Two-dimensional molybdenum disulfide (MoS2) as a novel semiconducting material has drawn tremendous interest in the research field. As a van der Waals layered material, the weak interlayer binding enables the preparation of ultra-thin (few- or monolayer) samples. Due to its ultra-thin nature and advanced optical and electronic properties, it is considered as one of the promising candidates for flexible optoelectronic devices such as transistors, photodetectors and light emitting diodes. In the past decade, numerous progresses have been made to improve the performance of MoS2 devices, where developing doping techniques and lowering the contact resistance have been two major approaches under study. In this thesis, a novel method was created to prepare two-dimensional molybdenum nitrides via chemical conversion of MoS2. Molybdenum nitrides are highly conductive and robust. More importantly, the method we developed holds the promise to integrate ultrathin molybdenum nitrides with MoS2 in lateral heterostructures with atomically seamless connection. Such structures are promising to form low resistance contact for MoS2. In addition, a method to achieve the tunable n-type doping of MoS2 via surface adsorption of carbon species was also developed. With a high tunability and stability of dopant concentration, this method is expected to facilitate the fabrication of MoS2-based semiconductor devices.