Dipolar excitons and interlayer coupling: Optics, dynamics and transport in Janus TMD heterostructures

原子薄的过渡金属二硫族化合物（TMDs）垂直堆叠成范德华异质结构，可以实现大量的物理现象和技术应用。这两种材料之间的界面决定了涌现特性，这些特性可以通过选择材料、扭曲和电场来调整。改变这些界面行为的一种激进方法是用化学方法修改其组成层。tmd单层由两层硫层之间的过渡金属层组成。因此，通过用另一层取代其中一层硫，可以生长出“双面”TMD，例如MoSe2变成semo。在Janus TMD异质结构（Janus HSs）中，固有的偶极子有望极大地改变它们的界面行为，调制层间距离、波函数杂化和扭曲系统中的摩尔势。内置偶极子的另一个结果是激子-声子相互作用被认为得到了增强。因此，我们期望Janus hs能够促进极高效的声子驱动层间激子的形成，最大限度地减少辐射损失。此外，我们预计在Janus hs中有一个非常有趣的激子输运：一方面，增强的moirise势和增加的激子-声子散射可能会阻碍激子的传播，但另一方面，固有的偶极子肯定会导致有效的激子-激子排斥，从而产生更快的激子输运。目前，人们对Janus hs知之甚少，只有少量的实验和理论研究。在这个提案中，我们将进行联合实验理论合作，将最先进的CVD生长与复杂的光学表征和微观多粒子理论结合起来，揭示Janus hs中激子光学，动力学和输运的有趣现象。特别是，我们将(i)探索堆叠顺序、材料选择、扭曲角、温度和电场对Janus hs层内和层间激子光学指纹的作用，（ii）揭示Janus hs中激子的形成、弛豫和衰变，其中长辐射寿命、增强声子散射和固有电偶极子将是至关重要的。(3)探索激子输运，并确定如何通过调整Janus hs中的堆叠顺序和扭转角来调节涡流势以捕获或释放激子。我们设想提供对Janus TMD异质结构中激子光学，动力学和输运的全面微观理解，揭示外部可调性和控制这些多粒子现象的途径，这些现象也与基于Janus TMD材料的光电应用高度相关。

The vertical stacking of the atomically thin transition metal dichalcogenides (TMDs) into van der Waals heterostructures allows the realization of a plethora of physical phenomena and technological applications. The interface between these two materials dictates the emergent properties, which can be tuned through choice of material, twisting and electric fields. A radical approach to alter the behaviour of these interfaces is to chemically modify the constituent layers. TMDs monolayers consist of a transition metal layer between two chalcogen layers. Therefore, by substituting one of these chalcogen layers with a different one, it is possible to grow a "Janus" TMD, as for example MoSe2 becomes SeMoS. The inherent dipole in Janus TMDs is expected to drastically alter their interfacial behaviour in Janus-based TMD heterostructures (Janus HSs), modulating the interlayer distance, the wave function hybridization, and the moiré potential in twisted systems. Another consequence of the built in-dipole is that the exciton-phonon interaction is believed to be enhanced. Therefore, we expect that Janus HSs could facilitate extremely efficient phonon-driven interlayer exciton formation, minimizing radiative losses. Furthermore, we expect a highly interesting exciton transport in Janus HSs: On one side the enhanced moiré potentials and the increased exciton-phonon scattering are likely to hinder the propagation of excitons, but on the other side the inherent dipole will certainly lead to an efficient exciton-exciton repulsion giving rise to a faster exciton transport. Currently, Janus HSs are poorly understood, with only a handful of experimental and theoretical studies. In this proposal we will undertake a joint experiment-theory collaboration combining state-of-the-art CVD growth with sophisticated optical characterization and microscopic many-particle theory to reveal intriguing phenomena in exciton optics, dynamics and transport in Janus HSs. In particular, we will (i) explore the role stacking order, choice of material, twist-angle, temperature and electric field have on the optical fingerprint of intra- and interlayer excitons in Janus HSs, (ii) reveal the exciton formation, relaxation and decay in Janus HSs, where the long radiative lifetime, enhanced phonon-scattering and inherent electric dipole will be of crucial importance, and (iii) explore the exciton transport and determine how the moiré potential can be modulated to either trap or release excitons through tuning the stacking order and twist angle in Janus HSs. We envision to offer a comprehensive microscopic understanding of exciton optics, dynamics and transport in Janus-based TMD heterostructures revealing pathways towards external tunability and control of these many-particle phenomena that are also highly relevant for optoelectronic applications based on Janus TMD materials.