Direct Optical Lithography Enabled Multispectral Colloidal Quantum-Dot Imagers from Ultraviolet to Short-Wave Infrared

Colloidal Quantum-Dot Heterojunction Imagers for Room-Temperature Thermal Imaging

Room-temperature operation or high-operation temperature (HOT) is essential for mid-wave infrared (MWIR) optoelectronics devices providing low-cost and compact systems for numerous applications. Colloidal quantum dots (CQDs) have emerged as a rising candidate to enable photodetectors to operate at HOT or room temperature and develop the next-generation infrared focal plane array (FPA) imagers. Here, band-engineered heterojunctions are demonstrated to suppress dark current with well-passivated mercury telluride (HgTe) CQDs enabling room-temperature MWIR imaging by single-pixel scanning and 640 × 512 FPA sensitive thermal imaging above 250 K. As a result, the room-temperature detectivity reaches as high as 1.26 × 1010 Jones, and the noise equivalent temperature difference (NETD) is as good as 25 mK up to 200 K.

PbS-based SWIR micro-spectrometer with on-chip Fabry-Perot filter array

Miniaturized and portable on-chip spectrometers have been widely explored to facilitate many applications including chemical analysis, environmental monitoring, medical diagnostics, and astronomical observations. However, the optical spectra of micro-spectrometers are mostly within the visible range. Here, we develop high-performance short-wave infrared (SWIR) micro-spectrometers through the integration of wafer-scale uniform lead sulfide (PbS) thin films with an on-chip Fabry-Perot filter array. The optoelectronic performance of PbS-based detectors could be markedly improved through the optimization of chemical bath deposition (CBD) conditions. The high-sensitivity PbS detectors based on the Fabry-Perot filter array demonstrate chemical analysis application.

Working Voltage Switching the Photo-/Thermo-Electric Effect for Distinct Ultraviolet and Infrared Signal Detection

The combination of sensing invisible ultraviolet photons and infrared radiation can significantly enhance target recognition by offsetting their own limit in a short sensing range and poor spatial resolution. However, the difference in their wavelength sets unique requirements for sensing materials and devices, which makes it hard to establish their implementation in a single detector. In this work, we present the design of a single detector with CH3NH3PbCl3 (MAPbCl3) for distinguishing ultraviolet and IR signals by switching its operating mode in the photo-/thermo-electric effect. The large optical band gap of ∼3.2 eV in MAPbCl3 ensures the response toward an ultraviolet photon, while its efficient thermoelectric effect allows the sensing of an IR signal. As a result, the detector exhibits a specific detectivity of 4.5 × 1012 Jones for 395 nm ultraviolet photons under 0 V, while under the working voltage of 2.5 V, it demonstrates a superior temperature coefficient of resistance of -3.7% K-1, a specific detectivity of 4.8 × 108 Jones, and a limit of detection of 0.58 mW/cm2 for 4 μm photons. The functionality of the switching response to ultraviolet or IR photons by working voltage allows background subtraction and enhances the target discrimination in the imaging.

Fabrication of broadband HgCdTe photodetectors with biomimetic insect corneal arrays

Broadband photodetectors are of great significance in a wide variety of technologically important areas. Inspired by bionics, insect cornea-mimicking microstructures could reduce surface reflection, thus enabling broadband detection. Here, we fabricate a broadband large-area (1280 × 1024) HgCdTe focal plane array photodetector based on biomimetic ZnS microarrays, which achieves high external quantum efficiency (> 60%, averaging 79%) across the broad wavelength range of 400 nm - 5000 nm. These results demonstrate that implementing biomimetic ZnS microstructures has effectively broadened the operational wavelength range of conventional HgCdTe infrared photodetectors to encompass the visible light spectrum. Our work achieves continuous visible-to-infrared spectral imaging and provides a beneficial route to fabricate broadband, large-area, high-performance photodetectors.

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.

Obviating Ligand Exchange Preserves the Intact Surface of HgTe Colloidal Quantum Dots and Enhances Performance of Short Wavelength Infrared Photodetectors

A large volume, scalable synthesis procedure of HgTe quantum dots (QDs) capped initially with short-chain conductive ligands ensures ligand exchange-free and simple device fabrication. An effective n- or p-type self-doping of HgTe QDs is achieved by varying cation-anion ratio, as well as shifting the Fermi level position by introducing single- or double-cyclic thiol ligands, that is, 2-furanmethanethiol (FMT) or 2,5-dimercapto-3,4-thiadiasole (DMTD) in the synthesis. This allows for preserving the intact surface of the HgTe QDs, thus ensuring a one order of magnitude reduced surface trap density compared with HgTe subjected to solid-state ligand exchange. The charge carrier diffusion length can be extended from 50 to 90 nm when the device active area consists of a bi-layer of cation-rich HgTe QDs capped with DMTD and FMT, respectively. As a result, the responsivity under 1340 nm illumination is boosted to 1 AW-1 at zero bias and up to 40 AW-1 under -1 V bias at room temperature. Due to high noise current density, the specific detectivity of these photodetectors reaches up to 1010 Jones at room temperature and under an inert atmosphere. Meanwhile, high photoconductive gain ensures a rise in the external quantum efficiency of up to 1000% under reverse bias.

Highly Responsive Switchable Broadband DUV-NIR Photodetector and Tunable Emitter Enabled by Uniform and Vertically Grown III-V Nanowire on Silicon Substrate for Integrated Photonics

Low-dimensional semiconductor nanostructures, particularly in the form of nanowire configurations with large surface-to-volume-ratio, offer intriguing optoelectronic properties for the advancement of integrated photonic technologies. Here, a bias-controlled, superior dual-functional broadband light detecting/emitting diode enabled by constructing the aluminum-gallium-nitride-based nanowire on the silicon-platform is reported. Strikingly, the diode exhibits a stable and high responsivity (R) of over 200 mAW-1 covering an extremely wide operation band under reverse bias conditions, ranging from deep ultraviolet (DUV: 254 nm) to near-infrared (NIR: 1000 nm) spectrum region. While at zero bias, it still possesses superior DUV light selectivity with a high off-rejection ratio of 106. When it comes to the operation of the light-emitting mode under forward bias, it can achieve large spectral changes from UV to red simply by coating colloid quantum dots on the nanowires. Based on the multifunctional features of the diodes, this study further employs them in various optoelectronic systems, demonstrating outstanding applications in multicolor imaging, filterless color discrimination, and DUV/NIR visualization. Such highly responsive broadband photodetector with a tunable emitter enabled by III-V nanowire on silicon provides a new avenue toward the realization of integrated photonics and holds great promise for future applications in communication, sensing, imaging, and visualization.

Stimuli-Responsive Surface Ligands for Direct Lithography of Functional Inorganic Nanomaterials

ConspectusColloidal nanocrystals (NCs) have emerged as a diverse class of materials with tunable composition, size, shape, and surface chemistry. From their facile syntheses to unique optoelectronic properties, these solution-processed nanomaterials are a promising alternative to materials grown as bulk crystals or by vapor-phase methods. However, the integration of colloidal nanomaterials in real-world devices is held back by challenges in making patterned NC films with the resolution, throughput, and cost demanded by device components and applications. Therefore, suitable approaches to pattern NCs need to be established to aid the transition from individual proof-of-concept NC devices to integrated and multiplexed technological systems.In this Account, we discuss the development of stimuli-sensitive surface ligands that enable NCs to be patterned directly with good pattern fidelity while retaining desirable properties. We focus on rationally selected ligands that enable changes in the NC dispersibility by responding to light, electron beam, and/or heat. First, we summarize the fundamental forces between colloidal NCs and discuss the principles behind NC stabilization/destabilization. These principles are applied to understanding the mechanisms of the NC dispersibility change upon stimuli-induced ligand modifications. Six ligand-based patterning mechanisms are introduced: ligand cross-linking, ligand decomposition, ligand desorption, in situ ligand exchange, ion/ligand binding, and ligand-aided increase of ionic strength. We discuss examples of stimuli-sensitive ligands that fall under each mechanism, including their chemical transformations, and address how these ligands are used to pattern either sterically or electrostatically stabilized colloidal NCs. Following that, we explain the rationale behind the exploration of different types of stimuli, as well as the advantages and disadvantages of each stimulus.We then discuss relevant figures-of-merit that should be considered when choosing a particular ligand chemistry or stimulus for patterning NCs. These figures-of-merit pertain to either the pattern quality (e.g., resolution, edge and surface roughness, layer thickness), or to the NC material quality (e.g., photo/electro-luminescence, electrical conductivity, inorganic fraction). We outline the importance of these properties and provide insights on optimizing them. Both the pattern quality and NC quality impact the performance of patterned NC devices such as field-effect transistors, light-emitting diodes, color-conversion pixels, photodetectors, and diffractive optical elements. We also give examples of proof-of-concept patterned NC devices and evaluate their performance. Finally, we provide an outlook on further expanding the chemistry of stimuli-sensitive ligands, improving the NC pattern quality, progress toward 3D printing, and other potential research directions. Ultimately, we hope that the development of a patterning toolbox for NCs will expedite their implementation in a broad range of applications.

Mercury telluride colloidal quantum-dot focal plane array with planar p-n junctions enabled by in situ electric field-activated doping

Colloidal quantum dot (CQD)-based photodetectors are promising alternatives to bulk semiconductor-based detectors to be monolithically integrated with complementary metal-oxide semiconductor readout integrated circuits avoiding high-cost epitaxial growth methods and complicated flip-bonding processes. To date, photovoltaic (PV) single-pixel detectors have led to the best performance with background-limit infrared photodetection performance. However, the nonuniform and uncontrollable doping methods and complex device configuration restrict the focal plane array (FPA) imagers to operate in PV mode. Here, we propose a controllable in situ electric field-activated doping method to construct lateral p-n junctions in the short-wave infrared (SWIR) mercury telluride (HgTe) CQD-based photodetectors with a simple planar configuration. The planar p-n junction FPA imagers with 640 × 512 pixels (15-μm pixel pitch) are fabricated and exhibit substantially improved performance compared with photoconductor imagers before activation. High-resolution SWIR infrared imaging is demonstrated with great potential for various applications including semiconductor inspection, food safety, and chemical analysis.

Colloidal Quantum Dots in Very-Long-Wave Infrared Detection: Progress, Challenges, and Opportunities

The very long wave infrared (VLWIR) is an electromagnetic wave with a wavelength range of 15-30 μm, which plays an important role in missile defense and weather monitoring. This paper briefly introduces the development of intraband absorption of colloidal quantum dots (CQDs) and investigates the possibility of using CQDs to produce VLWIR detectors. We calculated the detectivity of CQDs for VLWIR. The results show that the detectivity is affected by parameters such as quantum dot size, temperature, electron relaxation time, and distance between quantum dots. The theoretical derivation results, combined with the current development status, show that the detection of VLWIR by CQDs is still in the theoretical stage.

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.