Author: Luk, Chi Man
Title: Fabrication and characterization of graphene quantum dots for electronic and optoelectronic applications
Degree: Ph.D.
Year: 2015
Subject: Quantum dots.
Hong Kong Polytechnic University -- Dissertations
Pages: 88 pages : color illustrations
Language: English
Abstract: Graphene quantum dots (GQDs) are nano-scale fragments of one or few-layer graphene,composed of regular hexagonal lattice sp2 carbon atoms edged with heteroatomic functional groups. They attract particular research interest because of their unique properties such as UV-visible absorption, tunable luminescence, biocompatibility, low toxicity, chemical stability and hot carrier generation. In this work,we investigate the synthesis and characterization of GQDs. The GQDs were synthesized by microwave-assisted hydrothermal technique. It is found that the size of the GQDs varied with the reaction time and pressure. The structural and optical properties of the GQDs were characterized by transmission electron microscopy (TEM),Fourier Transform Infrared (FT-IR) spectroscopy, UV-visible absorbance spectroscopy and photoluminescence (PL). The high resolution transmission electron microscopy (HR-TEM) revealed that the GQDs consist of graphitic structure. As identified by electron energy loss spectroscopy (EELS), the GQDs contain various functional groups on the GQD surface, making the GQDs water soluble. Room temperature PL showed excitation wavelength dependent emission. The tunable luminescence was attributed to the emissive traps resulted from the functional groups at the surface of GQD. Because of strong PL emissions of GQDs,we fabricated GQD-agar composite and utilized as a light conversion material in a commercial blue LED. The luminous efficiency of the composite-coated LED is 42.2 lm W-1. The colour rendering index (CRI) and correlated colour temperature (CCT) of the composite coated LED were determined to be 72.0 and 5532 K, corresponding to cool white colour. The light conversion efficiency of the WLED is 61.1 % under 20 mA of forward current.GQDs-polyaniline (PANI-GQD) composite was synthesized by chemical oxidation polymerization. The PL spectra were found to be tuned by varying the mole concentration of PANI and sizes of the GQDs. The Au/PANI-GQD/ITO sandwich device was fabricated to study the electrical transport of the composite film. Electrical hysteresis was observed in response to the applied voltage. Both the tunable PL and electrical hysteresis behavior were attributed to the surface states of the GQDs. The two-photon luminescence of nitrogen doped GQDs (N-GQDs) has been demonstrated. The strongest PL was located at 517 nm when the excitation wavelength was 800 nm. The power-dependent emission of the N-GQDs also showed the quadratic relationship between the emission intensity and laser power. The intensity of two-photon emission of the N-GQDs was found to be altered with various N/C atomic ratios. It showed that N doping play an important role in two-photon luminescence. The emissive properties of both undoped and N doped GQDs were characterized at single particle level. The relative particle population of red and NIR emitting GQDs were compared. It was shown that N-GQDs exhibited larger proportion of particles with near infrared (NIR) emission compared with undoped GQDs.Both undoped and N-GQDs showed size dependent photophysical properties. Doping of GQDs leaded to significant changes in the NIR emissive properties.
Access: open access

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