NSRRC Activity Report 2022

Physics and Materials Science 021 Figure 2(a) shows an SEM image of three selected WS 2 single crystals (triangles) for SHG spatial mapping. The top two triangular crystals are covered by graphene, while the bottom triangular crystal is exposed for Cr doping. Figure 2(b) shows the corresponding SHG images recorded at three different excitation wavelengths. The images showed clear differences at wavelengths of 1260, 1220 and 840 nm as described in the following: (i) For Gr/WS 2 , the SHG image obtained under excitation at 1260 nm exhibits a uniform intensity indicating absence of trion contribution. On the other hand, for Cr:WS 2 under excitation at 1260 nm, SHG is strongly enhanced in the triangular region, and this is a signature of the trion resonance. (ii) In addition, weak exciton resonances are observed at 1220 nm, which show a spatial distribution contrary or opposite to the trions in Cr:WS 2 . However, the exciton resonance intensity is relatively uniform in Gr/WS 2 . (iii) Finally, at 840 nm, the SHG intensity is enhanced by the band-nesting energy resonance, but a uniform intensity in both Cr:WS 2 and Gr/WS 2 is observed. Thus, the trion and exciton signals confirm the spatial presence and absence of Cr doping, respectively, as expected. Figure 3 shows the SPEM and μ-XPS characterizations of pristine and Cr-doped WS 2 carried out at TLS 09A1 at the NSRRC. Figure 3(a) shows the SEM image of a WS 2 single crystal covered with a pre-patterned Fig. 3 : SPEM and μ-XPS characterizations of pristine and Cr-doped WS 2 . (a) SEM image of a WS 2 single crystal covered with a pre-patterned graphene mask. (b) SPEM spatial mapping of W 4f. (c) SPEM spatial mapping of C 1s. (d) μ-XPS spectra of W 4f and S 2p taken from the Gr/WS 2 (circles) and Cr:WS 2 (squares) areas. Cyan and dark yellow areas in the spectra represent the fittings of Gr/WS 2 and Cr:WS 2 , respectively. The inset corresponds to the SPEM chemical mapping image combined from (b) and (c). The scale bars in the figures are all 10 μm. [Reproduced from Ref. 4] sublattice. They are hopeful that this method will meet the demand for both n- and p-channel transistors in future integrated circuits based on such a layered semiconductor. (Reported by Ashish Chainani) This report features the work of Po-Wen Chiu and his collaborators published in Nanoscale 14 , 16968 (2022). TLS 09A1 SPEM • SPEM and μ-XPS • Materials Science, Condensed-matter Physics References 1. A. A. Tedstone, D.J. Lewis, P. O’Brien, Chem. Mater . 28 , 1965 (2016), 2. Z. Qin, L. Loh, J. Wang, X. Xu, Q. Zhang, B. Haas, C. Alva- rez, H. Okuno, J. Z. Yong, T. Schultz, N. Koch, J. Dan, S. J. Pennycook, D. Zeng, M. Bosman, G. Eda, ACS Nano 13 , 10768 (2019). 3. H. Cai, Y. Yu, Y. C. Lin, A. A. Puretzky, D. B. Geohegan, K. Xiao, Nano Res. 14 , 1625 (2021). 4. Y.-C. Lin, Y.-P. Chang, K.-W Chen, T.-T. Lee, B.-J. Hsiao, T.-H. Tsai, Y.-C. Yang, K.-I. Lin, K. Suenaga, C.-H. Chen, P.-W. Chiu, Nanoscale 14 , 16968 (2022). 5. T. H. Tsai, Z. Y. Liang, Y. C. Lin, C. C. Wang, K. I. Lin, K. Suenaga, P. W. Chiu, ACS Nano 14 , 4559 (2020). (a) (b) (c) (d) graphene mask. Figures 3(b) and 3(c) show the SPEM spatial mapping of W 4f and C 1s, respectively. The spatial details of the single crystal and pre-patterned mask as observed in the SEM image are confirmed. Figure 3(d) shows the μ-XPS spectra of the W 4f core level and S 2p core level taken from the Gr/WS 2 (circles) and Cr:WS 2 (squares) areas. In order to distinguish the doping effects of the masked and unmasked regions, peak deconvolution of the two areas was performed for both W 4f and S 2p levels. The peak areas of Gr/WS 2 and Cr:WS 2 are represented in yellow and cyan, respectively. The W 4f and S 2p binding energies exhibit an energy shift of 0.25 eV between the masked and unmasked regions. The red-shift of binding energy in the unmasked area indicates that the Fermi level moves closer toward the valence-band maximum of Cr:WS 2 and confirms a p-type doping effect caused by the substitutional Cr atoms. 4 Thus, the authors demonstrated an area-selective doping scheme for single-layer TMD materials by using a pre-patterned graphene mask, so that WS 2 could be substitutionally doped with Cr in unmasked areas. The authors believe the core mechanism of this doping scheme lies in the controlled sublimation of chalcogen via a devised CVD process, during which dopant impurities can be embedded and substituted into the transition-metal