Interlayer Van Der Waals Forces Research Articles

Dion-Jacobson perovskite solar cells: Further optimize the performance by SCAPS-1D simulation techniques

With rising global energy demands and the emergence of novel energy forms, perovskite solar cells (PSCs) are becoming increasingly prominent due to their exceptionally high efficiency in converting power. The advantage of Dion-Jacobson (DJ) type 2D layered perovskite materials lies in their lack of interlayer van der Waals forces, coupled with hydrogen bonds at the extremities, necessitating additional external energy to break through their two-dimensional layered configuration, thereby greatly enhancing the materials' structural integrity. This study utilizes DJ type 2D layered perovskite material to tackle the stability problem. This study aimed to uncover the inherent effects of DJ type 2D perovskite substances by simulating the devices with SCAPS-1D (Solar Cells Capacitance Simulator) and modifying appropriate physical parameters to align closely with the outcomes reported in experiments. Simulation results indicate that factors such as the thickness of the absorber layer, the band gap, the operating temperature, and the thickness of the interfacial defect layer uniquely influence the device's performance. The optimal thickness for the absorbing layer was determined by its ability to absorb light. The aim is to boost the sensitivity of DJ type 2D layered perovskite materials to light across various wavelengths and to refine the PCE. During the simulation, the maximum PCE (26.57%) was attained by enhancing both the band gap width in the absorber layer and the hole transport layer. To delve deeper into the fluctuations in device efficiency in environments with low temperatures, this study established the temperature spectrum between 270 and 300 Kelvin. Investigations revealed that at 272 K, the PCE and FF stood at 28.33% and 83.79% respectively, indicating the effectiveness of DJ type 2D layered PSC in low-temperature applications, preserving structural integrity and superior efficiency relative to other PSCs. Consequently, this study offers a viable approach for creating DJ type 2D layered PSC, ensuring high efficiency and enduring stability in cooler conditions.

Morphology design and electronic configuration of MoSe2 anchored on TiO2 nanospheres for high energy density sodium-ion half/full batteries

Molybdenum selenide (MoSe2) has shown potential sodium storage properties due to its large layer spacing (0.646 nm) and high theoretical capacity and narrow band gap. However, as the anode material of sodium ion batteries (SIBs), the MoSe2’s performance is not ideal, especially due to the layer agglomeration and stacking caused by volume expansion and low intrinsic conductivity. Hence, morphology design and electronic configuration of MoSe2 is proposed via building MoSe2 nanosheets and auxiliary sulfur doping on the surface of the TiO2 hollow nanosphere (S-MoSe2@TiO2). The hierarchical shaped S-MoSe2@TiO2 effectively overcomes the shortcomings of high surface energy and weak interlayer van der Waals force of MoSe2. As anode for SIBs, S-MoSe2@TiO2 delivers enhanced cycling life and rate capability (308 mAh/g at 10 A/g after 1000 cycles) with the comparison of MoSe2@TiO2 or pure MoSe2 and TiO2. Such excellent sodium storage performance is due to the fast diffusion kinetics of Na+. When it is applied in sodium ion full batteries, the S-MoSe2@TiO2 anode based cell can reach a high energy density of 187.8 W h kg−1 at 148.3 W kg−1. The design of the new MoSe2-based hybrid provides a novel scheme for the preparation of advanced anode in SIBs.

Preparation of highly dispersed Ni single-atom doped ultrathin g-C3N4 nanosheets by metal vapor exfoliation for efficient photocatalytic CO2 reduction

Single-atom photocatalysts can modulate the utilization of photons and facilitate the migration of photogenerated carriers. However, the preparation of single-atom uniformly doped photocatalysts is still a challenging topic. Herein, we propose the preparation of Ni single-atom doped g-C3N4 photocatalysts by metal vapor exfoliation. The Ni vapor produced by calcining nickel foam at high temperature accumulates in between g-C3N4 layers and poses a certain vapor pressure to destroy the interlayer van der Waals forces of g-C3N4. Individual metal atoms are doped into the structure while exfoliating g-C3N4 into nanosheets by metal vapor. Upon optimization of Ni content, the Ni single atom doped g-C3N4 nanosheets with 2.81 wt% Ni exhibits the highest CO2 reduction performance in the absence of sacrificial agents. The generation rates of CO and CH4 are 19.85 and 1.73 μmol g−1h−1, respectively. The improved photocatalytic performance is attributed to the anchoring Ni of single atoms on g-C3N4 nanosheets, which increases both carrier separation efficiency and reaction sites. This work provides insight into the design of photocatalysts with highly dispersed single-atom.

WS2 nanosheets vertically grown on Ti3C2 as superior anodes for lithium-ion batteries

Tungsten disulfide (WS2) is considered as a promising anode material for high-performance lithium-ion batteries (LIBs) result from its inherent characteristics such as high theoretical capacity, large interlayer spacing and weak interlayer Van der Waals force. Nevertheless, WS2 has the drawbacks of easy agglomeration, severe volume expansion and high Li+ migration barrier, which lead to rapid capacity degradation and imperfect rate ability. In this work, a novel two-dimensional (2D) hierarchical composite (Ti3C2/WS2) consisting of WS2 nanosheets vertically grown on titanium carbide (Ti3C2) nanosheets is prepared. Thanks to this distinctive hierarchical structure and synergy between WS2 and Ti3C2, the Ti3C2/WS2 composite demonstrates exceptional electrochemical performance in LIBs. In addition, we investigate the effect of the mass proportion of WS2 in Ti3C2/WS2 composite on the electrochemical performance, and find that the optimal mass ratio of WS2 is 60%. As expected, the optimal electrode exhibits a high specific capacity (650 mAh/g at 0.1 A/g after 100 cycles) and ultra-long cycle stability (400 mAh/g at 1.0 A/g after 5000 cycles).

A Universal Chelation-Induced Selective Demetallization Strategy for Bioceramic Nanosheets (BCene).

The emergence of nanosheet materials like graphene and phosphorene, which are created by breaking the interlayer van der Waals force, has revolutionized multiple fields. Layered inorganic materials are ubiquitous in materials like bioceramics, semiconductors, superconductors, etc. However, the strong interlayer covalent or ionic bonding in these crystals makes it difficult to fabricate nanosheets from them. In this study, we present a simple technique to produce nanosheets from layered crystals by selectively exfoliating their interlayer metal atoms using the metal-chelation reaction. As a proof of concept, we successfully produced bioceramic nanosheets (BCene) by extracting Ca layers from Akermanite (AKT). The 3D-printed BCene scaffolds exhibited superior mechanical strength and in vitro bioactivity compared to the scaffolds made from AKT nanopowders. Our findings demonstrate the outstanding potential of BCene nanosheets in tissue engineering. Additionally, the selective demetallization technique for nanosheet production could be applied to other inorganic layered crystals to optimize their performance.

G-C3N4 sheet nanoarchitectonics with island-like crystalline/amorphous homojunctions towards efficient H2 and H2O2 evolution

Photocatalystic evolution of H2O2 from water and oxygen has attracted significant attention because of environmentally friendly. The absorption in visible and hydrophilic feature of graphitic carbon nitride (g-C3N4) make it a good candidate. In this paper, a rapid post-treatment at high temperature was developed to obtain g-C3N4 nanosheets with abundant crystalline/amorphous interfaces to form homojunctions, which optimized uniplanar carrier mobility dynamics. The conversion from bulk to two-dimensional g-C3N4 resulted from the breakage of interplanar hydrogen bonds and interlayer Van der Waals force. The unique morphology not only rendered photocatalyst with larger specific surface area but also inhibited the robust volume recombination of charge carriers. The accelerated charge carriers flow at the interface, interplane and interlayer together ameliorated the separation and transfer of electrons and holes. A new-emerged n→π* transition ameliorated the poor light utilization efficiency. Beyond the increased photocatalytic H2 evolution property (779.2 μmol g−1 h−1), optimized sample displayed a H2O2 evolution activity as high as 4877.1 μM g−1 h−1 under visible light illumination, which was ∼5.8 times of that of bulk g-C3N4. Detailed photocatalytic mechanism investigation manifested that the two-step single-electron oxygen reduction process occupied the dominant status in H2O2 evolution. This work proposed a novel strategy for obtaining g-C3N4 homojunctions as a promising bi-functional metal-free catalyst to be applied in clean energy production field.

Monolayer MoS2 Fabricated by In Situ Construction of Interlayer Electrostatic Repulsion Enables Ultrafast Ion Transport in Lithium-Ion Batteries

HighlightsIn-situ construction of electrostatic repulsion between MoS2 interlayers is first proposed to successfully prepare Co-doped monolayer MoS2 under high vapor pressure.The doped Co atoms radically decrease bandgap and lithium ion diffusion energy barrier of monolayer MoS2 and can be transformed into ultrasmall Co nanoparticles (~2 nm) to induce strong surface-capacitance effect during conversion reaction.The Co doped monolayer MoS2 shows ultrafast ion transport capability along with ultrahigh capacity and outstanding cycling stability as lithium-ion-battery anodes.High theoretical capacity and unique layered structures make MoS2 a promising lithium-ion battery anode material. However, the anisotropic ion transport in layered structures and the poor intrinsic conductivity of MoS2 lead to unacceptable ion transport capability. Here, we propose in-situ construction of interlayer electrostatic repulsion caused by Co2+ substituting Mo4+ between MoS2 layers, which can break the limitation of interlayer van der Waals forces to fabricate monolayer MoS2, thus establishing isotropic ion transport paths. Simultaneously, the doped Co atoms change the electronic structure of monolayer MoS2, thus improving its intrinsic conductivity. Importantly, the doped Co atoms can be converted into Co nanoparticles to create a space charge region to accelerate ion transport. Hence, the Co-doped monolayer MoS2 shows ultrafast lithium ion transport capability in half/full cells. This work presents a novel route for the preparation of monolayer MoS2 and demonstrates its potential for application in fast-charging lithium-ion batteries.

Single‐Layered MoS2 Fabricated by Charge‐Driven Interlayer Expansion for Superior Lithium/Sodium/Potassium‐Ion‐Battery Anodes

Single-layered MoS2 is a promising anode material for lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and potassium-ion batteries (PIBs) due to its high capacity and isotropic ion transport paths. However, the low intrinsic conductivity and easy-agglomerated feature hamper its applications. Here, a charge-driven interlayer expansion strategy that Co2+ replaces Mo4+ in the doping form to endow MoS2 layers with negative charges, thus inducing electrostatic repulsion, together with the insertion of gaseous groups, to drive interlayer expansion which once breaks the confinement of interlayer van der Waals force, single-layered MoS2 is obtained and uniformly dispersed into carbon matrix arising from the transformation of carbonaceous gaseous groups under high vapor pressure, is proposed. Co atom doping helps enhance the intrinsic conductivity of single-layered MoS2 . Carbon matrix effectively prevents agglomeration of single-layered MoS2 . The doped Co atoms can be fully transformed into ultrasmall Co nanoparticles during conversion reaction, which enables strong spin-polarized surface capacitance and thus significantly boosts ion transport and storage. Consequently, the prepared material delivers superb Li/Na/K-ion storage performances, which are best in the reported MoS2 -based anodes. The proposed charge-driven interlayer expansion strategy provides a novel perspective for preparing single-layered MoS2, which shows huge potential for energy storage.

Synthesis of Crystalline Phosphine-Graphdiyne with Self-Adaptive p-π Conjugation.

"Dynamic" behavior materials with high surface activity and the ability of chemical bond conversion are the frontier materials in the field of renewable energy. The outstanding feature of these materials is that they have adaptive electronic properties that external stimuli can adjust. An original discovery in a new crystalline two-dimensional phosphine-graphdiyne (P-GDY) material is described here. Although the p-π conjugation of most trivalent phosphorus π-systems is insignificant because of the pyramidal configuration, the lone pair electrons of phosphorus atoms participate strongly in the delocalization under the influence of the interlayer van der Waals forces in P-GDY. Due to the dynamically reversible nature of noncovalent interactions (p-π conjugation), P-GDY exhibits a specific adaptive behavior and realizes the responsive reversible transport of a lithium ion by regulating p-π interactions. Our findings would provide the potential to develop a new family of responsive materials with tunable structures.

Recent progress in mid-infrared photodetection devices using 2D/nD (n=0, 1, 2, 3) heterostructures

Mid-infrared (MIR) photodetection plays an important role in a variety of modern technologies, such as biomedical devices, free-space communication and security. In the past decade, 2D materials and heterostructures have been widely utilized in MIR photodetectors because of the tunable bandgaps, high carrier mobility and strong light absorption. Apart from 2D/2D heterostructures, 2D material can also form mixed-dimensional heterostructures with other dimensional materials thanks to the interlayer van der Waals forces, which presents a much broader group and greatly expands the research of MIR photodetectors due to the synergistic advantages. In this review, the recent progress of MIR photodetectors based on van der Waals (vdWs) heterostructures, including all-2D and mixed-dimensional heterostructures is systematically summarized. First, the common synthesis strategies to realize vdWs heterostructures are presented. Then, the motivation, device design, mechanism and performance of MIR photodetectors are presented and discussed in classification of various kinds of vdWs heterostructures. Finally, the current challenges and some future prospects in this research field are suggested.

Dimethyl 2‐Methylglutarate (Iris): A Green Platform for Efficient Liquid‐Phase Exfoliation of 2D Materials

AbstractLiquid‐phase exfoliation of bulk crystals of layered materials, held together by weak interlayer van der Waals forces, is an ideal platform for scalable synthesis of nanosheets. However, it is mandatory to substitute existing solvents, regrettably displaying severe limitations due to toxicity. Here, dimethyl 2‐methylglutarate (Rhodiasolv Iris) is validated for efficient liquid‐phase exfoliation of selected van der Waals materials, namely, MoS2, WS2, GeSe, and graphite. Here, we show that Iris‐assisted liquid phase exfoliation provides high yield (up to 52%) of flakes of 2D materials with aspect ratio as high as 500. Considering the various advantages of Iris over the state‐of‐the‐art solvents, including the absence of toxicity and its biodegradability, this work opens new possibilities for the ecofriendly production of 2D materials and for their extensive usage in industrial processes hitherto semi‐unexplored, owing to the toxicity of state‐of‐the‐art solvents (including the production of drinkable water or fruit juice concentration). Accordingly, the validation of Iris for sustainable liquid‐phase exfoliation of van der Waals crystals has intrinsic outstanding potential commercial impact. Moreover, here the values of the surface tension, Hansen solubility parameter, and viscosity of Rhodiasolv Iris are reported for the first time, which will be relevant also for any other sustainable process based on this new green solvent.

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.

Universal Principle for Large-Scale Production of a High-Quality Two-Dimensional Monolayer via Positive Charge-Driven Exfoliation

On the basis of the intrinsic characteristics of the layered materials, here we report a universal principle for the production of intact monolayers via layer-by-layer exfoliation from their bulk via positive charge doping. At experimental accessible densities (nc) of ∼1014 cm-2, various multilayer crystals, including graphite, hexagonal boron nitride, transition metal dichalcogenides, MXenes, and black phosphorus, can be exfoliated into the corresponding monolayers through ab initio density functional theory stimulations. The carrier critical thresholds for exfoliating are found to be nearly independent of thickness but dependent on surface size. The universality of positive charge-driven exfoliation originates from the common intrinsic characteristics of electronic structures for layered materials. The positively doped charges that preferentially accumulate near the surface induce interlayer repulsion, leading to layer-by-layer exfoliation when repulsion surpasses interlayer van der Waals force. This strategy may open the possibility of producing diverse high-quality two-dimensional monolayers with a small number of defects toward large-scale manufacturing.

A discrete-continuum mosaic model for the buckling of inner tubes of double-walled carbon nanotubes under compression

In this paper, the discrete-continuum mosaic (DCM) method is applied for predicting buckling of inner tubes of double-walled carbon nanotubes (DWNTs). The moving least squares (MLS) approximation is used for linking discrete atomic model and the corresponding continuum counterpart. Actual atomic positions are used as interaction points for the interlayer van der Waals (vdW) force of DWNTs, which can accurately capture the discreteness of non-bonded interactions, and a DCM contact model is constructed. A variationally consistent meshless computational scheme based on the DCM contact model is developed for simulating the buckling of inner tubes of DWNTs upon compression. Results show that both the lengths and radii of outer tubes can influence the buckling patterns of the corresponding inner tubes. The inner tubes of DWNTs with the interlayer spacing of 0.2034 nm have strongest buckling resistance for the case that the outer tubes are not extremely short. The local critical axial compression ratio of the inner tube increases with the lengths of the inner and outer tube increasing given that the interlayer spacing of a DWNT is less than 0.34 nm.

Theoretical understanding of electronic and mechanical properties of 1T' transition metal dichalcogenide crystals.

Transition metal dichalcogenides (TMDs) with a 1T′ layer structure have recently received intense interest due to their outstanding physical and chemical properties. While the physicochemical behaviors of 1T′ TMD monolayers have been widely investigated, the corresponding properties of layered 1T′ TMD crystals have rarely been studied. As TMD monolayers do not have interlayer interactions, their physicochemical properties will differ from those of layered TMD materials. In this study, the electronic and mechanical characteristics of a range of 1T′ TMDs are systematically examined by means of density functional theory (DFT) calculations. Our results reveal that the properties of 1T′ TMDs are mainly affected by their anions. The disulfides are stiffer and more rigid, diselenides are more brittle. In addition, the 1T′ polytype is softer than 2H TMDs. Comparison with the properties of the monolayers shows that the interlayer van der Waals forces can slightly weaken the TM–X covalent bonding strength, which can further influence the mechanical properties. These insights revealed by our theoretical studies may boost more applications of 1T′ TMD materials.

On electrically tunable stacking domains and ferroelectricity in moir\xe9 superlattices

It is well known that stacking domains form in moiré superlattices due to the competition between the interlayer van der Waals forces and intralayer elastic forces, which can be recognized as polar domains due to the local spontaneous polarization in bilayers without centrosymmetry. We propose a theoretical model which captures the effect of an applied electric field on the domain structure. The coupling between the spontaneous polarization and field leads to uneven relaxation of the domains, and a net polarization in the superlattice at nonzero fields, which is sensitive to the moiré period. We show that the dielectric response to the field reduces the stacking energy and leads to softer domains in all bilayers. We then discuss the recent observations of ferroelectricity in the context of our model.

Enhanced carriers separation in novel in-plane amorphous carbon/g-C3N4 nanosheets for photocatalytic environment remediation

Although carbon-based materials/g-C3N4 heterostructure with an up-down structure in space can inhibit the recombination of charge carriers, the electron transfer is still suppressed by the interlayer van der Waals force. Herein, amorphous carbon is successfully introduced into the g-C3N4 nanosheet (CNS) by a self-conversion process to form an in-plane heterostructure of amorphous carbon/g-C3N4 (CNSC1). Kelvin probe atomic force microscopy (KPFM) and density functional theory (DFT) confirm that g-C3N4 and amorphous carbon are in the same plane, which can generate the surface electric field of CNSC1, providing a driving force for the transfer of electrons from g-C3N4 to amorphous carbon. Meanwhile, the sp2-hybridized π conjugation bond of amorphous carbon can rapidly capture and store photogenerated electrons, inhibiting charge carrier recombination and thus generating more electrons to facilitate the yield of hydroxyl radicals. The photocatalytic activity of CNSC1 for the degradation of tetracycline and rhodamine B is 2.7 times and 4.8 times higher than that of CNS, respectively, due to the efficient interface charge separation. This work is expected to provide a new idea for the combination of carbon materials and g-C3N4.

Recent progress on van der Waals heterojunctions applied in photocatalysis

Progress on the applications of van der Waals heterojunctions in photocatalysis.

KMnO4-assisted synthesis of oxygen-containing porous graphene with high gravimetric and volumetric performances for supercapacitor

• The samples use KMnO 4 as activator precursor. • The samples display high gravimetric and volumetric capacitance. • The optimized electrode shows good electrochemical performance. • The symmetric supercapacitor shows high energy density and cycling stability. Graphene becomes a promising electrode material for supercapacitor because of their high theoretical specific surface area and good conductivity. Nevertheless, the main disadvantage of graphene is the irreversible agglomeration because of the strong interlayer van der Waals force, which is significantly decreases the surface area, thus resulting in poor electrochemical performance. Herein, we report a new and facile strategy to synthesize rich oxygen-containing porous graphene (OPG) using KMnO 4 as activator precursor. The optimized OPG-600 samples possess crumpled porous framework with a suitable surface area and massive oxygen-containing functional groups. Benefiting from their synergistic influence, the OPG-600 electrode displays a high gravimetric and volumetric capacitance of 363.3 F g −1 and 342.5 F cm −3 at 0.5 A g −1 and good rate performance (240 F g −1 and 226.3 F cm −3 at 30 A g −1 ). More interestingly, the OPG-600 symmetric supercapacitor displays a high volumetric energy density of 19.2 Wh L −1 and superior electrochemical stability.

Nondestructive Picosecond Ultrasonic Probing of Intralayer and van der Waals Interlayer Bonding in α‐ and β‐In2Se3

AbstractThe interplay between the strong intralayer covalent‐ionic bonds and the weak interlayer van der Waals (vdW) forces between the neighboring layers of vdW crystals gives rise to unique physical and chemical properties. Here, the intralayer and interlayer bondings in α and β polytypes of In2Se3 are studied, a vdW material with potential applications in advanced electronic and optical devices. Picosecond ultrasonic experiments are conducted to probe the sound velocity in the direction perpendicular to the vdW layers. The measured sound velocities are different in α‐ and β‐In2Se3, suggesting a significant difference in their elastic properties. Density functional theory and an effective spring model are used to calculate the elastic stiffness of the layer and vdW gap in α‐ and β‐In2Se3. The calculated elastic moduli show good agreement with experimental values and reveal the dominant contribution of interlayer atomic bonding to the different elastic properties of the two polytypes. The findings show the power of picosecond ultrasonics for probing the fundamental elastic properties of vdW materials. The data and analysis also provide a reliable description of the intra‐ and interlayer forces in complex crystal structures, such as the polytype phases of In2Se3.