Novel Carbon Quantum Dots Structure And Nanoscale Carbon Materials With Rotational Stacking Graphene Structure

Carbon Quantum dots are one of the most popular luminescent materials in the field of nanomaterials.Due to various synthesis methods,the structures of carbon quantum dots are diverse.In particular,crystalline structures of carbon nanodots are usually different from many reported graphene quantum dots.Many carbon nanodots with a crystal structure have not been studied in detail,and there may be many new carbon structures not having been discovered.In this paper,the carbon quantum dots was synthesized by hydrothermal method with citric acid.It have a face centered cubic(fcc)carbon structure core and good crystallinity.Its size is within 5 nm and average size is 2.8 nm.This carbon structure is mainly composed of C-C bonds(sp3 carbon).At the same time,the presence of oxygen-containing functional groups on the surface of CQDs causes a small amount of sp2 carbon in the CQDs,and CQDs exhibit weak fluorescence characteristics in the aqueous solution,PL uantum yield is lower than 1%.However,after N doping,we have obtained nitrogen-doped carbon quantum dots(fluorescent quantum yield higher than 40%)with high fluorescence characteristics and the similiar core structure.The nitrogen-doped carbon quantum dots emitting range from orange to red with the face center cubical(FCC)cores were prepared by simple hydrothermal methods.Their luminescence mechanism was related to their face center cubical cores.Moreover,the EELS results show sp2 N atoms(weakly bonded or non-bonded nitrogen atoms)forming conjugated sp2-domian structure with the surrounding carbon atoms of the fcc carbon matrix either in the cores or at the interface between the core and the surface of the CQDs.It affected the bond gap and caused the long wavelengrth emission.Furthermore,with the content of sp2N atoms increasing,CQDs showed the red-shifted long wavelength emission.Graphene can obtain the similar bandgap of the semiconductor that can be controlled by the external electric field through layers stacking.Different stacking methods bring different properties and various properties can be obtained by changing the rotational angle between graphene layers.Nanoscale Carbon Materials with Rotational Stacking Graphene Structure was prepared by graphene quantum dots(using their structural characteristics).The low-voltage aberration corrected HRTEM with an electron accelerating voltage of 80 kV was used and the results show that there are a large nu,mber of nanoscale pore structures in carbon nanomaterials.Their structures are mainly composed of several graphene stacking layers with random rotational angles and form the superlattice structure(Moire pattern).The edge of the pore exhibits two typical graphene edges:zigzag and armchair.We distinguish each graphene layer by the Fast Fourier Transform(FFT)analysis of the sample's HRTEM images.The results show that these Moire patterns come from two ways:One is a rotational stack between graphene sheets,which also can be stacked by multi-layers of graphene to form a more complex Moire pattern;the other is that the same layer of graphene sheet is floded itself and we have observed this phenomenon through in-situ HRTEM.This folding phenomenon mostly exists at the edge of the sample.The single layer of graphene is folded to form two layers of graphene with arbitrary rotational angles.Moreover,the statistical results show that the relative rotational angles between graphene layers in different Moire patterns are arbitrary.Nanoscale regional superlattice(Moire patterns)and pore structures cause a large number of structural defects.Random rotational angles between different layers means that carbon materials have a variety of different bandgaps and various types of stacking methods,which may cause various photoelectric properties and chemical catalytic properties.