Interlayer Charge Transfer Research Articles

Building Asymmetric Zn−N3 Bridge between 2D Photocatalyst and Co‐catalyst for Directed Charge Transfer toward Efficient H2O2 Synthesis

AbstractTwo‐dimensional (2D) polymeric semiconductors are a class of promising photocatalysts; however, it remains challenging to facilitate their interlayer charge transfer for suppressed in‐plane charge recombination and thus improved quantum efficiency. Although some strategies, such as π–π stacking and van der Waals interaction, have been developed so far, directed interlayer charge transfer still cannot be achieved. Herein, we report a strategy of forming asymmetric Zn−N3 units that can bridge nitrogen (N)‐doped carbon layers with polymeric carbon nitride nanosheets (C3N4−Zn−N(C)) to address this challenge. The symmetry‐breaking Zn−N3 moiety, which has an asymmetric local charge distribution, enables directed interfacial charge transfer between the C3N4 photocatalyst and the N‐doped carbon co‐catalyst. As evidenced by femtosecond transient absorption spectroscopy, charge separation can be significantly enhanced by the interfacial asymmetric Zn−N3 bonding bridges. As a result, the designed C3N4−Zn−N(C) catalyst exhibits dramatically enhanced H2O2 photosynthesis activity, outperforming most of the reported C3N4‐based catalysts. This work highlights the importance of tailoring interfacial chemical bonding channels in polymeric photocatalysts at the molecular level to achieve effective spatial charge separation.

Al-N3 Bridge Site Enabling Interlayer Charge Transfer Boosts the Direct Photosynthesis of Hydrogen Peroxide from Water and Air.

Manipulating the electronic environment of the reactive center to lower the energy barrier of the rate-determining water oxidation step for boosting the direct generation of H2O2 from water, air, and sunlight is fascinating yet remains a grand challenge. Driven by a first-principles screening across a series of metal single atoms in carbon nitride, we report a class of an Al-N3 bridge site enabling interlayer charge transfer in carbon nitride nanotubes (CNNT-Al) for the highly efficient photosynthesis of H2O2 directly from water, oxygen, and sunlight. We demonstrate that the interlayered Al-N3 bridge site in CNNT-Al is able to activate the neighboring surface N atom for promoting the rate-determining step of the two-electron water oxidation to H2O2. It is also able to act as a bridge for enhancing the vertical interlaminar charge transfer due to the hybridization between the 3s and 3p states of the interstitial Al atom and the conduction band of two adjacent carbon nitride layers. Collectively, these factors lead to a highest photocatalytic mass activity of 1410.2 μmol g-1 h-1 (with a photocatalyst concentration of 1 g L-1) for direct photosynthesis of H2O2 out of all CN-based photocatalysts and a 7-fold higher solar-to-chemical conversion efficiency (0.73%) compared to that of the natural photosynthesis of typical plants (∼0.1%). Most importantly, the CNNT-Al-based flow reactor can steadily produce H2O2 for 200 h and be directly used for the on-site degradation of organic dye in water. The CNNT-Al-based flow reactor can also kill a 10 times higher concentration of bacteria in deionized water than that in natural water with 100% efficiency, which makes our design economically appealing for practical water treatment.

Stacking-order independent inter-layer charge transfer in MBE-grown MoSe2 and WSe2 heterostructures

Stacking-order independent inter-layer charge transfer in MBE-grown MoSe2 and WSe2 heterostructures

Enhanced spintronic and electronic properties in MTe2-GdCl2 (M=Mo, W) heterojunctions

Two-dimensional ferromagnetic heterojunctions with direct bandgaps and high Curie temperatures (TC) are promising candidates for applications in nanoelectronics and spintronics. Here, we design a Mo(W)Te₂-GdCl₂ heterojunction by integrating experimentally synthesized monolayer Mo(W)Te₂ with recently predicted ferromagnetic monolayer GdCl2. Our first-principles calculations reveal that these heterojunctions display a type-II (staggered) band alignment with a narrow direct bandgap ranging from 1.10 to 1.23 eV, using the HSE06 method. We have incorporated spin-orbit coupling and conducted Monte Carlo simulations to precisely estimate the magnetic ground states of these systems. We find that spin-orbit coupling plays a crucial role in the valence band splitting at the K-point in both heterojunctions. The magnetic anisotropy, TC, and optical performance of individual monolayers are enhanced within these heterojunctions. Under compressive strain, the TC of the heterojunctions increases significantly, reaching about 278 K at -7 % strain, due to strengthened super-exchange interactions between Gd-d and Cl-p orbitals and enhanced interlayer charge transfer. In addition, the band alignment shifts from type-II to type-I under -3 % compressive strain, while tensile strain maintains the type-II band alignment. Our findings offer a viable pathway for developing ferromagnetic semiconducting heterojunctions suitable for nanoelectronic and spintronic applications.

The coupling effect of polarization and lattice strain on charge transfer between quintuple-layer Al2X3/Al2Y3 (X, Y = O, S, Se, Te; X ≠ Y) interfaces

The electronic properties of two-dimensional (2D) polar semiconductor heterostructures are closely related to the degree of interlayer charge transfer. Taking 2D quintuple-layer (QL)-Al2S3 and QL-Al2O3 semiconductor as examples, we study the electronic properties and interlayer charge transfer of QL-Al2S3/Al2O3 interfaces by first-principles calculations. The driving force of the directional interlayer charge transfer is the polarization electric field. The kinetic process of interface charge transfer is related to the charge (strain) distribution of QL-Al2S3 and QL-Al2O3. By changing the strain distribution of QL-Al2S3/Al2O3 interfaces with same polarization arrangement, the electronic properties and interfacial charge transfer of heterojunctions can be adjusted. Our work is an important indicator for understanding the dynamic process of interlayer charge transfer between 2D polar semiconductors. In addition, we propose one method for regulating the electronic properties of heterojunctions by coupling polarization and lattice strain.

Building Asymmetric Zn-N3 Bridge between 2D Photocatalyst and Co-catalyst for Directed Charge Transfer toward Efficient H2O2 Synthesis.

Two-dimensional (2D) polymeric semiconductors are a class of promising photocatalysts; however, it remains challenging to facilitate their interlayer charge transfer for suppressed in-plane charge recombination and thus improved quantum efficiency. Although some strategies, such as π-π stacking and van der Waals interaction, have been developed so far, directed interlayer charge transfer still cannot be achieved. Herein, we report a strategy of forming asymmetric Zn-N3 units that can bridge nitrogen (N)-doped carbon layers with polymeric carbon nitride nanosheets (C3N4-Zn-N(C)) to address this challenge. The symmetry-breaking Zn-N3 moiety, which has an asymmetric local charge distribution, enables directed interfacial charge transfer between the C3N4 photocatalyst and the N-doped carbon co-catalyst. As evidenced by femtosecond transient absorption spectroscopy, charge separation can be significantly enhanced by the interfacial asymmetric Zn-N3 bonding bridges. As a result, the designed C3N4-Zn-N(C) catalyst exhibits dramatically enhanced H2O2 photosynthesis activity, outperforming most of the reported C3N4-based catalysts. This work highlights the importance of tailoring interfacial chemical bonding channels in polymeric photocatalysts at the molecular level to achieve effective spatial charge separation.

Tunable Interlayer Interactions in Exfoliated 2D van der Waals Framework Fe(SCN)2(Pyrazine)2.

2D materials can be isolated as monolayer sheets when interlayer interactions involve weak van der Waals forces. These atomically thin structures enable novel topological physics and open chemical questions of how to tune the structure and properties of the sheets while maintaining them as isolated monolayers. Here, this work investigates 2D electroactive sheets that exfoliate in solution into colloidal nanosheets, but aggregate upon oxidation, giving rise to tunable interlayer charge transfer absorption and photoluminescence. This optical behavior resembles interlayer excitons, now intensely studied due to their long-lived emission, but which remain difficult to tune through synthetic chemistry. Instead, the interlayer excitons of these framework sheets can be modulated through control of solvent, electrolyte, oxidation state, and the composition of the framework building blocks. Compared to other 2D materials, these framework sheets display the largest known interlayer binding strengths, attributable to specific orbital interactions between the sheets, and among the longest interlayer exciton lifetimes. Taken together, this study provides a microscopic basis for manipulating long-range opto-electronic behavior in van der Waals materials through molecular synthetic chemistry.

Piezo-photocatalysis synergy in γ-GeSe for highly efficient oxygen evolution reaction

Solar-driven semiconductor photocatalysts are highly appealing in applications of environmental remediation and energy conversion. However, photocatalytic reactions, particularly oxygen evolution reaction (OER), are often constrained by the swift recombination of electron–hole pairs, thereby resulting in low reaction efficiency. Although it is effective to separate charge carriers by constructing heterojunctions to form built-in electric field, the lattice mismatch and inefficient interlayer charge transfer of heterojunctions in the photocatalysts limit their further development. Here, we propose a new strategy by constructing an internal electric field for OER through an individual piezoelectric two-dimensional material. The results indicate that the piezoelectric effect regulates the electronic structure, reduces bandgap, improves light absorption efficiency, and that the displacement of positive and negative charge centers is the key factor in the enhanced OER. This research indicates the feasibility of combining piezoelectric properties of two-dimensional materials with OER (1.19 eV), providing new insights and guidance for applying the piezoelectric effect in the OER and opening up a way to promote efficient separation of charge carriers.

Interlayer Friction and Adhesion Effects in Penta-PdSe2-Based van der Waals Heterostructures.

Due to their inherent lattice mismatch characteristics, 2D heterostructure interfaces are considered ideal for achieving stable and sustained ultralow friction (superlubricity). Despite extensive research, the current understanding of how interface adhesion affects interlayer friction remains limited. This study focused on graphene/MoS2 and graphene/PdSe2 heterostructure interfaces, where extremely low friction coefficients of ≈10-3 are observed. In contrast, the MoS2/PdSe2 heterostructure interfaces exhibit higher friction coefficients, ≈0.02, primarily due to significant interfacial interactions driven by interlayer charge transfer, which is closely related to the ionic nature of 2D material crystals. These findings indicate that the greater the difference in ionicity between the two 2D materials comprising the sliding interfaces is, the lower the interlayer friction, providing key criteria for designing ultralow friction pairs. Moreover, the experimental results demonstrate that interlayer friction in heterostructure systems is closely associated with the material thickness and interface adhesion strength. These experimental findings are supported by molecular dynamics simulations, further validating the observed friction behavior. By integrating experimental observations with simulation analyses, this study reveals the pivotal role of interface adhesion in regulating interlayer friction and offers new insights into understanding and optimizing the frictional performance of layered solid lubricants.

Magnetic ferroelectric metal in bilayer Fe3GeTe2 under interlayer sliding

The inherent interlayer freedom in van der Waals stacked materials provides an excellent opportunity to investigate ferroelectric-like behavior through interlayer translation. Based on first-principles calculations, we find that the interlayer sliding in Fe3GeTe2 (FGT) bilayer enables the coexistence of polarization, metallicity, and ferromagnetism. We find that the polarization is induced by the uncompensated vertical interlayer charge transfer, and can be switched by an in-plane interlayer sliding. A moderate biaxial strain can reverse the polarization direction of the sliding FGT bilayer. The vertical polarization disentangles with the in-plane conductivity as was previously seen in the sliding ferroelectric WTe2 bilayer. Our work proposes an extremely rare magnetic ferroelectric metal phase that is useful for magnetoelectric and spintronic applications.

Bi-MoSe2 Contacts in the Ultraclean Limit: Closing the Theory-Experiment Loop.

Achieving robust electrical contacts is crucial for realizing the promise of monolayer 2D semiconductors such as semiconducting transition metal dichalcogenides (s-TMDs) in electronics. Despite recent breakthroughs, a gap remains between the experimental and theoretical understanding of metal-s-TMDs contacts. This study explores bismuth semimetal contacts to monolayer MoSe2, using a platform that minimizes experimental sources of uncertainty; we combine contact-front and contact-end measurements to measure key parameters like specific resistivity (ρc) and transfer length (Lt). We find that the resistivity of MoSe2 under the contacts is enhanced due to charge transfer that can be modeled using a self-consistent approach. In contrast, ab initio calculations of the interlayer charge transfer rate are inconsistent with the measured value of ρc, highlighting the need for new theoretical approaches.

Enhancing Thermoelectric and Cooling Performance of Bi0.5Sb1.5Te3 through Ferroelectric Polarization in Flexible Ag/PZT/PVDF/Bi0.5Sb1.5Te3 Film.

Bi2Te3-based thin films are gaining recognition for their remarkable room temperature thermoelectric performance. Beyond the conventional "process-composition-performance" paradigm, it is highly desirable to explore new methods to enhance their performance further. Here, we designed a sandwich-structured Ag/PZT/PVDF/Bi0.5Sb1.5Te3(BST) thin film device and effectively regulated the performance of the BST film by controlling the polarization state of the PZT/PVDF layers. Results indicate that polarization induces interlayer charge redistribution and charge transfer between PZT/PVDF and BST, thereby achieving the continuous modulation of the electrical transport characteristics of BST films. Finally, following polarization at a saturation voltage of 3 kV, the power factor of the BST film increased by 13% compared to the unpolarized condition, reaching 20.8 μW cm-1 K-2. Furthermore, a device with 7 pairs of P-N legs was fabricated, achieving a cooling temperature difference of 11.0 K and a net cooling temperature difference of 2.4 K at a current of 10 mA after the saturation polarization of the PZT/PVDF layer. This work reveals the critical effect of introducing ferroelectric layer polarization to achieve excellent thermoelectric performance of the BST film.

Ti3C2Tx MXene-mediating near- and long-range electronic effect on atomically dispersed Co for efficient lithium-sulfur batteries

Sluggish sulfur conversion kinetics pose an ongoing challenge in lithium-sulfur batteries (LSBs). Here, we present a solution through far-reaching long-range electronic regulation (LRER) on single-atom active sites. N-doped carbons (Co-NC) are implanted with densely-distributed Co single atoms, and supported on Ti3C2Tx MXene substrates to assemble 3D Co-NC/MXene catalyst. MXene effectively mediates interlayer charge transfer (∼0.70 |e|) contrasted with popular carbon materials (∼0.06 |e|) to produce LRER through surrounding carbon atoms. The synergy of LRER with near-range electronic regulation (NRER) tunes electronic structures, and enhances heterostructural stability, thus provoking desirous catalytic kinetics of Co single atoms in sulfur reduction. Thereby, the Co-NC/MXene/S cathodes exhibit impressive rate performance and excellent cycling stability (only 0.015% capacity decay per cycle over 600 cycles at 4 C) in LSBs, surpassing state-of-the-art sulfur cathodes. This work reveals the importance of LRER for improved catalysis, and provides new guidance to tailor heterostructures to achieve high-efficient catalysts in various process.

Twisted MoSe2 Homobilayer Behaving as a Heterobilayer.

Heterostructures (HSs) formed by the transition-metal dichalcogenide materials have shown great promise in next-generation (opto)electronic applications. An artificially twisted HS allows us to manipulate the optical and electronic properties. In this work, we introduce the understanding of the energy transfer (ET) process governed by the dipolar interaction in a twisted molybdenum diselenide (MoSe2) homobilayer without any charge-blocking interlayer. We fabricated an unconventional homobilayer (i.e., HS) with a large twist angle (∼57°) by combining the chemical vapor deposition (CVD) and mechanical exfoliation (Exf.) techniques to fully exploit the lattice parameter mismatch and indirect/direct (CVD/Exf.) bandgap nature. These effectively weaken the interlayer charge transfer and allow the ET to control the carrier recombination channels. Our experimental and theoretical results explain a massive HS photoluminescence enhancement due to an efficient ET process. This work shows that the electronically decoupled MoSe2 homobilayer is coupled by the ET process, mimicking a "true" heterobilayer nature.

Interplanar spacing and nanosheet thickness regulation of carbon nitride and its “cold” catalytic hydrogen production performance under 10 W LED irradiation

Investigation of hydrogen production catalysis under low-temperature and low-power light irradiation, such as LEDs, is important for energy conservation in cold natural conditions. Interlayer charge transfer in g-C3N4 nanosheets is influenced by the crystal face spacing and nanosheet thickness, due to rapid photogenerated carrier recombination and slow charge transfer, thus enhancing hydrogen production capabilities. Few studies have controlled both crystal face spacing and nanosheet thickness simultaneously. In this research, copper-phosphorus co-doped g-C3N4 was synthesized through one-step thermal polymerization, achieving both control over nanosheet thickness and crystal surface spacing. The hydrogen production rate of 0.5 %Cu/1 %P doped g-C3N4 was increased by 5.25 times and reached 958.98 μmol/g/h under 10 °C and 10 W LED illumination. Copper and phosphorus doping that reduced (002) surface spacing of g-C3N4, resulting in thinner nanosheets. This structural modification facilitated interlayer charge transfer and suppressed photogenerated carrier recombination. This research provides a novel insight and strategy for designing hydrogen production photocatalyst at low temperatures and with low energy consumption.

Enhanced near infrared and gate tunable photoresponse of MoSe2 transistor enabled by 2D hetero contact engineering

The excellent physical features of two-dimensional (2D) layered materials make them very promising for electronic and optoelectronic applications. Here, we investigated the gate-tunable and broadband (220, 365, 460, 510, 840 and 1020 nm) photoresponse of MoSe2 transistor with metallic (Cr–Au/MoSe2) and van der Waals (vdWs) hetero MoSe2/WTe2 contacts. Our study intends to investigate the underlying mechanism of photogeneration and carrier's transport process by Cr–Au/MoSe2 and hetero MoSe2/WTe2 contacts in the MoSe2 transistor. Since, our research revealed that MoSe2 devices with hetero MoSe2/WTe2 contacts exhibited remarkable performance, including high responsivity (R) of about 11.23 × 105 mA/W, exceptional external quantum efficiency (EQE) of 6.3 × 105 %, and detectivity (D*) of 4.61 × 1010 Jones compared to metallic contacts. This implies that the enhanced performance of our devices with 2D hetero-contacts could be attributed to reduced contact resistance, interlayer charge transfer and interlayer excitons in MoSe2/WTe2 heterostructure. Also, the gate-controlled response and broadband capabilities of van der Waals 2D heterostructures hold significant promises for a wide range of optoelectronic applications, including photodetectors, optical sensors, and visual imaging systems.

Interlayer Electric Multipoles Induced by In-Plane Field from Quantum Geometric Origins.

We show that interlayer charge transfer in 2D materials can be driven by an in-plane electric field, giving rise to electrical multipole generation in linear and second order in-plane field. The linear and nonlinear effects have quantum geometric origins in the Berry curvature and quantum metric, respectively, defined in extended parameter spaces characteristic of layered materials. We elucidate their symmetry characters and demonstrate sizable dipole and quadrupole polarizations, respectively, in twisted bilayers and trilayers of transition metal dichalcogenides. Furthermore, we show that this effect is strongly enhanced during the topological phase transition tuned by interlayer translation. The effects point to a new electric control on the layer quantum degree of freedom.

Exciton-Phonon Coupling Induces a New Pathway for Ultrafast Intralayer-to-Interlayer Exciton Transition and Interlayer Charge Transfer in WS2-MoS2 Heterostructure: A First-Principles Study.

Despite the weak, van der Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50 fs time scales. An early theoretical understanding of charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate optical properties of such materials. We employ an ab initio many-body perturbation theory approach, which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS2/MoS2 heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photoexcited excitons at the K valley of MoS2 and WS2 is 67 and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways that are otherwise inaccessible to noninteracting electrons and holes.

Energy band engineering in GaS/InS and GaSe/InS van der Waals bilayers by interlayer stacking design and applied vertical electric field - An ab-initio theoretical calculation based approach

In this work, for the first time, an extensive theoretical investigation of the structural and electronic properties of the van der Waals (vdW) bilayer heterostructures of two-dimensional (2D) Gallium Sulphide (GaS)/Indium Sulphide (InS) and Gallium Selenide (GaSe)/Indium Sulphide (InS), is performed using density functional theory calculation. In this context, the effects of natural and artificial interlayer stacking configurations, and externally applied out-of-plane direction electric field are emphasized. The structural, thermal, and dynamic stabilities of individual bilayers are assessed from the cohesive energy/interlayer interaction energy, phonon spectrum, and molecular dynamics evolution, respectively. The effects of the vdW bilayer environment on the in-plane structural parameters like lattice vector, bond length, and bond angle of the constituent monolayers are comprehensively investigated. The electronic properties are estimated from the band edge energy, the density of states, energy bandgap, and interlayer energy band alignment, which are systematically analysed from the interlayer interactions in terms of interlayer distance, out-of-plane charge distribution, and interlayer charge transfer. The key findings demonstrate that the interlayer stacking engineering and applied out-of-plane electric field can induce unique and tailor-made modulations in the overall electronic properties of GaS/InS and GaSe/InS bilayer, which can be exploited in favour of electronic/optoelectronic applications.

Enhancing Single Photon Emission Purity via Design of van der Waals Heterostructures.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.