Synthetic Engineering of Morphology and Electronic Band Gap in Lateral Heterostructures of Monolayer Transition Metal Dichalcogenides

Synthesis and Modulation of Low-Dimensional Transition Metal Chalcogenide Materials via Atomic Substitution

In recent years, low-dimensional transition metal chalcogenide (TMC) materials have garnered growing research attention due to their superior electronic, optical, and catalytic properties compared to their bulk counterparts. The controllable synthesis and manipulation of these materials are crucial for tailoring their properties and unlocking their full potential in various applications. In this context, the atomic substitution method has emerged as a favorable approach. It involves the replacement of specific atoms within TMC structures with other elements and possesses the capability to regulate the compositions finely, crystal structures, and inherent properties of the resulting materials. In this review, we present a comprehensive overview on various strategies of atomic substitution employed in the synthesis of zero-dimensional, one-dimensional and two-dimensional TMC materials. The effects of substituting elements, substitution ratios, and substitution positions on the structures and morphologies of resulting material are discussed. The enhanced electrocatalytic performance and photovoltaic properties of the obtained materials are also provided, emphasizing the role of atomic substitution in achieving these advancements. Finally, challenges and future prospects in the field of atomic substitution for fabricating low-dimensional TMC materials are summarized.

Patterned growth of two-dimensional atomic layer semiconductors

Transition metal dichalcogenides (TMDCs), which are representative of two-dimensional (2D) semiconductors, have attracted tremendous attention over the last two decades. TMDCs are regarded as potential candidates in modern nano- and optoelectronic applications due to their unique crystal structures and outstanding electronic and optoelectronic properties. For practical use, 2D semiconductors need to be fabricated with diverse morphologies for integration into electronic devices and to perform different functionalities. Controlled patterning synthesis with programmable geometries is therefore highly desired. We review state-of-the-art strategies for the patterned growth of atomic layer TMDCs and their heterostructures, including additive manufacturing and subtractive manufacturing for patterning single TMDC materials and the introduction of other low-dimensional nanomaterials as growth templates or hetero-atoms for element conversion in patterning TMDC heterostructures. The optoelectronic and electronic applications of the as-grown monolayer TMDC patterns are introduced. Future challenges and the prospects for the patterned growth of 2D semiconductors are discussed based on present achievements.

Bioheterojunction-Engineered Polyetheretherketone Implants With Diabetic Infectious Micromilieu Twin-Engine Powered Disinfection for Boosted Osteogenicity

Diabetic infectious micromilieu (DIM) leads to a critical failure rate of osseointegration by virtue of two main peculiarities: high levels of topical glucose and inevitable infection. To tackle the daunting issue, a bioheterojunction-engineered orthopedic polyetheretherketone (PEEK) implant consisting of copper sulfide/graphene oxide (CuS/GO) bioheterojunctions (bioHJs) and glucose oxidase (GOx) is conceived and developed for DIM enhanced disinfection and boosted osseointegration. Under hyperglycemic micromilieu, GOx can convert surrounding glucose into hydrogen peroxide (H₂ O₂ ). Then, upon infectious micromilieu, the bioHJs enable the catalyzation of H₂ O₂ to highly germicidal hydroxyl radical (·OH). As a result, the engineered implants massacre pathogenic bacteria through DIM twin-engine powered photo-chemodynamic therapy in vitro and in vivo. In addition, the engineered implants considerably facilitate cell viability and osteogenic activity of osteoblasts under a hyperglycemic microenvironment via synergistic induction of copper ions (Cu²⁺ ) and GO. In vivo studies using bone defect models of diabetic rats at 4 and 8 weeks further authenticate that bioHJ-engineering PEEK implants substantially elevate their osseointegration through biofilm elimination and vascularization, as well as macrophage reprogramming. Altogether, the present study puts forward a tactic that arms orthopedic implants with DIM twin-engine powered antibacterial and formidable osteogenic capacities for diabetic stalled osseointegration.

Electronic property modulation in two-dimensional lateral superlattices of monolayer transition metal dichalcogenides

Fabricating lateral heterostructures (HSs) and superlattices (SLs) provides a unique degree of freedom for modulating the physical properties of two-dimensional (2D) materials by varying the chemical component, geometric size and interface structure in the ultra-thin atomic thickness limit. While a variety of 2D lateral HSs/SLs have been synthesized, especially for transition metal dichalcogenides (TMDs), how such structures affect quantitatively the physical properties of 2D materials has not yet been established. We herein explore electronic property modulation in 2D lateral SLs of monolayer TMDs through first-principles high-throughput calculations. The dependence of the electronic structure, bandgap, carrier effective masses, charge density overlap on chemical components, interface type, and sub-lattice size of lateral TMD-SLs are investigated. We find that by comparison with their random alloy counterparts, the lateral TMD-SLs exhibit generally type-II band alignment, a wider range of bandgap tunability, larger carrier effective masses, and stronger electron-hole charge separation tendency. The bandgap variation with a sub-lattice size shows larger bowing parameters for the SLs with heterogeneous anions, by comparison with the homogeneous anion cases. A similar behavior is observed for the SLs with an armchair-type interface, by comparison with the zigzag-type interface cases. Further analyses reveal that the underlying physical mechanism can be attributed to the synergistic interplay among the band offset of sub-lattices, quantum confinement effect, and existing internal strain.

Strategies for Controlled Growth of Transition Metal Dichalcogenides by Chemical Vapor Deposition for Integrated Electronics

In recent years, transition metal dichalcogenide (TMD)-based electronics have experienced a prosperous stage of development, and some considerable applications include field-effect transistors, photodetectors, and light-emitting diodes. Chemical vapor deposition (CVD), a typical bottom-up approach for preparing 2D materials, is widely used to synthesize large-area 2D TMD films and is a promising method for mass production to implement them for practical applications. In this review, we investigate recent progress in controlled CVD growth of 2D TMDs, aiming for controlled nucleation and orientation, using various CVD strategies such as choice of precursors or substrates, process optimization, and system engineering. We then survey different patterning methods, such as surface patterning, metal precursor patterning, and postgrowth sulfurization/selenization/tellurization, to mass produce heterostructures for device applications. With these strategies, various well-designed architectures, such as wafer-scale single crystals, vertical and lateral heterostructures, patterned structures, and arrays, are achieved. In addition, we further discuss various electronics made from CVD-grown TMDs to demonstrate the diverse application scenarios. Finally, perspectives regarding the current challenges of controlled CVD growth of 2D TMDs are also suggested.

Band Alignment Engineering by Twist Angle and Composition Modulation for Heterobilayer

Atomically thin monolayer semiconducting transition metal dichalcogenides (TMDs), exhibiting direct band gap and strong light-matter interaction, are promising for optoelectronic devices. However, an efficient band alignment engineering method is required to further broaden their practical applications as versatile optoelectronics. In this work, the band alignment of two vertically stacked monolayer TMDs using the chemical vapor deposition (CVD) method is effectively tuned by two strategies: 1) formulating the compositions of MoS_2(1-x) Se_2x alloys, and 2) varying the twist angles of the stacked heterobilayer structures. Photoluminescence (PL) results combined with density functional theory (DFT) calculation show that by changing the alloy composition, a continuously tunable band alignment and a transition of type II-type I-type II band alignment of TMD heterobilayer is achieved. Moreover, only at moderate (10°-50°) twist angles, a PL enhancement of 28%-110% caused by the type I alignment is observed, indicating that the twist angle is coupled with the global band structure of heterobilayer. A heterojunction device made with MoS_0.76 Se_1.24 /WS₂ of 14° displays a significantly high photoresponsivity (55.9 A W^-1 ), large detectivity (1.07 × 10¹⁰ Jones), and high external quantum efficiency (135%). These findings provide engineering tools for heterostructure design for their application in optoelectronic devices.

Lateral layered semiconductor multijunctions for novel electronic devices

Layered semiconductors, represented by transition metal dichalcogenides, have attached extensive attention due to their unique and tunable electrical and optical properties. In particular, lateral layered semiconductor multijunctions, including homojunctions, heterojunctions, hybrid junctions and superlattices, present a totally new degree of freedom in research on electronic devices beyond traditional materials and their structures, providing unique opportunities for the development of new structures and operation principle-based high performance devices. However, the advances in this field are limited by the precise synthesis of high-quality junctions and greatly hampered by ambiguous device performance limits. Herein, we review the recent key breakthroughs in the design, synthesis, electronic structure and property modulation of lateral semiconductor multijunctions and focus on their application-specific devices. Specifically, the synthesis methods based on different principles, such as chemical and external source-induced methods, are introduced stepwise for the controllable fabrication of semiconductor multijunctions as the basics of device application. Subsequently, their structure and property modulation are discussed, including control of their electronic structure, exciton dynamics and optical properties before the fabrication of lateral layered semiconductor multijunction devices. Precise property control will potentially result in outstanding device performances, including high-quality diodes and FETs, scalable logic and analog circuits, highly efficient optoelectronic devices, and unique electrochemical devices. Lastly, we focus on several of the most essential but unresolved debates in this field, such as the true advantages of few-layer vs. monolayer multijunctions, how sharp the interface should be for specific functional devices, and the superiority of lateral multijunctions over vertical multijunctions, highlighting the next-phase strategy to enhance the performance potential of lateral multijunction devices.

Incommensurate transition-metal dichalcogenides via mechanochemical reshuffling of binary precursors

A new family of heterostructured transition-metal dichalcogenides (TMDCs) with incommensurate ("misfit") spatial arrangements of well-defined layers was prepared from structurally dissimilar single-phase 2H-MoS₂ and 1T-HfS₂ materials. The experimentally observed heterostructuring is energetically favorable over the formation of homogeneous multi-principle element dichalcogenides observed in related dichalcogenide systems of Mo, W, and Ta. The resulting three-dimensional (3D) heterostructures show semiconducting behavior with an indirect band gap around 1 eV, agreeing with values predicted from density functional theory. Results of this joint experimental and theoretical study open new avenues for generating unexplored metal-dichalcogenide heteroassemblies with incommensurate structures and tunable physical properties.

Two-Dimensional Lateral Heterostructures Made by Selective Reaction on a Patterned Monolayer MoS2 Matrix

Two-dimensional (2D) heterostructures have attracted widespread attention for their promising prospects in the fields of electronics and optoelectronics. However, in order to truly realize 2D-material-based integrated circuits, precisely controllable fabrication of 2D heterostructures is crucial and urgently needed. Here, we demonstrate an ex situ growth method of MoSe₂/MoS₂ lateral heterostructures by selective selenization of a laser-scanned, ultrathin oxidized region (MoO_x) on a monolayer MoS₂ matrix. In our method, monolayer MoS₂ is scanned by a laser with a pre-designed pattern, where the laser-scanned MoS₂ is totally oxidized into MoO_x. The oxidized region is then selenized in a furnace, while the unoxidized MoS₂ region remains unchanged, delivering a MoSe₂/MoS₂ heterostructure. Unlike in situ laser direct growth methods, our method separates the laser-scanned process from the selenization process, which avoids the long time of point-by-point selenization of MoS₂ by laser, making the efficiency of the synthesis greatly improved. The formation process of the heterostructure is studied by Raman spectroscopy and Auger electron spectroscopy. This simple and controllable approach to lateral heterostructures with desired patterns paves the way for fast and mass integration of devices based on 2D heterostructures.

Perfect absorption for monolayer transition-metal dichalcogenides by critical coupling

Due to the inherent atomical size, transition-metal dichalcogenides (TMDCs) have a weak coupling with free-space light and exhibit poor optical absorption, which restricts some practical applications. Exploring an effective way to boost their absorption is important and highly desired. In this work, based on the temporal coupled-mode theory, a generic guide to approach 100% absorption of an absorber is presented. From such a theory, the perfect absorption of TMDCs integrated with dielectric photonic structure is both theoretically and numerically demonstrated. This proposed structure has advantageous tunability at the wavelengths of interest by adjusting structure parameters. Moreover, the angular dependence of the light absorption of such a structure for different polarizations is also investigated. The present work of boosting light absorption would be particularly favorable for applications in advanced photodetectors and modulators.

Topochemical synthesis of low-dimensional nanomaterials

Over the past several decades, nanomaterials have been extensively studied owing to having a series of unique physical and chemical properties that exceed those of conventional bulk materials. Researchers have developed a lot of strategies for the synthesis of low-dimensional nanomaterials. Among them, topochemical synthesis has attracted increasing attention because it can provide more new nanomaterials by improving and upgrading inexpensive and accessible nanomaterials. In this review, we summarize and analyze many existing topochemical synthesis methods, including selective etching, liquid phase reactions, high-temperature atmosphere reactions, electrochemically assisted methods, etc. The future direction of topochemical synthesis is also proposed.