Mid-Infrared Intraband Photodetector via High Carrier Mobility HgSe Colloidal Quantum Dots

Intraband cascade electroluminescence with weakly n-doped HgTe colloidal quantum dots

Room temperature 6 μm intraband cascade electroluminescence (EL) is demonstrated with lightly n-doped HgTe colloidal quantum dots of ∼8 nm diameter deposited on interdigitated electrodes in a metal-insulator-metal device. With quantum dot films of ∼150 nm thickness made by solid-state-ligand-exchange, the devices emit at 1600 cm-1 (6.25 μm), with a spectral width of 200 cm-1, determined by the overlap of the 1Se-1Pe intraband transition of the quantum dots and the substrate photonic resonance. At the maximum current used of 20 mA, the bias was 30 V, the external quantum efficiency was 2.7%, and the power conversion efficiency was 0.025%. Adding gold nano-antennas between the electrodes broadened the emission and increased the quantum efficiency to 4.4% and the power efficiency to 0.036%. For these films, the doping was about 0.1 electron/dot, the electron mobility was 0.02 cm2 V-1 s-1, and the maximum current density was 0.04 kA cm-2. Higher mobility films made by solution ligand exchange show a 20-fold increase in current density and a 10-fold decrease in EL efficiencies. Electroluminescence with weak doping is interesting for eventually achieving electrically driven stimulated emission, and the requirements for population inversion and lasing are discussed.

Artificial Amacrine Retinal Circuits

Event-based imaging represents a new paradigm in visual information processing that addresses the speed and energy efficiency shortcomings inherently present in the current complementary metal oxide semiconductor-based machine vision. Realizing such imaging systems has previously been sought using very large-scale integration technologies that have complex circuitries consisting of many photodiodes, differential amplifiers, capacitors, and resistors. Here, we demonstrate that event-driven sensing can be achieved using a simple one-resistor, one-capacitor (1R1C) circuit, where the capacitor is modified with colloidal quantum dots (CQDs) to have a photoresponse. This sensory circuit emulates the motion-tracking function of the biological retina, in which the amacrine cells in the bipolar-to-ganglion synaptic pathway produce a transient spiking signal only in response to changes in light intensity but remain inactive under constant illumination. When extended to a 2D imaging array, the individual sensors work independently and output signals only when a change in the light intensity is detected; hence, the concept of the frame in image processing is thereby removed. In this work, we present the fabrication and characterization of a CQD photocapacitor-based 1R1C circuit that has a spectral response at 1550 nm in the short-wave infrared (SWIR). We report on the key performance parameters including peak responsivity, noise, and optical noise equivalent power and discuss the operating mechanism that is responsible for spiking responses in these artificial retinal circuits. The present work sets the foundation for expanding the bioinspired vision sensor capability toward midwave infrared (MWIR) and long-wave infrared (LWIR) spectral regions that are invisible to human eyes and mainstream semiconductor technologies.

Extending the detection limit: innovations in infrared quantum dot photodetectors reaching up to 18 μm

A regrowth method was used to synthesize large-sized colloidal quantum dots (CQDs). With the assistance of doping engineering, the synthesized CQD detectors demonstrate exceptional long-wavelength infrared detection performance, reaching up to 18 μm, significantly extending the spectral response limit for CQD-based infrared detectors. These detectors also achieve a reasonably high detectivity of 6.6 × 108 Jones.

Colloidal Quantum Dots for Nanophotonic Devices

Colloidal quantum dots (CQDs) have unique advantages in the wide tunability of visible-to-infrared emission wavelength and low-cost solution processibility [...].

Progress in Advanced Infrared Optoelectronic Sensors

Infrared optoelectronic sensors have attracted considerable research interest over the past few decades due to their wide-ranging applications in military, healthcare, environmental monitoring, industrial inspection, and human-computer interaction systems. A comprehensive understanding of infrared optoelectronic sensors is of great importance for achieving their future optimization. This paper comprehensively reviews the recent advancements in infrared optoelectronic sensors. Firstly, their working mechanisms are elucidated. Then, the key metrics for evaluating an infrared optoelectronic sensor are introduced. Subsequently, an overview of promising materials and nanostructures for high-performance infrared optoelectronic sensors, along with the performances of state-of-the-art devices, is presented. Finally, the challenges facing infrared optoelectronic sensors are posed, and some perspectives for the optimization of infrared optoelectronic sensors are discussed, thereby paving the way for the development of future infrared optoelectronic sensors.

Heterojunctions of Mercury Selenide Quantum Dots and Halide Perovskites with High Lattice Matching and Their Photodetection Properties

Heterojunction semiconductors have been extensively applied in various optoelectronic devices due to their unique carrier transport characteristics. However, it is still a challenge to construct heterojunctions based on colloidal quantum dots (CQDs) due to stress and lattice mismatch. Herein, HgSe/CsPbBrxI3-x heterojunctions with type I band alignment are acquired that are derived from minor lattice mismatch (~1.5%) via tuning the ratio of Br and I in halide perovskite. Meanwhile, HgSe CQDs with oleylamine ligands can been exchanged with a halide perovskite precursor, acquiring a smooth and compact quantum dot film. The photoconductive detector based on HgSe/CsPbBrxI3-x heterojunction presents a distinct photoelectric response under an incident light of 630 nm. The work provides a promising strategy to construct CQD-based heterojunctions, simultaneously achieving inorganic ligand exchange, which paves the way to obtain high-performance photodetectors based on CQD heterojunction films.

Very long wave infrared quantum dot photodetector up to 18 μm

Colloidal quantum dots (CQDs) are of interest for optoelectronic devices because of the possibility of high-throughput solution processing and the wide energy gap tunability from ultraviolet to infrared wavelengths. People may question about the upper limit on the CQD wavelength region. To date, although the CQD absorption already reaches terahertz, the practical photodetection wavelength is limited within mid-wave infrared. To figure out challenges on CQD photoresponse in longer wavelength, would reveal the ultimate property on these nanomaterials. What's more, it motivates interest in bottom-up infrared photodetection with less than 10% cost compared with epitaxial growth semiconductor bulk. In this work, developing a re-growth method and ionic doping modification, we demonstrate photodetection up to 18 μm wavelength on HgTe CQD. At liquid nitrogen temperature, the responsivity reaches 0.3 A/W and 0.13 A/W, with specific detectivity 6.6 × 108 Jones and 2.3 × 109 Jones for 18 μm and 10 μm CQD photoconductors, respectively. This work is a step toward answering the general question on the CQD photodetection wavelength limitation.

A near-infrared photodetector based on carbon nanotube transistors exhibits ultra-low dark current through field-modulated charge carrier transport

Near-infrared photodetectors (NIR PDs) are devices that convert infrared light signals, which are widely used in military and civilian applications, into electrical signals. However, a common problem associated with PDs is a high dark current. Interestingly, gate voltage can regulate carrier migration in the channels. In this study, a PbS quantum dot heterojunction combined with a carbon nanotube (CNT) field effect transistor (FET) is designed and described. Significantly, this NIR PD achieves field-modulated carrier transport in a CNT transistor, in which the dark current is effectively regulated by the gate voltage. In this PD, an ultra-low dark current of 8 pA is obtained by gate voltage regulation. Moreover, the device shows a fast response speed of 6.5 ms and a high normalized detectivity of 4.75 × 1011 Jones at 0.085 W cm-2 power density and -0.2 V bias voltage. Overall, this work details a novel strategy for the fabrication of a PD with an ultra-low dark current based on a FET.

Multiresonant Grating to Replace Transparent Conductive Oxide Electrode for Bias Selected Filtering of Infrared Photoresponse

Optoelectronic devices rely on conductive layers as electrodes, but they usually introduce optical losses that are detrimental to the device performances. While the use of transparent conductive oxides is established in the visible region, these materials show high losses at longer wavelengths. Here, we demonstrate a photodiode based on a metallic grating acting as an electrode. The grating generates a multiresonant photonic structure over the diode stack and allows strong broadband absorption. The obtained device achieves the highest performances reported so far for a midwave infrared nanocrystal-based detector, with external quantum efficiency above 90%, detectivity of 7 × 1011 Jones at 80 K at 5 μm, and a sub-100 ns time response. Furthermore, we demonstrate that combining different gratings with a single diode stack can generate a bias reconfigurable response and develop new functionalities such as band rejection.

Colloidal Ag2Se intraband quantum dots

With the emergence of the Internet of Things, wearable electronics, and machine vision, the exponentially growing demands for miniaturization, energy efficiency, and cost-effectiveness have imposed critical requirements on the size, weight, power consumption and cost (SWaP-C) of infrared detectors. To meet this demand, new sensor technologies that can reduce the fabrication cost associated with semiconductor epitaxy and remove the stringent requirement for cryogenic cooling are under active investigation. In the technologically important spectral region of mid-wavelength infrared, intraband colloidal quantum dots are currently at the forefront of this endeavor, with wafer-scale monolithic integration and Auger suppression being the key material capabilities to minimize the sensor's SWaP-C. In this Feature Article, we provide a focused review on the development of sensors based on Ag2Se intraband colloidal quantum dots, a heavy metal-free colloidal nanomaterial that has merits for wide-scale adoption in consumer and industrial sectors.

Room Temperature Bias-Selectable, Dual-Band Infrared Detectors Based on Lead Sulfide Colloidal Quantum Dots and Black Phosphorus

A single photodetector capable of switching its peak spectral photoresponse between two wavelength bands is highly useful, particularly for the infrared (IR) bands in applications such as remote sensing, object identification, and chemical sensing. Technologies exist for achieving dual-band IR detection with bulk III-V and II-VI materials, but the high cost and complexity as well as the necessity for active cooling associated with some of these technologies preclude their widespread adoption. In this study, we leverage the advantages of low-dimensional materials to demonstrate a bias-selectable dual-band IR detector that operates at room temperature by using lead sulfide colloidal quantum dots and black phosphorus nanosheets. By switching between zero and forward bias, these detectors switch peak photosensitive ranges between the mid- and short-wave IR bands with room temperature detectivities of 5 × 10⁹ and 1.6 × 10¹¹ cm Hz^1/2 W^-1, respectively. To the best of our knowledge, these are the highest reported room temperature values for low-dimensional material dual-band IR detectors to date. Unlike conventional bias-selectable detectors, which utilize a set of back-to-back photodiodes, we demonstrate that under zero/forward bias conditions the device's operation mode instead changes between a photodiode and a phototransistor, allowing additional functionalities that the conventional structure cannot provide.

Intraband Transition of HgTe Nanocrystals for Long-Wave Infrared Detection at 12 μm

The synthesis of n-doped HgTe colloidal quantum dots was optimized to produce samples with a 1S_e-1P_e intraband transition in the long-wave infrared (8-12 μm). The spin-orbit splitting of 1P_e states places the 1S_e-1P_e1/2 transition around 10 μm. The narrow line width of 130 cm^-1 at 300 K is limited by the size distribution. This narrowing leads to an absorption coefficient about 5 times stronger than is possible with the HgTe CQD interband transition at similar energies. From 300 to 80 K, the intraband transition blueshifts by 90 cm^-1, while the interband transition redshifts by 350 cm^-1. These shifts are assigned to the temperature dependence of the band structure. With ∼2 electrons/dot doping at 80 K, a photoconductive film of 80 nm thickness on a quarter wave reflector substrate showed a detectivity (D*) of ∼10⁷ Jones at 500 Hz in the 8-12 μm range.

Plasmonic Enhanced Nanocrystal Infrared Photodetectors

Low-dimensional nanomaterials are widely investigated in infrared photodetectors (PDs) due to their excellent optical and electrical properties. To further improve the PDs property like quantum efficiency, metallic microstructures are commonly used, which could squeeze light into sub-diffraction volumes for enhanced absorption through surface plasma exciton resonance effects. In recent years, plasmonic enhanced nanocrystal infrared PDs have shown excellent performance and attracted much research interest. In this paper, we summarize the progress in plasmonic enhanced nanocrystal infrared PDs based on different metallic structures. We also discuss challenges and prospects in this field.

Mercury chalcogenide colloidal quantum dots for infrared photodetection: from synthesis to device applications

Commercial infrared (IR) photodetectors based on epitaxial growth inorganic semiconductors, e.g. InGaAs and HgCdTe, suffer from high fabrication cost, poor compatibility with silicon integrated circuits, rigid substrates and bulky cooling systems, which leaves a large development window for the emerging solution-processable semiconductor-based photo-sensing devices. Among the solution-processable semiconductors, mercury (Hg) chalcogenide colloidal quantum dots (QDs) exhibit unique ultra-broad and tuneable photo-responses in the short-wave infrared to far-wave infrared range, and have demonstrated photo-sensing abilities comparable to the commercial products, especially with advances in high operation temperature. Here, we provide a focused review on photodetectors employing Hg chalcogenide colloidal QDs, with a comprehensive summary of the essential progress in the areas of synthesis methods of QDs, property control, device engineering, focus plane array integration, etc. Besides imaging demonstrations, a series of Hg chalcogenide QD photodetector based flexible, integrated, multi-functional applications are also summarized. This review shows prospects for the next-generation low-cost highly-sensitive and compact IR photodetectors based on solution-processable Hg chalcogenide colloidal QDs.

Micro Spectrometers Based on Materials Nanoarchitectonics

Spectral analysis is an important tool that is widely used in scientific research and industry. Although the performance of benchtop spectrometers is very high, miniaturization and portability are more important indicators in some applications, such as on-site detection and real-time monitoring. Since the 1990s, micro spectrometers have emerged and developed. Meanwhile, with the development of nanotechnology, nanomaterials have been applied in the design of various micro spectrometers in recent years, further reducing the size of the spectrometers. In this paper, we review the research progress of micro spectrometers based on nanomaterials. We also discuss the main limitations and perspectives on micro spectrometers.

Mid-wave infrared sensitized InGaAs using intraband transition in doped colloidal II-VI nanocrystals

Narrow bandgap nanocrystals (NCs) are now used as infrared light absorbers, making them competitors to epitaxially grown semiconductors. However, these two types of materials could benefit from one another. While bulk materials are more effective in transporting carriers and give a high degree of doping tunability, NCs offer a larger spectral tunability without lattice-matching constraints. Here, we investigate the potential of sensitizing InGaAs in the mid-wave infrared throughout the intraband transition of self-doped HgSe NCs. Our device geometry enables the design of a photodiode remaining mostly unreported for intraband-absorbing NCs. Finally, this strategy allows for more effective cooling and preserves the detectivity above 10⁸ Jones up to 200 K, making it closer to cryo-free operation for mid-infrared NC-based sensors.

The Historical Development of Infrared Photodetection Based on Intraband Transitions

The infrared technology is entering widespread use as it starts fulfilling a growing number of emerging applications, such as smart buildings and automotive sectors. Majority of infrared photodetectors are based on interband transition, which is the energy gap between the valence band and the conduction band. As a result, infrared materials are mainly limited to semi-metal or ternary alloys with narrow-bandgap bulk semiconductors, whose fabrication is complex and expensive. Different from interband transition, intraband transition utilizing the energy gap inside the band allows for a wider choice of materials. In this paper, we mainly discuss the recent developments on intraband infrared photodetectors, including 'bottom to up' devices such as quantum well devices based on the molecular beam epitaxial approach, as well as 'up to bottom' devices such as colloidal quantum dot devices based on the chemical synthesis.

High-operating-temperature mid-infrared photodetectors via quantum dot gradient homojunction

Due to thermal carriers generated by a narrow mid-infrared energy gap, cooling is always necessary to achieve ideal photodetection. In quantum dot (QD), the electron thermal generation should be reduced with quantum confinement in all three dimensions. As a result, there would be a great potential to realize high-operating-temperature (HOT) QD mid-IR photodetectors, though not yet achieved. Taking the advantages of colloidal nanocrystals' solution processability and precise doping control by surface dipoles, this work demonstrates a HOT mid-infrared photodetector with a QD gradient homojunction. The detector achieves background-limited performance with D^* = 2.7 × 10¹¹ Jones on 4.2 μm at 80 K, above 10¹¹ Jones until 200 K, above 10¹⁰ Jones until 280 K, and 7.6 × 10⁹ Jones on 3.5 μm at 300 K. The external quantum efficiency also achieves more than 77% with responsivity 2.7 A/W at zero bias. The applications such as spectrometers, chemical sensors, and thermal cameras, are also approved, which motivate interest in low-cost, solution-processed and high-performance mid-infrared photodetection beyond epitaxial growth bulk photodetectors.