Observations on the citation networks often confirm a core-periphery structure: A clustered group of artifacts possess the core knowledge to the field, which is widely cited by artifacts at periphery. We explain this as an outcome resulted from decentralized knowledge contributions from individuals who maximize their own utilities. Our model sheds insights on how knowledge creation, knowledge citation, and knowledge heterogeneity affect the emergence of knowledge core, in both cases of direct and indirect citations. We find through simulations that the core-periphery architecture of citation networks is robust to generalizations on knowledge heterogeneity and knowledge creativity. By studying the incentive rationale that underlies the growth of citation networks, our research has potential implications on the design and administration of intellectual communities.