Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals

Colloidal nanocrystals are an interesting platform for the design of low cost optoelectronic devices especially in the infrared range of wavelengths. Mercury chalcogenides have reached high maturity to address wavelengths above the telecom range (1.5 μm). However, no screening of the surface chemistry influence has been conducted yet. In this paper, we systematically probe the influence of a series of ligands, Cl^–, SCN^–, 1,2-ethanedithiol, 1,4-benzenedithiol, 1-octanethiol, 1-butanethiol, As₂S₃, and S^2–, on the photoconductive properties of HgTe nanocrystal thin films. A high bandwidth, large dynamic transient photocurrent setup is used to determine the photocarrier dynamics. Two regimes are clearly identified. At the early stage (few nanoseconds) a fast decay of the photocurrent is resulting from recombination and trapping. Then transport enters in a multiple trapping regime where carriers present a continuously decreasing effective value of their mobility. The power law dependence of the conductance can be used to estimate the trap carrier density and determine the value of the Urbach energy (35–50 meV). We demonstrate that a proper choice of ligand is necessary for a trade-off between the material performance (μτ product) and the quality of the surface passivation (to keep a low Urbach energy)

Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals

Mercury chalcogenide nanocrystals and especially HgTe appear as an interesting platform for the design of low cost mid-infrared (mid-IR) detectors. Nevertheless, their electronic structure and transport properties remain poorly understood, and some critical aspects such as the carrier relaxation dynamics at the band edge have been pushed under the rug. Some of the previous reports on dynamics are setup-limited, and all of them have been obtained using photon energy far above the band edge. These observations raise two main questions: (i) what are the carrier dynamics at the band edge and (ii) should we expect some additional effect (multiexciton generation (MEG)) as such narrow band gap materials are excited far above the band edge? To answer these questions, we developed a high-bandwidth setup that allows us to understand and compare the carrier dynamics resonantly pumped at the band edge in the mid-IR and far above the band edge. We demonstrate that fast (>50 MHz) photoresponse can be obtained even in the mid-IR and that MEG is occurring in HgTe nanocrystal arrays with a threshold around 3 times the band edge energy. Furthermore, the photoresponse can be effectively tuned in magnitude and sign using a phototransistor configuration

Coupled HgSe Colloidal Quantum Wells through a Tunable Barrier: A Strategy To Uncouple Optical and Transport Band Gap

Among semiconductor nanocrystals (NCs), 2D nanoplatelets (NPLs) are a special class of nanomaterials with well controlled optical features. So far, most of the efforts have been focused on wide band gap materials such as cadmium chalcogenide semiconductors. However, optical absorption can be pushed toward the infrared (IR) range using narrow band gap materials such as mercury chalcogenides. Here we demonstrate the feasibility of a core/shell structure made of a CdSe core with two HgSe external wells. We demonstrate that the optical spectrum of the heterostructure is set by the HgSe wells and this, despite the quasi type II band alignment, makes the band edge energy independent of the inner core thickness. On the other hand, these core/shell NPLs behave, from a transport point of view, as a wide band gap material. We demonstrate that the introduction of a wide band gap CdSe core makes the material less conductive and with a larger photoresponse. Hence, the heterostructure presents an effective electric band gap wider than the optical band gap. This strategy will be of utmost interest to design infrared effective colloidal materials for which the reduction of the carrier density and the associated dark current is a critical property

Wave-Function Engineering in HgSe/HgTe Colloidal Heterostructures To Enhance Mid-infrared Photoconductive Properties

The use of intraband transition is an interesting alternative path for the design of optically active complex colloidal materials in the mid-infrared range. However, so far, the performance obtained for photodetection based on intraband transition remains much smaller than the one relying on interband transition in narrow-band-gap materials operating at the same wavelength. New strategies have to be developed to make intraband materials more effective. Here, we propose growing a heterostructure of HgSe/HgTe as a means of achieving enhanced intraband-based photoconduction. We first tackle the synthetic challenge of growing a heterostructure on soft (Hg-based) material. The electronic spectrum of the grown heterostructure is then investigated using a combination of numerical simulation, infrared spectroscopy, transport measurement, and photoemission. We report a type-II band alignment with reduced doping compared with a core-only object and boosted hole conduction. Finally, we probe the photoconductive properties of the heterostructure while resonantly exciting the intraband transition by using a high-power-density quantum cascade laser. Compared to the previous generation of material based on core-only HgSe, the heterostructures have a lower dark current, stronger temperature dependence, faster photoresponse (with a time response below 50 μs), and detectivity increased by a factor of 30

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