少數層二維過渡金屬二硒化鉬超快載子動力學

Abstract

本論文主要為利用飛秒時間解析激發-探測技術，研究過渡金屬二硫化物之二硒化鉬薄膜的超快動力學性質。我們知道過渡金屬二硫化物同層電子間的強平面共價鍵和層間作用力弱的凡德瓦力耦合，使得二硒化鉬具有層狀結構。另一個廣為人知的特性是二維過渡金屬二硫化物在多層與塊狀結構下時具有間接能隙，而當材料為單層結構時，則轉變為直接能隙半導體。其因奈米尺寸造成強量子侷限效應的獨特性質，導致緊密束縛的激子形成，使得過渡金屬二硫化物產生極大的結合能。在本論文研究中，我們利用化學氣相沉積的方法生長少數層二硒化鉬，在A激態的超快載子動力演化過程，瞬態穿透訊號結果顯示，當在激子躍遷吸收峰附近，其時間零點呈現正訊號，而當開始遠離吸收峰時，初始訊號轉變為負訊號。隨著時間延遲我們觀察到光激發載子在0.7 ps內快速鬆弛，這可能是由於通過載子-聲子散射或者缺陷捕獲而導致的載子冷卻，並且將訊號由負（正）翻轉至正（負）信號，訊號在20-30 ps內逐漸放鬆，並非單純的指數衰減達到平衡。這種二次吸收我們認為是由於化學氣相沉積過程中，所產生的相關缺陷，誘導探測光吸收。另外，透過探測光波長相依，我們由文獻中分析，相鄰的兩子帶可以互相交換動量，造成子帶間散射增加，誘導探測光吸收，可能是造成訊號翻轉的原因。

Ultrafast dynamic properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) molybdenum diselenide (MoSe2) films were investigated using femtosecond pump-and-probe technique. Strong in-plane covalent bonding and weak van der Waals coupling between TMDC layers enables MoSe2 to have layered structure. Bulk and multilayer MoSe2 is known to have an indirect bandgap, while monolayer MoSe2 is a direct bandgap semiconductor. Its unique property of strong quantum confinement leads to the formation of tightly bound excitons with extremely large binding energy for atomically thin TMDCs. In this work, we have studied the ultrafast dynamic evolution of A-excitons in multilayer (2–4 layers) MoSe2 grown by chemical vapor deposition (CVD). The transient transmission shows the initial negative signals around time zero for both far below and above the A-exciton absorption edge, whereas it shows the positive signals near the exciton transition peak. The photoexcited carriers relax quickly within 0.7 ps, which can be attributed to either carrier cooling via carrier-phonon scattering or defect capturing. The fast relaxing negative (positive) signals change its sign to positive (negative) instead of simple exponential decay to equilibrium and slowly relax within 20 – 30 ps. This secondary absorption may be defect-induced absorption related to the CVD deposition process. The band-broadening due to carrier collision in closely neighboring A and B excitons may be responsible for the sign flipping of initial photo-induced absorption.