Fig. 2. Distribution of SnO_x in untreated and purified polycrystalline SnSe samples obtained by TOF–SIMS.

The surface of both SPS-processed specimens was sputtered to generate SnOH⁺ complex that is a relevant quantity to tin-bound oxygen. Accordingly, the SnOH⁺ map clearly represents the distribution of surface SnO_x on SnSe samples. a, The SnOH⁺ image for the untreated SnSe sample. b, The SnOH⁺ image for the purified SnSe sample. The red spots correspond to SnO_x. The white dotted lines indicate GBs, which were defined with optical images taken on the corresponding regions. Scale bars are 10 μm. c, The concentration of SnO_x across the GB by a line profile (yellow solid line in a) for the untreated SnSe sample. d, The concentration of SnO_x across the GB by a line profile (yellow solid line in b) for the purified SnSe sample. The width of a line profile is 3 μm, in which the concentrations of SnO_x were averaged. The substantial decrease in surface SnO_x is clearly observed by our purification process.

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