Transport Dynamics in Ordered Bilayer Assemblies of the n-Alkanes on Pt(111)
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- 作者：Adeana R. Bishop ; Michael J. Hostetler ; Gregory S. Girolami ; and Ralph G. Nuzzo
- 刊名：Journal of the American Chemical Society
- 出版年：1998
- 出版时间：April 15, 1998
- 年：1998
- 卷：120
- 期：14
- 页码：3305 - 3315
- 全文大小：268K
- 年卷期：v.120,no.14(April 15, 1998)
- ISSN：1520-5126
文摘
The energetics of self-diffusion within ordered bilayer assembliesof linear hydrocarbons on Pt(111)(layer-to-layer) have been characterized using isothermal molecularbeam-surface scattering in conjunctionwith temperature programmed desorption (TPD) and reflection-absorptioninfrared (RAIR) spectroscopies.The bilayers are prepared by layering a perdeuteratedn-alkane on top of a perprotio n-alkane (or viceversa).The exchange of molecules between the two layers is weaklyactivated, less so than is either desorption of themultilayer from the substrate (the monolayer is more strongly bound) orthe various phase transitions whichlead to the loss of two-dimensional order in a correspondingdensely-packed monolayer of the n-alkanes.Theexchange process is further characterized by substantial size-asymmetryand isotope substitution effects whichresult in a preference for the selective retention of the longer and(for identical chain lengths) the protiohydrocarbon at the surface regardless of the initial deposition order.Layer-to-layer exchange occurs by adisplacive mechanism which follows simple mass action principles:increasing the coverage of the post-absorbed species increases the extent of exchange. The differencein the activation energy for desorption(from the bilayer) and for exchange is ~1.5 kcal/mol for both aC8 and C10 perdeuterated n-alkanedisplacingan adsorbed (protio) chain of equal length. Thus, although theactivation energy for self-diffusion increaseswith chain length, it is always less than the activation energy forsublimation by a constant amount. Theimplications of these results for energy dissipation mechanisms andrelaxation dynamics in organic thin filmsare discussed and analogies to the properties of the so-calledplastic-crystalline state are developed.