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The rates of mechanochemical reactions are generally found to increase exponentially with applied stress. However, a buckling theory analysis of the effect of a normal stress on an adsorbate that is oriented perpendicularly to the surface that reacts by tilting suggests that a critical value of the stress should be required to initiate a mechanochemical reaction. This concept is verified by using density functional theory calculations to simulate the effect of compressing a homologous series of alkyl thiolate species on copper by a hydrogen-terminated copper counter-face. This predicts that a critical stress is indeed needed to initiate methyl thiolate decomposition, which has a perpendicular C–CH 3 bond. In contrast, no critical stress is found for ethyl thiolate with an almost horizontal C–CH 3 bond, while a critical stress is required to isomerize propyl thiolate from a trans to a cis configuration. These predictions are tested by measuring the mechanochemical reaction rates of these alkyl thiolates on a Cu(100) substrate by sliding an atomic force microscope tip over the surface and finding a critical stress of ∼0.43 GPa for methyl thiolate, ∼0.33 GPa for propyl thiolate, but no evidence of a critical stress for ethyl thiolate, in accord with the predictions. These results provide insights not only into mechanochemical reaction mechanisms on surfaces, but also on the origin of critical phenomena in stress-induced processes in general. It also suggests novel approaches to designing robust surface films that can resist wear and damage.
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Comparison of 2D crystals formed by dissociative adsorption of fluorinated and nonfluorinated alkyl iodides on Cu(111)
We report the comparison of a series of 2D molecular crystals formed from the intermediates of the dehalogenation reaction of iodoethane versus various fluorinated iodoalkanes on Cu(111). High-resolution scanning tunneling microscopy enables us to distinguish the alkyl groups from the iodine atoms, and we find that the ethyl groups and iodine atoms formed from the dissociation of ethyl iodide are well mixed. However, fluorination of the alkyl tail changes this behavior and leads to local segregation of the two species on the surface. We postulate that the low-polarizability and relatively large dipole moment of the fluorinated species drive the ordered assemblies of the fluorinated alkyl species on the surface and discuss this in the context of how solvophobicity can drive the clustering of fluorinated groups and, hence, phase separation.
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- Award ID(s):
- 2034911
- PAR ID:
- 10483405
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films
- Volume:
- 39
- Issue:
- 6
- ISSN:
- 0734-2101
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1116/6.0001316
