A new mechanism for the thermal desorption of molecular hydrogen from the monohydride phase on Si(100) has been identified. The unusual first-order desorption kinetics that are observed are due to the irreversible excitation of a hydrogen adatom into a delocalized, two-dimensional band state on the surface with an activation energy of 47 kcal/mol. The desorption reaction occurs between this excited hydrogen adatom and a second, localized hydrogen adatom.

The kinetics of the thermal recombinative desorption of hydrogen from the monohydride phase on the Si(100) surface has been studied by laser-induced thermal desorption (LITD). A rate law that is first order in the atomic hydrogen coverage with an activation energy of 45 kcal/mol gives an accurate fit to the data over a temperature range of 685–790 K and a coverage range of 0.006 to 1.0 monolayer. A new mechanism is proposed to explain these surprising results, namely, that the rate limiting step of the reaction is the promotion of a hydrogen atom from a localized bonding site to a delocalized band state. The delocalized atom then reacts with a localized atom to produce molecular hydrogen which desorbs. Evidence to support these conclusions comes from isotopic mixing experiments. Studies of recombinative desorption from other surfaces of silicon, which had been assumed to obey second-order kinetics, are discussed in the light of these results.

Nanoscale DNA surfaces are manipulated by coating ultraflat gold with short thiolated single-stranded (ss) and double-stranded (ds) DNA. Hybridization and 6-mercapto-1-hexanol (MCH) are used to modulate the thickness of the layers, all the changes being monitored using atomic force microscopy (AFM) in situ. Short thiolated ssDNA forms irregular 3.5 nm thick coatings, which are converted into more uniform 6.0 nm thick layers upon hybridization with complementary DNA. Holes formed in a monolayer of short thiolated dsDNA can be partially filled with a longer length thiolated ssDNA, which then protrudes from the surface after hybridization with a complementary strand. MCH is used to modulate the depth of a ssDNA layer and improve hybridization to it.

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