Anomalous behaviors of visible luminescence from graphene quantum dots: interplay between size and shape

Transcription

1 Supporting Information for Anomalous behaviors of visible luminescence from graphene quantum dots: interplay between size and shape Sung Kim, 1 * Sung Won Hwang, 2 * Min-Kook Kim, 3 Dong Yeol Shin, 1 Dong Hee Shin, 1 Chang Oh Kim, 1 Seung Bum Yang, 1 Jae Hee Park, 1 Euyheon Hwang, 1 Suk-Ho Choi, 1 Geunwoo Ko, 2 Sunghyun Sim, 2 Cheolsoo Sone, 2 Hyoung Joon Choi, 3 Sukang Bae, 4 Byung Hee Hong 4 1 Department of Applied Physics, Kyung Hee University, Yongin , Korea. 2 Advanced Development Department, Samsung Electronics Co., Ltd, Yongin , Korea. 3 Department of Physics and IPAP, Yonsei University, Seoul , Korea 4 Department of Chemistry, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon , Korea. * These authors contributed equally to this work. To whom correspondence should be addressed. sukho@khu.ac.kr 1

2 Figures for Supporting Information 2

3 Figure S1. (a) Typical HRTEM images of GQDs with particular shapes and their populations (%) for various average sizes of GQDs. Average sizes of GQDs estimated from the HRTEM images at each d are written in the first column. The sizes of individual GQDs are indicated in the corresponding HRTEM images. (b) Average size of GQDs as a function of d. Figure S2. AFM image and height profile of GQDs at average sizes of (a) 12 nm and (b) 35 nm. (c) Population of the number of graphene layers constituting GQDs as a function of average size. 3

4 Figure S3. EELS spectra of GQDs with d a = 12, 27, and 35 nm (a) in the π+σ plasmon region, indicating one, five, and more layers, respectively [S1,S2] and (b) in the carbon K-edge region exhibiting two major peaks at 285 and 291 ev, corresponding to the transitions from 1s to π* states in sp 2 -bonded carbon and from 1s to σ* states in sp 3 -bonded carbon, respectively [S3], suggesting that the GQDs consist of graphene. These results are consistent with the HRTEM and AFM data. Figure S4. Absorption spectra of various-size GQDs in DI water and a graphene sheet. 4

5 Figure S5. Excitation-wavelength-dependent PL spectra of (a) DI water and (b) GQDs with an average size of 17 nm. Figure S6. (a) XPS spectra of GQDs for various average sizes. (b) XPS peak intensities of sp 2 C=C, C-O, C=O, and COOH bonds as functions of GQD size. 5

6 Figure S7. Size-dependent (a) absorption-peak and (b) averaged-pl energies calculated for the three types of GQDs with 2 12 nm sizes: (i) hexagonal GQDs with armchair edges, (ii) hexagonal GQDs with zigzag edges, and (iii) circular GQDs with armchair and zigzag edges The PL was calculated by using a excitation wavelength of 300 nm. The maximum limit of the calculation ( 12 nm) available in this work is smaller than that of the experimental measurements, but these results suggest that the anomalous PL peak shifts could originate from the sizedependent shape and edge-state variations of GQDs. To obtain electronic structures of GQDs, we performed ab-initio pseudopotential density-functional calculations using the SIESTA code [S4]. The absorption spectra were obtained by considering all optical transitions from occupied to unoccupied states in GQDs. The optical transition rate between any pair of states was calculated using the Fermi golden rule and the dipole matrix element estimated from their electronic wavefunctions. The theoretical PL spectra were obtained by calculating the optical transition rates from unoccupied states to the two highest occupied states without considering the exciton effects, assuming that the unoccupied states are equally populated up to the excitation energy from the two highest occupied states. References for Supporting Information [S1] T. Eberlein, U. Bangert, R. R. Nair, R. Jones, M. Gass, A. L. Bleloch, K. S. Novoselov, A. Geim, and P. R. Briddon, Phys. Rev. B 77, (2008). [S2] V. Borka Jovanović, I. Radović, D. Borka, and Z. L. Mišković, Phys. Rev. B 84, (2011). [S3] A. Dato, V. Radmilovic, Z. Lee, J. Phillips, M. Frenklach, Nano Lett. 8, 2012 (2008). [S4] D. Sánchez-Portal, P. Ordejón, E. Artacho, and J. M. Soler, Int. J. Quantum Chem. 65, 453 (1997). 6