Location-selective growth of two-dimensional metallic/semiconducting transition metal dichalcogenide heterostructures

Epitaxial Growth of Large-Area Monolayers and van der Waals Heterostructures of Transition-Metal Chalcogenides via Assisted Nucleation

The transition-metal chalcogenides include some of the most important and ubiquitous families of 2D materials. They host an exceptional variety of electronic and collective states, which can in principle be readily tuned by combining different compounds in van der Waals heterostructures. Achieving this, however, presents a significant materials challenge. The highest quality heterostructures are usually fabricated by stacking layers exfoliated from bulk crystals, which - while producing excellent prototype devices - is time consuming, cannot be easily scaled, and can lead to significant complications for materials stability and contamination. Growth via the ultra-high vacuum deposition technique of molecular-beam epitaxy (MBE) should be a premier route for 2D heterostructure fabrication, but efforts to achieve this are complicated by non-uniform layer coverage, unfavorable growth morphologies, and the presence of significant rotational disorder of the grown epilayer. This work demonstrates a dramatic enhancement in the quality of MBE grown 2D materials by exploiting simultaneous deposition of a sacrificial species from an electron-beam evaporator during the growth. This approach dramatically enhances the nucleation of the desired epi-layer, in turn enabling the synthesis of large-area, uniform monolayers with enhanced quasiparticle lifetimes, and facilitating the growth of epitaxial van der Waals heterostructures.

Space-Confined One-Step Growth of 2D MoO2 /MoS2 Vertical Heterostructures for Superior Hydrogen Evolution in Alkaline Electrolytes

2D material-based heterostructures are constructed by stacking or spicing individual 2D layers to create an interface between them, which have exotic properties. Here, a new strategy for the in situ growth of large numbers of 2D heterostructures on the centimeter-scale substrate is developed. In the method, large numbers of 2D MoS₂ , MoO₂ , or their heterostructures of MoO₂ /MoS₂ are controllably grown in the same setup by simply tuning the gap distance between metal precursor and growth substrate, which changes the concentration of metal precursors feed. A lateral force microscope is used first to identify the locations of each material in the heterostructures, which have MoO₂ on the top of MoS₂ . Noteworthy, the creation of a clean interface between atomic thin MoO₂ (metallic) and MoS₂ (semiconducting) results in a different electronic structure compared with pure MoO₂ and MoS₂ . Theoretical calculations show that the charge redistribution at such an interface results in an improved HER performance on the MoO₂ /MoS₂ heterostructures, showing an overpotential of 60 mV at 10 mA cm^-2 and a Tafel slope of 47 mV dec^-1 . This work reports a new strategy for the in situ growth of heterostructures on large-scale substrates and provides platforms to exploit their applications.

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.

One-Step Synthesis of NbSe2/Nb-Doped-WSe2 Metal/Doped-Semiconductor van der Waals Heterostructures for Doping Controlled Ohmic Contact

van der Waals heterostructures (vdWHs) of metallic (m-) and semiconducting (s-) transition-metal dichalcogenides (TMDs) exhibit an ideal metal/semiconductor (M/S) contact in a field-effect transistor. However, in the current two-step chemical vapor deposition process, the synthesis of m-TMD on pregrown s-TMD contaminates the van der Waals (vdW) interface and hinders the doping of s-TMD. Here, NbSe₂/Nb-doped-WSe₂ metal-doped-semiconductor (M/d-S) vdWHs are created via a one-step synthesis approach using a niobium molar ratio-controlled solution-phase precursor. The one-step growth approach synthesizes Nb-doped WSe₂ with a controllable doping concentration and metal/doped-semiconductor vdWHs. The hole carrier concentration can be precisely controlled by controlling the Nb/(W + Nb) molar ratio in the precursor solution from ∼3 × 10¹¹/cm² at Nb-0% to ∼1.38 × 10¹²/cm² at Nb-60%; correspondingly, the contact resistance R_C value decreases from 10 888.78 at Nb-0% to 70.60 kΩ.μm at Nb-60%. The Schottky barrier height measurement in the Arrhenius plots of ln(I_sat/T²) versus q/K_BT demonstrated an ohmic contact in the NbSe₂/W_xNb_1-xSe₂ vdWHs. Combining p-doping in WSe₂ and M/d-S vdWHs, the mobility (27.24 cm² V^-1 s^-1) and on/off ratio (2.2 × 10⁷) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe₂ contact (0.022 cm² V^-1 s^-1 and 5 × 10³, respectively). Together, the R_C value using the NbSe₂ contact shows 2.46 kΩ.μm, which is ∼29 times lower than that of using a metal contact. This method is expected to guide the synthesis of various M/d-S vdWHs and applications in future high-performance integrated circuits.

Conformal Growth of Nanometer-Thick Transition Metal Dichalcogenide TiS x -NbS x Heterostructures over 3D Substrates by Atomic Layer Deposition: Implications for Device Fabrication

The scalable and conformal synthesis of two-dimensional (2D) transition metal dichalcogenide (TMDC) heterostructures is a persisting challenge for their implementation in next-generation devices. In this work, we report the synthesis of nanometer-thick 2D TMDC heterostructures consisting of TiS x -NbS x on both planar and 3D structures using atomic layer deposition (ALD) at low temperatures (200-300 °C). To this end, a process was developed for the growth of 2D NbS x by thermal ALD using (tert-butylimido)-tris-(diethylamino)-niobium (TBTDEN) and H2S gas. This process complemented the TiS x thermal ALD process for the growth of 2D TiS x -NbS x heterostructures. Precise thickness control of the individual TMDC material layers was demonstrated by fabricating multilayer (5-layer) TiS x -NbS x heterostructures with independently varied layer thicknesses. The heterostructures were successfully deposited on large-area planar substrates as well as over a 3D nanowire array for demonstrating the scalability and conformality of the heterostructure growth process. The current study demonstrates the advantages of ALD for the scalable synthesis of 2D heterostructures conformally over a 3D substrate with precise thickness control of the individual material layers at low temperatures. This makes the application of 2D TMDC heterostructures for nanoelectronics promising in both BEOL and FEOL containing high-aspect-ratio 3D structures.

Tailoring Domain Morphology in Monolayer NbSe2 and WxNb1-xSe2 Heterostructure

Domain morphology plays a pivotal role not only for the synthesis of high-quality 2D transition metal dichalcogenides (TMDs) but also for the further unveiling of related physical and chemical properties, yet little has been divulged to date, especially for metallic TMDs. In addition, solid precursor as a transition metal source has been conventionally introduced for the synthesis of TMDs, which leads to an inhomogeneous distribution of local domains with the substrate position, making it difficult to obtain a reliable film. Here, we tailor the domain morphologies of metallic NbSe₂ and NbSe₂/WSe₂ heterostructures using liquid-precursor chemical vapor deposition (CVD). We find that triangular, hexagonal, tripod-like, and herringbone-like NbSe₂ flakes are constructed through control of growth temperature and promoter and precursor concentration. Liquid-precursor CVD ensures domain morphologies that are highly reproducible over repeated growth and uniform along the gas-flow direction. A domain coverage of ∼80% is achieved at a high precursor concentration, starting with tripod-like NbSe₂ domain and evolving to the herringbone fractal. Furthermore, mixing liquid W and Nb precursors results in sea-urchin-like heterostructure domains with long-branch-shaped NbSe₂ at low temperature, whereas protruded hexagonal heterostructure domains grow at high temperature. Our liquid precursor approach provides a shortcut for tailoring the domain morphologies of metallic TMDs as well as metal/semiconductor heterostructures.

Polypyrrole coated niobium disulfide nanowires as high performance electrocatalysts for hydrogen evolution reaction

Developing an environmentally friendly and low-cost approach to improve electrocatalytic activity for hydrogen evolution reaction (HER) has drawn wide attention due to its significant value and challenge. NbS₂-based materials exhibit high performance catalytic activity in electrochemical area, but its poor stability and synthetic difficulty limits its development and application. This work reports on the enhancement of HER performance through the utilization of conductive polymer polypyrrole (Ppy) on NbS₂ nanowires as electrocatalysts, which can be easily prepared. The Ppy coated NbS₂ nanowires obtain excellent catalytic activity for HER with low onset potential (-56 mV) and much lower overpotential (-219 mV) at a current of -10 mA cm^-2 compared with bare NbS₂ nanowires.