Weak vdW Interactions Research Articles

Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2

PtSe2 monolayer is previously predicted to be a two-dimensional water-splitting photocatalyst. However, the weak van der Waals (vdW) interaction between H2O and the basal surface of PtSe2 significantly undermines its photocatalytic water-splitting activities. In this work, we explore the possibility of various intrinsic defects of PtSe2 in remedying this deficiency on the basis of first-principles calculations. It is interesting to find that the introduction of Pt@Se, Se@Pt, and Se interstitial defect not only fully retain the water redox abilities of pure PtSe2 and realize spatial separation of photogenerated electrons and holes, but also can extend optical absorption range and absorption coefficients. Moreover, introduction of the three kinds of defects increase the initial weak vdW interactions between H2O and the PtSe2 surface to different extent. In particular, Pt@Se anti-site defect transform the initial weak vdW to strong chemical interaction between H2O and PtSe2 surface, and function as active reaction site. These insights demonstrate that introduction of intrinsic defects, especially the Pt@Se anti-site defect, are effective means for improving the photocatalytic water-splitting activities of PtSe2 monolayer.

Tuning Schottky barrier in graphene/InSe van der Waals heterostructures by electric field

The contacts between semiconductor and metal are vital in the fabrication of nano electronic and optoelectronic devices. The contact type has a great influence on the function realization and performance of the device. In order to prepare multifunctional devices with high performance, it is necessary to modulate the barrier height and contact type at the interface. First-principles calculations based on the density functional theory (DFT) are implemented in the VASP package. The generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) with van der Waals (vdW) correction proposed by Grimme (DFT-D3) is chosen due to its good description of long-range vdW interactions. It is demonstrated that weak vdW interactions dominate between graphene and InSe with their intrinsic electronic properties preserved. We find that the n-type ohmic contact is formed at the graphene/InSe interface with the Fermi level through the conduction band of InSe (<i>Φ</i><sub>Bn</sub> < 0). The Fermi level of graphene/InSe heterostructure moves down to below the Dirac point of graphene layer, which results in p-type (hole) doping in graphene. Moreover, the external electric field is effective to tune the Schottky barrier, which can control not only the Schottky barrier height but also the type of contact. With the negative external electric field varying from 0 to –1 V/nm, the conduction band minimum of InSe below the Fermi level declines gradually but the n-type ohmic contact is still preserved. Nevertheless, with the positive external electric field varying from 0 to 0.8 V/nm, the conduction band minimum of InSe shifts upward and across the Fermi level, the conduction band minimum of InSe is closer to the Fermi level than the valence band maximum, which indicates that the n-type Schottky contact is formed. The Fermi level moves from the the conduction band minimum to the valence band maximum of InSe when the positive external electric field increases from 0.8 V/nm to 2 V/nm. The n-type Schottky barrier height exceeds the p-type Schottky barrier height gradually, which demonstrates that the positive external electric field transforms the n-type Schottky contact into the p-type Schottky contact at the graphene/InSe interface. When the positive external electric field exceeds 2 V/nm, the valence band of InSe moves upward and cross the Fermi level (<i>Φ</i><sub>Bp</sub> < 0), the ohmic contact is obtained again. Meanwhile, p-type (hole) doping in graphene is enhanced under negative external electric field and a large positive external electric field is required to achieve n-type (electron) doping in graphene. The external electric field can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/InSe interface.

Graphene/WSeTe van der Waals heterostructure: Controllable electronic properties and Schottky barrier via interlayer coupling and electric field

The formation of the graphene-based van der Waals (vdW) heterostructures has shown great potential for designing novel electronic and optoelectronic nanodevices. Here, we construct the Graphene/WSeTe heterostructure and investigate its structural, electronic, optical and transport properties through first-principles calculations. We find that the electronic properties of both graphene and Janus WSeTe are well preserved in Graphene/WSeTe heterostructure because of the weak vdW interactions. The optical absorption of the Graphene/WSeTe heterostructure is enhanced in both regions of the visible and UV lights in comparison with that of the graphene and Janus WSeTe monolayers. The absorption coefficient of the Graphene/WSeTe heterostructure for the visible light can reach 5 × 104 cm−1, which is as twice as that of Janus WSeTe monolayer. Whereas, for the UV light, the absorption coefficient of such heterostructure can reach up to 105 cm−1. Moreover, the Graphene/WSeTe heterostructure tends to own a high carrier mobility for both electrons and holes as compared with single-layered Graphene. Especially, a band gap of about 10 meV at the Dirac cone of graphene in such heterostructures can be opened. Depending on the stacking configurations, the Graphene/WSeTe heterostructure can form the p-type Ohmic contact or p-type Schottky contact with a small Schottky barrier of 0.35 eV. Furthermore, our results demonstrate that the electric fields and vertical strains can be effectively used to tune both the contact types and the Schottky barrier height of Graphene/WSeTe heterostructure from the p-type Schottky contact to n-type one or to p-type Ohmic contact. Our results could provide a significant guidance for understanding the physical properties of the Graphene/WSeTe heterostructure towards nanoelectronic and optoelectronic devices.

Corrugated graphene exposes the limits of a widely used ab initio van der Waals DFT functional

Theoretical formulations capable of modeling chemical interactions over 3--4 orders of magnitude of bond strength, from covalent to van der Waals (vdW) forces, are one of the primary goals in materials physics, and chemistry. Development of vdW corrections for density-functional theory has thus been a major research field for two decades. While many of these corrections are semiempirical, more theoretically rigorous ab initio functionals have been developed. The ab initio functional vdW-DF2, when coupled with the reoptimized B86 exchange function (vdW-DF2-rB86), has typically performed as well, if not better than most semiempirical formulations. Here we present a system, Co intercalation of graphene on Ir(111), for which a semiempirical correction predicts local corrugation maxima in locations at which the vdW-DF2-rB86 functional predicts global minima. Sub-angstrom precision quantitative structural measurements show better agreement with the semiempirical correction. We posit that it is balancing the weak vdW interaction with the stronger, even covalent, interactions that proves a challenge for the vdW-DF2-rB86 functional.

Exfoliation, lattice vibration and air stability characterization of antiferromagnetic van der Waals NiPS3 nanosheets

Layered NiPS3 compound belongs to interesting two dimensional magnetic van der Waals (vdW) crystals, which provide the ideal platform for understanding two dimensional magnetism. Here high-quality NiPS3 single crystals of millimeter size are synthesized through the chemical vapor transport method. The layered structure and weak vdW interaction between layers lead to exfoliation down to a single layer. The atomic force microscope (AFM) and optical contrast are combined to determine the layer thickness. The lattice vibrations are further probed by Raman spectroscopy in detail and exhibit significant dependence on thickness of nanosheets. By Raman spectroscopy and AFM, the stability of monolayer NiPS3 has been discussed when exposed to air at room temperature. It is estimated that the exposure to air induces degradation gradually and the monolayer NiPS3 is roughly stable within 3 days after exfoliation. The surface suffers from oxidation in air, as confirmed by XPS measurements.

Computational understanding of the band alignment engineering in PbI2/PtS2 heterostructure: Effects of electric field and vertical strain

In this work, a new type of two-dimensional-based heterostructure PbI2/PtS2 has been constructed for the first time and its electronic properties and band alignment have been investigated systematically through first-principles calculations. Our results show that at the ground state, the PbI2/PtS2 heterostructure is mainly characterized by a weak vdW interaction. Moreover, such heterostructure forms a type-II band alignment and shows a semiconducting character with an indirect band gap of 1.90 eV. The band alignment in the PbI2/PtS2 heterostructure can be controlled under the application of the electric field and vertical strain, which can result on the semiconductor to metal transition. These findings provide an effective approach for designing new electronic and optoelectronic nanodevices based on PbI2/PtS2 heterostructure.

Strain and electric field engineering of band alignment in InSe/Ca(OH)2 heterostructure

In this work, we investigate the structural and electronic properties of the combined InSe/Ca(OH)2 heterostructure through density functional theory. It suggests that a combination of InSe and Ca(OH)2 tends to a significant decrease in the band gap of the heterostructure, which may result from the vacuum energy difference of the monolayers. The InSe/Ca(OH)2 heterostructure mediates by the weak vdW interactions and possesses a type-II semiconductor with a direct band gap of 0.55 eV, which can also be engineered by applying electric field or vertical strains. The semiconductor-to-metal and direct-to-indirect transitions can also emerge, which make InSe/Ca(OH)2 heterostructure promising material for electronic nanodevices.

Computational understanding of electronic properties of graphene/$${\mathrm{{PtS}}_2}$$ heterostructure under electric field

Graphene-based two-dimensional van der Waals heterostructures (vdWH) have recently shown a great potential for high-performance nanodevices. Here, we design a novel vdWH, consisting of the graphene and a noble transition metal dichalcogenide $${\mathrm{{PtS}}_2}$$ monolayer and investigate its electronic properties as well as the effect of electric field. We find that the key intrinsic characteristics of both the graphene and $${\mathrm{{PtS}}_2}$$ monolayers are well preserved due to the weak vdW interactions. The graphene is found to form an n-type Schottky contact with the $${\mathrm{{PtS}}_2}$$ monolayer with a small barrier height of 0.11 eV. Moreover, this small barrier height of the $$\mathrm{{G}}/{\mathrm{{PtS}}_2}$$ vdWH is known to be very sensitive to the external electric field. It can be controlled and turned to the Ohmic contact under electric field. These findings could provide significant knowledge for designing novel electronic and optoelectronic devices of such heterostructure.

Tri-layered van der Waals heterostructures based on graphene, gallium selenide and molybdenum selenide

In this work, we propose ultrathin trilayered heterostructures (TL-HTSs) of graphene (G), gallium selenide (GaSe), and molybdenum selenide (MoSe2) monolayers and investigate their structural and electronic properties in the framework of first-principles calculations. By calculating the binding energies and interlayer distances and comparing them with those of the typical vdW HTSs, we find that the systems we consider are energetically stable and are characterized by weak vdW interactions. The formation of G, GaSe, and MoSe2 monolayers to form G/GaSe/MoSe2, GaSe/G/MoSe2, and G/MoSe2/GaSe HTSs leads to the opening of a sizable bandgap in graphene at the Dirac point and shows the p-type Schottky contact. Among these kinds of TL-HTSs, the G/GaSe/MoSe2 has many more advantages than the others due to the lowest binding energy of −29.47meV/Å2, the biggest bandgap opening in G of 84.7 meV, and the smallest Schottky barrier height of 0.63 eV. Furthermore, we find that the p-type Schottky contact of G/GaSe/MoSe2 HTS can be turned into an n-type one or into an Ohmic contact when vertical strain or electric field is applied. These results show a potential candidate of the combined HTSs of G, GaSe, and MoSe2 monolayers for developing high speed nanoelectronic and optoelectronic devices.

Synthesis, Crystal Structure and Non-covalent Interactions Analysis of Novel N-substituted Thiosemicarbazone

(E)-1-(4-Fluorobenzylidene)-4-(4-ethylphenyl)thiosemicarbazone was synthesized via the reaction of 4-(4-ethylphenyl)thiosemicarbazide and 4-fluorobenzaldehyde. The title compound was characterized by FTIR, 1H and 13C NMR, mass spectrometry and elemental analysis techniques. Structural property of the title compound was displayed by the X-ray single crystal diffraction. The title compound crystallized in triclinic space group $P\overline 1$ with a=0.6494(4) nm, b=0.7971(5) nm, c=1.5492(10) nm, α=83.690(11)°, β=84.185(10)°, γ=84.348(11)°, molecular formula C16H16FN3S, Mr=301.39, V=0.7868(9) nm3, Z=2, Dc=1.272 g/cm3, F(000)=316, μ=0.213 mm−1, S=1.02, R=0.0513, and ωR[I>2σ(I)]=0.1662. The intermolecular interactions in the crystal structure were explained using the Hirshfeld surface and their associated two-dimensional fingerprint plots. The title compound showed C—H⋯S(1−x, −y, −z) and N—H⋯S(1−x, −y, −z) intermolecular interactions, and formed the supramolecular self-assemblies through $$\overline G $$ and $${\rm{R}}_2^2(8)$$ ring motifs. Shape index and curvedness were performed to further understand some unique weak interactions, for instance, the weak π…π stacking contacts in molecular structure with different characteristic regions. Besides, the reduced density gradient(RDG) function provided a real-space function for discussing non-covalent interactions within molecule, such as hydrogen bonds, weak vdW interactions and attractive or repulsive effects.

Application of van der Waals density functionals to two dimensional systems based on a mixed basis approach

A van der Waals (vdW) density functional was implemented with the charge density obtained by the mixed basis approach previously developed for studying two dimensional systems, in which the vdW interaction plays an important role. The basis functions here are taken to be the localized B-splines for the finite non-periodic dimension and plane waves for the two periodic directions. This approach will reduce the size of the basis set, especially for large systems, and therefore is computationally efficient for the diagonalization of the Kohn–Sham Hamiltonian. The nonlocal vdW correlation was treated appropriately according to the layered geometry structure. We applied the present algorithm to calculate the binding energy for graphene and hexagonal boron nitride and the results are consistent with data reported earlier. We also found that, due to the relatively weak vdW interaction, the charge density obtained self-consistently for the whole bilayer system is not significantly different from the simple addition of those for the two individual monolayer systems, except when the interlayer separation is close enough that the strong electron-repulsion dominates. This finding suggests an efficient way to calculate the vdW interaction for large complex systems involving Moiré pattern configurations.

Promoting effect of Ni on the structure and electronic properties of NixMo(1−x)S2 catalyst and benzene adsorption: A periodic DFT study

Aromatic hydrogenation (e.g., benzene) over MoS2-based catalyst harbours essential importance in hydrotreating process due to stringent environmental and fuel requirement. In this work, the promoting effect of Ni on structure and electronic properties of NixMo(1−x)S2(1 0 0) surface (x = 0, 0.25, 0.5, 1.0) and benzene adsorption has been investigated by DFT calculations using PW91-OBS and PW91 functions. It is found that the adsorption of benzene requires at least three coordinatively unsaturated sites (CUS), which can improve the activation of CC bond. The introduction of Ni facilitates the formation of stable CUS sites, and enhances the eletrophilicity of the Mo atoms for adsorption. Furthermore, the volcanic-shape relationship between Ni content and benzene adsorption ability is then established. With the increase of Ni percentage from 0 to 50%, the adsorption energy and electrons transferred from benzene molecules to the surface gradually increase due to the increased ligand effect for Mo atoms. However, overmuch Ni leads to the sharp decrease of benzene adsorption energy. Dispersion correction analysis indicate that the interplay between benzene and Ni is dominated by weak vdW interactions. The results reported herein are beneficial to the design of efficient NiMoS2 catalysts for hydrotreating.

Interlayer-Decoupled Sc-Based Mxene with High Carrier Mobility and Strong Light-Harvesting Ability.

Two-dimensional (2D) van der Waals (vdW) layered materials offer a unique combination of electronic and structural properties attractive for technological applications. Most of them show strong vdW interactions, which lead to interlayer-coupled optoelectronic properties due to quantum confinement. Here we present a systematic computational study of one Mxene, 2D double-metal-layered scandium chloride carbides (Sc2CCl2). Unlike conventional quantum-confined nanosystems, 2D Sc2CCl2 exhibits weak vdW interactions with robust interlayer-decoupled optoelectronic properties and extremely high and anisotropic carrier mobilities of about 1-4.5 × 104 cm2 V-1 s-1 that consequently produce comparatively large drain currents. Specifically, the 2D Sc2CCl2 family has strong light-harvesting ability and could be utilized as efficient donor materials in excitonic solar cells. Overall, in combination with high structural stability against ambient conditions, interlayer-decoupled robust optoelectronic properties potentially relax the requirements for the fabrication of high-quality monolayers and for selection of suitable substrates and suggest promising next-generation optoelectronic applications.

Structural, electronic, vibrational, and elastic properties of graphene/ MoS2 bilayer heterostructures

Graphene/MoS$_2$ van der Waals (vdW) heterostructures have promising technological applications due to their unique properties and functionalities. Many experimental and theoretical research groups across the globe have made outstanding contributions to benchmark the properties of graphene/MoS$_2$ heterostructures. Even though some research groups have already made an attempt to model the graphene/MoS$_2$ heterostructures using {\it first-principles} calculations, there exists several discrepancies in the results from different theoretical research groups and the experimental findings. In the present work, we revisit this problem by first principles approach and address the existing discrepancies about the interlayer spacing between graphene and MoS$_2$ monolayers in graphene/MoS$_2$ heterostructures, and the location of Dirac points near Fermi-level. We find that the Tkatchenko--Scheffler method efficiently evaluates the long-range vdW interactions and accurately predicts interlayer spacing between graphene and MoS$_2$ sheets. We further investigate the electronic, mechanical and vibrational properties of the optimized graphene/MoS$_2$ heterostructures created using 5$\times$5/4$\times$4 and 4$\times$4/3$\times$3 supercell geometries having different magnitudes of lattice mismatch. The effect of the varying interlayer spacing on the electronic properties of heterostructures is discussed. Our phonon calculations reveal that the interlayer shear and breathing phonon modes, which are very sensitive to the weak vdW interactions, play vital role in describing the thermal properties of the studied systems. The thermodynamic and elastic properties of heterostructures are further discussed. A comparison between our results and the results reported from other research groups is presented.

Electronic properties of GaSe/MoS2 and GaS/MoSe2 heterojunctions from first principles calculations

In this work, we theoretically investigate electronic properties of GaSeMoS2 and GaSMoSe2 heterojunctions using density functional theory based on first-principles calculations. The results show that both GaSeMoS2 and GaSMoSe2 heterojunctions are characterized by the weak vdW interactions with a corresponding interlayer distance of 3.45 Å and 3.54 Å, and the binding energy of −0.16 eV per GaSeGaS cell. Furthermore, one can observe that both the GaSeMoS2, and GaSMoSe2 heterojunctions are found to be indirect band gap semiconductors with a corresponding band gap of 1.91 eV and 1.23 eV, respectively. We also find that the band gaps of these semiconductors belong to type II band alignment. A type–II band alignment in both GaSeMoS2 and GaSMoSe2 heterojunctions open their potential applications as novel materials such as in designing and fabricating new generation of photovoltaic and optoelectronic devices.

Bandgap modulation of partially chlorinated graphene (C4Cl) nanosheets via biaxial strain and external electric field: a computational study

Density functional theory calculations with van der Waals (vdW) correction were used to investigate the structural and electronic properties of monolayer and bilayer of partially chlorinated graphene (C4Cl); the effects of biaxial strain and external electric field (E-field) on the electronic structure of C4Cl nanosheets were also examined. The C4Cl monolayer is a semiconductor with an indirect bandgap of 1.51 eV. Interestingly, the weak vdW interactions between two C4Cl monolayers induce a much smaller bandgap in C4Cl bilayer than that in the single monolayer. More interestingly, the electronic structure of C4Cl monolayer and bilayer can be feasibly modulated by applying an external E-field and a biaxial strain, and the modulation can even result in semiconductor–metal transition. These findings present very interesting possibilities for tailoring the electronic properties of C4Cl nanosheets for future electronic devices.

Van der Waals graphene/g-GaSe heterostructure: Tuning the electronic properties and Schottky barrier by interlayer coupling, biaxial strain, and electric gating

Graphene-based van der Waals heterostructures are expected recently to design and fabricate many novel electronic and optoelectronic devices. The combination of the electronic structures of graphene and graphene-like GaSe monolayer (g-GaSe) in an ultrathin heterostructure has been realized experimentally, such as graphene/g-GaSe field effect transistor and dual Schottky diode device. In the present work, we investigate the electronic properties of the graphene/g-GaSe heterostructures under the applied electric field, in-plane strains, and interlayer coupling. Our results show that the electronic properties of the graphene/g-GaSe heterostructures are well preserved owing to a weak vdW interaction. Especially, a tiny band gap of 13 meV has opened in the presence of the g-GaSe monolayer. We found that the n-type Schottky contact is formed in the graphene/g-GaSe heterostructure with a Schottky barrier height of 0.86 eV, which can be efficiently modulated by applying the electric field, in-plane strains, and interlayer coupling. Furthermore, a transformation from the n-type to p-type Schottky contact is observed when the applied electric field is larger than 0.1 V/Å or the interlayer distance is smaller than 3.2 Å. Our results may provide helpful information to design and fabricate the future graphene-based vdW heterostructures, such as graphene/g-GaSe heterostructure and understand the physics mechanism in the graphene-based 2D vdW heterostructures.

Tuning electronic properties of silicane layers by tensile strain and external electric field: A first-principles study

The structural and electronic properties of silicane layers and bulk, the effects of a biaxial tensile strain and an external electric field (E-field) on the electronic properties of silicane monolayer and bilayer are investigated using density functional theory computations with the van der Waals (vdW) correction. It is demonstrated that the weak vdW interaction between silicane layers can efficiently tune the electronic properties of silicane multilayers. The silicane multilayers (up to 5) are indirect bandgap semiconductors whose bandgap slightly decreases with the number of layers, whereas bulk silicane is a direct bandgap semiconductor. The bandgaps of both silicane monolayer and bilayer can be flexibly modulated by applying a biaxial tensile strain, and indirect–direct transition occurs when the biaxial tensile strain reaches 4% and 2% respectively. Besides, the bandgaps of the silicane monolayer and bilayer can also be continuously modulated by an external E-field, with an indirect–direct transition observed when its magnitude reaches 0.5 and 0.7 V/Å respectively. A larger E-field can trigger semiconductor–metal transition at approximately 0.8 V/Å for both silicane monolayer and bilayer. Our results provide rather effective and flexible approaches to tuning the electronic properties of silicane layers for application in silicane-based electronic and optoelectronic devices.

Recent progress in Van der Waals (vdW) heterojunction-based electronic and optoelectronic devices

The rediscovery of graphene in 2004 triggered an explosive expansion of research on various van der Waals (vdW) materials. The atomic layers of these vdW materials do not have surface crystal defects and are bonded by weak vdW interactions, thus the vdW materials can be stacked onto each other to form vdW heterojunction structures without needing to consider the lattice mismatch issue. In addition, the broad library of vdW materials makes it possible to design diverse types of heterojunctions with a wide range of band alignments, bandgaps, and electron affinities. Vertical vdW heterostructures especially offer numerous possibilities for the realization of high-performance electronic and optoelectronic devices. Therefore, these vdW heterostructures have received significant attention, and extensive relevant experimental results have been reported in the past few years. In this review, we first introduce the transfer techniques to form vdW heterojunction structures. Next, we discuss recent progress in vdW heterostructure-based electronic and optoelectronic devices, including vertical field effect transistors, negative differential resistance devices, memories, photodetectors, photovoltaic devices, and light-emitting diodes. Finally, we conclude this review by discussing the current challenges facing vdW heterojunction structure-based devices and our perspective on future research directions.

Tunable Schottky contacts in the antimonene/graphene van der Waals heterostructures

Electronic structures modulation in the antimonene/graphene van der Waals(vdW) heterostructure with an external electric field(Eext) are investigated by density functional theory calculations. It is demonstrated that weak vdW interactions dominate between antimonene and graphene with their intrinsic electronic properties preserved. Furthermore, the vertical Eext can control not only the Schottky barrier but also the Schottky contacts (n-type and p-type) and Ohmic contacts (n-type) at the antimonene/graphene interface. Meanwhile, the negative Eext can shifts the Dirac point of graphene above the Fermi level, resulting in p-type doping in graphene because electrons can easily transfer from the Dirac point of graphene to the conduction band of antimonene. The present study would open a new avenue for application of ultrathin antimonene/graphene heterostructures in future nano- and optoelectronics.