Carbon dots: materials, synthesis, properties and approaches to long-wavelength and multicolor emission.

Herein, we summarize recent research developments and progress of carbon dots (CDs), which have been attracting considerable attention as a new type of photoluminescent material. Raw materials, from single carbonaceous compounds to colorful natural substances, for the synthesis of CDs are discussed. A range of diverse synthetic methodologies to achieve better photoluminescence performance and more advanced functions are summarized, and these are basically divided into two classes: top-down and bottom-up. The inspiring properties, mainly including composites, optical properties and cytotoxicity, are listed. In particular, the luminescence mechanism and surface functionalization of the CDs are briefly discussed. Moreover, on the basis of the above, the long-wavelength and multicolor emission properties of CDs and ways to achieve these goals including surface state and size controlled by synthesis strategies, proper precursors, chemical doping and modification, solvatochromic effects and energy transfer are reviewed in detail.