Research On The Transient Photophysical Mechanism Of Carbon Materials By Ultrafast Spectroscopy

With the advancement of ultra-short pulse laser, the high time-resolved spectral probe system has been realized. It plays a very important role in the research of physical process at the molecule and atom level, such as the separation and recombination of electron-hole pair, the absorption, relaxation and radiation of the excited electron, and the investigation of the interaction of light and matter, the electronic structure of materials and the life time of excited molecule. In this paper, we focus our attention on the study of luminescent mechanism, electronic structure and ultrafast dynamics of graphene oxide(GO), graphene oxide nanoribbon(GOR) and graphene quantum dot(GQD) by ultrafast spectroscopy, which contains four parts as follows:In the first part, we have accomplished the setup of femtosecond laser pump-probe system and the time-correlated single-photon counting(TCSPC) system. Meanwhile, we gave a brief introduction about the properties and research status of GO, GOR and GQD.In the second part, we have investigated the untreated GO, GO treated with HNO3 and Na OH by femtosecond transient absorption spectroscopy. By analysis, we discovered a surprising result. In all these different GO, there is a new hybrid state. We ascribed it to the amorphous carbon atoms matrix in the boundaries of sp2 domains and sp3 matrix, which was named sp2/sp3 state. Meanwhile, we also noticed the rapid carrier relaxation dynamics in GO treated by Na OH. We guess that the GO treated by Na OH decreased some functional groups and weakened the interaction between the sp2 domains and sp3 matrix, which accelerated the carries combinationIn the third part, we used the single-walled carbon nanotubes to synthesize quasi-one-dimensional GOR and CX-72 carbon black to prepare quasi-zero-dimensional GQD. From the transient absorption spectra of GO, GOR and GQD, we discovered that these three carbon-based materials have the completely different electronic structure. Two-dimensional GO has not only the hexagonally arrayed sp2-bonded carbon atoms domains, but also contains sp3-hybridized carbon atoms that are covalently bonded with abundant oxygen-containing functional groups. Besides, there are amorphous carbon atoms sp2/sp3 matrix that exist in the boundaries between the sp2 domains and sp3 matrix. Quasi-one-dimensional GOR is very similar to GO, but there are much less sp2/sp3 matrix. More surprisingly, with regard to quasi-zero-dimensional GQD, there are no the sp2/sp3 matrix at all. The zero-dimensional GQD are merely some sp2 hybrid carbon atoms enveloped by bountiful oxygen-containing functional groups. Therefore, we have validated the evolution process of the sp2/sp3 state with the changes of GO size. As the size of carbon-based materials alters from two-dimensional GO, quasi-onedimensional GOR to quasi-zero-dimensional GQD, the sp2/sp3 state is apparent in GO, becomes less in GOR and disappears completely in GQD.In conclusion, we can gain great insight to the optic and electronic properties to study the ultrafast physical process of GO, GOR and GQD. It may also provide theoretical basis to the practical application in electronic device, biomedical science and so on.