Real Device Applications Research Articles

Metasurface with Nanostructured Ge2Sb2Te5 as a Platform for Broadband‐Operating Wavefront Switch

AbstractBroadband‐operating active devices within a small‐footprint are highly on demand in various nanophotonic fields such as fiber‐optic communication systems and chip‐based integrated optical circuits. As pioneering approaches, diverse platforms of active metasurfaces (AMs) have been proposed due to their superior tunable functionality and ultra‐compact size. However, most of previous researches provide only limited operating bandwidth because they generally rely on resonant light–matter interaction between active material and plasmonic antenna. In this study, an active wavefront switching metasurface that can operate over 500 nm bandwidth at near‐infrared spectral bands is experimentally realized by utilizing nonresonant U‐shaped Ge2Sb2Te5 nanoantennas. Two different sizes of the U‐shaped antenna are designed to exhibit large transmittance contrast and their optical phases are determined by imposing the orientation angle variation. As an example of the functionality, anomalous refraction angle switching and dispersionless active hologram are demonstrated. The devices provide high signal‐to‐noise ratio (>7 dB) for overall operation bandwidth. It is believed that the proposed AMs can be an innovative platform for real device application thanks to their not only broadband and low‐noise operation but also fast speed, low power consumption switching within a small‐footprint.

(Invited) Expansion in proton-Conducting Ceramic Based Devices

High temperature proton conductors such as barium zirconate/cerate (BaCexZr0.9-xY0.1O3- δ referred to as BCZY) have become increasingly more popular, with a large number of potential applications including protonic ceramic fuel/electrolysis cells (PCFC/PCEC) and membrane reactors for steam methane reforming, methane dehydroaromatization or ammonia synthesis. It was also demonstrated that PCFCs can operate with high efficiency while remaining cost competitive in the temperature range 500-600 °C compared to traditional solid oxide fuel cells at 800 °C [1]. Contrary to traditional oxygen ion conductors (such as yttria stabilized zirconia) that display only thermal expansion, chemical expansion is also present in high temperature proton conductors upon hydration (equation 1) where protonic defects are incorporated into the oxygen sublattice [2-4]. (1) H2O (g) + OO x + VO°° ↔ 2 OHO° First, techniques for thermal and chemical expansion measurements will be presented together with the analysis/interpretation challenges. Then, an overview of the literature data on the expansion in high temperature proton conductors will be provided. While all these measurements are performed in a single atmosphere, it is important to be able to predict the expansion with different gas compositions on both sides of the membrane to mimic a real device application. To do so, we developed a computational model based on a Nernst-Planck-Poisson formulation and included a chemo-thermo-mechanical component. Defect concentrations and mobilities were taken from previous conductivity measurement modelling [5], whereas mechanical properties (Young modulus, Poisson ratios) were determined experimentally.

Doping-induced structural phase transition in cobalt diselenide enables enhanced hydrogen evolution catalysis

Transition metal dichalcogenide materials have been explored extensively as catalysts to negotiate the hydrogen evolution reaction, but they often run at a large excess thermodynamic cost. Although activating strategies, such as defects and composition engineering, have led to remarkable activity gains, there remains the requirement for better performance that aims for real device applications. We report here a phosphorus-doping-induced phase transition from cubic to orthorhombic phases in CoSe2. It has been found that the achieved orthorhombic CoSe2 with appropriate phosphorus dopant (8 wt%) needs the lowest overpotential of 104 mV at 10 mA cm−2 in 1 M KOH, with onset potential as small as −31 mV. This catalyst demonstrates negligible activity decay after 20 h of operation. The striking catalysis performance can be attributed to the favorable electronic structure and local coordination environment created by this doping-induced structural phase transition strategy.

The effect of the bottom electrode on ferroelectric tunnel junctions based on CMOS-compatible HfO2

Ferroelectric tunnel junctions (FTJs) have attracted research interest as promising candidates for non-destructive readout non-volatile memories. Unlike conventional perovskite FTJs, hafnia FTJs offer many advantages in terms of scalability and CMOS compatibility. However, so far, hafnia FTJs have shown poor endurance and relatively low resistance ratios and these have remained issues for real device applications. In our study, we fabricated HfZrO(HZO)-based FTJs with various electrodes (TiN, Si, SiGe, Ge) and improved the memory performance of HZO-based FTJs by using the asymmetry of the charge screening lengths of the electrodes. For the HZO-based FTJ with a Ge substrate, the effective barrier afforded by this FTJ can be electrically modulated because of the space charge-limited region formed at the ferroelectric/semiconductor interface. The optimized HZO-based FTJ with a Ge bottom electrode presents excellent ferroelectricity with a high remnant polarization of 18 μC cm−2, high tunneling electroresistance value of 30, good retention at 85 °C and high endurance of 107. The results demonstrate the great potential of HfO2-based FTJs in non-destructive readout non-volatile memories.

Stability and Activity of Non‐Noble‐Metal‐Based Catalysts Toward the Hydrogen Evolution Reaction

AbstractA fundamental understanding of the behavior of non‐noble based materials toward the hydrogen evolution reaction is crucial for the successful implementation into practical devices. Through the implementation of a highly sensitive inductively coupled plasma mass spectrometer coupled to a scanning flow cell, the activity and stability of non‐noble electrocatalysts is presented. The studied catalysts comprise a range of compositions, including metal carbides (WC), sulfides (MoS2), phosphides (Ni5P4, Co2P), and their base metals (W, Ni, Mo, Co); their activity, stability, and degradation behavior was elaborated and compared to the state‐of‐the‐art catalyst platinum. The non‐noble materials are stable at HER potentials but dissolve substantially when no current is flowing. Through pre‐ and post‐characterization of the catalysts, explanations of their stability (thermodynamics and kinetics) are discussed, challenges for the application in real devices are analyzed, and strategies for circumventing dissolution are suggested. The precise correlation of metal dissolution with applied potential/current density allows for narrowing down suitable material choices as replacement for precious group metals as for example, platinum and opens up new ways in finding cost‐efficient, active, and stable new‐generation electrocatalysts.

Stability and Activity of Non-Noble-Metal-Based Catalysts Toward the Hydrogen Evolution Reaction.

A fundamental understanding of the behavior of non-noble based materials toward the hydrogen evolution reaction is crucial for the successful implementation into practical devices. Through the implementation of a highly sensitive inductively coupled plasma mass spectrometer coupled to a scanning flow cell, the activity and stability of non-noble electrocatalysts is presented. The studied catalysts comprise a range of compositions, including metal carbides (WC), sulfides (MoS2 ), phosphides (Ni5 P4 , Co2 P), and their base metals (W, Ni, Mo, Co); their activity, stability, and degradation behavior was elaborated and compared to the state-of-the-art catalyst platinum. The non-noble materials are stable at HER potentials but dissolve substantially when no current is flowing. Through pre- and post-characterization of the catalysts, explanations of their stability (thermodynamics and kinetics) are discussed, challenges for the application in real devices are analyzed, and strategies for circumventing dissolution are suggested. The precise correlation of metal dissolution with applied potential/current density allows for narrowing down suitable material choices as replacement for precious group metals as for example, platinum and opens up new ways in finding cost-efficient, active, and stable new-generation electrocatalysts.

Barium strontium oxide functionalized carbon nanotubes thin film thermionic emitter with superior thermionic emission capability

Despite their potential, thin film thermionic emitters are yet to appear in real device applications. The main shortcoming for thin film thermionic emitters is their weak emission capability as compared to conventional thermionic cathodes. In this study, a high performance thin film thermionic emitter with emission capability on par with that of a conventional thermionic cathode is presented. This thin film emitter combines a large Schottky effect induced by the carbon nanotubes with a low work function oxide surface coating, resulting in a dramatic increase of thermionic emission. Emission current density as high as 4.5 A/cm2 was obtained at a typical thermionic emission temperature of 1380 K. The growth and fabrication techniques for this emitter are also compatible with the silicon process, making it possible to incorporate this thin film thermionic emitter into other semiconductor devices for potential new device applications.

High thermionic emission from barium strontium oxide functionalized carbon nanotubes thin film surface

Thermionic cathodes are widely used in applications where strong electron emission is essential. Thin film thermionic emitters, despite their potential, are yet to appear in real device applications. The main shortcoming of thin film thermionic emitters is their weak emission capability as compared to the bulky conventional thermionic cathodes. A high performance thermionic thin film emitter with emission capability on par with that of a conventional thermionic cathode is presented in this study. This thin film emitter is based on carbon nanotubes (CNTs) with their surface further functionalized with low work function oxide materials. The low-work-function barium strontium oxide coating combined with a large Schottky effect induced by the carbon nanotubes leads to a dramatic increase in thermionic emission. Emission current as high as 325 mA is obtained from an emission surface area of 0.0727 cm2 at 1380 K, which is equivalent to a current density of 4.5 A/cm2 at a modest thermionic emission temperature. Plasma enhanced chemical vapor deposition was used to grow the carbon nanotubes, while the magnetron sputtering technique was used to functionalize the CNT surface with a thin layer of low-work-function oxide coating. The whole growth and fabrication process of this thin film emitter are compatible with semiconductor fabrication processes, making it possible to incorporate this thermionic thin film into other semiconductor devices for other potential applications.

Black Phosphorus Based All-Optical-Signal-Processing: Toward High Performances and Enhanced Stability

Two-dimensional (2D) black phosphorus (BP) shows thickness dependent direct energy band-gaps in association with strong light-matter interaction and broadband optical response, rendering it with promising optoelectronic advantages particularly at the telecommunication band. However, intrinsic BP suffers from irreversible oxidization, restricting its competences toward real device applications. As one potential of 2D materials, all-optical signal processing sensitively depends on the strength of light–matter interaction. BP can be utilized as a novel optical medium. Herein, few-layer BP is synthesized with metal-ion-modification against oxidation and degradation, and then the feasibility of BP-coated microfiber as an optical Kerr switcher and a four-wave-mixing-based wavelength converter is demonstrated. The wavelength-tuning, long-term stability, wide-band RF frequency, and time-repeating measurements confirm that this optical device can operate as a broadband all-optical processor. It is further anticipa...

Defect concentration in nitrogen-doped graphene grown on Cu substrate: A thickness effect

Tuning the band-gap of graphene is a current need for real device applications. Copper (Cu) as a substrate plays a crucial role in graphene deposition. Here we report the fabrication of in-situ nitrogen (N) doped graphene via chemical vapor deposition (CVD) technique and the effect of Cu substrate thickness on the growth mechanism. The ratio of intensities of G and D peaks was used to evaluate the defect concentration based on local activation model associated with the distortion of the crystal lattice due to incorporation of nitrogen atoms into graphene lattice. The results suggest that Cu substrate of 20µm in thickness exhibits higher defect density (1.86×1012cm−2) as compared to both 10 and 25µm thick substrates (1.23×1012cm−2 and 3.09×1011cm−2, respectively). Furthermore, High Resolution -X-ray Photoelectron Spectroscopy (HR-XPS) precisely affirms ~0.4at% of nitrogen intercalations in graphene. Our results show that the substitutional type of nitrogen doping dominates over the pyridinic configuration. In addition, X-ray diffraction (XRD) shows all the XRD peaks associated with carbon. However, the peak at ~24° is suppressed by the substrate peaks (Cu). These results suggest that nitrogen atoms can be efficiently incorporated into the graphene using thinner copper substrates, rather than the standard 25µm ones. This is important for tailoring the properties by graphene required for microelectronic applications.

Antibacterial Activities of Graphene Oxide-Molybdenum Disulfide Nanocomposite Films.

Two-dimensional (2D) nanomaterials, such as graphene-based materials and transition metal dichalcogenide (TMD) nanosheets, are promising materials for biomedical applications owing to their remarkable cytocompatibility and physicochemical properties. On the basis of their potent antibacterial properties, 2D materials have potential as antibacterial films, wherein the 2D nanosheets are immobilized on the surface and the bacteria may contact with the basal planes of 2D nanosheets dominantly rather than contact with the sharp edges of nanosheets. To address these points, in this study, we prepared an effective antibacterial surface consisting of representative 2D materials, i.e., graphene oxide (GO) and molybdenum disulfide (MoS2), formed into nanosheets on a transparent substrate for real device applications. The antimicrobial properties of the GO-MoS2 nanocomposite surface toward the Gram-negative bacteria Escherichia coli were investigated, and the GO-MoS2 nanocomposite exhibited enhanced antimicrobial effects with increased glutathione oxidation capacity and partial conductivity. Furthermore, direct imaging of continuous morphological destruction in the individual bacterial cells having contacts with the GO-MoS2 nanocomposite surface was characterized by holotomographic (HT) microscopy, which could be used to detect the refractive index (RI) distribution of each voxel in bacterial cell and reconstruct the three-dimensional (3D) mapping images of bacteria. In this regard, the decreases in both the volume (67.2%) and the dry mass (78.8%) of bacterial cells that came in contact with the surface for 80 min were quantitatively measured, and releasing of intracellular components mediated by membrane and oxidative stress was observed. Our findings provided new insights into the antibacterial properties of 2D nanocomposite film with label-free tracing of bacterial cell which improve our understanding of antimicrobial activities and opened a window for the 2D nanocomposite as a practical antibacterial film in biomedical applications.

Zero-Dimensional Cs4PbBr6 Perovskite Nanocrystals.

Perovskite nanocrystals (NCs) have become leading candidates for solution-processed optoelectronics applications. While substantial work has been published on 3-D perovskite phases, the NC form of the zero-dimensional (0-D) phase of this promising class of materials remains elusive. Here we report the synthesis of a new class of colloidal semiconductor NCs based on Cs4PbBr6, the 0-D perovskite, enabled through the design of a novel low-temperature reverse microemulsion method with 85% reaction yield. These 0-D perovskite NCs exhibit high photoluminescence quantum yield (PLQY) in the colloidal form (PLQY: 65%), and, more importantly, in the form of thin film (PLQY: 54%). Notably, the latter is among the highest values reported so far for perovskite NCs in the solid form. Our work brings the 0-D phase of perovskite into the realm of colloidal NCs with appealingly high PLQY in the film form, which paves the way for their practical application in real devices.

Correlation of dielectric, electrical and magnetic properties near the magnetic phase transition temperature of cobalt zinc ferrite.

Multiferroic composite structures, i.e., composites of magnetostrictive and piezoelectric materials, can be envisioned to achieve the goal of strong room-temperature ME coupling for real practical device applications. Magnetic materials with high magnetostriction, high Néel temperature (TN), high resistivity and large magnetization are required to observe high ME coupling in composite structures. In continuation of our investigations on suitable magnetic candidates for multiferroic composite structures, we have studied the crystal structure, dielectric, transport, and magnetic properties of Co0.65Zn0.35Fe2O4 (CZFO). Rietveld refinement of X-ray diffraction patterns confirms the phase purity with a cubic crystal structure with the (Fd3[combining macron]m) space group; however, we have found a surprisingly large magneto-dielectric anomaly at the Néel temperature, unexpected for a cubic structure. The presence of mixed valences of Fe2+/Fe3+ cations is probed by X-ray photoelectron spectroscopy (XPS), which supports the catonic ordering-mediated large dielectric response. Large dielectric permittivity dispersion with a broad anomaly is observed in the vicinity of the magnetic phase transition temperature (TN) of CZFO suggesting a strong correlation between dielectric and magnetic properties. The evidence of strong spin-polaron coupling has been established from temperature dependent dielectric, ac conductivity and magnetization studies. The ferrimagnetic-paramagnetic phase transition of CZFO has been found at ∼640 K, which is well above room temperature. CZFO exhibits low loss tangent, a high dielectric constant, large magnetization with soft magnetic behavior and magnetodielectric coupling above room temperature, elucidating the possible potential candidates for multiferroic composite structures as well as for multifunctional and spintronics device applications.

Chemically modulated graphene quantum dot for tuning the photoluminescence as novel sensory probe.

A band gap tuning of environmental-friendly graphene quantum dot (GQD) becomes a keen interest for novel applications such as photoluminescence (PL) sensor. Here, for tuning the band gap of GQD, a hexafluorohydroxypropanyl benzene (HFHPB) group acted as a receptor of a chemical warfare agent was chemically attached on the GQD via the diazonium coupling reaction of HFHPB diazonium salt, providing new HFHPB-GQD material. With a help of the electron withdrawing HFHPB group, the energy band gap of the HFHPB-GQD was widened and its PL decay life time decreased. As designed, after addition of dimethyl methyl phosphonate (DMMP), the PL intensity of HFHPB-GQD sensor sharply increased up to approximately 200% through a hydrogen bond with DMMP. The fast response and short recovery time was proven by quartz crystal microbalance (QCM) analysis. This HFHPB-GQD sensor shows highly sensitive to DMMP in comparison with GQD sensor without HFHPB and graphene. In addition, the HFHPB-GQD sensor showed high selectivity only to the phosphonate functional group among many other analytes and also stable enough for real device applications. Thus, the tuning of the band gap of the photoluminescent GQDs may open up new promising strategies for the molecular detection of target substrates.

Growth of epitaxial orthorhombic YO1.5-substituted HfO2 thin film

YO1.5-substituted HfO2 thin films with various substitution amounts were grown on (100) YSZ substrates by the pulsed laser deposition method directly from the vapor phase. The epitaxial growth of film with different YO1.5 amounts was confirmed by the X-ray diffraction method. Wide-area reciprocal lattice mapping measurements were performed to clarify the crystal symmetry of films. The formed phases changed from low-symmetry monoclinic baddeleyite to high-symmetry tetragonal/cubic fluorite phases through an orthorhombic phase as the YO1.5 amount increased from 0 to 0.15. The additional annular bright-field scanning transmission electron microscopy indicates that the orthorhombic phase has polar structure. This means that the direct growth by vapor is of polar orthorhombic HfO2-based film. Moreover, high-temperature X-ray diffraction measurements showed that the film with a YO1.5 amount of 0.07 with orthorhombic structure at room temperature only exhibited a structural phase transition to tetragonal phase above 450 °C. This temperature is much higher than the reported maximum temperature of 200 °C to obtain ferroelectricity as well as the expected temperature for real device application. The growth of epitaxial orthorhombic HfO2-based film helps clarify the nature of ferroelectricity in HfO2-based films (186 words/200 words).

Aggregation-Induced Emission Active Metal-Free Chemosensing Platform for Highly Selective Turn-On Sensing and Bioimaging of Pyrophosphate Anion.

We report the synthesis of a metal-free chemosensor for highly selective sensing of pyrophosphate (PPi) anion in physiological medium. The novel phenylbenzimidazole functionalized imine containing chemosensor (L; [2,6-bis(((4-(1H-benzo[d]imidazol-2-yl)phenyl)imino) methyl)-4 methyl phenol]) could sense PPi anion through "turn-on" colorimetric and fluorimetric responses in a very competitive environment. The overall sensing mechanism is based on the aggregation-induced emission (AIE) phenomenon. Moreover, a real time in-field device application was demonstrated by sensing PPi in paper strips coated with L. Interestingly, detection of intracellular PPi ions in model human cells could also be possible by fluorescence microscopic studies without any toxicity to these cells.

Assembly and alignment of conjugated polymers: materials design, processing, and applications

Abstract

Free-standing boron and oxygen co-doped carbon nanofiber films for large volumetric capacitance and high rate capability supercapacitors

Carbon-based materials are the most common and important supercapacitor electrode materials, and have been attracting much attention for researchers. Although much work has focused on increasing the gravimetric capacitance of carbon materials, it is highly needed to obtain high volumetric capacitance for real compact device application. Therefore, a finely tuned carbon material structure with both optimal gravimetric and volumetric capacitances has been becoming a considerable challenge. In this work, we synthesized free-standing boron and oxygen co-doped carbon nanofiber (BO-CNF) films for the first time. Both high gravimetric and volumetric capacitances (192.8Fg−1 and 179.3Fcm−3 at 1Ag−1) can be obtained by an optimized design with regulating the heteroatom content and packing density. Meanwhile, the BO-CNF film with a relatively high packing density exhibits an excellent rate capability (78.5% capacitance retention from 1 to 100Ag−1), which is due to the formation of continuous electrolyte ion diffusion network as well as good electrical conductivity. Such BO-CNF film provides an excellent platform for depositing polyaniline active materials and the boron dopant can be recycled to reduce the cost for the possibly scalable application.

Mesoporous ceramic oxides as humidity sensors: A case study for gadolinium-doped ceria

Mesoporous materials have been studied as high performance sensing materials due to their singular microstructure and extremely high surface-to-volume ratio. However, the lack of stability of these nanostructures is assumed as one of the major drawbacks toward their application in real devices. In this work, this limitation is overcome by the synthesis of thermally stable mesoporous gadolinium doped ceria. Humidity sensors were fabricated and tested under different (i.e. humidity and temperature) conditions. The mesoporous layers were attached to the substrate at 900°C preserving mesoporous structure intact. This process at high temperature provides the layer with a mechanical strength and allows self-cleaning cycles at high temperatures if required. The humidity sensing mechanism is presented and discussed in detail by means of impedance spectroscopy. An ionic type of conduction mechanism is corroborated. Fast response and recovery, as well as very low hysteresis and no drift are observed.It was also shown that the response of the devices can be straightforwardly tuned by changing layer thickness or pore size, allowing to fulfill sensing needs of different applications. All the mentioned properties joined to the simplicity of the fabrication and the flexibility of the used fabrication route for synthesizing any other metal oxide make this kind of devices a potential group for developing high performance and fast gas sensors.

Functionalized Polyelectrolytes Assembling on Nano‐BioFETs for Biosensing Applications

A new surface functionalization scheme for nano‐Bio field effect transistors (FETs) using biocompatible polyelectrolyte thin films (PET) is developed. PET assemblies on Si nanowires (Si‐NWs) are driven by electrostatic interactions between the positively charged polymer backbone and negatively charged Si/SiO2 surface. Such assemblies can be directly coated from PET aqueous solutions and result in a uniform nanoscale thin film, which is more stable compared to the conventional amine silanization. Short oligo‐ethylene glycol chains are grafted on the PETs to prevent nonspecific protein binding. Moreover, the reactive groups of the polymer chains can be further functionalized to other chemical groups in specific stoichiometry for biomolecules detection. Therefore, it opens a new strategy to precisely control the functional group densities on various biosensor surfaces at the molecular level. In addition, such assemblies of the polymers together with the bound analytes can be removed with the pH stimulation resulting in regeneration of a bare sensor surface without compromising the integrity and performance of the Si‐NWs. Thus, it is believed that the developed PET coating and sensing systems on Si‐NW FETs represent a versatile, promising approach for regenerative biosensors which can be applied to other biosensors and will benefit real device applications, enhancing sensor lifetime, reliability, and repeatability.