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