Fabrication and Surface Engineering of Two-Dimensional SnS Toward Piezoelectric Nanogenerator Application
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.404
Fabrication and Surface Engineering of TwoDimensional SnS Toward Piezoelectric Nanogenerator Application Naoki Higashitarumizu1, Hayami Kawamoto1, Keiji Ueno2 and Kosuke Nagashio1,3 1
Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan 2
Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 ShimoOkubo, Sakura, Saitama 338-8570, Japan 3
PRESTO, Japan Science and Technology Agency (JST), Tokyo 113-8656, Japan
ABSTRACT Mechanical exfoliation is performed to fabricate ultrathin SnS layers, and chemical/thermal stability of SnS layers is discussed in comparison with GeS, toward piezoelectric nanogenerator application. Both SnS and GeS are difficult to be exfoliated under 10 nm using tape exfoliation due to strong interlayer ionic bonding by lone pair electrons in Sn or Ge atoms. Au-mediated exfoliation enables to fabricate larger-scale ultrathin SnS and GeS layers thinner than 10 nm owing to strong semi-covalent bonding between Au and S atoms, but GeS surface immediately degrades during Au etching in an oxidative KI/I2 solution. Although the surface of SnS after the Au-mediated exfoliation reveals several-nm oxide layer of SnOx, the surface morphology retains the flatness unlike the case of GeS. The SnS layers are more robust than GeS against the thermal annealing as well as the chemical treatment, suggesting that SnOx works as a passivation layer for SnS. Self-passivated SnS monolayer can be obtained by a controlled post-oxidation.
INTRODUCTION Two-dimensional tin sulfide (SnS), one of the group-IV monochalcogenides (MXs; M=Sn/Ge and X=S/Se), has recently attracted interests in the application to flexible piezoelectric generator, because it is theoretically predicted that SnS has a large piezoelectric coefficient d11=145 pm/V that is comparable to conventional lead zirconate titanate (PZT, d33~300 pm/V) [1]. For MXs, the piezoelectricity exists only in the oddnumber layers, since the broken inversion symmetry disappears in the even-number layers. The piezoelectric coefficient increases with decreasing the layer number, and is
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maximized in the monolayer [2]. However, the fabrication of monolayer SnS has only been achieved by liquid phase exfoliation with smaller lateral size than 500 nm [3]. Although relatively large SnS layers can be fabricated by mechanical exfoliation and thin film growth methods, SnS layers thinner than 5.5 nm have not been realized yet [4]. This is probably due to the strong interlayer interaction by lone pair electrons in Sn atoms [5]. Figure 1(a) shows the relationship between the activation energy barrier for O2 molecule to chemisorb on the monolayer MXs or black phosphorus (BP) [6], an
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