Electron-phonon coupling-assisted universal red luminescence of o-phenylenediamine-based carbon dots
Carbon dots (CDs) are novel carbon-based photoluminescence (PL) nanomaterials with a core-shell motif. Due to their fascinating advantages such as chemical inertness, high quantum yields (QYs), high water solubility, thermal stability and excellent biocompatibility, CDs have attracted much attention in various research applications such as cancer diagnosis, phototherapy and optoelectronic devices. However, the underlying PL phenomena of CDs remain a mystery due to the polydispersity of the products and the difficulty of establishing their atomic structures.
In a new article published in Light science and applicationA team of scientists led by Professor Siyu Lu and Yuxi Tian of the College of Chemistry, Zhengzhou University, and School of Chemistry and Chemical Engineering, Nanjing University, China, and colleagues has developed an innovative approach to investigate the formation process and fluorescence mechanism of o-phenylenediamine-based red emission CDs.
To this end, they designed six red emission CDs with different preparation methods, and they all showed the same absorption and PL spectra after purification. The characterization of their structures through a series of tests shows that they have similar carbon core structures, while spectral characterization confirmed the different surface states of the CDs. Moreover, transient absorption (TA) spectroscopy combined with single-particle PL spectroscopy technology confirmed that the red emission came from the transition of different vibrational energy levels in the same PL center.
Finally, theoretical calculations in combination with thermogravimetric analysis confirmed the formation process of such CDs. Therefore, this work proposes a systematic way to analyze the emission mechanism of red-emission CDs, which can be used as a guide for the analysis of the structure and mechanism of other types of CDs.
The presence of aromatic regions and some non-conjugated regions in CDs proves the hybrid structure, that is, these conjugated and non-conjugated structures exist both in the carbon core and in a polymer shell. These scientists summarize that there is no clear boundary between the carbon core and the surface shell of CDs, but the “density” of the two parts is different. There are more conjugated structures in the carbon core and more polymer chains in the shell.
Furthermore, these scientists confirmed their claims using multiple characterizations:
- The decay-dependent difference spectroscopy shows the same results from four samples, proving that the photophysical processes are the same for all four CDs. The fit results of the CDs were comparable and the carriers passed through the same relaxation channel.
- The single-particle PL spectra showed that all spectra of individual CDs were very consistent with the ensemble spectrum, indicating that the multimodal emission of CDs arose by itself, rather than the superposition of multiple luminescent species.
- Temperature-dependent Raman spectroscopy directly proves that CDs are a new species, unlike quantum dots and organic molecules, which possess the properties of both materials.
- Calculation of the density functional theory shows that in the hydrothermal process, oPD tends to aggregate and form planar structures. Then the planar structures self-assemble to form spherical CDs.
“This paper clarifies the general PL mechanism and the formation process of oPD-based red-emission CDs. This unified mechanism provides a new method for analyzing the structure-property relationship of CDs, paving the way for analyzing other kinds of CDs, leaving untapped opportunities,” say the scientists.
Many colors from a single dot
Boyang Wang et al, Electron-phonon coupling supported universal red luminescence of o-phenylenediamine-based carbon dots, Light: Science and Applications (2022). DOI: 10.1038/s41377-022-00865-x
Quote: Electron-phonon coupling assisted universal red luminescence of o-phenylenediamine-based carbon dots (2022, June 9,), retrieved June 9, 2022 from https://phys.org/news/2022-06-electron-phonon-coupling-assisted -universal-red-luminescence.html
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