## 3 Results
With the data obtained in the CPD and APS measurements, the values of the valence band maximum (VBM, corresponding to \(E_{\rm V}\)), surface Fermi level position (\(E_{\rm F}\)) and conduction band minimum (CBM) could be calculated using the common room temperature band gap energy for silicon of 1.12 eV. Figure 3d shows the difference between the energy levels of the surface valence band edge and the Fermi level (\(E_{\rm F}\)-\(E_{\rm V}\)) over immersion, as deduced from the reconstructed electronic surface band structure of n-doped SiNWs when immersed in PBS. Day 0 corresponds to the band structure of n-type SiNW in air. The measurements showed that the Fermi level was shifted towards the conduction band before the immersion. This was in accordance with the high donor doping concentration. Figure 3: **(a)** Schematic of the \(I\)-\(V\) measurement set-up with the kinked SiNW in red (p-type branch) and blue (n-type branch) integrated on a chip (black) as a p-n junction. **(b)** Schematic of the working principle of the surface band structure (upper left, pale yellow area) reconstruction. Using CPD measurements and APS, it is possible to determine the surface Fermi level and the ionization potential. With these two values and the knowledge of the material’s band gap energy, the surface band structure can be reconstructed. The APS method uses UV irradiation (violet) to liberate photoelectrons (blue) from the material, which are then registered by the detector (gold). **(c)** Decay of the electrical properties of a single SiNW p-n junction plotted as \(I\)-\(V\) curves on a logarithmic scale. For better legibility, only selected time points are shown. The circle highlights the increase in forward current during the first days of immersion. The complete measurement series is shown in Figure S2. **(d)** Time dependent changes in electronic surface band structure of n-type SiNWs during immersion in PBS at 37 \({}^{\circ}\)C. For comparison the conduction band minimum (CBM) for the common band gap energy of Si of 1.12 eV is given as a red line. Immersing the SiNWs led to considerable increases in the distance between the valence band edge and Fermi level. The Fermi level intermittently even rose above the CMB level. After seven days of immersion, however, the Fermi level shifted back toward the valence band and stabilized at around 0.8 eV above the valence band, still well above the middle of the band gap expected at 0.56 eV considering the silicon band gap of 1.12 eV. As recent literature shows, coating SiNW devices with Al\({}_{2}\)O\({}_{3}\) could enhance their lifetime significantly [11]. Therefore, we compared this dissolution rate to the one of SiNWs with a 3.5 nm Al\({}_{2}\)O\({}_{3}\) coating deposited by atomic layer deposition (ALD). Additionally, we tested a submonolayer coating of Al\({}_{2}\)O\({}_{3}\) to verify if a mixed oxide from aluminium and silicon could prevent the dissolution reaction, as suggested by [PERSON] [20]. Again, we measured the SiNW height using AFM (Figure S4). In this experiment, the tested Al\({}_{2}\)O\({}_{3}\) coatings did not have a considerable impact on the longevity of the SiNWs. However, optimizing the ALD deposition might yield improvements, which will be the subject of future studies.