Surface Diffusion and Epitaxial Self-Planarization for Wafer-Scale Single-Grain Metal Chalcogenide Thin Films

Wafer scale growth of single crystal two-dimensional van der Waals materials

Two-dimensional (2D) van der Waals (vdW) materials, including graphene, hexagonal boron nitride (hBN), and metal dichalcogenides (MCs), form the basis of modern electronics and optoelectronics due to their unique electronic structure, chemical activity, and mechanical strength. Despite many proof-of-concept demonstrations so far, to fully realize their large-scale practical applications, especially in devices, wafer-scale single crystal atomically thin highly uniform films are indispensable. In this minireview, we present an overview on the strategies and highlight recent significant advances toward the synthesis of wafer-scale single crystal graphene, hBN, and MC 2D thin films. Currently, there are five distinct routes to synthesize wafer-scale single crystal 2D vdW thin films: (i) nucleation-controlled growth by suppressing the nucleation density, (ii) unidirectional alignment of multiple epitaxial nuclei and their seamless coalescence, (iii) self-collimation of randomly oriented grains on a molten metal, (iv) surface diffusion and epitaxial self-planarization and (v) seed-mediated 2D vertical epitaxy. Finally, the challenges that need to be addressed in future studies have also been described.

Wafer-scale synthesis of two-dimensional ultrathin films

Two-dimensional (2D) materials, consisting of atomically thin layered crystals, have attracted tremendous interest due to their outstanding intrinsic properties and diverse applications in electronics, optoelectronics, and catalysis. The large-scale growth of high-quality ultrathin 2D films and their utilization in the facile fabrication of devices, easily adoptable in industrial applications, have been extensively sought after during the last decade; however, it remains a challenge to achieve these goals. Herein, we introduce three key concepts: (i) the microwave assisted quick (∼1 min) synthesis of wafer-scale (6-inch) anisotropic conducting ultrathin (∼1 nm) amorphous carbon and 2D semiconducting metal chalcogenide atomically thin films, (ii) a polymer-assisted deposition process for the synthesis of wafer-scale (6-inch) 2D metal chalcogenide and pyrolyzed carbon thin films, and (iii) the surface diffusion and epitaxial self-planarization induced synthesis of wafer-scale (2-inch) single crystal 2D binary and large-grain 2D ferromagnetic ternary metal chalcogenide thin films. The proposed synthesis concepts can pave a new way for the manufacture of wafer-scale high quality 2D ultrathin films and their utilization in the facile fabrication of devices.

Orietation-controlled synthesis and Raman study of 2D SnTe

Tin telluride (SnTe), as a narrow bandgap semiconductor material, has great potential for developing photodetectors with wide spectra and ultra-fast response. At the same time, it is also an important topological crystal insulator material, with different topological surface states on several common surfaces. Here, we introduce different Sn sources and control the growth of regular SnTe nanosheets along the (100) and (111) planes through the atmospheric pressure chemical vapor deposition method. It has been proven through various characterizations that the synthesized SnTe is a high-quality single crystal. In addition, the angular resolved Raman spectra of SnTe nanosheets grown on different crystal planes are first demonstrated. The experimental results showed that square SnTe nanosheets grown along the (100) plane exhibit in-plane anisotropy. At the same time, we use micro-nanofabrication technology to manufacture SnTe-based field effect transistors and photodetectors to explore their electrical and optoelectronic properties. It has been confirmed that transistors based on grown SnTe nanosheets exhibit p-type semiconductor characteristics and have a high response to infrared light. This work provides a new approach for the controllable synthesis of SnTe and adds new content to the research of SnTe-based infrared detectors.

First-Principles Investigations on the Semiconductor-to-Metal Phase Transition of 2D Si2Te3 for Reversible Resistive Switching

Si₂Te₃ is attracting attention due to its compatibility with Si technology while still showing advantages as a two-dimensional layered material. Although recent experimental studies have observed the resistive switching process in Si₂Te₃-based memristors, the mechanism has not been clearly identified. In this study, first-principles density functional theory calculations are employed to understand the relationship between the phase transition of Si₂Te₃ and the reversible resistive switching of the Si₂Te₃-based memristor. Our calculation results show that although semiconducting Si₂Te₃ is energetically more stable than two metallic Si₂Te₃ phases (α and β), two metallic Si₂Te₃ can be energetically stabilized by excess holes. The enhanced energetic preference of two metallic Si₂Te₃ by excess holes is explained by the reduced occupation of antibonding states between Si and Te. Our study finds that the energy barrier for the phase transition between semiconducting Si₂Te₃ and α-metallic Si₂Te₃ varies significantly by excess charge carriers so the phase transition can be directly connected to the reversible resistive switching of the Si₂Te₃-based memristor under external bias. Our finding will serve as a cornerstone for optimizing the resistive switching process of the Si₂Te₃-based memristor.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

New Approaches to Produce Large-Area Single Crystal Thin Films

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

Large-Area Epitaxial Film Growth of van der Waals Ferromagnetic Ternary Chalcogenides

Following the first experimental realization of intrinsic ferromagnetism in 2D van der Waals (vdW) crystals, several ternary metal chalcogenides with unprecedented long-range ferromagnetic order have been explored. However, the synthesis of large-area 2D ternary metal chalcogenide thin films is a great challenge, and a generalized synthesis has not been demonstrated yet. Here, a quick and scalable synthesis of epitaxially aligned ferromagnetic ternary metal chalcogenide thin films (Cr₂ Ge₂ Te₆ , Cr₂ Si₂ Te₆ , Mn₃ Si₂ Te₆ ) is reported. The synthesis is based on the flux-controlled surface diffusion of Te on metal (Cr, Mn)-deposited wafer (Ge, Si) substrates. Magnetic anisotropy study of the epitaxial ternary thin films reveals the intrinsic magnetic easy axis; out-of-plane direction for Cr₂ Ge₂ Te₆ and Cr₂ Si₂ Te₆ , and in-plane direction for Mn₃ Si₂ Te₆ . In addition to the synthesis, this work creates an opportunity for transfer-free device fabrication for realizing magnetoelectronics based on the electrical control of both charge and spin degrees of freedom in 2D ferromagnetic semiconductors.