Interlayer Transfer Research Articles

Highly Tunable Interlayer Coupling and Electronic Structures of Few-Layer Graphene with Pressure.

The interlayer electronic coupling is responsible for the electronic structure evolution from monolayer graphene to graphite and for the moiré potential in twisted bilayer graphene. Here we demonstrate that the interlayer transfer integral (hopping parameter) increases nearly 40% with a quite moderate pressure of ∼3.5 GPa, manifested by the resonance peak shift in the infrared spectra of all 2-10 L graphene. A simple model based on the Morse potential enabled us to establish the relationship between the transfer integral and pressure. Our work provides fundamental insights into the dependence of the electronic coupling on the interlayer distance.

Predicting the compressive strength of self-compacting concrete by developed African vulture optimization algorithm-Elman neural networks

The compressive strength of concrete depends on various factors. Since these parameters can be in a relatively wide range, it is difficult for predicting the behavior of concrete. Therefore, to solve this problem, an advanced modeling is needed. The aim of the literature is to achieve an ideal and flexible solution for predicting the behavior of concrete. Therefore, it is necessary to develop new approaches. Artificial Neural Networks (ANNs) have evolved from a theoretical method to a widely utilized technology by successful applications for a variety of issues. Actually, ANNs are a strong computing tool that provides the right solutions to problems that are difficult to use conventional methods. Inspired by the biological neural system, these networks are now widely used for solving a wide range of complicated problems in civil engineering. This study’’s target is evaluating the performance of developed African vulture optimization algorithm (DAVOA)-Elman neural networks (ENNs) by considering different input parameters in predicting the self-compacting concrete compressive strength. Hence, once 8 parameters and again to get as close as possible to the prediction conditions in the laboratory, 140 parameters entered to the improved version of Elman Neural Networks as input. According to the results, the element network has the lowest mean squares of the test error in predicting the compressive strength of 7 and 28 days in 100 repetitions. Further, in predicting both compressive strengths, the element grid with the Logsig-Purelin interlayer transfer function has the lowest test error, which determines the optimal transfer function. Moreover, the results showed that DAVOA as a reliable tool with time and cost savings have high power in predicting the desired characteristics. Also, in predicting both 7-day and 28-day compressive strength, networks built with 140 parameters have a 74.54 and 70.44% improvement in test error over 8-parameter networks, respectively, which directly affects this effect. Further parameters are considered as input to the network error rate in predicting the desired properties.

Photoluminescence properties of two-dimensional semiconductor heterointerfaces

Two-dimensional metal-sulfur compounds have attracted much attention due to their novel physical properties, such as layered structure, ultrathin physical dimensions, and continuously tunable bandgap. The vertical stacking of different 2D semiconductors enables the heterojunction to retain the excellent properties of its constituent materials and has physical properties such as interlayer energy transfer and interlayer carrier transfer. In this paper, we utilize the carrier interlayer transfer properties of p-n heterojunctions and form heterojunctions using p-type Te and PdSe2 prepared with n-type monolayer WS2 using the microzone transfer technique. We found that the PL spectrum of monolayer WS2 is purer after heterojunction formation. The photoluminescence peaks representing exciton recombination are sharper, while the peaks represented by trions almost disappear. These phenomena indicate that we can utilize p-n junctions to capture the PL spectra of excitons in WS2, which is important for the further study of the optical properties of 2D metal-sulfur compounds.

Interlayer interactions and electron transfer effects on sodium adsorption on 2D heterostructures surfaces

Interlayer interactions and electron transfer effects on sodium adsorption on 2D heterostructures surfaces

Achieving a solar-to-chemical efficiency of 3.6% in ambient conditions by inhibiting interlayer charges transport

Efficiently converting solar energy into chemical energy remains a formidable challenge in artificial photosynthetic systems. To date, rarely has an artificial photosynthetic system operating in the open air surpassed the highest solar-to-biomass conversion efficiency (1%) observed in plants. In this study, we present a three-dimension polymeric photocatalyst achieving a solar-to-H2O2 conversion efficiency of 3.6% under ambient conditions, including real water, open air, and room temperature. The impressive performance is attributed to the efficient storage of electrons inside materials via expeditious intramolecular charge transfer, and the fast extraction of the stored electrons by O2 that can diffuse into the internal pores of the self-supporting three-dimensional material. This construction strategy suppresses the interlayer transfer of excitons, polarizers and carriers, effectively increases the utilization of internal excitons to 82%. This breakthrough provides a perspective to substantially enhance photocatalytic performance and bear substantial implications for sustainable energy generation and environmental remediation.

Enhanced interlayer electron transfer by surface treatments in mixed-dimensional van der Waals semiconductor heterostructures

We investigate the excitonic species in WS2 monolayers transferred onto III–V semiconductor substrates with different surface treatments. When the III–V substrates were covered with amorphous native oxides, negatively charged excitons dominated the spectral weight in low-temperature near-resonance photoluminescence (PL) measurements. However, when the native oxides of the III–V substrates were reduced, neutral excitons began to dominate the spectral weight, indicating a reduction in the electron density in the WS2 monolayers. The removal of the native oxides enhanced the electron transfer from the WS2 monolayer to the III–V substrate. In addition, an additional shoulder-like PL feature appeared ∼50 meV below the emission of neutral excitons, which can be attributed to the emission of localized excitons. When the III–V substrate surface was passivated by sulfur after the reduction of the native oxides, neutral excitons still dominated the spectral weight. However, the low-energy PL shoulder disappeared again, suggesting the effective delocalization of excitons through substrate surface passivation. Surface engineering of the semiconductor substrates for two-dimensional (2D) materials can provide a novel approach to control the carrier density of the 2D materials, implement deterministic carrier localization or delocalization for the 2D materials, and facilitate the interlayer transfer of charge, spin, and valley currents. These findings open the avenue for novel device concepts and phenomena in mixed-dimensional semiconductor heterostructures.

Ferroelectricity in two-dimensional bilayers and multilayers of MgAl2S4

The promising applications of two-dimensional (2D) ferroelectrics have intrigued lots of studies. The van der Waals stacking as a successful method has been applied to obtain the 2D ferroelectrics from non-ferroelectric parent compounds. Applying this strategy to the new 2D MA2Z4 materials, We predict the sliding ferroelectricity in bilayer and multilayer MgAl2S4 structures by the first-principles calculations. The stacking order shows a significant impact on the physical properties, where staggered stacking in the bilayer or multilayer MgAl2S4 can induce ferroelectricity but direct stacking can not. The polarization of the bilayer or multilayer MgAl2S4 arises from the interlayer transfer of uncompensated charge between the faces and the outer layers, which can be switched by interlayer interface sliding. To manipulate the polarization strength, the external strain was applied and the material properties can be modulated. In addition, similar to the monolayer behavior in bulk ReS2, the electronic structures show little difference from monolayer to six-layer MgAl2S4. However, the polarization in MgAl2S4 is more than fifteen times as strong as that of the ReS2. Our work paves the way for further exploration into van der Waals semiconductor ferroelectric systems and the development of novel high-efficiency nonvolatile memory devices.

Interlayer stress transfer improvement in bi-layer graphene oxide /calcium silicate hydrate via CNTs

Previous studies have shown that the experimental values of mechanical properties of graphene oxide/calcium silicate hydrate (GO/CSH) are lower than theoretical values. The strengthening degree of graphene oxide (GO) on mechanical properties of calcium silicate hydrate (CSH) is quite different. This is mainly due to the weak interface between GO layer and CSH matrix. In order to further enhance the interlayer stress transfer and interfacial interaction between GO and CSH nanostructures, a series of CNTs are created in double GO layers to form a new double GO/CSH with different number of CNTs configuration (D-GO-CNTs/CSH nanostructures). The results show that the normalized pull-out energies and normalized shear stress values of double GO/CSH with hydroxyl groups (D-GO-OH/CSH) composites are enhanced by 26.58% and 33.33%, while the normalized pull-out energies and normalized shear stress values of double GO/CSH with epoxy groups (D-GO-Oo/CSH) are only enhanced by 14.29% and 15.38%. In addition, the CNTs leads to a greatly enhancement (about 2–3 times) in normalized interlayer stress transfer of D-GO/CSH nanostructures, while exerts a little adverse effect (about 10%) on the failure strain of D-GO/CSH nanostructures. According strain energy ratio and molecular dynamics analysis, the enhancement effects of interlayer stress transfer in D-GO/CSH model are the best when the number of CNTs is 3. It provides a new approach for actively enhancing the interlayer stress transfer and interfacial interaction of D-GO/CSH nanostructures.

Interlayer sp3 Bonds and Chirality at Bilayer Graphene Oxide/Calcium Silicate Hydrate Abnormally Enhance Its Interlayer Stress Transfer.

Graphene oxide (GO) is an ideal reinforcing material with super design capability, which can achieve the combination of strength and toughness. However, the actual effect of GO is far below the theoretical prediction. This is mainly due to the weak interface between the nanofiller and the matrix. In this paper, a controllable method for improving interlayer stress transfer of double-layer graphene oxide/C-S-H (D-GO-CSH)-layered nanostructures is proposed by using interlayer sp3 bond and chirality. The results show that, compared with the control group, the normalized shear stress and normalized pull-out energy of the OH-sp3 model are increased by 44.93 and 49.25%, respectively, while those of the OO-sp3 model are increased by 32.26 and 31.03%, respectively. The interlayer sp3 bonds lead to a great enhancement (more than 3 times) in normalized interlayer stress transfer of D-GO-CSH-layered nanostructures while exerting a little opposite effect (about 5%). The improvement effects induced by the interlayer sp3 bonds are also strongly dependent on their distributions and the chirality of GO. According to the fracture mechanic theory and molecular dynamics results, the strain energy percentage difference (bond length and bond angle) of the zigzag-cen model is 34.8% lower than that of the control group model, which proves that the interlayer sp3 bonds have a remarkably positive effect on the interlayer stress transfer of D-GO-CSH-layered nanostructures. This provides a new way to further improve the interlayer stress transfer, pull-out energy, and interlayer shear stress of D-GO-CSH-layered nanostructures.

Hidden phonon highways promote photoinduced interlayer energy transfer in twisted transition metal dichalcogenide heterostructures.

Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS2/WS2 at 4° twist angle and WSe2/MoSe2 heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations.

Carbon pre-protected iron strategy to construct Fe, C-codoped g-C3N4 for effective photodegradation of organic pollutants via hole oxidation mechanism

Improving photocatalytic activity in graphitic carbon nitride (g-C3N4) by enhancing charge transfer and light absorption remains a challenge. Herein, iron-loaded carbon (FeC) was first prepared at low temperature, and then the Fe, C-codoped g-C3N4 (FeC–CN) was synthesized by high temperature thermal copolymerization of FeC with melamine. The carbon shell of FeC prevented the formation of iron oxide during high temperature thermal copolymerization, and Fe as electron channels existed in the interlayer, meanwhile, C atoms of FeC replaced part of the N atoms in the triazine ring. This designed structure facilitates in-plane and interlayer transfer of photogenerated carriers, as well as the separation of photogenerated carriers. Additionally, the UV–visible absorption spectra have demonstrated that the substitution of C atoms can greatly enhance visible light absorption. The catalyst exhibits significantly increased photocatalytic activity and excellent stability compared to g-C3N4. Interestingly, different from the reactive oxygen species-based g-C3N4 catalysis reaction, the degradation of organic pollutants was found to initiate from a hole-involved non-radical pathway. The increased electron transfer facilitates the reduction of O2 via a one-step two-electron pathway to generate hydrogen peroxide.

Enhanced electron–phonon coupling by delocalizing phonon states for desirable interlayer transfer of excited charges in MoSSe/WS2 heterobilayer

Regulating the interlayer transfer of excited charges is challenging but crucial for high-efficiency photoelectric conversion devices based on semiconductor heterojunction. In this work, the interlayer transfer and recombination of excited charges are investigated for the heterobilayers based on monolayer MoSSe and WS2 by combining density functional theory calculation with nonadiabatic molecular dynamics simulation. Our results reveal that the heterobilayers possess type-II band alignments and the interface engineering from S–Se to S–S stacking configuration reverses the spatial distribution of valence and conduction bands from MoSSe and WS2 to WS2 and MoSSe layers, respectively, which produces interlayer transfer of excited charges in opposite direction. The interface engineering also causes the delocalization of out-of-plane phonon states from the WS2 layer to both WS2 and MoSSe layers. This delocalized character in S–S configuration facilitates the simultaneous coupling of out-of-plane phonon states with the localized donor and acceptor electronic states, accelerates the motion of interface atoms, and reduces the band energy differences, which synergistically promote interlayer transfer of excited charges. As a result, the interlayer transfer of excited charges in S–S configuration is faster than that in S–Se configuration. Our investigation demonstrates that delocalizing phonon states through interface engineering can regulate electron–phonon coupling and interlayer transfer of excited charges.

Carbon nanotubes at the Graphene/hBN interface abnormally enhance its fracture toughness

Different from Van der Waals heterojunction, defects often appear at the interface of the planar heterostructures due to the low-dimensional structure characteristics. These defects lead to complex mechanical problems, and seriously affect application of planar heterostructures in flexible electronics devices. The most commonly used method to improve the toughness is to reduce/eliminate the interfacial structural defects. Using a defective Graphene/hBN interface, we show surprisingly that Carbon Nanotubes (CNTs) are able to enhance the toughness at the interface. It is found that interlayer stress transfer, bonding strain energies and energy release rate of the defective Graphene/hBN interface with CNTs are higher than those of free-defect Graphene/hBN interface, which is in strong contrast to the general view that interfacial defects reduce toughness. By combining the analysis of atomic mechanics and fracture mechanics theory, we find that the abnormal enhancement in interfacial toughness comes from the stress field localization arising from sp2 + sp3 hybrid state and out-of-plane deformation at the interface. In addition, the stress field at the distal end of the simulated models gradually decays with the CNTs height, resulting in the stress fields to be controlled at the interface. This abnormal mechanism provides a new dimension for enhancing toughness in two-dimensional heterostructures.

Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe2/MoSe2 heterobilayers

Transition metal dichalcogenides (TMDs) have attracted much attention in the fields of valley- and spintronics due to their property of forming valley-polarized excitons when illuminated by circularly polarized light. In TMD-heterostructures it was shown that these electron-hole pairs can scatter into valley-polarized interlayer exciton states, which exhibit long lifetimes and a twist-angle dependence. However, the question how to create a valley polarization of free charge carriers in these heterostructures after a valley selective optical excitation is unexplored, despite its relevance for opto-electronic devices. Here, we identify an interlayer transfer mechanism in twisted WSe2/MoSe2 heterobilayers that transfers the valley polarization from excitons in WSe2 to free charge carriers in MoSe2 with valley lifetimes of up to 12 ns. This mechanism is most efficient at large twist angles, whereas the valley lifetimes of free charge carriers are surprisingly short for small twist angles, despite the occurrence of interlayer excitons.

PMP: A partition-match parallel mechanism for DNN inference acceleration in cloud–edge collaborative environments

To address the challenges of delay-sensitive deep learning tasks, Deep Neural Network (DNN) models are often partitioned and deployed to the cloud–edge environment for parallel and collaborative inference. However, existing parallel coordination mechanisms are not suitable for the cloud–edge environment, as the high inter-layer dependence of DNNs can increase transmission latency and wait times for inference, which contradicts the advantage of low latency in edge computing. To resolve this contradiction, the PMP mechanism takes into account the inter-layer transfer dependence of partitioning solutions and employs a multi-objective equalization algorithm to derive DNN model partitioning strategies suitable for multi-way parallel computing. Moreover, the mechanism establishes a DNN inference time prediction model based on these partitions and utilizes an iterative matching algorithm to approximate an optimal DNN inference workflow. Extensive evaluations of the proposed mechanism are conducted using various DNN models, and the results demonstrate its superiority over existing schemes, including local, CoEdge, and EdgeFlow. Notably, PMP achieves significant reductions in total inference latency compared to these schemes, with reductions of 80.9%, 37.9%, and 9.1%, respectively.

Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly challenging task. Using monolayer MoS2 sandwiched between top and bottom graphene, here we demonstrate fully valley-coherent excitons by observing ~100% degree of linear polarization in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and Maialle-Silva-Sham equations) and a careful choice of design of experiments using four different stacks with systematic variation of screening and exciton lifetime. To the best of our knowledge, this is the first report in which the excitons are found to be valley coherent in the entire lifetime in monolayer semiconductors, allowing optical readout of valley coherence possible.

Thickness-Dependent nonlinear optical absorption of InSe/Graphene van der Waals heterostructures

InSe/Graphene van der Waals heterostructures emerge as promising candidates for optoelectronic applications. In this work, we systematically examine the saturable absorption properties of InSe/Graphene heterostructures with varying thickness of InSe. We establish a positive correlation between the third-order nonlinear absorption coefficient and heterostructure thickness. Furthermore, we uncover the interplay of carrier dynamics within the heterostructures, revealing prolonged interlayer transfer times and slowed carrier relaxation as InSe thickness increases. These insights shed light on the potential of InSe/Graphene heterostructures for advanced optoelectronic devices, fostering further advancements in this field.

Strong phonon mode induced by carbon vacancy accelerating hole transfer in SiC/MoS2 heterostructure

The van der Waals SiC/MoS2 heterostructure with a type-II band alignment is promising for potential applications in photocatalytic and optoelectronic devices. By performing time-dependent nonadiabatic molecular dynamics simulations, we study the dynamics of single sulfur and carbon vacancy-mediated of photoexcited hole transfer at the interface of SiC/MoS2. The results indicate that single C-vacancy in SiC atomic layer promotes interlayer hole transfer more sufficiently in a shorter timescale of 68 fs than those of S-vacancy and pristine heterostructures. This promotion is attributed to the decreased energy gap related to intralayer hole transfer in SiC layer, resulting from the split of acceptor state, the excitation of a stronger in-plane carbon atom vibration mode at 600 cm−1, and the enhancement of electron–phonon coupling. Meanwhile, electron-hole recombination is three orders of magnitude slower than hole transfer, ensuring an effective charge separation in SiC/MoS2 with C-vacancy. This work provides theoretical insights into the dynamics of photoexcitation hole transfer in MoS2/SiC heterostructure.

Interfacial stress transfer in monolayer and few-layer MoS2 nanosheets in model nanocomposites

Understanding the stress transfer mechanisms from a polymer matrix to two-dimensional (2D) reinforcements is essential for the preparation of high performance nanocomposites. In this study, the interfacial stress transfer from a flexible polymer substrate to monolayer and few-layer molybdenum disulfide (MoS2) under tension has been investigated. Layer-dependent and strain-dependent photoluminescence (PL) spectroscopy were used to examine the stress transfer efficiency. The interlayer stress transfer efficiency of MoS2 was determined to be in the range of 0.76–0.86, higher than that of graphene. The transfer of strain from the polymer substrate to the flakes was derived through strain-dependent band shifts. With progressive loading, the strain distribution in monolayer MoS2 can be described by the shear-lag, partial-debonding and total-debonding models. The interfacial shear and frictional stresses were calculated to quantify the strength of the MoS2/polymer interface. It was found that the strength of the interface is similar to the strength of a graphene-polymer interface. Strain mapping was performed at different strain levels and it was found that the strain distribution in bilayer MoS2 is similar to the case of a monolayer sample. The interfacial shear strength remains almost unaffected, while the stress transfer length increases with increasing layer number.

Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer.

Photoinduced charge transfer in van der Waals heterostructures occurs on the 100 fs timescale despite weak interlayer coupling and momentum mismatch. However, little is understood about the microscopic mechanism behind this ultrafast process and the role of the lattice in mediating it. Here, using femtosecond electron diffraction, we directly visualize lattice dynamics in photoexcited heterostructures of WSe2/WS2 monolayers. Following the selective excitation of WSe2, we measure the concurrent heating of both WSe2 and WS2 on a picosecond timescale-an observation that is not explained by phonon transport across the interface. Using first-principles calculations, we identify a fast channel involving an electronic state hybridized across the heterostructure, enabling phonon-assisted interlayer transfer of photoexcited electrons. Phonons are emitted in both layers on the femtosecond timescale via this channel, consistent with the simultaneous lattice heating observed experimentally. Taken together, our work indicates strong electron-phonon coupling via layer-hybridized electronic states-a novel route to control energy transport across atomic junctions.