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Recently, new states of matter like superconducting or topological quantum states were found in transition metal dichalcogenides (TMDs) and manifested themselves in a series of exotic physical behaviors. Such phenomena have been demonstrated to exist in a series of transition metal tellurides including MoTe2, WTe2, and alloyed MoxW1-xTe2. However, the behaviors in the alloy system have been rarely addressed due to their difficulty in obtaining atomic layers with controlled composition, albeit the alloy offers a great platform to tune the quantum states. Here, we report a facile CVD method to synthesize the MoxW1-xTe2 with controllable thickness and chemical composition ratios. The atomic structure of a monolayer MoxW1-xTe2 alloy was experimentally confirmed by scanning transmission electron microscopy. Importantly, two different transport behaviors including superconducting and Weyl semimetal states were observed in Mo-rich Mo0.8W0.2Te2 and W-rich Mo0.2W0.8Te2 samples, respectively. Our results show that the electrical properties of MoxW1-xTe2 can be tuned by controlling the chemical composition, demonstrating our controllable CVD growth method is an efficient strategy to manipulate the physical properties of TMDCs. Meanwhile, it provides a perspective on further comprehension and sheds light on the design of devices with topological multicomponent TMDC materials.

In this letter, a dual-polarized frequency-selective absorptive structure is presented with two wideband transmission windows. The transmission windows are achieved within a wide –10-dB reflection band. At the frequencies of the passbands, the out-of-phase currents are excited without flowing through the lossy elements. Therefore, the passbands can be obtained with high transmittance. Moreover, only four resistors are loaded on each unit cell to consume the out-of-band reflective wave. By combining lossy flower-shaped and lossless arrays together, the desired dual-band and low reflection performances can be realized. Within the operating band, the reflection is below –10 dB with the fractional bandwidth of 127%. Meanwhile, around 6.5 and 12 GHz, the passband widths are 39% and 40% and the corresponding insertion losses are 0.07 and 0.1 dB, respectively. Finally, the fabricated prototype is measured to demonstrate the feasibility of the presented method.

Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronics devices. However, the synthesis of 2D magnetic crystals, especially the direct growth on SiO2/Si substrate, is just in its infancy. Here, we report a chemical vapor deposition (CVD)-based rational growth approach for the synthesis of ultrathin FeTe crystals with controlled structural and magnetic phases. By precisely optimizing the growth temperature (Tgrowth), FeTe nanoplates with either layered tetragonal or non-layered hexagonal phase can be controlled with high-quality. The two controllable phases lead to square and triangular morphologies with a thickness down to 3.6 and 2.8 nm, respectively. More importantly, transport measurements reveal that tetragonal FeTe is antiferromagnetic with a Neel temperature (TN) about 71.8 K, while hexagonal FeTe is ferromagnetic with a Curie temperature (TC) around 220 K. Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from a concomitant lattice distortion and the spin-lattice coupling. This study represents a major step forward in the CVD growth of 2D magnetic materials on SiO2/Si substrates and highlights on their potential applications in the future spintronic devices., Comment: 15 pages, 5 figures

In recent years, two‐dimensional (2D) ternary materials have attracted wide attention due to their novel properties which can be achieved by regulating their chemical composition with a very great degree of freedom and adjustable space. However, as for the precise synthesis of 2D ternary materials, great challenges still lie ahead that hinder their further development. In this work, we demonstrated a simple and reliable approach to synthesize 2D ternary‐layered BiOCl crystals through a microwave‐assisted space‐confined process in a short time (

Incorporating a truncation of the complex-frequency-shifted perfectly matched layer (CFS-PML), the direct-splitting- based Crank-Nicolson finite-difference time-domain (CNDS-FDTD) is developed and applied to the infrared two-dimensional layered material (2DLM) black phosphorous (BP) metasurface implementations on the all-dielectric nanostructure. To improve extremely low efficiencies in solving infrared terahertz (THz) problems with the few-atomic-layer thickness of 2DLMs, the CFS-CNDS-FDTD is proposed in demand due to the fact that it possesses capabilities of implicit FDTD method and unsplit-field CFS-PML truncation, respectively, in completely conquering the Courant-Friedrich-Levy condition (CFL) limit and holding good performance. The temporal incremental in the CFS-CNDS-FDTD can reach 1000 times larger than that in the regular FDTD for infrared nanoscale problems centered at the 2.5 THz and then keep accurate. Three-dimensional (3D) numerical cases have been carried out to corroborate the proposed method. The CFS-CNDS- FDTD can not only achieve high accuracies and then saves several dozen times of CPU time as compared to the regular FDTD, but also pave the way for designing all-dielectric nanostructures with other 2DLM metasurfaces.

A carbon microtube aerogel (CMA) with hydrophobicity, strong adsorption capacity, and superb recyclability was obtained by a feasible approach with economical raw material, such as kapok fiber. The CMA possesses a great adsorption capacity of 78-348 times its weight. Attributed to its outstanding thermal stability and excellent mechanical properties, the CMA can be used for many cycles of distillation, squeezing, and combustion without degradation, which suggests a potential practical application in oil-water separation. In addition, the adsorption capacity still retained 98% by distillation, 97% by squeezing, and 90% by combustion after 10 cycles. Therefore, the obtained CMA has a broad prospect as an economical, efficient, and environmentally friendly adsorbent. National Research Foundation (NRF) This work was financially supported by Singapore National Research Foundation under NRF RF Award No. MOE2016- T2-1-131, Tier 1 2017-T1-001-075.

Room temperature Na-ion batteries (SIBs) show great potential for use as renewable energy storage systems. However, the large-scale application of SIBs has been hindered by the lack of an ideal SIBs anode material. We synthesized MoS2 on carbonized graphene-chitosan (G-C) using the hydrothermal method. The strong interaction between the MoS2 and the G-C greatly improved the electron transport rate and maintained the structural stability of the electrode, which lead to both an excellent rate capability and long cycle stability. The G-C monolith was proven to enhance the electrical conductivity of the composites and served as a matrix for uniformly dispersing active MoS2 nanosheets (NSs), as well as being a buffer material to adapt to changes in volume during the cycle. Serving as an anode material for SIBs, the MoS2-G-C electrode showed good cycling stability (527.3 mAh g−1 at 100 mA g−1 after 200 cycles), excellent rate capability, and a long cycle life (439.1 mAh g−1 at 1A g−1 after 200 cycles). Keywords: Na-ion batteries, Carbon-based materials, MoS2, Long cycle life

In this letter, a novel absorptive structure with a high-transmittance bandpass behavior is proposed based on the electromagnetically induced transparency-like effect. The desired response is achieved by employing cascaded lossy π-shaped and lossless dual-band reflective arrays. The lossy array consists of radiative and subradiant resonators. A resistor is loaded on the radiative resonator to consume the excited current within the stopband. The bright-dark mode coupling contributes to a transparency window with an insertion loss of 0.07 dB. By combining with the reflective array, two absorption bands are obtained at both sides of the passband, and the maximum absorptivity is above 98%. Meanwhile, the 10 dB reflection reduction behavior is realized within the operating band, and a good stealth performance is provided. The fabricated prototype is measured, and the simulation and experimental results are in a good agreement.

Graphene, an innovated 2D material with atomic thickness, is a very promising candidate and has drawn great attentions in various applications. Graphene metasurface enables dynamic control of various wavefronts, achieving distinguished functionalities. The flexibility of graphene metasurface makes it possible to implement multifunctional devices with ease. In this work, a novel design of multifunctional graphene metasurface, which can combine the functionalities of generating and steering vortex waves, has been proposed. The multifunctional graphene metasurface consists of a large array of graphene reflective unit cells. Each unit cell is controlled independently by its size and external static gate voltage. By scrutinizing the reflective property of the graphene cell, the graphene metasurface is designed to realize multi-functionalities. Simulation results show that vortex wave can be generated and steered. This work can establish a methodology to design multifunctional graphene metasurfaces, and the tunability of graphene opens the gate to the design and fabrication of reconfigurable graphene devices.

This paper presents a Ka-band power divider with microstrip branching lines (MBLs) based on a dielectric substrate (RT/duroid 5880 with a thickness of 0.254 mm, permittivity of 2.2, and loss tangent of 0.0009). The proposed power divider consists of two microstrip branching lines and a thin-film resistor of 100 ohm connected to the microstrip branching lines, which leads to high isolation and matching between the two output ports of this power divider. This designed power divider is fabricated and measured. Both simulated and measured results agree well with each other, and show that the insertion loss is less than 4.0 dB, the return loss is higher than 19 dB, and the isolation between the two output ports is higher than 30 dB in the band from 35 – 40 GHz.

The limited memory retention for a ferroelectric field-effect transistor has prevented the commercialization of its nonvolatile memory potential using the commercially available ferroelectrics. Here, we show a long-retention ferroelectric transistor memory cell featuring a metal-ferroelectric-metal-insulator-semiconductor architecture built from all van der Waals single crystals. Our device exhibits 17 mV dec

In this letter, a new frequency-selective absorber is presented that has wideband transmission/absorption/reflection reduction responses. The presented absorber is completed by lossy cross-dog-bone and lossless second-order frequency-selective surfaces. The modal interaction pole generated by the lossy array is employed to achieve a wideband transmission. Three absorption bands are combined to realize a continuous stopband with high absorptivity. Furthermore, the improved absorber structure is investigated to reduce the strong reflection between the absorption and transmission bands. The 10 dB reflection reduction is obtained with a fractional bandwidth of 126.4%. Meanwhile, the –3 dB fractional passband width is 41.3% with minimum insertion loss of 0.37 dB. The absorption bandwidth of 91.5% is provided with at least 90% absorptivity. The distance between the lossy and lossless layers is only 0.08 $\lambda _{L}$ at the lowest operating frequency. Finally, the proposed design is validated by a good agreement between the numerical simulations and experimental measurements.

The Boltzmann distribution of electrons sets a fundamental barrier to lowering energy consumption in metal-oxide-semiconductor field-effect transistors (MOSFETs). Negative capacitance FET (NC-FET), as an emerging FET architecture, is promising to overcome this thermionic limit and build ultra-low-power consuming electronics. Here, we demonstrate steep-slope NC-FETs based on two-dimensional molybdenum disulfide and CuInP2S6 (CIPS) van der Waals (vdW) heterostructure. The vdW NC-FET provides an average subthreshold swing (SS) less than the Boltzmann’s limit for over seven decades of drain current, with a minimum SS of 28 mV dec−1. Negligible hysteresis is achieved in NC-FETs with the thickness of CIPS less than 20 nm. A voltage gain of 24 is measured for vdW NC-FET logic inverter. Flexible vdW NC-FET is further demonstrated with sub-60 mV dec−1 switching characteristics under the bending radius down to 3.8 mm. These results demonstrate the great potential of vdW NC-FET for ultra-low-power and flexible applications., The adaptation to atomically thin 2D semiconductors and van der Waals layered ferroelectrics can enable negative capacitance transistors with superior performance and bendability. Here, the authors report flexible negative capacitance transistors based on MoS2 and a ferroelectric dielectric CIPS with a minimum sub-threshold slope of 28 mV/dec and high gain logic invertors.

Since the very recent discovery of ferroic ordering under two dimensional limit, novel devices based on 2D ferroelectricity or ferromagnetism has attracted a lot of interests and are promising for next generation electronic and optoelectronic applications. We find that, below the transition temperature Tc ~320 K, ultrathin CuInP2S6 (CIPS) nanoflakes down to bilayer are still pyroelectric. Changing the temperature will lead to charge modification at the surface of CIPS. 2D pyroelectric nanogenerator is fabricated which can efficiently convert the pyroelectric charges into electrical current. This work will facilitate novel applications of pyroelectric and ferroelectric materials, including pyroelectric devices, thermal sensors, ultrathin nanogenerators and various van der Waals (vdW) heterostructure at room temperature. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version This work was supported by the National Research Foundation Singapore under NRF award number NRF-RF2013-08, Tier 2 MOE2017-T2-2-136, Tier 2 MOE2016-T2-2-153, MOE2015-T2-2-007, Tier 1 RG7/18, Tier 1 RG4/17. X.F.L thanks the support from the Ministry of Science and Technology (2017YFA0205004 and 2016YFA0200700), National Natural Science Foundation of China (21673054 and 11874130), Natural Science Foundation of Beijing Municipality (4182076 and 4184109).

Several configurations of multi-beam reconfigurable THz antennas based on graphene have been investigated. Two modulation mechanisms of graphene-based THz antenna are introduced, one is the reflector-transmission window model, and the other is the reflector-director model (Yagi-Uda antenna). The main parameters, such as main beam direction, resonance frequency, peak gain, and the front-to-back ratio of the proposed antenna can be controlled by adjusting the chemical potentials of the graphene in the antenna. Moreover, this paper provides an easy way to obtain complex graphene-based multi-beam antennas, showing strong potential in the design of other complex graphene-based systems, enabling nanoscale wireless communications and sensing devices for different applications.

The limited memory retention for a ferroelectric field-effect transistor has prevented the commercialization of its nonvolatile memory potential using the commercially available ferroelectrics. Here, we show a long-retention ferroelectric transistor memory cell featuring a metal-ferroelectric-metal-insulator-semiconductor architecture built from all van der Waals single crystals. Our device exhibits 17 mV dec−1 operation, a memory window larger than 3.8 V, and program/erase ratio greater than 107. Thanks to the trap-free interfaces and the minimized depolarization effects via van der Waals engineering, more than 104 cycles endurance, a 10-year memory retention and sub-5 μs program/erase speed are achieved. A single pulse as short as 100 ns is enough for polarization reversal, and a 4-bit/cell operation of a van der Waals ferroelectric transistor is demonstrated under a 100 ns pulse train. These device characteristics suggest that van der Waals engineering is a promising direction to improve ferroelectronic memory performance and reliability for future applications., The memory retention for a ferroelectric field-effect transistor is limited by the depolarization effects and carrier charge trapping. Here, the authors fabricate a long-retention memory cell with a metal-ferroelectric-metal-insulator-semiconductor architecture built from all van der Waals single crystals.

Novel 2D semiconductors play an increasingly important role in modern nanoelectronics and optoelectronics. Herein, a novel topology designer based on component fusion is introduced, featured by the submolecular component integration and properties inheritance. As expected, a new air-stable 2D semiconductor PdPSe with a tailored puckered structure is successfully designed and synthesized via this method. Notably, the monolayer of PdPSe is constructed by two sublayers via PP bonds, different from 2D typical transition metal materials with sandwich-structured monolayers. With the expected orthorhombic symmetry and intralayer puckering, PdPSe displays a strong Raman anisotropy. The field-effect transistors and photodetectors based on few-layer PdPSe demonstrate good electronic properties with high carrier mobility of ≈35 cm2 V−1 s−1 and a high on/off ratio of 106, as well as excellent optoelectronic performance, including high photoresponsivity, photogain, and detectivity with values up to 1.06 × 105 A W−1, 2.47 × 107%, and 4.84 × 1010 Jones, respectively. These results make PdPSe a promising air-stable 2D semiconductor for various electronic and optoelectronic applications. This work suggests that the component-fusion-based topology designer is a novel approach to tailor 2D materials with expected structures and interesting properties. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Z.L. acknowledges supports from Singapore National Research Foundation–Competitive Research Program NRF-CRP22-2019-0007 and NRF-CRP21-2018-0007. This research is also supported by A*STAR under its AME IRG Grant (Project No. A2083c0052)

FeTe1-xSe x, a promising layered material used to realize Majorana zero modes, has attracted enormous attention in recent years. Pulsed laser deposition (PLD) and molecular-beam epitaxy (MBE) are the routine growth methods used to prepare FeTe1-xSexthin films. However, both methods require high-vacuum conditions and polished crystalline substrates, which hinder the exploration of the topological superconductivity and related nanodevices of this material. Here we demonstrate the growth of the ultrathin FeTe1-xSex superconductor by a facile, atmospheric pressure chemical vapor deposition (CVD) method. The composition and thickness of the two-dimensional (2D) FeTe1-xSex nanosheets are well controlled by tuning the experimental conditions. The as-prepared FeTe0.8Se0.2 nanosheet exhibits an onset superconducting transition temperature of 12.4 K, proving its high quality. Our work offers an effective strategy for preparing the ultrathin FeTe1-xSex superconductor, which could become a promising platform for further study of the unconventional superconductivity in the FeTe1-xSex system. Accepted version

The design of a compact circular polarization (CP) electrically small antenna (ESA) is carried out and presented in this paper. The designed antenna consists of a planar circuit board (PCB) on which two 3-dB hybrid couplers are fabricated for providing left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) waves, and two ceramic substrates covered by thin metal films with the high dielectric constants of 20.5 and 39 and the sizes of 25x25x6 mm and 18×18×4 mm, respectively. This compact CP ESA can realize a coverage of ±75° elevation angle with gain above -4dB in the frequency band of 1518 - 1525 MHz for RHCP and in the band of 1668 - 1675 MHz for LHCP, and a coverage of ±60°elevation angle with gain above - 2dB in the band of 1575.42±1.023 MHz for RHCP.

In this letter, A pattern reconfigurable antenna is researched. Using the varactor diode which could be controlled by the voltage, the phase of the circulator could be changed. The principle of phase shift is discussed. The four elements antenna array is designed. Connecting with the phase shifter, radiation pattern is measured. The phase is shifted continuously by changing the voltage, which is add on varactor diode, continuously. The direction of radiation pattern could be changed continuously at the same time.

A rectangular filtering dielectric resonator antenna (FDRA) based on the metamaterial (MMT) with a low profile and high gain is investigated in this paper. The proposed antenna consists of a FDRA fed by a microstrip-coupled slot from bottom and a 3×3 array of the near-zero-index MMT on the top of FDRA. The reflection coefficient of the antenna is better than -10 dB in the band from 5.1 to 5.9 GHz, and it has an average gain of 8.3 dB over the band of 5.4~5.9GHz.

Mid-infrared (MIR) light-emitting devices play a key role in optical communications, thermal imaging, and material analysis applications. Two-dimensional (2D) materials offer a promising direction for next-generation MIR devices owing to their exotic optical properties, as well as the ultimate thickness limit. More importantly, van der Waals heterostructures—combining the best of various 2D materials at an artificial atomic level—provide many new possibilities for constructing MIR light-emitting devices of large tuneability and high integration. Here, we introduce a simple but novel van der Waals heterostructure for MIR light-emission applications built from thin-film BP and transition metal dichalcogenides (TMDCs), in which BP acts as an MIR light-emission layer. For BP–WSe2 heterostructures, an enhancement of ~200% in the photoluminescence intensities in the MIR region is observed, demonstrating highly efficient energy transfer in this heterostructure with type-I band alignment. For BP–MoS2 heterostructures, a room temperature MIR light-emitting diode (LED) is enabled through the formation of a vertical PN heterojunction at the interface. Our work reveals that the BP–TMDC heterostructure with efficient light emission in the MIR range, either optically or electrically activated, provides a promising platform for infrared light property studies and applications., Thin-film semiconductors enhance black phosphorus mid-infrared light emission Layering thin films of semiconducting transition metal dichalcogenides with thin-film black phosphorus enhances its ability to emit mid-infrared (MIR) light, which could facilitate MIR light emission investigations and applications. Xiaolong Chen of China’s Southern University of Science and Technology and colleagues layered thin flakes of black phosphorus (BP) with one or the other of a monolayer of tungsten diselenide (WSe2) or a thin film of molybdenum disulfide (MoS2). Thin-film BP is considered a promising MIR material, with potential applications in MIR photodetection and optical modulation. WSe2 efficiently transferred light energy to the BP, enhancing its MIR photoluminescence by up to 192%. On the other hand, the interface created by layering BP with MoS2 enabled the formation of a MIR light-emitting diode at room temperature.

Lateral heterostructures of two-dimensional transition metal dichalcogenides (TMDs) have offered great opportunities in the engineering of monolayer electronics, catalysis and optoelectronics. To explore the full potential of these materials, developing methods to precisely control the spatial scale of the heterostructure region is crucial. Here, we report the synthesis of ultra-long MoS2 nano-channels with several micrometer length and 2–30 nanometer width within the MoSe2 monolayers, based on intrinsic grain boundaries (GBs). First-principles calculations disclose that the strain fields near the GBs not only lead to the preferred substitution of selenium by sulfur but also drive coherent extension of the MoS2 channel from the GBs. Such a strain-driven synthesis mechanism is further shown applicable to other topological defects. We also demonstrate that the spontaneous strain of MoS2 nano-channels can further improve the hydrogen production activity of GBs, paving the way for designing GB based high-efficient TMDs in the catalytic application., Controlled growth of heterostructures within 10 nm scale is crucial for potential applications of transition metal dichalcogenides. Here, the authors report strain-driven synthesis of ultra-long MoS2 nano-channels having several micrometers length and 2–30 nm width embedded within MoSe2 monolayer.

Organic-inorganic hybrid perovskites have been considered as promising gain materials for lasing. Despite previous reports of lasing from nanocrystals, thin films and single crystals, the stability of perovskite lasers has been a challenge for its practical applications. Herein, a scalable strategy to prepare ultrastable perovskite@polymer hybrid fibers by employing a facile emulsion electrospinning approach is demonstrated. During the electrospinning process, polymethyl methacrylate (PMMA) first solidifies into an outer shell layer. Meanwhile, emulsion drops containing poly(vinylidene fluoride) (PVDF) and perovskite precursor are pushed inward and evolve into perovskite nanocrystals covered by PVDF. The PMMA with smooth surface benefits the light transport and the water-resistant PVDF blocks the moisture. The methylammonium lead bromide perovskite-embedded fibers can emit intensive light after storage in humid ambient environment (relative humidity >60%) or even in water. Amplified spontaneous emissions from the fibers network and waveguide lasing from chopped single fiber is demonstrated. Agency for Science, Technology and Research (A*STAR) Accepted version Z. Wang, H. He, and S. Liu contributed to the work equally. This work is supported from the Agency for Science, Technology, and Research (A*STAR) by AME Individual Research Grant (Grant number: A1883c0004).

Atom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs., Z.L. gratefully acknowledges funding supports from Ministry of Education (MOE) under AcRF Tier 1 (M4011782.070 RG4/17 and M4011993.070 RG7/18), AcRF Tier 2 (2015-T2-2-007, 2016-T2-1-131, 2016-T2-2-153, and 2017-T2-2-136), AcRF Tier 3 (2018-T3-1-002), National Research Foundation – Competitive Research Program (NRF-CRP21-2018-0092), and A*Star QTE programme. P.T., S.M.S., J.R.M., and J.A. acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and 1246 and the Spanish MINECO coordinated projects between IREC and ICN2 VALPEC (ENE2017-85087-C2-C3). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2017-0706). IREC and ICN2 are funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. S.M.S. acknowledges funding from ‘Programa Internacional de Becas “la Caixa”-Severo Ochoa’. J.R.M. recognizes also its affiliation to University of Barcelona. Part of the electron microscopy aspects of this work were supported by the EPSRC (UK), as the SuperSTEM Laboratory is the EPSRC National Research Facility for Advanced Electron Microscopy. Q.J.W. acknowledges the supports from MOE, Singapore grant (MOE2016-T2-2-159, MOE2016-T2-1-128, and MOE Tier 1 RG164/15) and National Research Foundation, Competitive Research Program (NRF-CRP18-2017-02) and NSFC (61704082) as well as Natural Science Foundation of Jiangsu Province (BK20170851). X.W. and B.K.T. gratefully acknowledge funding support from MOE, Singapore (grant no. MOE2015-T2-2-043). H.Z. acknowledges the supports from MOE under AcRF Tier 2 (MOE2015-T2-2-057; MOE2016-T2-2-103; and MOE2017-T2-1-162), AcRF Tier 1 (2016-T1-002-051; 2017-T1-001-150; and 2017-T1-002-119), Agency for Science, Technology and Research (A*STAR) under its AME IRG (Project No. A1783c0009), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. The authors would like to acknowledge the Facility for Analysis, Characterization, Testing, and Simulation, Nanyang Technological University, Singapore, for their electron microscopy and X-ray facilities. H.Z. also thanks the support from ITC via Hong Kong Branch of National Precious Metals Material Engineering Research Center, and the Start-Up Grant from City University of Hong Kong. Theory and simulations work at Rice university (Z.H., L.W., and B.I.Y.) was supported by the Office of Naval Research grant N00014-18-1-2182.

Controlling the fragmentation of atomically thin and brittle materials is of critical importance for both fundamental interest and technical purposes in fracture mechanics. However, the fragmentation of graphene is often random and uncontrollable because of the presence of grain boundaries and numerous defects. Here, by harnessing the strong localized strain during the necking process of thermoplastic polymers, we introduce a simple yet controllable method to tear apart a monolayer polycrystalline graphene (MPG) sheet into ordered graphene ribbons. More importantly, we show that the presence of active edges helps the graphene ribbons in exhibiting a field-effect characteristic pH response and improves the introduction of dopants. Furthermore, we demonstrate an optically transparent (∼98%), ultrathin (∼70 ± 15 nm), and skin-conformal pressure sensor for real-time tactile sensing. We believe that our results lead to further understanding of the fracture mechanics of graphene and offer unique advantages for practical applications, such as flexible electronics, chemical sensing, and biosensing. Ministry of Education (MOE) National Research Foundation (NRF) This work was supported in part by the Singapore Ministry of Education Academic Research Fund Tier 2 (MOE2015-T2-2-010) and the Singapore Ministry of Education Academic Research Fund Tier 1 (MOE 2019-T1-001-103 and MOE2019-T1-001- 111). This work was supported in part by the EEE Ignition Research Grant. This work was supported in part by the National Nature Science Foundation of China: 11804354. M.T. and K.F. acknowledge the Air Force Office of Scientific Research (AFOSR) grant 17RT0244. T.Z. acknowledges the Bureau of International Cooperation of Chinese Academy of Sciences, International Partnership Program grant 182211KYSB20170029. This work was supported in part by the Singapore National Research Foundation under NRF award numbers NRF-NRFF2013-08 and NRFCRP13-2014-05.

In this paper, we present the first millimeter-wave GaN HEMT monolithic integrated circuit (MMIC) amplifier with an operating frequency at 220 GHz. This MMIC has a small-signal gain well above 11 dB at 222 GHz. Moreover, the use of gallium nitride technology allows for achieving state-of-the-art saturated output power, up to 14dBm at 222 GHz. To our knowledge, the measured gain and output power levels are the best among any of the GaN MMICs beyond 200 GHz reported to date.

In communication systems, the nonlinear effect of waveguide flanges, coaxial connectors, antennas and other passive devices produces passive intermodulation (PIM). The mechanism for the nonlinear effect of the PIM is very complicated, which mainly comes from two sources: contact nonlinearity and material nonlinearity. PIM caused by contact nonlinearity at metal junctions of microwave devices is studied in this paper. According to the geometrical configuration and mechanical properties to characterize the contact surface, and the mechanical model of the metal contact surface is established. The microscopic physical effect of metal contact surfaces is analyzed and the equivalent circuit model is obtained. Moreover, the power series model is used to describe the nonlinear voltage-current relationship in the circuit. The model is verified by two-tone simulation, and the spectrum characteristics of PIM are obtained. Finally, the multi-carrier simulation of the equivalent circuit combined with the search method is conducted, predicting the intermodulation products under multi-carrier quickly and accurately.

This paper describes a beam-switching antenna with high gain and flexible control of beam numbers based on frequency selective surfaces (FSSs). The proposed antenna is composed of an omnidirectional monopole antenna, a hexagon FSS screen, and six metallic sheets that surround the monopole antenna. The hexagon FSS screen contains six columns, each is made up of two FSS unit-cells. The beam-switching antenna is divided into six equal portions by six metallic sheets, which are employed here to improve the gain of the antenna. The proposed FSS unit-cell consists of the two metallic crosses connected by a pin-diode. The transmission characteristics of the FSS unit-cell are investigated for different pin-diode states. Therefore, by changing the states of the pin-diodes in the hexagon FSS screen, the proposed antenna can sweep six directions with gain enhancement in the azimuth plane. Moreover, it can also flexibly operate at multiple beams modes, including two-beam mode and three-beam mode with low power. This proposed antenna is fabricated and measured, showing good performances at 5.2 GHz. The maximum enhancement of the antenna gain is up to 7 dB, which is achieved by using the metallic sheets and verified with simulated and measured results, which show good agreement.

Increasing the length of azaacene derivatives through quinone bridges is very important because these materials could have deep LUMO energy levels and larger overlapping in the solid state, which would have great applications in organic semiconducting devices. Here, two fully characterized large quinone-fused azaacenes Hex-CO and Hept-CO prepared through a novel palladium-catalyzed coupling reaction are reported. Our research clearly proved that the quinone unit can be employed as a bridge to extend the molecular conjugation length, increase the molecular overlapping, and engineer the molecular stacking mode. Hex-CO shows lamellar 2-D π-stacking modes, while Hept-CO shows 1-D π-stacking and adopts a 3-D interlocked stacking mode with the adjacent molecular layers vertical to each other. With the deep LUMO energy levels (∼−4.27 eV), Hex-CO and Hept-CO were both demonstrated to be electron-transport layers. Their charge transport properties were investigated through OFETs and theoretical calculations. Due to the different stacking modes, Hex-CO shows a higher electron mobility of 0.22 cm2 V−1 s−1 than Hept-CO (7.5 × 10−3 cm2 V−1 s−1) in a single-crystal-based OFET. Our results provide a new route for structure engineering through extending the azaacene derivatives by quinone bridges, which can be of profound significance in organic electronics. MOE (Min. of Education, S’pore)

This paper presents a frequency-selective rasorber whose transmission window locates at the higher frequency of absorption band. The accomplished rasorber is composed of dipole-like and slot arrays, and has realized the transmissive/absorptive performance. In every unit cell, each pair of dipole-like elements connected by vias is printed on the two sides of the substrate, and the coupling between long and short dipoles is suppressed by this structure. A guiding circuit is studied based on the analysis of the current path, and the insertion loss of transmission window is significantly reduced by the surface current at the pass-band that is hindered to pass through lossy elements. The presented rasorber acts as an absorber at the low frequencies, while providing a high transmittance window at 5.6 GHz. This design is elaborately optimized to achieve low reflection and angle-insensitive performance. Finally, the presented structure is validated by numerical simulations and experimental measurements. This rasorber could be used for secrecy communications among stealth facilities while providing stable broad-band absorptive properties.

This paper presents a design method of wideband Fabry–Perot resonator antennas (FPRAs) with two layers of electrically thin dielectric superstrates. The bandwidth enhancement mainly benefits from the superstrate structure with reflection phase increasing with frequency, which is used as a partially reflective surface. The proposed method is validated by a design example in ${X}$ -band. Experimental results of the FPRA prototype demonstrate that the 3-dB gain bandwidth ranges from 8.5 to 11.2 GHz (relatively 28%) with a peak gain of 14 dBi and coincides with the impedance band for ${S} _{11}\leqslant -10$ dB. Compared with the existing methods of gain bandwidth enhancement, the one presented in this paper does not need dielectric substrates with a large or fixed electrical thickness, improving cost performance and design flexibility.

A low-profile metamaterial loaded antenna array with anti-interference and polarization reconfigurable features is proposed for base-station communication. Owing to the dual notches etched on the radiating electric dipoles, an impedance bandwidth of 75.6% ranging from 1.68 to 3.72 GHz with a notch band from 2.38 to 2.55 GHz can be achieved. By employing the metamaterial loadings that are arranged in the center of the magnetic dipole, the thickness of the proposed antenna can be decreased from 28 to 20 mm. Furthermore, a serial feeding network that consists of several Wilkinson power dividers and phase shifters is introduced to attain the conversion between dual-linear polarization and triple-circular polarization. Hence, the antenna could meet the demand of the future 5G intelligent application.

The 2D transitional metal oxides/transition-metal dichalcogenides vertical heterostructures of MoO3–MoS2 are successfully synthesized on SiO2/Si substrates via one-step chemical vapor deposition process. The vertical MoO3–MoS2 heterostructures exhibit the average size of ∼20 μm and the thickness down to ∼10 nm. Moreover, the phototransistor device based on MoO3–MoS2 heterostructures presents responsivity of 5.41 × 103 A W−1, detectivity of 0.89 × 1010 Jones and external quantum efficiency of 1263.4%, respectively, under a 532 nm wavelength light. This study affords a new path to simplify process of fabricating MoO3–MoS2 vertical heterostructures for electronic and optoelectronic applications.

In this article, a novel broadband end-fire circularly polarized antenna is presented. It comprises a diamond-shaped shorted parallel-plate cavity (DSPC) and planar symmetrical conductor arms (PSCA). The DSPC and PSCA serve as the electric and magnetic dipole, respectively. By adjusting the aperture of DSPC and bending angle of PSCA, the impedance bandwidth and the axial-ratio bandwidth are greatly improved. Meanwhile, the end-fire circles are employed to improve the gain of an antenna. The overall size of the antenna is 35.5 mm × 38.2 mm × 2 mm, equal to 0.98 λ0 × 1.05 λ0 × 0.055 λ0. The measured results show that the realized impedance bandwidth for S11 ≤ –10 dB is 14.1% (7.63–8.79 GHz) and 3-dB AR bandwidth is 16.4% (7.57–8.92 GHz). A good agreement between the simulated and measured results is obtained.

A novel wideband antenna array with coplanar waveguide series feed is proposed in this letter. By etching a symmetrical circular slot (SCS) structure on the bottom metallic layer, the impedance characteristic of the antenna array is greatly improved. Meanwhile, the gain of the antenna array at the lower band is improved by inserting another SCS in the center of the two array elements. The measured results show that the impedance bandwidth of the proposed antenna array is from 2.04 to 3.74 GHz (the relative bandwidth is 58.8%). Moreover, the maximum gain reaches 6.9 dBi.

A graphene-based terahertz tunable negative refractive index metasurface has been demonstrated. The proposed metasurface unit-cell comprises of a metallic fractal cross resonant ring and embedded graphene, which presents a dynamic negative refractive index in the corresponding bands by modulating the chemical potential of the embedded graphene.

Modeling of terahertz multiplier GaAs Schottky diodes was realized by divided into nonlinear part and linear parasitic part. The nonlinear SPICE parameters of Schottky diode was obtained by the DC measurement. The linear parasitic of GaAs Schottky diode was extracted by simulation in the electromagnetic field software of HFSS. The circuit model of diodes was realized in the circuit software of ADS. The non-linear junction part of the diode was modified to take into account the self-heating effect during actual operation. A frequency multiplier operating at 170 GHz was designed and implemented based on the mode. The measured and simulated frequency multiplier efficiency trends are in good agreement.

The semiconductor–electrolyte interface dominates the behaviours of semiconductor electrocatalysis, which has been modelled as a Schottky-analogue junction according to classical electron transfer theories. However, this model cannot be used to explain the extremely high carrier accumulations in ultrathin semiconductor catalysis observed in our work. Inspired by the recently developed ion-controlled electronics, we revisit the semiconductor–electrolyte interface and unravel a universal self-gating phenomenon through microcell-based in situ electronic/electrochemical measurements to clarify the electronic-conduction modulation of semiconductors during the electrocatalytic reaction. We then demonstrate that the type of semiconductor catalyst strongly correlates with their electrocatalysis; that is, n-type semiconductor catalysts favour cathodic reactions such as the hydrogen evolution reaction, p-type ones prefer anodic reactions such as the oxygen evolution reaction and bipolar ones tend to perform both anodic and cathodic reactions. Our study provides new insight into the electronic origin of the semiconductor–electrolyte interface during electrocatalysis, paving the way for designing high-performance semiconductor catalysts. The semiconductor–electrolyte interface dominates the behaviour of semiconductor electrocatalysts. Inspired by ion-controlled electronics a universal self-gating phenomenon is now proposed to explain transport modulation during electrocatalytic reaction.

2D materials are considered as intriguing building blocks for next-generation optoelectronic devices. However, their photoresponse performance still needs to be improved for practical applications. Here, ultrasensitive 2D phototransistors are reported employing chemical vapor deposition (CVD)-grown 2D Bi2 O2 Se transferred onto silicon substrates with a noncorrosive transfer method. The as-transferred Bi2 O2 Se preserves high quality in contrast to the serious quality degradation in hydrofluoric-acid-assisted transfer. The phototransistors show a responsivity of 3.5 × 104 A W-1 , a photoconductive gain of more than 104 , and a time response in the order of sub-millisecond. With back gating of the silicon substrate, the dark current can be reduced to several pA. This yields an ultrahigh sensitivity with a specific detectivity of 9.0 × 1013 Jones, which is one of the highest values among 2D material photodetectors and two orders of magnitude higher than that of other CVD-grown 2D materials. The high performance of the phototransistor shown here together with the developed unique transfer technique are promising for the development of novel 2D-material-based optoelectronic applications as well as integrating with state-of-the-art silicon photonic and electronic technologies. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

The primary challenge for the widespread applications of two-dimensional (2D) electronics is to achieve satisfactory electrical contacts because, during traditional metal integration process, difficulties arise due to inevitable physical damages and selectively doping. The two-dimensional metal-semiconductor junctions have attracted captivated attention for the potential applications to achieve reliable electrical contacts in future atomically thin electronics. Here we demonstrate the van der Waals epitaxial growth of 2D NiTe2-MoS2 metal-semiconductor vertical junctions which the upper NiTe2 selectively nucleates at the edge of underlying MoS2. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning transmission electron microscope (STEM) studies confirm that NiTe2-MoS2 metal-semiconductor vertical junctions are successfully synthesized. Electrical properties of the NiTe2-contacted MoS2 field-effect transistors (FETs) show higher field-effect mobilities (μFE) than those with deposited Cr/Au contacts. This study demonstrates an effective pathway to improved MoS2 transistors performance with metal-semiconductor junctions. Ministry of Education (MOE) This work was supported by the National Research Foundation–Competitive Research Program NRF-CRP21-2018-0007 and CRP22-2019-0060, MOE Tier 2 MOE2017-T2-2-136, Tier 3 MOE2018-T3-1-002, and A*Star QTE programme. Z. Z. acknowledge supports from the National Natural Science Foundation of China (grants 61701402, 61804125, 61974120 and 61904148), the Key Program for International Science and Technology Cooperation Project of Shaanxi Province (grants 2018KWZ-08 and 2019KW-029), the National Key Research and Development Program of China (2019YFC1520904), the Natural Science Foundation of Shaanxi Province (grants 2017JM5135 and 2018JM6046), the Key Research and Development Program of Shaanxi Province (2018GY-025), the Foundation of the Education Department of Shaanxi Province (grans 18JK0772 and 18JK0780) and the Technology Innovation Program of Xi'an 201805041YD19CG25(1).

Carrier doping is the basis of the modern semiconductor industry. Great efforts are put into the control of carrier doping for 2D semiconductors, especially the layered transition metal dichalcogenides. Here, the direct laser patterning of WSe2 devices via light‐induced hole doping is systematically studied. By changing the laser power, scan speed, and the number of irradiation times, different levels of hole doping can be achieved in the pristine electron‐transport‐dominated WSe2, without obvious sample thinning. Scanning transmission electron microscopy characterization reveals that the oxidation of the laser‐radiated WSe2 is the origin of the carrier doping. Photocurrent mapping shows that after the same amount of laser irradiation, with increasing thickness, the laser patterned PN junction changes from the pure lateral to the vertical‐lateral hybrid structure, accompanied by the decrease in the open circuit voltage. The vertical‐lateral hybrid PN junction can be tuned to a pure lateral one by further irradiation, showing possibilities to construct complex junction profiles. Moreover, a NOR gate circuit is demonstrated by direct patterning of p‐doped channels using laser irradiation without introducing passive layers and metal electrodes with different work functions. This method simplifies device fabrication procedures and shows a promising future in large scale logic circuit applications. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Accepted version

2D noble-transition-metal chalcogenides (NTMCs) are emerging as a promising class of optoelectronic materials due to ultrahigh air stability, large bandgap tunability, and high photoresponse. Here, a new set of 2D NTMC: Ta2 PdS6 atomic layers is developed, displaying the excellent comprehensive optoelectronic performance with an ultrahigh photoresponsivity of 1.42 × 106 A W-1 , detectivity of 7.1 × 1010 Jones and a high photoconductive gain of 2.7 × 106 under laser illumination at a wavelength of 633 nm with a power of 0.025 W m-2 , which is ascribed to a photogating effect via study of the device band profiles. Especially, few-layer Ta2 PdS6 exhibits a good broadband photoresponse, ranging from 450 nm in the ultraviolet region to 1450 nm in the shortwave infrared (SIR) region. Moreover, this material also delivers an impressive electronic performance with electron mobility of ≈25 cm2 V-1 s-1 , Ion /Ioff ratio of 106 , and a one-year air stability, which is better than those of most reported 2D materials. Our studies underscore Ta2 PdS6 as a promising 2D material for nano-electronic and nano-optoelectronic applications.

The charge density wave (CDW) in two-dimensional (2D) materials is attracting substantial interest because of its magnificent many-body collective phenomena. Various CDW phases have been observed in several 2D materials before they reach the phase of superconductivity. However, to date, the atomically thin CDW materials were mainly fabricated by mechanically exfoliating from their bulk counterparts, which leads to low production yield and small sample sizes. Here, we report the controlled synthesis of atomically thin 1T-TaS2, a typical CDW material, by a chemical vapor deposition (CVD) method. The high quality of as-grown 1T-TaS2 has been confirmed by complementary characterization technologies. Moreover, the thickness-dependent CDW phase transitions have been revealed in these ultrathin flakes by temperature-dependent Raman spectra. This work opens up a new window for the large-scale synthesis of ultrathin CDW materials and sheds light on the fabrication of next-generation electronic devices.

The facile and sustainable production of hydrogen through water reduction requires high performance catalyst consisting of earth abundant elements for the hydrogen evolution reaction (HER). Here we report the synthesis of ultrathin molybdenum disulfide/g-carbon nitride (MoS 2 / g -C 3 N 4 ) vdW layers via an in situ interfacial engineering method. Such MoS 2 / g -C 3 N 4 vdW layers show outstanding HER activity with comparable potential and Tafel slope to commercial Pt catalysts. Our experimental data and density functional theory (DFT) based calculations revealed that the unpreceded electrocatalytic performances originate from the strong interfacial coupling of vdW layers through out-of-plane Mo-N bonding, which therefore enhances hydrogen adsorption/reduction kinetics for HER. These findings will shed light on the programmable catalysts for low cost and highly active water reduction.