A polarization-sensitive, high on/off ratio and self-powered photodetector based on Nb2CTxand Nd2CTx@MoS2.

MXene two-dimensional materials have been widely used in energy storage, catalysis, sensing and other fields, Nb2C as a typical two-dimensional MXene material, its exploration in the field of optoelectronics is still in its infancy, especially Nb2C-based photodetectors are still to be developed. This paper demonstrates that two-dimensional films based on few-layer Nb2C have a photoelectric response in the wavelength range from visible to near-infrared. We have found that the light response performance can be easily adjusted by controlling the thickness of the spin-coated film, and that Nb2C photodetectors show great advantages in terms of wide bandwidth, polarization response, high switching ratio, etc. By adjusting the material concentration and sample thickness, the photocurrent can reach up to 330 nA, the switching ratio can reach 410, and the responsivity can reach 8.3 × 10-4A W-1. In the polarization characteristic test, an extinction ratio of 7.6 can be obtained. By adjusting the content of that doped MoS2quantum dot, the dark current can reach 7.6 × 10-13A, and the switching ratio can reach 3 × 105, which can be increased by 700 times. The above results show that the few-layer Nb2C nanosheets can be used as optoelectronic detectors in the visible to near-infrared bands, which further broadens the application prospects of two-dimensional MXene.

Pressure-Induced Superconductivity in HgTe Single-Crystal Film.

HgTe film is widely used for quantum Hall well studies and devices, as it has unique properties, like band gap inversion, carrier-type switch, and topological evolution depending on the film thickness modulation near the so-called critical thickness (63.5 Å), while its counterpart bulk materials do not hold these nontrivial properties at ambient pressure. Here, much richer transport properties emerging in bulk HgTe crystal through pressure-tuning are reported. Not only the above-mentioned abnormal properties can be realized in a 400 nm thick bulk HgTe single crystal, but superconductivity is also discovered in a series of high-pressure phases. Combining crystal structure, electrical transport, and Hall coefficient measurements, a p-n carrier type switching is observed in the first high-pressure cinnabar phase. Superconductivity emerges after the semiconductor-to-metal transition at 3.9 GPa and persists up to 54 GPa, crossing four high-pressure phases with an increased upper critical field. Density functional theory calculations confirm that a surface-dominated topologic band structure contributes these exotic properties under high pressure. This discovery presents broad and efficient tuning effects by pressure on the lattice structure and electronic modulations compared to the thickness-dependent critical properties in 2D and 3D topologic insulators and semimetals.

Enhanced terahertz shielding by adding rare Ag nanoparticles to Ti3C2TxMXene fiber membranes.

Polyacrylonitrile/Ti3C2TxMXene/silver nanoparticles fiber membranes with different silver nanoparticles contents and thickness of porous structure have been successfully prepared by electrospinning. Through the measurement of terahertz time domain spectrum, the shielding effect of the fiber membrane with 1% silver nanoparticles content can reach up to 12 dB. Moreover, the thickness of the spinning fiber membranes is controlled by adjusting the spinning time, so as to better analyze the influence of the thickness of the shielding performance in terahertz band. We attribute this excellent phenomenon to porous structure of the spun fiber membrane and combination of Ti3C2TxMXene with few-layers and silver nanoparticles to increase the absorption and conductivity of the fiber membrane, thereby enhancing the shielding effect in terahertz range. Meanwhile, the prepared polyacrylonitrile/Ti3C2TxMXene/silver nanoparticles fiber membranes show good stability and little change in terahertz shielding effect after high temperature annealing. This may provide potential ideas about the development of high-performance terahertz shielding materials, which are of great significance of terahertz electromagnetic shielding.

Tunable 3D Dirac-semimetals supported mid-IR hybrid plasmonic waveguides.

Based on 3D Dirac-semimetal (DSM) modified hybrid waveguides, tunable propagation properties have been investigated, including the effects of Fermi levels, structural parameters, and operation frequency. The results show that if the operation frequency is smaller (larger) than the transition frequency (ℏω≈2|µc|), the proposed hybrid waveguides indicate strong (weak) confinement because the DSM layer manifests a high plasmonic (dielectric low) loss property. The dielectric fiber shape affects the propagation property obviously, as the elliptical parameter decreases, the confinement and figure of merit increase, and the loss reduces. With the increase in Fermi level, the propagation constant increases, and the frequency (amplitude) modulation depth is 32.31% (12.93%) if the Fermi level changes in the range of 0.01-0.15 eV. The results are very helpful in understanding the tunable mechanisms of hybrid waveguides and designing novel plasmonic devices in the future, e.g., modulators, filters, lasers, and resonators.

MXene-based ultra-thin film for terahertz radiation shielding.

We have successfully fabricated Ti-based MXenes flakes, Ti3C2Tx, by chemical etching, then prepared it as an organic dispersion and finally spin-coated it on polyimide plastic substrate for terahertz wave shielding. The shielding effectivity of the 12 µm ultra-thin film can reach up to 17 dB measured by the terahertz time-domain spectra. We can attribute the excellent phenomenon to the intrinsic absorption of triple-layered Ti3C2, due to the similar double-peak type refraction curves, which have been respectively observed from the experimental samples and the simulation ones. High conductivity and strong THz absorption indicate the Ti3C2Tx MXene is the absorptive electromagnetic shielding material. Comparing with other kinds of THz shielding materials, the Ti-based MXenes might be a potential candidate for the next generation of ultra-thin and lightweight THz shielding.

Enhanced photovoltaic response of lead-free ferroelectric solar cells based on (K,Bi)(Nb,Yb)O3 films.

Herein, we firstly present the (K,Bi)(Nb,Yb)O3 inorganic ferroelectric photovoltaic (FPV) film, in which a nearly ideal bandgap of ∼1.45 eV in the center of the solar spectrum and the co-existence of oxygen vacancies as well as ferroelectric polarization were confirmed. Furthermore, a novel cell structure is successfully fabricated by combining charge-transporting TiO2 nanoparticles, the perovskite sensitizer and a light-absorbing oxide hole p-type NiO conductor to realize a 1 V open circuit voltage, which can be increased to 1.56 V by adjusting the test bias near the coercive voltage. Additionally, under simulated standard AM 1.5G illumination, a fill factor of 86% and a power conversion efficiency of 0.85% are achieved via oxygen vacancy electromigration and polarization switching modulation. It is shown that the obtained power conversion efficiency is one to three orders of magnitude higher than those of pure BiFeO3 and Pb(Zr,Ti)O3. The enhanced PV effects are well elucidated using the transformation from a Schottky-like barrier to Ohmic contacts caused by polarization switching and oxygen vacancies. Building upon the above studies, deep insights into the bandgap tunability and PV effects in ferroelectric films with high oxygen vacancy concentration are provided and will facilitate a new versatile route for exploring high PV performance based on inorganic ferroelectric films.

Investigation of terahertz all-dielectric metamaterials.

The propagation properties of Si-based all-dielectric metamaterials (ADMs) structures were investigated systematically, taking into account the effects of structural parameters, operation frequencies, and graphene Fermi levels. The results manifested that ADMs indicated sharp resonant curves with large Q-factors of more than 60, and a figure of merit of approximately 20. Compared with that of thin metal metamaterial counterparts, the thickness of ADMs (in the range of tens of micrometers) required to excite obvious resonant curves was much larger. By introducing an asymmetrical structure, an obvious Fano-resonant peak was observed, which also became stronger with increasing asymmetrical degree. In addition, by unitizing a uniform graphene layer, the Fano-resonant curves can be flexibly modulated over a wide range, and the amplitude-modulation depth of the Fano peak was approximately 40% when the Fermi level varied in the range of 0.01-1.0 eV. These results are very useful for the design of high Q-factor dielectric devices in the future (e.g., biosensors, modulators, and filters).

Terahertz electromagnetically-induced transparency of self-complementary meta-molecules on Croatian checkerboard.

A terahertz (THz) electromagnetically-induced transparency (EIT) phenomenon is observed from two types of self-complementary meta-molecules (MMs) based on rectangular shaped electric split-ring resonators (eSRR) on Croatian checkerboard. Each MM contains a couple of identical size eSRRs and a couple of structural inversed eSRRs twisted π/2 in checkerboard pattern. In the first type of MM (type-I), the gap is in the middle line of eSRR. In the second type of MM (type-II), the gap is on the two arms of eSRR. Both types of MMs exhibit EIT effect. A maximum 20 ps group delay is observed at the transparency window of 0.63 THz in type-I MM; while a maximum 6.0 ps group delay is observed at the transparent window of 0.60 THz in type-II MM. The distribution of surface currents and electrical energy reveals that only CeSRR contribute to the transparency window as well as the side-modes in type-I MM, where the current leakage via contact point contributes to the low-frequency side-mode, and the coupled local inductive-capacitive (LC) oscillation in CeSRRs contributes to the high-frequency side-mode. In type-II MM, however, the localized dipolar oscillator of CeSRR contributes to the low-frequency side-mode; while the hybridization of dipole oscillation on eSRR and LC resonance on CeSRR contributes to the high-frequency side-modes. Our experimental findings manifest a new approach to develop THz slow-light devices.

Selective determination of acetone by carbon nanodots based on inner filter effect.

Fluorescent sensing of acetone has been achieved taking advantage of the unique optical property of acetone. However, the complicated synthesis process of the fluorescent probes limited their application. Here, carbon nanodots (CNDs) derived from glucose were chosen as the sensing material for the first time, which could be obtained by a one-pot microwave-assisted synthesis within 2 min. The CND had ultraviolet excitation spectrum, resulting in an obvious overlap between its excitation band with the absorption band of acetone. The fluorescent quenching occurred via inner filter effect with fast response. The CNDs showed sensitive response to acetone with a detection limit of 0.09 vol%, which was comparable to most previously reported MOF probes. The results of our work indicated the feasibility of IFE-based sensing method, and demonstrated an efficient and convenient way for acetone detection.

Tunable MoS2 modified hybrid surface plasmon waveguides.

The tunable propagation properties of MoS2 supported hybrid surface plasmon waveguides based on dielectric fiber-gap-metal substrate structures have been investigated by using the finite element method, including the effects of structural parameters, the dielectric fiber shape and carrier concentration of the MoS2 layer. The results reveal that as the dielectric fiber radius increases, the confinement of the hybrid mode increases, and the losses show a peak. The shape of the dielectric fiber affects the propagation properties obviously, with an optimum structural parameter (a large value of the elliptical parameter) the confinement and figure of merits increase, and the dissipation decreases simultaneously. In addition, as the carrier concentration of the MoS2 layer increases, the modulation depth of absorption reaches more than 40%, and the propagation constants manifest obvious double peaks at wavelengths of 610 nm (2.03 eV) and 660 nm (1.88 eV), coming from the excitons' absorption of the MoS2 layer. The results are very useful in helping one to understand the tunable mechanisms of hybrid mode waveguide structures and for the design of novel surface plasmonic devices in the future, e.g. absorbers, modulators, lasers, and resonators.

Tunable high Q-factor terahertz complementary graphene metamaterial.

Based on the complementary graphene asymmetric double bars patterns, the tunable Fano resonances with large Q-factors have been investigated in the terahertz regime, including the effects of Fermi levels, structural parameters and operation frequency. The results reveal that compared with existed graphene tunable devices, the Fano resonant curve is very narrow and indicates a large Q-factor of about 60. The strong Fano resonant curves can be convenient tailored. As Fermi level increases, the amplitude of the Fano dip decreases, and the resonant peak position shifts to high frequency. The amplitude modulation depth (MD) of the Fano curve is more, about 90%, if the Fermi level changes in the scope of 0.2-1.0 eV. With the increase of the sample refractive index, the resonant Fano dip shifts low frequency, and the dip amplitude MD can reach more than 40%. The results are very helpful to understand the tunable mechanisms of graphene based Fano systems and to design high sensitivity functional devices, e.g. sensors, modulators, and antenna.

Graphene patterns supported terahertz tunable plasmon induced transparency.

The tunable plasmonic induced transparency has been theoretically investigated based on graphene patterns/SiO2/Si/polymer multilayer structure in the terahertz regime, including the effects of graphene Fermi level, structural parameters and operation frequency. The results manifest that obvious Fano peak can be observed and efficiently modulated because of the strong coupling between incident light and graphene pattern structures. As Fermi level increases, the peak amplitude of Fano resonance increases, and the resonant peak position shifts to high frequency. The amplitude modulation depth of Fano curves is about 40% on condition that the Fermi level changes in the scope of 0.2-1.0 eV. With the distance between cut wire and double semi-circular patterns increases, the peak amplitude and figure of merit increases. The results are very helpful to develop novel graphene plasmonic devices (e.g. sensors, modulators, and antenna) and find potential applications in the fields of biomedical sensing and wireless communications.

Localized terahertz electromagnetically-induced transparency-like phenomenon in a conductively coupled trimer metamolecule.

We experimentally investigate the terahertz (THz) electromagnetically-induced transparency (EIT)-like phenomenon in a metamolecule (MM) of three-body system. This system involves a couple of geometrically identical split-ring resonators (SRRs) in orthogonal layout conductively coupled by a cut-wire resonator. Such a three-body system exhibits two frequency response properties upon to the polarization of incident THz beam: One is the dark-bright-bright layout to the horizontally polarized THz beam, where there is no EIT-like effect; the other is bright-dark-dark layout to the vertically polarized THz beam, where an EIT-like effect is observable. The transparency window can be tuned from 0.71 THz to 0.74 THz by the displacement of cut-wire inside the trimer MM. A maximum of 7.5 ps group delay of THz wave is found at the transparent window of 0.74 THz. When the cut-wire moved to the mid-point of lateral-side of SRR, the EIT-like phenomenon disappears, this leads to a localized THz slow-light effect. The distribution of surface currents and electric energy reveals that the excited inductive-capacitive (LC) oscillation of bright-SRR dominates the high frequency side-mode, which is isolated to the displacement of cut-wire resonator. However, the low frequency side-mode originates from the constructive hybridization of LC resonance in dark-SRR coupled with a localized S-shaped dipole oscillator, which is tunable by the displacement of cut-wire. As a consequence, the group delay as well as the spectral configuration of transparency window can be manipulated by tuning one side-mode while fixing the other. Such an experimental finding reveal the EIT-like effect in a conductively coupled three-body system and manifests a novel approach to achieve tunable THz slow-light device.

Terahertz tunable graphene Fano resonance.

By depositing graphene circular double rings (DR) on a SiO2/Si/polymer substrate, the tunable Fano resonance has been theoretically investigated in the terahertz regime, including the effects of the graphene Fermi level, structural parameters and operation frequency. The results demonstrate that the obvious Fano peak can be efficiently modulated because of strong coupling between the incident waves and graphene ribbons. As the Fermi level increases, the peak amplitude of the Fano curve increases, and the resonant peak position shifts to a high frequency. The amplitude modulation depth of the Fano curves is about 30% if the Fermi level changes in the scope of 0.1-1.0 eV. The optimum gap distance between the DR is about 8-12 µm, where the value of the figure of merit shows a peak. The results are very helpful in order to develop novel graphene plasmonic devices, e.g. sensors and modulators.

A further comparison of graphene and thin metal layers for plasmonics.

Which one is much more suitable for plasmonic materials, graphene or metal? To address this problem well, the plasmonic properties of thin metal sheets at different thicknesses have been investigated and compared with a graphene layer. As demonstration examples, the propagation properties of insulator-metal-insulator and metamaterials (MMs) structures are also shown. The results manifest that the plasmonic properties of the graphene layer are comparable to that of thin metal sheets with the thickness of tens of nanometers. For the graphene MMs structure, by using the periodic stack structure in the active region, the resonant transmission strength significantly improves. At the optimum period number, 3-5 periods of graphene/SiO2, the graphene MMs structure manifests good frequency and amplitude tunable properties simultaneously, and the resonant strength is also strong with large values of the Q-factor. Therefore, graphene is a good tunable plasmonic material. The results are very helpful to develop novel graphene plasmonic devices, such as modulators, antenna and filters.

Note: High-power piezoelectric transformer fabricated with ternary relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3-Pb(In(1/2)Nb(1/2))O3-PbTiO3 single crystal.

A plate-shaped piezoelectric transformer was designed and fabricated using ternary relaxor ferroelectric single crystal Pb(Mg(1/3)Nb(2/3))O3-Pb(In(1/2)Nb(1/2))O3-PbTiO3. Both the input and output sections utilized the transverse-extensional vibration mode. The frequency and load dependences of the electrical properties for the proposed transformer were systematically studied. Results indicated that under a matching load resistance of 14.9 kΩ, a maximum output power of 2.56 W was obtained with the temperature rise less than 5 °C. The corresponding power density reached up to 50 W/cm(3). This ternary single-crystal transformer had potential applications in compact-size converters requiring high power density.

Gravure-printed interdigital microsupercapacitors on a flexible polyimide substrate using crumpled graphene ink.

In this paper, we demonstrate gravure printing of crumpled graphene ink to obtain a highly porous pattern of interdigitated electrodes, leading to an interdigital microsupercapacitor (MSC) on a flexible polyimide substrate. During the process of synthesizing crumpled graphene ink, Mg(OH)2 nanosheets as nanospacers were inserted into graphite oxide layers, resulting in sufficient crumples in graphene nanosheets to prevent the graphene sheets from restacking to enhance the ion transport and expose the electrochemical active area with oxygen-containing groups to provide more pseudo-capacitance. The gravure-printed interdigital MSCs achieved a high energy density (1.41 mW h cm(-3) at 25 mW cm(-3)) and high power density (0.35 mW h cm(-3) at 300 mW cm(-3)), respectively. Additionally, a liquid crystal display was driven by the two serial and two parallel connected MSCs for 35 s after charging for 3 s.

Band structure and dispersion engineering of strongly coupled plasmon-phonon-polaritons in graphene-integrated structures.

We theoretically investigate the polaritonic band structure and dispersion properties of graphene using transfer matrix methods, with strongly coupled graphene plasmons (GPs) and molecular infrared vibrations as a representative example. Two common geometrical configurations are considered: graphene coupled subwavelength dielectric grating (GSWDG) and graphene nanoribbons (GNR). By exploiting the dispersion and the band structure, we show the possibility of tailoring desired polaritonic behavior in each of the two configurations. We compare the strength of coupling occurring in both structures and find that the interaction is stronger in GNR than that of GSWDG structure as a result of the stronger field confinement of the edge modes. The band structure and dispersion analysis not only sheds light on the physics of the hybridized polariton formation but also offers insight into tailoring the optical response of graphene light-matter interactions for numerous applications, such as biomolecular sensing and detection.

Single Crystal Formamidinium Lead Iodide (FAPbI3): Insight into the Structural, Optical, and Electrical Properties.

5 mm-scale large FAPbI 3 single crystals and corresponding photoconductive properties are shown. The phase transition of FAPbI3 between the α-phase and δ-phase is studied. The carrier mobility is 4.4 cm(2) V(-1) s(-1) with a lifetime of 484 ns in the bulk of the single crystal. Finally, photodetectors based on single-crystal FAPbI3 are demonstrated.

Ultrafast properties of femtosecond-laser-ablated GaAs and its application to terahertz optoelectronics.

We report on the first terahertz (THz) emitter based on femtosecond-laser-ablated gallium arsenide (GaAs), demonstrating a 65% enhancement in THz emission at high optical power compared to the nonablated device. Counter-intuitively, the ablated device shows significantly lower photocurrent and carrier mobility. We understand this behavior in terms of n-doping, shorter carrier lifetime, and enhanced photoabsorption arising from the ablation process. Our results show that laser ablation allows for efficient and cost-effective optoelectronic THz devices via the manipulation of fundamental properties of materials.