Sparked by the 2D graphene, advanced 2D transition metal dichalcogenides have captured enough attention due to their extraordinary properties and are promising enough for future high speed flexible electronic and optoelectronic devices. Among all the transition metal dichalcogenides, molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are explored most extensively since the last few years because of their complementary nature to metallic graphene. These thin 2D materials are semiconducting in nature, and moreover, bandgap also changes from indirect to direct as these materials are thinned down from bulk to monolayer form. In this study, a stabilized and large area growth of MoS2 monolayers has been established on oxide and semiconducting substrates such as (0001) sapphire, (100) p-type SiO2/Si, GaN and Ga2O3 using low pressure chemical vapor deposition. The quality and crystalline nature of grown MoS2 is deeply investigated optically by micro-photoluminescence and micro-Raman spectroscopy. Topography and morphology are characterized by scanning electron and atomic force microscopy. The applications of as grown MoS2 monolayers have been studied by the fabrication of large area photodetector. Also, the gas sensing ability of MoS2 has been explored by using CO2 gas, and the minimum detection limit found is 200ppm. In-addition one step growth of ternary alloys Mo1-xWxS2 has been achieved by LPCVD. Different compositions of W in MoS2 have been investigated by micro-photoluminescence and micro-Raman spectroscopy. In-plane heterojunctions of atomic-thick (2D) semiconductors (MoS2/WS2) are novel structures that can potentially pave the way for efficient ultrathin and flexible optoelectronics, such as light sources and photovoltaics. Such heterostructures are very rare and not much is known about their characteristics. They can only be achieved through a synthetic growth process such as chemical vapor deposition (CVD). This is unlike vertical heterostructures, for which the materials can be mechanically stacked one layer on top of the other. Here, we report a one-step CVD growth of monolayer thick MoS2/WS2 in-plane heterostructures. We have characterized their morphological and optical properties using micro-Raman and photoluminescence spectroscopy. Kelvin probe force microscope was used to extract the contact potential difference profile across the MoS2/WS2 heterojunction boundary. The junction region of these heterostructures are observed to be a ternary alloy Mo1-xWxS2. Moreover, through the tip enhanced Raman spectroscopy (TERS), the minimum junction width is extracted out to be pixel limited 25nm. Also, some novel Raman modes are detected through TERS in MoS2, and WS2 monolayers, which were not elaborated before.