Machine Learning-Assisted Large-Area Preparation of MoS2 Materials.

Molybdenum disulfide (MoS2) is a layered transition metal-sulfur compound semiconductor that shows promising prospects for applications in optoelectronics and integrated circuits because of its low preparation cost, good stability and excellent physicochemical, biological and mechanical properties. MoS2 with high quality, large size and outstanding performance can be prepared via chemical vapor deposition (CVD). However, its preparation process is complex, and the area of MoS2 obtained is difficult to control. Machine learning (ML), as a powerful tool, has been widely applied in materials science. Based on this, in this paper, a ML Gaussian regression model was constructed to explore the growth mechanism of MoS2 material prepared with the CVD method. The parameters of the regression model were evaluated by combining the four indicators of goodness of fit (r2), mean squared error (MSE), Pearson correlation coefficient (p) and p-value (p_val) of Pearson's correlation coefficient. After comprehensive comparison, it was found that the performance of the model was optimal when the number of iterations was 15. Additionally, feature importance analysis was conducted on the growth parameters using the established model. The results showed that the carrier gas flow rate (Fr), molybdenum sulfur ratio (R) and reaction temperature (T) had a crucial impact on the CVD growth of MoS2 materials. The optimal model was used to predict the size of molybdenum disulfide synthesis under 185,900 experimental conditions in the simulation dataset so as to select the optimal range for the synthesis of large-size molybdenum disulfide. Furthermore, the model prediction results were verified through literature and experimental results. It was found that the relative error between the prediction results and the literature and experimental results was small. These findings provide an effective solution to the preparation of MoS2 materials with a reduction in the time and cost of trial and error.

Machine Learning-Assisted Synthesis of Two-Dimensional Materials.

Two-dimensional (2D) materials have intriguing physical and chemical properties, which exhibit promising applications in the fields of electronics, optoelectronics, as well as energy storage. However, the controllable synthesis of 2D materials is highly desirable but remains challenging. Machine learning (ML) facilitates the development of insights and discoveries from a large amount of data in a short time for the materials synthesis, which can significantly reduce the computational costs and shorten the development cycles. Based on this, taking the 2D material MoS2 as an example, the parameters of successfully synthesized materials by chemical vapor deposition (CVD) were explored through four ML algorithms: XGBoost, Support Vector Machine (SVM), Naïve Bayes (NB), and Multilayer Perceptron (MLP). Recall, specificity, accuracy, and other metrics were used to assess the performance of these four models. By comparison, XGBoost was the best performing model among all the models, with an average prediction accuracy of over 88% and a high area under the receiver operating characteristic (AUROC) reaching 0.91. And these findings showed that the reaction temperature (T) had a crucial influence on the growth of MoS2. Furthermore, the importance of the features in the growth mechanism of MoS2 was optimized, such as the reaction temperature (T), Ar gas flow rate (Rf), reaction time (t), and so on. The results demonstrated that ML assisted materials preparation can significantly minimize the time spent on exploration and trial-and-error, which provided perspectives in the preparation of 2D materials.

Water-assisted controllable growth of atomically thin WTe2nanoflakes by chemical vapor deposition based on precursor design and substrate engineering strategies.

WTe2nanostructures have intrigued much attention due to their unique properties, such as large non-saturating magnetoresistance, quantum spin Hall effect and topological surface state. However, the controllable growth of large-area atomically thin WTe2nanostructures remains a significant challenge. In the present work, we demonstrate the controllable synthesis of 1T' atomically thin WTe2nanoflakes (NFs) by water-assisted ambient pressure chemical vapor deposition method based on precursor design and substrate engineering strategies. The introduction of water during the growth process can generate a new synthesized route by reacting with WO3to form intermediate volatile metal oxyhydroxide. Using WO3foil as the growth precursor can drastically enhance the uniformity of as-prepared large-area 1T' WTe2NFs compared to WO3powders. Moreover, highly oriented WTe2NFs with distinct orientations can be obtained by using a-plane and c-plane sapphire substrates, respectively. Corresponding precursor design and substrate engineering strategies are expected to be applicable to other low dimensional transition metal dichalcogenides, which are crucial for the design of novel electronic and optoelectronic devices.

Controllable epitaxial growth of GeSe2nanostructures and nonlinear optical properties.

Germanium diselenide (GeSe2) has emerged as a new member of anisotropic two-dimensional (2D) materials and gained increasing attention because of its excellent air stability, wide band gap and unique anisotropic properties, which exhibits promising applications in the fields of electronics, optoelectronics and polarized photodetection. However, the controllable epitaxial growth of large-scale and high-quality GeSe2nanostructures to date remains a big challenge. Herein, GeSe2nanofilms with lateral size up to centimeter scale have been successfully prepared on mica substrate by employing chemical vapor deposition technique. Experimental results demonstrated that hydrogen is the key factor for the controllable growth of GeSe2nanostructures and GeSe2-based heterostructures. Corresponding growth mechanism was proposed based on systematical characterizations. The nonlinear optical properties of as-prepared GeSe2were investigated by employing open-aperture z-scan technique exhibiting significant saturable and reverse saturable absorption behaviors at wavelengths of 400 nm and 800 nm, respectively. This study provides a new and robust route for fabricating GeSe2nanostructures and 2D heterostructures, which will benefit the development of GeSe2-based nonlinear optical and optoelectronic devices.

Space-confined and substrate-directed synthesis of transition-metal dichalcogenide nanostructures with tunable dimensionality.

Atomically thin transition-metal dichalcogenide (TMDC) nanostructures are predicted to exhibit novel physical properties that make them attractive candidates for the fabrication of electronic and optoelectronic devices. However, TMDCs tend to grow in the form of two-dimensional nanoplates (NPs) rather than one-dimensional nanoribbons (NRs) due to their native layered structure. Herein, we have developed a space-confined and substrate-directed chemical vapor deposition strategy for the controllable synthesis of WS2, WSe2, MoSe2, MoS2, WS2(1-x)Se2x NPs and NRs. TMDC NRs with lengths ranging from several micrometers to 100 µm have been obtained and the widths of TMDC NRs can be effectively tuned. Moreover, we found that TMDC NRs show different growth behaviors on van der Waals (vdW) and non-vdW substrates. The micro-nano structures, optical and electronic properties of synthesized TMDC NRs have been systematically investigated. This approach provides a general strategy for controllable synthesis of TMDC NRs, which makes these materials easily accessible as functional building blocks for novel optoelectronic devices.

Lateral and Vertical MoSe2-MoS2 Heterostructures via Epitaxial Growth: Triggered by High-Temperature Annealing and Precursor Concentration.

Atomically thin transition-metal dichalcogenide (TMDC) heterostructures have attracted increasing attention because of their unprecedented potential in the fields of electronics and optoelectronics. However, selective growth of either lateral or vertical TMDC heterostructures remains challenging. Here, we report that lateral and vertical MoS2/MoSe2 epitaxial heterostructures can be successfully fabricated via a one-step growth strategy, which includes triggering by the concentration of sulfur precursor vapor and a high-temperature annealing process. Vertically stacked MoS2/MoSe2 heterostructures can be synthesized via control of the nucleation and growth kinetics, which is induced by high sulfur vapor concentration. The high-temperature annealing process results in the formation of fractured MoSe2 and in situ epitaxial growth of lateral MoSe2-MoS2 heterostructures. This study has revealed the importance of sulfur vapor concentration and high-temperature annealing processes in the controllable growth of MoSe2-MoS2 heterostructures, paving a new route for fabricating two-dimensional TMDC heterostructures.

Controllable epitaxial growth of MoSe2-MoS2 lateral heterostructures with tunable electrostatic properties.

Recently, two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have attracted much attention due to their promising applications in the fields of electronics and optoelectronics. Controllable growth of TMDC heterostructures stimulates new interest by tuning their optical and electronic properties. Herein, large-scale lateral MoSe2-MoS2 and MoSe2(1-x)S2x -MoS2 heterostructures have been synthesized through one-step epitaxial ambient-pressure chemical vapor deposition method and we found that the growth time plays an important role in the formation of lateral heterostructures. Lateral MoSe2-MoS2 heterostructures have been systematically characterized by using atomic force microscopy, Raman spectroscopy and photoluminescence spectroscopy. Corresponding surface potential and charge distributions of MoSe2-MoS2 heterostructures have been investigated by employing Kelvin probe force microscopy. We found that the electrostatic properties of MoSe2-MoS2 heterostructures can be effectively tuned by injecting charges through conductive atomic force microscopy. Our results pave a new route for constructing 2D lateral heterostructures toward electronic and optoelectronic applications.

Three-dimensional holey-graphene/niobia composite architectures for ultrahigh-rate energy storage.

Nanostructured materials have shown extraordinary promise for electrochemical energy storage but are usually limited to electrodes with rather low mass loading (~1 milligram per square centimeter) because of the increasing ion diffusion limitations in thicker electrodes. We report the design of a three-dimensional (3D) holey-graphene/niobia (Nb2O5) composite for ultrahigh-rate energy storage at practical levels of mass loading (>10 milligrams per square centimeter). The highly interconnected graphene network in the 3D architecture provides excellent electron transport properties, and its hierarchical porous structure facilitates rapid ion transport. By systematically tailoring the porosity in the holey graphene backbone, charge transport in the composite architecture is optimized to deliver high areal capacity and high-rate capability at high mass loading, which represents a critical step forward toward practical applications.

Porous Fe2O3 Nanoframeworks Encapsulated within Three-Dimensional Graphene as High-Performance Flexible Anode for Lithium-Ion Battery.

Integrating nanoscale porous metal oxides into three-dimensional graphene (3DG) with encapsulated structure is a promising route but remains challenging to develop high-performance electrodes for lithium-ion battery. Herein, we design 3DG/metal organic framework composite by an excessive metal-ion-induced combination and spatially confined Ostwald ripening strategy, which can be transformed into 3DG/Fe2O3 aerogel with porous Fe2O3 nanoframeworks well encapsulated within graphene. The hierarchical structure offers highly interpenetrated porous conductive network and intimate contact between graphene and porous Fe2O3 as well as abundant stress buffer nanospace for effective charge transport and robust structural stability during electrochemical processes. The obtained free-standing 3DG/Fe2O3 aerogel was directly used as highly flexible anode upon mechanical pressing for lithium-ion battery and showed an ultrahigh capacity of 1129 mAh/g at 0.2 A/g after 130 cycles and outstanding cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is the best results that have been reported so far. This study offers a promising route to greatly enhance the electrochemical properties of metal oxides and provides suggestive insights for developing high-performance electrode materials for electrochemical energy storage.

Synthesis of WS2xSe2-2x Alloy Nanosheets with Composition-Tunable Electronic Properties.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently emerged as a new class of atomically thin semiconductors for diverse electronic, optoelectronic, and valleytronic applications. To explore the full potential of these 2D semiconductors requires a precise control of their band gap and electronic properties, which represents a significant challenge in 2D material systems. Here we demonstrate a systematic control of the electronic properties of 2D-TMDs by creating mixed alloys of the intrinsically p-type WSe2 and intrinsically n-type WS2 with variable alloy compositions. We show that a series of WS2xSe2-2x alloy nanosheets can be synthesized with fully tunable chemical compositions and optical properties. Electrical transport studies using back-gated field effect transistors demonstrate that charge carrier types and threshold voltages of the alloy nanosheet transistors can be systematically tuned by adjusting the alloy composition. A highly p-type behavior is observed in selenium-rich alloy, which gradually shifts to lightly p-type, and then switches to lightly n-type characteristics with the increasing sulfur atomic ratio, and eventually evolves into highly n-doped semiconductors in sulfur-rich alloys. The synthesis of WS2xSe2-2x nanosheets with tunable optical and electronic properties represents a critical step toward rational design of 2D electronics with tailored spectral responses and device characteristics.

Formation of ripples in atomically thin MoS2 and local strain engineering of electrostatic properties.

Ripple is a common deformation in two-dimensional materials due to localized strain, which is expected to greatly influence the physical properties. The effects of the ripple deformation in the MoS2 layer on their physics, however, are rarely addressed experimentally. We here grow atomically thin MoS2 nanostructures by employing a vapor phase deposition method without any catalyst and observed the ripples in MoS2 nanostructures. The MoS2 ripples exhibit quasi-periodical ripple structures in the MoS2 surface. The heights of the ripples vary from several angstroms to tens of nanometers and the wavelength is in the range of several hundred nanometers. The growth mechanism of rippled MoS2 nanostructures is elucidated. We have also simultaneously investigated the electrostatic properties of MoS2 ripples by using Kelvin probe force microscopy, which shows inhomogeneous surface potential and charge distributions for MoS2 ripple nanostructures with different local strains.

Electric tuning of the surface and quantum well states in Bi2Se3 films: a first-principles study.

Based on first-principles calculations in the framework of van der Waals density functional theory, we find that giant, Rashba-like spin splittings can be induced in both the surface states and quantum well states of thin Bi2Se3 films by application of an external electric field. The charge is redistributed so that the Dirac cones of the upper and lower surfaces become nondegenerate and completely gapless. Interestingly, a momentum-dependent spin texture is developed on the two surfaces of the films. Some of the quantum well states, which reside in the middle of the Bi2Se3 film under zero field, are driven to the surface by the electric field. The Rashba splitting energy has a highly non-linear dependence on the momentum and the electric field due to the large contribution of the high-order Rashba terms, which suggests complex spin dynamics in the thin films of Bi2Se3 under an electric field.