In Situ Tracking of Water Oxidation Generated Nanoscale Dynamics in Layered Double Hydroxides Nanosheets.

Layered double hydroxides (LDHs) are potential catalysts for water oxidation, and it is recognized that they undergo dynamic evolution during the operation. However, little is known about the interfacial behaviors at the nanoscale under working conditions nor the underlying effects on electrocatalytic performance. Herein, using electrochemical atomic force microscopy, we in situ visualize the heterogeneous evolution of LDH nanosheets during oxygen evolution reaction (OER). By further combining density functional theory calculations, we elucidate the origin of the heterogeneous dynamics and their impact on the OER efficiency. Our findings demonstrate that NiCo LDHs transform to the catalytically active NiCoOx(OH)2-x phase during OER, and the redox transition between is accompanied by compressive and tensile strain, leading to in-plane contraction and reversible expansion of the nanosheets. Nonisotropic strain and out-of-plane strain relaxation due to defects and interparticle interactions result in cracking and wrinkling in the nanostructure, which is responsible for the partial activation and long-term deterioration of LDH electrocatalysts toward the OER. With this knowledge, we suggest and validate that engineering defects can precisely tune these dynamic behaviors, improving the OER activity and stability among LDH-based electrocatalysts.

Shape-Preserved CoFeNi-MOF/NF Exhibiting Superior Performance for Overall Water Splitting across Alkaline and Neutral Conditions.

This study reported a multi-functional Co0.45Fe0.45Ni0.9-MOF/NF catalyst for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting, which was synthesized via a novel shape-preserving two-step hydrothermal method. The resulting bowknot flake structure on NF enhanced the exposure of active sites, fostering a superior electrocatalytic surface, and the synergistic effect between Co, Fe, and Ni enhanced the catalytic activity of the active site. In an alkaline environment, the catalyst exhibited impressive overpotentials of 244 mV and 287 mV at current densities of 50 mA cm-2 and 100 mA cm-2, respectively. Transitioning to a neutral environment, an overpotential of 505 mV at a current density of 10 mA cm-2 was achieved with the same catalyst, showing a superior property compared to similar catalysts. Furthermore, it was demonstrated that Co0.45Fe0.45Ni0.9-MOF/NF shows versatility as a bifunctional catalyst, excelling in both OER and HER, as well as overall water splitting. The innovative shape-preserving synthesis method presented in this study offers a facile method to develop an efficient electrocatalyst for OER under both alkaline and neutral conditions, which makes it a promising catalyst for hydrogen production by water splitting.

Sm and Gd Contacts in 2D Semiconductors for High-Performance Electronics and Spintronics.

Two-dimensional (2D) semiconductors have attracted great attention due to their rich electronic properties and even been considered to have the potential to extend Moore's Law. However, the Schottky barrier between the metal and 2D semiconductor is formed due to the metal-induced gap states (MIGS), which greatly hinder the development of 2D semiconductor transistors in large-scale integrated circuits. Meanwhile, most air-stable 2D semiconductors are nonmagnetic, limiting the possibility of spintronic application. Here, we report a new strategy to suppress the MIGS and reduce the Schottky barrier height on 2D semiconductors (MoS2, WS2, and WSe2) by using lanthanide metal (Sm and Gd) contacts. It was found the lanthanide contacts exhibit a good Ohmic property with a near-zero Schottky barrier. As a result, the carrier mobility of MoS2 transistors reaches 118 cm2/(V s). Furthermore, Gd-contact MoS2 transistors show the typical magnetic property where the magnetoresistance reaches 2.7% at 5 K. By studying its spin valve effect, it was demonstrated that the nonlocal magnetoresistance is 4.1% and spin polarization is 3.25%. This study provides a promising pathway for high-performance 2D electronic and spintronics, which may open a new strategy for future computing-in-memory architecture.

Electrically tunable interlayer recombination and tunneling behavior in WSe2/MoS2 heterostructure for broadband photodetector.

Electrically tunable band structure and light-matter interaction are of great importance in designing novel devices and constructing high-integrated and high-performance photodetector systems in the future. However, tunable mechanisms on the layered semiconductor, especially the heterojunction, are still unclear. Herein, the WSe2/MoS2 phototransistor with dual-gated configuration is fabricated, and its electrical and photoelectrical conversion has been studied to show large tunability. It was found that conduction and rectification characteristics can be tuned by dual gates showing four states, p-i, p-n, i-n, and n-n, as a result of the charging and depletion of WSe2 and MoS2. The rectifying ratio can be modulated across a large range from 102.5 to 10-3.2. Its photoelectronic characteristics were observed to exhibit bipolar and wavelength-dependent behaviors. The interlayer recombination of charge carriers dominates the photoresponse of the device under the illumination of visible light, while it is dominated by interlayer tunneling under the illumination of near-infrared wavelengths. This bipolar photoresponse is associated with different states of band alignment, which can be switched by dual-gating modulation. Finally, by tuning the gate voltage, responsivities reach 27 445 A W-1 and 2827 A W-1 at wavelengths of 400 and 1010 nm at room temperature, respectively, which directly extends the response region from visible light to near-infrared.

Flexible Ag2Se Thermoelectric Films Enable the Multifunctional Thermal Perception in Electronic Skins.

Skin is critical for shaping our interactions with the environment. The electronic skin (E-skin) has emerged as a promising interface for medical devices to replicate the functions of damaged skin. However, exploration of thermal perception, which is crucial for physiological sensing, has been limited. In this work, a multifunctional E-skin based on flexible thermoelectric Ag2Se films is proposed, which utilizes the Seebeck effect to replicate the sensory functions of natural skin. The E-skin can enable capabilities including temperature perception, tactile perception, contactless perception, and material recognition by analyzing the thermal conduction behaviors of various materials. To further validate the capabilities of constructed E-skins, a wearable device with multiple sensory channels was fabricated and tested for gesture recognition. This work highlights the potential for using flexible thermoelectric materials in advanced biomedical applications including health monitoring and smart prosthetics.

Strong Anisotropic Two-Dimensional In2Se3 for Light Intensity and Polarization Dual-Mode High-Performance Detection.

Detecting the light from different freedom is of great significance to gain more information. Two-dimensional (2D) materials with low intrinsic carrier concentration and highly tunable electronic structure have been considered as the promising candidate for future room-temperature multi-functional photodetectors. However, current investigations mainly focus on intensity-sensitive detection; the multi-dimensional photodetection such as polarization-sensitive photodetection is still in its early stage. Herein, the intensity- and polarization-sensitive photodetection based on α-In2Se3 is studied. By using angle-resolved polarized Raman spectroscopy, it is demonstrated that α-In2Se3 shows an anisotropic phonon vibration property indicating its asymmetric structure. The α-In2Se3-based photodetector has a photoelectric performance with a responsivity of 1936 A/W and a specific detectivity of 2.1 × 1013 Jones under 0.2 mW/cm2 power density at 400 nm. Moreover, by studying the polarized angle-resolved photoelectrical effect, it is found that the ratio of maximum and minimum photocurrent (dichroic ratio) reaches 1.47 at 650 nm suggesting good polarization-sensitive detection. After post-annealing, α-In2Se3 in situ converts to ß-In2Se3 which has similar in-plane anisotropic crystallinity and exhibits a dichroic ratio of 1.41. It is found that the responsivity of ß-In2Se3 is 6 A/W, much lower than that of α-In2Se3. The high-performance light intensity- and polarization-detection of α-In2Se3 enlarges the 2D anisotropic materials family and provides new opportunities for future dual-mode photodetection.

Emerging MoS2 Wafer-Scale Technique for Integrated Circuits.

As an outstanding representative of layered materials, molybdenum disulfide (MoS2) has excellent physical properties, such as high carrier mobility, stability, and abundance on earth. Moreover, its reasonable band gap and microelectronic compatible fabrication characteristics makes it the most promising candidate in future advanced integrated circuits such as logical electronics, flexible electronics, and focal-plane photodetector. However, to realize the all-aspects application of MoS2, the research on obtaining high-quality and large-area films need to be continuously explored to promote its industrialization. Although the MoS2 grain size has already improved from several micrometers to sub-millimeters, the high-quality growth of wafer-scale MoS2 is still of great challenge. Herein, this review mainly focuses on the evolution of MoS2 by including chemical vapor deposition, metal-organic chemical vapor deposition, physical vapor deposition, and thermal conversion technology methods. The state-of-the-art research on the growth and optimization mechanism, including nucleation, orientation, grain, and defect engineering, is systematically summarized. Then, this review summarizes the wafer-scale application of MoS2 in a transistor, inverter, electronics, and photodetectors. Finally, the current challenges and future perspectives are outlined for the wafer-scale growth and application of MoS2.

Highly Tunable, Broadband, and Negative Photoresponse MoS2 Photodetector Driven by Ion-Gel Gate Dielectrics.

Revealing the light-matter interaction of molybdenum disulfide (MoS2) and further improving its tunability facilitate the construction of highly integrated optoelectronics in communication and wearable healthcare, but it still remains a significant challenge. Herein, polyvinylidene fluoride and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PVDF-EMIM-TFSI) ion-gel are employed to replace the oxide to fabricate a MoS2-based phototransistor. The high capacitance enables a large tunability of the carrier concentration that results in ambipolar transport of MoS2. It is found that the photoelectrical effect of the MoS2 ion-gel phototransistor can be greatly tuned by the gate voltage including its photoresponsivity, detectivity, and response wavelength. An abnormal negative photoelectrical effect in both the electron branch and the hole branch is observed which is due to the adsorption/desorption of the C2F6NO4S2- ion. By tuning the carrier concentration, the photoresponse can be extended from the visible region to the short infrared region. At 1200 nm, the photoresponse and detectivity can be tuned as large as 0.90 A/W and 1.88 × 1011 Jones, respectively. Ultimately, by combining the tunability of gate voltage and wavelength, it is demonstrated that the photoelectrical effect is dominated by the photogating effect in the hole carrier, while it is coregulated by a photogating and photothermal effect in electron carrier. This study provides new insights for developing a highly tunable broadband photodetector with low consumption.

Si/SnSe-Nanorod Heterojunction with Ultrafast Infrared Detection Enabled by Manipulating Photo-Induced Thermoelectric Behavior.

Photothermal detectors have attracted tremendous research interest in uncooled infrared imaging technology but with a relatively slow response. Here, Si/SnSe-nanorod (Si/SnSe-NR) heterojunctions are fabricated as a photothermal detector to realize high-performance infrared response beyond the bandgap limitation. Vertically standing SnSe-NR arrays are deposited on Si by a sputtering method. Through manipulating the photoinduced thermoelectric (PTE) behavior along the c-axis, the Si/SnSe-NRs heterojunction exhibits a unique four-stage photoresponse with a high photoresponsivity of 106.3 V W-1 and high optical detectivity of 1.9 × 1010 cm Hz1/2 W-1 under 1342 nm illumination. Importantly, an ultrafast infrared photothermal response is achieved with the rise/fall time of 11.3/258.7 µs. Moreover, the coupling effect between the PTE behavior and external thermal excitation enables an improved response by 288.4%. The work not only offers a new strategy to develop high-speed photothermal detectors but also performs a deep understanding of the PTE behavior in a heterojunction system.

Layer-by-Layer Growth of AA-Stacking MoS2 for Tunable Broadband Phototransistors.

The stacking configuration has been considered as an important additional degree of freedom to tune the physical property of layered materials, such as superconductivity and interlayer excitons. However, the facile growth of highly uniform stacking configuration is still a challenge. Herein, the AA-stacking MoS2 domains with a ratio up to 99.5% has been grown by using the modified chemical vapor deposition through introducing NaCl molecules in the confined space. By tuning the growth time, MoS2 domains would transit from an AA-stacking bilayer to an AAAAA-stacking five-layer. The epitaxial growth mechanism has been insightfully studied, revealing that the critical nucleation size of the AA-stacking bilayer is 5.0 ± 3.0 µm. Through investigation of the photoluminescence, the photoemission, especially the indirect photoexcitation, is dependent on both the stacking fashion and layer number. Furthermore, by studying the gate-tuned MoS2 phototransistors, we found a significant dependence on the stacking configuration of MoS2 of the photoexcitation and a different gate tunable photoresponse. The AAA-stacking trilayer MoS2 phototransistor delivers a photoresponse of 978.14 A W-1 at 550 nm. By correction of the external quantum efficiency with external field and illumination power density, it has been found that the photoresponse tunability is dependent on the layer number due to the strong photogating effect. This strategy provides a general avenue for the epitaxial growth of van der Waals film which will further facilitate the applications in a tunable photodetector.

Directional copper dewetting to grow graphene ribbon arrays.

The V-groove confines the anisotropic dewetting of Cu film to form ribbons. The influence mechanism of film thickness and annealing procedure on the confined dewetting, structural and morphological evolution has been investigated. Thus, the synthesized graphene ribbons by CVD have uniform width, regular edges and good crystallinity, and deliver obvious room-temperature PL emission.

The Effect of Fe Dopant Location in Co(Fe)OOHx Nanoparticles for the Oxygen Evolution Reaction.

The addition of iron (Fe) can in certain cases have a strong positive effect on the activity of cobalt and nickel oxide nanoparticles in the electrocatalytic oxygen evolution reaction (OER). The reported optimal Fe dopant concentrations are, however, inconsistent, and the origin of the increased activity due to Fe dopants in mixed oxides has not been identified so far. Here, we combine density functional theory calculations, scanning tunneling microscopy, and OER activity measurements on atomically defined Fe-doped Co oxyhydroxide nanoparticles supported on a gold surface to establish the link between the activity and the Fe distribution and concentration within the oxyhydroxide phase. We find that addition of Fe results in distinct effects depending on its location on edge or basal plane sites of the oxyhydroxide nanoparticles, resulting in a nonlinear OER activity as a function of Fe content. Fe atom substitution itself does not lead to intrinsically more active OER sites than the best Co sites. Instead, the sensitivity to Fe promoter content is explained by the strong preference for Fe to locate on the most active edge sites of oxyhydroxide nanoparticles, which for low Fe concentrations stabilizes the particles but in higher concentrations leads to a shell structure with less active Fe on all edge positions. The optimal Fe content thereby becomes dependent on nanoparticle size. Our findings demonstrate that synthesis strategies that adjust not only the Fe concentration in mixed oxides but also its distribution within a catalyst nanoparticle can lead to enhanced OER performance.

Outstanding Catalytic Effects of 1T'-MoTe2 Quantum Dots@3D Graphene in Shuttle-Free Li-S Batteries.

It is still challenging to develop sulfur electrodes for Li-S batteries with high electrical conductivity and fast kinetics, as well as efficient suppression of the shuttling effect of lithium polysulfides. To address such issues, herein, polar MoTe2 with different phases (2H, 1T, and 1T') were deeply investigated by density functional theory calculations, suggesting that the 1T'-MoTe2 displays concentrated density of states (DOS) near the Fermi level with high conductivity. By optimization of the synthesis, 1T'-MoTe2 quantum dots decorated three-dimensional graphene (MTQ@3DG) was prepared to overcome these issues, and it accomplished exceptional performance in Li-S batteries. Owing to the chemisorption and high catalytic effect of 1T'-MoTe2 quantum dots, MTQ@3DG/S exhibits highly reversible discharge capacity of 1310.1 mAh g-1 at 0.2 C with 0.026% capacity fade rate per cycle over 600 cycles. The adsorption calculation demonstrates that the conversion of Li2S2 to Li2S is the rate-limiting step where the Gibbs free energies are 1.07 eV for graphene and 0.97 eV for 1T'-MoTe2, revealing the importance of 1T'-MoTe2. Furthermore, in situ Raman spectroscopy investigation proved the suppression of the shuttle effect of LiPSs in MTQ@3DG/S cells during the cycle.

Recent Progress in Emerging Two-Dimensional Transition Metal Carbides.

As a new member in two-dimensional materials family, transition metal carbides (TMCs) have many excellent properties, such as chemical stability, in-plane anisotropy, high conductivity and flexibility, and remarkable energy conversation efficiency, which predispose them for promising applications as transparent electrode, flexible electronics, broadband photodetectors and battery electrodes. However, up to now, their device applications are in the early stage, especially because their controllable synthesis is still a great challenge. This review systematically summarized the state-of-the-art research in this rapidly developing field with particular focus on structure, property, synthesis and applicability of TMCs. Finally, the current challenges and future perspectives are outlined for the application of 2D TMCs.

Recent progress in Van der Waals 2D PtSe2.

As a new member in two-dimensional (2D) transition metal dichalcogenides (TMDCs) family, platinum diselenium (PtSe2) has many excellent properties, such as the layer-dependent band gap, high carrier mobility, high photoelectrical coupling, broadband response, etc, thus it shows good promising application in room temperature photodetectors, broadband photodetectors, transistors and other fields. Furthermore, compared with other TMDCs, PtSe2is chemical inert in ambient, showing nano-devices potential with higher performance and stability. However, up to now, the synthesis and its device applications are in its early stage. This review systematically summarized the state of the art of PtSe2from its structure, property, synthesis and potential application. Finally, the current challenges and future perspectives are outlined for the applications of 2D PtSe2.

In Situ Construction of Mo2 C Quantum Dots-Decorated CNT Networks as a Multifunctional Electrocatalyst for Advanced Lithium-Sulfur Batteries.

The slow redox kinetics during cycling process and the serious shuttle effect caused by the solubility of lithium polysulfides (LiPSs) dramatically hinder the practical application of Li-S batteries. Herein, a facile and scalable spray-drying strategy is presented to construct conductive polar Mo2 C quantum dots-decorated carbon nanotube (CNT) networks (MCN) as an efficient absorbent and electrocatalyst for Li-S batteries. The results reveal that the MCN/S electrode exhibits a high specific capacity of 1303.3 mAh g-1 at 0.2 C, and ultrastable cycling stability with decay of 0.019% per cycle even at 1 C. Theoretical simulation uncovers that Mo2 C exhibits much stronger binding energies for S8 and Li2 Sn . The energy barrier for the conversion between Li2 S4 and Li2 S2 decreases from 1.02 to 0.72 eV when hybriding with Mo2 C. Furthermore, in situ discharge/charge-dependent Raman spectroscopy shows that long-chain Li2 S8 configuration is generated via S8 ring opening near the first plateaus at ≈2.36 V versus Li/Li+ and the S6 2- configuration in CNT/S electrode is maintained below the potential of ≈2.30 V versus Li/Li+ , indicating that the shuttle of soluble LiPSs happens during the whole discharge process. This work provides deep insights into the polar nanoarchitecture design and scalable fabrication for advanced Li-S batteries.

Long-distance electron transfer in a filamentous Gram-positive bacterium.

Long-distance extracellular electron transfer has been observed in Gram-negative bacteria and plays roles in both natural and engineering processes. The electron transfer can be mediated by conductive protein appendages (in short unicellular bacteria such as Geobacter species) or by conductive cell envelopes (in filamentous multicellular cable bacteria). Here we show that Lysinibacillus varians GY32, a filamentous unicellular Gram-positive bacterium, is capable of bidirectional extracellular electron transfer. In microbial fuel cells, L. varians can form centimetre-range conductive cellular networks and, when grown on graphite electrodes, the cells can reach a remarkable length of 1.08 mm. Atomic force microscopy and microelectrode analyses suggest that the conductivity is linked to pili-like protein appendages. Our results show that long-distance electron transfer is not limited to Gram-negative bacteria.

In Situ Resistive Switching Effect Scrutinization on Co-Designed Graphene Sensor.

Resistive switching (RS), an electric property based on the forming and rupture of conductive filaments in metal-insulator-metal structures, has attracted intensive attention due to its potential application in next generation energy-efficient and area-efficient memory devices. In situ studies of the RS effect are urgently needed for its mechanism understanding and memristive performance improvement. Here investigations of both the RS effect as well as the gate tunable conductance quantization effect are realized by co-designing an Ag/SiO2 based memory structure on a graphene local sensor. This design enables self-monitoring of the working states of the memristor in real-time by virtue of the graphene sensor. These findings pave the way for further investigations of on-chip electronics and quantum physics.

Electronic Modulation of Hierarchical Spongy Nanosheets toward Efficient and Stable Water Electrolysis.

The energy conversion efficiency of water electrolysis is determined by the activity of selected catalysts. Ideal catalysts should possess not only porous architecture for high-density assembly of active sites but also a subtle electronic configuration for the optimized activity at each site. In this context, the development of stable porous hosting materials that allow the incorporation of various metal elements is highly desirable for both experimental optimization and theoretical comparison/prediction. Herein, MOF-derived spongy nanosheet arrays constructed by assembly of carbon encapsulated hetero-metal doped Ni2 P nanoparticles is presented as a superior bifunctional electrocatalyst for water splitting. This hierarchical structure can be stably retained when secondary metal dopants are introduced, providing a flexible platform for electronic modulation. The catalytic origin of activity enhancement via metal (Fe, Cr, and Mn) doping is deciphered through experimental and theoretical investigations. Combining the advantages in both morphological and electronic structures, the optimized catalyst NiMn-P exhibits remarkable activity in both hydrogen and oxygen evolution in the alkaline media, with an ultrasmall cell voltage of 1.49 V (at 10 mA cm-2 ) and high durability for at least 240 h.

Photothermoelectric SnTe Photodetector with Broad Spectral Response and High On/Off Ratio.

A broadband photodetector with high performance is highly desirable for the optoelectric and sensing application. Herein, we report a "photo-thermo-electric" (PTE) detector based on an ultrathin SnTe film. The (001)-oriented SnTe films with the wafer size scale are epitaxially grown on the surface of sodium chloride crystals by a scalable sputtering method. Due to the giant PTE effect under laser spot excitation on the asymmetric position between two terminals, a built-in electrical field is produced to drive bulk carriers for a self-powered photodetector, leading to a broad spectral response in the wavelength range from 404 nm to 10.6 µm far beyond the limitation of the energy band gap. Significantly, the photodetector displays a high on/off photoswitching ratio of over 105 with a suppressed dark current, which is 4-5 orders of magnitude higher than that of other reported SnTe-based detectors. Under zero external bias, the device yields the highest detectivity of ∼1.3 × 1010 cm Hz1/2 W-1 with a corresponding responsivity of ∼3.9 mA W-1 and short rising/falling times of ∼78/84 ms. Furthermore, the photodetector transferred onto the flexible template exhibits excellent mechanical flexibility over 300 bending cycles. These findings offer feasible strategies toward designing and developing low-power-consumption wearable optoelectronics with competitive performance.