Which one of the following is used to convert sunlight into electricity in photovoltaic material?

In the following reaction,assign the correct category for the underlined atom based on the product of complete hydrolysis at $R.T.$:
$\underline{Si}F_4 + 2H_2O \longrightarrow H_4SiO_4 + 4HF$

Monosilane,on coming in contact with air,burns with a luminous flame producing vortex rings. These rings are of:

Which one of the following phosphorus halides contains both tetrahedral and octahedral shaped ions in its solid state?

Silicon dissolves in excess of $HF$ due to the formation of:

Explain the following:
$(a)$ $CO_2$ is a gas whereas $SiO_2$ is a solid.
$(b)$ Silicon forms $SiF_6^{2-}$ ion whereas corresponding fluoro compound of carbon is not known.