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