介電環境對於二硒化鎢單層內的激子及其維谷去極化之影響

Abstract

介電環境對於二維材料的特性 - 例如激子結合能、激子半徑以及交換作用強度 - 都有很大的影響力；已經有眾多發表文獻利用介電材料來讓二維材料展現具有應用價值的特性。在這篇論文我們建立受控制的介電環境來研究二維材料中，由於電子-電洞交換作用造成的激子去極化現象。所建立的介電環境透過低溫光致發光 (PL) 光譜來確認除了介電性質以外，材料沒有其他明顯的特性轉變。在不同的介電環境下，我們測量圓偏振極化強度 (極化強度) 來觀察其對於極化強度的影響。為了量化介電強度，我們使用了鏡像電荷法 (相容於 Keldysh 位能) 來估計出環境介電常數 (可用於計算交換作用) 以及總介電常數 (可用於計算激子類氫能階)。我們的極化強度實驗結果顯現出和理論預估不同的介電常數關係，且不能直接被兩種常見的模型：交換作用模型 Maialle-Silva-Sham (MSS) 及動量散射模型解釋。我們在最後提出可能的機制來解釋在實驗設置下觀察到的極化強度結果，以及偏離模型預估的冪次關係。

Dielectric environment plays an important role in determining 2D materials?exciton properties, such as binding energy, exciton radius, and exchange interaction. Numerous reports have realized utilization of dielectrics as a tool to manipulate 2D materials for desired characteristics. Here we use controlled dielectric environment to study the depolarization of exciton in exfoliated WSe2 monolayers due to electron-hole exchange interaction. To verify the uncontrolled variations when modifying dielectric environment, low temperature photoluminescence (PL) spectrum is taken to monitor exciton properties throughout the process. Circular polarization is measured to reveal its relation to dielectric environments. For estimation of effective dielectric constant, we use image charge method, compatible with Keldysh potential, giving both environmental and total dielectric contribution for energy level and exchange interaction estimations. Our result shows unexpected depolarization dependence on dielectric strength that cannot be explained by either momentum scattering or exchange interaction from Maialle-Silva-Sham (MSS) mechanism. We give a possible mechanism for the observed polarization and its deviation from theoretical estimations under practical polarized PL setups.