ABSTRACT
Organic afterglow materials based on carbon dots (CDs) have aroused extensive attention for their potential applications in sensing, photoelectric devices, and anticounterfeiting. Effective methods to control the CD structure and modulate the energy levels are critical but still challenging. Here, we demonstrate a method to modulate the afterglow emission of CDs@SiO2 composites by controlling the carbonization degree of CDs with variable calcining temperatures. The CDs@SiO2-Raw prepared with a hydrothermal bottom-up synthesis method shows a more polymerized structure of CDs with low carbonization degree, which emits long-lived thermally activated delayed fluorescence (TADF) with the lifetime of 252 ms. After calcination at 550 °C, CDs@SiO2-550 exhibits a larger conjugated π-domain structure with higher carbonization degree, thus inducing room-temperature phosphorescence (RTP) emission with a lifetime of 451 ms. The transformation of the carbonization degree of CD structures leads to changes in energy levels and ΔEST, which affect their afterglow luminescence behaviors. This work proposes a new concept to modulate the afterglow emission of CDs@SiO2 composites and forecasts potential applications of CD-based afterglow materials.
ABSTRACT
In this study, the effects of tubificid bioturbation near the water-sediment interface on pore structures and the migration of sediment particles were evaluated using a series of simulations. In these experiments, the distribution and variation of the tubificid burrows and the macropores in the sediment were investigated by X-ray computed tomography (CT) and digital image collecting, without sampling or disturbing the sediment. The migration of the sediment particles was also determined using CT by adding BaSO4 microspheres to the sediment as a tracer. The effects of tubificid bioturbation on the distribution and migration of contaminants in the sediment were verified by adding Pb-containing sediment layers to the sediment. The results indicate that after the addition of the tubificids, both the burrows and the macropores in the sediments increased with time, and the rate of increase slowed gradually. With the increased worm density, the burrows and the pore structures also increased. The in-depth distribution of the burrows and macropores was determined by the settlement time of the worms: with the settlement time increasing from 3 to 19 days, the depth of the zone with the highest density of burrows and macropores increased from 0-30 to 30-50 mm and from 0-10 to 30-60 mm, respectively. The distribution of the burrows and macropores was closely related to the distribution of the tubificids. Thickening of the oxidized zones in the superficial sediments in the presence of tubificid bioturbation was also observed. The main action of tubificids on the sediment particles was the transport of particles from the inner sediment (especially in the range of 30-50 mm in depth) to the water-sediment interface. The migration of Pb in the contaminated sediment with tubificid bioturbation could be interpreted by the variation in the burrows and macropores and the migration of sediment particles. Both the formation and the variation in the burrows and macropores, as well as the transport of particles from the inner sediment to the interface, would affect the behaviors of contaminants in the sediment and overlaying water near the water-sediment interface.