At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to ...summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance.
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•This article summarizes the significant advances achieved in the past few years on synthesis, classification, mechanism and applications of CDs and CDs-based photocatalysts to give a ...thorough summary.•CDs-derived photocatalysts are devided into pure CDs and CDs composite photocatalysts and the photocatalytic efficiencies are compared.•Various kinds of mechanism during photocatalytic process in typical systems are concluded.•Applications in the fields of degradation of organics, water splitting for H2 and O2 evolution, CO2 conversion and photocatalytic chemical reactions are also reviewed to confirm the great potential of CDs-derived photocatalysts for environmental and energy issues.
With the rapid development of science and technology, environmental pollution and energy shortage become more and more prominent. To solve these problems, photocatalytic technology is regarded as one of the most efficient methods, allowing for both pollutant degradation and energy conversion. Compared with traditional group IIVI, IIIV quantum dots (QDs), carbon dots (CDs), as a newly emerging kind of fluorescent carbon-based material, possess many excellent properties, such as high aqueous solubility, low cost, low toxicity, abundant surface functional groups and good biocompatibility. In particular, the unique up-converted photoluminescence (PL) behavior and photo-induced electron transfer ability of CDs provide the new route to achieve efficient metal-free photocatalysts. This article reviews recent progress on CDs utilized for photocatalysis from different perspectives, including the following three parts: classification and synthesis, mechanism of CDs-derived photocatalysts as well as the applications for environmental issues (up-converted PL process) and energy conversion (photo-induced electron transfer process).
Shine on you crazy dots: A rapid and high‐output strategy allows the fabrication of polymer‐like carbon dots (CDs) with quantum yields as high as ca. 80 %. This value is the highest reported to date ...for fluorescent carbon‐based materials, and gives promise for their application in multicolor‐patterning and biosensors.
Polymer carbon dots (PCDs) are proposed as a new class of room‐temperature phosphorescence (RTP) materials. The abundant energy levels in PCDs increase the probability of intersystem crossing (ISC) ...and their covalently crosslinked framework structures greatly suppress the nonradiative transitions. The efficient methods allow the manufacture of PCDs with unique RTP properties in air without additional metal complexation or complicated matrix composition. They thus provide a route towards the rational design of metal‐free RTP materials that may be synthesized easily. Furthermore, we find that RTP is associated with a crosslink‐enhanced emission (CEE) effect, which provides further routes to design improved PCDs with diverse RTP performance. Our results show the potential of PCDs as a universal route to achieve effective metal‐free RTP.
Room‐temperature phosphorescence: Polymer carbon dots (PCDs) showing metal‐free room‐temperature phosphorescence (RTP) have been constructed by using a facile method. The contribution of the cross‐link‐enhanced emission effect to the generation of RTP is verified and proposed as a guideline to forecast and synthesize a series of PCDs with diverse RTP performance (ISC=intersystem crossing).
The crosslink enhanced emission (CEE) in a new type of non-conjugated polymer dots (PDs) is proved. The enhanced PL originates from the decreased vibration and rotation of amino-based chromophores. ...Furthermore, the cellular uptake mechanism and internalization of PDs were investigated in detail.
A large amount of emerging research on carbon dots (CDs) has been gradually improving the understanding of their structures, properties and emission mechanism. Distinct from the dominating status of ...quantum confinement effect in quantum dots, CDs always suffer from the complicated optical properties, deriving from the large differences in raw materials and synthesis methods. The diverse concepts and species puzzle researchers and hinder the further study. Thus, there is an urgent need to unify the definition and clarify the confused relation of CDs. Herein, we classify the raw materials of CDs synthesis into small molecules and polymers, and discuss CDs from the aspects of raw materials. We believe that the polymer-like structures reserved in CDs are universal no matter from the condensation of small molecules or the direct inheritance of polymers. Moreover, many similarities are concluded between CDs and polymers through serious comparisons and enough evidences. The formation processes of CDs are mostly polymerization and the obtained CDs always possess polymeric characteristics, such as abundant reactive functional groups, polydispersity of products, highly crosslinked network structure and other similar properties to non-conjugated fluorescent polymers. Therefore, the new concept, polymer carbon dots (PCDs), is put forward to generalize all kinds of CDs based on the summary of related reports. Besides, the complicated influence factors of photoluminescence (PL) are discussed and mainly classified as molecule state, carbon core state, surface state and crosslink enhanced emission (CEE) effect. In general, this review puts forward PCDs as a unified definition of reported CDs, and summarizes the polymeric characteristics of PCDs from formation process and product properties, as well as simultaneously illustrates the PL mechanism.
•Put forward a new concept as polymer carbon dots (PCDs) to unify the confused definitions of reported CDs.•Summarize the polymeric characteristics of PCDs to offer a new angle to understand this luminescent material.•Classify the PL mechanism as molecule state, carbon core state, surface state and crosslink enhanced emission effect.•Pointed out the present challenges and provide with further research directions.
In this paper, several exact expressions for the mean heat flux at the wall (
q
w
) for the compressible turbulent channel flows are derived by using the internal energy equation or the total energy ...equation. Two different routes, including the FIK method and the RD method, can be applied. The direct numerical simulation data of compressible channel flows at different Reynolds and Mach numbers verify the correctness of the derived formulas. Discussions related to the different energy equations, and different routes are carried out, and we may arrive at the conclusion that most of the formulas derived in the present work are just mathematical ones and that they generally are lacking in clear physical interpretation in our opinion. They can be used to estimate
q
w
, but might not be suitable for exploring the underlying physics.
Lately, there has been a rapid increase in the use of software-based deep learning neural networks (S-DNN) for the analysis of unstructured data consumption. For implementation of the S-DNN, ...synapse-device-based hardware DNN (H-DNN) has been proposed as an alternative to typical Von-Neumann structural computing systems. In the H-DNN, various numerical values such as the synaptic weight, activation function, and etc., have to be realized through electrical device or circuit. Among them, the synaptic weight that should have both positive and negative numerical values needs to be implemented in a simpler way. Because the synaptic weight has been expressed by conductance value of the synapse device, it always has a positive value. Therefore, typically, a pair of synapse devices is required to realize the negative weight values, which leads to additional hardware resources such as more devices, higher power consumption, larger area, and increased circuit complexity. Herein, we propose an alternative simpler method to realize the negative weight (named weight shifter) and its hardware implementation. To demonstrate the weight shifter, we investigated its theoretical, numerical, and circuit-related aspects, following which the H-DNN circuit was successfully implemented on a printed circuit board.
The graphene quantum dots (GQDs) are attractive nanomaterials with excellent optical and optoelectronic properties. However, the clear photoluminescence (PL) was difficult to be investigated due to ...the complicated structures of the reported GQDs. In this work, four kinds of GQDs were synthesized by organic methods with certain chemical structures. The PL mechanism of the GQDs was investigated by ultrafast spectroscopy. In these organic synthesized GQDs, intrinsic state depends on size, while the energy level offset between intrinsic state and edge state decides their optical properties. As a result, the green fluorescence of the C42H18, C96H30 not only depends on the size, but also results from bright edge state. For large GQDs, the energy level of intrinsic state is lower than the edge state, which lead to the weak PL. Furthermore, the PL polymer dots (PDs) were prepared by assembling GQDs and polymeric surfactant. The PDs possessed perfect solubility in water and kept the PL behavior of the organic synthesized GQDs. The present methods and results will lead a new direction for investigating the PL mechanism of the GQDs.
The dust originating from the extinct lake of the Aral Sea poses a considerable threat to the surrounding communities and ecosystems. The accurate location of these wind erosion areas is an essential ...prerequisite for controlling sand and dust activity. However, few relevant indicators reported in this current study can accurately describe and measure wind erosion intensity. A novel wind erosion intensity (WEI) of a pixel resolution unit was defined in this paper based on deformation due to the wind erosion in this pixel resolution unit. We also derived the relationship between WEI and soil InSAR temporal decorrelation (ITD). ITD is usually caused by the surface change over time, which is very suitable for describing wind erosion. However, within a pixel resolution unit, the ITD signal usually includes soil and vegetation contributions, and extant studies concerning this issue are considerably limited. Therefore, we proposed an ITD decomposition model (ITDDM) to decompose the ITD signal of a pixel resolution unit. The least-square method (LSM) based on singular value decomposition (SVD) is used to estimate the ITD of soil (SITD) within a pixel resolution unit. We verified the results qualitatively by the landscape photos, which can reflect the actual conditions of the soil. At last, the WEI of the Aral Sea from 23 June 2020, to 5 July 2020 was mapped. The results confirmed that (1) based on the ITDDM model, the SITD can be accurately estimated by the LSM; (2) the Aral Sea is experiencing severe wind erosion; and (3) the middle, northeast, and southeast bare areas of the South Aral Sea are where salt dust storms may occur.
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