Ultrasensitive Monolithic Dopamine Microsensors Employing Vertically Aligned Carbon Nanofibers.

Brain-on-Chip devices, which facilitate on-chip cultures of neurons to simulate brain functions, are receiving tremendous attention from both fundamental and clinical research. Consequently, microsensors are being developed to accomplish real-time monitoring of neurotransmitters, which are the benchmarks for neuron network operation. Among these, electrochemical sensors have emerged as promising candidates for detecting a critical neurotransmitter, dopamine. However, current state-of-the-art electrochemical dopamine sensors are suffering from issues like limited sensitivity and cumbersome fabrication. Here, a novel route in monolithically microfabricating vertically aligned carbon nanofiber electrochemical dopamine microsensors is reported with an anti-blistering slow cooling process. Thanks to the microfabrication process, microsensors is created with complete insulation and large surface areas. The champion device shows extremely high sensitivity of 4.52× 104 µAµM-1·cm-2, which is two-orders-of-magnitude higher than current devices, and a highly competitive limit of detection of 0.243 nM. These remarkable figures-of-merit will open new windows for applications such as electrochemical recording from a single neuron.

Colloidal quantum dot materials for next-generation near-infrared optoelectronics.

Colloidal quantum dots (CQDs) are a promising class of materials for next-generation optoelectronic devices, such as displays, LEDs, lasers, photodetectors, and solar cells. CQDs can be obtained at low cost and in large quantities using wet chemistry. CQDs have also been produced using various materials, such as CdSe, InP, perovskites, PbS, PbSe, and InAs. Some of these CQD materials absorb and emit photons in the visible region, making them excellent candidates for displays and LEDs, while others interact with low-energy photons in the near-infrared (NIR) region and are intensively utilized in NIR lasers, NIR photodetectors, and solar cells. In this review, we have focused on NIR CQD materials and reviewed the development of CQD materials for solar cells, NIR lasers, and NIR photodetectors since the first set of reports on CQD materials in these particular applications.

Silicon Surface Passivation for Silicon-Colloidal Quantum Dot Heterojunction Photodetectors.

Sensitizing crystalline silicon (c-Si) with an infrared-sensitive material, such as lead sulfide (PbS) colloidal quantum dots (CQDs), provides a straightforward strategy for enhancing the infrared-light sensitivity of a Si-based photodetector. However, it remains challenging to construct a high-efficiency photodetector based upon a Si:CQD heterojunction. Herein, we demonstrate that Si surface passivation is crucial for building a high-performance Si:CQD heterojunction photodetector. We have studied one-step methyl iodine (CH3I) and two-step chlorination/methylation processes for Si surface passivation. Transient photocurrent (TPC) and transient photovoltage (TPV) decay measurements reveal that the two-step passivated Si:CQD interface exhibits fewer trap states and decreased recombination rates. These passivated substrates were incorporated into prototype Si:CQD infrared photodiodes, and the best performance photodiode based upon the two-step passivation shows an external quantum efficiency (EQE) of 31% at 1280 nm, which represents a near 2-fold increase over the standard device based upon the one-step CH3I passivated Si.

Unusual Surface Ligand Doping-Induced p-Type Quantum Dot Solids and Their Application in Solar Cells.

Doping quantum dots (QDs) is a problem that has been haunting researchers in the QD research community for years, even though doping techniques have been utilized for decades in conventional semiconductors. For the "self-purification" in colloidal QDs, engineering the surface ligands has emerged as an effective way to alter free carrier concentrations and doping types in colloidal QD solids. Halide-atomic ligands are the most popular ligands in producing PbS QD solids since they provide minimal dot-to-dot distance while maintain low in-gap trap states. However, previously reported halide surface treatment could only produce n-type QD solids. Here, we report the fabrication of p-type PbS QD solids using proton-assisted surface ligand exchange. We unveiled the origin of p-type doing in PbS QD solids, and it came from an unusual surface ligand; the HOH+ group formed using NH4X (X = Cl, Br, I) in methanol. We further fabricated QD solar cells using PbS-NH4Cl, a p-type QD solid predicted and proved by our theory and experiments. The champion device shows a high power conversion efficiency of 7.49%.

Nanocrystal-filled polymer for improving angular color uniformity of phosphor-converted white LEDs.

Improvement in angular color uniformity is of significant importance to reach high illumination quality for a white LED. In this demonstration, we show potential applications of cellulose-nanocrystal (CNC)-filled polymer for enhancing color uniformity of white LEDs. The excellent optical diffusion capability provided by CNC and the mechanical flexibility offered by the polymer matrix render it a highly efficient color mixer. The CNC-filled polymer was applied on the outer surface of a conventional white LED module to enhance both color and illumination uniformities. It reduced 71.4% of angular color deviation and improved illumination uniformity by 35.5% while maintaining over 85% of light energy transmission. We also demonstrate that for a specific application purpose, such as downlight illumination, one can simultaneously achieve high color uniformity and downlight illumination with reduced glare by constructing the CNC-filled polymer into a CNC-doped lens. In this case, a 0.03%-CNC-filled lens can reduce angular color deviation by 74.0% and achieve a light energy transmission of 85.5%. The light energy transmission can be further improved by advanced lens designs for energy-saving purposes.

Surface-Modified Substrates for Quantum Dot Inks in Printed Electronics.

Printed electronics fill the niches for low-cost, flexible devices in electronics. Developing substrates suitable for various printable electronic inks becomes an important topic in both academia and industry. Because of their extraordinary properties like solution processability, colloidal quantum dots (QDs) are gradually emerging in this field as promising candidates for electronic inks. In recent years, researchers have successfully produced high quality PbS QD inks in polar solvents. However, the incorporation of electronic inks onto a well-passivated substrate remains challenging due to the processing incompatibility between polar solvents and hydrophobic substrates. Here, we propose a surface modification strategy by using chlorine to achieve both trap-site suppression and a hydrophilic surface. The chlorine can effectively passivate the surface dangling bonds and charged hydroxyls while creating a hydrophilic surface. On this modified substrate, the contact angle between the water droplet and the SiO2 substrate can be as small as 20° and this strategy is also feasible for other polymer and inorganic substrates. For a proof-of-concept demonstration, we fabricated a PbS QD ink-based field-effect transistor on a Cl-passivated substrate, and the device showed a mobility as high as 4.36 × 10-3 cm2/V s, which indicates effective trap-site suppression. This device also enables the potential of the Cl-passivated substrates for QD inks with water or other polar solvents.

Reducing shadowing losses in silicon solar cells using cellulose nanocrystal: polymer hybrid diffusers.

Gridline shadowing is one of the main factors affecting the performance of silicon solar cells. In this demonstration, a straightforward, scalable approach is reported to reduce shadowing losses from metallic contacts on silicon solar cells by employing cellulose nanocrystals (CNC) mixed in a polymer- polydimethylsiloxane. The method is highly compatible with current solar cell module manufacturing. The CNC:polymer (CNP) hybrid diffusers, offering highly efficient broadband light diffusion, are applied atop the metallization areas to deflect the light impinging on metallic gridlines toward uncovered active areas on the solar cell. Simulations showed that the CNP diffuser is an excellent candidate for reducing shadowing losses within a wide range of incident angles, as it can reduce more than 30% of shadowing losses at normal incidence, and nearly 50% of the lost light can be recycled at the incident angle of 60°. Taking advantage of reduced shadowing losses, a new 6-busbar technology based on the CNP diffusers is proposed with lower manufacturing complexity and higher overall efficiency.

On-chip colloidal quantum dot devices with a CMOS compatible architecture for near-infrared light sensing.

Solution-processed semiconductors that exhibit tunable light absorption and can be directly integrated into state-of-the-art silicon technologies are attractive for near-infrared (NIR) light detection in applications of medical imaging, night vision cameras, hyperspectral sensing, etc. Colloidal quantum dot (CQD) is regarded as a promising candidate for its solution-processability and superior optoelectronic properties. Here we propose an on-chip CQD photodetector, photodiode-oxide-semiconductor field-effect transistor, for NIR light sensing. This CMOS compatible device architecture utilizes silicon as a channel for carrier transport and PbS CQD as the light absorbing material controlling the channel conductivity. While the light with a wavelength longer than about 1100 nm cannot excite a photocurrent in commercial silicon-based photodetectors due to the absorption cutoff of silicon, the proposed photodetector can have responses owing to the usage of a PbS CQD photodiode. Simulations showed that the photodiode could provide photovoltage to the semiconductor, forming an inversion layer at the oxide-semiconductor interface, and the electron density at the interface is significantly enhanced. As a result, currents could flow through this layer with ease between the source and drain electrodes. For a proof-of-concept demonstration, we experimentally connected a CQD photodiode with a commercial silicon transistor and proved that the current from the transistor could be increased by photovoltage provided by the photodiode under NIR light illumination. The device shows a responsivity of 5.9A/W at the wavelength of 1250 nm.

Improved response time of flexible microelectromechanical sensors employing eco-friendly nanomaterials.

Flexible force/pressure sensors are of interest for academia and industry and have applications in wearable technologies. Most of such sensors on the market or reported in journal publications are based on the operation mechanism of probing capacitance or resistance changes of the materials under pressure. Recently, we reported the microelectromechanical (MEM) sensors based on a different mechanism: mechanical switches. Multiples of such MEM sensors can be integrated to achieve the same function of regular force/pressure sensors while having the advantages of ease of fabrication and long-term stability in operation. Herein, we report the dramatically improved response time (more than one order of magnitude) of these MEM sensors by employing eco-friendly nanomaterials-cellulose nanocrystals. For instance, the incorporation of polydimethysiloxane filled with cellulose nanocrystals shortened the response time of MEM sensors from sub-seconds to several milliseconds, leading to the detection of both diastolic and systolic pressures in the radial arterial blood pressure measurement. Comprehensive mechanical and electrical characterization of the materials and the devices reveal that greatly enhanced storage modulus and loss modulus play key roles in this improved response time. The demonstrated fast-response flexible sensors enabled continuous monitoring of heart rate and complex cardiovascular signals using pressure sensors for future wearable sensing platforms.

Digital microelectromechanical sensor with an engineered polydimethylsiloxane (PDMS) bridge structure.

Functional electronic devices integrated on flexible substrates are of great interest in both academia and industry for their potential applications in wearable technologies. Recently, there have been an increasing number of investigations on developing new materials for flexible strain sensors and pressure sensors, with the aim of achieving better sensitivity and detection ranges. However, the analog signal outputs of these sensors are accompanied with challenges regarding device reproducibility and reliability. Here we designed and fabricated a new class of sensors-digital microelectromechanical (MEM) sensors for wearable technologies. Our digital MEM sensors were implemented with the polydimethysiloxane (PDMS) bridge on flexible substrates, and provided digital signal outputs based on electrical insulating-to-conducting transitions. By engineering the PDMS bridge structure, we could tune the sensitivity of the digital MEM sensor for various applications. These digital MEM sensors were used in bending tests: they were integrated on glove fingers and used to detect gestures. These sensors were also used as force sensors: they were used on human wrists to monitor heart rates. The device was experimentally found to maintain its performance level even after 10 000 cycles of bending or pressing. The digital output of our devices allows a higher tolerance for device fabrication to be set. Furthermore, our devices can be engineered for desired specifications in various potential applications.

Observation of localized surface plasmons and hybridized surface plasmon polaritons on self-assembled two-dimensional nanocavities.

Large-area patterning of periodic nanostructures using self-assembled nanospheres is of interest for fabricating low-cost plasmonic substrates, such as two-dimensional (2D) metallic gratings. Surface plasmon polaritons (SPPs) excited on metallic gratings have applications in biosensors, thin-film photovoltaics, photoelectrochemical cells, and photodetectors. Here we fabricated large-area metallic gratings using nanosphere lithography, and the geometry of gratings was controlled by the sphere size and distance between nanospheres. Both forward and backward propagating SPPs were observed using the grating coupling geometry. Furthermore, we reported the first observation of localized surface plasmons (LSPs) on this large-area metallic grating by both simulation and experimental studies. Such an LSP mode was confined in the 2D nanocavities and was not supported by dielectric gratings with the same 2D geometry.

In-plane coassembly route to atomically thick inorganic-organic hybrid nanosheets.

Control over the anisotropic assembly of small building blocks into organized structures is considered an effective way to design organic nanosheets and atomically thick inorganic nanosheets with nonlayered structure. However, there is still no available route so far to control the assembly of inorganic and organic building blocks into a flattened hybrid nanosheet with atomic thickness. Herein, we highlight for the first time a universal in-plane coassembly process for the design and synthesis of transition-metal chalcogenide-alkylamine inorganic-organic hybrid nanosheets with atomic thickness. The structure, formation mechanism, and stability of the hybrid nanosheets were investigated in detail by taking the Co9S8-oleylamine (Co9S8-OA) hybrid nanosheets as an example. Both experimental data and theoretical simulations demonstrate that the hybrid nanosheets were formed by in-plane connection of small two-dimensional (2D) Co9S8 nanoplates via oleylamine molecules adsorbed at the side surface and corner sites of the nanoplates. X-ray absorption fine structure spectroscopy study reveals the structure distortion of the small 2D Co9S8 nanoplates that endows structural stability of the atomically thick Co9S8-OA hybrid nanosheets. The brand new atomically thick nanosheets with inorganic-organic hybrid network nanostructure will not only enrich the family of atomically thick 2D nanosheets but also inspire more interest in their potential applications.