Ultrathin 2D Materials Research Articles

Theoretical Prediction of Heterogeneous Integration of Dissimilar Semiconductor with Various Ultra-Thin Oxides and 2D Materials

In this paper, we build a numerical p-n Si/GaAs heterojunction model using quantum-mechanical tunneling theory with various quantum tunneling interfacial materials including two-dimensional (2D) materials such as hexagonal boron nitride (h-BN) and graphene, and ALD-enabled oxide materials such as HfO2, Al2O3, and SiO2. Their tunneling efficiencies and tunneling currents with different thicknesses were systematically calculated and compared. Multiphysics modeling was used with the aforementioned tunneling interfacial materials to analyze changes in the strain under different temperature conditions. Considering the transport properties and thermal-induced strain analysis, Al2O3, among three oxide materials, and graphene in 2D materials are favorable material choices that offer the highest heterojunction quality. Overall, our results offer a viable route in guiding the selection of quantum tunneling materials for a myriad of possible combinations of new heterostructures that can be obtained with an ultra-thin tunneling intermediate layer.

Low‐Energy Oxygen Plasma Injection of 2D Bi2Se3 Realizes Highly Controllable Resistive Random Access Memory

AbstractResistive random access memory (RRAM) based on ultrathin 2D materials is considered to be a very feasible solution for future data storage and neuromorphic computing technologies. However, controllability and stability are the problems that need to be solved for practical applications. Here, by introducing a damage‐less ion implantation technology using ultralow‐energy plasma, the transport mechanisms of space charge limited current and Schottky emission are successfully realized and controlled in RRAM based on 2D Bi2Se3 nanosheets. The memristors exhibit stable resistive switching behavior with a high resistive switching ratio (>104), excellent cycling endurances (300 cycles), and great retention performance (>104 s). The reliability and controllability of Bi2Se3 memory endowed by oxygen plasma injection demonstrate the great potential of this ultralow‐energy ion implantation technology in the application of 2D RRAM.

Two-dimensional coordination polymers containing permethylated motifs - promising candidates for 2D emerging materials. Structural, behavioral and functional particularities

Although three-dimensional metal-organic frameworks (3D MOFs) currently enjoy great attention in the scientific world due to their utility to address challenges concerning energy, environment cleaning and not only, very recently two-dimensional version of these, rather considered two-dimensional coordination polymers (2D CPs), have become of interest from the perspective of their approach as ultra-thin layered 2D materials. Different from the emerging 2D materials that are generally inorganic in nature (graphene or 2D allotropes of various elements, MXenes, etc.), 2D CPs contain both inorganic and organic blocks, offering wider possibilities for structural diversification and area of applicability. However, in the crystallization process, 2D CP layers often stack on the basis of intermolecular interactions, leading to higher dimensional materials, which limit the manifestation of the size-effect properties. Their isolation individually or in multilayers with thickness/surface aspect ratio as small as possible is a challenge that seeks solutions through top-down or bottom-up approaches, each of them having certain limits. An original pathway consists in the use of a permethylated ligand with lower surface energy which, through its orientation in the coordination process to the metal ion, favors the anisotropic growth of the polymer and ensures the shielding of the formed nanosheet structure preventing the establishment of noticeable interlayer interactions. Such unique two-dimensional coordination polymers recently reported by the authors and some new data about them are reviewed here from the perspective of their structural particularities, properties, reactivity, functionality, and reusability/recyclability potential.

Wafer-Scale 2D Hafnium Diselenide Based Memristor Crossbar Array for Energy-Efficient Neural Network Hardware.

Memristor crossbar with programmable conductance could overcome the energy consumption and speed limitations of neural networks when executing core computing tasks in image processing. However, the implementation of crossbar array (CBA) based on ultrathin 2D materials is hindered by challenges associated with large-scale material synthesis and device integration. Here, a memristor CBA is demonstrated using wafer-scale (2-inch) polycrystalline hafnium diselenide (HfSe2 ) grown by molecular beam epitaxy, and a metal-assisted van der Waals transfer technique. The memristor exhibits small switching voltage (0.6V), low switching energy (0.82 pJ), and simultaneously achieves emulation of synaptic weight plasticity. Furthermore, the CBA enables artificial neural network with a high recognition accuracy of 93.34%. Hardware multiply-and-accumulate (MAC) operation with a narrow error distribution of 0.29% is also demonstrated, and a high power efficiency of greater than 8-trillion operations per second per Watt is achieved. Based on the MAC results, hardware convolution image processing can be performed using programmable kernels (i.e., soft, horizontal, and vertical edge enhancement), which constitutes a vital function for neural network hardware.

Field-induced half-metallic phase in epitaxial TcO[formula omitted] bilayer on rutile TiO[formula omitted] surface

For spintronic applications, it is highly desirable to realize 100%-spin-polarized two-dimensional (2D) electron systems in field-controllable epitaxial ultrathin films or 2D materials on semiconductor substrates. Through systematic first-principles investigation, we find that one epitaxial TcO2 bilayer on rutile TiO2 (001) substrate is an antiferromagnet-like narrow-gap semiconductor, and its electronic and magnetic properties can be manipulated through electric field. When electric field reaches 0.03 V/Å, it transits to a half-metallic ferrimagnet with 100% spin polarization. Our analysis indicates that in both phases the magnetization density and the electronic states near the Fermi level originate mainly from the TcO2 bilayer, and across the interface the bond lengths and angles quickly converge to the corresponding values of the bulk TiO2. Therefore, the heterostructure actually hosts a 2D electron system determined by the TcO2 bilayer and the TiO2 substrate. Because the half-metallic phase can be achieved from a nonmetallic phase, such epitaxial 2D electron systems should be usable in spintronic devices.

S defect-rich ultrathin 2D MoS2: The role of S point-defects and S stripping-defects in the removal of Cr(VI) via synergistic adsorption and photocatalysis

In the field of photocatalysis, one focus is on high-performance visible light catalysis. For this study, which follows the defect engineering strategy, ultrathin two-dimensional (2D) S defect-rich MoS2 nanosheets were created in situ by ball-milling MoS2 nanosheets with ascorbic acid and then used for the removal of Cr(VI) from wastewater. The results show that ascorbic acid increases both the specific surface area of MoS2 nanosheets and the concentration of S stripping-defects significantly. Of the samples, D-MoS2-3 (i.e., S defect-rich ultrathin 2D MoS2 nanosheets) exhibited the best Cr(VI) adsorption capacity and photocatalytic activity thanks to its large specific surface area and a high concentration of total S defects (18.5%), 311.1% better than for P-MoS2 (i.e., pristine MoS2 nanosheets) (4.5%). The concentration of S point-defects in D-MoS2-3 is only a little greater than in P-MoS2, but the concentration of S stripping-defects is significantly greater. S point-defects at such a high concentration readily act as recombination centers for photogenerated carriers. By contrast, S stripping-defects that lack dangling Mo-S bonds trap photogenerated holes and add to the separation efficiency of photogenerated electron-hole pairs. As a consequence, the photocatalytic performance of D-MoS2-3 in removing Cr(VI) is significantly better. Given this finding, the present study offers a new design pathway and a reference for the practical application of defect engineering to ultrathin 2D materials.

Nanodots Derived from Layered Materials: Synthesis and Applications.

Layered 2D materials, such as graphene, transition metal dichalcogenides, transition metal oxides, black phosphorus, graphitic carbon nitride, hexagonal boron nitride, and MXenes, have attracted intensive attention over the past decades owing to their unique properties and wide applications in electronics, catalysis, energy storage, biomedicine, etc. Further reducing the lateral size of layered 2D materials down to less than 10nm allows for preparing a new class of nanostructures, namely, nanodots derived from layered materials. Nanodots derived from layered materials not only can exhibit the intriguing properties of nanodots due to the size confinement originating from the ultrasmall size, but also can inherit some unique properties of ultrathin layered 2D materials, making them promising candidates in a wide range of applications, especially in biomedicine and catalysis. Here, a comprehensive summary on the materials categories, advantages, synthesis methods, and potential applications of these nanodots derived from layered materials is provided. Finally, personal insights about the challenges and future directions in this promising research field are also given.

Highly Accurate Thickness Determination of 2D Materials

AbstractDetermining the thickness of two‐dimensional (2D) materials accurately and reliably is highly necessary for multiple investigations, but at the same time it can be quite complex. Most studies in this field measure a topographic map at the edge of the 2D material using an atomic force microscope (AFM), and plot a single‐line cross‐section using the software of the AFM. However, this method is highly inaccurate and can result in high relative errors due to surface roughness and line‐to‐line variability. This is even more important in ultrathin (<4 nm) 2D materials grown by chemical vapor deposition, as these exhibit a larger surface roughness (compared to mechanically exfoliated) due to the high density of local defects. Here it is shown that the thickness of ultrathin 2D materials can be determined statistically with high accuracy and reliability in a very easy way by plotting the histogram height plot. Using this method should enhance the reliability of investigations and research papers in the field of 2D materials.

Vacancy Engineering of Ultrathin 2D Materials for Photocatalytic CO2Reduction

AbstractPhotocatalytic carbon dioxide (CO2) reduction is a sustainable and green strategy for the conversion of CO2into hydrocarbon solar fuels, whereas its large‐scale application is severely restricted by lack of highly effective photocatalysts. Ultrathin 2D materials with tunable electronic structure display great potential towards photocatalytic CO2reduction. However, the photocatalytic performance still remains unsatisfied due to high activation energy of CO2molecules on catalytic sites. To this end, surface vacancy engineering can endow coordinately unsaturated sites as actively catalytic sites for CO2molecules chemisorption and activation. In this review, we focus on vacancy‐engineered ultrathin materials for CO2photoreduction. Different vacancies with classified anion vacancies, cation vacancies, vacancy pairs, voids, and their corresponding role in CO2photoreduction are proposed. The different strategies based on vacancy engineering, including direct modulation of vacancy concentrations, refining vacancy states by heteroatom, and vacancy‐engineered heterostructure, are presented. Finally, the future developments and their associated challenges concerning defective ultrathin 2D materials are discussed.

Ultrathin two-dimensional Fe-doped cobaltous oxide as a piezoelectric enhancement mechanism in quartz crystal tuning fork (QCTF) photodetectors.

An innovative ultrathin two-dimensional (2D) Fe-doped cobaltous oxide (Fe-CoO) coated quartz crystal tuning fork (QCTF) was introduced for the purpose of developing a low-cost photoelectric detector with a simple configuration. The enhancement mechanism of the piezoelectric signal in the ultrathin 2D Fe-CoO-coated QCTF detector is assumed to be the synergetic photocarrier transfer and photothermal effect of ultrathin 2D Fe-CoO. The ultrathin 2D nanosheet structure of Fe-CoO with a large specific surface area can efficiently absorb and convert light into heat in the QCTF, and the photocarrier transfer from the Fe-CoO nanosheet to the electrode of the QCTF contributes to the enhancement in electricity given the shortened diffusion distance of carriers to the surfaces of the 2D nanosheet. Finite element modeling was adopted to simulate the thermoelastic expansion and mechanical resonance of the QCTF with 2D Fe-CoO coating to support experimental results and analyses. Moreover, the effects of 2D Fe-CoO on the performance of QCTF-based photoelectric detectors were investigated. This Letter demonstrates that ultrathin 2D materials have great potential in applications such as costly and tiny QCTF detectors, light sensing, biomedical imaging, and spectroscopy.

Cationic intermediates assisted self-assembly two-dimensional Ti3C2Tx/rGO hybrid nanoflakes for advanced lithium-ion capacitors

Two-dimensional (2D) material MXenes have been intensively concerned in energy-storage field due to these unique properties of metallic-like conductivity, good hydrophilicity and high volumetric capacity. However, the self-restocking of ultra-thin 2D materials seriously hinders these performances, which significantly inhibits the full exploitation of MXenes in the field of energy storage. To solve this issue, a strategy to prepare delaminated Ti3C2Tx (MXene) nanoflakes/reduced graphene oxide (rGO) composites is proposed using the electrostatic self-assembly between positively charged Ti3C2Tx with tetrabutylammonium ion (TBA+) modification and negatively charged graphene. The nanoflakes of Ti3C2Tx/rGO are well dispersed and arranged in a face-to-face structure to effectively alleviate the self-restacking and provide more electroactive sites for accessible of electrolyte ions. The prepared delaminated Ti3C2Tx/rGO anode shows a high reversible capacity up to 1394 mAh g−1 at a current density of 50 mA g−1. Moreover, a lithium-ion capacitor (LIC) was assembled with delaminated Ti3C2Tx/rGO anode and activated carbon (AC) cathode which can exhibit a specific capacity of 70.7 F g−1 at a current density of 0.1 A g−1 and deliver an ultrahigh energy density of 114 Wh kg−1 at a relatively high power density of 3125 W kg−1. These good electrochemical performances demonstrate the potential of delaminated Ti3C2Tx/rGO as an anode material for lithium-ion capacitors.

Engineering of Amorphous Structures and Sulfur Defects into Ultrathin FeS Nanosheets to Achieve Superior Electrocatalytic Alkaline Oxygen Evolution.

Integration of amorphous structures and anion defects into ultrathin 2D materials has been identified as an effective strategy for boosting the electrocatalytic performance. However, the in-depth understanding of the relationship among the amorphous structure, vacancy defect, and catalytic activity is still obscure. Herein, a facile strategy was proposed to prepare ultrathin and amorphous Mo-FeS nanosheets (NSs) with abundant sulfur defects. Benefited from the ultrathin, amorphous nanostructure, and synergy effect of Mo-doping and sulfur defect, the Mo-FeS NSs manifested excellent electrocatalytic activity toward oxygen evolution reaction (OER) in alkaline medium, as shown by an ultralow overpotential of 210 mV at 10 mA cm-2, a Tafel slope of 50 mV dec-1, and retaining such good catalytic stability over 30 h. The efficient catalytic performance for Mo-FeS NSs is superior to the commercial IrO2 and most reported top-performing electrocatalysts. Density functional theory calculations revealed that the accelerated electron/mass transfer over the oxygen-containing intermediates can be attributed to the amorphous structure and sulfur-rich defects caused by structural reconfiguration. Furthermore, the S vacancies could enhance the activity of its neighboring Fe-active sites, which was also beneficial to their OER kinetics. This work integrated both amorphous structures and sulfur vacancies into ultrathin 2D NSs and further systematically evaluated the OER performance, providing new insights for the design of amorphous-layered electrocatalysts.

Absorption spectroscopy gas sensor using a low-cost quartz crystal tuning fork with an ultrathin iron doped cobaltous oxide coating

A proof-of-concept photoelectric detector is proposed for use in low-cost, compact, and highly sensitive gas sensors. This detector is based on a quartz crystal tuning fork (QCTF) coated with an ultrathin two-dimensional (2D) iron doped cobaltous oxide (Fe-CoO) film. The Fe-CoO film enhances the photoelectric conversion efficiency of the detector. This detector is combined with wavelength modulation spectroscopy, and its suitability for trace gas sensing is demonstrated using CH4. A minimum detection limit of 0.88 ppmv is achieved with an integration time of 1 s. This corresponds to a normalized noise equivalent absorption coefficient of 2.2 × 10−10 cm-1·W Hz-1/2. Allan variance analysis is used to evaluate the stability of the proposed system, and the results show that an optimum precision of 20 ppbv can be achieved with an integration time of 220 s. Ultrathin 2D materials may be used to develop new photoelectric detectors that have an ultrawide wavelength response range and high photoelectric conversion efficiency. This would provide flexibility and chemical stability at low cost for applications in harsh environments.

Polymer “Tape”‐Assisted Ball‐Milling Method Fabrication Few‐Atomic‐Layered Bismuth for Improving K+/Na+ Storage

Few‐layered 2D analogs exhibit new physical/chemical properties, leading to a strong research interest and broad areas of application. Recently, lots of methods (such as ultrasonic and electrochemical methods) have already used to prepared 2D materials. However, these methods suffer from the drawbacks of low yield, high cost, or precarious state, which limit the large‐scale applications. Inspired by the famous Scotch tape method, we develop a ball‐milling with polymer “tape” method, fabricating few‐atomic‐layered material, showing the high‐yield, low‐cost, and much stability. As electrode material, ultrathin 2D materials can shorten the ion transfer pathway, contributing to the development of high‐power batteries. Meanwhile, few‐atomic‐layered structure can expose more active sites to increase their capacity, showing special energy storage mechanism. We use the as‐prepared few‐atomic‐layered Bi (FALB) and reduced oxide graphene composites as the anode for potassium/sodium‐ion batteries (KIBs/NIBs). The sample achieves a high reversible capacity of 395 mAh g−1 for KIBs, of which FALB contributes 438 mAh g−1 (higher than the theoretical capacity of Bi, 386 mAh g−1), and it carries outstanding cycle and rate performance in KIBs/NIBs.

Universal Access to Two‐Dimensional Mesoporous Heterostructures by Micelle‐Directed Interfacial Assembly

AbstractTwo‐dimensional (2D) mesoporous heterostructures combining ultrathin nanosheet morphology, periodic porous surface structures, and diverse hybrid compositions have become increasingly important for renewable energy storage and electronics. However, it remains a great challenge to develop a universal method to prepare 2D mesoporous heterostructures. Herein, we report a composite‐micelle‐directed interfacial assembly method to synthesize heterostructures of an ultrathin 2D material covered with mesoporous monolayers assembled on both sides. To demonstrate the concept, we first fabricated a new sandwichlike carbon@MXene@carbon mesoporous heterostructure through the self‐assembly of exfoliated MXene nanosheets and block copolymer F127/melamine‐formaldehyde resin composite micelles and subsequent thermal treatment. Finally, we demonstrate that the carbon@MXene@carbon mesoporous heterostructured nanosheets manifest remarkably enhanced electrochemical performance as a cathode material for lithium–sulfur batteries.

Precisely Controlled Two-Dimensional Rhombic Copolymer Micelles for Sensitive Flexible Tunneling Devices

Nanoscale two-dimensional (2D) organic materials have attracted significant interest on account of their unique properties, which result from their ultrathin and flat morphology. Supramolecular 2D ...

2D Boron Imidazolate Framework Nanosheets with Electrocatalytic Applications for Oxygen Evolution and Carbon Dioxide Reduction Reaction.

Ultrathin 2D materials possess unique properties that translate to enhanced efficiency as electrocatalysts, stimulating research toward methodologies that support their preparation. Herein, a two-step strategy is reported that involves the preparation of the new boron imidazolate framework (BIF-73) which is subsequently utilized as a precursor to yield the crystalline 2D nanosheet material (Fe@BIF-73-NS) via post-synthetic modification. This new electrocatalytic material stabilizes ultra-small (Fe2 O3 ) fragments resulting in an excellent electrocatalytic performance for the oxygen evolution reaction (OER: lower overpotential with 291 mV at the current density of 10 mA cm-2 ) and carbon dioxide reduction reaction (faradaic efficiency of CO reaching 88.6% at -1.8 V vs Ag/AgCl) without the need for noble metals. Additionally, theoretical calculations and microscopy reveal that the superior OER performance can be attributed to the increased exposure of binding sites within the material to which the catalytically active Fe3+ centers are bound through a post-synthetic modification procedure. A red-shift of the Fermi level around the valence band is observed and is proposed to be a result of the aforementioned interactions. This work opens an avenue toward the development of 2D functional metal organic framework nanosheets for energy conversion applications.

Atomic-Scale Imaging of a Free-Standing Monolayer Clay Mineral Nanosheet Using Scanning Transmission Electron Microscopy.

Although aberration-corrected scanning transmission electron microscope (STEM) enables the atomic-scale visualization of ultrathin 2D materials such as graphene, imaging of electron-beam sensitive 2D materials with structural complexity is an intricate problem. We here report the first atomic-scale imaging of a free-standing monolayer clay mineral nanosheet via the annular dark field (ADF) STEM. The monolayer clay nanosheet was stably observed under optimal conditions, and we confirmed that the hexagonal contrast pattern with a pore of ∼4 Å corresponds to the atomic structure of clay mineral that consisted of adjacent Si, Al, Mg, and O atoms by comparison with simulations. The findings offer the usefulness of ADF-STEM techniques for the atomic scale imaging of clay minerals and various 2D materials having electron-beam sensitivity and structural complexity than few-atom-thick graphene analogues.

Defect-Mediated Transport in Self-Powered, Broadband, and Ultrafast Photoresponse of a MoS2/AlN/Si-Based Photodetector

By combining unique properties of ultrathin 2D materials with conventional 3D semiconductors, devices with enhanced functionalities can be realized. Here, we report a self-powered and ultrafast pho...

Ultrasonic-Ball Milling: A Novel Strategy to Prepare Large-Size Ultrathin 2D Materials.

Large-size ultrathin 2D materials, with extensive applications in optics, medicine, biology, and semiconductor fields, can be prepared through an existing common physical and chemical process. However, the current exfoliation technologies still need to be improved upon with urgency. Herein, a novel and simple "ultrasonic-ball milling" strategy is reported to effectively obtain high quality and large size ultrathin 2D materials with complete lattice structure through the introduction of moderate sapphire (Al2 O3 ) abrasives in a liquid phase system. Ultimately numerous high-quality ultrathin h-BN, graphene, MoS2 , WS2 , and BCN nanosheets are obtained with large sizes ranging from 1-20 µm, small thickness of ≈1-3 nm and a high yield of over 20%. Utilizing shear and friction force synergistically, this strategy provides a new method and alternative for preparing and optimizing large size ultrathin 2D materials.