Actively variable-spectrum optoelectronics with black phosphorus.

Room-temperature optoelectronic devices that operate at short-wavelength and mid-wavelength infrared ranges (one to eight micrometres) can be used for numerous applications1-5. To achieve the range of operating wavelengths needed for a given application, a combination of materials with different bandgaps (for example, superlattices or heterostructures)6,7 or variations in the composition of semiconductor alloys during growth8,9 are used. However, these materials are complex to fabricate, and the operating range is fixed after fabrication. Although wide-range, active and reversible tunability of the operating wavelengths in optoelectronic devices after fabrication is a highly desirable feature, no such platform has been yet developed. Here we demonstrate high-performance room-temperature infrared optoelectronics with actively variable spectra by presenting black phosphorus as an ideal candidate. Enabled by the highly strain-sensitive nature of its bandgap, which varies from 0.22 to 0.53 electronvolts, we show a continuous and reversible tuning of the operating wavelengths in light-emitting diodes and photodetectors composed of black phosphorus. Furthermore, we leverage this platform to demonstrate multiplexed nondispersive infrared gas sensing, whereby multiple gases (for example, carbon dioxide, methane and water vapour) are detected using a single light source. With its active spectral tunability while also retaining high performance, our work bridges a technological gap, presenting a potential way of meeting different requirements for emission and detection spectra in optoelectronic applications.

A biomimetic eye with a hemispherical perovskite nanowire array retina.

Human eyes possess exceptional image-sensing characteristics such as an extremely wide field of view, high resolution and sensitivity with low aberration1. Biomimetic eyes with such characteristics are highly desirable, especially in robotics and visual prostheses. However, the spherical shape and the retina of the biological eye pose an enormous fabrication challenge for biomimetic devices2,3. Here we present an electrochemical eye with a hemispherical retina made of a high-density array of nanowires mimicking the photoreceptors on a human retina. The device design has a high degree of structural similarity to a human eye with the potential to achieve high imaging resolution when individual nanowires are electrically addressed. Additionally, we demonstrate the image-sensing function of our biomimetic device by reconstructing the optical patterns projected onto the device. This work may lead to biomimetic photosensing devices that could find use in a wide spectrum of technological applications.

Mid-Infrared, Optically Active Black Phosphorus Thin Films on Centimeter Scale.

Black phosphorus (BP) is a narrow bandgap (∼0.3 eV) semiconductor with a great potential for optoelectronic devices in the mid-infrared wavelength. However, it has been challenging to achieve a high-quality scalable BP thin film. Here we present the successful synthesis of optically active BP films on a centimeter scale. We utilize the pulsed laser deposition of amorphous red phosphorus, another allotrope of phosphorus, followed by a high-pressure treatment at ∼8 GPa to induce a phase conversion into BP crystals. The crystalline quality was improved through thermal annealing, resulting in the observation of photoluminescence emission at mid-infrared wavelengths. We demonstrate high-pressure conversion on a centimeter scale with a continuous film with a thickness of ∼18 nm using a flat-belt-type high-pressure apparatus. This synthesis procedure presents a promising route to obtain optical-quality BP films, enabling the exploration of integrated optoelectronic device applications such as light-emitting devices and mid-infrared cameras on a chip scale.

A Color-Tunable Alternating Current Organic Light Emitting Capacitor.

We demonstrate an alternating current (AC) driven light emitting capacitor in which the color of the emission spectra can be changed via an applied AC frequency. The device has a simple metal-oxide-semiconductor (MOS) capacitor structure with an organic emissive layer, enabling facile fabrication processing. The organic emissive layer comprises a thin, submonolayer low energy dye layer underneath a thick host matrix (∼30 nm) with higher energy emitting dyes. The emission of the lower energy dyes dominates at low frequency, while the higher energy emission of the host matrix dominates at high frequency. This simple color tunable device could be used for full-color displays and lighting in the future.

Gate Controlled Excitonic Emission in Quantum Dot Thin Films.

Formation of charged trions is detrimental to the luminescence quantum efficiency of colloidal quantum dot (QD) thin films as they predominantly undergo nonradiative recombination. In this regard, control of charged trion formation is of interest for both fundamental characterization of the quasi-particles and performance optimization. Using CdSe/CdS QDs as a prototypical material system, here we demonstrate a metal-oxide-semiconductor capacitor based on QD thin films for studying the background charge effect on the luminescence efficiency and lifetime. The concentration ratio of the charged and neutral quasiparticles in the QDs is reversibly controlled by applying a gate voltage, while simultaneous steady-state and time-resolved photoluminescence measurements are performed. Notably, the photoluminescence intensity is modulated by up to 2 orders of magnitude with a corresponding change in the effective lifetime. In addition, chip-scale modulation of brightness is demonstrated, where the photoluminescence is effectively turned on and off by the gate, highlighting potential applications in voltage-controlled electrochromics.

Single-pixel reconstructive mid-infrared micro-spectrometer.

Miniaturized spectrometers in the mid-infrared (MIR) are critical in developing next-generation portable electronics for advanced sensing and analysis. The bulky gratings or detector/filter arrays in conventional micro-spectrometers set a physical limitation to their miniaturization. In this work, we demonstrate a single-pixel MIR micro-spectrometer that reconstructs the sample transmission spectrum by a spectrally dispersed light source instead of spatially grated light beams. The spectrally tunable MIR light source is realized based on the thermal emissivity engineered via the metal-insulator phase transition of vanadium dioxide (VO2). We validate the performance by showing that the transmission spectrum of a magnesium fluoride (MgF2) sample can be computationally reconstructed from sensor responses at varied light source temperatures. With potentially minimum footprint due to the array-free design, our work opens the possibility where compact MIR spectrometers are integrated into portable electronic systems for versatile applications.

Large-area epitaxial growth of InAs nanowires and thin films on hexagonal boron nitride by metal organic chemical vapor deposition.

Large-area epitaxial growth of III-V nanowires and thin films on van der Waals substrates is key to developing flexible optoelectronic devices. In our study, large-area InAs nanowires and planar structures are grown on hexagonal boron nitride templates using metal organic chemical vapor deposition method without any catalyst or pre-treatments. The effect of basic growth parameters on nanowire yield and thin film morphology is investigated. Under optimised growth conditions, a high nanowire density of 2.1×109cm-2is achieved. A novel growth strategy to achieve uniform InAs thin film on h-BN/SiO2/Si substrate is introduced. The approach involves controlling the growth process to suppress the nucleation and growth of InAs nanowires, while promoting the radial growth of nano-islands formed on the h-BN surface. A uniform polycrystalline InAs thin film is thus obtained over a large area with a dominant zinc-blende phase. The film exhibits near-band-edge emission at room temperature and a relatively high Hall mobility of 399 cm-2/(Vs). This work suggests a promising path for the direct growth of large-area, low-temperature III-V thin films on van der Waals substrates.

Shape-controlled single-crystal growth of InP at low temperatures down to 220 °C.

III-V compound semiconductors are widely used for electronic and optoelectronic applications. However, interfacing III-Vs with other materials has been fundamentally limited by the high growth temperatures and lattice-match requirements of traditional deposition processes. Recently, we developed the templated liquid-phase (TLP) crystal growth method for enabling direct growth of shape-controlled single-crystal III-Vs on amorphous substrates. Although in theory, the lowest temperature for TLP growth is that of the melting point of the group III metal (e.g., 156.6 °C for indium), previous experiments required a minimum growth temperature of 500 °C, thus being incompatible with many application-specific substrates. Here, we demonstrate low-temperature TLP (LT-TLP) growth of single-crystalline InP patterns at substrate temperatures down to 220 °C by first activating the precursor, thus enabling the direct growth of InP even on low thermal budget substrates such as plastics and indium-tin-oxide (ITO)-coated glass. Importantly, the material exhibits high electron mobilities and good optoelectronic properties as demonstrated by the fabrication of high-performance transistors and light-emitting devices. Furthermore, this work may enable integration of III-Vs with silicon complementary metal-oxide-semiconductor (CMOS) processing for monolithic 3D integrated circuits and/or back-end electronics.

Efficiency Roll-Off Free Electroluminescence from Monolayer WSe2.

Exciton-exciton annihilation (EEA) is a nonradiative process commonly observed in excitonic materials at high exciton densities. Like Auger recombination, EEA degrades luminescence efficiency at high exciton densities and causes efficiency roll-off in light-emitting devices. Near-unity photoluminescence quantum yield has been demonstrated in transition metal dichalcogenides (TMDCs) at all exciton densities with optimal band structure modification mediated by strain. Although the recombination pathways in TMDCs are well understood, the practical application of light-emitting devices has been challenging. Here, we demonstrate a roll-off free electroluminescence (EL) device composed of TMDC monolayers tunable by strain. We show a 2 orders of magnitude EL enhancement from the WSe2 monolayer by applying a small strain of 0.5%. We attain an internal quantum efficiency of 8% at all injection rates. Finally, we demonstrate transient EL turn-on voltages as small as the band gap. Our approach will contribute to practical applications of roll-off free optoelectronic devices based on excitonic materials.

Bright Mid-Wave Infrared Resonant-Cavity Light-Emitting Diodes Based on Black Phosphorus.

The mid-wave infrared (MWIR) wavelength range plays a central role in a variety of applications, including optical gas sensing, industrial process control, spectroscopy, and infrared (IR) countermeasures. Among the MWIR light sources, light-emitting diodes (LEDs) have the advantages of simple design, room-temperature operation, and low cost. Owing to the low Auger recombination at high carrier densities and direct bandgap of black phosphorus (bP), it can serve as a high quantum efficiency emitting layer in LEDs. In this work, we demonstrate bP-LEDs exhibiting high external quantum efficiencies and wall-plug efficiencies of up to 4.43 and 1.78%, respectively. This is achieved by integrating the device with an Al2O3/Au optical cavity, which enhances the emission efficiency, and a thin transparent conducing oxide [indium tin oxide (ITO)] layer, which reduces the parasitic resistance, both resulting in order of magnitude improvements to performance.

Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis.

Wearable sensor technologies are essential to the realization of personalized medicine through continuously monitoring an individual's state of health. Sampling human sweat, which is rich in physiological information, could enable non-invasive monitoring. Previously reported sweat-based and other non-invasive biosensors either can only monitor a single analyte at a time or lack on-site signal processing circuitry and sensor calibration mechanisms for accurate analysis of the physiological state. Given the complexity of sweat secretion, simultaneous and multiplexed screening of target biomarkers is critical and requires full system integration to ensure the accuracy of measurements. Here we present a mechanically flexible and fully integrated (that is, no external analysis is needed) sensor array for multiplexed in situ perspiration analysis, which simultaneously and selectively measures sweat metabolites (such as glucose and lactate) and electrolytes (such as sodium and potassium ions), as well as the skin temperature (to calibrate the response of the sensors). Our work bridges the technological gap between signal transduction, conditioning (amplification and filtering), processing and wireless transmission in wearable biosensors by merging plastic-based sensors that interface with the skin with silicon integrated circuits consolidated on a flexible circuit board for complex signal processing. This application could not have been realized using either of these technologies alone owing to their respective inherent limitations. The wearable system is used to measure the detailed sweat profile of human subjects engaged in prolonged indoor and outdoor physical activities, and to make a real-time assessment of the physiological state of the subjects. This platform enables a wide range of personalized diagnostic and physiological monitoring applications.

Universal Inverse Scaling of Exciton-Exciton Annihilation Coefficient with Exciton Lifetime.

Be it for essential everyday applications such as bright light-emitting devices or to achieve Bose-Einstein condensation, materials in which high densities of excitons recombine radiatively are crucially important. However, in all excitonic materials, exciton-exciton annihilation (EEA) becomes the dominant loss mechanism at high densities. Typically, a macroscopic parameter named EEA coefficient (CEEA) is used to compare EEA rates between materials at the same density; higher CEEA implies higher EEA rate. Here, we find that the reported values of CEEA for 140 different materials is inversely related to the single-exciton lifetime. Since during EEA one exciton must relax to ground state, CEEA is proportional to the single-exciton recombination rate. This leads to the counterintuitive observation that the exciton density at which EEA starts to dominate is higher in a material with larger CEEA. These results broaden our understanding of EEA across different material systems and provide a vantage point for future excitonic materials and devices.

Extreme In-Plane Thermal Conductivity Anisotropy in Titanium Trisulfide Caused by Heat-Carrying Optical Phonons.

High in-plane anisotropies arise in layered materials with large structural difference along different in-plane directions. We report an extreme case in layered TiS3, which features tightly bonded atomic chains along the b-axis direction, held together by weaker, interchain bonding along the a-axis direction. Experiments show thermal conductivity along the chain twice as high as between the chain, an in-plane anisotropy higher than any other layered materials measured to date. We found that in contrast to most other materials, optical phonons in TiS3 conduct an unusually high portion of heat (up to 66% along the b-axis direction). The large dispersiveness of optical phonons along the chains, contrasted to many fewer dispersive optical phonons perpendicular to the chains, is the primary reason for the observed high anisotropy in thermal conductivity. The finding discovers materials with unusual thermal conduction mechanism, as well as provides new material platforms for potential heat-routing or heat-managing devices.

Flexible Electronics toward Wearable Sensing.

Wearable sensors play a crucial role in realizing personalized medicine, as they can continuously collect data from the human body to capture meaningful health status changes in time for preventive intervention. However, motion artifacts and mechanical mismatches between conventional rigid electronic materials and soft skin often lead to substantial sensor errors during epidermal measurement. Because of its unique properties such as high flexibility and conformability, flexible electronics enables a natural interaction between electronics and the human body. In this Account, we summarize our recent studies on the design of flexible electronic devices and systems for physical and chemical monitoring. Material innovation, sensor design, device fabrication, system integration, and human studies employed toward continuous and noninvasive wearable sensing are discussed. A flexible electronic device typically contains several key components, including the substrate, the active layer, and the interface layer. The inorganic-nanomaterials-based active layer (prepared by a physical transfer or solution process) is shown to have good physicochemical properties, electron/hole mobility, and mechanical strength. Flexible electronics based on the printed and transferred active materials has shown great promise for physical sensing. For example, integrating a nanowire transistor array for the active matrix and a conductive pressure-sensitive rubber enables tactile pressure mapping; tactile-pressure-sensitive e-skin and organic light-emitting diodes can be integrated for instantaneous pressure visualization. Such printed sensors have been applied as wearable patches to monitor skin temperature, electrocardiograms, and human activities. In addition, liquid metals could serve as an attractive candidate for flexible electronics because of their excellent conductivity, flexibility, and stretchability. Liquid-metal-enabled electronics (based on liquid-liquid heterojunctions and embedded microchannels) have been utilized to monitor a wide range of physiological parameters (e.g., pulse and temperature). Despite the rapid growth in wearable sensing technologies, there is an urgent need for the development of flexible devices that can capture molecular data from the human body to retrieve more insightful health information. We have developed a wearable and flexible sweat-sensing platform toward real-time multiplexed perspiration analysis. An integrated iontophoresis module on a wearable sweat sensor could enable autonomous and programmed sweat extraction. A microfluidics-based sensing system was demonstrated for sweat sampling, sensing, and sweat rate analysis. Roll-to-roll gravure printing allows for mass production of high-performance flexible chemical sensors at low cost. These wearable and flexible sweat sensors have shown great promise in dehydration monitoring, cystic fibrosis diagnosis, drug monitoring, and noninvasive glucose monitoring. Future work in this field should focus on designing robust wearable sensing systems to accurately collect data from the human body and on large-scale human studies to determine how the measured physical and chemical information relates to the individual's specific health conditions. Further research in these directions, along with the large sets of data collected via these wearable and flexible sensing technologies, will have a significant impact on future personalized healthcare.

Autonomous sweat extraction and analysis applied to cystic fibrosis and glucose monitoring using a fully integrated wearable platform.

Perspiration-based wearable biosensors facilitate continuous monitoring of individuals' health states with real-time and molecular-level insight. The inherent inaccessibility of sweat in sedentary individuals in large volume (≥10 µL) for on-demand and in situ analysis has limited our ability to capitalize on this noninvasive and rich source of information. A wearable and miniaturized iontophoresis interface is an excellent solution to overcome this barrier. The iontophoresis process involves delivery of stimulating agonists to the sweat glands with the aid of an electrical current. The challenge remains in devising an iontophoresis interface that can extract sufficient amount of sweat for robust sensing, without electrode corrosion and burning/causing discomfort in subjects. Here, we overcame this challenge through realizing an electrochemically enhanced iontophoresis interface, integrated in a wearable sweat analysis platform. This interface can be programmed to induce sweat with various secretion profiles for real-time analysis, a capability which can be exploited to advance our knowledge of the sweat gland physiology and the secretion process. To demonstrate the clinical value of our platform, human subject studies were performed in the context of the cystic fibrosis diagnosis and preliminary investigation of the blood/sweat glucose correlation. With our platform, we detected the elevated sweat electrolyte content of cystic fibrosis patients compared with that of healthy control subjects. Furthermore, our results indicate that oral glucose consumption in the fasting state is followed by increased glucose levels in both sweat and blood. Our solution opens the possibility for a broad range of noninvasive diagnostic and general population health monitoring applications.

Gate Quantum Capacitance Effects in Nanoscale Transistors.

As the physical dimensions of a transistor gate continue to shrink to a few atoms, performance can be increasingly determined by the limited electronic density of states (DOS) in the gate and the gate quantum capacitance (CQ). We demonstrate the impact of gate CQ and the dimensionality of the gate electrode on the performance of nanoscale transistors through analytical electrostatics modeling. For low-dimensional gates, the gate charge can limit the channel charge, and the transfer characteristics of the device become dependent on the gate DOS. We experimentally observe for the first time, room-temperature gate quantization features in the transfer characteristics of single-walled carbon nanotube (CNT)-gated ultrathin silicon-on-insulator (SOI) channel transistors; features which can be attributed to the Van Hove singularities in the one-dimensional DOS of the CNT gate. In addition to being an important aspect of future transistor design, potential applications of this phenomenon include multilevel transistors with suitable transfer characteristics obtained via engineered gate DOS.

Wearable Sweat Band for Noninvasive Levodopa Monitoring.

Levodopa is the standard medication clinically prescribed to patients afflicted with Parkinson's disease. In particular, the monitoring and optimization of levodopa dosage are critical to mitigate the onset of undesired fluctuations in the patients' physical and emotional conditions such as speech function, motor behavior, and mood stability. The traditional approach to optimize levodopa dosage involves evaluating the subjects' motor function, which has many shortcomings due to its subjective and limited quantifiable nature. Here, we present a wearable sweat band on a nanodendritic platform that quantitatively monitors levodopa dynamics in the body. Both stationary iontophoretic induction and physical exercise are utilized as our methods of sweat extraction. The sweat band measures real-time pharmacokinetic profiles of levodopa to track the dynamic response of the drug metabolism. We demonstrated the sweat band's functionalities on multiple subjects with implications toward the systematic administering of levodopa and routine management of Parkinson's disease.

Increasing Photoluminescence Quantum Yield by Nanophotonic Design of Quantum-Confined Halide Perovskite Nanowire Arrays.

High-photoluminescence quantum yield (PLQY) is required to reach optimal performance in solar cells, lasers, and light-emitting diodes (LEDs). Typically, PLQY can be increased by improving the material quality to reduce the nonradiative recombination rate. It is in principle equally effective to improve the optical design by nanostructuring a material to increase light out-coupling efficiency (OCE) and introduce quantum confinement, both of which can increase the radiative recombination rate. However, increased surface recombination typically minimizes nanostructure gains in PLQY. Here a template-guided vapor phase growth of CH3NH3PbI3 (MAPbI3) nanowire (NW) arrays with unprecedented control of NW diameter from the bulk (250 nm) to the quantum confined regime (5.7 nm) is demonstrated, while simultaneously providing a low surface recombination velocity of 18 cm s-1. This enables a 56-fold increase in the internal PLQY, from 0.81% to 45.1%, and a 2.3-fold increase in OCEy to increase the external PLQY by a factor of 130, from 0.33% up to 42.6%, exclusively using nanophotonic design.

Transistor-Based Work-Function Measurement of Metal-Organic Frameworks for Ultra-Low-Power, Rationally Designed Chemical Sensors.

A classic challenge in chemical sensing is selectivity. Metal-organic frameworks (MOFs) are an exciting class of materials because they can be tuned for selective chemical adsorption. Adsorption events trigger work-function shifts, which can be detected with a chemical-sensitive field-effect transistor (power ≈microwatts). In this work, several case studies were used towards generalizing the sensing mechanism, ultimately towards our metal-centric hypothesis. HKUST-1 was used as a proof-of-principle humidity sensor. The response is thickness independent, meaning the response is surface localized. ZIF-8 is demonstrated to be an NO2 -sensing material, and the response is dominated by adsorption at metal sites. Finally, MFM-300(In) shows how standard hard-soft acid-base theory can be used to qualitatively predict sensor responses. This paper sets the groundwork for using the tunability of metal-organic frameworks for chemical sensing with distributed, scalable devices.

Cation-Dependent Light-Induced Halide Demixing in Hybrid Organic-Inorganic Perovskites.

Mixed cation metal halide perovskites with increased power conversion efficiency, negligible hysteresis, and improved long-term stability under illumination, moisture, and thermal stressing have emerged as promising compounds for photovoltaic and optoelectronic applications. Here, we shed light on photoinduced halide demixing using in situ photoluminescence spectroscopy and in situ synchrotron X-ray diffraction (XRD) to directly compare the evolution of composition and phase changes in CH(NH2)2CsPb-halide (FACsPb-) and CH3NH3Pb-halide (MAPb-) perovskites upon illumination, thereby providing insights into why FACs-perovskites are less prone to halide demixing than MA-perovskites. We find that halide demixing occurs in both materials. However, the I-rich domains formed during demixing accumulate strain in FACsPb-perovskites but readily relax in MA-perovskites. The accumulated strain energy is expected to act as a stabilizing force against halide demixing and may explain the higher Br composition threshold for demixing to occur in FACsPb-halides. In addition, we find that while halide demixing leads to a quenching of the high-energy photoluminescence emission from MA-perovskites, the emission is enhanced from FACs-perovskites. This behavior points to a reduction of nonradiative recombination centers in FACs-perovskites arising from the demixing process and buildup of strain. FACsPb-halide perovskites exhibit excellent intrinsic material properties with photoluminescence quantum yields that are comparable to MA-perovskites. Because improved stability is achieved without sacrificing electronic properties, these compositions are better candidates for photovoltaic applications, especially as wide bandgap absorbers in tandem cells.