 nm. Compared to the interface states that define the fundamental energy gap of the QD, the four bulk states of the QD converge much slower, following the usual 1/length behavior of confined particles32. This indicates that the energy splitting of the QD-barrier interface states provides a new and more efficient way of tuning the fundamental energy gap of GQDs than the 1D quantum confinement responsible for the intrinsic QD bulk states. While DFT and TB electronic structures qualitatively agree with experiments very well, they quantitatively underestimate the band gaps. This is a well-known issue of DFT (and of TB parameterized on DFT) using local or semi-local exchange-correlation functionals. To obtain quantitative understanding, we calculate quasiparticle band structures of 7- and 14-AGNRs using the GW approach, a well-established methodology to calculate accurate electronic excitation energies in a broad range of materials33. We obtain quasiparticle band gaps of 3.94 eV and 0.71 eV for infinitely long 7- and 14-AGNRs in gas phase, respectively. In addition, the presence of the substrate induces a band gap reduction, due to polarization effects. The change in the quasiparticle energy levels induced by the substrate screening is estimated using a classical image-charge model34,35. We find that substrate screening significantly lowers the band gaps in both GNRs, as summarized in Table S1. Band gap reduction by substrate screening is more pronounced for AGNRs on Au(111) compared to AGNRs on NaCl/Au(111) as the presence of the NaCl monolayer reduces the substrate screening effects by increasing the distance of the GNR and the metallic substrate by ~3 Å. However, the long-range nature of screening effects34


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leads to significant renormalization even in the GNRs supported on the composite NaCl/Au(111) substrate. The calculated quasiparticle band gaps including substrate screening effects agree well with experiments for the 7-AGNR barrier segments, however the agreement for the central 14-AGNR QD segment appears to be rather poor (cf. Table S1). The calculated quasiparticle band gaps of the infinitely long 14-AGNR supported by Au and NaCl/Au are 0.17 eV and 0.24 eV respectively, which are significantly smaller than the experimental values of 0.84 eV and 1.17 eV (experimental gap is referred to the energy difference between states 2 and 5 in Figure 5b). We attribute this mismatch between calculation and experiment to quantum confinement effects, because the 14-AGNR segments are only 6.5 nm long in experiment, as opposed to the theoretical calculations on infinitely long GNRs. We also note that band gap convergence of 14AGNRs with length is significantly slower than that of 7-AGNRs. For finite length 7-AGNRs, a 6 nm long ribbon already shows a band gap that is within 0.1 eV of that of an infinitely long 7AGNR ribbon, while gap convergence of 14-AGNRs requires lengths beyond 30 nm (cf. Figure 5b). This significant difference derives from the fact that charge carriers in 7-AGNRs have a larger effective mass than those of 14-AGNRs (see details in Supporting Figure S6). Experimentally, we could not determine the band gap of very long 14-AGNR segments due to the overlap of gold surface states with ribbon states. The longest 14-AGNR segment that could be characterized has a length of 10.2 nm and a band gap of 0.56 eV (cf. Figure S5). In conclusion, we have demonstrated the feasibility of GQDs fabricated bottom-up via crossdehydrogenative coupling of 7-AGNRs, with electron/hole states that are tunable over a wide energy range by the length of the central 14-AGNR QD segment. Based on STM/STS investigations, we determined sharp transitions from an energy gap of 2.5 eV (2.9 eV) in the


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wide band gap 7-AGNR barrier segments to 0.15 eV (0.2 eV) in the central 14-AGNR QD for the 7-14-7 heterostructure supported on a Au (NaCl/Au) substrate. An in-depth understanding of electronic properties has been gained by combined DFT and TB calculations, confirming the presence of a pair of low energy interface states at the atomically sharp 7-14 AGNR interfaces. The exponential dependence of the energy splitting of these interface states on the 14-AGNR QD length allows a wide tunability of the fundamental gap of the GQDs by one order of magnitude within few nanometers length range, which opens exciting perspectives for the design of GQDbased devices. Compared with recently reported GQDs in continuous monolayer graphene 11,12,45, our bottom-up fabricated GQDs are much smaller with a size of only a few nanometers but with comparable energy gaps. In addition, the established bottom-up method yields GQDs with atomically precise atomic structure and hence precise energy gaps, and holds promise to further tune their properties by designing different molecular precursors and/or using different onsurface reactions.

METHODS: A commercial low-temperature STM (Scienta Omicron) was used for sample preparation and characterization in situ under ultra-high vacuum condition with a base pressure below 1×10-10 mbar. The Au(111) single crystal was cleaned by standard argon sputtering and annealing cycles. Following the recipe by Cai et al., we first synthesized 7-AGNRs on Au(111)19, and then post-annealed the sample to 725 K to activate edge fusion, yielding atomically precise GQDs. In order to obtain isolated short GQDs for manipulation experiments, annealing at much lower temperature of 525 K for 3 hours has been used to slowly activate edge fusion. The realized isolated short QDs can be easily manipulated both laterally and vertically due to the weak adhesion of defect-free heterostructures to the underlying Au(111) substrate.


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This allows to use a STM-manipulation procedure to transfer isolated ribbons onto NaCl monolayer islands to characterize the decoupled electronic structure28. The dI/dV spectra and dI/dV maps were recorded using the lock-in technique (Urms = 20 mV). DFT calculations are performed with the Quantum ESPRESSO36 code using the TroullierMartins norm-conserving pseudopotentials37 and the PBE functional38. We used a 60 Ry planewave cutoff and a 1×1×32 k-point grid. A plane-wave cutoff of 45 Ry is used. The k-point sampling is restricted to the Γ point. A 10 Å vacuum space is included to minimize interactions between the periodic images. All atomic coordinates are relaxed using a conjugate-gradient algorithm until all forces are smaller in magnitude than 0.05 eV/Å. GW calculations are carried out using the BerkeleyGW package39,40. The quasiparticle energies are calculated using the G0W0 approach within the generalized plasmon-pole (GPP) model39. We used a vacuum space of 8 Å between the periodic images and employed a truncated Coulomb interaction to nullify the interaction between periodic images41. To obtain the static dielectric matrix, we used a plane-wave cutoff of 8 Ry and 400 (600) bands for 7- (14-) AGNR, covering the energy range up to 40 eV above the highest occupied band. We also employed the staticremainder method to accelerate convergence with respect to unoccupied states42. Tight-binding calculations are performed using a model TB+U Hamiltonian of the form H = H + H , where H is the usual π-band tight-binding Hamiltonian with first-, second-, and third-nearest-neighbor hopping integrals given by t1=3.2, t2=0 eV and t3= 0.3 eV, respectively43. The hydrogen-termination at the edges is modeled by including a ∆t1 = 0.2 eV correction to the t1 parameter for the edge atoms43. H = U ∑ n ↑ n ↓ is the Hubbard term, where U = 2.944 eV is the on-site repulsion44 and n = c c is the site occupation operator. The TB+U Hamiltonian is

solved iteratively until the self-consistent solution is obtained.


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FIGURES

Figure 1. On-surface fabrication of graphene quantum dots. (a) Schematic illustration of the formation of a 14-AGNR quantum dot by edge fusion of two 7-AGNRs. (b) STM image showing several 7-14-7 quantum dot heterostructures on Au(111). Scale bar: 5 nm. (c) nc-AFM frequency shift image of a long 14-AGNR segment acquired with a CO functionalized tip. Scale bar: 2 nm. (d) nc-AFM image of a short 7-14-7 AGNR quantum dot. Scale bar: 2 nm. (e) Schematic energy level diagram of the 7-14-7 AGNR quantum dot in (d). Two red lines indicate a pair of low-


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energy interface states, and the black lines indicate the levels arising from longitudinal quantum confinement of electrons/holes within the 14-AGNR quantum dot segment.


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Figure 2. Quantized states with discrete energies of a short graphene quantum dot. (a) STM image of a short 7-14-7 AGNR quantum dot heterostructure on Au(111). Scale bar: 2 nm. (b) dI/dV spectra taken at the positions indicated by the color dots in (a). The black spectrum was taken nearby on the clean Au(111) surface as a reference. The spectra are offset vertically for clarity. (c) dI/dV maps of states 1-6 marked in (b). (d) Corresponding DFT-calculated LDOS maps of states 1-6 sampled at a height of 3.5 Å above the carbon plane.


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Figure 3. Energy gap transition within a 7-14-7 AGNR quantum dot heterostructure. (a) STM image (-1 V, 20 pA) of a 7-14-7 AGNR quantum dot heterostructure on Au(111). (b-c) Colorcoded representation of dI/dV spectra taken along two armchair edges of the heterostructure; (b), along the red dashed line and (c), along the black dashed line. (d) DFT-simulated STM image. (e) DFT-based energy resolved LDOS map along the heterostructure (integrated across the width and sampled at a height of 3.5Å above the carbon plane). Scale bars: 2 nm.


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Figure 4. Electronic decoupling of a short graphene quantum dot heterostructure via STM manipulation onto an insulating NaCl island. (a) Transfer of a 7-14-7 quantum dot heterostructure onto a NaCl monolayer island. Scale bar: 5 nm. (b) dI/dV spectra taken at the positions indicated by the color dots in the inset STM image (-0.2 V, 0.1 nA). The spectra are vertically offset for clarity. (c) dI/dV maps of states 1-6 marked in (b) and STM topographic image (-0.2 V, 0.1 nA) of the decoupled heterostructure. (d) STM image (top panel, -0.2 V, 0.1 nA) and color-coded representation of dI/dV spectra (bottom panel) taken along the upper armchair edge of the heterostructure. (e) Top panel: DFT-calculated STM image. Bottom panel:


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Energy resolved LDOS map along the heterostructure (integrated across the width and sampled at a height of 3.5Å above the carbon plane).


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Figure 5. Length-dependent energy level scheme for graphene quantum dots. (a) Tight-binding calculated LDOS plots of three graphene quantum dots with 14-AGNR segment length of 5, 15 and 30 nm, respectively. (b) Energies of the six states marked in (a) as a function of 14-AGNR segment length. The horizontal dotted lines depict the valence band maximum (VBM) and the conduction band minimum (CBM) of the infinitely long 14-AGNR. (c-d) Energy levels of two graphene quantum dots with 14-AGNR segment length of L1=2 nm and L2=10 nm, respectively.


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ASSOCIATED CONTENT Supporting Information: DFT calculated height-dependent LDOS maps are shown in Figure S1 and S2. DFT-LDOS maps of 14-AGNR QD with and without two side 7-AGNR barriers are shown in Figure S3. DFT and parameterized tight-binding calculations on the same 7-14-7 heterostructure are compared in Figure S4. Energy gaps of 14-AGNR quantum dots with different lengths are shown in Figure S5. Band structure of periodic 7-AGNR and 14-AGNR is given in Figure S6. Band alignment calculations of 7-AGNR and 14-AGNR with respect to supporting substrates are given in Figure S7. Additionally, experimental and theoretical values of band gaps of isolated and substrate-supported GNRs are summarized in supporting table 1. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Pascal Ruffieux †

E-mail: [email protected] (PR).

Author Contributions R.F. and P.R. conceived and supervised the experiments. S.W. performed the scanning probe experiments. N.K. and E.C.G. performed the simulations under supervision of V.M.. The Mainz and Dresden groups synthesized the molecular precursors. S.W., N.K., and P.R. wrote the paper. All authors discussed the results and implications and commented on the manuscript at all stages. S.W. and N.K. contributed equally to this work.


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Funding Sources ACKNOWLEDGMENT We acknowledge financial support by the Swiss National Science Foundation (Projects 200020_162857 and 20PC21_155644), the Office of Naval Research BRC Program (award N00014-12-1-1009), and the European Commission Graphene Flagship (No. CNECT-ICT604391). E.C.G. acknowledges support from CNPq (Process No. 473714/2013-2 and Process No. 306378/2014-0). E.C.G. and V.M. acknowledge support from CAPES through the Science without Borders program (Project No. A085/2013). Notes The authors declare no competing financial interest.

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