As a Chemistry student, you are expected to find and use high quality literature. You'll need to use a range of different sources and demonstrate your ability to find good results in order to impress your tutors and to develop your research skills.
To start a more detailed search for research papers, you'll need to use online databases. These are collections of resources that allow you to search from multiple journals at the same time.
University of York PhD/MPhil theses and Masters dissertations, plus theses and dissertations from other institutions.