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Wafer-scale monolayer two-dimensional (2D) materials have been realized by epitaxial chemical vapor deposition (CVD) in recent years. To scale up the synthesis of 2D materials, a systematic analysis of how the growth dynamics depend on the growth parameters is essential to unravel its mechanisms. However, the studies of CVD-grown 2D materials mostly adopted the control variate method and considered each parameter as an independent variable, which is not comprehensive for 2D materials growth optimization. Herein, we synthesized a representative 2D material, monolayer hexagonal boron nitride (hBN), on single-crystalline Cu (111) by epitaxial chemical vapor deposition and varied the growth parameters to regulate the hBN domain sizes. Furthermore, we explored the correlation between two growth parameters and provided the growth windows for large flake sizes by the Gaussian process. This new analysis approach based on machine learning provides a more comprehensive understanding of the growth mechanism for 2D materials. © 2023 American Chemical Society.

Tin (II) selenide (SnSe) is an emerging 2D material with many intriguing properties, such as record-high thermoelectric figure of merit (ZT), purely in-plane ferroelectricity, and excellent nonlinear optical properties. To explore these functional properties and related applications, a crucial step is to develop controllable routes to synthesize large-area, ultrathin, and high-quality SnSe crystals. Physical vapor deposition (PVD) constitutes a reliable method to synthesize 2D SnSe, however, effects of various growth parameters have not yet been systematically investigated, and current PVD-synthesized flakes are often thick (>10 nm) with small lateral sizes (<10 µm). In this work, high-quality 2D SnSe crystals are synthesized via low-pressure PVD, which display in-plane ferroelectric domains observed by piezoresponse force microscopy and polarization-dependent reflection spectroscopy. Detailed studies regarding the roles of various parameters are further carried out, including substrate pre-annealing, growth duration, temperature, and pressure, which enable to rationally optimize the growth and obtain 2D SnSe crystals with lateral sizes up to ≈23.0 µm and thicknesses down to ≈2.0 nm (3–4 layers). This work paves the way for the controlled growth of large-area 2D SnSe, facilitating the future exploration of many interesting multiferroic properties and applications with atomic thickness. © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH.

2D transition metal dichalcogenides (TMDCs) with intense and tunable photoluminescence (PL) have opened up new opportunities for optoelectronic and photonic applications such as light-emitting diodes, photodetectors, and single-photon emitters. Among the standard characterization tools for 2D materials, Raman spectroscopy stands out as a fast and non-destructive technique capable of probing material's crystallinity and perturbations such as doping and strain. However, a comprehensive understanding of the correlation between photoluminescence and Raman spectra in monolayer MoS2 remains elusive due to its highly nonlinear nature. Here, the connections between PL signatures and Raman modes are systematically explored, providing comprehensive insights into the physical mechanisms correlating PL and Raman features. This study's analysis further disentangles the strain and doping contributions from the Raman spectra through machine-learning models. First, a dense convolutional network (DenseNet) to predict PL maps by spatial Raman maps is deployed. Moreover, a gradient boosted trees model (XGBoost) with Shapley additive explanation (SHAP) to bridge the impact of individual Raman features in PL features is applied. Last, a support vector machine (SVM) to project PL features on Raman frequencies is adopted. This work may serve as a methodology for applying machine learning to characterizations of 2D materials.

Artificial heterojunctions formed by vertical stacking of dissimilar two-dimensional (2D) transition metal dichalcogenide (TMD) monolayer materials in a chosen sequence hold tantalizing prospects for futuristic atomically thin circuits. The emergence of 2D topological insulators (TI), including Bi2Te3, Bi2Se3, and Sb2Te3, represents a new class of 2D building blocks and can complement the existing artificial heterojunctions as a result of their intriguing surface states protected by the time-reversal symmetry. However, the determination of band alignments of such 2D TI/TMD vertical heterojunctions, the key parameter for designing HJ-based electronic/photonic devices, which lies in the development of epitaxy growth, remains in its infancy. Here, we demonstrate the epitaxy growth of 2D TI/TMD vertical heterojunctions comprised of Bi2Te3/WSe2 with atomically clean interfaces that are spectroscopically accessible, and theoretically tractable. Cross-sectional scanning transmission electron microscopy (STEM) images and the presence of interlayer-coupled characteristics from Raman spectroscopy collectively confirm the neat stacking of Bi2Te3/WSe2 with the absence of unwanted containments. Microbeam X-ray photoelectron spectroscopy (ΌXPS) measurement coupled with the density functional theory (DFT) calculations and electrical characteristics of field effect transistors quantitatively reveals the type-II alignment of vertically stacked of quintuple layers (QL) Bi2Te3/WSe2. Meanwhile, the type-III band emerges when transitioning to multi-quintuple layer (MQL) Bi2Te3/WSe2. The finding here provides a well-defined example of the epitaxy growth paradigm, the interlayer coupling-electronic properties relationship, for these emerging 2D TI/TMDs vertical heterojunctions. © 2020 American Chemical Society.

Tungsten diselenide (WSe2) is an attractive transition metal dichalcogenide material, since its Fermi energy close to the mid gap makes it an excellent candidate for realizing p-n junction devices and complementary digital logic applications. Doping is one of the most important technologies for controlling the Fermi energy in semiconductors, including 2D materials. Here we present a simple, stable and controllable p-doping technique on a WSe2 monolayer, where a more p-typed WSe2 field effect transistor is realized by electron transfer from the WSe2 to the gold (Au) decorated on the WSe2 surfaces. Related changes in Raman spectroscopy are also reported. The p-doping caused by Au on WSe2 monolayers lowers the channel resistance by orders of magnitude. The effective hole mobility is ~100 (cm2/Vs) and the near ideal subthreshold swing of ~60 mV/decade and high on/off current ratio of >106 are observed. The Au deposited on the WSe2 also serves as a protection layer to prevent a reaction between the WSe2 and the environment, making the doping stable and promising for future scalable fabrication.