Two-dimensional Layered Materials Research Articles

Layer-dependent fracture strength of few-layer WS2 induced by interlayer sliding: a molecular dynamics study

Understanding the relationship between interlayer interactions and the mechanical properties and behaviors of two-dimensional layered materials is critical in the development of related nanodevices. Nevertheless, it is still challenging due to difficulties in experiments. In this work, nanoindentation simulations on few-layer WS2 were conducted by varying the tip radius, suspended membrane radius, and membrane size using a molecular dynamics simulation. Consistent with our previous experimental results, few-layer WS2 exhibited a layer-dependent reduction in fracture strength owing to the uneven stress distribution among individual layers induced by interlayer sliding under out-of-plane deformation. Furthermore, apparent curve hysteresis was observed due to interlayer sliding in the supported region when a large tip radius and membrane radius were employed. However, instead of the supported part, the interlayer sliding within the suspended part resulted in reduced fracture strength with the increase of layer number. These findings not only provide an in-depth comprehension of the influence of interlayer sliding on fracture strength of few-layer WS2, but also suggest that the role of interlayer interactions should be seriously considered during nanodevice design.

Alternately Intercalated Superstructure of MoS2 and C Monolayers for High-Performance Sodium-Ion Energy Storage and Efficient Removal of Cr(VI)

Two-dimensional layered materials with a rational design have displayed great promise for applications of energy storage and removal of Cr(VI) from an aqueous environment. Herein, we report an ultrathin-layered MoS2-based hybrid with an interlayer-expanded structure by inserting a N-doped carbon layer into adjacent MoS2 monolayers. The superstructure not only shortens the Na+ diffusion pathway and provides abundant channels for Na+ diffusion but also elevates the electrical conductivity of the material significantly. When applied as a negative electrode in sodium-ion batteries (SIBs), the as-prepared MoS2/carbon hybrid delivers an outstanding cycle performance of 338.31 mAh g–1 at 1000 mA g–1 after 3000 cycles. In addition, due to the abundant pore structure and adsorption sites, the composite delivers an excellent adsorption performance of 128.80 mg g–1 for Cr(VI). This study may inspire developments of SIBs and adsorbent materials.

Visualizing Line Defects in non-van der Waals Bi2O2Se Using Raman Spectroscopy.

Atomic-layered materials, such as high-quality bismuth oxychalcogenides, which are composed of oppositely charged alternate layers grown using chemical vapor deposition, have attracted considerable attention. Their physical properties are well-suited for high-speed, low-power-consumption optoelectronic devices, and the rapid determination of their crystallographic characteristics is crucial for scalability and integration. In this study, we introduce how the crystallographic structure and quality of such materials can be projected through Raman spectroscopy analysis. Frequency modes at ∼55, ∼78, ∼360, and ∼434 cm-1 were detected, bearing out theoretical calculations from the literature. The low-frequency modes positioned at 55 and 78 cm-1 were activated by structural defects, such as grain boundaries and O-rich edges in the Bi2O2Se crystals, accompanied by sensitivity to the excitation energy. Furthermore, the line defects at ∼55 cm-1 exhibited a strong 2-fold polarization dependence, similar to graphene/graphite edges. Our results can help illuminate the mechanism for activating the Raman-active mode from the infrared active mode by defects, as well as the electronic structures of these two-dimensional layered materials. We also suggest that the nanoscale width line defects in Bi2O2Se can be visualized using Raman spectroscopy.

MoS2 hybrid integrated micro-ring resonator phase shifter based on a silicon nitride platform

We demonstrate a low-power, compact micro-ring phase shifter based on hybrid integration with atomically thin two-dimensional layered materials, and experimentally establish a low-loss silicon nitride platform. Using a wet transfer method, a large-area few-layer MoS2 film is hybrid integrated with a micro-ring phase shifter, leading to a tuning efficiency of 5.8 pm V-1 at a center wavelength of 1545.294 nm and a half-wave-voltage-length product as low as 0.09 V cm. Our device is designed to provide a hybrid-integration-based active phase modulation scheme for integrated optical communication networks with large-cross-section silicon nitride waveguides.

Emergent Fabry-Pérot Interference for Light-Matter Interaction in van der Waals WS2/SiP2 Heterostructures.

Fabry-Pérot interference plays an important role in modulating the spectral intensity of optical response originating from light-matter interactions. Examples of such interference occurring in the substrate as the resonating cavity have been demonstrated and probed by two-dimensional layered materials. Similarly, the Fabry-Pérot interference can occur and modulate the optical response in the heterostructure; however, this remains elusive. Herein, we observe the Fabry-Pérot interference on photoluminescence (PL) and Raman spectra in monolayer WS2/SiP2 heterostructures by varying the thickness of bottom SiP2 from 2 to 193 nm, which serves as the Fabry-Pérot cavity. Both the intensities of the PL spectra and the E2g1 Raman mode of WS2/SiP2 heterostructures first decrease to almost zero while displaying an interference increase at a SiP2 thickness of 75 nm. Our findings clearly demonstrate the Fabry-Pérot interference in the optical response of heterostructures, providing crucial information to optimize the optical response and paving the way toward photodetector applications.

L-lysine functionalized Ti3C2Tx coated polyurethane sponge for high-throughput oil–water separation

Creating an efficient method that can provide simple, practical and high-throughput separation of oil–water mixtures has proved extremely challenging. Two-dimensional layered Ti3C2Tx MXene material with excellent hydrophilicity has attracted substantial attention for oil–water separation. However, conventional Ti3C2Tx separation membranes are susceptible to contamination by oil due to its relatively high adhesion to oil, hindering its practical applications in high-efficiency oil–water separation. To make Ti3C2Tx have underwater superoleophobicity and low oil adhesion, the surface of Ti3C2Tx can be modified by introducing hydrophilic groups. Herein, L-lysine functionalized Ti3C2Tx (Ti3C2Tx-Ly) was firstly prepared by decorating L-lysine on Ti3C2Tx through electrostatic interaction and hydrogen bonding. A facile dip-coating method is then used to coat polyurethane (PU) sponge with Ti3C2Tx-Ly, obtaining a superhydrophilic/underwater superoleophobic Ti3C2Tx-Ly @PU sponge. Owing to the excellent underwater superoleophobicity resulting from hydrophilic groups (–OH、–NH3) on the surface of Ti3C2Tx-Ly nanosheets and the enlarged interlayer spacing of Ti3C2Tx, the Ti3C2Tx-Ly @PU sponges demonstrated outstanding separation performance for a series of oil–water mixtures. Specifically, the Ti3C2Tx-Ly @PU sponge exhibits low oil adhesion and superhydrophilic (WCA = 0°)/submerged superoleophobic (UOCA > 155°) properties. The Ti3C2Tx-Ly @PU sponges displayed the best separation performance for cyclohexane/water mixture, and the maximum permeation flux is up to 221892 L m–2 h–1, accompanied by a remarkable separation efficiency of higher than 99.2% after 100 cycles. The permeation flux has exceeded most of the previous reported multifunctional separation membranes. In addition, the Ti3C2Tx-Ly @PU sponge also showed excellent separation performance for high-density oil dichloromethane/water with a separation efficiency over 99.4%. Of note, the Ti3C2Tx-Ly @PU sponge features excellent chemical stability to corrosive media, such as acidic, alkaline, and high salt solutions, possessing great potential for real-world applications. Furthermore, the as-prepared Ti3C2Tx-Ly @PU sponge exhibits rapid and efficient separation of oil-water mixtures through a fully gravity-driven process, which makes it a good candidate for industrial oil-contaminated water treatment and oil spill clean-up, and also provides new insights into the design and development of functional Ti3C2Tx through L-lysine modification. Therefore, this work reports a facile and promising way to fabricate scalable superhydrophilic/underwater superoleophobic sponge material for highly efficient oil–water separation.

Selective Detection of Carbon Monoxide on P-Block Doped Monolayers of MoTe2.

CO and CO2 are among the most commonly monitored gases. However, the currently available semiconductor sensors require heating to ∼400 °C in order to operate effectively. This increases the power demand and shortens their lifespan. Consequently, new material prospects are being investigated. The adoption of novel two-dimensional layered materials is one of the pursued solutions. MoS2 and MoTe2 sheets have already been shown sensitive to NO2 and NH3 even at room temperature. However, their response to other compounds is limited. Hence, this work investigates, by employing density functional theory (DFT) calculations, the doping of Al, Si, P, S, and Cl atoms into the Te vacancy of MoTe2, and its impact on the sensing characteristics for CO and CO2. The computations predict that P doping significantly enhances the molecule-sheet charge transfer (up to +436%) while having only a little effect on the adsorption energy (molecular dynamics show that the molecule can effectively diffuse at 300 K). On the other hand, the doping has a limited impact on the adsorption of CO2. The relative (CO/CO2) response of P-doped MoTe2 is 5.6 compared to the 1.5 predicted for the pristine sheet. Thus, the doping should allow for more selective detection of CO in CO/CO2 mixtures.

Nonlinear saturable properties of indium selenide film fabricated by molecular beam epitaxy method in near infrared region and Q-switched laser performance for Nd:GdYNbO laser

Two-dimensional layered materials have caught considerable attention for their intrinsic value. Much effort has been made to excavate novel 2D materials with more distinctive characteristics as optical modulators. Single-crystal indium selenide (In2Se3) film with few layers fabricated by molecular beam epitaxy method, for the first time, is employed in this study as a saturable absorber for an Nd3+ doped Gd:GdYNbO infrared laser. Our experimental results demonstrate that the high-quality In2Se3 film achieves a desirable modulation depth of 18.4%, but only 5.9% for the LPE one. The shortest pulse duration and maximum single pulse energy of In2Se3 film are both much better than those of MoS2 and WS2 films in the same laser system. It further confirms the distinguishing properties of the MBE-growth In2Se3 film for optical modulation in infrared region.

Epitaxial Growth of GaAs Nanowires on Synthetic Mica by Metal-Organic Chemical Vapor Deposition.

The epitaxial growth of III-V nanowires with excellent optoelectronic properties on low-cost, light-weight, and flexible substrates is a key step for the design and engineering of future optoelectronic devices. In our study, GaAs nanowires were grown on synthetic mica, a two-dimensional layered material, via vapor-liquid-solid growth using metal-organic chemical vapor deposition. The effect of basic epitaxial growth parameters such as temperature and V/III ratio on the vertical yield of the nanowires is investigated. A vertical yield of over 60% is achieved at an optimum growth temperature of 400 °C and a V/III ratio 18. The structural properties of the nanowires are investigated using various techniques including scanning electron microscopy, high-resolution transmission electron microscopy, and high-angle annular dark-field imaging. The vertical nanowires grown at a low temperature and a high V/III ratio are found to have a zincblende phase with a [111] B polarity. The optical properties are investigated by photoluminescence (PL) and time-resolved PL measurements. First-principles electronic structure calculations within the framework of density functional theory elucidate the van der Waals nature of the nanowire/mica interface. Our results also show that these nanowires can be easily lifted off the bulk 2D mica template, providing a pathway for flexible nanowire devices.

Highly Sensitive NO2 Detection by TVS-Grown Multilayer MoS2 Films

Two-dimensional layered materials have been investigated for sensor applications over the last decade due to their very high specific surface area and excellent electrical characteristics. Although grain boundaries are inevitably present in polycrystalline-layered materials used for real applications, few studies have investigated their effects on sensing properties. In this study, we demonstrate the growth of two distinct MoS2 films that differ in grain size by means of chemical vapor deposition (CVD) and thermal vapor sulfurization (TVS) methods. Transistor-based sensors are fabricated using these films, and their NO2 sensing properties are evaluated. The adsorption behavior of NO2 on MoS2 is considered in terms of the Langmuir isotherm, and the experimental results can be well fitted by the equation. The CVD-grown film exhibits electrical properties 1–2 orders of magnitude superior to those of the TVS-grown one, which is attributed to the large grain size of the CVD-grown film. In contrast, the sensitivity to NO2 is unexpectedly found to be higher in the TVS-grown film and is of the same order of a previously reported record value. Transmission electron microscopy observations suggest that the TVS-grown film consists of multiple rotationally oriented grains that are connected by mirror twin grain boundaries. Theoretical calculation results reveal that the adsorption of NO2 on the grain boundary that we modeled is equal to that on the ideal basal plane surface of MoS2. In addition, the porous structure in the TVS-grown film may also contribute to enhancing the sensor response to NO2. This study suggests that a highly sensitive MoS2 sensor can also be fabricated by using a polycrystalline film with small grain size, which can possibly be applied to other two-dimensional materials.

Research progress of neuromorphic devices based on two-dimensional layered materials

In recent years, the development of artificial intelligence has increased the demand for computing and storage. However, the slowing down of Moore’s law and the separation between computing and storage units in traditional von Neumann architectures result in the increase of power consumption and time delays in the transport of abundant data, raising more and more challenges for integrated circuit and chip design. It is urgent for us to develop new computing paradigms to meet this challenge. The neuromorphic devices based on the in-memory computing architecture can overcome the traditional von Neumann architecture by Ohm’s law and Kirchhoff’s current law. By adjusting the resistance value of the memristor, the artificial neural network which can mimic the biological brain will be realized, and complex signal processing such as image recognition, pattern classification and decision determining can be carried out. In order to further reduce the size of device and realize the integration of sensing, memory and computing, two-dimensional materials can provide a potential solution due to their ultrathin thickness and rich physical effects. In this paper, we review the physical effects and memristive properties of neuromorphic devices based on two-dimensional materials, and describe the synaptic plasticity of neuromorphic devices based on leaky integrate and fire model and Hodgkin-Huxley model in detail, including long-term synaptic plasticity, short-term synaptic plasticity, spiking-time-dependent plasticity and spiking-rate-dependent plasticity. Moreover, the potential applications of two-dimensional materials based neuromorphic devices in the fields of vision, audition and tactile are introduced. Finally, we summarize the current issues on two-dimensional materials based neuromorphic computing and give the prospects for their future applications.

A first-principles prediction of novel Janus T'-RuXY (X/Y = S, Se, Te) monolayers: structural properties and electronic structures.

Due to the breaking of the mirror symmetry, two-dimensional layered Janus materials possess many extraordinary mechanical and electronic properties that cannot exist in symmetric structures. In this paper, we propose and investigate the structural and electronic properties of Janus T′-RuXY (X/Y = S, Se, and Te) monolayers using the first-principles simulations. Our calculated results indicate that the T′-RuXY is found to be dynamically and mechanically stable through the phonon dispersion analysis and examination of elastic properties. The T′-RuXY exhibits high anisotropic elastic characteristics due to its in-plane anisotropic atomic structure. Besides, the vertical asymmetry of T′-RuXY leads to the appearance of a difference in the vacuum level between its two different surfaces. At the ground state, all three structures of the Janus T′-RuXY are semiconductors with indirect bandgaps. The bandgaps of T′-RuXY can be modulated by a biaxial strain. Particularly, the semiconductor-to-metal phase transitions are observed in all studied structures at a large compressive strain. Our calculation results not only provide important structural and electronic features of the Janus T′-RuXY monolayers but also show the prospect of their application in nanoelectromechanical devices.

Application of two-dimensional layered materials in surface-enhanced Raman spectroscopy (SERS).

The subject of this perspective is the application of two-dimensional (2D) layered materials in surface-enhanced Raman spectroscopy (SERS). 2D materials are regarded as promising SERS substrates because of their inexpensive and non-toxic characteristics. However, compared with the traditional precious metal substrates, its enhancement factor and detection limit are lower. In this perspective, seven advanced strategies that can improve the SERS properties of 2D materials are highlighted, including layer-dependent strategy, defect engineering, regulation of excitation laser, electric field modulation, phase engineering, arrangement mode and heterostructure design. In addition, the enhancement mechanism, synthesis strategy and application status of typical examples of molybdenum disulfide (MoS2) and MXenes as SERS substrates are discussed in detail. Finally, the prospect of future progress in SERS and possible challenges of 2D layered materials have been put forward.

Effective high-throughput screening of two-dimensional layered materials for potential lithium-ion battery anodes.

Lithium-ion batteries (LIBs) are considered the promising next-generation advanced energy storage devices. It is very important to quickly screen out ideal anode materials for LIBs with excellent performance. In this work, an effective procedure is designed for the high-throughput screening of the three kinds of LIB anode materials from 131 613 inorganic compounds in the Materials Project database. The high throughput screen procedure was not only reliable but was also easily realized. Three ideal anode materials were obtained by considering remarkable thermodynamic stability, Li capacity larger than 372 mA h g-1, band gap smaller than 1.0 eV, and two-dimensional constraint. Furthermore, open-circuit voltage, volume expansion ratio, and the diffusion energy barrier were calculated by the DFT-D corrected density functional method. We believe that our high throughput screen procedure can effectively and accurately search for other kinds of anode materials, which can strongly support the theoretical basis for experimental research.

Positron charge sensing using a double-gated graphene field effect transistor

We utilize a high-mobility double-gated graphene field-effect transistor to measure the accumulated charge created by positron annihilation in its back-gate. The device consists of an exfoliated graphene flake stacked between two hexagonal boron nitride flakes placed on a 1 cm2 substrate of 500 μm thick conducting p-doped Si capped by 285 nm-thick SiO2. The device is placed in close proximity to a 780 kBq 22Na positron source emitting a constant flux of positrons. During the measurement, positrons annihilate within the back-gate, kept floating using a low-capacitance relay. The accumulated positive charge capacitively couples to the graphene device and builds a positive voltage, detectable through a shift in the top-gate dependent graphene resistance characteristic. The shift in the position of the top-gate Dirac peak is then used for extracting the exact voltage buildup and quantitative evaluation of the accumulated charge. Reaching a positron current sensitivity of ∼1.2 fA/Hz, detected over 20min, our results demonstrate the utility of two-dimensional layered materials as probes for charging dynamics of positrons in solids.

Two-dimensional layer materials for highly efficient molecular sensing based on surface-enhanced Raman scattering

Two-dimensional layer materials for highly efficient molecular sensing based on surface-enhanced Raman scattering

Recent Progress on Nanomaterial-Based Membranes for Water Treatment.

Nanomaterials have emerged as the new future generation materials for high-performance water treatment membranes with potential for solving the worldwide water pollution issue. The incorporation of nanomaterials in membranes increases water permeability, mechanical strength, separation efficiency, and reduces fouling of the membrane. Thus, the nanomaterials pave a new pathway for ultra-fast and extremely selective water purification membranes. Membrane enhancements after the inclusion of many nanomaterials, including nanoparticles (NPs), two-dimensional (2-D) layer materials, nanofibers, nanosheets, and other nanocomposite structural materials, are discussed in this review. Furthermore, the applications of these membranes with nanomaterials in water treatment applications, that are vast in number, are highlighted. The goal is to demonstrate the significance of nanomaterials in the membrane industry for water treatment applications. It was found that nanomaterials and nanotechnology offer great potential for the advancement of sustainable water and wastewater treatment.

Manipulation of the SnSe2 gas sensing properties via Au nanoparticles decoration

Gas sensing devices play a key role in safety production, environment protection and human health. Recently, two-dimensional layered materials with large surface area and flexible optoelectronic properties have been widely used for gas sensor design. One of major concern in this field is the sensitivity and selectivity. Here, we show that the sensitivity and selectivity of SnSe2 gas sensor can be manipulated by Au nanoparticles decoration. By choosing NO2 (electron acceptor) and NH3 (electron donor) as target gas, we reveal that the “space charge layer” at the Au-SnSe2 Schottky junction interface plays a critical role in manipulating the sensitivity and selectivity. For NO2, the “space charge layer” hampers the electron transfer from SnSe2 to molecules, thus depressing the sensitivity. While, the electron transfer direction is the opposite when considering NH3, the “space charge layer” would facilitate the electron transfer, thus greatly improving the sensitivity of NH3. Especially, we show that the Au-decorated SnSe2 manifests a sensitivity about -22% toward 80 ppm NH3, which is about three times larger than that measured on pristine SnSe2 under UV illumination. These findings can not only deep our understanding on the mechanism of SnSe2 gas sensing, but also provide an effective strategy to manipulate the sensitivity and selectivity of SnSe2 detector for high performance and low cost gas sensor design.

Synergistic effect of green phosphorus-containing bio-based material and two-dimensional layered material composite on flame-retardant property of polyvinyl alcohol

A novel environment-friendly and efficient flame retardant (phytic acid [PA] - layered double hydroxide [LDH]) has been synthesized to enhance the flame retardancy of PVA. PA-LDH possesses better char formation ability with a high char residue of 68.7 wt% at 600 °C. Peak release rate (pHRR) and heat release capacity (HRC) values of PVA/PA-LDH15% are 180 W/g and 310 J/g-K, with the descent of 69.4% and 31.7%, respectively. The burning rate of PVA/PA-LDH decreased visibly, presented that higher fire retardancy can be obtained for PVA/PA-LDH. It is worth noting that PA-LDH could restrain the melt-dripping of PVA during the combustion process, which is of great significance in fire-spread control. Furthermore, the synergistic flame-retardant mechanism of phosphorus-contained PA and inorganic LDH is proposed based on microscopic analysis on char residues. In addition, PVA/PA-LDH composites exhibit good mechanical properties, the green flame retardant is of considerable significance to the flame-retardant treatment of polymers.

Non-monotonic thickness dependent and anisotropic in-plane thermal transport in layered titanium trisulphide

Due to the potential applications in many fields such as thermoelectric, transistor, and electrode materials, a novel two-dimensional layered material TiS3 has attracted tremendous attention recently. In this article, the thickness dependent and anisotropic in-plane thermal transport behavior is explored by first-principles calculation with Boltzmann transport equation. The calculation results show that when the thickness is smaller than phonon confinement size (PCS), the phonon confinement effect is dominant and the in-plane thermal conductivity has a negative dependence on thickness due to the increase of phonon–phonon scattering; when the thickness is larger than PCS, the confinement effect is weak and the dependence becomes positive due to the decrease of phonon–boundary scattering. Through further analysis, we find this non-monotonic dependence or phonon confinement phenomenon is mainly caused by the low-frequency phonons. The calculation results also show strong thermal anisotropy within the basal plane. The coupling strength, bond length, phonon group velocity, dispersiveness of optical phonon branches, iso-energy surface and iso-surface of electron density distribution are analyzed to explain the anisotropy. Based on the 1D diatomic chain model, a unit cell having similar atomic mass and strong intracell bond can enhance the dispersiveness of optical phonon branches. Those findings here can give physical insights into the thickness dependent and anisotropic in-plane thermal transport in layered materials.