Fig. 4: Theory and experimental verification of the CM as the dominant effect on the PCNA substrate.

a Energy level diagram (obtained by our theoretical analysis based on density functional theory with Gaussian16) that shows the molecular orbitals of a test molecule (R6G) on a carbon sheet that approximates the surface of the PCNA. The diagram shows two charge-transfer pathways from the HOMO to the PCNA-R6G hybrid states, enabling the R6G molecule on the surface of the PCNA to resonantly excited at the wavelengths of 785 nm (1.58 eV) and 532 nm (2.33 eV), whereas the R6G molecule alone cannot be resonantly excited at 785 nm due to the absence of excited states between the HOMO and the LUMO and can be resonantly excited at 532 nm, but with a very strong fluorescence background that obscures the Raman spectrum. b Raman spectra of R6G on the silicon and PCNA substrates at excitation wavelengths of 532 and 785 nm. c Comparison in electric field magnitude distribution between the single CNA nanowire and the single PCNA nanowire to visualize the small contribution of the EM. d Absorption spectrum of the PCNA substrate with and without β-lactoglobulin on it and the charge-transfer band obtained by taking the difference of the two absorption spectra. e Raman spectrum of β-lactoglobulin up to the high Raman shift region, showing no appreciable peaks of overtones and combination bands. f Raman spectra of β-lactoglobulin at different excitation wavelengths of 532 and 785 nm.