TEM imaging contrast mainly depends upon: Mass contrast, Thickness contrast.

Here you have prepared a kind of nanocomposite having two different compounds and they have difference in their mass so the lighter on( C- dots) will become slightly brighter than the heavier one (ZnO NWs) .

For more details:

http://www.sfu.ca/~kavanagh/StrucAnallMat/ImageContrast2010.pdf

Dear Sharma

Thanks for your answer, based on the answer, every place on the ZnO surface which GQDs exist, we have two compounds (GQDs and ZnO). Therefore, electrons beam of TEM observes both compounds (no only GQDs) and as a result, their mass is heavier. Thus, they must have slightly darker than ZnO. If I'm wrong, please give me more details.

When you have a carbon based material it isn't supposed to have much contrast by TEM or SEM, because you are using carbon as your conductor!