Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe2-WSe2 van der Waals Heterojunction-Based Field-Effect Transistors

Controllable and Gradient Wettability of Bilayer Two-Dimensional Materials Regulated by Interlayer Distance

Surfaces with controllable and gradient wettability often require an elaborate design of the microstructure or its response under electrical, thermal, optical, pH, and other stimuli. Generally, the wettability change under these physical or chemical effects relies on a complex mechanism that is difficult to be quantitatively described. In this study, an online controlling strategy for surface wettability and the corresponding theoretical model are put forward based on a bilayer graphene-like atomic structure. Molecular dynamics results indicate that the surface wettability varies toward hydrophilicity after sticking a bottom material regardless of its wettability. But such an influence becomes weak with increasing interlayer distance, and the overall wettability approaches that of the upper layer material gradually. This variation is elucidated by the increase of the work of adhesion, providing new insight into the wetting transparency of graphene. A theoretical model of the governing relationship is established based on the work of adhesion, which correlates the overall surface wettability with the interlayer distance and the wettabilities of individual materials. Moreover, a surface with a uniform wettability gradient is achieved by inclining the bottom material. The spontaneous and steady motion of droplets can be induced by this gradient wettability. The relevant speedup behavior is evaluated through a theoretical model considering the varying interlayer distance, which reveals the critical role of the lower layer. This study proposes a novel strategy for controllable wetting and relevant gradient surfaces using prevailing two-dimensional materials, paving new routes to many applications such as microfluidic chips, virus diagnosis, and intelligent sensors.

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.

Ultrabroadband and High-Detectivity Photodetector Based on WS2/Ge Heterojunction through Defect Engineering and Interface Passivation

Broadband photodetectors are of great importance for numerous optoelectronic applications. Two-dimensional (2D) tungsten disulfide (WS₂), an important family member of transition-metal dichalcogenides (TMDs), has shown great potential for high-sensitivity photodetection due to its extraordinary properties. However, the inherent large bandgap of WS₂ and the strong interface recombination impede the actualization of high-sensitivity broadband photodetectors. Here, we demonstrate the fabrication of an ultrabroadband WS₂/Ge heterojunction photodetector through defect engineering and interface passivation. Thanks to the narrowed bandgap of WS₂ induced by the vacancy defects, the effective surface modification with an ultrathin AlO_x layer, and the well-designed vertical n-n heterojunction structure, the WS₂/AlO_x/Ge photodetector exhibits an excellent device performance in terms of a high responsivity of 634.5 mA/W, a large specific detectivity up to 4.3 × 10¹¹ Jones, and an ultrafast response speed. Significantly, the device possesses an ultrawide spectral response spanning from deep ultraviolet (200 nm) to mid-wave infrared (MWIR) of 4.6 μm, along with a superior MWIR imaging capability at room temperature. The detection range has surpassed the WS₂-based photodetectors in previous reports and is among the broadest for TMD-based photodetectors. Our work provides a strategy for the fabrication of high-performance ultrabroadband photodetectors based on 2D TMD materials.

Van der Waals Epitaxial Growth of Mosaic-Like 2D Platinum Ditelluride Layers for Room-Temperature Mid-Infrared Photodetection up to 10.6 µm

Mid-infrared (MIR) photodetection, covering diverse molecular vibrational regions and atmospheric transmission windows, is vital to civil and military purposes. Versatile use of MIR photodetectors is commonly dominated by HgCdTe alloys, InSb, and quantum superlattices, which are limited by strict operation demands, high-cost, and environmental toxicity. Despite the rapid advances of black phosphorus (BP)-based MIR photodetectors, these are subject to poor stability and large-area integration difficulty. Here, the van der Waals (vdW) epitaxial growth of a wafer-scale 2D platinum ditelluride (PtTe₂ ) layer is reported via a simple tellurium-vapor transformation approach. The 2D PtTe₂ layer possesses a unique mosaic-like crystal structure consisting of single-crystal domains with highly preferential [001] orientation along the normal direction, reducing the influence of interface defects and ensuring efficient out-of-plane carrier transportation. This characteristic, combined with the wide absorption of PtTe₂ and well-designed vertical device architecture, makes the PtTe₂ /Si Schottky junction photodetector capable of sensing ultra-broadband light of up to 10.6 µm with a high specific detectivity. Also, the photodetector exhibits an excellent room-temperature infrared-imaging capability. This approach provides a new design concept for high-performance, room-temperature MIR photodetection based on 2D layered materials.

Wafer-Scale Growth of 2D PtTe2 with Layer Orientation Tunable High Electrical Conductivity and Superior Hydrophobicity

Platinum ditelluride (PtTe₂) is an emerging semimetallic two-dimensional (2D) transition-metal dichalcogenide (TMDC) crystal with intriguing band structures and unusual topological properties. Despite much devoted efforts, scalable and controllable synthesis of large-area 2D PtTe₂ with well-defined layer orientation has not been established, leaving its projected structure-property relationship largely unclarified. Herein, we report a scalable low-temperature growth of 2D PtTe₂ layers on an area greater than a few square centimeters by reacting Pt thin films of controlled thickness with vaporized tellurium at 400 °C. We systematically investigated their thickness-dependent 2D layer orientation as well as its correlated electrical conductivity and surface property. We unveil that 2D PtTe₂ layers undergo three distinct growth mode transitions, i.e., horizontally aligned holey layers, continuous layer-by-layer lateral growth, and horizontal-to-vertical layer transition. This growth transition is a consequence of competing thermodynamic and kinetic factors dictated by accumulating internal strain, analogous to the transition of Frank-van der Merwe (FM) to Stranski-Krastanov (SK) growth in epitaxial thin-film models. The exclusive role of the strain on dictating 2D layer orientation has been quantitatively verified by the transmission electron microscopy (TEM) strain mapping analysis. These centimeter-scale 2D PtTe₂ layers exhibit layer orientation tunable metallic transports yielding the highest value of ∼1.7 × 10⁶ S/m at a certain critical thickness, supported by a combined verification of density functional theory (DFT) and electrical measurements. Moreover, they show intrinsically high hydrophobicity manifested by the water contact angle (WCA) value up to ∼117°, which is the highest among all reported 2D TMDCs of comparable dimensions and geometries. Accordingly, this study confirms the high material quality of these emerging large-area 2D PtTe₂ layers, projecting vast opportunities employing their tunable layer morphology and semimetallic properties from investigations of novel quantum phenomena to applications in electrocatalysis.

Synthesis of NbSe2 single-crystalline nanosheet arrays for UV photodetectors

Single-crystalline NbSe₂ nanosheet arrays were synthesized via a CVD method. The NbSe₂ nanosheet arrays based photodetectors show very high responsivity and external quantum efficiency to UV light.

Twist-Angle-Dependent Optoelectronics in a Few-Layer Transition-Metal Dichalcogenide Heterostructure

Lattice matching has been supposed to play an important role in the coupling between two materials in a vertical heterostructure (HS). To investigate this role, we fabricated a heterojunction device with a few layers of p-type WSe₂ and n-type MoSe₂ with different crystal orientation angles. The crystal orientations of WSe₂ and MoSe₂ were estimated using high-resolution X-ray diffraction. Heterojunction devices were fabricated with twist angles of 0, 15, and 30°. The I- V curve of the sample with the twist angle of 0° under the dark condition showed a diodelike behavior. The strong coupling due to lattice matching caused a well-established p-n junction. In cases of 15 and 30° samples, the van der Waals gap was built because of lattice mismatching, which resulted in the formation of a potential barrier. However, when the light-emitting diode light of 365 nm (3.4 eV) was illuminated, it was possible for excited electrons and holes to jump beyond the potential barrier and the current flowed well in both forward and reverse directions. The effects of the twist angle were analyzed by spectral responsivity and external quantum efficiency, where it was found that the untwisted HS exhibited higher sensitivity under IR illumination, whereas the twisting effect was not noticeable under UV illumination. From photoluminescence and Raman spectroscopy studies, it was confirmed that the twisted HS showed a weak coupling because of the lattice mismatch.