Sub‐Millimeter‐Scale Monolayer p‐Type H‐Phase VS2

2D H‐phase vanadium disulfide (VS₂) is expected to exhibit tunable semiconductor properties as compared with its metallic T‐phase structure, and thus is of promise for future electronic applications. However, to date such 2D H‐phase VS₂ nanostructures have not been realized in experiment likely due to the polymorphs of vanadium sulfides and thermodynamic instability of H‐phase VS₂. Preparation of H‐phase VS₂ monolayer with lateral size up to 250 µm, as a new member in the 2D transition metal dichalcogenides (TMDs) family, is reported. A unique growth environment is built by introducing the molten salt‐mediated precursor system as well as the epitaxial mica growth platform, which successfully overcomes the aforementioned growth challenges and enables the evolution of 2D H‐phase structure of VS₂. The honeycomb‐like structure of H‐phase VS₂ with broken inversion symmetry is confirmed by spherical aberration‐corrected scanning transmission electron microscopy and second harmonic generation characterization. The phase structure is found to be ultra‐stable up to 500 K. The field‐effect device study further demonstrates the p‐type semiconducting nature of the 2D H‐phase VS₂. The study introduces a new phase‐stable 2D TMDs materials with potential features for future electronic devices.     

Tunable Room‐Temperature Synthesis of ReS2 Bicatalyst on 3D‐ and 2D‐Printed Electrodes for Photo‐ and Electrochemical Energy Applications

The advancement in 3D‐printing technologies conveniently offers boundless opportunities for the customization of a practical substrate or electrode for diverse functionalities. ReS₂ is an attractive transition metal dichalcogenide (TMD), showing strong photoelectrochemical activities. Two advanced systems are merged for the next step in electrochemistry—the limits of the prevailing synthesis techniques of TMDs operating at high temperature or low pressure, which are not compatible with 3D‐printed polymer electrodes that can withstand only comparatively low temperatures, are overcome. A unique NH₄ReS₄ precursor is separately prepared to conduct subsequent ReS₂ electrodeposition at room temperature on 3D‐printed carbon and 2D‐printed carbon electrodes. The deposited ReS₂ is investigated as a dual‐functional electro‐ and photocatalyst in hydrogen evolution reaction and photoelectrochemical oxidation of water. Moreover, the electrodeposition conditions can be adjusted to optimize the catalytic activities. These encouraging outcomes demonstrate the simplicity yet versatility of TMDs based on electrodeposition technique on a rationally designed conductive platform, which creates numerous possibilities for other TMDs and on other low‐temperature substrates for electrochemical energy devices.     

Controlled Vapor Growth and Nonlinear Optical Applications of Large‐Area 3R Phase WS2 and WSe2 Atomic Layers

2D layered 3‐rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, a direct growth of large‐area WS₂ and WSe₂ atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route is reported. Large‐area triangular 3R phase layers are synthesized at a lower deposition temperature. Steady state and time‐resolved photoluminescence spectroscopy and Raman spectroscopy are used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarization‐resolved SHG, a uniform polarization preference is observed in bilayer and trilayer 3R phase WS₂, which could be a benefit for practical applications. The results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to promising nonlinear optical devices.     

Role of Charge Density Wave in Monatomic Assembly in Transition Metal Dichalcogenides

The charge density wave (CDW) in transition metal dichalcogenides (TMDs) has drawn tremendous interest due to its potential for tailoring their surface electronic and chemical properties. Due to technical challenges, however, how the CDW could modulate the chemical behavior of TMDs is still not clear. Here, this work presents a study of applying the CDW of NbTe₂, with a high transition temperature above room temperature, to generate the assembling adsorption of Sn adatoms on the surface. It is shown that highly ordered monatomic Sn adatoms with a quasi‐1D structure can be obtained under regulation by the single‐axis CDW of the substrate. In addition, the CDW modulated superlattices could in turn change the surface electronic properties from semimetallic to metallic. These results demonstrate an effective approach for tuning the surface chemical properties of TMDs by their CDWs, which could be applied in exploring them for various practical applications, such as heterogeneous catalysis, epitaxial growth of low‐dimensional materials, and future nanoelectronics.     

Dual Functionalization of Liquid‐Exfoliated Semiconducting 2H‐MoS2 with Lanthanide Complexes Bearing Magnetic and Luminescence Properties

Liquid exfoliated, atomically thin semiconducting transition metal dichalcogenides (TMDs), as inorganic equivalents of graphene, have attracted great interest due to their distinctive physical, optoelectronic, and chemical properties. Functionalization of 2D TMDs brings new prospects for applications in optoelectronics, quantum technologies, catalysis, and medicine. In this report, dual functionalization of 2D semiconducting 2H‐MoS₂ nanosheets through simultaneous incorporation of magnetic and luminescent properties is demonstrated. A facile method is proposed for tuning the properties of the TDM semiconductors and accessing multimodal platforms, consisting in covalent grafting of lanthanide complexes onto the surface of 2D TMDs. Dual functionalization of liquid‐exfoliated MoS₂ nanosheets is demonstrated simultaneously with both europium (III) and gadolinium (III) complexes to form a colloidally stable luminescent (with millisecond lifetimes) and paramagnetic MoS₂‐based nanohybrid material. This work is the first example of transition metal dichalcogenide nanosheets functionalized with preformed lanthanide complexes. These findings open new prospects for covalent functionalization of TMDs with molecular species bearing specific functionalities as a means to tune the optoelectronic properties of the semiconductors, in order to create advanced materials and devices with a wide range of functionalities.     

3D Visible-Light-Driven Plasmonic Oxide Frameworks Deviated from Liquid Metal Nanodroplets

Eutectic gallium-indium (EGaIn) liquid metal droplets have been considered as a suitable platform for producing customized 3D composites with functional nanomaterials owing to their soft and highly reductive surface. Herein, the synthesis of a 3D plasmonic oxide framework (POF) is reported by incorporating the ultra-thin angstrom-scale-porous hexagonal molybdenum oxide (h-MoO₃) onto the spherical EGaIn nanodroplets through ultrasonication. Simultaneously, a large number of oxygen vacancies form in h-MoO₃, boosting its free charge carrier concentration and therefore generating a broad surface plasmon resonance across the whole visible light spectrum. The plasmonic chemical sensing properties of the POF is investigated by the surface-enhanced Raman scattering detection of rhodamine 6G (R6G) at 532 nm, in which the minimum detectable concentration is 10⁻⁸ m and the enhancement factor reached up to 6.14 × 10⁶. The extended optical absorption of the POF also allowed the efficient degradation of the R6G dye under the excitation of ultraviolet-filtered simulated solar light. Furthermore, the POF exhibits remarkable photocurrent responses towards the entire visible light region with the maximum response of ≈ 1588 A W⁻¹ at 455 nm. This work demonstrates the great potential of the liquid metal-based POFs for high-performance sensing, catalytic, and optoelectronic devices.     

Atomic Insight into Thermolysis‐Driven Growth of 2D MoS2

Understanding and controlling the transformations of transition metal dichalcogenides (TMDs) from amorphous precursors into two‐dimensional (2D) materials is important for guiding synthesis, directing fabrication, and tailoring functional properties. Here, the combined effects of thermal energy and electron beam irradiation are explored on the structural evolution of 2D MoS₂ flakes through the thermal decomposition of a (NH₄)₂MoS₄ precursor inside an ultrahigh vacuum (10^?9 Torr) scanning transmission electron microscope (STEM). The influence of reaction temperature, growth substrate, and the initial precursor morphology on the resulting 2D MoS₂ flake morphology, edge structures, and point defects are explored. Although thermal decomposition occurs extremely fast at elevated temperatures and is difficult to capture using current STEM techniques, electron beam irradiation can induce local transformations at lower temperatures, enabling direct observation and interpretation of critical growth steps including oriented attachment and transition from single‐ to multilayer structures at atomic resolution. An increase in the number of layers of the MoS₂ flakes from island growth is investigated using electron beam irradiation. These findings provide insight into the growth mechanisms and factors that control the synthesis of few‐layer MoS₂ flakes through thermolysis and toward the prospect of atomically precise control and growth of 2D TMDs.     

Back Contact Plasma Treatment Enables 14.5% Efficient Solution-Processed CuIn(S,Se)2 Solar Cells

Solution-processed chalcopyrite thin film solar cells have made great progress but still lag behind the vacuum-based ones due to inferior absorber quality and non-ideal interfaces. Here, plasma treatment of the molybdenum (Mo) substrate is reported to modify the back interface and improve the efficiency of solution-processed copper indium sulfoselenide (CuIn(S,Se)₂, CISSe) solar cells. The results show plasma treatment oxides the surface Mo/MoO₂ to high oxidation oxides MoO_3-x (0 ≤ x < 1). The higher surface polarity greatly improves the wettability of the substrate to the precursor solution and results in formation of more compact and uniform precursor film, which grows to void-free larger grains absorber film with better adhesion to the substrate. In addition, the much higher chemical stability of MoO_3-x than Mo significantly reduces the thickness of high resistive MoSe₂ layer. The improved absorber quality and back interface property decreases charge carrier recombination in the absorber and back contact interface, resulting in 14.53% efficiency solution-processed CISSe solar cell, which is improved by 13% compared to device without plasma treatment.     

Towards Sub‐Microscale Liquid Metal Patterns: Cascade Phase Change Mediated Pick‐n‐Place Transfer of Liquid Metals Printed and Stretched over a Flexible Substrate

Eutectic gallium indium (EGaIn) is actively investigated toward wearable and stretchable electronic devices due to the high fluidity, high electrical conductivity, and low toxicity. However, high surface tension along with spontaneous oxidation makes fine patterning below ≈10 µm challenging. In this paper, a novel manufacturing technique that enables EGaIn patterns of single‐digit micrometer widths on planar elastomeric substrates is presented. First, a custom direct printing setup is constructed for continuous and uniform printing of EGaIn by feedback control of the distance between the dispensing needle and the substrate. With this setup, a 120 µm wide linear pattern is printed on the Ecoflex, a stretchable elastomer. Then, the initial printed line is stretched, frozen with deionized water, and transferred to an unstretched Ecoflex substrate. Upon gentle heating after the pick‐n‐place of the EGaIn line frozen with deionized water, only the stretched EGaIn line is left on the new Ecoflex substrate. The aforementioned pick‐n‐place transfer of the stretched EGaIn frozen with water is cascaded multiple times until a target width is obtained. Finally, a 2 µm wide linear pattern, 60‐fold reduction with respect to the initial dimension, is acquired. For practical applications, strain and tactile sensors are demonstrated by width‐reduced EGaIn patterns.     

Space‐Confined Chemical Vapor Deposition Synthesis of Ultrathin HfS2 Flakes for Optoelectronic Application

Due to the predicted excellent electronic properties superior to group VIB (Mo and W) transition metal dichalcogenides (TMDs), group IVB TMDs have enormous potential in nanoelectronics. Here, the synthesis of ultrathin HfS₂ flakes via space‐confined chemical vapor deposition, realized by an inner quartz tube, is demonstrated. Moreover, the effect of key growth parameters including the dimensions of confined space and deposition temperature on the growth behavior of products is systematically studied. Typical as‐synthesized HfS₂ is a hexagonal‐like flake with a smallest thickness of ≈1.2 nm (bilayer) and an edge size of ≈5 µm. The photodetector based on as‐synthesized HfS₂ flakes demonstrates excellent optoelectronic performance with a fast photoresponse time (55 ms), which is attributed to the high‐quality crystal structure obtained at a high deposition temperature and the ultraclean interface between HfS₂ and the mica substrate. With such properties HfS₂ holds great potential for optoelectronics applications.     

Sulfur‐Mastery: Precise Synthesis of 2D Transition Metal Dichalcogenides

Chemical vapor deposition (CVD) has been developed as the most promising method for the growth of transition metal dichalcogenides (TMDs). In this work, the key factor determining the growth of TMDs is ascertained. A straightforward method is devised to directly achieve a holistic control of thickness, shape, and size of WS₂ flakes via a single parameter control, namely, the status of the S‐precursor. The thickness‐dependent growth of WS₂ flakes from mono‐ to quad‐layers is achieved by precise control of the feeding rate of elemental S‐precursor. Moreover, the explicit control over amount and exposure time of S‐precursor determines the most optimum combination of these parameters to tune the shape of the crystals from triangular to hexagonal with appropriate size. Hence, the experimental findings provide a promising strategy to engineer the growth evolution of WS₂ atomic layers by fine tuning of the sulfur supply, paving a pathway to scalable electronic and photonic devices.     

Surface‐Shielding Nanostructures Derived from Self‐Assembled Block Copolymers Enable Reliable Plasma Doping for Few‐Layer Transition Metal Dichalcogenides

Precise modulation of electrical and optical properties of 2D transition metal dichalcogenides (TMDs) is required for their application to high‐performance devices. Although conventional plasma‐based doping methods have provided excellent controllability and reproducibility for bulk or relatively thick TMDs, the application of plasma doping for ultrathin few‐layer TMDs has been hindered by serious degradation of their properties. Herein, a reliable and universal doping route is reported for few‐layer TMDs by employing surface‐shielding nanostructures during a plasma‐doping process. It is shown that the surface‐protection oxidized polydimethylsiloxane nanostructures obtained from the sub‐20 nm self‐assembly of Si‐containing block copolymers can preserve the integrity of 2D TMDs and maintain high mobility while affording extensive control over the doping level. For example, the self‐assembled nanostructures form periodically arranged plasma‐blocking and plasma‐accepting nanoscale regions for realizing modulated plasma doping on few‐layer MoS₂, controlling the n‐doping level of few‐layer MoS₂ from 1.9 × 10¹¹ cm^?2 to 8.1 × 10¹¹ cm^?2 via the local generation of extra sulfur vacancies without compromising the carrier mobility.     

Mass Production of High‐Quality Transition Metal Dichalcogenides Nanosheets via a Molten Salt Method

2D transition metal dichalcogenides (TMDs) are well suited for energy storage and field–effect transistors because of their thickness‐dependent chemical and physical properties. However, as current synthetic methods for 2D TMDs cannot integrate both advantages of liquid‐phase syntheses (i.e., massive production and homogeneity) and chemical vapor deposition (i.e., high quality and large lateral size), it still remains a great challenge for mass production of high‐quality 2D TMDs. Here, a molten salt method to massively synthesize various high‐crystalline TMDs nanosheets (MoS₂, WS₂, MoSe₂, and WSe₂) with the thicknesses less than 5 nm is reported, with the production yield over 68% with the reaction time of only several minutes. Additionally, the thickness and size of the as‐synthesized nanosheets can be readily controlled through adjusting reaction time and temperature. The as‐synthesized MoSe₂ nanosheets exhibit good electrochemical performance as pseudocapacitive materials. It is further anticipates that this work will provide a promising strategy for rapid mass production of high‐quality nonoxides nanosheets for energy‐related applications and beyond.     

A General Vapor-Induced Coating Approach for Layer-controlled Organic Single Crystals

2D organic semiconductor crystals (2D OSCs) are vital for high-performance electronic and optoelectronic devices owing to their unique material merits. However, it is still challenging to fabricate high-quality and large-scale ultrathin 2D OSCs with controllable molecular layers due to the disordered molecular deposition and uncontrollable mass transport in solution-processing fabrication. Here, a vapor-induced meniscus modulating strategy for preparing unidirectional and stable Marangoni flow to guide contactless meniscus evolution is reported, which ensures uniform mass transport and ordered molecular deposition to achieve high-quality ultrathin 2D OSCs. Both the surface tension difference and the substrate wettability are critical to meniscus formation, which results in various meniscus deformation states and film morphologies. Based on the optimized vapor-solvent system, ultrathin 2D OSCs of C₈-BTBT with precise layer definition are prepared controllably. The discrepancies in liquid film height and solute concentration are decisive in controlling the molecular scale thickness ranging from mono to a few layers. Moreover, the layer-dependent electronic and optoelectronic properties of the ultrathin films are systematically investigated. Notably, high-performance polarization-sensitive solar-blind photodetectors are achieved with a dichroic ratio of photocurrent up to 2.26, and the corresponding polarimetric image sensor exhibits superior solar-blind polarization imaging capability thanks to the high crystalline quality.     

Understanding Dopant–Host Interactions on Electronic Structures and Optical Properties in Ce-Doped WS2 Monolayers

Substitutional lanthanide doping of 2D transition metal dichalcogenides (TMDs) is expected to be a promising strategy to engineer optical, electronic, and optoelectronic properties of TMDs. Understanding the interactions between lanthanide dopants and 2D TMDs host is one of the key problems to be resolved for their profound research studies. Herein, the interactions between Ce dopants and monolayer WS₂ in a physical vapor deposition grown Ce-doped WS₂ monolayer are studied by combining scanning tunneling microscopy with optical characterizations with high spatial and temporal resolution. It is found that the highly anisotropic crystal field can effectively split the energy levels of the Ce dopants’ f orbital. The electrons in the split energy levels can bind the holes in the valence band maximum of the Ce-doped WS₂, forming optical bright excitons. These excitons collide with the free A excitons when increasing the pump fluences, reducing the A exciton's lifetime. This study may be beneficial for the design and fabrication of optical, electronic, and optoelectronic devices based on lanthanide-doped TMDs.     

Theoretical and Experimental Insight into the Mechanism for Spontaneous Vertical Growth of ReS2 Nanosheets

Rhenium disulfide (ReS₂) differs fundamentally from other group‐VI transition metal dichalcogenides (TMDs) due to its low structural symmetry, which results in its optical and electrical anisotropy. Although vertical growth is observed in some TMDs under special growth conditions, vertical growth in ReS₂ is very different in that it is highly spontaneous and substrate‐independent. In this study, the mechanism that underpins the thermodynamically favorable vertical growth mode of ReS₂ is uncovered. It is found that the governing mechanism for ReS₂ growth involves two distinct stages. In the first stage, ReS₂ grows parallel to the growth substrate, consistent with conventional TMD growth. However, subsequent vertical growth is nucleated at points on the lattice where Re atoms are “pinched” together. At such sites, an additional Re atom binds with the cluster of pinched Re atoms, leaving an under‐coordinated S atom protruding out of the ReS₂ plane. This under‐coordinated S is “reactive” and binds to free Re and S atoms, initiating growth in a direction perpendicular to the ReS₂ surface. The utility of such vertical ReS₂ arrays in applications where high surface‐to‐volume ratio and electric‐field enhancement are essential, such as surface enhanced Raman spectroscopy, field emission, and solar‐based disinfection of bacteria, is demonstrated.     

Selective Vertical and Horizontal Growth of 2D WS2 Revealed by In Situ Thermolysis using Transmission Electron Microscopy

Direct observation of the growth dynamics of 2D transition metal dichalcogenides (TMDs) is of key importance for understanding and controlling the growth modes and for tailoring these intriguing materials to desired orientations and layer thicknesses. Here, various stages and multiple growth modes in the formation of WS₂ layers on different substrates through thermolysis of a single solid-state (NH₄)₂WS₄ precursor are revealed using in situ transmission electron microscopy. Control over vertical and horizontal growth is achieved by varying the thickness of the drop-casted precursor from which WS₂ is grown during heating. First depositing platinum (Pt) and gold (Au) on the heating chips much enhance the growth process of WS₂ resulting in an increased length of vertical layers and in a self-limited thickness of horizontal layers. Interference patterns are formed by the mutual rotation of two WS₂ layers by various angles on metal deposited heating chips. This shows detailed insights into the growth dynamics of 2D WS₂ as a function of temperature, thereby establishing control over orientation and size. These findings also unveil the important role of metal substrates in the evolution of WS₂ structures, offering general and effective pathways for nano-engineering of 2D TMDs for a variety of applications.     

Irradiation of Transition Metal Dichalcogenides Using a Focused Ion Beam: Controlled Single‐Atom Defect Creation

Manipulation and structural modifications of 2D materials for nanoelectronic and nanofluidic applications remain obstacles to their industrial‐scale implementation. Here, it is demonstrated that a 30 kV focused ion beam can be utilized to engineer defects and tailor the atomic, optoelectronic, and structural properties of monolayer transition metal dichalcogenides (TMDs). Aberration‐corrected scanning transmission electron microscopy is used to reveal the presence of defects with sizes from the single atom to 50 nm in molybdenum (MoS₂) and tungsten disulfide (WS₂) caused by irradiation doses from 10¹³ to 10¹⁶ ions cm^?2. Irradiated regions across millimeter‐length scales of multiple devices are sampled and analyzed at the atomic scale in order to obtain a quantitative picture of defect sizes and densities. Precise dose value calculations are also presented, which accurately capture the spatial distribution of defects in irradiated 2D materials. Changes in phononic and optoelectronic material properties are probed via Raman and photoluminescence spectroscopy. The dependence of defect properties on sample parameters such as underlying substrate and TMD material is also investigated. The results shown here lend the way to the fabrication and processing of TMD nanodevices.     

Metalorganic chemical vapor deposition of silver thin films for future interconnects by direct liquid injection system

Deposition of Ag films by direct liquid injection-metal organic chemical vapor deposition (DLI-MOCVD) was chosen because this preparation method allows precise control of precursor flow and prevents early decomposition of the precursor as compared to the bubbler-delivery. Silver(I)-2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedionato-triethylphosphine [Ag(fod)(PEt₃)] as the precursor for Ag CVD was studied, which is liquid at 30 °C. Ag films were grown on different substrates of SiO₂/Si and TiN/Si. Argon and nitrogen/hydrogen carrier gas was used in a cold wall reactor at a pressure of 50–500 Pa with deposition temperature ranging between 220 °C and 350 °C. Ag films deposited on a TiN/Si diffusion barrier layer have favorable properties over films deposited on SiO₂/Si substrate. At lower temperature (220 °C), film growth is essentially reaction-limited on SiO₂ substrate. Significant dependence of the surface morphology on the deposition conditions exists in our experiments. According to XPS analysis pure Ag films are deposited by DLI-MOCVD at 250 °C by using argon as carrier gas.     

Flexible High-Performance Photovoltaic Devices based on 2D MoS2 Diodes with Geometrically Asymmetric Contact Areas

Optoelectronic performance of 2D transition metal dichalcogenides (TMDs)-based solar cells and self-powered photodetectors remain limited due to fabrication challenges, such as difficulty in doping TMDs to form p–n junctions. Herein, MoS₂ diodes based on geometrically asymmetric contact areas are shown to achieve a high current rectification ratio of ≈10⁵, facilitating efficient photovoltaic charge collection. Under solar illumination, the device demonstrates a high open-circuit voltage (V_oc) of 430 mV and a short-circuit current density (J_sc) of −13.42 mA cm⁻², resulting in a high photovoltaic power conversion efficiency (PCE) of 3.16%, the highest reported for a lateral 2D solar cell. The diodes also show a high photoresponsivity of 490.3 mA W⁻¹, and a large photo detectivity of 4.05 × 10¹⁰ Jones, along with a fast response time of 0.8 ms under 450 nm wavelength at zero bias for self-powered photodetection applications. The device transferred on a flexible substrate shows a high photocurrent and PCE retentions of 94.4%, and 88.2% after 5000 bending cycles at a bending radius of 1.5 cm, respectively, demonstrating robustness for flexible optoelectronic applications. The simple fabrication process, superior photovoltaic properties, and high flexibility suggests that the geometrically asymmetric MoS₂ device architecture is an excellent candidate for flexible photovoltaic and optoelectronic applications.