Estudo da fotoluminescência de pontos quânticos de carbono aminofuncionalizados submetidos à tratamento térmico em estado sólido

Abstract

Carbon quantum dots (CQDs) are zero-dimensional luminescent carbon based nanoparticles, with a sp2 carbon core and a surface composed by functional groups and heteroatoms, known as defects. The luminescence of CQDs are the most explored photoluminescent properties by the researchers, because it is affected by many parameters, also the temperature. In this work, nitrogen carbon quantum dots (N-CDs), synthesized via hydrothermal from chitosan, and their structural and photoluminescents properties were studied in detail face with the post-synthesis heat treatment (TTS) in the solid state. Moreover, a better understanding of basic physics was sought behind the fluorescence mechanisms of these nanoparticles before and after TTS in the solid state, that is still considered a field open to discussions. The synthesized PQCs-N showed a quantum yield of 4.36 % and were subjected to heating cycles ranging from 30 circ C to 180 circ C while their fluorescence was analyzed. The physicochemical and morphological properties of nanoparticles were also studied using the techniques of UV-Vis absorption, infrared spectroscopy with Fourier transform (FTIR) and atomic force microscopy (AFM). From the obtained results, it was seen that the heating caused irreversible changes in the fluorescence of the N-CDs. When performing the first TTS cycle on solid PQCs-N, a redshift of approximately 20 nm was observed in the emission fluorescence (from 633 to 655 nm) with an excitation of 532 nm. On the other hand, for the second heating cycle, no shift was observed and the maximum emission remained at 655 nm. These changes reflected in the absorption spectrum of the N-CDs after the TTS that started to present a new absorption component at 350 nm, suggesting the presence of new surface states related to nitrogen groups. This behavior of deviation of emission due to heat treatment was also observed in N-CDs in suspension after TTS in muffle. Before the TTS they exhibited an emission dependent on the excitation wavelength with maximum emission fluorescence at 436 nm when excited at 340 nm and started to have a dependent emission with a maximum at 456 nm when excited at 380 nm. In addition, AFM images indicated that the size of the PQCs-N did not change with the TTS, pointing out that the emission mechanism is not related to the quantum confinement effect. Thus, the fluorescence mechanism of PQCs-Nestudates occurs mainly due to surface groups. Thermal treatment in these nanoparticles proved to be a pathway capable of modifying their surface and causing changes in CDs-N fluorescence.