Supporting Information. Enhanced Raman Scattering on In-Plane Anisotropic Layered Materials

Transcription

1 Supporting Information Enhanced Raman Scattering on In-Plane Anisotropic Layered Materials Jingjing Lin 1, Liangbo Liang 2,3, Xi Ling 4, Shuqing Zhang 1, Nannan Mao 1, Na Zhang 1, Bobby G. Sumpter 2,5, Vincent Meunier 3, Lianming Tong 1, *, Jin Zhang 1, * 1 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 2 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States. 3 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, United States. 4 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States. 5 Computer Science & Mathematics Division, Oak Ridge National Laboratory, Oak Ridge Tennessee 37831, United States. *Corresponding author tonglm@pku.edu.cn; jinzhang@pku.edu.cn Part I: Angle-resolved polarized Raman spectra of CuPc on BP samples Figure S1. (a, b) Normalized Raman spectra of BP and corresponding CuPc molecules on BP sample with different sample rotated angles from 0 to 360 at 10 apart under cross S1

2 polarization configuration. As stated in the main text of the manuscript, the CuPc molecules with a thickness of 2-3 Å, were first deposited on a clean blank 300 nm SiO 2 /Si substrate by vacuum thermal deposition and then BP was transferred on top of the molecules by mechanical exfoliation. Following this procedure, the CuPc molecules can make contact the BP substrate immediately after the peeling off process and assures that the interface of CuPc molecules and BP are clean. This may also avoid destroying the BP sheets during the thermal deposition process. Figure S1 shows normalized Raman spectra of the corresponding few-layer BP with different sample rotation angles from 0 to 360 at 10 apart under the cross polarization configuration. From Fig. S1a and S1b, it can be seen that the relative intensities of both BP vibrational modes and CuPc vibrational modes change significantly with the sample rotation angle under cross polarization configuration, which seems similar with the results under the parallel polarization configuration. Especially for CuPc, most of the vibrational modes exhibit similar angle dependent polarization behaviors compared to that under parallel polarization configuration, except that the mode at 682 cm -1 became weaker, which is coincident with the Raman selection rules. However, for BP, which is different from that under parallel polarization configuration, the intensity variation period of the three modes are 90º, which makes it is difficult to identify the crystalline orientation of BP in this situation. Thus, to investigate the interaction between CuPc and S2

3 BP (or other anisotropic 2D materials), the parallel polarization configuration should be preferred, because it can be used to identify the crystalline orientation of anisotropic 2D layered materials 1 in this work. Part II: Angle-resolved polarized Raman spectra of CuPc on ReS 2 samples Figure S2. (a) Schematic illustration of the sample preparation process (CuPc on few-layer ReS 2 sheet). (b) Atomic structure of a monolayer ReS 2 with distorted 1T crystal structure, where the blue and purple atoms represent the S and Re atoms, respectively. (c) Raman spectra of CuPc molecules on blank 300 nm SiO 2 /Si substrate and on few-layer ReS 2 substrate, the inset picture is the AFM image of corresponding few-layer ReS 2 sheet, where the scale bar is 4 m. (d) Raman spectra of CuPc molecules on few-layer ReS 2 substrate with different polarization angles, respectively. S3

4 Figure S3. (a, b) Normalized Raman spectra of the corresponding ReS 2 (a) and CuPc on ReS 2 (b) with different sample rotation angles from 0 to 360 at 10 apart. All the Raman spectra are collected under parallel polarization configurations with 633 nm laser as excitation wavelength. The corresponding polar plots are shown in Fig. 2g-2j in the main text. ReS 2, as a new member of the transition-metal dichalcogenide (TMD) family, exhibited extraordinary optoelectronics properties 2, 3. Different from other TMD 2D materials, such as MoS 2 and WSe 4, 5 2, probably owing to its distorted 1T structure with low symmetry (Fig. S2b), the interlayer coupling of ReS 2 is very weak and its appealing photoelectric properties can be reserved even in its bulk form 2. Thus, it has been declared that ReS 2 would be an ideal platform to study 2D phenomena. Here, we utilized few-layer ReS 2 as a SERS substrate, mainly owing to its anisotropic structure and 2D layered materials nature. Since few-layer ReS 2 is very stable in air, the samples of CuPc molecules on a few-layer ReS 2 sheet were prepared in two ways, which were illustrated by the S4

5 procedures as shown in Fig. S2a and S5a (similar to the procedure in Fig. 1a for CuPc on BP sample). Following the first procedure, as shown in Fig. S2a, the ReS 2 sheet was transferred on a clean blank 300 nm SiO 2 /Si substrate first by mechanical exfoliation and then the CuPc molecules with a thickness of 2-3 Å, were deposited on the top of the ReS 2 sheet by vacuum thermal deposition. Figure S2d shows the polarized Raman spectra of CuPc molecules on a few-layer ReS 2 sample with a different polarization angle θ under parallel polarization configuration. Similar to CuPc molecules on BP shown in Fig. 1f, it was found that the Raman intensity of CuPc molecules on few-layer ReS 2 also exhibited strong polarization dependence. Even the relative intensities of vibrational modes changed significantly with polarization angle θ. Figure S3 shows normalized Raman spectra of the corresponding few-layer ReS 2 with different sample rotation angles from 0 to 360 at 10 apart under the parallel polarization configuration. It can be seen that three main first-order vibrational modes of ReS 2 can be distinguished, that is, 152, 162 and 212 cm -1, respectively. The detailed assignment of these vibrational modes can be found in the Ref.S2. From Fig. S3a, different vibrational modes of ReS 2 exhibit different polarization dependent behaviors. Based on Ref.S6, the zigzag Re atom chain (ZZ) direction of ReS 2 can be identified with the vibrational mode of 212 cm -1 under the parallel polarization configuration. Thus, in the main text, only the results under the parallel polarization configuration were demonstrated. S5

6 Figure S4. (a, b) Normalized Raman spectra of few-layer ReS 2 and corresponding CuPc molecules on few-layer ReS 2 sample with different sample rotation angles from 0 to 360 at 10 apart under cross polarization configuration. The sample used here is the same as shown in Fig. 2g-2j. To perform a relative comprehensive investigation on CuPc on a few-layer ReS 2 sheet, angle-dependent polarized Raman spectra of CuPc on ReS 2 (shown in Fig. 2g-2j) under the cross polarization configuration were also collected and the results are shown in Fig. S4. All the results are similar to that on BP. For CuPc, most of the vibrational modes exhibit similar angle dependent polarization behaviors compared to that under the parallel polarization configuration, except that the mode of 682 cm -1 became weaker. However, different from that under the parallel polarization configuration, the intensity variation period of the three modes are 90º, which makes it difficult to identify the crystalline orientation of ReS 2 in this situation. S6

7 Figure S5. (a) Schematic illustration of the sample (ReS 2 /CuPc/SiO 2 /Si) preparation procedure. (b) Optical Microscopy (OM) image of the few-layer ReS 2 sheet on 300 nm SiO 2 /Si substrate, where the scale bar is 8 m. (c, d) Normalized Raman spectra of ReS 2 and corresponding CuPc molecules on ReS 2 sample with different sample rotation angles from 0 to 360 at 10 apart under cross polarization configuration. Another method to prepare CuPc molecules with ReS 2 is shown as Fig. S5a. Similar to that on BP, the CuPc molecules with a thickness of 2-3 Å, were deposited on a clean blank 300 nm SiO 2 /Si substrate first by vacuum thermal deposition and then ReS 2 was transferred on the top of the molecule layer by mechanical exfoliation. In this way, the CuPc molecules should be randomly distributed and the corresponding angle dependent polarization Raman spectra under the parallel polarization configuration were shown in Fig. S5c and S5d. Compared to that in Fig. S3 and Fig. 2f, no apparent difference was observed. This indicates that the induced anisotropic optical properties of CuPc S7

8 molecules on ReS 2 were not a result of directional arrangements of the CuPc molecules possibly induced by the ReS 2 sheet as a template in the deposition progress. Part III: Simulated polarization dependence for the A 1g, B 2g and B 1g vibrational modes of CuPc Figure S6. (a-c) Simulated polarization dependence for the A 1g, B 2g and B 1g vibrational modes of CuPc, respectively. Note that 0 in the simulation (indicated by the purple double arrow) was defined as the direction of the primary axis of CuPc, which is not necessarily the same to 0 used in the experimental measurements. However, it is clear that the maximum intensity angles of the B 1g mode correspond to the primary axis of CuPc, regardless of the 0 reference. Figure S6 shows the simulated polarization dependence for the A 1g, B 2g and B 1g vibrational modes of CuPc. Compared with the data in Fig. 2c-2e for the CuPc molecules on the few-layer BP sheet, it can be seen that the simulated results can be perfectly matched with the measured polarization dependence of 682, 1450 and 1530 cm -1 for S8

9 CuPc molecules respectively if rotated by 45. Since the crystalline orientation of BP was unknown when the polarized Raman measurements were performed, this difference of 45 should be attributed to the relative orientation of BP and the original polarized direction of the excitation laser. Part IV: DFT Calculations Figure S7. The total energy versus the CuPc molecular orientation for (a) CuPc/BP, (b) CuPc/ReS 2, and (c) CuPc/graphene system. The angle 0 corresponds to the primary axis of CuPc along (a) the BP armchair direction, (b) ReS 2 ZZ Re atomic chain direction and (c) graphene armchair direction. Note that 0 is equivalent to 90 since CuPc has four symmetric branches (axes). Total energy is normalized with respect to the value at 0. It is lowest for 0 and 90, but the energy difference between different angles is not significant, particularly for CuPc/BP (less than 40 mev) and CuPc/graphene (less than 50 mev), since the molecule-substrate interaction is dominated by weak van der Waals forces. S9

10 Figure S8. Charge distributions (in yellow) of electronic bands near the Fermi-level for (a) isolated BP surface, (b) CuPc/BP with 0 molecular orientation, (c) CuPc/BP with 45 molecular orientation, and (d) CuPc/BP with 60 molecular orientation. VBM and CBM stand for valence band maximum and conduction band minimum, respectively. VBM-1 and CBM+1 are the second electronic bands below valence band maximum (VBM) and S10

11 above conduction band minimum (CBM). (a) For the isolated BP surface, the charges are uniformly distributed across the surface. But upon CuPc adsorption, the charges are redistributed into 1D chains along AC direction, regardless the CuPc molecular orientation (0, 45 or 60 ). Similar anisotropic charge redistributions also occur to BP surface for the CuPc/BP system with 30 molecular orientation (not shown here). Note that the AC direction is also the direction corresponding to the highest carrier mobility for BP S11

12 Figure S9. Charge distributions (in yellow) of electronic bands near the Fermi-level for (a) isolated ReS2 surface, (b) CuPc/ReS2 with 0 molecular orientation, (c) CuPc/ReS2 with 45 molecular orientation, and (d) CuPc/ ReS2 with 60 molecular orientation. (a) For isolated ReS2 in the distorted 1T phase2, it forms 1D zigzag (ZZ) Re atomic chains (see the purple double-arrow in (b)) and subsequently the charges are primarily localized along the ZZ Re chain direction, while S atoms between ZZ Re chains have little or no S12

13 charge accumulation. Such anisotropic charge distributions remain after CuPc adsorption, regardless of the CuPc molecular orientation (0, 45 or 60 ). Notably, similar to the BP case, the ZZ Re chain direction is also the direction corresponding to the highest carrier mobility of ReS 3 2. Figure S10. Charge distributions (in yellow) of electronic bands near the Fermi-level for the CuPc/graphene system. For graphene surface, even with CuPc presence, the charge distributions remain isotropic. Part V: Experimental Section Sample preparation and characterization All the few layered 2D materials used throughout this work were attained using the standard mechanical cleavage method from a bulk crystal and deposited on the target substrates. The morphology and the thicknesses of all the samples were characterized by optical microscopy (OM) and atomic force microscopy (AFM). Deposition of CuPc molecules The CuPc molecules were deposited on the substrate using a standard thermal evaporator. The pressure in the vacuum chamber for deposition was about Torr and the current S13

14 for evaporation was about 50 A. The thickness of deposited CuPc molecules was around 2-3 Å, monitored by a quartz crystal monitor. Angle-resolved Polarized Raman Spectroscopy (ARPRS) Measurement Raman spectra were collected by a Jobin Yvon Horiba HR800 Raman system with a 633 nm laser, where the spectral resolution is about 1 cm -1 with a 100 objective and 600 lines/mm grating. To avoid damage to the samples, the laser power on the samples was kept below 300 W. ARPRS measurements were performed similarly to our previous work 1. In brief, two polarizers were placed in the incident light path and scattered light path to attain the parallel and cross polarization configurations, respectively. The samples were rotated 360 about the microscope optical axis at 10 step for the angular dependence measurement. At every step, the sample position was adjusted carefully in order to ensure that the same spot was probed. DFT calculations Plane-wave DFT calculations were performed using VASP 11 within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional 12. The optb86b-vdw functional was used to account for the vdw interactions between the CuPc molecule and the substrate (including BP, ReS 2 and graphene) 13,14. The projector augmented wave (PAW) pseudopotentials were used with a cutoff energy set at 400 ev. For single-layer orthorhombic BP, its optimized unit cell has in-plane lattice constants of about 4.34 Å (along the armchair direction) and 3.33 Å (along the zigzag direction 15 ). The z lattice constant was set as Å to avoid S14

15 spurious interactions with replicas. The BP substrate was built by a periodic slab geometry with a supercell (the size about Å Å Å) to ensure at least 14 Å vacuum separations in all directions for the molecule adsorbed on the substrate. For single-layer triclinic ReS 2, its optimized unit cell has in-plane lattice constants of about 6.55 Å and 6.49 Å along the two primary axes (the angle between them is around ), consistent with previous works 2,3. The z lattice constant was also set as Å to avoid spurious interactions with replicas. Then the ReS 2 substrate was built by a periodic slab geometry with a supercell (the size about Å Å Å) to ensure ~14 Å vacuum separations in all directions for the molecule adsorbed on the substrate. Similarly, for hexagonal graphene, its orthorhombic unit cell has in-plane lattice constants of about 4.26 Å (along the armchair direction) and 2.46 Å (along the zigzag direction). The graphene substrate was built by a periodic slab geometry with a supercell (the size as Å Å Å) to ensure at least 14 Å vacuum separations in all directions for the molecule adsorbed on graphene. For each molecule-substrate system, the supercell is large enough so that a single k-point at the Gamma point of the Brillouin zone is sufficient for the k-point sampling. All atoms were relaxed until the residual forces were below 0.02 ev/å. References S1. Wu, J.; Mao, N.; Xie, L.; Xu, H.; Zhang, J. Angew. Chem.-Int. Edit. 2015, 54, S2. Tongay, S.; Sahin, H.; Ko, C.; Luce, A.; Fan, W.; Liu, K.; Zhou, J.; Huang, Y.-S.; Ho, C.-H.; Yan, J.; S15

16 Ogletree, D. F.; Aloni, S.; Ji, J.; Li, S.; Li, J.; Peeters, F. M.; Wu, J. Nat. Commun. 2014, 5, S3. Liu, E.; Fu, Y.; Wang, Y.; Feng, Y.; Liu, H.; Wan, X.; Zhou, W.; Wang, B.; Shao, L.; Ho, C.-H.; Huang, Y.-S.; Cao, Z.; Wang, L.; Li, A.; Zeng, J.; Song, F.; Wang, X.; Shi, Y.; Yuan, H.; Hwang, H. Y.; Cui, Y.; Miao, F.; Xing, D. Nat. Commun. 2015, 6, S4. Jariwala, D., Sangwan, V. K., Lauhon, L. J., Marks, T. J. & Hersam, M. C. ACS Nano 2014, 8, S5. Li, H., Wu, J., Yin, Z. & Zhang, H. Acc. Chem. Res. 2014, 47, S6. Chenet, D.; Aslan, O. B.; Huang, P. Y.; Fan, C.; van der Zande, A. M.; Heinz, T. F.; Hone, J. Nano Lett. 2015, 15, S7. Qiao, J., Kong, X., Hu, Z.-X., Yang, F. & Ji, W. Nat. Commun. 2014, 5, S8. Xia, F., Wang, H. & Jia, Y. Nat.Commun. 2014, 5, S9. Wang, X.; Jones, A. M.; Seyler, K. L.; Vy, T.; Jia, Y.; Zhao, H.; Wang, H.; Yang, L.; Xu, X.; Xia, F. Nat. Nanotech. 2015, 10, 517. S10. He, J.; He, D.; Wang, Y.; Cui, Q.; Bellus, M. Z.; Chiu, H.-Y.; Zhao, H. ACS Nano 2015, 9, S11. Kresse, G.; Furthmuller, J. Comp. Mat. Sci. 1996, 6, 15. S12. Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, S13. Huang, S.; Ling, X.; Liang, L.; Song, Y.; Fang, W.; Zhang, J.; Kong, J.; Meunier, V.; Dresselhaus, M. S. Nano Lett. 2015, 15, S14. Liang, L.; Wang, J.; Lin, W.; Sumpter, B. G.; Meunier, V.; Pan, M. Nano Lett. 2014, 14, S15. Ling, X.; Liang, L.; Huang, S.; Puretzky, A. A.; Geohegan, D. B.; Sumpter, B. G.; Kong, J.; Meunier, V.; Dresselhaus, M. S. Nano Lett. 2015, 15, S16