Nonlocal dielectric function and nested dark excitons in MoS2

Abstract

Their exceptional optical properties are a driving force for the persistent interest in atomically thin transition metal dichalcogenides such as MoS 2. The optical response is dominated by excitons. Apart from the bright excitons, which directly couple to light, it has been realized that dark excitons, where photon absorption or emission is inhibited by the spin state or momentum mismatch, are decisive for many optical properties. However, in particular the momentum dependence is difficult to assess experimentally and often remains elusive or is investigated by indirect means. Here we study the momentum dependent electronic structure experimentally and theoretically. We use angle-resolved photoemission as a one-particle probe of the occupied valence band structure and electron energy loss spectroscopy as a two-particle probe of electronic transitions across the gap to benchmark a single-particle model of the dielectric function ϵðq; ωÞ against momentum dependent experimental measurements. This ansatz captures key aspects of the data surprisingly well. In particular, the energy region where substantial nesting occurs, which is at the origin of the strong light-matter interaction of thin transition metal dichalcogenides and crucial for the prominent C-exciton, is described well and spans a more complex exciton landscape than previously anticipated. Its local maxima in ðq ≠ 0; ωÞ space can be considered as dark excitons and might be relevant for higher order optical processes. Our study may lead to a more complete understanding of the optical properties of atomically thin transition metal dichalcogenides.

Synthesis and Photoluminescence Properties of MoS2/Graphene Heterostructure by Liquid-Phase Exfoliation

“… 14 Unlike graphene, MoS 2 is a direct-band-gap semiconductor with tunable electronic properties as a function of layer thickness and size. 8 , 15 , 16 MoS 2 shows potential applications in photocatalysis, energy storage, bioimaging, and biosensor applications. 4 , 10 , 17 − 19 Lateral and vertical heterostructures of graphene and TMDCs have attracted much interest due to their improved performance and stability in various fields ranging from energy conversion to biomedical applications.…”

“…Specifically, the inherent properties of two-dimensional (2D) materials are altered with a decrease in layer numbers to a single layer or a few layers, making them promising candidates in various fields of science. ,− Moreover, these 2D materials show enhanced biosensing and catalytic applications owing to their large surface area. , In particular, graphene is a widely known 2D material, which has become the research focus due to its excellent mechanical, thermal, electrical, and optical properties and its high specific surface area since its discovery in 2004. − It has shown potential applications in biosensors, batteries, photovoltaics, and supercapacitor applications . Unlike graphene, MoS 2 is a direct-band-gap semiconductor with tunable electronic properties as a function of layer thickness and size. ,, MoS 2 shows potential applications in photocatalysis, energy storage, bioimaging, and biosensor applications. ,,− Lateral and vertical heterostructures of graphene and TMDCs have attracted much interest due to their improved performance and stability in various fields ranging from energy conversion to biomedical applications. − The characteristic, high-quality heterointerface strongly affects the built-in electric field and hence the charge transfer and dissociation of excitons. This results in the manipulation of the optical and electronic properties of 2D materials with improved performance toward specific applications in catalysis, sensing, and optoelectronic devices. − …”

Synthesis and Photoluminescence Properties of MoS2/Graphene Hetero-Structure by Liquid Phase Exfoliation

“…10 Unlike graphene, MoS 2 is a direct band-gap semi-conductor with tunable electronic properties as a function of layer thickness and size. 8,11,12 MoS 2 shows potential applications in photo-catalysis, energy storage, bio-imaging and biosensor applications. 4,9,13,14 Lateral and vertical hetero-structures of graphene and TMDC's attracted much interest due to their improved performance and stability in various fields ranging from energy conversion to biomedical applications.…”

Inherently high valley polarizations of momentum-forbidden dark excitons in transition-metal dichalcogenide monolayers

“…In spite of violating the momentum selection rules, those MFDXs in TMD-MLs have drawn massive attention recently because of their essential involvement in various optical and dynamics phenomena. [15][16][17][18][19][20][21][22][23][24][25][26][27] Very recently, direct generation and probe of finitemomentum excitons in WSe 2 -MLs have been realized by using the integrated technology of optical pump-probe and angle-resolved photo-emission spectroscopies [28,29]. Moreover, recent cryogenic photo-luminescence (PL) measurements on high-quality tungstenbased TMD-ML samples have revealed the pronounced optical signatures of the inter-valley long-lived and optically accessible MFDXs are attractive for the quantum applications and realizing excitonic Bose-Einstein condensation (BEC) [31][32][33][34].…”