Scheme 1. Different types of CDs and role of aggregation in their formation, morphology, luminescence properties and applications.
As a novel kind of fluorescent nanomaterial, carbon dots (CDs) have enormous attention since they were first reported in 2004.[13] CDs are zero-dimensional (0D) carbon or 0D carbon-based nanomaterials.[14-18] Due to their excellent optical properties, low toxicity, good biocompatibility, environment-friendly, and facile preparation, and has shown exciting advantage in various application fields. [19, 20]Due to the variety of synthesis routes (top-down and bottom-up) and the abundance of raw materials (graphite, small organic molecules, polymers, and natural materials), a variety of CDs have mushroomed. The abundance of precursors, synthesis methods, and unknown reaction processes increase the difficulty of elucidating precise chemical structure of CDs. Researchers have tried to classify and name different CDs according to their structure and performance characteristics, but there is still a wide range of debate.[21-23] The formation of CDs mainly involves the aggregation of C, N, and O elements.[24-27] The proportions of these elements vary greatly among different types of CDs. The conjugated carbon sp2 domains (chromophores) inside CDs are proposed to have planar fused aromatic ring structures similar to fluorescent organic molecules.[28] Similar to organic fluorescent molecules, most of the initially synthesized CDs showed an aggregation-induced quenching effect. However, later research revealed that some CDs showed aggregation-induced (i.e. enhanced) luminescence, including concentration dependent luminescence[29-31] and solid-state luminescence.[32-34] In both the synthesis of CDs and their emission characteristics, aggregation plays an important role. Understanding such aggregation processes is of vital importance when designing CDs for particular applications.
This review aims to summarize recent research relating to synthesis and optical properties of CDs, placing particular emphasis on the role of aggregation in the formation of different kinds of CDs, including graphene quantum dots (GQDs), carbon quantum dots (CQDs), and carbonized polymer dots (CPDs), and subsequently how the aggregation of CDs with specific morphologies can realized special luminescent properties (Scheme 1).
2. Aggregation in different types of CDs
CDs are a general term for 0D carbon nanomaterials. However, owing to the diversity of their nanostructures, it is difficult to accurately define and classify them, resulting in widespread debate. At present, CDs can be roughly divided into three categories: GQDs, CQDs and CPDs (Figure 1A). Aggregation is a crucial part of the formation of each type of CDs.[21, 23] The aggregation of elements in CDs formation is very different for each. The aggregation of elements in CDs formation is very different for each. Below we examine the formation process, structure, classification of GQDs, CQDs and CPDs.
2.1. GQDs