Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons.
Carbon dots (CDs) have attracted rapidly growing interest in recent years due to their unique and tunable optical properties, the low cost of fabrication, and their widespread uses. However, due to the complex structure of CDs, both the molecular ingredients and the intrinsic mechanisms governing photoluminescence of CDs are poorly understood. Among other features, a large Stokes shift of over 100 nm and a photoluminescence spectrally dependent on the excitation wavelength have so far not been adequately explained. In this Letter we investigate CDs and develop a model system to mimic their optical properties. This system comprised three types of polycyclic aromatic hydrocarbon (PAH) molecules with fine-tuned concentrations embedded in a polymer matrix. The model suggests that the Stokes shift in CDs is due to the self-trapping of an exciton in the PAH network. The width and the excitation dependence of the emission comes from a selective excitation of PAHs with slightly different energy gaps and from energy transfer between them. These insights will help to tailor the optical properties of CDs and help their implementation into applications, e.g., light-emitting devices and biomarkers. This could also lead to "artificial" tunable carbon dots by locally modifying the composition and consequently the optical properties of composite PAH films.
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München , Amalienstr. 54, 80799 Munich, Germany.
Ming
M
Fu
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München , Amalienstr. 54, 80799 Munich, Germany.
Florian
F
Ehrat
Department of Physics and Materials Science and Center for Functional Photonics (CFP), City University of Hong Kong , Hong Kong SAR.
Yu
Y
Wang
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München , Amalienstr. 54, 80799 Munich, Germany.
Karolina Z
KZ
Milowska
Department of Physics and Materials Science and Center for Functional Photonics (CFP), City University of Hong Kong , Hong Kong SAR.
Claas
C
Reckmeier
Department of Physics and Materials Science and Center for Functional Photonics (CFP), City University of Hong Kong , Hong Kong SAR.
Andrey L
AL
Rogach
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München , Amalienstr. 54, 80799 Munich, Germany.
Jacek K
JK
Stolarczyk
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München , Amalienstr. 54, 80799 Munich, Germany.
Alexander S
AS
Urban
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München , Amalienstr. 54, 80799 Munich, Germany.
Jochen
J
Feldmann
Carbon dots; energy transfer; excitation-wavelength dependent photoluminescence; optical spectroscopy; polycyclic aromatic hydrocarbons; self-trapped exciton