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In the present study, we demonstrated a novel process of in-situ fabrication of graphene sheets by adopting the chemical vapor deposition (CVD) method. Fabricating such a layer of vertical graphene sheets (VGs) on the surface of microcrystalline diamond (MCD) films could decrease the stable coefficient of friction (COF) by 20–30%. Besides, such lubricity showed excellent load capacity and durability. With normal loads ranging from 3 N to 9 N (Hertz contact pressure of 2.1–3.1 GPa), the MCD-VGs films exhibited COFs stabilized at 0.105 and wear rates less than 5 × 10−6 mm3 N−1 m−1. Additionally, the produced VGs displayed an excellent service lifetime with 72,000 sliding cycles. During the friction tests, an equilibrium graphene self-mated contact formed on the sliding interface as graphene fragments adhered onto the counterpart ball and resulted in the reduction of friction and low wear rates. Additionally, the mixtures of Si3N4/SiO2 particles and graphene sheets residual within the wear tracks would promote the transformation of the sliding regime to three-body abrasion, beneficial for friction reduction. The findings presented in this study provide a novel and effective approach to enhance the lubricity of any engineering diamond surface.

Pristine monolayer molybdenum disulfide (MoS2) demonstrates predominant and persistent n-type semiconducting polarity due to the natural sulfur vacancy, which hinders its electronic and optoelectronic applications in the rich bipolarity area of semiconductors. Current doping strategies in single-layer MoS2 are either too mild to reverse the heavily n-doped polarity or too volatile to create a robust electronic device meeting the requirements of both a long lifetime and compatibility for mass production. Herein, we demonstrate that MoS2 can be transferred onto polytetrafluoroethylene (PTFE), one of the most electronegative substrates. After transfer, the MoS2 photoluminescence exhibits an obvious blueshift from 1.83 to 1.89 eV and a prolonged lifetime, from 0.13 to 3.19 ns. The Fermi level of MoS2 experiences a remarkable 510 meV decrease, transforming its electronic structure into that of a hole-rich p-type semiconductor. Our work reveals a strong p-doping effect and charge transfer between MoS2 and PTFE, shedding light on a new nonvolatile strategy to fabricate p-type MoS2 devices.

Coherent longitudinal lattice vibrations of black phosphorus provide unique access to the out-of-plane strain coupled in-plane optical properties. In this work, polarization-resolved femtosecond transient absorption microscopy is applied to study the anisotropic coherent phonon responses. Multiorder phonon harmonics were observed with thickness dependence well explained by the linear chain model, allowing rapid optical mapping of phonon frequency distributions. More interestingly, exotic coherent phonon oscillations occourred with a π-phase jump between the armchair and zigzag polarizations, which reveals opposite signs of photoelasticity under the longitudinal strain. Specifically, compressive strain reduces the imaginary refractive index in the armchair polarization but increases the real refractive index in the zigzag polarization, as confirmed by the ab initio calculations and thin film model. These fundamental properties of black phosphorus hold potential for applications in ultrafast and polarization-sensitive photoacoustic/photoelastic modulators.

2D semiconductors, such as molybdenum disulfide (MoS

Wrinkling occurs on the surfaces of large-area graphene ubiquitously. Despite that the wrinkled structures are found to degrade the lubricous property, the behind mechanisms remain less understood. Here, atomic force microscopy is adopted to characterize the friction and wear properties of graphene wrinkles (GWs) with different heights by nanoscratch tests. We verify the phenomena of high friction and reduced load-carrying capacity of wrinkles and report the observation of lubrication deterioration with increased heights. Using molecular dynamics simulations, we reveal that the contact quality at the interface is a dominant role in the friction evolution of wrinkles. The high friction of wrinkles is determined by the increased contact area and commensurability caused by the wrinkle deformation and topography changes. The wrinkle failure initiates near the root of the formed bilayer configuration due to the increased lateral stiffness and reduced atomic distance between the wrinkle layers. The increased interlocking effect results in a local shear stress of 91 GPa and induces the phase transitions of carbon atoms easily. As the wrinkle height decreases, the unstable local configuration weakens the interlocking effects and cannot fail even at a high load. This investigation sheds light on the microscopic frictional contact of GWs and provides guidance for tuning the tribological properties of graphene by controlling the wrinkle structures.

From chemical vapor deposition grown film to nanoscale lithographic devices, etching is a critical procedure to define two-dimensional (2D) materials’ geometry and edge type. Many reported etching procedures could potentially damage the activity of the edge, while a living edge is capable to heal and regrow back. Here we successfully demonstrated a continuous growth-etching-regrowth procedure to achieve the precise lateral control of graphene basal plane. The etching process leaves graphene with dominant zigzag edges. Up to 85% of the new graphene domains can be regrown from the living edges, replicating the graphene’s crystallographic orientation before the etching. At 1050 °C, the etching and regrowth can be linearly controlled at a rate of −2.7 μm/min and 2.4 μm/min, respectively. This represents a promising technique for precisely tailoring the lateral structure of graphene with selective edge type.

Electrocatalytic reduction of N2 to NH3 under an ambient atmosphere is highly desirable and extremely critical for energy-efficient nitrogen utilization. Inspired by the natural MoFe protein-based ...

Electrocatalysts are evolving toward chemically tunable atomic structures, among which the catalyst engineering from a defect perspective represents one of the mainstream technical genres. However,...

To reverse the ever-increasing trend of the CO2 concentration in the atmosphere, human needs to develop a killer “catalyst”, which could convert the anthropogenic CO2 back into high value-added or energy-storage molecules, not under the natural photosynthesis Calvin cycle, but through an artificial CO2 reduction pathway. In the race to this emergent technology, the electrochemical reduction of CO2 (CO2RR) with carbon based catalyst becomes one of the frontrunners. However, the carbon-based catalysts are genetically complex, with edge, strain, intrinsic topological defect, and doping effects, coexisting and affecting the catalysis behavior. Without a relative activity ranking system under the same metrics, unveiling the mechanism and improving the catalyst would become formidable. Here we assessed different carbon materials’ CO2RR performance under standard experimental conditions. The catalytic activities were evaluated and sequenced by their cathodic potential, characterized with the peak CO faradic efficiency. The doping effect is leading in the sequence, followed with the other three factors, all of which are superior to the pristine sp2 carbon material.

It is of great significance to explore unique and diverse chemical pathways to convert CO

Pristine monolayer molybdenum disulfide (MoS

Recently, research on two-dimensional (2D) semiconductors has begun to translate from the fundamental investigation into rudimentary functional circuits. In this work, we unveil the first functional MoS2 artificial neural network (ANN) chip, including multiply-and-accumulate (MAC), memory and activation function circuits. Such MoS2 ANN chip is realized through fabricating 818 field-effect transistors (FETs) on a wafer-scale and high-homogeneity MoS2 film, with a gate-last process to realize top gate structured FETs. A 62-level simulation program with integrated circuit emphasis (SPICE) model is utilized to design and optimize our analog ANN circuit. To demonstrate a practical application, a tactile digit sensing recognition was demonstrated based on our ANN circuits. After training, the digit recognition rate exceeds 97%. Our work not only demonstrates the protentional of 2D semiconductors in wafer-scale integrated circuits, but also paves the way for its future application in AI computation.

In-memory computing may enable multiply-accumulate (MAC) operations, which are the primary calculations used in artificial intelligence (AI). Performing MAC operations with high capacity in a small area with high energy efficiency remains a challenge. In this work, we propose a circuit architecture that integrates monolayer MoS2 transistors in a two-transistor–one-capacitor (2T-1C) configuration. In this structure, the memory portion is similar to a 1T-1C Dynamic Random Access Memory (DRAM) so that theoretically the cycling endurance and erase/write speed inherit the merits of DRAM. Besides, the ultralow leakage current of the MoS2 transistor enables the storage of multi-level voltages on the capacitor with a long retention time. The electrical characteristics of a single MoS2 transistor also allow analog computation by multiplying the drain voltage by the stored voltage on the capacitor. The sum-of-product is then obtained by converging the currents from multiple 2T-1C units. Based on our experiment results, a neural network is ex-situ trained for image recognition with 90.3% accuracy. In the future, such 2T-1C units can potentially be integrated into three-dimensional (3D) circuits with dense logic and memory layers for low power in-situ training of neural networks in hardware., In standard computing architectures, memory and logic circuits are separated, a feature that slows matrix operations vital to deep learning algorithms. Here, the authors present an alternate in-memory architecture and demonstrate a feasible approach for analog matrix multiplication.

Compared with the semimetallic single layer graphene (SLG), large-sized bilayer graphene with a desired stacking order is extremely useful in the twilight of two-dimensional electronics era. Howeve...

Recent advances in van der Waals heterostructures have extensively promoted the development of new-generation electronic devices. However, the normally utilized mechanical exfoliation method for preparing two-dimensional semiconductors is not scalable for circuit-level application. Herein, the fabrication and characterization of wafer-scale heterostructure arrays composed of multilayer 2H-MoTe2 and single-layer 2H-MoS2 are demonstrated. Owing to the type-II band alignment facilitating efficient electron–hole separation, the devices fabricated by the p-MoTe2/n-MoS2 heterostructure exhibit an excellent gate-tunable PN diode behavior, with a rectification ratio of over 103 and a self-powered photocurrent with a remarkable on–off ratio of ∼103 at a zero bias voltage. Complementary inverter arrays based on p-MoTe2/n-MoS2 are also demonstrated. The scalable production of p–n junction devices and complementary inverters paves the way for future integrated platforms in photoelectric detection and logic computation.

Catalyst doped with a single-atom noble metal displays distinctive catalytic behavior from the bulk counterparts, with tunable electronic structures and spatial versatilities, which excels in today’s heterogeneous catalysis. To deposit noble metals in a single atomic level requires a restricted chemical environment and precise thermodynamic control. Electroplating methods are commercially used to deposit uniform and conformal metal thin films on different hardware surfaces. Yet the atomic level electroplating has never been achieved. Herein we demonstrate a voltage gauged electrochemical deposition method to synthesize single-atom Pt, Au, and Pd on MoS2 and other two-dimensional (2D) materials. The surface atomic doping level for Pt, Au, and Pd can reach 1.1, 7.0, and 14%, respectively, and the doping sites are precisely positioned at Mo- and S-vacancies. The monodispersed noble atoms show enhanced hydrogen evolution activity and saturated CO tolerance, as explained by density functional theory calculatio...

The gaseous product concentration in direct electrochemical CO2 reduction is usually hurdled by the electrode’s Faradaic efficiency, current density, and inevitable mixing with the unreacted CO2. A concentrated gaseous product with high purity will greatly lower the barrier for large-scale CO2 fixation and follow-up industrial usage. Here, we developed a pneumatic trough setup to collect the CO2 reduction product from a precisely engineered nanotwinned electrocatalyst, without using ion-exchange membrane. The silver catalyst’s twin boundary density can be tuned from 0.3 to 1.5 × 104 cm−1. With the lengthy and winding twin boundaries, this catalyst exhibits a Faradaic efficiency up to 92% at −1.0 V and a turnover frequency of 127 s−1 in converting CO2 to CO. Through a tandem electrochemical-CVD system, we successfully produced CO with a volume percentage of up to 52%, and further transformed it into single layer graphene film., Isolating purified electrosynthesis product is a major challenge in electrochemical carbon dioxide reduction. Here, the authors report a nanotwinned silver electrocatalyst and a pneumatic-trough cell system to produce a 52% concentrated CO, which is further utilized as a carbon feedstock for graphene production.

We demonstrate the fabrication and characterization of large-area van der Waals heterostructure array composed of chemical vapor deposition (CVD) grown multilayer (ML) 2H-MoTe 2 and single-layer (SL) 2H-MoS 2 , which exhibits excellent rectifier properties. The transfer characteristics of field-effect transistors (FETs) are also investigated. Our work acquires excellent homogeneity and device performance, which facilitates the application of two-dimensional (2D) materials in electronics.

Triggered by the pioneering research on graphene, the family of two-dimensional layered materials (2DLMs) has been investigated for more than a decade, and appealing functionalities have been demonstrated. However, there are still challenges inhibiting high-quality growth and circuit-level integration, and results from previous studies are still far from complying with industrial standards. Here, we overcome these challenges by utilizing machine-learning (ML) algorithms to evaluate key process parameters that impact the electrical characteristics of MoS2 top-gated field-effect transistors (FETs). The wafer-scale fabrication processes are then guided by ML combined with grid searching to co-optimize device performance, including mobility, threshold voltage and subthreshold swing. A 62-level SPICE modeling was implemented for MoS2 FETs and further used to construct functional digital, analog, and photodetection circuits. Finally, we present wafer-scale test FET arrays and a 4-bit full adder employing industry-standard design flows and processes. Taken together, these results experimentally validate the application potential of ML-assisted fabrication optimization for beyond-silicon electronic materials., Here, the authors demonstrate the application of machine learning to optimize the device fabrication process for wafer-scale 2D semiconductors, and eventually fabricate digital, analog, and optoelectrical circuits.

The observation of electrically tunable and highly confined plasmons in graphene has stimulated the exploration of interesting properties of plasmons in other two-dimensional materials. Recently, hyperbolic plasmon resonance modes have been observed in exfoliated ${\mathrm{W}\mathrm{Te}}_{2}$ films, a type-II Weyl semimetal with layered structure, providing a platform for the assembly of plasmons with hyperbolicity and exotic topological properties. However, the plasmon modes were observed in relatively thick and small-area films, which restrict the tunability and application for plasmons. Here, large-area (approximately cm) ${\mathrm{W}\mathrm{Te}}_{2}$ films with different thicknesses are grown by the chemical vapor deposition method, in which plasmon resonance modes are observed in films with different thicknesses down to about 8 nm. Hybridization of plasmon and surface polar phonons of the substrate is revealed by mapping the plasmon dispersion. The plasmon frequency is demonstrated to be tunable by changing the temperature and film thickness. Our results facilitate the development of a tunable and scalable ${\mathrm{W}\mathrm{Te}}_{2}$ plasmonic system for revealing topological properties and towards various applications in sensing, imaging, and light modulation.

Two-dimensional semiconductors can be used to build integrated circuits for running artificial neural networks (ANN) with higher energy efficiency. The implementation of an ANN with 2D semiconductors has been held back by the large-scale and high-quality transistors required for running machine learning algorithms. Here we demonstrate the first functional MoS2 analog ANN integrated circuit, including memory, multiply-and-accumulate (MAC), activation function, and weight update circuits. The ANN integrated circuit is realized through 818 field effect transistors (FETs) with wafer-scale and high-homogeneity MoS2 film. The large current on/off ratio and output linearity of these MoS2 FETs allow the realization of convolutional and activation function circuits with a few number of transistors. This ANN can be used for recognizing tactile digit, showing the recognition rate exceeding 97%. Our work demonstrates wafer-scale processing of a 2D semiconductor for building integrated circuits with the functions of AI computation.

The nano-friction behaviors of surfaces with a graphene fold (GF) oriented ~90° to the scanning direction were studied via an atomic force microscope. The GF exhibited dramatic friction increases at folding edges and modest friction augments at the folded area. Assisted with molecular dynamics simulations, we reproduced the experimental phenomena and elucidated that the substantial friction augments at the folding edge are attributed to enhanced pinning effects in front of the tip resulted from geometry barriers. The contact quality of interfaces at the fold-over structures is improved by an increase in the number of local pinning atoms in front of the tip or strengthened attraction of graphene to the back of the tip, leading to modest friction increases.

Carbon aerogels with biomimetic structures have shown excellent physicochemical properties and brought great potential applications to a wide range of fields. The utilization of renewable resources as the carbon precursors offers a low-cost and scalable way to fabricate biomimetic carbon aerogels with intriguing properties such as ultralight weight, superelasticity, and high conductivity. Inspired by the unique hierarchical mineral bridge structure of Thalia dealbata stem, we fabricated an ultralight, superelastic, highly conductive carbon aerogel (KGA) by using konjac glucomannan and graphene oxide as the carbon precursors. The unique mineral-bridged layered structure not only endows the carbon aerogel with a low density of 4.2 mg cm–3 but also a high electrical conductivity (12.9 S m–1). In addition, the carbon aerogel also exhibits a superelastic property of 80% maximal strain and no obvious degradation after 1000 cycles of compression. We demonstrated that this Thalia dealbata inspired carbon aerogel ...

Advances in microelectronic system technology have necessitated the development and miniaturization of energy storage devices. Supercapacitors are an important complement to batteries in microelectronic systems; and further reduction of the size of micro-supercapacitors is challenging. Here, a novel strategy is demonstrated to break through the resolution limit of micro-supercapacitors by preparing nano-supercapacitors (NSCs) with interdigital nanosized electrodes using focused ion beam technology. The minimization of the size of the NSCs leads to a large increase in capacitance, with a high areal capacitance of 9.52 mF cm-2 and a volumetric capacitance of 18 700 F cm-3 , far superior to those of other reported works. Size reduction and the narrowing of the physical separation between nanoelectrodes are proved to be the most crucial factors in the enhancement of capacitive performances. New charge-storage mechanisms are discovered with a remarkable nonfaradaic double-layer capacitance that exists due to the considerable inner electric field force at the nanoscale. The developed strategy and the first set of data provided here shed light on the design and fabrication of flexible interdigitated NSCs that rival state-of-the-art supercapacitors in performance.

Due to the strong quantum confinement effect, few-layer {\gamma}-InSe exhibits a layer-dependent bandgap, spanning the visible and near infrared regions, and thus recently draws tremendous attention. As a two-dimensional material, the mechanical flexibility provides an additional tuning knob for the electronic structure. Here, for the first time, we engineer the band structures of few-layer and bulk-like InSe by uniaxial tensile strain, and observe salient shift of photoluminescence (PL) peaks. The shift rate of the optical gap is approximately 90-100 meV per 1% strain for 4- to 8-layer samples, which is much larger than that for the widely studied MoS2 monolayer. Density functional calculations well reproduce the observed layer-dependent bandgaps and the strain effect, and reveal that the shift rate decreases with increasing layer number for few-layer InSe. Our study demonstrates that InSe is a very versatile 2D electronic and optoelectronic material, which is suitable for tunable light emitters, photo-detectors and other optoelectronic devices., Comment: submitted

Two-dimensional (2D) materials have attracted broad research interests across various nonlinear optical (NLO) studies, including nonlinear photoluminescence (NPL), second harmonic generation (SHG), transient absorption (TA), and so forth. These studies have unveiled important features and information of 2D materials, such as in grain boundaries, defects, and crystal orientations. However, as most research studies focused on the intrinsic NLO processes, little attention has been paid to the substrates underneath. Here, we discovered that the NLO signal depends significantly on the thickness of SiO2 in SiO2/Si substrates. A 40-fold enhancement of the NPL signal of graphene was observed when the SiO2 thickness was varied from 270 to 125 nm under 800 nm excitation. We systematically studied the NPL intensity of graphene on three different SiO2 thicknesses within a pump wavelength range of 800–1100 nm. The results agreed with a numerical model based on back reflection and interference. Furthermore, we have ext...

As the graphene is transforming from a laboratory niche material into a mass-produced industrial commodity, maintaining high quality standard across different chemical vapor deposition (CVD) systems becomes the first priority. In this paper, we investigated the carbon diffusion behavior involving two kinds of annealing gases, which reduced the nucleation density in different rate. Compared to the time-consuming and hazardous high-pressure H 2 annealing, O 2 annealing represents a more efficient route to reduce the nucleation density down to ∼10 nuclei cm −2 in half an hour. We also proposed a well-defined carbon depletion and refilling mechanism, with which we can precisely gauge the minute carbon concentration inside Cu (less than 10 ppm), and control the nucleation density in a reproducible manner. Moreover, we find that O 2 displays an impressive carbon depletion activity, ∼6 order more efficient than H 2 . The graphene single crystals' quality was confirmed with Raman spectroscopy and field effect transistor (FET) devices.

Semiconductor p–n junctions are the elementary building blocks of most electronic and optoelectronic devices. The need for their miniaturization has fuelled the rapid growth of interest in two-dimensional (2D) materials. However, the performance of a p–n junction considerably degrades as its thickness approaches a few nanometres and traditional technologies, such as doping and implantation, become invalid at the nanoscale. Here we report stable non-volatile programmable p–n junctions fabricated from the vertically stacked all-2D semiconductor/insulator/metal layers (WSe2/hexagonal boron nitride/graphene) in a semifloating gate field-effect transistor configuration. The junction exhibits a good rectifying behaviour with a rectification ratio of 104 and photovoltaic properties with a power conversion efficiency up to 4.1% under a 6.8 nW light. Based on the non-volatile programmable properties controlled by gate voltages, the 2D p–n junctions have been exploited for various electronic and optoelectronic applications, such as memories, photovoltaics, logic rectifiers and logic optoelectronic circuits. Stable, nonvolatile, programmable 2D p–n junctions enable realization of high-performance memories, photovoltaics, logic rectifiers and logic optoelectronic circuits.

Achieving tailorable gated adsorption by tuning the dynamic behavior of a host porous material is of great interest because of its practical application in gas adsorption and separation. Here we devise a unique cation-exchange approach to tune the dynamic behavior of a flexible anionic framework, [Zn2(bptc)(datrz)]− (denoted as MAC-6, where H4bptc = [1,1′-biphenyl]-3,3′,5,5′-tetracarboxylic acid and Hdatrz = 3,5-diamine-1H-1,2,4-triazole), so as to realize the tailorable gated adsorption. The CO2 adsorption amount at 273 K can be enhanced by exchanging the counterion of protonated dimethylamine (HDMA+) with tetraethylammonium (TEA+), tetrabutylammonium (TBA+), and tetramethylammonium (TMA+), where the adsorption behavior is transferred from nongated to gated adsorption. Interestingly, the Pgo for gate-opening adsorption can be further tuned from 442 to 331 mmHg by simply adjusting the ratio of HDMA+ and TMA+. The origin of this unique tunable property, as revealed by X-ray diffraction experiments and stru...

Hexagonal boron nitride (h-BN) is an important member of two-dimensional (2D) materials with a large direct bandgap, and has attracted growing interest in ultraviolet optoelectronics and nanoelectronics. Compared with graphene and graphite, h-BN has weak Raman effect because of the far off-resonance excitation; hence, it is difficult to exploit Raman spectroscopy to characterize important properties of 2D h-BN, such as thickness, doping, and strain effects. Here, we applied stimulated Raman scattering (SRS) to enhance the sensitivity of the E2g Raman mode of h-BN. We showed that SRS microscopy achieves rapid high resolution imaging of h-BN with a pixel dwell time 4 orders of magnitude smaller than conventional spontaneous Raman microscopy. Moreover, the near-perfect linear dependence of signal intensity on h-BN thickness and isotropic polarization dependence allow convenient determination of the flake thickness with SRS imaging. Our results indicated that SRS microscopy provides a promising tool for high-speed quantification of h-BN and holds the potential for vibrational imaging of 2D materials.