In this dissertation, I report the fine size tuning of PbSe nanocrytals (NCs) with atomic-level precision and the morphologically controlled 2-dimensional PbSe nanosheets (NSs). The approach involves slow injection of precursors with excess amounts of oleic acid. Re-tarded growth, due to both the slow supply of precursors and the surfeit of oleic acid, caus-es the attachment of PbSe NCs through the (110) planes, the most reactive planes, resulting in the formation of two-dimensional NSs. In contrast, such an attachment process can be prevented by impurities, e.g., Cd chalcogenide (CdSe or CdS) NCs. Instead, PbSe NCs are formed with a uniform size so slowly that it is possible to obtain the targeted size of the NCS.