In recent years, colloidal semiconductor nanocrystals (NCs) with narrow band gaps have emerged that offer advantages such as highly tunable optical and electronic properties, low-temperature solution processing to make thin film devices, and compatibility with silicon integrated circuits and flexible substrates. In this thesis, I will review some results of the synthesis of colloidal quantum dot (CQDs) materials, arrange them in decreasing order of their band gap, i.e., beginning from cadmium telluride (CdTe) QDs emitting in the visible (Vis) region through near-infrared (NIR) emitters such as cadmium mercury telluride (CdxHg1-xTe) alloyed QDs, to narrow energy gap mercury telluride (HgTe) QDs. This will be followed by an overview of their optoelectronic application perspectives. Starting with CdTe QDs, we investigate the ion-exchange process using the continuous micro-flow reactor to prepare bright and compact CdxHg1-xTe alloy QDs with equalized particle size and tunable NIR fluorescence emission in a short reaction time circle in the wavelength beyond 0.9 μm. Afterward, focusing on HgTe QDs, I will describe the synthesis of an aqueous-grown HgTe QDs in the wavelength range 0.8 to 1.3 μm at room temperature by introducing a fully automatic controlled growth set-up. The photoluminescence quantum yield (PL QY) reached up to 45%, however the aqueous synthesis appeared limited to smaller sizes. Based on the similar procedures, a new aprotic method has been developed to synthesize HgTe QDs that allows scalable fabrication of larger sized QDs with high quality, and exhibiting a high PL QY up to 16 % in the wavelength range 2 to 3 μm. To the best of my knowledge, this PL QY value is the highest among the reported values for HgTe QDs that emit at the same wavelength. This new method allows a good insight into reaction kinetics at a slow reaction rate suitable for characterization alongside the synthesis and giving some chance of being able to use feedback for controlling the synthesis. In addition, some phase transfer methods have also been investigated, bring further flexibility by allowing transfer of HgTe QDs from dimethylsulfoxide (DMSO)/ furanmethanethiol (FMT) solutions into either organic solvent/dodecanethiol (DDT) solution or into aqueous solution with 1-thioglycerol (TG) as the ligand. Finally, a brief overview of the infrared (IR) photodetector device technologies that motivated the work on narrow bandgap QDs is given. The HgTe QDs are incorporated as solid films in bottom contact/bottom gate transistor structures on an oxide coated heavily doped silicon substrate using a simple multi-pass spray-coating process and show excellent operational stability in ambient conditions. With these devices, our collaborators at Chinese University of Hong Kong demonstrated the first room-temperature operated QD photodetector with > 1010 Jones detectivity in the mid infrared MIR (>2 µm) range, which is close to the typical detectivity of commercial liquid nitrogen or Peltier cooled MIR detectors demonstrating a potential low-cost alternative to today’s IR imaging technologies.