單層過渡金屬二硫化物激子能態之研究

Temperature Dependent Photoluminescence Spectroscopy of Excitonic States in Transition Metal Dichalcogenide Monolayers

吳威霖(Wei-Lin Wu)

指導教授 ： 果尚志

國立清華大學/理學院/物理學系/碩士(2019年)

摘要

自從科學上發現單一原子層的石墨烯擁有獨特的能帶特徵，科學家們便開始對於二維材料的研究產生極大的興趣。其中過渡金屬二硫化合物(transiton metal dichalcogenide, TMDC)，例如MoS2、MoSe2、WS2以及WSe2等等，因為具有從塊材的非直接能隙半導體轉變成原子級單層厚度時，會轉變成直接能隙半導體的驚奇特性，所以也帶來極大的關注。由於單層材料維度的限制，使得過渡金屬二硫化合物材料擁有非常大束縛能的激子(exciton)產生，束縛能大約0.5 eV左右，以及產生許多特別的準粒子(quasiparticle)效應，然而對於準粒子間存在的交互作用以及形成的基本物理機制仍再深入的調查中。
而本文的實驗主要是探討過渡金屬二硫化合物材料中存在的暗激子(dark exciton)效應，暗激子為光學選擇率(selection rule)所禁止的能態。我們藉由使用間接金原子物理剝離法(gold-mediated exfoliation)，此方法是運用金原子與硫族原子之間能形成強而有力的共價鍵，因此能夠從塊材分離出表面單層的材料，產生大約50-100 μm大小左右的單層過渡金屬二硫化合物，做為本文樣品的製備方式。我們使用改變溫度的光激發螢光光譜(temperature-dependent photoluminescence)，探討暗激子對於低溫光激發螢光光譜發光效率的影響，並且證實鉬(Mo)系列與鎢(W)系列的過渡金屬二硫化合物具有不同的暗激子效應。這些使得我們對於材料的能帶結構以及多體效應(many-body effects)有了更深入的探索。

關鍵字

二維材料 ； 過渡金屬二硫屬化合物 ； 光激發螢光光譜 ； 暗激子能態

並列摘要

Atomically thin transition metal dichalcogenide (TMDC) semiconductors such as MoS2, MoSe2, WS2, WSe2, owning to the monolayer limit, have extraordinarily large exciton binding energies and exhibit many interesting excitonic effects. Here, we use temperature-dependent photoluminescence (PL) spectroscopy to investigate the existence of optically forbidden dark exciton states. We show that the intensity of low-temperature PL can be affected by the presence of dark states in TMDC monolayers. However, the dark state is lower than the optically bright state only 17 meV in the case WSe2 monolayer and only 26 meV in WS2 monolayer. Furthermore, we confirm that Mo-series and W-series TMDC monolayers have different temperature-dependent behavior. These temperature dependences allow us to have an in-depth exploration on the many-body effects of monolayer TMDC materials.

並列關鍵字

2D material ； Transition metal dichalcogenide (TMDC) ； Photoluminescence (PL) spectroscopy ； Dark exciton states

參考文獻

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