Chemical Exfoliation of Tin Monosulfides for Thin Film FETs on Flexible Substrates

Low crystal symmetry and structural in-plane anisotropic orthorhombic group IV_A-VI metal monochalcogenides (MMCs) have recently emerged as novel electronic materials. [1-3] They have a chemical formula, MX (M=Si, Ge, Sn and X=S/Se), with uniquely distorted layered structures. Among them, earth-abundant tin (II) monosulfides (SnS) show rich fundamental physics and anisotropic optical, electrical and mechanical responses originating from their low crystal symmetry [1]. However, the successful isolation of atomically thin SnS single/ultrathin layers is challenging due to strong interlayer interactions attributed to the lone-pair electrons of sulfur (S), which, in turn, makes it difficult to realize low-cost, printed field-effect transistors (FETs).<br/>Here, we present a top-down liquid-phase exfoliation (LPE) approach to overcome the challenge by synergistically utilizing the thermal and ultrasound energies that induce hydrodynamic force in the solution, giving rise to the systematic isolation of highly crystalline SnS nanosheets [4, 5]. In the LPE method, tin (II) sulfide granular (&gt;99.99%) trace metals were dissolved in acetone in a glass vial sealed with Teflon tape, followed by cavitation using ultrasonication at 50 W and 40 kHz for 20 hours. After the sonication process, the obtained dark brown solution was centrifuged at 8000 rpm for 15 minutes. The isolated SnS nanosheets were characterized using an atomic force microscope (AFM), transmission electron microscope and Raman/PL spectroscopy. The AFM analysis of isolated SnS dimensionality revealed Gaussian distribution in thicknesses. The average sheet thickness of the nanosheets was ~0.90 nm, affirming the isolation of monolayers or bilayers with high crystallinity. The room temperature Raman spectra of SnS nanosheets exhibited four optically active phonon modes consistent with those of ultrathin layers. The phonon modes peaked at ~100, and ~190, ~210 cm^-1 correspond to A_g (1), A_g (2), A_g (3) vibrational modes, respectively, while the peak at ~160 cm^-1 to B_3g orthorhombic mode.<br/>The SnS nanosheets formulate a functional ink that can be subsequently deposited by spin/spray coating onto Si/SiO₂ and PET substrates. This chemical exfoliation technique enables large-scale isolation of electronic grade SnS nanolayers for a wide range of applications, including extended area nanoelectronic devices printed from solution to create field-effect transistors (FETs) and phototransistors for flexible electronic applications. Currently, we are fabricating these FET arrays, which will be included in the presentation at the meeting.<br/><br/><b>References</b><br/>[1] A. S. Sarkar and E. Stratakis, <b><i>Adv. Sci.</i></b>, 2020, 7, 2001655<br/>[2] F. Xia, H. Wang, J. C. M. Hwang, A. H. Castro Neto and Li Yang, <b><i>Nat. Rev. Phys.</i></b>, 2019, 1, 306–317.<br/>[3] X. Li, H. Liu, C. Ke, W. Tang, M. Liu, F. Huang, Y. Wu, Z. Wu and J. Kang, <b><i>Laser Photon. Rev.</i></b><i>,</i> 2021, 15, 2100322<br/>[4] A. S. Sarkar et al., Under revision in <b><i>Adv. Sci.</i></b>, 2022<br/><u>[</u>5] A. S. Sarkar, S. Erickson and E. H. Yang, <b><i>Work in progress</i></b>, 2022