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Band Depopulation of Graphene Nanoribbons Induced by Chemical Gating with Amino Groups Journal: ACS Nano Manuscript ID nn-2019-08162c.R2 Manuscript Type: Article Date Submitted by the n/a Author: Complete List of Authors: Li, jingcheng; CIC nanoGUNE, Brandimarte, Pedro; Centro de Fisica de Materiales, Vilas-Varela, Manuel; Universidade de Santiago de Compostela, Organic Chemistry Merino-Díez, Nestor; Donostia International Physics Center, Moreno, Cesar; Catalan Institute of Nanoscience and Nanotechnology Mugarza, Aitor; Institut Catalá de Nanotecnologia, mollejo, Jaime saez ; Donostia International Physics Center Sanchez-Portal, Daniel; CSIC, CFM CSIC-UPV/EHU Garcia de Oteyza, Dimas ; Donostia International Physics Center Corso, Martina; Centro de Fisica de Materiales, (CSIC-UPV/EHU) Garcia-Lekue, Aran; Donostia International Physics Center Peña, Diego; Universidade de Santiago de Compostela, CIQUS Pascual, Jose; CIC nanoGUNE Consolider, Tolosa Hiribidea, 76 ACS Paragon Plus Environment Page 1 of 19 ACS Nano 1 2 3 4 5 6 7 8 Band Depopulation of Graphene Nanoribbons 9 10 11 12 Induced by Chemical Gating with Amino Groups 13 14 15 Jingcheng Li,† Pedro Brandimarte,‡ Manuel Vilas-Varela,¶ Nestor 16 17 18 Merino-Dı́ez,†,‡ César Moreno,§ Aitor Mugarza,§,k Jaime Sáez Mollejo,‡ Daniel 19 20 Sánchez Portal,⊥,‡ Dimas G. de Oteyza,⊥,‡,# Martina Corso,⊥,‡ Aran 21 22 23 Garcia-Lekue,∗,‡,# Diego Peña,∗,¶ and Jose Ignacio Pascual ∗,†,# 24 25 26 †CIC nanoGUNE BRTA, Tolosa Hiribidea 76, 20018 Donostia-San Sebastian, Spain 27 ‡Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain 28 29 ¶Centro Singular de Investigación en Quı́mica Biolóxica e Materiais Moleculares (CiQUS), 30 31 and Departamento de Quı́mica Orgánica, Universidade de Santiago de Compostela,15782 32 33 Santiago de Compostela, Spain 34 35 §Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona 36 37 Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona Spain 38 39 kICREA Institució Catalana de Recerca i Estudis Avancats, 08193 Barcelona, Spain 40 41 ⊥Centro de Fı́sica de Materiales MPC (CSIC-UPV/EHU), 20018 Donostia-San Sebastián, 42 43 Spain 44 45 #Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain 46 47 48 E-mail: wmbgalea@ehu.eus; diego.pena@usc.es; ji.pascual@nanogune.eu 49 50 51 52 53 54 55 56 57 58 59 1 60 ACS Paragon Plus Environment ACS Nano Page 2 of 19 1 2 3 Abstract 4 5 The electronic properties of graphene nanoribbons (GNRs) can be precisely tuned 6 by chemical doping. Here we demonstrate that amino (NH2 ) functional groups attached 7 8 at the edges of chiral GNRs (chGNRs) can efficiently gate the chGNRs and lead to 9 the valence band (VB) depopulation on a metallic surface. The NH2 -doped chGNRs 10 are grown by on-surface synthesis on Au(111) using functionalized bianthracene pre- 11 12 cursors. Scanning tunneling spectroscopy resolves that the NH2 groups significantly 13 up-shift the bands of chGNRs, causing the Fermi level crossing of the VB onset of 14 chGNRs. Through density functional theory simulations we confirm that the hole- 15 16 doping behavior is due to an upward shift of the bands induced by the edge NH2 17 groups. 18 19 20 21 22 Keywords 23 24 scanning tunneling microscope, density functional theory, chiral graphene nanoribbons, dop- 25 ing, amino, chemical gating, band depopulation 26 27 28 29 30 31 32 Graphene nanoribbons (GNRs) have emerged as a promising material for nanoelectron- 33 ics, 1 since they combine many of the extraordinary properties of the parent graphene 2,3 with 34 35 a high tunability of their electronic band structure. 4 Contrary to graphene, GNRs are fre- 36 37 quently semiconducting materials, thus offering excellent perspectives for their utilisation as 38 electronic components such as diodes 5 or transistors. 1,3,6,7 A fascinating aspect of narrow 39 40 GNRs is that their electronic properties can be tuned through the precise control of their 41 atomic structure, which can be achieved thanks to the recent development of bottom-up 42 43 on-surface synthesis (OSS) strategies. 8,9 These strategies rely on the careful design of suit- 44 45 able molecular precursors with specific shape and chemical composition to steer a step-wise 46 reaction on a metal surface, leading to extended and atomically precise GNRs. An ample 47 48 library of precursors and reaction pathways has been constructed in the last years, incorpo- 49 50 rating successful examples of precise control over the GNR’s width, 10–17 orientation and edge 51 topology, 18–25 which resulted in large variations of their electronic structure. Furthermore, 52 53 OSS strategies allow to easily combine several precursors on a surface for fabricating complex 54 55 structures such as atomically sharp graphene heterojunctions, 26,27 quantum dots, 28,29 or hy- 56 brid systems composed of metal-organic molecules embedded in GNRs, 30–32 demonstrating 57 58 59 2 60 ACS Paragon Plus Environment Page 3 of 19 ACS Nano 1 2 3 their potential role for GNRs based nanoelectronics. 4 5 Since most GNRs are semiconducting, a promising method for tuning their band struc- 6 7 ture is the electrostatic gating effect induced by doping. 33 Chemical doping of GNRs has 8 been achieved through modification of the molecular precursors to either incorporate sub- 9 10 stitutional heteroatoms in the carbon backbone 26,28,34–41 or by adding functional groups at 11 12 the edges. 42 However, the band structure of the substitutionally-doped systems could not 13 always be simply related to the pristine one (CB); 28,35,36,38,39 the embedded heteroatoms 14 15 have a profound impact on the bands’ symmetries and character. 43 Instead, chemical func- 16 17 tionalization of the GNR edges promisingly behaved as an efficient method to fine-tune the 18 electronic structure of pristine GNRs. In the most simple scenarios, the effect of attached 19 20 chemical groups can be described as an effective electrostatic gating of the native band 21 22 structure. 26,40,42 The magnitude of edge chemical gating is small, to date barely restricted 23 to sub-gap down-shifts of the band structure. 24 25 Here, we report on the very efficient chemical gating of amino(NH2 ) functional groups 26 27 attached at the edges of narrow chiral GNRs with a sequence of 3 zigzag and 1 armchair sites 28 ((3,1) chGNRs). The electron donating character of NH2 is inherited by the GNR, resulting 29 30 in an effective GNR charging by hole injection and valence band depopulation on a Au(111) 31 32 surface. Experimentally, NH2 end groups were substituted at the edges of pristine chGNRs 33 through bottom-up synthesis with modified precursors. Our results satisfactorily demon- 34 35 strate that the electropositive affinity of the dopant is transmitted to the chGNR carbon 36 37 backbone, 42 which on a Au(111) surface becomes strongly hole doped. Through a combina- 38 tion of scanning tunnelling microscopy (STM) and spectroscopy (STS), X-ray photoelectron 39 40 spectroscopy (XPS) measurements and density functional theory (DFT) calculations we show 41 42 that the NH2 groups induce a substantial up-shift of the GNR bands, and cause their VB to 43 cross the Fermi level. The effective depopulation of the VB depends on the number of NH2 44 45 groups per unit cell surviving the reaction, as well as on their relative alignment. 46 47 48 49 Results/Discussion 50 51 Synthesis of the molecular precursor: To functionalize (3,1) chGNRs with NH2 groups 52 53 at the edge (named as NH2 -chGNRs), we prepared the molecular precursor 2,2’-dibromo- 54 [9,9’-bianthracene]-10,10-diamine (3, Fig. 1) in two steps from 2,2’-dibromo-9,9’-bianthracene 55 56 (1). First, selective double nitration of compound 1 led to the synthesis of compound 2, 57 58 59 3 60 ACS Paragon Plus Environment ACS Nano Page 4 of 19 1 2 3 which was isolated with 64% yield. Then, Fe-promoted reduction of bianthracene 2 in acetic 4 5 acid afforded the GNR precursor 3 with 98% yield (see Supporting Information for details). 6 7 8 9 10 11 12 13 1 2 3 14 15 16 Figure 1: Synthetic route to obtain 2,2’-dibromo-[9,9’-bianthracene]-10,10-diamine (3). 17 18 19 a Precursor 3 b c d N1s e 20 200 °C 100 400ºC 21 NH2 NH2 22 H 2N H 2N 200ºC Atomic Ratio N/N(RT) (%) 80 23 175ºC Intensity (a.u) 24 NH2 H2N NH2 150ºC 60 25 H2N 260 °C 100ºC 26 NH2 50ºC 40 NH2 27 H2N H2N High 20 28 RT 29 NH2 NH2 0 30 H 2N H 2N 5 nm 404 402 400 398 396 0 200 400 31 Low Binding Energy (eV) Sample temperature (ºC) 32 33 Figure 2: Synthesis of NH2 -chGNRs. (a),(b) Reaction scheme of the polymerization and 34 cyclodehydrogenation of molecular precursors respectively. For simplicity, only one chirality 35 of NH2 -chGNRs is shown here. (c) STM overview image of NH2 -GNRs grown on Au(111), 36 37 taken at Vs = 0.2 V, It = 20 pA. The dashed lines show the preferred orientations of 38 NH2 -chGNRs on the substrate. Same color scale is used for all the STM images. (d,e) 39 Characterization of NH2 groups at different annealing temperatures by N1s peaks in XPS 40 measurements. 41 42 43 The molecular precursor 3 was sublimated from a Knudsen cell at 212 ◦ C onto a clean 44 45 Au(111) substrate kept at room temperature. The precovered sample was gradually annealed 46 first to 200 ◦ C to induce the polymerization of compound 3 through Ullmann-like coupling 47 48 reactions (Fig. 2(a)). Subsequently, the sample temperature was further raised to 260 ◦ C to 49 50 trigger the cyclodehydrogenation step causing the planarization of the polymers into NH2 - 51 chGNRs (Fig. 2(b)). Fig. 2(c) shows a STM overview image of the resulting NH2 -chGNRs on 52 53 a Au(111) surface. The doped chGNRs, as the pristine ones, display preferential orientations 54 55 on the substrate, but notably shorter than pristine chGNRs, 21,24 reaching lengths of up to 15 56 nm. We speculate that the attachment of NH2 groups lowers the diffusion of precursors and 57 58 59 4 60 ACS Paragon Plus Environment Page 5 of 19 ACS Nano 1 2 3 thus hinders their polymerization processes 44 . The edges of the resulting NH2 -chGNRs are 4 5 quite inhomogeneous, and their width is larger than pristine chGNRs, suggesting the presence 6 7 of a certain amount of NH2 groups attached to the edges (see Supporting Information). 8 To prove the survival of NH2 groups during the on-surface synthesis (OSS) process, we 9 10 characterized the thermal evolution during the OSS with X-ray photoelectron spectroscopy 11 12 (XPS) measurements (Fig. 2(d,e)). We find a gradual decrease of the N-1s XPS peak in- 13 tensity with increasing annealing temperatures, which completely vanishes at 400◦ C. Since 14 15 the cyclodehydrogenation step completing the synthesis of chGNRs occurs at temperatures 16 17 slightly above 200◦ C, 21,24 the XPS results suggest that about 60% of the NH2 groups survive 18 the OSS. 19 20 Fig. 3(a) shows the constant current STM image of a NH2 -chGNR which shows irregular 21 22 edges. The superimposed chemical structure of the NH2 -chGNR indicates the loss of some 23 NH2 groups on the edges. To further prove this, we acquired high resolution STM image of 24 25 the same GNR with a CO-terminated tip, 45,46 as shown in Fig. 3(b). The high resolution 26 27 image resolves the backbone structure of the ribbon as composed of zig-zag edged segments, 24 28 and also shows distinctive triangular features at many of the zigzag edges (indicated by 29 30 arrows in Fig. 3(b)), which we attribute to surviving NH2 groups as shown in Fig. 3(a). 31 32 The carbon atoms with missing NH2 groups are then passivated by hydrogen atoms during 33 the on-surface reactions 47 . Most of the chGNR segments maintain at least one of the NH2 34 35 edge groups attached at one side, and some appear with both of them intact. From such 36 37 high resolution images on more than 80 GNRs, we extract that about 75% of the NH2 38 groups survived the on-surface reaction. This value is even larger than the one obtained 39 40 from the XPS results, most probably due to the different annealing processes (see methods). 41 42 This is a remarkable doping value when compared with the chemical doping surviving the 43 reaction for cyano(CN) functionalization of 7-AGNRs. 42 We note that edge functionalization 44 45 in such chiral ribbons is so efficient due to the lower temperatures required for their complete 46 47 synthesis, in contrast with e.g. 7-AGNRs, which need more than 300◦ C for their OSS. 48 Electronic structure of doped chGNRs: NH2 groups have an electron donating 49 50 character. To study their effect on the electronic properties of the chiral nanoribbons, we 51 52 recorded differential conductance plots (dI/dV) directly over singly and doubly NH2 -doped 53 segments, and compare them with measurements on pristine chGNRs. As shown in Fig. 3(c), 54 55 the characteristic 0.7 eV energy gap of pristine chGNRs 24 is severely modified on the NH2 - 56 57 58 59 5 60 ACS Paragon Plus Environment ACS Nano Page 6 of 19 1 2 3 a b c g VB Pristine d d e f 4 cGNRs 5 H2N 5 0.70 eV 1 6 NH2 7 4 NH2 8 9 0.51 eV 1 H2N NH2 3 10 dI/dV (a.u.) 11 NH2 h CB H2N 2 2 12 NH2 2 13 H2N 14 NH2 1 0.37 eV H2N 15 Au(111) NH2 16 0 17 -1.0 -0.5 0.0 0.5 1.0 100 mV 1 nm 2 mV 0 mV 200 mV 530 mV 18 Bias (V) 19 20 Figure 3: Characterization of the atomic and electronic structures of NH2 -chGNRs. (a) 21 Constant current image of a single NH2 -chGNRs with its chemical structure superimposed 22 on top. (b) Constant height current image (Vs = 2 mV) of the same NH2 -chGNRs in (a) 23 24 taken with CO-terminated tip. (c) dI/dV spectra taken on the locations as indicated with 25 numbers in (b). For comparison, dI/dV spectra taken on pristine chGNRs (black curve on 26 top) are also shown here. Blue and red dashed lines indicate the energy locations of VB 27 and CB, respectively. The values of the band gaps are noted in the figure. (Open-feedback 28 parameters: Vs = 1 V, It = 1 nA; modulation voltage Vrms = 10 mV). (d-f) Constant height 29 30 dI/dV maps of the ribbon in (a) taken at different bias values as indicated in the figures 31 respectively (modulation voltage Vrms = 10 mV). All STM images share the same scale bar 32 indicated in (b). (g),(h) DFT simulations of the wave function corresponding to states at 33 the onset of VB and CB (at the Γ point) of NH2 -chGNRs with all NH2 groups attached. 34 Red and blue colors represent isosurfaces of positive and negative wave function amplitudes, 35 36 for an isovalue of 0.015 Å−3/2 . 37 38 39 substituted segments, which exhibit instead two pronounced peaks, indicated by red and 40 blue dashed lines in Fig. 3(c). Over the singly doped segment, one of the peaks is pinned at 41 42 the Fermi level (zero bias), while the other appears at 500 mV above. On the doubly-doped 43 44 units, both peaks appear slightly shifted upwards, and their separation noticeably decreases. 45 To elucidate the origin of the peaks, we acquired dI/dV maps at bias values corresponding 46 47 to the peak energies, which unveil the spatial distribution of these states (Fig. 3(d-f)). dI/dV 48 49 maps at 0 mV and 200 mV (the bias values of the lower peaks in each case) resemble the 50 VB shape of pristine chGNRs at the singly- and doubly-doped segments, respectively. In 51 52 contrast, the dI/dV map at 530 mV (in coincidence with the higher-bias peak in both 53 54 cases) reproduces a wave-front pattern that is characteristic of the CB of pristine chGNRs. 24 55 Furthermore, these dI/dV maps show very good agreement with the density functional theory 56 57 58 59 6 60 ACS Paragon Plus Environment Page 7 of 19 ACS Nano 1 2 3 a b H 2N c CB d e 4 1 7 VB 5 NH2 1 6 2 6 2 7 8 H 2N 3 5 3 9 NH2 dI/dV (a.u.) 10 4 4 4 11 NH2 12 3 5 13 6 NH2 2 14 5 15 16 NH2 6 1 7 17 7 0 18 NH2 -1.0 -0.5 0.0 0.5 1.0 19 200 mV 1 nm 2 mV Bias (V) 0 mV 550 m V 20 21 Figure 4: Electronic structures of NH2 -chGNR with half NH2 groups lost. (a) Constant 22 current image of a single NH2 -chGNRs with its chemical structure superimposed on top. 23 (b) Constant height current image (Vs = 2 mV) of the same NH2 -chGNRs in (a) taken 24 with CO-terminated tip. (c) dI/dV spectra taken on the locations indicated with numbers 25 26 in (b). Blue and red dashed lines indicate the energy onsets of VB and CB, respectively. 27 (Open-feedback parameters: Vs = 1 V, It = 1 nA, modulation voltage Vrms = 10 mV). (d-e) 28 Constant height dI/dV maps of the ribbon in (a) taken at different bias values as indicated 29 in the figures respectively (modulation voltage Vrms = 10 mV). All STM images share the 30 same scale bar indicated in (b). 31 32 33 simulations of wave function amplitude of valence and conduction bands along a fully NH2 - 34 35 doped ribbon (Fig. 3(g,h)). Based on these observations, we ascribe the two dI/dV peaks to 36 37 the VB and CB onsets of NH2 -chGNRs, respectively. 38 Compared to pristine chGNRs, both VB and CB onsets of the NH2 -doped segments 39 40 appear shifted upwards, and the VB-CB energy gap is significantly reduced (as labelled in 41 42 Fig. 3(c)). This upwards shift of the frontier bands can be attributed to a significant hole- 43 doping introduced by the edge NH2 groups, whose effect increases with the number of groups 44 45 surviving in each segment. Particularly striking is the alignment of the VB onset with the 46 47 Fermi-level for the singly-doped segments, and its further shift to ∼0.18 eV when a second 48 NH2 is present, unveiling a substantial band depopulation by edge-doping, and underpinning 49 50 of chGNRs on Au(111). 51 52 Intriguingly, the number of NH2 -dopants per unit is not the only parameter tuning the 53 bands’ onset, but also their relative arrangement. This was concluded from the inspection 54 55 of ribbons with only one edge group per unit. In order to induce the more diluted function- 56 57 58 59 7 60 ACS Paragon Plus Environment ACS Nano Page 8 of 19 1 2 3 a b c d 0.125 4 charge balance (e) 5 6 0.0 7 8 9 10 0.125 11 e f g h i 6 VH 2NH2 -VH pris 12 1 4 13 x 14 2 2 15 ~0.7 eV 16 0 17 3 VH (eV) E (eV) 18 0.36 eV 0.37 eV 0.66 eV 2 0.31 eV 0.39 eV 0.78 eV 0.41 eV 19 4 20 4 21 5 6 22 23 8 24 6 25 10 26 /a /a /a /2a 0 10 20 30 x (Å) 27 28 Figure 5: Simulated electronic structure and charges. Charge distribution calculated with a 29 30 Hirshfeld population analysis for: (a) pristine chGNR, (b) chGNR with NH2 groups on one 31 side (1a-NH2 -chGNR), (c) NH2 groups on both sides (2-NH2 -chGNR), and (d) on alternating 32 sides (1b-NH2 -chGNR). Corresponding band structures obtained for (e) pristine chGNR, (f) 33 1a-NH2 -chGNR, (g) 2-NH2 -chGNR and (h) 1b-NH2 -chGNR. The shift in energy of the top 34 of the VB and the bottom of the CB as compared to the pristine chGNR are marked in green 35 36 and in red, respectively. (i) Electrostatic potential difference between the 2-NH2 -chGNR and 37 a pristine chGNR, taken at the plane of the ribbon and averaged along the periodic direction 38 (y). 39 40 41 alization observed in Fig. 4, we annealed the sample to a slightly higher temperature during 42 43 the OSS process (e.g. to 280 ◦ C for the data shown in Fig. 4). The result was a reduction of 44 the edge functionalization to about 50%, with a nice preference to maintain one group per 45 46 unit cell rather than two or none. We found that NH2 groups could either line-up along the 47 48 same edge (e.g. as between segments 4 and 7 in Fig. 4(a,b), or alternate at opposite sides 49 (as between 1 and 4 in Fig. 4(a,b)), what had an apparent impact in their corresponding 50 51 band alignment. While the dI/dV spectra (Fig. 4(c)) and maps (Fig. 4(d) and (e)) appear 52 53 in every case similar to the ones shown in Fig. 3, both VB and CB peaks in regions with 54 alternating NH2 groups (spectra 1 to 4 in Fig. 4(c)) appear shifted upwards by ∼ 50 meV 55 56 with respect to the regions where NH2 groups line up in the same side (spectra 5 to 7). 57 58 59 8 60 ACS Paragon Plus Environment Page 9 of 19 ACS Nano 1 2 3 Theoretical analysis: Further insight into the doping effects of NH2 groups is obtained 4 5 by DFT calculations of free-standing nanoribbons (see Methods for details on the simula- 6 7 tions). In particular, we studied the band structure of infinite (3,1) chGNRs with the edge 8 structures shown in Fig. 5(a-d), namely, the three experimentally observed scenarios: NH2 9 10 groups only on one side (1a-NH2 -chGNR), on both sides (2-NH2 -chGNR), and in alternating 11 12 positions (1b-NH2 -chGNR), as well as the pristine chGNRs for comparison. The correspond- 13 ing band structures (Fig. 5(e-h)) show that, in all three cases, the presence of NH2 groups 14 15 at the edge induces an upward shift of the bands with respect to the pristine case. For the 16 17 singly-doped cases, the VB band shifts about 0.4 eV. When both edges are doped (2-NH2 - 18 chGNR), the upwards shift of the VB is almost doubled (0.78 eV), showing that it scales 19 20 with the number of dopants. 21 22 The band structure also reproduces the clear reduction of the VB-CB energy gap with 23 doping seen in the experiment. In particular, we find that the band gap of the singly-doped 24 25 GNR decrease ≈ 10% with respect to the pristine case, while this reduction amounts to ≈ 26 27 40% for the doubly-doped species. A similar band gap closure with doping in GNRs was 28 attributed to the extension of the π-conjugated electron cloud outside the graphene backbone 29 30 with the addition of the functional groups. 42 On the other hand, we note that the calculated 31 32 shifts of the frontier bands are larger than the measured ones, and the gap reduction obtained 33 numerically is smaller than in experiments. These discrepancies might be related to screening 34 35 effects of the metallic substrate, which are not included in our calculations. Yet, our DFT 36 37 results are in very good qualitative agreement with the experimental measurements. 38 To explain the origin of the band shifts, we studied the charge redistribution induced by 39 40 the NH2 edge groups in the chGNRs. As seen from the Hirshfeld plots in Fig. 5(a-d), the 41 42 NH2 groups redistribute their charges by the covalent bonding to the edge and induce a small 43 electron depletion at the connecting C and accumulation right at the middle of the ribbon. 44 45 Interestingly, the lower electron density at the edge coincides with the higher current of these 46 47 rings in the chGNR backbone (Fig. 3(a) and Fig. 4(a))). We speculate that the higher current 48 signal over the edge phenyl rings bonding to NH2 and lower current over the neighbour rings 49 50 at the center are related to the calculated electron depletion and accumulation, respectively, 51 52 and to their influence on the effective tunneling. 53 As a result of the charge redistribution, a strong edge dipole of ∼3.6 D per unit cell 54 55 pointing outwards from the ribbon backbone emerges around the NH2 group. Considering 56 57 58 59 9 60 ACS Paragon Plus Environment ACS Nano Page 10 of 19 1 2 3 the edge dipole per unit cell of the pristine chGNR (∼2.1 D), and since both CH and NH2 4 5 dipoles are pointing in the same direction, we can estimate that each NH2 group contributes 6 7 with a dipole of about 2.0 D. This edge dipole increases the internal electrostatic potential, 8 and consequently, shifts their occupied bands upwards. For example, for the 2-NH2 -chGNR 9 10 case (Fig. 5(i)), we find that the ∼0.7 eV higher electrostatic potential is the main responsible 11 12 of the ∼0.7 eV up-shift of its VB. The result is that the NH2 groups endow their electron- 13 donating character to the ribbons. On the Au(111) surface, this leads to a substantial 14 15 hole-doping from the metal, reaching the point where the VB is partially depopulated. We 16 17 note that this scenario differs from the previously reported case of armchair GNRs doped 18 with CN groups. CN induced a dipole pointing opposite, i.e. towards the ribbon’s backbone, 19 20 leading to a total dipole smaller in magnitude and opposite in sign to the one presented 21 22 here. This caused a small band down shift, contrary to the situation found here, but the 23 semiconducting character remained unchanged. 42 24 25 26 27 28 Conclusions 29 30 31 Our results demonstrate a chemical-gating strategy of graphene nanoribbons, by which in- 32 corporating electron donating chemical groups to the edge of GNRs causes a significant 33 34 hole-doping on a Au(111) surface. As found previously with CN groups, 42 the GNR inherits 35 36 the electron affinity of the attached chemical group. Favored by the lower reaction tempera- 37 tures for the synthesis of chiral GNRs, we found that a large fraction of NH2 groups survived 38 39 the on-surface reaction, and caused the depopulation of its VB, signaling its charging. 40 Based on DFT simulations, we explained the enhanced electron-donating character of the 41 42 doped ribbons as arising from a substantial charge redistribution induced by the dopant. 43 44 From these results, we foresee that combination of edge terminations with different electron 45 affinity character can be a promising route to engineer hybrid GNRs with active electronic 46 47 properties. 48 49 50 51 52 Methods/Experimental 53 54 The experiments were performed on a home-made low temperature (5 K) STM under ultra- 55 56 high vacuum (UHV) conditions. The Au(111) substrate was cleaned in UHV by repeated 57 58 59 10 60 ACS Paragon Plus Environment Page 11 of 19 ACS Nano 1 2 3 cycles of Ne+ ion sputtering and subsequent annealing to 460 ◦ C. The molecular precursor 4 5 was sublimated at 210 ◦ C from a Knudsen cell onto the clean Au(111) substrate kept at 6 7 room temperature. Then the sample was first annealed at 200 ◦ C for 15 minutes and then 8 annealed at 260 ◦ C for 5 minutes. A tungsten tip functionalized with a CO molecule was 9 10 used for high resolution images. The high resolution images were acquired in constant height 11 12 mode, at very small voltages, and junction resistances of typically 20 MΩ. The dI/dV signal 13 was recorded using a lock-in amplifier with a bias modulation of Vrms = 10 mV at 760 Hz. 14 15 XPS measurements were carried out using a Specs PHOIBOS 150 hemispherical energy an- 16 17 alyzer using a monochromatic X-ray source (Al Kα line with an energy of 1486.6 eV and 400 18 W) and energy referenced to the Fermi level. Surface cleanliness was confirmed with x-ray 19 20 photoelectron spectroscopy (XPS) prior molecules deposition. Organic molecules were sub- 21 22 limated in UHV from a home-made Knudsen-cell type evaporator with an alumina crucible, 23 with the sample kept at room temperature. Then the sample was step-wisely annealed to 24 25 400 ◦ C. The sample was kept at each temperature step in Fig. 2d for 10 minutes for the 26 27 XPS measurements. 28 29 30 Computational Procedures 31 32 The optimized geometry and electronic structure of free-standing pristine and NH2 doped 33 ch-GNRS were calculated using density functional theory, as implemented in the SIESTA 34 35 code. 48 The nanoribbons were relaxed until forces on all atoms were < 0.01 eV/Å, and the 36 37 dispersion interactions were taken into account by the nonlocal optB88-vdW functional. 49 38 The basis set consisted of double-ζ plus polarization (DZP) orbitals for all species, with 39 40 an energy shift parameter of 0.01 Ry. A 1×1×100 Monkhorst-Pack mesh was used for the 41 42 k-point sampling of the Brillouin zone, where the 100 k-points are taken along the direction 43 of the ribbon. A cutoff of 300 Ry was used for the real-space grid integrations. The atomic 44 45 population analysis was performed using the Hirshfeld scheme for partitioning the electron 46 47 density. 50 The dipole moments were calculated integrating the valence electron density at 48 each point times the distance of that point to the ribbon backbone (central axis). The 49 50 corresponding neutralizing contribution due to the nuclei is also added. More details can be 51 52 find in Carbonell-Sanromà et al. 42 . 53 54 55 56 57 58 59 11 60 ACS Paragon Plus Environment ACS Nano Page 12 of 19 1 2 3 4 Supporting Information 5 6 The Supporting Information includes the detailed methods for the synthesis of molecualr 7 8 precursor; the characterization of molecular precursor by NMR spectra and the comparison 9 of line profiles between NH2 -chGNRs and pristine chGNRs. 10 11 12 13 14 Acknowledgement 15 16 17 We acknowledge financial support from: i) AEI/FEDER-EU through grants no. MAT2016- 18 78293-C6, FIS2017-83780-P (AEI/FEDER,EU), the Maria de Maeztu unit of excellence 19 20 MDM-2016-0618, and the Severo Ochoa program (ICN2) SEV-2017-0706; ii) the European 21 22 Research Council (grant agreement no. 635919); iii) the Xunta de Galicia (Centro singular de 23 investigación de Galicia, accreditation 20162019, ED431G/09); iv) the EU project SPRING 24 25 (863098); v) the European Regional Development Fund (ERDF) under the program Inter- 26 27 reg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNI), vi) the CERCA Pro- 28 gram/Generalitat de Catalunya, and vii) the Gobierno Vasco-UPV/EHU (project IT1246- 29 30 19). 31 32 33 34 References 35 36 37 1. Llinas, J. P.; Fairbrother, A.; Borin Barin, G.; Shi, W.; Lee, K.; Wu, S.; Yong Choi, B.; 38 39 Braganza, R.; Lear, J.; Kau, N.; Choi, W.; Chen, C.; Pedramrazi, Z.; Dumslaff, T.; 40 Narita, A.; Feng, X.; Müllen, K.; Fischer, F.; Zettl, A.; Ruffieux, P. et al. 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