After more than 30 years of research on colloidal quantum dots (CQDs), interband transitions have been extensively studied, while intraband transitions— excitations between energy levels within the same band — are underexplored and have not found any application yet. However, intraband transitions have a huge advantage in the application in infrared region (low energy): because the intraband transitions are not limited to narrow bandgap materials, more semiconductor and semimetal materials become potential candidates for infrared application. In this thesis, I will report our progress in i) the synthesis of HgS, HgSe, and HgSe/CdS core/shell CQDs; ii) their optical and electronic properties and their sensitivity to the CQD surface; and iii) intraband transitions and infrared photodetection with them. For the first time, we have successfully achieved intraband infrared photodetection in the mid-infrared region (between 3-5 microns), using intraband transitions of air-stable n-doped HgS and HgSe CQDs. Precise control of the doping level is essential to minimize the noise current and to increase the device performance. We also found that the doping level can be influenced by changing the surface condition of the CQDs and the surface gating could be used as a new method to tune the doping level of CQDs. To reduce nonradiative relaxation and improve thermal stability, we also made HgSe/CdS CQDs and studied the shell’s effect on the property of the HgSe QDs. It is demonstrated that the shell could enhance the intraband PL and protect the core QDs during annealing process at 200 ℃.