Monolayer (ML) transition metal dichalcogenides (TMDs) are a class of materials that have become the focus of intense research in the field of nanomaterials in recent years due to their wide spectrum of optical, electrical and physical properties. Materials in the TMD family include insulators, semiconductors, metals and superconductors. Combined with their atomic thinness, these three-atom-thick materials are ideal candidates for building circuitry in the atomically-thin limit, which is crucial for future electronic device miniaturization. Their thinness also leads to low bending stiffness, which makes monolayer TMDs particularly attractive for flexible electronic applications. The realization of industrial-scale applications in modern electronics based on these atomically-thin films requires overcoming numerous challenges. Here we will discuss two of these, namely, (1) efficient charge carrier injection into the ML TMD channels with atomically-thin contacts and (2) tuning the electrical properties of the ML TMD channel. Our approaches to address these two challenges, which are based on a well-controlled material synthesis technique, are presented in this thesis. Specifically, we create atomically-thin ohmic contacts to semiconducting ML TMDs via the edge of the materials for efficient charge carrier injection, while maintaining the small device volume. We then demonstrate the control of the electrical conductivity of semiconducting ML TMDs through substitutional doping, in which the conductivity is tunable over seven orders of magnitude. In addition, our systematic study further elucidates that the dopant behaviors in two-dimensional (2D) semiconductors are different from their three-dimensional (3D) counterparts. Lastly, we discuss our approach of conformally growing monolayer TMDs over arbitrary 3D surfaces that has the potential to generate ultrasmall devices with the smallest dimensions approaching the atomic size.