單層二維材料與量子點異質結構之載子動力學研究

Probing the carrier transfer dynamics in low dimensional heterostructure: monolayer 2D materials and quantum dots

李榮展(Jung-Chan Lee)

指導教授 ： 呂宥蓉

國立臺灣大學/理學院/物理學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202104160

摘要

層狀過渡金屬二硫族化物(TMDC) 材料是近年來發現具有優異光電性質的半導體之一，當被剝離至單層時，其能帶結構會由不直接能隙轉變為直接能隙，使得發光效率明顯提升。同時，由於較弱的介電屏蔽效應(dielectric screening)，導致電子與電洞具有較強的庫侖作用力，使得單層TMDC材料擁有很大的激子束縛能，因此能在室溫中穩定發光。但不幸的是，它的螢光量子產率(PLQY)很低，使其不易用來作為發光元件的實際應用。因此本論文嘗試藉由形成異質結構，利用兩材料的接面會因能帶結構不同，而需能帶對齊，藉此調整載子傳遞機制，進而改善二維材料之發光效率。
在這項工作中，我們在單層TMDC 材料(WS2)上旋塗鈣鈦礦量子點(CsPbI3、CsPbBr3)，以形成垂直堆疊的異質結構。由於全無機鹵化鉛鈣鈦礦膠體量子點具有高吸收截面和極高的螢光量子產率(PLQY)，因此可以透過載子轉移的機制來增強TMDC的光致發光強度。為了比較不同的能帶對齊形式，我們製備了 CsPbI3 / WS2 和 CsPbBr3 / WS2的異質結構，分別對應於電荷轉移和能量轉移機制。為了研究它們的載流子轉移機制，我們使用光致發光光譜和時間解析光譜技術來分析穩態激子的特性、隨溫度變化的能隙改變、激子與聲子間的相互作用，以及生命期等等。有趣的是，我們發現這兩種異質結構皆可在低溫下增強 WS2的光致發光強度。透過異質結構的形成，我們發現CsPbI3 / WS2在低溫與高溫的發光強度上，可以增強超過100倍，CsPbBr3 / WS2亦有增強的情形，符合我們使用高吸收材料來改善另一種發光材料發光效率的目的，期望未來能應用在光電元件、感測器、發光元件的設計上。此外，在研究的過程中，我們亦使用激子間轉換的三能階模型來解釋壓力對單層WS2 發光特性的改變，以利我們對於單層TMDC材料有更深入的了解。

關鍵字

鈣鈦礦量子點 ； 過渡金屬二硫族化物 ； 異質結構 ； 變溫螢光光譜 ； 光致發光光譜 ； 時間解析螢光光譜 ； 載子轉移 ； 電荷轉移 ； 能量轉移

並列摘要

Transition metal dichalcogenide (TMDC) 2D materials are semiconductors with excellent optoelectronic properties discovered in recent years. Unfortunately, the low photoluminescence quantum yield (PLQY) of monolayer TMDC has become a challenge in access to optoelectronic devices. Thus, finding a way to improve the PLQY of monolayer TMDC is essential.
In this work, we placed perovskite quantum dots (CsPbI3, and CsPbBr3), all-inorganic colloidal quantum dots that have high absorption cross-section, and ultrahigh PLQY, on a monolayer TMDC (WS2) flake to study the carrier transfer dynamics. The idea is to enhance TMDC photoluminescence through a carrier transfer mechanism between the quantum dots (QDs) and monolayer TMDC. To compare different energy band alignment models, we prepared CsPbI3 / WS2 and CsPbBr3 / WS2, corresponding to charge transfer and energy transfer model. We performed temperature-dependent micro-photoluminescence (μ-PL) and time-resolved PL (TRPL) spectroscopy to investigate their carrier transfer dynamics. For example, we studied the characteristics of excitons in the steady-state, temperature dependency of energy bandgap, emission peaks, and photon lifetime. We further discuss the relation between excitons, biexcitons, trions, and defect states in the monolayer WS2. In particular, we found that both types of band alignment in the heterostructure can enhance the PL emission of WS2 at low temperatures. We observed a 109-fold PL emission enhancement from the WS2 in CsPbI3 QDs / WS2 heterostructure at 8 K compared with the one measured at room temperature.
In contrast, we only obtained a weak PL enhancement from WS2 in the case of CsPbBr3 QDs / WS2 heterostructure, which is in agreement with our proposed model. As a result, we successfully prove the concept by demonstrating that using a high absorption material could improve the optical properties of another luminescent material. In addition, we use a three-level model of exciton species conversion to explain the pressure effect in the luminescence of monolayer WS2. These results could provide physical insights that are valuable in the design and development of hybrid dimensional 2D optoelectronic devices.

並列關鍵字

perovskite quantum dots ； TMDC ； heterostructure ； temperature-dependent ； photoluminescence (PL) ； time-resolved photoluminescence (TRPL) ； carrier transfer ； charge transfer ； energy transfer

參考文獻

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