Interlayer Force Research Articles

Anisotropic phonon and magnon vibration and gate-tunable optoelectronic properties of nickel thiophosphite

Transition metal phosphorus trichalcogenides retain spin-charge coupling and lattice vibrations in different layers, which are useful for spintronic and optoelectronic devices. The phonon, magnons and excitonic properties of two-dimensional ternary nickel-phosphorus trisulfides (NiPS3) are investigated using Raman spectroscopy and photoluminescence (PL) study. With magnetic exchange interaction, an exotic phonon scattering degenerates the optical phonons into in-plane A g and B g modes. We have observed eight Raman modes with two acoustic anisotropic magnon modes (M 1, M 2) below the critical temperature for co-(XX), while only M 1 at cross (XY) polarizations. The M 1 mode is coupled with the phonon B g mode that can survive after transition temperature. The phonon and magnon modes soften with variations in temperature, which is attributed to anharmonic phonon–phonon coupling and interlayer forces. The polarized Raman shows the two-fold and four-fold symmetry orientations of the phonon and magnon modes, respectively, which exhibit strong in-plane anisotropic phonon/magnon. The PL spectra revealed the existence of bound excitonic features and ensemble emitters in NiPS3. The robust interlayer excitation and structural stability further revealed the optothermal properties. Moreover, the fabricated field-effect transistor on NiPS3 reveals p-type semiconducting nature with an ON/OFF ratio of 5 × 106 and mobility of ∼16.34 cm2 V−1 s−1. In contrast, the rectification ratio indicates their diode characteristics. Similarly, the photocurrent is enhanced by changing the wavelength of light, which shows the potential for optoelectronics. The strong spin-charge interaction provides new insights into these materials’ magneto-optical and thermal properties for memory devices.

Preparation of boron nitride nanosheets by glucose-assisted ultrasonic cavitation exfoliation.

Boron nitride nanosheets (BNNSs) have been widely used in many fields due to their excellent properties. However, low preparation rates and difficulty in functionalization hinder their further development. This study proposes a novel glucose-assisted ultrasonic cavitation exfoliation (GAUCE) method with glucose as an auxiliary solution to prepare BNNSs. Results show that the method has a high preparation yield of 55.58%, which is higher than the average preparation yield of 33.86%. The mechanism of preparing BNNSs by GAUCE was also investigated. The exfoliation of BNNSs was achieved using the energy of ultrasonic cavitation bubble collapse, which will break the interlayer forces in h-BN. The grafting of hydroxyl groups decomposed by glucose on the edge and surface of BNNSs during cavitation prevented the re-aggregation of the nanosheets, thereby increasing the exfoliation yield of BNNSs. In addition, the contact angle of BNNSs prepared by GAUCE was reduced, and the hydrophilicity was greatly improved.

CO2 Induces Symmetry Breaking in Layered Dipeptide Crystals

AbstractControl of symmetry breaking of materials provides large opportunities to regulate their properties and functions. Herein, we report breaking the symmetry of layered dipeptide crystals by utilizing CO2 to induce the adjacent monomolecular layers to stack from the opposite to the same direction. The role of CO2 is to cover the interlayer interaction sites and force the dipeptides to adsorb at asymmetric positions. Further, the dipeptide crystals exhibit far superior piezoelectricity after symmetry breaking and the piezoelectric voltage generated from the dipeptide‐based generators becomes more than 500 % higher than before. This work reveals a potential route to engineer structures and properties of layered materials and provides a deep insight into the control of non‐covalent interactions.

CO2 Induces Symmetry Breaking in Layered Dipeptide Crystals.

Control of symmetry breaking of materials provides large opportunities to regulate their properties and functions. Herein, we report breaking the symmetry of layered dipeptide crystals by utilizing CO2 to induce the adjacent monomolecular layers to stack from the opposite to the same direction. The role of CO2 is to cover the interlayer interaction sites and force the dipeptides to adsorb at asymmetric positions. Further, the dipeptide crystals exhibit far superior piezoelectricity after symmetry breaking and the piezoelectric voltage generated from the dipeptide-based generators becomes more than 500 % higher than before. This work reveals a potential route to engineer structures and properties of layered materials and provides a deep insight into the control of non-covalent interactions.

Stability assessment for hard anti-inclined bedded rock slopes using a limit equilibrium method

The failure mechanism of hard anti-inclined bedded rock slopes with the possibility of undergoing flexural toppling is very complex so that it is difficult to effectively perform their stability assessment. In this study, an attempt was made to accurately predict the stability factor and the failure surface of such slopes: establishing a new failure zone model and developing a limit equilibrium method based on this model. In this model, the failure zones of such a slope were divided strictly according to the failure mechanisms of the rock layers. In the presented method, the failure surface was considered to be a bilinear-type surface as observed in field investigations and laboratory tests, and the non-dimensional parameter indicating the position of application of the interlayer force was revised by deriving the distribution and the equivalent substitution of interlayer force. Then, a comparative study on Yangtai slope was performed to prove the presented method, and the effect of the non-dimensional parameter on the stability was also investigated. The results reveal that the presented method can accurately determine the failure surface and precisely evaluate the slope stability factor. In addition, the presented method has higher predictive accuracy compared with other analytical methods. With the decrease of the non-dimensional parameter, the stability of the slope is reinforced, but the larger landslide with more serious damage effect will occur if the slope undergoes the overall failure.

Ultrafast coherent interlayer phonon dynamics in atomically thin layers of MnBi2Te4

The atomically thin MnBi2Te4 crystal is a novel magnetic topological insulator, exhibiting exotic quantum physics. Here we report a systematic investigation of ultrafast carrier dynamics and coherent interlayer phonons in few-layer MnBi2Te4 as a function of layer number using time-resolved pump-probe reflectivity spectroscopy. Pronounced coherent phonon oscillations from the interlayer breathing mode are directly observed in the time domain. We find that the coherent oscillation frequency, the photocarrier and coherent phonon decay rates all depend sensitively on the sample thickness. The time-resolved measurements are complemented by ultralow-frequency Raman spectroscopy measurements, which both confirm the interlayer breathing mode and additionally enable observation of the interlayer shear mode. The layer dependence of these modes allows us to extract both the out-of-plane and in-plane interlayer force constants. Our studies not only reveal the interlayer van der Waals coupling strengths, but also shed light on the ultrafast optical properties of this novel two-dimensional material.

Recent Progress in Fabrication and Application of BN Nanostructures and BN-Based Nanohybrids.

Due to its unique physical, chemical, and mechanical properties, such as a low specific density, large specific surface area, excellent thermal stability, oxidation resistance, low friction, good dispersion stability, enhanced adsorbing capacity, large interlayer shear force, and wide bandgap, hexagonal boron nitride (h-BN) nanostructures are of great interest in many fields. These include, but are not limited to, (i) heterogeneous catalysts, (ii) promising nanocarriers for targeted drug delivery to tumor cells and nanoparticles containing therapeutic agents to fight bacterial and fungal infections, (iii) reinforcing phases in metal, ceramics, and polymer matrix composites, (iv) additives to liquid lubricants, (v) substrates for surface enhanced Raman spectroscopy, (vi) agents for boron neutron capture therapy, (vii) water purifiers, (viii) gas and biological sensors, and (ix) quantum dots, single photon emitters, and heterostructures for electronic, plasmonic, optical, optoelectronic, semiconductor, and magnetic devices. All of these areas are developing rapidly. Thus, the goal of this review is to analyze the critical mass of knowledge and the current state-of-the-art in the field of BN-based nanomaterial fabrication and application based on their amazing properties.

An Optimized KBe2BO3F2‐Like Structure: The Unity of Deep‐Ultraviolet Transparency, Nonlinear Optical Property, and Ferroelectricity

AbstractAt present, only KBe2BO3F2 (KBBF) can effectively realize a deep‐ultraviolet (DUV) coherent output, while the layered growth habit seriously hinders its application. Here, a new design strategy with an optimized KBBF‐like structure based on the quasi‐phase matching principle is proposed. Thus, the first KBBF‐like ferroelectric, Li2KRb(SO4)2, is designed. It exhibits a short DUV absorption cutoff edge of 163 nm, a large nonlinear optical (NLO) effect of 1.7 × KH2PO4 under a 1064 nm laser, and typical ferroelectricity. More importantly, it completely eliminates the layered structure by replacing K‐F bonds with strong ‐Li‐O‐S‐ ones as the interlayer force. Therefore, this work finds a new promising DUV NLO candidate, a unity of the DUV transparency, NLO response, and ferroelectricity, which is advantageous to explore new DUV NLO materials.

Shear Modes in a 2D Polar Metal.

Low-frequency shear and breathing modes are important Raman signatures of two-dimensional (2D) materials, providing information on the number of layers and insights into interlayer interactions. We elucidate the nature of low-frequency modes in a 2D polar metal-2D Ga covalently bonded to a SiC substrate, using a first-principles Green's function-based approach. The low-frequency Raman modes are dominated by surface resonance modes, consisting primarily of out-of-phase shear modes in Ga, coupled to SiC phonons. Breathing modes are strongly coupled to the substrate and do not give rise to peaks in the phonon spectra. The highest-frequency shear mode blue-shifts significantly with increasing thickness, reflecting both an increase in the number of Ga layers and an increase in the effective interlayer force constant. The surface resonance modes evolve into localized 2D Ga modes as the phonon momentum increases. The predicted low-frequency modes are consistent with Raman measurements on 2D polar Ga.

Molecular beam epitaxy of PtSe2 using a co-deposition approach

The structural properties of co-deposited ultrathin PtSe2 films grown at low temperatures by molecular beam epitaxy on c-plane Al2O3 are studied. By simultaneously supplying a Se flux from a Knudsen cell and Pt atoms from an electron-beam evaporator, crystalline (001)-oriented PtSe2 films were formed between 200 °C and 300 °C. The long separation between substrate and electron beam evaporator of about 60 cm ensured minimal thermal load. At optimum deposition temperatures, a ten times or even higher supply rate of Se compared to Pt ensured that the pronounced volatility of the Se was compensated and the PtSe2 phase was formed and stabilized at the growth front. Postgrowth anneals under a Se flux was found to dramatically improve the crystalline quality of the films. Even before the postgrowth anneal in Se, the crystallinity of PtSe2 films grown with the co-deposition method was superior to films realized by thermal assisted conversion. Postgrowth annealed films showed Raman modes with narrower peaks and more than twice the intensity. Transmission electron microscopy investigations revealed that the deposited material transitioned to a two-dimensional layered structure only after the postgrowth anneal. PtSe2 growth was found to start as single layer islands that preferentially nucleated at atomic steps of the substrate and progressed in a layer-by-layer like fashion. A close to ideal wetting behavior resulted in coalesced PtSe2 films after depositing about 1.5 PtSe2 layers. Detailed Raman investigation of the observed PtSe2 layer breathing modes of films grown under optimized co-deposition conditions revealed an interlayer coupling force constant of 5.0–5.6 × 1019 N m−3.

Centrifuge Model Test on Anti-dip Rock Slopes with Unequal Thicknesses Subjected to Flexural Toppling Failure

Flexural toppling failure of anti-dip rock slopes (ADRSs) with layers of equal thicknesses has been investigated by many researchers, however, in natural slopes, rock layers in ADRSs are often of unequal thicknesses. To analyze this condition, six slope models were manufactured with glass plates and tests were conducted in a geotechnical drum centrifuge. In these experiments, deformation and interlayer force characteristics were obtained with cameras and film pressure sensors. The centrifuge test results show that the failure process can be divided into three stages: failure of the slope toe, development of fractures and failure of the slope. Initial toe failure has little effect on the stability of the upper rock mass. Through centrifuge test results, it is determined that slopes with only thick layers therein can be used in the evaluation of the stability of slopes with layers of unequal thicknesses. This is because thicker strata have a greater bending stiffness and bending strength than thin strata, thus influencing slope deformation and stability.

Thermal conductivity and mechanical durability of graphene composite films containing polymer-filled connected multilayer graphene patterns

Due to its high thermal conductivity, graphene has received much attention as a thermal interface material (TIM) for dissipating heat, which would otherwise be accumulated in heat sources, to heatsinks. However, the weak interlayer force induces tearing of graphene under repeated deformation; hence, the application of graphene as a TIM in the manufacturing of flexible electronics has been limited. To overcome this hurdle, the graphene composite (GC) films, in which thermoplastic polymers were infiltrated into connected multilayer graphene (MLG) patterns, were fabricated in this study. While the connected MLG patterns attained high in-plane thermal conductivity (κx), the polymers prevented tearing of the graphene. To investigate the effect of the graphene content on the κx of the GC films, the area of MLG patterns was carefully adjusted by coating a graphene solution through metal masks with various opening sizes. The κx of the GC-4 film was calculated as 53 W/m·K, which was slightly changed after 10,000 folding test cycles with a 1.5-mm bending radius.

A study on shear transfer behavior between fractured layers of a concrete composite structure

Aiming at problems of interlayer continuous destruction and shear transfer weakening of concrete composite structures caused by fracturing of the former, three-dimensional measurements and digital reconstruction of a composite structure cracked interlayer interface appearance were carried out, through shear modeling tests and reverse engineering. Additionally, the shear transfer behavior and its origins were analyzed and explored quantitatively and discretely. The results showed that a digital reconstruction based on reverse engineering can precisely implement an analysis of the interlayer interface appearance and force features. Shear capacity between structural layers was a result of coordination and balance of bulge fracture and surface friction on the interface. The differences of shear behavior between different layers were mainly due to the shape and quantity of bulges on the interface. The interlayer shear transfer ability of a longitudinal and transverse (bidirectional) roughness design was better than that of a longitudinal or transverse (unidirectional) roughness design, but the latter was more stable. Mattock and Barton's interlayer shear transfer formula was partly modified since in theory the normal force vs. shear force correlation curve of shear behavior does not start from zero.

In-situ chemical formation of strong stability GO/rGO hybrid membranes for efficient treatment of organic pollutant

In this study, a strong stability graphene oxide (GO)/reduced graphene oxide (rGO) hybrid membranes were successfully prepared by in-situ chemical reduction method. As the reducing agent, ammonia can effectively promote the deoxygenation of GO surface and the restoration of π-conjugation to produce rGO sheets. Moreover, the weakening of hydration and the enhancement of π-π conjugation promoted the interlayer force of GO/rGO membranes; as a result, the hybrid membranes exhibited extraordinary stability in strong acid and alkali solutions. In organic dyes separation application, the hybrid membranes displayed excellent rejection performance, and the rejection rate up to 100%. In addition, they also showed higher water flux (23.72 L m-2h−1 bar−1) comparing to pristine GO membranes (14.35 L m-2h−1 bar−1) and rGO membrane (8.66 L m-2h−1 bar−1). This work will promote the application scope of graphene-based membranes and provide better performance in water treatment.

Design of 3D lightweight Ti3C2Tx MXene porous film with graded holes for efficient electromagnetic interference shielding performance

Ti3C2Tx MXene exhibits excellent prospects in the field of electromagnetic interference (EMI) shielding because of its high conductivity and special 2D structure. However, Ti3C2Tx MXene flakes tend to be stacked due to the interlayer force which makes it difficult to exert its advantages. In this work, 3D lightweight Ti3C2Tx MXene porous film with graded holes were designed and fabricated with a simple template method to overcome the agglomeration and enlighten the inherent advantages of Ti3C2Tx MXene flakes. First, oligo-layered Ti3C2Tx MXene with an average thickness of about 1.96 nm was delaminated, which was further used to construct Ti3C2Tx MXene@PMMA hybrids. Afterward, 3D lightweight Ti3C2Tx MXene porous films with graded holes were controllably fabricated by adjusting the diameter of the PMMA spheres, and in-situ pyrolysis of the template. The relationship between conductivity, structure design, and their EMI shielding properties in K-band (18–26.5 GHz) was investigated. The characterizations revealed that the P1:1Ti3C2Tx with graded holes exhibited high conductivity (297 S/cm) and lightweight (merely 0.1 g/cm3) properties, which displayed a superior EMI shielding effectiveness (SE). The average SET value of P1:1Ti3C2Tx was 60.3 dB with the peak value of 71.4 dB, which was 1.5 times higher than that of Ti3C2Tx flakes in the same condition. The EMI shielding mechanism can be understood from the synergy of polarization loss, conduction loss, impedance mismatch multiple reflection and scattering, and ohmic loss.

Study on the inter-laminar shear properties of carbon fiber reinforced epoxy composite materials with different interface structures

To explore relationship between the inter-laminar shear properties and the interfacial structure of carbon fiber (CF) reinforced epoxy resin (EP) composite materials (CFRPs), four CFRPs with different interfacial structures were prepared through introducing polar groups on the surface of the CF and then combining the modified CF with the EP, including CF/EP (where no polar group was introduced), CF-COOH-EP (where carboxyl group –COOH was chemically introduced), CF-COOH/MXene /EP (where –COOH was chemically introduced while MXene was physically introduced and the MXene was non-covalently bonded to CF and to EP), and CF-CONH-MXene-EP (where both of –COOH and MXene were chemically introduced and the MXene was covalently bonded to CF and to EP). Results of inter-laminar shear strength (ILSS) of the materials and morphology of transverse and longitudinal cross-sections of the damaged materials show that with enhancing the interfacial interaction between CF and EP, the ILSS increases and the interlayer shear failure gradually changes from a single interface debonding (for CF/EP) to a combination of interface debonding and matrix fragmentation. The interfacial structure of CF-COONH-MXene-EP with higher interfacial interaction could bear more interlayer shear forces, being beneficial for improving the service life of CFRPs.

The effect of the modification of graphene oxide with γ- aminopropyltriethoxysilane (KH550) on the properties and hydration of cement

In this study, the effect of the incorporation of GO and KH550 modified GO(MGO) on the microstructure, mechanical properties and hydration process of the cement-based composites has been discussed. It is observed that the MGO surface is grafted with the functional groups, which reduce the interlayer force, thus, the dispersibility in the cement paste is improved. In addition, compared with GO, the addition of MGO can further promote the hydration of cement, and optimize the content of the various hydration products owing to no impact on the types of the hydration products, thus, make the microstructure of the cement more uniform. At the same time, the incorporation of MGO can refine the CH crystal size, reduce the total porosity of the cement material, and promote the conversion of harmful pores to harmless pores, thus, making the cement-based materials more compact and effectively improving the mechanical properties.

Insight into the stacking and the species-ordering dependences of interlayer bonding in SiC/GeC polar heterostructures

Two-dimensional (2D) polar materials experience an in-plane charge transfer between different elements due to their electron negativities. When they form vertical heterostructures, the electrostatic force triggered by such charge transfer plays an important role in the interlayer bonding beyond van der Waals (vdW) interaction. Our comprehensive first principle study on the structural stability of the 2D SiC/GeC hybrid bilayer heterostructure has found that the electrostatic interlayer interaction can induce the π–π orbital hybridization between adjacent layers under different stacking and out-of-plane species ordering, with strong hybridization in the cases of Si–C and C–Ge species orderings but weak hybridization in the case of the C–C ordering. In particular, the attractive electrostatic interlayer interaction in the cases of Si–C and C–Ge species orderings mainly controls the equilibrium interlayer distance and the vdW interaction makes the system attain a lower binding energy. On the contrary, the vdW interaction mostly controls the equilibrium interlayer distance in the case of the C–C species ordering and the repulsive electrostatic interlayer force has less effect. Interesting finding is that the band structure of the SiC/GeC hybrid bilayer is sensitive to the layer-layer stacking and the out-of-plane species ordering. An indirect band gap of 2.76 eV (or 2.48 eV) was found under the AA stacking with Si–C ordering (or under the AB stacking with C–C ordering). While a direct band gap of 2.00–2.88 eV was found under other stacking and species orderings, demonstrating its band gap tunable feature. Furthermore, there is a charge redistribution in the interfacial region leading to a built-in electric field. Such field will separate the photo-generated charge carriers in different layers and is expected to reduce the probability of carrier recombination, and eventually give rise to the electron tunneling between layers.

Large change of interlayer vibrational coupling with stacking in Mo1−xWxTe2

Stacking variations in quasi-2D materials can have an important influence on material properties, such as changing the topology of the band structure. Unfortunately, the weakness of van der Waals interactions makes it difficult to compute the stacking dependence of properties, and even in a material as simple as graphite the stacking energetics remain unclear. Mo$_{1-x}$W$_{x}$Te$_{2}$ is a material in which three differently-stacked phases are conveniently accessible by temperature changes: $1T^{\prime}$, $T^*_d$, and the reported Weyl semimetal phase $T_d$. The transitions proceed via layer sliding, and the corresponding interlayer shear mode (ISM) is relevant not just for the stacking energetics, but for understanding the relationship between the Weyl physics and structural changes. However, the interlayer interactions of Mo$_{1-x}$W$_{x}$Te$_{2}$ are not well understood, with wide variation in computed properties. We report inelastic neutron scattering of the ISM in a Mo$_{0.91}$W$_{0.09}$Te$_{2}$ crystal. The ISM energies are generally consistent with the linear chain model (LCM), as expected given the weak interlayer interaction, though there are some discrepancies from predicted intensities. However, the interlayer force constants $K_x$ in the $T^*_d$ and $1T^{\prime}$ phases are substantially weaker than that of $T_d$, at 76(3)% and 83(3)%, respectively. Considering that the relative positioning of atoms in neighboring layers is approximately the same regardless of overall stacking, our results suggest that longer-range influences, such as stacking-induced band structure changes, may be responsible for the substantial change in the interlayer vibrational coupling. These findings should elucidate the stacking energetics of Mo$_{1-x}$W$_{x}$Te$_{2}$ and other van der Waals layered materials.

Modified montmorillonite synergizes with Co-MOF@PBO fabric to improve the wear resistance of PBO/phenolic resin composites

Co-MOF (cobased metal-organic frame) nanosheets were developed onto the surface of PBO (poly(p-phenylene benzobisoxazole)) fabric, and OMMT (modified montmorillonite) was incorporated into phenolic resin synergistically to improve the wear resistance of PBO/phenolic resin composites. Co-MOF nanosheets with a large specific surface area exhibited strong interlocking and excellent compatibility between the fabric and resin. In addition, OMMT possessed excellent affinity with phenolic resin and a larger lamellar space, and then polymer chains could be conveniently entangled into interlayers, which further confined the movement of molecular chains caused by friction heat. In addition, a weak interlayer force was conducive to facilitating the formation of a uniform and robust transfer-film on the counterpart. It was demonstrated that the Co-MOF@PBO/OMMT composites presented optimal tribological behavior due to the synergistic effect between interfacial modification and OMMT reinforcement.