Supplementary information for: Making graphene. nanoribbons photoluminescent

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1 Supplementary information for: Making graphene nanoribbons photoluminescent B.V. M. S.K. A. Bliesener, J. Zhu, S. Michel, A.V. Fedorov,,, R. German, D. Hertel, D. Haberer, L. Petaccia, # F.R. Fischer, K. Meerholz, P.H.M. van Loosdrecht, K. Lindfors,, and A. Grüneis, II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, Köln, Germany, Department für Chemie, Universität zu Köln, Luxemburger Strasse 116, Köln, Germany, Institut für Angewandte Physik der Universität Bonn, Wegeler Strasse 8, Bonn, Germany, St Petersburg State University, Ulianovskaya 1, St Petersburg , Russia, IFW Dresden, P.O. Box , Dresden D-01171, Germany, Department of Chemistry, University of California at Berkeley, Tan Hall 680, Berkeley, CA 94720, USA, and Elettra Sincrotrone Trieste, Strada Statale 14 km 163.5, Trieste, Italy senkovskiy@ph2.uni-koeln.de; klas.lindfors@uni-koeln.de; grueneis@ph2.uni-koeln.de To whom correspondence should be addressed II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, Köln, Germany Department für Chemie, Universität zu Köln, Luxemburger Strasse 116, Köln, Germany Institut für Angewandte Physik der Universität Bonn, Wegeler Strasse 8, Bonn, Germany St Petersburg State University, Ulianovskaya 1, St Petersburg , Russia IFW Dresden, P.O. Box , Dresden D-01171, Germany Department of Chemistry, University of California at Berkeley, Tan Hall 680, Berkeley, CA 94720, USA # Elettra Sincrotrone Trieste, Strada Statale 14 km 163.5, Trieste, Contributed equally to this work 1

2 Sample characterization Figure S1(a) depicts a characterization of 7-AGNR samples on Au(788) by scanning tunneling microscopy (STM). The STM images reveal excellent nanoribbon alignment over the scan region as has been shown previously 1. Figure S1(b) depicts angle-resolved photoemission (ARPES) spectra of 7-AGNRs on Au(788). The ARPES data display a set of parabolic sub-bands that are well explained by zone-folding of the graphene π band structure 1. We perform a fit using a parabolic function of the effective mass of the highest valence band which is displayed in Figure S1(b). Notably, the value we obtain for the effective mass is in agreement to a previously published value 1. Figure S1(c) depicts low energy electron diffraction (LEED), which corroborates the GNR alignment seen in STM. LEED also reveals a structure rotated relatively to the Au diffraction spots by an angle of 30 degrees. This can be expressed in terms of the parent graphene unit cell, which forms a R30 orientation, as depicted in Figure S1(d). For transfer to other substrates, the 7-AGNRs/Au(788) sample was coated with polymethyl methacrylate (PMMA) and placed in an electrochemical cell with an NaOH aqueous solution (1 mol/l). A sketch of the electrochemical cell is shown in Figure S1(e). Samples transferred in this way were used for polarized Raman and Photoluminescence spectroscopy. Figure S1(f) depicts the Raman intensity of the radial-breathing like and the D modes as a function of light polarization (the polarization for both incident and Raman scattered light was identical). Figure S1(g) depicts a photograph of the GNR film after transfer. The region covered by GNRs is visible as a darker area. The Raman spectra for different excitation wavelengths between 510 nm an 670 nm nm is depicted in Figure S2 (both excitation and Raman scattered light was polarized along the GNR axis). The GNR film was transferred onto a quartz wafer in this case to avoid interference effects that could affect the Raman intensity for GNRs on a thin film of SiO 2. The integrated Raman signal of the Raman active RBLM, G and D modes was evaluated from these data. To that end we have normalized the spectra to the incident power, exposure time and setup sensitivity. 2

3 Figure S1: (a) Scanning tunneling microscopy topographic image of 7-AGNRs on Au(788). The parameters were Vs = 0.6 V and It = 40 pa. The samples were kept at 4.5 K. (b) Angle-resolved photoemission (ARPES) scan together with a prabolic fit of the effective mass of the first sub band. (c) Low-energy electron diffraction (LEED) patterns of 7-AGNRs/Au(788) with an electron beam energy of 57 ev. (d) Sketch of the 7-AGNR oriented along the (111) terraces of Au(788). The parent graphene unit cell (black) is rotated with respect to the unit cell of the Au surface (red) by 30 degrees. (e) Sketch of the bubbling transfer of 7-AGNRs from the vicinal surface of Au(788) crystal (f) The Raman intensity of the RBLM and D modes as a function of laser polarization (the wavelength was 532 nm). (g) Photograph of the transferred sample on a SiO2 /Si wafer. 3

4 Figure S2: The intensity profile of Raman spectra of transferred 7-AGNRs on SiO 2 as a function of excitation energy. The energies are depicted on the left side of the viewgraph. 4

5 Figure S3: (a) Spectrum (532nm laser) with Raman and photoluminescence features from graphene nanoribbons before and after defects have been induced. (b) Raman spectra from the same sample as in (a). The two peaks correspond to D (at 573 nm) and G (at 582 nm) lines, respectively. Photoluminescence In order to observe modifications to the nanoribbon lattice due to defect induction we compare Raman spectra before and after the induction. As one can see in Figure S3a there is an increase in intensity around 1.8 ev which comes from a brightening of the photoluminescence emission due to defect induction. Raman modes show a decrease after light exposure but there is no change in the position of the spectral features. To investigate spectral modifications of the Raman modes due to the light induced defects, we recorded spectra with a higher spectral resolution (see Figure S3b). Although it is expected to see an increase for D mode due to increasing defect density both modes show a reduction in intensities of approximately 25 %. Furthermore, one can see from the symmetric difference spectra that there is no significant shift of the modes after defect induction. To investigate the temporal dynamics of the emission processes we apply time-correlated single-photon counting. Figure S4a shows the time dependence of the PL signal. To reduce the instantaneous background from Raman modes, we excite the sample with 440 nm light and filter the emission with a 532 nm optical long pass filter. For comparison we record the instrument response function (IRF) as a time trace of a strongly attenuated reflection of the laser light without spectral filtering. From a comparison of the measurement with the instrument response we deduce 5

6 that the recombination timescales are shorter, or at maximum similar to the temporal resolution of our setup (FWHM of IRF approximately 500 ps and lifetime approximately 200 ps). Note: The instrument response is taken for a wavelength of 440 nm while the emission is at about 700 nm. This results in a slightly different temporal response function for the single photon avalanche diode. Additionally, the pulse length of the source is slightly different. The emission from the nanoribbons does not show bleaching over time scales of minutes. This is observed in Figure S4b where we show the emission of GNRs over several minutes and observe no photo-bleaching. Here the excitation wavelength is 532 nm for an illumination power of approximately 140 µw for a diffraction limited laser spot. Figure S4: (a) Time-resolved fluorescence (red solid line) and instrument response function (IRF, black solid line). (b) Photoluminescence signal over several minutes shows no significant change. As we have shown in the main paper in Figure 2, the emission is much brighter for light polarized along the ribbon. Here we discuss the polarization anisotropy of the PL. Figure S5a shows P EM, the polarization anisotropy of the emission process. P EM is defined as the difference in emission using excitation light polarized along and perpendicular to the ribbons normalized with their sum, i.e. P EM = I + I (I + I ) I + I + I + I (1) For the excitation process the degree of polarization anisotropy is defined similarly for the polarization dependence of the detected excitation. The polarization anisotropy in the excitation is 6

7 defined as P EXC = I + I (I + I ) I + I + I + I (2) For the above two equations, the first (second) arrow in the subscript to I denotes the polarization of the exciting (analyzed) light. A vertical arrow means polarization along the nanoribbon and a horizontal arrow means polarization perpendicular to the nanoribbon. For example, I means that both the exciting and analyzed light are polarized along the nanoribbon and I means that the exciting light is polarized perpendicular to the nanoribbon and the analyzed light is polarized along the nanoribbon and so on. In Figure S5a we show spatial maps of P EM and P EXC. It can be seen that the polarization anisotropies P EM and P EXC are similar to each other. Interestingly, the value of the degree of polarization anisotropy is almost 0.8, which is the value determined from the Raman spectroscopy measurements (see Figure 1 of the manuscript). The anisotropy thus seems to be limited by the alignment accuracy of the emitters. In Figure S5b we also compare the spectral polarization dependence on and beside the areas where the defects were induced. It is seen that one obtains almost the same spectral polarization dependence for defected and non-defected areas. Additionally, the degree of polarization anisotropy is high throughout the spectral region of the emission. Hydrogenation of 7-AGNRs As discussed in the main text, hydrogenation induces bright PL due to the presence of sp 3 defects. Here we show that the chemical bonding of H atoms to the basal plane of GNRs induces substantial modification in their core-level spectra which allow for an estimation of the H/C stoichiometry. These experiments were carried out on 7-AGNRs on Au(111). Experiments on SiO 2 have been tried but they are problematic because GNRs can become charged under the beam and their intrinsic line widths becomes broader after the transfer. We have used Au(111) as a substrate because no parallel sample alignment is needed in this experiment. All spectra were calibrated 7

8 Figure S5: (a) Left: Spatial distribution of the photoluminescence intensity. Middle and right: the polarization anisotropies for excitation and emission in the same area as the photoluminescence. (b) Polarization anisotropy of the emission spectra for each two spots on (red) and off (green) illumination defected areas. A spectrum for both polarizers aligned parallel to the ribbons is shown in black. 8

9 using the Au 4f 7/2 line (84.0 ev). Figure S6a shows the X-ray Photoelectron Spectroscopy (XPS) measurements of of 7-AGNRs/Au(111) before and after 40 minutes exposure to a beam of atomic hydrogen. The hydrogenation parameters, i.e. flux of H atoms and distance from the H-gun to the sample, was the same as for the 7-AGNRs/SiO 2 sample, which PL spectra are shown in the manuscript in Figure 3d. The C 1s spectra of pristine 7-AGNRs consist of two components [Figure S6a]. First component labeled as C 1 belongs to the carbon atoms bonded with three neighboring carbons and centered at ev. The second component labeled as C 2 ( ev) arises from the edge carbons, which have one C-H bond. The intensity ratio of C 1 :C 2 = 5:2, as dictated by the stoichiometry of 7-AGNRs unit cell (see the inset in Figure S6a). After hydrogenation the C 1s line shape changes, as seen from the top spectrum in Figure S6a. For the graphene/au system the binding energy of C 1s level is also founded at ev and in hydrogenated graphene two new peaks were identified 2. First peak at 0.5 ev higher binding energy corresponds to the sp 3 carbon atoms with out-of-plane C-H bond and the second one at 0.3 ev lower binding energy belongs to the C atoms next to the hydrogenated carbon 2,3. We can assume the same chemical shift for corresponding components of C 1 atoms in the case of hydrogenated 7-AGNRs (H-7-AGNRs). These components are labeled as C 3 and C 4 in Figure S6a and schematically shown in the top sketch in Figure S6b. The presence of the hydrogen terminated edges makes the fit analysis of XPS spectra for H-7AGNRs more complicated than for graphene. In our case one has to take into account additional components appearing in the spectrum when H atom attached to the edged C atoms, as illustrated in the bottom sketch in Figure S6b. Nevertheless, keeping the stoichiometry we are able to make a reasonable fit of the experimental data for H-7-AGNRs, as shown in the Figure S6a. Interesting, we find that about 25% of the C atoms can be saturated by H in agreement to previous works on graphene 3. To manifest the sp 3 type bounding of the H atoms to the 7-AGNRs basal plane we also performed near edge x-ray absorption fine structure (NEXAFS) measurements. The C K-edge absorption spectra of 7-AGNRs/Au(111) before and after hydrogenation are shown in Figure S6c. It can be seen that for the case of normal incidence (θ = 90 ) the hydrogenation induces a larger 9

10 intensity in the π component relative to the pristine sample. In the case of grazing incidence (θ = 10 ) the hydrogenation induces a larger intensity in the σ component relative to the pristine sample. This is a direct proof that the formation of carbon-hydrogen bonds induces a buckling in the nanoribbon. Figure S6: (a) XPS spectra of the C 1s core level of 7-AGNRs on Au(111) in the pristine state (bottom) and after hydrogenation (top). (b) Sketches of hydrogenated unit cell of 7-AGNRs denote the origin of the extra components in the C 1s. (c) NEXAFS spectra of 7-AGNRs on Au(111) at the C K-edge in the pristine and hydrogenated state for grazing and normal incidence of linearly polarized radiation, as illustrated in the inset. Quantification of photoluminescence intensity increase In order to quantify the increase in photoluminescence upon hydrogenation we perform a fit of the data shown in the main text (Figure 3) excluding the Raman contribution. The fits are depicted in Figure S7 and summarized in Table S1. They indicate that the photluminescence increases by a factor 5 upon 10 min hydrogenation. After 40 minutes of hydrogenation the sample contains more sp3 bonded carbon atoms and the PL intensity is decreased in absolute values, however the PL/Raman ratio is increased. 10

11 Table S1: PL and Raman intensity of pristine and hydrogenated 7-AGNRs/SiO 2. 7-AGNRs/SiO 2 PL intensity Raman intensity Ratio of PL/Raman intensities pristine Hydrogenated 10 min Hydrogenated 40 min Figure S7: PL and Raman spectra of transferred 7-AGNRs/SiO 2 measured inside a UHV chamber before (pristine) and after the hydrogenation (10 and 40 minutes). Spectra are accumulated using 532nm laser along the direction of 7-AGNRs alignment. The black dashed line is the base line used to extract the Raman intensity. The spectra are offset for clarity and measured with the same laser power and exposure time. 11

12 References (1) Ruffieux, P.; Cai, J.; Plumb, N. C.; Patthey, L.; Prezzi, D.; Ferretti, A.; Molinari, E.; Feng, X.; Müllen, K.; Pignedoli, C. A.; Fasel, R. ACS Nano 2012, 6, , PMID: (2) Haberer, D.; Vyalikh, D. V.; Taioli, S.; Dora, B.; Farjam, M.; Fink, J.; Marchenko, D.; Pichler, T.; Ziegler, K.; Simonucci, S.; Dresselhaus, M. S.; Knupfer, M.; Büchner, B.; Grüneis, A. Nano Letters 2010, 10, , PMID: (3) Haberer, D. et al. Advanced Materials 2011, 23,