vdW Forces Research Articles

Adsorption behavior and quantum chemical analysis of surface functionalized polystyrene nano-plastics on gatifloxacin.

In this paper, the adsorption of gatifloxacin (GAT) by three types of polystyrene nano-plastics (PSNPs), including 400 nm polystyrene (PS), amino-modified PS (PS-NH2), and carboxyl-modified PS (PS-COOH) was studied and the adsorption mechanism were assessed. Experimental findings revealed that the equilibrium adsorption capacity of PSNPs to GAT followed the order PS-NH2 > PS-COOH > PS. The adsorption was regulated by both physical and chemical mechanisms, with intra-particle and external diffusion jointly controlling the adsorption rate. The adsorption process was heterogeneous, spontaneous, and entropy-driven. Sodium chloride (NaCl), alginic acid, copper ions (Cu2+), and zinc ions (Zn2+) inhibited adsorption, with Cu2+ and Zn2+ having the strongest effect on PS-NH2. Theoretical computations indicated that π-π and electrostatic interactions dominated PS adsorption of GAT, while PS-COOH and PS-NH2 adsorbed GAT through electrostatic interactions, hydrogen bonds, and van der Waals (vdW) forces. The surface electrostatic potential of PS-COOH and PS-NH2 was considerably higher than that of PS, with the maximum vdW penetration distance of GAT-PS-NH2 being 1.20 Å. This study's findings provide a theoretical foundation for the migration and synergistic removal of antibiotics, micro-plastics (MPs), and nano-plastics (NPs).

Van der Waals epitaxial growth of single-crystal molecular film.

Epitaxy is the cornerstone of semiconductor technology, enabling the fabrication of single-crystal film. Recent advancements in van der Waals (vdW) epitaxy have opened new avenues for producing wafer-scale single-crystal 2D atomic crystals. However, when it comes to molecular crystals, the overall weak vdW force means that it is a significant challenge for small molecules to form a well-ordered structure during epitaxy. Here we demonstrate that the vdW epitaxy of Sb2O3 molecular crystal, where the whole growth process is governed by vdW interactions, can be precisely controlled. The nucleation is deterministically modulated by epilayer-substrate interactions and unidirectional nuclei are realized through designing the lattice and symmetry matching between epilayer and substrate. Moreover, the growth and coalescence of nuclei as well as the layer-by-layer growth mode are kinetically realized via tackling the Schwoebel-Ehrlich barrier. Such precise control of vdW epitaxy enables the growth of single-crystal Sb2O3 molecular film with desirable thickness. Using the ultrathin highly oriented Sb2O3 film as a gate dielectric, we fabricated MoS2-based field-effect transistors that exhibit superior device performance. The results substantiate the viability of precisely managing molecule alignment in vdW epitaxy, paving the way for large-scale synthesis of single-crystal 2D molecular crystals.

Stacking-dependent structural and electronic properties of trilayer γ-graphyne: An approach for new 2D carbon allotropes.

Vertical stacks of two-dimensional (2D) materials with interlayer van der Waals (vdW) force have provided a versatile approach for creating hybrid materials and modulating various properties. In this work, the structural and electronic properties of trilayer γ-graphyne, involving different stacking patterns, have been investigated through first-principles approaches. The result indicates that a metal-to-semiconducting transition can be triggered simply by switching the stacking order of trilayer γ-graphyne. More interestingly, in addition to typical vdW homostructures, new 2D carbon allotropes with novel carbon networks can be achieved on the basis of trilayer γ-graphyne, arising from the absence of intralayer acetylene linkages during the structural relaxation. One of the new 2D carbon allotropes possesses an intrinsic semiconducting nature with a wide bandgap of 1.827 eV, coupled with superior structural stability beyond single-layer γ-graphyne. Moreover, the biaxial strain effect on the new 2D carbon allotrope, as well as the trilayer vdW stacks, has also been revealed in this work. Correspondingly, the in-plane tensile strain is demonstrated to further enlarge the electronic bandgaps in these carbon sheets. Therefore, the results of this work imply the great potential of few-layer graphyne in future carbon-based nanoelectronic devices, and simultaneously provide a new approach for developing and synthesizing novel 2D carbon allotropes via the vertical stacking of graphyne with inherent acetylene linkages.

Praseodymium doped ceria: A new sight on coupled thermo-mechanical properties and oxygen vacancy diffusion via statistical moment approach

Applications of praseodymium doped ceria (PDC) in solid oxide fuel cells require a deep insight into the thermal expansion, stresses, and ionic conduction. Nonetheless, the investigation of thermo-mechanical properties and oxygen vacancy diffusion remains incomplete. Based on free Helmholtz energy, we formulate analytical expressions to directly connect thermal expansion coefficient, elastic moduli, oxygen vacancy–dopant binding and oxygen vacancy migration energies by employing statistical moment method (SMM). The SMM calculations capture the full anharmonicity of lattice thermal vibrations. Our analyses report the significant influences of temperature and dopant content on the thermal expansion coefficient, bulk and Young’s moduli, oxygen vacancy–dopant binding and oxygen vacancy migration energies, and ionic conductivity. The role of Pr dopant on the oxygen vacancy diffusion is fully described by combining the symmetric energy profiles related to blocking effect and asymmetric energy profiles generated from trapping effect. By switching van der Waals (vdW) forces on and off in the SMM calculations, we first show that the vdW forces cause a positive effect on the mechanical properties but reduce the thermal expansion and resist the oxygen vacancy diffusion. Our findings are discussed concerning the results from other calculations and experiments in the literature. The insights on the thermo-mechanical properties and oxygen vacancy diffusion for PDC crystal can be generalized and applied to other ceramic materials that are essential for solid state batteries.

Individual mono and co-interactions of butylated hydroxytoluene and its metabolite with pepsin: Multi-pronged research strategies

In this study, the interactions between butylated hydroxytoluene (BHT) and its metabolite 2,6-Di-tert-butyl-p-benzoquinone (BHT-Q) with pepsin (PEP) were explored using multispectral measurements and computer prediction techniques. UV–vis absorption spectra, fluorescence lifetime, and Stern-Volmer quenching analysis showed static fluorescence quenching of PEP by BHT/BHT-Q. Negative thermodynamic parameters indicated that the spontaneous formation of complexes was primarily driven by van der Waals (vdW) forces and hydrogen bonds (HB). Synchronous fluorescence and circular dichroism spectroscopy revealed conformational changes induced by BHT/BHT-Q on PEP. Furthermore, BHT and BHT-Q inhibited PEP's enzymatic activity, while PEP suppressed their antioxidant activity. Interestingly, BHT-Q weakened BHT's binding strength to PEP, affecting the enzyme inhibition rate. Computer predictions highlighted the integral role of hydrophobic interactions. Moreover, BHT and BHT-Q exhibited different effects on the stability and compactness of PEP, the residue environment of PEP became more flexible or rigid in the presence of BHT and BHT-Q. Changes in the hydrophobic solvent accessible surface area (SASA) elucidated that the microenvironment of hydrophobic residues of PEP was changed after binding with BHT and BHT-Q. Ultimately, BHT's stronger binding affinity to PEP than BHT-Q was attributed mainly to its larger negative surface area, facilitating interactions with more amino acid residues.

Probing the Interfacial Interaction Mechanisms of Nitrogen with Dodecane/Toluene: Implications for Foam Flooding.

Interfacial interactions between deformable bubbles and oil drops have attracted much attention in foam flooding. However, interactions involving nitrogen bubbles have not been reported. In this work, the interaction forces between nitrogen and dodecane/toluene in aqueous solutions were quantified using the atomic force microscopy bubble probe technique. The effects of the solution pH, ionic type, and solution concentration on the interactions were analyzed. The van der Waals (vdW), electric double layer (EDL), and hydrophobic (HB) interactions were involved in the low-concentration solutions. The EDL repulsion in NaCl increased with solution pH, while in CaCl2 and MgCl2, the EDL repulsion in general decreased and then increased with pH, attributed to the adsorption of OH- and divalent cations and their hydration products. The adsorption of divalent cations at the toluene/water interface was pronounced by cation-π interactions. At pH 10, precipitated divalent cation hydroxides at the bubble/water and oil/water interfaces adsorbed more cations, causing the increase of the surface potential. At high salinity, the EDL interaction was suppressed and the vdW repulsion became predominant. The vdW force of nitrogen with toluene was stronger than that with dodecane. Under all of the solution conditions, the attractive interaction could not overcome the total repulsive interaction at the minimum separation, and thus no bubble attachment was observed, which implied that a stable bubble/liquid/oil film was essential for maintaining foam stability. This work provides useful insights into the interfacial interaction mechanisms in nitrogen foam flooding. The findings can be readily extended to other engineering systems such as oil flotation and bubble-oil-water emulsions.

Low Dielectric Medium for Hyperbolic Phonon Polariton Waveguide in van der Waals Heterostructures.

Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons-quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS2. The low dielectric medium serves a dual purpose-it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS2 through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc.

Self-Scrolling of a Graphyne Ribbon Near a CNT in Multiphysical Environments.

Graphyne nanoscrolls (GNSs) have attracted significant research interest because of their wide-ranging applications. However, the production of GNSs via a self-scrolling approach is environment dependent. Here, molecular dynamics simulations are conducted to evaluate the self-scrolling behavior of an α-graphyne (α-GY) ribbon on a carbon nanotube (CNT) within various multiphysical environments, accounting for the interactions among temperature, electric field, and argon gas. The results demonstrate that the fabrication of an α-GNS lies in the interplay of van der Waals (vdW) forces among the components in a vacuum. Notably, the α-GY ribbon is easier to scroll onto a thicker CNT. The electric field attenuates the vdWinteraction, necessitating thicker CNTs for successful self-scrolling under a stronger electric field. In argon, both the vdW interaction and nanoscale pore contribute to the overlap formation. At 300 K, increasing argon density prolongs the time required for α-GNS formation, with self-scrolling failing beyond a critical gas density threshold. Moreover, the self-scrolling becomes easier at higher temperatures. In multiphysical environments, the interplay between the electric field and the gas density dictates the self-scrolling at low temperatures. Finally, reasonable suggestions are given for successful self-scrolling. The conclusions offer valuable insights for the practical fabrication of α-GNS.

Molecular dissociation of nitric oxide (NO) on VO2(010) surface : A DFT study including vdW forces

The adsorption and coadsorption of O and N atomic species, as well as the dissociation of NO molecule on VO2(010) surface have been studied using the density functional theory (DFT) with GGA-PBE as the exchange–correlation, including van der Waals (vdW) corrections of the form PBE+vdW-DF2. We found that the short-bridge (sb) site is the energetically most stable for both N and O atoms. The inclusion of PBE+vdW-DF2 corrections does not change the preferred adsorption site, however it reduces the value of the adsorption energy relative to ordinary GGA-PBE functional. Also, while the GGA-PBE functional results in finite magnetization on the O and N atoms when adsorbed on the VO2 surface, the inclusion of the vdW forces results in zero magnetization of the atoms. In addition, our Löwdin charge transfer analysis shows electron transfer from the VO2(010) surface to the O atom, which is further supported by an accumulation of charge density around the O atom as well as the charge depletion from the V surface atoms. At a lower coverage θ= 0.25 ML, the N atom behaves like a charge acceptor, that is, electrons are transferred from the VO2 surface to it. At a higher coverage θ= 0.50 ML and 0.75 ML, the N atom behaves like a charge donor, i.e., there is a transfer of electronic charges to the VO2 surface. We emphasize that charge transfer between the adsorbate (O and N) and the surface establishes adsorbate-surface bonding which enhances the stability of the adsorbate. Furthermore, our nudged elastic band calculations (NEB) for determining the minimum energy path (MEP) and activation barrier energy (Ea) for NO dissociation into N and O atoms show Ea= 2.18 eV (1.78 eV) as obtained using the GGA-PBE (PBE+vdW-DF2) functionals. Regarding potential application, this study thus contributes to reducing the amount and harmful effect of NO gas in the atmosphere.

High pressure phase transition and its influence on structural, mechanical properties and elastic anisotropy of [formula omitted]: An ab-initio study

Lithium carbide (Li2C2), a promising tritium breeding material with potential applications in magnetic confinement-based fusion reactor blankets, has drawn significant attention due to its recent utilization in lithium-ion batteries. In the current study, we employed first-principles calculations with van der Waals (vdW) force correction to explore the structural and mechanical properties of Immm and Pnma phases of Li2C2 subjected to external hydrostatic pressure. It is demonstrated that inclusion of vdW effects accurately predicts Immm→Pnma phase transition at 16 GPa, aligning closely with experimental transition pressure. In contrast, PBE-GGA calculations underestimate the same by about 21%. Current paper is the first comprehensive study of pressure evolution of mechanical properties and strength determining parameters of Li2C2. Interestingly, this inherently brittle material exhibits ductile characteristics beyond 7.5 GPa. The paper also offers a quantitative comparison of its strength parameters with other prominent tritium breeding materials. Finally, the strong directional dependence due to the presence of C2 dimer is investigated by analyzing the pressure variation of elastic anisotropy. The study has its relevance in assessing structural integrity of Li2C2 during inadvertent buildup of tritium and helium that would generate stress in the solid breeder.

Direct Selective Epitaxy of 2D Sb2Te3 onto Monolayer WS2 for Vertical p-n Heterojunction Photodetectors.

Two-dimensional transition metal dichalcogenides (2D-TMDs) possess appropriate bandgaps and interact via van der Waals (vdW) forces between layers, effectively overcoming lattice compatibility challenges inherent in traditional heterojunctions. This property facilitates the creation of heterojunctions with customizable bandgap alignments. However, the prevailing method for creating heterojunctions with 2D-TMDs relies on the low-efficiency technique of mechanical exfoliation. Sb2Te3, recognized as a notable p-type semiconductor, emerges as a versatile component for constructing diverse vertical p-n heterostructures with 2D-TMDs. This study presents the successful large-scale deposition of 2D Sb2Te3 onto inert mica substrates, providing valuable insights into the integration of Sb2Te3 with 2D-TMDs to form heterostructures. Building upon this initial advancement, a precise epitaxial growth method for Sb2Te3 on pre-existing WS2 surfaces on SiO2/Si substrates is achieved through a two-step chemical vapor deposition process, resulting in the formation of Sb2Te3/WS2 heterojunctions. Finally, the development of 2D Sb2Te3/WS2 optoelectronic devices is accomplished, showing rapid response times, with a rise/decay time of 305 μs/503 μs, respectively.

Adaptive structural modification of Zr-based MOF-808 via solvent and ligand engineering for enhanced fluoride ion adsorption efficiency

The rapid development of industries such as new energy and photovoltaic has led to the generation of a large amount of industrial fluorine wastewater, posing serious threats to water environment and human health. To address this issue, MOF-808 adsorbent was synthesized by controlling the synthesis conditions and modifying the ligand structure. Batch adsorption experiments were conducted by varying experimental conditions, including type of adsorbent, pH, wastewater concentration, adsorption time, temperature, and interfering anions. The adsorption behavior of MOF-808-AA towards fluoride ions was well fitted by the Elovich non-linear kinetic model, as well as the Freundlich and Temkin non-linear isotherm models, indicating that the adsorption behavior was influenced by multiple mechanisms other than just intraparticle diffusion. Thermodynamic results suggested a spontaneous exothermic monolayer chemical adsorption process. Under the conditions of pH 6.0 and T = 298 K, MOF-808-AA exhibited a maximum adsorption capacity of 84.65 mg/g for fluoride ions. Characterization techniques, as well as quantum chemical analysis was employed to analyze the adsorption behavior and predict the reaction sites. The potential mechanism can be summarized as the formation of new bonds between MOF-808-AA and the adsorbed fluoride ions, as well as the weak intermolecular interactions such as hydrogen bonding and vdW forces. Furthermore, MOF-808-AA demonstrated satisfactory reusability and excellent chemical stability during the cyclic process. These findings suggest the potential application of MOF-808-AA for removing fluoride ions from wastewater.

Ionic liquid-assisted extractive distillation for separating of ethyl acetate/isopropanol from chemical wastewater: Molecular insights and process intensification

The effective separation of ethyl acetate (EAC)/isopropanol(IPA)/water azeotrope from industrial wastewater significantly reduces environmental risks and recycling resources. An azeotrope separation technique was proposed, combining ionic liquid mixed entrainer and dividing wall column extractive distillation (DWC-ED). The selectivity and solubility of 225 ionic liquids were calculated based on the COSMO-RS model. The molecular polarity of EAC, IPA, H2O, dimethyl sulfoxide (DMSO), and [EMIM][AC] was studied, and 1-ethyl-3-methylimidazolium acetate ([EMIM][AC]) was determined as the best solvent. In addition, the interaction mechanism between [EMIM][AC] (or DMSO) and EAC/IPA/H2O was studied using the quantum chemistry (QC) theory. Independent gradient model (IGM), interaction energy, and atoms in molecules (AIM) analysis showed that hydrogen bonds and vdW forces were formed between [AC]− anion and H2O/IPA/EAC, and the interaction energy was gradually weakened, which significantly improved the performance of extractive distillation process. The results showed that the total annual cost (TAC) and energy consumption of IL-based mixed solvent extractive distillation were reduced by 36.04 % and 37.72 %, respectively, compared to the single DMSO extractive distillation process. The TAC and energy consumption of the DWC-ED process were 44.61 % and 51.43 % lower than the benchmark processes.

Molecular mechanism and extraction performance evaluation of choline chloride-based deep eutectic solvents for phenol separation from oil: Different glycols as hydrogen bond donors

Highly efficient separation of phenols from coal-to-liquids can enhance the economics of the coal conversion process. The selection of extractants with high extraction efficiency and low neutral oil entrainment is significant for energy development strategy. In this study, four ChCl-based DESs with different glycols as hydrogen bond donors (HBDs) were designed for the separation of phenol/toluene mixtures and the experimental results were analyzed by theoretical calculations. The experimental results showed that ChCl-EG had the highest extraction efficiency (94.02 %) and the least entrainment of toluene (0.449 g). Meanwhile, it could be regenerated and reused four times with no significant change in extraction capacity. The formation of hydrogen bonds between DESs and phenol was elucidated by σ-profile based on COSMO-SAC model. Multi-scale simulations and comparative analyses of the micro-mechanisms of the DESs system were carried out using quantum chemical (QC) and molecular dynamics (MD). It was shown that the attraction of DESs to phenol was mainly due to hydrogen bonds and vdW forces, with hydrogen bond interaction between Cl− in ChCl and H atom in phenol hydroxyl group being dominant. As the chain length of HBDs increased, the interaction of DESs with phenol was enhanced, while the entrainment of toluene also increased. Overall, the effect on the separation of phenol by different chain lengths of glycols as HBDs is elucidated. This study provides a theoretical reference for selecting and designing other DES extractants to separate phenol/toluene systems and similar systems.

Highly efficient separation of phenol with tetraethylammonium chloride-based deep eutectic solvents: Experiments and theoretical calculations

To gain more insight into the mechanism of the separation of phenols by deep eutectic solvents (DESs), four DESs: tetraethylammonium chloride (TEAC)-ethylene glycol (EG), TEAC-diethylene glycol (DEG), TEAC-triethylene glycol (TEG) and TEAC-tetraethylene glycol (TTEG) were prepared for the extraction of phenols from model oil. Experimental parameter optimization results indicated that four DESs were effective in extracting phenols, and TEAC-EG had the highest extraction efficiency (EE), with EE up to 98.95%. Subsequently, the formation mechanism of DESs and the extraction mechanism were revealed at the molecular/atomic level by quantum calculations using density functional theory (DFT). The most stable configurations, hydrogen bonding lengths, and interaction energies of DESs and corresponding systems have been calculated, and the weak interactions in the systems were visualized and further analyzed. The results showed that DESs formation were mainly due to a combined effect of hydrogen bonding and van der Waals (vdW) forces. Besides, hydrogen bonding and vdW forces between DESs and phenol were the main forces acting to extract phenol, and the hydrogen bonding within DESs was weakened when DESs interacted with phenol.

Van der Waals Colloidal Crystals

A general guiding principle for colloidal crystallization is to tame the attractive enthalpy such that it slightly overwhelms the repulsive interaction. As-synthesized colloids are generally designed to retain a strong repulsive potential for the high stability of suspensions, encoding appropriate attractive potentials into colloids has been key to their crystallization. Despite the myriad of interparticle attractions for colloidal crystallization, the van der Waals (vdW) force remains unexplored. Here, it is shown that the implementation of gold cores into silica colloids and the resulting vdW force can reconfigure the pair potential well depth to the optimal range between -1 and -4 kB T at tens of nanometer-scale colloidal distances. As such, colloidal crystals with a distinct liquid gap can be formed, which is evidenced by photonic bandgap-based diffractive colorization.

Ion Transport Behavior in van der Waals Gaps of 2D Materials.

2D materials, with advantages of atomic thickness and novel physical/chemical characteristics, have emerged as the vital building blocks for advanced lamellar membranes which possess promising potential in energy storage, ion separation, and catalysis. When 2D materials are stacked together, the van der Waals (vdW) force generated between adjacent layered nanosheets induces the construction of an ordered lamellar membrane. By regulating the interlayer spacing down to the nanometer or even sub-nanometer scale, rapid and selective ion transport can be achieved through such vdW gaps. The further improvement and application of qualified 2D materials-based lamellar membranes (2DLMs) can be fulfilled by the rational design of nanochannels and the intelligent micro-environment regulation under different stimuli. Focusing on the newly emerging advances of 2DLMs, in this review, the common top-down and bottom-up synthesis approaches of 2D nanosheets and the design strategy of functional 2DLMs are briefly introduced. Two essential ion transport mechanisms within vdW gaps are also involved. Subsequently, the responsive 2DLMs based on different types of external stimuli and their unique applications in nanofluid transport, membrane-based filters, and energy storage are presented. Based on the above analysis, the existing challenges and future developing prospects of 2DLMs are further proposed.

Flexoelectricity‐Enhanced Self‐Powered Photodetection in 2D van der Waals Heterojunctions With Large Curvatures

AbstractThe unique morphology of 2D van der Waals materials enables them to withstand large deformations and significant nonuniform strain, potentially inducing a strong flexoelectric effect. Despite the size‐dependent flexoelectric effect showing potential for modulating the optoelectronic performance of 2D van der Waals materials, it is far from being fully exploited owing to various challenges. Herein, the use of nanowires with different diameters as the bending media to fabricate 2D α‐In2Se3/β‐InSe heterojunctions with large curvatures of 0.1–1 µm−1 is proposed. The significant band alignment modulation in α‐In2Se3 resulting from the bending‐induced flexoelectric effect is verified through Kelvin probe force microscopy. The strain‐induced piezoelectric effect can be negated because of the weak vdW forces at the interface. The flexoelectric polarization in β‐InSe is screened via the accumulated electrons in the unilateral depleted heterojunction. Compared to the flat heterojunction, the curved heterojunction with an average curvature of 0.9 µm−1 shows 2.48‐fold and 7.62‐fold increases in open‐circuit voltage and zero‐biased responsivity, respectively. This study demonstrates the first successful modulation of photodetection in 2D heterojunctions by exploiting the flexoelectric effect, providing a new perspective for high‐performance 2D vdW optoelectronic devices.

Crucial role of interfacial interaction in 2D polar SiGe/GeC heterostructures

The planar charge transfer is a distinctive characteristic of the two-dimensional (2D) polar materials. When such 2D polar materials are involved in vertical heterostructures (VHs), in addition to the van der Waals (vdW) interlayer interaction, the interfacial interaction triggered by the in-plane charge transfer will play a crucial role. To deeply understand such mechanism, we conducted a comprehensive theoretical study focusing on the structural stability and electronic properties of 2D polar VHs built by commensurate SiGe/GeC bilayers with four species ordering patterns (classified as a C-group with patterns I and II and a Ge-group with patterns III and IV, respectively). It was found that the commensurate SiGe/GeC VHs are mainly stabilized by interfacial interactions (including the electrostatic interlayer bonding, the vdW force, as well as the sp 2/sp 3 orbital hybridization), with the Ge-group being the most energetically favorable than the C-group. A net charge redistribution occurs between adjacent layers, which is significant (∼0.23–0.25 e cell−1) in patterns II and IV, but slightly small (∼0.05–0.09 e cell−1) in patterns I and III, respectively, forming spontaneous p–n heterojunctions. Such interlayer charge transfer could also lead to a polarization in the interfacial region, with the electron depletion (accumulation) close to the GeC layer and the electron accumulation (depletion) close to the SiGe layer in the C-group (the Ge-group). This type of interface dipoles could induce a built-in electric field and help to promote photogenerated electrons (holes) migration. Furthermore, a semi-metal nature with a tiny direct band gap at the SiGe layer and a semiconducting nature at the GeC layer indicate that the commensurate SiG/GeC VHs possess a type-I band alignment of heterojunction and have a wide spectrum of light absorption capabilities, indicating its promising applications for enhancing light-matter interaction and interfacial engineering.

Engineering piezoelectricity at vdW interfaces of quasi-1D chains in 2D Tellurene

Low-dimensional piezoelectrics have drawn attention to the realization in nano-scale devices with high integration density. A unique branch of 2D Tellurene bilayers formed of weakly interacting quasi-1D chains via van der Waals forces is found to exhibit piezoelectricity due to the semiconducting band gap and spatial inversion asymmetry. Various bilayer stackings are systematically examined using density functional theory, revealing optimal piezoelectricity when dipole arrangements are identical in each layer. Negative piezoelectricity has been observed in two of the stackings AA′ and AA″ while other two stackings exhibit the usual positive piezoelectric effect. The layer-dependent 2D piezoelectricity ( ∣ e 222D ∣ ) increases with an increasing number of layers in contrast to the odd–even effect observed in h-BN and MoS2. Notably, the piezoelectric effect is observed in even-layered structures due to the homogeneous stacking in multilayers. Strain is found to enhance in-plane piezoelectricity by 4.5 times (−66.25 × 10−10 C m−1 at −5.1% strain) due to the increasing difference in Born effective charges of positively and negatively charged Te-atoms under compressive biaxial strains. Moreover, out-of-plane piezoelectricity is induced by applying an external electric field, thus implying Tellurene is a promising candidate for piezoelectric sensors.