**3.8 Stability of functionalized Si NW**

It was found that the stability of grafted molecules on the Si NWs is the function of several factors mainly the (i) molecular chain length, (ii) coverage level, and (iii) surface energy and diameter.

#### **3.9 Effect of coverage and chain length**

The molecular surfaces (C1–C10) were exposed to ambient air for 100 hours at room temperature, as shown in **Figure 15**. In the first days, all the alkylated Si NW show high oxide resistivity. However, after 8 days the oxide intensity became considerable and found to be dependent on the chain length and the coverage level. For example, in the case of C3–C6, the oxide level rose to ~0.13. However, at the same time, C1 shows only 0.03, which is twofold higher oxidation resistance than that of C3▬C6▬Si NWs. This implies that stability of the Si NWs is dependent on the molecule coverage [28].

#### **3.10 Effect of surface energy and diameter**

Different diameters of Si NW have been used to explore the impact of the diameter: 2D (100), 2D (111), Si NW 50 nm in diameter (Si NW50nm), and Si NW 25 nm in diameter (Si NW25nm). To make a proper comparison, we used the same molecule (CH3) in all the different samples. Then we exposed them to ambient air for same periods (see **Figure 16**).

Interestingly, the stability of methyl groups on Si NW is dependent on the surface. For example, the CH3 molecule on 2D (111) was more stable than 2D (100). For example, they show the same oxidation level, but after 40 and 20 days, the Si (111) and (100), respectively, i.e., (111), show double stability than (100). The higher stability of the 2D (111) relative to the 2D (100) structure is understandable since it naturally has a 15–20% higher coverage than the 2D (100) case [45, 46].

Compering to the NWs, the NWs show almost threefold *lower* oxidation than the Si (111) and (100). These observations can be attributed to the stronger Si▬C

#### **Figure 15.**

*Observed oxidation intensity (SiO2/Si2p peak ratio) of alkyl-terminated Si NWs at different exposure times to ambient air. Reproduced with permission from [28].*

*Heterojunction-Based Hybrid Silicon Nanowires Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.84794*

**Figure 16.**

*Ratio of the oxidized to bulk Si2p peak areas for the methyl modification of NW25nm and NW50nm and 2D Si (111), exposed to air over extended time periods.*

bonds on Si NW surfaces. This is supported by the shift in the Si▬C bond in the NWs from 284.33 ± 0.02 eV (Si NW25nm) and 284.22 ± 0.02 eV (Si NW50nm) to 284.11 ± 0.02 eV for planar 2D Si. The ~0.11 ± 0.02 eV to higher binding energy ascribed to the higher reactivity of atop sites.

### **3.11 Effect of bonds type:** *π***-***π* **vs. σ-σ interactions**

Not only the coverage degree and surface may affect the stability of the molecules on the Si surface. It was found that bond type interactions (*π*-*π* vs. σ-σ) can tune the stability. To check this, Si NW were embedded with methyl CH3 and propenyl (CH3▬CH〓CH▬ Si NWs). **Figure 17** shows the oxidation of CH3▬CH 〓 CH▬Si and CH3▬Si NWs. The oxidation began for the two molecules after only ∼100 hours of exposure. However, after 100 hours, the propenyl shows higher stability, i.e., less oxidation. This became more clear after 180 hours; the propenyl

#### **Figure 17.**

*Ratio of the SiO2 to Si2p peak areas for the different surface modifications of Si NWs, exposed to air over extended time periods. Reproduced with permission from [47].*

shows much lower intensity of 0.015 ± 0.005, that is, almost 8 times less than the methyl 0.11 ± 0.017. The high stability of the CH3▬CH 〓 CH▬Si NW can be attributed to the *π*-*π* interactions between the adjacent molecules [47–49].
