Incorporation of oxygen atoms as a mechanism for photoluminescence enhancement of chemically treated MoS2.

Chemical treatments to enhance photoluminescence (PL) in MoS2 have been explored extensively by experimental means in recent years. However, satisfactory theoretical explanations of the underlying mechanisms remain elusive. In this work, the surface reactions of the superacid bis(trifluoromethane)-sulfonimide (TFSI), hydrogen peroxide (H2O2), molecular oxygen (O2), and sulfuric acid (H2SO4) on a defective MoS2 monolayer have been studied using first principles calculations. An oxygen transfer reaction into a sulfur vacancy with a low activation barrier and thus significant reaction rates already at room temperature has been found. Band structure unfolding techniques show that the incorporation of oxygen atoms into sulfur vacancies restores the band structure of pristine MoS2, which is predicted to have a high PL quantum yield. PL spectroscopy is used to examine the effect of chemical treatment on PL intensity. Our experimental findings support our theoretical predictions, as PL in MoS2 is enhanced by up to a factor 20 after treatment with H2O2 or H2SO4, while the spectral shape is only slightly altered.

Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T'-ReSe2.

Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe2 regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.

Supercontinuum second harmonic generation spectroscopy of atomically thin semiconductors.

Two-dimensional semiconductors have recently emerged as promising materials for novel optoelectronic devices. In particular, they exhibit favorable nonlinear optical properties. Potential applications include broadband and ultrafast light sources, optical signal processing, and generation of nonclassical light states. The prototypical nonlinear process second harmonic generation (SHG) is a powerful tool to gain insight into nanoscale materials because of its dependence on crystal symmetry. Material resonances also play an important role in the nonlinear response. Notably, excitonic resonances critically determine the magnitude and spectral dependence of the nonlinear susceptibility. We perform ultrabroadband SHG spectroscopy of atomically thin semiconductors by using few-cycle femtosecond infrared laser pulses. The spectrum of the second harmonic depends on the investigated material, MoS2 or WS2, and also on the spectral and temporal shape of the fundamental laser pulses used for excitation. Here, we present a method to remove the influence of the laser by normalization with the flat SHG response of thin hexagonal boron nitride crystals. Moreover, we exploit the distinct angle dependence of the second harmonic signal to suppress two-photon photoluminescence from the semiconductor monolayers. Our experimental technique provides the calibrated frequency-dependent nonlinear susceptibility χ(2)(ω) of atomically thin materials. It allows for the identification of the prominent A and B exciton resonances, as well as excited exciton states.

Interlayer excitons in bilayer MoS2 under uniaxial tensile strain.

Atomically thin semiconducting transition metal dichalcogenides (TMDCs) have unique mechanical and optical properties. They are extremely flexible and exhibit a strong optical absorption at their excitonic resonances. Excitons in TMDC monolayers are strongly influenced by mechanical strain. Their energy shifts and even their line widths change. In bilayers, intralayer excitons with electrons and holes residing in the same layer also shift their energy with the applied strain. Recently, interlayer excitons with electrons and holes in different layers have been observed in bilayer MoS2 at room temperature. Here, we report on the behavior of interlayer excitons in bilayer MoS2 under uniaxial tensile strain of up to 1.6%. By recording the differential transmission spectra for different strain values, we derive a gauge factor of -47 meV per % for the energy shift of the interlayer exciton, which is similar to -49 meV per % for the intralayer A and B excitons. Our finding confirms the origin of the interlayer exciton at the K point in the Brillouin zone, with the electron located in one layer and the hole delocalized over two layers. Furthermore, our work paves the way for future straintronic devices based on interlayer excitons.

Nanoscale Positioning of Single-Photon Emitters in Atomically Thin WSe2.

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.