Exciton States in Monolayer MoSe2 and MoTe2 Probed by Upconversion Spectroscopy
Exciton States in Monolayer MoSe2 and MoTe2 Probed by Upconversion Spectroscopy
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DOI：
10.1103/physrevx.8.031073
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发表时间：
2018-09-18
影响因子：
12.5
通讯作者：
Urbaszek, B.
中科院分区：
文献类型：
--
作者：
Han, B.;Robert, C.;Urbaszek, B.
过渡金属二硫属化物（TMD）是单层（ML）极限的直接带隙半导体，具有迷人的光学和自旋谷特性。对于单个ML，高达20%的强光学吸收由激子控制，激子是由库仑吸引力束缚的电子-空穴对。MoSe 2和MoTe 2单分子膜中的激发激子态由于其较低的振子强度和较强的非均匀加宽而至今难以获得。在这里，我们表明，封装在六方氮化硼的结果在A：1 s激子的发射线宽低于1.5毫电子伏和3毫电子伏，在我们的MoSe 2和MoTe 2单层样品，分别。这使我们能够调查两个单层材料的光致发光上转换光谱的激发激子状态。激发激光被调谐到与A：1 s跃迁的共振，我们观察到激发激子态的发射高达200毫电子伏以上的激光能量。我们证明了这种非线性光学过程的效率的偏置控制。我们讨论了上转换效应的起源。我们的模型计算表明一个激子-激子（俄歇）散射机制特定的TMD ML涉及一个激发的导带，从而产生高能量激子与小波矢量。通过白色光反射率、光致发光激发和共振拉曼散射进一步研究了光学跃迁，证实了它们起源于单层薄半导体中的激发激子态。
Transitions metal dichalcogenides (TMDs) are direct gap semiconductors in the monolayer (ML) limit with fascinating optical and spin-valley properties. The strong optical absorption of up to 20% for a single ML is governed by excitons, electron-hole pairs bound by Coulomb attraction. Excited exciton states in MoSe2 and MoTe2 monolayers have so far been elusive because of their low oscillator strength and strong inhomogeneous broadening. Here, we show that encapsulation in hexagonal boron nitride results in an emission line width of the A:1s exciton below 1.5 meV and 3 meV in our MoSe2 and MoTe2 monolayer samples, respectively. This allows us to investigate the excited exciton states by photoluminescence upconversion spectroscopy for both monolayer materials. The excitation laser is tuned into resonance with the A:1s transition, and we observe emission of excited exciton states up to 200 meV above the laser energy. We demonstrate bias control of the efficiency of this nonlinear optical process. We discuss the origin of the upconversion effect. Our model calculations suggest an exciton-exciton (Auger) scattering mechanism specific to TMD MLs involving an excited conduction band, thus generating high-energy excitons with small wave vectors. The optical transitions are further investigated by white light reflectivity, photoluminescence excitation, and resonant Raman scattering, confirming their origin as excited excitonic states in monolayer thin semiconductors.