The Photoluminescence Characteristics Of Carbon Dots Under High Pressure

When carbon dots were first prepared,they attracted the attention of scholars for their bright photoluminescence intensity.With the further study of zero-dimensional carbon dots,more characteristics of carbon dots have been found,such as low toxicity,good chemical stability,low preparation cost,and excellent biocompatibility,etc.These excellent properties make carbon dots a promising new type of fluorescent nanomaterials.In recent years,researchers have been devoted to regulating the optical properties of carbon dots.Many precursors and preparation processes have been developed to achieve different color of carbon dots.However,in most studies,the optimum emission peak of carbon dots is located in the blue-green region.Although red emissive carbon dots have been realized in a few studies,their photoluminescence quantum yield is extremely low,and the realization of ideal red emissive carbon dots is still in its infancy,which tremendously limits the wide application of carbon dots in bioimaging,medical diagnosis and light-emitting devices.This is mainly because the structure of carbon dots is complex due to the diversified precursors and preparation processes.Accordingly,it is difficult to elucidate the relationship between the optical properties and structure of carbon dots,resulting in many disputes over the luminescence mechanism of carbon dots.Due to the ambiguous pohtoluminescence mechanism,it is difficult to design a reasonable structure to obtain the red emissive carbon dots with high photoluminescence quantum yield from the perspective of chemical preparation.Therefore,exploring the luminescence mechanism of the red emissive carbon dots plays a key role in realizing the red emissive carbon dots with high photoluminescence quantum yield.High pressure technology has become an important means to explore the optical properties of materials.Diffrerent from the commonly used chemical modification technology,high pressure technology has a unique advantage.It can change the electronic structure and optical properties of materials without changing the chemical composition.In this paper,high pressure technology is used to study the photoluminescence characteristics of red emissive carbon dots,which is helpful to explore the factors affecting the photoluminescence properties and photoluminescence mechanism of red emissive carbon dots.The main research are as follows:(1)Most carbon dots exhibit photoluminescence quenching underpressure and the mechanism of photoluminescence quenching is lacking.Photoluminescence quenching induced by pressure was also observed in red emissive carbon dots.The dependence of photoluminescence intensity on excitation light power and the structural change of carbon dots under pressure proved that the lattice relaxation induced by pressure destroyed the crystal symmetry of carbon core and triggered a sharp photoluminescence quenching.Our results emphasize the importance of well-crystalline carbon core for high photoluminescence intensity of carbon dots,and the study of photoluminescence quenching mechanism under pressure can help guide photoluminescence enhancement under atmospheric or high pressure.(2)Using the high pressure technique.the photoluminescence peak splitting induced by pressure in red emissive carbon dots was first discovered.The photoluminescence peak split into an irreversible blue shift peak and a reversible red shift peak under pressure,indicating that there are two photoluminescence mechanisms in the red carbon dots,from the surface group and the carbon core respectively,the blue shift peak is attributed to the pressure induced surface functional group oxidation and the red shift peak is attributed to pressure induced enhancement of π-π stacking effect of red emissive carbon dots.The dependence of high pressure photoluminescence spectra on concentration and excitation light energy,high pressure absorption spectra and time resolved photoluminescence spectra further support the conclusion of multiple emission centers.Our results provide a new method for the exploration of photoluminescence mechanism,that is,to distinguish different photoluminescence origins by pressure treatment based on the different pressure responsiveness of photoluminescence origin.(3)The relationship between the optical properties and the structure of red emissive carbon dots is not clear.Using the high pressure technique,we found a pressure-driven structure transformation from sp2 into sp3 hybridized domain in red emissive carbon dots.In different pressure transfer media,water and N,N-dimethylformamide,the photoluminescence spectra and absorption spectra of red emissive carbon dots under high pressure,as well as the Raman spectra of nano-sized graphite,have given sufficient evidence for the structural transformation of red emissive carbon dots under pressure.The pressure interval of structural transformation is affected by the pressure transfer medium.When water is the pressure transfer medium,the structure changes within the pressure interval of 5-7 GPa.While N,N-dimethylformamide was used as the pressure transfer medium,the structural transformation occurred at 15-20 GPa.Comparing the results of protic solvents(water)and aprotic solvents(N,N-dimethylformamide),it is shown that the presence of hydroxyl groups can induce the structural transformation to be achieved at lower pressures.Most importantly,the photoluminescence spectra in different pressure transfer media exclude the possibility of blue shift induced by the surface state,further confirming that the transformation of sp2to sp3 hybridized structure leads to blue shift and quenching of photoluminescence of red emissive carbon dots under pressure.Our results not only provide strong evidence that there is pressure driving the transformation of sp2 into sp3 hybridized structure in red emissive carbon dots,but also show the effect of the transformation of sp2 to sp3 hybridized structure on the photoluminescence properties of red emissive carbon dots,which is helpful to understand the relationship between the structure and optical properties of carbon dots.