Electron occupation in the lowest quantized state of the conduction band (1Se) in the colloidal quantum dot leads to the intraband transition in steady-state (1Se-1Pe). The intraband transition, solely originating from the quantum confinement effect, is the unique property of semiconducting nanocrystals. To achieve the electron occupation in 1Se state in the absence of impurity ions, non-thiol ligand passivated HgS colloidal quantum dots are synthesized. The non-thiol ligand passivated HgS quantum dot exhibits strong steady-state intraband transition in ambient condition and enables a versatile ligand replacement to oxide, acid and halide functional ligands, which was not achievable from conventional HgS or HgSe quantum dots. Surprisingly, the atomic ligand passivation to HgS colloidal quantum dot solution efficiently maintains the electron occupation at 1Se of HgS CQDs in ambient condition. The electron occupation in 1Se of HgS CQD solid film is controlled by surface treatment with charged ions, which is confirmed by the mid-IR intraband absorption (1Se-1Pe) intensity imaged by the FTIR microscope. Furthermore, a novel second intraband transition (1Pe-1De) is observed from the HgS CQD solid. The observation of the second intraband transition (1Pe-De) allows us to utilize the higher quantized states that were hidden for the last three decades. The use of the intraband transition with narrow bandwidth in mid-IR would enable to choose an optimal electronic transition occurring in the nanocrystal for a number of applications: wavelength-selective low-energy consuming electronics, space-communication light source, mid-infrared energy sensitized electrode and catalyst, infrared photodetector and infrared filter.
