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Published February 7, 2019 | Supplemental Material
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Probing Surface Chemistry at an Atomic Level; Decomposition of 1-Propanethiol on GaP(001)(2×4) Investigated by STM, XPS, and DFT


The adsorption and decomposition mechanisms for 1-propanethiol on a Ga-rich GaP(001) (2 × 4) surface are investigated at an atomic level using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Using a combination of experimental and theoretical tools, we probe the detailed structures and energetics of a series of reaction intermediates in the thermal decomposition pathway from 130 to 773 K. At 130 K, the propanethiolate adsorbates are observed at the edge gallium sites, with the thiolate–Ga bonding configuration maintained up to 473 K. Further decomposition produces two new surface features, Ga–S–Ga and P-propyl species at 573 K. Finally, S-induced (1 × 1) and (2 × 1) reconstructions are observed at 673–773 K, which are reportedly associated with arrays of surface Ga–S–Ga bonds and subsurface diffusion of S. To understand the observed site-selectivity on the hydrogen dissociation of the thiol molecule at 130 K, the two most likely dissociation pathways (Ga–P vs Ga–Ga dimer sites) are investigated using DFT Gibbs energy calculations. While the theory predicts the kinetic advantage for the dissociation reaction occurring on the Ga–P dimer (Lewis acid–base combination), we only observed dissociation products on the Ga–Ga dimer (Lewis acid). The DFT calculations clarify that the reversible thiolate diffusion along the Ga dimer row prevents recombinative desorption, which is probable on the Ga–P dimer. Together with experimental and theoretical results, we suggest a thermal decomposition mechanism for the thiol molecule with atomic-level structural details.

Additional Information

© 2019 American Chemical Society. Received: November 13, 2018; Revised: December 27, 2018; Published: January 4, 2019. This work was supported by Department of Energy. S.J. thanks Kwanjeong Educational Foundation for support. XPS measurement was carried out in the Molecular Materials Research Center of the Beckman Institute of Caltech. S.J. thanks Liangbo Liang at CNMS, Oak Ridge National Laboratory for sharing the STM simulation code that he developed. M.K. and H.K. acknowledges the support by the Global Frontier R&D Program (2013M3A6B1078884) and the Creative Materials Discovery Program (grant 2017M3D1A1039378) granted through the National Research Foundation of Korea (NRF). Author Contributions: S.J. and M.K. contributed equally. STM and XPS experiments were carried out by S.J. DFT calculations were carried out by M.K., H.K., and S.J. STM simulations were conducted by P.D. and S.J. The manuscript was written through contributions of all authors. The authors declare no competing financial interest.

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August 19, 2023
October 19, 2023