van der Waals Self-Epitaxial Growth of Inch-Sized Superconducting Niobium Diselenide Films.

Ultrathin superconducting films are the basis of superconductor devices. van der Waals (vdW) NbSe2 with noncentrosymmetry exhibits exotic superconductivity and shows promise in superconductor electronic devices. However, the growth of inch-scale NbSe2 films with layer regulation remains a challenge because vdW structural material growth is strongly dependent on the epitaxial guidance of the substrate. Herein, a vdW self-epitaxy strategy is developed to eliminate the substrate driving force in film growth and realize inch-sized NbSe2 film growth with thicknesses from 2.1 to 12.1 nm on arbitrary substrates. The superconducting transition temperature of 5.1 K and superconducting transition width of 0.30 K prove the top homogeneity and quality of superconductivity among all of the synthetic NbSe2 films. Coupled with a large area and substrate compatibility, this work paves the way for developing NbSe2 superconductor electronics.

Homologous post-treatment strategy enabling phase-pureα-FAPbI3films.

Formamidinium lead triiodide (FAPbI3) is considered as the prospective light-absorbing layer on account of the close-to-ideal bandgap of theα-phase, wide optical absorption spectrum and good thermal stability. Therefore, how to realizeδtoα-phase transition to obtain phase-pureα-FAPbI3without additives is important for FAPbI3perovskite films. Herein, a homologous post-treatment strategy (HPTS) without additives is proposed to prepare FAPbI3films with pureα-phase. The strategy is processed along with dissolution and reconstruction process during the annealing. The FAPbI3film has tensile strain with the substrate, and the lattice keeps tensile, and the film maintains in anα/δhybrid phase. The HPTS process releases the tensile strain between the lattice and the substrate. The process of strain release realizes the phase transition fromδtoα-phase during this process. This strategy can accelerate the transformation from hexagonalδ-FAPbI3to cubicα-FAPbI3at 120 °C. As a result, the acquiredα-FAPbI3films exhibit better film quality in optical and electrical properties, accordingly achieving device efficiency of 19.34% and enhanced stability. This work explores an effective approach to obtain additive-free and phase-pureα-FAPbI3films through a HPTS to fabricate uniform high-performanceα-FAPbI3perovskite solar cells.

Single Crystal Halide Perovskite Film for Nonlinear Resistive Memory with Ultrahigh Switching Ratio.

Morre's law is coming to an end only if the memory industry can keep stuffing the devices with new functionality. Halide perovskite acts as a promising candidate for application in next-generation nonvolatile memory. As is well known, the switching ratio is the key device requirement of resistive memory to improve recognition accuracy. Here, the authors introduce an all-inorganic halide perovskite CsPbBr3 single crystal film (SCF) into resistive memory as an active layer. The Ag/CsPbBr3 /Ag memory cells exhibit reproducible resistive switching with an ultrahigh switching ratio (over 109 ) and a fast switching speed (1.8 µs). It is studied that the Schottky barrier of metal/CsPbBr3 SCF contact follows the tendency of Schottky-Mott theory, and the Fermi level pinning effect is effectively reduced. The interface S parameter of metal/CsPbBr3 SCF contact is 0.50, suggesting a great interface contact is formed. The great interface contact contributes to the steady high resistance state (HRS), and then the steady HRS leads to an ultrahigh resistive switching ratio. This work demonstrates high performance from halide perovskite SCF-based memory. The introduction of halide perovskite SCF in resistive random access memory provides great potential as an alternative in future computing systems.

Efficient molecular ferroelectric photovoltaic device with high photocurrent via lewis acid-base adduct approach.

Traditional inorganic oxide ferroelectric materials usually have band gaps above 3 eV, leading to more than 80% of the solar spectrum unavailable, greatly limiting the current density of their devices just atµA cm-2level. Therefore, exploring ferroelectric materials with lower band gaps is considered as an effective method to improve the performance of ferroelectric photovoltaic devices. Inorganic ferroelectric materials are often doped with transition metal elements to reduce the band gap, which is a complex doping and high temperature fabrication process. Recently, molecular ferroelectric materials can change the symmetry and specific interactions of crystals at the molecular level by chemically modifying or tailoring cations with high symmetry, enabling rational design and banding of ferroelectricity in the framework of perovskite simultaneously. Therefore, the molecular ferroelectric materials have a great performance for both excellent ferroelectricity and narrow band gap without doping. Here, we report a ferroelectric photovoltaic device employing an organic-inorganic hybrid molecular ferroelectric material with a band gap of 2.3 eV to obtain high current density. While the poor film quality of molecular ferroelectrics still limits it. The Lewis acid-base adduct is found to greatly improve the film quality with lower defect density and higher carrier mobility. Under standard AM 1.5 G illumination, the photocurrents of ∼1.51 mA cm-2is achieved along with a device efficiency of 0.45%. This work demonstrates new possibilities for the application of molecular ferroelectric films with narrow band gaps in photovoltaic devices, and lays a foundation for Lewis acid-base chemistry to improve the quality of molecular ferroelectric thin films to obtain high current densities and device performance.

Ferroelectric Polymer Drives Performance Enhancement of Non-fullerene Organic Solar Cells.

Enhancing the built-in electric field to promote charge dynamitic process is of great significance to boost the performance of the non-fullerene organic solar cells (OSCs), which has rarely been concerned. In this work, we introduced a cheap ferroelectric polymer as an additive into the active layers of non-fullerene OSCs to improve the device performance. An additional and permanent electrical field was produced by the polarization of the ferroelectric dipoles, which can substantially enhance the built-in electric field. The promoted exciton separation, significantly accelerated charge transport, reduced the charge recombination, as well as the optimized film morphology were observed in the device, leading to a significantly improved performance of the PVDF-modified OSCs with various active layers, such as PM6 : Y6, PM6 : BTP-eC9, PM6 : IT-4F and PTB7-Th : Y6. Especially, a record efficiency of 17.72 % for PM6 : Y6-based OSC and an outstanding efficiency of 18.17 % for PM6 : BTP-eC9-based OSC were achieved.

Immobilized Precursor Particle Driven Growth of Centimeter-Sized MoTe2 Monolayer.

Molybdenum ditelluride (MoTe2) has attracted ever-growing attention in recent years due to its novel characteristics in spintronics and phase-engineering, and an efficient and convenient method to achieve large-area high-quality film is an essential step toward electronic applications. However, the growth of large-area monolayer MoTe2 is challenging. Here, for the first time, we achieve the growth of a centimeter-sized monoclinic MoTe2 monolayer and manifest the mechanism of immobilized precursor particle driven growth. Microscopic characterizations reveal an obvious trend of immobilized precursor particles being consumed by the monolayer and continuing to provide a source for the growth of the monolayer. Time-of-flight secondary ion mass spectrometry verifies the attachment of hydroxide ions on the surface of the MoTe2 monolayer, thereby realizing the inhibition of crystal growth along the [001] zone axis and the continuous growth of the MoTe2 monolayer. The first-principles DFT calculations prove the mechanism of immobilized precursor particles and the absorption of hydroxide ions on the MoTe2 monolayer. The as-grown MoTe2 monolayer exhibits a surface roughness of 0.19 nm and average conductivity of 1.5 × 10-5 S/m, which prove the smoothness and uniformity of the MoTe2 monolayer. Temperature-dependent electrical measurements together with the transfer characteristic curves further demonstrate the typical semimetallic properties of monoclinic MoTe2. Our research elaborates the microscopic process of immobilized precursor particles to grow large-area MoTe2 monolayer and provides a new thinking about the growth of many other two-dimensional materials.

Room-temperature processed high-quality SnO2 films by oxygen plasma activated e-beam evaporation.

Recently, SnO2 is considered to be one of the most promising materials as electron transport layer in perovskite solar cells (PSCs). Low-temperature processed SnO2 films are crucial for SnO2-based PSCs and flexible devices. However, it is difficult to prepare stoichiometric SnO2 films by e-beam evaporation at low-temperature. Herein, SnO2 films are fabricated by oxygen plasma activated e-beam evaporation technique at room-temperature. Oxygen plasma shows strong oxidation activity, which is essential to adjust the stoichiometry of SnO x in the evaporation process. The SnO2 films exhibit uniformity (R q  = 3.05 nm), high transmittance (T > 90%), high hall mobility (µ e  = 10.8 cm2 V -1 s-1) and good hydrophilic (water contact angle =19°). This work will promote the application of SnO2 films in PSCs and flexible devices.

Sub-Nanometer Thick Wafer-Size NiO Films with Room-Temperature Ferromagnetic Behavior.

Adding ferromagnetism into semiconductors attracts much attentions due to its potential usage of magnetic spins in novel devices, such as spin field-effect transistors. However, it remains challenging to stabilize their ferromagnetism above room temperature. Here we introduce an atomic chemical-solution strategy to grow wafer-size NiO thin films with controllable thickness down to sub-nanometer scale (0.92 nm) for the first time. Surface lattice defects break the magnetic symmetry of NiO and produce surface ferromagnetic behaviors. Our sub-nanometric NiO thin film exhibits the highest reported room-temperature ferromagnetic behavior with a saturation magnetization of 157 emu/cc and coercivity of 418 Oe. Attributed to wafer size, the easily-transferred NiO thin film is further verified in a magnetoresistance device. Our work provides a sub-nanometric platform to produce wafer-size ferromagnetic NiO thin films as atomic layer magnetic units in future transparent magnetoelectric devices.

Overcoming the Anisotropic Growth Limitations of Free-Standing Single-Crystal Halide Perovskite Films.

It is extremely challenging to grow single-crystal halide perovskite films (SCHPFs) with not only desired transport properties but also large lateral size with much thinner thickness. Here, we report the growth of freestanding single crystal CsPbBr3 SCHPFs with thickness less than 100 nm and a lateral size close to centimeter for the first time. A new model for growth kinetics (Ψ=Aexp[-(EA -Es )/(kB T)]) is proposed to address the surface energy and temperature effect on the growth rate of ultrathin CsPbBr3 single-crystal film. The experimental results and DFT calculations both demonstrated that the surfactant plays a critical role in modifying the surface energy and achieving anisotropic growth. This work opens new opportunities for high-quality SCHPFs with large lateral size and controllable thickness that may find wide applications for optoelectronic devices.

One-Pot Selective Epitaxial Growth of Large WS2/MoS2 Lateral and Vertical Heterostructures.

Controllable nucleation sites play a key role in the selective growth of heterostructures. Here, we are the first to report a one-pot strategy to realize the confined and selective growth of large MoS2/WS2 lateral and vertical heterostructures. A hydroxide-assisted process is introduced to control the nucleation sites, thereby realizing the optional formation of lateral and vertical heterostructures. Time-of-flight secondary ion mass spectrometry verifies the critical role of hydroxide groups toward the controllable growth of these heterostructures. The size of the as-grown MoS2/WS2 lateral heterostructures can be as large as 1 mm, which is the largest lateral size reported thus far. The obtained MoS2/WS2 heterostructures have a high carrier mobility of ∼58 cm2 V-1 s-1, and the maximum on/off current ratio is >108. This approach provides not only a pathway for the selective growth of large MoS2/WS2 lateral and vertical heterostructures but also a fundamental understanding of surface chemistry for controlling the selective growth of transition-metal dichalcogenide heterostructures.

Amorphous RuS2 electrocatalyst with optimized active sites for hydrogen evolution.

Transition metal chalcogenides have attracted much attention as high-performance electrocatalysts for hydrogen evolution reaction (HER). Here, we synthesized an efficient HER electrocatalyst of amorphous ruthenium sulfide (A-RuS2), exhibiting an overpotential of 141 mV at the current density of 10 mA cm-2 and a Tafel slope of 65.6 mV dec-1. Experiments demonstrate amorphous RuS2 has much better catalytic activity than that of its crystalline counterparts. Our study shows that amorphous RuS2 has increased intrinsic activity and active sites. This work provides a feasible strategy for the development of HER electrocatalysts in amorphous state.

Fully stoichiometric Cu2BaSn(S1-x Se x )4 solar cells via chemical solution deposition.

Cu2BaSn(S1-x Se x )4 has shown great prospects in the photoelectric field due to Earth-abundance, low toxicity, cost efficiency, direct bandgap, high absorption coefficient (>104 cm-1) and reduced anti-site disorder relative to Cu2ZnSn(S1-x Se x )4. A fully-tunable ratio of S/Se is the key to broaden the bandgap of Cu2BaSn(S1-x Se x )4. Here, we introduce a thionothiolic acid metathesis process to readily tune the stoichiometry of Cu2BaSn(S1-x Se x )4 films for the first time. Different stoichiometric Se/(S + Se) of Cu2BaSn(S1-x Se x )4 from zero to one can vary the bandgap range from 2 to 1.68 eV. The grain size of Cu2BaSn(S1-x Se x )4 films can be grown more than 10 µm. The optimized bandgap and high-quality growth of Cu2BaSn(S1-x Se x )4 films ensure the best power conversion efficiency of 2.01% for solution-processed Cu2BaSn(S1-x Se x )4 solar cells. This method provides an alternative solution-processed way for the synthesis of fully stoichiometric Cu2BaSn(S1-x Se x )4.

An ultrathin MoSe2 photodetector with near-perfect absorption.

An ultrathin near-perfect MoSe2 absorber working in the visible regime is demonstrated theoretically and experimentally, and it consists of a MoSe2/Au bi-layer film. The polymer-assisted deposition method is used to synthesize MoSe2 films, which can reduce the roughness and thus improve the film absorption. Simulation results show that the absorption of the absorber with 22 nm MoSe2 reaches to larger than 90% between 628.5 nm and 718 nm with a peak value up to 99.5% at 686 nm. Moreover, the measured absorption also shows near-perfect absorption of this simple absorber. Finally, an ultrathin photodetector is fabricated based on this perfect absorber and shows on/off reproducibility and remarkable photocurrent, which is three orders of magnitude higher than the dark current.

Fabrication of Nickel Oxide Nanopillar Arrays on Flexible Electrodes for Highly Efficient Perovskite Solar Cells.

Semiconductor nanomaterials with controlled morphologies and architectures are of critical importance for high-performance optoelectronic devices. However, the fabrication of such nanomaterials on polymer-based flexible electrodes is particularly challenging due to degradation of the flexible electrodes at a high temperature. Here we report the fabrication of nickel oxide nanopillar arrays (NiO x NaPAs) on a flexible electrode by vapor deposition, which enables highly efficient perovskite solar cells (PSCs). The NiO x NaPAs exhibit an enhanced light transmittance for light harvesting, prohibit exciton recombination, promote irradiation-generated hole transport and collection, and facilitate the formation of large perovskite grains. These advantageous features result in a high efficiency of 20% and 17% for the rigid and flexible PSCs, respectively. Additionally, the NaPAs show no cracking after 500 times of bending, consistent with the mechanic simulation results. This robust fabrication opens a new opportunity for the fabrication of a large area of high-performance flexible optoelectronic devices.

Molecular Ferroelectrics-Driven High-Performance Perovskite Solar Cells.

The nonradiative recombination of electrons and holes has been identified as the main cause of energy loss in hybrid organic-inorganic perovskite solar cells (PSCs). Sufficient built-in field and defect passivation can facilitate effective separation of electron-hole pairs to address the crucial issues. For the first time, we introduce a homochiral molecular ferroelectric into a PSC to enlarge the built-in electric field of the perovskite film, thereby facilitating effective charge separation and transportation. As a consequence of similarities in ionic structure, the molecular ferroelectric component of the PSC passivates the defects in the active perovskite layers, thereby inducing an approximately eightfold enhancement in photoluminescence intensity and reducing electron trap-state density. The photovoltaic molecular ferroelectric PSCs achieve a power conversion efficiency as high as 21.78 %.

MoS2-OH Bilayer-Mediated Growth of Inch-Sized Monolayer MoS2 on Arbitrary Substrates.

Due to remarkable electronic property, optical transparency, and mechanical flexibility, monolayer molybdenum disulfide (MoS2) has been demonstrated to be promising for electronic and optoelectronic devices. To date, the growth of high-quality and large-scale monolayer MoS2 has been one of the main challenges for practical applications. Here we present a MoS2-OH bilayer-mediated method that can fabricate inch-sized monolayer MoS2 on arbitrary substrates. This approach relies on a layer of hydroxide groups (-OH) that are preferentially attached to the (001) surface of MoS2 to form a MoS2-OH bilayer structure for growth of large-area monolayer MoS2 during the growth process. Specifically, the hydroxide layer impedes vertical growth of MoS2 layers along the [001] zone axis, promoting the monolayer growth of MoS2, constrains growth of the MoS2 monolayer only in the lateral direction into larger area, and effectively reduces sulfur vacancies and defects according to density functional theory calculations. Finally, the hydroxide groups advantageously prevent the MoS2 from interface oxidation in air, rendering high-quality MoS2 monolayers with carrier mobility up to ∼30 cm2 V-1 s-1. Using this approach, inch-sized uniform monolayer MoS2 has been fabricated on the sapphire and mica and high-quality monolayer MoS2 of single-crystalline domains exceeding 200 µm has been grown on various substrates including amorphous SiO2 and quartz and crystalline Si, SiC, Si3N4, and graphene This method provides a new opportunity for the monolayer growth of other two-dimensional transition metal dichalcogenides such as WS2 and MoSe2.

Recent Progress on Surface Reconstruction of Earth-Abundant Electrocatalysts for Water Oxidation.

As one important electrode reaction in electrocatalytic and photoelectrochemical cells for renewable energy circulation, oxygen catalysis has attracted considerable research in developing efficient and cost-effective catalysts. Due to the inevitable formation of oxygenic intermediates on surface sites during the complex reaction steps, the surface structure dynamically evolves toward reaction-preferred active species. To date, transition metal compounds, here defined as TM-Xides, where "X" refers to typical nonmetal elements from group IIIA to VIA, including hydroxide as well, are reported as high-performance oxygen evolution reaction (OER) electrocatalysts. However, more studies observe at least exterior oxidation or amorphization of materials. Thus, whether the TM-Xides can be defined as OER catalysts deserves further discussion. This Review pays attention to recent progress on the surface reconstruction of TM-Xide OER electrocatalysts with an emphasis on the identification of the true active species for OER, and aims at disseminating the real contributors of OER performance, especially under long-duration electrocatalysis.

Ultraflexible and Lightweight Bamboo-Derived Transparent Electrodes for Perovskite Solar Cells.

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light-weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo-based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass-based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.

Nanostructured hexagonal ReO3 with oxygen vacancies for efficient electrocatalytic hydrogen generation.

We report oxygen vacancies (OVs) rich hexagonal ReO3 nanostructured electrocatalysts for efficient hydrogen generation. Through a simple argon plasma exposure, OVs are introduced into the ReO3 nanoparticles (NP) and nanosheets to enhance electrocatalytic activities with decreasing overpotentails from 157 mV and 178 mV to 138 mV and 145 mV at the current density of 10 mA cm-2, respectively. As-processed OVs rich ReO3 NP exhibit a good stability during electrochemical measurements for 20 h in acidic electrolyte. The huge active surface area, abundant OVs and excellent conductivity contribute to the performance according to the experimental data. Further theoretical calculations show that the abundant OVs adsorb H with lower Gibbs free energy facilitating hydrogen evolution.

Neutral Mononuclear Copper(I) Complexes: Synthesis, Crystal Structures, and Photophysical Properties.

Neutral green-emitting four-coordinate Cu(I) complexes with general formula POPCu(NN), where POP = bis[2-(diphenylphosphino)phenyl]ether and NN = substituted 2-pyridine-1,2,4-triazole ligands, were synthesized. The crystal structures of (POPCuMeCN)(+)(PF6)(-) (1), POPCuPhPtp (2a, PhPtp = 2-(5-phenyl-2H-[1,2,4]triazol-3-yl)-pyridine), and POPCu(3,5-2FPhPtp) (2d, 3,5-2FPhPtp = 2-(5-(3,5-difluorophenyl)-2H-1,2,4-triazol-3-yl)pyridine) were determined by single-crystal X-ray diffraction analysis. The electronic and photophysical properties of the complexes were examined by UV-vis, steady-state, and time-resolved spectroscopy. At room temperature, weak emission was observed in solution, while in the solid state, all complexes exhibit intense green emission with quantum yield up to 0.54. The electronic and photophysical properties were further supported by calculation performed at the (time-dependent) density functional theory level. One of the complexes was also tested as dopant in electroluminescent devices.