Single-photon emitters in monolayer WSe2 are created at the nanoscale gap between two single-crystalline gold nanorods. The atomically thin semiconductor conforms to the metal nanostructure and is bent at the position of the gap. The induced strain leads to the formation of a localized potential well inside the gap. Single-photon emitters are localized there with a precision better than 140 nm.

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton-phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe, WSe, WS, and MoS under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS. For MoS monolayers, the line width increases. These effects are due to a modified exciton-phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton-phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.

Due to their unique band structure, single layers of transition metal dichalcogenides are promising for new atomic-scale physics and devices. It has been shown that the band structure and the excitonic transitions can be tuned by straining the material. Recently, the discovery of single-photon emission from localized excitons has put monolayer WSe 2 in the spotlight. The localized light emitters might be related to local strain potentials in the monolayer. Here, we measure strain-dependent energy shifts for the A, B, C, and D excitons for uniaxial tensile strain up to 1.4% in monolayer WSe 2 by performing absorption measurements. A gauge factor of -54 , meV % -50 , meV % +17 , meV % and -22 meV % is derived for the A, B, C, and D exciton, respectively. These values are in good agreement with ab initio GW-BSE calculations. Furthermore, we examine the spatial strain distribution in the WSe 2 monolayer at different applied strain levels. We find that the size of the monolayer is crucial for an efficient transfer of strain from the substrate to the monolayer. RECEIVED

Excitons dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). Besides optically accessible bright exciton states, TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we show that efficient exciton-phonon scattering couples bright and dark states and gives rise to an asymmetric excitonic line shape. The observed asymmetry can be traced back to phonon-induced sidebands that are accompanied by a polaron redshift. We present a joint theory-experiment study investigating the microscopic origin of these sidebands in different TMD materials taking into account intra- and intervalley scattering channels opened by optical and acoustic phonons. The gained insights contribute to a better understanding of the optical fingerprint of these technologically promising nanomaterials.

Atomically thin transition metal dichalcogenides (TMDCs) are an emerging class of two-dimensional semiconductors. Recently, first opto-electronic devices featuring photodetection as well as electroluminescence have been demonstrated using monolayer TMDCs as active material. However, the light-matter coupling for atomically thin TMDCs is limited by their small absorption length and low photoluminescence quantum yield. Here, we significantly increase the light-matter interaction in monolayer tungsten disulphide (WS 2 ) by coupling the atomically thin semiconductor to a plasmonic nanoantenna. Due to the plasmon resonance of the nanoantenna, strongly enhanced optical near-fields are generated within the WS 2 monolayer. We observe an increase in well as the photoluminescence spectrum are modified by the longitudinal plasmon resonance. The robust hybrid nanoantenna-monolayer system lights the way to efficient photodetectors, solar cells, light emitting and conceptually new valleytronic devices based on two-dimensional materials. Note added: During the review process we became aware of a related study 33 of Ag nanodisc arrays fabricated by electron-beam lithography on monolayer MoS 2 . ASSOCIATED CONTENT Supporting Information. Photoluminescence enhancement of monolayer WS 2 with both excitation and emission polarization matched to the nanoantenna; Absorption spectrum of a WS 2 monolayer; Numerically calculated scattering spectra of the antenna-WS 2 hybrid as a function of the distance between antenna and WS 2 ; Photoluminescence of the WS 2 monolayer without nanoantenna; Photoluminescence intensity depending on excitation power; Details of the simulation technique. This material is available free of charge via the Internet at

This is an author produced version of a paper subsequently published in 2D Mater. Tonndorf et al, Single-photon emitters in GaSe (2017) 4 021010, https://doi.org/10. 1088/2053-1583/aa525b eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request.

Large spin–orbit coupling in combination with circular dichroism allows access to spin-polarized and valley-polarized states in a controlled way in transition metal dichalcogenides. The promising application in spin-valleytronics devices requires a thorough understanding of intervalley coupling mechanisms, which determine the lifetime of spin and valley polarizations. Here we present a joint theory–experiment study shedding light on the Dexter-like intervalley coupling. We reveal that this mechanism couples A and B excitonic states in different valleys, giving rise to an efficient intervalley transfer of coherent exciton populations. We demonstrate that the valley polarization vanishes and is even inverted for A excitons, when the B exciton is resonantly excited and vice versa. Our theoretical findings are supported by energy-resolved and valley-resolved pump-probe experiments and also provide an explanation for the recently measured up-conversion in photoluminescence. The gained insights might help to develop strategies to overcome the intrinsic limit for spin and valley polarizations.

Here, we propose a method to determine the thickness of the most common transition metal dichalcogenides (TMDCs) placed on the surface of transparent stamps, used for the deterministic placement of two-dimensional materials, by analyzing the red, green and blue channels of transmission-mode optical microscopy images of the samples. In particular, the blue channel transmittance shows a large and monotonic thickness dependence, making it a very convenient probe of the flake thickness. The method proved to be robust given the small flake-to-flake variation and the insensitivity to doping changes of MoS2. We also tested the method for MoSe2, WS2 and WSe2. These results provide a reference guide to identify the number of layers of this family of materials on transparent substrates only using optical microscopy. MANUSCRIPT TEXT.Since the isolation of graphene in 2004[1], the mechanical exfoliation method (also called Scotch tape method) has established itself as one of the most used techniques to produce 2D materials [2][3][4][5]. Its facile implementation combined with the high quality of the produced samples are most likely the reasons behind the success of this technique. Mechanical exfoliation, nonetheless, yields randomly distributed flakes of various thicknesses and sizes on the surface of the substrate. This limitation has been overcome to a great extent through the development of rapid methods to find and identify thin flakes based on the observation of the apparent color when they are transferred onto a SiO2/Si surface [6][7][8][9][10][11][12]. On this substrate, there is a dependency of the apparent color of the flake with its thickness due to thin-film interference effects and many