Anisotropic and Layer-dependent Properties of Chemically grown PdSe2 Single Crystals
Li-Syuan Lu1*, Hui-Yu Cheng1, Guan-Hao Chen1, Chia-Hao Chen2, Tzu-Hung Chuang2, Der-Hsin Wei2, Kuan-Cheng Lu1, Ming-Yang Li3, Chih-Piao Chuu3, Yu-Ming Chang4, Ming-Yen Lu5, Wen-Bin Jian1, Lian-Jong Li3, Wen-Hao Chang1,6
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan
3Corporate Research and Chief Technology Office, Taiwan Semiconductor Manufacturing Company (TSMC), Hsinchu, Taiwan
4Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
5Department of Materials and Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan
6Center for Emergent Functional Matter Science (CEFMS), National Chiao Tung University, Hsinchu, Taiwan
* Presenter:Li-Syuan Lu,
Noble metal dichalcogenides (NMDs), such as PtSe2 and PdSe2, are currently being considered as a new-generation two-dimensional layered semiconductors. Owing to the strong interlayer coupling in NMDs, the strongly layer-dependent band gap provide a possible solution to improve the metal contact resistance. So far, experimental studies of PdSe2 are mostly based on mechanical exfoliations from bulk crystals. Here we demonstrate that single-crystalline PdSe2 flakes can be synthesized by chemical vapor deposition (CVD) at a low growth temperature (400˚C). The chemical stoichiometry of the CVD-grown PdSe2 has been confirmed by x-ray photoemission spectroscopy. High-angle annular-dark-field scanning transmission electron microscopy (HAAD-STEM) analyses reveal that the PdSe2 crystals exhibit puckered pentagonal structure with crystallographic axes rotated by about 45° with respect to the flake edges. Low-frequency Raman measurements reveals a strong layer-dependent breathing vibration modes, which can also be used for identifying the layer numbers [1]. Absorption measurements shows a significant bandgap shrinkage with increasing layer number from bilayer (1.4 eV) to 7 layer (0.9 eV), in consistent with the calculated bandgap based on density functional theory.[2] The CVD-grown PdSe2 have also been fabricated into field-effect transistors, showing high current on-off ratios, sizable carrier mobility and anisotropic transport properties due to the anisotropy in its crystalline structure.

1. A. A. Puretzky, A. D. Oyedele, K. Xiao, A. V. Haglund, B. G. Sumpter, D. Mandrus, D. B. Geohegan and L. Liang, 2D Materials 5 (3), 035016 (2018).
2. A. D. Oyedele, S. Yang, L. Liang, A. A. Puretzky, K. Wang, J. Zhang, P. Yu, P. R. Pudasaini, A. W. Ghosh, Z. Liu, C. M. Rouleau, B. G. Sumpter, M. F. Chisholm, W. Zhou, P. D. Rack, D. B. Geohegan and K. Xiao, J Am Chem Soc 139 (40), 14090-14097 (2017).

Keywords: 2D materials, transition metal dichalcogenides