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

Abstract

Self-doped colloidal quantum dots (CQDs) attract a strong interest for the design of a new generation of low-cost infrared (IR) optoelectronic devices because of their tunable intraband absorption feature in the mid-IR region. However, very little remains known about their electronic structure which combines confinement and an inverted band structure, complicating the design of optimized devices. We use a combination of IR spectroscopy and photoemission to determine the absolute energy levels of HgSe CQDs with various sizes and surface chemistries. We demonstrate that the filling of the CQD states ranges from 2 electrons per CQD at small sizes (<5 nm) to more than 18 electrons per CQD at large sizes (≈20 nm). HgSe CQDs are also an interesting platform to observe vanishing confinement in colloidal nanoparticles. We present lines of evidence for a semiconductor-to-metal transition at the CQD level, through temperature-dependent absorption and transport measurements. In contrast with bulk systems, the transition is the result of the vanishing confinement rather than the increase of the doping level.

Effect of Pressure on Interband and Intraband Transition of Mercury Chalcogenide Quantum Dots

“…The electronic spectrum of such heterostructure is nevertheless quite difficult to appreciate because of the combination of inverted band structure from the bulk material, presence of quantum confinement, strain due to the lattice mismatch, nature of the heterostructure and possible sensitivity to the surface chemistry. 21,22 In order to uncouple these different effects, each contribution needs to be quantified separately. The effect of the size (ie the quantum confinement) on the band alignment of HgSe 22 and HgTe 4 has already been reported.…”

Investigation of the Self‐Doping Process in HgSe Nanocrystals

“…At high energy, a broad feature is the result of interband transition. At low energy ( λ > 3 μm) a narrow absorption peak is the result of intraband transition, see Figure b and c. Thanks to the large size tunability, the wavelength associated to the mid IR feature can be adjusted from 3 to 26 μm . Similar mid‐IR transition where previously observed in optically and electrochemically charged wide band gap CQD such as CdSe.…”

“…d) Spectrum of HgSe CQD as a function of the surface dipole magnitude. Reproduced with permission . Copyright American Chemical Society, 2016.…”

“…d) Current modulation for thin film of HgSe CQD capped with S 2− ligands as a function of time while light is turned on and off. Reproduced with permission . Copyright American Chemical Society, 2016.…”

Designing Photovoltaic Devices Using HgTe Nanocrystals for Short and Mid‐Wave Infrared Detection

“…Because doping drives the presence of the intraband transition and the dark current magnitude, it must be controlled precisely to be exploited in IR‐sensing materials. The level of doping has been determined using methods such as (spectro) electrochemistry or photoemission and appears to be strongly dependent on the capping molecules …”