Direct Imprinting of Quasi-3D Nanophotonic Structures into Colloidal Quantum-Dot Devices

Micropattern Fabricated by Acropetal Migration Controlled through Sequential Photo and Thermal Polymerization

Bottom-up patterning technology plays a significant role in both nature and synthetic materials, owing to its inherent advantages such as ease of implementation, spontaneity, and noncontact attributes, etc. However, constrained by the uncontrollability of molecular movement, energy interaction, and stress, obtained micropatterns tend to exhibit an inevitable arched outline, resulting in the limitation of applicability. Herein, inspired by auxin's action mode in apical dominance, a versatile strategy is proposed for fabricating precision self-organizing micropatterns with impressive height based on polymerization-induced acropetal migration. The copolymer containing fluorocarbon chains (low surface energy) and tertiary amine (coinitiator) is designed to self-assemble on the surface of the photo-curing system. The selective exposure under a photomask establishes a photocuring boundary and the radicals would be generated on the surface, which is pivotal in generating a vertical concentration difference of monomer. Subsequent heating treatment activates the material continuously transfers from the unexposed area to the exposed area and is accompanied by the obviously vertical upward mass transfer, resulting in the manufacture of a rectilinear profile micropattern. This strategy significantly broadens the applicability of self-organizing patterns, offering the potential to mitigate the complexity and time-consuming limitations associated with top-down methods.

Ultra-Smooth Polishing of Single-Crystal Silicon Carbide by Pulsed-Ion-Beam Sputtering of Quantum-Dot Sacrificial Layers

Single-crystal silicon carbide has excellent electrical, mechanical, and chemical properties. However, due to its high hardness material properties, achieving high-precision manufacturing of single-crystal silicon carbide with an ultra-smooth surface is difficult. In this work, quantum dots were introduced as a sacrificial layer in polishing for pulsed-ion-beam sputtering of single-crystal SiC. The surface of single-crystal silicon carbide with a quantum-dot sacrificial layer was sputtered using a pulsed-ion beam and compared with the surface of single-crystal silicon carbide sputtered directly. The surface roughness evolution of single-crystal silicon carbide etched using a pulsed ion beam was studied, and the mechanism of sacrificial layer sputtering was analyzed theoretically. The results show that direct sputtering of single-crystal silicon carbide will deteriorate the surface quality. On the contrary, the surface roughness of single-crystal silicon carbide with a quantum-dot sacrificial layer added using pulsed-ion-beam sputtering was effectively suppressed, the surface shape accuracy of the Ø120 mm sample was converged to 7.63 nm RMS, and the roughness was reduced to 0.21 nm RMS. Therefore, the single-crystal silicon carbide with the quantum-dot sacrificial layer added via pulsed-ion-beam sputtering can effectively reduce the micro-morphology roughness phenomenon caused by ion-beam sputtering, and it is expected to realize the manufacture of a high-precision ultra-smooth surface of single-crystal silicon carbide.

Room-Temperature 15% Efficient Mid-Infrared HgTe Colloidal Quantum Dot Photodiodes

Mid-infrared HgTe colloidal quantum dot photovoltaic devices previously achieved background-limited infrared photodetection at cryogenic temperatures but also decreased from 20 to 1% efficiency from 150 to 300 K. The reduced quantum efficiency was tentatively attributed to the carrier diffusion length being much shorter than the device thickness of ∼400 nm at room temperature. Here, the carrier diffusion length is measured and is found to peak at 215 nm at 200 K and decrease only to 180 nm at 295 K. It is therefore not the cause of the much larger reduced quantum efficiency. Instead, it is shown that the efficiency drops due to the series resistance. With the device size reduced to 50 by 50 μm, the room-temperature quantum efficiency reaches 10 and 15% for HgTe colloidal quantum dot devices with 2400 cm^-1 (4.2 μm) and 2675 cm^-1 (3.7 μm) cutoff, respectively. These small-area devices achieve background-limited photodetection at 150 K and a detectivity above 10⁹ Jones at room temperature with a cutoff at 2675 cm^-1 (3.7 μm).

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.

Ultrahigh-resolution quantum dot patterning for advanced optoelectronic devices

Quantum dots have attracted significant scientific interest owing to their optoelectronic properties, which are distinct from their bulk counterparts. In order to fully utilize quantum dots for next generation devices with advanced functionalities, it is important to fabricate quantum dot colloids into dry patterns with desired feature sizes and shapes with respect to target applications. In this review, recent progress in ultrahigh-resolution quantum dot patterning technologies will be discussed, with emphasis on the characteristic advantages as well as the limitations of diverse technologies. This will provide guidelines for selecting suitable tools to handle quantum dot colloids throughout the fabrication of quantum dot based solid-state devices. Additionally, epitaxially fabricated single-particle level quantum dot arrays are discussed. These are extreme in terms of pattern resolution, and expand the potential application of quantum dots to quantum information processing.

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.

Helmholtz Resonator Applied to Nanocrystal-Based Infrared Sensing

While the integration of nanocrystals as an active medium for optoelectronic devices progresses, light management strategies are becoming required. Over recent years, several photonic structures (plasmons, cavities, mirrors, etc.) have been coupled to nanocrystal films to shape the absorption spectrum, tune the directionality, and so on. Here, we explore a photonic equivalent of the acoustic Helmholtz resonator and propose a design that can easily be fabricated. This geometry combines a strong electromagnetic field magnification and a narrow channel width compatible with efficient charge conduction despite hopping conduction. At 80 K, the device reaches a responsivity above 1 A·W^-1 and a detectivity above 10¹¹ Jones (3 μm cutoff) while offering a significantly faster time-response than vertical geometry diodes.

Nondestructive Photopatterning of Heavy-Metal-Free Quantum Dots

Electroluminescence from quantum dots (QDs) is a suitable photon source for futuristic displays offering hyper-realistic images with free-form factors. Accordingly, a nondestructive and scalable process capable of rendering multicolored QD patterns on a scale of several micrometers needs to be established. Here, nondestructive direct photopatterning for heavy-metal-free QDs is reported using branched light-driven ligand crosslinkers (LiXers) containing multiple azide units. The branched LiXers effectively interlock QD films via photo-crosslinking native aliphatic QD surface ligands without compromising the intrinsic optoelectronic properties of QDs. Using branched LiXers with six sterically engineered azide units, RGB QD patterns are achieved on the micrometer scale. The photo-crosslinking process does not affect the photoluminescence and electroluminescence characteristics of QDs and extends the device lifetime. This nondestructive method can be readily adapted to industrial processes and make an immediate impact on display technologies, as it uses widely available photolithography facilities and high-quality heavy-metal-free QDs with aliphatic ligands.

Generalised optical printing of photocurable metal chalcogenides

Optical three-dimensional (3D) printing techniques have attracted tremendous attention owing to their applicability to mask-less additive manufacturing, which enables the cost-effective and straightforward creation of patterned architectures. However, despite their potential use as alternatives to traditional lithography, the printable materials obtained from these methods are strictly limited to photocurable resins, thereby restricting the functionality of the printed objects and their application areas. Herein, we report a generalised direct optical printing technique to obtain functional metal chalcogenides via digital light processing. We developed universally applicable photocurable chalcogenidometallate inks that could be directly used to create 2D patterns or micrometre-thick 2.5D architectures of various sizes and shapes. Our process is applicable to a diverse range of functional metal chalcogenides for compound semiconductors and 2D transition-metal dichalcogenides. We then demonstrated the feasibility of our technique by fabricating and evaluating a micro-scale thermoelectric generator bearing tens of patterned semiconductors. Our approach shows potential for simple and cost-effective architecturing of functional inorganic materials.

Optical 3D printing techniques are low-cost mask-less patterning methods, but their application is limited by the number of printable materials. Here, the authors report a generalized optical method to print 2D or micrometre-thick 2.5D architectures based on metal chalcogenides inks, showing the realization of micro-scale thermoelectric generators.

Simulation and Design of HgSe Colloidal Quantum-Dot Microspectrometers

In recent years, colloidal quantum dots (CQD) have been intensively studied in various fields due to their excellent optical properties, such as size-tunable absorption features and wide spectral tunability. Therefore, CQDs are promising infrared materials to become alternatives for epitaxial semiconductors, such as HgCdTe, InSb, and type II superlattices. Here, we report a simulation study of a microspectrometer fabricated by integrating an intraband HgSe CQD detector with a distributed Bragg reflector (DBR). Intraband HgSe CQDs possess unique narrowband absorption and optical response, which makes them an ideal material platform to achieve high-resolution detection for infrared signatures, such as molecular vibration. A microspectrometer with a center wavelength of 4 µm is studied. The simulation results show that the optical absorption rate of the HgSe CQD detector can be increased by 300%, and the full-width-at-half-maximum (FWHM) is narrowed to 30%, realizing precise regulation of the absorption wavelength. The influence of the incident angle of light waves on the microspectrometer is also simulated, and the results show that the absorption rate of the HgSe quantum dot detector is increased 2–3 times within the incident angle of 0–23 degrees, reaching a spectral absorption rate of more than 80%. Therefore, we believe that HgSe CQDs are a promising material for realizing practical HgSe microspectrometers.

Room-Temperature Infrared Photodetectors with Zero-Dimensional and New Two-Dimensional Materials

Infrared photodetectors have received much attention for several decades due to their broad applications in the military, science, and daily life. However, for achieving an ideal signal-to-noise ratio and a very fast response, cooling is necessary in those devices, which makes them bulky and costly. Thus, room-temperature infrared photodetectors have emerged as a hot research direction. Novel low-dimensional materials with their easy fabrication and excellent photoelectronic properties provide a possible solution for room-temperature infrared photodetectors. This review aims to summarize the preparation methods and characterization of several low-dimensional materials (PbS, PbSe and HgTe, new two-dimensional materials) with great concern and the room-temperature infrared photodetectors based on them.

Integration of Colloidal Quantum Dots with Photonic Structures for Optoelectronic and Optical Devices

Colloidal quantum dot (QD), a solution-processable nanoscale optoelectronic building block with well-controlled light absorption and emission properties, has emerged as a promising material system capable of interacting with various photonic structures. Integrated QD/photonic structures have been successfully realized in many optical and optoelectronic devices, enabling enhanced performance and/or new functionalities. In this review, the recent advances in this research area are summarized. In particular, the use of four typical photonic structures, namely, diffraction gratings, resonance cavities, plasmonic structures, and photonic crystals, in modulating the light absorption (e.g., for solar cells and photodetectors) or light emission (e.g., for color converters, lasers, and light emitting diodes) properties of QD-based devices is discussed. A brief overview of QD-based passive devices for on-chip photonic circuit integration is also presented to provide a holistic view on future opportunities for QD/photonic structure-integrated optoelectronic systems.

MXene-Based Humidity-Responsive Actuators: Preparation and Properties

Water is a significant and abundant resource as well as a pure natural energy source. Many researchers have been reported on humidity-responsive actuators that mimick the humidity responsive behavior that widely exists in nature. Benefiting from advantages such as hydrophilicity, high electrical conductivity, and good dispersibility, MXenes (Ti₃ C₂ T_x ) show promising performance when applied to humidity-responsive actuators. This Minireview describes the preparation methods and structural characteristics of MXenes, and the mechanism of humidity-responsive actuators. Recent important advances of MXene materials in actuators are objectively reviewed and evaluated, and existing issues are discussed. In addition, the development of these systems is outlined from the aspects of MXene preparation, structure control, design and assembly, and applications, and provides new ideas and guidance for the development of the next generation of high-performance MXene-based humidity-responsive actuators.

Simulation of Monolithically Integrated Meta-Lens with Colloidal Quantum Dot Infrared Detectors for Enhanced Absorption

Colloidal quantum dots (CQDs) have been intensively investigated over the past decades in various fields for both light detection and emission applications due to their advantages like low cost, large-scale production, and tunable spectral absorption. However, current infrared CQD detectors still suffer from one common problem, which is the low absorption rate limited by CQD film thickness. Here, we report a simulation study of CQD infrared detectors with monolithically integrated meta-lenses as light concentrators. The design of the meta-lens for 4 μm infrared was investigated and simulation results show that light intensity in the focused region is ~20 times higher. Full device stacks were also simulated, and results show that, with a meta-lens, high absorption of 80% can be achieved even when the electric area of the CQD detectors was decreased by a factor of 64. With higher absorption and a smaller detector area, the employment of meta-lenses as optical concentrators could possibly improve the detectivity by a factor of 32. Therefore, we believe that integration of CQD infrared detectors with meta-lenses could serve as a promising route towards high performance infrared optoelectronics.

Advances of Sensitive Infrared Detectors with HgTe Colloidal Quantum Dots

The application of infrared detectors based on epitaxially grown semiconductors such as HgCdTe, InSb and InGaAs is limited by their high cost and difficulty in raising operating temperature. The development of infrared detectors depends on cheaper materials with high carrier mobility, tunable spectral response and compatibility with large-scale semiconductor processes. In recent years, the appearance of mercury telluride colloidal quantum dots (HgTe CQDs) provided a new choice for infrared detection and had attracted wide attention due to their excellent optical properties, solubility processability, mechanical flexibility and size-tunable absorption features. In this review, we summarized the recent progress of HgTe CQDs based infrared detectors, including synthesis, device physics, photodetection mechanism, multi-spectral imaging and focal plane array (FPA).

Spectrally tunable nanocomposite metamaterials as near-perfect emitters for mid-infrared thermal radiation management

Nanocomposite absorbers possess a simple structure and tailorable absorption in the mid-infrared (MIR) region achieved by manipulating structural parameters.