Van Der Waals Type Research Articles

Promising 2D Carbon Allotropes with Intrinsic Bandgaps Based on Bilayer α‐Graphyne

2D carbon allotropes with moderated electronic bandgaps are highly desirable for next‐generation carbon‐based nanoelectronics beyond graphene. Herein, the structural and electronic properties of bilayer α‐graphyne have been systematically investigated by using first‐principles calculations. Consequently, in addition to typical van der Waals (vdW) stacks of single‐layer α‐graphyne, new 2D carbon allotropes with purely sp2 or sp3 hybridizations can be achieved, arising from the absence of intralayer acetylene linkages during structural relaxation. Compared to the Dirac semimetallic behavior of monolayer α‐graphyne, the electronic structure of vdW bilayer α‐graphyne can be further modulated by stacking patterns, yet retains metallic characteristics. In contrast, intrinsic semiconducting characteristics can be observed in the proposed new 2D carbon allotropes with tunable bandgaps dependent on the in‐plane biaxial strain. Correspondingly, a pristine 2D carbon sheet with only sp2 hybridization exhibits a promising direct bandgap of 1.062 eV, while the sp3‐hybridized carbon network possesses an indirect bandgap of 1.708 eV, which can be further converted to the direct type under small compressive strains, implying great potential in future carbon‐based nanoelectronic applications beyond graphene. Also, this work provides a new approach for developing novel carbon allotropes via the vertical stacking of graphyne with acetylenic linkages.

Application of the significant structure theory for the viscosity modeling of ionic fluids

The present work introduces the application of a modified significant structure theory (SST) in order to obtain improved representations of the dynamic viscosity of several representative last-generation ionic fluids: pure ionic liquids (ILs) and deep eutectic solvents (DESs). The activated-state variables present in the resulting SST-based model were related to well-known thermodynamic potentials (residual internal energy, liquid and solid molar volumes) which in turn were estimated from two simple cubic equations of state of the van der Waals type: Soave-Redlich-Kwong or Peng-Robinson. The modifications introduced to the SST approach were successfully verified during the correlation and prediction of experimental dynamic viscosities of 3 families of imidazolium-based ILs ([CXmim][BF4], [CXmim][PF6] and [CXmim][Tf2N]), one pyridinium-based IL ([b3mpy][BF4]), one pyrrolidinium-based IL ([P14][Tf2N]), one ammonium-based IL ([N1114][Tf2N]) and four ILs having nonfluorinated anions ([dmim][MeSO4], [bmim][EtSO4], [bmim][Ac] and [b3mpy][dca]) over a temperature range varying from 273.15 to 438.15 K and at pressures from 1 to 3,000 bar We also considered three archetypal choline chloride-based DESs for model validation: Reline, Ethaline and Glyceline within a temperature range varying from 293.15 to 373.15 K and at pressures from 1 to 1,000 bar

Insights on the Bonding Mechanism, Electronic and Optical Properties of Diamond Nanothread-Polymer and Cement-Boron Nitride Nanotube Composites.

The success of composite materials is attributed to the nature of bonding at the nanoscale and the resulting structure-related properties. This study reports on the interaction, electronic, and optical properties of diamond nanothread/polymers (cellulose and epoxy) and boron nitride nanotube/calcium silicate hydrate composites using density functional theory modeling. Our findings indicate that the interaction between the nanothread and polymer is due to van der Waals-type bonding. Minor modifications in the electronic structures and absorption spectra are noticed. Conversely, the boron nitride nanotube-calcium silicate hydrate composite displays an electron-shared type of interaction. The electronic structure and optical absorption spectra of the diamond nanothread and boron nitride nanotube in all configurations studied in the aforementioned composite systems are well maintained. Our findings offer an electronic-level perspective into the bonding characteristics and electronic-optical properties of diamond nanothread/polymer and boron nitride nanotube/calcium silicate hydrate composites for developing next-generation materials.

Tunable electronic and optical properties of a type-II GaP/SiH van der Waals heterostructure as photocatalyst: A first-principles study

In this study, a novel GaP/SiH heterostructure was successfully predicted and constructed. Subsequently, an in-depth and systematic investigation was conducted to explore its structural, electronic, transport and optical properties. The calculated structural performance results revealed that the GaP/SiH heterostructure was a typical type-II van der Waals heterostructure, exhibiting excellent energy stability, mechanical stability and thermodynamic stability. In particular, the formation of the GaP/SiH heterostructure significantly reduced the bandgap of the materials to 2.24eV, and the type-II energy band arrangement effectively suppressed the recombination of photogenerated carriers. Furthermore, the biaxial and vertical strains could finely modulate the electronic structure of the GaP/SiH heterojunction. The electron mobility of the GaP/SiH heterostructure was measured to be 1573.91 cm2 V⁻1 s⁻1, indicating its superior charge carrier transport capabilities. Meanwhile, the GaP/SiH heterostructure also exhibited excellent optical absorption performance in the visible and UV region up to 2.34 × 106 cm−1. Thus, the GaP/SiH heterostructure will be a strong candidate for photocatalytic applications due to its excellent light absorption properties, electrical properties, and stability.

Magnetic Properties of Self-Assemble Naphthalene Diimide Radical Aggregates.

The concept of creating room-temperature ferromagnets from organic radicals proposed nearly sixty years ago, has recently experienced a resurgence due to advances in organic radical chemistry and materials. However, the lack of definitive design paradigms for achieving stable long-range ferromagnetic coupling between organic radicals presents an uncertain future for this research. Here, an innovative strategy is presented to achieve room-temperature ferromagnets by assembling π-conjugated radicals into π-π stacking aggregates. These aggregates, with ultra-close π-π distances and optimal π-π overlap, provide a platform for strong ferromagnetic (FM) interaction. The planar aromatic naphthalene diimide (NDI) anion radicals form nanorod aggregates with a π-π distance of just 3.26 Å, shorter than typical van der Waals distances. The suppressed electron paramagnetic resonance (EPR) signal and emergent near-infrared (NIR) absorption of the aggregates confirm strong interactions between the radicals. Magnetic measurements of NDI anion radical aggregates demonstrate room-temperature ferromagnetism with a saturated magnetization of 1.1 emug-1, the highest among pure organic ferromagnets. Theoretical calculations reveal that π-stacks of NDI anion radicals with specific interlayer translational slippage favor ferromagnetic coupling over antiferromagnetic coupling.

Phase transitions and composite order in U(1)N lattice London models

The phase diagrams and the nature of the phase transitions in multicomponent gauge theories with an Abelian gauge field are important topics with various physical applications. While an early renormalization-group-based study indicated that the direct transition from a fully ordered to a fully disordered state is continuous for N=1 and N>183, recently it was demonstrated that the transition is discontinuous for N=2. We quantitatively study the dependence on N of the degree of discontinuity of this transition. Our results suggest that the transition is discontinuous at least up to N=7. Furthermore, we demonstrate that, at increased coupling strength, the phase transitions of the neutral and charged sectors of the model split, which for N>2 yields a new phase with composite order. The transition from the composite-order phase to the fully disordered phase is then also discontinuous, at least for N=3 and N=4. Via a duality argument, this indicates that van der Waals–type interaction between directed loops may be responsible for the discontinuous phase transitions in these models. Published by the American Physical Society 2024

Interplay between the Lyapunov exponents and phase transitions of charged AdS black holes

We study the relationship between the standard or extended thermodynamic phase structure of various anti–de Sitter black holes and the Lyapunov exponents associated with the null and timelike geodesics. We consider dyonic, Bardeen, Gauss-Bonnet, and Lorentz-symmetry breaking massive gravity black holes and calculate the Lyapunov exponents of massless and massive particles in unstable circular geodesics close to the black hole. We find that the thermal profile of the Lyapunov exponents exhibits distinct behavior in the small and large black hole phases and can encompass certain aspects of the van der Waals type small/large black hole phase transition. We further analyze the properties of Lyapunov exponents as an order parameter and find that its critical exponent is 1/2, near the critical point for all black holes considered here. Published by the American Physical Society 2024

Structural phase transition and potential superconductivity initiated by pressure-driven in 1T–CrSe2

The exploration of the superconducting properties of antiferromagnetic parent compounds containing transition metals under pressure provides a unique idea for finding and designing superconducting materials with better performance. In this paper, the close relationship between the possible superconductivity and structure phase transition of the typical van der Waals layered material 1 T–CrSe2 induced by pressure is studied by means of electrical transport and x-ray diffraction for the first time. We introduce the possibility of pressure-induced superconductivity at 20 GPa, with a critical T c of approximately at 4 K. The superconductivity persists up to the highest measured pressure of 70 GPa, with a maximum T c ∼ 5 K at 24 GPa. We observed a structure phase transition from P-3 m1 to C2/m space group in the range of 9.4–11.7 GPa. The results show that the structural phase transition leads to the metallization of 1 T–CrSe2 and the further pressure effect makes the superconductivity appear in the new structure. The material undergoes a transition from a two-dimensional layered structure to a three-dimensional structure under pressure. This is the first time that possible superconductivity has been observed in 1 T–CrSe2.

Two-dimensional platinum borides: A Dirac nodal line quantum electrocatalyst for efficient hydrogen evolution reaction

Two-dimensional (2D) catalysts exhibiting high carrier mobility, near-zero Gibbs free energy (ΔGH*), and an active basal plane are highly desirable for optimizing the hydrogen evolution reaction (HER). However, fusing these attributes within a solitary material encounters profound difficulties. Utilizing density functional theory (DFT) calculations, we have identified the PtB2 monolayer is such a candidate, demonstrating exceptional dynamical, thermal, and mechanical stability. The band structure analysis reveals that the PtB2 monolayer features double Dirac points along its high-symmetry lines. The estimated Fermi velocity of the PtB2 monolayer is as high as 7.8×105m/s, close to that of graphene. By scanning the Dirac points in the first Brillouin Zone, the Dirac points constitute a nodal-line semimetal with a loop centered at the Γ point. This nodal line is anticipated to facilitate charge transfer between the catalyst surfaces and reaction intermediates, potentially enhancing catalytic efficiency. Importantly, PtB2 monolayer boasts superior HER performance, with a nearly zero ΔGH* of 0.057 eV at its boron-dominant sites. The application of a subtle 1% strain serves to reduce its ΔGH* to an even more favorable −0.003 eV, suggesting significantly improved HER capabilities. When layered in a bilayer form, the PtB2 exhibits van der Waals (vdW) type interlayer interactions. Remarkably, we found that varying the stacking order can effectively alter the nodal line's shape and, accordingly, the PtB2 layer's catalytic properties. The distinctive combination of a Dirac nodal line, ideal Gibbs free energy, and expansive active sites positions the PtB2 monolayer as a competitive 2D quantum catalyst for hydrogen production.

Understanding the origins of reversible and hysteretic pathways of adsorption phase transitions in metal-organic frameworks

Phase behavior of nanoconfined fluids adsorbed in metal–organic frameworks is of paramount importance for the design of advanced materials for energy and gas storage, separations, electrochemical devices, sensors, and drug delivery, as well as for the pore structure characterization. Phase transformations in adsorbed fluids often involve long-lasting metastable states and hysteresis that has been well-documented in gas adsorption–desorption and nonwetting fluid intrusion-extrusion experiments. However, theoretical prediction of the observed nanophase behavior remains a challenging problem. The mesoscopic canonical, or mesocanonical, ensemble (MCE) is devised to study the nanophase behavior under conditions of controlled fluctuations to stabilize metastable and labile states. Here, we implement and apply the MCE Monte Carlo (MCEMC) simulation scheme to predict the origins of reversible and hysteric adsorption phase transitions in a series of practical MOF materials, including IRMOF-1, ZIF-412, UiO-66, Cu-BTC, IRMOF-74-V, VII, and IX. The MCEMC method, called the gauge cell method, allows to produce Van der Waals type isotherms with distinctive swings around the phase transition regions. The constructed isotherms determine the positions of phase equilibrium and spinodals, as well as the nucleation barriers separating metastable states. We demonstrate the unique capabilities of the MCEMC method in quantitative predictions of experimental observations compared with the conventional grand canonical and canonical ensemble simulations. The MCEMC method is implemented in the open-source RASPA and LAMMPS software packages and recommended for studies of adsorption behavior and pore structure characterization of MOFs and other nanoporous materials.

Mesoporous magnetic biochar derived from common reed (Phragmites australis) for rapid and efficient removal of methylene blue from aqueous media

A novel mesoporous magnetic biochar (MBC) was prepared, using a randomly growing plant, i.e., common reed, as an exporter of carbon, and applied for removal of methylene blue (MB) from aqueous solutions. The prepared sorbent was characterized by nitrogen adsorption/desorption isotherm, saturation magnetization, pH of point of zero charges (pHPZC), Fourier-transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The obtained MBC has a specific surface area of 94.2 m2 g−1 and a pore radius of 4.1 nm, a pore volume of 0.252 cm3 g−1, a saturation magnetization of 0.786 emu g−1, and a pHPZC of 6.2. Batch adsorption experiments were used to study the impact of the physicochemical factors involved in the adsorption process. The findings revealed that MB removal by MBC was achieved optimally at pH 8.0, sorbent dosage of 1.0 g L−1, and contact time of 30 min. At these conditions, the maximum adsorption was 353.4 mg g−1. Furthermore, the adsorption isotherm indicated that the Langmuir pattern matched well with the experimental data, compared to the Freindlich model. The ∆G was − 6.7, − 7.1, and − 7.5 kJ mol−1, at 298, 308, and 318 K, respectively, indicating a spontaneous process. The values of ∆H and ∆S were 5.71 kJ mol−1 and 41.6 J mol−1 K−1, respectively, suggesting endothermic and the interaction between MB and MBC is van der Waals type. The absorbent was regenerated and reused for four cycles after elution with 0.1 mol L−1 of HCl. This study concluded that the magnetic biochar generated from common reed has tremendous promise in the practical use of removing MB from wastewater.Graphic

Topological interpretation of extremal and Davies-type phase transitions of black holes

Topological arguments are currently being used as a novel scheme to discern the properties of black holes while ignoring their detailed structure and specific field equations. Among various avenues of black hole physics, where this novel approach is being utilized, the phase transition in black hole thermodynamics lies at the forefront. There are several types of phase transition in black holes; such as the van der Waals type phase transition, Davies-type phase transition, extremal phase transition, and Hawking-Page (HP) transition. So far, the topological interpretation, where the critical point has been identified with the non-zero topological charge, has been obtained only for the van der Waals type phase transition and HP transition in different spacetimes. To complete the picture, here we provide the same interpretation for two other phase transitions: Davies-type phase transition and extremal phase transition. The entire analysis is general and is valid for any spacetime where these types of phase transitions are observed. More importantly, our analysis suggests that amid the apparent differences in these phase transitions, they share the same topological characteristics, i.e. non-zero topological charge arising from different thermodynamic potentials in different types of phase transition.

Mn-S chemical bond mediated Z-scheme photocatalyst boosting efficient H2 evolution with apparent quantum yield exceeding 32% under visible light

Developing a highly effective solar-to-H2 conversion system is a promising strategy for carbon neutrality. Here, we report a chemically bonded Z-scheme photocatalyst by specifically linking the conduction band site of ZnIn2S4 with the valence band site of Mn0.3Cd0.7S1-x. Deviates from the typical van der Waals interactions between two semiconductors, the in-built robust binding force (Mn-S bond) and charge transfer channel facilitates the electron mobility, minimizes detrimental deep charge trapping, and prolongs the active charge lifetime from 983 ps to 4250 ps. The sulfur vacancies in Mn0.3Cd0.7S1-x are partially refilled by atoms of the same element in ZnIn2S4 during the in-situ formation of heterojunctions, which promotes and solidates the establishment of interfacial chemical bonds. The optimal photocatalyst achieves a high quantum efficiency of 32.71 % at 420 nm. This study offers a new insight into the development of Z-Scheme heterojunctions via a synergistic approach involving doping, vacancy filling and strong interfacial chemical bonds.

Weakly bound complexes of 1,2,3-triazole with nitrogen and carbon dioxide isolated in solid argon: A combined FT-IR matrix isolation and theoretical investigation

Matrix isolation FT-IR spectroscopy was combined with quantum-chemical calculations in order to characterize complexes of 1,2,3-triazole (3TR) with nitrogen and carbon dioxide. Geometries of the possible 1:1 and 1:2 complexes, as well as 3TR dimers, were optimized at the DFT (B3LYP-D3) level of theory with the 6–311++G(3df,3pd) basis set. Six different 3TR⋯N2 structures of the 1:1 stoichiometry were optimized which are characterized by weak hydrogen bonds (N-H⋯N and C-H⋯N) and/or Van der Waals type interactions (N⋯C, N⋯N, N⋯π). Two the most stable geometries, both containing a N-H⋯N bridge, were identified experimentally in solid argon. As for 3TR⋯CO2 complexes, out of two minima located on the potential energy surface, only one with a strongly bent N-H⋯O hydrogen bond was detected in the matrix after deposition. In both cases, only annealing experiments at 32 K resulted in the formation of small amounts of 1:2 structures.

Experimental Investigation of Superconductivity in PdSSe Under High Pressure

AbstractAnalysis of the superconducting properties of transition metal dichalcogenides (TMDs) under high pressures offers valuable insights to guide the design and synthesis of high‐performance superconducting materials. Herein, the effect of pressure is investigated on the superconductivity of a typical van der Waals layered TMDs material, PdSSe, by measuring its transport properties. After initially increased pressure, superconductivity emerges at 10.2 GPa, with a critical superconducting temperature (Tc) of ≈5.1 K, accompanied by the diminishing charge density wave (CDW) that is originally strengthening. Then, the Tc gradually increases with increasing pressure, reaching 12.1 K at the maximum pressure. The study provides experimental evidence for the superconductivity of PdSSe, and to the best of the knowledge, this is the first report on the observation of amplified CDW phenomena under increasing pressure in nonmagnetic TMDs. The abnormal enhancement of CDW transition temperature at low pressure is consistent with the upward trend of resistance, which is related to the electron–electron interaction. Moreover, synchrotron X‐ray diffraction experiments reveal two additional structural phase transitions.

Van der Waals heterostructures of AlAs and InSe: Stacking-dependent Raman spectra and electric field dependence of electronic properties

In the present work, the electronic and vibrational properties of a van der Waals type heterostructure, composed of single layers of AlAs and InSe, are investigated using density functional theory (DFT)-based first-principles calculations. Vibrational analyses reveal that dynamically stable single layers of AlAs and InSe form van der Waals type heterostructure which is shown to exhibit stacking-dependent Raman spectra by means of the frequency shifts. According to our findings, a type-II band alignment with a direct band gap of 1.84 eV is found in the ground state stacking of AlAs/InSe vertical heterostructure, in contrast to the indirect band gap behaviors of each individual layer. Moreover, the application of an external vertical electric field shows that the both band alignment type and the electronic behavior of the heterostructure can be tuned. The heterostructure is found to exhibit direct to indirect band gap transition under negative electric field as well as a transition from type-II to type-I heterojunction under negative fields up to 0.3 V/Å. The stronger fields along the same direction results in overlapping of valence states of each layer and lead to a non-linear change of the energy band gap. Overall, the predicted van der Waals type heterobilayer of InSe and AlAs with stacking-dependent vibrational features and well-controlled electronic properties under external field is shown to be potential candidate for optical and optoelectronic applications.

Chlorpropamide-cyclodextrin inclusion complexes, theoretical basis of stability

A study of the stability of inclusion complexes of chlorpropamide and α, β, γ, and HP-β cyclodextrin, has been carried out using molecular dynamics simulations. The contribution of weak interactions to the stability of the complexes through DFT has been analyzed. The conventional molecular dynamics analysis suggests stable complexes for the inclusion of chlorpropamide. As expected, the complex stabilization is driven by hydrophobic interactions between cyclodextrin and chlorpropamide aromatic ring; similar mobility of chlorpropamide in the cyclodextrin cavities was observed. Molecular dynamics results indicate that intermolecular hydrogen bonds appeared between 17.70 and 22.52 % of the simulation time; consequently, the crucial complex stabilizing interactions encompass more than just hydrogen bonds, revealing a more intricate network of stabilizing forces at play. For the first time, based on quantum chemistry, a correlation between the electron density contributions to weak interactions and the glucopyranose units in each CD is presented. The most relevant stabilizing interactions are of the van der Waals type, followed by small contributions from hydrogen bonds. Non-covalent interactions are more intense in the chlorpropamide/HP-β cyclodextrin complex than in other systems. The inclusion complexes formed with β-type cyclodextrins are the most stable (due to the number and intensity of the contributions), in agreement with the molecular dynamics simulation results. In addition, MMPBSA and QM energies predict CPD/αCD as unstable and least stable complex, respectively. These results highlight the importance of β-type cyclodextrins in the formation of creating reliable inclusion complexes, which could be potentially applied to molecular encapsulation and drug delivery systems.

Assessment of the nanodelivery capacity of antineoplastic jacaranone by B12N12 nanocages: A DFT study

Cancer is a malignant disease characterized by uncontrolled multiplication of cancer cells in certain organs. As an alternative therapy, Jacaranone could be used as an anticancer. The use of B12N12 nanocages for hosting and transporting Jacaranone is an appealing strategy. In this context, a theoretical study was conducted to evaluate the interactions between Jacaranone and B12N12. Different positions were evaluated and four possible complexes were found to be stable. The structural analysis revealed small interaction distances for the boron–oxygen interaction, 1.54 and 1.58 Å for complexes 3 and 4, respectively. The energies calculated for the complexes showed that all complexes can occur (Eads < 0), they are exothermic and the complexes 3 and 4 are spontaneous. The quantum theory of atoms in molecules (QTAIM) characterized the B‧‧‧O interactions as partially covalent, and others are electrostatic. Just like QTAIM, the interaction region indicator (IRI) analysis indicates a relatively high concentration of electron density between the oxygen of the Jacaranone and the boron of the nanocage. Non-covalent-interaction and reduced-density gradient plots showed that weak interactions were van der Waals type.

Improved data analysis for molecular dynamics in liquid CCl4

Previously reported inelastic x-ray scattering spectra of a typical van der Waals molecular liquid CCl4 were reanalyzed using a generalized Langevin formalism with a memory function including a thermal and two viscoelastic relaxation processes and simple sparse modeling. The obtained excitations of longitudinal acoustic phonons show a largely positive deviation from the hydrodynamic value by about 57%, much larger than about 37% by the previously reported damped harmonic oscillator result. Such large values of fast sounds in molecular liquids larger than 15-20% of typical liquid metals are interpreted as extra energy losses for terahertz phonons by molecules' vibrational and rotational motions. The rates of the fast and slow viscoelastic relaxations in the memory function at low Q indicate large values, about 0.5 and 2 ps, which correspond to the vibrational and rotational motions of CCl4 molecules, respectively. These values are larger than those of the typical polar molecular liquid acetone, which may reflect the heavier atomic mass of CCl4. The Q dependences of the viscoelastic relaxation rates are discussed in terms of the lifetime and propagating length of the terahertz phonon oscillations. The microscopic kinematic longitudinal viscosity was obtained from the viscoelastic relaxation magnitudes and times, rapidly decreasing with Q from a macroscopic value at Q→0.

On the nanoscale interface, electronic structure, and optical properties of nanocarbon-reinforced calcium silicate hydrates

This study presents results from quantum chemical simulations of the synergetic interaction, electronic structure, and optical properties of calcium-silicate hydrates (C-S-H) reinforced by graphene-nanoribbons and single-walled carbon nanotubes (SWCNT). The calculations show that C-S-H/graphene-nanoribbon and C-S-H/SWCNT composites are stabilized by electrostatic interaction due to the charge transfer from Ca ions at the interface of C-S-H to the nearby C atoms of the graphene-nanoribbon and SWCNT. Removing Ca ions at the interface drastically decreases the strength of interaction into a weak van der Waals type. The Bader charge transfer analysis and electron distribution topology further confirm these results. Generally, the electronic states of the graphene-nanoribbon and SWCNT are shifted to lower energy in the complex. The electronic structure of graphene-nanoribbon and SWCNT is susceptible to the Ca ions-rich C-S-H environment. The composites’ overall absorption spectra can be considered superimposed of the isolated nanocarbon and C-S-H except in the lower energy region due to charge transfer and realignment of energy states. The results presented here reveal the bonding mechanism of the C-S-H with nanocarbon at the fundamental level. This work serves as a reference for the nanoengineering cement-based material with nanocarbon for the next-generation smart infrastructure.