二維材料堆疊異質結構之製備與應用

Abstract

二維材料領域自成功製得石墨烯起便蓬勃發展，近年來除研究單種二維材料之性質及應用外，由二維材料層狀堆疊而成的原子級薄異質結構亦極受關注。藉由不同二維材料疊層以自由調變光電性質，可提供探索新現象及開發新元件。 我們發展了自上而下的轉印堆疊技術，將從塊材上機械剝離而得的原子級薄二維晶體，重新堆疊組裝成異質結構。此技術以黏彈性高分子膜作為轉印支持層，利用黏彈性體於表面處能量耗散之速率相依性，控制高分子膜上二維晶體之附著與脫附，取代多數文獻中以有機溶劑去除高分子支持層的製程。除了乾式製程之優點外，藉此技術我們可由隨機雜亂分布於基板上的機械剝離之晶體中，選擇性地挑出欲進行後續製程的晶體，將其轉至支持層後，轉印至另一乾淨或經前處理之基板。 於第四章中，我們以少數層石墨烯為例，可將原子層薄的二維晶體從基板上轉至支持層，而後轉印至基板上選定之位置。而後我們更進一步，先於基板上作出石墨烯電極，再將硒化銦與二硫化鉬晶體自支持層轉印至其上，製作成場效電晶體並量測其電性。在第五章中，我們選用兩二維半導體作為異質堆疊的原料，以二硫化鉬及二硒化鎢製備垂直異質結構。異質結構製得後，層間存在交互作用與否相當關鍵，我們以拉曼和光致螢光光譜探討其層間耦合對振動模態與電子結構之影響。在第六章裡，我們應用此異質結構於電化學催化。由於二硫化鉬及二硒化鎢垂直堆疊之異質接面形成第二型能帶排列，當層間交互作用足夠強時，於雷射激發下激態電荷進行層間電荷轉移，可增進二硫化鉬在產氫反應的催化效果。於光沉積實驗中，由金原子沉積量的多寡，可得知相同時間內異質接面處催化了較多的金離子還原於其上，相較於二硫化鉬展現了更高的催化能力。

The research on two-dimensional materials rises since the successful isolation and identification of graphene. Recently, van der Waals heterostructures have emerged as a new field that draws extensive interest. The richness of two-dimensional material library provides us with a large possibility for electronic properties manipulation to design new classes of devices with novel properties and functionalities. We developed a top-down transfer method for fabricating vertical heterostructures based on distinct atomically thin crystals from mechanical exfoliation. With the assistance of an elastomeric stamp, we are able to pick up a particular flake from randomly distributed ones on the substrate, or drop it down to a chosen location on another substrate. The controlled transfer is monitored under optical microscope and is achieved by the viscoelastic nature of the stamp. In Chapter 4, we demonstrate the kinetic control of flake transfer, and print a semiconducting flake on pre-patterned few-layered graphene electrodes to fabricate a field effect transistor. In Chapter 5, we further use this technique to stack a MoS2 flake and a WSe2 flake on one another, forming a vertical heterojunction. In such a van der Waals heterostructure, interlayer interaction is the prime issue. In order to ensure that the as-fabricated device is a heterostructure but not two additive bilayers, we investigate the structural and electronic evolution before and after thermal annealing. In Chapter 6, as we verify the existence of interlayer coupling in the vertical heterojunction, we wonder if the type-II band alignment enhances the catalytic ability in hydrogen evolution reaction. We compare the performance of basal MoS2 and a MoS2/WSe2 heterostructure, with similar thickness of MoS2. The MoS2/WSe2 heterostructure shows higher current density under laser illumination, indicating that photoexcited electrons transfer and accumulate in MoS2 layer and participate in the hydrogen reduction. In the photodeposition of Au particles experiment, the photoexcited electrons transfer to the top layer MoS2, and then reduce the platinum cations to Au particles at the heterojunction. The apparent difference of deposited Au particles in amount between heterojunction and other places also indicates the better catalytic activity.