## 3 Results and Discussion
### Variations of PNA Signals with Bias  
As a negative tip bias is applied, both PNA amplitude and phase signals exhibit variations. In Figure 2b, a \(-\)2.5 V DC tip bias leads to a decrease (increase) in the amplitude (phase) signal. This suggests a sensitivity of SMM and the ability of S\({}_{11}\) to detect variations of electrical properties of P3 HT as negative DC tip bias favors hole injection in the P3 HT from the bottom electrode. Comparative measurements displayed in Figure S2 but with f\({}_{\text{OP}}>\) f\({}_{0}\) exhibit reversed amplitude and phase variations with \(-\)3 V DC bias. It is worth noting that in addition to f\({}_{\text{OP}}>\) f\({}_{0}\), the 180\({}^{\circ}\) shift at the interferometric frequency is also opposite that in Figure 2a. In other words, because the amplitude and phase profiles are both decreasing functions with frequency when f\({}_{\text{OP}}<\) f\({}_{0}\) like in Figure 2a, if the biasing was inducing only a frequency shift of f\({}_{0}\), one should observe this frequency shift with in phase amplitude and phase variations (i.e., increasing or decreasing together). A similar reasoning can be made in Figure S2 where the amplitude and phase profiles are both increasing around f\({}_{\text{OP}}>\) f\({}_{0}\). Therefore, a frequency shift upon bias cannot be the only signature of the bias on the PNA signals. To further investigate the origin of the amplitude and phase variations at f\({}_{\text{OP}}<\) f\({}_{0}\) upon biasing, Figure 3a,b evidence the impact of the bias onto the PNA amplitude and phase interferometric profiles, respectively. At the inset of both figures, a broad range display of the PNA signal is shown, highlighting in green the narrower spectral range in which f\({}_{\text{OP}}\) is usually set and which corresponds to the main display. Several interferometric profiles were successively captured varying the bias progressively from 0 V to \(-\)6 V. A last record at 0 V allows for a determination of the inherent drift towards higher frequencies as mentioned before and confirmed in Figure 3a between the black and grey profile. Despite the drift, one can clearly observe the significant decrease (increase) in the amplitude (phase) signal with the bias consistent with what observed in Figure 2a in similar f\({}_{\text{OP}}<\) f\({}_{0}\) operating conditions. The inset in Figure 3a also shows an increase in the interferometric fringe at \(-\)6 V that could be responsible for the decrease in the amplitude signal. Meanwhile, the apparent decrease in the amplitude signal at \(-\)6 V is more pronounced for f\({}_{\text{OP}}<\) f\({}_{0}\) compared to f\({}_{\text{OP}}>\) f\({}_{0}\) operating conditions. This indicates that DC bias induces both a frequency shift and variations in the fringe amplitude of the interferometric profiles. These mechanisms are likely to be cross linked in the time profiles observed before, unless f\({}_{\text{OP}}\) is properly chosen, so that frequency shift and fringe amplitude variations more specifically impact the PNA amplitude and phase, respectively. Figure 2: (**a**) PNA Amplitude and Phase spectra captured at t\({}_{0}\). The spectra at t\({}_{1}\) and t\({}_{1}\) + \(\Delta\)t corresponding to the interferometric response prior to and after the capture of time profiles displayed in (**b**) are reconstructed from the spectra at t\({}_{0}\) taking only into account the drift of f\({}_{0}\) evaluated \(\sim\)1.7 Hz s\({}^{-1}\) (**b**) Variations of PNA Amplitude and Phase upon DC gate bias for f\({}_{\text{OP}}\)\(\sim\)10.4925 GHz. The applied DC tip bias is also shown associated with a vertical blue scale. Figure S3 shows interferometric and associated time profiles of PNA phase and amplitude captured upon the time at the same fringe around 10.46944 GHz. Once the probe came into contact with the sample, a first interferometric profile was captured (top black and red curves in Figure S3a, t\({}_{0}\)), followed by two time profiles with a step bias of \(-\)1 V and f\({}_{0}>\) f\({}_{\rm{OP}}\) (Figure S3b).