van der Waals Epitaxial Growth of One-Unit-Cell-Thick Ferroelectric CuCrS2 Nanosheets.

Ferroelectric two-dimensional (2D) materials with a high transition temperature are highly desirable for new physics and next-generation memory electronics. However, the long-range polar order of ferroelectrics will barely persist when the thickness reaches the nanoscale. In this work, we synthesized 2D CuCrS2 nanosheets with thicknesses down to one unit cell via van der Waals epitaxy in a chemical vapor deposition system. A combination of transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements confirms the R3m space group and noncentrosymmetric structure. Switchable ferroelectric domains and obvious ferroelectric hysteresis loops were created and visualized by piezoresponse force microscopy. Theoretical calculation helps us understand the mechanism of ferroelectric switching in CuCrS2 nanosheets. Finally, we fabricated a ferroelectric memory device that achieves an on/off ratio of ∼102 and remains stable after 2000 s, indicating its applicability in novel nanoelectronics. Overall, 2D CuCrS2 nanosheets exhibit excellent ferroelectric properties at the nanoscale, showing great promise for next-generation devices.

Controllable van der Waals gaps by water adsorption.

Van der Waals (vdW) gaps with ångström-scale heights can confine molecules or ions to an ultimately small scale, providing an alternative way to tune material properties and explore microscopic phenomena. Modulation of the height of vdW gaps between two-dimensional (2D) materials is challenging due to the vdW interaction. Here we report a general approach to control the vdW gap by preadsorption of water molecules on the material surface. By controlling the saturation vapour pressure of water vapour, we can precisely control the adsorption level of water molecules and vary the height of the vdW gaps of MoS2 homojunctions from 5.5 Å to 53.6 Å. This technique can be further applied to other homo- and heterojunctions, constructing controlled vdW gaps in 2D artificial superlattices and in 2D/3D and 3D/3D heterojunctions. Engineering the vdW gap has great practical potential to modulate the device performance, as evidenced by the vdW-gap-dependent diode characteristics of the MoS2/gap/MoS2 junction. Our work introduces a general strategy of molecular preadsorption that can extend to various precursors, creating more tunability and variability in vdW material systems.

Compositional Engineering of Cu-Doped SnO Film for Complementary Metal Oxide Semiconductor Technology.

Metal oxide semiconductor (MOS)-based complementary thin-film transistor (TFT) circuits have broad application prospects in large-scale flexible electronics. To simplify circuit design and increase integration density, basic complementary circuits require both p- and n-channel transistors based on an individual semiconductor. However, until now, no MOSs that can simultaneously show p- and n-type conduction behavior have been reported. Herein, we demonstrate for the first time that Cu-doped SnO (Cu:SnO) with HfO2 capping can be employed for high-performance p- and n-channel TFTs. The interstitial Cu+ can induce an n-doping effect while restraining electron-electron scatterings by removing conduction band minimum degeneracy. As a result, the Cu3 atom %:SnO TFTs exhibit a record high electron mobility of 43.8 cm2 V-1 s-1. Meanwhile, the p-channel devices show an ultrahigh hole mobility of 2.4 cm2 V-1 s-1. Flexible complementary logics are then established, including an inverter, NAND gates, and NOR gates. Impressively, the inverter exhibits an ultrahigh gain of 302.4 and excellent operational stability and bending reliability.

Building Stable Solid-State Potassium Metal Batteries.

Solid-state potassium metal batteries (SPMBs) are promising candidates for the next generation of energy storage systems for their low cost, safety, and high energy density. However, full SPMBs are not yet reported due to the K dendrites, interfacial incompatibility, and limited availability of suitable solid-state electrolytes. Here, stable SPMBs using a new iodinated solid polymer electrolyte (ISPE) are presented. The functional ions reconstruct ion transport channels, providing efficient potassium ion transport. ISPE shows a combination of high ionic conductivity, superior interfacial compatibility, and electrochemical stability. In situ alloying and iodinated interlayer increase K metal compatibility for prolonged cycling with low polarization. Moreover, the ISPE enables SPMBs with Prussian blue cathode stable operation at a high voltage of 4.5 V, a superior rate capability, and long-term cycling over 3000 cycles (4.2 V vs K+ /K) with an ultra-high coulombic efficiency of 99.94%. More importantly, a classic solid-state potassium metal pouch cell achieves 4.2 V stable cycling over 800 cycles with a high retention of 93.6%, presenting a new development strategy for secure and high-performance rechargeable solid-state potassium metal batteries.

Co-activation for enhanced K-ion storage in battery anodes.

The relative natural abundance of potassium and potentially high energy density has established potassium-ion batteries as a promising technology for future large-scale global energy storage. However, the anodes' low capacity and high discharge platform lead to low energy density, which impedes their rapid development. Herein, we present a possible co-activation mechanism between bismuth (Bi) and tin (Sn) that enhances K-ion storage in battery anodes. The co-activated Bi-Sn anode delivered a high capacity of 634 mAh g-1, with a discharge plateau as low as 0.35 V, and operated continuously for 500 cycles at a current density of 50 mA g-1, with a high Coulombic efficiency of 99.2%. This possible co-activation strategy for high potassium storage may be extended to other Na/Zn/Ca/Mg/Al ion battery technologies, thus providing insights into how to improve their energy storage ability.

An Evaluation Study of a New Designed Oscillating Hydraulic Trainer of Neck.

In view of the importance of neck strength training and the lack of adequate training equipment, this study designed a new oscillating hydraulic trainer (OHT) of neck based on oscillating hydraulic damper. We used surface electromyography (sEMG) and subjective ratings to evaluate the neck OHT and compared the results with a simple hat trainer (HATT) and traditional weight trainer (TWT) to verify the feasibility and validity of the OHT. Under similar exercise conditions, 12 subjects performed a set of neck flexion and extension exercise with these 3 trainers. The sEMG signals of targeted muscles were collected in real time, and subjects were asked to complete subjective evaluations of product usability after exercise. The results showed that the root mean square (RMS%) of sEMG indicated that the OHT could provide two-way resistance and train the flexors and extensors simultaneously. The overall degree of muscle activation with OHT was higher than that with the other two trainers in one movement cycle. In terms of resistance characteristics exhibited by the sEMG waveform, duration (D) with OHT was significantly longer than HATT and TWT when exercising at a high speed, while Peak Timing (PT) was later. The ratings of product usability and performing usability of OHT were remarkably higher than that of HATT and TWT. Based on the above results, the OHT was proved to be more suitable for strength training, such as neck muscles, which were getting more attention gradually, but lacked mature and special training equipment.

Insights into electronic properties of strained two-dimensional semiconductors by out-of-plane bending.

Strain engineering is an important strategy to modulate the electronic and optical properties of two-dimensional (2D) semiconductors. In experiments, an effective and feasible method to induce strains on 2D semiconductors is the out-of-plane bending. However, in contrast to the in-plane methods, it will generate a combined strain effect on 2D semiconductors, which deserves further explorations. In this work, we theoretically investigate the carrier transport-related electronic properties of arsenene, antimonene, phosphorene, and MoS2under the out-of-plane bending. The bending effect can be disassembled into the in-plane and out-of-plane rolling strains. We find that the rolling always degrades the transport performance, while the in-plane strain could boost carrier mobilities by restraining the intervalley scattering. In other words, pursuing the maximum in-plane strain at the expense of minimum rolling should be the primary strategy to promote transports in 2D semiconductors through bending. Electrons in 2D semiconductors usually suffer from the serious intervalley scattering caused by optical phonons. The in-plane strain can break the crystal symmetry and separate nonequivalent energy valleys at band edges energetically, confining carrier transports at the Brillouin zone Γ point and eliminating the intervalley scattering. Investigation results show that the arsenene and antimonene are suitable for the bending technology, because of their small layer thicknesses which can relieve the rolling burden. Their electron and hole mobilities can be doubled simultaneously, compared with their unstrained 2D structures. From this study, the rules for the out-of-plane bending technology towards promoting transport abilities in 2D semiconductors are obtained.

Ferroelectric Wide-Bandgap Metal Halide Perovskite Field-Effect Transistors: Toward Transparent Electronics.

Transparent field-effect transistors (FETs) are attacking intensive interest for constructing fancy "invisible" electronic products. Presently, the main technology for realizing transparent FETs is based on metal oxide semiconductors, which have wide-bandgap but generally demand sputtering technique or high-temperature (>350 °C) solution process for fabrication. Herein, a general device fabrication strategy for metal halide perovskite (MHP) FETs is shown, by which transparent perovskite FETs are successfully obtained using low-temperature (<150 °C) solution process. This strategy involves the employment of ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the dielectric, which conquers the challenging issue of gate-electric-field screening effect in MHP FETs. Additionally, an ultra-thin SnO2 is inserted between the source/drain electrodes and MHPs to facilitate electron injection. Consequently, n-type semi-transparent MAPbBr3 FETs and fully transparent MAPbCl3 FETs which can operate well at room temperature with mobility over 10-3  cm2  V-1  s-1 and on/off ratio >103 are achieved for the first time. The low-temperature solution processability of these FETs makes them particularly attractive for applications in low-cost, large-area transparent electronics.

A Molybdenum Disulfide Nanozyme with Charge-Enhanced Activity for Ultrasound-Mediated Cascade-Catalytic Tumor Ferroptosis.

The deficient catalytic activity of nanozymes and insufficient endogenous H2 O2 in the tumor microenvironment (TME) are major obstacles for nanozyme-mediated catalytic tumor therapy. Since electron transfer is the basic essence of catalysis-mediated redox reactions, we explored the contributing factors of enzymatic activity based on positive and negative charges, which are experimentally and theoretically demonstrated to enhance the peroxidase (POD)-like activity of a MoS2 nanozyme. Hence, an acidic tumor microenvironment-responsive and ultrasound-mediated cascade nanocatalyst (BTO/MoS2 @CA) is presented that is made from few-layer MoS2 nanosheets grown on the surface of piezoelectric tetragonal barium titanate (T-BTO) and modified with pH-responsive cinnamaldehyde (CA). The integration of pH-responsive CA-mediated H2 O2 self-supply, ultrasound-mediated charge-enhanced enzymatic activity, and glutathione (GSH) depletion enables out-of-balance redox homeostasis, leading to effective tumor ferroptosis with minimal side effects.

Interlayer Transition Induced Infrared Response in ReS2/2D Perovskite van der Waals Heterostructure Photodetector.

The emerging Ruddlesden-Popper two-dimensional perovskite (2D PVK) has recently joined the family of 2D semiconductors as a potential competitor for building van der Waals (vdW) heterostructures in future optoelectronics. However, to date, most of the reported heterostructures based on 2D PVKs suffer from poor spectral response that is caused by intrinsic wide bandgap of constituting materials. Herein, a direct heterointerface bandgap (∼0.4 eV) between 2D PVK and ReS2 is demonstrated. The strong interlayer coupling reduces the energy interval at the heterojunction region so that the heterostructure shows high sensitivity with the spectral response expanding to 2000 nm. The large type-II band offsets exceeding 1.1 eV ensure fast photogenerated carriers separation at the heterointerface. When this heterostructure is used as a self-driven photodetector, it exhibits a record high detectivity up to 1.8 × 1014 Jones, surpassing any reported 2D self-driven devices, and an impressive external quantum efficiency of 68%.

Enhanced performance of p-type SnOxthin film transistors through defect compensation.

Due to the unique outermost orbitals of Sn, hole carriers in tin monoxide (SnO) possess small effective mass and high mobility among oxide semiconductors, making it a promising p-channel material for thin film field-effect transistors (TFTs). However, the Sn vacancy induced field-effect mobility deterioration and threshold voltage (Vth) shift in experiments greatly limit its application in complementary metal-oxide-semiconductor (CMOS) transistors. In this study, the internal mechanism of vacancy defect compensation by aluminum (Al) doping in SnOxfilm is studied combining experiments with the density functional theory (DFT). The doping is achieved by an argon (Ar) plasma treatment of Al2O3deposited onto the SnOxfilm, in which the Al2O3provides both the surface passivation and Al doping source. Experimental results show a wideVthmodulation range (6.08 to -19.77 V) and notable mobility enhancement (11.56 cm2V-1s-1) in the SnOxTFTs after the Al doping by Ar plasma. DFT results reveal that the most possible positions of Al in SnO and SnO2segments are the compensation to Sn vacancy and interstitial. The compensation will create an n-type doping effect and improve the hole carrier transport by reducing the hole effective mass (mh*), which is responsible for the device performance variation, while the interstitial in the SnO2segment can hardly affect the valence transport of the film. The defect compensation is suitable for the electronic property modulation of SnO towards the high-performance CMOS application.

Ferrocene-Containing Nucleic Acid-Based Energy-Storage Nanoagent for Continuously Photo-Induced Oxidative Stress Amplification.

Regulation of cellular oxidative stress plays a critical role in revealing the molecular mechanisms of cellular activities and thus is a potential strategy for tumor treatment. Optical methods have been employed for intelligent regulation of oxidative stress in tumor regions. However, long-time continuous irradiation inevitably causes damage to normal tissues. Herein, a ferrocene-containing nucleic acid-based energy-storage nanoagent was designed to achieve the continuous photo-regulation of cellular oxidative stress in the dark. Specifically, the photoenergy stored in the agent could convert effectively and accelerate Fenton-like reaction continuously, augmenting cellular oxidative stress. This nanoagent could also silence oxidative damage repair genes to further amplify oxidative stress. This strategy not only provides oxidative stress regulation for studying the molecular mechanisms of biological activities, but also offers a promising step toward tumor microenvironment modulation.

Flexible SnO Optoelectronic Memory Based on Light-Dependent Ionic Migration in Ruddlesden-Popper Perovskite.

Nonvolatile optoelectronic memories based on organic-inorganic hybrid perovskites have appeared as powerful candidates for next-generation soft electronics. Here, ambipolar SnO transistor-based nonvolatile memories with multibit memory behavior (11 storage states, 120 nC state-1) and ultralong retention time (>105 s) are demonstrated for which an Al2O3/two-dimensional Ruddlesden-Popper perovskite (2D PVK) heterostructure dielectric architecture is employed. The unique storage features are attributed to suppressed gate leakage by Al2O3 layer and hopping-like ionic transport in 2D PVK with varying activation energy under different light intensities. The photoinduced field-effect mechanism enables top-gated transistor operation under illumination, which would not be achieved under dark. As a result, the device exhibits remarkable photoresponsive characteristics, including ultrahigh specific detectivity (2.7 × 1015 Jones) and broadband spectrum distinction capacity (375-1064 nm). This study offers valuable insight on the PVK-based dielectric engineering for information storage and paves the way toward multilevel broadband-response optoelectronic memories.

Facet Selectivity Guided Assembly of Nanoarchitectures onto Two-Dimensional Metal-Organic Framework Nanosheets.

Facet-selective nanostructures in living systems usually exhibit outstanding optical and enzymatic properties, playing important roles in photonics, matter exchange, and biocatalysis. Bioinspired construction of facet-selective nanostructures offers great opportunities for sophisticated nanomaterials, but remains a formidable task. We have developed a macromolecule-mediated strategy for the assembly of upconversion nanoparticles (UCNPs)/two-dimensional metal-organic frameworks (2DMOFs) heterostructures with facet selectivity. Both experimental and theoretical results demonstrate that polyvinylpyrrolidone (PVP) can be utilized as an interface-selective mediator to further promote the facet-selective assembly of MOFs onto the surface of UCNPs. The UCNPs/2DMOFs nanostructures with facet selectivity display specific optical properties and show great advantages in anti-counterfeiting. Our demonstration of UCNPs/2DMOFs provides a vivid example for the controlled fabrication of facet-selective nanostructures and can promote the development of advanced functional materials for applications in biosensing, energy conversion, and information assurance.

Rational design of Al2O3/2D perovskite heterostructure dielectric for high performance MoS2 phototransistors.

Two-dimensional (2D) Ruddlesden-Popper perovskites are currently drawing significant attention as highly-stable photoactive materials for optoelectronic applications. However, the insulating nature of organic ammonium layers in 2D perovskites results in poor charge transport and limited performance. Here, we demonstrate that Al2O3/2D perovskite heterostructure can be utilized as photoactive dielectric for high-performance MoS2 phototransistors. The type-II band alignment in 2D perovskites facilitates effective spatial separation of photo-generated carriers, thus achieving ultrahigh photoresponsivity of >108 A/W at 457 nm and >106 A/W at 1064 nm. Meanwhile, the hysteresis loops induced by ionic migration in perovskite and charge trapping in Al2O3 can neutralize with each other, leading to low-voltage phototransistors with negligible hysteresis and improved bias stress stability. More importantly, the recombination of photo-generated carriers in 2D perovskites depends on the external biasing field. With an appropriate gate bias, the devices exhibit wavelength-dependent constant photoresponsivity of 103-108 A/W regardless of incident light intensity.

Efficient strain modulation of 2D materials via polymer encapsulation.

Strain engineering is a promising method to manipulate the electronic and optical properties of two-dimensional (2D) materials. However, with weak van der Waals interaction, severe slippage between 2D material and substrate could dominate the bending or stretching processes, leading to inefficiency strain transfer. To overcome this limitation, we report a simple strain engineering method by encapsulating the monolayer 2D material in the flexible PVA substrate through spin-coating approach. The strong interaction force between spin-coated PVA and 2D material ensures the mechanical strain can be effectively transferred with negligible slippage or decoupling. By applying uniaxial strain to monolayer MoS2, we observe a higher bandgap modulation up to ~300 meV and a highest modulation rate of ~136 meV/%, which is approximate two times improvement compared to previous results achieved. Moreover, this simple strategy could be well extended to other 2D materials such as WS2 or WSe2, leading to enhanced bandgap modulation.

A transport isolation by orbital hybridization transformation toward graphene nanoribbon-based nanostructure integration.

Among feature-rich graphene-related materials, graphene nanoribbon (GNR)-based nanostructures are particularly attractive because they can provide tunable and excellent electronic properties. However, the integration of high-quality GNR-based nanostructures on a large scale is still an open area. In this paper, a novel idea is proposed: transport isolation. By a construction of different orbital hybridizations of the carbon atoms in graphene, the GNR regions and functionalized graphene regions are integrated. In the hybrid system, the functionalized graphene regions play the role of the isolation barrier. Based on the first principle calculation, it is demonstrated that about 0.6 nm wide hydrogenated graphene is enough to reliably isolate the GNR regions. Besides, it is revealed that once the armchair GNRs (AGNRs) are fully isolated by functionalized graphene, their band gaps are basically maintained and are weakly dependent on the width of functionalized graphene regions. In addition, the transport characteristics of those isolated AGNRs are verified to be similar to the pristine AGNRs at the device level. The above virtues infer our method can effectively produce a reliable isolation, verified by a simulation of device integration demo. We hope it can provide an intriguing option for the integration of GNR-based nanostructures.

Scaling Effect of Phosphorene Nanoribbon - Uncovering the Origin of Asymmetric Current Transport.

In this paper, phosphorene nanoribbons (PNRs) are theoretically studied using a multiscale simulation flow from the ab initio level to the tight binding (TB) level. The scaling effects of both armchair PNRs (aPNRs) and zigzag PNRs (zPNRs) from material properties to device properties are explored. The much larger effective mass of holes compared to that of electrons in zPNR is responsible for its asymmetric transport. However, in aPNR, not only the effective mass difference but also the non-equal density of state (DOS) distributions near valence band maximum (VBM) and conduction band minimum (CBM) lead to the asymmetric transport. This non-equal distribution phenomenon is caused by energy band degeneracies near the VBM. Based on these two different mechanisms, PNRs' asymmetric transport characteristics at the device level are explained, and it is shown that this behaviour can be ameliorated well by reducing the ribbon width in an aPNR MOSFET. Calculation results also indicate that aPNR's effective mass is comparable to that of a graphene nanoribbon (GNR) at the same bandgap; however, aPNR's band gap variation is more stable and regular than that of GNR, making it a good candidate for use in low-dimensional nano devices.