Well-Defined and High Resolution Pt Nanowire Arrays for a High Performance Hydrogen Sensor by a Surface Scattering Phenomenon

Rare Earth Material for Hydrogen Gas Sensing: PtGd Alloy Thin Films as a Promising Frontier

At the focus of our investigation lies the precision fabrication of ultrathin platinum-gadolinium (PtGd) alloy films, with the aim to use these films for resistive hydrogen gas sensing. The imperative for sensitive and selective sensors to harness hydrogen's potential as an alternative energy source drives our work. Applying rare earth materials, we enhance the capabilities of hydrogen gas sensing applications. Our study pioneers PtGd alloy thin films for hydrogen gas sensing, addressing a gap in existing literature. Here, we demonstrate the functional characteristics of 2 nm thick PtxGd100'x (x = 25, 50 and 75) alloy films, analyzing their hydrogen gas sensing properties, comprehensively examining the interplay between alloy composition, temperature fluctuation and hydrogen concentration. The effect of composition and structural properties on the sensing response were assessed using EDX and XPS. The films are tested at a temperature range between 25 °C and 150 °C with hydrogen gas concentrations ranging from 10 ppm to 5%. Hydrogen gas sensing mechanisms in PtGd alloy ultrathin films are explained by surface scattering. The unique combination of Pt and Gd offers promising characteristics for gas sensing applications, including high reactivity with hydrogen gas and tunable sensitivity based on the alloy composition.

Critical Sensing Modalities for Hydrogen: Technical Needs and Status of the Field to Support a Changing Energy Landscape

Decarbonization of the energy system is a key aspect of the energy transition. Energy storage in the form of chemical bonds has long been viewed as an optimal scheme for energy conversion. With advances in systems engineering, hydrogen has the potential to become a low cost, low emission, energy carrier. However, hydrogen is difficult to contain, it exhibits a low flammability limit (>40000 ppm or 4%), low ignition energy (0.02 mJ), and it is a short-lived climate forcer. Beyond commercially available sensors to ensure safety through spot checks in enclosed environments, new sensors are necessary to support the development of low emission infrastructure for production, transmission, storage, and end use. Efficient scalable broad area hydrogen monitoring motivates lowering the detection limit below that (10 ppm) of best in class commercial technologies. In this perspective, we evaluate recent advances in hydrogen gas sensing to highlight technologies that may find broad utility in the hydrogen sector. It is clear in the near term that a sensor technology suite is required to meet the variable constraints (e.g., power, size/weight, connectivity, cost) that characterize the breadth of the application space, ranging from industrial complexes to remote pipelines. This perspective is not intended to be another standard hydrogen sensor review, but rather provide a critical evaluation of technologies with detection limits preferably below 1 ppm and low power requirements. Given projections for rapid market growth, promising techniques will also be amenable to rapid development in technical readiness for commercial deployment. As such, methods that do not meet these requirements will not be considered in depth.

Sub-50 nm patterning of alloy thin films via nanophase separation for hydrogen gas sensing

A novel patterning method achieves two-dimensional nano-patterning of metal nanofibers by depositing a platinum-cerium alloy film on a silicon wafer and inducing phase separation in an oxygen-carbon monoxide atmosphere. The resulting nano-patterned thin film, Pt#CeO2/Si, consists of platinum and cerium oxide with an average pattern width of 50 nm and exhibits potential as a hydrogen sensor with sensitive electrical responses to hydrogen ad/desorption. The patterning method introduced herein addresses the challenge of wavelength limitations in traditional optical lithography, offering a scalable approach for sub-50 nm patterns, which are crucial for advanced sensor and electronic applications.

A Platinum Nanowire Sensor for Ethylene in Air

A single platinum nanowire (PtNW) chemiresistive sensor for ethylene gas is reported. In this application, the PtNW performs three functions: (1) Joule self-heating to a specified temperature, (2) in situ resistance-based temperature measurement, and (3) detection of ethylene in air as a resistance change. Ethylene gas in air is detected as a reduction in nanowire resistance by up to 4.5% for concentrations ranging from 1 to 30 ppm in an optimum NW temperature range from 630 to 660 K. This response is rapid (30-100 s), reversible, and reproducible for repetitive ethylene pulses. A threefold increase in signal amplitude is observed as the NW thickness is reduced from 60 to 20 nm, commensurate with a signal transduction mechanism involving surface electron scattering.

AAO-Assisted Nanoporous Platinum Films for Hydrogen Sensor Application

The effects of the porosity and the thickness on the ability of hydrogen sensing is demonstrated through a comparison of compact and nanoporous platinum film sensors. The synthesis of anodic aluminum oxide (AAO) nanotubes with an average pore diameter of less than 100 nm served as the template for the fabrication of nanoporous Pt films using an anodization method. This was achieved by applying a voltage of 40 V in 0.4 M of a phosphoric acid solution at 20 °C. To compare the film and nanoporous Pt, layers of approximately 3 nm and 20 nm were coated on both glass substrates and AAO templates using a sputtering technique. FESEM images monitored the formation of nanoporosity by observing the Pt layers covering the upper edges of the AAO nanotubes. Despite their low thickness and the poor long-range order, the EDX and XRD measurements confirmed and uncovered the crystalline properties of the Pt films by comparing the bare and the Pt deposited AAO templates. The nanoporous Pt and Pt thin film sensors were tested in the hydrogen concentration range between 10–50,000 ppm H2 at room temperature, 50 °C, 100 °C and 150 °C. The results reveal that nanoporous Pt performed higher sensitivity than the Pt thin film and the surface scattering phenomenon can express the hydrogen sensing mechanism of the Pt sensors.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

Scalable Superior Chemical Sensing Performance of Stretchable Ionotronic Skin via a π-Hole Receptor Effect

Skin-attachable gas sensors provide a next-generation wearable platform for real-time protection of human health by monitoring environmental and physiological chemicals. However, the creation of skin-like wearable gas sensors, possessing high sensitivity, selectivity, stability, and scalability (4S) simultaneously, has been a big challenge. Here, an ionotronic gas-sensing sticker (IGS) is demonstrated, implemented with free-standing polymer electrolyte (ionic thermoplastic polyurethane, i-TPU) as a sensing channel and inkjet-printed stretchable carbon nanotube electrodes, which enables the IGS to exhibit high sensitivity, selectivity, stability (against mechanical stress, humidity, and temperature), and scalable fabrication, simultaneously. The IGS demonstrates reliable sensing capability against nitrogen dioxide molecules under not only harsh mechanical stress (cyclic bending with the radius of curvature of 1 mm and cyclic straining at 50%), but also environmental conditions (thermal aging from -45 to 125 °C for 1000 cycles and humidity aging for 24 h at 85% relative humidity). Further, through systematic experiments and theoretical calculations, a π-hole receptor mechanism is proposed, which can effectively elucidate the origin of the high sensitivity (up to parts per billion level) and selectivity of the ionotronic sensing system. Consequently, this work provides a guideline for the design of ionotronic materials for the achievement of high-performance and skin-attachable gas-sensor platforms.

Pt nanoparticles utilized as efficient ultraviolet plasmons for enhancing whispering gallery mode lasing of a ZnO microwire via Ga-incorporation

Introducing nanostructured metals with ultraviolet plasmonic characters has attracted much attention for fabricating high performance optoelectronic devices in the shorter wavelength spectrum. In this work, platinum nanoparticles (PtNPs) with controlled plasmonic responses in ultraviolet wavelengths were successfully synthesized. To demonstrate the promising availability, PtNPs with desired sizes were deposited on a hexagonal ZnO microwire via Ga-doping (PtNPs@ZnO:Ga MW). Under ultraviolet illumination, typical near-band-edge emission of ZnO:Ga MW was considerably enhanced; meanwhile, the photocurrent is much larger than that of the bare MW. Thereby, the enhanced phenomena of a ZnO:Ga MW is related to localized surface plasmon resonances of the decorated PtNPs. A single MW with a hexagonal cross-section can be a potential platform to construct a whispering gallery mode (WGM) cavity due to its total inner wall reflection. Given this, the influence of PtNPs via ultraviolet plasmons on lasing features of the ZnO:Ga MW was tested. The lasing characteristics are significantly enhanced, including lasing output enhancement, a clear reduction of the threshold and the improvement of the quality factor. To exploit the working principle, PtNPs serving as powerful ultraviolet plasmons can couple with ZnO:Ga excitons, accelerating radiative recombination. Since fabricating stable, typical nanostructured metals with ultraviolet plasmons remains a challenging issue, the results illustrated in the work may offer a low-cost and efficient scheme for achieving plasmon-enhanced wide-bandgap semiconductor based ultraviolet optoelectronic devices with excellent performances.

Metallic one-dimensional heterostructure for gas molecule sensing

We have investigated a new metallic core–shell nanowire (NW) geometry of that could be obtained experimentally, that is silicon (Si) and germanium (Ge) NWs with cores constituted by group-10 elements palladium (Pd) and platinum (Pt). These NWs are optimized with two different diameters of 1.5 Å and 2.5 Å. The nanowires having diameter of 1.5 Å show semi-metallic nature with GGA-PBE calculation and metallic nature while spin orbit interaction (SOC) is included. The quantum conductance of the NWs increases with the diameter of the nanowire. We have investigated current–voltage (IV) characteristics for the considered NWs. It has been found that current values in accordance with applied voltage show strong dependence on the diameter of the NWs. The optical study of the NWs shows that absorption co-efficient peak moves to lower energies; due to quantum confinement effect. Furthermore, we have extensively studied optical response of Pd and Pt based core–shell NWs in O₂ and CO₂ environment. Our study on Si and Ge based metallic core/shell NW show a comprehensive picture as possible electron connector in future nano-electronic devices as well as nano gas detector for detecting O₂ gas.

Chemiresistive Hydrogen Sensors: Fundamentals, Recent Advances, and Challenges

Hydrogen (H₂) is one of the next-generation energy sources because it is abundant in nature and has a high combustion efficiency that produces environmentally benign products (H₂O). However, H₂/air mixtures are explosive at H₂ concentrations above 4%, thus any leakage of H₂ must be rapidly and reliably detected at much lower concentrations to ensure safety. Among the various types of H₂ sensors, chemiresistive sensors are one of the most promising sensing systems due to their simplicity and low cost. This review highlights the advances in H₂ chemiresistors, including metal-, semiconducting metal oxide-, carbon-based materials, and other materials. The underlying sensing mechanisms for different types of materials are discussed, and the correlation of sensing performances with nanostructures, surface chemistry, and electronic properties is presented. In addition, the discussion of each material emphasizes key advances and strategies to develop superior H₂ sensors. Furthermore, recent key advances in other types of H₂ sensors are briefly discussed. Finally, the review concludes with a brief outlook, perspective, and future directions.

Recent Progress in Simple and Cost-Effective Top-Down Lithography for ≈10 nm Scale Nanopatterns: From Edge Lithography to Secondary Sputtering Lithography

The development of a simple and cost-effective method for fabricating ≈10 nm scale nanopatterns over large areas is an important issue, owing to the performance enhancement such patterning brings to various applications including sensors, semiconductors, and flexible transparent electrodes. Although nanoimprinting, extreme ultraviolet, electron beams, and scanning probe litho-graphy are candidates for developing such nanopatterns, they are limited to complicated procedures with low throughput and high startup cost, which are difficult to use in various academic and industry fields. Recently, several easy and cost-effective lithographic approaches have been reported to produce ≈10 nm scale patterns without defects over large areas. This includes a method of reducing the size using the narrow edge of a pattern, which has been attracting attention for the past several decades. More recently, secondary sputtering lithography using an ion-bombardment technique was reported as a new method to create high-resolution and high-aspect-ratio structures. Recent progress in simple and cost-effective top-down lithography for ≈10 nm scale nanopatterns via edge and secondary sputtering techniques is reviewed. The principles, technical advances, and applications are demonstrated. Finally, the future direction of edge and secondary sputtering lithography research toward issues to be resolved to broaden applications is discussed.

Coulomb Blockade Effect in Well-Arranged 2D Arrays of Palladium Nano-Islands for Hydrogen Detection at Room Temperature: A Modeling Study

The fast growth of hydrogen usage as a clean fuel in civil applications such as transportation, space technology, etc. highlights the importance of the reliable detection of its leakage and accumulation under explosion limit by sensors with a low power consumption at times when there is no accumulation of hydrogen in the environment. In this research, a new and efficient mechanism is presented for hydrogen detection-using the Coulomb blockade effect in a well-arranged 2D array of palladium nano-islands-which can operate at room temperature. We demonstrated that under certain conditions of size distribution and the regularity of palladium nano-islands, with selected sizes of 1.7, 3 and 6.1 nm, the blockade threshold will appear in current-voltage (IV) characteristics. In reality, it will be achieved by the inherent uncertainty in the size of the islands in nano-scale fabrication or by controlling the size of nanoparticles from 1.7 to 6.1 nm, considering a regular arrangement of nanoparticles that satisfies single-electron tunneling requirements. Based on the simulation results, the threshold voltage is shifted towards lower ones due to the expansion of Pd nanoparticles exposed to the environment with hydrogen concentrations lower than 2.6%. Also, exploring the features of the presented structure as a gas sensor, provides robustness against the Gaussian variation in nano-islands sizes and temperature variations. Remarkably, the existence of the threshold voltage in the IV curve and adjusting the bias voltage below this threshold leads to a drastic reduction in power consumption. There is also an improvement in the minimum detectable hydrogen concentration as well as the sensor response.

Selective growth of platinum nanolines by helium ion beam induced deposition and atomic layer deposition

Helium ion beam induced deposition (HIBID) is an attractive technique capable of precise fabrication of nanostructures. However, the damage caused by helium ion irradiation is the major drawback of conventional HIBID. In this study, area-selective atomic layer deposition (ALD) accompanied with the HIBID technique is explored to solve this problem. A platinum (Pt) seed layer was prepared by HIBID with a helium dose much lower than that of the conventional HIBID to reduce the damage due to the bombardment of energetic ions. Afterwards, Pt was selectively deposited on the seed layer to achieve area-selective ALD. Accordingly, the Pt nanolines with a feature size of ~15 nm are accomplished by the area-selective ALD and the HIBID technique under the condition of the damage-free does.

Rationally Designed PdAuCu Ternary Alloy Nanoparticles for Intrinsically Deactivation-Resistant Ultrafast Plasmonic Hydrogen Sensing

Hydrogen sensors are a prerequisite for the implementation of a hydrogen economy due to the high flammability of hydrogen-air mixtures. They are to comply with the increasingly stringent requirements set by stakeholders, such as the automotive industry and manufacturers of hydrogen safety systems, where sensor deactivation is a severe but widely unaddressed problem. In response, we report intrinsically deactivation-resistant nanoplasmonic hydrogen sensors enabled by a rationally designed ternary PdAuCu alloy nanomaterial, which combines the identified best intrinsic attributes of the constituent binary Pd alloys. This way, we achieve extraordinary hydrogen sensing metrics in synthetic air and poisoning gas background, simulating real application conditions. Specifically, we find a detection limit in the low ppm range, hysteresis-free response over 5 orders of magnitude hydrogen pressure, subsecond response time at room temperature, long-term stability, and, as the key, excellent resistance to deactivating species like carbon monoxide, notably without application of any protective coatings. This constitutes an important step forward for optical hydrogen sensor technology, as it enables application under demanding conditions and provides a blueprint for further material and performance optimization by combining and concerting intrinsic material assets in multicomponent nanoparticles. In a wider context, our findings highlight the potential of rational materials design through alloying of multiple elements for gas sensor development, as well as the potential of engineered metal alloy nanoparticles in nanoplasmonics and catalysis.

Metal-polymer hybrid nanomaterials for plasmonic ultrafast hydrogen detection

Hydrogen-air mixtures are highly flammable. Hydrogen sensors are therefore of paramount importance for timely leak detection during handling. However, existing solutions do not meet the stringent performance targets set by stakeholders, while deactivation due to poisoning, for example by carbon monoxide, is a widely unsolved problem. Here we present a plasmonic metal-polymer hybrid nanomaterial concept, where the polymer coating reduces the apparent activation energy for hydrogen transport into and out of the plasmonic nanoparticles, while deactivation resistance is provided via a tailored tandem polymer membrane. In concert with an optimized volume-to-surface ratio of the signal transducer uniquely offered by nanoparticles, this enables subsecond sensor response times. Simultaneously, hydrogen sorption hysteresis is suppressed, sensor limit of detection is enhanced, and sensor operation in demanding chemical environments is enabled, without signs of long-term deactivation. In a wider perspective, our work suggests strategies for next-generation optical gas sensors with functionalities optimized by hybrid material engineering.

Polyelemental Nanolithography via Plasma Ion Bombardment: From Fabrication to Superior H2 Sensing Application

The development of complex nanostructures containing a homo- and heteromixture of two or more metals is a considerable challenge in nanotechnology. However, previous approaches are considerably limited to the number of combinations of metals depending on the compatibility of elements, and to the complex shape control of the nanostructure. In this study, a significant step is taken toward resolving these limitations via the utilization of a low-energy argon-ion bombardment. The multilayer films are etched and re-sputtered on the sidewall of the pre-pattern, which is a secondary sputtering phenomenon. In contrast to the precursor mixing method, most metallic combinations can be fabricated. The degree of mixing is tuned by the control of the sequence and thickness of multilayers. In addition, the feature shape and dimensions are controlled by changing the pre-pattern or by controlling the ion-beam angle. Using this method, the shortest response time (2 s to 1% H₂ ) in comparison with those of Pd-based H₂ sensors reported previously and a limit of detection below 1 parts per million (ppm) for Pd/Au and Pd/Pt bimetallic line arrays are achieved. This study is expected to realize a family of polyelements that can be used in various applications.

Selective Functionalization of High-Resolution Cu₂O Nanopatterns via Galvanic Replacement for Highly Enhanced Gas Sensing Performance

Recently, high-resolution patterned metal oxide semiconductors (MOS) have gained considerable attention for enhanced gas sensing performance due to their polycrystalline nature, ultrasmall grain size (~5 nm), patternable properties, and high surface-to-volume ratio. Herein, we significantly enhanced the sensing performance of that patterned MOS by galvanic replacement, which allows for selective functionalization on ultrathin Cu₂O nanopatterns. Based on the reduction potential energy difference between the base channel material (Cu₂O) and the decorated metal ion (Pt²⁺), Pt could be selectively and precisely decorated onto the desired area of the Cu₂O nanochannel array. Overall, the Pt-decorated Cu₂O exhibited 11-fold higher NO₂ (100 ppm) sensing sensitivity as compared to the non-decorated sensing channel, the while the channel device with excessive Pt doping showed complete loss of sensing properties.

Free-standing Pt and Pd nanowires: strain-modulated stability and magnetic and thermoelectric properties

We studied the Lagrangian strain-induced colossal magnetism and thermoelectric performance of platinum (Pt) and palladium (Pd) nanowires (NWs) using first-principles density functional calculations. Pt and Pd NWs were found to be dynamically stable for both strain-free and strained situations. Their cohesive energy and magnetic moment showed decrease and increase, respectively, with an increase in tensile Lagrangian strain (2% to 10%) in the (001) plane. Furthermore, we analyzed the thermodynamic properties using the quasi-harmonic approximation (QHA), heat capacity and internal energy of both NWs originating at 0 K, where their internal energy (E) remained high. For the NWs with the (100) and (010) planes, magnetism exist in the strain-free case, whereas it decreases rapidly on increasing the value of strain. Our results predict the excellent stability, colossal magnetism, and thermoelectric properties of the studied NWs; therefore, these NWs can be used as potential thermoelectric materials for device applications.

Distinct Mechanosensing of Human Neural Stem Cells on Extremely Limited Anisotropic Cellular Contact

Human neural stem cells (hNSCs) can alter their fate choice in response to the biophysical cues provided during development. In particular, it has been reported that the differentiation of neural stem cells (NSCs) is enhanced by anisotropic contact, which facilitates focal adhesion (FA) formation and cytoskeletal organization. However, a biomolecular mechanism governing how the cells process the biophysical cues from these anisotropic geometries to their fate commitment is still poorly understood due to the limited availability of geometrical diversities (contact width above 50 nm) applicable to cell studies. Here, we firstly demonstrate that the biomolecular mechanism for enhanced neurogenesis on an anisotropic nanostructure is critically dependent on the resolution of a contact feature. We observed a totally different cellular response to anisotropic geometries by first utilizing a high-resolution nanogroove (HRN) structure with an extremely narrow contact width (15 nm). The width scale is sufficiently low to suppress the integrin clustering and enable us to elucidate how the contact area influences the neurogenesis of hNSCs at an aligned state. Both the HRN and control nanogroove (CN) pattern with a contact width of 1 μm induced the spontaneous topographic alignment of hNSCs. However, intriguingly, the focal adhesion (FA) formation and cytoskeletal reorganization were substantially limited on the HRN, although the cells on the CN showed enhanced FA formation compared with flat surfaces. In particular, the hNSCs on the HRN surface exhibited a strikingly lower fraction of nuclear yes-associated protein (YAP) than on the CN surface, which was turned out to be regulated by Rho GTPase in the same way as the cells sense the mechanical properties of the environment. Considering the previously reported role of YAP on neurogenesis, our finding newly substantiates that YAP and Rho GTPase also can be transducers of hNSCs to process topographical alternation to fate decision. Furthermore, this study with the unprecedented high-resolution nanostructure suggests a novel geometry sensing model where the functional crosstalk between YAP signaling and Rho GTPase integrally regulate the fate commitment of the hNSCs.

Ultrasmall Grained Pd Nanopattern H2 Sensor

Precise control of the size and interfaces of Pd grains is very important for designing a high-performance H₂ sensing channel because the transition of the Pd phase from α to β occurs through units of single grains. However, unfortunately, the grain controllability of previous approaches has been limited to grains exceeding 10 nm in size and simple macroscopic channel structures have only shown monotonic response behavior for a wide concentration range of H₂. In this work, for the first time, we found that Pd channels that are precisely grain-controlled show very different H₂ sensing behavior. They display dual-switching response behavior with simultaneous variation of the positive and negative response direction within single sensor. The Pd nanopattern channel having smallest grain size/interface among previous works could be fabricated via unique lithographic approaches involving low-energy plasma (Ar⁺) bombardment. The ultrasmall grain size (5 nm) and narrow interface gap (<2 nm) controlled by Ar⁺ plasma bombardment enabled both the hydrogen-induced lattice expansion (HILE) (Δ R_H2 < 0) and surface electron scattering (Δ R_H2 > 0) mechanisms to be simultaneously applied to the single Pd channel, thereby inducing dual-switching response according to the H₂ concentration range. In addition, the unique high-aspect-ratio high-resolution morphological characteristics made it possible to achieve highly sensitive H₂ detecting performance (limit of detection: 2.5 ppm) without any hysteresis and irreversible performance degradation. These noteworthy new insights are attributed to high-resolution control of the grain size and the interfaces with the Pd nanostructure channel.

Rational Design of Aminopolymer for Selective Discrimination of Acidic Air Pollutants

Strong acidic gases such as CO₂, SO₂, and NO₂ are harsh air pollutants with major human health threatening factors, and as such, developing new tools to monitor and to quickly sense these gases is critically required. However, it is difficult to selectively detect the acidic air pollutants with single channel material due to the similar chemistry shared by acidic molecules. In this work, three acidic gases (i.e., CO₂, SO₂, and NO₂) are selectively discriminated using single channel material with precise moiety design. By changing the composition ratio of primary (1°), secondary (2°), and tertiary (3°) amines of polyethylenimine (PEI) on CNT channels, unprecedented high selectivity between CO₂ and SO₂ is achieved. Using in situ FT-IR characterizations, the distinct adsorption phenomenon of acidic gases on each amine moiety is precisely demonstrated. Our approach is the first attempt at controlling gas adsorption selectivity of solid-state sensor via modulating chemical moiety level within the single channel material. In addition, discrimination of CO₂, SO₂, and NO₂ with the single channel material solid-state sensor is first reported. We believe that this approach can greatly enhance air pollution tracking systems for strong acidic pollutants and thus aid future studies on selective solid-state gas sensors.

An Ultrastable Ionic Chemiresistor Skin with an Intrinsically Stretchable Polymer Electrolyte

Ultrastable sensing characteristics of the ionic chemiresistor skin (ICS) that is designed by using an intrinsically stretchable thermoplastic polyurethane electrolyte as a volatile organic compound (VOC) sensing channel are described. The hierarchically assembled polymer electrolyte film is observed to be very uniform, transparent, and intrinsically stretchable. Systematic experimental and theoretical studies also reveal that artificial ions are evenly distributed in polyurethane matrix without microscale phase separation, which is essential for implementing high reliability of the ICS devices. The ICS displays highly sensitive and stable sensing of representative VOCs (including toluene, hexane, propanal, ethanol, and acetone) that are found in the exhaled breath of lung cancer patients. In particular, the sensor is found to be fully operational even after being subjected to long-term storage or harsh environmental conditions (relative humidity of 85% or temperature of 100 °C) or severe mechanical deformation (bending to a radius of curvature of 1 mm, or stretching strain of 100%), which can be an effective method to realize a human-adaptive and skin-attachable biosensor platform for daily use and early diagnosis.

Carrier Mobility-Dominated Gas Sensing: A Room-Temperature Gas-Sensing Mode for SnO2 Nanorod Array Sensors

Adsorption-induced change of carrier density is presently dominating inorganic semiconductor gas sensing, which is usually operated at a high temperature. Besides carrier density, other carrier characteristics might also play a critical role in gas sensing. Here, we show that carrier mobility can be an efficient parameter to dominate gas sensing, by which room-temperature gas sensing of inorganic semiconductors is realized via a carrier mobility-dominated gas-sensing (CMDGS) mode. To demonstrate CMDGS, we design and prepare a gas sensor based on a regular array of SnO₂ nanorods on a bottom film. It is found that the key for determining the gas-sensing mode is adjusting the length of the arrayed nanorods. With the change in the nanorod length from 340 to 40 nm, the gas-sensing behavior changes from the conventional carrier-density mode to a complete carrier-mobility mode. Moreover, compared to the carrier density-dominating gas sensing, the proposed CMDGS mode enhances the sensor sensitivity. CMDGS proves to be an emerging gas-sensing mode for designing inorganic semiconductor gas sensors with high performances at room temperature.

Self-assembly of an electronically conductive network through microporous scaffolds

Electron transfer spanning significant distances through a microporous structure was established via the self-assembly of an electronically conductive iridium oxide nanowire matrix enveloping the pore walls. Microporous formations were simulated using two scaffold materials of varying physical and chemical properties; paraffin wax beads, and agar gel. Following infiltration into the micropores, iridium nanoparticles self-assembled at the pore wall/ethanol interface. Subsequently, cyclic voltammetry was employed to electrochemically crosslink the metal, erecting an interconnected, and electronically conductive metal oxide nanowire matrix. Electrochemical and spectral characterization techniques confirmed the formation of oxide nanowire matrices encompassing lengths of at least 1.6mm, 400× distances previously achieved using iridium nanoparticles. Nanowire matrices were engaged as biofuel cell anodes, where electrons were donated to the nanowires by a glucose oxidizing enzyme.

Internal stress induced natural self-chemisorption of ZnO nanostructured films

The energetic particles bombardment can produce large internal stress in the zinc oxide (ZnO) thin film, and it can be used to intentionally modify the surface characteristics of ZnO films. In this article, we observed that the internal stress increased from −1.62 GPa to −0.33 GPa, and the naturally wettability of the textured ZnO nanostructured films changed from hydrophobicity to hydrophilicity. According to analysis of surface chemical states, the naturally controllable wetting behavior can be attributed to hydrocarbon adsorbates on the nanostructured film surface, which is caused by tunable internal stress. On the other hand, the interfacial water molecules near the surface of ZnO nanostructured films have been identified as hydrophobic hydrogen structure by Fourier transform infrared/attenuated total reflection. Moreover, a remarkable near-band-edge emission peak shifting also can be observed in PL spectra due to the transition of internal stress state. Furthermore, our present ZnO nanostructured films also exhibited excellent transparency over 80% with a wise surface wetting switched from hydrophobic to hydrophilic states after exposing in ultraviolet (UV) surroundings. Our work demonstrated that the internal stress of the thin film not only induced natural wettability transition of ZnO nanostructured films, but also in turn affected the surface properties such as surface chemisorption.

Hydrogen gas sensors from polysilicon nanobelt devices selectively modified with sensing materials

Double-junction n⁺/n^-/n⁺ polysilicon nanobelts featuring selectively deposited sensing materials have been investigated for application as H₂ gas sensors. The selective modification of the devices was performed through a combination of localized ablation of a resist and lift-off of a previous catalyst material deposited through e-beam evaporation. Four nanobelt devices, differentiated by their doping concentrations at the n^- region (from 2.5 × 10¹³ to 2.5 × 10¹⁴ cm^-2), were analyzed in terms of the responses to H₂ and their self-heating effects. A low doping concentration improved the response at room temperature, owing to a longer Debye length. The variation in the H₂-induced surface potential associated with temperature, accounting for degradation in the response of the nanobelts with Joule heating bias, was analyzed in terms of the I-V characteristics of the double-junction device. Among various catalysts (Pt, Pd, Pt/Pd) evaluated for their H₂ sensing characteristics, an ultrathin film of Pt/Pd was most favorable.

High-Resolution p-Type Metal Oxide Semiconductor Nanowire Array as an Ultrasensitive Sensor for Volatile Organic Compounds

The development of high-performance volatile organic compound (VOC) sensor based on a p-type metal oxide semiconductor (MOS) is one of the important topics in gas sensor research because of its unique sensing characteristics, namely, rapid recovery kinetics, low temperature dependence, high humidity or thermal stability, and high potential for p-n junction applications. Despite intensive efforts made in this area, the applications of such sensors are hindered because of drawbacks related to the low sensitivity and slow response or long recovery time of p-type MOSs. In this study, the VOC sensing performance of a p-type MOS was significantly enhanced by forming a patterned p-type polycrystalline MOS with an ultrathin, high-aspect-ratio (∼25) structure (∼14 nm thickness) composed of ultrasmall grains (∼5 nm size). A high-resolution polycrystalline p-type MOS nanowire array with a grain size of ∼5 nm was fabricated by secondary sputtering via Ar(+) bombardment. Various p-type nanowire arrays of CuO, NiO, and Cr2O3 were easily fabricated by simply changing the sputtering material. The VOC sensor thus fabricated exhibited higher sensitivity (ΔR/Ra = 30 at 1 ppm hexane using NiO channels), as well as faster response or shorter recovery time (∼30 s) than that of previously reported p-type MOS sensors. This result is attributed to the high resolution and small grain size of p-type MOSs, which lead to overlap of fully charged zones; as a result, electrical properties are predominantly determined by surface states. Our new approach may be used as a route for producing high-resolution MOSs with particle sizes of ∼5 nm within a highly ordered, tall nanowire array structure.

Highly Enhanced Fluorescence Signals of Quantum Dot-Polymer Composite Arrays Formed by Hybridization of Ultrathin Plasmonic Au Nanowalls

Enhancement of the fluorescence intensity of quantum dot (QD)-polymer nanocomposite arrays is an important issue in QD studies because of the significant reduction of fluorescence signals of such arrays due to nonradiative processes in densely packed polymer chains in solid films. In this study, we enhance the fluorescence intensity of such arrays without significantly reducing their optical transparency. Enhanced fluorescence is achieved by hybridizing ultrathin plasmonic Au nanowalls onto the sidewalls of the arrays via single-step patterning and hybridization. The plasmonic Au nanowall induces metal-enhanced fluorescence, resulting in a maximum 7-fold enhancement of the fluorescence signals. We also prepare QD nanostructures of various shapes and sizes by controlling the dry etching time. In the near future, this facile approach can be used for fluorescence enhancement of colloidal QDs with plasmonic hybrid structures. Such structures can be used as optical substrates for imaging applications and for fabrication of QD-LED devices.