| dc.description.abstract | The quantum dots (QDs) are known as artificial atoms due to their size of the order of 2-10 nm. After their discovery in 1980, QDs have attracted significant attention due to their enhanced optical, electronic, and chemical properties which makes them a potential candidate in optoelectronic applications such as light-emitting diodes (LEDs), photodetectors, sensors, and energy storage devices. In recent years, researchers have explored QDs for integrated quantum photonics applications due to their controllable optical properties. The tunability of optical properties by engineering the size of QDs where confinement of charge carriers in all three dimensions makes them important for optical applications in comparison to other nanostructures where the confinement is in one or two dimensions. Molybdenum disulfide (MoS2) has become a research interest from the last decade due to its large carrier mobilities, high current carrying capacity, and edge effects. With the reduction in size, the bandgap transition from indirect (bulk MoS2) to direct (monolayer MoS2) has opened its way for optoelectronic applications. The optical properties of MoS2 monolayers are reported with very low photoluminescence quantum yield (PLQY). As three-dimensional confinement of charge carriers (electron/holes) leads to enhancement in optical properties (photoluminescence), MoS2 QDs are expected to have enhanced PLQY value. A few reports are available on MoS2 QDs where QDs are synthesized using different fabrication methods. The colloidal method is a vastly studied method for QDs synthesis such as CdSe, PbS, etc due to its short reaction time and good controllability over size distribution. There is one report available on MoS2 QDs using the colloidal method with PLQY of 4%. The objective of this thesis is to enhance the optical properties of colloidal MoS2 QDs and their application in temperature sensors. The thesis mainly focuses on colloidal synthesis and characterization of MoS2 QDs, nanosheets, and nanorods.
Chapter 1 deals with the introduction to quantum dots. In this chapter, we have discussed different nanostructures 0D, 1D, 2D and how size affects the optical and electronic properties. In this chapter, we have discussed properties change in MoS2 QDs from bulk and monolayer counterparts. Here, we have discussed different
methods of synthesis of MoS2 QDs.
Chapter 2 presents the role of reaction temperature on the morphology and PLQY of MoS2 nanostructures. Interesting observations were made in the form of nanorods and nanosheet formation apart from QDs. This study could be used as a benchmark for controlling the morphology of MoS2 nanostructures. The morphology transition of colloidal MoS2 nanostructures with a change in reaction temperature from QDs to nanorods resulted in a maximum PLQY of 4.4% for MoS2 QDs with blue emission. With the reported value being highest with the colloidal method, further studies were carried out to increase the quantum yield beyond 4.4%.
Chapter 3 presents the role of dielectric media on the optical characteristics of colloidal MoS2 QDs in which we investigated the effect of three organic solvents that are used to disperse MoS2 QDs. As a result, we have achieved a PLQY of 4.4 %, which would require further enhancement for LED application.
Chapter 4 discusses the optimization of surface group concentration to enhance the fluorescence properties of MoS2 QDs, and we have successfully enhanced quantum yield to 13% with blue emission, which is the highest value achieved to date for colloidal MoS2 QDs.
Chapter 5 presents the work on practical applications of the synthesized MoS2 QDs. MoS2 QDs-based resistive and optical temperature sensors were demonstrated. We have achieved the sensitivity of 0.81%/◦C in the range of 10-50 ◦C and 0.6%/◦C in 25-100 ◦C, which is higher than other reported QDs. This is a first time demonstration of MoS2 QDs based temperature sensor.
The appendix presents the other possible applications using colloidal MoS2 QDs as an active material. In particular, we have studied the nonlinear second and third harmonic generation in MoS2 QDs and nanorods. MoS2 QDs, nanosheets and nanorods show strong second and third harmonic signals and can have applications in biomedical applications. | en_US |
