Higher Quantum State Transitions in Colloidal Quantum Dot with Heavy Electron Doping

Abstract

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, nonthiol ligand passivated HgS colloidal quantum dots are synthesized. The nonthiol 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.

HgTe Nanocrystal Inks for Extended Short‐Wave Infrared Detection

“…Since mercury has a strong affinity with thiols, a short thiol (ethanedithiol or mercapto alcohol) was used as a transfer agent to disperse the nanocrystals in the polar phase solvent dimethyl formamide (DMF) from the initial toluene solvent. The addition of HgCl 2 was found to further enhance electronic passivation and allowed good conductivity . We observed that the nanoparticle spectrum was barely affected by this ligand exchange procedure, see Figure b.…”

“…The addition of HgCl 2 was found to further enhance electronic passivation and allowed good conductivity. [25,[36][37][38] We observed that the nanoparticle spectrum was barely affected by this ligand exchange procedure, see Figure 1b. This contrasts with the solid state ligand exchange (SSLE) based on ethanedithiol (EDT) which led to a redshift and broadening of the band edge transition.…”

HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement

“…In the mid IR range, mercury chalcogenides (HgX) have become the most investigated materials because of their strong absorption and photoconductivity [8][9][10] . While HgTe CQDs appear nearly intrinsic, HgS 11,12 and HgSe 1,13 are self-doped nanocrystals, presenting excess electrons after their synthesis. As a result, their absorption spectrum is characterized by an additional feature in the mid-IR, resulting from transitions within the conduction band.…”

Investigation of the Self‐Doping Process in HgSe Nanocrystals

“…For larger CQD, the narrower band gap nature makes that the reduction by the environment is even more important . It has been shown in the case of HgS that higher order states get filled and that little by little the transition switches from its intraband behavior to a plasmonic behavior …”

“…While HgTe behaves as an intrinsic material for which the mid‐IR transition is an interband transition, HgS and HgSe present self‐doping properties. As a result, they come under a stable n‐doped form with intraband transition within the conduction band.…”