Interlayer Van Der Waals Interactions Research Articles

Nonlinear vibrations of helical graphene resonators in the dynamic nano-indentation testing

sHelical graphene nanoribbons (HGNRs) are a special structure made of single-layer graphene, which are of interest in the nanotechnology because of their unique mechanical features. Here we propose an asymmetrical nonlinear spring model (ANSM) for designing different HGNRs used in the dynamic nano-indentation testing. Both nonlinear static and dynamics behaviors of HGNRs are studied using molecular dynamics (MD) simulations and the ANSM, respectively. The interlayer van der Waals interactions, which play a key role in the mechanical behaviors of HGNRs, are quantitatively considered in the ANSM with quadratic and cubic nonlinearities. The response-frequency curves of forced vibrations in HGNRs show clear softening-type nonlinearity. In particular, a remarkable transformation from softening-type to hardening-type nonlinearity occurs with increasing contact stiffness in nonlinear dynamics behaviors of HGNRs. Checking against present static and linear dynamics results of MD simulations shows that the ANSM has high accuracy. The present study provides valuable physical insights for designing and assembling HGNR-based resonators.

Spacing dependent and cation doping independent superconductivity in intercalated 1T 2D SnSe2

The weak van der Waals interlayer interactions in the transition metal dichalcogenide (TMD) materials have created a rich platform to study their exotic electronic properties through chemical doping or physical gating techniques. We reported bulk superconductivity up to 7.6 K through careful manipulation of the charge carrier density and basal spacing d in the chemically intercalated 2D 1T-SnSe2 phase. We found, for the first time in the 2D SnSe2, that polar organic molecules cointercalated with the alkali metal Li into the basal layers, thus significantly enhancing the superconducting Tc. We observed that the Tc scales with the basal spacing distance, meanwhile being almost independent of x in Lix(THF)ySnSe2 system. Our results offers a new general chemical route to study the rich electron correlations and the interplay of charge density wave and unconventional superconductivity in the 2D material.

Axial and torsional buckling analysis of single- and multi-walled carbon nanotubes: finite element comparison between armchair and zigzag types

The present work deals with the axial and torsional buckling analysis and design of carbon nanotubes (CNTs) by finite element. Computer-aided design, computer-aided engineering simulation, and finite element modeling are applied for the calculation of axial and torsional displacement and von Mises stresses. The smallest diameter of single-walled, double-walled, and multi-walled carbon nanotubes have been selected to survey armchair and zigzag types of CNTs. Nanotube diameters, covalent and van der Waals bonding, interlayer spacing, and chirality is considered in nanotubes construction based on actual dimensions. This paper illustrates that armchair-type nanotube is more resistant than zigzag-type against axial buckling for single-walled carbon nanotubes, whereas, in the case of torsional loading, zigzag-type show superior performance against buckling. By increasing the diameter and number of layers, due to van der Waals interaction, elastic buckling properties of nanotubes can be improved significantly. Data analysis for double-walled carbon nanotubes has shown that armchair-type is a better choice under combined loading, whereas, in the meantime, zigzag-type for triple-walled carbon nanotubes is preferred in the comparative analysis under combined loading. The results demonstrate the vital role of covalent intermolecular forces for single-walled nanotubes and van der Waals interlayer interactions for multi-walled nanotubes under buckling loading. The present study investigated the effect of covalent and van der Waals bonding for small diameter nanotubes.

Wrinkling behavior of graphene on substrates with different surface morphologies

The wrinkling of graphene can strongly affect its stability and reliability in graphene-based nanodevices. However, it is still a tremendous challenge to accurately predict the wrinkling behavior of graphene on substrates with different surface morphologies. In this study, the wrinkling behavior of graphene on substrates with one-dimensional (1D) and two-dimensional (2D) sinusoidal surface morphologies is studied by continuum modeling and molecular dynamics (MD) simulations, where three deformation forms of graphene (non-conformal, transitional conformal and conformal) are theoretically determined. Our results show that the wrinkling behavior and interfacial shear force of graphene strongly depend on the wavelength and amplitude of the substrate surface and the elastic constants of the substrate, where the explicit expressions of the interfacial cohesive energy between graphene and the substrate are derived by using Gaussian quadrature based on interlayer van der Waals (vdW) interactions. The bistable state of the graphene wrinkling can be found when the wavelength and amplitude of the substrate surface are in a given range. Checking against our MD simulations shows that the present theoretical model has high accuracy. This study provides valuable physical insights for designing and assembling graphene-based flexible nanodevices on substrates.

Ultrathin Ruddlesden-Popper Perovskite Heterojunction for Sensitive Photodetection.

Thanks to their unique optical and electric properties, 2D materials have attracted a lot of interest for optoelectronic applications. Here, the emerging 2D materials, organic-inorganic hybrid perovskites with van der Waals interlayer interaction (Ruddlesden-Popper perovskites), are synthesized and characterized. Photodetectors based on the few-layer Ruddlesden-Popper perovskite show good photoresponsivity as well as good detectivity. In order to further improve the photoresponse performance, 2D MoS2 is chosen to construct the perovskite-MoS2 heterojunction. The performance of the hybrid photodetector is largely improved with 6 and 2 orders of magnitude enhancement for photoresponsivity (104 A W-1 ) and detectivity (4 × 1010 Jones), respectively, which demonstrates the facile charge separation at the interface between perovskite and MoS2 . Furthermore, the contribution of back gate tuning is proved with a greatly reduced dark current. The results demonstrated here will open up a new field for the investigation of 2D perovskites for optoelectronic applications.

In Situ Electron Microscopy Investigation of Sodiation of Titanium Disulfide Nanoflakes.

Two-dimensional (2D) metal sulfides show great promise for their potential applications as electrode materials of sodium ion-batteries because of the weak interlayer van der Waals interactions, which allow the reversible accommodation and extraction of sodium ions. The sodiation of metal sulfides can undergo a distinct process compared to that of lithiation, which is determined by their metal and structural types. However, the structural and morphological evolution during their electrochemical sodiation is still unclear. Here, we studied the sodiation reaction dynamics of TiS2 by employing in situ transmission electron microscopy and first-principles calculations. During the sodium-ion intercalation process, we observed multiple intermediate phases (phase II, phase Ib, and phase Ia), different from its lithiation counterpart, with varied sodium occupation sites and interlayer stacking sequences. Further insertion of Na ions prompted a multistep extrusion reaction, which led to the phase separation of Ti metal from the Na2S matrix, with its 2D morphology expanded to a 3D morphology. In contrast to regular conversion electrodes, TiS2 still maintained a compact structure after a full sodiation. First-principles calculations reveal that the as-identified phases are thermodynamically preferred at corresponding intercalation/extrusion stages compared to other possible phases. The present work provides the fundamental mechanistic understanding of the sodiation process of 2D transition metal sulfides.

Electronic and optical properties of GaN–MoS2 heterostructure from first-principles calculations**Project supported by the National Natural Science Foundation of China (Grant No. 11864011), the Hubei Provincial Natural Science Foundation of China (Grant No. 2018CFB390), and the Doctoral Fund Project of Hubei Minzu University, China (Grant No. MY2017B015).

Heterostructures (HSs) have attracted significant attention because of their interlayer van der Waals interactions. The electronic structures and optical properties of stacked GaN–MoS2 HSs under strain have been explored in this work using density functional theory. The results indicate that the direct band gap (1.95 eV) of the GaN–MoS2 HS is lower than the individual band gaps of both the GaN layer (3.48 eV) and the MoS2 layer (2.03 eV) based on HSE06 hybrid functional calculations. Specifically, the GaN–MoS2 HS is a typical type-II band HS semiconductor that provides an effective approach to enhance the charge separation efficiency for improved photocatalytic degradation activity and water splitting efficiency. Under tensile or compressive strain, the direct band gap of the GaN–MoS2 HS undergoes redshifts. Additionally, the GaN–MoS2 HS maintains its direct band gap semiconductor behavior even when the tensile or compressive strain reaches 5% or -5%. Therefore, the results reported above can be used to expand the application of GaN–MoS2 HSs to photovoltaic cells and photocatalysts.

1T′‐ReS2 Confined in 2D‐Honeycombed Carbon Nanosheets as New Anode Materials for High‐Performance Sodium‐Ion Batteries

AbstractReS2 (rhenium disulfide) is a new transition‐metal dichalcogenide that exhibits 1T′ phase and extremely weak interlayer van der Waals interactions. This makes it promising as an anode material for sodium‐ion batteries. However, achieving both a high‐rate capability and a long‐life has remained a major research challenge. Here, a new composite is reported, in which both are realized for the first time. 1T′‐ReS2 is confined through strong interfacial interaction in a 2D‐honeycombed carbon nanosheets that comprise an rGO inter‐layer and a N‐doped carbon coating‐layer (rGO@ReS2@N‐C). The strong interfacial interaction between carbon and ReS2 increases overall conductivity and decreases Na+ diffusion resistance, whilst the intended 2D‐honeycombed carbon protective layer maintains structural morphology and electrochemical activity during long‐term cycling. These findings are confirmed by advanced characterization techniques, electrochemical measurement, and density functional theory calculation. The new rGO@ReS2@N‐C exhibits the greatest rate performance reported so far for ReS2 of 231 mAh g−1 at 10 A g−1. Significantly, this is together with ultra‐stable long‐term cycling of 192 mAh g−1 at 2 A g−1 after 4000 cycles.

Thermo-breathing vibration of carbon nanoscrolls

In this study, the thermo-breathing vibration of carbon nanoscrolls (CNSs) is studied by molecular dynamics (MD) simulations and continuum modeling. The breathing vibration strongly depends on the competition between the cohesive energy among the graphene layers of CNSs and the bending energy of CNSs, where the explicit expressions for the total cohesive energy of CNSs are derived by using Gaussian quadrature based on van der Waals interlayer interactions. In particular, the interlayer amplitudes on the breathing vibration of CNSs at different temperatures are obtained by continuum modeling with the equipartition theorem. Checking against our MD simulations shows that the present theoretical model has high accuracy. The analytical solutions show that the temperatures have no effect on the nature frequency of CNSs, but have a great effect on the vibration amplitude of CNSs. The obtained results should be of great importance for understanding the thermo-mechanical vibration of nanosized devices.

Pressure dependence of direct optical transitions in ReS2 and ReSe2

The ReX2 system (X = S, Se) exhibits unique properties that differ from other transition metal dichalcogenides. Remarkably, its reduced crystal symmetry results in a complex electronic band structure that confers this material in-plane anisotropic properties. In addition, multilayered ReX2 presents a strong 2D character even in its bulk form. To fully understand the interlayer interaction in this system, it is necessary to obtain an accurate picture of the electronic band structure. Here, we present an experimental and theoretical study of the electronic band structure of ReS2 and ReSe2 at high-hydrostatic pressures. The experiments are performed by photoreflectance spectroscopy and are analyzed in terms of ab initio calculations within the density functional theory. Experimental pressure coefficients for the two most dominant excitonic transitions are obtained and compared with those predicted by the calculations. We assign the transitions to the Z k-point of the Brillouin zone and other k-points located away from high-symmetry points. The origin of the pressure coefficients of the measured direct transitions is discussed in terms of orbital analysis of the electronic structure and van der Waals interlayer interaction. The anisotropic optical properties are studied at high pressure by means of polarization-resolved photoreflectance measurements.

The preparation of flexible Ni-Mn-In thin films on mica substrates by pulsed laser deposition

Abstract The large volume variation of ferromagnetic shape memory alloy is strongly restricted in the film format on rigid substrates. Due to the high growth temperature (>673 K), it is difficult to grow Heusler alloys with high crystalline quality on flexible polymers. We successfully synthesize high-quality 100 nm thick Ni51.6Mn33.5In14.9 films on transparent flexible mica substrates by pulsed laser deposition, and compare the crystalline structure, morphology and magnetic properties with the films grown on Si (1 0 0) substrates. Due to the weak Van der Waals inter-layer interaction in mica substrates, the clamping effect of substrate can be significantly weakened by thinning the thickness of substrates using the simple mechanical exfoliation. This makes the final sample with Ni-Mn-In film on ultrathin mica substrate highly flexible with smallest curvature radius down to 0.745 cm. In addition, the mechanical properties of flexible Ni-Mn-In films were tested by bending in various curvature radiuses repeatedly. The resistance only changes slightly after more than 10,000 cycles, indicating the highly stable flexibility. These results demonstrate that flexible ferromagnetic shape memory alloy film can be obtained by using mica substrates and has great potential applications in flexible electronics.

Electronic properties and interlayer coupling of twisted MoS2/NbSe2 heterobilayers

In this paper we present first-principles calculations, based on both density functional theory and maximally localized Wannier functions, to study the electronic properties and interlayer coupling of twisted ${\mathrm{MoS}}_{2}/{\mathrm{NbSe}}_{2}$ heterobilayers. We accurately investigate different stacking configurations and commensurate twist angles by including an in-depth analysis of the interlayer van der Waals interaction. The metallic character of the investigated heterostructures is dominated, at the Fermi energy, by the ${\mathrm{NbSe}}_{2}$ atomic orbitals and shows a strong dependence on the twist angle. Notably, at the smallest considered twist angle, band structure flattening at the Fermi energy shows up, which should result in a lower conductivity of the metallic heterobilayer. The electrostatic potential analysis reveals no significant modification of the potential pattern with respect to the potentials of the isolated layers, with the exception of the interface region. A moderate electronic charge redistribution, compatible with electronically weakly coupled layers, is set up following the formation of the interface. The dependence of the electronic structure on the twist angle acts as a new degree of freedom for tuning properties relevant in electronic device applications.

The Atomic and Electronic Structure of 0° and 60° Grain Boundaries in MoS2

We have investigated atomic and electronic structure of grain boundaries in monolayer MoS2, where relative angles between two different grains are 0 and 60 degree. The grain boundaries with specific relative angle have been formed with chemical vapor deposition growth on graphite and hexagonal boron nitride flakes; van der Waals interlayer interaction between MoS2 and the flakes restricts the relative angle. Through scanning tunneling microscopy and spectroscopy measurements, we have found that the perfectly stitched structure between two different grains of MoS2 was realized in the case of the 0 degree grain boundary. We also found that even with the perfectly stitched structure, valence band maximum and conduction band minimum shows significant blue shift, which probably arise from lattice strain at the boundary.

Electron transfer and cascade relaxation dynamics of graphene quantum dots/MoS2 monolayer mixed-dimensional van der Waals heterostructures

We realize the efficient modulation of relaxation channels in MoS 2 monolayers and interfacial electron transfer by forming mixed-dimensional VdW heterostructures with graphene quantum dots (GQDs) and find that the cascaded relaxation of hot electrons in GQDs that originate from the quantum confinement effect is the ultimate reason. With femtosecond pump–probe spectroscopy, the biphasic electron injection model and modified rate equations are employed to quantitatively solve the interfacial dynamics. Van der Waals (VdW) heterostructures have emerged as promising materials for atomically thin optoelectronic and photovoltaic applications, where the efficient charge separation after photo-excitation is significant to enhance device performances. However, modulating the interfacial charge transfer is still challenging due to the weak interlayer VdW interaction. Revealing electron transfer and relaxation processes in heterostructures lays foundation to tune the interfacial dynamics and optoelectronic properties. Here, we realize the efficient modulation of relaxation channels in MoS 2 monolayers and interfacial electron transfer by forming mixed-dimensional VdW heterostructures with graphene quantum dots (GQDs). With femtosecond pump–probe spectroscopy, the biphasic electron injection model and modified rate equations are employed to quantitatively solve interfacial transfer rates of mixed-dimensional VdW heterostructures. We find that the cascaded relaxation of hot electrons in GQDs that originates from the quantum confinement effect can intensively affect the interfacial dynamics. Our established model is instructive to optimize performances of future photon-harvesting devices based on mixed low-dimensional heterostructures.

Finite element buckling analysis of double-layered graphene nanoribbons

In this paper, the molecular structural mechanics method is applied to assess the critical buckling stress of double-layered graphene nanoribbons (DLGNRs) for different types of chirality, stacking modes and aspect ratios. In the developed atomistic model, the DLGNRs are considered as space-frame structures, in which the interlayer van der Waals (vdW) interactions and the in-plane covalent bonds are modeled as truss and beam elements, respectively. The beam sectional properties are obtained by establishing a conjunction between the structural and molecular mechanics. The Lennard-Jones potential is used to derive the elastic properties of the truss rod elements. The finite element method is employed for an eigenvalue-based buckling analysis. The results show that the critical buckling stress of the DLGNRs increases with increasing the mode shape number and decreases with increasing the aspect ratio for both the inphase and the antiphase modes for different types of chirality and stacking modes. Also, it is found that for the inphase modes, the critical buckling stress reaches its maximum value for the configuration with AA stacking mode and armchair chirality for all the aspect ratios. In the antiphase modes, the configuration with the highest stability is dependent on the aspect ratio. For the aspect ratios below 10, the configuration with AB stacking mode and armchair chirality has the highest stability; however, on the other hand, by increasing the aspect ratio above 10, the configuration with AB stacking mode and zigzag chirality represents the highest stability.

Thickness-dependent bandgap of transition metal dichalcogenides dominated by interlayer van der Waals interaction

Based on a bandgap thermodynamic approach, the thickness-dependent bandgap of two-dimensional transition metal dichalcogenides is modeled without any adjustable parameter. An efficient expansion in bandgap upon lowering the thickness is predicted. The thickness-dependence of bandgap is believed originated from the interlayer van der Waals (vdW) interaction, while the surface effect is characterized by the difference in atomic thermal vibration between the surface and the interior. Due to the suppression role of the interlayer vdW interaction on the thermal vibration of interior chalcogen atoms, the surface effect is variable, which changes from monotonic increase for sulfides to decrease for tellurides. The role of the interlayer vdW interaction depends on the polarity of metal-chalcogen bonds. The model predictions agree with available experiment and simulation results.

Factors influencing thermal transport across graphene/metal interfaces with van der Waals interactions.

We implement non-equilibrium Green's function (NEGF) calculations to investigate thermal transport across graphene/metal interfaces with interlayer van der Waals interactions to understand the factors influencing thermal conductance across the interface. It is found that interfaces with a smaller interfacial lattice mismatch, lighter metal substrate and stronger interfacial bonding strength will show better interfacial thermal transport abilities. Strain induced by the interfacial lattice mismatch in graphene is the key factor for the decrease of interfacial phonon transmission in the main frequency range of metals, which finally results in a decrease of interfacial thermal conductance. A comprehensive interfacial influencing factor is proposed combining the factors of graphene density, metal density and interfacial binding energy to realize the prediction of interfacial thermal conductance across the graphene/metal interface. The results are hoped to promote the understanding of the thermal transport mechanism and design of graphene based 2D/3D materials interfaces.

Atomically Thin Two-Dimensional Nanosheets with Tunable Spin-Crossover Properties.

Combining the fascinating advantages of ultrathin two-dimensional (2D) nanosheets with the nanostructuration of spin-crossover (SCO) materials represents an attractive target of controlled fabrication of SCO nano-objects at the device level. Here, we demonstrate that through facile-operating ultrasonic force-assisted liquid exfoliation technology the three-dimensional (3D) van der Waals SCO bulk precursor {[Fe(1,3-bpp)2(NCS)2]2 (1, 1,3-bpp = 1,3-di(4-pyridyl)-propane)} can be exfoliated into single-layered 2D nanosheets (NS-1). As a consequence, the magnetism has been tuned from complete paramagnetic (bulk precursors) to SCO transition at around 250 K (2D nanosheets). In addition, the metal-to-ligand charge transition (MLCT), the intraligand π-π* transition and the color display also have been altered both in colloidal suspension and in the solid state. These dramatic changes of physical-chemical properties at different forms and states can be attributed to the efficient cooperativity derived from the interlayer van der Waals interactions within the curly or vertically stacked 2D building blocks.

Aqueous Stable Ti3C2 MXene Membrane with Fast and Photoswitchable Nanofluidic Transport.

High, stable, and modulatable ionic conductivity is important for many nanofluidic applications of layered two-dimensional (2D) membranes. In this study, we demonstrate a proton and ionic conductivity of the Ti3C2T x membrane that is orders of magnitude higher than that of bulk solution at low concentrations. Importantly, the membrane is highly stable in aqueous solution without any modification, due to the strong and attractive interlayer van der Waals interaction and weak electrostatic repulsive interaction. Furthermore, by exploiting the intrinsic photothermal property of MXene, we demonstrate that the ionic conductivity can be reversely, rapidly, and completely switched on or off with laser light. This study should prove MXene membrane as a suitable platform to study and utilize nanofluidic ion transport. It should also inspire future studies to manipulate the mass transport through 2D membranes using their inherent physicochemical properties.

Disassembly of 2D Vertical Heterostructures.

As one of the most widely discussed fields, the assembly of nanomaterials has always been extensively studied. However, its inverse process, namely disassembly, is still limited in the ambit of biomolecules. Specifically, in the emerging 2D research field, disassembly still remains unexplored. Inspired by the disassembly of DNA molecules via breaking intermolecular hydrogen bonds, the disassembly of 2D vertical heterostructures (2DVHs) is first achieved through the weakening of the interlayer van der Waals interactions. As a demonstration, ReS2 /WS2 VHs is successfully disassembled into individual building blocks. Density functional theory calculations are performed to study the disassembly of the 2DVHs, which simulate that 2DVHs are first activated by the disassembly promoters and then disassembled with weakened interlayer van der Waals interactions. Such a disassembly process demonstrates that it has great potential to be expanded as a general strategy to achieve the disassembly of other 2D superstructures.