Ultrathin, High-Aspect-Ratio Bismuth Sulfohalide Nanowire Bundles for Solution-Processed Flexible Photodetectors.

In this study, a novel synthesis of ultrathin, highly uniform colloidal bismuth sulfohalide (BiSX where X = Cl, Br, I) nanowires (NWs) and NW bundles (NBs) for room-temperature and solution-processed flexible photodetectors are presented. High-aspect-ratio bismuth sulfobromide (BiSBr) NWs are synthesized via a heat-up method using bismuth bromide and elemental S as precursors and 1-dodecanethiol as a solvent. Bundling of the BiSBr NWs occurs upon the addition of 1-octadecene as a co-solvent. The morphologies of the BiSBr NBs are easily tailored from sheaf-like structures to spherulite nanostructures by changing the solvent ratio. The optical bandgaps are modulated from 1.91 (BiSCl) and 1.88 eV (BiSBr) to 1.53 eV (BiSI) by changing the halide compositions. The optical bandgap of the ultrathin BiSBr NWs and NBs exhibits blueshift, whose origin is investigated through density functional theory-based first-principles calculations. Visible-light photodetectors are fabricated using BiSBr NWs and NBs via solution-based deposition followed by solid-state ligand exchanges. High photo-responsivities and external quantum efficiencies (EQE) are obtained for BiSBr NW and NB films even under strain, which offer a unique opportunity for the application of the novel BiSX NWs and NBs in flexible and environmentally friendly optoelectronic devices.

An ultra-sensitive colloidal quantum dot infrared photodiode exceeding 100 000% external quantum efficiency via photomultiplication.

In this study, we present ultrasensitive infrared photodiodes based on PbS colloidal quantum dots (CQDs) using a double photomultiplication strategy that utilizes the accumulation of both electron and hole carriers. While electron accumulation was induced by ZnO trap states that were created by treatment in a humid atmosphere, hole accumulation was achieved using a long-chain ligand that increased the barrier to hole collection. Interestingly, we obtained the highest responsivity in photo-multiplicative devices with the long ligands, which contradicts the conventional belief that shorter ligands are more effective for optoelectronic devices. Using these two charge accumulation effects, we achieved an ultrasensitive detector with a responsivity above 7.84 × 102 A W-1 and an external quantum efficiency above 105% in the infrared region. We believe that the photomultiplication effect has great potential for surveillance systems, bioimaging, remote sensing, and quantum communication.

Ultra-Low Noise Level Infrared Quantum Dot Photodiodes with Self-Screenable Polymeric Optical Window.

Quantum dot photodiodes (QPDs) have garnered significant attention because of their unparalleled near-infrared (NIR) detection capabilities, primarily attributable to their size-dependent bandgap tunability. Nevertheless, the broadband absorption spectrum of QPD engenders substantial noise floor within superfluous visible light regions, notably hindering their use in several emerging applications necessitating the detection of faint micro-light signals. To overcome these hurdles, a self-screenable NIR QPD featuring an internal optical filter with a thick polymeric interlayer to reduce electronic noise is demonstrated. This effectively screens out undesirable visible light regions while reducing the ionized defect owing to decreased density of state, yielding an extremely low dark current (≈1010 A cm-2 at V = -1 V). Consequently, the electronic noise spectral density is attained at levels below ≈10-27 -10-28 A2 Hz-1 , and responsivity (R) dropped to 92% within the visible light spectrum.

Defect Engineering of Metal Halide Perovskite Nanocrystals via Spontaneous Diffusion of Ag Nanocrystals.

Perovskite nanocrystals (NCs) have emerged as a promising building block for the fabrication of optic-/optoelectronic-/electronic devices owing to their superior characteristics, such as high absorption coefficient, rapid ion mobilities, and tunable energy levels. However, their low structural stability and poor surface passivation have restricted their application to next-generation devices. Herein, a drug delivery system (DDS)-inspired post-treatment strategy is reported for improving their structural stability by doping of Ag into CsPbBr3 (CPB) perovskite NCs; delivery to damaged sites can promote their structural recovery slowly and uniformly, averting the permanent loss of their intrinsic characteristics. Ag NCs are designed through surface-chemistry tuning and structural engineering to enable their circulation in CPB NC dispersions, followed by their delivery to the CPB NC surface, defect-site recovery, and defect prevention. The perovskite-structure healing process through the DDS-type process (with Ag NCs as the drug) is analyzed by a combination of theoretical calculations (with density functional theory) and experimental analyses. The proposed DDS-inspired healing strategy significantly enhances the optical properties and stability of perovskite NCs, enabling the fabrication of white light-emitting diodes.

High-Performance Self-Powered Quantum Dot Infrared Photodetector with Azide Ion Solution Treated Electron Transport Layer.

The demand for self-powered photodetectors (PDs) capable of NIR detection without external power is growing with the advancement of NIR technologies such as LIDAR and object recognition. Lead sulfide quantum dot-based photodetectors (PbS QPDs) excel in NIR detection; however, their self-powered operation is hindered by carrier traps induced by surface defects and unfavorable band alignment in the zinc oxide nanoparticle (ZnO NP) electron-transport layer (ETL). In this study, an effective azide-ion (N3 - ) treatment is introduced on a ZnO NP ETL to reduce the number of traps and improve the band alignment in a PbS QPD. The ZnO NP ETL treated with azide ions exhibited notable improvements in carrier lifetime and mobility as well as an enhanced internal electric field within the thin-film heterojunction of the ZnO NPs and PbS QDs. The azide-ion-treated PbS QPD demonstrated a increase in short-circuit current density upon NIR illumination, marking a responsivity of 0.45 A W-1 , specific detectivity of 4 × 1011 Jones at 950 nm, response time of 8.2 µs, and linear dynamic range of 112 dB.

Understanding Colloidal Quantum Dot Device Characteristics with a Physical Model.

Colloidal quantum dots (CQDs) are finding increasing applications in optoelectronic devices, such as photodetectors and solar cells, because of their high material quality, unique and attractive properties, and process flexibility without the constraints of lattice match and thermal budget. However, there is no adequate device model for colloidal quantum dot heterojunctions, and the popular Shockley-Quiesser diode model does not capture the underlying physics of CQD junctions. Here, we develop a compact, easy-to-use model for CQD devices rooted in physics. We show how quantum dot properties, QD ligand binding, and the heterointerface between quantum dots and the electron transport layer (ETL) affect device behaviors. We also show that the model can be simplified to a Shockley-like equation with analytical approximate expressions for reverse saturation current, ideality factor, and quantum efficiency. Our model agrees well with the experiment and can be used to describe and optimize CQD device performance.

Highly Conductive and Sensitive Wearable Strain Sensors with Metal/Nanoparticle Double Layer for Noninterference Voice Detection.

Human voice recognition via skin-attachable devices has significant potential for gathering important physiological information from acoustic data without background noise interference. In this study, a highly sensitive and conductive wearable crack-based strain sensor was developed for voice-recognition systems. The sensor was fabricated using a double-layer structure of Ag nanoparticles (NPs) and Ag metal on a biocompatible polydimethylsiloxane substrate. The top metal layer acts as a conducting active layer, whereas the bottom Ag NP layer induces channel cracks in the upper layer, effectively hindering current flow. Subsequently, the double-layer film exhibits a low electrical resistance value (<5 × 10-5 Ω cm), ultrahigh sensitivity (gauge factor = 1870), and a fast response/recovery time (252/168 µs). A sound wave was detected at a high frequency of 15 kHz with a signal-to-noise ratio (SNR) over 40 dB. The sensor exhibited excellent anti-interference characteristics and effectively differentiated between different voice qualities (modal, pressed, and breathy), with a systematic analysis revealing successful detection of the laryngeal state and glottal source. This ultrasensitive wearable sensor has potential applications in various physiological signal measurement methods, personalized healthcare systems, and ubiquitous computing.

Ultrasensitive Near-Infrared InAs Colloidal Quantum Dot-ZnON Hybrid Phototransistor Based on a Gradated Band Structure.

Amorphous metal oxide semiconductor phototransistors (MOTPs) integrated with colloidal quantum dots (QDs) (QD-MOTPs) are promising infrared photodetectors owing to their high photoconductive gain, low off-current level, and high compatibility with pixel circuits. However, to date, the poor mobility of conventional MOTPs, such as indium gallium zinc oxide (IGZO), and the toxicity of lead (Pb)-based QDs, such as lead sulfide and lead selenide, has limited the commercial applications of QD-MOTPs. Herein, an ultrasensitive QD-MOTP fabricated by integrating a high-mobility zinc oxynitride (ZnON)-based MOTP and lead-free indium arsenide (InAs) QDs is demonstrated. A new gradated bandgap structure is introduced in the InAs QD layer that absorbs infrared light, which prevents carriers from moving backward and effectively reduces electron-hole recombination. Chemical, optical, and structural analyses confirm the movement of the photoexcited carriers in the graded band structure. The novel QD-MOTP exhibits an outstanding performance with a responsivity of 1.15 × 105 A W-1 and detectivity of 5.32 × 1016 Jones at a light power density of 2 µW cm-2 under illumination at 905 nm.

Charge transport transition of PEDOT:PSS thin films for temperature-insensitive wearable strain sensors.

In this study, a temperature-insensitive strain sensor that detects only the strain without responding to the temperature was designed. The transport mechanism and associated temperature coefficient of resistance (TCR) of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film were modified through secondary doping with dimethyl sulfoxide (DMSO). Upon DMSO-doping, the carrier transport mechanism of the PEDOT:PSS thin film transitioned from hopping to band-like transport, with a morphological change. At the DMSO doping level, which caused the critical point of the transport transition, the resistance of the thin film was maintained with a change in temperature. Consequently, the TCR of the optimized PEDOT:PSS thin film was less than 9 × 10-5 K-1, which is 102 times lower than that of the as-prepared films. The carrier mobility of the PEDOT:PSS thin film was effectively improved with the morphological change due to DMSO doping and was investigated through combinational analysis. Ultimately, the wearable strain sensor prepared using the optimized PEDOT:PSS thin film responded stably to the applied strain with a gauge factor of 2 and exhibited excellent temperature anti-interference.

Acid-Base Reaction-Assisted Quantum Dot Patterning via Ligand Engineering and Photolithography.

The integration of quantum dots (QDs) into device arrays for high-resolution display and imaging sensor systems remains a significant challenge in research and industry because of issues associated with the QD patterning process. It is difficult for conventional patterning processes such as stamping, inkjet printing, and photolithography to employ QDs and fabricate high-resolution patterns without degrading the properties of QDs. Here, we introduce a novel strategy for the QD patterning process by treating QDs with a bifunctional ligand for acid-base reaction-assisted photolithography. Bifunctional ligands, such as MPA (mercaptopropionic acid) or TGA (thioglycolic acid), have a carboxyl group on one side that allows the QDs to be etched along with the photoresist (PR) by the base developer, while on the opposite side the ligands have a thiol group that passivates the QD surface. Passivating MPA ligands on QDs facilitates patterning of QD films and makes them compatible with harsh photolithography processes. We successfully achieved the patterning of QDs down to 5 µm. We also fabricated high-resolution patterned QD light-emitting diodes (LEDs) and QD photodetector arrays. Our patterning process provides precise control for the fabrication of highly integrated QD-based optoelectronic devices.

Stretchable and Directly Patternable Double-Layer Structure Electrodes with Complete Coverage.

Stretchable electrodes are widely used in next-generation wearable electronics. Recent studies incorporated designs that help rigid electrodes attain stretchability. However, these structures exhibited unsatisfactory charge/signal extraction efficiency because of their low areal fill factor. Additionally, they cannot be photolithographically patterned on polymer substrates because of their low adhesion, requiring additional complicated fabrication steps. We developed photolithographically patternable stretchable electrodes with complete coverage and enhanced charge-extraction efficiency. The electrodes, comprising double layers, included a chemically treated Ag nanowire mesh and Au thin film. The interfacial linker role of polyvinylpyrrolidone chemically strengthened the interfacial bonds, and the reinforced concrete structure of nanowire-embedded metal thin films enhanced the mechanical properties. Therefore, the electrodes provided superior efficiency and stability in capturing physical, electromagnetic, and electrophysiological signals while exceeding the existing stretchable electrode limits. A broad range of applications are foreseen, such as electrocardiogram sensing electrodes, strain sensors, temperature sensors, and antennas.

Ink-Lithography for Property Engineering and Patterning of Nanocrystal Thin Films.

Next-generation devices and systems require the development and integration of advanced materials, the realization of which inevitably requires two separate processes: property engineering and patterning. Here, we report a one-step, ink-lithography technique to pattern and engineer the properties of thin films of colloidal nanocrystals that exploits their chemically addressable surface. Colloidal nanocrystals are deposited by solution-based methods to form thin films and a local chemical treatment is applied using an ink-printing technique to simultaneously modify (i) the chemical nature of the nanocrystal surface to allow thin-film patterning and (ii) the physical electronic, optical, thermal, and mechanical properties of the nanocrystal thin films. The ink-lithography technique is applied to the library of colloidal nanocrystals to engineer thin films of metals, semiconductors, and insulators on both rigid and flexible substrates and demonstrate their application in high-resolution image replications, anticounterfeit devices, multicolor filters, thin-film transistors and circuits, photoconductors, and wearable multisensors.

Noninterference Wearable Strain Sensor: Near-Zero Temperature Coefficient of Resistance Nanoparticle Arrays with Thermal Expansion and Transport Engineering.

In this study, non-temperature interference strain gauge sensors, which are only sensitive to strain but not temperature, are developed by engineering the properties and structure from a material perspective. The environmental interference from temperature fluctuations is successfully eliminated by controlling the charge transport in nanoparticles with thermally expandable polymer substrates. Notably, the negative temperature coefficient of resistance (TCR), which originates from the hopping transport in nanoparticle arrays, is compensated by the positive TCR of the effective surface thermal expansion with anchoring effects. This strategy successfully controls the TCR from negative to positive. A near-zero TCR (NZTCR), less than 1.0 × 10-6 K-1, is achieved through precisely controlled expansion. Various characterization methods and finite element and transport simulations are conducted to investigate the correlated electrical, mechanical, and thermal properties of the materials and elucidate the compensated NZTCR mechanism. With this strategy, an all-solution-processed, transparent, highly sensitive, and noninterference strain sensor is fabricated with a gauge factor higher than 5000 at 1% strain, as demonstrated by pulse and motion sensing, as well as the noninterference property under variable-temperature conditions. It is envisaged that the sensor developed herein is applicable to multifunctional wearable sensors or e-skins for artificial skin or robots.

Janus-like Jagged Structure with Nanocrystals for Self-Sorting Wearable Tactile Sensor.

In this study, a self-sorting sensor was developed with the ability to distinguish between different pressure regimes and translate the pressure to electrical signals. Specifically, the self-sorting sensor can distinguish between soft and hard pressure like the human skin, without any software assistance and complicated circuits. To achieve the self-sorting property, Janus-like jagged structures were prepared via an all-solution process of spontaneous chemical patterning; they comprised electrically semi-insulating vertices and highly conductive valleys. This unique structure facilitates the detection and determination of the intensities and types of pressure by providing a significant gap between the current levels of two types of states, similar to the function of fibers in the human tactile system. The fabricated sensors also exhibit high sensitivity and durability as well as low power consumption, as demonstrated by the electronic skin and ternary Morse signal applications. Compared with conventional wearable pressure sensors, this sensor can detect signals without additional programming; thus, it is highly suitable for delay-sensitive, energy-efficient sensor applications such as driverless vehicles, autonomous artificial intelligence technology, and prosthetic devices.