The Impact of Localized Hot Spots on the Raman Response of Monolayer MoS2

Abstract

Abstract Body: Transition-metal dichalcogenides (TMDs) have attracted a lot of interest owing to their direct bandgap as they are reduced down to thickness of a monolayer (ML) [1]. Thus, they have potential for numerous applications in electronic and optoelectronic devices such as transistors, photodetection and sensing [2]. MoS2 is probably one of most investigated TMD material due to the ability to tune its band structure with applied strain or electric field [3, 4]. As a naturally occurring crystalline mineral, tuning the properties of MoS2 as a representative TMD material provides further insights into the effects of strain and doping on its properties before moving on to investigate rare and more complex TMD materials. The strain and doping in MoS2 can be modulated in various ways such as mechanical bending [5], use of different substrates [6] or by introducing a metal- MoS2 interface [7]. A metal-semiconductor interface provides an interesting platform to investigate the plasmon-exciton interactions and the charge transfer mechanism between the metal and the semiconductor. In this work, we create plasmonic hot spots between gold nanoparticles to modify the Raman and luminescence response of ML MoS2. Nominal 2, 4 and 7 nm of Au and 2nm of Al were evaporated on SiO2/Si (oxidized silicon). Au and Al do not wet well SiO2, resulting in the formation of nanoparticles. Au and Al nanoparticle films exhibit a plasmonic band centered around 550 nm and 300 nm respectively. Plasmonic hot spots are created upon excitation within the band of plasmonic response of the nanoparticles. Monolayers of MoS2 were mechanically exfoliated directly on the nanostructured metallic layers and on SiO2/Si as a reference sample. Raman spectroscopy and photoluminescence were performed to characterize the optical response of the MoS2 to the plasmonic hot spots and compared to the reference sample. By varying the thickness of the Au film from 2nm to 7nm, we observe a shift and splitting of the E12g Raman modes into E12g and E12g’. The A1g mode also red shifts. The shift and splitting are only observed when the plasmonic resonance is excited during the optical measurements, indicating a plasmonic hot spot effect. We also investigate the effect on the photoluminescence (PL). In comparison to the reference ML MoS2, the PL intensity from the ML MoS2 on Au is drastically reduced. As the thickness of the Au film increases, the PL emission from A and B excitonic peaks is quenched until the B-exciton is no longer seen for ML MoS2 on a 7nm Au film. This can be explained by the electron transfer from MoS2 to Au, which hinders the recombination of electron−hole pairs created by the photoexcitation. As the work function of MoS2 is lower than that of Au, the electrons in the excited state of MoS2 transfer to Au, leaving a hole behind, which causes p-doping in MoS2. The charge transfer phenomena, along with the strain effects on optoelectronic properties of MoS2 MLs will be presented and dicussed. This work shows the potential of combining plasmonics with 2D materials to modify their optical and phonon response. [1] Choi, W. et al. Mater. Today 20, 116–130 (2017). [2] Lorenz, T., Joswig, J.-O. & Seifert, G. 2D Mater. 1, 011007 (2014). [3] Castellanos-Gomez, A. et. Al. Nano Lett. 13, 5361–5366 (2013). [4] Chakraborty, B. et al. Phys. Rev. B 85, 161403 (2012). [5] Conley, H. J. et al. Nano Lett. 13, 3626–3630 (2013). [6] Sun, Y., Wang, R. & Liu, K. Appl. Phys. Rev. 4, 011301 (2017). [7] Moe, Y. A., Sun, Y., Ye, H., Liu, K. & Wang, R. ACS Appl. Mater. Interfaces 10, 40246–40254 (2018). Acknowledgement: This work is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 765075 (Project LIMQUET). Funding from Swiss National Science Foundation (project 196948) is also acknowledged.