**Figure 11.**

*Si2p emission of SiO2*▬*Si NW and H*▬*Si NW.*

#### **Figure 12.**

*XPS data from the C1 s emission region before and after alkyl-terminated Si NWs. (a) H*▬*Si NWs show two peaks: C*▬*C at 285.20 ± 0.02 eV and C*▬*O at 286.69 ± 0.02 eV. (b) Methyl-Si NWs show three peaks: C*▬*Si at 284.11 ± 0.02 eV, C*▬*C at 285.20 ± 0.02 eV, and C*▬*O at 286.69 ± 0.02 eV.*

286.69 ± 0.02 eV which may belong to adventitious hydrocarbons. After termination, the CH3 is chemically bonded to the Si and the Si▬C is observed. Therefore, the emission of the C1 s fitted to three peaks: C▬Si at 284.11 ± 0.02 eV, C▬C at 285.20 ± 0.02 eV, and C▬O at 286.69 ± 0.02 eV. The deconvolution method is described in [27].

#### **3.6 Calculating the molecular density on the Si NW surface**

In order to address this issue, we used a "model" molecule. In our case we chose the methyl (i.e., CH3) since it is the smallest organic alkyl molecule with a van der Waals diameter (VDW) of 2.5 Å lower than the internuclear distance between adjacent Si atoms (3.8 Å). To this end, theoretically, the molecule should give nearly full surface density (100%), i.e., coverage (see **Figure 13**). The molecular coverage can be obtained by dividing the area under the C▬Si peak to the area under the Si2p peak (sum of Si2p1/2 and Si2p3/2). Subsequently, we ratioed all the molecular coverage to the methyl, i.e., "(C▬Si/Si2p)alkyl/(C▬Si/Si2p)max.methyl" [27].

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

**Figure 13.** *Schematic diagram of methyl connected to adjacent Si atoms.*

### **3.7 Termination of the Si NW with different molecules**

Here we use alkyl molecules since they have the same structure as in methyl. The molecular coverage was found to be dependent on the steric effects which caused by the lateral interactions between the molecules. Steric effects decrease the coverage level for any molecule longer than methyl. This is due to the fact that longer molecules have higher VDW diameter (4.5–5.0 Å) than the diameter of the Si atoms. In our case, we used molecule with the form of C*n*H2*n* + 1 (where *n =* 1–10) and represented by C*n*. For example, methyl and decyl are represented by C1 and C10, respectively. As the VDW diameter increases, low coverage decreases as shown in **Figure 14**.

We compare the coverage between the Si NWs and the 2D silicon. We found a similar decay but lower coverage of 10% in average than the 1Si NWs. This is maybe due to the couverture effect, which causes the molecule to be normal to the surface, and therefore, the steric effect can be lower in the case of the Si NWs. However, after > C6, an inconsistency is observed. Based on this we can consider two main factors for grafting:


The first factor can play a role in the short molecules (C1–C5), since they exhibit liquid-like behavior and thermal fluctuations; the determining factor is the vertical

#### **Figure 14.**

*Γmax-alkyl versus alkyl chain length on Si NWs and, for comparison, on 2D Si(100) surfaces. Note: (Γmaxalkyl) (C*▬*Si/Si2p)max-alkyl/(C*▬*Si/Si2p)max-C1. Reproduced with permission from [28].*

interaction [43]. The second factor may play a role in the longer molecule, i.e., (C6–C10) that forms a solid-like phase, and therefore, the lateral interactions become dominant in this regime [44].
