Interlayer Van Der Waals Forces Research Articles

Nondestructive Picosecond Ultrasonic Probing of Intralayer and van der Waals Interlayer Bonding in α‐ and β‐In2Se3

AbstractThe interplay between the strong intralayer covalent‐ionic bonds and the weak interlayer van der Waals (vdW) forces between the neighboring layers of vdW crystals gives rise to unique physical and chemical properties. Here, the intralayer and interlayer bondings in α and β polytypes of In2Se3 are studied, a vdW material with potential applications in advanced electronic and optical devices. Picosecond ultrasonic experiments are conducted to probe the sound velocity in the direction perpendicular to the vdW layers. The measured sound velocities are different in α‐ and β‐In2Se3, suggesting a significant difference in their elastic properties. Density functional theory and an effective spring model are used to calculate the elastic stiffness of the layer and vdW gap in α‐ and β‐In2Se3. The calculated elastic moduli show good agreement with experimental values and reveal the dominant contribution of interlayer atomic bonding to the different elastic properties of the two polytypes. The findings show the power of picosecond ultrasonics for probing the fundamental elastic properties of vdW materials. The data and analysis also provide a reliable description of the intra‐ and interlayer forces in complex crystal structures, such as the polytype phases of In2Se3.

Three-Dimensional MOFs@MXene Aerogel Composite Derived MXene Threaded Hollow Carbon Confined CoS Nanoparticles toward Advanced Alkali-Ion Batteries.

MXene combining high metal-like conductivity, high hydrophilicity, and abundant surface functional groups has been recognized as a class of versatile two-dimensional materials for many applications. However, the aggregation of MXene nanosheets from interlayer van der Waals force and hydrogen bonds represents a major problem that severely limits their practical use. Here, we report an aerogel structure of MOFs@MXene, in which the in situ formed MOF particles can effectively prevent the accumulation of MXene, enabling a three-dimensional (3D) hierarchical porous conductive network to be composed with an ultralight feature. Subsequently, a 3D porous MXene aerogel threaded hollow CoS nanobox composite ((CoS NP@NHC)@MXene) derived from the MOFs@MXene aerogel precursor was synthesized, and the highly interconnected MXene network and hierarchical porous structure coupled with the ultrafine nanocrystallization of the electrochemically active phase of CoS yield the hybrid system with excellent electron and ion transport properties. Benefiting from the synergistic effect of the components, the (CoS NP@NHC)@MXene composite manifests outstanding electrochemistry properties as electrode materials for all of the lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and potassium-ion batteries (PIBs). It demonstrated the excellent cycle stability and high capacities of 1145.9 mAh g-1 at 1 A g-1 after 800 cycles and 574.1 mAh g-1 at 5 A g-1 after 1000 cycles for LIBs, 420 mAh g-1 at 2 A g-1 after 650 cycles for SIBs, and 210 mAh g-1 at 2 A g-1 after 500 cycles for PIBs. First-principle calculations confirmed that the (CoS NP@NHC)@MXene hybrid could enhance the charge transfer reaction kinetics, particularly at the interface. More importantly, the excellent rate performance under high mass loading and the high volumetric energy and power density of the entire electrode represent the potential of (CoS NP@NHC)@MXene composites for applications to practical electrochemical energy storage devices. The synthesis method reported in this Article is versatile and can be easily extended to produce other porous MXene-aerogel-based materials for various applications.

The spatially oriented redistribute of photogenerated carriers and photocatalytic hydrogen evolution mechanism research on polymeric carbon nitride Van der Waals homojunction

The weaker interlayer Van der Waals force of polymeric carbon nitride (PCN) increases the potential barrier of electron transfer between layers, which greatly inhibits the carrier separation efficiency of PCN. Herein, we designed and prepared Van der Waals homojunction of PCN (VHPCN), which the interlayer Van der Waals force forces were instead by Coulomb force. DFT calculated reveals that the equilibrium distance and the interface adhesion energy of VHPCN were significantly decreased and enhanced, respectively, under the function of Coulomb force. The equilibrium distance decreased from 3.27 Å to 2.49 Å and the interface adhesion energy enhanced from −0.35 eV to −1.78 eV. This structure not only the transfer distance of electrons at the interface is effectively decreased, but also the transfer barrier of electrons at the interface is reduced. Thus, the electrons/holes pairs in the VHPCN photocatalyst present the phenomenon of oriented interlayer transfer and ultrahigh carrier separation efficiency under the synergistic effect between the built-in electric field and the interlayer Coulomb force of the Van der Waals homojunction. The optimal VHPCN exhibited the photocatalytic H2 evolution rate and apparent quantum yield (AQY), which are 7.9 mmol·g−1·h−1 and 32.4% (>420 nm), respectively. This work put forward a promising strategy to construct novel Van der Waals homojunction within 2D materials for the desirable photogeneration carrier spatially distribution controlling, which could be generalized to other 2D materials for photocatalytic applications in the advanced energy and environmental field.

Electronic and optical properties of vertical borophene/MoS2 heterojunctions

Electronic and optical properties of stacked heterojunction based on borophene and MoS2 are studied by density functional theory. It is found that the different stack of 1/6 vacancy borophene β12 and MoS2 can induce different electronic properties through interlayer van der Waals force. The external electric field can tune the Schottky contact types of the heterojunction. The stacked heterojunction enlarges the range of light response compared with MoS2. It suggests that the stack changes the electronic and optical properties of sing layer MoS2 and external electric field induces the changed Schottky contact type, providing a way to design optoelectronic device with tunable Schottky contact and good optical properties.

Aligned monolayer MoS2 ribbons growth on sapphire substrate via NaOH-assisted chemical vapor deposition

This study reports the growth of aligned monolayer molybdenum disulfide (MoS2) ribbons on a sapphire substrate via NaOH-assisted chemical vapor deposition. The length of MoS2 ribbon is up to 400 μm. The MoS2 ribbon has excellent single crystal properties, a carrier mobility of {150 cm2 V−1 s−1, and a optical response of 103 mA W−1 at a wavelength of 550 nm. The growth model of MoS2 ribbons was given. NaOH reacts with MoO3 to form sodium molybdate droplets, which increase the fluidity of the molybdenum source on the substrate, realizing the vapor–liquid–solid growth of MoS2 on sapphire. The monolayer MoS2 ribbons have two kinds of arrangements on the sapphire substrate, one is the oriented growth affected by the interlayer van der Waals force and the lattice, and the other is the aligned growth constrained by the sapphire step. Our results promote the basic research and device applications of MoS2, and introduce a new way of synthesizing other one dimensional (1D) and 2D nanostructures.

Preparation of a Highly Stable Dispersion of Graphene in Water with the Aid of Graphene Oxide

Utilization of graphene (GE) in water is physically limited by its lack of functional groups, high hydrophobicity, and large interlayer van der Waals forces. Easy agglomeration of GE makes the preparation of GE-filled composites in an aqueous environment very difficult. Conventional methods solve this problem by introducing a surfactant to the system. However, the addition of a large amount of surfactant may hinder the performance of the composite. In this study, highly stable GE aqueous suspensions (remaining stable for more than 30 days) with a high concentration (1.5 mg/mL) were conveniently prepared by using graphene oxide (GO) as a stabilizer. A GE- and GO-filled rubber composite prepared by this GO–GE water suspension in an aqueous environment was also prepared and analyzed. It was found that a dual-network with dual functions was formed in the composite matrix: the GE network enhanced the electrical conductivity of the composite, while the GO network enhanced its mechanical properties. Our method i...

Modeling of anisotropic elastic properties of multi-walled zigzag carbon nanotubes

Non-linear three dimensional finite element model of double wall carbon nanotube has been developed to evaluate its anisotropic elastic properties. Carbon nanotube was modeled as a frame-like structure and its elastic properties have been considered in the sense of the average over volume (effective) response on the axial and radial loadings. The principal bonds between two adjacent atoms were simulated using equivalent pseudo-rectangular beam elements which exhibit different flexural rigidity along the two principal centroid axes of the cross-section. The simulation of the interlayer van der Waals force with intrinsic nonlinearity and complicated applying region has been performed. Resulting Young's moduli of double walled carbon nanotube of diameter of 0.7–2 nm was found to vary from 0.2 to 0.5 TPa in axial direction and from 1.4 to 43.3 GPa in radial direction. The radial Young's modulus exponentially decays with the tube diameter.

Co doping induced photocurrent enhancement in photocatalyst MoS2

A set of Co doped MoS2 photocatalysts were prepared by hydrothermal method and are used in photocatalytic degradation of methylene blue. A high consistency of the photocurrents and photocatalytic MB degradation are found. The photocurrents in MoS2 is improved greatly by Co doping, resulting in a high MB degradation up to 74% by 16% Co in molar ratio doped MoS2. A higher or lower Co doping increase the out-of plane interaction in MoS2, which reduces the active sites and the photocatlytic activity of these catalysts. The Co promotion in MoS2 are efficient in both decreasing the interlayer Van der Waals force and increasing the electron generation and transfer, thus the photoefficiency of MoS2.

A gas bubble exfoliation method to prepare g-C3N4 nanosheets with enhanced photocatalytic activities

The exotic physicochemical properties of graphitic carbon nitride (g-C3N4) make it widely investigated and applied in photocatalysis. However, its low specific surface area, limited active sites and fast electron-hole pair recombination rate limit its further application. In this work, g-C3N4 nanosheets (CNNs) were fabricated by a simple and convenient gas bubble exfoliation method. To the best of our knowledge, this is the first time that bulk g-C3N4 (b-CN) was exfoliated by the generation and expansion of gas bubbles obtained from the electrolysis of water, which provides essential and dominant driving force to overcome the interlayer van der Waals force of b-CN, and achieve the exfoliation. The thin layer structure of CNNs was verified by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The surface area of CNNs was about 2.57 times that of b-CN. In addition, the photocatalytic performance of CNNs was proved by the degradation of methylene blue (MB) under visible light irradiation. Thus, gas bubble exfoliation is a new strategy to improve the photocatalytic performance of b-CN.

Nanostructure-induced performance degradation of WO3·nH2O for energy conversion and storage devices.

Although 2D layered nanomaterials have been intensively investigated towards their application in energy conversion and storage devices, their disadvantages have rarely been explored so far especially compared to their 3D counterparts. Herein, WO3·nH2O (n = 0, 1, 2), as the most common and important electrochemical and electrochromic active nanomaterial, is synthesized in 3D and 2D structures through a facile hydrothermal method, and the disadvantages of the corresponding 2D structures are examined. The weakness of 2D WO3·nH2O originates from its layered structure. X-ray diffraction and scanning electron microscopy analyses of as-grown WO3·nH2O samples suggest a structural transition from 2D to 3D upon temperature increase. 2D WO3·nH2O easily generates structural instabilities by 2D intercalation, resulting in a faster performance degradation, due to its weak interlayer van der Waals forces, even though it outranks the 3D network structure in terms of improved electronic properties. The structural transformation of 2D layered WO3·nH2O into 3D nanostructures is observed via ex situ Raman measurements under electrochemical cycling experiments. The proposed degradation mechanism is confirmed by the morphology changes. The work provides strong evidence for and in-depth understanding of the weakness of 2D layered nanomaterials and paves the way for further interlayer reinforcement, especially for 2D layered transition metal oxides.

Effect of interlayer space on the structure and Poisson's ratio of a graphene/MoS2 tubular van der Waals heterostructure

We propose a tubular van der Waals heterostructure by rolling up graphene and MoS2 atomic layers into a tubular form. We show that the interlayer space of this heterostructure can be varied in a specific range. This specific range is related to the interlayer van der Waals force, the ultrahigh in-plane stiffness and small bending modulus of graphene, and the brittle characteristic properties of MoS2. This variability of the interlayer space can be utilized to efficiently tune the mechanical properties of the tubular van der Waals heterostructure. More specifically, we demonstrate that the Poisson's ratio of the tubular van der Waals heterostructure can be manipulated by a factor of two by varying the interlayer space in the range 1.44–4.44 Å. This work provides a basis for applications of a new member of the van der Waals heterostructure family with a tunable Poisson's ratio.

Atom-by-Atom Fabrication of Monolayer Molybdenum Membranes.

Fabrication of materials in the monolayer regime to acquire fascinating physical properties has attracted enormous interest during the past decade, and remarkable success has been achieved for layered materials adopting weak interlayer van der Waals forces. However, the fabrication of monolayer metal membranes possessing strong intralayer bonding remains elusive. Here, suspended monolayer Mo membranes are fabricated from monolayer MoSe2 films via selective electron beam (e-beam) ionization of Se atoms by scanning transmission electron microscopy (STEM). The nucleation and subsequent growth of the Mo membranes are triggered by the formation and aggregation of Se vacancies as seen by atomic resolution sequential STEM imaging. Various novel structural defects and intriguing self-healing characteristics are unveiled during the growth. In addition, the monolayer Mo membrane is highly robust under the e-beam irradiation. It is likely that other metal membranes can be fabricated in a similar manner, and these pure metal-based 2D materials add to the diversity of 2D materials and introduce profound novel physical properties.

Combining density functional theory-finite element multi-scale method to predict mechanical properties of polypropylene/graphene nanocomposites: Experimental study

In this paper a multi-scale finite element model is developed to study the tensile modulus of graphene reinforced polypropylene (PP/graphene) nanocomposites and compare the results with experimental tensile tests. Herein, the graphenes are modeled and optimized using density functional theory (DFT). By treating graphene as space-frame structures, in which the discrete nature of graphene is preserved, they are modeled using three-dimensional elastic beam elements for the C-C covalent bonds and point mass elements for the atoms. Our calculations reveal that the force constants of stated elastic beam obtained from DFT method are in best agreement comparing to the results of previous MD study which was equal to kr = 6.41 × 10−7 N/nm, kθ = 8.47 × 10−10 N nm/rad2 and kτ=2.7×10-10 N nm/rad2. The interlayer van der Waals forces between graphene and PP matrix are represented by Lennard–Jones potential and simulated by a nonlinear truss rod model. According to our DFT results we found that interaction energy between graphene and PP matrix decreased by increasing in the number of polymer matrix monomers and reached to a constant value when six monomer of polymer adsorbed onto the surface of graphene which was equal to −0.0044 eV/monomer. Our FE results showed that the moduli of the nanocomposites were close to the experimental results until 1 wt% of graphene and also were predictable. The Young's modulus obtained from combined model and experimental test were about 2.85 Mpa and 2.21 Mpa for 0.25 wt%, 3.88 Mpa and 3.22 Mpa for 0.5 wt%, 4.83 Mpa and 4.30 Mpa for 0.75 wt% and also 5.86 Mpa and 5.19 Mpa for 1 wt% of graphene, respectively.

Observation of Optical and Electrical In-Plane Anisotropy in High-Mobility Few-Layer ZrTe5.

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However, the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows a significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the Γ point is explained by theoretical calculations. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with a reduced thickness. A low-temperature Hall experiment sheds light on more intrinsic anisotropic electrical transport, with a hole mobility of 3000 and 1500 cm2/V·s along the a-axis and c-axis, respectively. Pronounced quantum oscillations in magnetoresistance are observed at low temperatures with the highest electron mobility up to 44 000 cm2/V·s.

Thickness tunable transport in alloyed WSSe field effect transistors

We report the field effect transistor characteristics of exfoliated transition metal dichalcogenide alloy tungsten sulphoselenide. WSSe is a layered material of strongly bonded S-W-Se atoms having weak interlayer van der Waals forces with a significant potential for spintronic and valleytronic applications due to its polar nature. The X-ray photoelectron spectroscopy measurements on crystals grown by the chemical vapor transport method indicate a stoichiometry of the form WSSe. We report flake thickness tunable transport mechanism with n-type behavior in thin flakes (≤11 nm) and ambipolarity in thicker flakes. The devices with flake thicknesses of 2.4 nm–54.8 nm exhibit a maximum electron mobility of ∼50 cm2/V s along with an ION/IOFF ratio >106. The electron Schottky barrier height values of 35 meV and 52 meV extracted from low temperature I–V measurements for 3.9 nm and 25.5 nm thick flakes, respectively, indicate that an increase in hole current with thickness is likely due to lowering of the bandgap through an increase in energy of the valence band maximum.

Thin tungsten telluride layer preparation by thermal annealing

We report a simple method to prepare a thin Tungsten Telluride (WTe2) flake with accurate thickness control, which allows preparing and studying this two dimensional material conveniently. First, the WTe2 flake, which is relatively thick due to its strong interlayer van der Waals forces, is obtained by a conventional mechanical exfoliation method. Then, the exfoliated flake is annealed at 600 °C under a constant Ar protecting flow. Raman and atomic force spectroscopy characterizations demonstrate that thermal annealing can effectively thin down the WTe2 flake and retain its original lattice structure, though its surface smoothness is slightly deteriorated. Additionally, systematical study indicates that the thinning process strongly depends on the initial thickness of the WTe2 flake before annealing: the thinning rate increases from 0.12 nm min−1 to 0.36 nm min−1 as the initial thickness increases from 10 nm to 45 nm, while the roughness of the final product also increases with the increase of its initial thickness. However, the method fails when it is applied to WTe2 flakes thicker than 100 nm, resulting in uneven or burnt surface, which is possibly caused by big cavities formed by a large amount of defects gathered at the top surface.

Strong Circularly Polarized Photoluminescence from Multilayer MoS2 Through Plasma Driven Direct-Gap Transition

We report circularly polarized photoluminescence spectra taken from few layer MoS2 after treatment with a remotely generated oxygen plasma. Here, the oxygen plasma decouples the individual layers in MoS2 by perturbing the weak interlayer van der Waals forces without damaging the lattice structure. This decoupling causes a transition from an indirect to a direct band gap material, which causes a strong enhancement of the photoluminescence intensity. Furthermore, up to 80% circularly polarized photoluminescence is observed after plasma treatment of few layer MoS2 flakes, consistent with high spin polarization of the optically excited carriers. A strong degree of polarization continues up to room temperature, further indicating that the quality of the crystal does not suffer degradation due to the oxygen plasma exposure. Our results show that the oxygen plasma treatment not only engineers the van der Waals separation in these TMDC multilayers for enhanced PL quantum yields, but also produces high quality mul...

Atomistic modeling of the stiffness, strength and charge-induced actuation of graphene nanofoams

To exploit the excellent electrical and mechanical properties of graphene, three-dimensional nanoporous graphene architectures have appeared recently. These graphene nanofoams are conductive and have very large internal surface areas, making them attractive for application as low-voltage electrochemical actuators. The key feature that connects the excess-charge-modified interatomic bond length to the overall strains is the three-dimensional spatial distribution of the graphene ribbons. This is governed by the competition between interlayer van der Waals forces and intralayer charge-affected covalent interactions. Here we explore this competition through atomistic modeling of ordered graphene hexagonal honeycombs. We modify the adaptive intermolecular reactive empirical bond order (AIREBO) potential to account for the excess-charge-induced change in equilibrium bond length of the carbon atoms. We use the charge-strain results of single graphene ribbons obtained by density functional theory (DFT) calculations to calibrate the atomistic AIREBO potentials. We investigate the effect of edges stresses and excess charge on the dimensional and morphological changes of free-standing graphene ribbons. We study the effect of internal surface area and relative density on the actuation stroke and work density and compare the graphene nanofoams with several nanoporous gold systems. Our results reveal that the graphene nanofoams have a smaller actuation stroke per unit charge density than random nanoporous gold, but that more mechanical work can be generated.

An analytical study on the nonlinear vibration of a double-walled carbon nanotube

In this paper, free vibrations of a clamped-clamped double-walled carbon nanotube (DWNT) under axial force is studied. By utilizing Euler-Bernoulli beam theory, each layer of DWNT is modeled as a beam. In this analysis, nonlinear form of interlayer van der Waals (vdW) forces and nonlinearities aroused from mid-plane stretching are also considered in the equations of motion. Further, direct application of multiple scales perturbation method is utilized to solve the obtained equations and to analyze free vibrations of the DWNT. Therefore, analytical expressions are found for vibrations of each layer. Linear and nonlinear natural frequencies of the system and vibration amplitude ratios of inner to outer layers are also obtained. Finally, the results are compared with the results obtained by Galerkin method.

Vibration analysis of nano-structure multilayered graphene sheets using modified strain gradient theory

In this paper, for the first time, the modified strain gradient theory is used as a new size-dependent Kirchhoff micro-plate model to study the effect of interlayer van der Waals (vdW) force for the vibration analysis of multilayered graphene sheets (MLGSs). The model contains three material length scale parameters, which may effectively capture the size effect. The model can also degenerate into the modified couple stress plate model or the classical plate model, if two or all of the material length scale parameters are taken to be zero. After obtaining the governing equations based on modified strain gradient theory via principle of minimum potential energy, as only infinitesimal vibration is considered, the net pressure due to the vdW interaction is assumed to be linearly proportional to the deflection between two layers. To solve the governing equation subjected to the boundary conditions, the Fourier series is assumed for w = w(x, y). To show the accuracy of the formulations, present results in specific cases are compared with available results in literature and a good agreement can be seen. The results indicate that the present model can predict prominent natural frequency with the reduction of structural size, especially when the plate thickness is on the same order of the material length scale parameter.