Mechanical Exfoliation Research Articles

Electron Beam‐Irradiated Au/h‐BN/Au Cross‐Point Memristors

Applying 2D materials to build memristors is a promising strategy that has attracted great interest in the semiconductor field in recent years. Although the study of memristors based on chemical vapor deposition (CVD) with lattice defects has been well established, the applications and detailed knowledge of the ones using mechanical exfoliation with good lattice structures are still scarce. Herein, the resistive switching behavior of an array of 3 μm × 3 μm cross‐point Au/25.7 nm mechanically exfoliated h‐BN/Au memristors is investigated after a controllable electron beam doping process. An electron beam irradiation strategy is first proposed to treat h‐BN memristors with intact lattice structure. Devices with moderate electron beam irradiation, which is about 1500 or 2000 mC cm−2, show good resistive switching curves. The memristor device has a repeatable resistive switching behavior, an on/off ratio of 108, a set voltage as low as 5 V, and a retention higher than 1000 s. This strategy provides an alternative method to improve the performance of memristor. Mechanical exfoliated h‐BN preserves the lattice structure, which is an immediate advantage to fabricate memristor using it compared to that fabricated by CVD.

Photoconduction Properties in Tungsten Disulfide Nanostructures.

We reported the photoconduction properties of tungsten disulfide (WS2) nanoflakes obtained by the mechanical exfoliation method. The photocurrent measurements were carried out using a 532 nm laser source with different illumination powers. The results reveal a linear dependence of photocurrent on the excitation power, and the photoresponsivity shows an independent behavior at higher light intensities (400-4000 Wm-2). The WS2 photodetector exhibits superior performance with responsivity in the range of 36-73 AW-1 and a normalized gain in the range of 3.5-7.3 10-6 cm2V-1 at a lower bias voltage of 1 V. The admirable photoresponse at different light intensities suggests that WS2 nanostructures are of potential as a building block for novel optoelectronic device applications.

Atomic Resolution Imaging of Highly Air-Sensitive Monolayer and Twisted-Bilayer WTe2.

Bulk Td-WTe2 is a semimetal, while its monolayer counterpart is a two-dimensional (2D) topological insulator. Recently, electronic transport resembling a Luttinger liquid state was found in twisted-bilayer WTe2 (tWTe2) with a twist angle of ∼5°. Despite the strong interest in 2D WTe2 systems, little experimental information is available about their intrinsic microstructure, leaving obstacles in modeling their physical properties. The monolayer, and consequently tWTe2, are highly air-sensitive, and therefore, probing their atomic structures is difficult. In this study, we develop a robust method for atomic-resolution visualization of monolayers and tWTe2 obtained through mechanical exfoliation and fabrication. We confirm the high crystalline quality of mechanically exfoliated WTe2 samples and observe that tWTe2 with twist angles of ∼5 and ∼2° retains its pristine moiré structure without substantial deformations or reconstructions. The results provide a structural foundation for future electronic modeling of monolayer and tWTe2 moiré lattices.

Bismuth telluride iodide monolayer flakes with nonlinear optical response obtained via gold-assisted mechanical exfoliation

Bismuth telluride iodide (BiTeI) is a layered material known as Rashba semiconductor, which is promising for several applications such as spintronics, nonlinear optics, and energy-related devices. Due to its layered structure and low cleavage energy, BiTeI is particularly suitable for producing two-dimensional (2D) BiTeI crystals, allowing for the development of devices at the nanoscale. However, traditional methods of mechanically exfoliating BiTeI have encountered significant challenges. To address this, we used the so-called gold-assisted mechanical exfoliation to successfully obtain 2D BiTeI flakes. Through extensive characterization employing a range of techniques including Raman spectroscopy and second-harmonic generation (SHG) measurements, we examine the morphological, structural, and chemical properties of the gold-assisted mechanically exfoliated BiTeI flakes. We confirm their crystalline nature while also providing information about strain and defects present within the flakes. Moreover, SHG measurements revealed a significant nonlinear optical response, and the non-centrosymmetric structure observed in these flakes can be attributed to quantum confinement effects and the absence of phase-matching requirements typically found in bulk nonlinear crystals.

Shearing-enhanced mechanical exfoliation with mild-temperature pretreatment for cathode active material recovery from spent LIBs

The conventional approaches for recovering valuable metals from spent lithium-ion batteries (LIBs) suffer from heavy dependence on chemical reagents, high energy consumption, and low recovery efficiencies. In this study, we developed a shearing-enhanced mechanical exfoliation combined with mild-temperature pretreatment (SMEMP) method. The method achieves high-efficiency exfoliation of the cathode active materials that remain strongly adhered to polyvinylidene fluoride after it melts during mild pretreatment. The pretreatment temperature was decreased from 500–550 °C to 250 °C, the duration was decreased to 1/4–1/6 of the traditional pretreatment duration, and the exfoliation efficiency and product purity reached 96.88% and 99.93%, respectively. Despite the weakening thermal stress, the cathode materials could be exfoliated by strengthened shear forces. Compared with other traditional methods, the superiority of this method in temperature reduction and energy saving was established. The proposed SMEMP method is environmentally friendly and economical, and it offers a new route for the recovery of cathode active materials from spent LIBs.

In English

In presented work we have reported that polyvinylpyrrolidone (PVP) is suitable organic solvent for mechanical exfoliation of crystalline graphite to graphene in aqueous solution through kitchen blender technique. Morphological, structural and optical, properties of graphene samples were characterized by scanning electron microscopy, Raman scattering and UV-Visible spectroscopy. Scanning electron microscopy demonstrates folded morphology of graphene flakes with ~4 nm thickness. Raman scattering was revealed the high-quality graphene with low levels of defects after the exfoliation process of graphite/PVP dispersions. The G band at ~1582 cm−1 on Raman spectra is related with stretching vibrations of carbon atoms. The broadening of the G band is not observed, that indicated on absence of the basal plane defects in the graphene structure, which can be introduced during exfoliation by the kitchen blender. The ratio of the intensity of 2D and G bands equal to 1.66 (I2D/IG>1) and the value of 2D band full width at half maximum is 79.88 cm–1, indicated a few-layer graphene (FLG) structure. The defects quantity identified by the intensity ratio of the D and G bands, ID/IG is 0.18, significantly lower than that reported for graphene oxide with basal plane defects (> 1) and for sonication dispersed graphene (~ 0.6). The graphene dispersions were characterized by UV–Vis spectroscopy, minimum transmittance observed at around 270 nm is attributed to the π→π* transitions of aromatic C–C bonds in graphene. The optical transmittance minimum and the overall high transmittance decreasing values observed between 270 and 800 nm strongly suggest the presence of two-dimensional material in the dispersion, thus confirming the successful exfoliation of graphite to graphene by blending. We demonstrate the simple and nontoxic method to product high-quality FLG sheets, free from defects, using a kitchen blender and eco-friendly polyvinylpyrrolidone as an organic exfoliant.

Impact performance on industrial scalable graphene reinforcement composites

Carbon fiber reinforced epoxy laminates are widely used in the design and manufacturing of aeronautical structures. Among different manufacturing procedures, resin transfer moulding (RTM) has emerged as a cost-effective alternative to prepregs manufactured in an autoclave. However, the laminate obtained by RTM exhibits approximately 20% lower mechanical properties. To address this limitation, the addition of nanoparticles has been suggested as a solution. In this study, graphene particles were obtained by in-situ mechanical exfoliation in the resin and directly injected into the mould. The major achievement of this study is that the entire manufacturing process was performed at an industrial scale using the same tooling used to produce aircraft structures. Laminates with varying concentrations and dispersion times were manufactured using RTM and subjected to compression after impact tests, resulting in a 7.6% improvement. Furthermore, analysis through SEM techniques revealed the role of graphene in the bear capacity of the laminates. Finally, to demonstrate the feasibility of the proposed methodology in an industrial environment, a real aircraft structure was manufactured with graphene, without any modifications to the actual industrial process.

Graphene Oxide Paper Manipulation of Micro-Reactor Drops.

Digital microfluidics, which relies on the movement of drops, is relatively immune to clogging problems, making it suited for micro-reactor applications. Here, graphene oxide paper of 100 μm thickness, fabricated by blade coating sedimented dispersions onto roughened substrates, followed by drying and mechanical exfoliation, was found to be relatively free of cracks and curling. It also exhibited high wettability and elasto-capillary characteristics. Possessing low enough stiffness, it could rapidly and totally self-wrap water drops of 20 μL volume placed 2 mm from its edge when oriented between 0 and 60° to the horizontal. This complete wrapping behavior allowed drops to be translated via movement of the paper over long distances without dislodgement notwithstanding accelerations and decelerations. An amount of 2 drops that were wrapped with separate papers, when collided with each other at speeds up to 0.64 m/s, were found to eschew coalescence. This portends the development of robust digital microfluidic approaches for micro-reactors.

The Progress on Magnetic Material Thin Films Prepared Using Polymer-Assisted Deposition.

Polymer-assisted deposition (PAD) has been widely used in the preparation of high-quality oxides and sulfides for basic research and applications. Specifically, diverse PAD-prepared magnetic material thin films such as ZnO, Ga2O3, SrRuO3, LaCoO3, LaMnO3, Y3Fe5O12, MoS2, MoSe2, and ReS2 thin films have been grown, in which thickness-dependent, strain-modulated, doping-mediated, and/or morphology-dependent room-temperature ferromagnetism (RTFM) have been explored. Inspired by the discovery of intrinsic low-temperature FM in two-dimensional (2D) systems prepared using mechanical exfoliation, the search for more convenient methods to prepare 2D ferromagnetic materials with high-temperature FM has seen explosive growth, but with little success. Fortunately, the very recent synthesis of 2D NiO by PAD has shed light on this challenge. Based on these abovementioned developments, the difficulties of PAD when preparing a-few-nanometer single-crystalline materials and the opportunities in PAD for novel materials such as chiral magnetic soliton material Cr1/3NbS2 are discussed.

Layer‐Dependent Optical Modulation and Field‐Effect‐Transistor in Two‐Dimensional 4H‐SnS2 Layers

AbstractTwo–dimensional (2D) 4H‐polytype tin disulfide (SnS2) flakes are synthesized using the chemical vapor transport technique. The weak Van der Waals force between the 2D SnS2 layers offers an easy exfoliation of flakes down to a bilayer of thickness ≈2.02 (±0.1) nm using a mechanical exfoliation technique. The optical and field effect transistor (FET) characteristics of the exfoliated 2D 4H‐SnS2 layers are studied. The exfoliated layers are used to fabricate the ≈13‐layered SnS2 FET. The 4H‐SnS2 exhibits a high on/off ratio of ≈106 and mobility ≈1–4 cm2 V−1 s−1. The low mobility of the 4H‐SnS2 FET devices shows an insulating state concordant with the 2D Motts variable range hopping mechanism at varying temperatures. Moreover, it is found that the optical bandgap of the 2D SnS2 single‐crystal layers is largely widened for the bilayers and tri‐layers. The optical bandgap energies vary in the range of 2.56–1.99 eV. The significant alteration in bandgap energies of ≈0.57 eV offers downscaling of the 2D nanoscale semiconducting devices. Such layer‐sensitive changes in optical transmittance, absorbance, and bandgap energies are reflected in Commission Internationale de L'Eclairage (CIE) chromaticity, showing the distinct color of transmittance through various 2D 4H‐SnS2 layers.

Optimizing the efficiency of solar thermal collectors and studying the effect of particle concentration and stability using nanofluidic analysis

The emission of greenhouse gases is widely acknowledged as the primary driver of global warming. The adoption of renewable energy sources is paramount to address the dependence on fossil fuels, which contribute significantly to this issue and account for 84.3% of current energy production. Solar thermal energy stands out as a prominent option, representing 54.1% of the world's solar energy derived from solar collectors. However, solar thermal energy encounters challenge due to the suboptimal thermal properties of the liquids used in these collectors. Incorporating particles into the liquids offers a potential solution to enhance absorption and thermal properties. Nanofluids, formed by reducing solid particles to nanoscale dimensions, provide an avenue for improvement. This study aimed to produce an Ag nanofluid through mechanical exfoliation and assess its impact on radiation absorption compared to a GO nanofluid. Under a simulated power of 1 unit, the Ag nanofluid demonstrated temperature differences of 4 to 7°C, while pure water showed no significant deviation. Moreover, the evaporation efficiency of the Ag nanofluid reached up to 40.8% for concentrations of 200 and 500 ppm, compared to 28.6% for pure water. These findings highlight the potential of Ag nanofluid as a promising option for direct absorption solar collectors, owing to its cost-effectiveness, low toxicity, and similar benefits to graphene. Incorporating nanofluids, particularly the Ag nanofluid produced through mechanical exfoliation, can significantly enhance the efficiency of direct absorption solar collectors.

Air-stable, all-dry transferred ReS2/GaSe heterostructure-based NO2 gas sensor

Two-dimensional (2D) materials have gained considerable attention in chemical sensing owing to their naturally high surface-to-volume ratio. However, the poor response time and incomplete recovery restrict their application in practical, high performance gas sensors. In this work, we fabricated air-stable ReS2/GaSe heterostructure-based NO2 gas sensors with excellent gas sensing response, recovery, selectivity and a low limit of detection (LOD) toward nitrogen dioxide (NO2). The ReS2/GaSe heterostructure was prepared via mechanical exfoliation and an all-dry transfer method. Before the sensing measurements, temperature-dependant transport measurements were carried out. The Schottky Barrier Height (SBH) of the ReS2/GaSe heterostructure was calculated and the corresponding transport mechanisms were discussed. The fabricated gas sensors showed a significant response enhancement with full reversibility toward ppm-level NO2 (response of ∼17% at 3 ppm, a LOD of ∼556 ppb) at an operating temperature of (33 °C). In particular, the total response and recovery time of the ReS2/GaSe was revealed to be less than 4 min (∼38 s and ∼174 s, respectively) for the 250 ppm concentration, which is one of the best response and recovery time toward ppm-level NO2. The excellent sensing performances and recovery characteristics of the ReS2/GaSe structure are attributed to its efficient charge separation, unique interlayer coupling and desirable band alignments. This atomically thin, ultrasensitive gas sensor that operates at room temperature is a strong technological contender to conventional metal oxide gas sensors, which often require elevated temperatures.

Bottom‐Up Synthesis of CoxSn1−xS Nanosheets: A Ferromagnetic and Photoconductive Semiconductor

Abstract2D ferromagnetic semiconductors are key to next‐generation spintronic devices in the post‐Moore era. The combination of ferromagnetic and optoelectronic properties offers exciting opportunities for advanced multifunctional devices in spin‐optoelectronic applications. Herein, the authors synthesize 2D van der Waals (vdW) CoxSn1‐xS with ferromagnetism and photoresponse through a bottom‐up reaction, which has a high yield compared to typical mechanical exfoliation. Ferromagnetic ordering is realized in 2D vdW semiconductor SnS by Co doping at the Sn sites. Magnetic properties are thoroughly studied at different doping concentrations, and first‐principles calculations are further performed to reveal the magnetism origin and spin interactions. In particular, a low Gilbert damping of 1.69 × 10−3 is obtained in vdW CoxSn1−xS through ferromagnetic resonance. In addition, photodetectors based on CoxSn1−xS quantum dots are demonstrated. These studies establish a promising semiconductor with both ferromagnetic ordering and photoelectric response, which provides unprecedented opportunities in spintronic‐photonic integrated applications.

Effect of Large-Area Exfoliation and Etching on the Electrical Behavior of Transition-Metal Dichalcogenide Field-Effect Transistors

Large-area layer transfer was used to fabricate multilayer transition-metal dichalcogenide (TMDC) thin-film transistor (TFT) arrays with areas of (40 × 40 μm2) from single-crystal flakes. The effects of plasma etching of the TFT backchannel surface and bulk defects in the layers on the electrical performance and stability of the n-channel depletion-mode TFTs were investigated. Few-layer structures (∼3 monolayers) were fabricated using a dry etching process to thin multilayer (∼60–90 nm thick) TMDC structures. The etching improved the threshold voltage of the TFTs, resulting in a positive threshold voltage shift of +40 V after etching the backchannel, correlating to a bulk trap density of approximately 1 × 1016 cm–3 eV–1 per monolayer. Etching the MoS2 surface resulted in a threshold voltage shift of 0.2 V per nanometer of MoS2 removed (for MoS2 thicknesses >15 nm). For MoS2 layers etched to <15 nm, the threshold voltage changed to ∼1.8 V per nanometer. An observed degradation of the carrier transport and electrical stability of these samples were found to be due to the proximity of the etched surface approaching the active channel region of the device. The results reveal the performance trade-offs of fabricating large-area arrays of few-layer TMDC TFTs using a mechanical exfoliation and dry etching approach.

Enhanced photocatalytic activity of liquid phase exfoliated WS2 nanosheets

Due to their large surface area and novel electronic properties, tungsten disulfide (WS2) nanosheets are promising materials for potential photocatalysis applications. The fabrication of large-scale WS2 nanosheets is a fundamentally important step to realize their applications. The liquid phase exfoliation method, which involves the mechanical exfoliation of layered materials in a solvent, has proven to be a feasible technique for fabricating nanosheets with high yields. Here, we systematically examine the optimization of the fabrication of WS2 nanosheets by exfoliating the WS2 powder in isopropyl alcohol, a low boiling point solvent, using a low-power probe sonicator and ball milling method. We show that prolonged sonication produces a concentration as high as 0.64 mg/ml, and the same concentration can be fabricated in less than half the time if the bulk powder is ball milled prior to the sonication. An atomic force microscopy study reveals that the proportions of monolayers and bilayers can be significantly increased if the nanosheet dispersions are centrifuged at a higher speed or sonicated for a long time. The photodegradation measurements reveal that the decomposition rate of methyl orange with nanosheets is at least 10% higher than that with WS2 powder, suggesting that the photocatalytic activity of WS2 can be enhanced if it is reduced to nanosheets.

Patching sulfur vacancies: A versatile approach for achieving ultrasensitive gas sensors based on transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) garner significant attention for their potential to create high-performance gas sensors. Despite their favorable properties such as tunable bandgap, high carrier mobility, and large surface-to-volume ratio, the performance of TMDCs devices is compromised by sulfur vacancies, which reduce carrier mobility. To mitigate this issue, we propose a simple and universal approach for patching sulfur vacancies, wherein thiol groups are inserted to repair sulfur vacancies. The sulfur vacancy patching (SVP) approach is applied to fabricate a MoS2-based gas sensor using mechanical exfoliation and all-dry transfer methods, and the resulting 4-nitrothiophenol (4NTP) repaired molybdenum disulfide (4NTP-MoS2) is prepared via a sample solution process. Our results show that 4NTP-MoS2 exhibits higher response (increased by 200 %) to ppb-level NO2 with shorter response/recovery times (61/82 s) and better selectivity at 25 °C compared to pristine MoS2. Notably, the limit of detection (LOD) toward NO2 of 4NTP-MoS2 is 10 ppb. Kelvin probe force microscopy (KPFM) and density functional theory (DFT) reveal that the improved gas sensing performance is mainly attributed to the 4NTP-induced n-doping effect on MoS2 and the corresponding increment of surface absorption energy to NO2. Additionally, our 4NTP-induced SVP approach is universal for enhancing gas sensing properties of other TMDCs, such as MoSe2, WS2, and WSe2.

A family of two-dimensional semiconductors with transition metal Kagome lattice, large power factor and ultralow lattice thermal conductivity

Two-dimensional (2D) materials hold promising application potential in the future, so it is worthy to explore new 2D materials with fascinating structures and functionals. Here we predict a series of 15 stable 2D semiconductors, A2B3C4 (A = K, Rb, Cs; B = Ni, Pd, Pt; C = S, Se), using first-principles calculations. The feasibility of mechanical exfoliation from their bulk phases has been confirmed by low cleavage energies. Interestingly, a perfect Kagome lattice formed by transition metal Ni/Pd/Pt atoms have been revealed in these fifteen monolayers. All A2B3C4 monolayers exhibit indirect band features (1.82 eV to 2.76 eV). More encouragingly, and it is theoretically proven, based on constant relaxation time (CRT) approximation, that the A2B3C4 monolayers possess large n-type power factors (1.77 ∼ 2.10 × 103 μWm-1K−2), which is mainly contributed from the loop formed by the conduction band minima in different directions in energy band structure. Furthermore, ultralow lattice thermal conductivity (0.20 Wm-1K−1 to 3.04 Wm-1K−1) has been confirmed in 2D A2B3C4 at room temperature. Finally, the thermoelectric figure of merit (ZT) of A2B3C4 monolayers has been evaluated based on CRT approximation. These interesting findings make A2B3C4 monolayers promising candidates for low-dimensional electronic devices.

High-Throughput Mechanical Exfoliation for Low-Cost Production of van der Waals Nanosheets.

A method is presented for scaling up the production of flakes of van der Waals materials via mechanical exfoliation. Using a roll-to-roll setup and an automatized, massive parallel exfoliation process, adhesive tapes with a high density of nanosheets of van der Waals materials are produced. The technique allows for obtaining a good trade-off between large lateral size and excellent area scalability, while also maintaining low cost. The potential of the method is demonstrated through the successful fabrication of field effect transistors and flexible photodetectors in large batches. This low-cost method to produce large area films out of mechanically exfoliated flakes is very general, and it can be applied to a variety of substrates and van der Waals materials and, moreover, it can be used to combine different van der Waals materials on top of each other. Therefore, it is believed that this production method opens an interesting avenue for fabrication of low-cost devices while maintaining a good scalability and performance.

Sonochemistry of Liquid-Metal Galinstan toward the Synthesis of Two-Dimensional and Multilayered Gallium-Based Metal-Oxide Photonic Semiconductors.

The scientific field of two-dimensional (2D) nanostructures has witnessed tremendous development during the last decade. To date, different synthesis approaches have been developed; therefore, various exceptional properties of this family of advanced materials have been discovered. It has recently been found that the natural surface oxide films of room-temperature liquid metals is an emerging platform for the synthesis of novel types of 2D nanostructures with numerous functional applications. However, most of the developed synthesis techniques for these materials are based on the direct mechanical exfoliation of 2D materials as research targets. This paper reports a facile and functional sonochemical-assisted approach for the synthesis of 2D hybrid and complex multilayered nanostructures with tunable characteristics. In this method, the intense interaction of acoustic waves with microfluidic gallium-based room-temperature liquid galinstan alloy provides the activation energy for synthesis of hybrid 2D nanostructures. The microstructural characterizations reveal the impact of sonochemical synthesis parameters, including the processing time and composition of the ionic synthesis environment, on the growth of GaxOy/Se 2D hybrid structures and InGaxOy/Se multilayered crystalline structures with tunable photonic characteristics. This technique shows promising potential for synthesis of various types of 2D and layered semiconductor nanostructures with tunable photonic characteristics.

Micro-Photoluminescence spectroscopy of two distinct types of WS2 monolayers using fs-Ti: Sapphire laser under vacuum and ambient conditions and multivariate calibration models

We utilized Micro-Photoluminescence (μPL) and time-resolved photoluminescence (TRPL) spectroscopies with a femtosecond laser (440 nm) to assess the optical properties of two types of WS2 monolayers (MLs) obtained through mechanical exfoliation of different crystals. Based on cryogenic μPL spectra (4 K), the monolayers were classified as low-defect-concentration (LDC) and high-defect-concentration (HDC). By subjecting the MLs to laser excitation at various photon fluxes under ambient and vacuum conditions at room temperature, we characterized their optical properties. The spectra exhibited two main regions corresponding to optical transitions of neutral excitons (X) and trions (T). Notably, two distinct regimes were observed for quasi-particle decay: X-X annihilation at low photon fluxes and Mott transition at high photon fluxes. The photoluminescence quantum yield (PLQY) was measured under ambient and vacuum conditions, with the presence of physisorbed O2 on the ML surface influencing the results. Additionally, we employed a novel analytical approach combining PL and TRPL spectra with chemometric methods for the regression analysis of the two types of WS2 MLs. Utilizing partial least squares regression and principal component regression models, we estimated the decay times of X and T quasi-particles based on the spectral datasets.