Research On The Ultrafast Dynamics Of Photoluminescent Carbon Based Nanomaterials

As new members of carbon material family,carbon based nanomaterials have attracted tremendous attentions.One of the most striking features is their strong photoluminescence,which has led to their use in bioimaging,light emitting diodes,laser and so on.The fluorescence of carbon based nanomaterials depend on excited wavelength and pH.Although there are a large number of research on the fluorescence of the carbon based nanomaterials with different technology,the actual mechanism of photoluminescence is still an open debate among researchers.Because of the diversity of materials,the origin of the PL have been assigned to several factors: quantum confinement effect,sp2 nanodomains,surface states,edge states,molecular states,crosslink enhanced emission effect and giant red-edge effect.The thesis in detail explores the change of carriers dynamics and unravels the mechanism of photoluminescence by ultrafast time resolved spectroscopy.Firstly,we built transient absorption specrtum detection system with femtosecond laser.On basis of this system,we developed two new sets of experiment setup:transient reflection system and transient absorption reflection system.But also,we built a set of time reolved optical kerr gate fluorescence spectroscopy detection system,which can detect 450-700 nm full spectrum fluorescence,the minimum instrument response function is 500 fs.Secondly,the data processing method of time resolved siganal is developed.The method of matrix recombination and variable projection least squares method is used to fit the dynamic decay curve.Through the comparative analysis,it is found that these two different algorithms can get the global optimal solution,and the fitted time of the variable projection least squares is shortened by nearly 10 times.For the analysis of transient absorption spectra,we develope several different processing and analysis methods,such as the number of components,determine the initial nonlinear parameters and two-dimensional correlation spectra.We have demonstrated the carrier relaxation of C-dots upon excitation in different pH environments by the femtosecond time-resolved transient absorption spectra.The experimental results indicates that the surface states composed of different oxygen contained functional groups(epoxy,carbonyl and carboxyl)and carbon atoms on the edge of carbon backbone.Through the global fitting analysis,the carriers of C-dots at different pH water dispersions experience the same four relaxation channels: electronelectron scattering and surface state trapping;optical phonon scattering;acoustic phonon scattering and electron-hole recombination.Deprotonation of carboxyl groups at high pH will change the distribution of ? electron cloud density between carbon backbone and surface states and consequently,compared with the excited electrons in the acidic and neutral environments,those in the alkaline environment can be more easily trapped by the surface within 1 ps.The dynamic change of fluorescence spectra and luminescence mechanisms about carbon nanodot under the strong acid,neutral and strong basic environment are investigated by time resolved optical kerr fluorescence spectra.The average lifetime of solvation dynamics are 11.9 ps,7.39 ps and 2.6 ps,when the carbon nanodot lie in strong acid,neutral and strong basic solutions.The peak shift of fluorescence spectra was caused by solvation dynamics.Under the strong acid water solution,the carbon nanodot will experience more solvation time because of the larger transition dipole moment of surface sates formed by carboxyl groups.With ionization of carboxyl groups,the transition dipole moment will decrease,which lead to short solvation time under strong basic conditions.By the global fitting,there are three transient fluorescence species in carbon nanodot.The deprotonation of carboxyl groups accelerate the transition of carriers into nonluminous surface states within the first lifetime.NIR-emissive carbon nanodot with two-photo fluorescence was prepared,the strongest emission of carbon nanodot in dilute ethanol solution was observed centered at 710 nm with a QY as high as ca.26.23%.The transient results demonstrated the carbon core and the surface state synergistically contributed to the PL.By different characterization technology,the carboxyl C and graphic N play a great role in the fluorescence.The two-photon luminescence under pulsed infrared laser excitation was confirmed.Then,we examined the origin of the two-photon fluorescence in the carbon nanodot via first-principles calculation.We calculate the two-photon absorption cross section based on the designed molecular model.The calculated results evidently demonstrated that the carbon nanodot should have a lager two-photon absorption cross section which is leading to two-photon fluorescence.