Fabrication of carbon nano-dots and their optical properties

Abstract

In this thesis, the realization of carbon nanodots (CDs) by various fabrication methods is described and the optical properties of the CDs are investigated. Firstly, a simple one-step laser irradiation method is proposed to fabricate laser-radiated CDs (L-CDs) which have size varied from 1.5 to 3.5 nm. By uniformly dispersing the L-CDs in N-Methylpyrrolidone (NMP), room-temperature white-light amplified spontaneous emission can be observed from the mixture under optical excitation at 266 nm. Peak wavelength and linewidth of the emission spectrum are found to be around 450 and 120 nm respectively. It is verified that the presence of NMP in the mixture enhances emission efficiency of L-CDs over the broad emission spectrum. This is because the presence of NMP absorbs the unused excitation light and transfers the power back to L-CDs for the excitation of the second time. Optical gain of the mixture at peak wavelength is found to be as high as ~0.92 cm⁻¹MW⁻¹, which is about 37% higher than that without the presence of NMP. Furthermore, lasing emission at visible wavelength is achieved from high-Q cylindrical microcavity, which can be realized by coating a layer of PEG functionalized CDs (P-CDs) in PEG200 onto the surface of an optical fiber, under 266 nm pulsed optical excitation. The enhancement of PL intensity can be obtained from the as-prepared P-CDs in PEG200 via esterification of the carboxylic groups on the surface of P-CDs. Whispering gallery modes are attributed to the support of lasing emission from the high-Q cylindrical microcavity. On the other hand, luminescent capability of few-layer graphene quantum dots (FL-GQDs) is investigated and compared with that of the CDs obtained from the same functionalization process. It is found that the optical gain of FL-GQDs is higher than that of CDs realized by the same surface functionalization process. This is due to the geometrical advantages of FL-GQDs such as larger surface area to volume ratio and smaller volume. Furthermore, under optical excitation at 266 nm, lasing emission is observed from a Fabry Perot cavity containing a mixture of FL-GQDs and TiO₂ nanoparticles disperse in ethanol. The lasing mechanism is attributed to random lasing action arisen from the high optical gain of FL-GQDs and the strong light scattering strength of TiO₂ nanoparticles. Organosilane functionalized carbon nanodots (O-CDs) are also fabricated by pyrolysis organic acid in organosilane at high temperature. Under optical excitation at 450 nm, wide bandwidth lasing is achieved from a Fabry Perot cavity using O-CDs dispersed in epoxy as the gain medium. Lasing threshold is found to be ~200 W/cm2 which is two orders of magnitude lower than that of the other CDs lasers. The enhancement of optical gain at long wavelength excitation regime is attributed to the maximization of organosilane functional groups on the surface of O-CDs. Furthermore, single-mode lasing with tunable bandwidth of ~60 nm is achieved from the proposed Fabry Perot O-CDs laser with external cavity in Littrow configuration. Hence, we have verified that the CDs is a potential optical gain medium to realize high-performance lasing devices.