Monolayer V_{2}MX_{4}: A New Family of Quantum Anomalous Hall Insulators.

We theoretically propose that the van der Waals layered ternary transition metal chalcogenide V_{2}MX_{4} (M=W, Mo; X=S, Se) is a new family of quantum anomalous Hall insulators with sizable bulk gap and Chern number C=-1. The large topological gap originates from the deep band inversion between spin-up bands contributed by d_{xz}, d_{yz} orbitals of V and spin-down band from d_{z^{2}} orbital of M at the Fermi level. Remarkably, the Curie temperature of monolayer V_{2}MX_{4} is predicted to be much higher than that of monolayer MnBi_{2}Te_{4}. Furthermore, the thickness dependence of the Chern number for few multilayers shows interesting oscillating behavior. The general physics from the d orbitals here applies to a large class of ternary transition metal chalcogenide such as Ti_{2}WX_{4} with the space group P-42m. These interesting predictions, if realized experimentally, could greatly promote the research and application of topological quantum physics.

Synergistic Effect of Electron Scattering and Space Charge Transfer Enabled Unprecedented Room Temperature NO2 Sensing Response of SnO2.

Metal oxide gas sensors have long faced the challenge of low response and poor selectivity, especially at room temperature (RT). Herein, a synergistic effect of electron scattering and space charge transfer is proposed to comprehensively improve gas sensing performance of n-type metal oxides toward oxidizing NO2 (electron acceptor) at RT. To this end, the porous SnO2 nanoparticles (NPs) assembled from grains of about 4 nm with rich oxygen vacancies are developed through an acetylacetone-assisted solvent evaporation approach combined with precise N2 and air calcinations. The results show that the as-fabricated porous SnO2 NPs sensor exhibits an unprecedented NO2 -sensing performance, including outstanding response (Rg /Ra  = 772.33 @ 5 ppm), fast recovery (<2 s), an extremely low detection limit (10 ppb), and exceptional selectivity (response ratio >30) at RT. Theoretical calculation and experimental tests confirm that the excellent NO2 sensing performance is mainly attributed to the unique synergistic effect of electron scattering and space charge transfer. This work proposes a useful strategy for developing high-performance RT NO2 sensors using metal oxides, and provides an in-depth understanding for the basic characteristics of the synergistic effect on gas sensing, paving the way for efficient and low power consumption gas detection at RT.

Electrically tunable graphene metamaterial with strong broadband absorption.

The coupling system with dynamic manipulation characteristics is of great importance for the field of active plasmonics and tunable metamaterials. However, the traditional metal-based architectures suffer from a lack of electrical tunability. In this study, a metamaterial composed of perpendicular or parallel graphene-Al2O3-graphene stacks is proposed and demonstrated, which allows for the electric modulation of both graphene layers simultaneously. The resultant absorption of hybridized modes can be modulated to more than 50% by applying an external voltage, and the absorption bandwidth can reach 3.55 µm, which is 1.7 times enhanced than the counterpart of single-layer graphene. The modeling results demonstrate that the small relaxation time of graphene is of great importance to realize the broadband absorption. Moreover, the optical behaviors of the tunable metamaterial can be influenced by the incident polarization, the dielectric thickness, and especially by the Fermi energy of graphene. This work is of a crucial role in the design and fabrication of graphene-based broadband optical and optoelectronic devices.

Porous and nanowire-structured NiO/AgNWs composite electrodes for significantly-enhanced supercapacitive and electrochromic performances.

NiO/AgNWs composite films which specially contain both porous and one-dimensional (1D) nanowire structures are prepared uniformly via a simple chemical bath deposition method. The supercapacitive electrodes constructed by the as-prepared NiO/AgNWs composite films exhibit a high specific capacitance (980 F g-1at 1 A g-1), much higher than that of the pure NiO films. Particularly, a large optical modulation (84.3% at 550 nm) and short switching times for the coloration and bleaching (5.4 and 6.5 s) are also observed if these NiO/AgNWs films serve as the electrochromic materials. The superior capacitive and electrochromic properties of the NiO/AgNWs composite films are attributed to the large electrochemically effective surface areas and enhanced conductivity induced by the addition of 1D AgNWs, which efficiently shorten the ions/electrons diffusion paths and accelerate the reversible redox reactions. Therefore, the NiO/AgNWs composite films hold a great potential for applications as a novel electrode material in supercapacitive and electrochromic devices.

Prevention of Hepatic Ischemia-Reperfusion Injury by Carbohydrate-Derived Nanoantioxidants.

Hepatic ischemia-reperfusion injury (IRI), which mainly results from excessive reactive oxygen species (ROS) generated by a reperfusion burst of oxygen, has long been a major cause of liver dysfunction and failure after surgical procedures. Here, a monodispersed hydrophilic carbohydrate-derived nanoparticle (C-NP) was synthesized as a nanoantioxidant that could effectively prevent hepatic IRI. The spherical C-NPs had a size of ∼78 ± 11.3 nm covered with polar surface groups. They were well dispersible in water with good colloidal stability, nontoxicity, and good ROS scavenging capability. The C-NPs also exhibited good circulation lifetime, effective delivery to liver, and gradual degradability with an ability to assist the IRI group maintaining a normal and healthy liver status. The pathology mechanism of C-NPs in hepatic IRI was confirmed to be scavenging of excessive ROS by C-NPs. The effective therapeutic treatment of C-NPs in living animals revealed a great potential in clinical prevention for hepatic IRI.

Anomalous redshift of graphene absorption induced by plasmon-cavity competition.

Anomalous redshift of the absorption peak of graphene in the cavity system is numerically and experimentally demonstrated. It is observed that the absorption peak exhibits a redshift as the Fermi level of graphene increases, which is contrary to the ordinary trend of graphene plasmons. The influencing factors, including the electron mobility of graphene, the cavity length, and the ribbon width, are comprehensively analyzed. Such anomalous redshift can be explained by the competition between the graphene plasmon mode and the optical cavity mode. The study herein could be beneficial for the design of graphene-based plasmonic devices.

Constructing Electrically and Mechanically Self-Healing Elastomers by Hydrogen Bonded Intermolecular Network.

One key limitation of artificial skin-like materials is the shortened service life caused by mechanical damages during practical applications. The ability to self-heal can effectively extend the material service life, reduce the maintenance cost, and ensure safety. Therefore, it is important and necessary to fabricate materials with simultaneously mechanical and electrical self-healing behavior in a facile and convenient way. Herein, we report a stretchable and conductive self-healing elastomer based on intermolecular networks between poly(acrylic acid) (PAA) and reduced graphene oxide (rGO) through a facile and convenient postreduction and one-pot method. The introduction of rGO provides the PAA-GO elastomers with good mechanical stability and electrical properties. Moreover, this material exhibited both electrical and mechanical self-healing properties. After cutting, the elastomers self-healed quickly (∼30 s) and efficiently (∼95%) at room temperature. The elastomers were accurate and reliable in detecting external strain even after healing. The elastomers were further applied for strain sensors, which were attached directly to human skin to monitor external movements, including finger bending and wrist twisting.

Facile and low-cost fabrication of a humidity sensor using naturally available sepiolite nanofibers.

Much effort has focussed on enhancing the humidity-sensing performances of humidity sensors, but their fabrication using facile and low-cost methods is also desirable. In this work, a humidity sensor based on a naturally available nanomaterial, sepiolite nanofibers (SNFs), was facilely fabricated without any expensive raw materials or complex processes. Characterization results show that SNFs have a natural slender nanofiber structure (diameter 20-50 nm) and abundant hydrophilic functional groups (-OH). The results of humidity-sensing tests show that the SNF humidity sensor has outstanding humidity-sensing properties (i.e. large response, good linearity and repeatability) within the relative humidity range from 10.9% to 91.5% at room temperature (25 °C). This work presents a moderate and cost-effective strategy for the fabrication of high-performance humidity sensors using the natural SNF nanomaterial.

Silicon-based PbS-CQDs infrared photodetector with high sensitivity and fast response.

Silicon-based photodetectors as the main force in visible and near-infrared detection devices have been deeply embedded in modern technology and human society, but due to the characteristics of silicon itself, its response wavelength is generally less than 1100 nm. It is an interesting study to combine the state-of-art silicon processing with emerging infrared-sensitive Lead sulfide colloidal quantum dots (PbS-CQDs) to produce a photodetector that can detect infrared light. Here, we demonstrated a silicon-compatible photodetector that could be integrated on-chip, and also sensitive to infrared light which is owing to a PbS-CQDs absorption layer with tunable bandgap. The device exhibit extremely high gain which reaches maximum detectivity [Formula: see text], fast response 211/558 µs, and extremely high external quantum efficiency [Formula: see text], which is owing to new architecture and reasonable ligand exchange options. The performance of the device originates from the new architecture, that is, using the photovoltaic voltage generated by the surface of PbS-CQDs to change the width of the depletion layer to achieve detection. Besides, the performance improvement of devices comes from the addition of PbS-CQDs (Ethanedithiol treated) layer, which effectively reduces the fall time and makes the device expected to work at higher frequencies. Our work paves the way for the realization of cost-efficient high-performance silicon compatible infrared optoelectronic devices.

High responsivity and fast UV-vis-short-wavelength IR photodetector based on Cd3As2/MoS2 heterojunction.

High responsivity, fast response time, ultra-wide detection spectrum are pursuing goals for state-of-art photodetectors. Cd3As2, as a three-dimensional (3D) Dirac semimetal, has a zero bandgap, high light absorption rate in broad spectral region, and higher mobility than graphene at room temperature. However, photoconductive detectors based Cd3As2 suffer low quantum efficiency due to the absence of high built-in field. Here, a Cd3As2 nanoplate/multilayer MoS2 heterojunction photodetector was fabricated which achieved a quite high responsivity of 2.7 × 103 A W-1 at room temperature. The photodetector exhibits a short response time of in broad spectra region from ultraviolet (365 nm) to short-wavelength-infrared (1550 nm) and reached 65 µs at 650 nm. This work provides a great potential solution for high-performance photodetector and broadband imaging by combining 3D Dirac semi-metal materials with semiconductor materials.

Percolation-amplified infrared sensitivity in titanium oxide-multi-walled carbon nanotube composite films.

Titanium dioxide (TiO2)-multi-walled carbon nanotube (MWCNT) thin films were prepared and studied systematically for the effects of the concentration of MWCNTs on the electro-optical properties. Result shows that the addition of MWCNTs not only improves the optical absorption and electrical conductivity, but also reduces the 1/f noise of the films. Percolation phenomenon is observed at MWCNT concentrations of 0.20-0.25 wt%. In this concentration range, the composite films exhibit an abrupt rise of the temperature coefficient of resistance value (-2.93% K-1) and large general thermal parameter, both of which are desirable for applications in uncooled infrared detectors.

Zero-Bias Visible to Near-Infrared Horizontal p-n-p TiO2 Nanotubes Doped Monolayer Graphene Photodetector.

We present a p-n-p monolayer graphene photodetector doped with titanium dioxide nanotubes for detecting light from visible to near-infrared (405 to 1310 nm) region. The built-in electric field separates the photo-induced electrons and holes to generate photocurrent without bias voltage, which allows the device to have meager power consumption. Moreover, the detector is very sensitive to the illumination area, and we analyze the reason using the energy band theory. The response time of the detector is about 30 ms. The horizontal p-n-p device is a suitable candidate in zero-bias optoelectronic applications.

Amplified molecular detection sensitivity in passive dielectric cavity.

Vibrational absorption spectroscopy presents an effective and direct way for molecular detection and identification. In this paper, we propose and demonstrate a simple strategy and structure to amplify molecular detection sensitivity via the example of a monolayer octadecanethiol (ODT). The underlying amplification mechanism operates on both the enhanced surface field in and the coupled-oscillators' energy transfer between the molecules and the cavity underneath. The structure is designed to be simple and free of lithography or patterning with the potential for large-scale uses. It is made of just a quarter wavelength thick dielectric (ZnSe) layer atop a metallic reflecting base. Both angle and polarization dependent reflection spectra reveal signatures of CH2 and CH3 vibrations in theory and experiment. A vibrational signal intensity of 8.54% reached in s-polarization at a large incident angle is comparable to those reported in plasmonic nanostructures with greater sophistications in structure.

Wide-angle broadband absorption in tapered patch antennas.

Strip array is a classical antenna structure, which provides an effective way to generate and explore new material properties and device functionalities. In this paper, we demonstrate wide-angle broadband absorption in patch antennas made of tapered strip arrays in the metal-insulator-metal geometry. By superimposing multiple resonances associated with the tapered width of the strips, near-perfect absorption is designed and realized over a wide bandwidth from 29.2 THz to 38 THz with efficiency exceeding 80% in the mid-infrared region. The strong absorption band is insensitive to incident angles up to 75°. The angle-independent absorption is attributed to the unique mechanism of coupling between relevant magnetic resonances and free-space incident light. Our tapered patch antenna design offers the advantage of simplicity, and therefore flexibility in engineering natural materials for strong omnidirectional absorption with a variable and wide bandwidth, which could be of interest in applications such as bolometric sensing, camouflaging, and spectral filtering.

Low-Dimensional Materials and State-of-the-Art Architectures for Infrared Photodetection.

Infrared photodetectors are gaining remarkable interest due to their widespread civil and military applications. Low-dimensional materials such as quantum dots, nanowires, and two-dimensional nanolayers are extensively employed for detecting ultraviolet to infrared lights. Moreover, in conjunction with plasmonic nanostructures and plasmonic waveguides, they exhibit appealing performance for practical applications, including sub-wavelength photon confinement, high response time, and functionalities. In this review, we have discussed recent advances and challenges in the prospective infrared photodetectors fabricated by low-dimensional nanostructured materials. In general, this review systematically summarizes the state-of-the-art device architectures, major developments, and future trends in infrared photodetection.

A terahertz study of taurine: Dispersion correction and mode couplings.

The low-frequency characteristics of polycrystalline taurine were studied experimentally by terahertz (THz) absorption spectroscopy and theoretically by ab initio density-functional simulations. Full optimizations with semi-empirical dispersion correction were performed in spectral computations and vibrational mode assignments. For comparison, partial optimizations with pure density functional theory were conducted in parallel. Results indicate that adding long-range dispersion correction to the standard DFT better reproduces the measured THz spectra than the popular partial optimizations. The main origins of the observed absorption features were also identified. Moreover, a coupled-oscillators model was proposed to explain the experimental observation of the unusual spectralblue-shift with the increase of temperature. Such coupled-oscillators model not only provides insights into the temperature dynamics of non-bonded interactions but also offers an opportunity to better understand the physical mechanisms behind the unusual THz spectral behaviors in taurine. Particularly, the simulation approach and novel coupled-oscillators model presented in this work are applicable to analyze the THz spectra of other molecular systems.

Experimental and Theoretical Study on Terahertz Spectra for Regenerated Cellulose.

In this work, regenerated cellulose films were prepared with an iced dissolution method, while the physical morphologies and crystal types of the products were systematically characterized with scanning electron microscope (SEM), Fourier transform infrared(FTIR), while X-Ray Diffraction (XRD). The results demonstrate that the as-prepared continuous and uniform films are indeed cellulose Ⅱ, whose morphology and crystal type are significantly different from those of the degreased cotton. Moreover, Terahertz time domain system (THz-TDS) and FTIR were employed to measure the THz spectra of the regenerated cellulose films. Accordingly, the THz characteristic peaks for the regenerated cellulose films are experimentally identified for the first time. In addition, the increase of the THz transmittance with the decrease of the wavenumber is attributed to the existence of amorphous components in the regenerated cellulose films. Although the shapes of Far-IR spectra in the range of 100～700 cm-1 are similar, the absorption peaks of the regenerated cellulose films move to lower wavenumbers (blue shift) compared with those of the degreased cotton. Based on this, we developed a new approach to distinguish the allomorphism of cellulose Ⅱ and cellulose Iß by Far-IR. Particularly, geometry optimization and IR calculation for the crystal structure of cellulose Ⅱ have been successfully processed by Density Functional Theory (DFT) using periodic boundary condition via CASTEP package. The calculated absorption peak positions are in good agreement with those experimentally measured. Consequently, the THz characteristic peaks of the regenerated cellulose films have been systematically and successfully assigned. Theoretical calculations reveal that the peaks at 42 and 54 cm-1 are assigned to the lattice vibration modes coupled with translational mode and rotational mode, respectively. Moreover, the absorption peaks in the range of 68～238 cm-1 are related with the torsion vibration of ­CH2OH group and deformation vibration of C­H bond and O­H bond, while those in the range of 351～583 cm-1 are assigned to the skeletal vibration of C­O­C bond and pyranoid ring, and those at 611 and 670 cm-1 are originated from the out-of-plane bending vibration of O­H bond. Each absorption peak is involved in more than single vibration mode. The THz spectra presented in this work, together with the theoretical simulations, indicate that the THz responses of regenerated cellulose are closely associated with both its chemical constituents and molecular structure. These results will be helpful not only for better understanding the relations between the molecular structure of the regenerated cellulose and its THz spectrum, but also for providing valuable information for future studies on the physical mechanisms of THz responses of other partially-crystalline polymers and organic biological macromolecules.

Wideband tunable mid-infrared cross polarization converter using rectangle-shape perforated graphene.

The strong plasmonic response and wide electrostatic tunability of graphene make it a promising material for developing infrared optoelectronic components. In this paper, we present a mid-infrared wideband tunable cross polarization converter using periodically perforated graphene. The polarization converter consists of a metal ground plane, an insulator layer, and a rectangle-shape periodically perforated graphene sheet. By superimposing two localized surface plasmon modes, the polarization converter transforms a linear polarization to its cross polarization over a bandwidth as wide as ~5% of the central frequency (46.8THz) with a peak conversion ratio exceeding 90%. The polarization conversion performance is maintained over a wide range of incident angles up to 50°, and is highly tunable by electrostatic tuning of the graphene Fermi energy. Our proposed device enables the manipulation of light polarization for potential mid-infrared applications.

Near-infrared photoresponse of femtosecond-laser processed Se-doped silicon n+ - n photodiodes.

Se-doped silicon was prepared using deposited Si-Se bilayer thin films followed by femtosecond-laser irradiation. n+-n photodiodes were fabricated from this material for the first time, to the best of our knowledge. The effects of the annealing temperature and the reverse bias voltage on the near-infrared responsivity were investigated. The photodiode exhibits optimal rectification and photoresponse at an annealing temperature of 500°C. At a 12 V bias, a responsivity of 2.41 A/W at 1064 nm is obtained. The linear increase at bias from 0 to 10 V and faster-than-linear increase at bias from 10 to 12 V for the responsivity are observed with the increase of the bias voltage. The results suggest that the gain mechanism is most likely to be a photoconductive gain.

Ammonia gas sensors based on poly (3-hexylthiophene)-molybdenum disulfide film transistors.

In this work, in order to enhance the recovery performance of organic thin film transistors (OTFTs) ammonia (NH3) sensors, poly (3-hexylthiophene) (P3HT) and molybdenum disulfide (MoS2) were combined as sensitive materials. Different sensitive film structures as active layers of OTFTs, i.e., P3HT-MoS2 composite film, P3HT/MoS2 bilayer film and MoS2/P3HT bilayer film were fabricated by spray technology. OTFT gas sensors based on P3HT-MoS2 composite film showed a shorter recovery time than others when the ammonia concentration changed from 4 to 20 ppm. Specifically, x-ray diffraction (XRD), Raman and UV-visible absorption were employed to explore the interface properties between P3HT and single-layer MoS2. Through the complementary characterization, a mechanism based on charge transfer is proposed to explain the physical originality of these OTFT gas sensors: closer interlayer d-spacing and better π-π stacking of the P3HT chains in composite film have ensured a short recovery time of OTFT gas sensors. Moreover, sensing mechanisms of OTFTs were further studied by comparing the device performance in the presence of nitrogen or dry air as a carrier gas. This work not only strengthens the fundamental understanding of the sensing mechanism, but provides a promising approach to optimizing the OTFT gas sensors.