**3.3 Prior termination**

Before any surface treatment, we removed the oxides by immersing the Si NWs in HF solution, and Si▬H can be formed. Obtaining Si▬H bonds has three main advantages: (i) it helps us explore oxidation mechanism since Si▬H bonds are stable for a few minutes (less than 5 min), (ii) it gives full monolayer, and (iii) H-terminated is the starting step for molecular grafting [33–36].

#### **Figure 7.**

*XPS spectrum of Si2p core-level emission showing two silicon and four oxide peaks.*

To follow the stability of the Si▬H bonds or in other words to follow the oxidation of the Si NWs, we followed the Si2p emission spectra. As you can see in **Figure 7**, the Si2p emission includes two silicon spin-splitting peaks: (i) Si 2*p*1/2 and (ii) Si 2*p*3/2.

We can follow the amount of each oxide state (*I*SiOx) by the relative integrated area under each peak. For example, we divide the integrated area under the oxide state (*A*SiOx) by the sum of the integrated area under the Si2p, i.e., the Si2*p*1/2 and Si2*p*3/2 peaks (*A*Si 2*p*1/2 + *A*Si 2*p*3/2). Therefore, the total oxidation (*I*ox) can be calculated by the sum of the all the oxide states, i.e., (*I*ox = *I*Si2O + *I*SiO + *I*Si2O3 + *I*SiO2). It is worth to mention that the oxidation rate is different at low or high temperature (**Figure 8**). For example, Bashouti and co-authors observed different mechanisms at low temperatures (from 25 to 150°C), in which the suboxide states are the main share of the total oxide state, while at high temperatures (200–400°C), the full oxide state (i.e., SiO2) is the main contributor to the total oxide [37, 38].

Each oxide state shows different shift and intensity relative to the Si2p. Therefore, each state has its own oxidation rate. To this end, we can calculate the respective activation energies (EA ox) of each state. Roughly speaking, since all the

#### **Figure 8.**

*(a) The sub- and full oxide distribution as function of binding energy shift and intensity per suboxide and (b) total oxide intensity of all oxide states in low and high temperature in Si NWs and 2D surfaces.*

suboxides show similar rate, the EA ox was 46.35 and 23.31 meV in high and low temperature, respectively [39, 40]. The differences in the activation energies of Si NW in the high and low temperatures reveal different oxidation kinetic mechanisms:


Understanding the oxidation mechanism will help us get high stable molecules on the Si NWs surface. In addition, since most of the electronic devices are operated in low temperature (below 200°C), the understanding of the low-temperature mechanism is very valuable [42].
