Hongwei Liu

The direct synthesis of wafer-scale single-crystal transition metal dichalcogenides (TMDs) remains challenging, albeit with enormous potential applications as semiconductors. In this work, we demonstrate the feasibility of using single-crystal 2H-MoTe₂ films as templates, followed by a sequential selenium substitution reaction to synthesize a variety of TMDs and their heterostructures. We also demonstrate the synthesis of a MoTe₂/MoSe₂ lateral heterostructure with various substitution temperatures for Se substitution in 1T' and 2H phase MoTe₂. Computational results illustrate that Se substitution is likely to start at Te vacancy sites, where generated strain lowers the energy barrier for further substitution, leading to a chain reaction that propagates until the entire layer is selenized. The obtained MoSe₂ shows a high hole mobility of 32 cm² V^-1 s^-1, comparable to the 2.8-31.6 range from mechanically exfoliated samples. Consequently, this MoSe₂-based photodetector shows a comparable responsivity of 41 mA W^-1 under near-infrared (1060 nm) illumination.

Visible-range single-photon emitters (SPEs), based on hexagonal boron nitride (hBN), with exceptional optical performance have become an outstanding candidate for quantum optical technology. However, the control of the carbon defect structures to obtain uniform and confined band structure remains elusive, restricting their integration into on-chip quantum devices. Here, we demonstrate tuning of the defect structure of hBN to precisely control the emission in SPEs. The defect structure engineering from CB (carbon substituted at the boron site) to C₂B-CN (carbon doped into two boron sites and one nitrogen site) carbon defect conversion in hBN is realized by regulating the carbon concentration from 0.0005 at % to 0.082 at % in Cu substrates to adjust the carbon diffusion during the CVD process. Meanwhile, the zero-phonon line exhibits a precise shift from the range of 600-610 nm to 630-640 nm; these shifts of the spectral features are further supported by density functional theory results, reflected in changes in the band structure, vibrational degrees of freedom, and electronic transitions. The SPE emission spectrum serves as a valuable tool for identifying the footprint of a carbon point defect structure change. Our project offers evidence of achieving structured defect engineering for tailored emission properties and showcases potential for the integration of advanced 2D material engineering into on-chip quantum devices.

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.

WSe₂ has a high mobility of electrons and holes, which is an ideal choice as active channels of electronics in extensive fields. However, carrier-type tunability of WSe₂ still has enormous challenges, which are essential to overcome for practical applications. In this work, the direct growth of n-doped few-layer WSe₂ is realized via in situ defect engineering. The n-doping of WSe₂ is attributed to Se vacancies induced by the H₂ flow purged in the cooling process. The electrical measurements based on field effect transistors demonstrate that the carrier type of WSe₂ synthesized is successfully transferred from the conventional p-type to the rarely reported n-type. The electron carrier concentration is efficiently modulated by the concentration of H₂ during the cooling process. Furthermore, homomaterial inverters and self-powered photodetectors are fabricated based on the doping-type-tunable WSe₂. This work reveals a significant way to realize the controllable carrier type of two-dimensional (2D) materials, exhibiting great potential in future 2D electronics engineering.

Vanadium diselenide (VSe₂) exhibits versatile electronic and magnetic properties in the trigonal prismatic (H-) and octahedral (T-) phases. Compared to the metallic T-phase, the H-phase with a tunable semiconductor property is predicted to be a ferrovalley material with spontaneous valley polarization. Herein we report an epitaxial growth of the monolayer 2D VSe₂ on a mica substrate via the chemical vapor deposition (CVD) method by introducing salt in the precursor. Our first-principles calculations suggest that the monolayer H-phase VSe₂ with a large lateral size is thermodynamically favorable. The honeycomb-like structure and the broken symmetry are directly observed by spherical aberration-corrected scanning transmission electron microscopy (STEM) and confirmed by giant second harmonic generation (SHG) intensity. The p-type transport behavior is further evidenced by the temperature-dependent resistance and field-effect device study. The present work introduces a new phase-stable 2D transition metal dichalcogenide, opening the prospect of novel electronic and spintronics device design.

We demonstrate the use of the machine learning (ML) tools to rapidly and accurately predict the electric field as a guide for designing core-shell Au-silica nanoparticles to enhance ¹O₂ sensitization and selectivity of organic synthesis. Based on the feature importance analysis, obtained from a deep neural network algorithm, we found a general and linear dependent descriptor (θ ∝ aD^0.25t^-1, where a, D, and t are the shape constant, size of metal nanoparticles, and distance from the metal surface) for the electric field around the core-shell plasmonic nanoparticle. Directed by the new descriptor, we synthesized gold-silica nanoparticles and validated their plasmonic intensity using scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) mapping. The nanoparticles with θ = 0.40 demonstrate an ∼3-fold increase in the reaction rate of photooxygenation of anthracene and 4% increase in the selectivity of photooxygenation of dihydroartemisinic acid (DHAA), a long-standing goal in organic synthesis. In addition, the combination of ML and experimental investigations shows the synergetic effect of plasmonic enhancement and fluorescence quenching, leading to enhancement for ¹O₂ generation. Our results from time-dependent density functional theory (TD-DFT) calculations suggest that the presence of an electric field can favor intersystem crossing (ISC) of methylene blue to enhance ¹O₂ generation. The strategy reported here provides a data-driven catalyst preparation method that can significantly reduce experimental cost while paving the way for designing photocatalysts for organic drug synthesis.

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.