Intraband Mid-Infrared Transitions in Ag₂Se Nanocrystals: Potential and Limitations for Hg-Free Low-Cost Photodetection

Infrared photodetection based on colloidal nanoparticles is a promising path toward low-cost devices. However, mid-infrared absorption relies on interband transitions in heavy metal-based materials, which is a major flaw for the development toward mass market. In the quest of mercury-free infrared active colloidal materials, we here investigate Ag₂Se nanoparticles presenting intraband transition between 3 and 15 μm. With photoemission and infrared spectroscopy, we are able to propose an electronic spectrum of the material in the absolute energy scale. We also investigate the origin of doping and demonstrate that it results from a cation excess under the Ag⁺ form. We demonstrate photoconduction into this material under resonant excitation of the intraband transition. However, performances are currently quite weak with (i) a slow photoresponse (several seconds) and (ii) some electrochemical instabilities at room temperature

Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances

Colloidal quantum dots (CQDs) are candidates of interest for the design of low cost IR detector, especially in the short wave infrared (SWIR; 0.8–3 μm), where the vicinity of the visible range makes the high cost of available technologies even more striking. HgTe nanocrystals are among the most promising candidates to address SWIR since their spectrum can be tuned all over this range while demonstrating photoconductive properties. However, several main issues have been swept under the rug, which prevents further development of active materials and devices. Here we address two central questions, which are (i) the stability of the device under ambient air condition and (ii) the reduction of dark current. Encapsulation of HgTe CQDs is difficult because of their extreme sensitivity to annealing, we nevertheless demonstrate an efficient encapsulation method based on a combination of O₂ and H₂O repellant layers leading to stability over >100 days. Finally, we demonstrate that the dark current reduction can be obtained by switching from a photoconductive geometry to a photovoltaic (PV) device, which is fabricated using solution and low temperature based approach. We demonstrate fast photoresponse (>10 kHz) and detectivity enhancement by 1 order of magnitude in the PV configuration at room temperature. These results pave the way for narrow bandgap CQD based cost-effective optoelectronic devices in developing next generation SWIR photonic systems

Material Perspective on HgTe Nanocrystal-Based Short-Wave Infrared Focal Plane Arrays

After the use of nanocrystals as light downconverters, infrared sensing appears to be one of the first market applications where they can be used while being both electrically and optically active. Over recent years, tremendous progress has been achieved, leading to an apparent rise in the technological-readiness level (TRL). So far, the efforts have been focused on PbS nanocrystals for operation in the near-infrared. Here, we focus on HgTe since its narrower band gap offers more flexibility to explore the extended short-wave and midwave infrared. We report a photoconductive strategy for the design of short-wave infrared focal plane arrays with enhanced image quality. An important aspect often swept under the rug at an early stage is the material stability. It appears that HgTe remains mostly unaffected by oxidation under air operation. The evaporation of Hg, a potentially dramatic aging process, only occurs at temperatures far beyond the focal plane array’s standard working temperature. The main bottleneck appears to be the particle sintering resulting from joule heating of focal plane arrays. This suggests that a cooling system is required, whose first role is to prevent the material from sintering even before targeting dark current reduction