Thanks Tiwari to answer

I mean accelerating voltage of SEM

If it's correct what I understood from your question: the dark material must be prepared by coating the samples with a very thin gold film by converting gold to spray before using a scanning electron microscope to get images .. This would help in getting the differences between the two materials.

thanks for your answer dear Mr Al-Badrawy A. Abo El-Nasr 

I found the best answer in the following link. you could download it.

It's really applicable for the researchers who use SEM apparatus for synthetic materials characterization.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwips6GhxZzRAhXFxxQKHS2CBIEQFggZMAA&url=http%3A%2F%2Fwww.jeolusa.com%2FRESOURCES%2FElectron-Optics%2FDocuments-Downloads%3FEntryId%3D597%26Command%3DCore_Download&usg=AFQjCNHXffXC5bfLyq97tM648aCpFwTfiw&sig2=jLs6x1l1u_Mzk67wyv4ebQ&bvm=bv.142059868,d.d2s

Regards