II–VI two-dimensional (2D) nanoplatelets (NPLs) exhibit the narrowest optical features among nanocrystals (NCs). This property remains true for Hg-based NPLs, despite a cation exchange procedure to obtain them from Cd-based NPLs, which leads to structural defects (poorly defined edges and voids) inducing inhomogeneous broadening. Here, we propose an optimized procedure for which a solvent, surface chemistry, and reaction conditions are rationally considered. The procedure is applied to the growth of alloyed HgSe1–x Te x NPLs with various compositions. We report a bright photoluminescence for all compositions. Structural properties being now well defined, it is possible to study the electronic properties of these objects. To do so, we combine k·p modeling of quantum-confined structures with X-ray photoemission. In particular, we clarify the origin of the similarity between CdTe and HgTe NPLs absorption spectra despite their vastly differing bulk band structures. Finally, static- and time-resolved photoemission unveil a crossover from n- to p-type behavior in HgSe1–x Te x NPLs while increasing the Te content.

As nanocrystals (NCs) gain maturity, they become central building blocks for optoelectronics in devices such as solar cells and, more recently, infrared focal plane arrays. Now that proof of concept...

Nanocrystals (NCs) have reached a high level of maturity, enabling their integration into optoelectronic devices. The next challenge is the combination of several types of devices into one complex system to achieve better on‐chip integration. Here, an all‐HgTe‐NC active imaging setup operating in the short‐wave infrared (IR) is focused on. First, the design of an optimized IR light‐emitting diode (LED) is focused on. It is shown that a halide technology processing enables an increase of the electroluminescence signal by a factor of 3, while preserving a low turn‐on voltage and a high brightness (3 W sr−1 m−2). Then the degradation mechanism of this LED under continuous operation is unveiled and a shift from band edge to trap emission is shown. This degradation process can be strongly reduced thanks to the encapsulation and the thermal control of the LED. Lastly, the IR emission of the LED is imaged using a focal plane array whose active layer is also made of HgTe NCs, paving the way for all‐NC‐based active imaging setups.

Thanks to their narrow band gap nature and fairly high carrier mobility, HgTe nanocrystals (NCs) are of utmost interest for optoelectronics beyond the telecom window (λ > 1.55 μm). In particular, they offer an interesting cost-effective alternative to the well-developed InGaAs technology. However, in contrast to PbS, far less work has been dedicated to the integration of this material in photodiodes. In the short-wave infrared region, HgTe NCs have a more p-type character than in the mid-wave infrared region, thus promoting the development of new electron transport layers with an optimized band alignment. As for perovskites, HgTe NCs present a fairly deep band gap with respect to vacuum. Thus, we were motivated by the strategy developed for perovskite solar cells, for which SnO2 has led to the best performing devices. Here, we explore the following stack made of SnO2/HgTe/Ag2Te, in which the SnO2 and Ag2Te layers behave as electron and hole extractors, respectively. Using X-ray photoemission, we show that SnO2 presents a nearly optimal band alignment with HgTe to efficiently filter the hole dark current while letting the photoelectrons flow. The obtained I–V curve exhibits an increased rectifying behavior, and the diode stack presents a high internal efficiency for the diode (above 60%) and an external quantum efficiency that is mostly limited by the absorption magnitude. Furthermore, we tackle a crucial challenge for the transfer of such a diode onto readout circuits, which prevents back-side illumination. We also demonstrate that the diode stack is reversible with a partially transparent conducting electrode on the top, while preserving the device’s responsivity. Finally, we show that such a SnO2 layer is also beneficial for electron injection and leads to an enhanced electroluminescence signal as the diode is operated under forward bias. This work is an essential step toward the design of a focal plane array with a HgTe NC-based photodiode.

The integration of photonic structures in nanocrystal (NC)-based photodetectors has been demonstrated to improve device performances. Furthermore, bias-dependent photoresponse can be observed in such devices as a result of the interplay between hopping transport and inhomogeneous electromagnetic field. Here, we investigate the main physical concepts leading to a voltage-dependent photoresponse. We first bring evidence of bias-dependent carrier mobilities in a NC array over a wide range of temperatures. Then, we fabricate an infrared sensing device using HgTe NCs, where the electrodes also play the role of a grating, inducing a spatially inhomogeneous absorption. The obtained device exhibits a significant bias-dependent photoresponse while possessing a competitive detection performance in the extended short-wave and mid-wave infrared, with detectivity reaching 7 × 1010 Jones at 80 K and a fast response time of around 70 ns. This work provides the foundation for further advancements in NC-based-active photonics devices.

Hopping transport associated with the granular nature of nanocrystal arrays has led to the thought that nanocrystal-based devices might be incompatible with fast operations. Here we explore the design of HgTe nanocrystal-based sensors operating in the short-wave infrared and with very fast time response down to a few ns. To reach this goal, the design relies on a planar geometry to reduce the device capacitance. A strong in-built electric field is tailored via electrostatic control from two bottom split-gate electrodes, which promotes the charge extraction. While using graphene electrodes patterned over the two gate electrodes, we optimize the control on the electrostatic design of the p-n junction inside the nanocrystal array. Taking advantage of a high-k dielectric spacer, we demonstrate that the device can be operated under low gate bias (<6 V). The split-gate photodetector appears to be versatile, and can be used either in phototransistor or diode modes, upon the two gates voltages that are set to design isotype or diode-type heterojunctions. We finally highlight that time response enabled by the planar diode configuration can be made much faster than the one associated with the conventional vertical geometry.

As the field of nanocrystal-based optoelectronics matures, more advanced techniques must be developed in order to reveal the electronic structure of nanocrystals, particularly with device-relevant conditions. So far, most of the efforts have been focused on optical spectroscopy, and electrochemistry where an absolute energy reference is required. Device optimization requires probing not only the pristine material but also the material in its actual environment (i.e., surrounded by a transport layer and an electrode, in the presence of an applied electric field). Here, we explored the use of photoemission microscopy as a strategy for operando investigation of NC-based devices. We demonstrate that the method can be applied to a variety of materials and device geometries. Finally, we show that it provides direct access to the metal–semiconductor interface band bending as well as the distance over which the gate effect propagates in field-effect transistors.

Nanocrystal integration into focal plane arrays requires the development of new photodiode designs combining an efficient charge dissociation with a low dark current. Previously reported architectures based on HgTe/Ag2Te stacks appear to be suboptimal for cutoff wavelengths below 2.5 μm. Here, we show that the introduction of a thin and strongly coupled CdSe layer acting as an electron transport layer and a unipolar barrier drastically improves the electrical performances. This diode achieves a responsivity as high as 0.8 A W–1, corresponding to an internal efficiency above 90% for a 2 μm cutoff wavelength. The specific detectivity is close to 1011 Jones at room temperature and reaches 9 × 1011 Jones at 200 K, the highest value reported for a HgTe nanocrystal-based photodiode with operation around 2 μm. The diode time response can be as short as 200 ns and appears to be limited by band bending dynamics as revealed by time-resolved photoemission measurements.