Excitons in emerging materials: atomically thin transition metal dichalcogenides and hybrid perovskite

Abstract
The talk will focus on the electronic properties of the excitons in new emerging materials as atomically thin transition metals dichalcogenides and solid-state perovskite investigated by magneto optics.

Reducing dimensionality of the dichalcogens from 3D to 2D leads to new interesting optical properties e.g. opening a direct gap in the visible range. I will discuss the optical properties of excitons in mono and tri – layer transitions metal dichalcogenides (TMDC). I will demonstrate a novel approach to neutralize the intrinsic defects of CVD-grown TMDCs, using flake transfer tools routinely employed in the fabrication of van-der-Waals heterostructures. We investigate the optical properties of trilayer stacks composed of external CVD-grown MoS2 flakes as capping layers and an internal CVD-grown MoSe2 flake which has a smaller band gap. Remarkably, this fabrication approach strongly suppresses the localized exciton emission in MoSe2 yielding a low temperature PL comparable to that observed in mechanically exfoliated samples. This striking result can be understood from density functional theory, which suggests that the more reactive MoS2 donates chalcogen atoms to heal vacancy defects in MoSe2. Our results pave the way for the production of large area high quality TMDCs. Furthermore, the investigation of the charge transfer between the MoS2/MoSe2 layers allows us to demonstrate a novel way to introduce the valley polarization in MoSe2. Tuning the excitation laser to the A-exciton resonance of the larger band gap MoS2 leads to a considerable charge transfer towards lower band gap MoSe2. Our results show that spin of the hole is conserved during charge transfer leading to non-zero steady state valley polarization in MoSe2, which has previously never been observed under non-resonant excitation. The conservation of spin during the charge transfer opens new possibilities for spintronics and spin injection [1]. Furthermore, I will discuss the electronic properties of the interlayer exciton in particular towards long lived valley polarization [2]

In the second part of the talk I will discuss the results of the measurement of the Exciton Binding Energy and effective masses for Charge carriers in Organic-Inorganic or fully inorganic Tri-halide Perovskites. I will show that for all the family of these materials, binding energy of the exciton is smaller or comparable with the thermal energy at 300K, explaining the excellent performance of the devices. Finally, I will demonstrate that morphology has negligible influence on the electronic properties of organic inorganic hybrid perovskite. [3]

[1] 2D materials,4, 025016 (2017), Nano Letters 17 2979 (2017)
[2] Nano Letters 17 6360 (2017)
[3] Nature Physics 11, 582 (2015), Energy Environ. Sci. 9, 962 (2016), Energy Environ. Sci. 10, 1358 (2017), J. Phys. Chem. Lett 8, 1851 (2017), ACS Energy Letters 2, 1621 (2017)