Molecular Beam Epitaxy of Highly Crystalline MoSe2 on Hexagonal Boron Nitride

Boosting Monolayer Transition Metal Dichalcogenides Growth by Hydrogen-Free Ramping during Chemical Vapor Deposition

The controlled vapor-phase synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is essential for functional applications. While chemical vapor deposition (CVD) techniques have been successful for transition metal sulfides, extending these methods to selenides and tellurides often faces challenges due to uncertain roles of hydrogen (H2) in their synthesis. Using CVD growth of MoSe2 as an example, this study illustrates the role of a H2-free environment during temperature ramping in suppressing the reduction of MoO3, which promotes effective vaporization and selenization of the Mo precursor to form MoSe2 monolayers with excellent crystal quality. As-synthesized MoSe2 monolayer-based field-effect transistors show excellent carrier mobility of up to 20.9 cm2/(V·s) with an on-off ratio of 7 × 107. This approach can be extended to other TMDs, such as WSe2, MoTe2, and MoSe2/WSe2 in-plane heterostructures. Our work provides a rational and facile approach to reproducibly synthesize high-quality TMD monolayers, facilitating their translation from laboratory to manufacturing.

Growth of Single Crystalline 2D Materials beyond Graphene on Non-metallic Substrates

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

Molecular Beam Epitaxy Growth of Cadmium Telluride Structures on Hexagonal Boron Nitride

We investigate the feasibility of the epitaxial growth of a three-dimensional semiconductor on a two-dimensional substrate. In particular, we report for the first time the molecular beam epitaxy growth of cadmium telluride (CdTe) quantum wells on hexagonal boron nitride (hBN). The presence of the quantum wells is confirmed by photoluminescence measurements conducted at helium temperatures. Growth of the quantum wells on two-dimensional, almost perfectly flat hBN appears to be very different from growth on bulk substrates; in particular, it requires 70-100 °C lower temperatures.

Dispersing small Ru nanoparticles into boron nitride remodified by reduced graphene oxide for high-efficient electrocatalytic hydrogen evolution reaction

Ruthenium (Ru) electrocatalysts suffer from excessive aggregation during the hydrogen evolution reaction (HER), which hinders their practical application for hydrogen production. Hexagonal boron nitride (h-BN) is a potential carrier that could solve the above problem, but its wide band gap and low conductivity become obstacles. Herein, we provide a new, facile, low-cost, and effective strategy (killing two birds with one stone) to overcome the above issues. After modifying h-BN with reduced graphene oxide (rGO), a small amount of Ru nanoparticles (NPs) (2.2 %) are dispersed into BN with approximately uniform distribution and size control of Ru nanoparticles (∼3.85 nm). The strong synergy between Ru NPs and BN@C in the optimal Ru/BN@C (Ru wt.% = 2.22 %) electrocatalyst endows it an outstanding HER activity, with small HER overpotentials (η₁₀ = 32 mV, 35 mV) and low Tafel slopes (33.89 mV dec^-1, 37.66 mV dec^-1) in both 1 M KOH and 0.5 M H₂SO₄ media, respectively, along with good long-term stability for 50 h. Based on density functional theory (DFT) calculations, the addition of Ru to BN has been successful in creating fresh active sites for H*, with good possible adsorption/desorption ability (ΔG_H* = -0.24 eV) while preserving low water dissociation (ΔG_b = 0.46 eV) in an alkaline environment. As a result, the Ru/BN composite exhibits outstanding HER activity in both acidic and alkaline conditions. Furthermore, this study provides, for the first time, a template-free strategy to develop a good and low-cost supporter (BN) for dispersing other noble metals and the formation of highly efficient HER/OER electrocatalysts.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Two-dimensional devices and integration towards the silicon lines

Despite technical efforts and upgrades, advances in complementary metal-oxide-semiconductor circuits have become unsustainable in the face of inherent silicon limits. New materials are being sought to compensate for silicon deficiencies, and two-dimensional materials are considered promising candidates due to their atomically thin structures and exotic physical properties. However, a potentially applicable method for incorporating two-dimensional materials into silicon platforms remains to be illustrated. Here we try to bridge two-dimensional materials and silicon technology, from integrated devices to monolithic 'on-silicon' (silicon as the substrate) and 'with-silicon' (silicon as a functional component) circuits, and discuss the corresponding requirements for material synthesis, device design and circuitry integration. Finally, we summarize the role played by two-dimensional materials in the silicon-dominated semiconductor industry and suggest the way forward, as well as the technologies that are expected to become mainstream in the near future.

Application of Two-Dimensional Materials towards CMOS-Integrated Gas Sensors

During the last few decades, the microelectronics industry has actively been investigating the potential for the functional integration of semiconductor-based devices beyond digital logic and memory, which includes RF and analog circuits, biochips, and sensors, on the same chip. In the case of gas sensor integration, it is necessary that future devices can be manufactured using a fabrication technology which is also compatible with the processes applied to digital logic transistors. This will likely involve adopting the mature complementary metal oxide semiconductor (CMOS) fabrication technique or a technique which is compatible with CMOS due to the inherent low costs, scalability, and potential for mass production that this technology provides. While chemiresistive semiconductor metal oxide (SMO) gas sensors have been the principal semiconductor-based gas sensor technology investigated in the past, resulting in their eventual commercialization, they need high-temperature operation to provide sufficient energies for the surface chemical reactions essential for the molecular detection of gases in the ambient. Therefore, the integration of a microheater in a MEMS structure is a requirement, which can be quite complex. This is, therefore, undesirable and room temperature, or at least near-room temperature, solutions are readily being investigated and sought after. Room-temperature SMO operation has been achieved using UV illumination, but this further complicates CMOS integration. Recent studies suggest that two-dimensional (2D) materials may offer a solution to this problem since they have a high likelihood for integration with sophisticated CMOS fabrication while also providing a high sensitivity towards a plethora of gases of interest, even at room temperature. This review discusses many types of promising 2D materials which show high potential for integration as channel materials for digital logic field effect transistors (FETs) as well as chemiresistive and FET-based sensing films, due to the presence of a sufficiently wide band gap. This excludes graphene from this review, while recent achievements in gas sensing with graphene oxide, reduced graphene oxide, transition metal dichalcogenides (TMDs), phosphorene, and MXenes are examined.

Uniform nucleation and epitaxy of bilayer molybdenum disulfide on sapphire

Two-dimensional transition-metal dichalcogenides (TMDs) are of interest for beyond-silicon electronics^1,2. It has been suggested that bilayer TMDs, which combine good electrostatic control, smaller bandgap and higher mobility than monolayers, could potentially provide improvements in the energy-delay product of transistors^3-5. However, despite advances in the growth of monolayer TMDs^6-14, the controlled epitaxial growth of multilayers remains a challenge¹⁵. Here we report the uniform nucleation (>99%) of bilayer molybdenum disulfide (MoS₂) on c-plane sapphire. In particular, we engineer the atomic terrace height on c-plane sapphire to enable an edge-nucleation mechanism and the coalescence of MoS₂ domains into continuous, centimetre-scale films. Fabricated field-effect transistor (FET) devices based on bilayer MoS₂ channels show substantial improvements in mobility (up to 122.6 cm² V^-1 s^-1) and variation compared with FETs based on monolayer films. Furthermore, short-channel FETs exhibit an on-state current of 1.27 mA μm^-1, which exceeds the 2028 roadmap target for high-performance FETs¹⁶.

Direct Z-scheme WTe2/InSe van der Waals heterostructure for overall water splitting

WTe2/InSe is a direct Z-scheme vdW heterostructure for water splitting. The Te-vacancy can effectively lower the energy of the HER, and the overall water splitting can proceed spontaneously on the surface of the WTe2/InSe heterostructure when pH > 7.

MoSSe/Hf(Zr)S2 heterostructures used for efficient Z-scheme photocatalytic water-splitting

HfS2/SMoSe, HfS2/SeMoS, ZrS2/SMoSe, and ZrS2/SeMoS heterostructures are promising overall water-splitting photocatalysts.

Thermal Conductivity of Large-Area Polycrystalline MoSe2 Films Grown by Chemical Vapor Deposition

It is of great importance to understand the thermal properties of MoSe₂ films for electronic and optoelectronic applications. In this work, large-area polycrystalline MoSe₂ films are prepared using a low-cost, controllable, large-scale, and repeatable chemical vapor deposition method, which facilitates direct device fabrication. Raman spectra and X-ray diffraction patterns indicate a hexagonal (2H) crystal structure of the MoSe₂ film. Ellipsometric spectra analysis indicates that the optical band gap of the MoSe₂ film is estimated to be ∼1.23 eV. From the analysis of the temperature-dependent and laser-power-dependent Raman spectra, the thermal conductivity of the suspended MoSe₂ films is found to be ∼28.48 W/(m·K) at room temperature. The results can provide useful guidance for an effective thermal management of large-area polycrystalline MoSe₂-based electronic and optoelectronic devices.

Heteroepitaxial Growth of High Optical Quality, Wafer-Scale van der Waals Heterostrucutres

Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers although only for materials of sufficient crystalline quality and so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow the growth of large-scale, high-quality TMD monolayers but also allow the growth to be performed directly on large-scale epitaxial hBN. In this work, we address this problem and demonstrate that MoSe2 of high optical quality can be directly grown on epitaxial hBN on an entire 2 in. wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by metal organic vapor phase epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by molecular beam epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole 2 in. wafer with only ±0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures.

Janus PtXO (X = S, Se) monolayers: the visible light driven water splitting photocatalysts with high carrier mobilities

Triggered by the recent experimental synthesis of the Janus PtSSe monolayer, we use the first-principles calculations to predict two new Janus photocatalysts PtXO (X = S, Se), based on the systematic investigations of the structural stabilities, electronic structures, band alignments, catalytic activity and optical absorption. The two Janus structures are found to be mechanically, dynamically and thermodynamically stable, and have suitable band edge positions for the overall water splitting. Owing to the high electron mobility (up to 2164.95 cm² V^-1 s^-1) and large disparity between the electron and hole mobilities, together with the indirect band gaps and the intrinsic dipole induced built-in electric fields, the photogenerated electrons/holes can be efficiently separated in PtXO. Moreover, the S/Se vacancy can effectively lower the free energy difference of the HER, making the catalytic reactions occur spontaneously under the potentials of photoexcited electrons and holes. Large optical absorption coefficients (10⁵ cm^-1) are also confirmed in the visible light range, and the biaxial tensile strain can further enhance the optical absorption while maintaining the capability of the overall water splitting. Our results not only propose two new Janus materials by demonstrating the possibility of experimental realization, but also indicate that PtXO are peculiar candidates for photocatalytic water splitting.

Scanning Thermal Microscopy and Ballistic Phonon Transport in Lateral Spin Valves

Using scanning thermal microscopy, we have mapped the spatial distribution of temperatures in an operating nanoscale device formed from a magnetic injector, an Ag connecting wire, and a magnetic detector. An analytical model explained the thermal diffusion over the measured temperature range (2-300 K) and injector-detector separation (400-3000 nm). The characteristic diffusion lengths of the Peltier and Joule heat differ remarkably below 60 K, a fact that can be explained by the onset of ballistic phonon heat transfer in the substrate.

Novel Two-Dimensional Janus MoSiGeN4 and WSiGeN4 as Highly Efficient Photocatalysts for Spontaneous Overall Water Splitting

Searching for highly efficient and eco-friendly photocatalysts for water splitting is essential for renewable conversion and storage of inexhaustible solar energy but remains a great challenge. Herein, based on the new emerging two-dimensional (2D) material of MoSi₂N₄, we report novel Janus MoSiGeN₄ and WSiGeN₄ structures with excellent stabilities and great potentials in photocatalytic applications through first-principles calculations. Comprehensive studies show that MoSi₂N₄, MoSiGeN₄, and WSiGeN₄ exhibit semiconductor characteristics with an indirect gap, appropriate band gaps, and strong optical absorbance in the visible spectrum. Excitingly, by constructing Janus structures, an intrinsic electric field is realized that enhances the spatial separation and anisotropic migration of photoexcited electrons and holes. Further, this strategy can also alter the band alignment to provide an adequate photoexcited carrier driving force for water redox reactions. Moreover, the surface N vacancy can effectively lower the energy demand of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) so that the catalytic process can be self-sustained under the potential provided by the photocatalyst alone. Particularly, the overall water splitting can proceed simultaneously and spontaneously on the surface of MoSiGeN₄ and WSiGeN₄ when pH is 3 or ≥8, respectively. These explorations offer new prospects for the design of highly efficient photocatalysts.

Synthesis of large-area monolayer and few-layer MoSe2 continuous films by chemical vapor deposition without hydrogen assistance and formation mechanism

Two dimensional (2D) MoSe2 with a layered structure has attracted extensive research due to its excellent electronic and optical properties. The controlled synthesis of large-scale and high-quality MoSe2 is highly desirable but still remains challenging. Ambient pressure chemical vapor deposition (APCVD) is an excellent method for the synthesis of 2D materials but the inevitable use of hydrogen during the growth and the easy formation of cracks in the ultrathin films still need to be solved. In the present work, we reported the synthesis of large-area continuous MoSe2 films with different layers by the APCVD method without the assistance of hydrogen on SiO2/Si substrates just by raising the reaction temperature of Se. The synthesized continuous MoSe2 films can reach several centimeters, which can be seen clearly by naked eyes, and, more importantly, the size of the monolayer film can reach up to 3 mm. The morphology, structural characteristics, and optical properties of the synthesized MoSe2 films have been investigated, demonstrating good performance and high crystallinity of the films. Raman spectra give the empirical expression of the frequency difference between E2g1 and A1g dependence of the layer number (N = 1-10 L) for CVD grown MoSe2, which is useful in layer number identification. Further, the formation mechanism of the MoSe2 continuous film is of interest as a fundamental scientific problem and needs to be studied. We proposed the wing model, boundary layer theory, and diffusion theory to account quantitatively for the formation behavior of the MoSe2 film. The presented facile growth method and theoretical model are useful to synthesize other ultrathin transition metal dichalcogenide films and understand the formation behaviors of the systems.

Chlorine doping of MoSe2 flakes by ion implantation

The efficient integration of transition metal dichalcogenides (TMDs) into the current electronic device technology requires mastering the techniques of effective tuning of their optoelectronic properties. Specifically, controllable doping is essential. For conventional bulk semiconductors, ion implantation is the most developed method offering stable and tunable doping. In this work, we demonstrate n-type doping in MoSe₂ flakes realized by low-energy ion implantation of Cl⁺ ions followed by millisecond-range flash lamp annealing (FLA). We further show that FLA for 3 ms with a peak temperature of about 1000 °C is enough to recrystallize implanted MoSe₂. The Cl distribution in few-layer-thick MoSe₂ is measured by secondary ion mass spectrometry. An increase in the electron concentration with increasing Cl fluence is determined from the softening and red shift of the Raman-active A_1g phonon mode due to the Fano effect. The electrical measurements confirm the n-type doping of Cl-implanted MoSe₂. A comparison of the results of our density functional theory calculations and experimental temperature-dependent micro-Raman spectroscopy data indicates that Cl atoms are incorporated into the atomic network of MoSe₂ as substitutional donor impurities.

Electron beam triggered single-atom dynamics in two-dimensional materials

Controlling atomic structure and dynamics with single-atom precision is the ultimate goal in nanoscience and nanotechnology. Despite great successes being achieved by scanning tunneling microscopy (STM) over the past a few decades, fundamental limitations, such as ultralow temperature, and low throughput, significantly hinder the fabrication of a large array of atomically defined structures by STM. The advent of aberration correction in scanning transmission electron microscopy (STEM) revolutionized the field of nanomaterials characterization pushing the detection limit down to single-atom sensitivity. The sub-angstrom focused electron beam (e-beam) of STEM is capable of interacting with an individual atom, thereby it is the ideal platform to direct and control matter at the level of a single atom or a small cluster. In this article, we discuss the transfer of energy and momentum from the incident e-beam to atoms and their subsequent potential dynamics under different e-beam conditions in 2D materials, particularly transition metal dichalcogenides (TMDs). Next, we systematically discuss the e-beam triggered structural evolutions of atomic defects, line defects, grain boundaries, and stacking faults in a few representative 2D materials. Their formation mechanisms, kinetic paths, and practical applications are comprehensively discussed. We show that desired structural evolution or atom-by-atom assembly can be precisely manipulated by e-beam irradiation which could introduce intriguing functionalities to 2D materials. In particular, we highlight the recent progress on controlling single Si atom migration in real-time on monolayer graphene along an extended path with high throughput in automated STEM. These results unprecedentedly demonstrate that single-atom dynamics can be realized by an atomically focused e-beam. With the burgeoning of artificial intelligence and big data, we can expect that fully automated microscopes with real-time data analysis and feedback could readily design and fabricate large scale nanostructures with unique functionalities in the near future.

Electrostatically Enhanced Electron-Phonon Interaction in Monolayer 2H-MoSe2 Grown by Molecular Beam Epitaxy

The enhancement of electron-phonon interaction provides a reasonable explanation for gate-tunable phonon properties in some semiconductors where multiple inequivalent valleys are simultaneously occupied upon charge doping, especially in few-layer transition metal dichalcogenides (TMDs). In this work, we report var der Waals epitaxy of 2H-MoSe₂ by molecular beam epitaxy (MBE) and gate-tunable phonon properties in monolayer and bilayer MoSe₂. In monolayer MoSe₂, we find that out-of-plane phonon mode A_1g exhibits a strong softening and shifting toward lower wavenumbers at a high electron doping level, while in-plane phonon mode E_2g¹ remains unchanged. The softening and shifting of the out-of-plane phonon mode could be attributed to the increase of electron-phonon interaction and the simultaneous occupation of electrons in multiple inequivalent valleys. In bilayer MoSe₂, no corresponding changes of phonon modes are detected at the same doping level, which could originate from the occupation of electrons only in single valleys upon high electron doping. This study demonstrates electrostatically enhanced electron-phonon interaction in monolayer MoSe₂ and clarifies the relevance between occupation of multiple valleys and phonon properties by comparing Raman spectra of monolayer and bilayer MoSe₂ at different doping levels.

Strain robust spin gapless semiconductors/half-metals in transition metal embedded MoSe2monolayer

The realization of spin gapless semiconductor (SGS) and half-metal (HM) behavior in two-dimensional (2D) transition metal (TM) dichalcogenides is highly desirable for their applications in spintronic devices. Here, using density functional theory calculations, we demonstrate that Fe, Co, Ni substitutional impurities can not only induce magnetism in MoSe2monolayer, but also convert the semiconducting MoSe2to SGS/HM system. We also study the effects of mechanical strain on the electronic and magnetic properties of the doped monolayer. We show that for all TM impurities we considered, the system exhibits the robust SGS/HM behavior regardless of biaxial strain values. Moreover, it is found that the magnetic properties of TM-MoSe2can effectively be tuned under biaxial strain by controlling the spin polarization of the 3dorbitals of Fe, Co, Ni atoms. Our findings offer a new route to designing the SGS/HM properties and modulating magnetic characteristics of the TM-MoSe2system and may also facilitate the implementation of SGS/HM behavior and realization of spintronic devices based on other 2D materials.

Narrow Excitonic Lines and Large-Scale Homogeneity of Transition-Metal Dichalcogenide Monolayers Grown by Molecular Beam Epitaxy on Hexagonal Boron Nitride

Monolayer transition-metal dichalcogenides (TMDs) manifest exceptional optical properties related to narrow excitonic resonances. However, these properties have been so far explored only for structures produced by techniques inducing considerable large-scale inhomogeneity. In contrast, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of the interesting optical effects. Here, we report for the first time on the MBE-grown TMD exhibiting narrow and resolved spectral lines of neutral and charged exciton. Moreover, our material exhibits unprecedented high homogeneity of optical properties, with variation of the exciton energy as small as ±0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth is presented for MoSe₂ and includes the use of atomically flat hexagonal boron nitride substrate. This recipe opens a possibility of producing TMD heterostructures with optical quality, dimensions, and homogeneity required for optoelectronic applications.

Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy

The precisely controlled growth of transition metal dichalcogenide (TMDC) monolayers requires sensitive and nondestructive techniques to monitor the morphology and coverage in situ and in real time. In the current work, differential reflectance spectroscopy (DRS) was applied to monitor the molecular beam epitaxy (MBE) growth of atomically thin MoSe₂ layers on mica. The optical evolution exhibits an oscillation with monolayer periodicity, revealing a two-dimensional (2D) layer-by-layer growth of the MoSe₂ thin films. The observed sensitivity of DRS to the step density is associated to the modified electronic structures at the edges of TMDC monolayers. As DRS works in any transparent ambient, we speculate that it could be of great use for realizing precisely controlled growth of TMDC monolayers using not only MBE but also chemical vapor deposition (CVD).

Controllable growth of transition metal dichalcogenide multilayer flakes with kirigami structures

2D TMDCs with triangular-shaped and hexagonal-shaped kirigami structures are grown on amorphous SiO₂ substrates by chemical vapor deposition (CVD).

Spontaneous full photocatalytic water splitting on 2D MoSe2/SnSe2 and WSe2/SnSe2 vdW heterostructures

Spontaneous full photocatalytic water splitting into hydrogen and oxygen under visible light irradiation without the need for sacrificial agents is a challenging task, because suitable band gaps, low overpotentials for both half-reactions and spatially-separated catalytic sites should be fulfilled simultaneously in a photocatalytic system. Here, we propose a promising strategy to achieve this goal by constructing van der Waals (vdW) heterostructures of two-dimensional (2D) materials. Using first-principles calculations, we predict two promising photocatalysts, MoSe2/SnSe2 and WSe2/SnSe2 heterostructures, with the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) taking place separately on the MoSe2 (WSe2) and SnSe2 layers. More excitingly, the Se-vacancy of the MoSe2 (WSe2) monolayer effectively lowers the HER overpotential, making the catalytic reactions occur spontaneously under the potentials solely provided by the photo-generated electrons and holes in pure water. The unique band alignment of these hetero-structured photocatalysts leads to high solar-to-hydrogen (STH) energy conversion efficiencies up to 10.5%, which is quite promising for commercial applications. This work opens up an avenue for the design of highly-efficient photocatalysts for full water splitting.

Molecular Beam Epitaxy Scalable Growth of Wafer-Scale Continuous Semiconducting Monolayer MoTe2 on Inert Amorphous Dielectrics

Monolayer MoTe₂ , with the narrowest direct bandgap of ≈1.1 eV among Mo- and W-based transition metal dichalcogenides, has attracted increasing attention as a promising candidate for applications in novel near-infrared electronics and optoelectronics. Realizing 2D lateral growth is an essential prerequisite for uniform thickness and property control over the large scale, while it is not successful yet. Here, layer-by-layer growth of 2 in. wafer-scale continuous monolayer 2H-MoTe₂ films on inert SiO₂ dielectrics by molecular beam epitaxy is reported. A single-step Mo-flux controlled nucleation and growth process is developed to suppress island growth. Atomically flat 2H-MoTe₂ with 100% monolayer coverage is successfully grown on inert 2 in. SiO₂ /Si wafer, which exhibits highly uniform in-plane structural continuity and excellent phonon-limited carrier transport behavior. The dynamics-controlled growth recipe is also extended to fabricate continuous monolayer 2H-MoTe₂ on atomic-layer-deposited Al₂ O₃ dielectric. With the breakthrough in growth of wafer-scale continuous 2H-MoTe₂ monolayers on device compatible dielectrics, batch fabrication of high-mobility monolayer 2H-MoTe₂ field-effect transistors and the three-level integration of vertically stacked monolayer 2H-MoTe₂ transistor arrays for 3D circuitry are successfully demonstrated. This work provides novel insights into the scalable synthesis of monolayer 2H-MoTe₂ films on universal substrates and paves the way for the ultimate miniaturization of electronics.

Healing of Planar Defects in 2D Materials via Grain Boundary Sliding

Understanding the mechanisms and kinetics of defect annihilations, particularly at the atomic scale, is important for the preparation of high-quality crystals for realizing the full potential of 2D transition metal dichalcogenides (TMDCs) in electronics and quantum photonics. Herein, by performing in situ annealing experiments in an atomic resolution scanning transmission electron microscope, it is found that stacking faults and rotational disorders in multilayered 2D crystals can be healed by grain boundary (GB) sliding, which works like a "wiper blade" to correct all metastable phases into thermodynamically stable phases along its trace. The driving force for GB sliding is the gain in interlayer binding energy as the more stable phase grows at the expanse of the metastable ones. Density functional theory calculations show that the correction of 2D stacking faults is triggered by the ejection of Mo atoms in mirror twin boundaries, followed by the collective migrations of 1D GB. The study highlights the role of the often-neglected interlayer interactions for defect repair in 2D materials and shows that exploiting these interactions has significant potential for obtaining large-scale defect-free 2D films.

Molecular-Beam Epitaxy of Two-Dimensional In2Se3 and Its Giant Electroresistance Switching in Ferroresistive Memory Junction

Ferroelectric thin film has attracted great interest for nonvolatile memory applications and can be used in either ferroelectric Schottky diodes or ferroelectric tunneling junctions due to its promise of fast switching speed, high on-to-off ratio, and nondestructive readout. Two-dimensional α-phase indium selenide (In₂Se₃), which has a modest band gap and robust ferroelectric properties stabilized by dipole locking, is an excellent candidate for multidirectional piezoelectric and switchable photodiode applications. However, the large-scale synthesis of this material is still elusive, and its performance as a ferroresistive memory junction is rarely reported. Here, we report the low-temperature molecular-beam epitaxy (MBE) of large-area monolayer α-In₂Se₃ on graphene and demonstrate the use of α-In₂Se₃ on graphene in ferroelectric Schottky diode junctions by employing high-work-function gold as the top electrode. The polarization-modulated Schottky barrier formed at the interface exhibits a giant electroresistance ratio of 3.9 × 10⁶ with a readout current density of >12 A/cm², which is more than 200% higher than the state-of-the-art technology. Our MBE growth method allows a high-quality ultrathin film of In₂Se₃ to be heteroepitaxially grown on graphene, thereby simplifying the fabrication of high-performance 2D ferroelectric junctions for ferroresistive memory applications.