以同調聲子探討二維二硫化鉬介面之凡德瓦力彈性耦合與光聲換能器之應用

Abstract

聲學現象是普遍存在的，而根據聲音的傳遞介質與震盪頻率的不同，許多應用也隨之而生。受惠於超快雷射技術的發明，操控兆赫頻段聲子的技術得以實現，人們稱其為皮秒超聲波技術。在其發展的這四十年間，該技術已幫助人們探明了許多新興材料的物理謎團。而在石墨烯單原子層薄膜首次被分離出來後，二維系統的獨特性質很快就吸引了全世界研究者的目光，如今新穎二維材料相關的研究風潮方興未艾，我們將利用皮秒超聲波技術探索一種備受關注的二維堆疊之過渡金屬硫化物──單層與數層的二硫化鉬。本論文的目標是揭露二硫化鉬的關鍵參數，並且實現其作為光聲換能器的潛力。 本論文從介紹皮秒超聲波的實驗技術出發，回顧了這十年間，人們在二維材料的兆赫聲學與載子動力學領域，利用該技術所達成的許多重要貢獻。其中，二維材料層與層之間的凡德瓦力耦合現象，影響了二維元件的諸多光電特性。藉由兆赫同調聲子頻譜學實驗，我們精確地測量了雙層與三層的二硫化鉬的所有呼吸模態。下一步，我們將線性彈簧連接模型，引入基板的凡德瓦力耦合效應與次近鄰交互作用，以計算二硫化鉬介面之間的凡德瓦力強度。最後，我們進一步考慮了二維材料的強共價鍵結與介面間的弱凡德瓦力的彈性串聯，以提供更加精確的修正模型。在該研究中我們發現，隨著二維堆疊的層數的增加，介面的凡德瓦作用力也隨之提升。藉由這項技術，我們提供了模型中所有彈性耦合強度的定量分析。 由於在半導體材料中，聲學聲子是熱的主要傳導媒介，因此聲學特性不只可以反映凡德瓦介面的品質，它更決定了二維電子元件與光電元件運作時不可避免的熱傳遞的行為。在第五章節，我們探討由二硫化鉬與氮化鎵基板形成的第二型異質接面，該特殊的異質結構在近年因為其在寬頻光偵測與光伏應用而受到重點關注。我們在載子動力學訊號中發現了布里淵震盪，這證實了該異質接面受到飛秒雷射的激發，往基板的面外方向發出了聲學脈衝。藉由具壓電效應的量子阱的幫助，我們在時域上量測了該介面產生的兆赫聲波的完整波形，其形似一個非對稱的雙極波形。為了解釋該音波的形成機制，我們建立了一系列的理論，並藉由擬合實驗數據與理論模擬，我們不只瞭解了該凡德瓦異質接面的聲學響應之特性，更提供了如電子-聲子耦合與電荷轉移等關鍵參數。我們相信下一代基於二維材料的元件開發將會受惠於本研究成果。

Acoustic phenomena are ubiquitous and show various applications highlighted by its frequency and medium. Benefited by the invention of the ultrafast laser, the picosecond ultrasonics has been developed nearly forty years. The technology of manipulating the terahertz phonon has helped people to unravel numerous physical mysteries of novel materials, such as 2D materials. When the first isolation of graphene was achieved, the uniqueness of the 2D system was brought into the spotlight, and since then, the research on 2D materials has experienced a vigorous upsurge. Here, we have used picosecond ultrasonics to investigate layered 2D molybdenum disulfide (MoS2), one of the most famous transition metal dichalcogenides. The aim of this thesis is to reveal some of the critical parameters and to discover the potential of converting the optical excitation into acoustic energy. The thesis starts with an overview of the experimental details of picosecond ultrasonics and reviews the important findings of THz acoustics and carrier dynamics for the 2D materials in the last decade. The 2D van der Waals interlayer coupling is considered crucial in determining the discrepancy for the properties of the 2D-based device. Our results demonstrated that all layer breathing modes can be excited and monitored by THz coherent phonon spectroscopy, and thus the interlayer vdWs elastic constants could be deduced using the linear chain model considering the substrate effect and the next nearest neighbor effect. Furthermore, we obtained a more accurate quantification for the actual vdWs bonding by considering the interplay of the strong intralayer covalent bonding as a correction term. We conclude that the vdWs coupling becomes stronger as the layer number increases, and we provide all the elastic coupling strengths included in the models. The acoustical behavior of the 2D materials not only reflects the quality of the vdWs interface, but it also determines the heat transfer pathway, which is also critical to the performance of the electronics and optoelectronics. In the following chapter, we studied a type-II heterojunction formed by MoS2 and GaN substrate, which stands out for the broadband photo-detection and photovoltaic applications. We successfully generated acoustic waves in the out-of-plane direction into the substrate, as evidenced by the Brillouin oscillation. In addition, we temporally retrieved the waveform of the THz acoustic wave generated by the vdWs heterojunction, and an asymmetric bipolar acoustic strain wave was observed. A theory explaining the generation and coupling of the strain wave was given. Our results provided a clear picture that also helped to determine critical parameters such as electron-phonon coupling and charge transfer time. We believed that this work would eventually facilitate the design of the next generation of 2D-based devices.