Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets exhibits a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of ultra-thin tin films therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.

Microfluidic-Assisted 3D Printing Zinc Powder Anode with 2D Conductive MOF/MXene Heterostructures for High-Stable Zinc-Organic Battery

Zinc powder (Zn-P) anodes have significant advantages in terms of universality and machinability compared with Zn foil anodes. However, their rough surface, which has a high surface area, intensifies the uncontrollable growth of Zn dendrites and parasitic side reactions. In this study, an anti-corrosive Zn-P-based anode with a functional layer formed from a MXene and Cu-THBQ (MXene/Cu-THBQ) heterostructure is successfully fabricated via microfluidic-assisted 3D printing. The unusual anti-corrosive and strong adsorption of Zn ions using the MXene/Cu-THBQ functional layer can effectively homogenize the Zn ion flux and inhibit the hydrogen evolution reaction (HER) during the repeated process of Zn plating/stripping, thus achieving stable Zn cycling. Consequently, a symmetric cell based on Zn-P with the MXene/Cu-THBQ anode exhibits a highly reversible cycling of 1800 h at 2 mA cm-2 /1 mAh cm-2 . Furthermore, a Zn-organic full battery matched with a 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl organic cathode riveted on graphene delivers a high reversible capacity and maintains a long cycle life.

Manipulating Interlayer Excitons for Near-Infrared Quantum Light Generation

Interlayer excitons (IXs) formed at the interface of van der Waals materials possess various novel properties. In parallel development, strain engineering has emerged as an effective means for creating 2D quantum emitters. Exploring the intersection of these two exciting areas, we use MoS2/WSe2 heterostructure as a model system and demonstrate how strain, defects, and layering can be utilized to create defect-bound IXs capable of bright, robust, and tunable quantum light emission in the technologically important near-infrared spectral range. Our work presents defect-bound IXs as a promising platform for pushing the performance of 2D quantum emitters beyond their current limitations.

Graphene-In2Se3 van der Waals Heterojunction Neuristor for Optical In-Memory Bimodal Operation

Functional diversification at the single-device level has become essential for emerging optical neural network (ONN) development. Stable ferroelectricity harnessed with strong light sensitivity in α-In2Se3 holds great potential for developing ultrathin neuromorphic devices. Herein, we demonstrated an all-2D van der Waals heterostructure-based programmable synaptic field effect transistor (FET) utilizing a ferroelectric α-In2Se3 nanosheet and monolayer graphene. The devices exhibited reconfigurable, multilevel nonvolatile memory (NVM) states, which can be successively modulated by multiple dual-mode (optical and electrical) stimuli and thereby used to realize energy-efficient, heterosynaptic functionalities in a biorealistic fashion. Furthermore, under light illumination, the prototypical device can toggle between volatile (photodetector) and nonvolatile optical random-access memory (ORAM) logic operation, depending upon the ferroelectric-dipole induced band adjustment. Finally, plasticity modulation from short-term to prominent long-term characteristics over a wide dynamic range was demonstrated. The inherent operation mechanism owing to the switchable polarization-induced electronic band alignment and bidirectional barrier height modulation at the heterointerface was revealed by conjugated electronic transport and Kelvin-probe force microscopy (KPFM) measurements. Overall, robust (opto)electronic weight controllability for integrated in-sensor and in-memory logic processors and multibit ORAM systems was readily accomplished by the synergistic ferrophotonic heterostructure properties. Our presented results facilitate the technological implementation of versatile all-2D heterosynapses for next-generation perception, optoelectronic logic systems, and Internet-of-Things (IoT) entities.

Hybrid Heterostructures of a Spin Crossover Coordination Polymer on MoS2 : Elucidating the Role of the 2D Substrate

Controlling the deposition of spin-crossover (SCO) materials constitutes a crucial step for the integration of these bistable molecular systems in electronic devices. Moreover, the influence of functional surfaces, such as 2D materials, can be determinant on the properties of the deposited SCO film. In this work, ultrathin films of the SCO Hofmann-type coordination polymer [Fe(py)2 {Pt(CN)4 }] (py = pyridine) onto monolayers of 1T and 2H MoS2 polytypes are grown. The resulting hybrid heterostructures are characterized by GIXRD, XAS, XPS, and EXAFS to get information on the structure and the specific interactions generated at the interface, as well as on the spin transition. The use of a layer-by-layer results in SCO/2D heterostructures, with crystalline and well-oriented [Fe(py)2 {Pt(CN)4 }]. Unlike with conventional Au or SiO2 substrates, no intermediate self-assembled monolayer is required, thanks to the surface S atoms. Furthermore, it is observed that the higher presence of Fe3+ in the 2H heterostructures hinders an effective spin transition for [Fe(py)2 {Pt(CN)4 }] films thinner than 8 nm. Remarkably, when using 1T MoS2 , this transition is preserved in films as thin as 4 nm, due to the reducing character of this metallic substrate. These results highlight the active role that 2D materials play as substrates in hybrid molecular/2D heterostructures.

Supercritical CO2 Directional-Assisted Synthesis of Low-Dimensional Materials for Functional Applications

Supercritical CO2 (SC CO2 ), as one of the unique fluids that possess fascinating properties of gas and liquid, holds great promise in chemical reactions and fabrication of materials. Building special nanostructures via SC CO2 for functional applications has been the focus of intense research for the past two decades, with facile regulated reaction conditions and a particular reaction field to operate compared to the more widely used solvent systems. In this review, the significance of SC CO2 on fabricating various functional materials including modification of 1D carbon nanotubes, 2D materials, and 2D heterostructures is stated. The fundamental aspects involving building special nanostructures via SC CO2 are explored: how their structure, morphology, and chemical composition be affected by the SC CO2 . Various optimization strategies are outlined to improve their performances, and recent advances are combined to present a coherent understanding of the mechanism of SC CO2 acting on these functional nanostructures. The wide applications of these special nanostructures in catalysis, biosensing, optoelectronics, microelectronics, and energy transformation are discussed. Moreover, the current status of SC CO2 research, the existing scientific issues, and application challenges, as well as the possible future directions to advance this fertile field are proposed in this review.

Efficient Light Coupling and Purcell Effect Enhancement for Interlayer Exciton Emitters in 2D Heterostructures Combined with SiN Nanoparticles

Optimal design of a silicon nitride waveguide structure composed of resonant nanoantennas for efficient light coupling with interlayer exciton emitters in a MoSe2-WSe2 heterostructure is proposed. Numerical simulations demonstrate up to eight times coupling efficiency improvement and twelve times Purcell effect enhancement in comparison with a conventional strip waveguide. Achieved results can be beneficial for development of on-chip non-classical light sources.

Graphene Spin Valves for Spin Logic Devices

An alternative to charge-based electronics identifies the spin degree of freedom for information communication and processing. The long spin-diffusion length in graphene at room temperature demonstrates its ability for highly scalable spintronics. The development of the graphene spin valve (SV) has inspired spin devices in graphene including spin field-effect transistors and spin majority logic gates. A comprehensive picture of spin transport in graphene SVs is required for further development of spin logic. This review examines the advances in graphene SVs and their role in the development of spin logic devices. Different transport and scattering mechanisms in charge and spin are discussed. Furthermore, the on/off switching energy between graphene SVs and charge-based FETs is compared to highlight their prospects for low-power devices. The challenges and perspectives that need to be addressed for the future development of spin logic devices are then outlined.

Moiré Synergy: An Emerging Playground by Coupled Moirés

Moiré superlattices of tunable wavelengths and the further developed coupled-moiré systems, by artificially assembling two-dimensional (2D) van der Waals (vdW) materials as designed, have brought up a versatile toolbox to explore fascinating condensed matter physics and their stimulating physicochemical functionalities. In this Perspective, we briefly review the recent progress in the emerging field of moiré synergy, highlighting the synergetic effects arising in distinct multi-moiré heterostructures of graphene and transition metal dichalcogenides (TMDCs). A spectrum of coupled-moiré configurations, the advanced characterization, and the exploitation efforts on the moiré-moiré interactions will be discussed. Finally, we look out for urgent challenges to be conquered in the community and some potential research directions in the near future.

Cation-Driven Assembly of Bilayered Vanadium Oxide and Graphene Oxide Nanoflakes to Form Two-Dimensional Heterostructure Electrodes for Li-Ion Batteries

Lithium preintercalated bilayered vanadium oxide (LVO or δ-Li_xV₂O₅·nH₂O) and graphene oxide (GO) nanoflakes were assembled using a concentrated lithium chloride solution and annealed under vacuum at 200 °C to form two-dimensional (2D) δ-Li_xV₂O₅·nH₂O and reduced GO (rGO) heterostructures. We found that the Li⁺ ions from LiCl enhanced the oxide/carbon heterointerface formation and served as stabilizing ions to improve structural and electrochemical stability. The graphitic content of the heterostructure could be easily controlled by changing the initial GO concentration prior to assembly. We found that increasing the GO content in our heterostructure composition helped inhibit the electrochemical degradation of LVO during cycling and improved the rate capability of the heterostructure. A combination of scanning electron microscopy and X-ray diffraction was used to help confirm that a 2D heterointerface formed between LVO and GO, and the final phase composition was determined using energy-dispersive X-ray spectroscopy and thermogravimetric analysis. Scanning transmission electron microscopy and electron energy-loss spectroscopy were additionally used to examine the heterostructures at high resolution, mapping the orientations of rGO and LVO layers and locally imaging their interlayer spacings. Further, electrochemical cycling of the cation-assembled LVO/rGO heterostructures in Li-ion cells with a non-aqueous electrolyte revealed that increasing the rGO content led to improved cycling stability and rate performance, despite slightly decreased charge storage capacity. The heterostructures with 0, 10, 20, and 35 wt % rGO exhibited capacities of 237, 216, 174, and 150 mAh g^-1, respectively. Moreover, the LVO/rGO-35 wt % and LVO/rGO-20 wt % heterostructures retained 75% (110 mAh g^-1) and 67% (120 mAh g^-1) of their initial capacities after increasing the specific current from 20 to 200 mA g^-1, while the LVO/rGO-10 wt % sample retained only 48% (107 mAh g^-1) of its initial capacity under the same cycling conditions. In addition, the cation-assembled LVO/rGO electrodes exhibited enhanced electrochemical stability compared to electrodes prepared through physical mixing of LVO and GO nanoflakes in the same ratios as the heterostructure electrodes, further revealing the stabilizing effect of a 2D heterointerface. The cation-driven assembly approach, explored in this work using Li⁺ cations, was found to induce and stabilize the formation of stacked 2D layers of rGO and exfoliated LVO. The reported assembly methodology can be applied for a variety of systems utilizing 2D materials with complementary properties for applications as electrodes in energy storage devices.

Heterojunction Transistors Printed via Instantaneous Oxidation of Liquid Metals

Semiconducting transparent metal oxides are critical high mobility materials for flexible optoelectronic devices such as displays. We introduce the continuous liquid metal printing (CLMP) technique to enable rapid roll-to-roll compatible deposition of semiconducting two-dimensional (2D) metal oxide heterostructures. We leverage CLMP to deposit 10 cm²-scale nanosheets of InO_x and GaO_x in seconds at a low process temperature (T < 200 °C) in air, fabricating heterojunction thin film transistors with 100× greater I_on/I_off, 4× steeper subthreshold slope, and a 50% increase in mobility over pure InO_x channels. Detailed nanoscale characterization of the heterointerface by X-ray photoelectron spectroscopy, UV-vis, and Kelvin probe elucidates the origins of enhanced electronic transport in these 2D heterojunctions. This combination of CLMP with the electrostatic control induced by the heterostructure architecture leads to high performance (μ_lin up to 22.6 cm²/(V s)) while reducing the process time for metal oxide transistors by greater than 100× compared with sol-gels and vacuum deposition methods.

The interface ofin-situgrown single-layer epitaxial MoS2on SrTiO3(001) and (111)

SrTiO₃(STO) is a versatile substrate with a high dielectric constant, which may be used in heterostructures with 2D materials, such as MoS₂, to induce interesting changes to the electronic structure. STO single crystal substrates have previously been shown to support the growth of well-defined epitaxial single-layer (SL) MoS₂crystals. The STO substrate is already known to renormalize the electronic bandgap of SL MoS₂, but the electronic nature of the interface and its dependence on epitaxy are still unclear. Herein, we have investigated anin-situphysical vapor deposition (PVD) method, which could eliminate the need for ambient transfer between substrate preparation, subsequent MoS₂growth and surface characterization. Based on this, we then investigate the structure and epitaxial alignment of pristine SL MoS₂in various surface coverages grown on two STO substrates with a different initial surface lattice, the STO(001)(4 × 2) and STO(111)-(9/5 × 9/5) reconstructed surfaces, respectively. Scanning tunneling microscopy shows that epitaxial alignment of the SL MoS₂is present for both systems, reflected by orientation of MoS₂edges and a distinct moiré pattern visible on the MoS₂(0001) basal place. Upon increasing the SL MoS₂coverage, the presence of four distinct rotational domains on the STO(001) substrate, whilst only two on STO(111), is seen to control the possibilities for the formation of coherent MoS₂domains with the same orientation. The presented methodology relies on standard PVD in ultra-high vacuum and it may be extended to other systems to help explore pristine two-dimensional transition metal dichalcogenide/STO systems in general.

Ultra-High Interfacial Thermal Conductance via Double hBN Encapsulation for Efficient Thermal Management of 2D Electronics

Heat dissipation is a major limitation of high-performance electronics. This is especially important in emerging nanoelectronic devices consisting of ultra-thin layers, heterostructures, and interfaces, where enhancement in thermal transport is highly desired. Here, ultra-high interfacial thermal conductance in encapsulated van der Waals (vdW) heterostructures with single-layer transition metal dichalcogenides MX₂ (MoS₂ , WSe₂ , WS₂ ) sandwiched between two hexagonal boron nitride (hBN) layers is reported. Through Raman spectroscopic measurements of suspended and substrate-supported hBN/MX₂ /hBN heterostructures with varying laser power and temperature, the out-of-plane interfacial thermal conductance in the vertical stack is calibrated. The measured interfacial thermal conductance between MX₂ and hBN reaches 74 ± 25 MW m^-2 K^-1 , which is at least ten times higher than the interfacial thermal conductance of MX₂ in non-encapsulation structures. Molecular dynamics (MD) calculations verify and explain the experimental results, suggesting a full encapsulation by hBN layers is accounting for the high interfacial conductance. This ultra-high interfacial thermal conductance is attributed to the double heat transfer pathways and the clean and tight vdW interface between two crystalline 2D materials. The findings in this study reveal new thermal transport mechanisms in hBN/MX₂ /hBN structures and shed light on building novel hBN-encapsulated nanoelectronic devices with enhanced thermal management.

Two Dimensional Heterostructures for Optoelectronics: Current Status and Future Perspective

Researchers have found various families of two-dimensional (2D) materials and associated heterostructures through detailed theoretical work and experimental efforts. Such primitive studies provide a framework to investigate novel physical/chemical characteristics and technological aspects from micro to nano and pico scale. Two-dimensional van der Waals (vdW) materials and their heterostructures can be obtained to enable high-frequency broadband through a sophisticated combination of stacking order, orientation, and interlayer interactions. These heterostructures have been the focus of much recent research due to their potential applications in optoelectronics. Growing the layers of one kind of 2D material over the other, controlling absorption spectra via external bias, and external doping proposes an additional degree of freedom to modulate the properties of such materials. This mini review focuses on current state-of-the-art material design, manufacturing techniques, and strategies to design novel heterostructures. In addition to a discussion of fabrication techniques, it includes a comprehensive analysis of the electrical and optical properties of vdW heterostructures (vdWHs), particularly emphasizing the energy-band alignment. In the following sections, we discuss specific optoelectronic devices, such as light-emitting diodes (LEDs), photovoltaics, acoustic cavities, and biomedical photodetectors. Furthermore, this also includes a discussion of four different 2D-based photodetector configurations according to their stacking order. Moreover, we discuss the challenges that remain to be addressed in order to realize the full potential of these materials for optoelectronics applications. Finally, as future perspectives, we present some key directions and express our subjective assessment of upcoming trends in the field.

Electric Field and Strain Tuning of 2D Semiconductor van der Waals Heterostructures for Tunnel Field-Effect Transistors

Heterostacks consisting of low-dimensional materials are attractive candidates for future electronic nanodevices in the post-silicon era. In this paper, using first-principles calculations based on density functional theory (DFT), we explore the structural and electronic properties of MoTe₂/ZrS₂ heterostructures with various stacking patterns and thicknesses. Our simulations show that the valence band (VB) edge of MoTe₂ is almost aligned with the conduction band (CB) edge of ZrS₂, and (MoTe₂)_m/(ZrS₂)_m (m = 1, 2) heterostructures exhibit the long-sought broken gap band alignment, which is pivotal for realizing tunneling transistors. Electrons are found to spontaneously flow from MoTe₂ to ZrS₂, and the system resembles an ultrascaled parallel plate capacitor with an intrinsic electric field pointed from MoTe₂ to ZrS₂. The effects of strain and external electric fields on the electronic properties are also investigated. For vertical compressive strains, the charge transfer increases due to the decreased coupling between the layers, whereas tensile strains lead to the opposite behavior. For negative electric fields a transition from the type-III to the type-II band alignment is induced. In contrast, by increasing the positive electric fields, a larger overlap between the valence and conduction bands is observed, leading to a larger band-to-band tunneling (BTBT) current. Low-strained heterostructures with various rotation angles between the constituent layers are also considered. We find only small variations in the energies of the VB and CB edges with respect to the Fermi level, for different rotation angles up to 30°. Overall, our simulations offer insights into the fundamental properties of low-dimensional heterostructures and pave the way for their future application in energy-efficient electronic nanodevices.

Epitaxial Atomic Substitution for MoS2-MoN Heterostructure Synthesis

Integrating different two-dimensional (2D) crystals is highly demanded for advancing their application in next-generation electronics. 2D transition metal carbides, nitrides, and carbonitrides (MXenes), as new members in the 2D family, are promising candidates for 2D electrodes because of their high conductivity and stability. However, integrating MXenes with other 2D semiconductors has been underdeveloped due to the limitation of top-down etching synthesis of MXenes. Our recent development of atomic substitution synthesis achieved ultrathin non-van der Waals (non-vdW) transition metal nitrides (TMNs) through the conversion of vdW transition metal dichalcogenides (TMDs), opening opportunities of combining TMDs with TMNs via controllable partial conversion. Here, we perform an in-depth study of the atomic substitution process from semiconducting MoS₂ to metallic MoN and realize both lateral and vertical MoN-MoS₂ heterostructures via edge and surface epitaxial conversion, respectively. The structural evolution investigation from MoS₂ to MoN using high-resolution transmission electron microscopy suggests atomically bonded interface for lateral heterostructures and moiré pattern in vertical heterostructures. Moreover, mask-assisted atomic substitution is applied to create patterned MoN-MoS₂-MoN lateral heterostructures. Electrical measurements reveal a Schottky barrier height of meV for a three-layer MoS₂-MoN interface, showcasing the potential of atomically bonded lateral heterostructures for MoS₂ electronics with MoN as contact electrodes.

Raman Fingerprint of Interlayer Coupling in 2D TMDCs

Vertical stacking of two-dimensional (2D) homo- and heterostructures are intriguing research objects, as they are essential for fundamental studies and a key towards 2D device applications. It is paramount to understand the interlayer coupling in 2D materials and to find a fast yet precise characteristic signature. In this work, we report on a Raman fingerprint of interlayer coupling in 2D transition metal dichalcogenides (TMDCs). We observed that the out-of-plane B2g vibrational mode is absent when two monolayers form a vertical stack yet remain uncoupled but emerges after strong coupling. Using systematic Raman, photoluminescence (PL), and atomic force microscopy (AFM) studies of WSe2/WSe2 homo-bilayers and MoSe2/WSe2 hetero-bilayers, we conclude that the B2g vibrational mode is a distinct Raman fingerprint of interlayer coupling in 2D TMDCs. Our results propose an easy, fast, precise, and reliable measure to evaluate the interlayer coupling in 2D TMDCs.

Wrinkle-mediated CVD synthesis of wafer scale Graphene/h-BN heterostructures

The combination of two-dimensional materials (2D) into heterostructures enables their integration in tunable ultrathin devices. For applications in electronics and optoelectronics, direct growth of wafer-scale and vertically stacked graphene/hexagonal boron nitride (h-BN) heterostructures is vital. The fundamental problem, however, is the catalytically inert nature of h-BN substrates, which typically provide a low rate of carbon precursor breakdown and consequently a poor rate of graphene synthesis. Furthermore, out-of-plane deformations such as wrinkles are commonly seen in 2D materials grown by chemical vapor deposition (CVD). Herein, a wrinkle-facilitated route is developed for the fast growth of graphene/h-BN vertical heterostructures on Cu foils. The key advantage of this synthetic pathway is the exploitation of the increased reactivity from inevitable line defects arising from the CVD process, which can act as active sites for graphene nucleation. The resulted heterostructures are found to exhibit superlubric properties with increased bending stiffness, as well as directional electronic properties, as revealed from atomic force microscopy measurements. This work offers a brand-new route for the fast growth of Gr/h-BN heterostructures with practical scalability, thus propelling applications in electronics and nanomechanical systems.

Microwave Facilitated Covalent Organic Framework/Transition Metal Dichalcogenide Heterostructures

Organic/inorganic heterostructures present a versatile platform for creating materials with new functionalities and hybrid properties. In particular, junctions between two dimensional materials have demonstrated utility in next generation electronic, optical, and optoelectronic devices. This work pioneers a microwave facilitated synthesis process to readily incorporate few-layer covalent organic framework (COF) films onto monolayer transition metal dichalcogenides (TMDC). Preferential microwave excitation of the monolayer TMDC flakes result in selective attachment of COFs onto the van der Waals surface with film thicknesses between 1 and 4 nm. The flexible process is extended to multiple TMDCs (MoS₂, MoSe₂, MoSSe) and several well-known COFs (TAPA-PDA COF, TPT-TFA-COF, and COF-5). Photoluminescence studies reveal a power-dependent defect formation in the TMDC layer, which facilitates electronic coupling between the materials at higher TMDC defect densities. This coupling results in a shift in the A-exciton peak location of MoSe₂, with a red or blue shift of 50 or 19 meV, respectively, depending upon the electron donating character of the few-layer COF films. Moreover, optoelectronic devices fabricated from the COF-5/TMDC heterostructure present an opportunity to tune the PL intensity and control the interaction dynamics within inorganic/organic heterostructures.

In Situ Measurements of Strain Evolution in Graphene/Boron Nitride Heterostructures Using a Non-Destructive Raman Spectroscopy Approach

The mechanical properties of engineered van der Waals (vdW) 2D materials and heterostructures are critically important for their implementation into practical applications. Using a non-destructive Raman spectroscopy approach, this study investigates the strain evolution of single-layer graphene (SLGr) and few-layered boron nitride/graphene (FLBN/SLGr) heterostructures. The prepared 2D materials are synthesized via chemical vapor deposition (CVD) method and then transferred onto flexible polyethylene terephthalate (PET) substrates for subsequent strain measurements. For this study, a custom-built mechanical device-jig is designed and manufactured in-house to be used as an insert for the 3D piezoelectric stage of the Raman system. In situ investigation of the effects of applied strain in graphene detectable via Raman spectral data in characteristic bonds within SLGr and FLBN/SLGr heterostructures is carried out. The in situ strain evolution of the FLBN/SLGr heterostructures is obtained in the range of (0-0.5%) strain. It is found that, under the same strain, SLG exhibits a higher Raman shift in the 2D band as compared with FLBN/SLGr heterostructures. This research leads to a better understanding of strain dissipation in vertical 2D heterostacks, which could help improve the design and engineering of custom interfaces and, subsequently, control lattice structure and electronic properties. Moreover, this study can provide a new systematic approach for precise in situ strain assessment and measurements of other CVD-grown 2D materials and their heterostructures on a large scale for manufacturing a variety of future micro- and nano-scale devices on flexible substrates.

MAX-phase Derived Tin Diselenide for 2D/2D Heterostructures with Ultralow Surface/Interface Transport Barriers toward Li-/Na-ions Storage

2D tin diselenide and its derived 2D heterostructures have delivered promising potentials in various applications ranging from electronics to energy storage devices. The major challenges associated with large-scale fabrication of SnSe₂ crystals, however, have hindered its engineering applications. Herein, a tin-extraction synthetic method is proposed for producing large-size SnSe₂ bulk crystals. In a typical synthesis, a Sn-containing MAX phase (V₂ SnC) and a Se source are heat-treated under a reducing atmosphere, by which Sn is extracted from the V₂ SnC phase as a rectified Sn source to form SnSe₂ crystals in the cold zone. After the following liquid exfoliation, the obtained 2D SnSe₂ nanosheets have a lateral size of a few centimeters and an atomic thickness. Furthermore, by coupling with 2D graphene to form 2D/2D SnSe₂ /graphene heterostructured electrodes, as validated by theoretical calculation and experimental studies, the superior Li-/Na-ion storage performance with ultralow surface/interface ion transport barriers are achieved for rechargeable Li-/Na-ion batteries. This innovative synthetic strategy opens a new avenue for the large-scale synthesis of selenides and offers more options into the practical application of emerging 2D/2D heterostructure for electrochemical energy storage.

Transfer-free, scalable vertical heterostructure FET on MoS2/WS2continuous films

Taking into account the novel layered structure and unusual electronic properties of MoS₂and WS₂on the side the lack of dangling bonds between these two components and donor-acceptor linkage effects, growth of the MoS₂/WS₂vertical heterojunction film on the amorphous SiO₂/Si substrate have created high demand. In this study, we reported the continuous, scalable, and vertical MoS₂/WS₂heterostructure film by using a sputtering without a transfer step. The WS₂film was continuously grown on MoS₂and eventually led to the formation of the MoS₂/WS₂vertical heterojunction film. Dozens of FETs fabricated on MoS₂/WS₂continuous heterojunction film were created on the same substrate in a single lithographic fabrication step, allowing them to be commercialized and not only used in research applications. RAMAN spectra proved the formation of the MoS₂/WS₂heterostructure film. In XPS measurements, it was shown that a separate MoS₂and WS₂layer was grown instead of the alloy structure. The polarity behavior of the MoS₂/WS₂heterostructure FET was found to be modulated with different drain voltages as p-type to ambipolar and finally n-type conductivity because of the transition of band structure and Schottky barrier heights at different drain voltages. Electron mobility (7.2 cm²V.s^-1) and on/off ratio (10⁴-10⁵) exhibited by the MoS₂/WS₂heterostructure FETs displayed a more improved electrical performance than that of individual WS₂, MoS₂devices. It was observed that the mobility value of MoS₂/WS₂FET was approximately 514 times greater than WS₂FET and 800 times greater than MoS₂FET. Additionally, the MoS₂/WS₂FET on/off ratio was larger than 2 order MoS₂FET and 1 order WS₂FET. The film of continuous vertical heterojunctions as in the MoS₂/WS₂currents in the study would be a promising candidate for nanoelectronics fields. This work demonstrated the progress towards realizing carrier-type controlled high-performance MoS₂/WS₂heterojunction-based FETs for future logic devices.

Role of Surface Adsorbates on the Photoresponse of (MO)CVD-Grown Graphene-MoS2 Heterostructure Photodetectors

A promising strategy toward ultrathin, sensitive photodetectors is the combination of a photoactive semiconducting transition-metal dichalcogenide (TMDC) monolayer like MoS₂ with highly conductive graphene. Such devices often exhibit a complex and contradictory photoresponse as incident light can trigger both photoconductivity and photoinduced desorption of molecules from the surface. Here, we use metal-organic chemical vapor deposition (MOCVD) to directly grow MoS₂ on top of graphene that is deposited on a sapphire wafer via chemical vapor deposition (CVD) for realizing graphene-MoS₂ photodetectors. Two-color optical pump-electrical probe experiments allow for separation of light-induced carrier transfer across the graphene-MoS₂ heterointerface from adsorbate-induced effects. We demonstrate that adsorbates strongly modify both magnitude and sign of the photoconductivity. This is attributed to a change of the graphene doping from p- to n-type in case adsorbates are being desorbed, while in either case, photogenerated electrons are transferred from MoS₂ to graphene. This nondestructive probing method sheds light on the charge carrier transfer mechanisms and the role of adsorbates in two-dimensional (2D) heterostructure photodetectors.

Scalable Synthesis of Monolayer Hexagonal Boron Nitride on Graphene with Giant Bandgap Renormalization

Monolayer hexagonal boron nitride (hBN) has been widely considered a fundamental building block for 2D heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, an hBN/graphene (hBN/G) interface-mediated growth process for the controlled synthesis of high-quality monolayer hBN is proposed and further demonstrated. It is discovered that the in-plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single-domain hBN, aligned to the underlying graphene lattice. Furthermore, it is identified that the deep-ultraviolet emission at 6.12 eV stems from the 1s-exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer-scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

Design of Vertically Aligned Two-Dimensional Heterostructures of Rigid Ti3C2TX MXene and Pliable Vanadium Pentoxide for Efficient Lithium Ion Storage

Designing a thick electrode with appropriate mass loading is a prerequisite toward practical applications for lithium ion batteries (LIBs) yet suffers severe limitations of slow electron/ion transport, unavoidable volume expansion, and the involvement of inactive additives, which lead to compromised output capacity, poor rate perforamnce, and cycling instability. Herein, self-supported thick electrode composed of vertically aligned two-dimensional (2D) heterostructures (V-MXene/V₂O₅) of rigid Ti₃C₂T_X MXene and pliable vanadium pentoxide are assembled via an ice crystallization-induced strategy. The vertical channels prompt fast electron/ion transport within the entire electrode; in the meantime, the 3D MXene scaffold provides mechanical robustness during lithiation/delithiation. The optimized electrodes with 1 and 5 mg cm^-2 of V-MXene/V₂O₅ respectively deliver 472 and 300 mAh g^-1 at a current density of 0.2 A g^-1, rate performance with 380 and 222 mAh g^-1 retained at 5 A g^-1, and reliability over 800 charge/discharge cycles.

Coupling of N-Doped Mesoporous Carbon and N-Ti3 C2 in 2D Sandwiched Heterostructure for Enhanced Oxygen Electroreduction

2D heterostructures provide a competitive platform to tailor electrical property through control of layer structure and constituents. However, despite the diverse integration of 2D materials and their application flexibility, tailoring synergistic interlayer interactions between 2D materials that form electronically coupled heterostructures remains a grand challenge. Here, the rational design and optimized synthesis of electronically coupled N-doped mesoporous defective carbon and nitrogen modified titanium carbide (Ti₃ C₂ ) in a 2D sandwiched heterostructure, is reported. First, a F127-polydopamine single-micelle-directed interfacial assembly strategy guarantees the construction of two surrounding mesoporous N-doped carbon monolayers assembled on both sides of Ti₃ C₂ nanosheets. Second, the followed ammonia post-treatment successfully introduces N elements into Ti₃ C₂ structure and more defective sites in N-doped mesoporous carbon. Finally, the oxygen reduction reaction (ORR) and theoretical calculation prove the synergistic coupled electronic effect between N-Ti₃ C₂ and defective N-doped carbon active sites in the 2D sandwiched heterostructure. Compared with the control 2D samples (0.87-0.88 V, 4.90-5.15 mA cm^-2 ), the coupled 2D heterostructure possesses the best onset potential of 0.90 V and limited density current of 5.50 mA cm^-2 . Meanwhile, this catalyst exhibits superior methanol tolerance and cyclic durability. This design philosophy opens up a new thought for tailoring synergistic interlayer interactions between 2D materials.

Spatiotemporally Coupled Electron-Hole Dynamics in Two Dimensional Heterostructures

Coulomb interactions play a crucial role in low-dimensional semiconductor materials, e.g., 2D layered semiconductors, dictating their electronic and optical properties. However, fundamental questions remain as to whether and how Coulomb interactions affect the charge or energy flow in 2D heterostructures, which is essential for their light-electricity conversions. Herein, using ultrafast spectroscopy, we report real space coupled electron-hole dynamics in 2D heterostructures. We show in (WSe₂/)WS₂/MoTe₂ with a controlled energy gradient for the hole and a near flat band for electron transfer, the fate of the electron is controlled by the hole in coupled dynamics. The interfacial electron transfer from WS₂ to MoTe₂ follows the hole closely and can be facilitated or suppressed by dynamic Coulomb interaction. In parallel to the band alignment, this study reveals the critical role of Coulomb interactions on the fate of photogenerated charges in 2D heterostructures, providing experimental evidence for coupled electron-hole dynamics and a new knob for steering nanoscale charge or energy transfer process.

Science of 2.5 dimensional materials: paradigm shift of materials science toward future social innovation

The past decades of materials science discoveries are the basis of our present society - from the foundation of semiconductor devices to the recent development of internet of things (IoT) technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science '2.5 dimensional (2.5D) materials,' to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of '2.5D materials' and review recent research developments based on this new scientific concept.

Dynamic Self-Rectifying Liquid Metal-Semiconductor Heterointerfaces: A Platform for Development of Bioinspired Afferent Systems

The assembly of geometrically complex and dynamically active liquid metal/semiconductor heterointerfaces has drawn extensive attention in multidimensional electronic systems. In this study the chemovoltaic driven reactions have enabled the microfluidity of hydrophobic galinstan into a three-dimensional (3D) semiconductor matrix. A dynamic heterointerface is developed between the atomically thin surface oxide of galinstan and the TiO₂-Ni interface. Upon the growth of Ga₂O₃ film at the Ga₂O₃-TiO₂ heterointerface, the partial reduction of the TiO₂ film was confirmed by material characterization techniques. The conductance imaging spectroscopy and electrical measurements are used to investigate the charge transfer at heterointerfaces. Concurrently, the dynamic conductance in artificial synaptic junctions is modulated to mimic the biofunctional communication characteristics of multipolar neurons, including slow and fast inhibitory and excitatory postsynaptic responses. The self-rectifying characteristics, femtojoule energy processing, tunable synaptic events, and notably the coordinated signal recognition are the main characteristics of this multisynaptic device. This novel 3D design of liquid metal-semiconductor structure opens up new opportunities for the development of bioinspired afferent systems. It further facilitates the realization of physical phenomena at liquid metal-semiconductor heterointerfaces.

Visualizing Van der Waals Epitaxial Growth of 2D Heterostructures

Understanding the growth mechanisms of 2D van der Waals (vdW) heterostructures is of great importance in exploring their functionalities and device applications. A custom-built system integrating physical vapor deposition and optical microscopy/Raman spectroscopy is employed to study the dynamic growth processes of 2D vdW heterostructures in situ. This allows the identification of a new growth mode with a distinctly different growth rate and morphology from those of the conventional linear growth mode. A model that explains the difference in morphologies and quantifies the growth rates of the two modes by taking the role of surface diffusion into account is proposed. A range of material combinations including CdI₂ /WS₂ , CdI₂ /MoS₂ , CdI₂ /WSe₂ , PbI₂ /WS₂ , PbI₂ /MoS₂ , PbI₂ /WSe₂ , and Bi₂ Se₃ /WS₂ is systematically investigated. These findings may be generalized to the synthesis of many other 2D heterostructures with controlled morphologies and physical properties, benefiting future device applications.

Ultrathin epitaxial Bi film growth on 2D HfTe2template

Among ultrathin monoelemental two-dimensional (2D) materials, bismuthene, the single layer of heavier group-VΑ element bismuth (Bi), has been predicted to have large non trivial gap. Here, we demonstrate the growth of Bi films by molecular beam epitaxy on 2D-HfTe₂template. At the initial stage of Bi deposition (1-2 bilayers, BL), both the pseudocubic Bi(110) and the hexagonal Bi(111) phases are formed. When reaching 3 BL Bi, a transformation to pure hexagonal Bi(111) occurs. The electronic band structure of 3 BL Bi(111) films was measured by angle-resolved photoemission spectroscopy showing very good matching with the density functional theory band structure calculations of 3 BL free standing Bi(111). The grown Bi(111) thin film was capped with a protective Al₂O₃layer and its stability under ambient conditions, necessary for practical applications and device fabrication, was confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy.

Memristive Behavior Enabled by Amorphous-Crystalline 2D Oxide Heterostructure

The emergence of memristive behavior in amorphous-crystalline 2D oxide heterostructures, which are synthesized by atomic layer deposition (ALD) of a few-nanometer amorphous Al₂ O₃ layers onto atomically thin single-crystalline ZnO nanosheets, is demonstrated. The conduction mechanism is identified based on classic oxygen vacancy conductive channels. ZnO nanosheets provide a 2D host for oxygen vacancies, while the amorphous Al₂ O₃ facilitates the generation and stabilization of the oxygen vacancies. The conduction mechanism in the high-resistance state follows Poole-Frenkel emission, and in the the low-resistance state is fitted by the Mott-Gurney law. From the slope of the fitting curve, the mobility in the low-resistance state is estimated to be ≈2400 cm² V^-1 s^-1 , which is the highest value reported in semiconductor oxides. When annealed at high temperature to eliminate oxygen vacancies, Al is doped into the ZnO nanosheet, and the memristive behavior disappears, further confirming the oxygen vacancies as being responsible for the memristive behavior. The 2D heterointerface offers opportunities for new design of high-performance memristor devices.

Nanoscale All-Oxide-Heterostructured Bio-inspired Optoresponsive Nociceptor

Retina nociceptor, as a key sensory receptor, not only enables the transport of warning signals to the human central nervous system upon its exposure to noxious stimuli, but also triggers the motor response that minimizes potential sensitization. In this study, the capability of two-dimensional all-oxide-heterostructured artificial nociceptor as a single device with tunable properties was confirmed. Newly designed nociceptors utilize ultra-thin sub-stoichiometric TiO₂-Ga₂O₃ heterostructures, where the thermally annealed Ga₂O₃ films play the role of charge transfer controlling component. It is discovered that the phase transformation in Ga₂O₃ is accompanied by substantial jump in conductivity, induced by thermally assisted internal redox reaction of Ga₂O₃ nanostructure during annealing. It is also experimentally confirmed that the charge transfer in all-oxide heterostructures can be tuned and controlled by the heterointerfaces manipulation. Results demonstrate that the engineering of heterointerfaces of two-dimensional (2D) films enables the fabrication of either high-sensitive TiO₂-Ga₂O₃ (Ar) or high-threshold TiO₂-Ga₂O₃ (N₂) nociceptors. The hypersensitive nociceptor mimics the functionalities of corneal nociceptors of human eye, whereas the delayed reaction of nociceptor is similar to high-threshold nociceptive characteristics of human sensory system. The long-term stability of 2D nociceptors demonstrates the capability of heterointerfaces engineering for effective control of charge transfer at 2D heterostructured devices.

Graphene Supported MoS2 Structures with High Defect Density for an Efficient HER Electrocatalysts

The development of novel efficient and robust electrocatalysts with sufficient active sites is one of the key parameters for hydrogen evolution reactions (HER) catalysis, which plays a key role in hydrogen production for clean energy harvesting. Recently, two-dimensional (2D) materials, especially those based upon transition metal dichalcogenides such as molybdenum disulfide (MoS₂), have gained attention for the catalysis of hydrogen production because of their exceptional properties. Innovative strategies have been developed to engineer these material systems for improvements in their catalytic activity. Toward this aim, the facile growth of MoS₂ clusters by sulfurization of molybdenum dioxide (MoO₂) particles supported on reduced graphene oxide (rGO) foams using the chemical vapor deposition (CVD) method is reported. This approach created various morphologies of MoS₂ with large edges and defect densities on the basal plane of rGO supported MoS₂ structures, which are considered as active sites for HER catalysis. In addition, MoS₂ nanostructures on the surface of the porous rGO network show robust physical interactions, such as van der Waals and π-π interactions between MoS₂ and rGO. These features result in an improved process to yield a suitable HER catalyst. In order to gain a better understanding of the improvement of this MoS₂-based HER catalyst, fully atomistic molecular dynamics (MD) simulations of different defect geometries were also performed.

Influence of Proximity to Supporting Substrate on van der Waals Epitaxy of Atomically Thin Graphene/Hexagonal Boron Nitride Heterostructures

Combining graphene and the insulating hexagonal boron nitride (h-BN) into two-dimensional heterostructures is promising for novel, atomically thin electronic nanodevices. A heteroepitaxial growth, in which these materials are grown on top of each other, will be crucial for their scalable device integration. However, during this so-called van der Waals epitaxy, not only the atomically thin substrate itself must be considered but also the influences from the supporting substrate below it. Here, we report not only a substantial difference between the formation of h-BN on single- (SLG) and on bi-layer epitaxial graphene (BLG) on SiC, but also vice versa, that the van der Waals epitaxy of h-BN at growth temperatures well below 1000 °C affects the varying number of graphene layers differently. Our results clearly demonstrate that the additional graphene layer in BLG enhances the distance to the corrugated, carbon-rich interface of the supporting SiC substrate and thereby diminishes its influence on the van der Waals epitaxy, leading to a homogeneous formation of a smooth, atomically thin heterostructure, which will be required for a scalable device integration of 2D heterostructures.

Controlling Photoluminescence Enhancement and Energy Transfer in WS2 :hBN:WS2 Vertical Stacks by Precise Interlayer Distances

2D semiconducting transition metal dichalcogenides (TMDs) are endowed with fascinating optical properties especially in their monolayer limit. Insulating hBN films possessing customizable thickness can act as a separation barrier to dictate the interactions between TMDs. In this work, vertical layered heterostructures (VLHs) of WS₂ :hBN:WS₂ are fabricated utilizing chemical vapor deposition (CVD)-grown materials, and the optical performance is evaluated through photoluminescence (PL) spectroscopy. Apart from the prohibited indirect optical transition due to the insertion of hBN spacers, the variation in the doping level of WS₂ drives energy transfer to arise from the layer with lower quantum efficiency to the other layer with higher quantum efficiency, whereby the total PL yield of the heterosystem is increased and the stack exhibits a higher PL intensity compared to the sum of those in the two WS₂ constituents. Such doping effects originate from the interfaces that WS₂ monolayers reside on and interact with. The electron density in the WS₂ is also controlled and subsequent modulation of PL in the heterostructure is demonstrated by applying back-gated voltages. Other influential factors include the strain in WS₂ and temperature. Being able to tune the energy transfer in the VLHs may expand the development of photonic applications in 2D systems.

Oxygen Vacancy-Rich In-Doped CoO/CoP Heterostructure as an Effective Air Cathode for Rechargeable Zn-Air Batteries

An efficient and low-cost electrocatalyst for reversible oxygen electrocatalysis is crucial for improving the performance of rechargeable metal-air batteries. Herein, a novel oxygen vacancy-rich 2D porous In-doped CoO/CoP heterostructure (In-CoO/CoP FNS) is designed and developed by a facile free radicals-induced strategy as an effective bifunctional electrocatalyst for rechargeable Zn-air batteries. The electron spin resonance and X-ray absorption near edge spectroscopy provide clear evidence that abundant oxygen vacancies are formed in the interface of In-CoO/CoP FNS. Owing to abundant oxygen vacancies, porous heterostructure, and multiple components, In-CoO/CoP FNS exhibits excellent oxygen reduction reaction activity with a positive half-wave potential of 0.81 V and superior oxygen evolution reaction activity with a low overpotential of 365 mV at 10 mA cm^-2 . Moreover, a home-made Zn-air battery with In-CoO/CoP FNS as an air cathode delivers a large power density of 139.4 mW cm^-2 , a high energy density of 938 Wh kg_Zn ^-1 , and can be steadily cycled over 130 h at 10 mA cm^-2 , demonstrating great application potential in rechargeable metal-air batteries.

Quantitative imaging of anion exchange kinetics in halide perovskites

Ion exchange, as a postsynthetic transformation strategy, offers more flexibilities in controlling material compositions and structures beyond direct synthetic methodology. Observation of such transformation kinetics on the single-particle level with rich spatial and spectroscopic information has never been achieved. We report the quantitative imaging of anion exchange kinetics in individual single-crystalline halide perovskite nanoplates using confocal photoluminescence microscopy. We have systematically observed a symmetrical anion exchange pathway on the nanoplates with dependence on reaction time and plate thickness, which is governed by the crystal structure and the diffusion-limited transformation mechanism. Based on a reaction-diffusion model, the halide diffusion coefficient was estimated to be on the order of [Formula: see text] This diffusion-controlled mechanism leads to the formation of 2D perovskite heterostructures with spatially resolved coherent interface through the precisely controlled anion exchange reaction, offering a design protocol for tailoring functionalities of semiconductors at the nano-/microscale.

In-Plane Isotropic/Anisotropic 2D van der Waals Heterostructures for Future Devices

Mono- to few-layers of 2D semiconducting materials have uniquely inherent optical, electronic, and magnetic properties that make them ideal for probing fundamental scientific phenomena up to the 2D quantum limit and exploring their emerging technological applications. This Review focuses on the fundamental optoelectronic studies and potential applications of in-plane isotropic/anisotropic 2D semiconducting heterostructures. Strong light-matter interaction, reduced dimensionality, and dielectric screening in mono- to few-layers of 2D semiconducting materials result in strong many-body interactions, leading to the formation of robust quasiparticles such as excitons, trions, and biexcitons. An in-plane isotropic nature leads to the quasi-2D particles, whereas, an anisotropic nature leads to quasi-1D particles. Hence, in-plane isotropic/anisotropic 2D heterostructures lead to the formation of quasi-1D/2D particle systems allowing for the manipulation of high binding energy quasi-1D particle populations for use in a wide variety of applications. This Review emphasizes an exciting 1D-2D particles dynamic in such heterostructures and their potential for high-performance photoemitters and exciton-polariton lasers. Moreover, their scopes are also broadened in thermoelectricity, piezoelectricity, photostriction, energy storage, hydrogen evolution reactions, and chemical sensor fields. The unique in-plane isotropic/anisotropic 2D heterostructures may open the possibility of engineering smart devices in the nanodomain with complex opto-electromechanical functions.

Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials

With a large number of researches being conducted on two-dimensional (2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that provides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high-quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of different types of 2D heterostructures, followed by the discussions on present challenges and perspectives of further investigations.

Site-Selective and van der Waals Epitaxial Growth of Rhenium Disulfide on Graphene

The surface property of growth substrate imposes significant influence in the growth behaviors of 2D materials. Rhenium disulfide (ReS₂ ) is a new family of 2D transition metal dichalcogenides with unique distorted 1T crystal structure and thickness-independent direct bandgap. The role of growth substrate is more critical for ReS₂ owing to its weak interlayer coupling property, which leads to preferred growth along the out-of-plane direction while suppressing the uniform in-plane growth. Herein, graphene is introduced as the growth substrate for ReS₂ and the synthesis of graphene/ReS₂ vertical heterostructure is demonstrated via chemical vapor deposition. Compared with the rough surface of SiO₂ /Si substrate with dangling bonds which hinders the uniform growth of ReS₂ , the inert and smooth surface nature of graphene sheet provides a lower energy barrier for migration of the adatoms, thereby promoting the growth of ReS₂ on the graphene surface along the in-plane direction. Furthermore, patterning of the graphene/ReS₂ heterostructure is achieved by the selective growth of ReS₂ , which is attributed to the strong binding energy between sulfur atoms and graphene surface. The fundamental studies in the role of graphene as the growth template in the formation of van der Waals heterostructures provide better insights into the synthesis of 2D heterostructures.

New Pathway for Hot Electron Relaxation in Two-Dimensional Heterostructures

Two-dimensional (2D) heterostructures composed of transition-metal dichalcogenide atomic layers are the new frontier for novel optoelectronic and photovoltaic device applications. Some key properties that make these materials appealing, yet are not well understood, are ultrafast hole/electron dynamics, interlayer energy transfer and the formation of interlayer hot excitons. Here, we study photoexcited electron/hole dynamics in a representative heterostructure, the MoS₂/WSe₂ interface, which exhibits type II band alignment. Employing time-dependent density functional theory in the time domain, we observe ultrafast charge dynamics with lifetimes of tens to hundreds of femtoseconds. Most importantly, we report the discovery of an interfacial pathway in 2D heterostructures for the relaxation of photoexcited hot electrons through interlayer hopping, which is significantly faster than intralayer relaxation. This finding is of particular importance for understanding many experimentally observed photoinduced processes, including charge and energy transfer at an ultrafast time scale (<1 ps).

A Perspective on the Application of Spatially Resolved ARPES for 2D Materials

In this paper, a perspective on the application of Spatially- and Angle-Resolved PhotoEmission Spectroscopy (ARPES) for the study of two-dimensional (2D) materials is presented. ARPES allows the direct measurement of the electronic band structure of materials generating extremely useful insights into their electronic properties. The possibility to apply this technique to 2D materials is of paramount importance because these ultrathin layers are considered fundamental for future electronic, photonic and spintronic devices. In this review an overview of the technical aspects of spatially localized ARPES is given along with a description of the most advanced setups for laboratory and synchrotron-based equipment. This technique is sensitive to the lateral dimensions of the sample. Therefore, a discussion on the preparation methods of 2D material is presented. Some of the most interesting results obtained by ARPES are reported in three sections including: graphene, transition metal dichalcogenides (TMDCs) and 2D heterostructures. Graphene has played a key role in ARPES studies because it inspired the use of this technique with other 2D materials. TMDCs are presented for their peculiar transport, optical and spin properties. Finally, the section featuring heterostructures highlights a future direction for research into 2D material structures.

Determining the Optimized Interlayer Separation Distance in Vertical Stacked 2D WS2 :hBN:MoS2 Heterostructures for Exciton Energy Transfer

The 2D semiconductor monolayer transition metal dichalcogenides, WS₂ and MoS₂ , are grown by chemical vapor deposition (CVD) and assembled by sequential transfer into vertical layered heterostructures (VLHs). Insulating hBN, also produced by CVD, is utilized to control the separation between WS₂ and MoS₂ by adjusting the layer number, leading to fine-scale tuning of the interlayer interactions within the VLHs. The interlayer interactions are studied by photoluminescence (PL) spectroscopy and are demonstrated to be highly sensitive to the input excitation power. For thin hBN separators (one to two layers), the total PL emission switches from quenching to enhancement by increasing the laser power. Femtosecond broadband transient absorption measurements demonstrate that the increase in PL quantum yield results from Förster energy transfer from MoS₂ to WS₂ . The PL signal is further enhanced at cryogenic temperatures due to the suppressed nonradiative decay channels. It is shown that (4 ± 1) layers of hBN are optimum for obtaining PL enhancement in the VLHs. Increasing thickness beyond this causes the enhancement factor to diminish, with the WS₂ and MoS₂ then behaving as isolated noninteracting monolayers. These results indicate how controlling the exciton generation rate influences energy transfer and plays an important role in the properties of VLHs.

Robust Stacking-Independent Ultrafast Charge Transfer in MoS2/WS2 Bilayers

Van der Waals-coupled two-dimensional (2D) heterostructures have attracted great attention recently due to their high potential in the next-generation photodetectors and solar cells. The understanding of charge-transfer process between adjacent atomic layers is the key to design optimal devices as it directly determines the fundamental response speed and photon-electron conversion efficiency. However, general belief and theoretical studies have shown that the charge transfer behavior depends sensitively on interlayer configurations, which is difficult to control accurately, bringing great uncertainties in device designing. Here we investigate the ultrafast dynamics of interlayer charge transfer in a prototype heterostructure, the MoS₂/WS₂ bilayer with various stacking configurations, by optical two-color ultrafast pump-probe spectroscopy. Surprisingly, we found that the charge transfer is robust against varying interlayer twist angles and interlayer coupling strength, in time scale of ∼90 fs. Our observation, together with atomic-resolved transmission electron characterization and time-dependent density functional theory simulations, reveals that the robust ultrafast charge transfer is attributed to the heterogeneous interlayer stretching/sliding, which provides additional channels for efficient charge transfer previously unknown. Our results elucidate the origin of transfer rate robustness against interlayer stacking configurations in optical devices based on 2D heterostructures, facilitating their applications in ultrafast and high-efficient optoelectronic and photovoltaic devices in the near future.

High Quality Factor Graphene-Based Two-Dimensional Heterostructure Mechanical Resonator

Ultralight mechanical resonators based on low-dimensional materials are well suited as exceptional transducers of minuscule forces or mass changes. However, the low dimensionality also provides a challenge to minimize resistive losses and heating. Here, we report on a novel approach that aims to combine different two-dimensional (2D) materials to tackle this challenge. We fabricated a heterostructure mechanical resonator consisting of few layers of niobium diselenide (NbSe₂) encapsulated by two graphene sheets. The hybrid membrane shows high quality factors up to 245,000 at low temperatures, comparable to the best few-layer graphene mechanical resonators. In contrast to few-layer graphene resonators, the device shows reduced electrical losses attributed to the lower resistivity of the NbSe₂ layer. The peculiar low-temperature dependence of the intrinsic quality factor points to dissipation over two-level systems which in turn relax over the electronic system. Our high sensitivity readout is enabled by coupling the membrane to a superconducting cavity which allows for the integration of the hybrid mechanical resonator as a sensitive and low loss transducer in future quantum circuits.

Cation-Eutectic Transition via Sublattice Melting in CuInP2S6/In4/3P2S6 van der Waals Layered Crystals

Single crystals of the van der Waals layered ferrielectric material CuInP₂S₆ spontaneously phase separate when synthesized with Cu deficiency. Here we identify a route to form and tune intralayer heterostructures between the corresponding ferrielectric (CuInP₂S₆) and paraelectric (In_4/3P₂S₆) phases through control of chemical phase separation. We conclusively demonstrate that Cu-deficient Cu_1-xIn_1+x/3P₂S₆ forms a single phase at high temperature. We also identify the mechanism by which the phase separation proceeds upon cooling. Above 500 K both Cu⁺ and In³⁺ become mobile, while P₂S₆^4- anions maintain their structure. We therefore propose that this transition can be understood as eutectic melting on the cation sublattice. Such a model suggests that the transition temperature for the melting process is relatively low because it requires only a partial reorganization of the crystal lattice. As a result, varying the cooling rate through the phase transition controls the lateral extent of chemical domains over several decades in size. At the fastest cooling rate, the dimensional confinement of the ferrielectric CuInP₂S₆ phase to nanoscale dimensions suppresses ferrielectric ordering due to the intrinsic ferroelectric size effect. Intralayer heterostructures can be formed, destroyed, and re-formed by thermal cycling, thus enabling the possibility of finely tuned ferroic structures that can potentially be optimized for specific device architectures.

Multipurpose Black-Phosphorus/hBN Heterostructures

Black phosphorus (BP) has recently emerged as a promising semiconducting two-dimensional material. However, its viability is threatened by its instability in ambient conditions and by the significant decrease of its band gap in multilayers. We show that one could solve all the aforementioned problems by interfacing BP with hexagonal boron nitride (hBN). To this end, we simulate large, rotated hBN/BP interfaces using linear-scaling density functional theory. We predict that hBN-encapsulation preserves the main electronic properties of the BP monolayer, while hBN spacers can be used to counteract the band gap reduction in stacked BP. Finally, we propose a model for a tunneling field effect transistor (TFET) based on hBN-spaced BP bilayers. Such BP TFETs would sustain both low-power and fast-switching operations, including negative differential resistance behavior with peak-to-valley ratios of the same order of magnitude as those encountered in transition metal dichalcogenide TFETs.

High-Tc Layered Ferrielectric Crystals by Coherent Spinodal Decomposition

Research in the rapidly developing field of 2D electronic materials has thus far been focused on metallic and semiconducting materials. However, complementary dielectric materials such as nonlinear dielectrics are needed to enable realistic device architectures. Candidate materials require tunable dielectric properties and pathways for heterostructure assembly. Here we report on a family of cation-deficient transition metal thiophosphates whose unique chemistry makes them a viable prospect for these applications. In these materials, naturally occurring ferrielectric heterostructures composed of centrosymmetric In4/3P2S6 and ferrielectrically active CuInP2S6 are realized by controllable chemical phase separation in van der Waals bonded single crystals. CuInP2S6 by itself is a layered ferrielectric with a ferrielectric transition temperature (Tc) just over room temperature, which rapidly decreases with homogeneous doping. Surprisingly, in our composite materials, the ferrielectric Tc of the polar CuInP2S6 phase increases. This effect is enabled by unique spinodal decomposition that retains the overall van der Waals layered morphology of the crystal, but chemically separates CuInP2S6 and In4/3P2S6 within each layer. The average spatial periodicity of the distinct chemical phases can be finely controlled by altering the composition and/or synthesis conditions. One intriguing prospect for such layered spinodal alloys is large volume synthesis of 2D in-plane heterostructures with periodically alternating polar and nonpolar phases.

Two-Step Growth of Two-Dimensional WSe2/MoSe2 Heterostructures

Two dimensional (2D) materials have attracted great attention due to their unique properties and atomic thickness. Although various 2D materials have been successfully synthesized with different optical and electrical properties, a strategy for fabricating 2D heterostructures must be developed in order to construct more complicated devices for practical applications. Here we demonstrate for the first time a two-step chemical vapor deposition (CVD) method for growing transition-metal dichalcogenide (TMD) heterostructures, where MoSe2 was synthesized first and followed by an epitaxial growth of WSe2 on the edge and on the top surface of MoSe2. Compared to previously reported one-step growth methods, this two-step growth has the capability of spatial and size control of each 2D component, leading to much larger (up to 169 μm) heterostructure size, and cross-contamination can be effectively minimized. Furthermore, this two-step growth produces well-defined 2H and 3R stacking in the WSe2/MoSe2 bilayer regions and much sharper in-plane interfaces than the previously reported MoSe2/WSe2 heterojunctions obtained from one-step growth methods. The resultant heterostructures with WSe2/MoSe2 bilayer and the exposed MoSe2 monolayer display rectification characteristics of a p-n junction, as revealed by optoelectronic tests, and an internal quantum efficiency of 91% when functioning as a photodetector. A photovoltaic effect without any external gates was observed, showing incident photon to converted electron (IPCE) efficiencies of approximately 0.12%, providing application potential in electronics and energy harvesting.