Engineering Interface Defects and Interdiffusion at the Degenerate Conductive In2O3/Al2O3 Interface for Stable Electrodes in a Saline Solution.

A low-temperature Al2O3 deposition process provides a simplified method to form a conductive two-dimensional electron gas (2DEG) at the metal oxide/Al2O3 heterointerface. However, the impact of key factors of the interface defects and cation interdiffusion on the interface is still not well understood. Furthermore, there is still a blank space in terms of applications that go beyond the understanding of the interface's electrical conductivity. In this work, we carried out a systematic experimental study by oxygen plasma pretreatment and thermal annealing post-treatment to study the impact of interface defects and cation interdiffusion at the In2O3/Al2O3 interface on the electrical conductance, respectively. Combining the trends in electrical conductance with the structural characteristics, we found that building a sharp interface with a high concentration of interface defects provides a reliable approach to producing such a conductive interface. After applying this conductive interface as electrodes for fabricating a field-effect transistor (FET) device, we found that this interface electrode exhibited ultrastability in phosphate-buffered saline (PBS), a commonly used biological saline solution. This study provides new insights into the formation of conductive 2DEGs at metal oxide/Al2O3 interfaces and lays the foundation for further applications as electrodes in bioelectronic devices.

Rational Strategy for Space-Confined Atomic Layer Deposition.

Atomic layer deposition (ALD) offers excellent controllability of spatial uniformity, film thickness at the Angstrom level, and film composition even for high-aspect-ratio nanostructured surfaces, which are rarely attainable by other conventional deposition methodologies. Although ALD has been successfully applied to various substrates under open-top circumstances, the applicability of ALD to confined spaces has been limited because of the inherent difficulty of supplying precursors into confined spaces. Here, we propose a rational methodology to apply ALD growths to confined spaces (meter-long microtubes with an aspect ratio of up to 10 000). The ALD system, which can generate differential pressures to confined spaces, was newly developed. By using this ALD system, it is possible to deposit TiOx layers onto the inner surface of capillary tubes with a length of 1000 mm and an inner diameter of 100 µm with spatial deposition uniformity. Furthermore, we show the superior thermal and chemical robustness of TiOx-coated capillary microtubes for molecular separations when compared to conventional molecule-coated capillary microtubes. Thus, the present rational strategy of space-confined ALD offers a useful approach to design the chemical and physical properties of the inner surfaces of various confined spaces.

Interfacial Molecular Compatibility for Programming Organic-Metal Oxide Superlattices.

Artificially programming a sequence of organic-metal oxide multilayers (superlattices) by using atomic layer deposition (ALD) is a fascinating and challenging issue in material chemistry. However, the complex chemical reactions between ALD precursors and organic layer surfaces have limited their applications for various material combinations. Here, we demonstrate the impact of interfacial molecular compatibility on the formation of organic-metal oxide superlattices using ALD. The effects of both organic and inorganic compositions on the metal oxide layer formation processes onto self-assembled monolayers (SAM) were examined by using scanning transmission electron microscopy, in situ quartz crystal microbalance measurements, and Fourier-transformed infrared spectroscopy. These series of experiments reveal that the terminal group of organic SAM molecules must satisfy two conflicting requirements, the first of which is to promptly react with ALD precursors and the second is not to bind strongly to the bottom metal oxide layers to avoid undesired SAM conformations. OH-terminated phosphate aliphatic molecules, which we have synthesized, were identified as one of the best candidates for such a purpose. Molecular compatibility between metal oxide precursors and the -OHs must be properly considered to form superlattices. In addition, it is also important to form densely packed and all-trans-like SAMs to maximize the surface density of reactive -OHs on the SAMs. Based on these design strategies for organic-metal oxide superlattices, we have successfully fabricated various superlattices composed of metal oxides (Al-, Hf-, Mg-, Sn-, Ti-, and Zr oxides) and their multilayered structures.

Breath odor-based individual authentication by an artificial olfactory sensor system and machine learning.

Breath odor sensing-based individual authentication was conducted for the first time using an artificial olfactory sensor system. Using a 16-channel chemiresistive sensor array and machine learning, a mean accuracy of >97% was successfully achieved. The impact of the number of sensors on the accuracy and reproducibility was also demonstrated.

Correction: Breath odor-based individual authentication by an artificial olfactory sensor system and machine learning.

Correction for 'Breath odor-based individual authentication by an artificial olfactory sensor system and machine learning' by Chaiyanut Jirayupat et al., Chem. Commun., 2022, DOI: https://doi.org/10.1039/D1CC06384G.

Nanocellulose Paper Semiconductor with a 3D Network Structure and Its Nano-Micro-Macro Trans-Scale Design.

Semiconducting nanomaterials with 3D network structures exhibit various fascinating properties such as electrical conduction, high permeability, and large surface areas, which are beneficial for adsorption, separation, and sensing applications. However, research on these materials is substantially restricted by the limited trans-scalability of their structural design and tunability of electrical conductivity. To overcome this challenge, a pyrolyzed cellulose nanofiber paper (CNP) semiconductor with a 3D network structure is proposed. Its nano-micro-macro trans-scale structural design is achieved by a combination of iodine-mediated morphology-retaining pyrolysis with spatially controlled drying of a cellulose nanofiber dispersion and paper-crafting techniques, such as microembossing, origami, and kirigami. The electrical conduction of this semiconductor is widely and systematically tuned, via the temperature-controlled progressive pyrolysis of CNP, from insulating (1012 Ω cm) to quasimetallic (10-2 Ω cm), which considerably exceeds that attained in other previously reported nanomaterials with 3D networks. The pyrolyzed CNP semiconductor provides not only the tailorable functionality for applications ranging from water-vapor-selective sensors to enzymatic biofuel cell electrodes but also the designability of macroscopic device configurations for stretchable and wearable applications. This study provides a pathway to realize structurally and functionally designable semiconducting nanomaterials and all-nanocellulose semiconducting technology for diverse electronics.

Edge-Topological Regulation for in Situ Fabrication of Bridging Nanosensors.

In situ fabrication of well-defined bridging nanostructures is an interesting and unique approach to three-dimensionally design nanosensor structures, which are hardly attainable by other methods. Here, we demonstrate the significant effect of edge-topological regulation on in situ fabrication of ZnO bridging nanosensors. When employing seed layers with a sharp edge, which is a well-defined structure in conventional lithography, the bridging angles and electrical resistances between two opposing electrodes were randomly distributed. The stochastic nature of bridging growth direction at the sharp edges inherently causes such unintentional variation of structural and electrical properties. We propose an edgeless seed layer structure using a two-layers resist method to solve the above uncontrollability of bridging nanosensors. Such bridging nanosensors not only substantially improved the uniformity of structural and electrical properties between two opposing electrodes but also significantly enhanced the sensing responses for NO2 with the smaller variance and the lower limit of detection via in situ controlled electrical contacts.

Impact of Lateral SnO2 Nanofilm Channel Geometry on a 1024 Crossbar Chemical Sensor Array.

We propose a rational strategy to fabricate thermally robust, highly integrated molecular and gas sensors utilizing a lateral SnO2 nanofilm channel geometry on a 1024 crossbar sensor array. The proposed lateral channel geometry substantially suppresses the detrimental effects of parasitic interconnect wire resistances compared with those of a conventional vertical sandwich-type crossbar array because of its excellent resistance controllability. A conductive oxide top-contact electrode on the lateral SnO2 nanofilm channel enhances the thermal stability at temperatures of up to 500 °C in ambient air. Integrating this lateral SnO2 nanofilm geometry with analog circuits enables the operation of a 1024 crossbar sensor array without selector devices to avoid sneak currents. The developed 1024 crossbar sensor array system detects the local spatial distribution of the molecular gas concentration. The spatial data of molecular concentrations include molecule-specific data to distinguish various volatile molecules based on their vapor pressures. Thus, this integrated crossbar sensor array system using lateral nanofilm geometry offers a platform for robust, reliable, highly integrated molecular and gas sensors.

Surface Dissociation Effect on Phosphonic Acid Self-Assembled Monolayer Formation on ZnO Nanowires.

Understanding the formation process of self-assembled monolayers (SAMs) of organophosphonic acids on ZnO surfaces is essential to designing their various applications, including solar cells, heterogeneous catalysts, and molecular sensors. Here, we report the significant effect of surface dissociation on SAM formation of organophosphonic acids on single-crystalline ZnO nanowire surfaces using infrared spectroscopy. When employing the most conventional solvent-methanol (relative permittivity εr = 32.6), the production of undesired byproducts (layered zinc compounds) on the surface was identified by infrared spectral data and microscopy. On the other hand, a well-defined SAM structure with a tridentate coordination of phosphonic acids on the surface was confirmed when employing toluene (εr = 2.379) or tert-butyl alcohol (εr = 11.22-11.50). The observation of layered zinc compounds as byproducts highlights that the degree of Zn2+ dissociation from the ZnO solid surface into a solvent significantly affects the surface coordination of phosphonic acids during the SAM formation process. Although the ZnO nanowire surface (m-plane) is hydrophilic, the present results suggest that a weaker solvent polarity is preferred to form well-defined phosphonic acid SAMs on ZnO nanowire surfaces without detrimental surface byproducts.

Water-Selective Nanostructured Dehumidifiers for Molecular Sensing Spaces.

Humidity and moisture effects, frequently called water poisoning, in surroundings are inevitable for various molecular sensing devices, strongly affecting their sensing characteristics. Here, we demonstrate a water-selective nanostructured dehumidifier composed of ZnO/TiO2/CaCl2 core-shell heterostructured nanowires for molecular sensing spaces. The fabricated nanostructured dehumidifier is highly water-selective without detrimental adsorptions of various volatile organic compound molecules and can be repeatedly operated. The thermally controllable and reversible dehydration process of CaCl2·nH2O thin nanolayers on hydrophilic ZnO/TiO2 nanowire surfaces plays a vital role in such water-selective and repeatable dehumidifying operations. Furthermore, the limitation of detection for sensing acetone and nonanal molecules in the presence of moisture (relative humidity ∼ 90%) was improved more than 20 times using nanocomposite sensors by operating the developed nanostructured dehumidifier. Thus, the proposed water-selective nanostructured dehumidifier offers a rational strategy and platform to overcome water poisoning issues for various molecular and gas sensors.

Mechanistic Approach for Long-Term Stability of a Polyethylene Glycol-Carbon Black Nanocomposite Sensor.

Polymer-carbon nanocomposite sensor is a promising molecular sensing device for electronic nose (e-nose) due to its printability, variety of polymer materials, and low operation temperature; however, the lack of stability in an air environment has been an inevitable issue. Here, we demonstrate a design concept for realizing long-term stability in a polyethylene glycol (PEG)-carbon black (CB) nanocomposite sensor by understanding the underlying phenomena that cause sensor degradation. Comparison of the sensing properties and infrared spectroscopy on the same device revealed that the oxidation-induced consumption of PEG is a crucial factor for the sensor degradation. According to the mechanism, we introduced an antioxidizing agent (i.e., ascorbic acid) into the PEG-CB nanocomposite sensor to suppress the PEG oxidation and successfully demonstrated the long-term stability of sensing properties under an air environment for 30 days, which had been difficult in conventional polymer-carbon nanocomposite 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.

Image Processing and Machine Learning for Automated Identification of Chemo-/Biomarkers in Chromatography-Mass Spectrometry.

We present a method named NPFimg, which automatically identifies multivariate chemo-/biomarker features of analytes in chromatography-mass spectrometry (MS) data by combining image processing and machine learning. NPFimg processes a two-dimensional MS map (m/z vs retention time) to discriminate analytes and identify and visualize the marker features. Our approach allows us to comprehensively characterize the signals in MS data without the conventional peak picking process, which suffers from false peak detections. The feasibility of marker identification is successfully demonstrated in case studies of aroma odor and human breath on gas chromatography-mass spectrometry (GC-MS) even at the parts per billion level. Comparison with the widely used XCMS shows the excellent reliability of NPFimg, in that it has lower error rates of signal acquisition and marker identification. In addition, we show the potential applicability of NPFimg to the untargeted metabolomics of human breath. While this study shows the limited applications, NPFimg is potentially applicable to data processing in diverse metabolomics/chemometrics using GC-MS and liquid chromatography-MS. NPFimg is available as open source on GitHub (http://github.com/poomcj/NPFimg) under the MIT license.

Fabrication of a Robust In2O3 Nanolines FET Device as a Biosensor Platform.

Field-effect transistors (FETs) are attractive biosensor platforms for rapid and accurate detection of various analytes through surface immobilization of specific bio-receptors. Since it is difficult to maintain the electrical stability of semiconductors of sensing channel under physiological conditions for long periods, passivation by a stable metal oxide dielectric layer, such as Al2O3 or HfO2, is currently used as a common method to prevent damage. However, protecting the sensing channel by passivation has the disadvantage that the distance between the target and the conductive channel increases, and the sensing signal will be degraded by Debye shielding. Even though many efforts use semiconductor materials directly as channels for biosensors, the electrical stability of semiconductors in the physiological environments has rarely been studied. In this work, an In2O3 nanolines FET device with high robustness in artificial physiological solution of phosphate buffered saline (PBS) was fabricated and used as a platform for biosensors without employing passivation on the sensing channel. The FET device demonstrated reproducibility with an average threshold voltage (VTH) of 5.235 V and a standard deviation (SD) of 0.382 V. We tested the robustness of the In2O3 nanolines FET device in PBS solution and found that the device had a long-term electrical stability in PBS with more than 9 days' exposure. Finally, we demonstrated its applicability as a biosensor platform by testing the biosensing performance towards miR-21 targets after immobilizing the phosphonic acid terminated DNA probes. Since the surface immobilization of multiple bioreceptors is feasible, we demonstrate that the robust In2O3 FET device can be an excellent biosensor platform for biosensors.

The impact of surface Cu2+ of ZnO/(Cu1-x Zn x )O heterostructured nanowires on the adsorption and chemical transformation of carbonyl compounds.

The surface cation composition of nanoscale metal oxides critically determines the properties of various functional chemical processes including inhomogeneous catalysts and molecular sensors. Here we employ a gradual modulation of cation composition on a ZnO/(Cu1-x Zn x )O heterostructured nanowire surface to study the effect of surface cation composition (Cu/Zn) on the adsorption and chemical transformation behaviors of volatile carbonyl compounds (nonanal: biomarker). Controlling cation diffusion at the ZnO(core)/CuO(shell) nanowire interface allows us to continuously manipulate the surface Cu/Zn ratio of ZnO/(Cu1-x Zn x )O heterostructured nanowires, while keeping the nanowire morphology. We found that surface exposed copper significantly suppresses the adsorption of nonanal, which is not consistent with our initial expectation since the Lewis acidity of Cu2+ is strong enough and comparable to that of Zn2+. In addition, an increase of the Cu/Zn ratio on the nanowire surface suppresses the aldol condensation reaction of nonanal. Surface spectroscopic analysis and theoretical simulations reveal that the nonanal molecules adsorbed at surface Cu2+ sites are not activated, and a coordination-saturated in-plane square geometry of surface Cu2+ is responsible for the observed weak molecular adsorption behaviors. This inactive surface Cu2+ well explains the mechanism of suppressed surface aldol condensation reactions by preventing the neighboring of activated nonanal molecules. We apply this tailored cation composition surface for electrical molecular sensing of nonanal and successfully demonstrate the improvements of durability and recovery time as a consequence of controlled surface molecular behaviors.

Maximizing Conversion of Surface Click Reactions for Versatile Molecular Modification on Metal Oxide Nanowires.

Click reactions (e.g., Huisgen cycloaddition) on metal oxide nanostructures offer a versatile and robust surface molecular modification for various applications because they form strong covalent bonds in a wide range of molecular substrates. This study reports a rational strategy to maximize the conversion rate of surface click reactions on single-crystalline ZnO nanowires by monitoring the reaction progress. p-Polarized multiple-angle incidence resolution spectrometry (pMAIRS) and Fourier-transformed infrared (FT-IR) spectroscopy were employed to monitor the reaction progress of an azide-terminated self-assembled monolayer (SAM) on single-crystalline ZnO nanowires. Although various reaction parameters including the concentration of Cu(I) catalysts, triazolyl ligands, solvents, and target alkynes were systematically examined for the surface click reactions, 10-30% of terminal azide on the nanowire surface remained unreacted. Temperature-dependent FT-IR measurements revealed that such unreacted residual azides deteriorate the thermal stability of the nanowire molecular layer. To overcome this observed conversion limitation of click reactions on nanostructure surfaces, we considered the steric hindrance around the closely packed SAM reaction points, then experimented with dispersing the azide moiety into a methyl-terminated SAM. The mixed-SAM method significantly improved the azide conversion rate to almost 100%. This reaction method enables the construction of spatially patterned molecular surface modifications on metal oxide nanowire arrays without detrimental unreacted azide groups.

Rational Strategy for Space-Confined Seeded Growth of ZnO Nanowires in Meter-Long Microtubes.

Seeded crystal growths of nanostructures within confined spaces offer an interesting approach to design chemical reaction spaces with tailored inner surface properties. However, such crystal growth within confined spaces tends to be inherently difficult as the length increases as a result of confinement effects. Here, we demonstrate a space-confined seeded growth of ZnO nanowires within meter-long microtubes of 100 µm inner diameter with the aspect ratio of up to 10 000, which had been unattainable to previous methods of seeded crystal growths. ZnO nanowires could be grown via seeded hydrothermal crystal growth for relatively short microtubes below the length of 40 mm, while any ZnO nanostructures were not observable at all for longer microtubes above 60 mm with the aspect ratio of 600. Microstructural and mass spectrometric analysis revealed that a conventional seed layer formation using zinc acetate is unfeasible within the confined space of long microtubes as a result of the formation of detrimental residual Zn complex compounds. To overcome this space-confined issue, a flow-assisted seed layer formation is proposed. This flow-assisted method enables growth of spatially uniform ZnO nanowires via removing residual compounds even for 1 m long microtubes with the aspect ratio of up to 10 000. Finally, the applicably of ZnO-nanowire-decorated long microtubes for liquid-phase separations was demonstrated.

Self-Anti-Stacking 2D Metal Phosphide Loop-Sheet Heterostructures by Edge-Topological Regulation for Highly Efficient Water Oxidation.

2D metal phosphide loop-sheet heterostructures are controllably synthesized by edge-topological regulation, where Ni2 P nanosheets are edge-confined by the N-doped carbon loop, containing ultrafine NiFeP nanocrystals (denoted as NiFeP@NC/Ni2 P). This loop-sheet feature with lifted-edges prevents the stacking of nanosheets and induces accessible open channels for catalytic site exposure and gas bubble release. Importantly, these NiFeP@NC/Ni2 P hybrids exhibit a remarkable oxygen evolution activity with an overpotential of 223 mV at 20 mA cm-2 and a Tafel slope of 46.1 mV dec-1 , constituting the record-high performance among reported metal phosphide electrocatalysts. The NiFeP@NC/Ni2 P hybrids are also employed as both anode and cathode to achieve an alkaline electrolyzer for overall water splitting, delivering a current density of 10 mA cm-2 with a voltage of 1.57 V, comparable to that of the commercial Pt/C||RuO2 couple (1.56 V). Moreover, a photovoltaic-electrolysis coupling system can as well be effectively established for robust overall water splitting. Evidently, this ingenious protocol would expand the toolbox for designing efficient 2D nanomaterials for practical applications.

Artificial visual systems enabled by quasi-two-dimensional electron gases in oxide superlattice nanowires.

Rapid development of artificial intelligence techniques ignites the emerging demand on accurate perception and understanding of optical signals from external environments via brain-like visual systems. Here, enabled by quasi-two-dimensional electron gases (quasi-2DEGs) in InGaO3(ZnO)3 superlattice nanowires (NWs), an artificial visual system was built to mimic the human ones. This system is based on an unreported device concept combining coexistence of oxygen adsorption-desorption kinetics on NW surface and strong carrier quantum-confinement effects in superlattice core, to resemble the biological Ca2+ ion flux and neurotransmitter release dynamics. Given outstanding mobility and sensitivity of superlattice NWs, an ultralow energy consumption down to subfemtojoule per synaptic event is realized in quasi-2DEG synapses, which rivals that of biological synapses and now available synapse-inspired electronics. A flexible quasi-2DEG artificial visual system is demonstrated to simultaneously perform high-performance light detection, brain-like information processing, nonvolatile charge retention, in situ multibit-level memory, orientation selectivity, and image memorizing.

Oxygen-Induced Reversible Sn-Dopant Deactivation between Indium Tin Oxide and Single-Crystalline Oxide Nanowire Leading to Interfacial Switching.

An impurity doping in semiconductors is an important irreversible process of manipulating the electrical properties of advanced electron devices. Here, we report an unusual reversible dopant activation/deactivation phenomenon, which emerges at an interface between indium tin oxide (ITO) and single-crystalline oxide channel. We found that the interface electrical resistance between ITO electrodes and single-crystalline oxide nanowire channel can be repeatedly switched between a metallic state and a near-insulative state by applying thermal treatments in air or vacuum. Interestingly, this electrical switching phenomenon disappears when the oxide nanowire changes from the single-crystalline structure to the lithography-defined polycrystalline structure. Atmosphere-controlled annealing experiments reveal that atmospheric oxygen induces repeatable change in the interfacial electrical resistance. Systematic investigations on metal cation species and channel crystallinity demonstrate that the observed electrical switching is related to an interface-specific reversible Sn-dopant activation/deactivation of ITO electrode in contact with a single-crystalline oxide channel.