Anisotropic Superconducting Nb2 CTx MXene Processed by Atomic Exchange at the Wafer Scale.

Superconductivty has recently been induced in MXenes through surface modification. However, the previous reports have mostly been based on powders or cold-pressed pellets, with no known reports on the intrinsic superconsucting properties of MXenes at the nanoale. Here, it is developed a high-temperature atomic exchange process in NH3 atmosphere which induces superconductivity in either singleflakes or thin films of Nb2 CTx MXene. The exchange process between nitrogen atoms and fluorine, carbon, and oxygen atoms in the MXene lattice and related structural adjustments are studied using both experiments and density functional theory. Using either single-flake or thin-film devices, an anisotropic magnetic response of the 2D superconducting transformation has been successfully revealed. The anisotropic superconductivity is further demonstrated using superconducting thin films uniformly deposited over a 4 in. wafers, which opens up the possibility of scalable MXene-based superconducting devices.

Peptide Gel Electrolytes for Stabilized Zn Metal Anodes.

The rechargeable aqueous Zn ion battery (AZIB) is considered a promising candidate for future energy storage applications due to its intrinsic safety features and low cost. However, Zn dendrites and side reactions (e.g., corrosion, hydrogen evolution reaction, and inactive side product (Zn hydroxide sulfate) formation) at the Zn metal anode have been serious obstacles to realizing a satisfactory AZIB performance. The application of gel electrolytes is a common strategy for suppressing these problems, but the normally used highly cross-linked polymer matrix (e.g., polyacrylamide (PAM)) brings additional difficulties for battery assembly and recycling. Herein, we have developed a gel electrolyte for Zn metal anode stabilization, where a peptide matrix, a highly biocompatible material, is used for gel construction. Various experiments and simulations elucidate the sulfate anion-assisted self-assembly gel formation and its effect in stabilizing Zn metal anodes. Unlike polymer gel electrolytes, the peptide gel electrolyte can reversibly transform between gel and liquid states, thus facilitating the gel-involved battery assembly and recycling. Furthermore, the peptide gel electrolyte provides fast Zn ion diffusion (comparable to conventional liquid electrolyte) while suppressing side reactions and dendrite growth, thus achieving highly stable Zn metal anodes as validated in various cell configurations. We believe that our concept of gel electrolyte design will inspire more future directions for Zn metal anode protection based on gel electrolyte design.

Large-Area Metal-Semiconductor Heterojunctions Realized via MXene-Induced Two-Dimensional Surface Polarization.

Direct MXene deposition on large-area 2D semiconductor surfaces can provide design versatility for the fabrication of MXene-based electronic devices (MXetronics). However, it is challenging to deposit highly uniform wafer-scale hydrophilic MXene films (e.g., Ti3C2Tx) on hydrophobic 2D semiconductor channel materials (e.g., MoS2). Here, we demonstrate a modified drop-casting (MDC) process for the deposition of MXene on MoS2 without any pretreatment, which typically degrades the quality of either MXene or MoS2. Different from the traditional drop-casting method, which usually forms rough and thick films at the micrometer scale, our MDC method can form an ultrathin Ti3C2Tx film (ca. 10 nm) based on a MXene-introduced MoS2 surface polarization phenomenon. In addition, our MDC process does not require any pretreatment, unlike MXene spray-coating that usually requires a hydrophilic pretreatment of the substrate surface before deposition. This process offers a significant advantage for Ti3C2Tx film deposition on UV-ozone- or O2-plasma-sensitive surfaces. Using the MDC process, we fabricated wafer-scale n-type Ti3C2Tx-MoS2 van der Waals heterojunction transistors, achieving an average effective electron mobility of ∼40 cm2·V-1·s-1, on/off current ratios exceeding 104, and subthreshold swings of under 200 mV·dec-1. The proposed MDC process can considerably enhance the applications of MXenes, especially the design of MXene/semiconductor nanoelectronics.

MoS2 -Mediated Epitaxial Plating of Zn Metal Anodes.

Metal-based anodes (Li, Zn, etc.) are regarded as promising solutions for next-generation advanced batteries due to their high theoretical specific capacities. However, most of these metal anodes suffer from dendrite growth, which severely restricts their practical applications. Recently, epitaxial anode metal deposition by choosing a suitable substrate has received tremendous attention as an effective strategy to suppress dendrites. However, the epitaxial relationship between plated metal and the substrate has been a subject of debate. Herein, large-area, mono-orientated 2D material (MoS2 ) is used, for the first time, to electrodeposit truly epitaxial Zn anodes. The continuous (without edges) mono-orientated MoS2 films are shown to be an effective strategy for suppressing metal dendrites. In addition, the epitaxial nature of the electrodeposited Zn anode is proven by pole figure analysis, which provides the first demonstration of truly epitaxial Zn anode growth over large area as metal anode protection strategy through epitaxy.

A Sustainable NH4 + Ion Battery by Electrolyte Engineering.

Aqueous ammonium ion battery is a promising sustainable energy storage system. However, the side reactions originating from electrolytes (the water decomposition and host material dissolution) preclude its practical applications. Unlike the metal-based aqueous batteries, the idea of "ultrahigh concentrated electrolyte" is not feasible due to the strong hydrolysis of ammonium ions. Therefore, we propose an effective and sustainable strategy for the water hydrogen bond network modulation by adding sucrose into the electrolytes. The sucrose can form sucrose-water hydrogen bond networks to break the continuous water hydrogen bond network, thereby inhibiting water decomposition significantly. Moreover, the weak hydrogen bond interaction between ammonium and sucrose facilitates rapid ion migration, leading to an improved ionic conductivity. This work presents a new electrolyte modulating strategy for the practical application of aqueous ammonium ion batteries.

MXenes for Energy Harvesting.

Energy harvesting modules play an increasingly important role in the development of autonomous self-powered microelectronic devices. MXenes (i.e., 2D transition metal carbide/nitride) have recently emerged as promising candidates for energy applications due to their excellent electronic conductivity, large specific surface area, and tunable properties. Herein, a perspective on using MXenes to harvest energy from various sources in the environment is presented. First, the characteristics of MXenes that facilitate energy capturing are systematically introduced and the preparation strategies of MXenes and their derived nanostructures tailored toward such applications are summarized. Subsequently, the harvesting mechanism of different energy sources (e.g., solar energy, thermoelectric energy, triboelectric energy, piezoelectric energy, salinity-gradient energy, electrokinetic energy, ultrasound energy, and humidity energy) are discussed. Then, the recent progress of MXene-based nanostructures in energy harvesting, as well as their applications, is introduced. Finally, opinions on the existing challenges and future directions of MXene-based nanostructure for energy harvesting are presented.

Growth of 2D Materials at the Wafer Scale.

Wafer-scale growth has become a critical bottleneck for scaling up applications of van der Waal (vdW) layered 2D materials in high-end electronics and optoelectronics. Most vdW 2D materials are initially obtained through top-down synthesis methods, such as exfoliation, which can only prepare small flakes on a micrometer scale. Bottom-up growth can enable 2D flake growth over a large area. However, seamless merging of these flakes to form large-area continuous films with well-controlled layer thickness and lattice orientation is still a significant challenge. This review briefly introduces several vdW layered 2D materials covering their lattice structures, representative physical properties, and potential roles in large-scale applications. Then, several methods used to grow vdW layered 2D materials at the wafer scale are reviewed in depth. In particular, three strategies are summarized that enable 2D film growth with a single-crystalline structure over the whole wafer: growth of an isolated domain, growth of unidirectional domains, and conversion of oriented precursors. After that, the progress in using wafer-scale 2D materials in integrated devices and advanced epitaxy is reviewed. Finally, future directions in the growth and scaling of vdW layered 2D materials are discussed.

High-Yield Ti3 C2 Tx MXene-MoS2 Integrated Circuits.

It is very challenging to employ solution-processed conducting films in large-area ultrathin nanoelectronics. Here, spray-coated Ti3 C2 Tx MXene films as metal contacts are successfully integrated into sub-10 nm gate oxide 2D MoS2 transistor circuits. Ti3 C2 Tx films are spray coated on glass substrates followed by vacuum annealing. Compared to the as-prepared sample, vacuum annealed films exhibit a higher conductivity (≈11 000 S cm-1 ) and a lower work function (≈4.5 eV). Besides, the annealed Ti3 C2 Tx film can be patterned through a standard cleanroom process without peeling off. The annealed Ti3 C2 Tx film shows a better band alignment for n-type transport in MoS2 channel with small work function mismatch of 0.06 eV. The MoS2 film can be uniformly transferred on the patterned Ti3 C2 Tx surface and then readily processed through the cleanroom process. A large-area array of Ti3 C2 Tx MXene-MoS2 transistors is fabricated using different dielectric thicknesses and semiconducting channel sizes. High yield and stable performance for these transistor arrays even with an 8 nm-thick dielectric layer are demonstrated. Besides, several circuits are demonstrated, including rectifiers, negative-channel metal-oxide-semiconductor (NMOS) inverters, and voltage-shift NMOS inverters. Overall, this work indicates the tremendous potential for solution-processed Ti3 C2 Tx MXene films in large-area 2D nanoelectronics.

Controlled Deposition of Zinc-Metal Anodes via Selectively Polarized Ferroelectric Polymers.

Aqueous zinc-ion batteries are regarded as ideal candidates for stationary energy-storage systems due to their low cost and high safety. However, zinc can readily grow into dendrites, leading to limited cycling performance and quick failure of the batteries. Herein, a novel strategy is proposed to mitigate this dendrite problem, in which a selectively polarized ferroelectric polymer material (poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))) is employed as a surface protective layer on zinc anodes. Such a polarized ferroelectric polymer layer can enable a locally concentrated zinc-ion distribution along the coated surface and thus enable the horizontal growth of zinc plates. As a result, symmetrical zinc batteries using such anodes exhibit long cycling lifespan at 0.2 mA cm-2 , 0.2 mAh cm-2 for 2000 h, and a high rate performance up to 15 mA cm-2 . Also, the full cell (including a Zn-MnO2 battery and a zinc-ion capacitor) based on this anode is demonstrated. This work provides a novel strategy to protect the zinc anode and even other metal anodes exploiting polymer ferroelectricity.

TiO2@TiO2-xHx core-shell nanoparticle film/Si heterojunction for ultrahigh detectivity and sensitivity broadband photodetector.

A simple hydrogenation treatment is used to synthesize unique oxygen-deficient TiO2 with a core/shell structure (TiO2@TiO2-xHx), superior to the high H2-pressure process (under 20 bar for five days). It is demonstrated that oxygen-deficient TiO2 nanoparticle film/Si heterojunction possesses improved photoresponse performance compared to the untreated TiO2 nanoparticle film/Si heterojunction. Particularly, under 900 nm of 0.5 µW cm-2, the oxygen-deficient TiO2 nanoparticle film (TiO2@TiO2-xHx core-shell nanoparticle film)/Si heterojunction shows high responsivity (R) of 336 A W-1, prominent sensitivity (S) of 1.3 × 107 cm2 W-1, accompanied with a fast rise and decay time of 6 and 5 ms, respectively. Significantly, the detectivity (D*) of the photodetector is up to 1.17 × 1014 cm Hz1/2 W-1, which is better than that reported in metal oxide nanomaterials/Si heterojunction photodetectors, and is 4-5 orders of magnitude higher than some 2D nanosheets/Si heterojunctions of 109-1010 cm Hz1/2 W-1, indicating the excellent ability to detect weak signals. The oxygen vacancies generated in amorphous shell TiO2-xHx make the Fermi level of TiO2-x shift near the conduction band minimum and can lead to reduced dark current. The high absorption and reduced dark current of the heterojunction ensure excellent photoresponse properties of oxygen-deficient TiO2 nanoparticle film/Si heterojunction. The H-reduced oxygen-deficient amorphous shell may be an excellent candidate to enhance the photoresponse performance of metal oxide/Si heterojunction.

A hierarchical structured steel mesh decorated with metal organic framework/graphene oxide for high-efficient oil/water separation.

A hierarchical structured steel mesh decorated with metal organic framework (UiO-66-NH2) nanoparticles/graphene oxide (GO) nanosheets was successfully prepared via a simple self-assemble method. Because water molecules tend to build hydrogen bonds with the amine, carboxyl and hydroxyl functional groups of UiO-66-NH2/GO hierarchical structure, the hierarchical structure can easily capture water and tightly lock the water to build a stable water layer on the steel mesh surface and block oil in contact with the steel mesh. Therefore, the obtained hierarchical structured steel mesh exhibits super-hydrophilicity, underwater super-oleophobicity, excellent oil resistance and outstanding oil/water separation performance with a superior high permeating flux (54,500 L m-2 h-1) and rejection (>99.9%) under gravity force, indicating the mesh possesses great potential for treating oily wastewater.

Layer-by-layer self-assembly of polyaniline nanofibers/TiO2 nanotubes heterojunction thin film for ammonia detection at room temperature.

In this paper, for the first time, polyaniline nanofibers/TiO2 nanotubes (PANI/TiO2) heterojunction thin film has been prepared on Pt interdigital electrodes by layer-by-layer self-assembly method and applied in room temperature NH3 detection. It is found that the optimal self-assembly layer number is three (PANI/TiO2-3) compared to one layer (PANI/TiO2-1) and five layers (PANI/TiO2-5). The PANI/TiO2-3 thin film sensor possesses superior response characteristics compared with our other PANI based sensors, including higher response value (336%@5 ppm NH3), acceptable response/recovery time (110 s/1 086 s@5 ppm NH3), low detection limit (0.5 ppm), and remarkable selectivity. The enhanced gas sensing performances could be ascribed to the tremendous variation of the carrier concentration caused by the p-n junctions as well as the increased specific surface area and pore volume. This work not only offers a superb strategy to fabricate heterojunction thin film but also accelerates the development of room-temperature operable NH3 sensors.

Synthesis of nanowire bundle-like WO3-W18O49 heterostructures for highly sensitive NH3 sensor application.

Heterojunctions are very promising structures due to their hybrid properties, which are usually obtained via a multistep growth process. However, in this paper, WO3-W18O49 heterostructures are synthesized via a novel one-step approach by using isopropanol as reaction media and are applied in NH3 gas detection for the first time. The obtained WO3-W18O49 heterostructures with loose nanowire bundle-like morphology show a response value of 23.3 toward 500 ppm NH3 at 250 °C, which is 5.63 times higher than that of pristine W18O49. In addition, the WO3-W18O49 sensor also exhibits great dynamic response/recovery characteristics (13 s/49 s @ 500 ppm NH3), superb selectivity and low detection limit of 460 ppb. The substantial improvement in the response of WO3-W18O49 heterostructures toward NH3 can be explained by the formation of n-WO3/n-W18O49 heterojunctions that facilitate the generation of a more extended depletion layer as well as the enhancement of specific surface area and pore volume. Our research results open an easy pathway for facile one-step preparation of heterojunctions with high response and low cost, which can be used for the development of other high-performance gas sensors.

Ultrahigh broadband photoresponse of SnO2 nanoparticle thin film/SiO2/p-Si heterojunction.

The SnO2/Si heterojunction possesses a large band offset and it is easy to control the transportation of carriers in the SnO2/Si heterojunction to realize high-response broadband detection. Therefore, we investigated the potential of the SnO2 nanoparticle thin film/SiO2/p-Si heterojunction for photodetectors. It is demonstrated that this heterojunction shows a stable, repeatable and broadband photoresponse from 365 nm to 980 nm. Meanwhile, the responsivity of the device approaches a high value in the range of 0.285-0.355 A W-1 with the outstanding detectivity of ∼2.66 × 1012 cm H1/2 W-1 and excellent sensitivity of ∼1.8 × 106 cm2 W-1, and its response and recovery times are extremely short (<0.1 s). This performance makes the device stand out among previously reported oxide or oxide/Si based photodetectors. In fact, the photosensitivity and detectivity of this heterojunction are an order of magnitude higher than that of 2D material based heterojunctions such as (Bi2Te3)/Si and MoS2/graphene (photosensitivity of 7.5 × 105 cm2 W-1 and detectivity of ∼2.5 × 1011 cm H1/2 W-1). The excellent device performance is attributed to the large Fermi energy difference between the SnO2 nanoparticle thin film and Si, SnO2 nanostructure, oxygen vacancy defects and thin SiO2 layer. Consequently, practical highly-responsive broadband PDs may be actualized in the future.

Positive Relationship Between Individuality and Social Identity in Virtual Communities: Self-Categorization and Social Identification as Distinct Forms of Social Identity.

Previous studies have reported conflicting results regarding the relationship between individuality and social identity, indicating this area requires further examination. This study constructed a research model to help understand the positive role of individualized behavior and social identity in virtual communities. The results of an online survey conducted to assess our theoretical research model indicated that social identity can be expressed in two ways: self-categorization and social identification. Furthermore, we found individualized behavior was positively related to social identification, while self-categorization was directly derived from social identification.