Fig. 5.

The size-dependent Raman enhancement for CuPc. a–c The ground state charge transfer between CuPc and graphene, GQDs (2.2 nm) or GQDs (6.2 nm). The electron states in the CuPc/graphene (or GQD) system, which have the possibility to contribute to the SERS, are marked by red shadows. d–f The calculated DOS and g–i the calculated molecular orbital (at the HOMO level of CuPc) of CuPc/graphene, CuPc/GQDs (2.2 nm), and CuPc/GQDs (6.2 nm), respectively. (g–i) The atomic models used in the density functional theory (DFT) calculations and the calculated charge transfer integrals (I). j Raman spectra of thermally evaporated CuPc on P-GQDs with different sizes. k Relative intensity of the characteristic peaks of CuPc on P-GQDs with different sizes, normalized to the signals on P-GQD-4. The largest enhancement of CuPc signal occurs on P-GQD-1