vdW Forces Research Articles

Modulation of chiral spectral deflection by van der Waals force-induced molecular electropolarization in catenane oligomers.

The striking chiral optical properties of carbon nanostructures are closely related to the precise three-dimensional spatial arrangement (interaction) of carbon atoms. This work investigated the chiral optical properties of three different structures of all-benzene catenane and trefoil knot regulated by van der Waals (vdW) forces using density functional theory (DFT) calculations and wave function analysis. We systematically illustrate how molecular electrical polarization modulates the chiral optical deflection of alkane oligomers under the induction of van der Waals forces. In this work, the UV-vis spectra, transition density matrices (TDM), and electron-hole density diagrams of three molecules have been studied. Combined with a visualization method to represent the effect of molecular polarization on transition electric/magnetic dipole moments (TEDMs\TMDMs), the results show that vdW interactions can induce chirality deflection in polymers. This mechanism provides a clear direction for designing polymers with specific chirality: by modifying the structure, vdW interactions can be generated in specific regions, and then the chirality of the molecule can be precisely regulated. This will help us to establish a strategy for precisely-oriented design of chiral optical materials, and provide guidance for the application and development of optoelectronic materials in specific fields.

Nanowires exfoliated from one-dimensional van der Waals transition metal trihalides and quadrihalides.

The exfoliation of van der Waals (vdW) materials has been widely used to fabricate two-dimensional (2D) materials. However, the exfoliation of vdW materials to isolate atomically thin nanowires (NWs) is an emerging research topic. In this letter, we identify a large class of transition metal trihalides (TMX3), which have one-dimensional (1D) vdW structures, i.e., they comprise columns of face-sharing TMX6 octahedral chains, whereas the chains are bound by weak vdW forces. Our calculations show that the single-chain and multiple-chain NWs constructed from these 1D vdW structures are stable. The calculated binding energies of the NWs are relatively small, suggesting that it is possible to exfoliate NWs from the 1D vdW materials. We further identify several 1D vdW transition metal quadrihalides (TMX4) that are candidates for exfoliation. This work opens a paradigm for exfoliating NWs from 1D vdW materials.

Robustness of heteroaggregates involving hydrophobic cholesterol and its mimetics.

Exploring the self and cross aggregation affinity of cholesterol (CHL) and some of its lookalikes, e.g., cholesteryl hemisuccinate (CHM), campesterol (CAM) and arjunic acid (ARJ), provides crucial understanding towards the influence of weak forces in inducing mixed micellization through heteroaggregation. Strongly hydrophobic CHL, with a benchmark inclination towards aggregation, often forms detrimental plaques in crucial human organs that are fairly difficult to disintegrate. Traditionally known anti-dyslipidemic agents like CAM and ARJ are known to interact strongly with CHL in the gut when ingested. They further form mixed micelles along with the bile components and interfere with the CHL absorption across the epithelial cell layer of the intestine. Some invariant questions like how robust are the heteroaggregates formed between these mimetics and CHL are very important to appreciate the efficacy of such anti-dyslipidemic agents. In this work using molecular dynamics simulations and varied structural analysis, we characterize the heteroaggregates. Simulations indicate that CHL-CHM mixed assemblies are comparatively bigger and significantly stabilized by strong electrostatic and favourable vdW forces. Small and diffused CHL-ARJ aggregates are observed in our simulations with a not so favourable energetics, indicating a possible attenuation pathway of CHL aggregation in the presence of ARJ.

Quasi-van der Waals Epitaxial Recrystallization of a Gold Thin Film into Crystallographically Aligned Single Crystals.

Heterointerfaces between two-dimensional (2D) materials and bulk metals determine the electrical and optical properties of their heterostructures. Although deposition of various metals on 2D materials has been studied, there is still a lack of studies on the interaction at the van der Waals (vdW) heterointerface between 2D materials and metals. Here, we report quasi-van der Waals (qvdW) epitaxial recrystallization of a gold thin film into crystallographically aligned single crystals by encapsulation annealing of a gold thin film with hexagonal boron nitride (hBN). When a polycrystalline gold thin film passivated with hBN was annealed, it was recrystallized into single gold crystals with a planar shape and crystallographic alignment with hBN due to a strong interaction between the gold film and hBN at the heterointerface. This reflects that a weak vdW force at the heterointerface is sufficiently strong to induce epitaxial recrystallization. Using this method, we fabricated a gold nanocrystal array with the same crystalline orientation and smooth top surface. Our work demonstrates a new method for epitaxial recrystallization of bulk crystals and provides a deep understanding of the interaction at the vdW heterointerface of 2D materials and metals.

Impact of Dispersion Force Schemes on Liquid Systems: Comparing Efficiency and Drawbacks for Well-Targeted Test Cases.

First-principles molecular dynamics (FPMD) calculations were performed on liquid GeSe4 with the aim of inferring the impact of dispersion (van der Waals, vdW) forces on the structural properties. Different expressions for the dispersion forces were employed, allowing us to draw conclusions on their performances in a comparative fashion. These results supersede previous FPMD calculations obtained in smaller systems and shorter time trajectories by providing data of unprecedented accuracy. We obtained a substantial agreement with experiments for the structure factor regardless of the vdW scheme employed. This objective was achieved by using (in addition to FPMD with no dispersion forces) a selection of vdW schemes available within density functional theory. The first two are due to Grimme, D2 and D3, and the third one is devised within the so-called maximally localized Wannier functions approach (MLWF). D3 results feature a sizeable disagreement in real space with D2 and MLWF in terms of the partial and total pair correlation functions as well as the coordination numbers. More strikingly, total and partial structure factors calculated with D3 exhibit an unexpected sharp increase at low k. This peculiarity goes along with large void regions within the network, standing for a phase separation of indecipherable physical meaning. In view of these findings, further evidence of unconventional structural properties found by employing D3 is presented by relying on results obtained for a complex ionic liquid supported on a solid surface. The novelty of our study is multifold: new, reliable FPMD data for a prototypical disordered network system, convincing agreement with experimental data and assessment of the impact of dispersion forces, with emphasis on the intriguing behavior of one specific recipe and the discovery of common structural features shared by drastically dissimilar physical systems when the D3 vdW scheme is employed.

Ursolic Acid and Oleanolic Acid Dissolved in Methanol/Acetone + Water Blends: Thermodynamic Solubility, Intermolecular Interactions, and Solvation Behavior

With the purpose of qualitative characterization of the electrostatic properties of the acidity and basicity of ursolic acid (UA) and oleanolic acid (OA), quantitative analyses of the molecular surface were employed in this contribution. The −OH and −COOH groups on the molecules of OA and UA took precedence over establishing hydrogen bonds with the solvents acetone/methanol/water. In addition, the intermolecular interactions of OA and UA with acetone/methanol/water were investigated using an independent gradient model based on the Hirshfeld partition of molecular density. In solutions of OA/UA dissolved in methanol/acetone/water, attractive interactions, including the hydrogen-bond force and van der Waals (vdW) force, occur. The shake-flask technique was applied to evaluate the equilibrium solubility of UA and OA in acetone + water and methanol + water at 101.2 kPa and elevated temperatures covering from 278.15 to 323.15 K. Solvent–solvent interactions accounted by the solubility parameter of solutions had a significant domination in the variability of solubility magnitudes. The van’t Hoff–Jouyban–Acree, Apelblat, and Jouyban–Acree models revealed a good correlation for the measured solubility, with a relative mean deviation of no more than 7.73%. The Kirkwood–Buff integral approach was utilized to determine the preferred solvation of UA and OA by solvent species. In both the rich- and intermediate-composition regions of methanol/acetone, UA or OA is preferentially solvated with positive preferential solvation parameters by methanol/acetone.

Multispectroscopic and Computational Investigations on the Binding Mechanism of Dicaffeoylquinic Acids with Ovalbumin.

Recently, studies on the interactions between ovalbumin (OVA) and polyphenols have received a great deal of interest. This study explored the conformational changes and the interaction mechanism of the binding between OVA and chlorogenic acid (CGA) isomers such as 3,4-dicaffeoylquinic acids (3,4-diCQA), 4,5-dicaffeoylquinic acids (4,5-diCQA), and 3,5-dicaffeoylquinic acids (3,5-diCQA) using multispectroscopic and in silico analyses. The emission spectra show that the diCQAs caused strong quenching of OVA fluorescence under different temperatures through a static quenching mechanism with hydrogen bond (H-bond) and van der Waals (vdW) interactions. The values of binding constants (OVA-3,4-diCQA = 6.123 × 105, OVA-3,5-diCQA = 2.485 × 105, OVA-4,5-diCQA = 4.698 × 105 dm3 mol-1 at 298 K) suggested that diCQAs had a strong binding affinity toward OVA, among which OVA-3,4-diCQA exhibits higher binding constant. The results of UV-vis absorption and synchronous fluorescence indicated that the binding of all three diCQAs to OVA induced conformational and micro-environmental changes in the protein. The findings of molecular modeling further validate the significant role of vdW force and H-bond interactions in ensuring the stable binding of OVA-diCQA complexes. Temperature-dependent molecular dynamics simulation studies allow estimation of the individual components that contribute to the total bound free energy value, which allows evaluation of the nature of the interactions involved. This research can provide information for future investigations on food proteins' physicochemical stability and CGA bioavailability in vitro or in vivo.

Formation of oriented layered MoS2 from amorphous thin film revealed by polarized x-ray absorption spectroscopy

MoS2 is a prototypical two-dimensional van der Waals (vdW) solid, where covalently bonded S–Mo–S triplets are held together by weaker vdW forces. In this work, we have studied structural transformation from a three-dimensional amorphous phase of MoS2 into a layered vdW crystal using S L2,3 edge x-ray absorption near-edge structure (XANES) spectroscopy with in-plane and out-of-plane polarized x-ray beam. The crystallization process, which starts from an isotropic amorphous phase, is accompanied by the establishment of vdW interaction between covalently bonded layers, resulting in the anisotropic nature of the crystalline phase. We have disclosed that the preferential growth of MoS2 layers along the (200) Bragg reflection commences immediately from the amorphous phase with no intermediate crystal orientations. We have additionally identified a unique signature in the S L2,3 edge spectrum that is associated with vdW bonds and can be possibly used to determine sulfur-based single-layered and multi-layered transition metal dichalcogenides.

In-silico and in-detail experimental interaction studies of new antitumor Zn(II) complex with CT-DNA and serum albumin

In this study, a novel Zn(II) complex with the formula [Zn(pyrr-ac)2] (pyrr-ac: pyrrolidineacetate) was synthesized and characterized through molar conductivity, elemental analysis, 1H Nuclear Magnetic Resonance (1H NMR), UV-Visible spectroscopy, and Fourier transform infrared (FT-IR) methods. B3LYP level of DFT method along with aug-cc-pVTZ-PP/6-311G(d,p) basis set was utilized to perform the geometry optimization and HOMO-LUMO analysis. In addition, MEP, NLO and NBO computations were also performed at the same level of theory. In vitro antitumor activity of the mentioned complex on leukemia cell line, K562, was investigated using the MTT assay which surprisingly revealed the effective antitumor activity of the studied zinc complex. Interaction of this compound with biological macromolecules viz., CT-DNA and BSA was studied via different spectroscopic methods. The results of fluorescence experiment displayed that the metal complex binds to both macromolecules through hydrogen bond (H-bond) and van der Waals (vdW) forces. UV-Vis tests indicated a decline in the absorption spectra of CT-DNA/BSA in the presence of the compound. The interaction was further corroborated for CT-DNA via gel electrophoresis, CD spectroscopy and viscosity experiments and for BSA using CD spectroscopy. Furthermore, molecular docking simulation was done to evaluate the nature of interaction between the aforementioned zinc complex and CT-DNA/BSA. These results were in agreement with experimental findings and demonstrated that the main interaction is hydrogen bonding. The above type of investigations may provide a pathway through which zinc complexes join the anticancer category. The in-silico and in-vitro results confirm that the newly made [Zn(pyrr-ac)2] complex interacts with CT-DNA than BSA. Communicated by Ramaswamy H. Sarma

Electronic and photocatalytic properties of ZnO/GaTe heterostructure from first principles calculations

Constructing a novel van der Waals (vdW) heterostructure by integrating diverse two-dimensional (2D) materials is significant to generate the more fascinating performances. In this present contribution, the electronic and photocatalytic properties of ZnO/GaTe heterostructure have been explored using first-principles calculations. The results reveal that the interaction between layers is dominated by the vdW forces. It displays an intrinsic type-II band alignment and a direct band gap of 1.730 eV under hybrid HSE06 functional. The valence band top and conduction band bottom are located at ZnO and GaTe layers, respectively, which guarantees the spatial separation of photogenerated electron-hole pairs and improves the photocatalytic efficiency. Moreover, the band edges all span the energy levels of both the reduction and oxidation potentials of H2O, indicating that its better photocatalytic performance. The modulated photocatalytic properties under varied pH circumstances imply that an acid environment is more favorable to the water splitting process. Furthermore, the band edge alignment exhibits the significant adjustability through varying vertical and in-plane biaxial strains. These theoretical findings indicate that the ZnO/GaTe heterostructure can offer an alternative strategy to design the efficient photocatalysts for water splitting.

Van der Waals lattice-induced colossal magnetoresistance in Cr2Ge2Te6 thin flakes

Recent discovery of two-dimensional (2D) magnets with van der Waals (vdW) gapped layered structure prospers the fundamental research of magnetism and advances the miniaturization of spintronics. Due to their unique lattice anisotropy, their band structure has the potential to be dramatically modulated by the spin configuration even in thin flakes, which is still unexplored. Here, we demonstrate the vdW lattice-induced spin modulation of band structure in thin flakes of vdW semiconductor Cr2Ge2Te6 (CGT) through the measurement of magnetoresistance (MR). The significant anisotropic lattice constructed by the interlayer vdW force and intralayer covalent bond induces anisotropic spin-orbit field, resulting in the spin orientation-dependent band splitting. Consequently, giant variation of resistance is induced between the magnetization aligned along in-plane and out-of-plane directions. Based on this, a colossal MR beyond 1000% was realized in lateral nonlocal devices with CGT acting as a magneto switch. Our finding provides a unique feature for the vdW magnets and would advance its applications in spintronics.

Atomistically-informed continuum modeling and isogeometric analysis of 2D materials over holey substrates

This work develops, discretizes, and validates a continuum model of a molybdenum disulfide (MoS2) monolayer interacting with a periodic holey silicon nitride (Si3N4) substrate via van der Waals (vdW) forces. The MoS2 layer is modeled as a geometrically nonlinear Kirchhoff–Love shell, and vdW forces are modeled by a Lennard-Jones (LJ) potential, simplified using approximations for a smooth substrate topography. Both the shell model and LJ interactions include novel extensions informed by close comparison with fully-atomistic calculations. The material parameters of the shell model are calibrated by comparing small-strain tensile and bending tests with atomistic simulations. This model is efficiently discretized using isogeometric analysis (IGA) for the shell structure and a pseudo-time continuation method for energy minimization. The IGA shell model is validated against fully-atomistic calculations for several benchmark problems with different substrate geometries. Agreement with atomistic results depends on geometric nonlinearity in some cases, but a simple isotropic St.Venant–Kirchhoff model is found to be sufficient to represent material behavior. We find that the IGA discretization of the continuum model has a much lower computational cost than atomistic simulations, and expect that it will enable efficient design space exploration in strain engineering applications. This is demonstrated by studying the dependence of strain and curvature in MoS2 over a holey substrate as a function of the hole spacing on scales inaccessible to atomistic calculations. The results show an unexpected qualitative change in the deformation pattern below a critical hole separation.

2D Magnetic Semiconductor Fe3GeTe2 with Few and Single Layers with a Greatly Enhanced Intrinsic Exchange Bias by Liquid-Phase Exfoliation.

A 2D van der Waals (vdW) magnet can get rid of the constraints of lattice matching and compatibility and then create a variety of vdW heterostructures, which provides a opportunity for spintronic devices. However, the ability to reliably exfoliate large, high-quality vdW ferromagnetic Fe3GeTe2 (FGT) nanoflakes in scaled-up production is severely limited. Herein, an efficient and stable three-stage sonication-assisted liquid-phase exfoliation was developed for mass preparation of high-structural-integrity few- and single-layer FGT nanoflakes with a greatly enhanced intrinsic exchange bias. The three stages include slicing crystals, weakening interlayer vdW forces, and using ultrasonic cavitation. The highest yield of FGT nanoflakes is 22.3 wt % with single layers accounting for 6%. The size is controllable, and several micrometers, tens of micrometers, and a maximum of 103 μm are available. The 200 mg level output has overcome the limitations of mechanical exfoliation and molecular beam epitaxy in economically amplificated production. An intrinsic exchange bias is observed in the restacked nanoflakes due to the magnetic proximity on the interface of the FGT/natural surface oxide layer. The material reaches 578 Oe (2 K) and 2300 Oe after further oxidation, at least 250% higher than other precisely tailored vdW magnetic heterostructures. In addition, the unusual semiconductivity of the liquid-phase exfoliated FGT nanoflakes is reported. This work skillfully utilizes oxidation to enhance the potential of FGT for large-scale spintronics, optoelectronics, efficient data storage, and various extended applications, and it is beneficial for exfoliating other promising magnetic vdW materials.

Modeling of pristine, Ir- and Au-decorated C60 fullerenes as sensors for detection of hydroxyurea and nitrosourea drugs

Even though Hydroxyurea (HU) and Nitrosourea (NU) are well known drugs efficient for chemotherapy, the drugs’ misuse and/or release of trace levels of un-metabolized drugs into the environment leads to diverse side effects. Herein, we investigated the potential applications of pristine, Au-, Ir-decorated C60 fullerenes in HU and NU drugs detection using Density Functional Theory (DFT) approach. Based on our results, the adsorption energies were in the range of HU/C60 < NU/C60 < NU/AuC59 < NU/IrC59 and HU/IrC59 corresponding to the calculated energies of − 5.674, − 12.767, − 27.065, − 31.423 and − 31.676 kcal/mol, respectively. The result indicates that Au- and Ir-decorated fullerenes have stronger interactions with the HU and NU drugs when compared with the pristine C60. Furthermore, the energy band gap (Eg) decreases from 4.49 eV (C60) to 4.45 (HU/C60) and 4.47 (NU/C60) eV indicates that HU and NU adsorption has insignificant effect on the electrical conductivity of pristine C60. On the other hand, Ir-metal decoration improved both the reactivity and electronic sensitivity of C60 towards the HU and NU drugs. The topology analysis analyses revealed that the studied complexes are stabilized by covalent bonding and/or weak van der Waals (vdW) forces, with HU/C60 having the least stability. The recovery time (τ) for the desorption of the NU drug from AuC59 fullerene surface was found to be 0.67 s while τ for HU and NU drugs desorption was 54.15 and 69.93 s, respectively, for the IrC59 fullerene surface. Though the AuC59 fullerene may not be an effective electronic sensor, it can be used as work-function based sensor for NU drug detection. Conversely, IrC59 fullerene may potentially be used as both electronic and work-function type sensor for HU and NU drugs detection.

Mapping the Binding Energy of Layered Crystals to Macroscopic Observables.

Van der Waals (vdW) integration of two dimensional (2D) crystals into functional heterostructures emerges as a powerful tool to design new materials with fine-tuned physical properties at an unprecedented precision. The intermolecular forces governing the assembly of vdW heterostructures are investigated by first-principles models, yet translating the outcome of these models to macroscopic observables in layered crystals is missing. Establishing this connection is, therefore, crucial for ultimately designing advanced materials of choice-tailoring the composition to functional device properties. Herein, components from both vdW and non-vdW forces are integrated to build a comprehensive framework that can quantitatively describe the dynamics of these forces in action. Specifically, it is shown that the optical band gap of layered crystals possesses a peculiar ionic character that works as a quantitative indicator of non-vdW forces. Using these two components, it is then described why only a narrow range of exfoliation energies for this class of materials is observed. These findings unlock the microscopic origin of universal binding energy in layered crystals and provide a general protocol to identify and synthesize new crystals to regulate vdW coupling in the next generation of heterostructures.

Boosting the solar conversion efficiency of MoSe2/PtX2 (X = O, S) vdW heterostructure by strain and electric field engineering

Vertically stacking two-dimensional materials via weak van der Waals (vdW) forces is an effective strategy for modulating optoelectronic performance of materials. To accelerate more novel MoSe2-based heterostructure design, the interlayer coupling effect in MoSe2/PtX2 (X = O, S) heterostructure has been systematically studied, from the atomic structure to the electronic and optical properties, on the basis of first-principles calculations and BSE model with scissor inclusion. Density functional theory (DFT) calculations unveil a type-II indirect bandgap measuring between 0.85 and 0.91 eV at HSE06 level, with Bader and charge density difference analyses suggesting occurrence of charge redistributions at the interface and electrons diffusion from MoSe2 to PtX2 layers, driven by large band offsets. The thermodynamic and thermal stabilities of the heterostructures are demonstrated by the negative binding energy and AIMD simulation. The heterostructure interface is influenced by the weak vdW coupling with an equilibrium interlayer distance of 3.01 to 3.08 Å and binding energy of −5.5 to −11.2 meV Å−2, indicating an exothermic process and steady adhesion at the interface. Reasonable lattice mismatch that ranges from 1.5 to 4.7% between the vdW heterostructure and separate monolayers suggests good structure compatibility. The optical performance of the heterostructure was examined using the real and imaginary components of dielectric function, where enhanced light absorption of 104–105 cm−1 and prominent peaks are observed encompassing the infrared to ultraviolet domains. Record high spectroscopic limited maximum efficiency (SLME) of ∼33% was also predicted. The absorption strength of MoSe2/PtO2 and MoSe2/PtS2 enhances with increasing negative external electric field (Eext) and compressive strain, individually, inferring their optical properties modulation by Eext and biaxial strain. Both heterostructures present high carrier mobility up to 1322.98 cm2 V−1 s−1 in zigzag direction.

Regulatable thermal conductivity and excellent mass transport of water-filled carbon nanotube as capillary wicks

• The heat and mass transfer of single wall carbon nanotubes as capillary wicks are studied by molecular dynamics simulations. • The regulatable thermal conductivity can be beneficial to both start-up and overheating operation of heat pipe. • The thermal conductivity of carbon nanotube is reduced due to the thermal motion of water molecules and vdW force. Passive liquid cooling of ultrathin heat pipe has received considerable attention due to its excellent performance of removing heat from hot spot of electronic devices. However, achieving controllable thermal conductivity of the capillary wick materials to reduce the start-up time and total thermal resistance remains a challenge. Here we propose the capillary wick of carbon nanotube array to regulate its heat transfer performance via the water filling ratio. Molecular dynamics simulation results found that the thermal conductivity of water-filled carbon nanotubes is reduced by 17.8%-23.8%, which is benefit to start-up operation. At overheating operation, the high thermal conductivity of empty carbon nanotube can reduce the temperature of the endothermic surface. Importantly, the thermophoretic movement speed of water can reach up to 74 m/s and radial mass exchange occurs driven by temperature gradient. Phonon spectral analysis and heat flow decomposition revealed that the decrease of water-filled carbon nanotube thermal conductivity originates from the low-energy coupling of van der Waals force between water molecules and carbon nanotube and the thermal motion of two water molecules layers. The regulatable thermal conductivity and excellent mass transport performance demonstrate the promising potential of carbon nanotube as next-generation capillary wick of ultrathin heat pipe.

Molecular Dynamics Simulation on In-Plane Thermal Conductivity of Graphene/Hexagonal Boron Nitride van der Waals Heterostructures.

Graphene, hexagonal boron nitride (h-BN), and their heterostructures are promising thermal interface materials due to the outstanding thermal properties of graphene and h-BN. For the heterostructures, extensive work has mainly focused on the thermal transport of two-dimensional (2D) graphene/h-BN (GBN) in-plane heterostructures in which graphene and h-BN are bonded at the interface. In this study, we investigate the thermal conductivity of three-dimensional (3D) GBN van der Waals (vdW) heterostructures by means of nonequilibrium molecular dynamics (NEMD) simulations. Unlike the 2D GBN in-plane heterostructure, the 3D GBN vdW heterostructure consists of three layers where graphene is sandwiched by two h-BN sheets via vdW forces. Various techniques, including hydrogen-functionalization, vacancy defects, tensile strain, interlayer coupling strength, layer numbers of h-BN, size effect, and temperature, are extensively explored to find an effective route for the modulation of the thermal conductivity. It is found that the thermal conductivity of the triple-layer GBN vdW heterostructure is very sensitive to these extrinsic factors. Of these, hydrogen-functionalization is the most effective method. A low hydrogen coverage of 1% in the sandwiched graphene can lead to 55% reduction in the thermal conductivity of the vdW heterostructure. Vacancy defects on graphene exert a more significant effect on the thermal conductivity reduction for the vdW heterostructure than B or N vacancies in the outer h-BN layers. This work reveals the physical mechanism for manipulating the thermal transport along the GBN vdW heterostructures via structural modification and provides a useful guideline for designing novel thermal management devices based on the GBN vdW heterostructures.

Constructing van der Waals heterostructures by dry-transfer assembly for novel optoelectronic device

Since the first successful exfoliation of graphene, the superior physical and chemical properties of two-dimensional (2D) materials, such as atomic thickness, strong in-plane bonding energy and weak inter-layer van der Waals (vdW) force have attracted wide attention. Meanwhile, there is a surge of interest in novel physics which is absent in bulk materials. Thus, vertical stacking of 2D materials could be critical to discover such physics and develop novel optoelectronic applications. Although vdW heterostructures have been grown by chemical vapor deposition, the available choices of materials for stacking is limited and the device yield is yet to be improved. Another approach to build vdW heterostructure relies on wet/dry transfer techniques like stacking Lego bricks. Although previous reviews have surveyed various wet transfer techniques, novel dry transfer techniques have been recently been demonstrated, featuring clean and sharp interfaces, which also gets rid of contamination, wrinkles, bubbles formed during wet transfer. This review summarizes the optimized dry transfer methods, which paves the way towards high-quality 2D material heterostructures with optimized interfaces. Such transfer techniques also lead to new physical phenomena while enable novel optoelectronic applications on artificial vdW heterostructures, which are discussed in the last part of this review.

AFM‐Based Hamaker Constant Determination with Blind Tip Reconstruction

AbstractParticle contamination of extreme ultraviolet (EUV) photomasks is one of the numerous challenges in nanoscale semiconductor fabrication, since it can lead to systematic device failures when disturbing patterns are projected repeatedly onto wafers during EUV exposure. Understanding adhesion of particle contamination is key in devising a strategy for cleaning of photomasks. In this work, particle contamination is treated as a particle‐plane problem in which surface roughness and the interacting materials have major influences. For this purpose, vacuum atomic force microscopy (AFM) contact measurements to quantify the van der Waals (vdW) forces between tip and sample are performed. This as a vacuum AFM‐based methodology that combines numerical Hamaker theory and blind tip reconstruction is introduced. The Hamaker constants of 15 × 10−20 J and 13 × 10−20 J for the material systems of a silicon (Si) tip with both aluminum oxide (Al2O3) and native silicon dioxide (SiO2) on Si substrates, respectively have been determined. These methodology allows an alternative, quick and low‐cost approach to characterize the Hamaker constant within the right order of magnitude for any material combination.