Hi Vahid,

I think that one of the reasons why transition metals are used as dopants more than others for ZnO, is because their atomic radius is quite similar to Zn. Furthermore, since Zn is a transition metal, he might share more characteristics with them than with others.

Because transition metals exhibit at least two oxidation states. From one oxidation state to another by an external stimuli, their optical and electrical properties change usually. Thus, they are good candidate for sensing applications.

For the first question, you should check the literature.