Supporting Information for Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors Yen-Chia Chen,1,2‡ Dimas G. de Oteyza,1,3‡ Zahra Pedramrazi,1 Chen Chen,4 Felix R. Fischer,2,4* Michael F. Crommie1,2* 1
Department of Physics, University of California at Berkeley, Berkeley, CA 94720, U.S.A.. 2
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A..
3
Centro de Física de Materiales CSIC/UPV-EHU-Materials Physics Center, San Sebastián, E-20018, Spain. 4
Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720, U.S.A..
‡
These authors contributed equally to this work.
* Address correspondence to
[email protected],
[email protected] 1
Contents 1. Synthesis and General Remarks 2. Scanning Tunneling Spectroscopy of 7-AGNRs 3. Additional STM dI/dV spectra and dI/dV maps of 13-AGNR 4. References
1. Synthesis and General Remarks Unless otherwise stated, all reactions were performed in oven-dried glassware, under an atmosphere of nitrogen. All solvents and reagents were purchased from Alfa Aesar, Spectrum Chemicals, Acros Organics, TCI America, Sigma-Aldrich, and Matrix Scientific and were used as received unless otherwise noted. Thin layer chromatography was performed using SiliCycle silica gel 60 Å F-254 precoated plates (0.25 mm thick) and visualized by UV irradiation. Flash chromatography was performed on SiliCycle silica gel (particle size 40–63 μm). All 1H and 13C NMR spectra were recorded on a Bruker AV-600 spectrometer, and are referenced to residual solvent peaks (CHCl3 1H NMR; δ = 7.26, 13C NMR; δ = 77.16). HR-Mass spectrometry was recorded on a Waters AutoSpec Premier (Waters) spectrometer.
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Scheme S1. Synthesis of 1. 2,2-di((1,1’-biphenyl)-2-yl)-9,9’-bianthracene (2) A 50 mL Schlenk tube with reflux condenser was charged under an atmosphere of nitrogen with 2,2’-dibromo-9,9’bianthracene1 (200 mg, 0.39 mmol), and (1,1’-biphenyl)-2-boronic acid (193 mg, 0.98 mmol) in an aqueous solution of 2N K2CO3 (10 mL) and THF (10 mL). The reaction mixture was thoroughly degassed before tetrakis(triphenylphosphine)palladium(0) (225 mg, 0.19 mmol) was added. The reaction mixture was heated to 80 °C for 4 h, cooled to 24 °C and extracted with CH2Cl2 (150 mL). The combined organic phases were washed with saturated aqueous NaHCO3 solution, saturated aqueous NaCl solution, dried over MgSO4 and concentrated on a rotary evaporator. Colum chromatography (SiO2; hexane/AcOEt 20:1) yielded 2 (247 mg, 0.38 mmol, 96%) as a pale yellow solid. M.p. 163 °C; 1H NMR (600 MHz, CDCl3) δ = 6.81–6.89 (m, 8 H), 6.91–6.99 (m, 2 H), 7.04 (d, J = 9.0, 2 H), 7.07– 7.14 (m, 8 H), 7.19–7.22 (m, 2 H), 7.26–7.31 (m, 4 H), 7.42–7.45 (m, 2 H), 7.88, (d, J = 9.6, 2 H), 8.11 (d, J = 8.4, 2 H), 8.56 (s, 2 H) ppm;
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C NMR (600 MHz, CDCl3) δ = 125.3,
125.9, 126.6, 127.0, 127.1, 127.3, 127.5, 127.6, 127.8, 127.9, 128.2, 128.7, 129.7, 130.5, 130.6, 130.9, 131.6, 131.8, 132.0, 133.1, 139.2, 140.7, 140.8, 141.2 ppm; MALDI-MS: m/z: 657.5106 ([M–H]+, C52H33+).
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2,2-di((1,1’-biphenyl)-2-yl)-10,10’-dibromo-9,9’-bianthracene (1) A 25 mL Schlenk tube was charged under an atmosphere of nitrogen with 2 (210 mg, 0.32 mmol) in CCl4 (7.5 mL) and cooled to 0 °C. A solution of Br2 (120 mg, 0.75 mmol) in CCl4 (7.5 mL) was added dropwise over 1 h. The reaction mixture was stirred for 12 h at 24 °C and concentrated on a rotary evaporator. Colum chromatography (SiO2; hexane/AcOEt 20:1) yielded 1 (234 mg, 0.29 mmol, 76%) as a yellow solid. M.p. 224 °C; 1H NMR (600 MHz, CDCl3) δ = 6.79–6.88 (m, 10 H), 6.93 (d, J = 8.4, 2 H), 6.99 (d, J = 1.2, 2 H), 7.09 (d, J = 7.2, 2 H), 7.12–7.15 (m, 2 H), 7.21–7.31 (m, 8 H), 7.56 (dd, J = 6.6, 7.8, 2 H), 8.46 (d, J = 9.6, 2 H), 8.65 (d, J = 9.0, 2 H) ppm; 13C NMR (600 MHz, CDCl3) δ = 123.9, 126.5, 126.7, 127.2, 127.3, 127.4, 127.6, 127.7, 127.8, 127.9, 128.3, 129.4, 129.5, 130.0, 130.7, 130.8, 130.9, 132.1, 132.6, 133.1, 139.6, 139.7, 140.8, 140.9 ppm; MALDI-MS: m/z: 815.1511 ([M–H]+, C52H31Br2+).
2. Scanning Tunneling Spectroscopy of 7-AGNRs We performed STS on 7-AGNRs synthesized from 10,10’-dibromo-9,9’-bianthracene (DBBA) building blocks2 as a reference for our 13-AGNR measurements. We observed a band gap of 2.5 eV ± 0.1 eV for 7-AGNRs. A characteristic dI/dV spectrum for a 7-AGNR is depicted in Fig. S1. The dI/dV spectrum of a 7-AGNR (green line) has two prominent resonances at –0.8 eV and 1.7 eV relative to the Fermi energy. Similar behavior was seen for four 7-AGNRs, each measured using different STM tips (the 7-AGNR energy gap value
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reported here is the average of these measurements, and the reported uncertainty arises from the standard deviation of these measurements). In previous work Koch et al. report the energy gap of 7-AGNRs to be in the range 2.6 2.7 eV using a similar gap definition as ours (i.e., the energy separating resonance peaks at the conduction and the valence band edges).3 Ruffieux et al. report a smaller gap for 7AGNRs, 2.3 ± 0.1 eV, but their gap was defined as the distance between the onsets of the conduction and the valence bands.4 If we apply the peak-to-peak gap definition used here to their data, then a 7-AGNR energy gap of 2.7 ± 0.1 eV results. Linden et al. have also derived an energy gap of 2.8 ± 0.4 eV for 7-AGNRs using combined photoemission and inverse photoemission techniques.5 An energy gap of approximately 2.6 eV for 7-AGNRs (peak-to-peak measurement) is thus consistent with all reported measurements.
3. Additional STM dI/dV spectra and dI/dV maps of 13-AGNR In this section we compare STM dI/dV spectra recorded at the edge and in the interior of a 13-AGNR that is distinct from the GNR shown in Fig. 2 of the paper, and we also show dI/dV maps taken at resonances other than the conduction and valence band edges. Fig. S2A depicts characteristic dI/dV spectra taken at a 13-AGNR edge (blue line), in the GNR interior (red line), and on bare Au(111) (green line). While the spectrum at the edge exhibits resonances at +1.22 V, –0.15 V, and –0.60 V, these resonances are substantially reduced in the interior of the 13-AGNR. The reduction of resonance intensity in the middle
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region of the 13-AGNR is consistent with dI/dV maps recorded at the valence and conduction band edges (Figs. S2C and S2D, respectively) which show band edge states to be most prominent near GNR edges. A dI/dV map obtained at a sample bias of –0.60 V, which corresponds to an occupiedstate resonance below the valence band edge, is shown in Fig. S2B. The dI/dV map exhibits a strong longitudinal nodal pattern in the LDOS along the edges (some edge-asymmetry can be seen in the LDOS, which we assume arises from surface imperfections). The fact that this nodal pattern has shorter wavelength oscillations than the longitudinal LDOS seen for the –0.15 V resonance (Fig. S2C) is consistent with the –0.60 V resonance representing a higher-energy hole-like excitation of the GNR. This further supports identification of the –0.15 V resonance (which is the lowest-energy hole-like excitation of the GNR) as the valence band edge. Similar behavior is also observed in the empty states. The 13-AGNR dI/dV spectrum in Fig. S3A exhibits resonances at 1.20 V, 1.60 V and 1.90 V. dI/dV maps obtained at these resonance energies (Figs. S3B–D) show smooth longitudinal edge intensity in the LDOS at 1.20 V (Fig. S3B), but strong longitudinal nodal variation in the edge intensity at 1.60 V and 1.90 V (Figs. S3C, D). The nodal oscillations of the 1.60 V and 1.90 V resonances are consistent with these resonances being electron-like excited states of the GNR (likely subbands). This supports identification of the 1.20 V resonance (which is the lowestenergy electron-like excitation of the GNR) as the conduction band edge.
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Figure S1. Normalized STM dI/dV spectra for a 13-AGNR versus a 7-AGNR. The 13AGNR spectrum (blue line) exhibits an energy gap of 1.4 eV, whereas the 7-AGNR spectrum (green line) shows a gap of 2.5 eV (open-feedback parameters: Vs = 1.00 V, It = 35 pA, and modulation voltage Vr.m.s. = 10 mV for 13-AGNR. Vs = 1.20 V, It = 15 pA, and Vr.m.s. = 5 mV for 7-AGNR).
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Figure S2. STM dI/dV spectra at different positions for a 13-AGNR. (A) dI/dV spectra recorded at 13-AGNR edge (blue line), mid-point (red line), and on bare Au(111) (green line). The 13-AGNR spectra recorded at the edge and at the middle are offset vertically by 3 and 1.5 a.u., respectively, for clarity (open-feedback parameters: Vs = 1.00 V, It = 35 pA; modulation voltage Vr.m.s. = 10 mV). Crosses in inset topographic image indicate positions
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where spectra were obtained (inset Vs = –0.80 V, It = 4.05 nA). (B–D) dI/dV maps of 13AGNR acquired at (B) the –0.60 V resonance (hole-like GNR excited state) (Vs = –0.60 V, It = 35 pA), (C) the –0.15 V resonance (valence band edge) (Vs = –0.15 V, It = 35 pA), and (D) the 1.22 V resonance (conduction band edge) (Vs = 1.22 V, It = 35 pA).
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Figure S3. Empty-state resonances in STM dI/dV measurement of 13-AGNR. (A) dI/dV spectra recorded on a 13-AGNR (blue line) and on bare Au(111) (green line). The 13AGNR spectrum is offset vertically by 1.5 a.u. for clarity (open-feedback parameters: Vs = 2.20 V, It = 105 pA; modulation voltage Vr.m.s. = 10 mV). Crosses in inset topographic 10
image indicate positions where spectra were obtained (inset Vs = 2.20 V, It = 105 pA). (B– D) dI/dV maps of 13-AGNR taken at (B) the 1.20 V resonance (conduction band edge) (Vs = 1.20 V, It = 105 pA), (C) the 1.60 V resonance (electron-like GNR excited state) (Vs = 1.60 V, It = 105 pA), and (D) the 1.90 V resonance (electron-like GNR excited state) (Vs = 1.90 V, It = 105 pA).
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Figure S4. Comparison of the on-ribbon and off-ribbon spectra depicted in Fig. 2 without any vertical offset (bottom panel), along with the ratio of on-ribbon spectrum to the offribbon spectrum (top panel). The ratio more clearly shows the energy splitting of the -0.2 V resonance described in the main text.
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4. References 1. Weiler-Feilchenfeld, H.; Bergmann, E. D.; Hirschfeld, A. The Conformation of 9,9’ Bianthryl. Tetrahedron Lett. 1965, 6, 4129–4131. 2. Cai, J.; Ruffieux, P.; Jaafar, R.; Bier, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.; et al. Atomically Precise Bottom-up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470–473. 3. Koch, M.; Ample, F.; Joachim, C.; Grill, L. Voltage-dependent Conductance of a Single Graphene Nanoribbon. Nat. Nanotechnol. 2012, 7, 713–717. 4. Ruffieux, P.; Cai, J.; Plumb, N. C.; Patthey, L.; Prezzi, D.; Ferretti, A.; Molinari, E.; Feng, X.; Müllen, K.; Pignedoli, C. A.; et al. Electronic Structure of Atomically Precise Graphene Nanoribbons. ACS Nano 2012, 6, 6930–6935. 5. Linden, S.; Zhong, D.; Timmer, A.; Aghdassi, N.; Franke, J. H.; Zhang, H.; Feng, X.; Müllen, K.; Fuchs, H.; Chi, L.; et al. Electronic Structure of Spatially Aligned Graphene Nanoribbons on Au(788). Phys. Rev. Lett. 2012, 108, 216801.
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