Correlating Structure and Detection Properties in HgTe Nanocrystal Films.

HgTe nanocrystals (NCs) enable broadly tunable infrared absorption, now commonly used to design light sensors. This material tends to grow under multipodic shapes and does not present well-defined size distributions. Such point generates traps and reduces the particle packing, leading to a reduced mobility. It is thus highly desirable to comprehensively explore the effect of the shape on their performance. Here, we show, using a combination of electron tomography and tight binding simulations, that the charge dissociation is strong within HgTe NCs, but poorly shape dependent. Then, we design a dual-gate field-effect-transistor made of tripod HgTe NCs and use it to generate a planar p-n junction, offering more tunability than its vertical geometry counterpart. Interestingly, the performance of the tripods is higher than sphere ones, and this can be correlated with a stronger Te excess in the case of sphere shapes which is responsible for a higher hole trap density.

Electroluminescence from HgTe Nanocrystals and Its Use for Active Imaging.

Mercury telluride (HgTe) nanocrystals are among the most versatile infrared (IR) materials with the absorption of lowest energy optical absorption which can be tuned from the visible to the terahertz range. Therefore, they have been extensively considered as near IR emitters and as absorbers for low-cost IR detectors. However, the electroluminescence of HgTe remains poorly investigated despite its ability to go toward longer wavelengths compared to traditional lead sulfide (PbS). Here, we demonstrate a light-emitting diode (LED) based on an indium tin oxide (ITO)/zinc oxide (ZnO)/ZnO-HgTe/PbS/gold-stacked structure, where the emitting layer consists of a ZnO/HgTe bulk heterojunction which drives the charge balance in the system. This LED has low turn-on voltage, long lifetime, and high brightness. Finally, we conduct short wavelength infrared (SWIR) active imaging, where illumination is obtained from a HgTe NC-based LED, and demonstrate moisture detection.

Pushing Absorption of Perovskite Nanocrystals into the Infrared.

To date, defect-tolerance electronic structure of lead halide perovskite nanocrystals is limited to an optical feature in the visible range. Here, we demonstrate that IR sensitization of formamidinium lead iodine (FAPI) nanocrystal array can be obtained by its doping with PbS nanocrystals. In this hybrid array, absorption comes from the PbS nanocrystals while transport is driven by the perovskite which reduces the dark current compared to pristine PbS. In addition, we fabricate a field-effect transistor using a high capacitance ionic glass made of hybrid FAPI/PbS nanocrystal arrays. We show that the hybrid material has an n-type nature with an electron mobility of 2 × 10-3 cm2 V-1 s-1. However, the dark current reduction is mostly balanced by a loss of absorption. To overcome this limitation, we couple the FAPI/PbS hybrid to a guided mode resonator that can enhance the infrared light absorption.

Field-Effect Transistor and Photo-Transistor of Narrow-Band-Gap Nanocrystal Arrays Using Ionic Glasses.

The gating of nanocrystal films is currently driven by two approaches: either the use of a dielectric such as SiO2 or the use of electrolyte. SiO2 allows fast bias sweeping over a broad range of temperatures but requires a large operating bias. Electrolytes, thanks to large capacitances, lead to the significant reduction of operating bias but are limited to slow and quasi-room-temperature operation. None of these operating conditions are optimal for narrow-band-gap nanocrystal-based phototransistors, for which the necessary large-capacitance gate has to be combined with low-temperature operation. Here, we explore the use of a LaF3 ionic glass as a high-capacitance gating alternative. We demonstrate for the first time the use of such ionic glasses to gate thin films made of HgTe and PbS nanocrystals. This gating strategy allows operation in the 180 to 300 K range of temperatures with capacitance as high as 1 µF·cm-2. We unveil the unique property of ionic glass gate to enable the unprecedented tunability of both magnitude and dynamics of the photocurrent thanks to high charge-doping capability within an operating temperature window relevant for infrared photodetection. We demonstrate that by carefully choosing the operating gate bias, the signal-to-noise ratio can be improved by a factor of 100 and the time response accelerated by a factor of 6. Moreover, the good transparency of LaF3 substrate allows back-side illumination in the infrared range, which is highly valuable for the design of phototransistors.

The effect of steamed potato-wheat bread intake on weight, lipids, glucose, and urinary Na+/K+: A randomized controlled trial in Chinese adults.

Steamed potato bread has received much attention from nutritionists and agriculturalists since it became a staple food of China in 2015. Epidemiological studies have indicated that potatoes may cause diabetes and hypertension, but few trials have evaluated this effect. Through a clinical trial, we evaluated the effect of steamed potato bread intake on adults. In total, 49 and 30 individuals were assigned to the intervention and control groups, respectively. Potato-wheat bread (raw wheat flour and cooked potato flour in the ratio 3:7) and steamed wheat bread (100% raw wheat flour) were provided to the intervention and control groups, respectively, once a day for 4 weeks. Compared with the control group, the intervention group showed significant net changes in weight (-0.6 kg; 95% confidence interval [CI]: -1.2, -0.1; p = 0.016), body mass index (BMI, -0.2 kg/m2; 95% CI: -0.4, -0.1; p = 0.020), low-density lipoprotein cholesterol (LDL-c, -0.22 mmol/L; 95% CI: -0.49, -0.01; p = 0.035), and the urinary level of Na+/K+ (-2.4; 95% CI: -4.1, -0.7; p = 0.007). In conclusion, the steamed potato-wheat bread intake for 4 weeks resulted in decreases in weight, BMI, LDL-c, and the urinary Na+/K+ level among Chinese adults.

Nanoplatelet-Based Light-Emitting Diode and Its Use in All-Nanocrystal LiFi-like Communication.

Now that colloidal nanocrystals (NCs) have been integrated as green and red sources for liquid crystal displays, the next challenge for quantum dots is their use in electrically driven light-emitting diodes (LEDs). Among various colloidal NCs, nanoplatelets (NPLs) have appeared as promising candidates for light-emitting devices because their two-dimensional shape allows a narrow luminescence spectrum, directional emission, and high light extraction. To reach high quantum efficiency, it is critical to grow core/shell structures. High temperature growth of the shells seems to be a better strategy than previously reported low-temperature approaches to obtain bright NPLs. Here, we synthesize CdSe/CdZnS core/shell NPLs whose shell alloy content is tuned to optimize the charge injection in the LED structure. The obtained LED has exceptionally low turn-on voltage, long-term stability (>3100 h at 100 cd m-2), external quantum efficiency above 5%, and luminance up to 35,000 cd m-2. We study the low-temperature performance of the LED and find that there is a delay of droop in terms of current density as temperature decreases. In the last part of the paper, we design a large LED (56 mm2 emitting area) and test its potential for LiFi-like communication. In such an approach, the LED is not only a lightning source but also used to transmit a communication signal to a PbS quantum dot solar cell used as a broadband photodetector. Operating conditions compatible with both lighting and information transfer have been identified. This work paves the way toward an all NC-based communication setup.

Halide Ligands To Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness.

Zinc blende II-VI semiconductor nanoplatelets (NPLs) are defined at the atomic scale along the thickness of the nanoparticle and are initially capped with carboxylates on the top and bottom [001] facets. These ligands are exchanged on CdSe NPLs with halides that act as X-L-type ligands. These CdSe NPLs are costabilized by amines to provide colloidal stability in nonpolar solvents. The hydrogen from the amine can participate in a hydrogen bond with the lone pair electrons of surface halides. After ligand exchange, the optical features are red-shifted. Thus, ligand tuning is another way, in addition to confinement, to tune the optical features of NPLs. The improved surface passivation leads to an increase in the fluorescence quantum efficiency of up to 70% in the case of bromide. However, for chloride and iodide, the surface coverage is incomplete, and thus, the fluorescence quantum efficiency is lower. This ligand exchange is associated with a decrease in stress that leads to unfolding of the NPLs, which is particularly noticeable for iodide-capped NPLs.

HgTe Nanocrystals for SWIR Detection and Their Integration up to the Focal Plane Array.

Infrared applications remain too often a niche market due to their prohibitive cost. Nanocrystals offer an interesting alternative to reach cost disruption especially in the short-wave infrared (SWIR, λ < 1.7 µm) where material maturity is now high. Two families of materials are candidate for SWIR photoconduction: lead and mercury chalcogenides. Lead sulfide typically benefits from all the development made for a wider band gap such as the one made for solar cells, while HgTe takes advantage of the development relative to mid-wave infrared detectors. Here, we make a fair comparison of the two material detection properties in the SWIR and discuss the material stability. At such wavelengths, studies have been mostly focused on PbS rather than on HgTe, therefore we focus in the last part of the discussion on the effect of surface chemistry on the electronic spectrum of HgTe nanocrystals. We unveil that tuning the capping ligands is a viable strategy to adjust the material from the p-type to ambipolar. Finally, HgTe nanocrystals are integrated into multipixel devices to quantize spatial homogeneity and onto read-out circuits to obtain a fast and sensitive infrared laser beam profile.

A colloidal quantum dot infrared photodetector and its use for intraband detection.

Wavefunction engineering using intraband transition is the most versatile strategy for the design of infrared devices. To date, this strategy is nevertheless limited to epitaxially grown semiconductors, which lead to prohibitive costs for many applications. Meanwhile, colloidal nanocrystals have gained a high level of maturity from a material perspective and now achieve a broad spectral tunability. Here, we demonstrate that the energy landscape of quantum well and quantum dot infrared photodetectors can be mimicked from a mixture of mercury selenide and mercury telluride nanocrystals. This metamaterial combines intraband absorption with enhanced transport properties (i.e. low dark current, fast time response and large thermal activation energy). We also integrate this material into a photodiode with the highest infrared detection performances reported for an intraband-based nanocrystal device. This work demonstrates that the concept of wavefunction engineering at the device scale can now be applied for the design of complex colloidal nanocrystal-based devices.

Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals.

We demonstrate the growth of 2D nanoplatelets (NPLs) made of a HgTe/CdS heterostructure, with an optical absorption reaching the shortwave infrared range. The material is an interesting platform to investigate the effect of dimensionality (0D vs. 2D) and confinement on the electronic spectrum and carrier dynamics in colloidal materials. We bring consistent evidence for the p-type nature of this material from transport and photoemission measurements. The majority carrier dynamics probed using pump-probe photoemission is found to be mostly dependent on the presence of a confinement barrier at the surface rather than on the material dimensionality. The minority carrier, on the other hand, is strongly affected by the material shape showing a longer lived minority carrier in 2D NPLs compared to their 0D equivalent with a similar band gap. Finally, we test the potential of this material for photodetection in the short-wave infrared range (SWIR) and show that fast photoresponse and detectivity reaching 109 Jones at room temperature can be achieved.

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.