resumo
Carbon dots (CDs) have become one of the most investigated nanomaterials in the last few years, since they offer excellent prospects to be used in a wide range of technological applications. Nonetheless, the understanding of the photoluminescence (PL) processes remains unclear and solely based on spectroscopic measurements at room temperature, which hampered a full discussion on the nature of recombination mechanisms. Insights into the CDs' PL and corresponding recombination models can be achieved from temperature, excitation energy and excitation density dependency studies. Thus, in this paper, CDs with a mean diameter of similar to 3.0 nm were embedded in poly(methyl methacrylate) (PMMA) and studied by such optical techniques. The recombination of CDs/PMMA composites was found to be due to an overlap of emitting centres, both in the blue and green regions, with intensity and peak position strongly dependent on the excitation density. Moreover, this emission was found to be preferentially populated by two overlapped excitation bands. Temperature-dependent PL measurements corroborate the hypothesis of overlapped emitting optical centres (e.g. small molecules at the CDs' surface), as no shift on peak position and no changes in spectral shape were observed, which allow to exclude the hypothesis of a conventional free excitonic transition and quantum confinement effects, as observed in typical nanosized semiconductors with dimensions lower than the exciton Bohr radius.
palavras-chave
FACILE PREPARATION; FLUORESCENCE; MECHANISM; EMISSION; NANOPARTICLES; COMPOSITES; NANODOTS; ORIGIN; ACCESS; CELLS
categoria
Materials Science
autores
Rodrigues, J; Pereira, SO; Teixeira, SS; Zhou, YQ; Peng, ZL; Liyanage, PY; Leblanc, RM; Barros-Timmons, AMMV; Costa, LC; Costa, FM; Monteiro, T
nossos autores
Ana Margarida Madeira Viegas de Barros Timmons
Professor associado
Sónia Oliveira Pereira
Bolseiro de investigaçãoProjectos
CICECO - Aveiro Institute of Materials (UID/CTM/50011/2013)
RMNE-UA-National Network of Electron Microscopy (REDE/1509/RME/2005 )
agradecimentos
The authors acknowledge the financial support from FEDER funds through the COMPETE 2020 Programme and National Funds through Portuguese Science and Technology foundation (FCT) under the project UID/CTM/50025/2013 and POCI-01-0145-FEDER-028755. This work was also developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. Additionally, J. Rodrigues and S. O. Pereira thank I3N for the BPD grants (BPD/UI96/4821/2018 and BPD/UI96/5808/2017) and S. Soreto Teixeira thanks FCT for the grant SFRH/BD/05211/2014. The authors also thank to the RNME - Pole University of Aveiro, FCT Project REDE/1509/RME/2005.