A misunderstanding about upconversion luminescence of carbon quantum dots

During the recent few years, a number of articles based on detecting the upconversion luminescence in graphene quantum dots (GQDs) or carbon quantum dots (CQDs) using spectrofluorimetry with non-coherent monochromatic light source have been published in trusted journals. Accordingly, some related applications of this phenomenon such as dye degradation with the advantage of using a higher wavelength range of light have been reported. Herein, we have selected four synthetic methods for preparation of CQDs to examine their upconversion properties in the presence and absence of an excitation filter. Independent of different synthetic strategies used, the results clearly indicated that the so-called upconversion phenomenon is, in fact, only a normal photoluminescence excited by the second-order diffraction light (lambda/2) passing from the excitation monochromator. In addition, CQDs were used to prepare TiO2/CQDs nanocomposites in order to check their potential applications in degradation of methyl orange using the visible range of light. From the results, no difference between the efficiency of pure TiO2 and TiO2/CQDs nanocomposites using this range of wavelengths were observed.