Towards Improving the Optical Properties of MoS₂ Quantum Dots

Konar, Arindam (2019) Towards Improving the Optical Properties of MoS₂ Quantum Dots. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

There are mainly three parts to this research work. In the first chapter, the time and work had been dedicated to literature survey. Second chapter involved experimental part with the optimization of synthesis conditions such as molar ratio, temperature and time related to Molybdenum Disulfide (MoS₂) quantum dots (QDs). This chapter involves the details of experimental and instrumentation part. The third chapter includes FESEM and TEM imaging and Energy dispersive X-ray Spectroscopy has also been carried out, which suggests that MoS₂ QD is spherical in size with an average size of 15nm. This part also includes photophysical studies including absorption and emission behaviour of MoS₂ QDs and lifetime measurements. Excitation wavelength dependent emission spectrum and photoluminescence quantum yield (PLQY) had been observed with a maximum value of 24% (this is the highest so far reported in the literature). Excited state photoluminescence (PL) decays were noted to be tri-exponential in nature, with the lifetimes 0.17 – 0.65 ns, 1.6 – 3.0 ns and 6 - 9 ns. The nature of PL decay curves depend on excitation / monitoring wavelength. In order to improve the optical behaviour of MoS₂ QD, we have carried out a few modifications. As part of the modification, the synthesis has been carried out in presence of triflic acid (organic super acid). This procedure yielded MoS₂ QD which exhibit hitherto unobserved excitation wavelength independent emission maxima. PLQY has been observed to be dependent on excitation wavelength. A maximum value of 34% PLQY has been noted. This value is so far the highest in literature. Photoluminescence decays of MoS₂ QDs were found to be bi-exponential in nature with the lifetimes of 1.6 - 3.0 ns and 11.2 – 12.2 ns.

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