Carbon nanodots (C-dots) synthesized by electrochemical ablation and small molecule carbonization, as well as graphene quantum dots (GQDs) fabricated by solvothermally cutting graphene oxide, are three kinds of typical green fluorescence carbon nanomaterials. Insight into the photoluminescence origin in these fluorescent carbon nanomaterials is one of the important matters of current debates. Here, a common origin of green luminescence in these C-dots and GQDs is unraveled by ultrafast spectroscopy. According to the change of surface functional groups during surface chemical reduction experiments, which are also accompanied by obvious emission-type transform, these common green luminescence emission centers that emerge in these C-dots and GQDs synthesized by bottom-up and top-down methods are unambiguously assigned to special edge states consisting of several carbon atoms on the edge of carbon backbone and functional groups with C═O (carbonyl and carboxyl groups). Our findings further suggest that the competition among various emission centers (bright edge states) and traps dominates the optical properties of these fluorescent carbon nanomaterials.
Three-dimensional photonic crystal structures were fabricated with laser microfabrication techniques through two-photon-absorption photopolymerization of resin. Significant band-gap effects in the infrared wavelength region were observed from “layer-by-layer” structures.
Actuators that can convert environmental stimuli into mechanical work are widely used in intelligent systems, robots, and micromechanics. To produce robust and sensitive actuators of different scales, efforts are devoted to developing effective actuating schemes and functional materials for actuator design. Carbon-based nanomaterials have emerged as preferred candidates for different actuating systems because of their low cost, ease of processing, mechanical strength, and excellent physical/chemical properties. Especially, due to their excellent photothermal activity, which includes both optical absorption and thermal conductivities, carbon-based materials have shown great potential for use in photothermal actuators. Herein, the recent advances in photothermal actuators based on various carbon allotropes, including graphite, carbon nanotubes, amorphous carbon, graphene and its derivatives, are reviewed. Different photothermal actuating schemes, including photothermal effect-induced expansion, desorption, phase change, surface tension gradient creation, and actuation under magnetic levitation, are summarized, and the light-to-heat and heat-towork conversion mechanisms are discussed. Carbon-based photothermal actuators that feature high light-to-work conversion efficiency, mechanical robustness, and noncontact manipulation hold great promise for future autonomous systems. and most have high photothermal conversion efficiencies. [55][56][57][58] With excellent thermal conduction characteristics, carbon materials can transfer the as-obtained thermal energy to the heat-sensitive materials, achieving effective photothermal actuation. [59,60] In addition, carbon materials have many advantages, such as excellent physical and chemical stability, high mechanical strength, conductivity, as well as tunable light absorption properties, which allow for broad application of carbon-based actuators. [61] In this review, we focus on the recent advancements in carbon-based photothermal actuators and highlight their unique advantages, good performance, and potential for future applications. Specific light-to-heat conversion mechanisms and strategies that enable photothermal actuation are discussed. The performance of various photothermal actuators focusing on energy conversion, response/recover time, stability, mechanical strength, and actuating manners has been reviewed in details. Figure 1 shows a summary of typical actuators constructed of different carbon-based materials and their possible light-to-work mechanisms, including photothermal expansion, desorption, phase change, Marangoni effect, and magnetic susceptibility change. In addition, the advantages and limitations of carbon-based photothermal actuators and the current challenges in this field are discussed in Section 4. The development of carbon-based photothermal actuators may stimulate rapid progress in various smart devices for cutting-edge applications.
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