vdW Interactions Research Articles

Computational investigations of the metal/semiconductor NbS2/boron phosphide van der Waals heterostructure: effects of an electric field.

In this work, we design computationally the metal-semiconductor NbS2/BP heterostructure and investigate its atomic structure, electronic properties and contact barrier using first-principles prediction. Our results show that the M-S NbS2/BP heterostructure is energetically stable and is characterized by weak vdW interactions. Interestingly, we find that the combination of the metallic NbS2 and semiconducting BP layers leads to the formation of a M-S contact. The M-S NbS2/BP heterostructure exhibits a p-type Schottky contact and a low tunneling-specific resistivity of 3.98 × 10-10 Ω cm2, indicating that the metallic NbS2 can be considered as an efficient 2D electrical contact to the semiconducting BP layer to design NbS2/BP heterostructure-based electronic devices with high charge injection efficiency. The contact barrier and contact type in the M-S NbS2/BP heterostructure can be adjusted by applying an external electric field. The conversion from p-type ShC to n-type ShC can be achieved by applying a negative electric field, while the transformation from ShC to OhC type can be achieved under the application of a positive electric field. The conversion between p-type and n-type ShC and ShC to OhC type in the NbS2/BP heterostructure demonstrates that it can be considered as a promising material for next-generation electronic devices.

Symmetry versus asymmetry game in vaporization enthalpies of imidazolium-based ionic liquids

The vaporization enthalpy as a function of alkyl chain length was studied using both experimental techniques and molecular dynamics simulations for imidazolium based ionic liquids (ILs) with symmetric and asymmetric imidazolium substitution. It was found that the vaporization enthalpy increases with increasing alkyl chain length due to the increased van der Waals (vdW) interaction. In addition, the vaporization enthalpies of the symmetric ILs ([CnCnIm][X], X = NTf2-, Cl-, Br-, I-) series were found to be systematically lower (by 5–10 kJ mol−1) in comparison with the corresponding asymmetric imidazolium-based ILs ([CmC1Im][X]). This energetic difference was attributed to the intramolecular dispersion interactions between the two alkyl side chains existing in the symmetric cation series.

Structural, electronic and magnetic properties of greigite Fe3S4 by GGA and GGA+U versus SCAN meta-GGA density functionals

The performance of exchange-correlation functional of density functional theory represented in generalized gradient approximation (GGA) and in the strongly constrained and appropriately normed (SCAN) meta-GGA scheme to study structural, electronic, and magnetic properties of greigite (Fe3S4) was investigated. The effects of inclusion of strong electron correlations represented by on-site Hubbard correction U, and nonlocality of the long-range van der Waals (vdW) interactions were also considered. Geometry optimization yielded the inverse spinel structure and lattice parameter of greigite in good agreement with experimental data. Calculated electronic structure revealed a half-metallic nature of the greigite bands for the applied functionals except for GGA, which predicts metallic behavior. Antiferromagnetic coupling of iron ions in tetrahedral and octahedral coordinations makes the overall crystal structure ferrimagnetic. In general the GGA+U and SCAN show comparable performance in prediction physical properties of greigite. Inclusion of the vdW correction does not change the character of the bands.

A study of 2D GeI2/InTe van der Waals hetero bilayer as a photocatalyst material

The computational study of the van der Waals hetero (vdW) bilayer GeI2/InTe has been carried out in present study. The isolated monolayer GeI2 and InTe have been studied first and the results were compared to the previous studies. The possible stackings are considered after the vdW interaction correction is applied in the structure relaxation. The vdW hetero bilayer stability has been checked from the phonon dispersion and ab initio Molecular Dynamics calculations. The charge transfer from InTe to GeI2 monolayer. Type-II indirect band gap (1.98, 2.01 eV) is verified by the projected band structure and band alignment calculations. The vdW hetero bilayer is a superior photocatalyst for the pH value up to pH = 0 to 11. The optical properties are calculated from the complex dielectric constant. The absorption coefficient shows the enhance absorption of light in the visible and ultraviolet regions. The vdW hetero bilayer has shown low reflectivity (37%) and a high refractive index (2.80) in the visible region. The enhanced optical properties have shown its possible applications in optoelectronic devices.

Reduction of thermal conductivity in carbon nanotubes by fullerene encapsulation from machine-learning molecular dynamics simulations

The carbon nano-peapod is a representative structure with interlayer van der Waals (vdW) interactions, in which encapsulated fullerene molecules play a critical role in modulating the transport properties of the carbon nanotubes (CNTs). In particular, their influence on the thermal transport characteristics has been the focal point of considerable attention. In this study, we trained an accurate machine learning potential for fullerene-encapsulated CNTs based on the efficient NEP model to investigate their thermal properties. Using equilibrium molecular dynamics simulation along with the spectral decomposition method for thermal conductivity, we find that the thermal conductivity of fullerene-encapsulated CNTs is roughly 55% lower than that of empty CNTs, aligning with experimental observations for CNT bundles with fullerene encapsulation [Kodama et al., Nat. Mater. 16, 892 (2017)]. The research suggests that weak vdW interactions between both the fullerene and CNTs, as well as between fullerene molecules themselves, hinder phonon propagation. The encapsulated fullerene contributes to an increase in phonon scattering within the CNTs, ultimately leading to a reduction in thermal conductivity. We utilized machine learning potential to investigate the structure of fullerene-encapsulated CNTs and their heat transport property. This approach provides valuable insights for performance research of complex systems featuring interlayer vdW interactions.

Programmable oscillation of C60 inside carbon nanotubes subjected to strain gradient

Programmable locomotion of molecules inside the carbon nanotube (CNT) has a significant role in controlling the reactions and delivery systems based on nanotubes. Using molecular dynamics (MD) simulations as well as the theoretical approach, we evaluate the oscillation of C60 inside the CNTs that are subjected to strain gradients from both sides. The molecular dynamics simulations are implemented by LAMMPS open-source software. Using this program, the van der Waals (vdW) interactions are established between C60 and nanotube, and the simulations are performed in canonical ensemble. The strain gradient applied on CNT provides the restoring force of the oscillation of C60. The potential energy of fullerene finds the minimum value at the unstrained region of CNT, which makes it the equilibrium point of oscillation. The amplitude of the oscillations is shown to be related to the thermal energy of C60. The frequency of the oscillations depends on the magnitude of the strain gradient applied on the nanotubes. At higher strain gradients of CNTs, we observe the increase in the frequency due to the increase in the restoring force acting on the fullerene molecule. We exploit the strained carbon nanotubes to control the position of C60 inside the nanotube. It has been shown that by changing the strain gradient of CNT, it is possible to steer the locomotion of C60 to different points inside the nanotube.

First principles insight on the stability and multiple properties of the Nb2CS2 MXene/MS2 (M=W, Mo, Ta) heterojunction structure

In the present study, the structural stability, mechanical, thermal physical, and electronic properties of the Nb2CS2/MS2 (M=Mo, W, Ta) vdW heterojunctions with three stacking modes were investigated by first principles calculations. We evaluated the structural stability with energetic, mechanical, and dynamical criteria. The thermal physical properties of the stable heterojunctions were obtained with the calculated phonon spectrums. Compared with the elastic properties of the single layers, the moduli of the heterojunctions are greater. The projected band structures show metallic character, while the Fermi energy level is mainly contributed by Nb. The ELF maps of the interfaces indicate that there only exists vdW interaction, and the asymmetric ELF could also be observed.

Probing chiral discrimination in biological systems using atomic force microscopy: The role of van der Waals and exchange interactions

We analyze from a theoretical perspective recent experiments where chiral discrimination in biological systems was established using Atomic Force Microscopy (AFM). Even though intermolecular forces involved in AFM measurements have different origins, i.e., electrostatic, bonding, exchange, and multipole interactions, the key molecular forces involved in enantiospecific biorecognition are electronic spin exchange and van der Waals (vdW) dispersion forces, which are sensitive to spin–orbit interaction (SOI) and space-inversion symmetry breaking in chiral molecules. The vdW contribution to chiral discrimination emerges from the inclusion of SOI and spin fluctuations due to the chiral-induced selectivity effect, a result we have recently demonstrated theoretically. Considering these two enantiospecific contributions, we show that the AFM results regarding chiral recognition can be understood in terms of a simple physical model that describes the different adhesion forces associated with different electron spin polarization generated in the (DD), (LL), and (DL) enantiomeric pairs, as arising from the spin part of the exchange and vdW contributions. The model can successfully produce physically reasonable parameters accounting for the vdW and exchange interaction strength, accounting for the chiral discrimination effect. This fact has profound implications in biorecognition where the relevant intermolecular interactions in the intermediate-distance regime are clearly connected to vdW forces.

Efficient DFT prediction of chemical and structural stability using van der Waals correction: application for A3B2Ga3O12 garnets (A = Lu, Y and B = Al, Sc)

Several seamless van der Waals (vdW) correction methods available for a wide range of systems could be expected to enhance stability predictions by accounting for the vdW effect. The stability of material can be evaluated using chemical potential phase diagram (CPD) which reveals the elemental chemical potential conditions for a successful synthesis. In this work, viability of various vdW correction approaches in improving the accuracy of stability prediction for A3B2Ga3O12 garnets (A = Lu, Y and B = Al, Sc) has been studied. From the results, we have found that vdW-df-cx, Grimme-D3, vdW-df-c09, and vdW-df2-c09 significantly improve ΔH prediction with MAPE of >5.0% lower than PBE, which exhibit their potential for stability prediction based on the CPD analysis. For CPD construction whose reliability is based on ΔH prediction, vdW-df-cx which can minimize the MAPE in ΔH, relative to experimental data, is selected as the best method among all studied vdW approaches. A more accurate description of total energy of O2 molecule and the competing compounds with layered structure can be also acquired by incorporating vdW interaction. However, the MAPE in lattice constant reveals that there is no significant improvement of lattice constant prediction for the studied garnets and their competing compounds. The vdW method which gives the MAPE in lattice constant slightly lower than that of PBE is vdW-df2-b86r. Although we found that the vdW corrections can improve material stability prediction, there is still room for the development of a novel DFT-based vdW method capable of accurately predicting both the lattice constant and ΔH of solids, including complex materials like garnets.

Non-axisymmetric modes in ultrathin annular liquid films coating a cylindrical fiber

This paper presents a detailed analysis of the three-dimensional stability properties of an annular liquid film coating a cylindrical fiber in the presence of van der Waals (vdW) interactions, whose influence depends on the wettability of the solid by the liquid. Under wetting conditions, vdW interactions can stabilize a uniform annular film when its thickness is smaller than a critical value that depends only on the fiber radius, the Hamaker constant, and the surface tension coefficient [Quéré et al., Science 249(4974), 1256–1260 (1990)]. In contrast, under non-wetting conditions, both surface tension and vdW forces contribute to destabilize the interface, and non-axisymmetric modes may become dominant depending on the thickness of the film and the relative strength of the surface tension and vdW forces. We perform temporal stability analyses of both the Stokes and lubrication equations of motion, allowing us to reveal the dominant azimuthal mode, as well as the optimal axial wavenumber and the corresponding temporal growth rate, as a function of the relevant governing parameters.

Enhanced stability of anthocyanins by cyclodextrin–metal organic frameworks: Encapsulation mechanism and application as protecting agent for grape preservation

Anthocyanins are promising naturally occurring food preservatives for enhancing the quality of food products due to their excellent antioxidant properties. However, their low stability hinders their food packaging application. Here, we propose a facile strategy to achieve the improved stability of anthocyanins encapsulated in γ-cyclodextrin metal-organic frameworks (CD-MOFs) with an in-depth exploration of their structure–property relationships. The adsorbed anthocyanins in CD-MOFs are stabilized by multiple cooperative non-covalent interactions including hydrogen bonding and van der Waals (vdW) interactions as demonstrated by density functional theory (DFT) calculations and spectroscopy analysis. Particularly, by ion-exchange of acetate ions into the pores of CD-MOFs, the resulting CD-MOFs (CD-MOF_OAc) shows a higher anthocyanins adsorption rate with a maximum loading capacity of 83.7 % at 1 min. Besides, CD-MOF_OAc possesses the more effective protecting effect on anthocyanins with at least two-fold enhancement of stability in comparison of free anthocyanins under heating and light irradiation. The anthocyanins encapsulated CD-MOFs films for fruit freshness was validated by the Kyoho experiment. This novel encapsulation system provides a new possibility for the potential use of CD-MOFs as the encapsulating material for anthocyanins in fruit preservation.

Structure property correlation study of energetic solid XeO3

In the realm of material science, xenon trioxide (XeO3) is characterized by an significant degree of shock sensitivity, resembling characteristics associated with primary explosives. To determine the underlying principles and quantify such behaviour we present a comprehensive computational study addressing the structural, vibrational, electronic and optical behaviour of energetic XeO3 by employing DFT calculations (Density Functional Theory). Our study demonstrates a good agreement between our computed structural parameters at equilibrium and experimental findings, particularly after incorporating vdW (van der Waals) interactions. Also, the mechanical stability of XeO3 is assessed by computing the Cij (elastic constants) and the B (bulk modulus). The evaluation of elastic constants in various directions indicates that (C33 > C22 > C11) XeO3 is highly shock sensitive along crystallographic a-axis. The computed (B) bulk modulus is determined to be 12.98 GPa, aligning closely with the values observed in other primary explosives. The phonon dispersion spectra computed within the harmonic approximation confirms the dynamical stability of XeO3 in P212121 structure. The spectra also reveal a notable interaction between the acoustic and optical branch, resulting in low thermal conductivity which is a trait for the energetic materials. The precise measurement of the energy band gap is crucial for establishing potential correlations with the photodecomposition process of energetic materials. To support this, we have conducted band structure calculations using the TB-mBJ (Tran-Blaha modified Becke-Johnson) potential, alongside the conventional GGA/LDA methods, generally known to underestimate band gaps. Our TB-mBJ based calculations yielded an energy band gap around 2.5 eV, which can also be compared with other energetic compounds. The rapid fluctuations of the optical constants suggest photodecomposition nature of XeO3 in the near UV region. The insights gained from our study would be of high interest for the study of other xenon (Xe) compounds and their oxides, delivering direction for potential applications across diverse fields.

Graphene quantum dots blocking the channel egresses of cytochrome P450 enzyme (CYP3A4) reveals potential toxicity

Graphene quantum dots (GQDs) have garnered significant attention, particularly in the biomedical domain. However, extensive research reveals a dichotomy concerning the potential toxicity of GQDs, presenting contrasting outcomes. Therefore, a comprehensive understanding of GQD biosafety necessitates a detailed supplementation of their toxicity profile. In this study, employing a molecular dynamics (MD) simulation approach, we systematically investigate the potential toxicity of GQDs on the CYP3A4 enzyme. We construct two distinct simulation systems, wherein a CYP3A4 protein is enveloped by either GQDs or GOQDs (graphene oxide quantum dots). Our results elucidate that GQDs come into direct contact with the bottleneck residues of Channels 2a and 2b of CYP3A4. Furthermore, GQDs entirely cover the exits of Channels 2a and 2b, implying a significant hindrance posed by GQDs to these channels and consequently leading to toxicity towards CYP3A4. In-depth analysis reveals that the adsorption of GQDs to the exits of Channels 2a and 2b is driven by a synergistic interplay of hydrophobic and van der Waals (vdW) interactions. In contrast, GOQDs only partially obstruct Channel 1 of CYP3A4, indicating a weaker influence on CYP3A4 compared to GQDs. Our findings underscore the potential deleterious impact of GQDs on the CYP3A4 enzyme, providing crucial molecular insights into GQD toxicology.

Influence of van der Waals Interactions between the Alkyl Chains of Surface Ligands on the Size and Size Distribution of Nanocrystals Prepared by the Digestive Ripening Process.

Thermal heating of polydispersed nanocrystals (NCs) with surface-active organic ligands in a solvent leads to the formation of monodispersed NCs, and this process is known as digestive ripening (DR). Here, by performing DR on Au NCs using different-chain-length amine and thiol ligands, we evidently show that ligands with C12 chain length result in the formation of NCs with narrow size distributions when compared to C8, C16, and C20 chain length ligands. Furthermore, our findings also show that in the case of alkyl thiol, the NC size remains more or less the same, while the size distribution gets altered significantly with the chain length. On the other hand, both size and size distribution are affected significantly when the alkyl amine chain length is varied. Fourier transform infrared (FTIR) studies indicate that the van der Waals (vdW) interactions are weakest when the amine with C12 carbon chain is used as the DR agent, while in the case of thiols, molecules with C8 and C12 chain lengths have nearly the same vdW interactions (with C12 slightly weaker than C8), which are weaker than those of C16 and C20. Molecular dynamics (MD) simulation results corroborate the experimental observations and suggest that due to more defects in the alkyl chain, the C8 and C12 (amine as well as thiol) ligands are disordered and less stable on Au(111) and Au(100) surfaces. This could result in efficient etching and redeposition, making the ligands with C8 and C12 chain lengths the better DR agents.

High carrier mobility ZrSSe/SnX (X=S, Se, S2, Se2) vdW heterostructures for photocatalytic overall water splitting: A first-principles study

Recently, the assembly of novel two-dimensional materials via van der Waals (vdW) interactions to form heterostructures is considered an effective strategy for producing materials with enhanced photocatalytic activities. This study systematically explored the optoelectronic and photocatalytic properties and carrier mobilities of novel ZrSSe/SnX (X = S, Se, S2, Se2) vdW heterostructures using density functional theory. The formation energies, elastic constants, phonon dispersions, and results of ab-initio molecular dynamic simulations indicate structural, mechanical, dynamic, and thermal stability. The electronic band structure and optical properties are calculated using hybrid functional HSE06 for accurate calculations. The results of calculations using the bandgap center technique based on density functional theory show that ZrSSe/SnSe, ZrSSe/SnS2, and ZrSSe/SnSe2 heterostructures possess suitable band edges for photocatalytic overall water splitting. The optical absorption spectrum covers a broad range from 104 in the visible region and increases to 105 in the UV region. Furthermore, the ZrSSe/SnX (X = S, Se, S2, Se2) heterostructures have high carrier mobilities of the order of 105. High mobilities help carriers to move quickly and reduce the recombination rate of holes and electrons. The findings indicate that ZrSSe/SnX (X = S, Se, S2, Se2) heterostructures are potentially efficient visible-light photocatalysts for overall water splitting.

Graphene oxide incorporated PSU/SPEKEKK for potential application as proton exchange membranes in fuel cells

This study aims to develop a new PEM for fuel cell applications consisting of PSU/SPEKEKK/SGO membranes with improved physiochemical properties to surmount the low power generation of Nafion. The DFT calculations using NCI plots show that there are network interactions between PSU and SPEKEKK, established by the hydrogen bonding and vdW interactions. SGO was incorporated into the as-prepared PSU/SPEKEKK membrane. A solution casting method was employed to fabricate SGO-loaded composite membranes in the range of 0.1% to 1.5% (wt%). Structural evidence of the SGO and composite membranes was characterized by ATR-FTIR and FESEM, respectively. Adding SGO to PSU/SPEKEKK shows a significant increase in proton conductivity due to the presence of more protonated sites (SO3H) and water-mediated interactions for proton conduction. Of all the compositions considered, PSU/SPEKEKK/SGO-0.6-1.5% loading was found to have the highest proton conductivity, 0.03Scm−1. Although adding SGO to the composite membrane has reduced water absorption, the hydrophilic oxygenated functional groups in SGO, the bound water molecules, and the increased –SO3H groups have successfully improved the proton conductivity. The findings reveal that all composite membranes show excellent oxidative stabilities for 32 h in Fenton's reagent. To summarize, all composite membranes fabricated in this study show acceptable water uptake and swelling ratio, high oxidation, excellent thermal stability, and high proton conductivity.

Approximating the van der Waals interaction potentials between agglomerates of nanoparticles

Van der Waals (vdW) are probably one of the most important interaction potentials or forces between nanoparticles and interfaces found in many applications of powder technology spanning filtration, impaction, coagulation, agglomerates restructuring, and resuspension. However, the vdW interactions are currently poorly modeled especially for raspberry-like and fractal-like agglomerates of nanoparticles due to their complex shape. In this work, we have developed both analytical and semi-analytical models to predict the agglomerate-molecule and agglomerate-agglomerate vdW interactions. These new models can find multiple applications in powder technology as those mentioned above.

Molecular Dynamics of Hydration Shells of Adsorbates in Entropy-Driven Adsorption (Hydrophobic Bonding) to Activated Carbon Surfaces

Hydrophobic bonding is a phenomenon wherein the adsorption of solutes from aqueous solutions is driven largely by the desire of solvent molecules to interact with each other, thus squeezing out solute molecules onto the adsorbent surface. A novel computational analysis of hydration shell water dynamics was used to study the driving force for the hydrophobic bonding of five small drug molecules to activated carbon. It was demonstrated that the solvation of these drug molecules produced hydration shells of lower density and molecular mobility than bulk water, up to 10.5–14 Å distance. Excellent correlations were found between the simulated water-water hydrogen bonding lifetimes in the hydration shell and the experimental capacity constants of hydrophobic bonding (KHB) obtained from the Two-Mechanism Langmuir-Like Equation. KHB also correlated well with the solute-solvent vdW interaction energies in a manner that could allow future predictions of KHB values from simple simulations. Such correlations were not found with the capacity constant of the well-known enthalpy-driven adsorption. The driving force for hydrophobic bonding has entropic origins due to the elimination of water structuring in the hydration shells. However, unlike a typical entropy-driven process, hydrophobic bonding to activated carbon was also associated with a large exothermic enthalpy change when studied with isoperibol calorimetry.

LibMBD: A general-purpose package for scalable quantum many-body dispersion calculations.

Many-body dispersion (MBD) is a powerful framework to treat van der Waals (vdW) dispersion interactions in density-functional theory and related atomistic modeling methods. Several independent implementations of MBD with varying degree of functionality exist across a number of electronic structure codes, which both limits the current users of those codes and complicates dissemination of new variants of MBD. Here, we develop and document libMBD, a library implementation of MBD that is functionally complete, efficient, easy to integrate with any electronic structure code, and already integrated in FHI-aims, DFTB+, VASP, Q-Chem, CASTEP, and Quantum ESPRESSO. libMBD is written in modern Fortran with bindings to C and Python, uses MPI/ScaLAPACK for parallelization, and implements MBD for both finite and periodic systems, with analytical gradients with respect to all input parameters. The computational cost has asymptotic cubic scaling with system size, and evaluation of gradients only changes the prefactor of the scaling law, with libMBD exhibiting strong scaling up to 256 processor cores. Other MBD properties beyond energy and gradients can be calculated with libMBD, such as the charge-density polarization, first-order Coulomb correction, the dielectric function, or the order-by-order expansion of the energy in the dipole interaction. Calculations on supramolecular complexes with MBD-corrected electronic structure methods and a meta-review of previous applications of MBD demonstrate the broad applicability of the libMBD package to treat vdW interactions.

Kinetic Delay in Cooperative Supramolecular Polymerization by Redefining the Trade-Off Relationship between H-Bonds and Van der Waals/π-π Stacking Interactions.

We here present how rebalancing the interplay between H-bonds and dispersive forces (Van der Waals/π-π stacking) may induce or not the generation of kinetic metastable states. In particular, we show that extending the aromatic content and favouring the interchain VdW interactions causes a delay into the cooperative supramolecular polymerization of a new family of toluene bis-amide derivatives by trapping the metastable inactive state.