In this paper, we discuss recent progress obtained on infrared nanocrystal based on mercury chalcogenides (HgTe and HgSe). These materials can become some key building blocks for the next generation of infrared optoelectronic devices. To reach this goal, the infrared nanocrystals need to combine fine control on the optical features and efficient electronic transport. Here, we report about (i) the development of HgTe NPL for enhanced optical features (narrower and faster PL) in the near IR and (ii) about the development of self-doped nanocrystals of HgSe to demonstrate tunable intraband absorption up to the THz range.
In this article we discuss the infrared properties of self-doped nanocrystals and in particular the case of HgSe. HgSe colloidal quantum dots have recently been reported for their tunable optical features all over the mid infrared from 3 to 20 μm. Their optical absorption is a combination of interband absorption at high energy and intraband absorption at low energy. The latter results from the self-doped character of HgSe. The origin of this self-doping is also discussed. We demonstrated that the doping results from the combination of the narrow band gap and high work function of HgSe, which leads to a reduction of the CQD by the water in the environment. In addition, we demonstrated that the doping density can be tuned over an order of magnitude thanks to the control of the capping ligands.
Quantum wells (QWs) are thin semiconductor layers than confine electrons and holes in one dimension. They are widely used for optoelectronic devices, particularly semiconductor lasers, but have so far been produced using expensive epitaxial crystal-growth techniques. This has motivated research into the use of colloidal semiconductor nanocrystals, which can be synthesized chemically at low cost, and can be processed in the solution phase. However, initial demonstrations of optical gain from colloidal nanocrystals involved high thresholds.
Recently, colloidal synthesis methods have been developed for the production of thin, atomically flat semiconductor nanocrystals, known as nanoplatelets (NPLs). We investigated relaxation of high-energy carriers in colloidal CdSe NPLs, and found that the relaxation is characteristic of a QW system. Carrier cooling and relaxation on time scales from picoseconds to hundreds of picoseconds are dominated by Auger-type exciton-exciton interactions. The picosecond-scale cooling of hot carriers is much faster than the exciton recombination rate, as required for use of these NPLs as optical gain and lasing materials.
We therefore investigated amplified spontaneous emission using close-packed films of NPLs. We observed thresholds that were more than 4 times lower than the best reported value for colloidal nanocrystals. Moreover, gain in these films is 4 times higher than gain reported for other colloidal nanocrystals, and saturates at pump fluences more than two orders of magnitude above the ASE threshold. We attribute this exceptional performance to large optical cross-sections, relatively slow Auger recombination rates, and narrow ensemble emission linewidths.
Quantum dots are nanometre-sized semiconductor particles exhibiting unique size-dependent electronic
properties. In order to passivate the nanocrystals surface and to protect them from oxidation, we grow a shell
composed of a second semiconductor with a larger bandgap on the core (for example a core / shell CdS / ZnS).
However, the lattice mismatch between the two materials (typically 7% between ZnS and CdS) induces
mechanical stress which can lead to dislocations. To better understand these mechanisms, it is important to be
able to measure the pressure induced on the semiconductor core. We used a nanocrystal doped with manganese
ions Mn<sup>2+</sup>, which provide a phosphorescence signal depending on the local pressure. A few dopant atoms per
nanoparticle were placed at controlled radial positions in a ZnS shell formed layer by layer. The experimental
pressure measurements are in very good agreement with a simple spherically symmetric elastic continuum
model. Using manganese as a pressure gauge could be used to better understand some structural phenomena
observed in these nanocrystals, such as crystalline phases transition, or shell cracking.