Understanding the Photoluminescence Mechanism of Carbon Dots
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Understanding the Photoluminescence Mechanism of Carbon Dots Zhoufeng Jiang* §, Marta J. Krysmann†, Antonios Kelarakis†, Petr Koutnik§, Pavel Anzenbacher Jr.§, Paul J. Roland¶, Randy Ellingson¶, Liangfeng Sun* § * Department of Physics and Astronomy, Bowling Green State University, Bowling Green, Ohio 43403, USA § Center of Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, USA † Centre for Materials Science, University of Central Lancashire, Preston PR12HE, U.K. ¶ Department of Physics and Astronomy, Wright Center for Photovoltaic Innovation and Commercialization, School of Solar and Advanced Renewable Energy, University of Toledo, Toledo, Ohio 43606, USA ABSTRACT The carbon dots were investigated to reveal their light-emitting mechanism. The fluorescence spectra of carbon dots show typically two different types of photoluminescence: the excitation-independent component in the short wavelength, and the excitation-dependent component in the longer wavelength. The UV-Vis spectrum of carbon dots shows the absorption maximum of 340 nm which should be accredited to the n-π* transition of the carbonyl group in carbon dots. Absolute quantum yields of carbon dots dispersed in Polyvinyl alcohol is around 15% when the excitation wavelength is less than 425 nm, but decreases continuously when the excitation wavelength increases. The decay lifetimes of the carbon dots also show an abrupt change at excitation wavelength 425 nm. Time resolved photoluminescence was implemented from 31K to 291K to study the photoluminescence decay dynamics of carbon dots, resulting in the continuously decreasing of the lifetime as the temperature increases. Introduction Fluorescent semiconductor quantum dots have generated broad promising applications including biological labeling and solid-state lighting. Nanometer-size carbon dots which are the counterparts of silicon nanoparticles now light up. They are non-toxic, abundant, stable and biocompatible. Although the typical photoluminescence quantum efficiency of carbon dots is not high yet, they are non-blinking1 and have large two-photon absorption cross section2 which are favorable for one-photon or two-photon bio-imaging. There are two schools of fabrication methods for carbon dots: top-down, bottom-up. Topdown method starts from bulk carbon, and using laser2 or electrochemical oxidation3 to break down the carbon into small pieces which have the size in nanometer scale. Graphene fragment carbon dots exhibit size tunable optical properties4. The bottom-up method starts from organic molecules and using chemical reaction, pyrolysis method to fabricate carbon dots, pioneered by Giannelis groups5. Following this direction, we can make mass production of carbon dots. Using these carbon dots, we investigate their optical properties and discovered an abrupt change of the photoluminescence lifetime as the excitation wavelength is scanned from UV to visible region. This abrupt change indicates a change of light emission mechanism of the carbon dots. Experiment
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