Dynamics of Photoexcitations in SnS₂ Single Crystals

Tin disulfide (SnS₂) is a layered material that is environmentally stable, composed of earth-abundant materials, has a moderate bandgap of ~2.3 eV and high absorption in the blue-green spectral range, making it attractive for optoelectronic and solar energy applications. We have previously shown that photoexcited carriers in single crystalline SnS₂ and in vertical nanoflakes have a high mobility of ~ 300 cm²/Vs and lifetimes in excess of 1 ns [1,2]. Additionally, above band gap excitation of single crystalline SnS₂ at normal incidence results in emission of THz radiation, a hallmark of a second order nonlinearity. This observation is surprising due to inversion symmetry being present in unexcited SnS₂ even in a monolayer limit, and we hypothesize that photoexcitation dynamically breaks the inversion symmetry by excitation of an in-plane phonon mode. Here, we use time-resolved THz spectroscopy, THz emission spectroscopy and transient optical absorption to uncover the complete picture of the ultrafast electronic responses following photexcitation. Our experimental results provide important checkpoints for theory of nonlinear effects in SnS₂ and other 2D dichalcogenide materials.<br/><br/>[1] Binod Giri, Maryam Masroor, Tao Yan, Kateryna Kushnir, Alexander D Carl, Curtis Doiron, Haochuan Zhang, Yanyan Zhao, Arthur McClelland, Geoffrey A Tompsett, Dunwei Wang, Ronald L Grimm, Lyubov V Titova, Pratap M Rao, Balancing Light Absorption and Charge Transport in Vertical SnS2 Nanoflake Photoanodes with Stepped Layers and Large Intrinsic Mobility, Adv. Energy Materials 9, 1901236 (2019)<br/>[2] K. Kushnir, E. Morissette, B. Giri, C.W. Doiron, R.L. Grimm, P.M. Rao and L.V. Titova, "Carrier Dynamics in SnS2 Single Crystals and Vertical Nanostructures: Role of Edges," 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Nagoya, Japan, 2018, pp. 1-2, doi: 10.1109/IRMMW-THz.2018.8509909.