To my knowledge, both FeO and Fe2O3 are lossy dielectrics in visible and near infrared. This means that the real part of their permittivity is positive and no plasmon can be excited on nanoparticles made of such material.

Precisely!

thank you very much....another thing ,which solvent is best for their colloidal dispersion?

The simplest way is to use water. But there are a lot of other issues. Look through these two sources, for example:

http://rubenmorones.com/people/cv/salksdjalskd.pdf

http://www.diva-portal.org/smash/get/diva2:618655/fulltext02.pdf

Sir,Alexandre V. Tishchenko ,can you please explain in more simple words the reason of their poor Lspr properties