Interlayer Van Der Waals Interactions Research Articles

Three‐Dimensional VTe2/MXene/CNT Ternary Architectures for the Development of High Performance Microsupercapacitors

AbstractRapid advancements in portable electronics have created a demand for ultrathin power sources. Microsupercapacitors (MSCs) are becoming a competitive and advantageous option for these applications. It is widely recognized that to develop MSCs with exceptional performance, electrode materials having two‐dimensonal (2D) permeable channels, structural scaffolds with high‐conductivity and large surface area are suitable. Vanadium ditelluride (VTe2) stands out as an ideal material platform in this context. Its unique combination of metallic properties and exfoliative characteristics‐stemming from the conducting Te–V–Te layers held together by weak van der Waals interlayer interactions‐ renders it highly promising for high‐performance MSCs. This study is the first to report that the restacking issues and electrochemical performance of VTe2 can be successfully avoided by the simultaneous incorporation of MXene and CNT to form a ternary hybrid. Here, a laser‐induced graphene (LIG)‐based MSC utilizing VTe2/MXene/CNT as the active electrode material is fabricated. This MSC achieve fabrications an outstanding maximum energy density of 6.84 µWh cm−2 and a power density of 304.7 µW cm−2. This significant achievement demonstrates the potential of this LIG‐based MSC to advance the design of high‐performance micro‐energy storage devices.

Synergistic effect of bonding heterogeneity and phonon localization in introducing excellent thermoelectric properties in layered heteroanionic NdZnSbO material

Layered rare-earth metal oxides, harnessing the dual properties of oxides and two-dimensional layered materials, exhibit remarkable thermal stability and quantum confinement effects. Therefore, this work adopts the first-principles calculation combined with the Boltzmann transport theory to predict the thermoelectric properties of NdZnSbO compound. The coexistence of weak interlayer van der Waals interactions, robust intralayer ionic bonding, and partial covalent bonding leads to remarkable bonding heterogeneity, which engenders pronounced phonon scattering and imposes constraints on thermal transport along the out-of-plane direction. The weakened chemical bonds induced by the antibonding states, together with the rattling-like behavior of the Zn atom, culminate in the profound anharmonicity in the layered NdZnSbO compound. The weakening bond and heavy element contribute to the softness of phonon modes, which significantly diminishes the phonon group velocity. The redistribution-dominated four-phonon scattering process spans a large optical gap, which effectively reduces the lattice thermal conductivity. The NdZnSbO compound exhibits direct semiconductor characteristic with a bandgap of 0.73 eV by adopting the Heyd-Scuseria-Ernzerhof (HSE06) functional in combination with spin–orbit coupling (SOC) effect. The multi-valley feature of NdZnSbO compound augur favorably for band degeneracy, thus amplifying the power factor. Consequently, an optimal figure-of-merit (ZT) of 3.40 at 900 K is achieved for the n-type NdZnSbO compound. The present study delves deeply insights into the origins for the low thermal conductivity of NdZnSbO compound and proposes an optimization scheme to enhance overall thermoelectric performance.

Mechanical strain effect on the optoelectronic properties and photocatalysis applications of layered AlN/GaN nanoheterostructure.

The aim of this work is to use first principles calculations to examine the effects of different mechanical strains on the optoelectronic and photocatalytic capabilities of the 2D/2D nanoheterostructure of AlN/GaN. By utilizing the lmBJ (Meta-GGA) and PBEsol (GGA) functional, the bandgap of the nanoheterostructure is calculated and found to be 4.89eV and 3.24eV. Simulated 2D AlN/GaN nanoheterostructure exhibits exceptional optical and electronic characteristics under applied biaxial tensile and compressive strains. The band gap changes from 4.89 to 3.77eV, while the energy gap nature transitions from direct to indirect during tensile strain fluctuations of 0% to 8%. Strain is also found to have a significant effect on the optical absorption peaks. And a 0-8% rise in tensile strain causes the initial absorption peak of the 2D AlN/GaN nanoheterostructure to shift from 4.88 to 4.20eV, which results in a 14% red shift in photon energy for every 2% change in strain. Furthermore, the optimum bandgap and band edge positions of the 2D AlN/GaN nanoheterostructure enable the water redox process to produce hydrogen and oxygen for wide range of pH. Thus, modification via strain may be an effective method for altering the optical as well as electronic characteristics of a 2D AlN/GaN nanoheterostructure, and this study may pave the way for new applications of this material in optoelectronic devices in the future. In the current work, density functional theory is used to explore every attribute of the 2D AlN/GaN nanoheterostructure. To characterize the electronic exchange-correlation, we used the PBEsol functional. In order to prevent any interlayer contact between periodicity of images, a vacuum is produced along the z-direction of approximately 10 Å. To increase the precision of bandgap prediction, the electronic and optical characteristics were computed using the meta-GGA lmBJ functional. To account for interlayer van der Waals interactions, nanoheterostructure computations were performed using the DFT-D3 functional.

Unveiling the Mechanism of Spontaneous Nanoscroll Formation from Janus Transition Metal Dichalcogenide Nanoribbons.

Due to the atomic asymmetry, Janus transition metal dichalcogenide monolayers possess spontaneous curling and can even form one-dimensional nanoscrolls. Unveiling this spontaneous formation mechanism of nanoscrolls is of great importance for precise structural control. In this paper, we successfully simulate the process of Janus MoSSe nanoscroll formation from flat nanoribbons, based on molecular dynamics (MD) simulations with hybrid potentials. The spontaneous scrolling is purely driven by the relaxation of intrinsic strain in Janus MoSSe. The final structure of nanoscroll is strongly affected by the length of nanoribbon with a nonmonotonous relation. To further understand the mechanism, we establish a thermodynamic model to determine the inner radius of MoSSe nanoscrolls, which is shown to be related to spontaneous curvature, bending stiffness, interlayer van der Waals interaction, interlayer distance, and length of initial nanoribbon. The results correspond well with MD simulations of nanoscrolls from flat nanoribbons and the molecular static simulations of directly built nanoscrolls. Moreover, the inner radii of MoSeTe and MoSTe nanoscrolls are predicted based on the model. Our results provide insights into the Janus TMD nanoscroll formation and a pathway for controllable fabrication of nanoscrolls.

2D Dion Jacobson/3D perovskite heterojunction solar cells without hole transport layer: Further optimize the performance by SCAPS-1D

The global energy demand is steadily increasing, prompting a widespread focus on renewable energy sources such as solar, wind, hydro, and biomass. Perovskite solar cells (PSCs) have gained considerable attention due to their remarkably high power conversion efficiency (PCE). The Dion-Jacobson (DJ) type two-dimensional layered perovskite material, lacking interlayer van der Waals interaction, enhances device stability significantly. Simultaneously, the heterojunction structure formed by MAPbI3 improves power conversion efficiency (PCE) while maintaining high stability. This study employs 2D Dion Jacobson/3D perovskite heterojunction solar cells for further optimization through simulation analysis using SCAPS-1D (Solar Cells capacitance simulator). The intrinsic impact of heterojunction perovskite solar cells is identified, and appropriate physical parameters are adjusted to approximate experimental results. Optimization variables include absorption layer thickness, absorption layer band gap, interface defect layer thickness, back contact material, and electron transport layer material, all influencing device performance differently. Determining the ideal absorption layer thickness based on absorption characteristics, a hole-free transport layer structure is adopted in this work to reduce production costs and process complexity. To enhance the responsiveness of DJ type two-dimensional layered perovskite materials to various light wavelengths and improve PCE, the simulation employs 3D graphics to optimize variables such as the absorption layer's band gap. The highest PCE achieved is 27.42 %. Additionally, the study explores device performance in a low-temperature operating environment (265 ∼ 325 K). At 265 K, the PCE and fill factor (FF) are 29.37 % and 85.45 %, respectively, indicating that 2D Dion Jacobson/3D perovskite heterojunction solar cells exhibit structural stability and high efficiency at low temperatures compared to other PSCs. These findings suggest promising practical applications in the fields of photovoltaics and optoelectronics.

Intercalation of quaternary ammonium cations as a key factor of electron storage in MoS2 thin films

Electrochemical intercalation of cations within two-dimensional transition metal dichalcogenides presents a promising route for tailoring their optoelectronic properties. We have now succeeded in modulating the optical properties of MoS2 thin films through electrochemical intercalation of quaternary ammonium cations. The spectroelectrochemical experiments conducted with varying sizes of the intercalant revealed the size-dependent stability of the intercalated MoS2 nanosheets. The observed absorption change of the exciton bands is reversible and arises from the storage of electrons in MoS2 nanosheets and the subsequent weakening of interlayer van der Waals interactions following cation intercalation. This structural change is evidenced by the emergence of A*1g out-of-plane Raman mode. Additionally, the photoelectron spectroscopy reveals the emergence of a lower binding energy component of Mo 3d and the shift in Fermi level to higher energies, confirming the presence of stored electrons in cation intercalated-MoS2. The underlying mechanism of intercalation-induced property modifications in MoS2 discussed in the present study is useful in developing strategies for energy conversion devices.

Controlled growth of 3R phase niobium diselenide and its properties

Inversion symmetry broken 3R phase transition metal dichalcogenides (TMDs) show fascinating prospects in spintronics, valleytronics, and nonlinear optics. However, the controlled synthesis of 3R phase TMDs is still a great challenge. In this work, two-dimensional 3R-NbSe2 single crystals up to 0.2 mm were synthesized for the first time through chemical vapor deposition method by designing a space-confined system. The crystal size and morphology can be controlled by the location of the stacked substrates and the amount of the Nb2O5 precursor. Scanning transmission electron microscopy and Raman measurements reveal the NbSe2 exhibits a pure 3R stacking mode with relatively weak interlayer van der Waals interactions. Importantly, 3R-NbSe2 shows obvious second harmonic generation signal which intensity intensified as thickness increases. Density functional theory calculations and optical absorption demonstrate the coexistence of metallic and semiconducting optical properties of 3R-NbSe2. We designed a NbSe2/WS2/NbSe2 photodetector utilizing the metallicity of 3R-NbSe2, which shows good performance especially an ultrafast response (6–7 μs, 0.5 ms − 7.9 s for Au electrodes in literature). The proposed strategy and findings are of great significance for the growth of many other 3R-TMDs and applications of nonlinear optical and ultrafast devices.

Dual carbon modification of few-layered ReS2 nanosheets to retard polysulfide shuttling for long-cycling sodium-ion batteries

Rhenium disulfide (ReS2) is promising to work as a robust host for sodium storage, owing to the extremely weak interlayer van der Waals interactions and large interlayer spacing. However, the low electronic conductivity and huge volume expansion seriously restrict high-rate capability and cycling lifespan. In this work, few-layered ReS2 nanosheets with carbon coating (ReS2/C) grown on the hollow mesoporous carbon spheres (HMCS) are designed to form the HMCS@ReS2/C architectures that have several merits for the sodium storage, attributed to the dual carbon modification and the hollow structure. (i) HMCS provides a large interior void to relieve the volume change during discharge/charge processes and superior structural stability for long-term cycling. (ii) The large hetero-interfacial contact can enhance electron/Na+ transport in ReS2, enabling accelerated reaction kinetics. (iii) The outer carbon layer formed on ReS2 surface is capable of inhibiting dissolution of polysulfide ions and their corrosion to the copper current collector, preventing the “under-voltage failure” phenomenon. Consequently, HMCS@ReS2/C delivers superior sodium storage performance, yielding excellent high-rate capability (211 mAh g−1 at 10 A g−1), long lifespan (4000 cycles at 10 A g−1) and stable cycling stability with an extremely low capacity decay rate of 0.0096% per cycle. The high-rate cyclability surpasses the record of ReS2-based anodes reported previously. Furthermore, the fabricated Na3V2(PO4)3║HMCS@ReS2/C full cells can also exhibit outstanding cycling performance.

Crystallization of molecular layers produced under confinement onto a surface

It is well known that molecules confined very close to a surface arrange into molecular layers. Because solid-liquid interfaces are ubiquitous in the chemical, biological and physical sciences, it is crucial to develop methods to easily access molecular layers and exploit their distinct properties by producing molecular layered crystals. Here we report a method based on crystallization in ultra-thin puddles enabled by gas blowing, which allows to produce molecular layered crystals with thickness down to the monolayer onto a surface, making them directly accessible for characterization and further processing. By selecting four molecules with different types of polymorphs, we observed exclusive crystallization of polymorphs with Van der Waals interlayer interactions, which have not been observed with traditional confinement methods. In conclusion, the gas blowing approach unveils the opportunity to perform materials chemistry under confinement onto a surface, enabling the formation of distinct crystals with selected polymorphism.

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.

Electrostatic Gating of Phosphorene Polymorphs.

The ability to directly monitor the states of electrons in modern field-effect transistors (FETs) could transform our understanding of the physics and improve the function of related devices. In particular, phosphorene allotropes present a fertile landscape for the development of high-performance FETs. Using density functional theory-based methods, we have systematically investigated the influence of electrostatic gating on the structures, stabilities, and fundamental electronic properties of pristine and carbon-doped monolayer (bilayer) phosphorene allotropes. The remarkable flexibility of phosphorene allotropes, arising from intra- and interlayer van der Waals interactions, causes a good resilience up to equivalent gate potential of two electrons per unit cell. The resilience depends on the stacking details in such a way that rotated bilayers show considerably higher thermodynamical stability than the unrotated ones, even at a high gate potential. In addition, a semiconductor to metal phase transition is observed in some of the rotated and carbon-doped structures with increased electronic transport relative to graphene in the context of real space Green's function formalism.

A study on the effect of defects on the buckling of double-walled carbon nanotubes under compression based on a new atomic-continuum coupling method

An atomic-continuum coupling (ACC) method is developed for the nonlinear mechanical analysis of defective double-walled carbon nanotubes (DWCNTs). The moving least squares (MLS) approximation is resorted to bridge the fully atomic discrete structures of defective DWCNTs and the corresponding virtual continuum solids. The intrinsic mechanic laws implied in nanostructures can be accurately mapped into the mechanical governing equations of the continuum models. Based on ACC method, a numerical computational scheme is developed for predicting the buckling and contact behaviors of defective DWCNTs, which do not need any numerical integration method to calculate potential functional and its derivatives. The numerical tests show that this method can furnish good predictions even with a small number of nodes. It is found that Stone–Wales (SW) defects can lead to greatly decrease in the buckling properties of DWCNTs. In contrary, the complex interlayer van der Waals (vdW) interactions can enhance the buckling resistance of DWCNTs.

Two-dimensional heavy fermions in the van der Waals metal CeSiI.

Heavy-fermion metals are prototype systems for observing emergent quantum phases driven by electronic interactions1-6. A long-standing aspiration is the dimensional reduction of these materials to exert control over their quantum phases7-11, which remains a significant challenge because traditional intermetallic heavy-fermion compounds have three-dimensional atomic and electronic structures. Here we report comprehensive thermodynamic and spectroscopic evidence of an antiferromagnetically ordered heavy-fermion ground state in CeSiI, an intermetallic comprising two-dimensional (2D) metallic sheets held together by weak interlayer van der Waals (vdW) interactions. Owing to its vdW nature, CeSiI has a quasi-2D electronic structure, and we can control its physical dimension through exfoliation. The emergence of coherenthybridization of f and conduction electrons at low temperature is supported by the temperature evolution of angle-resolved photoemission and scanning tunnelling spectra near the Fermi level andby heat capacity measurements. Electrical transport measurements on few-layer flakes reveal heavy-fermion behaviour and magnetic order down to the ultra-thin regime. Our work establishes CeSiI and related materials as a unique platform for studying dimensionally confined heavy fermions in bulk crystals and employing 2D device fabrication techniques and vdW heterostructures12 to manipulate the interplay between Kondo screening, magnetic order and proximity effects.

Magneto-electric coupling beyond van der Waals interaction in two-dimensional multiferroic heterostructures

Exploring two-dimensional (2D) multiferroic systems with strong magneto-electric coupling properties holds significant application value in nanoscale spintronics devices. However, due to the weak interlayer van der Waals interactions, strong magneto-electric coupling in 2D heterostructures is relatively rare. By using first-principles simulations, we demonstrate that in the NiPS3/Sc2CO2 heterostructure, the ferroelectric polarization switching of the Sc2CO2 layer induces a transition in the magnetic ground state of the NiPS3 layer from the ferromagnetic state to antiferromagnetic ordering, accompanied by a transformation from a semiconductor to a half-metallic state. This magnetic phase transition is caused by a novel magneto-electric coupling mechanism: the polarization switching changes the band alignment between the two materials and then induces a significant interlayer charge transfer, leading to the emergence of Stoner itinerant ferromagnetism. In addition, the polarization switching can also change the magnetic anisotropy from an easy magnetization plane to an easy magnetization axis. These results not only offer a promising multiferroic heterostructure for nonvolatile memory devices and magnetic sensors but also provide a feasible approach for designing multiferroic system with strong magneto-electric coupling.

Reduction of thermal conductivity in carbon nanotubes by fullerene encapsulation from machine-learning molecular dynamics simulations

The carbon nano-peapod is a representative structure with interlayer van der Waals (vdW) interactions, in which encapsulated fullerene molecules play a critical role in modulating the transport properties of the carbon nanotubes (CNTs). In particular, their influence on the thermal transport characteristics has been the focal point of considerable attention. In this study, we trained an accurate machine learning potential for fullerene-encapsulated CNTs based on the efficient NEP model to investigate their thermal properties. Using equilibrium molecular dynamics simulation along with the spectral decomposition method for thermal conductivity, we find that the thermal conductivity of fullerene-encapsulated CNTs is roughly 55% lower than that of empty CNTs, aligning with experimental observations for CNT bundles with fullerene encapsulation [Kodama et al., Nat. Mater. 16, 892 (2017)]. The research suggests that weak vdW interactions between both the fullerene and CNTs, as well as between fullerene molecules themselves, hinder phonon propagation. The encapsulated fullerene contributes to an increase in phonon scattering within the CNTs, ultimately leading to a reduction in thermal conductivity. We utilized machine learning potential to investigate the structure of fullerene-encapsulated CNTs and their heat transport property. This approach provides valuable insights for performance research of complex systems featuring interlayer vdW interactions.

Bandgap engineering and tuning of optoelectronic properties of 2D NbSe2/MoS2 heterostructure using first principle computations

This study performed first-principles calculations based on density functional theory to study the interlayer electronic and optical properties of NbSe2/MoS2 heterostructures. Bandgap in 2H-MoS2 is often quite large typically around 1.8 eV, showing slow response time and low photoresponsivity (R); however, a slight bandgap variation can improve the properties of semiconducting and conducting heterostructures. Different stacking configurations of the interlayer van der Waals interaction were precisely investigated. Due to their unique properties, atomically thin NbSe2/MoS2 based heterostructures hold great potential for future electronic and optoelectronic devices. LDA, GGA, GGA with SOC, and HSE06 are used to study the monolayers of MoS2, NbSe2, and their T and H stacking structures. Our results demonstrate that the metallic NbSe2 effect on the semi-metallic MoS2 reduces the band gap of MoS2 up to 140 meV. Moreover, these heterostructures exhibit outstanding absorption properties from visible to ultraviolet regions, which makes them ideal candidates for optoelectronic applications, particularly in photodetectors.

Strain-induced effects on the optoelectronic properties of ZrSe2/HfSe2 heterostructures.

Two-dimensional semiconductor materials have received much attention in recent years due to their wide variety of applications in the field of nano-optoelectronic devices. In this project, we applied stresses ranging from -6 to +6% to the ZrSe2/HfSe2 heterostructure and systematically investigated its electrical and optical properties. It is discovered that stress can effectively modulate the forbidden bandwidth of the ZrSe2/HfSe2 heterojunction; whereas, under compressive stress, the forbidden bandwidth of the material decreases further until the bandgap is zero, leading to the material's transformation from semiconductor to metal. The forbidden band gap of the ZrSe2/HfSe2 heterojunction increases with increasing horizontal biaxial tensile strain. We discovered that the light absorption performance of this heterostructure is significantly better than that of its similar monomolecular layer and that its light absorption intensity can reach an order of magnitude of 104. Under compressive and tensile stresses, the ZrSe2/HfSe2 heterojunctions exhibit different degrees of red or blue shift. The results indicate that constructing ZrSe2/HfSe2 heterojunctions and applying horizontal biaxial stresses to them can significantly modulate the optoelectronic properties of the materials. ZrSe2/HfSe2 heterojunction is a new type of high-performance photogenerated carrier transport device with a wide range of applications. The calculations in this study are carried out the first principles approach of density functional theory, as implemented in the CASTEP module of Materials-Studio2019. The researchers used an ultrasoft reaction potential to calculate the interactions between the ion core and the electrons and applied the Perdew-Burke-Ernzerhof (PBE) and the generalized gradient approximation (GGA) to perform the calculations. The Monkhorst-Pack technique was employed to create the k-point samples utilized for integration on the Brillouin zone, and the k-point grid was uniformly 6 × 6 × 1. In addition, in order to avoid interactions between the atomic layers affecting the properties and stability of the material, such interactions were prevented by adding a 30 Å vacuum layer. Using a plane-wave energy cutoff of 500 eV and the convergence accuracy of the iterative process was set to 1 × 10-5 eV to ensure the accuracy of the computational results, and in addition. The maximum stress in the lattice was limited to less than 0.05 GPa or the interaction force between neighboring atoms was lower than 0.03 eV/Å. For the calculation of the properties of the optical properties, a k-point grid of 18 × 18 × 1 is used for optimization, and the polarization direction of the material is not taken into account, considering that the material is isotropic. This study proposes to apply the Tkatchenko-Scheffler (TS) dispersion correction method in DFT-D to appropriately represent the interlayer van der Waals interaction forces to solve inaccuracies in the computation of van der Waals interactions via density functional theory.

Interlayer expanded VS2 nanosheet for efficient sodium storage

An interlayer-expanded vanadium disulfide (VS2) electrode has been developed to enhance the performance of sodium ion batteries. The interlayer expansion of the (001) plane from the standard 5.7 Å to 9.5 Å improves capacity by 2–3 times at various current densities even over long cycles. This advancement is credited to the optimized kinetics of Na+ diffusion, as the intrinsic structure of VS2 with weak van der Waals interlayer interactions leads to minimal resistance along 2D ionic diffusion routes. Our findings highlight the significance of interlayer engineering in designing efficient energy storage electrodes.

The pressure response of SnSxSe2-x tin dichalcogenide alloys studied by Raman spectroscopy

High-pressure Raman spectroscopic experiments were conducted in a diamond anvil cell to investigate the pressure response of the phonon modes of SnSxSe2-x (x = 0.6, 0.8, 1) tin dichalcogenide alloys up to 8 GPa. Owing to the two-mode behavior of the Eg and A1g modes in these ternary alloys, up to four Raman bands are observed at ambient conditions and the frequency evolution of three of them {Eg(SnSe2-like), A1g(SnSe2-like) and A1g(SnS2-like)} is followed with pressure. With increasing pressure, all the Raman bands shift quasi-linearly and reversibly to higher frequencies, reflecting the bond stiffening due to the volume reduction and the absence of any phase transition. Despite the stronger intralayer covalent bonding compared to the much weaker interlayer van der Waals interactions along the c-axis, the in-plane Eg(SnSe2-like) mode has larger pressure coefficient than those of the two stronger in intensity A1g modes along the c-axis. The pressure coefficient of the A1g(SnS2-like) mode gradually increases from 3.60 to 3.93 cm−1GPa−1 with increasing sulfur content (x). On the other hand, the pressure coefficient of the A1g(SnSe2-like) mode decreases from 3.08 to 2.72 cm−1GPa−1 with increasing x. The extracted Grüneisen parameters of all the A1g Raman peaks indicate the stronger Sn–S interaction along the c-axis compared to that of Sn–Se.

Angular resolved ultra-low frequency Raman spectroscopy of few-layer black phosphorous

Few-layer black phosphorus (FLBP) exhibits valuable mechanical, optical, and electronic properties for potential applications in nanodevices, ranging from insulators to superconductors. In particular, their band structure and optical properties are closely related to layer numbers, stacking sequences, and twist angles. However, the mechanism behind these factors is not fully understood, as it is quite difficult to obtain stacking details using non-destructive probes. This article provides direct experimental evidence of the weak interlayer van der Waals (vdW) interaction of FLBP using a state-of-the-art, angular resolved, ultra-low frequency Raman spectroscopy system. A modified linear-chain model is presented to evaluate the layer number dependence on the extremely low-frequency breathing modes (Ag3 and Ag4). The polarization analysis illustrates that most bi-layer BP samples prefer the AB stacking sequence. Furthermore, the stacking angle of bi-layer BP was assigned based on its minimum intensity of Ag3 and Ag4 modes. This investigation on angular resolved extremely low-frequency Raman spectra sheds light on a new approach for analyzing the stacking details of FLBP, which is of great significance for developing novel FLBP-based nanodevices.