Fig. 4: Raman evolution for PEDOT films and dependence on molecular orientation.

a Average eNPoM SERS spectra vs shell thickness \({\boldsymbol{d}}\). b–d Peak intensity ratios \({\boldsymbol{\eta }}\) vs shell thickness \({\boldsymbol{d}}\) for b 1540:1456 cm⁻¹, c 1043:993 cm⁻¹, and d 526:574 cm⁻¹ peaks, labelled in a. Errors cover range from NPoMs. e 2^nd order polynomial fits of \({\boldsymbol{\eta }}\) vs PEDOT shell thickness. f SERS spectra for NPoMs with \({\boldsymbol{d}}\) = 2, 18 nm PEDOT shells. Inset to right: single PEDOT²⁺ tetramer structure used in DFT for optimized SERS spectra, axes as labelled. g DFT simulated Raman spectra of PEDOT²⁺ tetramer separated into components for different optical field polarizations along \({\boldsymbol{a}}{\boldsymbol{,}}{\boldsymbol{b}}{\boldsymbol{,}}{\boldsymbol{c}}\) directions of PEDOT tetramer, as in Fig. 3a. \({{\boldsymbol{E}}}_{{\boldsymbol{z}}}\): Dominant vertical optical field in eNPoM nanogap