Insights into photoluminescence mechanisms of carbon dots:advances and perspectives

Carbon dots(CDs) are potentially useful in many areas such as bioimaging, light-emitting diodes, and sensing because of their excellent optical properties, high biocompatibility, and low toxicity.Knowledge of their photoluminescence(PL) mechanisms, which have been widely studied, is of significance in guiding the synthesis and promoting applications of CDs with tunable PL emissions. However,the intrinsic mechanism of PL emission remains unclear, and a unified mechanism has not been found because of differences in particle structures. This review generalizes the categories of CDs, noting their structural diversity. Three types of PL mechanism pertaining to structural differences are outlined: internal factors dominated emission(including the conjugation effect, the surface state, and the synergistic effect), external factors dominated emission(including the molecular state and the environment effect),and crosslink-enhanced emission. Optical applications of CDs are also briefly mentioned. Finally, the prospects for research into PL mechanisms are discussed, noting the remaining challenges and directions for future work.

Aggregation and luminescence in carbonized polymer dots

The aggregate luminescence behavior of organic luminescent materials has been studied extensively.As a new kind of luminescent nanomaterials,carbonized polymer dots(CPDs)not only inherit the stability and biocompatibility of carbon materials,but also possess the luminescence tunability,water solubility,and high photoluminescence quantum yield of organic luminescent materials,rendering them a strong candidate for the next generation of light-emitting materials.Previously,people mainly understood its luminescence from the perspective of carbon materials,but some luminescence mechanisms are still unclear.In this review,we discuss the luminescence mechanism by referring to organic luminescent materials with emphasis on their aggregation behavior.Firstly,three representative aggregate luminescence phenomena of organic luminescent materials are briefly introduced.Chromophores present in CPDs are elaborated to further discuss the potential interactions between them,with emphasis on the role of crosslinked polymer networks.On this basis,some special luminescence phenomena of CPDs in the aggregate state are summarized,and relevant mechanisms are discussed in detail to consolidate relevant statements.

Cross-linking enhanced room-temperature phosphorescence of carbon dots

Currently,there is a strong drive to discover alternative materials that exhibit room-temperature phosphorescence(RTP)for displays,bioimaging,and data security.Ideally,these materials should be nontoxic,cheap,and possess controllable photoluminescent properties.Carbon dots(CDs)possess each of these characteristics,but to date,less attention has been paid to their RTP mechanism.Herein,we synthesized a series of CDs by self-crosslinking and carbonization of precursor.The resultant CDs were luminescent and exhibited a bright,micro-second afterglow lifetime.To increase the RTP,a second microwave processing step was used to coat the CDs with polyvinyl alcohol(PVA),polyacrylamide(PAM),or tetraethyl orthosilicate(TEOS),producing CDs@PVA,CDs@PAM,and CDs@TEOS composites.The core-shell structure acted to enhance crosslinking at the surface of the CDs to boost the RTP,creating abundant energy levels for intersystem crossover.In situ X-ray photoelectron spectroscopy verified electron transfer during luminescence.Finally,we present a design rule that can be used to tune the quantum yields and RTP lifetime of CDs,based on the effective stabilization of triplet excited states through the extent and strength of cross-linking.This simple strategy provides a flexible route for guiding the further development of CDs with tailored RTP properties for various applications.