As you can see in this image you will find the agglomeration of different field related to Materials Science. Material Science is basically deals with the materials. In the 21st Century world is changing so fast it needs more and more new materials and composites day by day. As you have not mentioned your background I will not be able to suggest best option for you but I will try to give a birds view on all the related field to material science. You can choose what best suits you according to your interest and background. Nanotechnology is a upcoming science which can be applied in material science to produce new smart materials which includes composites of glass, ceramics, plastics and polymers. Plastics and polymers are the new members which will be replacing the metals, ceramics and glass. Nanotechnology will help in it's processing. Applications of these smart materials include aviation, transportation and space. energy sector is also using these type of smart materials in the making of solar panels, fuel cells, and non-conventional applications like wind-mills. Bio-materials are also increasing important due to some new recent discoveries. These bio-materials have applications in bio-medical field as well as in medicines. advance materials characterization is also needed equally importantly to characterize and detect faults and fractures in these materials. These characterization techniques also includes the microscopy.