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Two-dimensional (2D) heterostructures have attracted widespread attention for their promising prospects in the fields of electronics and optoelectronics. However, in order to truly realize 2D-material-based integrated circuits, precisely controllable fabrication of 2D heterostructures is crucial and urgently needed. Here, we demonstrate an ex situ growth method of MoSe2/MoS2 lateral heterostructures by selective selenization of a laser-scanned, ultrathin oxidized region (MoOx) on a monolayer MoS2 matrix. In our method, monolayer MoS2 is scanned by a laser with a pre-designed pattern, where the laser-scanned MoS2 is totally oxidized into MoOx. The oxidized region is then selenized in a furnace, while the unoxidized MoS2 region remains unchanged, delivering a MoSe2/MoS2 heterostructure. Unlike in situ laser direct growth methods, our method separates the laser-scanned process from the selenization process, which avoids the long time of point-by-point selenization of MoS2 by laser, making the efficiency of the synthesis greatly improved. The formation process of the heterostructure is studied by Raman spectroscopy and Auger electron spectroscopy. This simple and controllable approach to lateral heterostructures with desired patterns paves the way for fast and mass integration of devices based on 2D heterostructures.

Two-dimensional (2D) heterostructures based on the combination of transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs) have aroused growing attention due to their integrated merits of both components and multiple functionalities. However, nondestructive approaches of constructing TMD-TMO heterostructures are still very limited. Here, we develop a novel type of lateral TMD-TMO heterostructure (NbS2-Nb2O5-NbS2) using a simple lithography-free, direct laser-patterning technique. The perfect contact of an ultrathin TMO channel (Nb2O5) with two metallic TMDs (NbS2) electrodes guarantee strong electrical signals in a two-terminal sensor. Distinct from sensing mechanisms in separate TMOs or TMDs, this sensor works based on the modulation of surface conduction of the ultrathin TMO (Nb2O5) channel through an adsorbed layer of water molecules. The sensor thus exhibits high selectivity and ultrahigh sensitivity for room-temperature detection of NH3 (ΔR/R = 80% at 50 ppm), superior to the reported NH3 sensors based on 2D materials, and a positive temperature coefficient of resistance as high as 15%–20%/°C. Bending-invariant performance and high reliability are also demonstrated in flexible versions of sensors. Our work provides a new strategy of lithography-free processing of novel TMD-TMO heterostructures towards high-performance sensors, showing great potential in the applications of future portable and wearable electronics.

Description, Wafer-scale freestanding VO2 films with high quality obtained in 6 min facilitate device integration., Vanadium dioxide (VO2), with well-known metal-to-insulator phase transition, has been used to realize intriguing smart functions in photodetectors, modulators, and actuators. Wafer-scale freestanding VO2 (f-VO2) films are desirable for integrating VO2 with other materials into multifunctional devices. Unfortunately, their preparation has yet to be achieved because the wafer-scale etching needs ultralong time and damages amphoteric VO2 whether in acid or alkaline etchants. Here, we achieved wafer-scale f-VO2 films by a nano-pinhole permeation-etching strategy in 6 min, far less than that by side etching (thousands of minutes). The f-VO2 films retain their pristine metal-to-insulator transition and intrinsic mechanical properties and can be conformably transferred to arbitrary substrates. Integration of f-VO2 films into diverse large-scale smart devices, including terahertz modulators, camouflageable photoactuators, and temperature-indicating strips, shows advantages in low insertion loss, fast response, and low triggering power. These f-VO2 films find more intriguing applications by heterogeneous integration with other functional materials.

Molybdenum dioxide (MoO2) and molybdenum disulfide (MoS2) are considered as promising non-precious metal electrocatalysts for the hydrogen evolution reaction (HER). However, the thermodynamic inactivity of MoO2 and poor conductivity of MoS2 result in unsatisfactory catalytic activities for individual materials. Here we developed a high-performance HER hetero-electrocatalyst of high-valence-cation doped MoS2/MoO2 composites incorporated in carbon nanotube (CNT) foam by a simple one-step synthesis. MoO2 is stably attached and partially sulfurized into MoS2 on CNT foam, which couples the features of abundant active sites of MoS2, good conductivity of MoO2, flexibility and porosity of the CNT foam matrix, and unique doping effects of high-valence cations to synergistically enhance the HER activity. This delicately designed hetero-electrocatalyst thus exhibits low overpotential (62 mV @ η10), small Tafel slope (44 mV dec−1), and extremely high double-layer capacitance (217.9 mF cm−2). The performance remains almost unchanged after hundreds/thousands of times of bending/CV scans, and shows long-term stability at both low and high currents. Experiments and density functional theory calculations suggest that the high-valence-cation doping improves both electrical conductivity and catalytic activity of the materials. This state-of-the-art electrocatalyst can be directly used as an easily processable, flexible, free-standing, and cost-effective electrode without binders in diverse environments.

Transition metal compounds are considered as competitive candidates for efficient noble-metal-free electrocatalysts in overall water splitting. However, a single material generally fails to maintain simultaneous activities in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we developed a hybrid electrocatalyst of a Fe-(NiS2/MoS2)/carbon nanotube (CNT) film by combining composite and doping strategies, followed by a microstructure design of laser-drilled micropores, for high-efficiency overall water splitting and pH-universal HER. The charge transfer between MoS2 and NiS2 synergistically enhances both the HER and OER, and the light Fe doping further promotes the synergistic effect. The microstructure with periodic pores favors the release of bubbles, ensuring long-term stabilities at both low and large current densities without activity degradation. Consequently, Fe-(NiS2/MoS2)/CNT exhibits an ideal voltage, even lower than that of noble-metal electrodes, of 1.51 V at 10 mA cm−2 for overall water splitting in 1 M KOH, with η10 of the HER/OER at 87/234 mV, respectively. As a cathode, it also delivers low η10 values of 98 mV in 0.5 M H2SO4 and 127 mV in 1 M PBS, showing a pH-universal HER activity. Our work provides a comprehensive strategy towards high-efficiency electrocatalysts for overall water splitting, which will find a broad range of green energy applications.

Structural asymmetry of materials plays a crucial role in developing multipurpose devices. Layered metallic transition metal dichalcogenides (MTMDCs) have been proposed as promising materials in electronics. However, they are still subject to native surface oxidation, limiting their practical applications. Combination of surface protection and native surface oxidation of MTMDCs will create asymmetric structures for devices but has yet to be explored. Here, we report a bifunctional NbS2-based vertical heterostructure derived from epitaxially grown NbS2 on MoS2 followed by a natural oxidation (MoS2-NbS2-NbOx), which simultaneously exhibits both high-efficiency tunneling conductive and memristive surfaces. With the tunneling conductive surface, the heterostructure functions as nearly ohmic contact electrodes with a two-dimensional (2D) channel in lateral transistors, delivering an enhanced mobility ∼140 times higher than that of the exfoliated NbS2-contacted device. With the memristive surface, the heterostructure can be used to build high-performance lateral or vertical memristors with low working voltages and synaptic functions. By combining both types of surfaces, a memristor array for nonvolatile memory is further developed. Moreover, the memristors show a good endurance for 2000 cycles of bending as flexible devices. The bifunctional heterostructure based on NbS2 offers a strategy toward the future applications of layered metallic materials.

Vanadium dioxide (VO2) is a strongly correlated electronic material and has attracted significant attention due to its metal-to-insulator transition and diverse smart applications. Traditional synthesis of VO2 usually requires minutes or hours of global heating and low oxygen partial pressure to achieve thermodynamic control of the valence state. Further patterning of VO2 through a series of lithography and etching processes may inevitably change its surface valence, which poses a great challenge for the assembly of micro- and nanoscale VO2-based heterojunction devices. Herein, we report an ultrafast method to simultaneously synthesize and pattern VO2 on the time scale of seconds under ambient conditions through laser direct writing on a V5S8 "canvas". The successful ambient synthesis of VO2 is attributed to the ultrafast local heating and cooling process, resulting in controlled freezing of the intermediate oxidation phase during the relatively long kinetic reaction. A Mott memristor based on a V5S8-VO2-V5S8 lateral heterostructure can be fabricated and integrated with a MoS2 channel, delivering a transistor with abrupt switching transfer characteristics. The other device with a VSxOy channel exhibits a large negative temperature coefficient of approximately 4.5%/K, which is highly desirable for microbolometers. The proposed approach enables fast and efficient integration of VO2-based heterojunction devices and is applicable to other intriguing intermediate phases of oxides.

A superhydrophobic material was prepared by a simple and easily accessed sol-gel method using epoxy resin (E-51) and γ-aminopropyltriethoxysilane (KH-550) as the precursors, aqueous ammonia (NH4OH) as the catalyst and hydrogenated silicone oil (PMHS) as the hydrophobic modifier, and then pelleting the final product. The morphologies, surface chemical properties and thermal stability of the superhydrophobic bulk materials were characterized by scanning electron microscopy, Fourier infrared spectrometry and thermal analyzer. The hydrophobic properties and repairability of the as-prepared materials were also studied. The results showed that the prepared epoxy resin-hydrogenated silicone oil bulk materials were composed of tightly bound nanoparticles with a size of 50–100 nm in diameter. The material showed excellent superhydrophobic properties with a surface contact angle of 152°. The material also had good thermal resistance with a heat-resistant temperature of 300 °C and showed good repairability. The epoxy resin-hydrogenated silicone oil bulk superhydrophobic material exhibited excellent performance and showed wide application prospects.

Thin films with large compressive residual stress and low interface adhesion can buckle and delaminate from relatively rigid substrates, which is a common failure mode of film/substrate interfaces....

The alumina toughened zirconia (ATZ) ceramic particle reinforced gray iron matrix surface composite was successfully manufactured by pressureless infiltration. The porous preform played a key role in the infiltrating progress. The microstructure was observed by scanning electron microscopy (SEM); the phase constitutions was analyzed by X-ray diffraction (XRD); and the hardness and wear resistance of selected specimens were tested by hardness testing machine and abrasion testing machine, respectively. The addition of high carbon ferrochromium powders leads to the formation of white iron during solidification. The wear volume loss rates of ATZ ceramic particle reinforced gray iron matrix surface composite decreases first, and then tends to be stable. The wear resistance of the composite is 2.7 times higher than that of gray iron matrix. The reason is a combination of the surface hardness increase of gray iron matrix and ATZ ceramic particles and alloy carbides protecting effect on gray iron matrix.

Polycarbonate (PC) and polymethyl methacrylate (PMMA) are the most widely used transparent matrices for the preparation of solid samples of optical limiting materials. It was observed that the polymer matrices (PC and PMMA) were damaged when exposed to laser radiation, and optical limiting effects emerged after laser damage. The damage thresholds and laser tolerance times of different commercial PCs and polymerized PMMA were systematically investigated through Z-scans. This study demonstrates that (1) the polymer matrix is not applicable for practical applications of optical limiters, and (2) the optical limiting performance of polymer matrix solid samples may be deviated.

The optical limiting performances of few-layer transitional metal dichalcogenides (TMDs) nanosheets in the VB group (VS2, VSe2, NbS2, NbSe2, TaS2, and TaSe2) were systematically investigated for the first time, to the best of our knowledge. It was found that these TMDs nanosheets showed a normalized transmittance in the range of 20%–40% at the input energy of 1.28 GW/cm2. Ultralow initial threshold FS (0.05–0.10 J/cm2) and optical limiting threshold FOL (0.82–2.23 J/cm2) were achieved in the TMDs nanosheets, which surpassed most of the optical limiting materials. This work showed the potential of TMDs beyond MoS2 in optical limiting field.

Van der Waals heterojunctions based on transition metal dichalcogenides (TMDs) show promising potential in optoelectronic devices, due to the ultrafast separation of photoexcited carriers and efficient generation of the photocurrent. Herein, this study demonstrated a high-responsivity photovoltaic photodetector based on a MoTe2/MoSe2 type-II heterojunction. Due to the interlayer built-in potential, the MoTe2/MoSe2 heterojunction shows obvious photovoltaic behavior and its photoresponse can be tuned by the gate voltage due to the ultrathin thickness of the heterojunction. This self-powered photovoltaic photodetector exhibits an excellent responsivity of 1.5 A W&minus, 1, larger than previously reported TMDs-based photovoltaic photodetectors. Due to the high-efficiency separation of electron-hole pairs and ultrafast charge transfer, the light-induced on/off ratio of current switching is larger than 104 at zero bias, and the dark current is extremely low (~10&minus, 13 A). These MoTe2/MoSe2 type-II heterojunctions are expected to provide more opportunities for future nanoscale optoelectronic devices.

Synaptic devices based on 2D-layered materials have emerged as high-efficiency electronic synapses and neurons for neuromorphic computing. Lateral 2D synaptic devices have the advantages of multiple functionalities by responding to diverse stimuli, but they consume large amounts of energy, far more than the human brain. Moreover, current lateral devices employ several mechanisms based on conductive filaments and grain boundaries (GBs), but their formation is random and difficult to control, also hindering their practical applications. Here, four-terminal, lateral synaptic devices with artificially engineered GBs are reported, which are made from monolayer MoS2 . With lithography-free, direct-laser-writing-controlled MoS2 /MoS2-x Oδ GBs, such synaptic devices exhibit short-term and long-term plasticity characteristics that are responsive to electric and light stimulation simultaneously. This enables detailed simulations of biological learning and cognitive processes as well as image perception and processing. In particular, the device exhibits low energy consumption, similar to that of the human brain and much lower than those of other lateral 2D synaptic devices. This work provides an effective way to fabricate lateral synaptic devices for practical application development and sheds light on controllable electrical state switching for neuromorphic computing.

Transition metal dichalcogenides (TMDs), an emerging optical limiting material, exhibit excellent optical limiting properties for high energy laser. Generally, TMDs materials showed better laser protection with the decrease of lateral sizes. However, in this work, TiS2 nanosheets showed exceptional lateral size-dependent optical limiting property contrary to the reported TMDs nanosheets. TiS2 nanosheets with similar thickness and varied lateral sizes of 50 nm, 100 nm, 150 nm, 350 nm were obtained. Larger size TiS2 nanosheets displayed better optical limiting performance and TiS2 nanosheets with ultra-low normalized transmittance (as low as 12%) were developed. It was interesting to find that both two-photon absorption (TPA) and reverse saturable absorption (RSA) were exhibited in large TiS2 nanosheets (350 nm) with a bandgap of 2.67 eV, while in small TiS2 nanosheets (50 nm) with a bandgap of 3.51 eV, only TPA existed. Furthermore, the TiS2 nanosheets incorporated polymethyl methacrylate (PMMA) sample with an extremely low content of 0.066 wt‰ TiS2 nanosheets exhibited a low normalized transmittance of 18%, along with a low initial threshold (0.03 J/cm2) and low optical threshold (0.57 J/cm2), which showed great potential for practical use of the TiS2 nanosheets in optical limiting field.

LZAS glass-ceramic composites toughened by 5, 10, 15 and 20 vol% 3-mol%-Y 2 O 3 -tetragonal-ZrO 2 -polycrystal (3Y-TZP) were prepared via pressureless sintering. Sinterability of composites was investigated in the temperature range of 520–720 °C using soaking time of 30 min. The sintered specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods. The results revealed that during sintering 3Y-TZP particles agglomerated between the glass powders and were not dissolved by glass-matrix. Mechanical properties of the sintered samples such as bending strength, Vickers micro-hardness and fracture toughness were also investigated. Measurements showed that the relative density of the samples decreased with increasing 3Y-TZP content. The composite containing 15 vol% 3Y-TZP has a best mechanical properties and it would be the optimum composition. It can be confirmed that crack deflection and transformation toughening are the dominant mechanisms for improving mechanical properties of the composites.

3 mol% yttria-stabilized zirconia (3Y-TZP) ceramic composites containing 0 to 50 vol% of micro-alumina were prepared. The effect and mechanism of micro-alumina addition on their mechanical properties were investigated in this paper. The results show that with less than 20 vol% micro-alumina, hardness and relative density vary slightly and flexural strength increases dramatically. For micro-alumina content over 20 vol%, hardness and relative density reduce severely and flexural strength varies slightly. Compared with 3Y-TZP ceramic, the hardness and relative density of the composite with 50 vol% micro-alumina decrease by about 29.4% and 5.8%, respectively, but the flexural strength of the composite with 20 vol% micro-alumina increases by ~15.8%. With increasing micro-alumina content from 0 to 50 vol%, the fracture toughness of the composite increases by approximately 32.7%, and the fracture mode of the 3Y-TZP grains changes gradually from intergranular to transgranular. While the transgranular fracture of 3Y‐TZP grains is mostly found around micro-alumina particles. The strengthening is mainly attributed to the dispersion strengthening effect of micro-alumina particles. The increase in fracture toughness should be due not only to the crack deflection by dispersed micro-alumina particles and the increase in transgranular fracture of the composites, but also by the phase transformation of t-ZrO 2 accelerated by the residual stress from the coefficient of thermal expansion (CTE) mismatch between 3Y-TZP and alumina. For micro-alumina content over 10 vol%, the increase in fracture toughness is mainly attributed to the transformation toughening of t-ZrO 2 .

Ag–21Cu–25Sn alloy ribbon as a promising intermediate temperature alloy solder (400–600 °C) was prepared by melt spinning technique in this paper. Rare earth La was added into Ag–21Cu–25Sn alloy to refine the microstructures and improve the wettabilities of as-prepared alloy solders. The phase constitutions, microstructures, melting temperatures and wettabilities of selected specimens were respectively tested. The results showed that the dominant phase constitutions of Ag–21Cu–25Sn–xLa alloy ribbons were Ag3Sn and Cu3Sn. The grain size of Ag–21Cu–25Sn–xLa alloy decreased with the addition of La increasing. La addition reduced the melting temperatures of Ag–21Cu–25Sn–xLa alloy ribbons, and effectively improved the wettabilities of the alloy ribbons. When the addition of La was 0.5 wt%, the wettability of as-prepared alloy solder achieved the optimal value of 158 cm2 g−1 under brazing temperature 600 °C and dwell time 15 min. In addition, raising brazing temperature and prolonging dwell time could improve the wettability of Ag–21Cu–25Sn–xLa alloy ribbon. Keywords: Ag–Cu–Sn alloy, Rare earth La, Melting temperature, Wettability, Melt spinning technique

The microstructure and melting properties of ternary Ag–Cu–In intermediate-temperature alloys (400–600 °C) prepared by electric arc melting were investigated in this work. The melting properties, phase compositions, microstructure and hardness were characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and micro-hardness tester, respectively. The results show that the melting properties, phase compositions, microstructure and hardness of Ag–Cu–In brazing alloys are substantially different when adding different levels of indium. Indium element could effectively reduce the melting temperatures of (Ag–Cu28)–xIn alloys, and the melting temperatures of (Ag–Cu28)–25In alloy are located at 497.86 and 617.48 °C. When the indium content varies from 5 wt% and 10 wt%, the dominant phases in the alloys are Ag-rich and Cu-rich phases, and their granular crystals are smaller than 0.5 µm. When the indium content is higher than 15 wt%, the phase compositions of the alloy are Ag4In and Cu11In9, and the microstructure exhibits dendritic crystals with a uniform distribution. The hardness of (Ag–Cu28)–xIn alloy decreases first and then increases with the content of indium increasing, and the highest hardness of (Ag–Cu28)–25In alloy is HV 266.0.

This paper reports the investigation on indium addition into Sn–0.7Cu–0.2Ni lead-free solder to improve its various performances. The effects of indium addition on melting temperature, coefficient of thermal expansion (CTE), wettability, corrosion resistance and hardness of the solder alloys were studied. The results showed that when the addition of indium was ⩽0.3 wt.%, the change in melting temperature of Sn–0.7Cu–0.2Ni–xIn solders was negligible, but the melting range of the solder alloy increased. The CTE and spreading area of Sn–0.7Cu–0.2Ni–xIn solders on copper both increased with the addition of indium. An optimal CTE was 17.5 × 10−6/°C by adding 0.3 wt.% indium. At this concentration, the spreading area of solder on copper was about 15.6% larger than that of Sn–0.7Cu–0.2Ni indium-free solder. The corrosion resistance also increased with the addition of indium increasing, and the corrosion rate of Sn–0.7Cu–0.2Ni–0.3In solder was reduced by 32.8% compared with Sn–0.7Cu–0.2Ni alloy after 14 days in 5% hydrochloric acid solution at room temperature. However, a decrease of 11.7% in hardness of the solder was found when 0.3 wt.% indium was added.

Photodetectors based on two-dimensional (2D) transition metal dichalcogenides (TMDs) can only be operated in a single photovoltaic or photoconductive mode, showing either high detectivity or high responsivity, but not both. An effort to develop a photodetector that can dynamically switch its working mode is indispensable for practical applications. In this article, we demonstrate a high performance, phase-transition modulated, dual-mode photodetector based on WSe2/VO2 heterojunction. Enabled by the insulator-to-metal transition of VO2, the WSe2/VO2 heterojunction can be regulated from a type-II heterojunction to a Schottky junction, showing a tunable built-in electric field at the heterojunction interface. This resulted in the dynamic switch of carrier transport and photoresponse in the heterojunction. With this dual-mode function, the new photodetector can have both a high detectivity and a large responsivity, surpassing the current performance of single mode 2D TMDs photodetectors. With a direct laser writing and erasing technique, the photoresponse of a WSe2/VO2 device can be locally modulated as desired. This dual mode detection of the WSe2/VO2 photodetector deepens the fundamental understanding of charge transfer in heterojunctions and favors versatile applications in photodetection.

The effect of NaOH solution on the formation of nanoparticles has been the subject of ongoing debate in selenium-based material research. In this project, the robust correlation between the mechanistic growth of zinc selenide/graphene oxide (ZnSe/GO) composite and the concentration of NaOH are elucidated. The ZnSe/GO composite was synthesized via microwave-assisted hydrothermal method and the concentrations of NaOH are controlled at 2 M, 3 M, 4 M, 5 M and 6 M. The XRD spectra show that the crystal phases of the samples exhibited a 100% purity of ZnSe when the concentration of sodium hydroxide (NaOH) was set at 4 M. The further increase of NaOH concentration leads to the formation of impurities. This result reflects the essential role of hydroxyl ions in modifying the purity state of ZnSe/GO composite. The optical band gap energy of ZnSe/GO composite also decreased from 2.68 eV to 2.64 eV when the concentration of NaOH increased from 2 M to 4 M. Therefore, it can be concluded that the optimum concentration of NaOH used in synthesizing ZnSe/GO composite is 4 M. This project provides an alternative green method in the formation of a high purity ZnSe/GO composite.

Ni and TiN coatings on alumina particles were achieved using a chemical vapor deposition (CVD) method. The coated alumina was then used as raw materials to prepare alumina/heat-resistant steel (HRS) composites. The effects of Ni/TiN coatings on the alumina/HRS interface strength and oxidation resistance at elevated temperature were investigated. It is found that the both coatings can improve the alumina/HRS interface bonding strength and Ni coating has a more significant effect. For the composite made from Ni-coated alumina and HRS, the interface exhibited a better oxidation resistance due to the dissolution of Ni coating into the HRS matrix. However, the composite of TiN-coated alumina/HRS has a poor oxidation resistance behavior because TiN still existed on the alumina/HRS interface.