Supporting Information. Molecular Selectivity of. Graphene-Enhanced Raman Scattering

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1 1 Supporting Information Molecular Selectivity of Graphene-Enhanced Raman Scattering Shengxi Huang,, Xi Ling,,, * Liangbo Liang, ǁ Yi Song, Wenjing Fang, Jin Zhang, Jing Kong, Vincent Meunier, ǁ and Mildred S. Dresselhaus,, *. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA ǁ. Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, USA. Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing , P.R. China. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. These authors contributed equally to this work. * Correspondence Authors: Prof. Mildred S. Dresselhaus Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Tel: millie@mgm.mit.edu Dr. Xi Ling Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Tel: xiling@mit.edu 1 / 13

2 Molecules with Small Raman Cross Sections Table SI lists the molecules with small Raman cross sections (smaller than cm 2 sr -1 under non-resonant conditions 1 ). These molecules show no Raman signals under the measurement conditions used in this work because of their negligible Raman scattering efficiency, which is proportional to the Raman cross section according to classical Raman scattering theory Table SI. Molecules with unobservable GERS Signals The Calculation of GERS EF and its Error In the preparation of our samples, the molecules are thermally evaporated on to the 2 / 13

3 sample surface, and the molecular coverage is uniform. 2,3,4,5,6,7 Moreover, the numbers of molecules evaporated on the graphene area and on a blank SiO 2 /Si substrate area (without graphene) are the same, as confirmed by XPS (X-ray Photoelectron Spectroscopy) measurements (see the later section). 7 Therefore, the calculation of the GERS EF can be obtained through the following expression: GERS EF = I on G I on sub Eqn. (S1) in which I on G and I on sub represent the molecular Raman peak intensity on the graphene area and on the blank SiO 2 /Si substrate area without graphene, respectively. The error of EF is determined by checking the variations of the peak intensities for molecules on SiO 2 /Si and on graphene. When molecules are on graphene, the Raman spectra of molecules at different sample positions are very uniform. When the molecules are on SiO 2 /Si, the variations of the peak intensities are all smaller than 5.5%. Figure S1 shows multiple Raman spectra of F 16 CuPc on each substrate, and the spectra on the same substrate appear to be almost the same, demonstrating the Raman signal uniformity and the small error in EF. According to Eqn. (S1), the variation of EF is (EF) EF = (I on sub) + (I on G) I on G I on sub in which (EF), (I on sub ), (I on G ) are the errors of each parameter. Therefore, the relative error of EF is the sum of the relative errors of I on sub and I on G. The relative error of I on G is negligible, and as mentioned above, the relative error of I on sub is at most ±5.5%, so the relative error of EF is no more than ±5.5%. For the EF values we 3 / 13

4 Intensity (a.u.) Intensity (a.u.) measured (most are listed in Table 1), the relative error of ±5.5% is very small, and does not affect the analyses in our manuscript. For example, the smallest EF we measured is 0.9 (for the 1450 cm -1 mode of sp2-tpd), and the variation of its EF is at most ±0.05; a relatively large EF we measured is 24.3 (for the 1451 cm -1 mode of CuPc under 532 nm laser excitation), and the variation of its EF is at most ±1.3, not to mention that the relative error of I on sub for this mode is smaller than ±5.5% (and is only ±4.5%, leading to the variation of EF to be only ±1.1). (a) F 16 CuPc on SiO 2 /Si 532 nm (b) F 16 CuPc on graphene 532 nm Raman Shift (cm -1 ) Raman Shift (cm -1 ) Figure S1. Raman spectra of F 16 CuPc on a SiO 2 /Si substrate (a) and on graphene (b). Both (a) and (b) show multiple spectra collected using the same conditions at different locations on the same sample, indicating the uniformity of Raman spectra across the sample surface GERS EF of CuPc, ZnPc and F 16 CuPc Figure S2 shows the GERS EF (Enhancement Factor) versus Raman Shift for CuPc and F16CuPc under 532 nm laser excitation, and for ZnPc under 633 nm and 532 nm laser 4 / 13

5 78 excitation Figure S2. EF versus Raman Shift for the (a) CuPc and (b) F 16 CuPc molecules under 532 nm laser excitation, and for ZnPc under (c) 633 nm and (d) 532 nm laser excitation. Laser Energy Dependence of GERS For the TCTA, sp2-npb and TTP molecules, under 532 nm laser excitation, the GERS enhancement effects are much weaker than under 633 nm laser excitation for both TCTA and sp2-npb (Figure S3), and the EFs even disappear for TTP. Although the HOMO/LUMO energy gaps of TTP, TCTA and sp2-npb are 3.1, 3.3 and 3.3 ev, respectively, in which situation the 532 nm (2.33 ev) laser excitation is closer to the 5 / 13

6 resonance condition of the molecules themselves, the fact is that these molecules show stronger Raman signals under 633 nm laser excitation than under 532 nm laser excitation. This is interpreted as indicating that the energy resonances were shifted from the molecular HOMO-LUMO energy gap to the graphene Fermi level-molecular energy levels. This resonance shift is due to the graphene-molecule coupling, which enhances the charge-transfer between graphene and the molecules, as well as mixing and shifting the 96 energy levels. 8 This phenomenon further supports the theoretical analysis discussed above in the case of CuPc, F 16 CuPc and ZnPc, and can be seen in the numerator of the Fermi s Golden Rule formula, which represents the coupling between graphene and the molecules. For sp2-npb, the parallel-connected benzene rings strengthen the coupling between the molecule and graphene. Such an enhanced coupling shifts the Raman scattering resonance condition from the molecular HOMO-LUMO to the graphene-molecule, which has been discussed in the case of TTP and TCTA and is shown in Figure 3 (b, c) of the main text. In sp2-tpd, the weak molecule-graphene coupling results in the Raman scattering resonance condition still being strongly dependent on the molecular HOMO-LUMO level separation. Due to the large HOMO-LUMO energy gap of 3.2 ev for sp2-tpd, the 633 nm laser energy (1.96 ev) is farther away from the resonant condition than for the 532 nm laser (2.33 ev) for both the molecule and the molecule-graphene system, which 6 / 13

7 Intensity (a.u.) Intensity (a.u.) explains the GERS enhancement disappearance of sp2-tpd under 633 nm excitation. (a) TCTA 532 nm * D 3h (b) sp2-npb 532 nm * S G SiO 2 /Si Raman Shift (cm -1 ) G SiO 2 /Si Raman Shift (cm -1 ) Figure S3. Raman spectra of (a) 5 A TCTA, (b) 5 A sp2-npb on graphene (green line) and on a blank SiO 2 /Si substrate (black line), both with the excitation laser wavelength of 532 nm. Insets of (a, b) show the molecular structure of TCTA and sp2-npb, respectively, with the symmetries as labeled. The * marked peaks denote the G-band of graphene. Other number marked peaks are for the corresponding molecules. UV-Visible Absorption Spectra of TPBi Figure S4 shows the UV-Vis absorption spectra of TPBi before and after contacting graphene. Similar to sp2-npb and sp2-tpd shown in Figure 5 of the main text, TPBi in Figure S4 shows no frequency shift after contacting graphene in its absorption peaks at approximately 601 nm. A slight frequency shift is observed near 220 nm. These small frequency shifts indicate the weak graphene-tpbi interaction, which is reflected in the small GERS EF (less than 3) of TPBi. 7 / 13

8 Transmission (%) nm 269nm Figure S4. The UV visible transmission spectra of TPBi (black dashed line) and TPBi on graphene (black solid line). The wavelengths of the transmission valleys (absorption peaks) are labeled with a green dotted vertical line and a cyan box. Uniformity of Molecule Coverage on Sample Surface 601 nm 646 nm Wavelength (nm) TPBi G-TPBi The molecule coverage on the sample surface is uniform, and the numbers of molecules on graphene and on a blank SiO 2 /Si substrate are the same. In order to prove this, we evaporated the main molecules mentioned in the manuscript. For each molecule, in the same batch, the evaporation was done on two SiO 2 /Si substrates with and without CVD graphene covered on top, and we measured the X-ray photoelectron spectra (XPS) on both substrates. Figure S5 shows the XPS spectra of 5 A -thick F 16 CuPc evaporated on a SiO 2 /Si substrate and on graphene, including the signals from N 1s (Figure R1a, ~ ev), F 1s (Figure R1b, ~ ev) and Cu 2p3/2 (Figure R1c, ~ ev). Six points are chosen randomly (three on the SiO 2 /Si substrate and three on graphene) for the XPS 8 / 13

9 measurements, and we can see experimentally that the intensities are similar for each of the peaks. The relative differences between them are below 8%, proving that the numbers of molecules are very similar on SiO 2 /Si and on graphene, and the molecular coverage is uniform. We also performed the same measurements for other molecules, and the results are shown in Figure S6: XPS spectra of C 1s of PTCDA (Figure S6a), N 1s of CuPc (Figure S6b), sp2-npb (Figure S6c) and TCTA (Figure S6d). The thicknesses of the molecules on the substrates are all 5 A. We performed the measurements on multiple spots both on the SiO 2 /Si substrate and on graphene, and the spectra are almost the same. Therefore, in order to avoid redundancy, for each molecule, only two spectra are shown: one on SiO 2 /Si and one on graphene. In Figure S6a, the spectra of PTCDA C1s are fitted with multiple peaks, and the main peaks are labeled with their peak positions. The peak at around ev (marked with * ) corresponds to C1s peak in PTCDA, and matches the intensity of the ev peak for PTCDA on SiO 2 /Si. The other peak at around ev is for graphene. For other molecules in Figure S6(b-d), the intensities of the peaks are almost the same for each of the molecules on SiO 2 /Si and on graphene. Therefore, the relative numbers of molecules on graphene and on the blank SiO 2 /Si substrate can be considered as the same, and the contribution to the different Raman enhancement factors is mainly from the difference of the molecular properties. 9 / 13

10 (a) N 1s on graphene-3 (b) F 1s on graphene-3 (c) Cu 2p3/2 on graphene-3 on graphene-2 on graphene-1 on graphene-2 on graphene-1 on graphene-2 on graphene-1 on Si/SiO 2-3 on Si/SiO 2-2 on Si/SiO 2-3 on Si/SiO 2-2 on Si/SiO 2-3 on Si/SiO 2-2 on Si/SiO 2-1 on Si/SiO 2-1 on Si/SiO Binding Energy (ev) Binding Energy (ev) Binding Energy (ev) Figure S5. XPS spectra of N 1s (a), F 1s (b), and Cu 2p3/2 (c) of 5 A F 16 CuPc evaporated on SiO 2 /Si substrates with and without graphene covered. Each of (a-c) shows six spectra of F 16 CuPc on blank SiO 2 /Si substrate and on graphene. 10 / 13

11 (a) PTCDA C 1s (b) CuPc N 1s on graphene * on graphene on Si/SiO 2 (c) on Si/SiO Binding Energy (ev) sp2-npb N 1s (d) Binding Energy (ev) TCTA N 1s on graphene on graphene on Si/SiO 2 on Si/SiO Binding Energy (ev) Binding Energy (ev) Figure S6. XPS spectra of the C 1s of PTCDA (a), and the N 1s of CuPc (b), of sp2-npb (c) and of TCTA (d), both on a blank SiO 2 /Si substrate and on graphene. The thicknesses of the molecules are all 5 A. In (a), the spectra are fitted with multiple peaks, and the main peaks are labeled with their peak positions. The peak at around ev (marked with * ) corresponds to C1s peak in PTCDA, and matches the intensity of the ev peak for PTCDA on a SiO 2 /Si substrate. 11 / 13

12 171 Atomic Structures of Nonplanar TCTA and NPB Molecules Figure S7. Top and side views of the atomic structures of (a) TCTA and (b) NPB. Carbon atoms are marked as black and nitrogen atoms are marked as blue. Clearly, these molecules have non-planar geometries, especially near the nitrogen atoms. For (a) TCTA, the three branches near the central N atom are twisted to different planes. According to our DFT calculations, compared to a planar TCTA molecule, the non-planar one is more energetically stable by ~3.7 ev. For (b) NPB, the three terminals connecting to the N atoms are located on different planes as well. Similar non-planar structures occur for TPD, sp2-npb and sp2-tpd, as reported in the literature. 9,10 References (1) Ru, E. Le; Etchegoin, P. Principles of Surface-Enhanced Raman Spectroscopy: and related plasmonic effects; Elsevier, 2008; pp (2) Ling, X.; Xie, L.; Fang, Y.; Xu, H.; Zhang, H.; Kong, J.; Dresselhaus, M. S.; Zhang, J.; Liu, Z. Nano Lett. 2010, 10, (3) Ling, X.; Moura, L. G.; Pimenta, M. A.; Zhang, J. J. Phys. Chem. C 2012, 116, / 13

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