Surface imaging and adsorption mechanisms

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Science  23 Jan 1998:
Vol. 279, Issue 5350, pp. 453
DOI: 10.1126/science.279.5350.453d

Gas-phase molecules can adsorb onto surfaces and form strong bonds with the surface atoms; this chemisorption process can be a multistep process and can require particular surface sites. Two reports show how the scanning tunneling microscope can reveal insights into these processes (see the commentary by King, p. 503). More weakly bound precursor states to the chemisorbed state have been invoked in many surface science studies to explain several observations, such as the apparent ability of a molecule to remain on the surface as it moves about before finding an appropriate adsorption site. These extrinsic precursors, which are blocked from surface sites by previously adsorbed molecules, have been directly observed in earlier studies. Brown et al. (p. 542) provide direct evidence for the formation of intrinsic precursors, which form on bare surface sites. The molecules can rest in the same surface sites as the chemisorbed molecules in a weakly bound state, and they can be switched from one state to the other by changes in temperature or with an electric field. Thin film growth by chemical vapor deposition requires reactive sites near one another on the surface. Semiconductors such as silicon are usually terminated with hydrogen atoms; heating desorbs some H2 and leaves reactive “dangling bonds,” but these same elevated temperature conditions can cause the dangling bonds to diffuse away from each other. McEllistrem et al. (p. 545) studied the Si(100)-2×1 deuterium-terminated surface at elevated temperatures with a scanning tunneling microscope and show that while the dangling bonds do diffuse, they appear to be attracted to one another and recombine. This process produces a greater population of reactive pairs of dangling bonds.

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