Enhanced Transport and Optoelectronic Properties of van der Waals Materials on CaF2 Films.

To achieve better properties of van der Waals (vdW) devices, vdW heterointerfaces with substrates such as hexagonal boron nitride (h-BN) were introduced to alleviate adverse substrate effects. However, the premature dielectric breakdown and its scale limitation make wider application of h-BN substrates challenging. Here we report a fluoride-based substrate that substantially improves optoelectronic and transport properties of dichalcogenide devices, with enhancement factors comparable to those of h-BN. A model system of wafer-scale fluoride calcium (CaF2) ultrathin films with the preferable growth direction along [111] is prepared by the magnetron sputtering method. Results show that the constructed SnS2/CaF2 and WS2/CaF2 devices exhibit 1 order of magnitude higher than devices based on the SiO2 substrate in electronic mobility and photoresponsivity. Theoretical calculations reveal that devices based on fluoride substrates are immune from the Coulomb impurity scattering by forming quasi-vdW interfaces, exhibiting great potential for high responsivity and mobility of photogenerated carriers in 2D vdW devices.

Boosting the Sensitivity of WSe2 Phototransistor via Janus Interfaces with 2D Perovskite and Ferroelectric Layers.

Hybrid systems have recently attracted increasing attention, which combine the special attributes of each constitute and create interesting functionalities through multiple heterointerface interactions. Here, we design a two-dimensional (2D) hybrid phototransistor utilizing Janus-interface engineering, in which the WSe2 channel combines light-sensitive perovskite and spontaneously polarized ferroelectrics, achieving collective ultrasensitive detection performance. The top perovskite (BA2(MA)3Pb4I13) layer can absorb the light efficiently and provide generous photoexcited holes to WSe2. WSe2 exhibit p-type semiconducting states of different degrees due to the selective light-operated doping effect, which also enables the ultrahigh photocurrent of the device. The bottom ferroelectric (Hf0.5Zr0.5O2) layer dramatically decreases the dark current, which should be attributed to the ferroelectric polarization assisted charge trapping effect and improved gate control. As a whole, our phototransistors show excellent photoelectric performances across the ultraviolet to near-infrared range (360-1050 nm), including an ultrahigh ON/OFF current ratio > 109 and low noise-equivalent power of 1.3 fW/Hz1/2, all of which are highly competitive in 2D semiconductor-based optoelectronic devices. In particular, the devices show excellent weak light detection ability, where the distinguishable photoswitching signal is obtained even under a record-low light intensity down to 1.6 nW/cm2, while showing a high responsivity of 2.3 × 105 A/W and a specific detectivity of 4.1 × 1014 Jones. Our work demonstrates that Janus-interface design makes the upper and lower interfaces complement each other for the joint advancement into high-performance optoelectronic applications, providing a picture to realize the integrated engineering on carrier dynamics by light irradiation, electric field, interfacial trapping, and band alignment.

Direct observation of contact resistivity for monolayer TMD based junctions via PL spectroscopy.

Monolayer transition metal dichalcogenides (mTMDs) possess a direct band gap and strong PL emission that is highly sensitive to doping level and interfaces, laying the foundation for investigating the contact between mTMD and metal via PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (RC), but they are complicated, time-consuming, high-cost and suffer from inevitable chemical disorders and Fermi level pinning. In addition, previously reported contact resistances comprise both Schottky barrier and tunnel barrier components. Here, we report a simple, rapid and low-cost method to study the tunnel barrier dominated contact resistance of mTMD based junctions through PL spectroscopy. These junctions are free from chemical disorders and Fermi level pinning. Excluding the Schottky barrier component, solely tunnel barrier dominated contact resistances of 1 L MoSe2/Au and 1 L MoSe2/graphene junctions were estimated to be 147.8 Ω µm and 54.9 Ω µm, respectively. Density functional theory (DFT) simulations revealed that the larger RC of the former was possibly due to the existence of intrinsic effective potential difference (Φbarrier) between mTMD and metal. Both junctions exhibit an increasing tendency of RC as temperature decreases, which is probably attributed to the thermal expansion coefficient (TEC) mismatch-triggered interlayer spacing (d) increase and temperature-induced doping. Remarkably, a significant change of RC was observed in 1 L MoSe2/Au junctions, which is possibly ascribed to the changes of their orbital overlaps. Our results open new avenues for exploring fundamental metal-semiconductor contact principles and constructing high-performance devices.

Controllable Edge Epitaxy of Helical GeSe/GeS Heterostructures.

Emerging twistronics based on van der Waals (vdWs) materials has attracted great interest in condensed matter physics. Recently, more neoteric three-dimensional (3D) architectures with interlayer twist are realized in germanium sulfide (GeS) crystals. Here, we further demonstrate a convenient way for tailoring the twist rate of helical GeS crystals via tuning of the growth temperature. Under higher growth temperatures, the twist angles between successive nanoplates of the GeS mesowires (MWs) are statistically smaller, which can be understood by the dynamics of the catalyst during the growth. Moreover, we fabricate self-assembled helical heterostructures by introducing germanium selenide (GeSe) onto helical GeS crystals via edge epitaxy. Besides the helical architecture, the moiré superlattices at the twisted interfaces are also inherited. Compared with GeS MWs, helical GeSe/GeS heterostructures exhibit improved electrical conductivity and photoresponse. These results manifest new opportunities in future electronics and optoelectronics by harnessing 3D twistronics based on vdWs materials.

Effect of Co Doping on Electrocatalytic Performance of Co-NiS2/CoS2 Heterostructures.

There are abundant water resources in nature, and hydrogen production from electrolyzed water can be one of the main ways to obtain green and sustainable energy. Traditional water electrolysis uses precious metals as catalysts, but it is difficult to apply in massive volumes due to low reserves and high prices. It is still a challenge to develop hydrogen electrocatalysts with excellent performance but low cost to further improve the efficiency of hydrogen production. This article reported a potential candidate, the Co-NiS2/CoS2 (material is based on NiS2, and after Co doping, The NiS2/CoS2 heterostructure is formed) heterostructures, prepared by hydrothermal method with carbon paper as the substrate. In a 0.5 M sulfuric acid solution, the hydrogen evolution reaction with Co-NiS2/CoS2 as the electrode showed excellent catalytic performance. When the Co (Cobalt) doping concentration is increased to 27%, the overpotential is -133.3 mV, which is a drop of 81 mV compared with -214.3 mV when it is not doped. The heterostructure formed after doping also has good stability. After 800 CV cycles, the difference in overpotential is only 3 mV. The significant improvement of the catalytic performance can be attributed to the significant changes in the crystal structure and properties of the doped heterostructures, which provide an effective method for efficient electrocatalytic hydrogen production.

Topotactic Growth of Free-Standing Two-Dimensional Perovskite Niobates with Low Symmetry Phase.

Here, we report a novel topotactic method to grow 2D free-standing perovskite using KNbO3 (KN) as a model system. Perovskite KN with monoclinic phase, distorted by as large as ∼6 degrees compared with orthorhombic KN, is obtained from 2D KNbO2 after oxygen-assisted annealing at relatively low temperature (530 °C). Piezoresponse force microscopy (PFM) measurements confirm that the 2D KN sheets show strong spontaneous polarization (Ps) along [101̅]pc direction and a weak in-plane polarization, which is consistent with theoretical predictions. Thickness-dependent stripe domains, with increased surface displacement and PFM phase changes, are observed along the monoclinic tilt direction, indicating the preserved strain in KN induces the variation of nanoscale ferroelectric properties. 2D perovskite KN with low symmetry phase stable at room temperature will provide new opportunities in the exploration of nanoscale information storage devices and better understanding of ferroelectric/ferroelastic phenomena in 2D perovskite oxides.

Fabrication of NiS2 Nanomaterials for Cd2+ Sensing.

Heavy metal Cadmium (Cd) will continuously pollute the atmosphere, soil and various water environments through material circulation, and even pose a threat to human safety. It has been designated as a first-class pollutant in sewage by China, therefore there is an urgent need to find new, more effective, and low-cost method to accurately detect Cadmium ion (Cd2+) concentration. We experimentally prepared a new Cd2+ sensor based on NiS2 nanomaterials capable of measuring Cd2+ concentration. The corresponding relationship between over potential of NiS2 nanomaterials in H2SO4 electrolyte solutions with different Cd2+ concentration and reduction peak with change of Cd2+ concentration was obtained by electrochemical method.

Hydrothermal Synthesis of Polyhedral Nickel Sulfide by Dual Sulfur Source for Highly-Efficient Hydrogen Evolution Catalysis.

Transition metal sulfides are cheap and efficient catalysts for water splitting to produce hydrogen; these compounds have attracted wide attention. Nickel sulfide (NiS2) has been studied in depth because of its simple preparation process, excellent performance and good stability. Here, we propose a modification to the hydrothermal synthesis method for the fabrication of a highly efficient and stable NiS2 electrocatalyst prepared by two different sulfur sources, i.e., sulfur powder and C3H7NaO3S2 (MPS), for application in hydrogen evolution reactions. The obtained NiS2 demonstrated excellent HER performance with an overpotential of 131 mV to drive -10 mA cm-1 in 0.5 M H2SO4 solution with 5mV performance change after 1000 cycles of stability testing. We believe that this discovery will promote the industrial development of nonprecious metal catalysts.

Fabrication and Electrical Properties of Silver Telluride Nanowires.

As a new topological insulator material, the ß-phase silver telluride (Ag2Te) nanowire is a narrow bandgap semiconductor, which is attractive for its excellent properties. In this study, Ag2Te nanowires were synthesized by one-step hydrothermalmethod. The nanowires showed good electrical properties with maximum drain-source voltage of 1.5 V, and the output current was up to 20 µA. The gate voltage has a significant effect on output current for the device. The Ag2Te nanowires will have more extensive and in-depth applications in the fields of optoelectronics and thermoelectricity.

Preparation of SnO Nanoshells with Enhanced Lithium-Storage Properties.

Tin monoxide is a kind of IV-VI metal monoxides that has attracted great deal of attention due to its wide optical band gap and high field effect mobility in the past decade. On the other hand, nanoshell is a unique porous structure. Its curved shell provides a shelter for the hollow core, as well as a much bigger special surface area. We in this study systematically prepared SnO nanoshells through a facile self-assembly method under different annealing conditions. The lithium ion batteries were fabricated immediately based on the as prepared nanoshells. The capacity of as fabricated lithium ion batteries was 559.3 mAhg-1 at rate performance of 0.1 Ag-1 and 497.5 mAhg-1 at 1 Ag-1 in 30th cycle. This work exhibited high application performance of SnO nanoshells. We hope this work will help study similar structure and applications of IV-VI metal monoxides.

Hydroxyl-Assisted Phosphorene Stabilization with Robust Device Performances.

Phosphorene (few-layer black phosphorus) has been widely investigated for its unique optical and electronic properties. However, it is challenging to synthesize and process stable phosphorene as it degrades rapidly upon exposure to oxygen and moisture under ambient conditions, which has limited its use in practical applications. Herein, we propose an alkali-assisted stabilization process to produce high-quality phosphorene nanosheets. Our morphology measurements show that alkali-treated phosphorene remains stable for over 7 days in air. Electrical measurements on alkali-treated BP devices further proved its stable electrical property under ambient conditions. We further demonstrate superior light-assisted electrochemical water splitting performance using stable phosphorene. We attribute the stabilization effect to the chemical modification of the surface of phosphorene with P-OH bond formation. This study paves the avenue for the implementation of phosphorene devices in ambient conditions.

Production of SnS2 Nanostructure as Improved Light-Assisted Electrochemical Water Splitting.

Tin disulfide (SnS2) has gained a lot of interest in the field of converting solar energy into chemical fuels in light-assisted electrochemical water splitting due to its visible-light band gap and high electronic mobility. However, further decreasing the recombination rate of electron-hole pairs and increasing the density of active states at the valence band edge of the photoelectrodes were a critical problem. Here, we were successful in fabricating the super-thin SnS2 nanostructure by a hydrothermal and solution etching method. The super-thin SnS2 nanostructure as a photo-electrocatalytic material exhibited low overpotential of 0.25 V at the current density of -10 mA·cm-2 and the potential remained basically unchanged after 1000 cycles in an H2SO4 electrolyte solution, which was better than that of the SnS2 nanosheet and SnS/SnS2 heterojunction nanosheet. These results show the potential application of super-thin SnS2 nanostructure in electrochemical/photo-electrocatalytic field.

Ultra-Sensitive Dopamine Sensor Using Stable Black Phosphorus Quantum Dots.

The use of various optical methods for detection of dopamine (DA), such as colorimetry, fluorometry, surface-enhanced Raman spectroscopy (SERS), and electrochemiluminescence (ECL), has progressively matured over the past decade. However, the development of a simple, inexpensive, and quick detection method for dopamine still remains a challenge. Herein, we used stable black phosphorus quantum dots (BPQDs) for sensitive and selective detection of dopamine by UV-Vis spectroscopy. The initial UV-Vis absorption peaks of the BPQDs in aqueous solution were effectively quenched upon the addition of DA. The quenching efficiency was proportional to the concentration of DA within the range of 1 nM to 70 nM or 1 nM to 1250 nM (encompassing physiological DA concentrations) with a low detection limit of 0.33 nM (pH 5˜9). This optical analysis method provides a platform for the detection of dopamine, which has many advantages such as high sensitivity, selectivity, stability, low cost, non-toxicity, and so on.

High-Sensitive Ammonia Sensors Based on Tin Monoxide Nanoshells.

Ammonia (NH3) is a harmful gas contaminant that is part of the nitrogen cycle in our daily lives. Therefore, highly sensitive ammonia sensors are important for environmental protection and human health. However, it is difficult to detect low concentrations of ammonia (≤50 ppm) using conventional means at room temperature. Tin monoxide (SnO), a member of IV⁻VI metal monoxides, has attracted much attention due to its low cost, environmental-friendly nature, and higher stability compared with other non-oxide ammonia sensing material like alkaline metal or polymer, which made this material an ideal alternative for ammonia sensor applications. In this work, we fabricated high-sensitive ammonia sensors based on self-assembly SnO nanoshells via a solution method and annealing under 300 °C for 1 h. The as fabricated sensors exhibited the response of 313%, 874%, 2757%, 3116%, and 3757% (∆G/G) under ammonia concentration of 5 ppm, 20 ppm, 50 ppm, 100 ppm, and 200 ppm, respectively. The structure of the nanoshells, which have curved shells that provide shelters for the core and also possess a large surface area, is able to absorb more ammonia molecules, leading to further improvements in the sensitivity. Further, the SnO nanoshells have higher oxygen vacancy densities compared with other metal oxide ammonia sensing materials, enabling it to have higher performance. Additionally, the selectivity of ammonia sensors is also outstanding. We hope this work will provide a reference for the study of similar structures and applications of IV⁻VI metal monoxides in the gas sensor field.

Multivariate Control of Effective Cobalt Doping in Tungsten Disulfide for Highly Efficient Hydrogen Evolution Reaction.

Tungsten Disulfide (WS2) is considered to be a promising Hydrogen Evolution Reaction (HER) catalyst to replace noble metals (such as Pt and Pd). However, progress in WS2 research has been impeded by the inertness of the in-plane atoms during HER. Although it is known that microstructure and defects strongly affect the electrocatalytic performance of catalysts, the understanding of such related catalytic origin still remains a challenge. Here, we combined a one-pot synthesis method with wet chemical etching to realize controlled cobalt doping and tunable morphology in WS2. The etched products, which composed of porous WS2, CoS2 and a spot of WOx, show a low overpotential and small Tafel slope in 0.5 M H2SO4 solution. The overpotential could be optimized to -134 mV (at 10 mA/cm2) with a Tafel slope of 76 mV/dec at high loadings (5.1 mg/cm2). Under N2 adsorption analysis, the treated WS2 sample shows an increase in macropore (>50 nm) distributions, which may explain the increase inefficiency of HER activity. We applied electron holography to analyze the catalytic origin and found a low surface electrostatic potential in Co-doped region. This work may provide further understanding of the HER mechanism at the nanometer scale, and open up new avenues for designing catalysts based on other transition metal dichalcogenides for highly efficient HER.

Fabrication of SnS2/SnS Heterojunction with Enhanced Light-Assisted Electrochemical Water Splitting Performance.

Semiconducting metal sulfides have raised strong research interest among researchers as a promising candidate for light-assisted electrochemical water splitting, because they have wide band gap. In order to harvest more light wavelengths for improvement of light-assisted electrochemical water splitting capacity, we fabricated SnS2/SnS heterojunction nanosheets via facile and environmental route. The SnS2/SnS heterojunction nanosheets were used as photo-electrocatalytic material which exhibited low over potential of -0.64 V at the current density of 10 mA·cm-2 in 0.5 M NaSO4 solution. Moreover, the SnS and SnS2 nanosheets displayed high over potential values of -0.80 and -0.88 V at the current density of 10 mA·cm-2, respectively. This research finding may therefore show the potential for use of SnS2/SnS heterojunction nanosheets as low cost and environmentally friendly photo-electrocatalysis.

Hierarchical Co-FeS2/CoS2 heterostructures as a superior bifunctional electrocatalyst.

The traditional method of preparing hydrogen and oxygen as efficient clean energy sources mainly relies on the use of platinum, palladium, and other precious metals. However, the high cost and low abundance limit wide application of such metals. As such, one challenging issue is the development of low-cost and high-efficiency electrocatalysts for such purposes. In this study, we synthesized Co-FeS2/CoS2 heterostructures via a hydrothermal method for efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Benefitting from their unique three-dimensional hierarchical nanostructures, Co-doped FeS2, and CoS2 formed heterostructures on Co-FeS2 petals, which bestowed remarkable electrocatalytic properties upon Co-FeS2/CoS2 nanostructures. Co-FeS2/CoS2 effectively catalyzed the OER with an overpotential of 278 mV at a current density of 10 mA cm-2 in 1 M KOH solution, and also is capable of driving a current density -10 mA cm-2 at an overpotential of -103 mV in 0.5 M H2SO4 solution. The overpotential of the OER and HER only decreased by 5 mV and 3 mV after 1000 cycles. Our Co-FeS2/CoS2 materials may offer a promising alternative to noble metal-based electrocatalysts for water splitting.