利用感應式耦合電漿輔助合成Janus層狀材料及其壓電特性之研究

Abstract

本研究旨在開發並優化一種新穎的Janus硫硒化鉬（Janus MoSSe）合成製程，並驗證其卓越的壓電性能，以應對次世代可撓式電子元件的需求。傳統二維材料如二硫化鉬（MoS2）因其結構對稱，壓電效應僅限於面內方向，從而限制了其應用潛力 。本研究的核心目標是透過將MoS2轉化為具有垂直不對稱結構（S-Mo-Se）的Janus MoSSe，以實現理論上預測的、優異的面外壓電特性。研究中採用感應式耦合電漿（ICP）輔助技術，在溫和的室溫條件下，選擇性地將MoS2頂層的硫原子置換為硒原子。初期發現直接在藍寶石基板上進行電漿製程，易因反應放熱和化學體積膨脹導致薄膜破損。為解決此關鍵問題，本研究創新地引入「AlOx/Al緩衝層」於基板上，成功克服了應力問題，實現了高品質、結構完整之Janus MoSSe薄片的穩定合成。透過一系列精密分析，成功合成的材料被證實為高品質的單晶六方晶格結構，其化學成分比例接近理想的Mo:S:Se = 1:1:1，且為能隙約1.72 eV的n型半導體。在壓電性能驗證上，壓電力顯微鏡（PFM）量測結果顯示，Janus MoSSe的有效面外壓電係數（d_33^eff）高達46.79 pm/V，是其母材MoS2（~13.24 pm/V）的3.5倍以上，且顯著優於多數已報導的二維材料。此外，本研究成功將Janus MoSSe整合於軟性基板上，製作出可撓式壓電奈米發電機（PENG）。在0.39%的應變下，元件可產生高達175 mV的開路電壓與1.4 nA的短路電流，其性能遠超過以相同製程製作的MoS2元件及相關文獻報導。本論文的研究成果不僅建立了一套穩定的合成方案，更從實驗上證明了Janus結構在壓電應用中的巨大潛力，為未來開發新一代高性能可撓式壓電元件奠定了堅實的基礎。

This study aims to develop and optimize a novel synthesis process for Janus molybdenum sulfoselenide (Janus MoSSe) and to verify its superior piezoelectric properties for next-generation flexible electronic devices. Conventional 2D materials, such as molybdenum disulfide (MoS2), are limited by their symmetric structure, which confines their piezoelectric effects to the in-plane direction. This research addresses this limitation by transforming MoS2 into Janus MoSSe, which possesses a vertically asymmetric structure (S-Mo-Se) predicted to exhibit a strong out-of-plane piezoelectric response. An inductively coupled plasma (ICP) assisted method was employed to selectively replace the top-layer sulfur atoms of MoS2 with selenium atoms under mild, room-temperature conditions. A critical challenge identified early in the study was film cracking and damage on sapphire substrates, attributed to thermal stress from the exothermic reaction and chemical volume expansion. This was overcome by introducing an innovative AlOx/Al buffer layer, which successfully mitigated the stress and enabled the synthesis of high-quality, structurally intact Janus MoSSe films. A comprehensive suite of characterization techniques, including PL, Raman, HRTEM, XPS, and UPS, confirmed the successful synthesis of high-quality, single-crystal Janus MoSSe with a Mo:S:Se atomic ratio near 1:1:1, a bandgap of approximately 1.72 eV, and intrinsic n-type semiconductor characteristics. The piezoelectric properties were validated at both the nanoscale and device level. Piezoelectric force microscopy (PFM) revealed that Janus MoSSe has an effective out-of-plane piezoelectric coefficient (d_33^eff) of 46.79 pm/V, which is over 3.5 times greater than that of its parent MoS2 (~13.24 pm/V) and superior to most 2D materials reported in the literature. Furthermore, a flexible piezoelectric nanogenerator (PENG) fabricated using Janus MoSSe generated a high open-circuit voltage of ~175 mV and a short-circuit current of ~1.4 nA under 0.39% strain. This performance significantly surpasses that of a control device made from MoS2 and previously reported values. This work establishes a robust synthesis route for high-quality Janus materials and provides definitive experimental evidence of their enhanced piezoelectric performance, highlighting their immense potential for future applications in flexible sensors, energy harvesters, and piezotronics.