Ultrafast energy relaxation and transfer dynamics in fluorescent carbon nanodots

Andrea Cannizzo^a, Thomas Feurer^a, Egmont Rohwer^a, Ariana Rondi^a, Maryam Akbarimoosavi^a, Maryam Nazari^a, Michela Gazzetto^a, Luisa Sciortino^b, Fabrizio Messina^b

^aUniversity of Bern, Institute of Applied Physics, Sidlerstrasse, 5, Bern, Switzerland

^bDepartment of Physics and Chemistry, University of Palermo, Via Archirafi 36, Palermo, 90123, Italy

Materials for Sustainable Development Conference (MATSUS)
Proceedings of September Meeting 2016 (NFM16)

Berlin, Germany, 2016 September 5th - 13th

Organizers: Marin Alexe, Enrique Cánovas, Celso de Mello Donega, Ivan Infante, Thomas Kirchartz, Maksym Kovalenko, Federico Rosei, Lukas Schmidt-Mende, Laurens Siebbeles, Peter Strasser, Teodor K Todorov, Roel van de Krol and Ulrike Woggon

Oral, Michela Gazzetto, presentation 422
Publication date: 14th June 2016

Carbon nanodots (CDs) are a novel family of optically-active carbon-based nanomaterials discovered only a few years ago. They are 1-10 nm nanoparticles composed by carbon, oxygen and hydrogen and endowed with a rather appealing combination of properties, such as “tunable” and strong fluorescence in the visible, excellent solubility in aqueous environments, photo- and chemical stability, sensitivity to perturbations like the presence of metal cations, and the capability of behaving as efficient photo-activated acceptors or donors of electrons and protons. Because of this unique combination of properties, CDs have the potential to complement or even replace semiconductor nanoparticles in several applications and domains, in particular where bio-toxicity is an issue. N-doped CDs are particularly interesting because of their enhanced fluorescence, and their fundamental properties are the subject of a strong debate in the literature. Understanding the optical properties of CDs at the fundamental level is crucial to move towards applications such as optoelectronics, bio-imaging, physicochemical sensors and markers.

We carried out an extensive study to characterize highly N-doped CDs, developing a versatile DUV-to-Vis femtosecond time-resolved transient absorption (TA) spectrometer with time resolution of 40 fs (standard deviation). Our study included varying pump wavelength and polarization, to gain a deeper understanding of the photophysical properties of CDs and of the dynamics of the higher excited states. Our results provide unanticipated insight on the photocycle of carbon nanodots in aqueous solution, unraveling the relaxation steps of photo-excited CDs and their characteristic timescales.In particular, we observed rotational diffusion of the transition dipole moments on timescales (10s ps) much faster than the rotational diffusion of the entire nanoparticles (10s ns). This finding reveals for the first time a diffusional energy transfer occurring through surface electronic states of the nanodots, making them appealing for chemical sensing or as light harvester antennas. Modelling and further experimental evidences are underway to rationalize such an unexpected behaviour.

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