Symposium EN15 : Nanomaterials for Sensing and Control of Energy Systems—Processing, Characterization and Theory

Perovskite metal oxides have been studied thoroughly in the literature as electrode materials for solid oxide fuel cells, which necessitate stability at high-temperature and in chemically harsh environments. The stability of the perovskite crystal structure under high concentrations of dopants allows for a great deal of versatility in material design, due to the unique electrical, magnetic, and optical properties exhibited by materials within this family. The incorporation of metallic nanoparticles into metal oxide thin films has also been previously demonstrated as a method for introducing a strong, surface-sensitive optical absorption, which can be tracked for harsh environment sensing on the optical fiber platform.
In this work, the gas and temperature sensing properties will be presented for several perovskite oxide thin films (i.e., LST, LSM, LSF, LSC) incorporated with gold nanoparticles, measured under both oxidizing and reducing conditions at high temperature (up to 800 C). On their own, the optical absorption spectrum of these perovskite oxide films is governed by oxygen-defect sensitive interband transitions, in some cases extending into the visible and NIR, and the free-carrier absorption characteristic of other conducting metal oxides. Using modeling and in-situ optical transmission measurements at high temperature, the interaction between the nanoparticle localized surface-plasmon resonance (LSPR) and the unique properties of the individual host films will be examined. The sensing response is further studied using principal component analysis (PCA) to demonstrate improved sensing discrimination of several parameters of interest (i.e., temperature and gas concentration). The combined versatility of the perovskite oxide class of materials and LSPR-based sensing will be discussed as a tool for developing optical sensors able to provide “electronic nose”-like discrimination, suitable for harsh environment

Commonly adopted resistive-type chemical sensors are based on metal oxide semiconductors, which usually display significant cross-sensitivities for different gaseous analytes, resulting in a low selectivity.¹ By collecting sensing signals from a number of different individual sensors, so called a sensor array, the selectivity could be remarkably enhanced.² However, complex and costly fabrication protocol is usually associated with integration of such an sensor array, limiting their applications , especially upon operation under extreme environments. Specifically, the complexity is due to the increased number of dissimilar sensor devices in need of complex electrical wiring, contacts, as well as electrodes, which may dramatically increase the risk of potential system malfunction. Besides, the bulky size and shortened service life resulted from complex circuitry may increase the cost of deployment, operation, and maintenance. In this study, a new pattern-recognition strategy has been developed based on correlation of resistance-metric mode and impedance-metric mode for differentiation of multiple gas analytes using a single ZnO nanorod array (nano-array) gas sensor. Uniform ZnO nano-arrays were grown on silicon substrate using a microwave-assisted hydrothermal synthesis method. The characteristic signature for the target analyte is successfully extracted by incorporating the resistive response and frequency-dependent dielectric response via global Electrochemical Impedance Spectroscopy (EIS). Targeting differentiation of three different oxidative gases, i.e., NO₂, SO₂ and O₂, a database was established using the sensitivities characterized from three sensing data sets from resistance-metric mode, real part and imaginary part of the impedance-metric mode. These three oxidative gas analytes have been successfully differentiated through a principal component analysis (PCA) practice. Linear and non-linear decision boundaries have been computationally built for pattern recognition and analytes quantification utilizing machine neural network training. A facile user interface (UI) for recognition of unknown gas analyte has been developed using the MATLAB codes, with the error of analyte concentration achieved as low as 2%. The identification of test analytes beyond the available training data sets could be accomplished using a deep learning training-set; the error of corresponding concentration prediction is around 4.5%. In addition to the single analyte analysis, the on-board interrogation could be achieved towards gas mixture (i.e. NO and NO₂) through correlation between the electrical and electrochemical responses. According to the in-situ DRIFTS spectra, no mutual interaction occurs between the NO and NO₂ in the co-adsorption scenario. As such, a pair of equations could be derived between gas mixture responses and components’ concentration on the basis of charge carrier concentration. Facilitated by a graphical method, the detailed concentration of component gas could be quantified using the measured responses of NO_x mixture in the bimodular sensing on such a single nano-array sensor platform.
Reference
[1] Palacios, Manuel A., et al. "Supramolecular chemistry approach to the design of a high-resolution sensor array for multi-anion detection in water." Journal of the American Chemical Society 129.24 (2007): 7538-7544.
[2]. Kumar, Vinod, et al. "Classification and quantification of binary mixtures of gases/odors using thick-film gas sensor array responses." IEEE Sensors Journal 15.2 (2015): 1252-1260.
[3]. Zhang, B.; Sun, J.; Gao, P.-X., to be submitted, 2019.

Air quality sensors and sensors in measuring systems detect toxic and explosive gases and warn against the escape of hazardous pollutants. Today's sensors have a comparatively low sensitivity and a large cross-sensitivity for many applications with high power consumption and high costs.
Actually, established nano-techniques use nanoparticles or nanowires that are processed at high temperatures separately from the substrate. Subsequently, these particles are suspended in liquids and applied to electrodes - the electrodes are arranged on the CMOS surfaces. However, the established nano-techniques provide only disordered structures. In addition, there are problems in the reproducibility of production.
"FunALD" is now taking a significant step further: the nano-electrodes are not produced by applying nanoparticles, but based on the ALD method (Atomic Layer Deposition). Ultrathin single and multiple layers with a layer thickness of less than 50 nm are deposited monolayer after monolayer. With the ALD method, layers with exact thicknesses and tunable electrical properties can be deposited. Ultrasensitive self-supporting nanowires, made of metal oxide and produced in a sacrificial layer process, are used as the sensing component.

As a first step we investigated the oxygen diffusion within tin oxide as sensing material. The sensor was heated up from room temperature to 313 K to evaporate surface water from the sensor surface. Resistance measurements reveal an increase of conductivity by a factor of about 10 during this process. After that process, the temperature was cycled between 313 K and 353 K and the resistance was measured. Within this temperature range an increase of conductivity with increasing temperature is observed as it is expected for a semi conducting material. An activation energy of 58 meV has been calculated from an Arrhenius representation and linear regression of the data for the charge carrier transport process.
The oxygen diffusion capability of the sensing material was tested under N₂ atmosphere at 428 K. For this purpose the sensitivity S=R_N2/R₀ was measured for 50 minutes, where R_N2 represents the resistance in dry N₂ und R₀ the initial value. In dry N₂, the resistance decreased reversibly by 1–2 orders of magnitude and this decrease can be attributed to increased electronic conduction associated with loss of O₂ on reducing oxygen partial pressure pO₂ in the surrounding atmosphere. S remains constant after 40 minutes, which can be attributed to a saturation of oxygen loss within the tin oxide. An immediate change of resistance was observed by introducing 400 ppm O₂ into the measurement chamber, which reveals an extremely fast increase of resistance by about 15 % with 5 s.
Finally, ALD processed tin oxide shows a remarkable high, reversible loss of oxygen, in N₂ and air at temperatures of 428 K and the changes can be attributed to sample surfaces effects, at least on short timescales.

Hydrogen-powered vehicles are part of a more global answer in order to develop a sustainable society. However, like with other energy sources, safety issues are crucial, and because of the high flammability of hydrogen gas, efficient hydrogen sensors are needed. Hydrogen sensors meeting performance targets for use in hydrogen-powered vehicles are scarce, and a lot of effort has been invested in material design to improve the sensitivity and response time of such sensors.
Palladium-based materials have been known for long for their capacity to easily form hydride phases, making them good candidates for hydrogen sensing applications. Those rely on two fundamental features of metallic palladium: the ability to dissolve hydrogen to form stable hydride and the possibility to easily release the stored hydrogen.
More than a bare protective layer, polymer coating of palladium nanoparticles has been shown to enhance sensing behavior. Using DFT, we shed light on how this phenomenon can be explained. We propose a large scope model for this system, going from the modelling of H₂ diffusion in a polymer matrix to the study of subsurface hydrogen diffusion in palladium hydride via the comprehension of the processes occuring at the polymer-metal interface.
In order to do this, the effect of several polymer coating on H₂ adsorption/desorption as well as subsurface diffusion is considered. Rationalization on how and why the polymers affect the metal NP is proposed. The diffusion of H₂ molecules in polymer environment is studied as to take into account a realistic medium for those nanoparticles. Finally, the challenging task of properly describing the metal NP-polymer interface is adressed in order to be able to give an as complete and as accurate as possible view of the whole phenomenon.

Silver nanocolloid (or nanoink), a dense suspension of ligand-encapsulated silver nanoparticles, is an important material for printing-based device production technologies. Here we present and discuss a groundbreaking printing technique that takes advantage of the unique nature of specific silver nanoinks that show self-sintering characteristics [1]. The technique, which we call as surface photo-reactive nanometal printing (SuPR-NaP), is based on chemisorption effect of silver nanoparticles on activated patterned perfluoro-polymer surfaces. The technique is extremely simple, only composed of two-step processes: fabrication of a patterned activated surface by masked vacuum ultraviolet (VUV) irradiation of the perfluoro-polymer (Cytop) layer surface, and then blade coating to expose the silver nanoinks on the surface for a short period of time at ambient conditions. Eventually, it allows easy, high-speed, and large-area manufacturing of ultrafine metal wiring with a minimum line width of 800 nm that is conductive without any post-heating treatment, and strongly adheres on substrate surfaces. These features are in striking contrast to the conventional printing technique which usually suffer from the limitations of the physisorption phenomena of the fluidic ink. The technique is applicable to the production of flexible and transparent touch screen sensor sheet composed of printed ultrafine metal mesh.
We found that the technique is possible only with a class of silver nanoinks, obtained by thermal decomposition of oxalate-bridging silver alkyl-amine complexes [2]. The peculiar nature of these nanoinks is that the high dispersion stability is preserved for several months at room temperature, whereas the silver nanoparticles are readily self-fused with each other, if the metal nanoinks are dried out, eventually exhibiting metallic conductivity at room temperature. Based on the confocal dynamic light scattering study, we recently demonstrated that these silver nanoinks are composed of a unique balance of ligand formulation and dispersant composition, and that the unique balance enables the rapid silver nanoparticle chemisorption [3,4]
[1] T. Yamada et al. Nature Commun. 7, 11402 (2016).
[2] M. Itoh et al., J. Nanosci. Nanotech. 9, 1 (2009).
[3] K. Aoshima et al. Sci. Rep. 8, 6133 (2018).
[4] Y. Hirakawa et al. under review.

Monitoring the levels of combustion gases such as H₂, CO and NO_xwithin high temperature (>300°C) environments requires multivariable sensing materials with demonstrated stability and resilience. Au nanoparticles (AuNPs) have shown potential in plasmonic gas sensing applications due to their catalytic activity, redox stability, and plasmonics sensitivity to changes in its surrounding environment. Encapsulation of the AuNPs in a variety of 2D metal oxide thin films enables both the sensitive and selective detection of target gases within a multivariable sensing array or a single sensing element system design. Au nanorods (AuNRs) embedded in metal oxides allows for the plasmonically active wavelength region to range from the visible to the NIR through variation of the AuNR aspect ratio. Furthermore, AuNRs can be designed to have both the transverse and longitudinal dipole modes as well as multipole plasmon modes for extended multivariable sensing applications within a single array element. Recent multivariable sensor platforms which utilizes Morphobutterfly wing dimensionality for bio-inspired inorganic 3D structures will also be detailed. Functionality and performance parameters of metal-metal oxide thin film coatings on 3D structures will be compared with 2D thin films within the framework of general sensor design as well as their application for the oxygen free detection of steam reformation gases.

A major challenge in environmental sensing is the detection of trace-level pollutants (e.g. carcinogenic formaldehyde¹) in complex gas mixtures. Devices based on chemo-resistive gas sensors would offer sufficient sensitivity and low detection limit down to ppb, but lack selectivity. Here, we present a highly sensitive, selective and compact sensor array for real-time quantification of formaldehyde at realistic conditions². This array consists of four nanostructured and highly porous Pt-, Si-, Pd- and Ti-doped SnO₂ sensing films directly deposited onto silicon wafer-based microsubstrates by flame spray pyrolysis (FSP). The constituent sensors offer stable responses and detection of formaldehyde down to 5 ppb (signal-to-noise ratio > 30) at breath-realistic 90% relative humidity. Each dopant induces different analyte selectivity enabling selective detection of formaldehyde in 2- and 4-analyte mixtures by multivariate linear regression. In simulated breath (FA with higher acetone, NH3 and ethanol concentrations), formaldehyde is detected with an average error ≤ 9 ppb using the present array.
In a next step, this can be improved further when combining the sensors with highly selective zeolite membranes pre-separating gas mixtures³. Zeolites - broadly applied in catalysis and gas separation - effectively separate molecules based on kinetic diameter, sorption and diffusion characteristics⁴. Therefore, zeolite membranes are suitable filters for gas sensors removing undesired species from indoor air. As proof-of-concept, a zeolite MFI/Al₂O₃ membrane is placed upstream a highly sensitive but weakly selective Pd-doped SnO₂ sensor. Their combination exhibits exceptional selectivity (>100) for formaldehyde down to 30 ppb at 90% relative humidity, outperforming state-of-the-art detectors by more than an order of magnitude. This novel concept is readily extendable to other tracers, as manifold combinations of widely tunable microporous membranes and gas sensor types can be realized in the modular sensing device. This could enable a new class of highly sensitive and selective environmental monitors.

(1) International Agency for Research on Cancer, IARC classifies formaldehyde as carcinogenic to humans. IARC: Lyon, 2004; Vol. 15.
(2) Güntner, A. T.; Koren, V.; Chikkadi, K.; Righettoni, M.; Pratsinis, S. E., ACS Sens. 2016, 1 (5), 528-535.
(3) Güntner, A. T.; Abegg, S.; Wegner, K.; Pratsinis, S. E., Sens. Actuators B 2018, 257, 916-923.
(4) Davis, M. E., Nature 2002, 417 (6891), 813-821.

1D LaFeO₃ nanofibers (NFs) have attracted research attention since they can provide higher surface area and pores for more gas diffusion channels, which allow ease of gas penetration of the sensing material. However; the gas sensing performance of pure LaFeO₃ is quite poor and doping with noble metals has shown to be an effective approach to enhance the sensing performance of LaFeO₃ . In this work; Ag-LaFeO₃ nanofibers were synthesized through the electrospinning method followed by annealing. The influence of Ag doping on crystallinity, morphology, surface area and gas sensing behavior was investigated using X-ray diffraction (XRD), Scanning electron microscope (SEM) and Brunauer-Emmett-Teller, respectively. The samples were further tested as gas sensors in ethanol atmospheres at concentrations of 2.5-50 ppm at an optimum operating temperature of 140 °C. The sensor response revealed concentration dependence behaviour with the Ag-LaFeO₃ based sensor showing higher response, respectively. The enhanced response of Ag-LaFeO₃ could be attributed to the porous fibre morphology of the LaFeO₃ NFs and the catalytic activity of Ag.

Atmospheric ammonia is recognized as a key environmental pollutant with respiratory damage being a primary adverse health indicator. Catalytic conversion is commonly used in automobiles to reduce gas pollution during vehicle operation. Unfortunately, ammonia is one of the byproducts of reduction of NOx during the process, which is often referred to in the industry as "ammonia slip". Released ammonia participates in the oxidation of SO2 into SO3, which reacts with atmospheric water vapor to form sulfuric acid fumes, and also forms ammonium sulfate which can clog flue lines and filters. In this work, we report on the development of a one-time electronic ammonia fuse. Formic and citric acids were reacted with ammonium vanadate precursor to synthesize vanadium oxides. Powder X-ray diffraction (PXRD) confirmed the formation of the orthorhombic and monoclinic crystal structure of V₂O₅ and VO₂ respectively. Energy dispersive x-ray analysis (EDX) confirms the purity of the samples. Using transmission electron microscopy (TEM), we inferred that the average diameter of vanadium oxides particles was in the range of 90-120 nm and some small size particles ~70-90nm are also observed. Chemical composition of the synthesized vanadium oxides was confirmed using Raman spectroscopy. V₂O₅ shows Raman shift at 993 cm⁻¹, 698 cm⁻¹, 478 cm⁻¹, 281 cm⁻¹ and 404 cm⁻¹which can be attributed to the V=O bonding, stretching modes of triple-bonded oxygen, stretching mode of V−O−V, and bending vibrations of the V=O bonds respectively. VO₂ show a weak hump at 224 cm^-1 which can be attributed to motions of V, highlighting the phase transition of monoclinic phase to rutile phase. Commercial silicon dioxide wafers (University Wafers) were cleaned (RCA) and a bottom metal stack was deposited using DC sputtering (Cr) and RF sputtering (Au). Using the synthesized V_xO_y (x=1,2, and y= 2,5) species, a film of the oxide was deposited using spin-coating. Top contact (Ag) was deposited using thermal evaporation to complete a metal insulator metal structure. Current-voltage (IV) characterization of devices (Keithley 4200-SCS) showed an on/off ratio of 10² and less than 10¹ respectively for V₂O₅ and VO_2. On exposing these devices to ammonia solution (boiling point 24^○C) by introducing 5-10 µl of ammonia (30%) solution at 60-70^○C, the IV characteristic of the device started exhibiting a distorted waveform, without any retention, possibly due to the effect of the corrosive ammonia. We are at present establishing the minimal exposure level of ammonia to achieve the destruction of the device. In deployment, the exhaust from the catalytic converter would be monitored by this sensor to provide feedback to the automotive control to reduce ammonia emissions from the vehicle.

Environmental pollution as a result of rapid urbanization and industrialization and its indirect effect on global warming, climate change, and ocean acidification has been of increasing scientific concern worldwide over the past few decades. Carbon dioxide (CO₂) is one of the many such toxic pollutants that contribute to environmental pollution, thus stimulating extensive research and concerted efforts for its detection, monitoring, capture, and storage. However, the development of metal oxide semiconductor (MOS) based heterostructures with controlled structural motifs such as a large number of active nanointerfaces for low temperature (sub 200^oC) CO₂ sensing applications continues to be an alluring task. Herein, we report on the highly selective CO₂ sensing performance of CaO-ZnO heterostructures achieved using an elegant strategy of calcium hydroxide (Ca(OH)₂) mediated transformation of zinc hydroxide carbonate (Zn₅(CO₃)₂(OH)₆) to zinc oxide (ZnO) at 50^oC. FE-SEM analysis of zinc hydroxide carbonate powder revealed hierarchical microspheres (referred here as ZMS) with diameters of 3–8 μm, primarily composed of numerous thin sheets (thickness ~10 nm) interleaved to form floral architectures. Feasibility of the synthetic route was probed by comparing CO₂ sensing characteristics of CaO-ZnO heterostructures achieved at 50^oC for 12 h with the ones obtained by heat treatment at 600^oC for 2 h and referred to as Ca-ZMS and CaO-ZnO, respectively.

Case 1: Synthesis of Ca-ZMS hierarchical heterostructures
Briefly, ca. 67 mg of as-synthesized ZMS was added to 40 ml of water in a beaker and sonicated for 60 min. To this solution, 33 mg of Ca(OH)₂ was added to the prepared solution containing 100 mg/40 mL Ca-Zn-C-O-H complex mixture. The calculated amount of Ca(OH)₂ varied based on weight percentages. The complex mixture solution was then continuously stirred for 12 h followed by drying overnight at 50^oC using a hotplate and will be referred to as “R-Ca-ZMS” (where R = Ca(OH)₂ concentration in wt.%).

Case 2: Synthesis of Ca-ZnO hierarchical heterostructures
Ca-Zn-C-O-H complex mixture (100 mg/40 mL) was sonicated for 30 mins followed by centrifuging and drying overnight at 70^oC in an ambient air atmosphere using a furnace. Subsequently, the dried Ca-Zn-C-O-H mixture was weighed (20 mg) and calcined at 600^oC for 2 h in a muffle furnace under air atmosphere and post-calcination will be referred to as “R-CaO-ZnO” (where R = Ca(OH)₂ concentration in wt.% added before the calcination). Calcination at high temperature was deemed necessary to completely convert Zn₅(CO₃)₂(OH)₆ to ZnO and Ca(OH)₂ to CaO with an increased adhesion among the singularities.

Pure ZnO, Zn₅(CO₃)₂(OH)₆, Ca(OH)₂ and CaO microsensors exhibited low sensitivity (≤20%) towards 500 ppm of CO₂, whereas the CaO-ZnO heterostructures from both the cases exhibited significant sensitivity (from 26 to 91%) towards CO₂ concentration in the 100-10000 ppm range, respectively, when operated at 150^oC. In this study, 25Ca-ZMS heterostructures exhibited promising sensitivity of 77% and a superior selectivity of 98%, while 25CaO-ZnO heterostructures revealed lower sensitivity of 59% and selectivity of 87% upon exposure to CO₂ gas (500 ppm). The gas sensing mechanism in dry and humid conditions underlying the high selectivity in presence of 6 analyte gases that commonly coexist with CO₂ (such as methane, ethanol, hexane, H₂, CO, NO₂, SO₂ etc) and faster kinetics (response and recovery times of 230 s and 280 s to 100 ppm) compared to control samples as well as to the ones reported in the literature is detailed. Improved performance is attributed to the low-temperature synthesis route which results in a gas-accessible hierarchical morphology. Additionally, the high CO₂ adsorption capacity of CaO combined with the compatibility of CaO-ZnO in forming highly active n-n nanointerfaces is a promising step towards developing a precise and cost-effective CO₂ gas microsensor.

The terahertz frequency range (0.1-10THz) provides the perfect probe for investigating electronic processes within these nanomaterials. The energy range of terahertz radiation extends over the energies of several typical quasiparticles, such as free electrons and holes, plasmons, magnons and polarons, and can stimulate collective excitations, such as optical phonons. Terahertz spectroscopy therefore forms a powerful, non-contact, non-destructive technique for measuring electrical conductivity in advanced functional nanomaterials.
In this talk, I will demonstrate how terahertz spectroscopy can reveal the ultrafast carrier dynamics of an ensemble of semiconductor nanowires on ultrafast (<1ps) timescales, allowing characterisation of photoconductivity lifetimes, mobility and doping levels1,2. This technique will directly be applied to InAsSb nanowires to demonstrate the effects of alloying on optoelectronic properties and elucidate the scattering mechanisms governing their carrier transport. I will show that these nanowires have a record-holding electron mobility due to a reduction in defect density and impurity scattering, making them promising candidates for MIR photodetectors and thermoelectric devices3. Following this, I will also introduce near-field THz microscopy, which allows direct imaging of key optoelectronic properties on nanometre length scales with surface sensitivity, as well as direct electrical probing within a device architecture4,5. I will present recent results on topological insulator thin films, highlighting the capability of this technique. Finally, I will show how these techniques can be used to develop novel THz devices based on semiconductor nanowires: single-nanowire THz detectors6,7 and THz modulators8. These devices are promising candidates for active components in THz imaging devices, shrinking down current THz technologies down by 3 orders of magnitude accelerating the development of nanoscale/microscale THz sensing systems.
1. Joyce, H. J., Boland, J. L., et al. A review of the electrical properties of semiconductor nanowires: Insights gained from terahertz conductivity spectroscopy. Semicond. Sci. Technol. 31, 1–21 (2016).
2. Boland, J. L. et al. Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility. Nano Lett. 15, 1336–1342 (2015).
3. Boland, J. L. et al. High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires. Nano Lett. 18, 3703–3710 (2018).
4. Mooshammer, F. et al. Nanoscale Near-Field Tomography of Surface States on (Bi0.5Sb0.5)2Te 3. Nano Lett. acs.nanolett.8b03008 (2018). doi:10.1021/acs.nanolett.8b03008
5. Eisele, M. et al. Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution. Nat. Photonics 8, 841–845 (2014).
6. Peng, K. et al. Broadband Phase-Sensitive Single InP Nanowire Photoconductive Terahertz Detectors. Nano Lett 16, 4925–4931 (2016).
7. Peng, K. et al. Single n + -i-n + InP nanowires for highly sensitive terahertz detection. Nanotechnology 28, 125202 (2017).
8. Boland, J.L. et al. An ultrafast switchable terahertz polarization modulator based on III--V semiconductor nanowires. Nano Lett. 17, 2603–2610 (2017).

–The organic electronic field is nowadays collecting increasing interest by both the scientific and industrial world. Compared to the inorganic counterparts, Organic Thin Film Transistors (OTFT), show some important advantages such as flexibility and stretchability but, most importantly, the compatibility with low-cost high-throughput printing-based manufacturing: all these advantages are mainly due to the use of Organic Semiconductors (OSCs).
OSCs’ properties like, for example, solution processability and easy molecular structure’s tailoring, open a wide range of applications that could not be covered by traditional electronic devices, such as into the medical field with biocompatible sensors, into the consumable electronic field with flexible and infrangible screens, into the wearable electronics field, or into the smart tags and distributed sensors fields.
On the other side, the stability of these materials and the overall performances of the Organic Field Effect Transistors (OFETs), mainly in terms of operational speed, should be improved. The figure of merit widely adapted to evaluate the operational speed of a single transistor is called Transition Frequency (f_t) and is defined as the frequency at which the current gain of a transistor, in short circuit condition, is equal to 1. At the state of the art, the highest f_t measured on planar OFET made by techniques compatible to large-printing is 20 MHz^[1], while on a vertical transistor is 40MHz^[2]. In this contest, the aim of this work is to overcome this limit, bringing the transistors to operational frequencies in the order of hundreds of MHz or of the GHz, keeping the flexibility requirements.
In this landscape, to maintain flexibility and guarantee good performances together, bottom-contacts top-gate OFETs have been fabricated, mainly on flexible and very thin substrates of Polyethylene Naphthalate (PEN) (around 100μm thin and a rms roughness around 50nm) by means of several different techniques: lithography and laser sintering has been used to make bottom contacts, bar coating and/or spin coating have been used to deposit the organic semiconductor and the dielectric layers, thermal evaporation or inkjet printing have been used to deposit the gate electrode. Once the lithography process and the laser sintering have been optimized on plastic substrate, different kind of organic semiconductors and dielectric materials have been used. For example, among the n–type semiconductor, the poly {[N,N′– bis (2–octyldodecyl) – 1,4,5,8 – naphthalene dicarboximide – 2,6 – diyl]–alt–5,5′–(2,2′–bithiophene)} ((P(NDI2OD–T2)) has been optimized and tested on short channel devices, obtaining a directly measured transition frequency of 11MHz. Among the p–type semiconductor materials, optimizations have been carried out on the diketopyrrolopyrrolethieno [3,2–b] thiophene copolymer (DPP–T–TT), obtaining a directly measured transition frequency of 22MHz. This represents an important result because it is the best frequency achieved until now on flexible substrate (with respect to the 14MHz obtained by Perinot et al.^[3]).

[1] “Direct-written polymer field-effect transistors operating at 20 MHz”; A. Perinot et al.; Scientific Reports; 2016; DOI: 10.1038/srep3894;
[2] “A Pulse-Biasing Small-Signal Measurement Technique Enabling 40 MHz Operation of Vertical Organic Transistors”; B. Kheradmand-Boroujeni et al.; Scientific Reports; 2018; DOI:10.1038/s41598-018-26008-0;
[3] “Accessing MHz Operation at 2 V with Field-Effect Transistors Based on Printed Polymers on Plastic”; A. Perinot et al.; Advanced Science; 2018; DOI: 10.1002/advs.201801566.

With the ever-growing number of applications demanding photodetectors in the Mid-IR, ranging from astronomy, pollution monitoring and fire sensing to healthcare, security and automotive, thermal photodetectors especially bolometers gained significant research interest over the last years. Per definition, a bolometer consists of an absorptive element, which is connected to a thermal reservoir through a thermal link and a thermometer. Any radiation, which is incident on the absorptive element, will raise its temperature above that of the reservoir proportional to the radiation intensity. Quasi 1D germanium (Ge) nanowires (NWs), embedded between two electrical contacts are perfect buildings blocks for bolometers according to the above definitions. They have an extremely small thermal mass, and in addition, the same highly temperature dependent electrical resistance as bulk Ge-bolometers, which are already used since decades. Here we present a Mid-IR sensing device based on an individual aluminum (Al) contacted Ge NW on a 40 nm thick Si₃N₄ membrane. By using a thermally induced exchange reaction between the polycrystalline Al contact pads and a single crystalline Ge NW, the length and thus the thermal mass of the NW can be tuned. The resulting self-aligned single crystalline Al leads provide a highly effective heat link to the surrounding macroscopic Al metallization, which acts as the thermal reservoir. The final quasi 1D monolithic NW heterostructure thus resembles an ultrascaled Ge NW segment contacted to crystalline Al leads with atomically sharp interfaces. An electrically contacted multi-layer graphene (MLG) flake placed on the opposite side, acts as the absorber and as an electrostatic gate controlling the Fermi level and thus the intrinsic resistivity of the Ge NW. The heat generated in the absorbing MLG flake is transferred via the ultrathin Si₃N₄ membrane to the NW thus controlling the temperature dependent electrical transport in the diffusive as well as quasiballistic/ballistic for ultrascaled devices with Ge NW length down to 20 nm. In a detailed analysis we investigated the temperature dependence and transient response as well as the the influence of surface traps defining the overall functionality of the bolometer device.

Wearable electronics are becoming increasingly significant owing to their widespread and far-reaching applications in the forthcoming Internet of Things (IoTs) era, including but not limited to health monitoring, motion tracking, and interactive entertainment. The rapid development and potential market of wearable technologies make it urgent to strategically re-aligning currently predominant power supplies, i.e. batteries, considering the limitations in terms of regular recharging and replacing, size and weight, as well as environmental pollution. In this regard, alternative energy supplying technology with the merits of sustainable, maintenance-free, and eco-friendly is highly desired. Triboelectric nanogenerators (TENGs) are proven to be efficient, simple, and environmentally friendly to generate power by daily human motion. Nevertheless, it remains a challenge to improve its unsatisfactory performance under the humid condition that obstructs the practical wearable applications, e.g. being integrated into clothes or shoes, since sweating is inevitable for the body to release excessive heat routinely.

In this work, we report a fluorinated polymer sponge-based TENG (FPS-TENG), which is capable of addressing the moisture-sensing issue benefiting from intrinsic hydrophobicity of the sponge structure and chemical introduction of hydrophobic terminated functional groups, achieving a relatively stable output performance over a wide range of ambient humidity. Concurrently, the FPS-TENG is super durable even under heavy abrasion (simulated via wearing away 1mm-thickness surface layer), indicating the capability in long-term operation under harsh conditions. Specifically, the FPS-TENG is primarily assembled with electropositive copper electrodes and a critical electronegative component, i.e. polydimethylsiloxane (PDMS) based functionalized polymer sponge (FPS), which is suitable for wearable applications for its excellent shock absorption and comfortability due to the elastic property. Under a relative humidity of 40%, the open-circuit (OC) voltage, volume current density, and transfer volume charge density can reach 181 V, 0.55 A/m³, and 17.5 mC/m³ by the FPS-TENG treated at 95°C for 60min during the fluorinated process, respectively, corresponding to a volume power density of 60 W/m³, providing up to 364 % increase compared with those of pristine PDMS sponge-based TENGs (PPS-TENGs) and pristine PDMS film-based TENGs (PPF-TENGs). With the remarkable humidity resistance of FPS-TENGs, 60% electrical output is reserved from 20% RH to 85% RH, comparing to only 10% for PPF-TENGs, demonstrating the feasibility of FPS-TENGs to be applied in potential wearable applications. Last but not least, quasi-bulk-phase functionalization brings about dramatic improvement in working stability and durability than surface functionalization, by which the improvement would vanish once the functionalized surface is worn away. Herein, the enhanced performance of FPSs from functionalization is certified to degrade by only 10% after wearing away 1mm-thickness surface layer. Additionally, the FPS-TENG charges a 50μF capacitor from zero to 5 V within 10min and successfully lights up a LED panel through tapping by hands.

In the modern world of an impending energy crisis, energy saving low powered devices will become a commodity of necessity to help prevent the total depletion of the present natural resources. One of the major chunks of energy being used in today’s world is the HVAC systems installed all around, accounting for ~ 10% of the total energy generated in the United States. Low powered smart window devices are a viable alternative to help saving this major part of the energy generated. In this regard, researchers all around the globe have been working on being able to tune the Metal-to-Insulator Transition (MIT) properties in VO₂ thin films, which, because of the MIT property is a potential candidate for smart window coating applications. In this work, we report successful fabrication of VO₂ thin films despite their highly metastable nature using pulsed laser deposition by systematic optimization of the deposition and annealing oxygen partial pressures. The as-grown films were able to demonstrate a transition temperature of ~ 64 ^oC, approximately 4-6 ^oC lower than the reported transition temperature for bulk VO₂. Moreover, in this work we were able to successfully fabricate a potential smart window device structure using aluminum doped zinc oxide (Al:ZnO) thin films as a resistive heating layer which has an added advantage of being transparent. Using a conventional VO₂/Al:ZnO structure resulted in certain degradation to the properties of the films, such as decrease in the magnitude of the MIT as well as a total loss of the Joule heating in the Al:ZnO films. The fabricated device using a modified structure was able to display the MIT transition successfully, all while maintaining the Joule heating effect in the Al:ZnO thin films and the magnitude and sharpness of the MIT in the VO₂ thin films along with reducing the transition temperature of the films to ~ 60 ^oC, approximately 10 ^oC lower than bulk VO₂.The results shown in this work demonstrate the structural, electrical and optical properties of all the fabricated films as well as the multi-layered films proving the effectiveness of the films to serve as a potential standalone smart window device. In addition, the results will open up doors to incorporate different structures to the multi-layered film in order to improve the performance of the device.
Acknowledgement:
This work has been supported by NSF-CREST Grant number HRD 1036494 and NSF-CREST Grant number HRD 1547771

Wearable electronics for detecting physiological and biomechanical signals of human body are key sensors for healthcare. The vision of non-invasive, automated personalized healthcare is a new and fast-growing multidisciplinary research area. To make the sensors work independently and sustainably, self-powered sensors that can extract energy from human body motions is particularly desirable. In this talk, i’ll present our research progress on the fabrication of piezoelectret for wearable healthcare sensor. For an integrated sensor device based on a piezoelectret film, continuous reliable heartbeat and respiration information is successfully detected and transmitted to a mobile phone. The study promisingly has great influence for professional motion detecting, touch screens, artificial intelligence, mobile healthcare, and wearable electronics.

[1] W. Li, N. Wu, J. Zhong, Q. Zhong, S. Zhao, B. Wang, X. Cheng, S. Li, K. Liu, B. Hu, J. Zhou, Theoretical Study of Cellular Piezoelectret Generators, Adv. Funct. Mater. 2016, 26, 1964-1974.
[2] Q. Zhong, J. Zhong, X. Cheng, X. Yao, B. Wang, W. Li, N. Wu, K. Liu, B. Hu and J. Zhou, Paper-Based Active Tactile Sensor Array, Adv. Mater. 2015, 27, 7130-7136.
[3] N. Wu, X. Cheng, Q. Zhong, J. Zhong, W. Li, B. Wang, B. Hu, J. Zhou, Cellular Polypropylene Piezoelectret for Human Body Energy Harvesting and Health Monitoring, Adv. Funct. Mater. 2015, 25, 4788-4794.
[4] B. Wang, C. Liu, Y. J. Xiao, J. W. Zhong, W. B. Li, Y. L. Cheng, B. Hu, L. Huang, and J. Zhou, Ultrasensitive Cellular Fluorocarbon Piezoelectret Pressure Sensor for Self-Powered Human Physiological Monitoring, Nano Energy, 2017, 32, 42-49.
[5] N. Wu, S. W. Chen, S. Z. Lin, W. B. Li, Z. S. Xu, F. Yuan, L. Huang, B. Hu, and J. Zhou, Theoretical Study and Structural Optimization of a Flexible Piezoelectret-Based Pressure Sensor, J. Mater. Chem. A, 2018, 6, 5065-507

Tribotronics has attracted great attentions as a new research field about the control and tuning of semiconductor transport by triboelectricity. Here, the tribotronics is firstly reviewed for active mechanosensation and human-machine interfacing. As the fundamental component, contact electrification field-effect transistor is analyzed, in which the triboelectric potential could be used to control the electrical transport in semiconductors. On the basis, several tribotronic functional devices have been developed including tribotronic smart skin, tactile sensing array and tuning diode, which has demonstrated triboelectricity-controlled electronics and established the active mechanosensation for external environment. In addition, the triboelectric power management strategy is also reviewed, in which the triboelectricity can be managed by electronics in reverse action. With the implantation of triboelectric power management module, the harvested triboelectricity by various kinds of human kinetic and environmental mechanical energy could be effectively managed as a power source for self-powered microsystems. By the research prospects for interactions between triboelectricity and semiconductor, tribotronics is expected for significant impacts and potential applications in MEMS/NEMS, flexible electronics, robotics, wireless sensor network, and Internet of Things.

Recent interest in the fields of human motion monitoring, electronic skin and human-machine interface technology demand strain sensors with high stretchability/compressibility (ε > 50%), high sensitivity (or gauge factor ( GF > 100)) and long-lasting electromechanical compliance. However, current metal and semiconductor-based strain sensors have very low (ε < 5%) stretchability and low sensitivity (GF < 2) indicating that a new generation of strain sensors are required where the stretchability is not sacrificed for high-sensitivity. We propose a simple one-step, low cost fabrication of mechanically compliant, physically robust metallic carbon nanotube (CNT, grown via photo thermal vapour deposition method)- polydimethylsiloxane (PDMS) strain sensors. The process allows the alignment of CNTs within the PDMS elastomer permitting directional sensing. Aligning CNTs horizontally (HA-CNTs) on the substrate before embedding in the PDMS reduces the number of CNT junctions as well as introduces scale-like features on the CNT film perpendicular to the tensile strain direction, resulting in improved sensitivity compared to vertically aligned CNT -(VA-CNT)-PDMS strain sensors under tension. The CNT alignment combined with the scale-like features with CNTs bridging-over modulate the electron conduction pathway affecting the electrical sensitivity. Resulting GF are 316.1 for HA-CNT-PDMS and 140.9 for VA-CNT-PDMS sensors under 50 % tensile strains. We further show that these sensors behave differently under compressive strains. VA-CNT-PDMS show more sensitivity to small-scale deformation than HA-CNT-PDMS due to the CNT orientation and the continuous morphology of the film demonstrating that the sensing ability can be improved by aligning the CNTs in certain directions. Furthermore, mechanical robustness and electromechanical durability are tested for over 6000 cycles up to 50 % tensile and compressive strains with good frequency response with negligible hysteresis. Finally, both sensors can detect human motion induced mechanical deformations multidirectionally, both at large and small-scale such as swallowing, frowning, wrist and finger bending with ability to distinguish the motions with reaction and recovery times as low as 130 ms and 240 ms, respectively. These sensors could help people with severe disabilities and limited movement to translate small muscle movements into electrical signals that can be developed into gestures/new alphabet to help communication.

Infrared spectroscopy is a versatile tool for the identification of trace levels of various volatile chemicals which act as common pollutants. However, for applications in environmental monitoring, trace quantities of a target analyte need to be detected to make a practically useful sensor, which can be difficult using simple transmission geometries. One of the routes to enhancing the sensitivity of infrared spectroscopy is to exploit the surface enhancement of the electric field at the surface of certain materials. This is called surface enhanced infrared absorption (SEIRA) spectroscopy, and typically exploits surface polaritons which form at the surface of certain materials. Surface polaritons form when light couples to coherently oscillating charges in a material, most commonly electrons (surface plasmon polaritons, SPPs) or phonons (surface phonon polaritons, SPhPs). This type of wave can be excited in nanostructured optical antennas, which support a series of resonant modes at a sense of defined frequency. To realize SEIRA using surface polaritons the analyte is typically coated onto the antenna which supports the SPP or SPhP mode. As a consequence, one of the challenges in realizing practical SEIRA is achieving an excellent coating over the optical antennas in order to get a predictable and reproducible response.
One appealing route to achieving repeatable SEIRA results is to perform measurements directly within a liquid environment. This gives a consistent overlap between the analyte and the polaritonic material, and provides compatibility with microfluidic applications. Here we fabricate deep gratings (up to 24 µm depth) of 4H-SiC, in order to perform SEIRA within liquid environments. Our gratings support SPhP modes in the spectral region between ~792 and 972 cm^-1, coincident with the frequencies of spectral signatures of fluorocarbon pollutants. Resonant modes exhibit strong absorption of incident infrared light (>80%) and high Q factors (>90). This is due to the formation of waveguide type modes in-between the grating teeth, with strong electric field enhancement. This provides an ideal platform for SEIRA, as the high Q factor provides excellent chemical selectivity for individual chemical bonds. After characterizing our sample in air, we then characterize our gratings when submerged in a range of solvents, including water, isopropyl alcohol, acetone and toluene. We find that resonant modes are still supported in these different environments, despite infrared absorption from the medium itself. We then assess the possibility of performing SEIRA on a target molecule with an absorption band in this frequency range. Our results suggest that deep gratings offer a versatile platform for SEIRA, which could be tuned using a range of different materials to target specific molecules.

Recently research on sense of touch has been carried out in a great deal of research, and research is being conducted to detect hand movement and body movement as well as accurate implementation of strength and touch. In order to realize this, modern science not only measures and utilizes motion of one axis or independent two axes, but also detecting the motion of three or more axes to measure human motion more accurately. Therefore, it is necessary to measure the movement as well as the feedback of the device.
Strain sensors, especially flexible sensors, can be attached to the surface of a moving object or any type of object to measure the deformation of the object, so that various types of elastic strain sensors can be developed in a variety of materials and structures. As part of this research trend, we have recently developed a sensor that is very flexible but sensitive, cost-effective, has a wide measurement range and is highly applicable. With this method, our noble strain sensors can measure up to 150% strain and have fast response, good linearity, repeatability, etc.
In general, however, polymers used in strain sensors have defects due to material limitations such as mullins effect. When the strain is restored after stretching in one direction, the mechanical response of the strain-stress curve varies with the maximum load, which is a fundamental limitation of the polymer-based sensor. In order to solve this problem, the resistance information received from the high-precision strain sensor can be used to learn the position of the current polymer according to the previous resistance change using machine learning. In this way, even if the mechanical characteristics and the resistance information change depending on the maximum load, it is possible to compensate for the more accurate position tracking.
In the case of the strain sensor developed in this way, when the device is attached to a specially manufactured device, the accurate movement of the device can be predicted according to the deformation of the device. In addition, learning algorithms can be used to learn and feedback on the exact position of the polymer. This approach allows precise motion measurement by attaching a stretchable strain sensor to a multi-axis deformable joystick. This is expected to play a useful role in medical surgical robots and virtual reality, which require precise movement.

In-situ distributed sensing capabilities in high temperature energy conversion applications have been pursued in order to set up the optimized operation strategy for the best efficiency, load variation and system maintenance. Finding proper sensing materials with good sensitivity and stability to detect the concentration of target gases is another topic for the harsh environment sensing research. In this study, the optical fiber sensing platform with various oxide films is demonstrated for the gas sensing in reducing and oxidizing atmospheres. For the oxygen sensing test, we conducted material/optical characterizations and sensitivity experiments with (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM), (La_0.8Sr_0.2)_0.95CoO_3-δ (LSC) and (La_0.8Sr_0.2)_0.95Co_0.2Fe_0.8O_3-δ (LSCF) films which have been known as representative material categories for cathodes of the solid oxide fuel cell application. We also deposited a different material which has better stability and proper conductivity in reducing atmosphere and evaluated the hydrogen sensing performance in material and optical points of view. Based on the obtained sensitivity results, distributed sensor tests have been performed to check the feasibility in the final applications such as solid oxide fuel cells.

A change in an electrical resistance is a facile platform for monitoring chemical reactions. However, a reaction-induced change in resistance is usually masked by a high bulk conductivity of the sensing layer. Therefore, an ultrathin sensing layer is desired to enhance the surface sensitivity. In this study, an atomically thin film of transition metal oxide nanosheets was employed as the sensing layer to monitor surface chemical reactions. Oxides of transition metals such as iridium and ruthenium are known to possess a hydrocarbon oxidation activity. A change in an electrical resistance was clearly observed upon methane exposure, which is ascribable to an electron transfer process associated with the oxidation of methane molecules adsorbed on the nanosheets.

This study examined the geometrical, charge density topologies, electronic band structures, optical and dynamical properties VO₂ and its Mg-substituted species in the rutile and monoclinic phases using density functional theory implemented in VASP.^[1] It was found that the strongly constrained and appropriately normed meta-GGA functional SCAN^[2] could correctly predict the insulating and metallic behaviors of these materials, in appreciable agreement with the photoemission data.^[3] In particular, it was found that the singly-substituted Mg atom in VO₂ promotes significant widening of the band gap in both the phases, which is not only consistent with experiment but also explains the enhanced optics of the Mg-VO₂. Substitution of Mg in VO₂ did not contribute directly to either the conduction band or the valence band extrema, which were primarily due to the orbital contributions from the constituent V and O atoms. Although the GGA functionals, such as PBE and PBEsol, provided erroneous density of states and electronic band structures for the low-temperature phase of these materials, these were efficient in revealing the dielectric properties of both VO₂ and Mg-VO₂ in the near infrared to the visible region. From phonon band structure analyses, it was found that VO₂ in both phases is dynamically stable, but this is not so for Mg-doped VO₂.



References:
[1] (a) The Vienna Ab initio Simulation Package (VASP), Available via: https://www.vasp.at/ (accessed on June 12, 2019); (b) G. Kresse and J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B, 47, 558, (1993); (c) G. Kresse and J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium, Phys. Rev. B, 49, 14251, (1994); (d) G. Kresse and J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mat. Sci., 6, 15, (1996); (e) G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B, 54,11169, (1996).
[2] J. Sun, A. Ruzsinszky, and J. P. Perdew, Strongly Constrained and Appropriately Normed Semilocal Density Functional, Phys. Rev. Lett.,115, 036402 (2015).
[3] S. Shin , S. Suga, M. Taniguchi, M. Fujisawa, H. Kanzaki, A. Fujimori, H. Daimon, Y. Ueda, K. Kosuge, and S. Kachi, Vacuum-ultraviolet reflectance and photoemission study of the metal-insulator phase transitions in VO₂ , V₆O₁₃ , and V₂O₃ , Phys. Rev. B, 41, 4993 (1990).

Acknowledgment:
The authors gratefully thank NEDO project (Grant no. P16010) for funding and support of a postdoctoral scholarship (A.V.).

The persistent photoconductivity (PPC) effect was studied in individual tin oxide (SnO₂) nanobelts as a function of temperature and gate-voltage, in air, helium, and vacuum atmospheres, and low temperature. Photoluminescence measurements were carried out to study the optical transitions and to determine the acceptor/donors levels and their best representation inside the band gap. Under ultraviolet (UV) illumination and at temperatures in the range of 200–400 K, we observed a fast and strong enhancement of the photoconductivity, and the maximum value of the photocurrent induced increases as the temperature or the oxygen concentration decreases. By turning off the UV illumination, the induced photocurrent decays with lifetimes up to several hours. The photoconductivity and the PPC results were explained by adsorption and desorption of molecular oxygen at the surface of the SnO₂ nanobelts. On the basis of the temperature dependence of the PPC decay, an activation energy of 230 meV was found, which corresponds to the energy necessary for thermal ionization of free holes from acceptor levels to the valence band, in agreement with the photoluminescence results presented. The molecular-oxygen recombination with holes is the origin of the PPC effect in metal oxide semiconductors, so that the PPC effect is not related to the oxygen vacancies, as commonly presented in the literature [1]. Was also observed that the gate-voltage V_G can control the maximum value of the photocurrent induced up to 10⁴.

[1] E. R. Viana et. al. J. Phys. Chem. C 117, 7844–7849 (2013).

An individual tin oxide (SnO₂) nanobelt was connected in a back-gate field-effect transistor (FET) configuration and the conductivity of the nanobelt was measured at different temperatures from 400 K to 4 K, in darkness and under UV illumination. In darkness, the SnO₂ nanobelts showed semiconductor behavior for the whole temperature range measured. However, when subjected to UV illumination the photoinduced carriers were high enough to lead to a metal-to-insulator transition (MIT), near room temperature, at T_MIT = 240 K. By measuring the current I_ds versus gate-voltage V_G curves, and considering the electrostatic properties of a non-ideal conductor, for the SnO₂ nanobelt on top of a gate-oxide substrate, we estimated the capacitance per unit length, the mobility and the density of carriers. In darkness, the density was estimated to be 5–10 × 10¹⁸ cm⁻³, in agreement with our previously reported result (Phys. Status Solid. RRL 6, 262-264 (2012)). However, under UV illumination the density of carriers was estimated to be 0.2–3.8 × 10¹⁹ cm⁻³ near TMIT, which exceeded the critical Mott density estimated to be 2.8 × 10¹⁹ cm⁻³ above 240 K. These results showed that the electrical properties of the SnO₂ nanobelts can be drastically modified and easily tuned from semiconducting to metallic states as a function of temperature and light.

Published in Nanotechnology 28, 445703 (2017).

People living in modern society in the 21st century live in a place where fire does not go out 24 hours a day. These lights are sometimes helpful to people, but sometimes there are lights that cause discomfort to humans, such as light pollution. Thus, as important as the generation of light is the detection of light. The range of applications of photodetectors is very diverse. Photodetectors are also used for image sensors, missile defense, daily UV detection, environmental pollution monitoring, and optical communication. Light sensing materials have various band gaps and various structures. Among them, we used a Layered Double Hydroxides (LDHs) material that is easy to synthesize and environmentally friendly. The general formula of a LDH is [M(II)_1−xM(III)_x (OH)₂]^x+[Aⁿ⁻_x/n°yH₂O]^x−, where M(II) and M(III) are divalent and trivalent metal cations, respectively, and Aⁿ⁻ is an exchangeable n-valent anion, such as CO₃²⁻, Cl⁻, NO₃⁻, and CH₃COO⁻. In this study, we fabricated the photodetector using a material that grows in a scroll structure. The ZnAl-LDH nanosheet scrolls exhibit highly resistive semiconducting properties with a band gap of 3.2 eV and work function of 3.64 eV. The photodetector grown in a scroll structure has shown a performance suitable for fabricating flexible and reliable devices.

Selective and wide near-infrared (NIR) absorption property realized by Cs-doped hexagonal tungsten bronze (Cs-HTB) in the form of nanoparticulate dispersion has been applied to various industrial use such as solar control windows, optical filters for imaging units, agricultural heat-shading sheet, photothermal conversion materials, and others. However, the understanding of the NIR absorption mechanism of Cs-HTB nanoparticles has been still controversial among preceding studies. In our recent report, optical absorption spectra of Cs-HTB nanoparticles were analyzed with a conclusion that the crucial factors determining the optical absorption property are the anisotropic dielectric property derived from the hexagonal crystal structure and the alteration in dielectric property of nanoparticles. In this study, high energy-resolution electron energy-loss spectroscopy (EELS) electron microscope was applied to measure the anisotropic dielectric properties of coarse and nano-sized Cs-HTB particles, and also examine the possibility of alteration in dielectric property.
A mixture of cesium carbonate and tungstic acid was calcinated under reductive atmosphere to synthesize coarse Cs-HTB powders of mm size. The powder was mixed with propylene glycol monomethylether and a dispersing agent, followed by bead-milling to be pulverized into nanoscale size.
For the coarse particles, EELS spectra with momentum transfer vectors (q) along with a- and c-axes of hexagonal structure, which reflect each component of dielectric tensor, have been measured. Those EELS spectra showed plasmon peaks at 1.2 eV and 1.8 eV respectively, whose energies are close to reported values obtained by an optical measurement on bulk Cs-HTB. By using anisotropic dielectric functions derived from those spectra of coarse particles, an optical extinction coefficient was calculated based on Mie’s scattering theory. However, the calculated extinction coefficient did not reproduce the optical absorption curve of nanoparticles well even though the size and shape effects were included, which suggested the alteration in the dielectric property of nanoparticles.
For the nanoparticle measurement, different EELS spectra were obtained depending on probe positions just outside a nanoparticle facing different surface orientations. Such a dependence on probe positions should reflect each dominant component of dielectric tensor. EELS spectra obtained along a- or c-axes showed plasmon peaks at different energies of around 0.8 and 1.2eV, respectively. Those anisotropic plasmon peak energies were smaller than those energies determined of μm-sized Cs-HTB particles, which provides evidence of the alteration in dielectric properties from bulk sample to pulverized nanoparticles. It was also confirmed that energy positions of the plasmon peaks varied within the ranges of 0.6-1.1 eV for a-axis and 0.9-1.5 eV for c-axis among nanoparticles. These various plasmon energies should be due to the different degree of dielectric alterations and/or the shape effect of individual nanoparticles.

It is well known that Morpho butterflies get their intense shimmering colors from the ordered, nanostructured arrays on their wings. Using these butterflies as inspiration, we are developing optical sensors with three-dimensional (3D) nanostructures using a conventional cost-effective fabrication process of photolithography and chemical etching. These 3D nanostructures are built with vertical ridges that support several horizontal lamella. These nanostructures are coated with materials designed to be sensitive to specific gases such as hydrogen or carbon monoxide. The concentrations of these gases can be measured through changes in the intensity of the reflected light at different wavelengths upon interactions of the 3D nanostructures with analyte gases.
Development of these new sensing paradigms are needed since mature analytical techniques, including vibrational spectroscopy, are currently used because of the need of accurate quantitation of these gases. The use of these techniques is unavoidable because existing sensors have several prominent limitations such as poor dynamic range, poor stability, slow response time, and inability to accurately detect one or several gases of interest in the presence of numerous interferences and contaminants. Therefore, in the present study Au nanoparticles embedded within cerium oxide thin films have been deposited on 3D nanostructures and are evaluated for the sensitive, stable and selective detection of hydrogen and carbon monoxide, at elevated temperatures.
The future of these sensors is to simultaneously detect multiple gases, such as for solid oxide fuel cell applications where numerous gases are present (such as hydrogen, carbon monoxide, carbon dioxide, methane, and water). These sensors will be used to complement existing analytical instruments in situations when multi-point or distributed measurements are needed and as such sensors with demonstrated stability, selectivity and sensitivity will ensure a series of parallel measurements for enhanced system control.

Triboelectric nanogenerators (TENGs) technology have been one of several candidates for energy harvesting. TENGs can convert external mechanical energy into electrical energy without other electrical power source. The invented TENG technology has rapidly developed with various approaches such as energy harvesting, mechanical/chemical sensors, self-powered systems, etc. Measuring the rotating motion in mechanical motions is one of common and essential technology in industrial and manufacturing systems. We believe that our triboelectric sensors provide high potential in terms of applicability. It shows that it can be used in many ways from a point of industry, not simply from a device. Through this, we have enhanced capability of self-powered triboelectric mechanical sensors.

In this work, we have roller-bearing structure applied to triboelectric sensors. Herein a simply fabricated, cost-effective, triboelectric sensor with a roller-bearing structure which is composed of rollers and electrode is presented. By integrating the structure of bearing and TENG technology, our triboelectric bearings would be applied to a variety of mechanical areas, particularly those involving mechanical movement. Based on the triboelectric effect, this smart bearing generates the output electrical signals in response to rotation movement or displacement of an object mounted with the bearing. Notably, our self-powered bearing exhibits high output performance, compared to other TENG based on the structure of bearing, of 45 V (open-circuit voltage) and 6 μA (short-circuit current) at a rotation rate of 400 r/min. Using the output signals of the smart bearing, the angular velocity and position as well as damage in the components can be continuously monitored. Furthermore, the smart bearing shows the sensing capabilities of various angle movements. We showed that our smart bearing mounted to the steering wheel can be utilized as an energy harvester, a rotation direction detector, and a driving style sensing system at the same time. This work greatly expands the applicability of triboelectric nanogenerators as self-powered sensing systems.

The synergistic effects among different nanomaterial induces new physical and chemical properties of the composite materials which can be applied for advanced applications. In the present work, CuO nanochain structure was fabricated via wet chemical method. Composite films of CuO/ graphene oxide (GO) with different ratio of CuO and GO were synthesised by drop casting method on glass substrate, where GO in the CuO/GO films was reduced by the thermal reduction treatment. The structures and morphologies of the CuO nanostructure and composite films were investigated by X-ray diffraction (XRD), field-emission electron scanning microscopy (FESEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD patterns revealed the presence of reduced graphene oxide (rGO) sheets in the CuO nanochains by showing a broad peak of rGO along with the peaks indexed to monoclinic CuO. FESEM and TEM images of CuO nanostructure exhibited nanoparticles arranged to form chain like structure. High resolution transmission electron microscope (HRTEM) image CuO nanochains displayed the lattice fringes of spacing 0.232 nm that correspond to (111) crystalline plane of the monoclinic CuO. Selected area electron diffraction (SAED) pattern showed several concentric diffraction rings masked by spots, thus, indicating the polycrystalline nature of the nanochains. FESEM image of the composite films showed well dispersed CuO nanochains affixed onto the rGO sheets. Furthermore, XPS spectroscopy confirmed Cu to be present in Cu²⁺ state. The study on the gas sensing properties of CuO at room temperature (RT~ 25 °C) was carried out by loading different amount of rGO in the CuO nanochain sample. A maximum sensor response value of 29.1% for 20 ppm NO₂ was obtained for the optimized material ratio of CuO and rGO (4:1) which was almost four times of the sensor with ratio of CuO and rGO (1:3). Furthermore, the optimal CuO/rGO film sensor exhibited fast response rate and excellent reversibility within the detection range of 5–100 ppm NO₂. Moreover, the sensor showed good repeatability when tested over five repeated cycles of 20 ppm NO₂ at RT. The presence of local p-n heterojunctions between rGO sheets and nanochain structure of CuO was attributed to the enhanced sensor response. The present study suggest that GO addition would be an effective approach for developing RT NO₂ sensors based on CuO.

Optics of metal-dielectric composites reveals variety of new effects: surface plasmon excitation,related resonant enhancement of the electromagnetic field through multiple optical antennas, poles and nearly zero values of the dielectric coefficient (epsilon near zero, ENZ), etc. Of great interest are so called hyperbolic metamaterials, which got this name from the hyperbolic dispersion low that is realized due to the uniaxial anisotropy of a nanostructured metal-dielectric composite medium. We suggest and implement one of the ways for the composition of the hyperbolic medium based on Ag and ferromagnetic metals incorporated in the pores of the nuclear filters. The main goal was the search of the proper design of the structure for the realization of the hyperbolic dispersion.

Metal-polymer composites (arrays of nanowires (NWs) embedded in a polymer matrix) were obtained and investigated. Two types of samples were grown: pure silver NWs and silver NWs with a short layer of iron on top. NWs were obtained by galvanic deposition into the pores of etched-track matrix (track membranes with pore diameter - 30-100 nm, pore length - 12 μm, pore density 1.5*10⁹, JINR, Dubna). The filling of the pores was carried out by the two-bath technique. Two solutions were used: silver iodide (AgCl - 20 g/l; KI – 300 g/l) and ferrous sulphate (FeSO₄ * 7H₂O – 120 g/l; H₃BO₃ - 45 g/l; SDS and C₆H₈O₆). The deposition potentials were 400 mV for silver and 1200 mV for iron; the dependence of the current on time was recorded. SEM investigation of obtained NWs confirmed the calculated length: for silver part it was 500 - 3500 nm, while for iron part (cap on the silver NW) it was 100 nm.

We performed the modelling of the optical spectra of the composed samples using the analythical expressions for the Maxwell Garnett anisotropic effective medium. It was found that for all types of structures should reveal strong absorption close to 400 nm wavelength due to the local plasmon excitations, and a deep valley in the red-IR wavelength at oblique incidence associated with ENZ spectral point. In particular, for the arrays of Ag NWs (L=3 mcm, diameter-70 nm) the ENZ point corresponds to the wavelength of 1450 nm. The theoretical predictions are consistent with the experiment obtained via the transmission spectroscopy measurements, which proves high homogeneity of the structures, which can be used as hyperbolic media with properties tunable by varying the design of the NWs. We also composed similar structures consist in part of ferromagnetic metals (Ag/Fe NWs) which reveal specific magnetooptical (MO) spectral features, while their resonant behavior is controlled mostly by the geometry of Ag nanowires. The data obtained for Ag/Fe NWs are also presented and discussed. Such materials are rather perspective as MO media with tunable resonance behavior.

Acknowledgements
Grants RFBR 18-02-00830 and MK-5704.2018.2; State Task of FNIC Crystallography and Photonics.

Palladium (Pd) is expected material as one of hydrogen gas detection and/or hydrogen storage material. Furthermore, by forming the nanoporous (NP) structure, the specific surface area is increased, and improvement in sensitivity can be expected. In our previous study, we succeeded in the preparation of NP-Pd films by dealloying of Al-Pd alloy films using chelating solution of citric acid [1]. However, there were problems, that is slow reaction rate and residual Al (about 13 at%Al) in the film after dealloying.
In this study, using ethylene-diamine-tetraacetic-acid (EDTA) as dealloying agent instead of citric acid speeds up the dealloying reaction. Furthermore, in order to increase the purity of Pd after dealloying the nitrogen-containing Al-Pd alloy (Al-Pd-N) film was used as the mother alloy film.
These 70-nm-thick Al_0.82-Pd_0.18 or Al_0.82-Pd_0.18-N mother alloy films were prepared by the rf-sputtering method in the ambient gas of pure Ar or in the mixture gas of Ar and N₂. The pH value was controlled to optimize the chelating ability of EDTA to Al ions in solution. EDTA dissociates four ionic states, i.e., EDTA-, EDTA²-, EDTA³-, and EDTA⁴- by increasing pH value from 4 to 10 by adding sodium carbonate (Na₂CO₃).
The dealloying process was performed at 368K and at the same time, in-situ observation of light transmittance change was performed to evaluate the dealloying rate. The membrane structure of dealloyed film was also evaluated by using SEM and STEM.
When the pH value of the EDTA solution was increased from 4 to 10, the ionization of EDTA proceeded, and the dealloying reaction saturation time (T_s) of the Al_0.82-Pd_0.18 mother alloy film was reduced to 1/33 from about 4000s to 120s. Furthermore, the T_s was reduced to 40s by dealloying the nitrogen- containing Al-Pd-N film in EDTA solution at pH=10 instead of the Al-Pd alloy film without containing nitrogen. As a result, the T_s was reduced to 1/100, and the composition of the NP-Pd film was purified to 99.5 at%Pd.

[1] T. Harumoto et al., AIP ADVANCES 5, 017146, (2015).

First, Thin anodized aluminum oxide nano-mask was prepared by facile self-assembly technique without using polymer buffer layer, which was utilized as direct-contact template for oxygen plasma etch to produce near periodic, small-neck-width NPG. This work also demonstrates that our direct-contact, self-assembled mask lithography is a pathway for low-cost, high throughput, large scale nanomanufacturing of graphene nano devices. The average neck width on graphene after controlled etching time is 25.0 ± 4.3 nm. It is expected that a neck width reducing process, such as utilizing a controlled oxygen plasma etch, could be utilized, which can lead to a substantially reduced neck width.
By using the fabrication of large-scale NPG network, its applications for gas sensing are reported. NPG network shows significantly enhanced sensitivity to ammonia gas compared to pristine graphene. The detection sensitivity of the nanoscale NPG network is even further improved by decorating NPG network with palladium (Pd) nanoparticles, which shows a relative resistance response of 65 % to 50 ppm of ammonia in nitrogen at room temperature as well as good reversibility in air.

Piezoelectric functional material is an vital for microelectromechanical systems (MEMS) such as physical sensor, energy harvester, ultrasonic transducer for fingerprint and resonator for radio frequency. Aluminum nitride (AlN) is a piezoelectric material. Because this material has good properties such as sensor sensitivity, thermal stability and elastic modulus, we can use in harsh environments such as industrial plants and geothermal power generators as well as cars and smartphones. Additionally, it was revealed that a doping scandium (Sc) significantly increases the piezo response of AlN. This finding made the doped AlN a candidate for advanced MEMS device applications but Sc is an expensive element for the industry. In this study, we investigated the effect of Yb doping into AlN piezo thin film as an alternative for Sc. The Yb doped AlN YbxAl1-xN films were prepared on the silicon substrates by a radio frequency co-sputtering. The concentration of Yb, x was controlled by the cathode power. The prepared films had a column texture less than 100 nm in diameter. The piezoelectric coefficient increased by Yb doping. The coefficient reached to about twice that of AlN at x= 0.33. Because the rate of increase at concentrations below 0.1 was comparable to that of Sc doping, Yb can replace Sc in this concentration region. However, Yb could not be doped as much as Sc. Although Sc can be doped at 0.4 or more and the film shows giant piezo response, Yb did not exhibit piezoelectric response at 0.37 or more. The decrease of piezo response would be caused by a phase transition. In case of Sc doping, it is known that the crystal structure changes from wurtzite (polar phase) to rock salt (non-polar phase). In this study, the lattice constant changed continuously until the Yb concentration of X = 0.37. However, the change in lattice constants was discontinuous above x= 0.4 according to the conventional XRD analysis. This result would indicate the phase transition. We analyzed the crystal structure of prepared films in detail by a wide area X-ray diffraction reciprocal space mapping.

To cope with surging demand for highly sensitive, responsive, long term stability and selective Volatile Organic Compounds (VOCs) and Humdity Sensor’s in IoT applications, requires the development and exploration of innovative materials which can be considered as stepping stone in next generation sensing applications.¹ Gas sensors with high sensitivity and selectivity at room temperature have been expected for practical applications. But the limitations of conventional metal oxide based sensing materials and their unstable outputs caused by ambient environment or harsh conditions, significantly restrict their practical applications.²
Here in this study, we are exploring a heptazine based microporous polymer sensor for ammonia detection at room temperature and ambient conditions. The chemiresistive sensor performance was investigated for ammonia over a wide concentration range (1-200 ppm) and in the presence of relative humidity range (23-85 %RH). The presence of active sites (C=N and NH groups) and porous network of heptazine gas sensor enables enhanced response (16.6 @ 70 %RH & 47 @ 84 %RH) towards 50 ppm of ammonia with response and recovery times of 65 and 9s, respectively. The electron withdrawing nature of heptazine enables better selectivity towards ammonia over other VOC analytes.
Therefore, we believe that the developed sensing material presents an excellent opportunity for the development of high performance, portable, easy-to-use device for disease diagnosis via breath analysis and environment monitoring in ambient conditions.

References:
[1] H. Singh, V. K. Tomer, N. Jena, I. Bala, N. Sharma, D. Nepak, A. De Sarkar, K. Kailasam and S. K. Pal, J. Mater. Chem. A, 2017, 5, 21820-21827.
[2] P. Rai, R. Khan, S. Raj, S. M. Majhi, K.-K. Park, Y.-T. Yu, I.-H. Lee and P. K. Sekhar, Nanoscale, 2014, 6, 581-588.

Thin films of Pb(Zr_xTi_1-x)O₃ have been most widely used for piezoelectric sensors and gyro applications due to their large piezoelectricity. These films have been also investigated for the vibration energy harvesters. In particular, the giant electromechanical response is obtained at the Zr/(Zr+Ti) ratio near the so-called morphotropic phase boundary, MPB. Most of studies have been only focused on the relationship between the crystal structure of the as-deposited films and the piezoelectric response. However, studies on the relationship between the crystal structure during the poling process and the piezoelectric response are very limited. Because the piezoelectric response is considered to accompany the crystal structural change under applying an electric field. Thus, it is crucial to investigate the change of crystal structure via the poling process and under applying an electric field after the poling .
In the present study, we investigated the change of crystal structure by the poling process and under applying an electrical filed using a micro-beam XRD study. The measurements were performed for {100}-one-axis -oriented Pb(Zr_xTi_1-x)O₃ films with 1.5 µm in thickness and near MPB composition. The crystal structures and their change by these treatments strongly depended on the Zr/(Zr+Ti) ratio of the films. Films with Zr/(Zr+Ti)=0.65 changed from (100)-oriented rhombohedral phase to (001)-oriented tetragonal one by the poling process. Furthermore, Zr-rich films with Zr/(Zr+Ti) = 0.67 changed from (100)-oriented rhombohedral phase to (001)-oriented tetragonal one only under an electric field. The present results suggest the change from (100)-oriented rhombohedral to (001)-oriented tetragonal phases by not only the poling process but also under applying an electric field. This study provided fundamental insights on the crystal structure change with an electric field for Pb(Zr,Ti)O₃ films with composition near phase boundary.

Vanadium dioxide has been intensively explored as a viable material for numerous applications such as smart windows coatings and miniaturised optoelectronic devices and a great progress has been achieved on laboratory-scale. To realise its commercialisation, industrially-viable synthetic techniques are highly required for large-area thin films that meet commercial demands. DC reactive sputtering is such a robust technique which offers high quality large-area thin films on large-scale with controllable stoichiometry and composition for various applications in optoelectronic and energy harvesting devices. Herein, we report synthesis, characterization and IR photoresponse properties of 150.8 nm thick VO₂(M1) thin films synthesized by DC reactive sputtering. Phase purity was confirmed by XRD and Raman spectroscopic studies. Morphological and microstructural analysis by atomic force microscope (AFM), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) revealed the polycrystalline nature of the nanosized films with rms roughness value of 8.69 nm. Electrical characterization showed a first order transition of the films with a resistance change of more than two orders of magnitude and a TCR of -1.24 % K^-1 at 30 ^oC. The fabricated IR photodetector based on VO₂(M1) thin films exhibited high photoresponse properties with excellent stability and reproducibility in the ambient conditions with a sensitivity of 1775 %, responsivity of 40.09 mA/W, quantum efficiency of 4.67 % and detectivity of 7.07 X 10¹¹ Jones upon illumination with a 1064 nm laser having a power density of 250 mW/cm² at 5 V bias voltage.

Silver nanowire (Ag NW) networks hold great potential to replace commercial transparent conducting oxides due to their superior properties in conjunction with their competitive cost, availability and flexibility. However, there are still challenges to overcome for the wide scale utilization of Ag NWs in devices due to oxidation/sulfidation of nanowires depending on the operating conditions, which leads to loss in performance. Here we develop a solution based strategy to deposit a thin platinum (Pt) shell layer to improve environmental stability of the Ag NWs. Ag NWs were synthesized by polyol method [1] and coated with a thin conformal Pt layer. In a typical coating process, ascorbic acid was used as the reducing agent, polyvinylpyrrolidone (PVP) was used for the directional growth, surface of Ag NWs acted as nucleation sites for Pt ions and deionized water was used as the solvent. Fabricated core-shell nanowires were than deposited onto glass substrates in the form of networks via spray coating. Environmental stability of the core-shell nanowire networks was monitored under different relative humidity conditions (43, 75 and 85%) and compared to that of bare Ag NWs. In order to further evaluate their stability and simulate organic electronics fabrication conditions, both electrodes were kept at 75 °C for 120 hours and 150 °C for 24 hours. Finally, hydrogen peroxide stability of core-shell nanowire networks was tested and compared to that of bare Ag NW networks. All in all, this highly effective and simple strategy to improve the stability of Ag NWs will certainly open new avenues for their large scale utilization in various optoelectronic devices. The applicability of the fabricated highly stable networks has been demonstrated both in resistive and capacitive touch screens and capacitive sensors are fabricated.
[1] S. Coskun, B. Aksoy, H.E. Unalan, Polyol Synthesis of Silver Nanowires: An Extensive Parametric Study, Crystal Growth & Design 11 (2011) 4963.
This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under grant no 117E539.

Helium(He) plasma exposure to various metals with low irradiation energy and high ion fluence greatly changes the surface structure of the metals. Among them, the unique structure induced on tungsten (W) is called tungsten nano-structure or fuzz because it has a fluff-like shape [1]. Since this fluff-like fibrous structure (fuzz) has a nano-order sized and complicated structure, its surface area is greatly increased compared with flat surfaces. Therefore, application of fuzz to various gas sensors is very attractive. In particular, with regard to an ethanol gas sensor, an experiment was conducted for a molybdenum (Mo) layer deposited on a quartz surface, which was irradiated with He plasma to form fuzz [2]. By using this Mo fuzz, significant improvement in sensitivity of ethanol was confirmed and the possibility of application to various gas sensor was shown. Presently development of hydrogen gas sensor with W fuzz is in progress for future hydrogen based energy society.
For the gas sensor applications of the fuzz structure, it is necessary to produce nanostructures with larger surface area and durability. For this purpose, understanding of formation mechanisms controlling the thickness and length of the fuzz structure must be known. However, although a lot of experimental and simulation researches have been performed, there still remain many uncertainties in formation mechanisms and the detailed control methods of fuzz. So far, in order to elucidate the formation mechanism, various 6th period transition metals were irradiated with He plasma under various conditions [3]. In this work, fuzz structures were observed for several 6th period transition metals except tantalum (Ta) in the region where the normalized temperature T/T_m is 0.3 or above (T: specimen temperature, T_m: melting temperature).
Based on the above mentioned research, He plasma irradiation to the 5th period metals have been also performed to systematically investigate the surface morphology changes and to acquire basic data for controlling fuzz structure. He plasma irradiation experiments have been conducted for zirconium (Zr), niobium (Nb), molybdenum (Mo), palladium (Pd), silver (Ag), and we found that thin fibers are formed in the region where the normalized temperature T/T_m is 0.3 or above for all of these except for Nb. Similar tendency was observed for the same group elements such as tantalum (Ta) and Nb (5th group), and tungsten (W) and molybdenum (Mo) (6th group) [4]. A thick rod-like structure was formed for Nb and Ta, while fibrous structure (fuzz) was formed for W and Mo. In the presentation, experimental results on alloys with two metals which have different responses to He irradiation (e.g. Mo and Nd) are also shown, and the formation mechanism of fuzz will be discussed in detail.

Graphite has excellent properties, such as high strength, high thermal conductivity, and chemical stability. A variety of applications such as high performance batteries and heat dissipation materials for highly integrated electronic devices are expected for graphene as the next-generation materials. Recently, focusing on the high thermal conductivity of graphite in particular, investigations for the mechanism of phonon-scattering at crystalline domain interfaces have attracted attention. In this study, we tried to microfabricate nano-sized graphite by oxygen plasma etching process for highly oriented pyrolytic graphite (HOPG) in order to clarify the size-dependent thermal conductance of graphitic materials.
By an ICP plasma etching system (RIE-101iPH, Samco), a HOPG plate (10×10×3mm) was etched by using a copper / nickel-made metal mesh, a SiO₂ (285 nm) / Si plate and micro-patterned Au / Cr islands (1 µm circle) as masks, in the conditions of ICP power of 100 W, Bias power of 50 W, and process pressure of 0.05 Pa. AFM measurement was performed by using SPA400-DFM, Seiko Instruments Inc. with the Si cantilever. SEM images were acquired by using SU-8020, HITACHI High-Technologies in the condition of acceleration voltage of 15.0 kV.
Both of copper and nickel mesh are deformed during etching process above 60 min. The longer etching time, the more significant deformation of the mask. On the other hand, SiO₂ (285 nm) / Si masks exhibit sufficient etching-proof nature. The etching rate gradually decreases as the etching time increases. It is known that a deposition layer is formed at etched step by the reattachment effect during plasma etching of graphite, where etching in the sample region just below the deposition layer is prevented [1]. This is why the etching rate becomes dropped in the initial etching process below 5 hours. By using micropatterned islands mask, well-aligned and uniform micro-fabricated graphite pattern is successfully obtained, where the horizontal size of the island was found to be slightly larger than that of the mask pattern.

References
[1] X. Lu et al, Appl. Phys. Lett., 75, 2, 193 (1999)

Pressure is one of the most important process variables for wide area applications. Various principles used to design pressure sensors include strain, piezoresistivity, piezoelectricity, capacitance and so on. Among these, capacitive pressure sensors have several advantages such as high measurement sensitivity, low temperature sensitivity, low power consumption and high stability. However, non-linearity of change in capacitance with respect to applied pressure is a major consideration in capacitive pressure sensors, thus necessitating the need for detailed modelling and analysis. An interesting characteristic of capacitive pressure sensors is insensitivity to harsh environments such as extreme temperature, strong chemicals and radiation. Further, for harsh environment applications, selection of materials with suitable properties is also very important. In literature, materials such as silicon nitride, silicon carbide, stainless steel, sapphire and diamond have been studied for applications in capacitive pressure sensors for harsh environment applications. In this work, we have designed and modelled capacitive pressure sensor membranes using composite membranes of silicon carbide and silicon nitride to reduce the dielectric constant while maintaining structural stability and manufacturability.

The basic capacitive pressure microsensor consists of two parallel plates, one is a movable diaphragm under applied pressure, and the other one is fixed. In this work, mathematical modelling of elliptical capacitive pressure sensors has been carried out. In order to perform mathematical modelling, the fourth order partial differential plate equation of clamped elliptical diaphragm was solved, following which the capacitance change was calculated using the area integral of the capacitance of the diaphragm. While developing the mathematical model, the small deflection Kirchoff’s plate theory and pull-in phenomena were considered. To follow the small deflection theory, the deflection in diaphragm should be less than 1/10th of diaphragm thickness and the maximum deflection was limited to 1/3rd of the separation gap between plates to avoid pull-in phenomena. While performing the mathematical modelling of elliptical pressure sensor, the effect of electric field was ignored. We also performed finite element analysis (FEA) to validate the mathematical model. Various performance parameters of capacitive pressure sensors such as diaphragm deflection, capacitance variation, mechanical sensitivity, electrical sensitivity, non-linearity of sensor have been studied as a function of diaphragm physical dimensions, flexural rigidity, applied pressure and separation gap.

To compare various sensor designs, we designed the elliptical diaphragm using the same total area with different eccentricities. The designs were also compared with a circular shape diaphragm of the same area. The sensors were designed for measurement of pressure underwater in marine environments. After study of elliptical sensors of the same overlapping area between plates, it was found that the deflection decreases as the semi-major axis increases, which results in decrease in change in capacitance, which, in turn, reduces the mechanical and electrical sensitivity. However, the responses become more linear as eccentricity is increased. It was observed that circular diaphragm pressure sensor shows more deflection, capacitance change, mechanical and electrical sensitivity, however the repose is more nonlinear than the elliptical-shaped diaphragm. Hence, there is a trade-off between linearity and sensitivity. Thus, the ideal eccentricity for elliptical (circular) pressure sensors can be calculated based on the particular requirements of the application.

Graphene is a two-dimensional material with a thickness of single atomic layer where carbon atoms are arranged into a honeycomb lattice. Molecular adsorption effects are significant in graphene because all of carbon atoms belong the surface, where the electronic properties are modulated by the interactions at the interface between graphene and other materials. Hydrogen molecule adsorption is one of the most important examples in terms of applications for the transistor devices accompanied with band gap opening technology and hydrogen storage materials. Meanwhile, introducing defects is one of the interesting strategies to give additional adsorption sites to graphene in view of enhancing the interactions between hydrogen molecule and graphene. In this study, we investigated carrier doping and scattering caused by hydrogen molecule adsorption and defects introduction into graphene by electric conductivity measurement and Raman spectroscopy.
A 4-terminal field effect transistor using single-layer graphene obtained by exfoliation of graphite (g-FET) was fabricated on SiO₂/Si substrate. Time dependence of the electrical conductivity for the g-FET was measured after annealing in a vacuum chamber under hydrogen atmosphere (1 atm) at room temperature. For Raman spectroscopy, epitaxial graphene grown on SiC was used, where atomic vacancies were introduced by sputtering with Ar ion beam after pre-annealing, followed by exposed to ambient atmosphere and hydrogen molecule and atomic hydrogen, respectively. The charge neutral point where the minimum conductivity appears in a g-FET exhibits no significant shift from 30 min after the hydrogen molecule adsorption up to 24 hr. This means little carrier doping occurs into graphene when hydrogen molecule adsorbed at 1 atm at room temperature. However, for a theoretical calculation, where hydrogenated atomic vacancies in graphene have a low energy barrier and little adsorption heat for additional hydrogen molecule [1]. Thus, we verified hydrogen molecule adsorption to defect-introduced graphene by Raman spectroscopy. The ratio of D-band to G-band in Raman spectroscopy is smaller for graphene with defects adsorbed by hydrogen molecule and atomic hydrogen than that exposed to ambient atmosphere containing oxygen. This might be explained by that holes are injected into graphene through oxygen-terminated defects and electrons are injected into graphene through hydrogen-terminated defects. The scattering probability of carriers at the defect site depends on the Fermi energy. Thus, D-band intensity which is related to carrier scattering processes in graphene is different between hydrogen-terminated defects and oxygen-terminated defects. Interestingly, even among defects terminated with hydrogen atom, the D-band intensity is slightly different due to the difference in the chemical structure of the adsorbed hydrogen atoms. This difference suggests that the reactivity with defects is different in atomic hydrogen and molecular hydrogen. These results suggest that the effect of defects introduction on the hydrogen molecule adsorption to graphene is important for the future application of graphene to devices and hydrogen storage materials.

References
[1] G, Sunnardianto, K. Kusakabe et al., Int. J. Hydrogen Energy, 42, 23691(2017).

In the last years, much effort has been dedicated to the development of high-performance transparent heaters (TFTHs) due to their versatile use in consumer electronics, automobiles, and smart windows. We successfully fabricated and studied the electrical, optical and thermal properties of Al:ZnO (AZO) / MS₂ multilayer thin film heaters by keeping the AZO layer fixed and varying the MoS₂ layer thickness. The multilayers (3 – 5 layers) of AZO and MS₂ films were grown at 200-400^oC temperature range on glass, kapton, and silicon substrates using radio-frequency magnetron sputtering. The transparent heaters show a stable and reproducible heating effect with low consuming power (<10 V) as well as maintaining a high optical transparency higher than 75%. The temperature dependent resistivity behavior of the films was investigated using linear four probe measurement technique. The AZO films exhibit good electrical conductivity. These heaters offer an alternative to conventional and expensive ITO electrodes at a much lower cost. The potential applications are automobile window defrosters, foldable and wearable electronics, pain/injury therapy, smart windows, and low-cost power electronics.

This work has been supported by NSF-CREST Grant number HRD 1036494 and NSF-CREST Grant number HRD 1547771

Chemiresistors based on metal oxide semiconducting (MOS) nanostructures have drawn researchers’ attention because of their several advantages related to gas sensor performance, such as stability, sensitivity, and reversibility [1]. Despite having wide-ranging applications, the gas-sensing performance of TiO₂ nanostructures, still needs to be better evaluated and TiO₂ in anatase phase has demonstrated interesting sensing properties related to gases such as CO, NO₂ or acetone [2-3]. Thereby, a facile and environmentally friendly synthesis approach for the production of M:TiO₂ (M=V, Nb and W) nanostructures was demonstrated via hydrothermal decomposition of their respective peroxo-complexes. The effect of metal ions addition on the structural, morphological and electronic properties of the as-prepared samples were investigated by X-ray diffraction (XRD), Raman Spectroscopy, UV-visible diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS), and electron microscopy techniques. XRD analysis confirm that all M_xTi_1−xO₂ (M=V, Nb and W) samples presented only the TiO₂ anatase phase, despite the different amounts or metallic ions. Despite the different amount or metallic ions, scanning electron microscopy (SEM) shows that the increasing of metallic ions contents leads to a morphological evolution, from anisotropic to isotropic nanostructures, reducing their size (at around 5nm) as the surface area. From the studied samples, W doped TiO₂ nanostructures presented interesting ozone (O₃) gas-sensing performance evidenced by their working temperature (below 200⁰C), sensor response, repeatability, as well as a good range of detection.
[1] Zhang, J.; Liu, X. H.; Neri, G.; Pinna, N.. Advanced Materials 2016, 28 (5), 795-831.
[2] Yang, Y.; Hong, A. J.; Liang, Y.; Xu, K.; Yu, T.; Shi, J.; Zeng, F. Y.; Qu, Y. H.; Liu, Y. T.; Ding, M. Q.; Zhang, W.; Yuan, C. L. Applied Surface Science 2017, 423, 602-610.
[3] Epifani, M.; Comini, E.; Diaz, R.; Force, C.; Siciliano, P.; Faglia, G.. Applied Surface Science 2015, 351, 1169-1173, DOI: 10.1016/j.apsusc.2015.06.080.

Tungsten oxide (WO_x) is attracting attention as an active material for many useful applications such as material for electrochromic device, electrode of Li-ion batteries, gas sensors and so on. Especially, amorphous tungsten oxide a-WO_x films are essential material for advanced thin film devices, for example, the recently developed electrochromic transistor (ECT) because a-WO₃ films show clear electrochromism as well as flexibility, and it can be fabricated at room temperature (RT). One can expect that practical smart windows, which can store the information both electronically and visually utilizing the electrochromism of a-WO₃. In order to improve the a-WO_x-based ECT performance, fundamental physical properties of a-WO_x are strongly required. Although the relationships between the electrical, optical and thermal properties and the chemical composition of crystalline WO_x have been investigated, there is no systematic report in the case of a-WO₃. Here we report the relationship between the valence state of W ion and the properties of oxygen deficient a-WO_x thin films continuously against x.^[1] We fabricated a-WO_x films with various x ranging from 2.511 to 2.982 by the pulsed laser deposition technique and measured the electrical, optical, and thermal properties systematically. Although the +6 dominant films were electrical insulators with optical transparency in the visible region, we found that both optical transmissivity and electrical resistivity decreased drastically with increasing the +5 concentrations, which also enhances the thermal conductivity due to the fact that heat can be carried by additional conduction electrons. As the +4 states became dominant in the film, the resistivity slightly increased whereas the low transmission was maintained in the visible range. We believe that the results would provide useful information to improve recently developed a-WO_x-based ECTs, which utilized reversible electrochromism of the active material to change the valence state of its transition metal ions, enabling the control of both the electrical conductance and visible transmission.
[1] G. Kim et al., J. Phys. Chem. C in press

Monodisperse Vanadium Dioxide nanoparticles were synthesized using a hydrothermal method and characterized using Transmission Electron Microscopy and Synchrotron X-ray Diffraction. The particles were then studied using Soft X-ray Absorption (XAS) and Resonant Inelastic X-ray Scattering (RIXS) at both above and below the metal insulator transition, in order to reveal the extent of particle size and lattice effects on nano-scale sample bandstructure. For the first time to our knowledge, the XAS peak indicating the prominent presence of vanadium dimers in the low-temperature monoclinic phase has been identified as a stand-alone peak in VO2 nanoparticles. Implications for extent of dimerization and the metal insulator transition will be discussed

Nowadays, thin film transistor (TFT) are utilized in various display application like TV, mobile phone, car, etc. Backplane contains an array of TFTs which operate for switching the individual pixels on and off. It determines performance when it comes to resolution and power consumption. Active channels of TFTs include amorphous silicon (a-Si), low-temperature polycrystalline silicon (LTPS) and indium gallium zinc oxide (IGZO). Especially, LTPS channel has an advantage of compatibility on a-Si fabrication processes.
In this study, we focus on electrical properties of LTPS TFT as a function of temperature in the range of 300 – 375 K. The characterization of LTPS TFTs fabricated on glass and plastic substrates is measured by using the semiconductor parameter analyzer (Keithley 4200) in vacuum chamber with thermal chuck.
Electrical performances (Threshold voltage V_TH, Subthreshold swing SS, Effective mobility μ_eff) of LTPS TFTs on glass and substrates are different. Although active channels have similar dimension, V_TH SS related to switching operation on plastic substrate (1.44 V/decade) is larger than glass substrate’s one (0.60 V/decade) at 300 K. SS is explained by equation SS=2.3nkT/q, (n = 1+(C_dep+C_it)/C_ox). C_dep is inverse proportional to temperature, also C_it exponentially decreases according to increasing temperature. Compared to LTPS, single crystalline Si can be the ideal 60 mV/decade at 300K because the effective C_dep and C_it are negligible. But, LTPS channel has many grains and grain boundaries which related to channel depletion capacitance and then glass and plastic substrates which have different interface quality are the cause of interface trap capacitance. It can affect μ_eff difference between glass (75 cm²/Vs) and plastic (59 cm²/Vs) substrates.
Ascending temperature in the range of 300 – 375 K, the characterization of LTPS TFTs on glass and plastic substrates get worse that SS is proportional to temperature. Compared to extracted SS and equation which applies the thermal dependency of depletion and interface capacitance, C_dep of plastic substrate among grains is near 2 times larger (0.33 μF/cm²) than one of glass substrate (0.16 μF/cm²). It might describe that small grain size and large number of grain boundaries for plastic substrate affect depletion capacitance and lower effective mobility. Meanwhile, glass substrate which has lower interface traps between gate oxide, active LTPS layer, and substrate than plastic substrate operates deeper and faster.
Consequently, active LTPS channel layers on glass and plastic substrate have different physical factors like grain size, grain boundaries, and trap densities among another layers (gate oxide, substrate). It has been shown that these factors affect switching operation and driving performances of TFTs, respectively.

Among various Metal Oxide Semiconductor (MOS) gas sensor materials, ZnO is an n-type semiconductor, which has a band gap of 3.37eV, is recognized as a noticeable sensing material due to its chemical stability, its high electron mobility, and the variation of donor density. Typically, optimal working temperature of ZnO based gas sensors is above 300^oC. However, ZnO has a problem in that it has no selectivity for target gases. Recently, as the hydrogen age comes, there is a growing interest in hydrogen gas sensors having selectivity to hydrogen gas.
In recent years, noble metal nanoparticles (NPs) have been used to improve the sensing properties of gas sensor. Among the various noble metal catalysts, palladium has excellent ability to adsorb hydrogen, so that it can increase the selectivity of the gas sensor to hydrogen gas when used as a catalyst. However, since palladium can be easily oxidized at a high temperature, when it is used as a catalyst of a gas sensor at a high operating temperature, it is oxidized so as to lose its function as a metallic catalyst. Therefore, palladium alloy needs to be considered as a catalyst. On the other hand, one of the new strategies for using noble metal NPs in MOS gas sensors is to design core-shell NPs. In the core-shell structure, the shell helps maintain the catalytic properties of the metal core.
In this study, Pd-Au alloy nanoparticles with different composition were synthesized to prevent oxidation of palladium. Subsequently, Pd-Au(alloy)@ZnO core-shell NPs were prepared using Au colloid, Zn(NO₃)₂ and ascorbic acid. To investigate the oxidation behavior of the Pd-Au alloy core, Pd-Au(alloy)@ZnO core shell nanoparticles were annealed at the range of 200 to 500°C. The synthesis of Pd-Au alloy nanoparticles was confirmed by using HRTEM, and the oxidation behavior of Pd-Au alloy core with different composition was observed through X-ray diffraction analysis. In addition, the gas sensing tests were performed at operating temperature of 200~500°C for various gases (H₂, CO, CH₄, C₂H₆O), and the composition effect of Pd-Au alloy on H2 gas sensing response was investigated.

In this talk, I will focus on Material Synthesis, Energy Application and Mechanism Study using Battery Intercalation Strategy. For material synthesis, I will discuss how to design battery intercalation “environments” to obtain a serials of high quality single layer transition metal dichalcogenides (TMD) nanosheets (MoS₂, WS₂, TaS₂, TiS₂ and ZrS₂)- the Li insertion can be monitored and finely controlled in the battery testing system, so that the galvanostatic discharge is stopped at a proper Li content to avoid decomposition of the intercalated compounds; Moreover, the battery intercalation strategy can be optimized and the controllable lithiation process can be extended to BN, NbSe₂, WSe₂, Sb₂Se₃, Bi₂Te₃ and so on. The prepared noble metal-2D TMD composites are promising candidate for sensing and hydrogen evolution reaction. For mechanism study, I am interested in failure mechanism study of Lithium/Sodium Ion Batteries at nanometer scale utilizing imaging and spectroscopy protocols. With the self-designed electrochemical liquid cell TEM, we can directly vapture the dynamic electrochemical lithiation and delithiation of electrode in a commercial LiPF₆/EC/DEC electrolyte, such as lithium metal dendritic growth, electrolyte decomposition, and solid-electrolyte interface (SEI) formation. This technique opened a venue by which looking inside the dynamic electrochemical reactions in real-time. Which render us to improve the electrode design for reducing short circuit failure and improving the performance of lithium/sodium ion batteries.

Nanowire aerogels (NWAs) possess an unusual conglomeration of many useful properties: lightweight, large surface area, high electrical conductivity and low thermal conductivity. They have been used in many applications such as electronics, energy generation/storage, thermal management, sensing, catalysis, etc. However, most synthesized NWAs are composed of single component that may produce unsatisfactory aggregated performance in mechanical strength, conductivity and electrochemistry. Therefore, it is hard to apply them in a multifunctional application.
To address this issue, we synthesized the reticulate dual-nanowire aerogels (rDNWAs) composed of FeS₂ nanowires and carbon nanotubes (CNTs) via a simple solvothermal method. In the typical synthesis process, highly porous CNT sponge was used as the matrix to allow the in-situ growth of nanowires and thus the two components densely intertwined with each other to form the reticulate structure. This structure is versatile and TiO₂ based rDNWA was also successfully fabricated via a similar process. The FeS₂ based rDNWA shows high porosity (> 98%), good conductivity (0.65 S cm^-1), and excellent mechanical properties (maximum Young’s modulus: 1.32 MPa). Due to its high compressibility, it can be used as the strain sensor. The sensor exhibits good sensitivity and enhanced stability compared to the CNT matrix. Besides, it can be directly compacted to produce a high-performance free-standing electrode for lithium ion batteries (LIBs) that exhibits high mass and areal specific capacities (10 mAh cm^-2 with the mass loading of 14.4 mg cm^-2). Further, the obtained TiO₂ based rDNWA shows not only similar properties and applications but also some uniqueness. When applied as the electrode for the LIBs, it can be utilized to fabricate the compressible LIB devices by controlling the voltage window to 1-3 V. The outstanding overall performance and multiple applications of our hybrid aerogel are derived from the synergistic effect of intertwined two components and the reticulate structure can be extended to fabricate more NWAs with novel multi-functional capabilities.

Healthcare monitoring devices assess the health status of a person by detecting the physiological signals such as heartbeat, respiration rate, and temperature measured directly or in relation to the deformation and vibration of the human body. Due to the needs for maintenance-free operation of wearable and wireless sensing units, the demand for self-powered sensors is continuously growing. Using various kinds of nanogenerators that convert thermal, mechanical or chemical energy into electricity, self-powered sensors can continuously measure physiological signals of the body. Among these power-generators, triboelectric nanogenerators (TENGs) have attracted interest for wearable sensing applications because they can scavenge energy from mechanical body motion with high energy conversion efficiency. There are four fundamental operation modes of TENG; vertical contact-separation, in-plane sliding, single-electrode, and free-standing operation mode. Compared with other paired-electrode modes, the single-electrode mode has a much simpler structure that does not need an auxiliary electrode on the surface. Hence, only the single-electrode mode TENG (SETENG) fulfills the energy harvesting conditions necessary for human-machine interfaces, such as touch panels, where human skin acts as the opposite triboelectric surface to the component surface of the TENG. In this work, we fabricated the SETENG using the electrospun silver nanowires (AgNWs)/P(VDF-TrFE) composite nanofibers (NFs). The inclusion of a small fraction of AgNWs in the NFs promoted the β-phase formation of P(VDF-TrFE), a higher surface charge density, and the output performances. The SETENG generated an output power density of up to 217 W/m² with repetitive contact and separation. To the best of our knowledge, this power density is one to two orders larger than that of other single-electrode-type TENGs (3.4 ~ 27 W/m²) reported by others. Conducting high-resolution X-ray diffraction (HR-XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and Kelvin probe force microscopy (KPFM) analyses, we could confirm that AgNWs create the β-phase and induce a much lower surface potential. Using highly efficient SETENG, we demonstrated a self-powered, transparent and wearable touch panel for a human-machine interface by arraying SETENG units as a touch sensing component. The SETENG array was connected to Arduino microcontroller for real-time communication using low-pass RC filters with 10 Mohm resistors and 10 nF capacitors for noise filtering. We could successfully type any text on the computer using the SETENG array. Additionally, we designed a self-powered strain sensor connecting SETENG with the strain sensor as a power source. Touching and detaching the SETENG, saturated open-circuit voltage signal by transferred charges was transformed by resistive transducer (the strain sensor). It showed the linearity with 16 gauge factor (GF) at 35% strain. It also had no hysteresis behavior up to 10% strain of and showed stable ε sensing performance during the repeated stretching test at ε of 10% for 10,000 cycles. Using self-powered strain sensor, we could successfully measure the strain on the finger without any power supply.

Recent developments in the areas of energy, environment and public health have made the role of chemical sensors more relevant than ever before. While efforts to increase process efficiency without environmental impact and new technologies emerging in the areas of energy conversion and storage demand development of reliable chemical sensors for process monitoring and safety, concerns over increasing pollutants in the environment and rapidly increasing mortality rates related to a wide range of diseases including cancer prompt their use in environment and health monitoring. Nevertheless, the concentrations of gases/vapors of interest are, in many of these cases, in the range of parts per million to trillion (ppm to ppt) levels and the environment is complex. Metal oxide semiconductor based chemical sensors are attractive for such applications because of their ability to detect a wide range of chemical species. These devices are generally low cost and fast responding and have the ability to operate in harsh environments. Inadequate sensitivity and selectivity are often problems with these sensors. Advent of new low dimensional metal oxides facilitated the development of sensors with unique functionalities. These materials offer high surface area and nanoscale features for improved receptor and transducer functions. As a result, highly selective sensors with ppm to ppt range detection ability can be developed using these materials. For example, we recently developed a zinc oxide nanomaterial based sensor that could detect parts per billion levels of certain volatile organic compounds (VOC) in air with reasonable selectivity. Such VOC sensors find applications in various areas including pollution monitoring and early diagnosis of diseases. This presentation will give an overview of the current status of the nanostructured oxides based chemiresistive sensor technology for environmental monitoring. The focus of discussion will be on trends in volatile organics detection technology.

Electromigration is a critical failure mode in the semiconductor industry, and in the emerging fields of flexible and wearable electronics. This failure can be attributed to the diffusion of metallic atoms due to momentum transfer of electrons flowing through a metallic interconnect when voltage is applied. The diffusion eventually leads to formation of voids and hence a time-dependent increase of electrical resistance of the interconnect. Since it is a diffusion process, the microstructure of the interconnect is one of the controlling factors of electromigration. Twin boundaries, in contrast to grain boundaries, can have the potential to reduce diffusion of atoms and consequentially improve electromigration performance. In flexible electronics, it is common to have twinned interconnects. Unfortunately, very few studies have been conducted about the diffusion performance of these interconnects. Even those studies consider a few twin boundaries in grain boundary diffusion. No literature is available about the collective effect of twin boundaries throughout the interconnect. Moreover, some of the studies said that twinning slows down the diffusion and hence void growth [1], [2] and some said that twinning does not necessarily hinders void growth [3]. Given these conflicting reports, there is clearly a need for a systematic study of the electromigration reliability of twinned interconnects.
In this work, we investigated electromigration failure of penta-twinned silver nanowires, with a nominal diameter of 100 nm and length ranging from 3 μm to 15 μm, to yield reliability estimates for nanowire- based flexible electronics. Samples were prepared by random deposition of nanowires on a substrate containing gold electrodes fabricated by photolithography. The nanowires were connected with the electrodes by e-beam lithography. The electrodes were then connected to a printed circuit board (PCB) through a wirebonding package. Under a fixed current density and temperature, the resistance was measured using 4-point measurement method so that it can be measured continuously without the influence of contact resistances. The currents were applied in an automated fashion to test a large number of nanowires. The failure criterion was set as 10% increase in resistance which is also widely used in the electronics industry. The median time to failure of nanowires was determined using Black’s equation. The experimental data not only provides a significant guideline about reliability of flexible electronics in terms of electromigration failure but also valuable information about diffusion parameters through twin boundaries. Comparison between the data obtained in this experiment and the published data for interconnects currently used in the electronics industry, depending on the variation in diameter and length, revealed promising prospect for twinned interconnects in the next generation electronic devices.

References

[1] K. Chen, W. Wu, C. Liao, L. Chen and K. Tu, "Observation of atomic diffusion at twin-modified grain boundaries in copper," Science, vol. 321, no. 5892, pp. 1066-1069, 2008.
[2] H. Chem, C. Huang, C. Wang, W. Wu, C. Liao, L. Chen and K. Tu, "Optimization of the nanotwin- induced zigzag surgace of copper by electromigration," Nanoscale, vol. 8, no. 5, pp. 2584-2588, 2016.
[3] Y. Oh, S. Kim, M. Kim, S. Lee and Y. Kim, "Preferred diffusion paths for copper electromigration by in situ transmission electron microscopy," Ultramicroscopy, vol. 181, no. Supplement C, pp. 160-164, 2017.

As one of the world’s favorite beverages, teas have been consumed by many generations. Due to their high health benefits, it is important to understand the quality of teas. The objective is therefore finding an economic way to identify aromas of tea that can eventually help with the purpose of tea quality control. Conventionally, similar to wine products, a tea-tasting expert will tell how good a tea is. However, it is very expensive and subjective as there is no measurable parameter. For that reason, a more reliable and easy-to-operate alternative approach is desirable. Based on earlier success using organo-functionalized nanoparticles to differentiate volatile organic compounds, we continue to investigate the performance of gold nanoparticle chemiresistive sensor arrays using more complex analytes - tea samples.

In this work, we demonstrated a nanoparticle-based electronic nose system that separates 35 flavors of teas, ranging from black teas, green teas, and herbal teas. Four types of gold nanoparticles (AuNPs) with chemically diverse ligands were chosen as sensing elements. The functionalization includes DMAP (4-dimethylaminopyridine), ODA (octadecylamine), 3-MPA (3-mercaptopropionic acid), and 4-ATP (4-aminothiophenol). To assemble the electrodes, nanoparticle solutions were drop-cast on the active sensing region that consists of castellated microstructures with 2-μm gap. The sensor chip was designed with separated active sensing regions so that each AuNP can have 12 working sensors maximum. This allowed diversity as well as redundancy for sensing experiment. Fresh tea powders were added to a glass syringe and the dosing was completed by a syringe pump with controlled delivery speed. Dry nitrogen gas was used for purging. Sensing responses, expressed as ΔR/R₀, were measured simultaneously for all sensors using a switch matrix/multimeter system. Raw data processing and statistical analysis were performed using MATLAB to evaluate the performance of the chemiresistive electronic nose array.

Depending on flavors, sensing elements, and baseline resistance difference, ΔR/R₀ of 1-20% were achieved. The resulting variation enables statistical analysis, such as principal component analysis (PCA), linear discriminant analysis (LDA), support vector machine (SVM), and random forest (RF). With five-fold cross validation, we have achieved 99% accuracy with LDA for all 35 flavors of teas. We also found that multi-particle assembly works better than using single type of particles. For example, by choosing two working sensors from each particle, we obtained 91.4 ± 3.3% accuracy for 13 black and green teas, and 88.6 ± 3.7% for 22 herbal teas. On the other hand, if we chose 8 sensors from each of the four particles, the classification accuracy dropped. This is a significant improvement (> 10%) from single particle assembly. The results showed accurate classification of 35 flavors of teas and provided a promising approach to monitor the quality and grade of tea products. Moreover, by increasing the chemical variation in ligands, it is possible to select the ligand combination that gives the best overall performance.

New nanoscale devices and protocols for molecular detection in aqueous environments at room temperature are desirable for their potential application in biomolecular analysis and in vivo cell studies. Due to their electromechanical properties, graphene and other 2D materials provide a platform for electromechanical sensing in noisy and complex environments. We have investigated this idea by analyzing representative tight-binding models and extended Huckel models for suspended graphene nanoribbons. In particular, we have derived analytic expressions for the current, the electromechanical susceptibility, and signal-to-noise ratio. These expressions reveal the relative importance of thermal fluctuations, strain, and geometric properties in the signal and electromechanical response. We find that as a result of the environmental fluctuations, electromechanical structures have an electron transmission function that follows a generalized Voigt profile, in close analogy to the inhomogeneous lineshapes found in spectroscopic and diffraction studies. These results allow us to formulate optimal sensing protocols in terms of detector parameters, and give the underlying mechanics and fundamental operational principles for the design of graphene-based nanoelectromechanical detectors.

The pH sensing applications have been widely used in various fields, and its redox reaction is the determining mechanism of sensitivity. However, regular pH sensing materials are limited to a pH sensing slope under -59 mV/pH due to the Nernst equation. In contrast, the IrO₂ is known to reveal a super-Nernstian response, a characteristic slope as high as -88.7 mV/pH. In our recent work, we have developed a facile method to synthesize robust IrO₂ thin films via a bipolar pulsing processing combining electrophoresis and electro-flocculation of IrO₂ nanoparticles in a colloidal suspension. The synthesized IrO₂ thin film exhibits a pH sensing slope of -73.6 mV/pH, with impressive surface uniformity and strong adhesion to the substrate.

A novel photoelectrochemical (PEC) immunosensor based on the ZnFe₂O₄/TiO₂/Au@Pt nanoarray functionalized paper working electrode (ZnFe₂O₄/TiO₂/Au@Pt-PWE) was developed and used for ultrasensitive detection of carcinoembryonic antigen (CEA). To achieve a boosted PEC response under visible light excitation, the ZnFe₂O₄ nanocrystals served as photosensitizer were grown in-situ on the surface of the paper fibers with modification of highly ordered TiO₂ nanotube arrays and Au@Pt alloy nanoparticles, which was further used as the 3D electron collector to boost the separation of the photogenerated electron-hole pair of the TiO₂ arrays. The probable PEC promoting mechanism was investigated by Mott-Schottky tests, phosphorescence decay measurements as well as X-ray photoelectron spectroscopy analyses. The obtained ZnFe₂O₄/TiO₂/Au@Pt-PWE was explored as the immunosensor substrate for fixing biomolecules. By virtue of sandwich-like immunoreaction, ultra-sensitive quantification of CEA was obtained. Under the optimized experimental conditions, the developed sensor exhibited a fast response, a broad linear range, and low detection limit. This work may provide a universal method for preparing multifunctional paper-based electrode and offer an innovative platform for the highly sensitive antigen-analysis.

Prof. Wen Shang, Qingchen Shen, Zhen Luo, Jiaqing He, Lingye Zhou, Porf. Tao Deng

School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China, 200240

With billions of years of nature evolution, biological systems have developed many elegant sensing structures with superior sensing performance. Inspired by such biological sensing systems, different man-made sensors have been designed and fabricated to match or surpass the performance of those biological counterparts. In this presentation I will discuss our effort in design, fabrication, and characterization of bioinspired nanostructured sensors for energy and environmental detection. In particular I will discuss the nanosensors for thermal energy detection. The detection of thermal energy is critical in many industrial applications, including the thermal energy distribution within both steam turbine and gas turbine, the thermal energy signal within the component of aerospace vehicles, and the thermal energy distribution within a solar-thermal energy conversion system (1). With the critical roles played by the thermal detection technologies in a growing range of applications, the need of portable and high performance thermal sensing systems drives up the technology development (2, 3). We explored an alternative thermal detection mechanism that is based on the desorption of vapor molecules from the butterfly-inspired nanosensor under the stimulation of incoming thermal radiation, and demonstrated the use of such stimulated desorption for both broadband and wavelength selective thermal radiation detection. This new detection mechanism enables more than an order of magnitude of improvement in both the response of relative reflectance and also the signal-to-noise ratio at the maximum heat-sink-free response speed. The stimulated desorption also provides the opportunity to achieve the wavelength selective thermal radiation detection due to the wavelength selective thermal absorption of the molecules. Based on the similar approach, a self-powered thermal radiation nanosensor that can convert thermal radiation into electric signals was also demonstrated. Besides the thermal energy detection, these bioinspired nanosensors can also be used for the detection of other environmental targets, including both vapor detection and noise detection (4, 5). Such detection approach thus offers a platform for a range of high performance sensing systems, including energy, environmental, chemical vapor, and biological sensing systems.

(1) Tao, P.; Ni, G.; Song, C. Y.; Shang, W.; Wu, J. B.; Zhu, J.; Chen, G.; Deng, T. “Solar-driven interfacial evaporation”, Nature Energy, 2018, 3,1031-1041.
(2) Shen, Q. C.; Luo, Z.; Ma, S.; Tao, P.; Song, C.; Wu, J. B.; Shang, W.; Deng, T. “Bioinspired infrared sensing materials and systems”, Advanced Materials, 2018, 30, 1707632.
(3) Zhang, F.; Shen, Q.; Shi, X.; Li, S.; Wang, W.; Luo, Z.; He, G.; Zhang, P.; Tao, P.; Song, C.; Zhang, W.; Zhang, D.; Deng, T.; Shang, W., “Infrared detection based on localized modification of Morpho butterfly wings”, Advanced Materials, 2015, 27, 1077-1082.
(4) Luo, Z.; Weng, Z.; Shen, Q.; An, S.; He, J.; Fu, B., Tang, R.; Tao, P.; Song, C.; Wu, J.; Deng, T.; Shang, W., “Vapor detection through dynamic process of molecule desorption from butterfly wings”, Pure and Applied Chemistry, 2019, DOI: 10.1515/pac-2019-0118.
(5) Zhou, L.; He, J.; Li, W.; He, P.; Ye, Q.; Fu, B.; Tao, P.; Song, C.; Wu, J.; Deng, T.; Shang, W., “Butterfly wing hears the sound: acoustic detection using biophotonic nanostructure”, Nano Letters, 2019, DOI: 10.1021/acs.nanolett.9b00468.

Liquid crystalline elastomers (LCE)^[1] are crosslinked polymeric systems with the ability to undergo temperature-dependent anisotropic shape changes. These changes are governed by the orientation of liquid crystal mesogens connected to the polymer backbone. The dynamic behavior of LCEs makes them interesting for actuators in soft robots and sensors.

Drawing inspiration from nature^[2,3] we can build materials with microstructures^[4,5] that can perform tasks for applications such as wetting, microfluidics, information processing and cargo transport, capture and release. When composed of LCEs, these arrays of microstructures can variously tilt, twist, stretch or contract upon heating depending on a director alignment encoded magnetically during fabrication.^[6] Upon incorporation of a molecular photoswitch (e.g. an azobenzene), they can be rendered light-responsive^[7] which further extends their range of deformations. Here, we use light-responsive microstructured LCE surfaces with unprecedented deformation behavior. In arrays of pillars of different geometry, reversible twisting of these pillars can be triggered with directionality governed by the angle of UV irradiation with respect to the director alignment. Achiral surfaces can thus be dynamically converted into surfaces with controlled chirality. Moreover, depending on the light intensity, deformation modes beyond twisting are accessible. This bioinspired LCE platform offers new modes of deformation, coupled motion, and energy conversion that may be interesting in contexts such as soft robotics and microfluidic devices.

Keywords: Actuators; Bio-Inspiration; Multi-Functionality; Light-Control; Autonomous Materials

References
[1] Ohm, C.; Brehmer, M.; Zentel, R. Adv. Mater. 2010, 22 (31), 3366–3387.
[2] Aizenberg, J. MRS Bull. 2010, 35 (04), 323–330
[3] Studart, A.R. Angew. Chem. Int. Ed. 2015, 54 (11), 3400–3416.
[4] Pokroy, B.; Kang, S.H.; Mahadevan, L.; Aizenberg, J. Science 2009, 323 (5911), 237–240.
[5] Sidorenko, A.; Krupenkin, T.; Taylor, A.; Fratzl, P.; Aizenberg, J. Science 2007, 315 (5811), 487–490.
[6] Yao, Y.; Waters, J.T.; Shneidman, A.V; Cui, J.; Wang, X.; Mandsberg, N.K.; Li, S.; Balazs, A.C.; Aizenberg, J. Proc. Natl. Acad. Sci. U.S.A. 2018, 115 (51), 12950–12955.
[7] Shang, Y.; Wang, J.; Ikeda, T.; Jiang, L. J. Mater. Chem. C, 2019, 7, 3413-3428

Early and reliable fire detection is essential to prevent thousands of human deaths and injuries every year in the US alone [1]. Modern fire (smoke) sensors detect the light scattered from carbonaceous particles (soot) emitted during open fires. Such particles have fractal-like structure formed by agglomeration and surface growth [2]. Yet, their optical properties are calculated typically by Rayleigh or Mie theories for spheres impeding their selective sensing among other airborne particles [3]. So, fire detectors are not selective enough for such particles producing false alarms costing up to 1 billion £/y in United Kingdom alone [4].
Here, carbonaceous agglomerate structure and light scattering are measured from premixed ethylene flames [5] and simulated by coupling Discrete Element Modeling (DEM) for surface growth and agglomeration with the Discrete Dipole Approximation (DDA) [6]. Using the Mie theory for spheres overestimates the measured particle mass up to 10 times. This results in 60 % larger differential light scattering cross-sections than those measured in premixed ethylene flames. In contrast, the DEM-derived particle structure and light scattering is in excellent agreement with those measured here in premixed flames. Thus, the DEM-DDA can be used to optimize the selective sensing of fire detectors.

References:
[1] Ahrens, M. (2019). http:// www.nfpa.org.
[2] Kelesidis, G. A., Goudeli, E., & Pratsinis, S. E. (2017). Carbon, 121, 527-535.
[3] Keller, A., Loepfe, M., Nebiker, P., Pleisch, R., & Burtscher, H. (2006). Fire Safety J., 41, 266-273
[4] Chagger, R., & Smith, D. (2014). http://www.bre.co.uk.
[5] Kelesidis, G. A., Kholghy, M. R., Zuercher, J., Robertz, J., Allemann, M., Duric, A., & Pratsinis, S. E. (2019). Powder Technol., doi.org/10.1016/j.powtec.2019.02.003.
[6] Kelesidis, G. A., & Pratsinis, S. E. (2019). Proc. Combust. Inst., 37, 1177-1184.

We demonstrate the self-powered wireless triboelectric vibration sensor as made from the naturally nanoporous SiO₂ particles for allowing the detection of the vibrations and movement in the underwater environment. The nanoporous SiO₂ particles are directly converted from rice husks (referred to as RH_SiO2), which exhibit strongly interacting surface hydroxyl groups. Through the enzymatic treatments, the surface potential of the RH_SiO2 can be modulated to obtain either an extremely low or strongly high electronegativity. Specifically, by adding fluorinated groups using fluoroalkylsilane (FOTS) treatment to obtain RH_SiO2-F, the charge density of the RH_SiO2-F triboelectric nanogenerator (TENG) can be enhanced ~ 56.67-fold as compared to the untreated RH_SiO2-TENG. The power density of the RH_SiO2-F TENG is increased from 0.077 mWm^-2 to 261 mWm^-2. The RH_SiO2-F particles are encapsulated in a quartz cube to fabricate a self-powered wireless sensor that can be stabilized for operating in water at various temperatures. The theoretical calculation further demonstrates that the triboelectric potential is dramatically established between the surface functionalized RH_SiO2-F particles and the quartz’s surface. With porous nature of rice husks covered with nano-Si is of a high functionality for designing a new-type TENG which has a great potential to apply in the environmental monitoring.

The reliable production of atomically-thin crystals with tailored properties is essential for exploring new science and implementing novel technologies in the 2-D limit. However, ongoing efforts are limited by the vague potential in scaling-up, restrictions on growth substrates and conditions, small sizes, and instability of synthesized materials. In this talk, I will discuss our recent progress in the discovery and production of tellurene (2-D form of elemental tellurium) with an intriguing chiral-chain structure. The solution-synthesized tellurene crystals exhibit process-tunable thickness from a monolayer to tens of nanometers, lateral sizes ~ 100 mm, and can be transferred to designer substrates. I will further discuss our findings in 2-D tellurene’s material properties, as well as our efforts in the prototypical device explorations with the acquired fundamental understandings of tellurene’s properties. Our results show that the air-stable tellurene, as an emerging 2-D material, exhibits a plethora of intriguing physical properties appealing for applications in electronics, optoelectronics, energy, sensors, and quantum devices.

Bulk photovoltaic (PV) effect has recently attracted an increasing interest for its potential in simplifying the structure of optical sensors and photovoltaic cells, and as a possible solution to overcome the Shockley – Queisser limit in the solar cell energy conversion efficiency. Until recently, the bulk PV effect has only been observed and exploited in non-centrosymmetric materials, strongly limiting the range of possible applications.

A possible way to overcome this limitation comes from the so-called flexo-PV effect, in which a gradient strain is used to induce an internal polarization within the material, enabling the collection of photo-generated carriers also in centrosymmetric materials. In this context flexo-PV has already been demonstrated in many large and intermediate bandgap materials, including titanium dioxide, strontium titanate, and silicon under visible and ultraviolet light illumination.
Low bandgap materials, such as Bismuth Telluride (Bi₂Te₃) are sensitive to infrared (IR) light, opening opportunities for the implementation of the bulk PV effect to develop new types of IR sensors. However, a comprehensive theoretical and experimental analysis of the strain-induced flexo-PV effect in this kind of materials is still missing. Furthermore, the flexo-PV effect has been manly demonstrated at the nanoscale, leaving doubts on its applicability in wider contexts and for macroscopic device footprints.

In this communication we present the experimental demonstration of flexo-PV effect in Bi₂Te₃ thin films, under different illuminations and conditions. Depositing Bi₂Te₃ thin films on flexible substrates, by magnetron sputtering, and developing ohmic contact and anti-reflective coating by e-beam evaporation, we were able to create a strain gradients in the material, and analyze its electrical response. We will show and discuss how the power output in our samples can be varied as a function of the applied strain, sample temperature, and illumination level at different wavelengths. Our results help to shed light on the relation between applied strain, material polarization, and the photon detector performance, and reveal new possible applications of the flexo-PV effect in low-gap materials under different illumination scenarios. The new flexo-electric photo-detectors can operate under real-life conditions, i.e., without cryogenic cooling and in the self-powered regime, and provide an opportunity to integrate energy-harvesting and conversion as well as environmental sensing capabilities on a single cheap, lightweight, and flexible platform.

Water, a solvent for a poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) solution, was solvent-exchanged with various solvents by an ultrafiltration method, while PEDOT:PSS continuously remained in a wet state for stable redispersion without aggregation. Among the tested solvents, we demonstrated for the first time that PEDOT:PSS was successfully and stably dispersed in ethanol (EtOH) and ethylene glycol (EG) by the facile ultrafiltration method. Compared to water, the EtOH based PEDOT:PSS exhibited enhanced wettability over Polydimethylsiloxane (PDMS) substrates without any additional surface treatments, as measured by the contact angle measurements. The solvent exchange process (SEP) did not alter the conductivity of PEDOT:PSS from its pristine form, but increased the pH from 2.4 to 4.5 by removing the excess PSS. An exceptionally uniform with conformal nano-coating of EtOH-based PEDOT:PSS solution on PDMS pyramids resulted in a pressure sensor exhibiting very high sensitivity in the low-pressure regime (-21 kPa^-1, < 100 Pa), a fast response time (90 ms) with a maximum load of 1kPa, and high mechanical stability over 10000 cycles. In addition, the pressure sensor functioned as a human pulse rate sensor that produced a precise pulse waveform with peaks clearly indicating incident, systolic, and diastolic pressure. Additionally, it exhibited excellent selective accuracy with little interference in a 4 x 4-pixel pressure mapping panel. We believe that the solvent-exchange process developed in this study provide the pressure sensor could deliver new opportunities in wearable electronic applications.

Nanosystems capable of reversible transitions between two states in response to applied stimuli are attracting more and more attention given the pursuit of miniaturization of trigger-type and switch-type elements. These elements are key units of many functional nanodevices (e.g., molecular and macromolecular switches, single molecular sensors, molecular actuators, electric- and optoelectric devices, as well as logic gates). The desire to create nanometer-scale switching devices has motivated an active search for bi-state macromolecular systems allowing for sharp conformational transitions in response to stimuli. Short oligomers possessing such mechanic-like characteristics are a promising platform for the design of nanodevices and molecular machines.
Using full-atomistic simulations, we demonstrate that oligomers of thermosensitive polymers, themselves only a few nanometers in size, possess conformational bistability, and react to power loads as non-linear mechanical systems. We establish the bifurcation effect, the hysteresis effect, spontaneous vibrations, as well as the stochastic resonances of these oligomers [1]. This study shows how such switching-like behavior of oligomers of thermosensitive polymers can be used in molecular nanosensing, in particular for single molecule detection.
[1] Vladik A. Avetisov, Anastasia A. Markina, Alexander F. Valov, “Oligomeric "Catastrophe Machines" with Thermally Activated Bistability and Stochastic Resonance”, submitted to the Journal of Physical Chemistry Letters (2019).

Yttrium oxy-hydride (YO_xH_y, cubic, a = 5.35 Å) is a promising material for optoelectronics applications such as smart windows and optical sensors, because optical transparency and electrical resistance can be reversibly switched by sunlight irradiation [1]. It is reported that the resistance of YO_xH_y is reduced by one order of magnitude on irradiation of sunlight [1]; however, the resistance change is smaller than that of the commercially available CdS (three orders of magnitude). A possible reason of the small response in the resistance is their polycrystalline nature of YO_xH_y films deposited on glass substrates [1, 2].

In this study, we fabricated epitaxial YO_xH_y thin films, and investigated the influence of crystallinity on the photochromic response. We observed a sunlight-induced resistance decrease by three orders of magnitude, comparable to that of CdS, and finally achieved reversible light-induced insulator-to-metal transition using UV-laser irradiation.

Epitaxial thin films of YO_xH_y were deposited on single-crystalline yttria-stabilized zirconia (YSZ) (111) substrates using reactive magnetron sputtering (thickness: ~150 nm). We monitored the four-probe resistance of the YO_xH_y epitaxial thin films during the light irradiation using a solar simulator (intensity: 1 mW/mm²) and a UV laser (wavelength: 375 nm, intensity: 15 mW/mm²).

Irradiation of sunlight for 30 min. reduced the resistance in YO_xH_y epitaxial thin film by three orders of magnitude (2×10⁸ W to 2×10⁵ W). Furthermore, we found that the irradiation of UV-laser (more intense light) for 90 min. induced insulator-to-metal transition in YO_xH_y epitaxial thin films. The original four-probe resistance of as-grown YO_xH_y epitaxial thin film was ~5×10⁷ W (high resistance state). After the UV laser irradiation, the resistance sharply dropped accompanying color change, and finally reached 2×10¹ W (low resistance state). In this low resistance state, surprisingly, the temperature dependence was metallic in the range of 4 – 300 K. We next annealed the UV irradiated sample at 125°C for 2 h in 4%-H₂/Ar gas atmosphere of 0.05 MPa. As a result, the optical transparency and the high resistance state recovered. We again irradiated UV-laser onto the YO_xH_y sample and confirmed the metallic behavior. These results demonstrate the reversible light-induced insulator-to-metal transition in YO_xH_y epitaxial thin films.

References:
[1] T. Mongstad et al., Sol. Energy Mater. Sol. Cells. 95 (2011) 3596.
[2] C. C. You et al., Materialia. 6 (2019) 100307

Two-dimensional (2D) nanomaterials, particularly when their thickness is just one or a few atomic layers, exhibit physical properties dissimilar to those of their bulk counterparts and other forms of nanostructures. Nonetheless, 2D nanostructures so far have been largely limited to naturally layered materials, i.e. the van der Waals solids. A much larger and diverse portfolio of 2D materials including non-layered compounds are desirable to meet the specific requirements of individual components in various devices. We demonstrate that surfactant monolayers could serve as a soft template supporting the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. Through this approach, 1 to 2 nm thick, single-crystalline free-standing ZnO nanosheets with sizes up to tens of microns were synthesized at the water-air interface. This technique was denoted as Ionic Layer Epitaxy (ILE) – the first solution-based technique for growing large-area ultrathin nanosheets without the support of crystalline substrates. Mimicking the biomineralization processing by using mix charge surfactants led a successful synthesis of single-crystalline nanosheets from a broad range of functional oxide materials, including CoO, MnO₂, Bi₂O₃, etc. New physical properties emerged from the ultrathin geometry. For example, stable p-type conductivity was observed from the ZnO nanosheets as a result of electron depletion. High concentration of cation or oxygen vacancies could be controlled by the surfactant modulation, leading to superior magnetic property and memristive behavior. Substantially enhanced electrochemical catalytic performance was also discovered from multiple ultrathin oxide systems. In general, ILE vastly broadens the range of 2D nanomaterials from layered van der Waals solids to oxide ceramics, opening up opportunities for discoveries of exciting transport, magnetic, photonic, and catalytic properties.

The superior ferroelectric and nonlinear property of LiNbO₃ makes it a popular and promising candidate for electro-optics, energy converter and memory devices. However, integration of LiNbO₃ on technologically important substrates has been a challenge. In this work, by pulsed laser deposition, we synthesize epitaxial LiNbO₃ thin films on muscovite mica through van der Waals epitaxy. We apply X-ray diffraction to characterize the impurity phase and optimize the growth condition. We reveal the in-plane epitaxy relation by pole figure/X-ray diffraction phi scan. We obtain freestanding LiNbO₃ thin films by mechanical exfoliation. Raman spectroscopy is used to confirm LiNbO₃ phase after transfer from muscovite mica. Ferroelectric and pyroelectric devices out of freestanding LiNbO₃ reveal giant polarization in film of a few tens of nanometers in thickness. Our work shows that van der Waals epitaxy may enable the feasibility of integrating high-quality LiNbO₃ onto semiconductor substrates such as Si, opening a window in pursing novel heterogenous devices and circuits.

Amorphous oxide semiconductor based thin film transistors (TFTs) using quantum dots (QDs) have great potential for photosensors because they show high sensitivity to light and excellent electrical properties (field effect mobility and on/off ratio) [1]. In most studies, characteristic evaluation of QD-based oxide phototransistors is the effect of interest. However, the interface and the charge transfer between the oxides and the QDs are rarely reported.
In this presentation, indium−gallium−zinc oxide (IGZO) TFTs with QDs are fabricated and characterized. We uses QDs with core and core/shell structures and with various surface ligands. In order to explore the change of electrical energy levels and chemical bonding between the IGZO and the QD according to different surface treatments, ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurement are carried out. Lastly, we perform time resolved photoluminescence (TRPL) measurement on the IGZO with various type of QDs to clarify the carrier transfer mechanism.

Acknowledgment
This work was supported by Institute for Information & communications Technology Promotion (IITP), Korea government (MSIT), project “Development of Core Technologies for Transparent Flexible Display Integrated Biometric Recognition Device (2018-0-00202)”.

References
[1] D. Kufer, G. Konstantatos, ACS Photonics, 3, 2197 (2016).

The excellent intrinsic properties of aligned nanofibers, such as carbon nanotubes (CNTs), and their ability to be easily formed into multifunctional 3D architectures motivates their use as shape-engineerable materials for a variety of commercial applications, such as batteries, chemical sensors for environmental monitoring, and energy harvesting devices. While controlling nanofiber adhesion to the growth substrate is essential for bulk-scale manufacturing, application-specific performance, and integration with advanced device platforms, limited experimental approaches and models to date have neglected to address the scaling of CNT array-substrate adhesion (F_a) with processing conditions, and a mechanistic understanding of this phenomenon is still unknown. In this work, we synthesize vertically aligned CNT arrays by chemical vapor deposition and combine experimental and modeling techniques to study a simple, post-growth annealing step we call 'cementation' and its effect on F_a as a function of temperature (T_c) up to 1000°C. This process, which increases F_a by two orders of magnitude (∼0.5–50 N/cm², as measured via uniform CNT array delamination from a flat growth substrate) enables the capillary densification of CNT arrays spanning three orders of magnitude in height (μm-mm scale) to create dense, patterned architectures whose geometries can be accurately predicted by elasto-capillary theory. Here, the non-monotonic scaling of F_a with T_c is quantified and modeled analytically based on contact mechanics theory and the atomic- and meso-scale evolution of CNT-catalyst-substrate interfaces. Extensive morphological, structural, and chemical characterization of the CNT arrays and CNT-substrate interfaces is performed via scanning and transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. These results show that higher T_c reduces the CNT-catalyst number density, graphitizes and stiffens the CNT roots to increase F_a, and alters the mechanical deformation mode during CNT-substrate separation, consistent with the model results. Using this approach, this work provides new insights into the interfacial interactions responsible for variable nanofiber-substrate adhesion and enables the creation of shape-tunable architectures for advanced sensors, energy devices, and emerging nanoscale technologies.

Nanoporous metals have drawn increasing interests due to their special three dimensional bicontinuous structure, which features high specific surface area, open porosity for gas or fluid channel and outstanding electrochemical performance. Among these metals, nanoporous copper (NPC) has several attractive advantages, including high electrical and thermal conductivity and relatively cost effective compared to other noble metals. CO₂ reduction reaction (CO₂RR) and hydrogen evolution reaction (HER) were evaluated by using NPC as electrocatalysts, and the results showed that the performance of NPC electrocatalysts is ten times higher than that of Cu foil one without nanostructure.
In this research, both bulk and sputtered thin film of Cu_x-Al_1-x (x=18-37 at%) precursor alloys were chemically dealloyed to synthesize NPC thin film and powder, with ligament sizes ranging from 23 to 168 nm. The ligament size effects on CO₂RR performances were observed in 0.1M KHCO₃ under the potential range of -0.3V to -1.6V (vs Ag/AgCl), while HER seems to present simultaneously. The various structures of nanoporous electrocatalysts suggest different selectivity between CO₂RR and HER. Our goal is to clarify the mechanism of the catalysts, and build a model between two forms of precursor alloys, ligament sizes and the resultant products. Finally, the electrochemical reactions within the NPC structures could be predicted and well-controlled.

Various types of transmembrane pores and ion channels in the range of 1-100 nm are found in each biological cell, playing crucial roles in varieties of significant physiological activities such as maintaining cell osmotic balance and stabilizing cell volume. Inspired by this natural phenomenon, different biomimetic nanodevices (nanopore/channel) with different characteristics have emerged as an attractive and powerful platform and have been used for a wide range of applications. Generally, the principle of analysis applications based on solid nanopore/channel can be described as follows: molecules access or attach on the inner surface of a nanopore/channel, change the effective diameter of the nanopore/channel, or affect the charge transfer as well as the wettability of the inner surface of the nanopore/channel, leading to ionic current changes that can be detected.

In this study, Au NPs and TiO₂ integrated into PANI are synthesized by facile and time-saving electrochemistry techniques of low waste emission. The stability of Au NPs-TiO₂/PANI, which superiors than most of reported results, reached up to 70 days. The sensitivity and detection limit are 355.8 μA mM^-1cm^-2 and 0.15 μM (S/N=3), respectively. It is also confirmed that Au NPs-TiO₂/PANI performs high selectivity by the interference test. The current response of the developed Au NPs-TiO₂/PANI electrodes were tested 5 times, and the relative standard deviation was obtained to be 1.30%, indicating a high reliability of the electrode preparation method. The evaluated results suggest the applicability of Au NPs-(0.05)TiO₂/PANI electrode for glucose sensors.
More than 285 million people worldwide are suffering from diabetes mellitus, which is a major public health problem today [1]. Therefore, accurate, fast, and reliable sensors for detecting glucose are attracting a great attention to prevent further development of diabetes mellitus. Electrochemical sensors are considered to be the most promising measure due to its good sensitivity, rapid response, outstanding selectivity, high reliability, portability, and user friendliness. Electrochemical glucose sensors are further divided into enzymatic and non-enzymatic types. Enzymatic sensors are considered to be less stable than non-enzymatic ones since its accuracy tend to fluctuate easily with temperature, humidity, pH, and toxic chemicals.
In the non-enzymatic glucose sensors, metals such as Pt, Pd, Au, Ag, Cu, Ni, CuO, NiO, and their derivatives are found to be intrinsically electroactive to the electrooxidation of glucose. Since the faradic current arising from the glucose oxidation can be enhanced by increasing the surface area, the aforementioned metals and metal oxides in nano scale have been considered as a promising candidate. In particular, Au is at the top of the list due to its high electrocatalytic activity and negative oxidation potential for the oxidation of glucose [2].
Oxides such as ZnO, CeO₂, and TiO₂ were utilized for achieving well distribution of nano-structure Au, since they have an effect to immobilize the Au nanoparticles (NPs) and impede the aggregation of Au NPs due to the anchoring effect. In this work, TiO₂ was chosen to control the Au NPs due to its low cost, biocompatibility, and ease of modification. TiO₂ also showed a great affinity to H₂O and OH^-, which is a critical factor for the oxidation of glucose in the aqueous solution. Electrooxidation of glucose, which is catalyzed by Au NPs, is benefited from the OH_ad on the adjacent TiO₂, and hence the current of electrooxidation of glucose is enhanced.
Polyaniline (PANI), which is a common electrically conductive polymer, was utilized as a supporting material in this study, for its low cost, good stability, and good electrical conductivity [3]. Nano-materials (i.e. metal NPs and/or oxides) integrated with PANI enhanced the electrochemical properties, such as augmented amount of nano-materials loading on electrode, accelerated electron transfer, and homogeneous distribution of nano-materials. These improvements accordingly result in improved electrocatalytic activity to the oxidation of glucose.
It has been known that the fabrication process for the non-enzymatic glucose sensor is tedious and time-consuming. In this study, however, a facile and time-saving procedure to fabricate Au NPs-TiO₂ decorated PANI was conducted. Furthermore, by virtue of the facile and time-saving procedure, little waste-solution emission was realized.
Reference
[1] J. E. Shaw, R. A. Sicree, and P. Z. Zimmet, Diabetes Res. Clin. Pract. 87 (2010) 4-14.
[2] F.-Y. Kong, X.-R. Li, W.-W. Zhao, J.-J. Xu, and H.-Y. Chen, Electrochem. Commun. 14(1) (2012) 59-62
[3] C. Dhand, M. Das, M. Datta, and B. D. Malhotra, Biosens. Bioelectron. 26 (2011) 2811-2821

Infrared thermal imaging is a key technology used in night vision, surveillance, medical diagnosis, firefighting, as well as in the chemical, metallurgical and petroleum industry, where the monitoring of thermal processes is fundamental for correct and safe operation. A microbolometer measures the temperature changes through the change in the resistance of a thermosensor material as a result of the absorption of infrared radiation. The operation of the uncooled microbolometers is carried out at room temperature, which is convenient and advantageous for systems of smaller size, weight, power, and cost (SWPaC). Microbolometers are generally made of vanadium oxide (VO_x) or amorphous silicon (a-Si:H), both types must meet the requirements of low resistivity to perform a coupling with the reading circuit and a large temperature coefficient of resistance (TCR). In addition, the material must have low electrical noise and, in order to reduce manufacturing costs, be compatible with standard integrated circuit (IC) fabrication processes. As a result of the current trend, it is evident that amorphous silicon is receiving a lot of attention in its application and improvement as the active sensing layer due to its compatibility with IC processes. However, it still has a high level of noise and electrical resistivity. By modifying the deposition parameters during the growth of a-Si:H, it can be turned into a nanostructured material. Hydrogenated polymorphous silicon-germanium (pm-Si_xGe_1-x:H) is a material composed mainly of an amorphous matrix with embedded nanocrystals (approximately 2-5 nm) produced in the plasma cloud and incorporated during growth. These nanocrystals improve transport properties and resistance to radiation-induced degradation, which translates into better performance and long-term stability. Here we present the characterization of a pm-Si_xGe_1-x:H alloy deposited by plasma enhanced chemical vapor deposition (PECVD) at 200 °C and the application of this in microbolometers. The material properties as surface roughness, TCR and resistivity have been investigated. Device performance having an area of 50x50 µm² was evaluated through responsivity and noise measurements.

Hierarchical nanoscale structure plays a vital role in achieving excellent property, which can successfully be used in preparing tailor-made materials for sensing applications. These hierarchical nanostructures can successfully be prepared with the help of structure directing agents (SDAs). This work reports the preparation of polyaniline (PANI) nanorods in the presence of anionic (sodium dodecyl sulfate, SDS) and nonionic surfactant (Pluronic F127) as the SDAs. We prepare a series of ternary composites with varying ratio of PANI and F127 for a given amount of SDS. The SDS and F127 play a pivotal role as SDAs in delivering a hierarchical nanostructure of PANI. Detailed characterization reveals that the in-situ polymerization of PANI confirms the formation of the well-disperse composite. The amphiphilic character of F127 facilitates the formation of a core-shell micellar structure, which in turn facilitates the polymerization of PANI in the core region. We have observed a composition based structure formation of the ternary composites. At the increasing amount of F127, the composite exhibits higher crystallinity (viz. less porosity) ensured via XRD and microscopy images. Among them the synthesized composite at the 1:1 ratio of PANI and F127 exhibits less porosity (viz., high crystallinity) explored by BET surface area measurements. The XRD analysis does not show a sharp peak, which signifies that the material acquires a polycrystalline structure, as revealed by HRTEM analysis. The polycrystalline nature of the nanorods facilitates the highest thermal stability, contributes significantly towards the enhanced electrochemical activity and thus, can successfully be used in sensing applications. The 1:1 material shows remarkably highest glucose sensitivity (~485.787 μΑ/cm² mM) compare to ternary composites under amperometric measurements. The sensor shows an excellent electrochemical performance within a range of 5 to 50 mM with a lower detection limit of ~3.202 μΜ.

Nanomechanical resonator offers the possibility of an ultrasensitive detection of various physical quantities, with emerging applications including signal processing, biological detection and fundamental tests of quantum mechanics. By exploiting a vibration of nanoscale cantilever, electromagnetic wave has been reported to be detected even with a nanoscale detector. However, the detector cannot correct the whole energy of the wave due to its ultra-small size, which results in the failure of the detection. Here, to improve the detection performance, we introduce digital signal processing for nanomechanical systems.
We first suggested the nanomechanical phase detector, which consists of single carbon nanotube (CNT) cantilever^[1]. Here, the digitized phase of the electromagnetic wave is to be detected. In the proposed detector, the tip of the CNT vibrates according to the phase. The amplitude of the vibration can be observed via field emission current; the oscillation of CNT varies the current, and the original phase information can be estimated by observing the current. Analytical investigation reveals an interesting point that the information is contained in two frequency components of the current. By introducing a simple signal processing that combines these components with optimum weights, our analysis shows that the detection performance is significantly enhanced.
One of the critical issues in the detector is the dependence on temperature. In traditional semiconductor physics, the current inherently depends on temperature, which affects on the detection performance. To analyze the influence of the temperature variation, a mathematical model of the mean and variance of the current was first derived based on experimental results^[2]. The obtained mathematical expression shows that both the mean and variance exponentially increase with increasing temperature. Based on this result, the detection performance was also analyzed; as the increase of the temperature, the variance is significantly increased, and then the detection is likely to be failed. This analysis indicates that to secure the reliable detection, temperature should be kept low like room temperature.
Above results provide a novel framework of detecting the electromagnetic wave in nanomechanical systems. The operational condition regarding temperature was also derived. Notable point is that the digitized phase of the electromagnetic wave can be detected in the nanoscale detector. This indicates that digitally modulated signals, which are often used in digital communications, can be detected even with nanomechanical systems. This opens up a new paradigm to transfer digital data, which contributes to the forthcoming age with IoT and AI-based systems.

[1] Y. Tadokoro, Y. Ohno, and H. Tanaka, IEEE Trans. Nanotech., 17, 1, 84-92 (2018).
[2] K. Funayama, H. Tanaka, J. Hirotani, K. Shimaoka, Y. Ohno, and Y. Tadokoro, IEEE Access, 7, 57820-57828 (2019).

Temperature is an important parameter that regulates chemical reactions of the environment and has been developed until recently for monitoring in various fields. Particularly, developing polymer positive temperature coefficient (PTC) thermistors is attracting much attention because they have several advantages including high sensitivity, flexibility, conformability, biocompatibility, and biodegradability. However, most polymer PTC thermistors including containing a high concentration of opaque filler materials still have issues such as low sensitivity, low optical transparency, and poor operational durability due to low electrical conductivity and inefficient hopping transport. Here, a highly sensitive, flexible, and transparent polymer thermistor based on silver fractal dendrites (AgFDs) and a polyacrylate (PA) matrix has been successfully demonstrated. AgFDs–PA composite films are prepared using a simple melt-mixing process for the AgFDs and molten PA. It exhibits a superior PTC effect (about 10⁶~10⁷ Ω) around body temperature because of the high electrical conductivity of the AgFDs and the quantum tunneling effect resulting from their unique structures. A flexible and transparent thermistor based on the AgFDs–PA composite shows excellent sensitivity, detection resolution, and fast sensing response through dramatic resistance changes in the human body temperature range. Moreover, it exhibits excellent optical transparency, mechanical flexibility, and operational durability. In addition, electrical impedance spectroscopy (EIS) analysis proved that the quantum tunneling effect amplified by the AgFD branch has a significant effect on the changes in resistance. Based on these characteristics, the thermistor was successfully demonstrated for real-time temperature monitoring of human body.

Calcium carbonate (CaCO₃) is a widely important mineral that is found extensively in biological and geological systems. The interaction of water, ions, organic molecules, macromolecules, and nanomaterials with calcite surfaces controls a broad range of natural and industrial processes such as ion exchange, contaminant migration, enhanced oil recovery, biomineralization, and flocculation. Previously, an atomic force microscopy (AFM) study and molecular dynamics (MD) simulations demonstrated that carboxylate alkanethiols functionalized AFM tips (as surrogates for nanomaterials) showed mitigated adhesion to calcite surfaces in fluids with specific ions. The MD study also showed that calcite crystallography affected adhesion as a result of surface calcium ion exposure. Here, we extended the MD simulations to explore the effects of calcite surface defects. Specifically, we focused on four line and point defects on the calcite (104) surfaces: (1) acute step, (2) obtuse step, (3) Ca²⁺ vacancy, and (4) CO₃^2- vacancy. In addition, we considered three fluids: (1) deionized water (DI), (2) seawater (SW; 0.7 M NaCl), and (3) brine (B; 1.28 M NaCl + 0.34 M CaCl₂), which the latter two are relevant in subsurface hydrocarbon reservoir applications. The simulation results showed that the calcite surface defects have the strongest effects on adhesion in DI compared to in SW and B, where in DI the adhesion were changed by -30% on obtuse step, -10% on Ca²⁺ vacancy, and +25% on CO₃^2- vacancy. On the other hand, in SW and B the adhesion changed within ±10% in most cases. Interestingly, we observed an inversion of the adhesiveness of vacancies compared to no-defect surfaces in different fluids, where the Ca²⁺ vacancies were less adhesive in DI but more adhesive in SW and B, and the CO₃^2- vacancies were more adhesive in DI but less adhesive in SW and B, due to counter ion accumulations on the vacancies. We believe the atomic-level understanding obtained in this work can be helpful for developing novel nanomaterials for subsurface applications and may further the understanding of wettability alteration mechanisms in ionic fluids.

Zinc oxide (ZnO) has attracted a great deal of interest because it has direct wide band gap of 3.37 eV at room temperature (RT) and large exciton binding energy of 60 meV, and these properties make ZnO a suitable material to fabricate a variety of optoelectronic devices, such as light-emitting diodes, laser diodes, optical communication, and solar cells. In addition, ZnO represents several other interesting advantages like high optical transparency in the visible region and high carrier mobility, so it can be applied as flexible and transparent UV photodetectors. But, ZnO has high electrical resistivity owing to its low carrier concentration induced by native donor defects, interstitial zinc, and oxygen vacancies which degrade the optical and electrical properties of ZnO thin films-based UV photodetector. Thus, many researchers have doped with group three elements such as aluminum, gallium, and indium acted as donor dopants for ZnO to reduce the electrical resistivity of ZnO thin films [1]. Particularly, the indium-doped ZnO (IZO) represents higher optical transmittance, better conductivity, and excellent surface roughness than the other dopants doped ZnO. IZO thin films can be synthesized by various techniques such as physical or chemical vapor deposition, pulsed laser deposition, atomic layer deposition, and sol-gel spin coating method. Among these methods, sol-gel spin coating method has some advantages including uniformity of thickness, controllability of composition, simplicity, and low cost [2]. However, since the sol-gel spin coated IZO thin films must be annealed at high temperature to crystallize the amorphous phase of IZO, the IZO thin films-based flexible and transparent UV photodetectors which utilize the polymeric substrate couldn’t have been fabricated by sol-gel spin coating method. Therefore, it is necessary to develop a new annealing method which prevents damage to polymeric substrate that are affected by high temperature during annealing process.
In this study, we annealed the sol-gel spin-coated IZO thin films by using a novel method, synchronized heat and cool annealing (SHCA) method, which anneals thin films on top with IR heaters and simultaneously removes the high light energy transferred to polymeric substrate using cold plate. To investigate the effect of indium on the structural and optical properties of IZO thin films annealed by SHCA method, we measured the IZO thin films by using X-ray diffraction (XRD), and photoluminescence (PL). As a result of the measurement, in the XRD, IZO thin films showed the three diffraction peaks corresponding to ZnO (100), (002), and (101) plane, and the diffraction peak of ZnO (002) plane was red-shifted. For PL, the intensity of the near-band-edge and deep-level emissions for IZO thin films significantly increased, which indicates that carrier concentration and the number of defects in the ZnO lattice increased, respectively.

By detecting gases, we can take early measures in keeping the safety and freshness of our life. So far, various types of highly sensitive and selective gas sensors have been reported using carbon nanotubes or nanowires as sensing materials. However, these previous reports have limits in their practical application due to the need of electrical power and high manufacturing cost. Here, we introduce a biomimetic colorimetric film that can be utilized for the detection of ethylene glycol. The film is fabricated by Chitin on a silicon surface by a thermal evaporation from a Chitin powder. The film enable to thin-film effect into stable color patterns. By controlling the evaporation times 1 to 3, the film thickness could be controlled from 50 to 180 nm, resulting in films with different colors in the visible range.
When the Ethylene glycol reacted with the amine groups on the Chitin-film by etherification, the films underwent both chemical and structural changes, resulting in color changes in sub-ppm concentrations.
We demonstrated that sensing data analysis interlocked with a phone camera and discriminant analysis algorithms can further discriminate gas concentration in air. We expect that our results will be utilized in providing a low-cost and low-power systems for the monitoring of gas.

There is an increased interest in thin film transparent heaters (TFTH) for applications in optoelectronics, military, de-icing of airplane wings, defogging of windshields, and a plethora of domestic usages such as smart windows, defrosters in automobiles and many other applications. In this work we present the growth and characterization of one of the potential TFTHs, Gallium doped zinc oxide (GZO). GZO thin films were deposited by pulsed laser deposition (PLD) on transparent substrates such as glass and flexible polymer. Ga doped (5 atomic weight percentage) ZnO target was made in the laboratory using Ga₂O₃and ZnO powder. GZO films with thickness 110 and 125 nm were deposited on glass and flexible polymer at substrate temperatures of 400°C and 120°C, respectively. Atomic force microscopy images reveal that the films are very smooth with excellent surface roughness (~1.18 nm). The temperature dependent resistivity measured was done using linear four probe measurement and the samples showed a very low sheet resistance value of about 2.6×10-4 Ω cm and also exhibited high optical transparency value of (>90%) from transmission data. GZO transparent heater also showed a stable and reproducible Joule heating effect and the temperature can reach easily 100°C by the application of low input (~8V) voltage. This research finding may be beneficial for the potential use of GZO as a transparent oxide material for possible applications in the emerging area of low-cost power electronics, and flexible and wearable optoelectronics devices.

This work has been supported by NSF-CREST Grant number HRD 1036494 and NSF-CREST Grant number HRD 1547771

Photodetectors in the short wavelength infrared region (SWIR: 0.9 to 2.5 μm) have attracted attention for use in environmental monitoring, night vision, automotive autonomous driving and also biological research. Recently, we thermally diffused Ag to an n-type Mg₂Si substrate to fabricate a Mg₂Si pn junction photodiode (PD), and succeeded in obtaining photoensitivity below a cutoff wavelength of about 2.1 μm. The lifetime of minority carrier is important to enhance the photosensitivity of PDs, but so far the lifetime of Mg₂Si has not been investigated. In this paper, we measured the lifetime of Mg₂Si pn junction PDs fabricated by the thermal diffusion method in the temperature range from 300 K to 77 K by the open circuit voltage method (OCVD) method. The observed voltage decay waveform which is a result of the time required for the minority carrier decay became longer with decreasing the temperature and reached maximum at about 160K. The minority carrier lifetimes determined from the OCVD measurements were 0.5 μs at 300 K, 5.3 μs at 160 K, and 3.8 μs at 77 K, respectively. Furthermore, the relationship between the measured minority carrier lifetime and the photosensitivity of the fabricated device was investigated.

LiDAR sensors, a primary sensor used on AVs, utilize emitted light pulses, typically at 905nm, to remotely detect the location of objects. Incident light from the LiDAR sensor is both specularly reflected by the clearcoat and absorbed/reflected by pigments in the basecoat. Paint colors that reflect a greater amount of light at 905 nm are more easily detected by LiDAR. Thus, technologies to enhance paint reflectivity at 905 nm, while not affecting the appearance of the paint, are desirable. However, most black and dark tone cars are painted with paints based on carbon black pigment that exhibits very low reflectivity not only for the visible but the infrared light. Alternately, autonomous vehicles need to raise the laser power to detect the black vehicles, which is highly energy consuming. As the automotive market changes, the development of new black paint with high infrared reflectivity has become necessary.
Here, we design a dark-tone multilayer with low reflectivity at the visible wavelengths and high reflectivity at 905nm at the same time. Various oxide materials including Fe₂O₃, CuO, TiO₂, SiO₂, and Al₂O₃ are investigated, and the reflectivity is calculated as a function of the multilayer’s composition, the number of layers, and the layer thicknesses. We find an optimized multilayer and deposit the constituents by using sputtering on a glass substrate alternately. Finally, the measured reflectivity shows a good agreement with the simulation. We expect it can be materialized as a pearl pigment that is a plate type particle with tens of aspect ratio by depositing the oxide materials on pulverized substrate materials or by pulverizing the substrate with deposited materials.

The advanced nanomaterials and fabrication methods for various biomarkers, including pH, glucose, uric acid, and lactate, are important to monitor the wound healing process. Numerous sensing techniques have been developed for effective wound monitoring. Cellulose nanocrystals (CNCs) have been attractive bio/nanomaterials, due to the high specific surface area and specific strength, hydrophilicity, biodegradability and surface functionalization. In this study, we have successfully developed hybrid CNC nanocomposite biosensors for detection of uric acid and lactate. The CNC and polypyrrole (Ppy) nanostructures were fabricated on the electrode by electropolymerization. Subsequently, the urease or lactate oxidase was immobilized on the CNC-Ppy modified electrode. The structural investigation was performed to examine the growing mechanism of the CNC-Ppy nanocomposite on the electrode surface using physical and chemical characterization methods, such as SEM, FTIR, EIS, and CV. The stability, reproducibility, repeatability, selectivity and linearity of the biosensor were investigated. The proposed biosensor exhibits high sensitivity and was fully selective. The developed hybrid nanocomposite biosensor is cost effective and the fabrication method is relatively simple, providing an excellent sensing platform for efficient electrochemical biosensing of uric acid and lactate.

Air conditioning consumes a significant amount of electricity in summer, especially in humid tropical regions. Passive radiative cooling can radiate heat to the outer space through the infrared atmospheric window (8-13 um) without any energy consumption, providing an attractive approach for cooling buildings. Most of the state-of-the-art daytime radiative coolers show high solar reflection but high emission in the whole mid-infrared (mid-IR) regime (5-25 um) without good spectral selectivity beyond the atmospheric window, and their cooling performances under humid climates are limited. Besides, most of them adopt organic components that face aging issues when exposed to sunlight. In this study, we developed a solution-processed inorganic passive radiative cooler with high mid-IR emissivity, good mid-IR selectivity and high solar reflectivity. It illustrates a high solar reflectivity up to 98% and a high infrared emissivity of 90% within the 8-13um atmospheric window. Compared with the state-of-the-art passive coolers, it can achieve high cooling power in both mid-altitude (>85W/m²) and humid area (>25W/m²) and better cooling temperature (-7.6^oC) in tropical areas, due to its good mid-IR selectivity. Its high cooling performance and excellent scalability render it promising for space cooling in humid tropical regions.

Designing next generation light-weight pulsed power sources hinges on understanding the factors influencing the performance of high energy density storage materials. We have demonstrated the use of Cold Zone Annealing with Soft Shear (CZA-SS) as a processing strategy to fabricate highly stratified lamellar block copolymer (L-BCP) films which results in ~50% enhancement in breakdown voltage (E_BD) or ~225% increase in stored energy density (U), compared to unordered as-cast films. Increasing L-BCP layer thickness, d by increasing Mw increases E_BD but kinetics of ordering is challenging at high L-BCP Mw. Modestly high molecular weight films of amorphous L-BCP of PS-b-PMMA with layered morphology exhibited U of 4.8 J/cm³, comparable to the industry standard of semi-crystalline BOPP films of ~5 J/cm³. Efforts continue to demonstrate the full potential of the method. Notably, E_BD was found to increase linearly with M_n of neat L-BCP and will be reported. Blending L-BCP with homopolymers or cyclics is an alternate approach to increase layer thickness. While blending increases L-BCP layer thickness, chain ends of linear homopolymers act as defect sites for promoting breakdown in the L-BCP polymer films, that may be avoided by addition of cyclic polymers. Neutron Reflectivity and X-ray scattering (GISAXS) measurements confirm stratified lamellar structure in blend systems for correlations to E_BD. Neutron reflectivity measurements show that the ‘wet brush regime’ swelling of L-BCP with cyclics and homopolymer are comparable while ‘dry brush regime’ swelling differ significantly. The breakdown strength of L-BCP swollen with cyclics will be reported. These findings can be vital in the selection of L-BCP for designing next generation high energy density, solid-state polymer capacitors for flexible electronics.

Evaporation techniques for making dense metal films typically utilize a high vacuum process. Through the conversion of the idea of high-pressure evaporation, it was possible to create highly porous structure of silver by increasing background working pressure to a few Torr. In this evaporation process, opposite to the typical thermal evaporation, evaporated silver atoms encounter repeated collision with the surrounding Ar gas molecules and this causes homogeneous nucleation and growth of silver. The nucleation and growth are controlled by adjusting the working pressure, temperature, evaporation rate, etc., resulting in different morphology and porosity. The higher the pressure, the higher the porosity and the darker the structure becomes absorbing light more. As evidenced by SEM and TEM, the porous structure consists of aggregates of crystalline nanoparticles of silver ranging tens of nanometer to hundreds of nanometers. Especially, a lot of nano-sized gaps are found between the particles. These nanogaps can act as a hot-spots for surface-enhanced Raman spectroscopy (SERS). Using rhodamine-6G, we show these nanoporous structures of silver outperform commercial test substrates of SERS. This new approch of metal deposition can contribute to other areas that need high surface-to-volume ratio such as catalysts and gas sensors.

With the development of internet of things (IoT), energy harvesting technologies are evolved to increase the operation time of small electronics without consuming more fossil fuels in recent years. In addition, natural materials have recently been attracting much attention due to their eco-friendly and abundant properties. Above all, cellulose, which plays a role of the primary cell wall of green plants, is the most abundant organic compound on earth. In this work, we investigate a cellulose nanofiber-based triboelectric nanogenerator (CN-TENG), which was developed to convert vertical moving energy into an electrical energy. The top and bottom parts of the CN-TENG consist of silver nanowires (AgNWs) and cellulose nanofiber (CNF), respectively. The electrical output of this CN-TENG was measured by contacting and separating with only two sheets of fabricated devices. The CN-TENG with a higher pressure and increased number of passes in the homogenizer during fabrication from the suspension represents highest open-circuit voltage of 21 V and short-circuit current of 25 µA, respectively. AgNWs layer with the concentration of 0.1 wt% showed saturated electrical outputs. Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) were utilized to analyze the components and surface morphology in order to investigate the correlation between the surface morphology and the electrical output. Additionally, a 693 mW/m2 power density was observed at an external resistance of 10 MΩ. Moreover, this fabricated film shows the characteristics of foldable, writable, erasable This fabricated CN-TENG can be developed as a paper-based electrical energy generator for various applications such as e-paper.

A triboelectric nanogenerator (TENG) can convert mechanical energies into electrical power with excellent conversion efficiency. TENGs can generate electrical energy based on contact- electrification and electrostatic induction. Among the various operating modes, a rotating-disk-based TENG harvesting wind energy is being studied extensively because it can produce high current. Deep analysis of the flow of the fluid is essential to improve the conversion efficiency of the TENG.
In this paper, the propeller TENG (P-TENG) was designed to demonstrate the relationship between TENG and fluid mechanics as a wind energy harvester and wind speed sensor. The P-TENG is mainly composed of three parts: a rotator (propeller) and a stator (electrode), and a main body. The P-TENG has the cylindrical structure—having a diameter of 8.8 cm and height of 3 cm—and an inlet and outlet on the side and top, respectively. The rotator (propeller) and main body were 3D-printed of acrylonitrile butadiene styrene (ABS). The ABS has outstanding resistance to chemicals, heat, and impact. When the wind energy is injected into the main body, it collided with the perpendicular plane of the propeller and cause the rotational motion.
The five factors (the pressure of the injected gas, the distance between the stator and rotator, the number of propeller wings, and the height and angle of the inlet) were changed to verifying the electrical characteristics of the disk-based TENG and to examine the gas flow within the P-TENG. The highly injected pressure of the fluid causes the higher rotating speed, as a result, it contributes to increasing the current. In this regard, the P-TENG can act as a wind speed sensor without an external power source. Also, the comprehensive analysis conducted based on these electrical output results. The COMSOL simulation was carried out to calculate the value of pressure to the propeller and the flow of fluid. The laminar flow was applied as physics condition and solved with the Navier–Stokes equations. The injected fluid was analyzed by dividing it into four flows in the vertical and horizontal directions and each flow influences the rotational motion in a way to improve or reduce.
Based on the comprehensive analysis, the optimized P-TENG consists of the 16-sector propeller, a low inlet height, a 60° inlet angle, and PTFE film with a nanostructured surface. Finally, the P-TENG can power 205 LEDs as well as a commercial stopwatch in real time, without any other power source. The P-TENG enabled Bluetooth transmission and reception and drove the sensing, processing, and data transmission of a commercial temperature and humidity sensor, so it can be used as a power source for a self-powered wireless sensor system. The investigation of P-TENG provides not only the possibility that using as a self-powered wind speed sensor and maximized energy conversion efficiency by optimizing the structure of the device based on depth analysis of the flow of fluid.

Of late, carbon-based nanomaterials have been prevalent research topic owing to their potential application and especially excitation-dependent diverse fluorescence emissions. The later property can be tremendously useful in sensing, imaging, and photovoltaics. However, applications using this unique fluorescence property of carbon-based nanomaterials have not been explored so far, which may be due to a significant drop in fluorescence intensity in longer wavelength region after photoexcitation of the nanomaterials. In this paper, we report a simple aqueous synthesis of ZnO-graphite nanocomposite, which displayed unusual excitation dependent fluorescence emission violating the Kasha’s rule, without significant shrink in the fluorescence intensity at longer wavelength region. Moreover, the fluorescence intensity of the nanocomposite monotonically increased with the addition of cyanide ion of millimolar concentration. The fluorescence enhancement followed Langmuir binding isotherm, which enables quantitative detection of cyanide. The phenomenon of unique excitation-dependent emission along with fluorescence enhancement in the presence of cyanide ion opens up a new avenue for the design of a multichannel array-based sensor to detect toxic and environmentally hazardous analytes.

Lead Sulfide (PbS) is a unique material system that is interesting for photovoltaic, thermoelectric, and sensing applications. In this work, we report a systematic study of syntheses of Manganese (Mn) doped lead sulfide (PbS) nanoparticles via varying growth temperature (50°C to 120°C), growth time (10min to 24hrs), with various precursors' ratios (Mn :Pb). The synthesized nanoparticles (2~5nm) were characterized by UV-VIS absorption, XRD, EDS, TEM, XRD, etc. The lifetimes of emissions of doped nanoparticles measured by ultrafast spectroscopy were found to be affected by Mn concentration, ranging from 5ns to 100ns. The DOS of nanoparticles simulated by DFT method were also found to be dependent upon the doping concentration. The band gaps were found out to be not affected by the doping concentrations. Through tuning of doping concentration, the material properties of Mn-PbS nanoparticles can be modified based on specific applications.

Vibration-based structural health monitoring (SHM) systems allow to evaluate the state of the structure and the presence of damage. [1] For the detection of damage by sensing strains under vibrations without external electrical energy supply, multifunctional mechano-luminescence-optoelectronic (MLO) composites were suggested. [2] MLO composites use two transformative materials – vibration-sensitive light-emitting mechano-luminescent (ML) material, and photosensitive electrical-response generating material. [3] As a result, MLO composites generate direct current (DC) by harvesting mechanical energy from ambient vibrations in tested structures viathe coupled two-step energy conversion mechanism (i.e., mechanical-radiant and radiant-electrical energy conversions). [4] The two functional constituents – ML copper-doped zinc sulfide (ZnS:Cu)-based elastomeric composites and mechano-optoelectronic (MO) poly(3-hexylthiophene) (P3HT)-based films – of the MLO composites are designed to selectively sense different modes of vibrations. [5] The MO P3HT-based thin films generate DC by utilizing light emitted from ML ZnS:Cu-based composites. Being combined together, our materials and devices represent a new class of self-powered and multi-modal sensor technology that can potentially enhance self-sustainability of, for example, unmanned aerial vehicles, or other remotely controlled mechanisms.
In our study, two functional constituents – ZnS:Cu-and P3HT-based composites – will be prepared and characterized. Opticalspectra of ZnS:Cu and PDMS; ZnS:Cu-PDMS composites will be obtained using Raman, FTIR, and photoluminescence spectroscopies. Depending on the Cu doping concentration, the PL spectra of ZnS:Cu-PDMS have a maximum of emission in green (low doping concentration) or blue (higher Cu concentration). Composites with ZnS, which is co-doped with Cu and Mn, show orange luminescence. In addition, all tested samples demonstrate broad-range room temperature luminescent signals in the range of 0.86-0.84 eV, which seems to be attributed to optical transitions via AN, AS and PS centers of Cu²⁺ in ZnS. This indicates possibilities of the coexistence of different structural forms and different particle sizes of ZnS in the ML ZnS:Cu-PDMS composites. ML spectrum from different repetition loading frequencies and strains will be obtained. Luminescence from ZnS:Cu-PDMS was found to be sensitive to the strain and strain rate. The photodetection spectra of MO P3HT-based composites show a sensitivity peak around 590-600 nm (2-2.1 eV); the experiments with different light modulation frequencies reveal the dynamical limitations of this type of photodetector. Overall, simultaneous optical characterization of ML and MO constituents of MLO composites opens up possibilities for “fine tuning” of both ML and MO constituents for the achievement of maximal mutual compatibility and optimal performance of the entire multifunctional MLO composites.
We would like to thank NASA’s Space Grant College and Fellowship Program and NASA EPSCoR CAN (grant #: 80NSSC17M0050) for supporting this study.

REFERENCES
1 Zagrai, A., D. Doyle, and B. Arritt. in Health Monitoring of Structural and Biological Systems 2008. International Society for Optics and Photonics, 2008.
2. Montalvao, D., N.M.M. Maia, and A.M.R. Ribeiro,.Shock and vibration digest, 2006. 38(4): 295-324.
3. Ryu, D. and K.J. Loh, Smart Materials and Structures, 2014. 23(8), 085011.
4. Ryu, D. and K.J. Loh, Smart Materials and Structures, 2012. 21(6), 065016.
5. Mongare, A., et al. Autonomous structural composites for self-powered strain sensing-enabled damage detection. in Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2019. 2019. International Society for Optics and Photonics.

CNTs can have the ability to act as compliant small-scale springs or as shock resistance micro-contactors. This work investigates the performance of vertically-aligned CNTs (VA-CNTs) as compliant, micro-contactors in electromechanical testing applications for testing at wafer-level chip-scale-packaging (WLCSP) and wafer-level-packaging (WLP). Fabricated on ohmic substrates, 500-μm-tall CNT-metal composite contact structures are electromechanically characterized. The probe design and architecture are scalable, allowing for the assembly of thousands of probes in short manufacturing times, with easy pitch control. We discuss the effects of the metallization morphology and thickness on the compliance and electro- mechanical response of the metal-CNT composite contacts. Pd-metallized CNT contactors show up to 25 μm of compliance, with contact resistance as low as 460mΩ (3.6 kΩ/mm) and network resistivity of 1.8 x10^-5 U cm, after 25000 touchdowns, with 50 μm of over-travel; they form reproducible and repeatable contacts, with less than 5% contact resistance degradation. Failure mechanisms are studied in-situ and after cyclic testing and show that, for top-cap-and-side metallized contacts, the CNT-metal shell provides stiffness to the probe structure in the elastic region, whilst reducing the contact resistance. The stable low resistance achieved, the high repeatability and endurance of the manufactured probes make CNT micro-contactors a viable candidate for WLP and WLCSP testing. Furthermore we show the addressability of individual probes for an array with 200 μm pitch by utilising ink-jet printing after on-chip CNT-metal hybrid probe fabrication allowing to address 144 individual probes. The stable low resistance achieved, the high repeatability and endurance of the manufactured probes make CNT micro-contacts a viable candidate for WLP and WLCSP testing.

Ref: Tas, M. O.; Baker, M. A.; Musaramthota, V.; Uppal, H.; Masteghin, M. G.; Bentz, J.; Boxshall, K.; Stolojan, V. Carbon Nanotube Micro-Contactors on Ohmic Substrates for on-Chip Microelectromechanical Probing Applications at Wafer Level. Carbon 2019, 150, 117–127

The quick upsurge in research and development of β-Ga₂O₃ material gives rise to its application for photodetectors working in solar-blind (SB) regime. SB photodetectors were fabricated on β-Ga₂O₃ thin films deposited by pulsed laser deposition method. These SB photodetectors were operated in the harsh environments of temperature and gamma radiations. The photo to dark current ratio of about 7100 was observed at room temperature and 2.3 at a high temperature of 250 °C with 10 V applied bias. The high temperature photocurrent mechanism for Ga₂O₃ were realized for future advancement in the technology. The electron-phonon interaction and self-trapped holes were found to influence the photoresponse in the devices. The excellent properties of Ga₂O₃ photodetectors pave the way for next-generation high temperature stable SB photodetectors.
The radiation hardness of Ni/Ga₂O₃/Ni metal-semiconductor-metal (MSM) SB photodetectors has also been investigated under the exposure of ⁶⁰Co γ-source. It was observed that the metal contacts were not degraded and the dark current of photodetector was slightly improved from 3.27 ×10⁻⁷ A to 1.88×10⁻⁷ A. The photo to dark current ratio (PDCR)was observed to increase from 5.1 to 14.1 with increasing γ-radiation exposure. The apparent Schottky barrier height (SBH) evaluated from current-voltage characteristics were found to increase with irradiation. The increased SBH was explained using image force induced barrier lowering. The obtained results reveal that the Ga₂O₃ solar blind photodetectors are relatively less susceptible to radiation environment.

Herein, the different chemical stratigies have been adopted for tuning the properties of nanocrystalline TiO₂ so as to form the ternary hybrids and thereafter these hybrids have been tested for efficient solar-to-electrical conversions through photovoltaic measurments. Initially, nanocrystalline TiO₂ have been anchored with the different systems such as dyad system, dye-quantum dot system, composites with carbon nanostructures sensitized through dye. Along with optical coverage for better absorption, the interconnectivity between the various components and their charge-transport properties of modified TiO₂ for solar energy conversions have been studied in detail.
In the dyad system, a supramolecular ruthenium (II) phthalocyanine (RuPc) with peryleneimide (PI) has been anchored on the surfaces of TiO₂ and thereafter these TiO₂-dyad sensitized solar cells have assembled in a bottom-up fashion. Upon photo-irradiation at air-mass (AM) 1.5, the dyad-based solar devices convert solar light to electricity of 2.1%; which is higher than that of individual dyes. In next strategies, the synthesized CdS quantum dots (QDs) have connected electrostatically to the surfaces of TiO₂ nanoparticles (NPs) and thereafter, this binary hybrid heterojunction is further self-assembled physically with N719 dye to form ternary TiO₂ NPs-CdS QDs-N719 photoelectrodes. These co-sensitization devices reported the solar energy conversion efficiency (η) up to 2.35 %. In addition, anatase TiO₂ NPs is anchored on the surface of functionalized carbon nanostructures (MWCNTs or RGO); which is further sensitized with Ru(II) dyes for solar devices having efficiency reached to 6.21%. In another modification, Cr(III) as well as Mo(VI) ions are inserted into TiO₂ host lattice and then their composites with CNs are further sensitized with dyes for boosting the solar energy conversions and reached up to 7.69 % of efficiency.

The abundance of mercury (II) in trace amounts throughout the ecosystem, combined with its ability to accrue in human tissue has motivated the need for highly sensitive and selective detection methods. Gold Nanorods (AuNR) have demonstrated a unique facility for the detection of mercury due to their plasmonic properties and the propensity for mercury and gold to form an amalgamation. Prevalent methods for the synthesis of AuNR’s utilize high concentrations of cetyl trimethylammonium bromide (CTAB) which forms a dense micelle bi-layer on the exterior of the AuNR’s. The sensitivity of AuNR’s to mercury has been shown to be extremely diminished with increasing concentration of CTAB, however reduction of CTAB concentration led to aggregation of the AuNR’s. CTAB was exchanged for polyethylene glycol thiol (PEG) as a capping agent on the AuNR’s resulting in much better stability and a significant increase in both sensitivity and selectivity for detection of mercury (II) over CTAB coated AuNR’s. The increased stability of PEG coated AuNR’s also allows for manipulation of the region of highest sensitivity to mercury (II) by changing the concentration of AuNR’s. It was also discovered that the sensitivity of PEG coated AuNR’s to mercury (II) could be as much as tripled with incubation of the PEG coated AuNR’s. This increased sensitivity has been attributed to the preferential removal of PEG from the tips of AuNR’s by the formation of a complex between PEG and mercury (II) which has a higher stability than the complex between gold and PEG.

Graphene oxide (GO) having oxygen-containing groups is expected as metal-free and environment-compatible catalyst. The research on GO catalyst progressed [1 - 3], although the detail mechanism for GO catalytic activity is not clear yet. Investigating Graphene Oxide (GO) properties before / after the catalytic reaction is a promising strategy for clarification of the mechanism for GO catalytic activity. Additionally, it is known that removing oxidative debris on GO by base-treatment is important factor to get the higher yield for the following catalytic reaction [1]. In this study, the principle for reactivity of GO is investigated by conducting the oxidative amine coupling reaction of benzylamine (1) to N-benzylidenebenzylamine (2) using baGO and thermally reduced one (rbaGO) as catalysts. baGO-Pristine was obtained by washing GO, synthesized by Hummers method, with NaOH. rbaGO was prepared by reducing baGO at 450C under Ar gas flow. After the reaction, the reaction media filtered for removing the catalyst was characterized by GC-MS. After the 1st cycle reaction, the recovered catalyst was used as that for the 2nd cycle (baGO-1st cycle, rbaGO-1st cycle). The chemical structure of each catalyst after the reaction was evaluated by XPS (Al Ka), where it was washed with ethanol after the reaction. The atomic ratio O/C of catalyst much decreases after the reduction or 1st cycle reaction and rbaGO shows poor yield, suggesting roles of baGO / rbaGO not as catalyst but as oxidant. However, baGO-1st cycle exhibits the highest yield in spite of C1s spectrum and O/C similar to that of rbaGO and rbaGO-1st cycle, which also indicates no more reduction for rbaGO in the 1st cycle reaction. Taking these results and appearance of nitrogen content after the 1st cycle reaction into consideration, a different mechanism in the 2nd cycle reaction is suggested, where nitrogen incorporated into rbaGO / baGO in the 1st cycle reaction plays an important role for the reactivity. To clarify the most responsible functional groups in rbaGO / baGO for the higher catalytic reactivity in the 2nd cycle reaction, we have derived the amount of nitrogen-containing functional group per 100 mg in each GO sample for the 2nd cycle reaction. By plotting the amount of nitrogen-containing functional group and the GC-yield of the product, the correlation coefficient r is calculated for each functional groups. In particular, the GC-yield tends to increase as Graphitic N (correlation coefficient r = 0.94) and Oxidized N (correlation coefficient r = 0.87) increase. The important roles of those functional groups in the oxidation amine coupling reaction could be explained by the enhancement in the oxygen adsorption ability, which is important to produce O2*- species from ambient oxygen molecules for proceeding the oxidative coupling reaction.

References
[1] C. Su, M. Acik, K. Takai, J. Lu, S. Hao, Y. Zheng, P. Wu, Q. Bao, T. Enoki, Y. J. Chabal, and K. P. Loh, Nature Communications, 3, 1298 (2012).
[2] A. Shaabani, M. Mahyari, and F. Hajishaabanha, Res. Chem. Intermed., 40, 8, 2799–2810 (2014).
[3] K. Savaram, M. Li, K. Tajima, K. Takai, T. Hayashi, G. Hall, E. Garfunkel, V. Osipov, and H. He, Carbon,139, 861-871(2018).

Cilostazol is a quinolone antiplatelet agent that contributes to vasodilatation. It can be used to relief the symptoms of intermittent claudication (decreased blood supply in the lower limps) that is attributed to ischemia. Also, it relieves the cramping pain that occurs during heavy exercises. Its main mode of action is the inhibition of both primary and secondary agglomeration of platelets, which is an important issue in drug monitoring. Herein, we report on the modification of carbon paste electrodes (CPE) with transition metal oxides (TMO), which showed a superior accuracy and sensitivity towards cilostazole than unmodified CPE. Characterization of the composite was done by scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform Infrared (FTIR), Cyclic voltammetry (CV), and square wave voltammetry (SWV). Moreover, the addition of reduced graphene oxide as a composite material has increased the current and sensitivity of the CPE to cilostazole. The ratio of the graphene to TMO in the composite was optimized to achieve the maximum response. The electrochemical sensor showed high linearity, sensitivity, and very low limit of detection in both Britton Robinson Buffer (BRB) and spiked plasma and urine samples.

GaN nanowire (NW)-based devices have gained increasing interest for applications in solar driven photocatalysis and sensing due to the intrinsically high surface-to-volume ratio, the excellent achievable crystal quality and possible electro-magnetic field enhancement within periodic array [1]. Many materials that are catalytically active and sensitive to environmental changes, suffer from insufficient stability under operational conditions. By investigating the accelerated corrosion of hexagonal, dodecagonal and round GaN NWs in deionized water under high intensity illumination, we find that the crystallographic c-plane facet is stable, while a- and m-plane are etched with rates up to 11 nm/min, depending on the applied light intensity and wavelength. We identify photo-excited holes that are driven to the NW surface by the upward band-bending present in not intentionally doped GaN as the driving force of this process. Consequently, structural defects decrease the etching rate by serving as recombination centers for photo-induced carriers, which is shown by low-temperature photoluminescence and scanning electron microscopy investigations. The gained fundamental insights allow the development of viable strategies for the stabilization of the nanostructures under light-driven reaction conditions and harsh environments, which include facet engineering, change of doping type and shell growth.

[1] Winnerl, J., Hudeczek, R., & Stutzmann, M. (2018). Journal of Applied Physics, 123(20), 203104.

Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. However, in order to achieve this elusive goal, we must first obtain fundamental understanding of the structural and chemical properties of these complex systems. In addition, the dynamic nature of the nanoparticle (NP) catalysts and their response to the environment must be taken into consideration. Despite the significant progress in experimental tools for NP characterization and theoretical NP modeling approaches, understanding the relation between intriguing properties of metal NPs (e.g., catalytic activity or unique thermodynamic characteristics) and their structure and surface composition is still a challenging task. The intrinsic complexity and heterogeneity of NPs, their interactions with the support, ligands and adsorbates, strong static disorder and anharmonic effects at the NP surface, or in situ transformations of their structure pose significant difficulties both for theoretical modeling and for the interpretation of experimental data.
In this talk, the complexity of real-world catalysts will be experimentally addressed by taking advantage of a variety of cutting-edge complementary methods (AFM, TEM, XPS, XAFS).
New insights into the thermal hydrogenation and electrocatalytic reduction of CO₂ will be provided. Important aspects that will be discussed are: (i) the design of size-and shape-controlled catalytically active NPs (Cu, Cu-Zn, CuNi) on C, Al₂O₃, ZnO, ZnOAl, SiO₂, and (ii) the investigation of structure/chemical state-reactivity correlations in situ and under realistic operando reaction conditions, i.e., at high pressure or under potential control. For example, time-dependent EXAFS data combined with neural network analysis will be used to gain insight into the structural evolution and brass alloy formation during the electrochemical reduction of CO₂ as well the its thermal hydrogenation over micelle-synthesized CuZn NPs. Overall, our results are expected to open up new routes for the reutilization of CO₂ through its direct conversion into valuable chemicals and fuels such as ethylene, ethanol, and methanol.

Due to its high stability against corrosion, low background capacitive current, mechanical durability and chemical inertness, boron-doped diamond (BDD) electrodes are applied in electroanalysis, electrosynthesis and spectroelectrochemistry [1]. Recently, electrochemical reduction of CO₂ to value-added products, for example, carbon monoxide, formic acid and formaldehyde, in water-based solutions (including seawater) using BDD electrode has been reported [2]. The abovementioned properties make BDD electrodes an ideal candidate for implementation in the currently developing sunlight-driven electrochemical systems and devices for solar fuel generation (hydrogen, formaldehyde, etc.).
In the present work, we investigate routes towards a direct photo-electrochemical water-splitting and conversion of CO₂ into useful chemicals via the integration of a light absorber and boron-doped diamond electrodes as cathode and anode material. In such a system, silicon photoelectode is responsible for the light absorption, charge carrier generation and separation (p-n junction), while BDD electrodes protect the Si surface from its inherent corrosion in an aqueous media.
Nanocrystalline BDD electrodes have been fabricated via microwave plasma-enhanced chemical vapour deposition process [3]. We show that nanocrystalline BDD layer (thickness of 150 - 300nm) deposited onto Si photoelectrode allows for photogenerated hole charge carrier transfer from the underlying Si towards the electrolyte solution to participate in water oxidation reaction. Addition of the water oxidation catalyts onto the BDD surface reduces the overpotential for the water oxidation. Nanocrystalline BDD electrodes of various thicknesses and electrical conductivity values have been investigated for the CO₂ reduction in water-based electrolytes to establish a relationship between the BDD layer structure/composition and the products observed from CO₂ conversion.
This work has been supported by the Grant Agency of the Czech Republic (GACR) contract 19-09784Y.

[1] J. V. Macpherson, Phys. Chem. Chem. Phys. 2015, 17, 2935 − 2949.
[2] Y. Einaga, Bull. Chem. Soc. Jpn. 2018, 91, 1752 - 1762
[3] P. Ashcheulov et al., ACS Appl. Mater. Interfaces 2018, 10, 35, 29552-29564

Achieving excellent electrochemical performance of electrodes at high mass loading holds significant importance to energy storage. Pseudocapacitive materials such as manganese oxide (MnO₂) deposited on current collectors have achieved outstanding gravimetric capacitances, sometimes even close to their theoretical values. Yet, this is only achievable with very small mass loading of active material typically less than 1 mg cm^-2. Increasing mass loading often leads to drastic decay of capacitive performance due to sluggish ion diffusion in bulk material. In this talk, I will demonstrate a 3D printed macroporous graphene aerogel electrode with MnO₂ loading of 182.2 mg cm^-2, which achieves a record-high areal capacitance of 44.13 F cm^-2. The engineered porous structure allows efficient ion diffusion and therefore enables the ultrahigh mass loading of pseudocapacitive materials without sacrificing their gravimetric and volumetric capacitive performance. Most importantly, this 3D printed graphene aerogel/MnO₂ electrode can simultaneously achieve excellent capacitance normalized to the area, gravimetry, and volume, which is not possible for traditional electrodes. This work successfully validates the feasibility of printing practical pseudocapacitive electrodes, which might innovate the conventional layer-by-layer stacking fabrication process of commercial supercapacitors.

The development of electrodes with high specific capacitance remains the main challenge for energy storage nowadays [1]. As an emerging technique, material extrusion 3D printing [2] provides an alternative pathway for designing and fabricating electrodes of supercapacitors by controlling the 3D macrostructures and nanocomposites.

In this work, MnO₂nanoparticles and multi-walled CNT are mixed with polyvinylpyrrolidone (PVP) and deionized water as designed inks for desired rheological properties, which allow material extrusion from the nozzle. The inks are then used to print the nanocomposite electrodes layer by layer, followed by thermal treatment in the argon atmosphere. The ink composition is studied for the printability. PVP and deionized water tune the viscosity of the ink so that the ink can be printed fluently and completely, while multi-walled CNTs, served as conductive networks and interconnects in the nanocomposites for the high efficiency of electron delivery [3].

The 3D printed electrodes effectively integrate MnO₂ and multi-walled CNT, which achieves more abundant interactions caused by the higher loading of MnO₂ without the traditional binder of polyvinylidene fluoride (PVDF) [4], In addition, the stacking and alignment of MnO₂ and multi-walled CNT via material extrusion 3D printing provides the flexible tunable thickness [5] of electrodes by controlling the printing layers (design and test one, two, four, eight layers with a 0.4 mm in diameter nozzle, respectively). This approach provides a promising way of manufacturing the needed electrodes in multiple aspects of size, shape, and dimension.

Keywords: MnO₂/multi-walled CNT electrodes, supercapacitors, material extrusion 3D printing.

Reference


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The potential solid-state electrolyte, MgZr₄P₆O₂₄ was synthesised by a modified sol-gel method. The structural, electrical and thermodynamic properties of this solid-state electrolyte were determined. DSC-TGA analysis indicated that pure dried xerogel powder when calcined at 800^oC and 900^oC, converts to single phase MgZr₄P₆O₂₄ nanopowder with good crystallinity. Pellets of 13-mm diameter and 3.8-mm thickness made by uniaxial compression were sintered at 1300^oC for 24h. XRD and HR-TEM indicated that the crystalline phase is monoclinic having crystallite sizes of approximately 32nm and 40nm, after calcining at both temperatures, respectively, showing positive temperature effect on crystallite size. The sintered pellets were stable in the temperature range from 1000 to 1300^oC, with minor extraneous peaks indicating traces of a coexistent second phase, Zr₂(PO₄)₂O at higher temperatures. Using electrochemical impedance spectroscopy, the electrical conductivity of MgZr₄P₆O₂₄ was determined as 7.23 x 10^-3 Scm^-1 at 725^oC [1]. MgZr₄P₆O₂₄ pellet was successfully employed for the design and fabrication of solid-state Mg-sensors for sensing Mg concentration in liquid Al at 700±5^oC, with a biphasic powder mixture of MgCr₂O₄+Cr₂O₃ as the ceramic reference electrode in air. A linear dependence of emf on logarithm of Mg concentration was obtained. Thermodynamic measurements using electrochemical method was deployed. The measured voltage of the sensor emf against the theoretical emf was achieved and compared, and an average transport number for Mg²⁺-cation in the MgZr₄P₆O₂₄ solid-state electrolyte is approximately 0.85(±0.03) at 700±5^oC, which is in agreement with an earlier study [2]. The thermodynamic activity of Mg in liquid Al shows a negative deviation from the Raoult's law. MgZr₄P₆O₂₄ solid-state electrolyte has useful applications in a stable, highly-sensitive, high-temperature electrochemical Mg-sensors during scrap metal recycling, refinning and virgin metals alloying.


References

[1] M. Adamu and G.M. Kale, Novel Sol-Gel Synthesis of MgZr₄P₆O₂₄ Composite Solid Electrolyte and Newer Insight into the Mg²⁺-Ion Conducting Properties Using Impedance Spectroscopy. J. Phys. Chem. C. 2016;120:17909-17915.

[2] G.R.K. Aghdam and M. Soltanieh, Study of the ThermodynamicProperties of the Al-Mg Binary System Between 973 - 1073 K by the EMF Method. Can. Metall. Quart. 2010;49:39-45.

Electronic and diffusion properties of hydrogen in ZnO have been extensively studied, since efficient defect engineering is essential to fabricate electronic, ferroelectric, and optical devices [1-2]. We introduced the protons in ZnO from chemical doping and/or proton implantation. To investigate the properties, we employed ¹H nuclear magnetic resonance (NMR) relaxometry, which is a powerful technique of atomic-scale access to probe ion hopping motion in solids. Herein, we unambiguously identify the proton species which were occupied at the surface or in the bulk of ZnO, from their distinct NMR relaxometry and resonance shifts. NMR relaxometry gives the activation barriers of interstitial H in the bulk to be 0.46 eV by means of the rotating-frame spin-lattice relaxation measurements. Besides, it turns out that correlated jump diffusion of the surface hydroxyl group of multiple lines at ~1 ppm indicates the presence of structural disorder at the ZnO surface [1]. Hence, ¹H NMR relaxometry may shed light on identifying proton species and investigating proton dynamics in hydrogen-incorporated systems.

[1] J. K. Park, H.-J. Kwon, C. E. Lee, Sci. Rep. 6, 23378 (2016)
[2] J. K. Park, K. W. Lee, C. E. Lee, Appl. Phys. Lett. 103, 023109 (2013)

(This work has been supported through KOMAC operation fund of KAERI by Ministry of Science ICT and Future Planning of Korean Government.)

The perovskite, methylammonium lead iodide (MAPI), changes from its black phase to white crystalline CH₃NH₃PbI₃●H₂O when exposed to humidity greater than ~85% RH at room temperature. While this degradation is undesirable for photovoltaic devices, or solar cells, the reaction is reversible and the material can be cycled between clear and dark states. This has potential for smart window applications, as there is a need for materials with color-neutral transitions that minimize color distortions in the appearance of objects viewed through tinted smart window glass. However, the transition in MAPI is too slow for smart window applications, with dark-to-clear transitions occurring over 12-24 hours, and is in fact, not fully reversible due to side reactions. Here, we report a deliquescent chromic effect in thin films of nickel (II) iodide. Like MAPI, NiI₂ undergoes a color neutral transition when exposed to moisture. Unlike MAPI, this deliquescent transition is relatively rapid, switching between dark and clear states within 2 min at 50% RH. This transition happens at humidity as low as ~25% RH and the film reverts back to its dark state after heating above 50 ^oC. Cooling the heated film to ~30 ^oC restores the clear state, and this thermal switching between optical states is reversible. FTIR and XRD demonstrate the clear state is an amorphous aqueous solution of nickel iodide with a H₂O:NiI₂ molar ratio of at least 10, while the dark state is crystalline nickel (II) iodide. The switching kinetics of the film were also studied as a function of heating rate and film thickness. Increased heating rates led to significant hysteresis in the optical switching, but the transition remains otherwise reversible. Films with thickness greater than 1 μm exhibit no optical modulation, showing this transition occurs exclusively in thin (<1 μm) films of NiI₂.

We present in this work the study of nano materials for hydrogen leak detection sensors. On example of a configuration that can be used is based on a original transducer layers deposited on the core of a multimode fiber optic. The reference transducer layer is a multilayer stack based on a silver, a silica and a palladium layer.
The spectral modulation of the light transmitted by the fiber allows to detect hydrogen. The sensor is only sensitive to the Transverse Magnetic polarized light and the Transverse Electric polarized light can be used as a reference signal.
The multilayer thickness defines the sensor performances in terms of sensitivity, SNR and time response. The silica thickness tunes the resonant wavelength, the silver (or gold) supports the plasmon and the palladium detects the hydrogen gas in the environment.
This study synthesizes the sensors performances as a function of different parameters such as the sensitive materials, different thicknesses, numerical apertures…

Advanced additive manufacturing has enabled an unprecedented material design space that spans over a wide range of structures and properties. While the additively manufactured nanomaterials have demonstrated fascinating mechanical properties, their thermal properties, and potential capabilities of controlling heat transfer have received little attention. The controlled thermal properties of additively manufactured nanomaterials could lead to efficient sensing and control of various energy systems, and the understanding will contribute to the emerging field of thermal metamaterials. Here we use a combination of two-photon polymerization direct laser writing (TPP-DLW) and pyrolysis to create glassy carbon nanowires and nanolattices. The pyrolysis process induces a high-strain shrinkage up to 80% and allows the formation of glass carbon nanowires with an aspect ratio as large as 640, in which the nanowire length is in an average of 48 µm, and the diameter is in an average of 75 nm. The large aspect ratio allows characterization of the heat capacity as well as the thermal conductivity. We use an electrical resistance thermometry technique known as the 3ω method to simultaneously characterize the electrical conductivity, thermal conductivity, and heat capacity of the glassy carbon nanowires over a wide range of temperature. The electrical conductivity of the glassy carbon nanowires increases from 22740 S/m at room temperature to 27757 S/m at 748 K, and their thermal conductivity increases from 2.4 Wm^-1K^-1 at room temperature to 7.0 Wm^-1K^-1 at 748 K. The increase in electrical and thermal conductivities is attributed to an increasing rate in graphitization ratio as a function of temperature. We measured the volumetric heat capacity of 2.1×10⁶ J/m³K for glassy carbon nanowires as compared to the amorphous carbon which has a specific heat capacity of 1.8×10⁶ J/m³K. The glassy carbon nanowires also provide excellent thermal stability, high Young’s modulus (20-30 GPa), and low density (1.3-1.5 gcm^-3), which would be useful for sensing and control systems in extreme environments. Furthermore, we use the same manufacturing and characterization methods to create glassy carbon nanolattices and control the porosity up to 99%. Our investigations show that thermal properties of nanolattices can range from that of bulk materials to nanowires and offer unique capabilities of decoupling mechanical and thermal properties. The outcomes of this work advance our understanding of size, shape, and temperature dependences of thermal transport in additively manufactured nanomaterials, and these findings will guide future designs of sensing and control of various energy systems.

Merging optics and nanotechnology has led to a remarkable fundamental insight into the interaction between light and matter at the nanoscale. The unique capacity of plasmonic nanostructures to concentrate electromagnetic fields, scatter electromagnetic radiation, makes them important in many scientific areas, including imaging, near field scanning optical microscopy, and chemical and biological sensing. Taking advantage of plasmonic nanoparticles especially those made of gold and silver is necessary to develop techniques to design responsive material with high sensitivity, selectivity, and high spectral resolution. Manipulation of the tight confinement of light across the gap of assembly of plasmonic nanoparticles could enable possibilities to achieve this idea. Plasmonic optical modes are very sensitive to the surrounding media, because of their electromagnetic field distribution around the particles. With considering a plasmonic nanoparticle as a thousand dipole points, when they are exposed to electromagnetic radiation, for nearby particles, dipole-dipole interaction has the most contribution in their plasmonic coupling. However, achieving even more tightly hot spots (confined fields) at the gap size of 2 nm or less requires to consider other multipole modes in the coupling phenomenon. This study develops new models of plasmonic equation that takes account the contribution of multipolar modes in the plasmonic coupling of pairs of silver nanoparticles at nanojunction separation. As the inter-dimer axis decreases, an increase in the plasmon dipolar and multipolar interaction takes place. It reveals that the plasmonic coupling depends on the density of the dipoles on the nanoparticle; as the dipole density increases, the average multipole density increases, resulting in higher dipole-multipole interactions. This research aims to design more sensitive optical sensors, which have many applications including single-molecule spectroscopy, biomedical and ultrafast optoelectronic applications.

Most acoustic sensors are based on rigid Micro-Electro-Mechanical Systems and are optimised for high-frequency applications (>10k Hz). These systems often distort the airflow during measurement and require an additional output power source.
Herein, we demonstrate a high performance, self-powered acoustic sensor platform. A dynamic Near-Field Electrospinning (dNFES) method is developed to fabricate suspended and interconnected sub-micro scale fibre webs. Permissive to flow, these interconnected and suspended sub-micro scale fibre webs can precisely imitate tiny variations triggered by the surrounding airflow motion. Electroactive Poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TRFE) copolymers are used as material to create the self-powered sensing ability of the fibre webs. Our dNFESFP process enables us to fabricate novel fibre webs to produce an acoustic sensing device in a one-step process. Due to the suspension and flexibility of the fibre webs, the acoustic sensing device shows particularly high sensitivity at most audible acoustic waves (between 200-5000 Hz). Furthermore, the dNFESFP process is an easy-to-use and stable technique that demonstrates a high reproducibility. Lastly, the process allows the tuning of fibre webs to optimise the sensitivity of the acoustic sensing device to the frequency range of interest.