Building SiO2 nanocluster in MS

Hello All,
I am graduate student working on MD simulations. Our aim is to simulate Nitrate, Carbonate salts with SiO2 nanoparticles added to them.

I have made a SiO2 nanocluster in Material Studio and inserted them into the Carbonate/Nitrate systems. But the properties that we are getting after simulation are not as expected. We believe this is happening due to the coarse structure of SiO2 nanocluster.

We needed a particular number of atoms in our nanostructure relating to the percentage added in the experiments. So after calculations, we needed around 234 atoms (78 Si, 156 O). To get this what I did was, make a alpha crystal first and then made a spherical nanocrystal with 10A D using the Build/Build Nanostructure/Nanocluster. But as it did not had the requires atoms, I deleted the extra manually as required. Which left me with a coarse nanocrystalline structure. I am not very familiar with material studio. So i might sound little confused. But I would be very thankfull if anyone could help me.

Thank-you,
Regards,
Vidula Pawar.